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We review the long-term climate variations during the last 540 million years (Phanerozoic Eon). We begin with a short summary of the relevant geological and geochemical datasets available for the reconstruction of long-term climate variations. We then explore the main drivers of climate that appear to explain a large fraction of these climatic oscillations. The first is the long-term trend in atmospheric CO

Plate tectonics, the principal vehicle for dissipation of planetary energy, is believed to buffer the δ18O of seawater at its near-modern value of 0‰ SMOW (Standard Mean Ocean Water) because the hot and cold cells of hydrothermal circulation at oceanic ridges cancel each other. The persistence of plate tectonics over eons apparently favors attribution of the well-documented oxygen isotope secular trends for carbonates (cherts, phosphates) to progressively warmer oceans, from 40–70 °C in the early Paleozoic to 60–100 °C in the Archean. We argue that these oceanic hydrothermal systems are dominated by low-temperature (

Porewater extractions and acid leachates of rock core from a 250 m thick sequence of low-permeability Ordovician-age shales and limestones, on the eastern flank of the Michigan Basin, were analysed for strontium isotope ratios in an attempt to infer porewater ages from observed 87Sr/86Sr enrichments. The porewaters originated as Ordovician seawater, which subsequently mixed with evaporated Silurian seawater infiltrating from above, and, to some extent, with a deep brine—with an enriched 87Sr/86Sr signature—from the underlying crystalline shield or deep basin. The porewater 87Sr/86Sr ratios are more radiogenic than contemporaneous seawater but show no obvious correlation to those leached from the solid rock phases. Accepting that the initial 87Sr/86Sr signatures in porewaters were dominated by Late Silurian brine, potentially with an additional deep brine component, the excess of radiogenic 87Sr appears to represent ingrowth from 87Rb decay over a time span of some 420 million years, approaching the depositional age of the rocks. Similarly, Rb/Sr errochron ages of acid leachates of solid phases, and the calculated initial 87Sr/86Sr isotopic ratios, are consistent with a proposition that the calcites inherited their Sr from Ordovician seawater and were dolomitized shortly afterwards by infiltrating Mg-enriched evaporative brine, indicating long-term conservative behaviour for the enclosing carbonate rocks. The errochron for leachates from (alumino)silicates yields a high initial 87Sr/86Sr, but with an errochron age of about 340 ± 48 Ma, likely owing to variable admixtures of diagenetic illite in the shales. Overall, the data provide evidence for a stable hydrologic regime since Paleozoic time.

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.

For almost fifty years, geochemists have been interpreting the clues from Pb isotopic ratios concerning mantle composition and evolution separately. The Pb isotopes of ocean island basalts (OIB) indicate that their mantle source is heterogeneous, most likely due to the presence of end-components derived from recycled crust and sediment. Some OIB have unusually high 206Pb/204Pb coming from one of the end-components with a long time-integrated high 238U/204Pb or μ (HIMU). Most OIB and many mid-ocean ridge basalts (MORB) also have high 206Pb/204Pb, indicating a HIMU-like source. Moreover, measured 232Th/238U (κ) for most MORB are lower than those deduced from their 208Pb/204Pb and 206Pb/204Pb. Such high μ and low κ features of oceanic basalts are inconsistent with the known geochemical behavior of U, Pb and Th and temporal evolution of the mantle; these have been respectively termed the 1st and 2nd Pb paradox. Here we show that subducted marine carbonates can be a source for HIMU and a solution to the Pb paradoxes. The results are consistent with the predictions of the marine carbonate recycling hypothesis that posits the Pb isotopes of oceanic basalts indicate a common origin and/or magma generation process.

