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The 40Ar/39Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra-marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between 14C and longer-lived isotopic systems that are too imprecise for dating young samples. However, 40Ar/39Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, 238U-230Th disequilibrium, to demonstrate that 40Ar/39Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous 40Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in 40Ar/39Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of 40Ar due to rapid loss to an "infinite reservoir" such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced. [ABSTRACT FROM AUTHOR]

New 238U-230Th and Sr isotope data from 40Ar/39Ar-dated mafic lavas of the composite cone, A.D. 1902 dacitic tephra, and 1922-present dacitic dome lavas from the Santa María-Santiaguito volcanic complex in northwestern Guatemala indicate that genesis of magma beneath the western end of the Central American volcanic arc requires sources much different than previously envisioned. Although U-series data from historical lavas along the Central American volcanic arc, including Guatemala, show variable but small degrees of either 230Th or 238U excess, 72-35 ka basalts and basaltic andesites from the Santa María cone have 15%-26% 238U excesses, among the largest measured in Central America. This implies that the mantle wedge beneath this sector of the arc was significantly modified by slab-derived fluids. A decrease in 87Sr/86Sr and (238U/230Th) ratios over the past 72 k.y. as basaltic andesite gave way to dacite is consistent with fractional crystallization coupled with progressive assimilation of crust that has relatively unradiogenic, mid-oceanic ridge basalt-like Sr isotope composition. Overall, our U-series data along with published 238U/230Th isotope results from Central America demonstrate that (1) slab-fluid flux can be high throughout Central America, including regions of relatively thick crust, and (2) the middle to lower crust beneath northwestern Guatemala may not be dominated by ancient metamorphic and granitic rocks. [ABSTRACT FROM AUTHOR]

Abstract: The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto-Earth and a Mars-sized impactor very early in solar system history. The identical W, O, K, and Cr isotope compositions between materials from the Earth and Moon require that the material from the two bodies were well-homogenized during the collision process. As such, the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the Earth. Two recent studies to obtain high-precision 142Nd/144Nd ratios of lunar mare basalts yielded contrasting results. In one study, after correction of neutron fluence effects imparted to the Nd isotope compositions of the samples, the coupled 142Nd–143Nd systematics were interpreted to be consistent with a bulk Moon having a chondritic Sm/Nd ratio [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science 312, 1369–1372]. The other study found that their data on the same and similar lunar mare basalts were consistent with a bulk Moon having a superchondritic Sm/Nd ratio [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett. 262, 505–516]. Delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the Moon and for constraining the types of materials available for accretion into large terrestrial planets such as Earth. To further examine this issue, the same six lunar mare basalt samples measured in Rankenburg et al. [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science 312, 1369–1372] were re-measured for high-precision Nd isotopes using a multidynamic routine with reproducible internal and external precisions to better than ±3ppm (2σ) for 142Nd/144Nd ratios. The measurements were repeated in a distinct second analytical campaign to further test their reproducibility. Evaluation of accuracy and neutron fluence corrections indicates that the multidynamic Nd isotope measurements in this study and the 3 in Boyet and Carlson [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett. 262, 505–516] are reproducible, while static measurements in the previous two studies show analytical artifacts and cannot be used at the resolution of 10ppm to determine a bulk Moon with either chondritic or superchondritic Sm/Nd ratios. The multidynamic data are best explained by a bulk Moon with a superchondritic Sm/Nd ratio that is similar to the present-day average for depleted MORB. Hafnium isotope data were collected on the same aliquots measured for their 142Nd/144Nd isotope ratios in order to assess if the correlation line for 142Nd–143Nd systematics reflect mixing processes or times at which lunar mantle sources formed. Based on the combined 142Nd–143Nd–176Hf obtained we conclude that the 142Nd–143Nd correlation line measured in this study is best interpreted as an isochron with an age of 229+24 −20Ma after the onset of nebular condensation. The uncertainties in the data permit the sources of these samples to have formed over a 44Ma time interval. These new results for lunar mare basalts are thus consistent with a later Sm–Nd isotope closure time of their source regions than some recent studies have postulated, and a superchondritic bulk Sm/Nd ratio of the Moon and Earth. The superchondritic Sm/Nd signature was inherited from the materials that accreted to make up the Earth–Moon system. Although collisional erosion of crust from planetesimals is favored here to remove subchondritic Sm/Nd portions and drive the bulk of these bodies to superchondritic in composition, removal of explosive basalt material via gravitational escape from such bodies, or chondrule sorting in the inner solar system, may also explain the compositional features that deviate from average chondrites that make up the Earth–Moon system. This inferred superchondritic nature for the Earth similar to the modern convecting mantle means that there is no reason to invoke a missing, subchondritic reservoir to mass balance the Earth back to chondritic for Sm/Nd ratios. However, to account for the subchondritic Sm/Nd ratios of continental crust, a second superchondritic Sm/Nd mantle reservoir is required. [Copyright &y& Elsevier]

