Isotope ratios of osmiun, carbon and oxygen of Miocene sediments and benthic foraminifera@en

Seawater 187Os/188Os ratios for the Middle Miocene were reconstructed by measuring the 187Os/188Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187Os/188Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187Os/188Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01/Myr increase in marine 187Os/188Os ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine 87Sr/86Sr ratios at 16 Ma. A large increase of 1 per mil in benthic foraminiferal delta18O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate delta13C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187Os/188Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187Os/188Os ratios and also nearly constant 87Sr/86Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin-New Guinea arc system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the arc terrane, for the maintenance of assumed low CO2 levels after 15 Ma. The temporal resolution (three samples/Myr) of the 187Os/188Os record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously published records. These high resolution data suggest oscillations with amplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variations in the 187Os/188Os record of this magnitude can be easily resolved analytically, this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent. […]本研究通过测定太平洋（深海钻探计划（Deep Sea Drilling Project, DSDP）598站位）与大西洋（DSDP 521站位）含金属碳酸盐的187Os/188Os比值，重建了中中新世海水187Os/188Os比值。大西洋与太平洋的187Os/188Os测量结果几乎无差异，且与此前已发表的太平洋岩芯Os同位素记录相一致。本次报道的大西洋数据首次直接证明，长期沉积187Os/188Os记录反映了海水Os同位素组成的全大洋变化。 太平洋与大西洋的Os同位素测量结果证实，海洋187Os/188Os比值以每百万年0.01的速率长期增长，且起始时间不晚于16 Ma。该Os同位素增长的起始时间，与16 Ma时海洋87Sr/86Sr比值的增长速率下降相吻合。 底栖有孔虫δ18O值大幅升高1‰，该信号被解释为反映全球变冷与冰盖扩张，其起始时间约为14.8 Ma，即晚于16 Ma约1百万年。而全岩碳酸盐δ13C值长期向低值偏移的起始时间则更晚，约在13.6 Ma，即滞后超2百万年。 16 Ma以来海洋187Os/188Os比值的增长，最可能由放射成因物质的风化作用驱动——这些物质要么是古老沉积物，要么是具沉积原岩的硅铝地壳。我们提出两种可解释该海洋187Os/188Os比值增长，同时维持87Sr/86Sr比值近乎恒定的中新世特有的地质事件：（1）北半球首次对覆盖沉积物的克拉通开展冰川侵蚀；（2）澳大利亚被动大陆边缘-新几内亚弧系的抬升剥蚀。后者通过弧地体中可溶性钙镁硅酸盐的富集，为15 Ma以来假定的低CO₂水平维持提供了机制。 598站位的187Os/188Os记录具备稳定同位素地层学框架，其时间分辨率（3个样品/百万年）显著高于此前已发表的记录。该高分辨率数据显示存在振幅为0.01~0.02、周期约1百万年的振荡信号。尽管该量级的187Os/188Os记录变化可通过分析手段轻易分辨，但该高频信号需在其他站位得到验证，才能被可靠认定为具有全球范围的意义。[…]