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      Quantifying trace element and isotope fluxes at the ocean–sediment boundary: a review

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          Abstract

          Quantifying fluxes of trace elements and their isotopes (TEIs) at the ocean's sediment–water boundary is a pre-eminent challenge to understand their role in the present, past and future ocean. There are multiple processes that drive the uptake and release of TEIs, and properties that determine their rates are unevenly distributed (e.g. sediment composition, redox conditions and (bio)physical dynamics). These factors complicate our efforts to find, measure and extrapolate TEI fluxes across ocean basins. GEOTRACES observations are unveiling the oceanic distributions of many TEIs for the first time. These data evidence the influence of the sediment–water boundary on many TEI cycles, and underline the fact that our knowledge of the source–sink fluxes that sustain oceanic distributions is largely missing. Present flux measurements provide low spatial coverage and only part of the empirical basis needed to predict TEI flux variations. Many of the advances and present challenges facing TEI flux measurements are linked to process studies that collect sediment cores, pore waters, sinking material or seawater in close contact with sediments. However, such sampling has not routinely been viable on GEOTRACES expeditions. In this article, we recommend approaches to address these issues: firstly, with an interrogation of emergent data using isotopic mass-balance and inverse modelling techniques; and secondly, by innovating pursuits of direct TEI flux measurements. We exemplify the value of GEOTRACES data with a new inverse model estimate of benthic Al flux in the North Atlantic Ocean. Furthermore, we review viable flux measurement techniques tailored to the sediment–water boundary. We propose that such activities are aimed at regions that intersect the GEOTRACES Science Plan on the basis of seven criteria that may influence TEI fluxes: sediment provenance, composition, organic carbon supply, redox conditions, sedimentation rate, bathymetry and the benthic nepheloid inventory.

          This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

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          Most cited references8

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          Iron Fertilization of the Subantarctic Ocean During the Last Ice Age

          John H. Martin, who discovered widespread iron limitation of ocean productivity, proposed that dust-borne iron fertilization of Southern Ocean phytoplankton caused the ice age reduction in atmospheric carbon dioxide (CO2). In a sediment core from the Subantarctic Atlantic, we measured foraminifera-bound nitrogen isotopes to reconstruct ice age nitrate consumption, burial fluxes of iron, and proxies for productivity. Peak glacial times and millennial cold events are characterized by increases in dust flux, productivity, and the degree of nitrate consumption; this combination is uniquely consistent with Subantarctic iron fertilization. The associated strengthening of the Southern Ocean's biological pump can explain the lowering of CO2 at the transition from mid-climate states to full ice age conditions as well as the millennial-scale CO2 oscillations.
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            Quantification of dissolved iron sources to the North Atlantic Ocean.

            Dissolved iron is an essential micronutrient for marine phytoplankton, and its availability controls patterns of primary productivity and carbon cycling throughout the oceans. The relative importance of different sources of iron to the oceans is not well known, however, and flux estimates from atmospheric dust, hydrothermal vents and oceanic sediments vary by orders of magnitude. Here we present a high-resolution transect of dissolved stable iron isotope ratios (δ(56)Fe) and iron concentrations ([Fe]) along a section of the North Atlantic Ocean. The different iron sources can be identified by their unique δ(56)Fe signatures, which persist throughout the water column. This allows us to calculate the relative contribution from dust, hydrothermal venting and reductive and non-reductive sedimentary release to the dissolved phase. We find that Saharan dust aerosol is the dominant source of dissolved iron along the section, contributing 71-87 per cent of dissolved iron. Additional sources of iron are non-reductive release from oxygenated sediments on the North American margin (10-19 per cent), reductive sedimentary dissolution on the African margin (1-4 per cent) and hydrothermal venting at the Mid-Atlantic Ridge (2-6 per cent). Our data also indicate that hydrothermal vents in the North Atlantic are a source of isotopically light iron, which travels thousands of kilometres from vent sites, potentially influencing surface productivity. Changes in the relative importance of the different iron sources through time may affect interactions between the carbon cycle and climate.
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              Oceanography. Centennial changes in North Pacific anoxia linked to tropical trade winds.

