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Environmental context: Mineral dust is an important external source of trace metals to the offshore ocean. Dust exposure to acids is a significant driver of the release of dissolved trace elements. This study provides an analysis of mineral dust interaction with acid, as a proxy for atmospheric processes. An insight is given into the processes that may occur in the atmosphere where desert dust may add nutrient or toxic metals to oceans. Trace metal concentrations in oceans are influenced by several factors including biogeochemical cycling effects on distributions, concentrations and speciation. The major input of trace metals (and P) to the surface waters of the offshore ocean is mineral dust, predominantly from desert regions. This dust can be subject to acid processing in the atmosphere due to the presence of anthropogenic acidic gases (oxides of nitrogen and sulfur), potentially making trace metals more bioavailable when dust is deposited in the oceans. Here we present a study on the release of trace metals from a desert dust when exposed to a series of acid addition treatments. Al, Fe, Co, Cu, Zn and Pb are preferentially leached from the dust only when the calcite phase has been exhausted and the pH is no longer buffered at circumneutral values. Further acid additions quickly released the majority of leachable trace metals, although lower concentrations of most metals continue to be leached with further acid addition cycles. This contrasts with the behaviour of Ca and P, where in prior work it had been shown that dissolution mirrors closely the addition of protons to mineral surfaces demonstrating the related but contrasting processes for trace element dissolution. Environmental context. Mineral dust is an important external source of trace metals to the offshore ocean. Dust exposure to acids is a significant driver of the release of dissolved trace elements. This study provides an analysis of mineral dust interaction with acid, as a proxy for atmospheric processes. An insight is given into the processes that may occur in the atmosphere where desert dust may add nutrient or toxic metals to oceans. This article belongs to the collection Dedication to Prof. Edward Tipping. [ABSTRACT FROM AUTHOR]

Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 50'N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 50'N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined.

Carbonate rocks provide unique and valuable sedimentary archives for secular changes in Earth’s physical, chemical, and biological processes. However, reading the stratigraphic record produces overlapping, nonunique interpretations that stem from the difficulty in directly comparing competing biological, physical, or chemical mechanisms within a common quantitative framework. We built a mathematical model that decomposes these processes and casts the marine carbonate record in terms of energy fluxes across the sediment–water interface. Results showed that physical, chemical, and biological energy terms across the seafloor are subequal and that the energetic dominance of different processes varies both as a function of environment (e.g., onshore vs. offshore) as well as with time-varying changes in seawater chemistry and with evolutionary changes in animal abundance and behavior. We applied our model to observations from the end-Permian mass extinction—a massive upheaval in ocean chemistry and biology—revealing an energetic equivalence between two hypothesized drivers of changing carbonate environments: a reduction in physical bioturbation increased carbonate saturation states in the oceans. Early Triassic occurrences of ‘anachronistic’ carbonates—facies largely absent from marine environments after the Early Paleozoic—were likely driven more by reduction in animal biomass than by repeated perturbations to seawater chemistry. This analysis highlighted the importance of animals and their evolutionary history in physically shaping patterns in the sedimentary record via their impact on the energetics of marine environments. [ABSTRACT FROM AUTHOR]

Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 500 N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 500 N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined. [ABSTRACT FROM AUTHOR]

• Ocean crustal veins reflect dynamic evolution in ocean chemistry and plate cooling. • Veins widths and strain increase with crustal age, while vein densities remain constant. • Elemental mapping reveals multiple precipitation and fracturing episodes. • Ocean plate cooling drives vein reactivation, sustaining fluid flow over tens of millions of years. • Excess vein material linked to higher CO 2 levels during crustal formation. As ocean crust traverses away from spreading ridges, low-temperature hydrothermal minerals fill cracks to form veins, transforming the physical and chemical properties of ocean crust whilst also modifying the composition of seawater. Vein width and frequency observations compiled from the International Ocean Discovery Program (IODP) South Atlantic Transect (∼31°S) and previous scientific ocean drilling holes show that vein width distributions progressively broaden and observed strain (Σ m veins / m core) increases with crustal age, whereas vein densities (# veins / m core) remain approximately constant. Elemental mapping and textural observations illuminate multiple precipitation and fracturing episodes that continue as the ocean crust ages. This challenges the existing notion that ocean crustal veins are passively filled; rather, they are dynamic features of ocean crust aging. These data, combined with thermal strain modelling, indicate a positive feedback mechanism where cooling of the ocean plate induces cracking and the reactivation of pre-existing veins, ultimately resulting in further cooling. Waning of this feedback provides a mechanism for the termination of the global average heat flow anomaly. Sites with total vein dilation greater than expected for their age correspond with crustal formation during periods of high atmospheric CO 2. The amount of vein material thus reflects the changing balance between ocean plate cooling, ocean chemistry, and the age of the ocean crust. Our results demonstrate that ocean crust endures as an active geochemical reservoir for tens of millions of years after formation. [ABSTRACT FROM AUTHOR]

