Modern Ocean Research Articles

Lithium isotope evidence for a plumeworld ocean in the aftermath of the Marinoan snowball Earth

The snowball Earth hypothesis predicts that continental chemical weathering diminished substantially during, but rebounded strongly after, the Marinoan ice age some 635 Mya. Defrosting the planet would result in a plume of fresh glacial meltwater with a different chemical composition from underlying hypersaline seawater, generating both vertical and lateral salinity gradients. Here, we test the plumeworld hypothesis using lithium isotope compositions in the Ediacaran Doushantuo cap dolostone that accumulated in the aftermath of the Marinoan snowball Earth along a proximal-distal (nearshore-offshore) transect in South China. Our data show an overall decreasing δ7Li trend with distance from the shoreline, consistent with the variable mixing of a meltwater plume with high δ7Li (due to incongruent silicate weathering on the continent) and hypersaline seawater with low δ7Li (due to synglacial distillation). The evolution of low δ7Li of synglacial seawater, as opposed to the modern oceans with high δ7Li, was likely driven by weak continental chemical weathering coupled with strong reverse weathering on the seafloor underneath silica-rich oceans. The spatial pattern of δ7Li is also consistent with the development and then collapse of the meltwater plume that occurred at the time scale of cap dolostone accumulation. Therefore, the δ7Li data are consistent with the plumeworld hypothesis, considerably reduced chemical weathering on the continent during the Marinoan snowball Earth, and enhanced reverse weathering on the seafloor of Precambrian oceans.

Stable and radiogenic strontium isotopes trace the composition and diagenetic alteration of remnant glacial seawater

A remnant of glacial seawater preserved in the pore fluids of sediment cores from the Maldives Inner Sea provided an opportunity to investigate the stable strontium isotopic composition (δ88/86Sr) of the ocean during the Last Glacial Maximum and explore the usefulness of δ88/86Sr as a tracer of early marine diagenesis. We used paired measurements of δ88/86Sr and radiogenic Sr isotope ratios (87Sr/86Sr) in pore fluids and surrounding carbonate sediments to constrain the diagenetic history of the preserved glacial water mass at IODP Sites U1466 and U1468. These pore fluid profiles document variability in δ88/86Sr in a shallow marine setting, revealing distinct diagenetic processes dominating within different depth intervals. We find evidence for isotope fractionation during secondary calcite precipitation at intermediate depths and observe that in aragonite-dominated settings, fractionation during recrystallization may be obscured by the dissolution of aragonite in the uppermost sediments. Correcting for the effect of carbonate recrystallization on pore fluid Sr concentration ([Sr]) and isotopic composition, we estimate that glacial seawater [Sr] was higher (∼98μM) and δ88/86Sr lower (∼0.32‰) compared to the modern ocean, consistent with hypotheses attributing the present-day disequilibrium of the ocean Sr budget to glacial/interglacial changes in shelf carbonate weathering and burial. Our results provide evidence that the ocean [Sr] and δ88/86Sr are sensitive to carbon cycle changes on timescales much shorter than its residence time (∼2 Myr) and demonstrate that pore fluid δ88/86Sr measurements are a useful addition to multi-tracer studies of diagenesis in complex marine systems.

Evolution of gas envelopes and outgassed atmospheres of rocky planets that formed via pebble accretion

In this work, we present results of numerical simulations of the formation and early evolution of rocky planets through pebble accretion, with an emphasis on hydrogen envelope longevity and the composition of the outgassed atmosphere. We modelled planets with a range in mass from 0.1 to 5 Earth masses that orbit between 0.7 and 1.7 AU. The composition of the outgassed atmosphere was calculated with the partial pressure of free oxygen fit to geophysical models of magma ocean self-oxidation. The combined X-ray and UV (XUV) radiation-powered photoevaporation is considered as the main driver of atmospheric escape. We modelled planets that remain below the pebble isolation mass and hence accrete tenuous envelopes only. We considered slow, medium, or fast initial stellar rotation for the temporal evolution of the XUV flux. The loss of the envelope is a key event that allows the magma ocean to crystallise and outgas its bulk volatiles. The atmospheric composition of the majority of our simulated planets is dominated by CO$_2$. Our planets accrete a total of 11.6 Earth oceans of water, the majority of which enters the core. The hydrospheres of planets lighter than the Earth reach several times the mass of the Earth's modern oceans, while the hydrospheres of planets ranging from 1 to 3.5 Earth masses are comparable to those of our planet. However, planets of 4-5 Earth masses have smaller hydrospheres due to the trapping of volatiles in their massive mantles. Overall, our simulations demonstrate that hydrogen envelopes are easily lost from rocky planets and that this envelope loss triggers the most primordial partitioning of volatiles between the solid mantle and the atmosphere.