The temperature of ancient oceans is an important constraint for understanding the climate history of our planet. The classical oxygen isotope paleothermometry on fossil shells, while very proficient when applied to the younger (Cenozoic) portion of the geologic record, is believed to yield only unreliable results for the Phanerozoic “deep time”, either because the empirically well documented secular trend to more negative δ 18 O values with increasing age was generated by post-depositional recrystallization processes or, if primary, implies ecologically unpalatable hot early oceans. Here we present a compilation of δ 18 O measurements for 58,532 low-Mg calcite marine shells that cover almost the entire Phanerozoic eon, argue that the secular decline of about − 6‰ is primary, propose that it reflects the changing oxygen isotopic composition of sea water, and define a new baseline trend for δ 18 O of paleo-sea water; the latter providing a new template for calculation of ambient habitat temperatures of fossil specimens. The resulting pattern for fossil taxa (foraminifera, brachiopods, belemnites and bivalves) mimics their modern counterparts in temperature ranges and modes. This conceptual framework enables application of actualistic concepts to ambient habitat temperatures of fossils and provides us with a long overdue tool for interpretation of “deep time” geologic history.

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 Permian was a period of waning large-scale continental glaciations in the southern Hemisphere. The waning of these ice sheets during the Early Permian led to discharge of 18O-depleted ice–melt water into the oceans. This, coupled with rising seawater temperatures, resulted in a concomitant decline of about 2.5‰ in the δ18O of seawater, as recorded by brachiopod shells from low-latitude (

The Sr, S, O and C isotope database of marine carbonates contains over 55,000 published isotope values of low-Mg calcite from diagenetically little altered Phanerozoic fossil shells as well as samples of whole rocks and calcite cements of Ordovician to Archean age. Carbon and oxygen isotope data for the shell material are divided into habitat subsets (high-, mid-, low-latitude and deep sea), and whole rock data are separated by mineralogy into calcite/dolomite subsets. Trend, correlation, wavelet, and spectral analyses on Gaussian-filtered isotope records were applied to detect and quantify similarities and patterns in temporal records with the following results: (1) Oxygen isotope trends from the “high-latitude” and “deep-sea” habitats are almost indistinguishable through the last 115 Ma, consistent with the existence of the “oceanic conveyor belt” throughout this interval; (2) All oxygen isotope habitat records show a strong, coherent 30–45 Ma (∼ 38 Ma) cyclicity throughout the Cretaceous and the Cenozoic (3) Up to 70% of the multi-million year variability in the δ18O record of the last 115 Ma can be simulated by the following equation: δ 18 O ( ‰ ) = 0.64 sin ( 2 π t / 120 Ma + 0.9 ) + X sin ( 2 π t / 38.3 Ma + 1.1 ) with X ranging from 0.4–0.6‰ for the “low-“, “high-latitude” and “deep-sea” habitats, to 0.8‰ for the “mid-latitude” realm. (1) A ∼ 120 ± 20 Ma cycle occurs in the Paleozoic and Neoproterozoic δ18O record, consistent with paleoclimate variability as interpreted from sedimentological and faunal records. (2) The offset of δ13C values between “deep water” and “high-latitude” vs. surficial habitats at lower latitudes is consistent with the operation of a biological pump in the oceans since at least the Cretaceous. (3) Sr and S isotope records exhibit a ∼ 60–70 Ma cyclicity throughout the Phanerozoic.

This study resolves the Great Lakes Basin (GLB) hydrologic cycle into its components using stable isotopes of hydrogen and oxygen together with long term meteorological data. The results are used to calculate basinwide Net Primary Production (NPP). Stoichiometric relations of carbon and water during photosynthesis form the basis of computing NPP. Mean annual discharge from the GLB is 29% of the precipitation flux and in δ D– δ 18 O space it has a slope of 5.6, distinctly lower than the 7.5 average slope for precipitation in the basin. Mean annual evaporation flux to the atmosphere from water bodies, soils, and wet canopies is 24% of the precipitation flux. Transpiration however, is the strongest pathway for loss of water from the basin, annually returning 342 Pg (petagram = 10 15 g) or 47% of the precipitation flux to the atmosphere. Transpiration and carbon assimilation during photosynthesis are coupled processes. For the GLB vegetation, approximately every mole of CO 2 fixation requires 850 mol of H 2 O loss by transpiration. Therefore, basinwide annual NPP is 0.268 Pg C. Heterotrophic respiration in soils and herbivory annually release 0.138 Pg C to the atmosphere. The surplus NPP or 0.130 Pg C year − 1 Net Ecosystem Productivity (NEP) is stored in GLB vegetation, consistent with the postulated missing North American carbon sink.