Abstract: A brief period of enhanced 10Be flux that straddles the interstadial warm period known as Dansgaard–Oeschger event 10 in Greenland and its counterpart in Antarctica, the Antarctic Isotope Maximum 10 is but one consequence of the weakening of Earth''s magnetic field associated with the Laschamp excursion. This 10Be peak measured in the GRIP ice core is dated at 41,250 y b2k (= before year 2000 AD) in the most recent GICC05 age model obtained from the NorthGRIP core via multi-parameter counting of annual layers. Uncertainty in the age of the 10Be peak is, however, no better than ±1630 y at the 95% confidence level, reflecting accumulated error in identifying annual layers. The age of the Laschamp excursion [Guillou, H., Singer, B.S., Laj, C., Kissel, C., Scaillet, S., Jicha, B., 2004. On the age of the Laschamp geomagnetic excursion. Earth Planet. Sci. Lett. 227, 331-343.] is revised on the basis of new 40Ar/39Ar, unspiked K–Ar and 238U–230Th data from three lava flows in the Massif Central, France, together with the 40Ar/39Ar age of a transitionally magnetized lava flow at Auckland, New Zealand. Combined, these data yield an age of 40,700±950 y b2k, where the uncertainty includes both analytical and systematic (40K and 230Th decay constant) errors. Taking the radioisotopic age as a calibration tie point suggests that the layer-counting chronologies for the NorthGRIP and GISP2 ice cores are more accurate and precise than previously thought at depths corresponding to the Laschamp excursion. [Copyright &y& Elsevier]

Abstract: A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (230Th/238U) >1 and span a large range in (230Th/232Th) ratios, and most basalts have higher (230Th/232Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~20–25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate 230Th excesses (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by 187Os/188Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large 230Th excesses (25–48%) and high 187Os/188Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization. [Copyright &y& Elsevier]

Sedimentary facies of the Eocene Green River Formation reflect a rapid increase in water supply to Lake Gosiute ca. 49 Ma, marked by a stratigraphic fill-to-spill surface. Deposits below this surface constitute repetitive lacustrine expansion-desiccation cycles, whereas those above consist of continuous profundal lacustrine mudstone, grading upward into volcaniclastic deltaic sandstone. Above the fill-to-spill surface, calcitic mudstone δ18O decreases from ∼+26‰ to +20‰ over an interval representing ∼100 k.y. We interpret this shift to have resulted from capture of a foreland river (or rivers) that drained higher topography north of Lake Gosiute, most likely in north-central Idaho. Accurate paleoelevation estimates derived from stable isotopic records in intermontane basins thus may require detailed knowledge of regional drainage systems. [ABSTRACT FROM AUTHOR]

Abstract: Eclogites from the Western Gneiss Region (WGR), Norway, that formed over a wide range in temperature (∼600 °C to >850 °C) and pressure (high- and ultrahigh-pressure conditions; UHP) have a diverse range of Lu–Hf and Sm–Nd ages that correlate with their P–T histories. Three eclogites (∼676–850 °C and ∼2.0–2.85 GPa) yield Lu–Hf ages of 419.5±4.3 Ma, 416.3±3.7 and 412.0±4.7 Ma, and two Sm–Nd ages of 402.7±4.6 and 398.3±5.5 Ma. The former are interpreted to date early garnet growth and the latter are interpreted to date near-peak metamorphism. These results support previous proposals that, for eclogites which recrystallized below Lu–Hf and Sm–Nd closure temperatures, the differences in ages for the two isotopic systems constrain the period of prograde garnet growth. In the case of the WGR, garnet growth is estimated to have occurred over a period of at least 20 m.y., beginning at ∼425–420 Ma. In contrast, two UHP eclogites (>800 °C and >3 GPa) yield significantly younger and overlapping Lu–Hf and Sm–Nd ages of 390–370 Ma; these ages are significantly younger than ages that are generally accepted for peak eclogite-facies conditions in the WGR, although they overlap 40Ar/39Ar ages that have been previously determined on white mica from the same samples or localities, indicating that the Lu–Hf and Sm–Nd ages may reflect rapid exhumation and cooling during the latest stages of HP and UHP metamorphism in the WGR. [Copyright &y& Elsevier]