UPPER CRETACEOUS–LOWER EOCENE MUDSTONES OF THE OUTER CARPATHIANS (PETROCHEMICAL AND PALEOGEODYNAMIC ASPECTS)

The mineral types of the Upper Cretaceous–Lower Eocene greenish-gray mudstones of the Skyba and Boryslav-Pokuttya nappes of the Outer Carpathians were studied. According to sedimentological features, these mudstones belong to hemipelagites and contain information about the background sedimentation conditions of the deep-sea oceanic basin. The purpose of the work is to clarify the paleogeodynamic features of the basin based on the results of studying the material composition of the Upper Cretaceous–Lower Eocene background mudstones, which in particular are part of the variegated-colored horizons. Research methods were geological mapping, X-ray structural, sedimentological, lithostratigraphic, petrogeochemical, geodynamic analysis. On the classification module diagram (Na2O+K2O)/Al2O3–(FeO*+MnO+MgO)/SiO2, the figurative points of the contents of the Upper Cretaceous–Lower Eocene mudstones of the Stryi, Yamna, and Manyava suites fall into the fields corresponding to two mineral types: a mixture with a predominance of montmorillonite and three-component mixture of chlorite+montmorillonite+hydromica composition. According to the obtained data, a gradual change in the conditions of the sedimentary environment from the Late Cretaceous to the Eocene can be traced, with a successive increase in the normalized alkalinity modulus and a decrease in the phemic modulus. On the paleogeodynamic discriminant diagrams (FeO*+MgO)–TiO2 and F1–F2, figurative points of Upper Cretaceous–Lower Eocene mudstones form petrochemical distribution trends that cover the fields of geodynamic conditions from passive to active margins of the sedimentation basin. The Cretaceous–Paleogene pelagic mudstones of the Sicilian domain of the Alpine Belt have a similar distribution of petrochemical components. The mineral assemblages found in the mudstones of the Skyba and Boryslav-Pokuttya nappes are typical for deep-sea clay sediments of modern oceans. The presence of montmorillonite and chlorite in the background argillites indicates a high probability of a contribution to the background petrofund of the sedimentatary basin by a femic magmatic component of endogenous origin, realized as a manifestation of volcanic and hydrothermal activity, synchronous to sedimentogenesis. An exogenous source could be the magmatic material transferred or changed by the processes of halmyrolysis, which is not synchronous with sedimentation. The Late Cretaceous–Early Eocene geodynamic events of the Outer Carpathian sedimentary basin developed in front of the ALCAPA and Tisza-Dacia terranes of the Alpine Tethys. Convergence, which caused the formation of the accretionary prism in front of the terranes, could contribute to the riftogenic (?) opening, deepening of the Outer Carpathian Basin, where the penetration of synsedimentary endogenous material, mainly of the basic composition, took place. This material could be the rock-forming source of the basin substrate, on which the mineral assemblages of the background hemipelagic mudstones were formed.

Early Devonian bioclastic contourites in the High Atlas: a plastered drift recording the convergence between Gondwana and Laurussia (Sub-Meseta Zone, Morocco)

Bottom-current deposits, formed in association with ancient oceanic gateways, preserve records of past global oceanic circulation, plate-tectonic events, and palaeogeographic change. This outcrop study showcases a Pragian to early Emsian carbonate-contourite drift, plastered on the northern upper margin of Gondwana, which recorded the convergence of Laurussia and Gondwana and the (temporal) closure of the nearby American Gateway. The results underscore the value of bed-scale features, including microfacies and primary sedimentary structures, in interpreting bottom-current dynamics. The documented contourite-specific bi-gradational sequences, 40 to 80 cm thick, are largely complete and well-comparable with the standard facies sequence identified in modern oceans. The bioclastic contourites show subtle variations in grain size and mud content in the lower part, distinct sequences of inverse-to-normal grading at different scales in the middle, and a large variety of traction structures in the upper part of the succession. This long-term coarsening-upward trend was caused by a successive increase in maximum bottom-current strength during drift growth. Traction structures such as current ripples and dunes document a westward-directed palaeocirculation of surface or intermediate water during the Pragian-Emsian. They provide evidence of enhanced bottom-current episodes towards the top of the drift, before the contourites were buried by late Emsian shales.

