Ocean and Sediment Biogeochemistry

2022

1. Nd

   Reactive-transport modeling of neodymium and its radiogenic isotope in deep-sea sediments: The roles of authigenesis, marine silicate weathering and reverse weathering

   Jianghui Du, Brian A. Haley, Alan C. Mix, April N. Abbott, James McManus, and Derek Vance

   Earth and Planetary Science Letters, Oct 2022

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   Dissolved Rare Earth Elements (REE) and radiogenic neodymium (epsilon(Nd)) isotope composition (ENd) of seawater are widely used geochemical tools in studying marine processes, but their modern ocean budgets are poorly understood. Recent discoveries of large benthic fluxes of REE with unique epsilon(Nd) signatures from marine sediments, particularly in the deep-sea, have led to a “bottom-up” hypothesis, which suggests that early diagenesis below the sediment-water interface (SWI) controls the ocean’s REE and epsilon(Nd) budgets. To investigate such sedimentary processes, we created a reactive-transport model for the biogeochemical cycling of Nd and epsilon(Nd) in marine sediments. Here, we attempt to quantify the roles of authigenesis, marine silicate weathering and reverse weathering in the diagenetic cycling of Nd and epsilon(Nd) at a deep-sea (3000 m) site on the Oregon margin. Our model predicts that, at this site, Nd carried by Fe/Mn oxides into sediments eventually transforms to authigenic Nd-phosphate, during which similar to 9% of the incoming solid Nd flux is released as a dissolved benthic flux back to the overlying bottom water. We also find that the classic reversible scavenging formulation applied to Nd co-cycling with Fe/Mn oxides is inconsistent with the data. Rather, a co-precipitation formulation, assuming Nd is structurally incorporated into Fe/Mn oxides, successfully simulates the data. The model also shows that authigenesis alone cannot explain the pore water and authigenic epsilon(Nd), which are both more radiogenic than bottom water at this site. However, the weathering of volcanic silicates sourced from the local subduction zone can successfully explain epsilon(Nd). We suggest that, because reverse weathering by authigenic clay formation maintains the under-saturation of primary silicates in pore water, marine silicate weathering can proceed. The processes we model likely affect the sedimentary cycling of many other trace elements and isotopes, with much broader implications for the understanding of ocean biogeochemistry. (C) 2022 The Author(s). Published by Elsevier B.V.

2. Nd

   Widespread lithogenic control of marine authigenic neodymium isotope records? Implications for paleoceanographic reconstructions

   A. N. Abbott, S. C. Lohr, A. Payne, H. Kumar, and Jianghui Du

   Geochimica et Cosmochimica Acta, Feb 2022

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   Our understanding of past ocean-climate dynamics is informed by multiple paleocirculation proxies including delta C-13, Pa-231/(230Th), and radiogenic neodymium isotopes (epsilon(Nd)). Of these, the epsilon(Nd) signature of marine authigenic phases is of particular importance as it is considered a robust circulation proxy applicable across timescales, permitting circulation reconstructions during periods of rapid, climatically-induced biological or chemical change (e.g. productivity, pH). However, growing evidence of non-conservative behavior and a widespread sedimentary source (benthic flux via pore water) of Nd to the global ocean suggests that authigenic epsilon(Nd) records do not strictly record a water mass signature, highlighting the need to reconsider interpretations of the authigenic record. To examine the impact of a sedimentary influence on the authigenic record, here we compile paired authigenic and detrital neodymium records from every major ocean basin and from 80 Ma to present. We then focus on just the North Atlantic Ocean basin to examine if this relationship holds up regionally and how authigenic epsilon(Nd) changes relate to sediment composition changes from two scientific ocean drill cores spanning the past 25 ka. We present a new conceptual framework to guide our discussion that examines the coupling or decoupling of authigenic and detrital epsilon(Nd) in terms of the relative importance of each of the three major potential controls as defined in the existing literature (bottom water, pore water, sediments) on the authigenic record. Our compilation reveals a strong linear relationship between detrital epsilon(Nd) and authigenic epsilon(Nd )(correlation coefficient = 0.86, n = 871), demonstrating a widespread influence of lithogenically sourced neodymium on authigenic epsilon(Nd). We find the same is true within the North Atlantic, with the authigenic records at both locations strongly influenced by the sediments and likely not recording bottom water neodymium values. Emerging evidence for a lithogenic or benthic flux influence on the budgets of a wide range of trace elements suggests that our interpretative framework will be broadly useful for understanding the behavior of trace elements and their isotopes at the sediment-water interface.(c) 2021 Elsevier Ltd. All rights reserved.

