Oceanic Anoxic Events (OAEs) are conspicuous intervals in the geologic record that are associated with the deposition of organic carbon (OC)-rich marine sediment, linked to extreme biogeochemical perturbations, and characterized by widespread ocean deoxygenation. Mechanistic links between the marine biological carbon pump (BCP), redox conditions, and organic carbon burial during OAEs, however, remain poorly constrained. In this work we reconstructed the BCP in the western Tethys Ocean across OAE1a (~120 Mya) using sediment geochemistry and OC mass accumulation rates (OC_Acc). We find that OC_Acc were between 0.006 and 3.3 gC m⁻² yr⁻¹, with a mean value of 0.79 ± 0.78 SD gC m⁻² yr⁻¹—these rates are low and comparable to oligotrophic regions in the modern oceans. This challenges longstanding assumptions that oceanic anoxic events are intervals of strongly elevated organic carbon burial. Numerical modelling of the BCP, furthermore, reveals that such low OC fluxes are only possible with either or both low to moderate OC export fluxes from ocean surface waters, with rates similar to oligotrophic (nutrient-poor, <30 gC m⁻² yr⁻¹) and mesotrophic (moderate-nutrients, ~50–100 gC m⁻² yr⁻¹) regions in the modern ocean, and stronger than modern vertical OC attenuation. The low OC fluxes thus reflect a relatively weak BCP. Low to moderate productivity is further supported by palaeoecological and geochemical evidence and was likely maintained through nutrient limitation that developed in response to the burial and sequestration of phosphorus in association with iron minerals under ferruginous (anoxic iron-rich) ocean conditions. Without persistently high productivity, ocean deoxygenation during OAE1a was more likely driven by other physicochemical and biological factors including ocean warming, changes in marine primary producer community composition, and fundamental shifts in the efficiency of the BCP with associated effects and feedbacks.

中文翻译：

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OAE1a 期间的碳泵动力学和有限的有机碳埋藏

海洋缺氧事件 (OAE) 是地质记录中的显着间隔，与富含有机碳 (OC) 的海洋沉积物的沉积有关，与极端生物地球化学扰动有关，其特征是广泛的海洋脱氧。然而，OAE 期间海洋生物碳泵 (BCP)、氧化还原条件和有机碳埋藏之间的机制联系仍然没有得到很好的约束。在这项工作中，我们使用沉积物地球化学和 OC 质量积累率 ( OC _Acc ) 重建了 OAE1a (~120 Mya) 西部特提斯洋的 BCP 。我们发现OC _Acc在 0.006 和 3.3 gC m ⁻²  yr ⁻¹之间，平均值为 0.79 ± 0.78 SD gC m ⁻²  yr⁻¹ —这些比率很低，可与现代海洋中的贫营养区相媲美。这挑战了长期以来的假设，即海洋缺氧事件是有机碳埋藏强烈升高的间隔。此外，BCP 的数值模型表明，只有来自海洋表面水域的低至中等 OC 输出通量中的一个或两个才有可能出现如此低的 OC 通量，其速率类似于贫营养（营养不良，<30 gC m ^-2  yr ^{- 1} ) 和中养（中等营养素，~50–100 gC m ⁻²  yr ⁻¹) 现代海洋中的区域，并且比现代垂直 OC 衰减更强。因此，低 OC 通量反映了相对较弱的 BCP。古生态学和地球化学证据进一步支持了低到中等生产力，并且可能是通过养分限制维持的，养分限制是在铁质（缺氧富铁）海洋条件下响应与铁矿物相关的磷的掩埋和封存而形成的。如果没有持续的高生产力，OAE1a 期间的海洋脱氧更有可能是由其他物理化学和生物因素驱动的，包括海洋变暖、海洋初级生产者群落组成的变化以及 BCP 效率的根本变化以及相关影响和反馈。