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In the modern ocean, U reduction and incorporation into anoxic sediments imparts a large isotopic fractionation of approximately +0.6‰ that shifts the seawater δ238U value (238U/235U, expressed as δ238U per mil deviation relative to CRM-112a) relative to continental runoff. Given the long residence time of U in the modern oceans (∼400 kyr), the isotopic composition of carbonates (taken as a proxy for seawater) reflects the global balance between anoxic and other sinks. The U isotopic composition of open-marine carbonates has thus emerged as a proxy for reconstructing past changes in the redox state of the global ocean. A tenet of this approach is that the δ 238U values of seawater and anoxic sediments should always be fractionated by the same amount. In order to test this hypothesis, we have measured the U concentrations and isotopic compositions of carbonates spanning ages from 3250 Ma to present. A first-order expectation for the Archean and possibly Proterozoic is that near-quantitative U removal to extensive anoxic sediments should have shifted the uranium isotopic composition of seawater and carbonates towards lower values. Instead, the measurements reveal that many Archean and Proterozoic carbonates have unfractionated δ238U values similar to those of continents and riverine runoff. These results are inconsistent with the view that the U isotopic composition of seawater simply reflects the areal extent of anoxic sediments in the past. We consider two plausible explanations for why the U isotopic composition of Archean and Proterozoic carbonates is not fractionated from the crustal and riverine composition: (1) the residence time of U could have been much shorter in the Precambrian oceans when anoxic settings were much more extensive, and (2) the process of incorporation of U into anoxic sediments in the Precambrian imparted a smaller U isotopic fractionation than in the modern because of differences in the efficiency or mechanism of uranium removal. This study highlights the challenges inherent to applying knowledge of the modern marine U isotopic cycle to periods of Earth’s history when ocean-floor anoxia was much more extended, anoxic basins were ferruginous, and atmospheric oxygen content was significantly lower than present.

The isotopic composition of Phanerozoic marine sediments provides important information about changes in seawater chemistry. In particular, the radiogenic strontium isotope (87Sr/86Sr) system is a powerful tool for constraining plate tectonic processes and their influence on atmospheric CO2 concentrations. However, the 87Sr/86Sr isotope ratio of seawater is not sensitive to temporal changes in the marine strontium (Sr) output flux, which is primarily controlled by the burial of calcium carbonate (CaCO3) at the ocean floor. The Sr budget of the Phanerozoic ocean, including the associated changes in the amount of CaCO3 burial, is therefore only poorly constrained. Here, we present the first stable isotope record of Sr for Phanerozoic skeletal carbonates, and by inference for Phanerozoic seawater (δ88/86Srsw), which we find to be sensitive to imbalances in the Sr input and output fluxes. This δ88/86Srsw record varies from ∼0.25‰ to ∼0.60‰ (vs. SRM987) with a mean of ∼0.37‰. The fractionation factor between modern seawater and skeletal calcite Δ88/86Srcc-sw, based on the analysis of 13 modern brachiopods (mean δ88/86Sr of 0.176±0.016‰, 2 standard deviations (s.d.)), is -0.21‰ and was found to be independent of species, water temperature, and habitat location. Overall, the Phanerozoic δ88/86Srsw record is positively correlated with the Ca isotope record (δ44/40Casw), but not with the radiogenic Sr isotope record ((87Sr/86Sr)sw). A new numerical modeling approach, which considers both δ88/86Srsw and (87Sr/86Sr)sw, yields improved estimates for Phanerozoic fluxes and concentrations for seawater Sr. The oceanic net carbonate flux of Sr (F(Sr)carb) varied between an output of -4.7x1010mol/Myr and an input of +2.3x1010mol/Myr with a mean of -1.6x1010mol/Myr. On time scales in excess of 100Myrs the F(Sr)carb is proposed to have been controlled by the relative importance of calcium carbonate precipitates during the “aragonite” and “calcite” sea episodes. On time scales less than 20Myrs the F(Sr)carb seems to be controlled by variable combinations of carbonate burial rate, shelf carbonate weathering and recrystallization, ocean acidification, and ocean anoxia. In particular, the Permian/Triassic transition is marked by a prominent positive δ88/86Srsw-peak that reflects a significantly enhanced burial flux of Sr and carbonate, likely driven by bacterial sulfate reduction (BSR) and the related alkalinity production in deeper anoxic waters. We also argue that the residence time of Sr in the Phanerozoic ocean ranged from ∼1Myrs to ∼20Myrs.

We collected samples in lakes Superior, Erie and Huron, including Georgian Bay, in mid-summer during the stratified period to determine the relative importance of production, respiration and atmospheric exchange to carbon dynamics in the lakes. In all lakes sampled, oxygen saturation averaged 107 ± 2%, significantly greater than the atmospheric equilibrium, and the isotopic composition of dissolved oxygen, δ18ODO, averaged 22.8 ± 0.1‰, significantly less than the isotopic composition of dissolved atmospheric oxygen, suggesting that biological production was greater than respiration in the epilimnion during the summer months. While lakes worldwide are commonly oversaturated in carbon dioxide, average pCO2 in the epilimnion of the lake studied was 342 ± 42 μatm and the isotopic composition of dissolved inorganic carbon, δ13CDIC, was 0.2 ± 0.2‰, both values not significantly different from expected atmospheric CO2 equilibration (n = 65, p = 0.89, n = 64, p = 0.81). This trend is atypical; approximately ninety percent of lakes globally are oversaturated in carbon dioxide. The atmospheric equilibrium observed in the Laurentian Great Lakes and other studies suggest that pCO2 in large lakes is depleted relative to smaller lakes. This is also the case for rivers that are sourced from or flow though large lakes and they can be expected to have lower pCO2 downstream from the lakes and lower per unit area carbon fluxes than other rivers. An improved understanding of the global carbon cycle mandates that this be taken into account when considering CO2 fluxes from aquatic ecosystems.

Rivers that drain the tectonically-active, high-relief islands of the Indo-Pacific Archipelago contribute disproportionately large amounts of water, solutes, organic matter and sediments to the oceans. The magnitude of these fluxes is principally related to prodigious amounts of precipitation and the steep, mountainous terrain of the islands that together cause vast amounts of water to move rapidly through many relatively small watersheds. In this study, rainfall and streamflow data and a comprehensive set of geochemical and stable isotope data collected from the Fly River (the largest river of New Guinea in terms of annual flow) are interpreted to constrain the magnitude of carbon fluxes from the watershed and the principal sources of carbon to the rivers. Results indicate that the export of dissolved carbon from the Fly River watershed is primarily dependent on rainfall variability related to the reception of moisture from the northern and southern hemispheres at different times of the year. Specifically, rainfall amounts (and hence the rate of carbon flux via rivers) were higher during the Austral summer when the Inter Tropical Convergence Zone is situated directly over New Guinea and lower during the period of moisture transport by trade-winds from the western Pacific Ocean. Carbon isotope data suggests that the primary source of inorganic carbon in the Fly River watershed is the dissolution of carbonate minerals by acidity related to biogenic soil CO 2 although considerable seasonality in δ 13 C values was attributed to biological processes that occur within the rivers themselves (i.e. the relative amounts of photosynthesis and respiration in the aquatic system at different times of the year). Photosynthesis, for instance, appears to dominate in the Austral summer whereas respiration tends to dominate in the winter.

The Paleoproterozoic interval is characterized by the first evidence for incontrovertibly free oxygen in the ocean/atmosphere system. In this study, carbonates from the Transvaal Group, the Silverton, Lucknow, Rooinekke and particularly the Duitschland Formations, were analyzed for isotopes of carbon, oxygen and strontium in order to constrain models dealing with the patterns of seawater evolution. Although the trace element data showed variable alteration of the carbonate samples, selected samples with a lesser degree of alteration yielded interpretable results. The “best” measured 87Sr/86Sr isotope value of 0.7031 at 2.35 Ga ago fits well with the published curve for Precambrian seawater confirming that Paleoproterozoic budget of Sr was dominated by input from continental sources. During the Paleoproterozoic times, carbonates with δ13C values of about +10‰ are known to have been present worldwide and they have been studied also during this research. The observation that in the Duitschland Formation such high isotope values and “normal marine” values were both present within a single formation, indeed within its single beds, suggested that the large spread of δ13C values reflects local or regional rather than global phenomena. The oxygen isotope data show a large spread of values, from −3‰ to −16‰, while their corresponding δ13C signals are almost unchanged over this entire range, with the Lucknow and Silverton Formations clustering around 10‰ and the Rooinekke Formation around −2‰. The Duitschland Formation has a bimodal distribution with both δ13C populations present. The last formation also contains samples with δ18O as low as −20‰, particularly in the vicinity of the Bushveld intrusive body. These highly 18O-depleted samples show homogenization of δ13C values towards +5‰. Modelling suggests that the trends could have resulted from post-depositional alteration by basinal fluids, at near-surface to about 50 °C temperatures, in a rock buffered system. The Duitschland Formation samples, particularly at Duitschland Farm, the locality closest to the Bushveld Complex, were then further altered at temperatures up to about 80 °C, in response to this intrusion.