• This work explores the roles of depositional and diagenetic processes that operated on an Archean carbonate platform. • Micro-analytical techniques (scanning XRF and LA-ICP-MS) were employed to investigate those controls on geochemistry. • Samples with proxies indicating the presence of free oxygen are the least diagenetically altered. • Rare earth elements and minor elements, e.g. Sr, Ba and Mn, were mobile during dolomitization. • The extent of diagenetic alteration was not apparent using only visual and microscopic examination. A plethora of proxies has been developed over the preceding two decades in attempts to investigate the geochemistry of the Archean ocean–atmosphere system, and in particular oxygen levels. Unfortunately the necessary parallel investigations of the effects that localized ocean chemistry and diagenesis can have on Archean sediments have commonly not kept pace. We used micro-analytical techniques (LA-ICP-MS and XRF scanning), to distinguish the effects of changes in water composition during precipitation and diagenesis on marine limestone precipitates at the margin and interior of a 2.8 Ga carbonate platform (Mosher Carbonate, Steep Rock Group) in western Superior Province, Canada. Platform margin meter-scale hybrid giant domes consist of centimetric interlayered couplets of (1) Sr-rich crystal fan fabric and cuspate fenestral microbialite, both with pronounced negative Ce-anomalies, and (2) net-like fenestral microbialite, rich in diagenetic cement with diminished concentrations of Sr and lacking negative Ce anomalies. The elevated Sr in the crystal fan fabric and cuspate fenestral microbialite is a general sign of less diagenetic alteration, as is preservation of millimeter-scale chemical differences. XRF mapping revealed that samples that otherwise appear pristine from a second site on the platform rim, near a zone of alteration in the limestone, have ferroan dolomite-filled micro-fractures with Mn flooding of the surrounding calcite and lack Ce anomalies. Platform interior silicified and ferroan dolomitized columnar stromatolites have some calcite laminae, but exceptionally low Sr contents indicate that they formed by dedolomitization. In several horizons REE patterns identical to those of offshore iron formation, replacement of calcite by iron carbonate, and the presence of iron oxides all suggest that short term flooding of the platform by offshore seawater episodically introduced ferroan dolomitizing fluids. Overall, these data indicate a restricted marine environment subject to periodic flooding by offshore waters that caused seafloor diagenetic alteration and precipitation of iron hydroxides. In this system the least altered limestone was the lithotype most likely to retain evidence of free oxygen. Samples with abundant phreatic cement and/or Mn alteration associated with micro-fractures were liable to have experienced REE mobilization. Detailed studies that integrate both depositional and diagenetic information are critical for the correct interpretation of geochemical data from sedimentary rocks. [ABSTRACT FROM AUTHOR]

Abstract Ocean chemistry and carbonate sedimentation link Earth's climate, carbon cycle, and marine pH. The carbonate system in seawater is complex and there are large uncertainties in key parameters in deep time. Here, we link sedimentary textures formed in arid coastal environments and preserved in the rock record to past seawater carbonate chemistry. Prior to the mid‐Mesozoic, tepee structures and pisoids – features associated with peritidal environments – co‐vary with available shelf area during cycles of supercontinent formation and rifting. In contrast, tepees and pisoids are consistently scarce after the mid‐Mesozoic, which coincides with a radiation in pelagic calcifiers as well as the breakup of Pangea. Numerical models suggest that the global and temporal abundances of tepee structures and pisoids are correlated with secular shifts in seawater chemistry, and that trends likely reflect the underlying influence of tectonics and biotic innovation on marine alkalinity and the saturation states of carbonate minerals. As independent sedimentary proxies, tepees and pisoids serve as benchmarks for global carbon cycle models and provide a new proxy record of seawater chemistry that can help discern links among tectonics, biotic innovation, and seawater chemistry.

The conversion of dissolved nutrients and carbon to organic matter by phytoplankton in the surface ocean, and its downward transport by sinking particles, produces a “biological pump” that reduces the concentration of atmospheric CO2. Global rates of organic matter export are a poor indicator of biological carbon storage however, because organic matter gets distributed across water masses with diverse pathways and timescales of return to the surface. Here we show that organic matter export and carbon storage can be related through a sequestration efficiency, which measures how long regenerated nutrients and carbon will be stored in the interior ocean before being returned to the surface. For the first time, we derive global maps of the sequestration efficiency of the biological pump at different residence time horizons. These maps reveal how regional patterns of organic matter export contribute to the biological pump, and how the biological pump responds to changes in biological productivity driven by climate change.