Rubidium-Sr and Sm-Nd isotope data and rare earth element (REE) concentrations of the metasedimentary rocks within the Zermatt-Saas (ZS) ophiolite complex of the western Alps are used to investigate element mobility and to determine the provenance of the metasediments in order to place constraints on the precoHisional paleogeography of the Piemont-Ligurian portion of the neo-Tethys ocean. Present-day 87Sr/86Sr variations for the ZS metasediments scatter about an early Tertiary Alpine metamorphic age, whereas local nappes that have been interpreted to reflect African/Apulian and European basements scatter about a Variscan-like age; this suggests 87Sr/86Sr isotope systematics were nearly completely homogenized for most of the ZS metasediments during early Tertiary metamorphism, probably because they were relatively wet prior to metamorphism. In contrast to the Sr isotope data, REE data and Nd isotope compositions of the ZS metasediments overlap those of average upper continental crust, average shale, and the local nappes, and Nd model ages of the ZS metasediments overlap with those of Variscan-age rocks. These relations suggest that the REEs of the ZS metasediments were not disturbed during high-to ultra-high pressure Alpine metamorphism. Based on REE data, Nd isotope compositions, and mixing models, the ZS metasediments comprise two groups that require distinct source terranes: one group (Group I) seems to be mixing of an old, crustal component, such as the paragneissic basement nappe samples, with metasediments similar to the second group (Group II). The source material for Group H is dominated by homogenization of the Variscan-like orthogneissic basement nappe samples. The provenance of Group I samples is interpreted to be local, where source nappes must have been proximal to the Piemont-Ligurian basin prior to Alpine convergence. The similarity in dispersion of Nd isotope compositions of the metasediments and likely source terranes suggests that the metasediments reflect deposition in small, isolated basins early in the formation of the Piemont-Ligurian ocean. [ABSTRACT FROM AUTHOR]

Abstract: Iron isotope fractionation between aqueous Fe(II) and biogenic magnetite and Fe carbonates produced during reduction of hydrous ferric oxide (HFO) by Shewanella putrefaciens, Shewanella algae, and Geobacter sulfurreducens in laboratory experiments is a function of Fe(III) reduction rates and pathways by which biogenic minerals are formed. High Fe(III) reduction rates produced 56Fe/54Fe ratios for Fe(II)aq that are 2–3‰ lower than the HFO substrate, reflecting a kinetic isotope fractionation that was associated with rapid sorption of Fe(II) to HFO. In long-term experiments at low Fe(III) reduction rates, the Fe(II)aq-magnetite fractionation is −1.3‰, and this is interpreted to be the equilibrium fractionation factor at 22°C in the biologic reduction systems studied here. In experiments where Fe carbonate was the major ferrous product of HFO reduction, the estimated equilibrium Fe(II)aq-Fe carbonate fractionations were ca. 0.0‰ for siderite (FeCO3) and ca. +0.9‰ for Ca-substituted siderite (Ca0.15Fe0.85CO3) at 22°C. Formation of precursor phases such as amorphous nonmagnetic, noncarbonate Fe(II) solids are important in the pathways to formation of biogenic magnetite or siderite, particularly at high Fe(III) reduction rates, and these solids may have 56Fe/54Fe ratios that are up to 1‰ lower than Fe(II)aq. Under low Fe(III) reduction rates, where equilibrium is likely to be attained, it appears that both sorbed Fe(II) and amorphous Fe(II)(s) components have isotopic compositions that are similar to those of Fe(II)aq. The relative order of δ56Fe values for these biogenic minerals and aqueous Fe(II) is: magnetite > siderite ≈ Fe(II)aq > Ca-bearing Fe carbonate, and this is similar to that observed for minerals from natural samples such as Banded Iron Formations (BIFs). Where magnetite from BIFs has δ56Fe >0‰, the calculated δ56Fe value for aqueous Fe(II) suggests a source from midocean ridge (MOR) hydrothermal fluids. In contrast, magnetite from BIFs that has δ56Fe ≤0‰ apparently requires formation from aqueous Fe(II) that had very low δ56Fe values. Based on this experimental study, formation of low-δ56Fe Fe(II)aq in nonsulfidic systems seems most likely to have been produced by dissimilatory reduction of ferric oxides by Fe(III)-reducing bacteria. [Copyright &y& Elsevier]