Lakes worldwide are commonly oversaturated with CO 2 , however the source of this CO 2 oversaturation is not well understood. To examine the magnitude of the C flux to the atmosphere and determine if an excess of respiration ( R ) over gross primary production (GPP) is sufficient to account for this C flux, metabolic parameters and stable isotopes of dissolved O 2 and C were measured in 23 Quebec lakes. All of the lakes sampled were oversaturated with CO 2 over the sampling period, on average 221 ± 25%. However, little evidence was found to conclude that this CO 2 oversaturation was the result of an excess of pelagic R over GPP. In lakes Croche and a l’Ours, where CO 2 flux, R and GPP were measured weekly, the annual difference between pelagic GPP and R , or net primary production (NPP), was not sufficient to account for the size of the CO 2 flux to the atmosphere. In Lac Croche average annual NPP was 14.4 mg C m −2 d −1 while the average annual flux of CO 2 to the atmosphere was 34 mg C m −2 d −1 . In Lac a l’Ours average annual NPP was −9.1 mg C m −2 d −1 while the average annual flux of CO 2 to the atmosphere was 55 mg C m −2 d −1 . In all of the lakes sampled, O 2 saturation averaged 104.0 ± 1.7% during the ice-free season and the isotopic composition of dissolved O 2 (δ 18 O DO ) was 22.9 ± 0.3‰, lower than atmospheric values and indicative of net autotrophy. Carbon evasion was not a function of R , nor did the isotopic signature of dissolved CO 2 in the lakes present evidence of excess R over GPP. External inputs of C must therefore subsidize the lake to explain the continued CO 2 oversaturation. The isotopic composition of dissolved inorganic C (δ 13 C DIC ) indicates that the CO 2 oversaturation cannot be attributed to in situ aerobic respiration. δ 13 C DIC reveals a source of excess C enriched in 13 C, which may be accounted for by anaerobic sediment respiration or groundwater inputs followed by kinetic isotope fractionation during degassing under open system conditions.

The Ca isotope compositions of 37 late Mesozoic skeletal carbonates, belemnites and brachiopods, from the Tethyan realm were analyzed by thermal (TIMS) and plasma (MC-ICP-MS) ionization mass spectrometry. A poor correlation between δ 44/40 Ca and δ 18 O values of belemnites suggests only a weak temperature dependency for the Ca isotope composition of belemnites, likely less than 0.02‰/°C. The δ 44/40 Ca record of belemnites was therefore used to reconstruct the Ca isotope composition of paleo-seawater ( δ 44/40 Ca SW ), based on an experimentally determined fractionation factor between seawater Ca and belemnite calcite ( α CC–SW ) of ∼ 0.9986. The inferred δ 44/40 Ca SW record, with an average stratigraphic resolution of 1 Ma, shows systematic temporal variation of ∼ 0.5‰ with the Middle/Late Jurassic (∼ 154 Ma) minimum of ∼ 1.4‰ and a subsequent general increase to the Early Cretaceous (∼ 124 Ma) maximum of ∼ 1.9‰. The global nature of the δ 44/40 Ca SW record is supported by identical Ca isotope compositions of coeval (Kimmeridgian) belemnites collected from two distinct paleogeographic regions, the southern (New Zealand) and northern (Germany) margin of the Tethys Ocean. The observed late Mesozoic δ 44/40 Ca SW record was simulated using a simple Ca isotope mass balance model, and the results indicate that the variation in δ 44/40 Ca SW record can be explained by changes in oceanic input fluxes of Ca that were independent of the carbonate ion fluxes, such as the hydrothermal Ca flux or the release of Ca to the oceans via dolomitization of marine carbonates.

A recurrent interpretation of ancient climate based on the oxygen isotopic composition of marine carbonates and cherts suggests that Earth's climate was substantially warmer in the distant past and remained so until as recently as 400 Myr ago. This interpretation is difficult to reconcile with the long-term glacial record, with evidence for modest weathering rates during most of Earth's history, with biomarker and fossil evidence for eukaryotes and even vertebrates at times of anomalously low δ18O values, and with the predicted faintness of the young Sun. We argue here, following earlier suggestions, that the low δ18O values in ancient rocks are a consequence of the low δ18O of ancient seawater. A modest increase in ocean depth with time, together with progressive increases in pelagic sedimentation on midocean ridge flanks since about 550 Ma, could account for the variation in seawater isotopic composition. The required change in ocean depth, coupled with thinning of the oceanic crust, is a natural consequence of the decline in heat flow over time. Contrary to previous assertions, such a model is not inconsistent with data from ophiolites. It seems likely that Earth's climate remained largely within Phanerozoic norms throughout the past 3.5 Ga.

Water mass balances on an isolated embayment (Hoople Bay) in the St. Lawrence River revealed a small stream (Hoople Creek), local groundwater and the St. Lawrence Main Channel as the 3 principal water sources. The latter had an average evaporative isotope signal that was inherited from the Great Lakes ( δ 18 O H 2 O =−7.0‰ ) and an average Cl− content of 0.55 mmol/l. Hoople Creek and Hoople Bay waters were more variable in their isotopic composition and Cl− contents, while local groundwater was assumed to have a homogeneous composition year around. These parameters constituted an equation system that was solved with matrix operations to yield monthly contributions of the 3 endmembers. Influx of groundwater and Hoople Creek dominated the embayment only after higher snowmelt discharges, while the Main Channel contributed more than 50% during the remainder of the year. Preliminary results suggest that potential pollution in the Main Channel would strongly affect Hoople Bay and similar ecosystems along the river. Nevertheless, more detailed data are needed for a better water balance over longer time periods. The 3-component mixing technique serves as a good tool to evaluate seasonal water fluxes and may also become useful in other mass balances.

The fringing reef in Tutum Bay on the west side of Ambitle Island, Papua New Guinea is the only presently known coral reef exposed to the extensive discharge of a hot, mineralized hydrothermal fluid. There, aragonite and ferrihydrite, a hydrous ferric oxide, are the prominent hydrothermal precipitates. Aragonite forms two distinct crystal habits, (a) euhedral (pseudohexagonal) crystals up to 2 cm long and (b) micro-crystals similar in appearance to ‘‘feather dendrite’’. Aragonite encrusts dead coral fragments, volcaniclastic boulders and pebbles, and fills secondary fracture and remaining primary intergranular porosity within volcaniclastic arenite. The hydrothermal aragonite has a distinctively different isotopic composition when compared to ‘‘normal’’ shallow-water marine inorganic and organic carbonate that precipitated from seawater. This difference arises from precipitation at high temperature from a mixture of seawater and hydrothermal fluid that has lower 87 Sr/ 86 Sr and d 18 O values than seawater. Based on a 87 Sr/ 86 Sr mixing model, aragonite precipitated from a hydrothermal fluid–seawater mixture of approximately 9:1. Precipitation from the hydrothermal solution is mainly caused by CO2 degassing, but mixing between hydrothermal fluid and seawater may have enhanced precipitation due to an increase in pH. The d 13 C of Tutum Bay hydrothermal aragonite ranges from 1.9xto 2.2x(VPDB). This range of values is in good agreement with experimental data [J. Phys. Chem. 72 (1968) 800; Geochim. Cosmochim. Acta 61 (1997) 3461], indicating that C-13-equilibirum has been reached during its formation. Values for d 18 O range from 14.2xto 14.7xand calculated isotopic equilibrium temperatures are approximately 20 jC lower than directly measured vent fluids and those temperatures obtained from fluid inclusion experiments and the 87 Sr/ 86 Sr mixing model. This indicates that either oxygen isotope equilibrium was not attained or that the calcite–water fractionation factor for oxygen isotopes is not applicable for the precipitation of Tutum Bay hydrothermal aragonite. Trace element concentrations, except for the REEs, Y and Sr are low. The REE patterns of aragonite are similar to those of Tutum Bay vent water, indicating the hydrothermal origin of the aragonite. Rare earth element concentrations are higher in the coarse than in the fine-grained aragonite, which might be caused by a change in precipitation rate and seawater mixing. D 2004 Elsevier B.V. All rights reserved.

The sulfur cycle plays a key role in the hydrochemical development of lignite and coal mining districts, which are often characterized by acid mine drainage. Identification of sulfur sources and transformations is essential if we are to understand the current status of acid lakes and propose successful remediation strategies. We utilized stable isotope (δ34Ssulfate, δ18Osulfate, δ34Sdissolved sulfide, δ18Owater) and hydrochemical data of lake and groundwater samples as well as the concentrations and isotope ratios of various sedimentary sulfur compounds (acid-soluble sulfate [ASS], acid-volatile sulfur [AVS], chromium-reducible sulfur [CRS]) in aquifer and lake sediments for assessing the biogeochemical sulfur cycle in a Lusatian acidic mining lake (ML111) and the surrounding area. Pyrite oxidation is the dominant source of sulfate and iron in the two aquifers west and east of mining lake ML111. Concentration and isotope data for dissolved sulfate suggest that bacterial (dissimilatory) sulfate reduction became an increasingly important process in the flow direction of the western aquifer, thereby improving water quality. In contrast, bacterial sulfate reduction was only of local importance in the dump aquifer on the eastern side of the lake. Oxygen isotope ratios of the lake water sulfate, and mass and isotope balances demonstrate that ∼80% of the sulfate in mining lake ML111 is derived via groundwater influx from the dump aquifer, with the remainder contributed by the western aquifer. In-lake pyrite oxidation or bacterial sulfate reduction in the water column are insignificant. The latter process is restricted to the monimolimnion, which represents less than 1% of the lake volume. Hence, the potential for natural generation of alkalinity by bacterial (dissimilatory) sulfate reduction is presently low. For successful remediation of ML111, it is essential to reduce significantly the continuous input of sulfate and acidity from inflowing dump groundwater.

