(Table 1) Bulk Fe isotope values and pyrite framboids sizes of Cenomanian-Turonian samples

Organic carbon-rich shales from localities in England, Italy, and Morocco, which formed during the Cenomanian-Turonian oceanic anoxic event (OAE), have been examined for their total organic carbon (TOC) values together with their carbon, nitrogen, and iron isotope ratios. Carbon isotope stratigraphy (d13Corg and d13Ccarb) allows accurate recognition of the strata that record the oceanic anoxic event, in some cases allowing characterization of isotopic species before, during, and after the OAE. Within the black shales formed during the OAE, relatively heavy nitrogen isotope ratios, which correlate positively with TOC, suggest nitrate reduction (leading ultimately to denitrification and/or anaerobic ammonium oxidation). Black shales deposited before the onset of the OAE in Italy have unusually low bulk d57Fe values, unlike those found in the black shale (Livello Bonarelli) deposited during the oceanic anoxic event itself: These latter conform to the Phanerozoic norm for organic-rich sediments. Pyrite formation in the pre-OAE black shales has apparently taken place via dissimilatory iron reduction (DIR), within the sediment, a suboxic process that causes an approximately -2 per mil fractionation between a lithogenic Fe(III)oxide source and Fe(II)aq. In contrast, bacterial sulfate reduction (BSR), at least partly in the water column, characterized the OAE itself and was accompanied by only minor iron isotope fractionation. This change in the manner of pyrite formation is reflected in a decrease in the average pyrite framboid diameter from ~10 to ~7 µm. The gradual, albeit irregular increase in Fe isotope values during the OAE, as recorded in the Italian section, is taken to demonstrate limited isotopic evolution of the dissolved iron pool, consequent upon ongoing water column precipitation of pyrite under euxinic conditions. Given that evidence exists for both nitrate and sulfate reduction during the OAE, it is evident that redox conditions in the water column were highly variable, in both time and space.对采自英格兰、意大利及摩洛哥多地的富有机质页岩开展研究，这些页岩形成于森诺曼期-土仑期海洋缺氧事件（Cenomanian-Turonian oceanic anoxic event, OAE），研究测定了其总有机碳（total organic carbon, TOC）含量，并同步分析了碳、氮、铁同位素比值。碳同位素地层学（δ¹³C_org与δ¹³C_carb）可精准识别出记录该海洋缺氧事件的地层，在部分案例中还可刻画OAE发生前、期间及发生后的同位素组分特征。在OAE形成的黑色页岩中，与TOC呈显著正相关的相对偏重氮同位素比值，指示了硝酸盐还原过程，该过程最终会引发反硝化作用与/或厌氧氨氧化。意大利地区OAE发生前沉积的黑色页岩，其整体δ57Fe值异常偏低，这与OAE期间沉积的黑色页岩（博纳雷利层，Livello Bonarelli）的地球化学特征截然不同；后者符合富有机质沉积物的显生宙（Phanerozoic）标准值范围。OAE发生前的黑色页岩内的黄铁矿形成，显然是通过沉积物内部的异化铁还原（dissimilatory iron reduction, DIR）完成的，该过程属于亚氧化环境下的地球化学过程，会使陆源Fe(III)氧化物源与水相Fe(II)之间产生约-2‰的同位素分馏。与之形成鲜明对比的是，OAE期间的黄铁矿形成以细菌硫酸盐还原（bacterial sulfate reduction, BSR）为主，该过程至少部分发生于水体柱中，仅伴随微弱的铁同位素分馏。这种黄铁矿形成方式的转变，体现在莓状黄铁矿的平均粒径从约10 μm降至约7 μm。意大利剖面记录的OAE期间铁同位素值呈现缓慢（尽管存在不规则波动）的上升趋势，该现象被认为反映了溶解铁库的同位素演化受限，这是硫化缺氧水体条件下持续发生黄铁矿水相沉淀的结果。鉴于已有证据表明OAE期间同时存在硝酸盐还原与硫酸盐还原作用，可见水体柱中的氧化还原条件在时间与空间维度均存在显著变化。