Composition Of Ancient Seawater Research Articles

How does weathering influence geochemical proxies in Paleoproterozoic banded iron formations? A case study from outcrop samples of 2.46 Ga banded iron formation, Hamersley Basin, Western Australia

Trace metal and rare earth element (REE) abundances in banded iron formations are critical for assessing the chemical composition of ancient seawater and the long-term evolution of the ocean-atmosphere system. Recent studies, however, have highlighted the potential effects of outcrop weathering, raising concerns about whether banded iron formation samples are suitable proxies for ancient redox conditions or if exposure to surficial weathering regimes may have altered key geochemical signals. Here, we present a detailed, high-resolution study of several banded iron formation outcrop samples from the Hamersley Basin, Western Australia, to investigate microscale differences in composition between banded iron formation and weathered surfaces (i.e., weathered crusts). Elemental mapping and bulk-rock geochemical analyses reveal that weathered crust is more enriched in most elements than the banded iron formation, except for silica, which is significantly depleted. There is also a significant loss of redox-sensitive elements (RSEs) in the weathered surface, which suggests that outcrop samples have been affected by higher degrees of chemical leaching than physical erosion. These results are significant, because we clearly show that the geochemical characteristics of the weathered surface—irrespective of how it formed—are distinct from those of the remainder of the sample. This means that with sufficient screening of samples for obvious signs of alteration, banded iron formation outcrop samples may indeed be used as a reliable proxy for the evolution of Earth’s coupled ocean-atmosphere system. This increases the volume of easily accessible Precambrian sample material, so that researchers no longer solely need to rely on core recovered through costly drilling programs.

The Paleoproterozoic Zhaigou banded iron formation in the Fuping Complex, North China Craton: Geochemistry, geochronology and implications for genesis and tectonic setting

Precambrian Banded iron formations (BIFs) provide important insights on the chemical composition of ancient seawater, as well as the tectonic setting of their formation. The Precambrian BIFs deposits in the North China Craton are widely distributed in the Archean greenstone belts and are composed predominantly of Neoarchean Algoma-type including those in the Anshan-Benxi and eastern Hebei areas of the Eastern Block, together with minor Paleoproterozoic Superior-type BIFs in the southern segment of the trans-North China Orogen. The Zhaigou BIF deposit is associated with Paleoproterozoic Wanzi supracrustal sequence and amphibolite in the Fuping Complex in central North China Craton. Here, we present petrological, geochronological and geochemical data of the meta-sedimentary rocks, BIF ores and amphibolites from the Zhaigou BIF deposit in the Fuping Complex for the first time to constrain the formation age, source characteristics and depositional setting, and evaluate the redox states of the Paleoproterozoic seawater. Whole-rock geochemical data show that the BIF ores are enriched in HREE with low (La/Yb)PAAS values (0.037–0.073), and exhibit positive Eu (3.34–6.21) and Pr (1.05–1.10) anomalies. The PAAS-normalized REY patterns resemble 1:100 mixture of high-temperature hydrothermal fluid and seawater, indicating the involvement of the two end members. In combination with whole-rock geochemical data of BIF ores, trace elements of magnetite and U-Pb ages of detrital zircons from the meta-sedimentary rocks, we propose that crustal components from the Fuping TTG gneiss also contributed to the BIFs. The geochemical features suggest that the protolith of amphibolite is basalt formed in island arc setting. The negative Ce anomaly (Ce/Ce* = 0.81 ∼ 0.90) indicates the deposition of the Zhaigou BIF ore in an oxidizing environment. Zircon U-Pb data on the BIF ore yield weighted mean 207Pb/206Pb ages of 2028 ± 33 Ma and 1874 ± 52 Ma, representing the formation age of the Zhaigou BIF deposit and the subsequent metamorphic event, respectively. The Zhaigou BIF deposit belongs to the Algoma-type which shows close association with the Paleoproterozoic sedimentary sequence and basaltic magmatism in oceanic island arc setting. The Paleoproterozoic seawater forming the Zhaigou deposit was oxidized after the Great Oxygenation Event (GOE) during the terrane assembly in the trans-North China Orogen.

Evaluation and optimisation of the dissolution method for bulk analysis of carbonate neodymium isotopic composition based on chemical leaching of carbonate certified reference materials