The Vindhyan Supergroup of central India records a substantial portion of Proterozoic time and contains some of the most disputed Precambrian/Cambrian fossil discoveries. Despite the significance of these “fossil” finds, many issues, including reliable geochronology, remained unresolved. Similarly, the geochemical aspects of the sedimentary sequences and their implications to the evolution of Proterozoic oceans, have been studied only at a reconnaissance level. Here we report the results of the Pb–Pb dating for the Rohtasgarh Limestone from the Lower Vindhyan Supergroup giving an age of 1601±130 Ma, an estimate that is in accord with the recent U–Pb zircon dating of the Lower Vindhyan Supergroup by Rasmussen et al. [Geology 30 (2002) 103] and Ray et al. [Geology 30 (2002) 131]. Sr isotope stratigraphy of carbonates from the Upper Vindhyan Supergroup suggests a Mid-Neoproterozoic (750–650 Ma) age for these formations. This, in turn, argues for a long hiatus between the deposition of the Lower and Upper Vindhyan Supergroups. Our results conflict with the reports of Cambrian age small shelly fossils and Ediacaran fossils from the Upper Vindhyan rocks. At the same time, they suggest that the alleged trace fossils in the Lower Vindhyan rocks must be of Paleoproterozoic age. The most common δ 18 O values for calcitic components from limestone formations of the Supergroup range from −10 to −7‰, similar to Paleo-Neoproterozoic carbonates worldwide. The modes of δ 13 C in the Lower and Upper Vindhyan Supergroups are 0±2 and 3±2‰, respectively, again consistent with the worldwide dataset. The secular patterns of carbon isotope trends do not support the earlier assumptions that the carbonate sequences at the southern margin correlate with those at the western or northern margins of the Vindhyan Basin. C and Sr isotope data provide new calibration points for the seawater evolutionary curves, particularly for the Paleoproterozoic. For strontium isotopes, the data from the Lower Vindhyan Supergroup suggest that seawater departed probably less rapidly from mantle values than was previously believed.

The 87Sr/86Sr values based on brachiopods and conodonts define a nearly continuous record for the Late Permian and Triassic intervals. Minor gaps in measurements exist only for the uppermost Brahmanian, lower part of the Upper Olenekian, and Middle Norian, and only sparse data are available for the Late Permian. These 219 measurements include 67 brachiopods and 114 conodont samples from the Tethyan realm as well as 37 brachiopods and one conodont sample from the mid-European Middle Triassic Muschelkalk Sea. The Late Permian/Lower Triassic interval is characterized by a steep 1.3 × 10−3 rise, from 0.7070 at the base of the Dzhulfian to 0.7082 in the late Olenekian, a rate of change comparable to that in the Cenozoic. In the mid-Triassic (Anisian and Ladinian), the isotope values fall to 0.7075, followed again by a rise to 0.7081 in the Middle/Late Norian. The 87Sr/86Sr values decline again in the Late Norian (Sevatian) and Rhaetian to 0.7076. The sharp rise in the 87Sr/86Sr values during the Late Permian/Early Triassic was coincident with widespread clastic sedimentation. Because of the paucity of tectonic uplifts, the enhanced erosion may have been due to intermittent humid phases, during mainly an arid interval, coupled with the absence of a dense protective land plant cover following the mass extinction during the latest Permian. The apex of the 87Sr/86Sr curve at the Olenekian/Anisian boundary coincides with cessation of the large-scale clastic sedimentation and also marks the final recovery of land vegetation, as indicated by the renewed onset of coal formation in the Middle Triassic. The rising 87Sr/86Sr values from the Middle Carnian to the Late Norian coincide with the uplift and erosion of the Cimmeride-Indosinian orogens marking the closure of the Palaeotethys. The subsequent Rhaetian decline that continues into Jurassic (Pliensbachian/Toarcian boundary), on the other hand, coincides with the opening of the Vardar Ocean and its eastern continuation in the Izmir-Ankara Ophiolitic Belt. Samples from the Upper Muschelkalk are more radiogenic than the global trend. This may reflect separation of the basin from the open ocean. Due to strong meteoric influx from a large land mass in the north, the Germanic Basin became increasing brackish up section in the north and east, but because of the high evaporation rates, the salt content was not much reduced in the southern and central basin where a rich, but increasingly endemic, marine fauna survived.

The upper Danube River and 19 of its major tributaries were monitored for 1046 km downriver and seasonally for Ca, Mg, Sr, Na, K, HCO3, CO2, Cl, PO4, SiO2, NO3, NO2, NH4, SO4, δ13CDIC, δ18OH2O, δDH2O, δ34SSO4, δ18OSO4, and 87Sr/86Sr. Hydrological considerations and δ18O/δD data show that the water balance in the river, particularly after its confluence with the Inn, is controlled by the southern tributaries draining the Mesozoic carbonate complexes of the Alps. As a result, chemical balance at Bratislava is mostly a conservative product of its tributaries. The concentrations of Ca, Mg, and Sr in the Danube are 6 to 9 times higher than in “pristine” rivers. Although all these cations are derived from dissolution of Mesozoic carbonates, the ultimate cause is likely the enhanced generation of soil CO2 due to agricultural and forestry practices. Dissolution of Triassic sulfates is an additional factor for Ca enrichment in the headwater section of the river. Dissolved sulfate, with a comparable enrichment factor to that of alkaline earths, appears to be derived mostly from atmospheric deposition, a proposition based on consideration of its sulfur and oxygen isotopic compositions. Na, K, and Cl are enriched by a factor of 2.5 to 4 times, mostly as a result of industrial and municipal pollution sources. In contrast to the above components, which behave mostly conservatively during the downriver flow of the Danube, biogenic elements such as nutrients and Si are influenced by in-river processes. The photosynthesis/respiration balance that impacts the carbon cycle and the oxygen balance has been discussed elsewhere (Pawellek and Veizer, 1994) . NO3, NO2, and NH4 are enriched by a factor of 10 to 16 times from point and diffuse sources along the watercourses, generating at times downflow nitrification plumes from NH4 to NO3. PO4 varies seasonally, chiefly as a result of biologic demand during the warm periods. For SiO2, the biological uptake (mostly for secretion of diatom frustules), combined with the deficiency of this compound that results from the predominantly carbonate lithology of the catchment, results in concentrations that are below those of pristine rivers. Overall, the present-day “salted” characteristics of the river are chiefly a consequence of the long habitation history of the upper Danube watershed.

The Vazante Formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized. Samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the Vazante carbonates. Dolomites can be classified, based on petrography and geochemistry, into four generations ranging in crystal size between 4 m (almost micritic) and 3 mm and occurring as both replacements and cements. The Sr/Ca molar ratios, calculated for the dolomitizing fluid (0.0006–0.0138), suggest a contribution from a non-marine, possibly meteoric, water component. The 18 Oa nd 13 C values vary from − 0.1 to − 14.3‰ (PDB) and 0.2 to − 2.3‰ (PDB), respectively. Fluid-inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 °C. A mixing-zone model of dolomitization for Dolomites I–III satisfies the constraints from elemental chemistry, 18 O, 87 Sr/ 86 Sr and fluid inclusions. The petrographic and chemical criteria of the latest generation, Dolomite IV, reflect conditions of deep burial environment at temperatures above 120–130 °C. The 13 C values show only small variations at the base of the formation, followed by a major negative plunge ( 4‰) at the top, this plunge correlated with the Sturtian glacial phase. In contrast to C-isotopes, only few samples may have retained their near-primary 87 Sr/ 86 Sr values of 0.70614–0.70734. The lowest 87 Sr/ 86 Sr value (0.70614), from fibrous cement in the upper part of the formation, correlates with the negative 13 C shift and is also consistent with the Sr-isotope signature proposed for the glacial Sturtian seawater. The 34 S values, obtained from sulfates trapped in carbonates, range between 10.8 and 16.9‰ with a jump to 21.3‰ in the overlying formation. All isotope signals are within the range suggested for the early Neoproterozoic seawater, but the Precambrian baseline is poorly known and additional work is required to confirm this tentative assignment. © 2001 Elsevier Science B.V. All rights reserved.

