Publications
40 results found
Vance D, Little SH, De Souza GF, et al., 2017, Silicon and zinc biogeochemical cycles coupled through the Southern Ocean, Nature Geoscience, Vol: 10, Pages: 202-206, ISSN: 1752-0908
Zinc is vital for the physiology of oceanic phytoplankton. The striking similarity of the depth profiles of zinc to those of silicate suggests that the uptake of both elements into the opaline frustules of diatoms, and their regeneration from these frustules, should be coupled. However, the zinc content of diatom opal is negligible, and zinc is taken up into and regenerated from the organic parts of diatom cells. Thus, since opaline frustules dissolve deep in the water column while organic material is regenerated in the shallow subsurface ocean, there is little reason to expect the observed close similarity between zinc and silicate, and the dissimilarity between zinc and phosphate. Here we combine observations with simulations using a three-dimensional model of ocean circulation and biogeochemistry to show that the coupled distribution of zinc and silicate, as well as the decoupling of zinc and phosphate, can arise in the absence of mechanistic links between the uptake of zinc and silicate, and despite contrasting regeneration length scales. Our simulations indicate that the oceanic zinc distribution is, in fact, a natural result of the interaction between ocean biogeochemistry and the physical circulation through the Southern Ocean hub. Our analysis demonstrates the importance of uptake stoichiometry in controlling ocean biogeochemistry, and the utility of global-scale elemental covariation in the ocean in understanding these controls.
van de Flierdt T, Griffiths AM, Lambelet M, et al., 2016, Neodymium in the oceans: a global database, a regional comparison and implications for palaeoceanographic research, Journal: Philosophical Transactions A: Mathematical, Physical and Engineering Sciences, Vol: 374, ISSN: 1471-2962
The neodymium (Nd) isotopic composition of seawater has been used extensively to reconstruct ocean circulation on a variety of time scales. However, dissolved neodymium concentrations and isotopes do not always behave conservatively, and quantitative deconvolution of this non-conservative component can be used to detect trace metal inputs and isotopic exchange at ocean–sediment interfaces. In order to facilitate such comparisons for historical datasets, we here provide an extended global database for Nd isotopes and concentrations in the context of hydrography and nutrients. Since 2010, combined datasets for a large range of trace elements and isotopes are collected on international GEOTRACES section cruises, alongside classical nutrient and hydrography measurements. Here, we take a first step towards exploiting these datasets by comparing high-resolution Nd sections for the western and eastern North Atlantic in the context of hydrography, nutrients and aluminium (Al) concentrations. Evaluating those data in tracer–tracer space reveals that North Atlantic seawater Nd isotopes and concentrations generally follow the patterns of advection, as do Al concentrations. Deviations from water mass mixing are observed locally, associated with the addition or removal of trace metals in benthic nepheloid layers, exchange with ocean margins (i.e. boundary exchange) and/or exchange with particulate phases (i.e. reversible scavenging). We emphasize that the complexity of some of the new datasets cautions against a quantitative interpretation of individual palaeo Nd isotope records, and indicates the importance of spatial reconstructions for a more balanced approach to deciphering past ocean changes.
Vance D, Little SH, Archer C, et al., 2016, The oceanic budgets of nickel and zinc isotopes: the importance of sulfidic environments as illustrated by the Black Sea, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 374, ISSN: 1364-503X
Isotopic data collected to date as part of the GEOTRACES and other programmes show that the oceanic dissolved pool is isotopically heavy relative to the inputs for zinc (Zn) and nickel (Ni). All Zn sinks measured until recently, and the only output yet measured for Ni, are isotopically heavier than the dissolved pool. This would require either a non-steady-state ocean or other unidentified sinks. Recently, isotopically light Zn has been measured in organic carbon-rich sediments from productive upwelling margins, providing a potential resolution of this issue, at least for Zn. However, the origin of the isotopically light sedimentary Zn signal is uncertain. Cellular uptake of isotopically light Zn followed by transfer to sediment does not appear to be a quantitatively important process. Here, we present Zn and Ni isotope data for the water column and sediments of the Black Sea. These data demonstrate that isotopically light Zn and Ni are extracted from the water column, probably through an equilibrium fractionation between different dissolved species followed by sequestration of light Zn and Ni in sulfide species to particulates and the sediment. We suggest that a similar, non-quantitative, process, operating in porewaters, explains the Zn data from organic carbon-rich sediments.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.