Y-Ho fractionation during basalt alteration in hydrothermal system: An implication for superchondritic Y/Ho signature recorded in Precambrian banded iron formations

The concentration of rare earth elements (REE) in Precambrian sedimentary rocks is widely used to estimate their depositional environment. Specifically, superchondritic Y/Ho value is a powerful indicator for determining whether a sedimentary rock was deposited in a marine environment. Currently, superchondritic Y/Ho value in seawater is believed to result from the preferential adsorption of Y onto Fe-Mn oxides. In modern oceans, Fe-Mn oxides are considered the sink for subchondritic Y/Ho value. On the contrary, Precambrian banded iron formations (BIFs) display superchondritic Y/Ho value, suggesting that Precambrian seawater also possessed a superchondritic Y/Ho value. However, Precambrian BIFs cannot account for the sink of subchondritic Y/Ho value. Thus, other processes must have been responsible for maintaining the superchondritic Y/Ho value of Precambrian seawater. A hydrothermal experiment involving mid-ocean ridge basalt (MORB) and a Cl-rich fluid was conducted at 300 °C and 500 bars to assess the role of basalt-hosted hydrothermal systems on the Y/Ho value of seawater. Over time, the fluid's Ca concentration increased while Na decreased, suggesting plagioclase albitization occurred. The REE pattern of the reacted fluids exhibited superchondritic Y/Ho values up to 63.2. Based on the presence of positive Eu anomaly and the degree of light REE depletion, the REE in the hydrothermal fluids was mainly sourced from plagioclase during albitization. The superchondritic Y/Ho values in the fluids can be explained by the preferential leaching of these elements due to differences in chloride complexing strength between REE. Subchondritic Y/Ho values observed in Precambrian hydrothermally altered basalts may be interpreted as remnants generating superchondritic Y/Ho values in coexisting Precambrian hydrothermal fluids. Therefore, we believe that the superchondritic Y/Ho value of Precambrian seawater was maintained by basalt-hosted hydrothermal system.

A benthic source of isotopically heavy Ni from continental margins and implications for global ocean Ni isotope mass balance

Nickel (Ni) is a bio-essential element for phytoplankton in the modern oceans, and yet, the global ocean mass balance of Ni has puzzled scientists for decades. Many estimates of total Ni output flux are larger than the total Ni input flux to the ocean. The measurement of Ni stable isotopes (δ60Ni) in earth materials may inform our understanding of ocean biogeochemistry and redox conditions in both the modern ocean and the geological past. An ocean Ni stable isotope imbalance has also concerned scientists, with a global ocean δ60Ni of +1.4 ‰ which is notably heavier than the major source of Ni to the oceans from rivers. Recent efforts to resolve Ni and δ60Ni imbalances have focused on the role which manganese (Mn) oxides play in marine Ni cycling, both in the water column and in marine sediments. Manganese oxide rich marine sediments can supply isotopically heavy dissolved Ni to porewater fluids and seawater, but the source of the isotopically heavy porewater Ni remains unclear. Here we present porewater trace metal concentrations and δ60Ni from two sites from the continental margin off southern California. Porewater Ni concentrations are up to roughly 100-fold higher than deep seawater concentrations (∼1 μM compared to 10 nM, respectively). Porewater δ60Ni is near 0 ‰ in the subsurface region where Ni concentrations are highest, suggesting dissolution of lithogenic material from sediments. Porewater δ60Ni increases dramatically towards the sediment-water interface with values of up to +2.66 ‰, which to our knowledge are the heaviest Ni isotope compositions ever reported for natural materials. Measurements of porewater and sediment Ni and Mn concentrations suggest that Ni is released to porewaters in the Mn-reducing zone, and then removed by newly precipitated Mn oxides as Mn and Ni move towards the oxygenated zone of sediments. A simple Rayleigh model suggests a Ni isotope fractionation of factor of -0.61 ‰ for Ni sorption onto Mn oxides, while a diffusion-reaction model suggests a Ni isotope fractionation of -1.80 to -0.96 ‰, always with preferential adsorption of lighter Ni isotopes. This flux of Ni toward the sediment-water interface, coupled with preferential removal of lighter Ni isotopes in marine sediments, provides evidence supporting an isotopically heavy benthic source of new Ni to the oceans, which we estimate has a magnitude of 0.65 × 108 to 1.66 × 108 moles/year. We find that a benthic flux such as measured here over just 0.27-2.70 % of the ocean seafloor could mix with the river δ60Ni of +0.8 ‰ to achieve the observed deep ocean δ60Ni of +1.4 ‰. This study reveals the similarities in how rivers and continental margin sediments supply heavy Ni isotopes to the ocean. In both continental rivers and margin sediments, terrigenous Ni is released with an δ60Ni near 0.1 ‰, but lighter isotopes are captured by precipitation onto oxides so that the dissolved flux to the ocean is isotopically heavier than the terrigenous material from which Ni was released, and in the case of margin sediments may be heavier than even bulk seawater δ60Ni. We thus recognize continental margin sediments with active Mn cycling as a ‘new’ source of isotopically heavy dissolved Ni to the ocean.