The Nd isotopic composition of ancient seawater recorded by marine carbonates is valuable for revealing temporal changes in continental weathering inputs and ocean circulation patterns. When analysing carbonate Nd isotopic composition, it is important to extract the primary carbonate portion without bringing in contaminative Nd released from non-carbonate phases, such as clay minerals in sample dissolution. However, there has been no specific research on the dissolution method required for neodymium isotope analysis of carbonates, whereas different dissolution methods vary significantly in selecting the appropriate acid and dissolution procedures. In this study, we conducted a set of chemical leaching experiments based on carbonate reference materials with high purity and excellent homogeneity, aiming to evaluate and optimise the dissolution method for bulk analysis of carbonate neodymium isotopes.We selected a suite of reference materials, including carbonate certified reference materials and non-carbonate minerals, such as kaolinite, montmorillonite, illite, and Fe-Mn nodules, and created a set of artificial carbonate mixed reference materials with different purities. A set of leaching experiments were carried out on these mixed reference materials. The results showed that dilute acetic acid should be preferentially selected over hydrochloric and nitric acids to dissolve carbonate rocks to obtain the Nd isotopic composition. Our study revealed that the carbonate Nd isotopes are not prone to being affected by clay minerals, as the Nd contamination released from clay minerals was limited, especially for samples with purity higher than 85% (m/m). Contrarily, the Nd contamination from Fe-Mn nodules is significant. Regarding the optimised protocol for carbonate Nd isotope analysis, we suggest a one-step leaching protocol to dissolve approximately 60% (m/m) of the total sample for sample purity above 85% (m/m). Specifically, 4 mL of 2.5% (v/v) HAc is used to dissolve 100 mg of the sample, and the leachate is analysed for Nd isotopes. For sample purity lower than 85%, we suggest a two-step leaching protocol in which 30–40% (m/m) of the sample is preleached and discarded, and another 30–40% (m/m) of the sample is dissolved to prepare for Nd isotope analysis. Specifically, 100 mg of the sample is first preleached with 2.5 mL 2.5% (v/v) HAc and then dissolved in 2.5 mL 2.5% (v/v) HAc to prepare for Nd isotope analysis. All the results indicate that accurate data of Nd isotopes can be easily obtained, which potentially broadens the application of carbonate Nd isotope. Based on the chemical leaching technique, we first report high-precision Nd isotope ratios for two carbonate standards (JDo-1 and IAEA-B-7) that can be used for quality control in Nd isotope analyses of carbonates.

Li-isotope exchange during low-temperature alteration of the upper oceanic crust at DSDP Sites 417 and 418

The record of the Li-isotopic composition of ancient-seawater has the potential to provide important insight into the Earth system. However, we still have an incomplete understanding of the controls on the magnitude and isotopic compositions of the fluxes of Li into and out of the ocean. An important sink for Li from seawater is through low-temperature hydrothermal alteration of the upper oceanic crust. Here we present a detailed study of three adjacent drill cores in 120 Myr old Atlantic crust (Holes 417A, 417D and 418A) that had very different sedimentation histories. Samples from Hole 417A, drilled on a topographic high and exposed to seawater for >20 Myr before sediment started to accumulate, have much higher Li contents (up to ∼70 ppm) than samples from the other sites where sediment began accumulating immediately after crustal accretion (up to ∼20 ppm). Despite this difference in Li content, the δ7Li of the altered basalts in all cores increases with depth over the upper ∼40 m from around −1‰ at the lava-sediment interface to 6 ± 2‰ at ∼40 m depth, remaining roughly constant with depth after this. If the uppermost lavas equilibrated with seawater, with a bulk fractionation factor of ∼0.980 ± 0.003, then the isotopic composition of seawater 120 Myr ago was ∼19 ± 3‰, within the range of other estimates for Cretaceous seawater. Fluid-rock reaction models, with the hydrological regime varying as a function of depth and sediment accumulation history, can explain the observed rock compositions. However, modelling shows that complexities related to the time-varying hydrological regime, along with the possibility of time-varying δ7Li of seawater, mean that the Li-isotopic composition of altered oceanic lavas cannot be uniquely interpreted. The bulk Li content and isotopic composition of the well-sampled 120 Myr crust in Holes 417A, 417D and 418A suggest that Cretaceous upper oceanic crust was a sink for ∼4×109 mol yr−1 of Li. Surprisingly, the bulk Li-isotopic composition of the lavas sampled at these sites (∼6‰) is not much higher than that of fresh MORB, and within uncertainty of that of high-temperature hydrothermal vent fluids; i.e., high- and low-temperature alteration processes approximately balanced one another for Li-isotopes in the late Cretaceous. This could be fortuitous; alternatively, if the ocean was at steady state the main source of Li to the ocean at this time may have had a δ7Li similar to that of high-temperature hydrothermal fluids.