There are significant questions about how the physiology and behavior of marine fishes will respond to elevated carbon dioxide concentrations in future ocean conditions, an issue referred to as ocean acidification. Marine fish are the most vulnerable to changes in ocean chemistry at the egg and larval stage because of their thinner skin and heightened exposure to environmental conditions. This study investigates the impacts of elevated carbon dioxide for the swimming behavior of larval white seabass, Atractoscion nobilis. White seabass larvae were reared to 7 days post fertilization under variable carbon dioxide concentrations with the control infused with 380ppm CO2 (close to current atmospheric conditions) and the treatment with 2500ppm CO2. The larvae were filmed in shooting session on days 4-7, and these films were analyzed using imagej particle tracker, an image analysis tool to attain information form the trajectories of each larval movement. Three categories were used to determine swimming conditions; distance traveled along each trajectory, mean swim speeds, and maximum swim speeds. Overall, the results from t-tests showed that there was not a signification difference between the control and treatment for all three categories. The elevated swimming activity of the treatment larvae compared to the control throughout the experiment was a surprising result. There was no observational evidence that the larvae were disoriented under the elevated carbon dioxide as compared to the control and there was no impact for the survival rate to day 7. These results indicate that marine fish larvae may express resilience to changes in swimming behavior from ocean acidification.

Marine tracer studies indicate that large volumes of saline groundwater discharge to the ocean in passive margin settings. These results have not found widespread recognition because the location and cause(s) of this submarine groundwater discharge (SGD) are unclear. Here we report observations from a new long‐term seafloor monitoring network in the South Atlantic Bight that support large‐scale SGD far from shore. In the study area near Charleston, South Carolina, we determined hydrostratigraphy via vibracoring and chirp seismic surveys, collected water samples from seafloor wells, and used heat as a tracer to monitor SGD. We detected significant pulses of saline SGD issuing from the seafloor 10–15 km from shore. These pulses coincided with abrupt sea level declines of up to 30 cm. Based on an analysis of marine conditions at the time, we propose that upwelling‐favorable winds depressed sea level in the region, causing saline groundwater to discharge from confined coastal aquifers that connect land and ocean. The combination of stacked confined aquifers and variations in sea level are nearly ubiquitous in passive coastal margins. This previously overlooked combination can explain a wide range of other published observations and promotes more dynamic flows than simple tidal fluctuations. This new mechanism may explain Ra tracer signals in the coastal Atlantic Ocean and supports significant nutrient inputs to the ocean. These large natural geochemical fluxes may be sensitive to groundwater usage on land. Plain Language Summary: Previous studies based on the chemistry of coastal ocean water show that saline groundwater must discharge to the ocean. In the Atlantic Ocean, the rate of groundwater discharge roughly equals river discharge, and the groundwater delivers more nutrients to the ocean than rivers. Studies of marine ecosystems largely ignore these inputs because it is not clear where or why these flows occur. We installed wells in the seafloor 10–20 km offshore to monitor the composition of groundwater and the temperature of the sediments below the seafloor. We used heat as a tracer to calculate groundwater flow rates. We found that groundwater discharged to the ocean in response to anomalous changes in sea level, likely associated with wind events. This can only happen if permeable sediment layers (aquifers) form a hydraulic connection between the land and the seafloor. Aquifers like this are extremely common in coastal regions, and wind‐related variations in sea level are equally common. The combination of stacked coastal aquifers and variations in sea level can explain many different observations. These large groundwater pulses probably occur widely, far beyond the southeastern United States. Groundwater pumping could affect this discharge, which could in turn alter coastal ecosystems. Key Points: We detected significant pulses of submarine groundwater discharge 10–15 km offshore, caused by wind‐driven variations in sea levelThe groundwater must issue from confined aquifers, common in coastal areas, that promote groundwater‐seawater exchange far offshoreThis mechanism explains prior observations and likely drives major fluxes of groundwater and nutrients in passive continental margins [ABSTRACT FROM AUTHOR]

Past changes in the halogen composition of seawater are anticipated based on the differing behavior of chlorine and bromine that are strongly partitioned into seawater, relative to iodine, which is extremely depleted in modern seawater and enriched in marine sediments due to biological uptake. Here we assess the use of chert, a chemical sediment that precipitated throughout the Precambrian, as a proxy for halide ratios in ancient seawater. We determine a set of criteria that can be used to assess the primary nature of halogens and show that ancient seawater Br/Cl and I/Cl ratios can be resolved in chert samples from the 2.5 Ga Dales Gorge Member of the Brockman Banded Iron Formation, Hamersley Group, Western Australia. The values determined of Br/Cl ~2 × 10-3 M and I/Cl ~30 × 10-6 M are comparable to fluid inclusions in hydrothermal quartz from the 3.5 Ga North Pole area, Pilbara Craton, Western Australia, that were the subject of previous reconstructions of ancient ocean salinity and atmospheric isotopic composition. While the similar Br/Cl and I/Cl values indicate no substantial change in the ocean halide system over the interval 2.5–3.5Ga, compared to modern seawater, the ancient ocean was enriched in Br and I relative to Cl. The I/Cl value is intermediate between bulk Earth (assumed chondritic) and the modern seawater ratio, which can be explained by a smaller organic reservoir because this is the major control on marine iodine at the present day. Br/Cl ratios are about 30% higher than both modern seawater and contemporary seafloor hydrothermal systems, perhaps indicating a stronger mantle buffering of seawater halogens during the Archean. [ABSTRACT FROM AUTHOR]

