Seawater Nd and Pb isotopic record in central Pacific and Caribbean sediments

The shoaling and final closure of the Central American Seaway (CAS) resulted in a major change of the global ocean circulation and has been suggested as an essential driver for strengthening of Atlantic Meridional Overturning Circulation (AMOC). The exact timing of CAS closure is key to interpreting its importance. Here we present a reconstruction of deep and intermediate water Nd and Pb isotope compositions obtained from fossil fish teeth and the authigenic coatings of planktonic foraminifera in the eastern equatorial Pacific (Ocean Drilling Program (ODP) Site 1241) and the Caribbean (ODP Sites 998, 999, and 1000) covering the final stages of CAS closure between 5.6 and 2.2 Ma. The data for the Pacific site indicate no significant Atlantic/Caribbean influence over this entire period. The Caribbean sites show a continuous trend to less radiogenic Nd isotope compositions during the Pliocene, consistent with an enhancement of Upper North Atlantic Deep Water (UNADW) inflow and a strengthening of the AMOC. Superimposed onto this long-term trend, shorter-term changes of intermediate Caribbean Nd isotope signatures approached more UNADW-like values during intervals when published reconstructions of seawater salinity suggested complete closure of the CAS. The data imply that significant deep water exchange with the Pacific essentially stopped by 7 Ma and that shallow exchange, which still occurred at least periodically until approximately 2.5 Ma, may have been linked to the strength of the AMOC but did not have any direct effect on the intermediate and deep Caribbean Nd isotope signatures through mixing with Pacific waters.中美洲海道（Central American Seaway, CAS）的淤浅与最终封闭，引发了全球大洋环流的重大变革，被认为是增强大西洋经向翻转环流（Atlantic Meridional Overturning Circulation, AMOC）的关键驱动因素。准确厘定CAS封闭的时间节点，是解读其环境影响重要性的核心所在。本研究依托东赤道太平洋（大洋钻探计划（Ocean Drilling Program, ODP）1241站位）与加勒比海区域（ODP 998、999及1000站位）的化石鱼齿及浮游有孔虫自生涂层样品，重建了5.6 Ma至2.2 Ma期间CAS最终封闭阶段的深水与中层水钕（Nd）、铅（Pb）同位素组成。太平洋站位的数据显示，在整个研究时段内未受大西洋/加勒比海区域的显著影响。加勒比海各站位的钕同位素组成在上新世（Pliocene）呈现出持续降低的放射成因特征，这与北大西洋上层深水（Upper North Atlantic Deep Water, UNADW）注入增强、AMOC强化的地质记录相一致。在该长期演化趋势之上，还叠加了短期的中层水钕同位素组成变化：当已有研究重建的海水盐度数据指示CAS完全封闭的时段，加勒比海中层水的钕同位素信号更趋近于UNADW的特征值。本研究数据表明，加勒比海与太平洋的深层水交换在约7 Ma前已基本终止；而浅层水交换虽至少周期性地持续至约2.5 Ma，但该过程可能仅与AMOC强度相关，并未通过与太平洋水体混合的方式对加勒比海中层及深水钕同位素组成产生直接影响。