The fluxes of water and carbon from terrestrial ecosystems are coupled via the process of photosynthesis. Constraining the annual water cycle therefore allows first order estimates of annual photosynthetic carbon flux, providing that the components of evapotranspiration can be separated. In this study, an isotope mass-balance equation is utilized to constrain annual evaporation flux, which in turn, is used to determine the amount of water transferred to the atmosphere by plant transpiration. The Volta River watershed in West Africa is dominated by woodland and savanna ecosystems with a significant proportion of C 4 vegetation. Annually, the Volta watershed receives ∼ 380 km 3 of rainfall, ∼ 50% of which is returned to the atmosphere via transpiration. An annual photosynthetic carbon flux of ∼ 170 × 10 12 g C yr − 1 or ∼ 428 g C m − 2 was estimated to be associated with this water vapor flux. Independent estimates of heterotrophic soil respiration slightly exceeded the NPP estimate from this study, implying that the exchange of carbon between the Volta River watershed and the atmosphere was close to being in balance or that terrestrial ecosystems were a small annual source of CO 2 to the atmosphere. In addition to terrestrial carbon flux, the balance of photosynthesis and respiration in Volta Lake was also examined. The lake was found to evade carbon dioxide to the atmosphere although the magnitude of the flux was much smaller than that of the terrestrial ecosystems.

Atmospheric carbon dioxide concentrations seem to have been several times modern levels during much of the Palaeozoic era (543-248 million years ago), but decreased during the Carboniferous period to concentrations similar to that of today. Given that carbon dioxide is a greenhouse gas, it has been proposed that surface temperatures were significantly higher during the earlier portions of the Palaeozoic era. A reconstruction of tropical sea surface temperatures based on the delta18O of carbonate fossils indicates, however, that the magnitude of temperature variability throughout this period was small, suggesting that global climate may be independent of variations in atmospheric carbon dioxide concentration. Here we present estimates of sea surface temperatures that were obtained from fossil brachiopod and mollusc shells using the 'carbonate clumped isotope' method-an approach that, unlike the delta18O method, does not require independent estimates of the isotopic composition of the Palaeozoic ocean. Our results indicate that tropical sea surface temperatures were significantly higher than today during the Early Silurian period (443-423 Myr ago), when carbon dioxide concentrations are thought to have been relatively high, and were broadly similar to today during the Late Carboniferous period (314-300 Myr ago), when carbon dioxide concentrations are thought to have been similar to the present-day value. Our results are consistent with the proposal that increased atmospheric carbon dioxide concentrations drive or amplify increased global temperatures.

In recent years the variability of the cosmic ray flux has become one of the main issues interpreting cosmogenic elements and especially their connection with climate. In this review, an interdisciplinary team of scientists brings together our knowledge of the evolution and modulation of the cosmic ray flux from its origin in the Milky Way, during its propagation through the heliosphere, up to its interaction with the Earth’s magnetosphere, resulting, finally, in the production of cosmogenic isotopes in the Earth’ atmosphere. The interpretation of the cosmogenic isotopes and the cosmic ray – cloud connection are also intensively discussed. Finally, we discuss some open questions.

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.

