Evaluating a primary carbonate pathway for manganese enrichments in reducing environments
Evaluating a primary carbonate pathway for manganese enrichments in reducing environments
复制标题
DOI：
10.1016/j.epsl.2020.116201
复制
发表时间：
2020-05
影响因子：
5.3
通讯作者：
C. Wittkop;E. Swanner;A. Grengs;N. Lambrecht;M. Fakhraee;A. Myrbo;A. Bray;S. Poulton;S. Katsev
中科院分区：
文献类型：
--
作者：
C. Wittkop;E. Swanner;A. Grengs;N. Lambrecht;M. Fakhraee;A. Myrbo;A. Bray;S. Poulton;S. Katsev
沉积岩记录中的大多数锰(锰)富集物赋存于碳酸盐矿物中，这些矿物被认为是由前体锰氧化物的成岩还原形成的，被认为是强氧化条件的诊断。在这里，我们探索了另一种模型，在这种模型中，碳酸锰在氧化还原分层水柱中形成，与碳酸钙的溶解有关。在美国明尼苏达州铁质的布朗尼湖，我们发现碳酸锰是一种存在于沉积物圈闭和水柱减少部分中的可提取HCl的相。在溶解氧浓度降至5μM以下的布朗尼湖化跃层中，碳酸锰处于过饱和状态，氧化锰还原增加了溶解锰的浓度。当地表水中的方解石溶解于化学跃层的酸性更强的水中时，过饱和度就会增强。在同一带内，硫酸盐还原和微氧甲烷氧化增加了溶解无机碳，δ为负13 C。这些观察表明，沉积富锰可能1)从原生碳酸盐相发育，2)可能发生在溶解氧浓度为<；5μM的环境中。原生碳酸锰很可能起源于溶解锰浓度较高的环境(>；200μM)，其中锰和铁被S旋回、光铁营养或微好氧铁氧化作用所分配。浅层溶跃层为晶体生长提供了额外的DIC和成核点，从而提高了碳酸锰的产量。这种碳酸盐富锰模型预计在富铁和富铁环境中都是可行的，并提供了关于锰和碳循环之间关系的更微妙的观点，在整个岩石记录中得到了应用。
Most manganese (Mn) enrichments in the sedimentary rock record are hosted in carbonate minerals, which are assumed to have formed by diagenetic reduction of precursor Mn-oxides, and are considered diagnostic of strongly oxidizing conditions. Here we explore an alternative model where Mn-carbonates form in redox-stratified water columns linked to calcium carbonate dissolution. In ferruginous Brownie Lake in Minnesota, USA, we document Mn-carbonates as an HCl-extractable phase present in sediment traps and in reducing portions of the water column. Mn-carbonate becomes supersaturated in the Brownie Lake chemocline where dissolved oxygen concentrations fall below 5 μM, and Mn-oxide reduction increases the dissolved Mn concentration. Supersaturation is enhanced when calcite originating from surface waters dissolves in more acidic waters at the chemocline. In the same zone, sulfate reduction and microaerobic methane oxidation add dissolved inorganic carbon (DIC) with negative δ 13 C. These observations demonstrate that sedimentary Mn enrichments may 1) develop from primary carbonate phases, and 2) can occur in environments with dissolved oxygen concentrations< 5 μM. Primary Mn-carbonates are likely to originate in environments with high concentrations of dissolved Mn (> 200 μM), and where Mn and Fe are partitioned by S cycling, photoferrotrophy, or microaerophilic Fe-oxidation. A shallow lysocline enhances Mn-carbonate production by providing additional DIC and nucleation sites for crystal growth. This carbonate model for Mn-enrichments is expected to be viable in both euxinic and ferruginous environments, and provides a more nuanced view of the relationships between Mn and carbon cycling, with applications throughout the rock record.