Rhenium-Osmium isotope systematics of an Early Mesoproterozoic SEDEX polymetallic pyrite deposit in the North China Craton: Implications for geological significance and the marine osmium isotopic record

The rhenium-osmium (Re-Os) isotopic system preserved in ancient seafloor hydrothermal sulfide deposits can be used to gain insight into the Os composition of ancient seawater. In particular, the age of sedimentary exhalative (SEDEX) deposits can be determined by measuring Re-Os isotopes in syn-depositional pyrite, a common constituent of SEDEX deposits. Here we report on the Gaobanhe sediment-hosted, polymetallic pyrite deposit which is hosted within the Gaoyuzhuang Formation in the central part of the northern margin of the North China Craton (NCC). Re-Os isotopic data from syn-depositional pyrite yield an isochron age of 1505 ± 55 Ma, constraining the timing of primary mineralization, as well as the minimum age of the host sediments to the Mesoproterozoic. Analyzed pyrite grains indicate an initial 187Os/188Os (Osi) value of 0.181 ± 0.066, suggesting that the mineralizing fluids were an admixture of seawater and submarine hydrothermal fluids, with the Os being primarily derived from seawater. The Osi value is slightly higher than that of Paleoproterozoic seawater (~0.13), suggesting that there was a radiogenic riverine flux to the oceans and that atmospheric oxygen must have been sufficiently high at that time to drive this increased, continental radiogenic Os input to the oceans. We propose that the Re-Os isotopic system, as recorded by these SEDEX deposits, may provide key insights when examining changes in the Os isotopic composition of seawater throughout the Precambrian.

Diagenetic controls on the isotopic composition of carbonate‐associated sulphate in the Permian Capitan Reef Complex, West Texas

AbstractLate Palaeozoic‐age strata from the Capitan Reef in west Texas show facies‐dependent heterogeneity in the sulphur isotopic composition of carbonate‐associated sulphate, which is trace sulphate incorporated into carbonate minerals that is often used to reconstruct the sulphur isotopic composition of ancient seawater. However, diagenetic pore fluid processes may influence the sulphur isotopic composition of carbonate‐associated sulphate. These processes variously modify the sulphur isotopic composition of incorporated sulphate from syndepositional seawater in shelf crest, outer shelf, shelf margin and slope depositional settings. This study used a new multicollector inductively‐coupled plasma mass spectrometry technique to determine the sulphur isotopic composition of samples of individual depositional and diagenetic textures. Carbonate rocks representing peritidal facies in the Yates and Tansill formations preserve the sulphur isotopic composition of Guadalupian seawater sulphate despite alteration of the carbon and oxygen isotopic compositions by meteoric and dolomitizing diagenetic processes. However, sulphur isotopic data indicate that limestones deposited in reef and slope facies in the Capitan and Bell Canyon formations largely incorporate sulphate from anoxic marine‐phreatic pore fluids isotopically modified from seawater by microbial sulphate reduction, despite generally preserving the carbon and oxygen isotopic compositions of Permian seawater. Some early and all late meteoric calcite cements have carbonate‐associated sulphate with a sulphur isotopic composition distinct from that of Permian seawater. Detailed petrographic and sedimentary context for carbonate‐associated sulphate analyses will allow for improved reconstructions of ancient seawater composition and diagenetic conditions in ancient carbonate platforms. The results of this study indicate that carbonate rocks that diagenetically stabilize in high‐energy environments without pore fluid sulphate gradients can provide a robust archive of ancient seawater's sulphur isotopic composition.

Global importance of oxic molybdenum sinks prior to 2.6 Ga revealed by the Mo isotope composition of Precambrian carbonates

Abstract Sedimentary molybdenum (Mo) isotope compositions are a promising paleoredox indicator because the Mo isotope composition of seawater reflects the balance between anoxic and oxic sinks. Most available data are from shales; however, the Mo isotope composition of carbonates also reflects the composition of ancient seawater. Here, we provide an expanded data set of carbonate Mo isotope compositions, including the first data for carbonates older than 2.64 Ga, which we evaluate against a compilation of published data for carbonates, shales, and iron formations spanning geological time. Archean carbonate samples reveal maximum δ98Mo values that are generally above 1‰. These heavy values indicate that Mn(IV)-oxide or Fe(III)-oxide sinks were sufficiently important to influence the Mo isotope composition of seawater as far back as 2.93 Ga. Comparison of Mo isotope and rare earth element data, as well as residence time considerations, indicates that this metal-oxide influence was likely global. Available Mo isotope data for shales over the same time period generally show crustal values, which we attribute to negligible authigenic enrichment of Mo from seawater due to low ambient concentrations and a paucity of euxinic conditions. Our work demonstrates that the carbonate record provides important new insights into marine paleoredox conditions, especially when shale records are absent or unsuitable, and reinforces the emerging paradigm that oxic Mo sinks were important in the marine realm prior to 2.7 Ga.

