Sr/Ca ratios of Cenozoic benthic foraminifera

A Cenozoic multi-species record of benthic foraminiferal calcite Sr/Ca has been produced and is corrected for interspecific offsets (typically less than 0.3 mmol/mol) and for the linear relationship between decreasing benthic foraminiferal Sr/Ca and increasing water depth. The water depth correction, determined from Holocene, Late Glacial Maximum and Eocene paleowater-depth transects, is ~0.1 mmol/mol/km. The corrected Cenozoic benthic foraminiferal Sr/Ca record ranges from 1.2 to 2.0 mmol/mol, and has been interpreted in terms of long-term changes in seawater Sr/Ca, enabling issues related to higher-resolution variability in Sr/Ca to be ignored. We estimate that seawater Sr/Ca was ~1.5 times modern values in the late Cretaceous, but declined rapidly into the Paleogene. Following a minimum in the Eocene, seawater Sr/Ca increased gradually through to the present day with a minimum superimposed on this trend centered in the late Miocene. By assuming scenarios for changing seawater calcium concentration, and using published carbonate accumulation rate data combined with suitable values for Sr partition coefficients into carbonates, the seawater Sr/Ca record is used to estimate global average river Sr fluxes. These fluxes are used in conjunction with the seawater strontium isotope curve and estimates of hydrothermal activity/tectonic outgassing to calculate changes in global average river 87Sr/86Sr through the Cenozoic. The absolute magnitude of Sr fluxes and isotopic compositions calculated in this way are subject to relatively large uncertainties. Nevertheless, our results suggest that river Sr flux increased from 35 Ma to the present day (roughly two-fold) accompanied by an overall increase in 87Sr/86Sr (by ~0 to 0.001). Between 75 and 35 Ma, river 87Sr/86Sr also increased (by ~0.001 to 0.002) but was accompanied by a decrease (two- to three-fold) in river Sr flux.

本研究构建了新生代底栖有孔虫方解石锶钙比（Sr/Ca）的多物种记录，并针对种间偏差（通常小于0.3 mmol/mol）以及底栖有孔虫Sr/Ca随水深增加而降低的线性关系进行了校正。本次水深校正基于全新世、末次盛冰期及始新世的古水深断面确定，校正系数约为0.1 mmol/mol/km。经校正的新生代底栖有孔虫Sr/Ca记录区间为1.2~2.0 mmol/mol，相关研究已通过海水Sr/Ca的长期变化对其进行解译，从而得以忽略Sr/Ca高分辨率变异性相关的问题。
研究估算，白垩纪晚期海水Sr/Ca约为现代值的1.5倍，随后快速下降至古近纪。在始新世出现最小值后，海水Sr/Ca逐渐回升至今，且该长期趋势上叠加了一个以晚中新世为中心的低值扰动。
通过假设海水钙浓度变化情景，并结合已发表的碳酸盐堆积速率数据与合适的碳酸盐相中锶分配系数，本研究利用海水Sr/Ca记录估算了全球平均河流锶通量。将该通量与海水锶同位素曲线以及热液活动/构造脱气的相关估算相结合，即可计算新生代以来全球平均河流87Sr/86Sr的变化。
通过该方法计算得到的锶通量绝对量级与同位素组成存在相对较大的不确定性。尽管如此，研究结果显示，自35 Ma至今，河流锶通量增长约一倍，同时87Sr/86Sr整体上升约0~0.001；而在75~35 Ma期间，河流87Sr/86Sr同样上升约0.001~0.002，但伴随河流锶通量下降了2~3倍。