High-resolution UK’37 sea surface temperature reconstruction in the southwestern Atlantic Ocean during the Holocene

Modern sea surface temperatures (SST) were assessed using the alkenone calibration index (UK′37; SST-UK′37) values from twenty-one core-top sediment samples collected along the Southwestern Atlantic Ocean and validated by comparison with satellite-derived SSTs. The SST-UK′37 values revealed an annual signal between 28 and 30°S and a winter signal (83% of the region samples) between 30 and 34° S, with deviations falling within the UK′37 calibration error range. The paleotemperature (SST-UK′37) reconstruction through the Holocene was performed using two high-resolution marine sediment cores from 33 to 34°S latitude band. Millennial-scale variations dominate most of the SST-UK′37 results with changes up to ∼6 °C. Considering the modern ocean circulation dynamics, the SST-UK’37 in our records likely stems from the combined influence of the Subtropical Shelf Front (STSF) position and the interaction between the warm, salty, and nutrient-poor Subtropical Shelf Water (STSW) and the cold, fresh, and nutrient-rich Subantarctic Shelf Water (SASW). Our SST-UK’37 results also suggest that the northernmost position limit of the STSF reached around 33°S during the mid-to-late Holocene, while after ∼5500 cal yr BP the Plata Plume Water became the dominant influence on the regional SST-UK’37 signal. Spectral analyses revealed statistically significant oscillations of ∼8600 and ∼ 2250 years in record MDBT 557, and ∼ 5300 and ∼ 2600 years in record MDBT 561 which suggests the influence of Southern Westerly Winds (SWW) on the SST-UK’37, and therefore, on the STSF dynamic through the Holocene.

Balancing the oceanic Zn isotope budget: The key role of deep-sea pelagic sediments

Abstract Oxygenated deep-sea pelagic sediments with Fe-Mn–oxide particles represent a key oceanic oxic sink for transition metals in the modern ocean. However, the isotopic composition of authigenic Zn in the pelagic zone remains poorly constrained, which hampers our understanding of the global budget of Zn isotopes. Here, we analyzed the Zn isotopic compositions of two deep-sea pelagic sediment columns collected from the Pacific Ocean. The results show that authigenic Zn in deep-sea sediments is primarily hosted by the Fe-Mn (oxyhydr)oxides. The light Zn isotopic signatures (δ66Zn: −0.02‰ to 0.34‰, n = 42; computed as the per mille deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard) observed in deep-sea sediments are completely different from the previously assumed values of ~1.0‰ based on the Zn isotopic compositions of Fe-Mn crusts and nodules. Based on this observation (Zn flux of deep-sea sediments = 5.3 × 108 mol yr–1), we propose a new, balanced global budget for Zn isotopes.

Heterogeneous marine response during the Toarcian Oceanic Anoxic Event (TOAE): The potential role of storminess