A review of O, C, Sr and S isotope trends for the entire Phanerozoic shows that the present-day values of isotope signals are similar to those at the Proterozoic termination. The sharp rise in 87Sr/86Sr since ∼65 Ma has been attributed to an uplift and subsequent metamorphism and erosion associated with the Himalayas and Tibet. This orogenic evolution has been postulated to have influenced the global organic and inorganic carbon cycles and climate as well. A similar large-scale orogeny, the Pan-African event, also dominated the Neoproterozoic (Vendian) times, and the similarity of modern and Neoproterozoic isotope values for seawater may therefore have had a comparable tectonic cause. In this contribution, we present the results of a numerical model of the coupled C–alkalinity–S–Sr cycles suggesting that the early Paleozoic (from early Cambrian to late Devonian) evolution of Sr, O, C and S seawater isotope signals could have been the consequence of progressive oxidation of a large reduced carbon reservoir exhumed during the Pan-African orogeny. The δ18O measured in brachiopod shells is used as a forcing of the model, postulating that any change in the oxygen isotopic composition of seawater is the result of a disequilibrium in the organic carbon subcycle through the coupling of the oxygen isotopic and carbon cycles. The calculated δ13C, 87Sr/86Sr and δ34S are in good agreement with the data, as is the reasonable calculated history for atmospheric pCO2 and its relation to global climate.

The oxygen isotope composition of nitrate is used increasingly for identifying the origin of nitrate in terrestrial and aquatic ecosystems. This novel isotope tracer technique is based on the fact that nitrate in atmospheric deposition, in fertilizers, and nitrate generated by nitrification in soils appear to have distinct oxygen isotope ratios. While the typical ranges of δ18O values of nitrate in atmospheric deposition and fertilizers are comparatively well known, few experimental data exist for the oxygen isotope composition of nitrate generated by nitrification in soils. The objective of this study was to determine δ18O values of nitrate formed by microbial nitrification in acid forest floors. Evidence from laboratory incubation experiments and field studies suggests that during microbial nitrification in acid forest floor horizons, up to two of the three oxygen atoms in newly formed nitrate are derived from water, particularly if ammonium is abundant and nitrification rates are high. It was, however, also observed that in ammonium-limited systems with low nitrification rates, significantly less than two thirds of the oxygen in newly formed nitrate can be derived from water oxygen, presumably as a result of heterotrophic nitrification. It can be concluded from the presented data that the δ18O values of nitrate formed by microbial nitrification in acid forest floors typically range between +2 and +14‰, assuming that soil water δ18O values vary between −15 and −5‰. Hence, oxygen isotope ratios of nitrate formed by nitrification in forest floors are usually distinct from those of other nitrate sources such as atmospheric deposition and synthetic fertilizers and, therefore, constitute a valuable qualitative tracer for distinguishing among these sources of nitrate. A quantitative source apportionment appears, however, difficult because of the wide range of δ18O values, particularly for atmospheric nitrate deposition and for nitrate from microbial nitrification.

We have analyzed 214 fossil apatite samples from nine stratigraphic sections worldwide that cover the lower Cambrian to lower Ordovician for their strontium isotope ratios. Of these samples, 180 from six sections that cover the Cambrian-Ordovician transition (

Rivers link terrestrial and marine biospheres by transporting carbon from land to ocean and by exchanging CO2 with the atmosphere. In an attempt to quantify the aquatic carbon cycle, we developed a technique based on carbon isotopic composition of dissolved inorganic carbon (δ13CDIC) and oxygen isotopic composition of dissolved oxygen (δ18ODO). The approach was tested on selected boreal ecosystems in the Ottawa area, Canada: Meech Lake; the Ottawa River; and Green Creek, the latter dewatering the Mer Bleue bog. The three ecosystems were monitored for 1 yr and the calculated aquatic respiration/photosynthesis ratios in general fell within the 1 to 3.5 ranges. The ecosystems were respiration dominated year-round, despite increased photosynthetic rates during the warm season, a development that was apparently matched by a comparable enhancement in respiration rates. The year-round predominance of respiration over photosynthesis supports the results of earlier studies, showing that boreal aquatic ecosystems, particularly the lakes, are not solely dissipation pathways for soil-generated CO2 introduced into surficial water bodies by groundwater input. To fuel this year-round “excess” respiration, the steady state balance consideration demands that the ecosystem must have a continuous supply of allochthonous reduced carbon.

This study deals with the major ions and isotope systematics for C, O, S, and Sr in the Indus River Basin (IRB). Major ion chemistry of the Indus, and most of its headwater tributaries, follow the order Ca2+>Mg2+>(Na++K+) and HCO3−>(SO42−+Cl−)>Si. In the lowland tributaries and in some of the Punjab rivers, however, (Na++K+) and (SO42−+Cl−) predominate. Cyclic salts, important locally for Na+ in dilute headwater tributaries, constitute about 5% of the annual solutes transported by the Indus. Weathering of two lithologies, sedimentary carbonates and crystalline rocks, controls the dissolved inorganic carbon (DIC) concentrations and its carbon isotope systematics throughout the Indus, but turbulent flow and lower temperatures in the headwaters, and storage in reservoirs in the middle and lower Indus promote some equlibration with atmospheric carbon dioxide. Combined evidence from sulfur and oxygen isotopic composition of sulfates refutes the proposition that dissolution of these minerals plays a significant role in the IRB hydrochemistry and suggests that any dissolved sulfates were derived by oxidation of sulfide minerals. In the upper Indus, silicate weathering contributes as much as 75% (or even higher in some tributaries) of the total Na+ and K+, declining to less than 40% as the Indus exits the orogen. In contrast, about two-thirds of Ca2+ and Mg2+ in the upper Indus (over 70% in some tributaries) and three-fourth in the lower Indus, are derived from sedimentary carbonates. The 87Sr/86Sr ratios tend to rise with increasing proportions of silicate derived cations in the headwater tributaries and in the upper and middle Indus, but are out of phase or reversed in the lower Indus. Finally, close to the river mouth, the discharge weighted average contribution of silicate derived Ca2++Mg2+ and silicate derived Na++K+ are, respectively, about one-fourth and two-thirds of their total concentrations.

Previous research on sea floor Fe(III) oxyhydroxide deposits has focused primarily on deep-sea, hydrothermal systems found along volcanically active portions of the mid-ocean ridges and on hydrogenetic deposits formed in deep basins and along continental shelves. There is, however, not much known about their formation in shallow-water settings associated with volcanic islands. The hydrothermal system at Ambitle Island, Papua New Guinea provides an excellent opportunity to study the formation of Fe(III) oxyhydroxides in a shallow-water setting. Precipitation from the hydrothermal solution is caused by mixing with seawater. Based on a 87 Sr / 86 Sr mixing model, the calculated minimum and maximum seawater fractions are approximately 11 and 57%, respectively. Thus, precipitation of Tutum Bay Fe(III) oxyhydroxides takes place at a temperature range between approximately 60 and 93°C. The chemical composition shows low Mn contents (Fe/Mn>600), and elements that are usually enriched in Fe(III) oxyhydroxides, such as Co and V are below crustal abundance and well below their concentrations in island-arc volcanics. Arsenic concentrations, on the other hand, are by two orders of magnitude higher than those in other marine deposits. Rare earth element (REE) concentrations reflect their concentration in the hydrothermal fluids rather than seawater. The crystallinity of the deposits increases with age, as protoferrihydrite is apparently altered to Fe-smectite and hematite, and As-bearing minerals are formed. Contact with seawater, and therefore oxidizing conditions, seems to be the factor increasing the crystallinity.

Carboniferous brachiopods from western Europe and the former USSR have been utilized as carriers of isotopic proxy signals for paleoceanography and paleoclimatology during the assemblage of Pangea. This is of particular interest, because this time interval coincides with the transition from greenhouse to icehouse conditions. The δ 18 O values (3 Ma running means) show an overall δ 18 O increase from about −7±1‰ to −3±1‰ during the Carboniferous. The rise in the δ 18 O is stepwise, with major shifts in the mid-Tournaisian and at the Visean/Serpukhovian transition. The comparable δ 13 C record shows a Tournaisian peak at 5‰, a Visean oscillation at about 2.5±1‰, a Serpukhovian rise of more than 3‰, a plateau around 5‰ during the Bashkirian and Moscovian, a 4‰ drop during the Kasimovian, and another rise, of about 3‰, during the Gzhelian. The 87 Sr / 86 Sr record shows an Early Carboniferous decline from ∼0.7083 to a mid-Visean minimum of 0.7077, followed by a rapid Serpukhovian rise to a Bashkirian and Moscovian plateau at 0.7087 and a slight decline during the Kasimovian and Gzhelian. The covariance of isotopic trends, and the coincidence with geological phenomena, indicates that the long term variations in ocean chemistry and climate may be coupled to the tectonic evolution of the earth system. The Namurian witnesses the onset of Hercynian orogeny, initiation of the main phase of Carboniferous glaciation, formation of the extensive coal deposits and the rise in 87 Sr / 86 Sr , δ 13 C and δ 18 O . All this suggests that a tectonically controlled climate change, resulting in cooling and increased carbon storage in the organic reservoir, was the major triggering factor for all the above phenomena. The late Carboniferous glacial episode was interrupted by a warm interval in the Stephanian. The carbon and oxygen isotope data indicate that this episode may have been restricted to the early Kasimovian, a proposition at odds with the poorly dated record of glacial deposits that are interpreted as indicating a warm climate for the entire Stephanian. In addition, the pattern of short-lived peaks in δ 13 C and δ 18 O in the Tournaisian (and possibly Visean) may be interpreted as a reflection of short-lived glacial episodes, a proposition consistent with geologic records.