Little SH, Vance D, McManus J, et al., 2016, Key role of continental margin sediments in the oceanic mass balance of Zn and Zn isotopes, Geology, Vol: 44, Pages: 207-210, ISSN: 0091-7613
Zinc is an essential micronutrient and its concentration and isotopic composition in marine sediments represent promising tracers of the ocean carbon cycle. However, gaps remain in our understanding of the modern marine cycle of Zn, including an explanation of the heavy Zn isotopic composition of seawater relative to the known inputs, and the identity of a required missing sink for light Zn isotopes. Here we present Zn isotope data for organic-rich and trace metal–rich continental margin sediments from the east Pacific margins that together provide the first observational evidence for the previously hypothesized burial of light Zn in such settings. In turn, this light Zn output flux provides a means to enrich the seawater dissolved pool in heavy isotopes. The size and isotopic composition of the margin sink are controlled by the uptake of Zn into organic matter in the photic zone and the fixation of this pool, probably in the form of Zn sulfides, in sediments. An estimate of its significance to the overall Zn oceanic mass balance, both in terms of flux and isotopic composition, indicates that such settings can fulfill the requirements of the missing Zn sink. Taken together, these observations have important implications for the interpretation of Zn isotope data for marine sediments in the geologic record.
Little SH, Vance D, Lyons TW, et al., 2015, Controls on trace metal authigenic enrichment in reducing sediments: insights from modern oxygen-deficient settings, American Journal of Science: an international earth science journal, Vol: 315, Pages: 77-119, ISSN: 0002-9599
Any effort to reconstruct Earth history using variations in authigenic enrichments of redox-sensitive and biogeochemically important trace metals must rest on a fundamental understanding of their modern oceanic and sedimentary geochemistry. Further, unravelling the multiple controls on sedimentary enrichments requires a multi-element approach. Of the range of metals studied, most is known about the behavior of Fe, Mn, and Mo. In this study, we compare the authigenic enrichment patterns of these elements with a group whose behavior is not as well defined (Cd, Cu, Zn, and Ni) in three oxygen-poor settings: the Black Sea, the Cariaco Basin (Venezuela), and the Peru Margin. These three settings span a range of biogeochemical environments, allowing us to isolate the different controls on sedimentary enrichment. Our approach, relying on the covariation of elemental enrichment factors [EF, defined for element X as: EFX = (X/Al)sample/(X/Al)lithogenic], has previously been applied to Mo and U to elucidate paleoenvironmental information on, for example, benthic redox conditions, the particulate shuttle, and the evolution of water mass chemistry. We find two key controls on trace metal enrichment. First, the concentration of an element in the lithogenic background sediment (used in calculating EFX) controls the magnitude of potential enrichment. Maximum enrichment factors of 376 and 800 are calculated for Mo (∼1 ppm in detrital sediments) and Cd (∼0.3 ppm), respectively, compared to values not greater than 17 in any setting for the other five metals (∼45 ppm to ∼4.5 wt.% in detrital sediments). Second, there is a relationship between the aqueous concentration of the element in overlying seawater and its degree of enrichment in the sediment. We further identify four important processes for delivery of trace metals to the sediment. These are: (1) cellular uptake (especially important for Zn and Cd), (2) interaction/co-precipitation with sulfide (Mo, Cu, and Cd)
Sherman DM, Little SH, Vance D, 2015, Reply to comment on "Molecular controls on Cu and Zn isotopic fractionation in Fe-Mn crusts", EARTH AND PLANETARY SCIENCE LETTERS, Vol: 411, Pages: 313-315, ISSN: 0012-821X
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Andersen MB, Romaniello S, Vance D, et al., 2014, A modern framework for the interpretation of <SUP>238</SUP>U/<SUP>235</SUP>U in studies of ancient ocean redox, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 400, Pages: 184-194, ISSN: 0012-821X