The Toarcian Oceanic Anoxic Event (TOAE; ∼183 Ma) represents an important hyperthermal and deoxygenation event in the Early Jurassic. However, TOAE marine records are spatially heterogeneous with regard to nutrient levels, primary productivity, redox conditions and organic enrichment. This non-uniform response to global hyperwarming is not readily accounted for by local variations in paleogeography, climate, or water depth. Largely overlooked to date is the intensified storm activity that characterized the TOAE, and the role that this may have played in controlling marine responses to that event. A review of TOAE studies from multiple marine environments suggests that storm intensity covaried with paleoceanographic conditions, such as nutrient availability, primary productivity, redox conditions, and organic-rich sedimentation. At mid-paleolatitude sites, relatively weak storm activity during the TOAE induced short-term watermass oxygenation, and marine settings were mainly characterized by enhanced anoxia (even euxinia), water-column stratification, increased primary productivity (fueled by terrestrial runoff and P regeneration in euxinic settings), and organic-rich sedimentation. At low-paleolatitude sites, TOAE storm activity was relatively strong, and contributed to marine environments characterized by oxic to suboxic conditions, reduced water-column stratification, decreased primary productivity (possibly due to limited P regeneration and upwelling), low sedimentary organic content, and locally high oolite abundance. TOAE marine sites at all paleolatitudes exhibit: i) sea-level rise and enhanced continental weathering fluxes linked to an intensified hydrological cycle; ii) reduced dinoflagellate and increased cyanobacterial activity; and iii) low δ15N values (mainly −1‰ to +3‰) linked to enhanced diazotrophic nitrogen fixation. The spatial heterogeneity of the response of TOAE marine systems is difficult to reconcile with scenarios linking increased terrestrial flux to marine eutrophication, primary productivity increase and organic-rich sedimentation. Consequently, we hypothesize that the intensity of storm activity influenced TOAE marine systems, and that this factor can, at least partially, account for heterogeneous patterns of environmental changes at middle versus low paleolatitudes and open versus restricted marine settings. Importantly, increased storm activity can induce pycnocline deepening via vertical water-column mixing, thereby promoting: i) enhanced aerobic degradation of organic matter (low sediment organic matter content) due to a reduced oxygen-minimum zone; ii) less nutrient upwelling from deep waters into the photic zone (nutrient-depleted upper ocean), and iii) blooms of nitrogen-fixing cyanobacteria (low δ15N) and calcification (ooid formation). Thus, the interaction between storminess and pycnocline depth is a potentially important factor affecting marine environmental changes during TOAE. These findings have implications for modern oceans now experiencing climatic warming and intensified tropical storm activity.

Tipping Points in Overturning Circulation Mediated by Ocean Mixing and the Configuration and Magnitude of the Hydrological Cycle: A Simple Model

Abstract In the modern ocean, the transformation of light surface waters to dense deep waters primarily occurs in the Atlantic basin rather than in the North Pacific or Southern Oceans. The reasons for this remain unclear, as both models and paleoclimatic observations suggest that sinking can sometimes occur in the Pacific. We present a six-box model of overturning that combines insights from a number of previous studies. A key determinant of the overturning configuration in our model is whether the Antarctic Intermediate Waters are denser than the northern subpolar waters, something that depends on the magnitude and configuration of atmospheric freshwater transport. For the modern ocean, we find that although the interbasin atmospheric freshwater flux suppresses Pacific sinking, the poleward atmospheric freshwater flux out of the subtropics enhances it. When atmospheric temperatures are held fixed, North Pacific overturning can strengthen with either increases or decreases in the hydrological cycle, as well as under reversal of the interbasin freshwater flux. Tipping-point behavior, where small changes in the hydrological cycle may cause the dominant location of densification of light waters to switch between basins and the magnitude of overturning within a basin to exhibit large jumps, is seen in both transient and equilibrium states. This behavior is modulated by parameters such as the poorly constrained lateral diffusive mixing coefficient. If hydrological cycle amplitude is varied consistently with global temperature, northern polar amplification is necessary for the Atlantic overturning to collapse. Certain qualitative insights incorporated in the model can be validated using a fully coupled climate model. Significance Statement Currently, the global overturning circulation involves the conversion of waters lighter than Antarctic Intermediate Water to deep waters denser than Antarctic Intermediate Water primarily in the North Atlantic, rather than in the North Pacific or Southern Oceans. Many different factors have been invoked to explain this configuration, with atmospheric freshwater transport, basin geometry, lateral mixing, and Southern Ocean winds playing major roles. This paper develops a simple theory that combines previous theories, presents the intriguing idea that alternate configurations might be possible, and identifies multiple possible tipping points between these states.

Palaeoenvironmental implications of a turritelline-dominated assemblage from Upper Cretaceous carbonate rocks of Narmada Basin, India

Turritelline-dominated assemblages (TDAs) occur in modern siliciclastic seas within shallow depths, under normal to below normal salinity, high nutrient flux, high ocean upwelling current and/or runoff and cool water conditions. Similar assemblages in carbonate sediments are rare in modern oceans. However, TDA in carbonate rocks are common during the Cretaceous. In this study, we report the presence of a TDA in the late Turonian Nodular Limestone Formation of the Bagh Group, Narmada Basin, central India. An unusually small species of Turritella, T. bardhani Halder, n. sp., is described from the TDA. The TDA, in terms of host rock microfacies and taphonomy, indicates shallow water to intertidal, low to moderate energy, eutrophic to mesotrophic conditions, that prevailed in restricted lagoonal to marginal lagoonal, back-reef environments. The turritellines, based on trace elements and Rare Earth Elements concentration, thrived under dysoxic conditions and experienced above-normal marine salinities. The preferences of modern TDAs do not fully represent such past ecological and lithological conditions, indicating an example of a non-uniformitarian shift in habitat.