The new set of 87 Sr / 86 Sr , δ 18 O and δ 13 C experimental data for Phanerozoic seawater, the “Bochum/Ottawa Isotope Dataset”, has been tested by wavelet, discontinuity and sliding window correlation dimension analyses for cyclicities and nonstationarities in the isotope signal. The tests indicate discontinuities in the strontium isotope signal at ∼500, 340, 288, 210, 65 and 28 Ma, while for the oxygen and carbon isotopes they are at ∼500, 385, 290, 210 and 65 Ma. These discontinuities, often coincident with major stage boundaries, reflect mostly single (likely tectonic) events that do not affect the structure of the underlying system. The two most pronounced nonstationarities in all isotope systematics are at ∼65 and 210 Ma, respectively, that is at the K/T and Norian/Rhaetian transitions. Wavelet analysis for all three isotope systems yields a long-term quasi-periodicity at ∼94–125 Ma, best developed during the Paleozoic, with superimposed intermittent 48–57 and 29–35 Ma oscillations, all likely a reflection of plate reorganizations within the Caledonian, Hercynian and Alpine tectonic cycles.

Seasonal studies of a cross section through the upper St. Lawrence River in 1995 and 1996 revealed strong biogeochemical gradients between a wetland, a creek, embayments and the main river near the city of Cornwall (Ontario), Canada. These local differences are much more pronounced than over the entire length of the main river, indicating the importance of near-shore ecosystems in the riverine carbon cycle. The aqueous pCO2 ranged from atmospheric equilibrium to values as high as 5450 ppmV, reflecting a gradient from the Great Lakes-dominated main river to the peripheral ecosystems with active biological recycling. The isotopic composition of dissolved inorganic carbon fits this observation, with δ 13 C DIC values ranging from +2.2 to −13.7‰ from the main river to near-shore ecosystems. Warm season samples of embayments revealed active photosynthesis, with pO2 values up to 50% higher than those of the atmosphere, and pCO2 below atmospheric equilibrium. While photosynthetic activity was mostly caused by phytoplankton, as indicated by chlorophyll-a up to 63.0 mg/l, an active periphyton and macrophyte community also contributed to oxygen oversaturations in the littoral zone. In situ respiration dominated in the wetland, particularly during winter and spring when dissolved organic carbon (up to 17.3 mg/l) was transformed into CO2, leading to δ 13 C DIC values as low as −12.3‰. The creek was the only ecosystem with a respiratory signal inherited from near-shore baseflow, as indicated by its δ 18 O H 2 O composition that was similar to local groundwater during peak runoff periods. Its peak DOC concentrations of 38.3 mg/l, the highest amongst the studied ecosystem, may have resulted in further enhanced respiratory activity.

Isotopic composition of dissolved inorganic carbon ( δ 13 C DIC ) in the Ottawa River basin is about −8 and −16‰ for lowland carbonate and upland silicate tributaries, respectively. This suggests that (1) the source of DIC to the Ottawa River is soil respiration and carbonate weathering, (2) exchange with the atmosphere is unidirectional or volumetrically unimportant, and (3) in-river respiration and photosynthesis are not significant influences on the river carbon budget. Accepting these constraints, chemical and isotopic data are used to reconstitute soil pCO2 for tributary catchments. Averages for upland silicate, mixed, and lowland carbonate basins are calculated to be roughly 2000, 5000, and 30,000 ppm, respectively. These values are used as input to model the pathway of carbon through the watershed—rain water to soil water to river water. The flux of carbon from the Ottawa River as DIC is calculated to be 4.3×1010 mol C/a. Utilizing carbon isotopes, 75% and 25% of the Ca2++Mg2+ flux is calculated to originate from carbonate and silicate weathering, respectively, and 61% of the DIC is calculated to originate from organic respiration. The latter represents some 6% of respired carbon in the basin, assuming an average respiration rate of 0.5 mmol C m−2 h−1. Based on a diffusion model, CO2 evasion to the atmosphere from the Ottawa River and its tributaries is estimated to be 1.3×1010 mol C/a or 30% of the DIC flux.

Seven sampling locations in the upper St. Lawrence River near the city of Cornwall (Ontario, Canada), including the main river and six near-shore ecosystems (a creek, embayments and a wetland) were studied in order to determine the origin of particulate organic carbon. Parameters studied included chlorophyll-a (chl-a), particulate organic carbon (POC) and dissolved inorganic carbon (DIC), as well as the isotopic compositions of the latter two (δ13CPOC, δ13CDIC). The results show that in situ photosynthesis and detrital inputs are both significant contributors to the POC pool in the isolated embayments. The former dominates during warm seasons, with POC concentrations up to 2663 μg/l and chl-a concentrations up to 26.1 μg/l. Near-shore ecosystems have a wide range of δ13CPOC values (−31.5 to −16.3‰), but this variability is not reflected in the `Main Channel'. There, the δ13CPOC signal is uniformly close to −27‰, in accord with estimates from earlier studies on the river's estuary. This suggests that the POC contribution from near-shore ecosystems is minor. Although the `Main Channel' has low chl-a concentrations, model calculations suggest that most of its POC originates from photosynthetic activity, probably within the Great Lakes.

The Una Group, Irece Basin (Bahia state) is correlative with the better known Bambui Group of the Sao Francisco Basin (Minas Gerais state); these basins are remnants of widespread carbonate platform that covers > 300 000 km2 of the Sao Francisco Craton in Brazil. The Irece area contains a well-controlled stratigraphic succession hosting economic phosphate deposits and sulfide mineralization. Una Group strata comprise a basal siliciclastic sequence, mostly composed of glacial diamictites (Bebedouro Formation), followed by a carbonate succession that is several hundred meters thick, the Salitre Formation. An angular unconformity separates the two units. The carbonates of the Salitre Formation are characterized by 87Sr/86Sr ratios ranging between 0.70745 and 0.71776, and δ13C values ranging between −4.4 and +9.4‰ PDB. The carbonates also contain beds and nodules of barite and gypsum with highly positive δ34S values (+25.2 to +32.8‰ CDT). The ‘best preserved’ carbonate samples selected on the basis of their Mn and Sr concentrations as well as their ratios, yield 87Sr/86Sr values of 0.70745–0.70765. Comparison of these values with the secular strontium isotopic curve for the Neoproterozoic seawater suggests a 600 (to 670) Ma age for the Salitre Formation and for its correlative units in the Sao Francisco Basin.

utilized for strontium isotope measurements. The results document a gradual decrease in 87 Sr/ 86 Sr, from 0.7091 to 0.7087, from Tremadoc to Llandeilo, a sharp decline to 0.7078 during the late Llandeilo-early Caradoc; little change during Caradoc and the Ashgil; and a steady rise to 0.7087 through the Silurian. These long-term (10 7 yr) variations, with magnitudes in the range of 10 23 , are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering. The gradual decrease in 87 Sr/ 86 Sr ratio during the Early Ordovician may record the reduction in uplift and weathering rates due to waning of the Pan-African orogenies. The rapid decline near the Llandeilo/Caradoc boundary suggests a strong hydrothermal flux likely due to increased sea-floor spreading and a possible superplume event. The latter may have caused the prominent transgressive phase, the largest in the Phanerozoic, which would have muted continental flux input. Although the Caradoc was the main interval for the Taconic Orogeny, its impact on the Sr continental flux may have been delayed until the early Llandovery. This effect, complemented by reworking of glacial deposits near the Ordovician-Silurian boundary and enhanced by phases of the Silurian Salinic Orogeny, may have combined to give the progressive increase in the strontium isotope ratio through the Silurian. The scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes. Copyright © 1998 Elsevier Science Ltd

The Geochemical Earth Reference Model (GERM) initiative is a grass-root effort with the goals of establishing a community consensus on a chemical characterization of the Earth, its major reservoirs, and the fluxes between them. The GERM initiative will provide a review of available scientific constraints for: (1) the composition of all major chemical reservoirs of the present-day Earth, from core to atmosphere; (2) present-day fluxes between reservoirs; (3) the Earth's chemical and isotopic evolution since accretion; and (4) the chemical and isotopic evolution of seawater as a record of global tectonics and climate. Even though most of the constraints for the GERM will be drawn from chemical data sets, some data will have to come from other disciplines, such as geophysics, nuclear physics, and cosmochemistry. GERM also includes a diverse chemical and physical data base and computer codes that are useful for our understanding of how the Earth works as a dynamic chemical and physical system. The GERM initiative is developed in an open community discussion on the World Wide Web (http://www-ep.es.llnl.gov/germ/germ-home.html) that is moderated by editors with responsibilities for different reservoirs, fluxes, data bases, and other scientific or technical aspects. These editors have agreed to lay out an initial, strawman GERM for their respective sections and to moderate community discussions leading to a first, preliminary consensus. The development of the GERM began with an initial workshop in Lyon, France in March, 1996. Since then, the GERM has continued to be developed on the Internet, punctuated by workshops and special sessions at professional meetings. A second GERM workshop will be held in La Jolla, CA USA on March 10–13, 1998.