A comparison of bulk versus laser ablation trace element analyses in banded iron formations: Insights into the mechanisms leading to compositional variability

Banded iron formations (BIF) represent one of the few chemical sedimentary archives available for exploring the redox state of the Precambrian atmosphere-ocean system and the marine availability of biologically critical trace nutrients through time. Temporal records compiled for a number of trace elements show a relatively wide spread in concentrations spanning several orders of magnitude within a single deposit. It has been suggested that this spread is present in both bulk rock digestion and laser ablation data, however, the comparability of data from the two methods has not been thoroughly evaluated. Here, we provide a statistical assessment of how comparable laser ablation data of individual BIF hematite and magnetite grains are to one another, as well as how closely averages of laser ablation data approximate bulk rock values. To do so, we use (laser ablation)-high resolution inductively coupled plasma mass spectrometry ((LA)-HR-ICP-MS) data from 14 BIF slabs and corresponding powders that span four different formations. We focus in particular on BIF from the ≥3.75 Ga Nuvvuagittuq Supracrustal Belt (Québec, Canada) and multiple BIF deposits of the 2.6–2.45 Ga Hamersley Group (North Western Australia), representing deposits separated by over a billion years and of differing metamorphic grade. The scaling between selected transition metals is also evaluated in light of expectations for possible delivery mechanisms of trace elements from seawater to BIF precursor sediments – the adsorption to hydrous ferric oxides, biological vectors, and detrital influence. Our findings show that average trace element concentrations between bulk versus laser ablation data typically vary within an order of magnitude. Furthermore, reproducibility between the two methods appears to be independent of formation age, metamorphic grade, or depositional setting. These findings underscore the overall fidelity of the BIF record at capturing seawater compositional signatures at various scales. We demonstrate that evaluations of ancient seawater chemistry using the BIF record are best served by a combined approach utilizing both bulk rock and laser ablation data, as bulk methods offer the most conservative proxy data for ancient seawater composition but may mask significant enrichments or depletions that are detectable at the grain-scale by laser ablation.

Low temperature stable mineral recrystallization of foraminiferal tests and implications for the fidelity of geochemical proxies

Fossilized foraminiferal tests are widely used as proxies to constrain paleoenvironments, including past sea surface and bottom water temperatures and the chemical composition of ancient seawater. However, relatively little is known about the extent to which calcareous foraminiferal tests exchange with seawater prior to burial. In this study, we used a 45Ca radiotracer approach to quantify the extent and rate of foraminiferal calcite–fluid exchange over a three-month period. Modern foraminiferal tests exhibited faster exchange with a 45Ca-spiked solution (2.6⋅10−4 to 2.0⋅10−5 mol/m2/d) than fossil foraminiferal tests (7.6⋅10−5 to 5.3⋅10−6 mol/m2/d) from ODP Site 807. The impact of dissolved Si on the exchange rate of modern foraminifera was minimal (2.6⋅10−4 to 9.2⋅10−6 mol/m2/d). Unlike foraminiferal calcite, inorganic calcite exchanged at remarkably slow rates (6.6⋅10−6 to 5.9⋅10−7 mol/m2/d) suggesting that biogenic calcite is more reactive than inorganic calcite. Increased Ca2+, Mg2+, and Sr2+ concentrations in fluids from parallel tracer-free reactors indicated preferential dissolution of tests, though there was no overt change in test morphology during experiments (as imaged by scanning electron microscopy and micro-CT scanning). Time-dependent box model simulations of 45Ca, as well as aqueous elemental chemistry, support the hypothesis that calcite–fluid exchange occurred via dissolution–reprecipitation of foraminiferal tests that was induced by intra-test chemical heterogeneity. The short-term exchange reactions investigated in this study are comparable to the reactions that can occur in the seawater column and at the sediment–seawater interface. This work suggests that dissolution–reprecipitation in such settings has the potential to overprint the chemistry of foraminiferal tests without overt physical alteration, which can lead to a potential bias in paleoenvironmental reconstructions. Model-based estimates suggest a maximum of 2–3°C alteration of Mg/Ca-based temperatures over experimental time scales. Model efforts also suggest that the diagenetic trajectories in Mg/Ca–δ18O space, for instance, are impacted by the partition coefficient of Mg in calcite (λMg) as well as by the mechanism of alteration. This is demonstrated for (i) the ratio of exchange to dissolution and (ii) the relative rate of high-Mg-calcite to low-Mg-calcite reaction. We have illustrated that the mechanism of alteration, in addition to water-rock ratios, determine diagenetic trajectories. Finally, surface reaction has implications for our ability to constrain fractionation factors in experimental and natural systems. The experimental observations suggest that exchange between tests and solution is nominal over ∼100 days. Experiments aimed at deducing fractionation factors rely on the equilibration of bulk solid and fluid, since these are the readily analyzed reservoirs. We quantify the fraction of carbonate (>5%) that must react so that surface reaction does not impact the interpretation of Ca isotopic fractionation factors, and discuss the implications for more “fluid-dominated” elements such as Mg and O.