Carbonate uranium isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event

The Toarcian Oceanic Anoxic Event (T-OAE; ~183 Mya) was a globally significant carbon-cycle perturbation linked to widespread deposition of organic-rich sediments, massive volcanic CO2 release, marine faunal extinction, sea-level rise, a crisis in carbonate production related to ocean acidification, and elevated seawater temperatures. Despite recognition of the T-OAE as a potential analog for future ocean deoxygenation, current knowledge on the severity of global ocean anoxia is limited largely to studies of the trace element and isotopic composition of black shales, which are commonly affected by local processes. Here, we present the first carbonate-based uranium isotope (δ238U) record of the T-OAE from open marine platform limestones of the southeastern Tethys Ocean as a proxy for global seawater redox conditions. A significant negative δ238U excursion (~0.4‰) is recorded just prior to the onset of the negative carbon isotope excursion comprised within the T-OAE, followed by a long-lived recovery of δ238U values, thus confirming that the T-OAE represents a global expansion of marine anoxia. Using a Bayesian inverse isotopic mass balance model, we estimate that anoxic waters covered ~6 to 8% of the global seafloor during the peak of the T-OAE, which represents 28 to 38 times the extent of anoxia in the modern ocean. These data, combined with δ238U-based estimates of seafloor anoxic area for other CO2-driven Phanerozoic OAEs, suggest a common response of ocean anoxia to carbon release, thus improving prediction of future anthropogenically induced ocean deoxygenation.

Extreme Planktic Foraminiferal Dwarfism Across the ETM2 in the Tethys Realm in Response to Warming

AbstractPronounced warming negatively impacts ecosystem resilience in modern oceans. To offer a long‐term geological perspective of the calcareous plankton response to global warming, we present an integrated record, from two Tethyan sections (northeastern Italy), of the planktic foraminiferal and calcareous nannofossil response to the Eocene Thermal Maximum 2 hyperthermal (ETM2, ∼54 Ma). Our study reveals pronounced changes in assemblage composition and a striking dwarfing of planktic foraminiferal tests of up to 40% during the event, impacting both surface and deeper dwellers. The increased abundance of small placoliths among calcareous nannofossils is interpreted as community size reduction. Literature and our foraminiferal size data from Sites 1263 and 1209 (Atlantic and Pacific Oceans) highlights that the pronounced dwarfism is restricted to the Tethyan area. The ETM2 is characterized by warm sea surface temperatures as indicated by our δ18O data, but this warming is of global extent and cannot explain the unique dwarfism. Excluding evolutionary modifications, other potential drivers of dwarfism (eutrophication, deoxygenation, metabolic adaptation) cannot explain the exceptional dwarfism by themselves. The smallest sizes are in close temporal association with peaks in volcanic derived Hg/Th‐Hg/Rb recorded just before and at the ETM2 which could not have been brought into our sections through weathering. In contrast, size reductions are absent below and above the ETM2 at Hg peaks where δ18O data do not show warm conditions. We speculate that the local input of toxic metals from submarine volcanic emissions could have acted synergistically to warming, causing the unique dwarfism.

Centennial-millennial scale global climate-linked monsoonal and non-monsoonal changes in the eastern Arabian Sea during the last 42,800 years

The Arabian Sea significantly impacts the global climate due to its hosting of one of the largest sedimentary bodies in the Modern Ocean basin and thickest oxygen minimum zone. It makes the study of fine-scale evolutionary changes in the Arabian Sea imperative to address the ongoing challenges in developing a strong and cohesive model for predicting rapid climate change in the future. Therefore, this study carried out environmental magnetic, grain size, stable isotope, total organic carbon (TOC), trace elements (TE), and rare earth elements (REE) investigations on a well-dated 2.68 m long sediment core from the eastern Arabian Sea to understand the fluctuation in monsoon and non-monsoon-driven sediment supply and associated primary productivity changes during the late Quaternary. The careful observations of chronological changes in the investigated parameters concerning coeval major global events enabled us to successfully identify the response of major global climatic events that occurred around 42.8–28 ka, 17 ka, 14.5 ka, 11.7 ka, 9.7 ka, 8.2 ka, 4.6–3.9, and 2–0.6 ka. These global events also played a crucial role in co-regulating the water column oxygen conditions in the Arabian Sea. Comparing our record with a sedimentary record from off Chennai, Bay of Bengal, suggests that opposite variations (anti-phasing) between southwest (SW) monsoon and northeast (NE) monsoon is a post-25 ka phenomenon. Pre-25 ka SW and NE monsoon showed similar variations (same phase), and we speculate that this anti-phasing between the SW and NE monsoon was cyclically driven by the earth's axial precession cycle.