Estimation of burial rates and duration of prograde metamorphism of ultra-high-pressure (UHP) rocks (T=590–630°C, P=2.7–2.9 GPa) of the Zermatt–Saas ophiolite from Lago di Cignana, Italy, may be made through combined Lu–Hf and Sm–Nd garnet geochronology in conjunction with petrologic estimates of the prograde P–T path. We report a Lu–Hf garnet–omphacite–whole-rock isochron age of 48.8±2.1 Ma from the UHP locality at Lago di Cignana, which stands in contrast to the Sm–Nd age of 40.6±2.6 Ma [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438] obtained from the same sample and mineral material. The Sm–Nd and Lu–Hf ages, as well as other ages determined on metamorphic garnet, zircon and white mica [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438; Mayer et al., Eur. Union Geosci. 10 (1999) Abstr. 809; Rubatto et al., Contrib. Mineral. Petrol. 132 (1998) 269–287; Dal Piaz et al., Int. J. Earth Sci. 90 (2001) 668–684] from Lago di Cignana and elsewhere in the Zermatt–Saas ophiolite, lie within a range of ∼50–38 Ma, which we interpret to encompass the duration of prograde metamorphism, and possibly the duration of garnet growth. The difference in measured Sm–Nd and Lu–Hf ages from Cignana can be accounted for by expected core to rim variations in Lu, Hf, Sm, and Nd. The measured yttrium content in garnet, which may be a proxy for Lu, is highest in garnet core and lowest in the mineral rim, generally following a profile that is predicted by Rayleigh fractionation. Preferential enrichment of Lu in the core produces a Lu–Hf age that is weighted toward the older garnet core. Sm–Nd ages, as predicted by Rayleigh fractionation of Sm and Nd during garnet growth, however, reflect later grown garnet as compared to Lu–Hf ages. The difference in Sm–Nd and Lu–Hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as Sm–Nd and Lu–Hf blocking temperatures were not exceeded for a long period of time. Based on the 12 Myr duration of prograde garnet growth estimated in this study, burial rates for rocks at Lago di Cignana were on the order of 0.23–0.47 cm/yr. These values correlate with continuous shortening rates of 0.4–1.4 cm/yr between the European plate and the African–Adriatic promontory between 50 and 38 Ma, which is on the order of that calculated for plate velocities from plate reconstructions, suggesting that the Zermatt–Saas ophiolite may have remained well-coupled to the down-going slab to UHP conditions. [Copyright &y& Elsevier]

Describes the role of strontium isotopes in providing records of the past changes in weathering and regional drainage patterns. Documentation of Eocene Green River Formation; Succession of transgressive stromatolite facies; Connection between radiogenic isotope with dolomicrite.

Tufa in the Little Mesa area of the northern Bridger Basin has been interpreted to record carbonate deposition via subaqueous and subaerial springs emanating near the shoreline of Eocene Lake Gosiute. Sedimentary facies record an overall transgression, culminating with mound structures that reach up to 9 m in height and 40 m in diameter. Mounds exhibit a strong positive, linear covariance between δ13C and δ18O, defining a slope of ~1. Similar trends occur in many other paleolake deposits, where they are interpreted to reflect changes in evaporation, atmospheric CO 2 exchange, and organic matter burial. However, δ13C and δ18O in this study also covary strongly with 87Sr/86Sr, a new finding that is inconsistent with previously proposed mechanisms. We conclude that Little Mesa isotopic trends reflect mixing of groundwater with low 87Sr/86Sr, δ18O and δ13C and lake water with opposite characteristics. Low 87Sr/86Sr in groundwater likely resulted from interaction with marine carbonate strata within the Sevier fold and thrust belt to the west, whereas drainage from Precambrian-cored uplifts that bounded Lake Gosiute to the north, east, and south was responsible for higher lake water ratios. Little Mesa carbonate facies are all less radiogenic than any time-equivalent facies near the center of the basin, implying horizontal and vertical gradients in Lake Gosiute 87Sr/86Sr. Previous studies have shown that the lowest 87Sr/86Sr in basin center deposits correspond to lake highstands. Results of this study support the hypothesis that climatic modulation of surface runoff and spring emanations from the Sevier belt were principally responsible for precessional-scale expansions and contractions of Lake Gosiute. More broadly, groundwater discharge may represent an important but underappreciated contributor to covariance between 87Sr/86Sr ratios, δ13C and δ18O in closed paleolake systems. • Spring-mound facies in the Little Mesa deposited during transgression. • Strong positive correlation between 87Sr/86Sr ratio, δ13C and δ18O • δ13C and δ18O covariance indicate groundwater as a contributor to paleolake. • Spring-mounds carbonate in the Little Mesa deposited during Wilkins Peak Member. [ABSTRACT FROM AUTHOR]