Chemical and stable isotope analyses of the St. Clair, Detroit, Niagara, and St. Lawrence rivers (“St. Lawrence” system) and their tributaries show that the chemical and isotopic compositions of the waters are strongly controlled by the geology of their drainage basins. Tributaries draining the Canadian Shield have very low TDS, HCO3−, SO42−, Ca2+, Mg2+, NO3−, Sr2+, higher Si and Fetotal, and high 87Sr/86Sr ratios (0.710–0.713). The Grand and Thames rivers that drain Paleozoic limestones, dolostones, and evaporites are characterized by opposite attributes. The “St. Lawrence” and the tributaries draining the Canadian Appalachians fall between these two endmembers. The St. Clair, Detroit, and Niagara rivers do not show any pronounced seasonal variations in major component chemistry due to buffering by the Great Lakes. In contrast, pronounced seasonal variations characterize the lower St. Lawrence mainly because of significant tributary inputs into the overall water budget. The δD and δ18O in the “St. Lawrence” range from −60.9 to −44.5‰ and from −8.5 to −6.1‰ SMOW, respectively, much heavier than the comparative values measured for the tributaries (−92.8 to −58.3‰ and −13.1 to −8.5‰). This is a consequence of evaporative loss that, over the residence time of water of 102 years, equals about 7% of the water volume in the Great Lakes. The strontium and sulfur isotopic values for the “St. Lawrence” system are relatively uniform, with measured values from 0.70927 to 0.71112 for 87Sr/86Sr and from 4.3 to 5.6‰ for sulfate δ34S. Their seasonal variations are also minor. The strontium and sulfur fluxes of the St. Lawrence river are calculated to be 7.84 × 108 and 1.09 × 1011 mol/a, respectively. The relative contributions of the Great Lakes, tributaries, and other sources to these fluxes are 73:16:11% for strontium and 64:13:23% for sulfur. Isotopic composition of dissolved inorganic carbon (δ13CDIC) in the “St. Lawrence” system ranges from −4.7 to +0.7‰, considerably heavier than the values for the tributaries (−16.5 to −6.7‰). The light δ13CDIC values for the tributaries suggest that CO2 from bacterial respiration plays an important role in the isotopic composition of riverine DIC. However, in the main stem river(s), this bacterial signal is masked by isotopic equilibration with atmospheric CO2 due to the long residence time of water in the Great Lakes. Seasonally, the main stem river(s) have heavier δ13C values in the fall than in the spring, a consequence of preferential 12C consumption by photosynthetic plants in the epilimnion of the Great Lakes during the growth season. In the down-stream portion of the St. Lawrence river, influx of isotopically light tributary waters causes progressive 13C depletion, from −1.3 to −2.0‰ and −1.4 to −3.0‰ in the fall and spring, respectively. The total DIC carbon flux of the St. Lawrence river is calculated to be 3.9 × 1011 mol/a. Mass balance calculations show that the relative contributions of the Great Lakes, tributaries, decay of organic matter, exchange with the atmosphere, and dissolution of carbonates to this total DIC flux are 81:13:2:-6:10% in the spring, and 83:15:-2:4:0% in the fall, respectively.

Carbonates from the ~1100–1450 Ma old Proterozoic Belt Supergroup were collected from stratigraphic sections throughout Montana and Idaho, USA. The sampled sequences, in ascending stratigraphic order, include the Newland, Altyn, Spokane/Greyson transition, Empire, Wallace, Helena, Siyeh, Snowslip, Shepard, and Libby formations. An increase in the degree of postdepositional alteration of Belt limestones is reflected in a diminution of Sr and Mg contents, an increase in Mn, and depletion in 13C and 18O. Two diagenetic trends can be resolved for the limestones. One, affecting the presumed originally aragonite-rich sediments, includes carbonates from the Lower Belt Newland Formation. In contrast, the Middle Belt Carbonate (Wallace, Helena, Siyeh formations) may have been originally of high-Mg calcitic mineralogy. Projection of the alteration trends for the Lower and Middle Belt limestones suggest ~21‰ SMOW and ~ +2.5‰ to +1.0‰ PDB as the best preserved values for the δ18O and δ13C of seawater, respectively; both comparable to results from other Mesoproterozoic carbonate sequences. The oxygen isotope data for limestones show a regional westward depletion of ~8‰ in 18O, possibly reflecting a higher temperature of postdepositional alteration in the western Belt basin. This depletion in 18O is accompanied by a comparable decrease in δ13C values, most likely because a higher proportion of carbon was incorporated from CO2 generated by thermal cracking of hydrocarbons at depth. Dolostones in the Belt basin are dominantly micritic, with good preservation of depositional textures. Chemically and isotopically, their alteration trends mimic those of limestones, leading to comparable projected “best” values for δ13C. 87Sr/86Sr values of Belt carbonates range between 0.70484 and 0.74991. Progressive diagenesis, as indexed by decreasing concentrations of Sr and depletions in 18O and 13C, results in an increase in 87Sr/ 86Sr values. The least radiogenic measurement, from a Lower Belt Newland limestone sample, appears to fit reasonably well into the general trend for Proterozoic seawater, as discussed in Mirota and Veizer (1994). This and the consistency of δ18O and δ13C in limestones with other coeval sequences suggests that the (Lower and Middle) Belt carbonate sections are marine, deposited in an environment that was not continuously separated from the open ocean.

A set of 145 strontium isotope ratios, based on skeletal components of Emsian to Frasnian age, reveals a detailed structure of the seawater 87Sr/86Sr curve that can ultimately serve as a tool for “high resolution isotope stratigraphy” in the Devonian. Brachiopod and conodont samples were collected in the Eifel region of Germany, the area around the global stratotype for the Middle Devonian, with an apparent average temporal resolution in the 105 y range. Preservation of the brachiopod shell material has been assessed by optical microscopy, SEM, and cathodoluminescence and only the better preserved internal (“secondary”) layer of the shell has been utilized for strontium isotope measurements. The 87Sr/86Sr curve shows a short decline from 0.7081 to 0.7078 through the late Emsian, followed by short-term fluctuations with an amplitude of up to 10−4 around a mean of ~0.7078 during the Eifelian and Givetian, and a rise from late Givetian into the Late Devonian. The above high frequency oscillations and spikes are likely a reflection of condensed sedimentary records, due to erosional and nondepositional events, but a direct confirmation for such a scenario can be claimed only for the latest Eifelian otomari-event. Insufficient stratigraphic resolution precludes testing for the other events and isotopic spikes. Conodont skeletal elements, even if well preserved at CAI < 2.5, yielded results that are mostly enriched in radiogenic 87Sr, if compared to coeval brachiopods. Although this enrichment is usually within the 10−5 range, conodonts, despite their superior stratigraphic resolution, should be utilized only as a material of second choice in cases where brachiopods are rare or absent.

The Fraser drains one quarter of British Columbia, but within its watershed there occurs a disproportionately large share of the province's economic activity. Agriculture, fishing, forestry, mining and tourism are all important, the Fraser being the world's largest source of salmon. To preserve this range of activities, the quality of its waters must be ensured. This study documents the chemistry of the waters. Samples from seventeen sites along the Fraser and its principal tributaries were analyzed for major anions and cations, for 28 dissolved trace and ultratrace elements, and for the isotope ratios of O, H, S, C and Sr. The data show that the primary control on chemistry is the diverse geological terranes that are drained. For example, S04 in the headwaters comes from dissolution of sedimentary sulphate minerals. Towards the coast, weathering of sulphide minerals become progressively more important as a source of SO4, including igneous sulphides that contribute a substantial flux of elements such as Se and Sb. Superimposed on the natural fluxes are anthropogenic contributions. Some, such as dissolved organic carbon and NaCl from pulp mills, can be readily identified. The sources of others are less certain. Molybdenum is much higher in the Fraser than in the Ottawa River. But it is not clear if this high level of Mo is natural or is the result of mining for this metal. During the period of peak flow in July, 1993, Fraser waters had an unusually high level of CO2. The CO2 is derived from the decay of vegetation on land and may have been influenced by timber operations.