Well-preserved brachiopods from the Niagara Gorge area, Anticosti Island, Britain, Gotland and Estonia were utilised to delineate a complex isotopic evolution for Llandovery–Wenlock seawater. The Sr-isotope record reflects the Salinic I tectophase of the Late Llandovery in the continuous increase in 87Sr/86Sr values from 0.708070 to 0.708346. The Salinic II tectophase is marked by relative constancy of Sr isotope values until the Late Wenlock when it rises from 0.708345 to 0.708430. The second tectonic phase was therefore likely only of a regional nature. The carbon isotopes during the Llandovery fall within a band of about − 1‰ to + 3‰, a range comparable to modern low-latitude brachiopods. A large positive δ13C excursion of about 3‰, identifies the Ireviken event/excursion, characterizes the Early Wenlock. The biotic crisis and the isotope excursion itself may be ultimately related to the onset and duration of the Cancaniri glaciation, although a direct causative scenario is as yet unknown. The oxygen isotopic trends of well-preserved brachiopods clearly reflect a warm climate interval during the latest Llandovery associated with the Silurian sea level highstand. Subsequently, in the Early Wenlock, the sea level fell with the onset of the Cancaniri glaciation in the southern hemisphere. This is reflected in a significant positive δ18O excursion, particularly in brachiopods from the Niagara Gorge area. Brachiopods from lower latitudes were awash in warm tropical currents and therefore exhibit somewhat more negative δ18O values, indicating a lesser cooling gradient.

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.

A population of 169 Permian articulate brachiopod shells was analysed for their 87 Sr/ 86 Sr ratios. 51 of these, characterised as well preserved and stratigraphically well defined, are utilized for delineation of the Permian seawater strontium isotope trend. The 87 Sr/ 86 Sr curve shows values of about 0.7080 in the lowermost Permian (Asselian), followed by a gradual decline to 0.70685 in the Capitanian and a minor rise in the Dzhulfian and lower Dorashamian to values of 0.70715 just below the Permian-Triassic boundary. The Early Permian decrease in the strontium isotope curve that commences in the early Sakmarian is coincident with the advancing deglaciation of the Gondwana and with the increased aridity in large parts of the Pangaea. These factors may have led to a reduced continental weathering of Rb-rich silicate rocks, and thus to the decline in seawater 87 Sr/ 86 Sr. Starting with the Artinskian, the opening of the Neotethys and the associated widespread basaltic volcanism supplied low radiogenic strontium to seawater from an enhanced hydrothermal flux. In the Capitanian, basaltic volcanism in the entire Palaeotethys ceased and this may have been the reason for a slightly more radiogenic seawater isotopic composition during the Lopingian. Higher input of riverine Sr due to expansion of humid areas may have been a contributory factor.

Primary δ18O signals of 97 brachiopod shells from the Lower Silurian (Llandovery) carbonate succession of Anticosti Island were used to test the hypothesis of water-depth and water-temperature gradient for the Silurian onshore–offshore benthic assemblages (BA1–BA5). The analyzed shells were from the Pentamerus palaformis, Pentamerus oblongus, Stricklandia planirostrata, Ehlersella davidsonii, and Triplesia anticostiensis communities, which have been interpreted to occupy different water depths. Screening of the shells, using petrographic and chemical criteria, confirmed good preservation of original material. Shells of P. palaformis, P. oblongus, E. davidsonii, and T. anticostiensis have very similar mean δ18O values (–5‰ Vienna Peedee Belemnite (VPDB)), while shells of S. planirostrata have a somewhat lighter value (–5.6‰). The δ18O values, in corroboration with taphonomic and paleoecologic data, suggest the following: (1) that the water mass of the Anticosti carbonate shelf was warm and well mixed vertically during the Early Silurian; (2) that the pentamerid brachiopod paleocommunities that lived at ~20–90 m of water-depth show little δ18O differentiation in their shell composition; and (3) that the Pentamerus,Stricklandia, and Triplesia communities need to be investigated at the species level, as both taphonomic and oxygen isotopic data indicate that the Stricklandia planirostrata Community most likely lived in a notably warmer, shallower water than the Pentamerus palaformis Community and that Triplesia anticostiensis (BA5) lived at water temperature comparable to that of the Pentamerus habitat (BA3). This may suggest either that, at the generic level, the Pentamerus, Stricklandia, and Triplesia (Clorinda equivalent) communities may have had substantial overlap in their spatial distribution in the BA3–BA4 paleobathymetric settings or that the isotope signals are too faint to record water depth differences on a tropical shelf.