• Alpine soils were studied in the Uinta Mountains (Utah, USA). • Soils are geochemically intermediate between rock and dust end members. • Mixing models were constructed from Sr and Nd isotope values. • Alpine soils contain ~50 to 80% dust. Profiles of alpine soils in the Uinta Mountains (Utah, USA) were investigated to determine how long-term dust deposition has influenced soil properties in this environment. Field studies were focused on four above-treeline sites, all of which were apparently beyond the reach of erosive glacial ice during the Pleistocene. Modern dust, soil A and B horizons, and local bedrock were compared in terms of major and trace element geochemistry, along with Sr and Nd isotope compositions. In all cases, soil samples are a mixture of dust and local bedrock end members, with A horizons more closely resembling dust, and B horizons more similar to bedrock. Calculations estimate that these soil profiles contain ~50 to 80% dust, which is effectively mixed downward into the solum by cryoturbation. Because these landscape positions were not glaciated, the total amount of dust contained within soil profiles is large relative to sites at lower elevation within the glacial limit. In addition to altering physical properties of these soil profiles, including horizonation and texture, deposition of dust rich in base cations positively influences soil fertility. [ABSTRACT FROM AUTHOR]

• High Fe isotope exchange between Fe(II) aq and ferrihydrite-organic matter (OM). • No mineralogical transformation of ferrihydrite. • Equilibrium 56Fe/54Fe fractionation between Fe(II)aq and ferrihydrite-OM is −2.36‰. • This value differs from equilibrium Fe(II) aq -pure ferrihydrite fractionation (−3.2‰). • Different equilibrium fractionation implies altered Fe bonds in ferrihydrite by OM. Low-temperature geochemical cycling of Fe, C, nutrients, and toxic metals in nature are largely regulated by the stability and reactivity of fine-grained ferrihydrite. The presence of impurities such as C and Si in ferrihydrite structure are found to inhibit the rapid transformation of ferrihydrite to more stable iron oxides upon interaction with aqueous Fe(II). Therefore, understanding the factors controlling the reactivity of ferrihydrite, especially in the presence of C, is critical to evaluate the role of ferrihydrite in geochemical cycles. Equilibrium stable isotope fractionations are fundamental thermodynamic properties and therefore equilibrium fractionation in 56Fe/54Fe ratios reflects the nature of Fe bonding in ferrihydrite and its reactivity. In this study, we investigated stable Fe isotope fractionation between aqueous Fe(II) and ferrihydrite-organic matter coprecipitates to evaluate whether previously documented inhibition of mineralogical transformation by organic C is accompanied by changes in Fe isotope fractionation. Experiments conducted using Suwannee River natural organic matter (SRNOM) that was coprecipitated with ferrihydrite (molar C:Fe = 1.2) produced an equilibrium 56Fe/54Fe fractionation of −2.36 ± 0.26‰ between aqueous Fe(II) and ferrihydrite-SRNOM coprecipitates. This fractionation factor significantly differs from that previously determined between Fe(II) aq and pure ferrihydrite (−3.20‰) but is similar to that measured for Fe(II) aq -Si-ferrihydrite (−2.58 ± 0.14‰, molar Fe:Si = 1). Furthermore, the addition of C in the ferrihydrite structure at molar C:Fe ∼1.2 markedly increased the extent of Fe isotope exchange to a similar degree as observed with Si-ferrihydrite with molar Fe:Si ∼1. These observations suggest similar effects on both bonding and reactivity of ferrihydrite upon addition of equimolar C and Si. Due to the coexistence of ferrihydrite with organic matter in nature (e.g., wetlands), these results are important for understanding Fe isotope fractionation and exchange kinetics during mineral-fluid interactions in natural ferrihydrite-bearing systems. [ABSTRACT FROM AUTHOR]