Fluid-rock interactions in hydrothermal systems at or near mid-oceanic ridges (MOR) play a major role in determining the composition of the oceanic crust and seawater. To quantify the processes that govern cation exchange in these environments we have experimentally studied the isotopic evolution of δ26/24Mg in the fluid phase during seawater-basalt interaction at 250 and 290 °C. Mass balance constraints indicate that isotopically heavy Mg was preferentially incorporated into non-exchangeable (octahedral) sites in secondary clay minerals such as saponite (Mg-rich smectite), leaving residual fluids enriched in light Mg isotopes. The magnitude of fractionation observed during smectite precipitation in our experiments (ε Smectite - Liquid 26 / 24 ) ranged from 0.35 ‰ to 0.42 ‰. This observation, which contrasts with the preferential uptake of light Mg isotopes into biogenic and inorganic marine carbonates, highlights the potential utility of Mg isotopes as tracers of the precipitation dynamics of authigenic Mg-silicate and Mg-carbonate phases. Furthermore, although Mg isotopic fractionation is often masked by the almost complete removal of Mg in high temperature marine hydrothermal systems, our experiments demonstrate that it does become significant at lower temperatures where Mg removal by clay formation is incomplete. Under such conditions, this fractionation will create isotopically light fluids due to smectite precipitation, thus potentially represents an important component of the marine Mg isotope inventory. [ABSTRACT FROM AUTHOR]

We consider the Si flux resulting from sand grain dissolution on beaches under the pressure of the intensive and continuous shaking by the waves, a potential source of oceanic DSi that is not currently considered. Today, DSi source and sink fluxes are balanced within large uncertainties, at ca. 10.4 ± 4.2 and 14.6 ± 7.8 × 1012 mol yr–1, respectively, underlining that some processes are not well constrained and possibly overlooked so far. To quantitatively explore this idea, we first realized an experimental dissolution of quartz grains in a stirred vessel designed to simulate the sediment orbital motion induced by the waves. These experiments lead to the calculation of a solid–liquid mass-transfer coefficient directly linked to the rotation speed of the shaker. This coefficient being itself related to the energy communicated to the liquid, we could apply the Nienow relationship to calculate a mass-transfer coefficient for beach sand exposed to 1 m height waves. Extrapolation of this value to the whole sandy beaches led us to conclude that this mechanism could be significant, shortening the calculated residence time of oceanic DSi by up to a factor 2.

Redox sensitive trace elements in pyrite, including nodules, are increasingly used to infer the chemical conditions of ancient oceans—but considerable uncertainty remains regarding the mechanism and timing of nodule formation. Resolving these uncertainties is important because pyrite nodules must form in connection with the overlying water column, rather than during late diagenesis, to reflect the composition of the global ocean. Existing models for pyrite nodule formation have been specific to pyrite textures from individual sites, and we lack a unified model that can explain the compositional and textural diversity observed in nodules from different localities. In this study we examine ten pyrite nodules from several geological periods (Neoarchean to Carboniferous) using in situ LA-ICP-MS and SHRIMP-SI analyses. We present transects of spot analyses of trace elements (As, Ag, Cu, Co, Ni, Sb and Se) and S isotope ratios for each nodule. The pyrite nodules can be classified according to three main categories: those with (1) little to no trace element or isotopic zonation of the nodule from core to margin, (2) strong zonation from core to margin, and (3) minor zonation near the core but more significant zonation near the margin of the nodule. We further illustrate this zonation with a NanoSIMS element map from an additional pyrite nodule. These results are interpreted to indicate nodule formation along a spectrum between two end-member mechanisms. We suggest that the absence of trace element or isotopic zonation reflects nodule growth by a pathway that is analogous to the pervasive growth mechanism for carbonate nodules. This model involves the production of many nucleation sites that are evenly distributed within the volume that the nodule eventually occupies. Consequently, this mechanism results in a chemically homogenous nodule. Pyrites formed this way are suitable for paleoceanographic reconstruction. The other end-member mechanism is analogous to the concentric growth model for carbonate concretions. In this scenario, the core of the nodule forms first and is followed by the addition of concentric layers—each with a progressively different trace element content and δ34S signature as diagenesis progresses. Despite having limited utility for reconstructing ancient seawater, these late forming nodules may track the evolving availability of bioessential trace elements for the subsurface biosphere with important implications for global biogeochemical cycles. Spatial trends in trace elements and S isotopes thus speak to the mechanisms of pyrite nodule formation and provide a framework for evaluating nodule suitability for a range of paleoenvironmental studies. [ABSTRACT FROM AUTHOR]