A dataset of 160 isotope δ13C and δ18O values from Anisian, Ladinian, Carnian and Rhaetian articulate brachiopod shells, complemented by 158 carbon and oxygen isotope values from whole rock carbonates, define the first continuous stable isotope baseline trends for the Triassic seawater. The carbon isotope data suggest the existence of several short-term high amplitude excursions in the Early Triassic, followed by a predominance of values around 0.5 ± 1‰ during the Middle Triassic, a rise to ∼ 3.5‰ during the Carnian, plateau at this level during the Late Carnian to Early Norian, and a 1.5‰ decline in the Middle Norian to values around 2‰ during the Late Norian–Rhaetian interval. The causation scenarios for these rapid oscillations are at present equivocal, but may in part reflect a biological instability of the carbon cycle following the recovery from the end-Permian extinction event and/or an input of “mantle”-derived CO2 from enhanced volcanic activity. The δ18O values from well-preserved brachiopods from the Tethyan realm range from − 3.9 to − 0.6‰ V-PDB. These values require open marine Triassic seawater δ18O values close to 0‰ V-SMOW for the calculated temperatures to be within the range of tolerance of the coexisting reef-building corals. A 2‰ δ18O increase within the uppermost Cordevolian and early Julian suggests either a distinct temperature decline in the Southern Alps during that time interval, an increase in seawater salinity, or their combination. Oxygen isotope values for the Muschelkalk brachiopods range between − 6.2 and − 2.0‰ and likely reflect a strong influx of meteoric waters into the Germanic Basin that shifted the oxygen isotopes to more negative values.

Two hundred sixty-four δ 18 O and δ 13 C values of Permian articulate brachiopod shells were analyzed and 81 of these values were characterized as well preserved and biostratigraphically well defined. These were then utilized for construction of baseline oxygen and carbon isotope curves for the Permian interval. In addition, 21 δ 13 C whole rock values are reported for the Wordian and Capitanian. The early Permian, Asselian to Artinskian, times are characterized by ∼ 2.5‰ decrease in oxygen isotope values, from ∼ − 0.7‰ to − 3.3‰ (V-PDB). This is attributed to a ∼ 4–7 °C increase in temperature in the Southern Urals, concomitant with the retreat of the Permo-Carboniferous ice sheets and return of the 18 O-depleted melt water into the oceans. The Late Permian samples from Iran (Jolfa at Kuh-e-Ali-Bashi) and China (Meishan) yield δ 18 O values, and presumably temperatures, similar to those that followed the termination of the large-scale glaciation in the Lower Permian. In between, the upper Kungurian to Capitanian samples from the Delaware Basin (Guadalupe Mountains) are enriched in 18 O, at − 1.5‰ to − 3‰. We have no definitive explanation for these data, but tentatively suggest that the “anomaly” can potentially be a result of evaporative enrichment of seawater in 18 O, due to intracratonic arid setting of the basin. The 18 O-enriched nature of the Zechstein samples (− 1.2‰ to + 2.5‰), on the other hand, is in all probability a reflection of the high evaporation rates in the Zechstein sea. The Permian interval is characterized by a relatively constant δ 13 C, at about 4‰. The exceptions are again the brachiopods from the Delaware Basin (Guadalupe Mountains), which show ∼ 1.6‰ increase in the Guadalupian, to values of up to 5.9‰ in the Wordian. A tentative explanation, as in the case of oxygen, is based on the proposition that the semi-enclosed Delaware Basin was likely stratified, with sequestration of the 13 C-depleted carbon to the deeper water layers and a complementary 13 C enrichment in the upper oxygenated layer. The coeval open ocean water DIC may have been similar to that of the remainder of the Permian interval, at ∼ 4‰, as indicated by whole rock carbonate samples from Oman, Sicily, and Iran. In the latest Permian, the trend mimics the well-known δ 13 C drop at the Permian/Triassic boundary.