One hundred and two articulate brachiopods and twenty-one marine cements from Ordovician strata were analyzed for oxygen and carbon isotopes. These, together with 200 previously published analyses of Ordovician brachiopods (mostly of Upper Ordovician age), define a general trend of increasing δ18O values with decreasing age, from a minimum of − 10.5%. in the earliest Ordovician to a maximum of −1.5%. at the Ordovician /Silurian boundary. The δ13C values show a general positive correlation with δ18O and increase from −2.5%. in the earliest Ordovician to as high as +7%. at the Ordovician/Silurian boundary. The magnitudes of Ordovician excursions for both δ18O and δ13C are similar to those reported previously for the entire Phanerozoic. Different genera of brachiopods from the same stratigraphie level did not yield any large differences in their isotopic compositions, suggesting that vital effects did not influence decisively the isotope values of the studied skeletal parts. Petrographic examination and trace element analyses indicate that brachiopod shells from Llandeilo to Ashgill strata are well preserved and thus, may have retained their primary isotopic signatures. The same criteria for the Tremadoc, Arenig and some Llanvirn brachiopods suggest partial diagenetic alteration of δ18O values in these samples, but this does not preclude their utility as a recorder of a primary δ13C signal. The observed δ18O trend for the well preserved portion of the record may reflect progressive cooling during Ordovician, perhaps complemented by a changing δ18O values of seawater. The latter could have been a consequence of a net increase in the rate of high-temperature water/rock interaction relative to its low-temperature counterpart, as indicated also by the large coeval decrease of marine 87Sr86Sr. Superimposed on the above long-term δ18O trend is a positive excursion of about 2%. at the end of the Ordovician, probably reflecting a large expansion of polar ice caps. The magnitude of this excursion indicates a degree of glaciation comparable to that at the height of the Quaternary glacial episodes. The secular δ13C trend may reflect a progressive increase in marine organic productivity and/or enhanced organic deposition in the Ordovician oceans, particularly noticeable at the time of the terminal Ordovician glaciation

The ~1.85 + 0.15 Ga old Albanel Formations of northern Quebec, Canada, an example of Paleoproterozoic carbonate buildups, are lithologically composed of cherty dolostones. Subordinate limestones are encountered only near the base of the sequence, coincident with maximal transgression. The chemistry of the bulk rocks is almost entirely controlled by noncarbonate phases, such as chert, quartz, plagioclase, illite (±K-spar), pyrite and Fe-Mn oxides, all abundant particularly in the basal part of the section. The chemistry of the carbonate portion of limestones is strongly reset by post-depositional phenomena, but the relict high Sr contents suggest that aragonite may have been an original precursor phase. Geochemically, the carbonate portion of dolostones can be subdivided into two populations, reflecting the stratigraphie bottom and top of the sequence. Trace element and oxygen isotope criteria suggest that the entire sequence was subjected to meteoric and subsequent burial diagenesis, with the less deeply buried top portion of the sequence having recrystallized and stabilized at lower temperatures, but in a more open system. The latter reflects the greater lithological variability and permeability of the younger strata. The δ13C varies up-section from about +3 to + 1‰, perhaps a reflection of progressively more open marine circulation. The associated four generations of post-depositional pyrites have overlapping δ34S, from -7.1 to +54‰, with a mean of +24‰; a reflection of dissimilatory bacterial sulfate reduction in a relatively closed diagenetic system with perhaps a limited sulfate availability and/or replenishment. The best measured 87 Sr 86 Sr of 0.70600, although possibly not a direct reflection of the coeval seawater, may still be a near-original value, as indicated by its coherency with the roughly co-stratigraphic Paleo-proterozoic sequences. Evaluation of the existing data indicates that strontium isotopic composition of the Precambrian seawater was characterized by oscillations similar to those of the Phanerozoic, the interval of ~0.8–1.5 Ga ago being dominated by relatively nonradiogenic values, followed by a radiogenic period at ~ 1.6–2.2 Ga ago, near-mantle values during the late Archean, and possibly another radiogenic peak at ~3.5 Ga ago.

This study presents the abundances of Ru, Ir, Pt, Pd, Rh, and Au (platinum-group elements; PGE) in the ejecta layer at the Cretaceous-Tertiary boundary and melt rocks from East Clearwater Lake, Brent, New Quebec, and Lake Wanapitei impact craters, all believed to have been formed by the impact of chondritic projectiles. The objective was to evaluate the pure application of PGE interelement ratios (e.g., Ru Ir ) for impactor identification, to compare the utility of both melt rocks and ejecta deposits for this purpose, and to expand the PGE database for these craters. The similarity of PGE interelement ratios within the chondrite and iron meteorite groups precludes their use in the fine classification of impactors of this composition. Introduction of indigenous PGE from country rock may contaminate the original meteoritic signature. In addition, the extent of PGE fractionation during impact and melt sheet cooling and of post-sedimentary PGE remobilization must be evaluated in each case. For East Clearwater, the PGE chondrite-normalized plot is flat, indicating a chondritic impactor. Based on Ru Ir and Cr Ir ratios, a Cl- or L-chondrite is most likely. However, variation of ratios with depth in the core makes any fine classification of the projectile tenuous. For the Brent and New Quebec craters, the flat chondrite-normalized PGE plot indicates a chondritic impactor, but finer classification is precluded by possible post/syn depositional PGE fractionation. At Lake Wanapitei, Ni Cr and Cr Ir ratios indicate a LL-chondrite impactor ( Wolf et al., 1980) but sparse PGE data for this chondrite-type does not allow further classification of the projectile. A flat chondrite-normalized PGE plot indicates a chondritic rather than an iron impactor at Lake Wanapitei. The global ejecta layer at the Cretaceous-Tertiary boundary has a chondritic PGE signature, but only when the integrated flux of PGE on a global scale is considered. Individual sites may not show chondritic ratios due to differential PGE remobilization resulting from local differences in depositional environment (continental vs. marine), and to vaporization and/or condensation PGE fractionation occurring during impact. The results for melt rocks and the K-T ejecta deposit indicate that PGE can be used for impactor identification but primarily in combination with other siderophile/chalcophile meteoritic elements and their ratios.

Ruthenium and Ir are the least mobile platinum group elements (PGEs) within the Cretaceous-Tertiary (K-T) boundary clay. The Ru/Ir ratio is therefore the most useful PGE interelement parameter for distinguishing terrestrial and extraterrestrial contributions to the boundary clay. The Ru/Ir ratio of European marine K-T sections (1.67 ± 0.38) is statistically different from that of the North American continental sections (0.76 ± 0.26). It is unlikely that this difference is due to secondary PGE remobilization, PGE input to the boundary clay during multiple impacts, or volcanic emissions. The global difference in Ru/Ir ratios in the boundary clay may therefore be primary. The positive correlation between distance from the Chicxulub impact structure, Yucatan, Mexico, and Ru/Ir ratio and the more than 1000°C difference in the condensation temperatures of these elements lead us to propose that fractionation of Ru from Ir during condensation from the ejecta cloud may have occurred, resulting in the global difference in Ru/Ir ratios.

[1] We examined carbon cycling in the Mississippi River using stable isotopes of inorganic carbon and dissolved oxygen. Eighteen sites were sampled along the river and its tributaries over 1 year. We estimate using a conservative approach that the flux of CO2 to the atmosphere (1 × 1013 g C yr−1) approximately equaled the flux of alkalinity to the Gulf of Mexico (9.7 × 1012 g C yr−1) and greatly exceeded the flux of dissolved organic carbon (1.5 × 1012 g C yr−1). Though only a first-order estimate, our work shows that the atmospheric flux of CO2 is significant and should not be ignored when examining the carbon budget of the Mississippi River. As expected, because of the large area covered by the Mississippi watershed, the isotopic composition of dissolved inorganic carbon, δ13CDIC, varied widely. In the Ohio and upper and lower Mississippi basins, δ13CDIC indicates that the source of inorganic carbon in the rivers is primarily from carbonate dissolution by soil CO2. Dissolved inorganic carbon in the Missouri River was enriched in 13C, and the isotopic composition of dissolved oxygen in this river suggests that this results from an excess of aquatic photosynthesis over respiration.

Isotopic analyses of trace sulfur in Precambrian carbonates and clastic sedimentary rocks support the results of previous studies which have shown that δ34S measurements of Archean sedimentary sulfur cluster mostly around mantle-like values and that a significant overall dispersion of 34 S 32 S ratios commenced at about 2.7 Ga ago. The outstanding feature of the Proterozoic record is the predominance of heavy δ34Ssulfide values, mostly in excess of 0%. CDM. Consideration of these data, in combination with the sparse δ534S record for Proterozoic sulfates, suggests that Δ34Ssulfide-sulfide may have been smaller than was the case during the Phanerozoic. A possible scenario that is compatible with the above data is based on a proposition that the terrestrial exogenic sulfur cycle evolved in a tripartite manner. In the Archean, it was coupled to and controlled by the magmatic endogenic cycle. The Proterozoic, for the most part, represents a transitional stage into a Phanerozoic-like exogenic cycle, the latter essentially decoupled from its endogenic counterpart and its redox balance maintained and mediated by the biosphere. The indication that the mean combined δ34S of Proterozoic sulfate and sulfide may have significantly exceeded the Phanerozoic mean value of +2%. CDM is enigmatic and poses problems for balance considerations.

A study of mineralogy, chemistry, and isotopic composition of the Coronation Supergroup (about 1.9 Ga, NWT), Canada, and the McArthur Group (about 1.65 NT), Australia, is reported in order to obtain better constrained data for the first- and second-order variations in the isotopic composition of late Paleoproterozoic (1.9 +/- 0.2 Ga) seawater. Petrologically, both carbonate sequences are mostly dolostones. The McArthur population contains more abundant textural features that attest to the former presence of sulfates and halite, and the facies investigated represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes. It is suggested that dolomitization was an early diagenetic event and that the O-18 depletion of the Archean to late Paleoproterozoic carbonates is not an artifact of postdepositional alteration.