Abstract We report Fe isotope compositions in banded iron formations (BIF) from three cores from the pre-GOE Transvaal Supergroup, South Africa, and one core from the pre-GOE Joffre Member of the Hamersley Group, Australia. The low abundances of detrital elements such as Al, Ti, Sc, and V suggest that these BIF were deposited in distal positions with respect to Precambrian continents, while the very low P abundances are incompatible with strong biological productivity at these localities. A combination of U–Pb chronology and cobalt accumulation rates is used to establish a high-resolution time scale and deduce chemical fluxes. The e-folding time of δ 56 Fe variations up stratigraphy is used to determine Fe oceanic residence times and Fe concentrations as well as the dissolved carbonate content of Early Proterozoic seas. Iron oceanic residence times increased from 0.2 to 2.3 Ma during the time interval between 2521 and 2394 Ma covered by the present cores, translating into ocean Fe concentrations increasing from 6.4 to 37 mmol kg−1. Massive BIF precipitation was triggered by release of CO 2 into the atmosphere and subsequent surges of alkalinity into the ocean due to the weathering of large subaerial volcanic systems. We argue that a suitable electron acceptor for Fe2+ oxidation to magnetite is the inorganic conversion of CO 2 (or dissolved inorganic carbon) to CH 4. In the process, H+ is produced, which is reinjected into oceanic hydrothermal systems liberating Fe2+. The couple Fe2+-magnetite may, in the Archean, have played the same buffering role as the couple Ca2+-calcite plays today. Massive injection of methane into the atmosphere would accompany BIF deposition and make the early Earth similar to modern Titan. Therefore, although biological processes may have assisted iron oxidation and precipitation, they are not a prerequisite for BIF deposition. Highlights • Time scales inferred from Co accumulation rate are consistent with U–Pb chronology. • Iron oceanic residence time increased from 0.2 to 2.3 Ma from 2521 to 2394 Ma. • Low contents of detrital elements indicate BIF deposition at distal sites. • Low P contents cast doubt on biological productivity. • BIF deposition induces massive inorganic reduction of atmospheric CO 2 to CH 4. [ABSTRACT FROM AUTHOR]

Finely laminated cherts enclosing nanoparticles of greenalite and apatite are ubiquitous in Archean-Paleoproterozoic (3.46–2.45 Ga) ferruginous cherts, jaspilites and Banded Iron Formations (BIFs) and the greenalite and apatite are considered to be primary deposits. The cherts and BIFs are chemical sedimentary rocks interpreted to have been precipitated in marine settings prior to the first permanent rise in atmospheric oxygen at the Great Oxidation Event (GOE) ca. 2.45–2.32 M.y. ago. As chemical sediments, they are potential archives of the solutions from which they precipitated, incorporating signals from hydrothermal fluids and ambient seawater. Previous studies of rare earth elements and Y (REY) in pre-GOE BIFs have mostly found an "Archean seawater signature" with positive Eu anomalies, attributed to the influence of high-temperature hydrothermal processes, and positive anomalies for La, Gd and Y, ascribed to seawater. REY abundances determined by in situ LA-ICP-MS are presented for well-preserved, laminated greenalite-bearing cherts from ten formations of pre-GOE ferruginous cherts and BIFs from Western Australia. The samples come from a wide range of depositional environments, e.g., submarine proximal volcanic environments, basin floor, slope and deep marine shelf, and are between 3.46 Ga to 2.45 Ga in age. Five groups with different REY patterns are identified, namely (i) mafic-volcanic-influenced vent-proximal chert from the Marble Bar Chert Member of the Duffer Formation of the Warrawoona Group; (ii) felsic volcanic- and sediment-associated chert from the Kangaroo Caves Formation of the Sulphur Springs Group and the Wilgie Mia Formation of the Murchison Supergroup, (iii) ferruginous cherts in shelf sediments from the Bee Gorge Member of the Wittenoom Formation of the Hamersley Group, (iv) BIFs from the Nammuldi Member of the Marra Mamba Iron Formation and Joffre Member of the Brockman Iron Formation, Hamersley Group, and (v) BIFs from the Dales Gorge Member of the Brockman Iron Formation. Of these, only the Dales Gorge Member BIF has a typical Archean seawater signature while the others have REY patterns likely reflecting differing source fluids and environments of deposition but not necessarily global ocean chemistry. Analyses of chert containing sub-micron-sized particles of greenalite and apatite indicate that the likely hosts of the REEs are apatite, siderite and possibly greenalite. The REY patterns of the greenalite-bearing cherts may differ from those of bulk samples of the same formations, perhaps reflecting a diagenetic overprint in the bulk samples, whereas the greenalite-bearing cherts likely preserve their depositional compositions, locked in by early silicification. • REE + Y analyses of 3.45–2.45 Ga greenalite-bearing cherts and BIFs. • Textures and chemistry preserved by early silicification. • Dales Gorge BIFs preserve seawater signature. • Joffre and Marra Mamba BIFs have hydrothermal fluid influence. • Greenstone-belt cherts and BIFs have dominant hydrothermal contribution. [ABSTRACT FROM AUTHOR]