Chocolate Pots hot spring (CP) is an Fe-rich, circumneutral-pH geothermal spring in Yellowstone National Park. Relic hydrothermal systems have been identified on Mars, and modern hydrothermal environments such as CP are useful for gaining insight into potential pathways for generation of biosignatures of ancient microbial life on Earth and Mars. Fe isotope fractionation is recognized as a signature of dissimilatory microbial iron oxide reduction (DIR) in both the rock record and modern sedimentary environments. Previous studies in CP have demonstrated the presence of DIR in vent pool deposits and show aqueous-/solid-phase Fe isotope variations along the hot spring flow path that may be linked to this process. In this study, we examined the geochemistry and stable Fe isotopic composition of spring water and sediment core samples collected from the vent pool and along the flow path, with the goal of evaluating whether Fe isotopes can serve as a signature of past or present DIR activity. Bulk sediment Fe redox speciation confirmed that DIR is active within the hot spring vent pool sediments (but not in more distal deposits), and the observed Fe isotope fractionation between Fe(II) and Fe(III) is consistent with previous studies of DIR-driven Fe isotope fractionation. However, modeling of sediment Fe isotope distributions indicates that DIR does not produce a unique Fe isotopic signature of DIR in the vent pool environment. Because of rapid chemical and isotopic communication between the vent pool fluid and sediment, sorption of Fe(II) to Fe(III) oxides would produce an isotopic signature similar to DIR despite DIR-driven generation of large quantities of isotopically light solid-associated Fe(II). The possibility exists, however, for preservation of specific DIR-derived Fe(II) minerals such as siderite (which is present in the vent pool deposits), whose isotopic composition could serve as a long-term signature of DIR in relic hot spring environments.

The causal effects among uplift, climate, and continental weathering cannot be fully addressed using presently available geochemical proxies. However, stable potassium (K) isotopes can potentially overcome the limitations of existing isotopic proxies. Here we report on a systematic investigation of K isotopes in dissolved load and sediments from major rivers and their tributaries in China, which have drainage basins with varied climate, lithology, and topography. Our results show that during silicate weathering, heavy K isotopes are preferentially partitioned into aqueous solutions. Moreover, δ 41 K values of riverine dissolved load vary remarkably and correlate negatively with the chemical weathering intensity of the drainage basin. This correlation allows an estimate of the average K isotope composition of global riverine runoff (δ 41 K = -0.22‰), as well as modeling of the global K cycle based on mass balance calculations. Modeling incorporating K isotope mass balance better constrains estimated K fluxes for modern global K cycling, and the results show that the δ 41 K value of seawater is sensitive to continental weathering intensity changes. Thus, it is possible to use the δ 41 K record of paleo-seawater to infer continental weathering intensity through Earth's history., Competing Interests: The authors declare no conflict of interest.

Banded iron formations (BIFs) record a time of extensive Fe deposition in the Precambrian oceans, but the sources and pathways for metals in BIFs remain controversial. Here, we present Fe- and Nd-isotope data that indicate two sources of Fe for the large BIF units deposited 2.5 billion y ago. High-εNd and -δ(56)Fe signatures in some BIF samples record a hydrothermal component, but correlated decreases in εNd- and δ(56)Fe values reflect contributions from a continental component. The continental Fe source is best explained by Fe mobilization on the continental margin by microbial dissimilatory iron reduction (DIR) and confirms for the first time, to our knowledge, a microbially driven Fe shuttle for the largest BIFs on Earth. Detailed sampling at various scales shows that the proportions of hydrothermal and continental Fe sources were invariant over periods of 10(0)-10(3) y, indicating that there was no seasonal control, although Fe sources varied on longer timescales of 10(5)-10(6) y, suggesting a control by marine basin circulation. These results show that Fe sources and pathways for BIFs reflect the interplay between abiologic (hydrothermal) and biologic processes, where the latter reflects DIR that operated on a basin-wide scale in the Archean.