The mineralogy, chemistry, and isotopic composition of the Malmani Dolomite, Duck Creek Dolomite, and Bruce 'Limestone' Member of the Espanola Formation are studied in an effort to restrict the first- and second-order variations in isotopic composition of Early Paleoproterozoic seawater. The diagenetic rank is found to increase in the order Duck Creek less than Bruce less than Malmani. The interpolation of alteration trends to 'best' value yields an estimate of 0.70550 for Sr-87/Sr-86. For delta C-13, the measured range of 0 +/- 1.5 percent PDB is similar to that observed for Phanerozoic marine carbonates, while the 'best' delta O-18 value for dolostones is -5 percent PDB, depleted in O-18 relative to Phanerozoic counterparts but comparable to estimates obtained for Archean facies. The isotope geochemistry and mineralogy of Bruce 'Limestone' Member is consistent with the proposition that the sequence was deposited in a lacustrine environment.

The isotope geochemistry of fracture calcites in three Precambrian plutons on the Canadian Shield has been investigated in order to understand the paleohydrogeological conditions in fractured crystalline rock. Fracture calcites of ancient hydrothermal origins in the Chalk River and East Bull Lake plutons exhibit 18O enrichment and 13C depletion trends resulting from recent low-temperature calcite recrystallization under open-system conditions for oxygen, but semiclosed for carbon, and under extremely variable time-integrated, water/rock (calcite) ratios. This process causes recycling of elements with calcite distribution coefficients > 1 (rare earths, manganese, and possibly iron) from the precursor calcite to younger calcites as well as calcite control over the 87 Sr 86 Sr ratio of the groundwater within the Chalk River pluton. The large but variable water/rock (calcite) mole ratios calculated from the shifts in the stable isotopic compositions of fracture calcites are compatible with fracture flow models that invoke flow channeling within single fractures that also contain regions of immobile porosity. Quantification of isotopic shifts resulting from recrystallization requires that the initial isotopic composition of the precursor hydrothermal calcite be well constrained. Unlike the Chalk River and East Bull Lake plutons, hydrothermal fracture calcites in the White Lake pluton, which intrudes marble-rich country rocks of the Grenville Supergroup, have highly variable δ13C and δ18O values. This is attributed to mixing of carbon from magmatic and sedimentary reservoirs, and to oxygen isotopic exchange between hydrothermal fluids and carbonate country rocks at the time of intrusion. Convective circulation of meteoric groundwater from the surrounding 13C-rich carbonate rocks (up to 6.6%.) resulted in δ13C values as heavy as 3.0%. for the fracture calcites in the pluton.

Three hundred and eleven articulate brachiopods, with extensive geographic coverage, spanning the Cambrian to Silurian stratigraphic interval, were analyzed for oxygen and carbon isotopic composition. Cambrian samples have δ18O ≤ −7%., Ordovician samples ≤ −2.4‰, and Silurian samples ≤ −1.9‰, confirming the previously established trend towards lighter isotopic compositions with increasing age throughout the Lower Paleozoic. Forty-nine “best preserved” Ordovician and Silurian samples were identified based on scanning electron microscopy and trace element analysis. They were found to bracket the isotopic compositions of over 85% of specimens from these stratigraphic intervals supporting widespread preservation of the isotopic signal. Some latest Ordovician and Lower Ludlovian samples associated with shale sequences are apparently “enriched” in 18O. These are interpreted as an environmental phenomenon, perhaps related to water temperature and glaciation. A number of Silurian samples of varying genera and stratigraphic levels are highly enriched in 13C, up to +6‰. Some are shale related, but some are associated with carbonate-bearing basins. These are also thought to represent “near-original” compositions, but a single environmental cause is unknown. The present data show that luminescence is not a decisive criterion for evaluating the degree of brachiopod preservation. Whole-shell values were isotopically similar to their nonluminescent portions for both oxygen and carbon.

The Nyong watershed, with an area of 27 800 km2 and a mean annual discharge of 390 m3 s− 1, is the second largest river in Cameroon. The Nyong watershed serves as an outstanding study area for the examination of carbon fluxes in humid tropical environments because of its limited anthropogenic impact and homogeneous silicate bedrock. Between April 2005 and April 2007, we sampled water at seven stations, from the small watershed of the Mengong (0.6 km2) to the Nyong at Edea (24 500 km2), and monitored temperature, pH, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) contents, as well as the isotopic composition of DIC (δ13CDIC) and DOC (δ13CDOC). We estimated terrestrial net ecosystem productivity in the Nyong River watershed and measured fluvial fluxes of carbon to the ocean and the atmosphere. The Nyong River basin sequesters significant amounts of carbon on an annual basis: ~ 7 920 000 t C year− 1 (300 g C m− 2 year− 1). The combined dissolved organic, dissolved inorganic and atmospheric fluxes of carbon from the Nyong River only export 3% of this flux from the basin on an annual basis. This includes a minimum CO2 outgassing of 1487 g C m− 2 year− 1, comparable to 115% of the annual flux of DOC and four times greater than the flux of DIC.

Samples from the Pangola and Ventersdorp Supergroups (Kaapvaal Craton, South Africa) and from the Fortescue and Hamersley Groups (Pilbara Block, Australia) were analyzed, using XRF, AAS, and isotope-analysis techniques to investigate the mineralogical, chemical, and isotopic features of these representatives of contemporary shelf carbonates (Pangola and Hamersley samples) and nonmarine carbonates (the Ventersdorp and Fortescue samples). Results show that, mineralogically, the shelf carbonates are almost exclusively dolostones, while the lacustrine facies are predominantly limestones. Geological, trace-element, and oxygen-isotope results of the shelf carbonates suggest that their original mineralogy may have been aragonite, and that the Pangola dolostones may represent a direct dolomitization product of this precursor. By contrast, the stabilization of the Hamersley carbonates may have involved an additional step of transformation of a metastable precursor into limestone.

[1] Terrestrial water vapor fluxes represent one of the largest movements of mass and energy in the Earth's outer spheres, yet the relative contributions of abiotic water vapor fluxes and those that are regulated solely by the physiology of plants remain poorly constrained. By interpreting differences in the oxygen-18 and deuterium content of precipitation and river water, a methodology was developed to partition plant transpiration (7) from the evaporative flux that occurs directly from soils and water bodies (E d ) and plant surfaces (In). The methodology was applied to fifteen large watersheds in North America, South America, Africa, Australia, and New Guinea, and results indicated that approximately two thirds of the annual water flux from the "water-limited" ecosystems that are typical of higher-latitude regions could be attributed to T. In contrast to "water-limited" watersheds, where T comprised 55% of annual precipitation, T in high-rainfall, densely vegetated regions of the tropics represented a smaller proportion of precipitation and was relatively constant, defining a plateau beyond which additional water input by precipitation did not correspond to higher T values. In response to variable water input by precipitation, estimates of T behaved similarly to net primary productivity, suggesting that in conformity with small-scale measurements, the terrestrial water and carbon cycles are inherently coupled via the biosphere. Although the estimates of T are admittedly first-order, they offer a conceptual perspective on the dynamics of energy exchange between terrestrial systems and the atmosphere, where the carbon cycle is essentially driven by solar energy via the water cycle intermediary.

The development of a palaeothermometer for ancient oceans has been a prime goal of stable isotope geochemistry since its inception in the middle of the last century. Following the calibration of the calcite oxygen isotope palaeothermometer by Epstein et al . (1953), and expanding on the seminal papers by Cesare Emiliani, the technique blossomed into a widely applied tool of palaeoceanography. These studies resulted in major advances in our understanding of Quaternary, and to some extent Tertiary oceans, but for older time periods the effort is mired by controversy and is at a standstill. In part this is due to the dearth of well-characterized carrier phases for the temperature signal, such as foraminifera shells, in the pre-Tertiary strata, but an intractable disagreement among practitioners as to the oxygen isotopic composition of ancient seawater is probably a more important stumbling block. The dearth of suitable carrier phases is being slowly rectified by utilizing other groups of organisms that secrete their shells from low-Mg calcite, such as oysters, belemnites and particularly brachiopods (Veizer et al ., 1999). Modern counterparts of ancient brachiopods, those inhabiting warm shelf environments, display up to 4‰ spread of …

Reply to comment by Chakraborty P.P. and Sarkar A. on “C, O, Sr and Pb isotope systematics of carbonate sequences of the Vindhyan Supergroup, India: age, diagenesis, correlations and implications for global events” Jyotiranjan S. Ray a,∗, J. Veizer b,c a Physical Research Laboratory, Navrangpura, Ahmedabad 380009, India b Department of Earth Sciences, Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa ON, Canada K1N 6N5 c Institut fur Geologie, Mineralogie und Geophysik, Ruhr Universitat, Bochum 44780, Germany