Abstract: The biogeochemical cycling of zinc (Zn) is intimately coupled with organic carbon in the ocean. Based on an extensive new sedimentary Zn isotope record across Earth's history, we provide evidence for a fundamental shift in the marine Zn cycle ~800 million years ago. We discuss a wide range of potential drivers for this transition and propose that, within available constraints, a restructuring of marine ecosystems is the most parsimonious explanation for this shift. Using a global isotope mass balance approach, we show that a change in the organic Zn/C ratio is required to account for observed Zn isotope trends through time. Given the higher affinity of eukaryotes for Zn relative to prokaryotes, we suggest that a shift toward a more eukaryote‐rich ecosystem could have provided a means of more efficiently sequestering organic‐derived Zn. Despite the much earlier appearance of eukaryotes in the microfossil record (~1700 to 1600 million years ago), our data suggest a delayed rise to ecological prominence during the Neoproterozoic, consistent with the currently accepted organic biomarker records. [ABSTRACT FROM AUTHOR]

The Kingston Peak Formation records glacial sedimentation during the Cryogenian in Death Valley, California, and contains iron formation horizons. These iron formations are part of a thick sedimentary succession containing glaciogenic diamictites, together with mass flow breccias, conglomerates, sandstones and siltstones. The Kingston Peak iron formations are mineralogically and sedimentologically simple, consisting of finely laminated hematitic siltstones that contain up to 50 wt% Fe. These iron formations have been the subject of controversy as they have previously been described as being either absent, or the product of volcanism, diagenesis, hydrothermalism or weathering. Here we present a detailed case study of the sedimentology, stratigraphy, geochemistry and iron isotope composition of the Kingston Peak iron formations. Unequivocal textural evidence indicates a synsedimentary origin for these iron formations. The iron isotopic signature (0.2 < δ 56 Fe < 1.65) is indicative of partial oxidation of a ferrous iron reservoir. Therefore the Kingston Peak iron formations are interpreted to be primary chemical sediments deposited by the mixing of oxygenated glaciogenic fluids with ferruginous seawater. Pulses of oxidants led to the precipitation of iron oxides, which became enriched under periods of sediment starvation in a basin influenced by episodic mass flows and glacial input. Sedimentological evidence implicates deposition in a range of glaciomarine environments, from ice-distal to relatively ice-proximal, at multiple stages throughout the glacial succession and well before the end of the glacial period. These iron formations have geochemical and sedimentological similarities to other glacially-associated Neoproterozoic iron formations that occur globally and have important implications for our understanding of ocean chemistry and glaciation in the Cryogenian. [ABSTRACT FROM AUTHOR]

Over 50% of Earth is covered by oceanic crust, the uppermost portion of which is a high-permeability layer of basaltic lavas through which seawater continuously circulates. Fluid flow is driven by heat lost from the oceanic lithosphere; the global fluid flux is dependent on plate creation rates and the thickness and distribution of overlying sediment, which acts as a low-permeability layer impeding seawater access to the crust. Fluid-rock reactions in the crust, and global chemical fluxes, depend on the average temperature in the aquifer, the fluid flux, and the composition of seawater. The average temperature in the aquifer depends largely on bottom water temperature and, to a lesser extent, on the average seafloor sediment thickness. Feedbacks between off-axis chemical fluxes and their controls may play an important role in modulating ocean chemistry and planetary climate on long timescales, but more work is needed to quantify these feedbacks. [ABSTRACT FROM AUTHOR]

Banded iron formations (BIFs) are marine chemical sedimentary rocks that were widely deposited before the start of the Great Oxidation Event (2.45–2.32 Ga). They represent an important archive into the chemistry of the Precambrian oceans and atmosphere, and the evolution of the marine biosphere. It is believed that BIFs were derived from ferric oxides/hydroxides that were deposited by iron-oxidizing bacteria or by reaction with oxygen released by Cyanobacteria. Hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide/hydroxide precipitate, whereas magnetite is thought to have formed from ferric oxides/hydroxides via bacterial iron reduction and oxidation of microbial biomass. However, studies are equivocal about when and how magnetite formed in BIFs. We present the results of a detailed petrographic study of magnetite in BIFs of the Hamersley Group, Western Australia. We find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction. Instead, most of the magnetite formed by replacement of siderite and other ferrous-rich minerals after burial. Our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism. Published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern Pilbara Craton (200–350 °C), the thermal decomposition of siderite could account for much of the magnetite in the Hamersley BIFs (3FeCO 3  + H 2 O → Fe 3 O 4  + 3CO 2  + H 2 ). The relationship between magnetite and the primary precipitates, which are preserved as greenalite nanoparticles and microgranules in chert bands, suggests that much of the magnetite formed in siderite-rich, iron-silicate bands that developed by compaction of non-silicified iron-silicate muds. These bands acted as reactive pathways for fluid flow and the formation of magnetite. If most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron. Our results are consistent with models for the deposition of BIFs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial. [ABSTRACT FROM AUTHOR]

