Geochemical conditions regulating chromium preservation in marine sediments
Geochemical conditions regulating chromium preservation in marine sediments
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DOI：
10.1016/j.gca.2023.03.003
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发表时间：
2023-03-28
影响因子：
5
通讯作者：
McManus, J.
中科院分区：
文献类型：
--
作者：
Bruggmann, S.;Severmann, S.;McManus, J.
在海洋沉积物记录中，铬（Cr）的浓度和同位素比值可以用来重建海洋生物地球化学条件。这些重建依赖于详细了解的化学途径，铬经历，因为它是从水柱转移到沉积物记录。我们研究了来自六个大陆边缘站点的海洋孔隙流体和沉积物中的Cr浓度，这些站点可以分为两个基本环境：（1）站点，沉积物是氧化的，富含固相Mn（在此称为好氧），和（2）站点，沉积物是有机C（Corg）-丰富和氧耗尽（缺氧）。我们发现铬浓度较低（最大12纳米的孔隙流体和124 ppm的沉积物固相）在好氧的网站相比，缺氧的网站（最大77纳米和184 ppm）。我们的发现证实了先前发表的关于孔隙流体中溶解的Cr的解释（Brumsack和Gieskes，1983; Shaw等人，1990年）。在好氧表层沉积物中，颗粒状Cr（III）可被Mn氧化物氧化，导致溶解Cr浓度升高，与沉积物中Mn浓度升高同时发生。在这些氧化条件下，下芯沉积物含有相对低的固相Cr浓度。在好氧沉积物中，Cr的形态表明，大部分的孔隙流体Cr是在Cr（VI）的状态。富锰氧化物沉积物的网站下面的好氧水柱休息，从沉积物到底层水的铬的氧化损失导致最低的估计铬埋葬效率的网站在这里检查。在缺氧富含Corg的条件下，孔隙流体和沉积物固相都含有高浓度的Cr，40 - 80%的溶解孔隙流体Cr以Cr（III）存在。这种富集的铬似乎是紧密相连的存在下，高的总有机碳（TOC）的含量和清除铬（有机）颗粒在水柱中。结合起来，这些数据突出了强烈的依赖性铬沉积氧化还原条件以及生物生产力。根据现代大陆边缘沉积物的数据，我们认为自生沉积物组分的铬浓度和同位素组成可能记录了水体中氧化还原条件和生物生产力的组合。如果得到Cr同位素分析的证实，这些发现将进一步支持Cr可能作为海洋生物和化学沉积条件的代用指标的观点。因此，在利用沉积记录重建氧化还原条件时，需要仔细评估有机质对Cr的影响。(c)2023年作者。由Elsevier Ltd.发布。这是CC BY许可下的开放获取文章（http：//creativecommons.org/licenses/by/4.0/）。
In the marine sediment record, concentrations and isotope ratios of chromium (Cr) can be used to recon-struct ocean biogeochemical conditions. These reconstructions rely on a detailed understanding of the chemical pathways that Cr undergoes as it is transferred from the water column to the sediment record. We examined Cr concentrations in marine pore fluids and sediments from six continental margin sites, which can be grouped into two basic environments: (1) sites where sediments are oxygenated and rich in solid phase Mn (herein termed oxic), and (2) sites where sediments are organic C (Corg)-rich and oxy-gen is depleted (anoxic). We found Cr concentrations to be lower (maximum of 12 nM in pore fluids and 124 ppm sediment solid phase) at oxic sites compared with anoxic sites (maximum of 77 nM and 184 ppm). Our findings confirm previously published interpretations of dissolved Cr in pore fluids (Brumsack and Gieskes, 1983; Shaw et al., 1990). In oxic surface sediments, particulate Cr(III) can be oxi-dised by Mn oxides, which leads to elevated concentrations of dissolved Cr co-occurring at the same depth as elevated Mn concentrations in the sediment. Under these oxidising conditions, down-core sed-iments contain relatively low solid-phase Cr concentrations. In oxic sediments, Cr speciation reveals that most of the pore fluid Cr is in the Cr(VI) state. At the site where Mn oxide-rich sediments rest below an oxic water column, oxidative loss of Cr from the sediment to the bottom water leads to the lowest esti-mated Cr burial efficiency of the sites examined here. Under anoxic Corg-rich conditions, both pore fluids and sediment solid phases contain high Cr concentrations, with 40-80% of dissolved pore fluid Cr present as Cr(III). This enrichment of Cr appears to be tightly linked to the presence of high total organic carbon (TOC) content and scavenging of Cr by (organic) particles in the water column. Combined, these data highlight the strong dependence of Cr on both sedimentary redox conditions as well as biological produc-tivity. Based on the data from modern continental margin sediments, we propose that Cr concentrations and isotope compositions of the authigenic sediment fraction may record a combination of redox condi-tions and biological productivity in the water column. If confirmed by Cr isotope analyses, these findings will add support for the notion that Cr may serve as a proxy for ocean biological and chemical sedimen-tological conditions. Thus, careful assessment of the impact of organic matter on Cr is required for recon-structions of redox conditions with sedimentary records. (c) 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).