Delta 44/40 Ca, Mg/Ca and Sr/Ca record of Globigerinoides sacculifer and calculated temperatures and salinity changes of ODP Hole 165-999A

The delta18O values of planktonic foraminifera increased in the Caribbean by about 0.5? relative to the equatorial East Pacific values between 4.6 and 4.2 Ma as a consequence of the closure of the Central American Gateway (CAG). This increase in delta18O can be interpreted either as an increase in Caribbean sea surface (mixed layer) salinity (SSS) or as a decrease in sea surface temperatures (SST). This problem represents an ideal situation to apply the recently developed paleotemperature proxy delta44/40Ca together with Mg/Ca and d18O on the planktic foraminifer Globigerinoides sacculifer from ODP Site 999. Although differences in absolute temperature calibration of delta44/40Ca and Mg/Ca exist, the general pattern is similar indicating a SST decrease of about 2-3 8C between 4.4 and 4.3 Ma followed by an increase in the same order of magnitude between 4.3 and 4.0 Ma. Correcting the delta18O record for this temperature change and assuming that changes in global ice volume are negligible, the salinity-induced planktonic delta18O signal decreased by about 0.4? between 4.4 and 4.3 Ma and increased by about 0.9? between 4.3 and 4.0 Ma in the Caribbean. The observed temperature and salinity trends are interpreted to reflect the restricted exchange of surface water between the Caribbean and the Pacific in response to the shoaling of the Panamanian Seaway, possibly accompanied by a southward shift of the Intertropical Convergence Zone (ITCZ) between 4.4 and 4.3 Ma. Differences in Mg/Ca- and delta44/40Ca-derived temperatures can be reconciled by corrections for secular variations of the marine Mg/Ca[sw] and delta44/40Ca, a salinity effect on the Mg/Ca ratio and a constant temperature offset of ~2.5 °C between both SST proxy calibrations.

在4.6 Ma至4.2 Ma期间，受中美洲通道（Central American Gateway, CAG）关闭的影响，加勒比海浮游有孔虫的氧同位素δ¹⁸O（delta18O）值较东赤道太平洋升高约0.5‰。该δ¹⁸O的升高既可以解释为加勒比海海表（混合层）盐度（sea surface salinity, SSS）的增加，也可归因于海表温度（sea surface temperature, SST）的降低。本研究以大洋钻探计划（Ocean Drilling Program, ODP）999站位的泡抱球虫（Globigerinoides sacculifer）为研究载体，结合镁钙比（Mg/Ca）与δ¹⁸O，采用新近开发的古温度代用指标δ⁴⁴/⁴⁰Ca（delta44/40Ca），为解决这一科学问题搭建了理想的研究框架。尽管δ⁴⁴/⁴⁰Ca与Mg/Ca的绝对温度校准存在差异，但二者的整体变化趋势高度一致：在4.4 Ma至4.3 Ma期间，海表温度下降约2~3℃，随后在4.3 Ma至4.0 Ma期间以相近幅度回升。针对该温度变化对δ¹⁸O记录进行校正，并假设全球冰量变化可忽略不计，加勒比海由盐度驱动的浮游有孔虫δ¹⁸O信号在4.4 Ma至4.3 Ma期间降低约0.4‰，在4.3 Ma至4.0 Ma期间升高约0.9‰。观测到的温度与盐度变化趋势被认为反映了两大过程：一是随着巴拿马海道逐渐变浅，加勒比海与太平洋之间的表层水交换受到显著限制；二是在4.4 Ma至4.3 Ma期间，可能伴随热带辐合带（Intertropical Convergence Zone, ITCZ）向南迁移。通过校正海水镁钙比（Mg/Ca[sw]）与δ⁴⁴/⁴⁰Ca的长期变化、盐度对Mg/Ca比值的影响，以及两种SST代用指标校准之间约2.5℃的恒定温度偏移，可合理解释Mg/Ca与δ⁴⁴/⁴⁰Ca反演得到的温度差异。