We have collected rock samples from the abandoned Konrad iron ore mine near Salzgitter/Germany, to elucidate the diagenetic history of the Korallenoolith formation in northwestern Germany. Petrographic and cathodoluminescence investigations showed that the rocks contain a wide range of particles, from primary biogenic material to various generations of interparticle cements and cements in fissures and fractures. The diagenetic environment of cement precipitation was reconstructed using in-situ trace element microanalyses with PIXE, using the Bochum proton microprobe. Furthermore, the different components were analysed for their radiogenic ( 87 Sr/ 86 Sr) isotope signatures. ‘Rim cements’ around echinoid fragments display several stages of cement precipitation. Early cement generations show high Sr concentrations and low 87 Sr/ 86 Sr values. In contrast, later generations have relatively low Sr values, around 300 ppm, and iron concentrations of up to 3%. Fissure cements represent an even later stage of diagenetic history. They consist of calcite, anhydrite and celestite and are characterised by significantly higher 87 Sr/ 86 Sr values. The results of combined trace element and isotopic measurements indicate that the biogenic material was deposited in a shallow marine environment with normal salinity and a relatively high primary availability of iron. The early generations of diagenetic cements have similar, marine, signatures and were therefore precipitated in a closed system, where the trace element and isotopic properties were inherited from the dissolved precursor phases. In contrast, the later generations of rim cements and particularly the fissure cements show completely different and variable chemical signatures. They were precipitated in an open system from formation waters.

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 end-Ordovician was an enigmatic interval in the Phanerozoic, known for massive glaciation potentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and a devastating extinction event. Ice-sheet volumes claimed to be twice those of the Last Glacial Maximum paradoxically coincided with oceans as warm as today. Here we argue that some of these remarkable claims arise from undersampling of incomplete geological sections that led to apparent temporal correlations within the relatively coarse resolution capability of Palaeozoic biochronostratigraphy. We examine exceptionally complete sedimentary records from two, low and high, palaeolatitude settings. Their correlation framework reveals a Cenozoic-style scenario including three main glacial cycles and higher-order phenomena. This necessitates revision of mechanisms for the end-Ordovician events, as the first extinction is tied to an early phase of melting, not to initial cooling, and the largest δ13C excursion occurs during final deglaciation, not at the glacial apex., Claims that the end-Ordovician Earth was characterized by giant ice sheets, yet paradoxically warm oceans and elevated CO2 levels are open to debate. Here, Ghienne et al. examine sedimentary records from low and high palaeolatitude settings and propose a revision of the mechanisms for end-Ordovician events.

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 (

Atmospheric carbon dioxide concentrations are believed to drive climate changes from glacial to interglacial modes1, although geological1,2,3 and astronomical4,5,6 mechanisms have been invoked as ultimate causes. Additionally, it is unclear7,8 whether the changes between cold and warm modes should be regarded as a global phenomenon, affecting tropical and high-latitude temperatures alike9,10,11,12,13, or if they are better described as an expansion and contraction of the latitudinal climate zones, keeping equatorial temperatures approximately constant14,15,16. Here we present a reconstruction of tropical sea surface temperatures throughout the Phanerozoic eon (the past ∼550 Myr) from our database17 of oxygen isotopes in calcite and aragonite shells. The data indicate large oscillations of tropical sea surface temperatures in phase with the cold–warm cycles, thus favouring the idea of climate variability as a global phenomenon. But our data conflict with a temperature reconstruction using an energy balance model that is forced by reconstructed atmospheric carbon dioxide concentrations18. The results can be reconciled if atmospheric carbon dioxide concentrations were not the principal driver of climate variability on geological timescales for at least one-third of the Phanerozoic eon, or if the reconstructed carbon dioxide concentrations are not reliable.