              Climate warming is expected to reduce oxygen (O2) supply to the ocean and expand its oxygen minimum zones (OMZs). We reconstructed variations in the extent of North Pacific anoxia since 1850 using a geochemical proxy for denitrification (δ(15)N) from multiple sediment cores. Increasing δ(15)N since ~1990 records an expansion of anoxia, consistent with observed O2 trends. However, this was preceded by a longer declining δ(15)N trend that implies that the anoxic zone was shrinking for most of the 20th century. Both periods can be explained by changes in winds over the tropical Pacific that drive upwelling, biological productivity, and O2 demand within the OMZ. If equatorial Pacific winds resume their predicted weakening trend, the ocean's largest anoxic zone will contract despite a global O2 decline. Copyright © 2014, American Association for the Advancement of Science.
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                Author and article information

                Journal
                Philos Trans A Math Phys Eng Sci
                Philos Trans A Math Phys Eng Sci
                RSTA
                roypta
                Philosophical transactions. Series A, Mathematical, physical, and engineering sciences
                The Royal Society
                1364-503X
                1471-2962
                28 November 2016
                28 November 2016
                : 374
                : 2081 , Discussion meeting issue ‘Biological and climatic impacts of ocean trace element chemistry’ organised and edited by Gideon Henderson, Ed Boyle, Maeve Lohan, Micha Rijkenberg and Géraldine Sarthou
                : 20160246
                Affiliations
                [1 ]Department of Earth Sciences, University of Oxford , South Parks Road, Oxford OX1 3AN, UK
                [2 ]School of Oceanography, University of Washington , 1503 NE Boat Street, Seattle, WA 98105, USA
                [3 ]Department of Earth Sciences, University of Southern California , Los Angeles, CA 90089, USA
                [4 ]Department of Earth Sciences, ETH Zürich , Clausiusstrasse 25, 8092 Zürich, Switzerland
                [5 ]College of Marine Science, University of South Florida , St Petersburg, FL 33701, USA
                [6 ]Laboratoire des Sciences du Climat et de l'Environnement (LSCE), IPSL, CEA–Orme des Merisiers , 91191 Gif-sur-Yvette, France
                [7 ]Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (LEGOS) , 14 Avenue Edouard Belin, 31400 Toulouse, France
                [8 ]Department of Marine and Coastal Sciences, Rutgers University , 71 Dudley Road, New Brunswick, NJ 08901, USA
                [9 ]School of Environmental Sciences, University of Liverpool , Jane Herdman Building, Liverpool L69 3GP, UK
                Author notes

                One contribution of 20 to a discussion meeting issue ‘ Biological and climatic impacts of ocean trace element chemistry’.

                Electronic supplementary material is available online at https://dx.doi.org/10.6084/m9.figshare.c.3491607.

                Author information
                http://orcid.org/0000-0002-9562-8591
                http://orcid.org/0000-0002-4445-6742
                http://orcid.org/0000-0002-3069-9786
                http://orcid.org/0000-0002-6279-7137
                http://orcid.org/0000-0002-3045-4949
                http://orcid.org/0000-0002-4915-4719
                http://orcid.org/0000-0002-3572-3634
                Article
                rsta20160246
                10.1098/rsta.2016.0246
                5069539
                bf669f92-0906-473e-a862-1f8d477453d7
                © 2015 The Authors.

                Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

                History
                : 24 August 2016
                Funding
                Funded by: UK Natural Environment Research Council;
                Award ID: NE/K009532/1
                Categories
                1002
                78
                135
                1005
                19
                140
                1006
                139
                Articles
                Discussion
                Custom metadata
                November 28, 2016

                ocean,sediment,trace element,isotope,benthic boundary layer,geotraces

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