Cretaceous seawater and hydrothermal fluid compositions recorded in abiogenic carbonates from the Troodos Ophiolite, Cyprus

We evaluate the potential of ophiolites as archives of paleoseawater and hydrothermal fluid compositions by analysing the chemical and isotopic composition of abiogenic carbonates, precipitated from fluids within the oceanic crust of the 91 Ma Troodos Ophiolite, Cyprus. Calculated variations in fluid Mg/Ca, Sr/Ca and 87Sr/86Sr with temperature within the upper sections of the ophiolite are similar to those from drilled oceanic crust, and yield literature values for late Cretaceous seawater Mg/Ca, Sr/Ca and 87Sr/86Sr. This indicates that carbonates from ophiolites could be used to estimate the composition of ancient seawater at times before the age of the oldest preserved in-situ oceanic crust. Whereas most carbonates recovered from in-situ oceanic crust were precipitated at temperatures <60 °C, abiogenic carbonates from the Troodos Ophiolite formed over a temperature range of 7 °C to 218 °C. These provide unique insights into the chemical and mineralogical processes that transform seawater into a high temperature hydrothermal fluid within the oceanic crust. We use ‘hydrothermal variation diagrams’ of Mg/Ca, Sr/Ca, 87Sr/86Sr and δ44/40Ca versus calculated temperature (δ18O) to trace this fluid evolution within the Troodos oceanic crust. We find that successive fluid-crust-interaction, the precipitation of Mg- and Ca-bearing minerals and the early formation of anhydrite (>44 °C) gradually transform Cretaceous seawater into a Troodos hydrothermal fluid. Comparison of the Troodos data with a global dataset of abiogenic carbonates from in-situ oceanic crust shows that the chemical pathways of low-temperature fluid evolution are similar for all Cretaceous sites. These different sites represent varied geotectonic settings (mid-ocean ridge vs. suprasubduction zone), with different basement composition (basalt, basaltic andesite/boninite) and situated in different ocean basins (Atlantic, Pacific, Mediterranean [Tethys]). The similarity in the carbonate record indicates that these differences do not significantly influence seafloor weathering and hydrothermal alteration at low temperatures. However, abiogenic carbonates from younger oceanic crust differ from the Cretaceous trends and follow different fluid evolution pathways. This indicates, that temporal variations in the composition of seawater may control the nature and the extent of seafloor weathering and hydrothermal alteration at low temperatures. A thermodynamic model of fluid-crust interaction, in which modern and Cretaceous seawater are heated to 200 °C while an average Troodos basaltic andesite is successively added under otherwise identical conditions predicts that fluid evolution and alteration of the oceanic crust were different in the Cretaceous than they are today, and that initial seawater chemistry affects the nature and the extent of seafloor alteration up to moderate fluid temperatures. For example, twice the amount of carbonate formed during alteration of the oceanic crust in the Cretaceous compared to modern times, indicating that the flux of CO2 from the hydrosphere-atmosphere system into the oceanic crust was greater in the Cretaceous than it is nowadays, and that it probably varied throughout geologic time.

Geochemistry, U-Pb zircon geochronology and Sm-Nd isotopes of the Xincai banded iron formation in the southern margin of the North China Craton: Implications on Neoarchean seawater compositions and solute sources

Banded iron formations (BIFs) that are distal from submarine volcanic activity and generally lack direct relationships with volcanic rocks have been extensively used as proxies to infer the chemical composition of ancient seawater and trace the effects of continental weathering at the time of their deposition. Here, we discuss the major and trace elements and Sm-Nd isotopic geochemistry of the poorly-studied Xincai BIF, located west of the Superior-type Huoqiu BIF along a NW-SE trending supracrustal belt in the southern margin of the North China Craton, to understand the compositions and solute sources of seawater during the Xincai BIF precipitation. U-Pb zircon dating of granitic intrusive rock and biotite schist interlayer of the BIF constrains the depositional age of the Xincai BIF to 2.5–2.7 Ga for the first time, indicating that the Xincai BIF could be part of the Huoqiu BIF or consist of contemporaneous sediments formed in a similar tectonic-depositional environment. Shale-normalized rare earth element and yttrium (REY) patterns of the bulk and individual band samples display features that are characteristic of other Archean BIFs with light REE depletion relative to heavy REE, positive LaSN, GdSN and YSN anomalies, indicative of pure chemical sedimentation. The marine origin of these samples is further supported by their super-chondritic Y/Ho ratios (average Y/Ho = 47). The very low Al, Ti and high field strength element contents of the Xincai BIF indicate an essentially detritus-free component. The bulk BIF samples show a narrow range of εNd(t) values from −1.5 to +1.0, and most of the samples have negative εNd(t) values, −0.3 on average, which is different from Archean BIFs with mantle-derived depleted Nd isotopes. Deviations toward negative εNd(t) values of the iron bands (−0.5 to −0.1) and the silica bands (−4.7 to −0.5) further indicate a continental Nd contribution. In addition, positive EuSN anomalies that were found in all of the bulk and individual band samples indicate that a high-T hydrothermal input also contributed to their REY signatures. It is considered that both high-T hydrothermal fluids and ambient seawater with solutes from erosion and weathering of a nearby landmass can provide their REY signatures.