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.

The coral reef in Tutum Bay, Ambitle Island, Papua New Guinea, is exposed to the vigorous discharge of hydrothermal fluids (up to 98°C). This study investigates eight Porites lobata samples that were collected throughout the area of active venting at varying distances from vent sites. A sample from a “non-hydrothermal” coral (C-29), collected 10 km north of Tutum Bay, was analyzed for comparative purposes. Density banding is moderately well developed in these corals and subannual bands are common. For corals from Tutum Bay, δ13C ranges from −4.5 to −1.0‰ and δ18O from −6.0 to −3.8‰, which are relatively depleted values for shallow water Porites. The comparison sample, C-29, has δ13C values ranging from −1.8 to −0.5‰, and δ18O values of −5.4 to −4.6‰. Concentrations of As, Co, Cr, Ga, Ge, Mo, Nb, Ni, Pb, Rb, Se, W, Y and Zr were always below their respective proton probe detection limits. Ba, Br, Cu, Fe, Mn, and Zn were detected in some samples. Sr was detected in all samples and concentrations ra...

A numerical model that couples carbon-sulfur-strontium and atmospheric oxygen cycles is used here to explore the impact of continental growth on the long term (> or =10 8 yrs) evolution of the isotopic composition of seawater. Three growth scenarios are tested: "big bang" generation of continents shortly after the accretion of the Earth and two more gradual scenarios, with a major growth episode around the Archean-Proterozoic boundary. The corresponding 87 Sr/ 86 Sr, delta 34 S, and delta 13 C of seawater and the sizes of the respective crustal sedimentary reservoirs are calculated for each scenario and compared to the available data. The gradual continental growth scenarios yield a better fit to the existing 87 Sr/ 86 Sr and delta 34 S isotope data for ancient seawater than does the "big bang" model and can be in agreement also with the measured seawater delta 13 C, providing C org participates in carbon subduction flux over the Earth history. These scenarios also generate a progressive oxygenation of the ocean/atmosphere system, with a large PO 2 rise coincident with (and due to) the major continental growth event around the Archean-Proterozoic transition, in accord with the geologic record that indicates a major oxidation event in the early Proterozoic. The advancing oxygenation of the planetary exogenic system may therefore be a consequence of tectonic evolution rather than of biological innovations such as the photosystem 2. The latter may have predated considerably the impact of oxygenation visible in the geologic record. In contrast to the above isotope systematics, the model does not approximate well experimental observations of large delta 18 O variations at 10 8 yrs time scales, at least during the Phanerozoic. The reason for this discrepancy may depend on the model structure that permits large variations in the oxygen isotopic composition of seawater only on time scales of 10 9 yrs.

The residual signals of the physical and chemical processes undergone in a river basin are stored in the isotopic composition of river water and are used here to isolate and quantify fluxes of water, energy and carbon for a large boreal river basin. The integrated nature of the river signal is exploited to provide meaningful basin-wide annual averages for fluxes difficult to quantify and extrapolate by studying highly variable interface exchanges at discrete locations. The slope of the linear regression of deuterium (δD) and oxygen (δ18O) isotopes for the Ottawa River is ∼6.0, considerably less than the slope of the local meteoric water line (7.7). This discrepancy is a consequence of evaporative loss from open water bodies and soils and, through a new method, is calculated to be 8.1% of annual precipitation. As well, on the basis of thirty years of daily meteorological and discharge data, annual evapotranspiration for the Ottawa River basin is calculated to be 53.1%. Combining the evaporation and evapotranspiration calculations apportions 45% of the water losses to transpiration. The energy required to drive these cycles is calculated to be 8% of annual solar radiation for total evapotranspiration and 13% of growing season solar radiation for transpiration. These energies are transformed into latent heat. The water use efficiency ratio is used to estimate total fixation of carbon (gross primary production (GPP)) for the basin at 15.6 mol C m−2 yr−1. This rate is substantially greater than the export of carbon via rivers plus rates estimated for carbon respiration in the literature, indicating that the boreal forest is a plausible component of the postulated “missing” carbon sink. Comparison of accumulation rates of C in peatlands and the rates required to account for the missing sink suggest that peat accumulation rates are ∼20 times too slow to account for the missing sink flux. Speculatively, the living biomass of the boreal forest is the dominant sink. Accepting this, the respiration rate needed for a steady state balance between the calculated boreal forest GPP and the missing global carbon sink is found to be around 5.6 mol C m−2 yr−1.