2017

1. Nd

   The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans

   Brian A. Haley, Jianghui Du, April N. Abbott, and James McManus

   Frontiers in Marine Science, Feb 2017

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   Neodymium (Nd) isotopes are considered a valuable tracer of modern and past ocean circulation. However, the promise of Nd isotope as a water mass tracer is hindered because there is not an entirely self-consistent model of the marine geochemical cycle of rare earth elements (REEs, of which Nd is one). That is, the prevailing mechanisms to describe the distributions of elemental and isotopic Nd are not completely reconciled. Here, we use published [Nd] and Nd isotope data to examine the prevailing model assumptions, and further compare these data to emergent alternative models that emphasize benthic processes in controlling the cycle of marine REEs and Nd isotopes. Our conclusion is that changing from a “top-down” driven model for REE cycling to one of a “bottom-up” benthic source model can provide consistent interpretations of these data for both elemental and isotopic Nd distributions. We discuss the implications such a benthic flux model carries for interpretation of Nd isotope data as a tracer for understanding modern and past changes in ocean circulation.

2016

1. Nd

   Neodymium isotopes in authigenic phases, bottom waters and detrital sediments in the Gulf of Alaska and their implications for paleo-circulation reconstruction

   Jianghui Du, Brian A. Haley, and Alan C. Mix

   Geochimica et Cosmochimica Acta, Nov 2016

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   The isotopic composition of neodymium (epsilon(Nd)) extracted from sedimentary Fe-Mn oxyhydroxide offers potential for reconstructing paleo-circulation, but its application depends on extraction methodology and the mechanisms that relate authigenic epsilon(Nd) to bottom water. Here we test methods to extract authigenic epsilon(Nd) from Gulf of Alaska (GOA) sediments and assess sources of leachate Nd, including potential contamination from trace dispersed volcanic ash. We show that one dominant phase is extracted via leaching of core-top sediments. Major and trace element geochemistry demonstrate that this phase is authigenic Fe-Mn oxyhydroxide. Contamination of leachate (authigenic) Nd from detrital sources is insignificant (<1%); our empirical results are consistent with established kinetic mineral dissolution rates and theory. Contamination of extracted epsilon(Nd) from leaching of volcanic ash is below analytical uncertainty. However, the epsilon(Nd) of core-top leachates in the GOA is consistently more radiogenic than bottom water. We infer that authigenic phases record pore water epsilon(Nd), and the relationships of epsilon(Nd) among bottom waters, pore waters, authigenic phases and detrital sediments are primarily governed by the exposure time of bottom water to sea-floor sediments, rate of exchange across the sediment-water interface and the reactivity and composition of detrital sediments. We show that this conceptual model is applicable on the Pacific basin scale and provide a new framework to understand the role of authigenic phases in both modern and paleo-applications, including the use of authigenic epsilon(Nd) as a paleo-circulation tracer.(C) 2016 Elsevier Ltd. All rights reserved.

2022

1. REE

   Dominance of benthic flux of REEs on continental shelves: implications for oceanic budgets

   K. Deng, S. Yang, Jianghui Du, E. Lian, and D. Vance

   Geochemical Perspectives Letters, Jun 2022

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   Rare earth elements (REEs) are powerful tools to track oceanic biogeochemical proc-esses. However, our understanding of REE sources is incomplete, leading to contro-versial interpretations regarding their oceanic cycling. Continental margin sediments are often assumed to be a major source, but the sediment pore water data required to understand the processes controlling that potential source are scarce. Here, we mea-sure and compile pore water and estuarine REE data from the Changjiang (Yangtze) estuary-East China Sea shelf. We show that release of REEs, from shallow pore water to overlying seawater, is coupled to Mn reduction. In contrast, REEs are removed in deep pore water, perhaps via formation of an authigenic REE-bearing phase. This sedimentary source can potentially explain REE addition in the estuary at mid-high salinity. Our calculations suggest that the benthic flux is the largest Nd source (similar to 40 %) on the East China Sea shelf. Globally, however, despite a higher benthic Nd flux on the advection-dominated shelf, the much more extensive deep ocean still domi-nates the total area-integrated benthic flux. Our results call for a more extensive investigation of the magnitude of the benthic flux of REEs to the oceans.