Overlooked shelf sediment reductive sinks of dissolved rhenium and uranium in the modern ocean

Rhenium (Re) and uranium (U) are essential proxies in reconstructing past oceanic oxygenation evolution. However, their removal in continental shelf sediments, hotspots of early diagenesis, were previously treated as quantitatively unimportant sinks in the ocean. Here we examine the sedimentary reductive removal of Re and U and their coupling with organic carbon decomposition, utilizing the 224Ra/228Th disequilibria within the East China Sea shelf. We identified positive correlations between their removal fluxes and the rates of sediment oxygen consumption or organic carbon decomposition. These correlations enable an evaluation of global shelf reductive sinks that are comparable to (for Re) or higher than (~4-fold for U) previously established suboxic/anoxic sinks. These findings suggest potential imbalances in the modern budgets of Re and U, or perhaps a substantial underestimation of their sources. Our study thus highlights shelf sedimentary reductive removal as critical yet overlooked sinks for Re and U in the modern ocean.

Biogeochemical Fluxes of Nickel in the Global Oceans Inferred From a Diagnostic Model

AbstractNickel (Ni) is a micronutrient that plays a role in nitrogen uptake and fixation in the modern ocean and may have affected rates of methanogenesis on geological timescales. Here, we present the results of a diagnostic model of global ocean Ni fluxes which addresses key questions about marine Ni cycling. Sparsely available observations of Ni concentration are first extrapolated into a global gridded climatology using tracers with better observational coverage such as macronutrients, and testing three different machine learning techniques. The physical transport of Ni is then estimated using the ocean circulation inverse model (OCIM2), revealing regions of net convergence or divergence. These diagnostics are not based on any assumption about Ni biogeochemical cycling, but their spatial patterns can be used to infer where biogeochemical processes such as biological Ni uptake and regeneration take place. Although Ni and silicate (Si) have similar concentration patterns in the ocean, we find that the spatial pattern of Ni uptake in the surface ocean is similar to phosphate (P) uptake but not to silicate (Si) uptake. This suggests that their similar distributions arise from different biogeochemical mechanisms, consistent with other evidence showing that Ni is not incorporated into diatom frustules. We find that Ni:P ratios at uptake do not decrease as Ni concentrations approach 2 nM, which challenges the hypothesis of a ∼2 nM pool of non‐bioavailable Ni in the surface ocean. Finally, we find that the net regeneration of Ni occurs deeper in the ocean than for P, though not as deeply as for Si.

Coupling and Decoupling Between Sedimentary Mercury and Organic Carbon Preservation in the Oxygenated Marine Environment

AbstractMercury (Hg) enrichment relative to total organic carbon (TOC) in sedimentary records has been widely used as a volcanism proxy. However, the depositional and diagenetic effects on Hg burial are not well understood, limiting the reliability of the proxy. Here, we report a systematic investigation of Hg sedimentation under well‐oxygenated bottom water. Most of the studied cores show total mercury content (THg) to TOC ratio between 50 and 300 ppb/%, including northwest Pacific ODP Site 1208, East Equatorial Pacific ODP Sites 677 and 1241, the Antarctic Zone ODP Site 1094, and East China Sea sediment core EC2005. The consistent THg/TOC ratio confirms the strong coupling between Hg and particulate organic matter (POM) despite large differences in geographic locations, sedimentation rates, TOC content, POM sources, and early diagenetic environments. Nevertheless, the THg/TOC ratio is higher in sediments under well‐oxygenated bottom water than in those under oxygen‐depleted waters, probably as a result of a higher degree of organic matter degradation in oxygenated sediments during early diagenesis. The THg in the high TOC variability section at Site 1208 is abnormally high, resulting in decoupling between Hg and POM. Mercury in this section is still leachable by oxidizing solution and has a similar trend of variability with easily reducible iron content, implying that the metal oxide‐organic matter association might act as Hg bounding phase. We suggest that the enriched Hg is probably supplied by the neighboring high TOC sediment. Therefore, while THg/TOC > 300 ppb/% could be considered as distinct Hg enrichment in sedimentary records, the diagenetic mobilization effect should be first excluded as the possible cause as demonstrated by the records of site 1208. Our study therefore provides new insights into the Hg cycle in the modern ocean and the utilization of Hg enrichment as a volcanism proxy.