GEOTRACES is an international study of the marine biogeochemical cycles of trace elements and their isotopes (TEIs), designed by marine geochemists to accelerate TEI research under a global program. Combining ocean sections, process studies, data synthesis, and modeling, GEOTRACES will identify and quantify the processes that supply TEIs at ocean boundaries as well as the physical and biological processes that redistribute TEIs within and between ocean basins. Constraining processes that remove TEIs from the ocean will enable complete mass budgets to be generated. Anticipated beneficiaries of GEOTRACES products include scientists studying the sustained health of marine ecosystems and their sensitivity to changes in micronutrient supply; paleoceanographers seeking to reconstruct past changes in the ocean environment, including the ocean's role in climate variability; and scientists and policymakers who seek a better understanding of the transport and fate of contaminants in the ocean. It is hoped that the experiences described here will provide helpful guidance to scientists in other disciplines who wish to advance their fields by organizing coordinated research programs

Sustained observations provide critically needed data and understanding not only about ocean warming and water cycle reorganization (e.g., salinity changes), ocean eutrophication, and ocean deoxygenation, but also about changes in ocean chemistry. As an example of changes in the global ocean carbon cycle, consistent changes in surface seawater CO2-carbonate chemistry are documented by seven independent CO2 time series that provide sustained ocean observations collected for periods from 15 to 30 years: (1) Iceland Sea, (2) Irminger Sea, (3) Bermuda Atlantic Time-series Study (BATS), (4) European Station for Time series in the Ocean at the Canary Islands (ESTOC), (5) CArbon Retention In A Colored Ocean sites in the North Atlantic (CARIACO), (6) Hawaii Ocean Time-series (HOT), and (7) Munida in the Pacific Ocean. These ocean time-series sites exhibit very consistent changes in surface ocean chemistry that reflect the impact of uptake of anthropogenic CO2 and ocean acidification. The article discusses the long-term changes in dissolved inorganic carbon (DIC), salinity-normalized DIC, and surface seawater pCO2 (partial pressure of CO2) due to the uptake of anthropogenic CO2 and its impact on the ocean's buffering capacity. In addition, we evaluate changes in seawater chemistry that are due to ocean acidification and its impact on pH and saturation states for biogenic calcium carbonate minerals.

The modern industrialized and urbanized world, dubbed the "Anthropocene" by Paul Crutzen (2006), includes the past 250 years of multiple human impacts. Nobel Prize winner and atmospheric chemist Crutzen states: During the past 3 centuries human population increased tenfold to 6,000 million, growing by a factor of four during the past century alone. More than half of all accessible fresh water is used by mankind. Fisheries remove more than 25% of the primary production of the oceans in the upwelling regions and 35% in the temperature continental shelf regions. 30–50% of the world's land surface has been transformed by human action. Coastal wetlands have lost 50% of the world's mangroves. More nitrogen is now fixed synthetically and applied as fertilizers in agriculture than fixed naturally in all terrestrial ecosystems. Many of the world's rivers have been dammed or diverted.

Changing atmospheric composition due to human activities, primarily carbon dioxide (CO2) emissions from fossil fuel burning, is already impacting ocean circulation, biogeochemistry, and ecology, and model projections indicate that observed trends will continue or even accelerate over this century. Elevated atmospheric CO2 alters Earth's radiative balance, leading to global-scale warming and climate change. The ocean stores the majority of resulting anomalous heat, which in turn drives other physical, chemical, and biological impacts. Sea surface warming and increased ocean vertical stratification are projected to reduce global-integrated primary production and export flux as well as to lower subsurface dissolved oxygen concentrations. Upper trophic levels will be affected both directly by warming and indirectly from changes in productivity and expanding low oxygen zones. The ocean also absorbs roughly one-quarter of present-day anthropogenic CO2 emissions. The resulting changes in seawater chemistry, termed ocean acidification, include declining pH and saturation state for calcium carbon minerals that may have widespread impacts on many marine organisms. Climate warming will likely slow ocean CO2 uptake but is not expected to significantly reduce upper ocean acidification. Improving the accuracy of future model projections requires better observational constraints on current rates of ocean change and a better understanding of the mechanisms controlling key physical and biogeochemical processes.