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.

The coral reef that circles Ambitle Island, Papua New Guinea, is exposed to the discharge of a hot, mineralized hydrothermal fluid. The hydrothermal fluids have a pH of {approximately}6 and are slightly reducing and rich in As. Seven individual vents discharge an estimated 1500 g of As per day into an area of approximately 50 x 100 m that has an average depth of 6 m. Despite the amount of As released into the bay, corals, clams, and fish do not show a response to the elevated values. The authors analyzed hydrothermal precipitates for their chemical and mineralogical composition in order to determine As sinks. Two mechanisms efficiently control and buffer the As concentration: (1) dilution by seawater and (2) incorporation in and adsorption on Fe(III) oxyhydroxides that precipitate when the hydrothermal fluids mix with ambient seawater. Fe(III) oxyhydroxides contain up to 76,000 ppm As, by an order of magnitude the highest As values found in a natural marine environment. Following adsorption, As is successfully retained in the Fe(III) oxyhydroxide deposits because oxidizing conditions prevail and high As activity allows for the formation of discrete As minerals, such as claudetite, arsenic oxide, and scorodite.

Submarine, hydrothermal venting occurs at Tutum Bay in shallow (5–10 m) water along the inner shelf that contains a patchy distribution of coral–algal reefs. Two types of venting are observed. (1) Focused discharge of a clear, two-phase fluid from discrete orifices, 10–15 cm in diameter. Discharge temperatures are between 89 and 98°C and estimated flow rates are as high as 300 to 400 l/min. (2) Dispersed or diffuse discharge that consists of streams of gas bubbles ubiquitous in the area. The composition of the gas is mainly CO 2 (92.6–97.9%) with minor amounts of N 2 (2.2–4.7%), O 2 (0.43–0.73%), CH 4 (0.6–2%) and He (∼0.01–0.02%). Based on their geographic position and chemical composition, the vents have been divided into two groups, A and B. Area B vents have higher K, Rb, Sb, Cs, Tl, and As and lower Ca, Li, Mn, Fe, and Sr concentrations. Their chemical difference is likely caused by subsurface mixing of a CO 2 -rich water with a deep reservoir neutral chloride fluid in varying proportions. A two- or possibly three-step process controls fluid evolution and final chemical composition: (1) phase separation in the deep reservoir beneath Ambitle Island produces a high temperature vapor that rises upward and subsequently reacts with cooler ground water to form a low pH, CO 2 -rich water of approximately 150–160°C. (2) The steep topography causes lateral movement of this CO 2 -rich fluid towards the margin of the hydrothermal system where it mixes with the marginal upflow from the deep reservoir. This produces a dilute chloride water of approximately 165°C. A third step may be the entrainment of minor amounts of ground or seawater during its final ascent. Based on a B–Rb/Cs mixing model, it has been estimated that approximately 10% of the deep reservoir fluid reaches the surface. Compared to seawater, the hydrothermal fluids are depleted in Cl, Br, SO 4 , Na, K, Ca, Mg, and Sr and enriched in HCO 3 , B, Si, Li, Mn, Fe, Rb, Cs, Sb, Tl and As. Although some elements are significantly enriched, they do not have a clear impact on ambient seawater composition because their concentration is buffered by mixing and uptake into secondary minerals. Only the surface water in Tutum Bay carries a clear imprint of the hydrothermal fluids.

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.