Combined cadmium and chromium isotopes record a collapse of bioproductivity across the Cretaceous—Paleogene boundary in the Danish basin

Cadmium (Cd) isotope signatures (δ114Cd) deduced from modern and ancient marine deposits are often used as a proxy for bioproductivity and for micronutrient cycling, whereas chromium (Cr) isotope signatures (δ53Cr) are used as a potential proxy for oxygenation. The Cretaceous—Paleogene (K/Pg) boundary records one of Earth's five big mass extinctions, which contains geochemical fingerprints of the Chixculub asteroid impact and is coeval with the Deccan Trap volcanism. Here we present for the first time a combined record of Cd and Cr isotopes from the Danish Basin to interrogate the effects on bioproductivity across the K/Pg boundary. The δ114Cd values range between −0.28‰ and 0.27‰ defining an average of 0.05‰ ± 0.24 (2σ, n = 87). Authigenic δ53Cr values range between 0.06‰ and 1.06‰ and define an average of 0.59‰ ± 0.51 (2σ, n = 81). An abrupt jump, across the K/Pg boundary layer, from consistently positively fractionated Cd and Cr isotope signatures recorded in Late Maastrichtian white chalks towards less positively fractionated δ114Cd values and heterogeneous δ53Cr values in the Early Danian carbonates, likely indicates a decrease in surface water bioproductivity. This decrease is accompanied by redox fluctuations in the water column as a consequence of environmental changes either related to the Chixculub impact event and/or to the Deccan volcanic eruptions coeval with this boundary. Assuming an efficient removal of Cd and Cr through consumption and/or adsorption by primary producers, or by incorporation into the skeleton/shells of calcifiers, and using the fractionation factor of Cd into carbonates, we reconstruct surface water conditions during the Late Masstrichtian—Early Danian that are compatible with those of modern oceans, with δ114Cd varying from 0.17 to 0.72‰. Overall, the combined Cr-Cd isotope fluctuations over the K/Pg boundary are consistent with a scenario characterized by climate-induced changes of nutrient availability and concomitant responses of primary production levels, ultimately related to the asteroid impact and to the proceeding volcanic activities that influenced the ocean's chemistry. We anticipate our results to point for further studies of other stratigraphic sections worldwide to reveal an extensive Cd-Cr record for the Cretaceous—Paleogene boundary and its corresponding collapse of bioproductivity and decreasing ocean oxygenation.

Geochemical Signatures of Mafic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Mafic Magmatism

Abstract Greenstone belts are dominated by mafic volcanic rocks with geochemical characteristics that indicate a range of possible geodynamic influences. Many analogies with modern tectonic settings have been suggested. Increasing exploration of the modern oceans and comprehensive sampling of volcanic rocks from the sea floor are now providing unique opportunities to characterize different melt sources and petrogenesis that can be more closely compared to greenstone belts. In this study, we have compiled high-quality geochemical analyses of more than 2,850 unique samples of submarine mafic volcanic rocks (<60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hotspot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical variability spanning the full range of compositions of basalts found in greenstone belts. This diversity is interpreted to be due to variable crustal thickness, dry melting versus wet melting conditions, mantle mixing, and contamination. In particular, different melting conditions have been linked to mantle heterogeneity, complex mantle flow regimes, and short-lived tectonic domains, such as those associated with diffuse spreading, overlapping spreading centers, and triple junctions. These are well documented in the microplate mosaics of the Western Pacific. Systematic differences in mafic volcanic rock compositions in modern oceanic settings are revealed by a combination of principal components analysis and unsupervised hierarchical clustering of the compiled data. Mafic volcanic rocks from most arc-back arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry such as large ion lithophile element (LILE) enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements (HREEs). This contrasts with mafic volcanic rocks in Archean greenstone belts, which show no, or at least weaker, subduction-related chemistry, a less depleted mantle, less wet melting, and variable crustal contamination. The differences are interpreted to be the result of the lower mantle temperatures, thinner crust, and subduction-related processes of present-day settings. However, mafic rocks that are geochemically identical to those in Archean greenstone belts occur in many modern back-arc basins, including the Lau basin, East Scotia ridge, Bransfield Strait, and Manus basin, which are characterized by fertile mantle sources, high heat flow, and complex spreading regimes typical of small-scale microplate mosaics. These types of settings are recognized as favorable for volcanogenic massive sulfide (VMS) deposits in modern and ancient greenstone belts, and therefore the particular geochemical signatures of the mafic volcanic rocks are potentially important for area selection in base metal exploration.