The first symposium on “The Ocean in a High-CO2 World” in 2004 proved to be a landmark event in our understanding of the seriousness of ocean acidification, as reported in Oceanography (Cicerone et al., 2004). The scientific community reunited in 2008 for a second symposium on “The Ocean in a High-CO2 World.” During the four years between the two symposia, more scientific papers were published on the topic of ocean acidification than during the preceding 55 years. Ocean acidification is now widely cited in the press and is familiar to many nonscientists. Participants at the 2008 symposium identified new research priorities and stressed the importance of improving international coordination to facilitate agreements on protocols, methods, and data reporting in order to optimize limited resources by greater sharing of materials, facilities, expertise, and data. Despite major uncertainties, the research community must find ways to scale up understanding of individual organisms’ responses to provide meaningful predictions of ocean acidification’s effects on food webs, fisheries, marine ecosystems, coastal erosion, and tourism. Easy-to-understand information, such as simple indicators of change and of thresholds beyond which marine ecosystems will not recover, is also needed for management and policymaking.

Space-based observations provide synoptic coverage of surface ocean temperature, winds, sea surface height, and color useful to a wide range of oceanographic applications. These measurements are increasingly applied to monitor large-scale environmental and climate processes that can have an impact on important managed marine resources. From observing the development of harmful algal blooms using ocean color, to tracking regions of thermal stress that can induce coral bleaching, satellites are routinely used for environmental monitoring. Here, we demonstrate an approach to monitoring changes in sea surface ocean chemistry in response to ocean acidification as applied to the Greater Caribbean Region. It is based on establishing regional empirical algorithms that combine parameters measured in situ and remotely sensed observables and then using the high-resolution remotely sensed products. This tool is important for exploring regional to basinwide trends in ocean acidification on seasonal to interannual time scales.

Conventional models for the deposition of banded iron formations (BIFs) envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide/hydroxide particles in surface waters that settled to form laterally extensive layers of iron-rich sediment. A fundamental tenet of this model is that fine-grained hematite (so-called dusty hematite) in least-altered BIFs represents the dehydration product of original oxide/hydroxide precipitates. However, this premise has never been proven. We have investigated the origin of the earliest-formed iron oxides in chert in well-preserved BIFs of the 2.63–2.45 billion-year-old Hamersley Group, Australia. We find that laminated chert in BIFs show progressive stages of in situ alteration from gray–green chert, containing iron-silicate nanoparticles, to red chert with abundant hematite dust. Analysis of textures by transmission electron microscopy of samples from the transition zone between gray–green and red chert reveals that dusty hematite formed after partial dissolution of iron-silicate nanoparticles by the precipitation of iron oxides in resulting cavities. These observations suggest that hematite dust is not a relict of an original seawater precipitate but the end-product of post-depositional oxidation. Our observations are consistent with paleomagnetic results from the Hamersley Group, which record two major phases of magnetic remanence carried by hematite that post-date deposition by more than 200 million years. Our results may provide an alternative explanation for the origin of jasper in BIFs deposited before the start of the Great Oxidation Event about 2.4 billion years ago. If correct, it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early Precambrian seawater. Furthermore, our results suggest that the primary iron precipitate in BIFs was iron-silicate mud that was silicified at or just below the sediment–water interface, a hypothesis that requires neither dissolved oxygen nor photosynthetic life, but was an inorganic, chemical process, reflecting anoxic oceans enriched in iron and silica. [ABSTRACT FROM AUTHOR]

The chemistry of the oceans has changed over the passage of geological time. Specifically, changes are recognized in the salt composition thereof, and in particular secular variations alternating between low magnesium calcite (LMC) and aragonite / high magnesium calcite (HMC) as the predominant polymorph of calcium carbonate. A calcitic sea, where LMC is precipitated, is associated with high rates of oceanic expansion, as well as with high levels of CO2 in the atmosphere and a global greenhouse effect. Chemically, it is related to an Mg / Ca <2. Meanwhile, in an ocean where Mg / Ca is usually> 2, aragonite or HMC are preferably precipitated. This type of "seas" are related to low oceanic expansion rates and also to lower atmospheric CO2 levels in comparison with calcitic seas. Furthermore, aragonitic seas are usually related to "icehouse effect" episodes. The changes in the chemistry of the oceans affects the organisms that inhabit them, and in particular, in reef-building organisms such as such as sponges, corals and coralline algae, among others. Fossils found in a calcitic interval are characterised by a LMC mineralogy, whereas those found in an aragonitic interval, have a mineralogy consisting either of aragonite/HMC. Nonetheless, it is interesting to pinpoint that both mineralogies can appear in a same clade, if this clade is encompassed in a succession of calcitic-aragonitic intervals. [ABSTRACT FROM AUTHOR]