Microbe-clay interactions as a mechanism for the preservation of organic matter and trace metal biosignatures in black shales

Organic-rich, fine-grained sedimentary rocks, such as black shales, are important geochemical archives providing information on the evolution of seawater composition and biological activity over the past 3billionyears. While biological productivity and sedimentation rates greatly affect the organic matter content in these rocks, mechanisms linking these two processes remain poorly resolved. Here, we examine the interactions of clay minerals with the marine planktonic cyanobacterium Synechococcus sp. PCC 7002. We suggest that clays settling through the water column could influence carbon and trace metal burial in three ways: (1) the interaction of reactive clay surfaces with the bacterial cells increases organic matter deposition via mass increase in a seawater growth medium by several orders of magnitude; (2) reactive bacterial cells become completely encased within a clay shroud, enhancing the preservation potential of this organic matter; and (3) the trace metal content of the biomass buried along with metals sorbed to the clay particles contributes to the trace metal concentrations of the black shale precursor sediments. Significantly, our findings imply that the chemical composition of ancient, organic-rich, fine-grained deposits are not only archives of ancient seawater composition and redox state, but they also provide a record of the degree of biological activity in the water column through geological time.

Experimental constraints on reconstruction of Archean seawater Ni isotopic composition from banded iron formations

The Ni isotopic systematics in banded iron formations (BIFs) potentially recorded the Ni isotopic composition of ancient seawater over Precambrian geological history. However, the utility of BIFs as proxies requires quantitative knowledge of how Ni isotopes fractionated as dissolved Ni was initially incorporated into iron-rich sediments and how diagenesis may have affected the Ni isotopic systematics. Here we report results of synthesis experiments to investigate the behavior of Ni isotopes during Ni coprecipitation with ferrihydrite and then transformation of ferrihydrite to hematite. Ferrihydrite coprecipitation experiments at neutral pH demonstrated that the dissolved Ni was variably heavier than coprecipitated Ni (likely a mixture of surface-adsorbed and structurally incorporated Ni), with the isotope fractionation becoming larger as the fraction of Ni associated with solid increased (Δ60/58Nisolution-solid=+0.08 to +0.50‰). Further experiments at lower pH (3.7–6.7), in which structurally incorporated Ni likely dominated in solids, documented a decrease in Δ60/58Nisolution-solid from +0.44‰ to −0.18‰ as the pH decreased. The negative value for Δ60/58Nisolution-solid at low pH indicates the enrichment of heavier isotopes in incorporated Ni relative to dissolved and adsorbed Ni, possibly as a result of the presence of a small amount of tetrahedral Ni2+ in addition to octahedral Ni2+ in the ferrihydrite structure. The results of the ferrihydrite experiments thus reflect equilibrium isotope fractionation between three pools of Ni, with δ60/58Ni values in the order of incorporated>dissolved>adsorbed. Hematite was synthesized by transformation of Ni-bearing ferrihydrite in aqueous solution at ∼100°C. A significant amount of Ni (up to 60%) was released (desorbed) from solids into solutions as pH dropped from ∼7 to 4.5–5.5 upon phase transformation. Rinsing of the synthesized hematite in 2M acetic acid released only very small amounts of Ni (<4% of total Ni, presumably surface-adsorbed) that were isotopically heavier (δ60/58Ni=+0.11±0.06‰) than the residues (presumably dominated by incorporated Ni), which had δ60/58Ni of −0.26±0.07‰. The preference of lighter isotopes for the incorporated Ni relative to the surface-adsorbed Ni after phase transformation (most had been released into solution) is probably due to distortion of NiO octahedra in the hematite structure, with weaker NiO bond strengths on average. Hence, the more variable Δ60/58Nisolution-solid values (−0.04 to +0.77‰) observed in hematite experiments most likely reflect thermodynamically driven Rayleigh fractionation, with incorporated Ni unavailable to exchange with dissolved Ni due to continuous reduction in size of the highly reactive surface pool of Ni, through which all solid-solution exchange must occur. Overall, the synthesized hematite was isotopically lighter than the ferrihydrite by ∼0.08‰ in δ60/58Ni, which is however within the current analytical uncertainties (±0.09‰). This implies that earliest diagenesis of BIFs results in very limited change in the isotopic composition of solid-associated Ni. Our experimental results, although conducted in a very simple system that differs from Archean seawater, represent an important step toward reconstruction of the Ni isotopic composition of ancient seawater from Ni isotopic signatures in BIFs.

