Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA.
Proc Natl Acad Sci U S A. 2013 Jul 9;110(28):11244-9. doi: 10.1073/pnas.1218874110. Epub 2013 Jun 3.
Phanerozoic levels of atmospheric oxygen relate to the burial histories of organic carbon and pyrite sulfur. The sulfur cycle remains poorly constrained, however, leading to concomitant uncertainties in O2 budgets. Here we present experiments linking the magnitude of fractionations of the multiple sulfur isotopes to the rate of microbial sulfate reduction. The data demonstrate that such fractionations are controlled by the availability of electron donor (organic matter), rather than by the concentration of electron acceptor (sulfate), an environmental constraint that varies among sedimentary burial environments. By coupling these results with a sediment biogeochemical model of pyrite burial, we find a strong relationship between observed sulfur isotope fractionations over the last 200 Ma and the areal extent of shallow seafloor environments. We interpret this as a global dependency of the rate of microbial sulfate reduction on the availability of organic-rich sea-floor settings. However, fractionation during the early/mid-Paleozoic fails to correlate with shelf area. We suggest that this decoupling reflects a shallower paleoredox boundary, primarily confined to the water column in the early Phanerozoic. The transition between these two states begins during the Carboniferous and concludes approximately around the Triassic-Jurassic boundary, indicating a prolonged response to a Carboniferous rise in O2. Together, these results lay the foundation for decoupling changes in sulfate reduction rates from the global average record of pyrite burial, highlighting how the local nature of sedimentary processes affects global records. This distinction greatly refines our understanding of the S cycle and its relationship to the history of atmospheric oxygen.
显生宙大气氧的水平与有机碳和黄铁矿硫的埋藏历史有关。然而，硫循环仍然受到很大的限制，导致氧气预算存在相应的不确定性。在这里，我们通过将多种硫同位素分馏的幅度与微生物硫酸盐还原速率联系起来，提出了一些实验。这些数据表明，这种分馏是由电子供体（有机物）的可用性控制的，而不是由电子受体（硫酸盐）的浓度控制的，这是一种在沉积埋藏环境中变化的环境限制。通过将这些结果与黄铁矿埋藏的沉积物生物地球化学模型相结合，我们发现过去 2 亿年来观察到的硫同位素分馏与浅海底环境的面积之间存在很强的关系。我们将这解释为微生物硫酸盐还原率与富含有机物海底环境的可用性之间的全球依赖性。然而，早古生代和中古生代的分馏与陆架面积无关。我们认为这种解耦反映了较浅的古氧化还原边界，主要局限于早显生宙的水柱中。这两种状态之间的转变始于石炭纪，大约在三叠纪-侏罗纪边界结束，表明对石炭纪氧气上升的反应时间较长。总之，这些结果为从黄铁矿埋藏的全球平均记录中解耦硫酸盐还原速率的变化奠定了基础，突出了沉积过程的局部性质如何影响全球记录。这种区别极大地提高了我们对 S 循环及其与大气氧历史关系的理解。