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- Citations: 143
Little SH, Sherman DM, Vance D, et al., 2014, Molecular controls on Cu and Zn isotopic fractionation in Fe-Mn crusts, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 396, Pages: 213-222, ISSN: 0012-821X
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- Citations: 70
Little SH, Vance D, Siddall M, et al., 2013, A modeling assessment of the role of reversible scavenging in controlling oceanic dissolved Cu and Zn distributions, Global Biogeochemical Cycles, Vol: 27, Pages: 780-791, ISSN: 0886-6236
The balance of processes that control elemental distributions in the modern oceans is important in understanding both their internal recycling and the rate and nature of their eventual output to sediment. Here we seek to evaluate the likely controls on the vertical profiles of Cu and Zn. Though the concentrations of both Cu and Zn increase with depth, Cu increases in a more linear fashion than Zn, which exhibits a typical “nutrient-type” profile. Both elements are bioessential, and biological uptake and regeneration has often been cited as an important process in controlling their vertical distribution. In this study, we investigate the likely importance of another key vertical process, that of passive scavenging on sinking particles, via a simple one-dimensional model of reversible scavenging. We find that, despite the absence of lateral or vertical water advection, mixing, diffusion, or biological uptake, our reversible scavenging model is very successful in replicating dissolved Cu concentration profiles on a range of geographic scales. We provide preliminary constraints on the scavenging coefficients for Cu for a spectrum of particle types (calcium carbonate, opal, particulate organic carbon, and dust) while emphasizing the fit of the shape of the modeled profile to that of the tracer data. In contrast to Cu, and reaffirming the belief that Zn behaves as a true micronutrient, the scavenging model is a poor match to the shape of oceanic Zn profiles. Modeling a single vertical process simultaneously highlights the importance of lateral advection in generating high Zn concentrations in the deep Pacific.
Little SH, Vance D, Walker-Brown C, et al., 2013, The oceanic mass balance of copper and zinc isotopes, investigated by analysis of their inputs, and outputs to ferromanganese oxide sediments, Geochimica et Cosmochimica Acta, Vol: 125, Pages: 673-693, ISSN: 1872-9533
The oceanic biogeochemical cycles of the transition metals have been eliciting considerable attention for some time. Manyof them have isotope systems that are fractionated by key biological and chemical processes so that significant informationabout such processes may be gleaned from them. However, for many of these nascent isotopic systems we currently know toolittle of their modern oceanic mass balance, making the application of such systems to the past speculative, at best. Here weinvestigate the biogeochemical cycling of copper (Cu) and zinc (Zn) isotopes in the ocean. We present estimates for the isotopiccomposition of Cu and Zn inputs to the oceans based on new data presented here and published data. The bulk isotopiccomposition of dissolved Cu and Zn in the oceans (d65Cu +0.9&, d66Zn +0.5&) is in both cases heavier than their respectiveinputs (at around d65Cu = +0.6& and d66Zn = +0.3&, respectively), implying a marine process that fractionates themand a resulting isotopically light sedimentary output. For the better-known molybdenum isotope system this is achieved bysorption to Fe–Mn oxides, and this light isotopic composition is recorded in Fe–Mn crusts. Hence, we present isotopic datafor Cu and Zn in three Fe–Mn crusts from the major ocean basins, which yield d65Cu = 0.44 ± 0.23& (mean and 2SD) andd66Zn = 1.04 ± 0.21&. Thus for Cu isotopes output to particulate Fe–Mn oxides can explain the heavy isotopic compositionof the oceans, while for Zn it cannot. The heavy Zn in Fe–Mn crusts (and in all other authigenic marine sediments measuredso far) implies that a missing light sink is still to be located. These observations are some of the first to place constraints on themodern oceanic mass balance of Cu and Zn isotopes.
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