Exploring the potential of clumped isotope thermometry on coccolith‐rich sediments as a sea surface temperature proxy

AbstractUnderstanding past changes in sea surface temperatures (SSTs) is crucial; however, existing proxies for reconstructing past SSTs are hindered by unknown ancient seawater composition (foraminiferal Mg/Ca and δ18O) or reflect subsurface temperatures (TEX86) or have a limited applicable temperature range ( ). We examine clumped isotope (Δ47) thermometry to fossil coccolith‐rich material as an SST proxy, as clumped isotopes are independent of original seawater composition and applicable to a wide temperature range and coccolithophores are widespread and dissolution resistant. The Δ47‐derived temperatures from &lt;63, &lt;20, &lt;10, and 2–5 μm size fractions of two equatorial Pacific late Miocene‐early Pliocene sediment samples (c1; c2) range between ∼18 and 29°C, with c1 temperatures consistently above c2. Removing the &gt;63 μm fraction removes most nonmixed layer components; however, the Δ47‐derived temperatures display an unexpected slight decreasing trend with decreasing size fraction. This unexpected trend could partly arise because larger coccoliths (5–12 μm) are removed during the size fraction separation process. The c1 and &lt;63 μm c2 Δ47‐derived temperatures are comparable to concurrent SSTs. The &lt;20, &lt;10, and 2–5 μm c2 Δ47‐derived temperatures are consistently cooler than expected. The Δ47‐ temperature offset is probably caused by abiotic/diagenetic calcite present in the c2 2–5 μm fraction (∼53% by area), which potentially precipitated at bottom water temperatures of ∼6°C. Our results indicate that clumped isotopes on coccolith‐rich sediment fractions have potential as an SST proxy, particularly in tropical regions, providing that careful investigation of the appropriate size fraction for the region and time scale is undertaken.

Robustness of fossil fish teeth for seawater neodymium isotope reconstructions under variable redox conditions in an ancient shallow marine setting

AbstractFossil fish teeth from pelagic open ocean settings are considered a robust archive for preserving the neodymium (Nd) isotopic composition of ancient seawater. However, using fossil fish teeth as an archive to reconstruct seawater Nd isotopic compositions in different sedimentary redox environments and in terrigenous‐dominated, shallow marine settings is less proven. To address these uncertainties, fish tooth and sediment samples from a middle Eocene section deposited proximal to the East Antarctic margin at Integrated Ocean Drilling Program Site U1356 were analyzed for major and trace element geochemistry, and Nd isotopes. Major and trace element analyses of the sediments reveal changing redox conditions throughout deposition in a shallow marine environment. However, variations in the Nd isotopic composition and rare earth element (REE) patterns of the associated fish teeth do not correspond to redox changes in the sediments. REE patterns in fish teeth at Site U1356 carry a typical mid‐REE‐enriched signature. However, a consistently positive Ce anomaly marks a deviation from a pure authigenic origin of REEs to the fish tooth. Neodymium isotopic compositions of cleaned and uncleaned fish teeth fall between modern seawater and local sediments and hence could be authigenic in nature, but could also be influenced by sedimentary fluxes. We conclude that the fossil fish tooth Nd isotope proxy is not sensitive to moderate changes in pore water oxygenation. However, combined studies on sediments, pore waters, fish teeth, and seawater are needed to fully understand processes driving the reconstructed signature from shallow marine sections in proximity to continental sources.

Silicon isotopes: from cosmos to benthos

Silicon is the second most abundant element on the Earth and one of the more abundant elements in our Solar System. Variations in the relative abundance of the stable isotopes of Si (Si isotope fractionation) in different natural reservoirs, both terrestrial (surface and deep Earth) as well as extra-terrestrial (e.g. meteorites, lunar samples), are a powerful tracer of present and past processes involving abiotic as well as biotic systems. The versatility of the Si isotope tracer is reflected in its wide-ranging applications from understanding the origin of early Solar System objects, planetary differentiation, Moon formation, mantle melting and magma differentiation on the Earth, ancient sea-water composition, to modern-day weathering, clay formation and biological fractionation on land as well as in the oceans. The application of Si isotopes as tracers of natural processes started over six decades ago and its usage has seen a sudden increase over the last decade due to improvements in mass spectrometry, particularly the advent of multi-collector inductively coupled plasma mass spectrometers, which has made Si isotope measurements safe and relatively easy while simultaneously improving the accuracy and precision of measurements.

Calcium isotopes in evaporites record variations in Phanerozoic seawater SO4 and Ca

Reconstructing variations in the major element chemistry of seawater provides constraints on how the global carbon and sulfur cycles have changed over geological time, but archives for this information are rare. This work generates a new independent record of ancient seawater composition, in particular the relative abundance of Ca and SO 4 , through Ca isotope measurements in marine evaporites. Evaporite sequences that reach halite saturation record large Ca isotope variability if SO 4 > Ca, whereas a small range in Ca isotope ratios is observed if Ca > SO 4 . Analyses of geological evaporites indicate SO 4 -rich seawater in the Neogene and Permian and Ca-rich seawater in the Cretaceous and Silurian. These results agree with previous reconstructions using fluid inclusions in halite, and demonstrate a new approach for extending our understanding of oceanic and geochemical evolution.