2023

1. Ni

   The nickel output to abyssal pelagic manganese oxides: A balanced elemental and isotope budget for the oceans

   Sarah Fleischmann, Jianghui Du, Aditi Chatterjee, James McManus, Sridhar D. Iyer, Ankeeta Amonkar, and Derek Vance

   Earth and Planetary Science Letters, 2023

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   The development of nickel isotopes as a chemical tracer of past ocean environments requires a sound understanding of the modern oceanic budget. Our current understanding of this budget implies a large elemental and isotope imbalance between inputs to and outputs from the dissolved pool of the ocean. This imbalance is mainly caused by the dominant oxic sink of Ni to Mn oxide-rich sediments. Though the Ni isotope composition of Fe-Mn crusts has previously been used as proxy for the Ni isotope composition of these sediments, crusts and nodules represent a very small part of the total Mn oxide output. Instead, Mn oxide microparticle supply to pelagic and hemi-pelagic sediments dominates the removal of Mn to sediments, but there are very few isotope data for such samples. Here we present the first extensive Ni concentration and isotope dataset from fully oxic Mn-rich pelagic sediments, from 6 different sites across the open Pacific and 10 closely-spaced sites in the Indian Ocean. We also present data for one hemi-pelagic site representing a suboxic setting on the California Margin. Abyssal Pacific and Indian Ocean sediments have a Ni/Mn ratio of 0.02 (similar to Fe-Mn crusts) and their authigenic Ni is isotopically lighter (δ60Ni = +0.26 to +1.08‰) than seawater (+1.33‰) and crusts (+1.55±0.38‰). Data presented here for organic carbon-rich suboxic sediments of the Californian margin have lower Ni/Mn ratios (0.004 to 0.014 for the oxic top of the core, where Mn oxide is present in abundance) and even lighter authigenic Ni isotope compositions (δ60Ni = -0.08±0.11‰). We show that the Ni isotopes of nearly all Mn-rich sediments and deposits analysed to date, including the new data presented here, are correlated with Co/Mn ratios, suggesting that both are controlled by accumulation rate, progressive incorporation of Ni into the metal oxide structure and isotopic re-equilibration between the solid and aqueous phase. At sites where sediments are diagenetically processed, such as the California Margin, differential diagenetic remobilisation of Mn, Ni and Co cause deviations from this correlation. We present a new mass balance calculation that recognises the importance of scavenging of oceanic Ni to Mn oxide-rich proximal hydrothermal sediments, with low Ni/Mn and light isotope compositions. The mass balance produces a budget that can be simultaneously balanced for both amounts and isotope compositions of Ni. This result provides a strong basis for the application of Ni isotopes as records of the evolution of the metal sink from the oxic oceans through Earth history.

2023

1. Be

   Dominance of benthic fluxes in the oceanic beryllium budget and implications for paleo-denudation records

   Kai Deng, Jörg Rickli, Tim Jesper Suhrhoff, Jianghui Du, Florian Scholz, Silke Severmann, Shouye Yang, James McManus, and Derek Vance

   Science Advances, 2023

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   The ratio of atmosphere-derived 10Be to continent-derived 9Be in marine sediments has been used to probe the long-term relationship between continental denudation and climate. However, its application is complicated by uncertainty in 9Be transfer through the land-ocean interface. The riverine dissolved load alone is insufficient to close the marine 9Be budget, largely due to substantial removal of riverine 9Be to continental margin sediments. We focus on the ultimate fate of this latter Be. We present sediment pore-water Be profiles from diverse continental margin environments to quantify the diagenetic Be release to the ocean. Our results suggest that pore-water Be cycling is mainly controlled by particulate supply and Mn-Fe cycling, leading to higher benthic fluxes on shelves. Benthic fluxes may help close the 9Be budget and are at least comparable to, or higher ( 2-fold) than, the riverine dissolved input. These observations demand a revised model framework, which considers the potentially dominant benthic source, to robustly interpret marine Be isotopic records. Benthic fluxes from ocean sediments dominate the oceanic Be budget, complicating the use of 10Be/9Be as a paleo-denudation tracer.