A master of the earth: H.D. Holland and his contributions to geochemistry

H.D. Holland was one of the great geochemists of modern times. As a Professor at Princeton University for 22years and at Harvard for 33, he mentored 24 graduate students and 24 postdoctoral researchers. He was a member of the U.S. National Academy of Sciences and a President of the Geochemical Society. He co-authored 179 publications in 64years from 1949 to 2012, including four books. He edited one other book and, with Karl Turekian, co-edited the ten-volume Treatise on Geochemistry. His contributions to geochemistry were mainly in economic geology, hydrothermal processes, and especially the chemistry and evolution of the Earth's atmosphere and oceans, a topic to which he devoted nearly half of his publications. His most significant contributions documented changes in the composition of seawater through geologic time and the rise of oxygen in Earth's atmosphere. He pioneered the use of mineral sequences in marine evaporite deposits and fluid inclusions in evaporite minerals to determine the composition of ancient seawater. He collected extensive data on paleosols and used them to calibrate the oxygen content of the ancient atmosphere, documenting its rise in the Great Oxidation Event that peaked between 2.41 and 2.32Ga, and writing extensively about the causes of this event, one of the most significant in Earth history.

Nd isotope systematics on ODP Sites 756 and 762 sediments reveal major volcanic, oceanic and climatic changes in South Indian Ocean over the last 35 Ma

We have analyzed the Nd isotopic composition of both ancient seawater and detrital material from long sequences of carbonated oozes of the South Indian Ocean which are ODP Site 756 (Ninety East Ridge (−30°S), 1518m water depth) and ODP Site 762 (Northwest Australian margin, 1360m water depth). The measurements indicate that the εNd changes in Indian seawater over the last 35Ma result from changes in the oceanic circulation, large volcanic and continental weathering Nd inputs. This highlights the diverse nature of those controls and their interconnections in a small area of the ocean.These new records combined with those previously obtained at the equatorial ODP Sites 757 and 707 in the Indian Ocean (Gourlan et al., 2008) established that the distribution of intermediate seawater εNd was uniform over most of the Indian Ocean from 35Ma to 10Ma within a geographical area extending from 40°S to the equator and from −60°E to 120°E. However, the εNd value of Indian Ocean seawater which kept an almost constant value (at about −7 to −8) from 35 to 15Ma rose by 3 εNd units from 15 to 10Ma. This sharp increase has been caused by a radiogenic Nd enrichment of the water mass originating from the Pacific flowing through the Indonesian Passage. Using a two end-members model we calculated that the Nd transported to the Indian Ocean through the Indonesian Pathway was 1.7 times larger at 10Ma than at 15Ma.The Nd isotopic composition of ancient seawater and that of the sediment detrital component appear to be strongly correlated for some specific events. A first evidence occurs between 20 and 15Ma with two positive spikes recorded in both εNd signals that are clearly induced by a volcanic crisis of, most likely, the St. Paul hot-spot. A second evidence is the very large εNd decrease recorded at ODP Sites 756 and 762 during the past 10Ma which has never been previously observed. The synchronism between the εNd decrease in seawater from 10 to 5Ma and evidences of desertification in the western part of the nearly Australian continent suggests enhanced weathering inputs in this ocean from this continent as a result of climatic changes.

Molybdenum evidence for expansive sulfidic water masses in ~ 750 Ma oceans

The Ediacaran appearance of large animals, including motile bilaterians, is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances (pO 2). To date, direct evidence for low oxygen in pre-Ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins, but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time. Because the abundance and isotopic composition of molybdenum (Mo) in organic-rich euxinic sediments can vary in response to changes in global redox conditions, Mo geochemistry provides independent constraints on the global evolution of well-oxygenated environments. Here, we establish a theoretical framework to access global marine Mo cycle in the past from the abundance and isotope composition of ancient seawater. Further, we investigate the ~ 750 Ma Walcott Member of the Chuar Group, Grand Canyon, which accumulated in a rift basin with open connection to the ocean. Iron speciation data from upper Walcott shales indicate that local bottom waters were anoxic and sulfidic, consistent with their high organic content (up to 20 wt.%). Similar facies in Phanerozoic successions contain high concentrations of redox-sensitive metals, but in the Walcott Member, abundances of Mo and U, as well as Mo/TOC (~ 0.5 ppm/wt.%) are low. δ 98 Mo values also fall well below modern equivalents (0.99 ± 0.13‰ versus ~ 2.35‰ today). These signatures are consistent with model predictions where sulfidic waters cover ~ 1–4% of the global seafloor, corresponding to a ~ 20–80 fold increase compared to the modern ocean. Therefore, our results suggest globally expansive sulfidic water masses in mid-Neoproterozoic oceans, bridging a nearly 700 million-year gap in previous Mo data. We propose that anoxic and sulfidic (euxinic) conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800–750 Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O 2 level, consistent with the hypothesis that pO 2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran Period.