Astronomically modulated Neogene sediment records of ODP Site 184-1148@en

General global cooling over the Neogene has been modulated by changes in Earth's orbital parameters. Investigations of deep-sea sediment sequences show that various orbital cycles can dominate climate records for different latitudes or for different time intervals. However, a comprehensive understanding of astronomical imprints over the entire Neogene has been elusive because of the general absence of long, continuous records extending beyond the Pliocene. We present benthic foraminiferal d18O and d13C records over the past 23 Ma at Ocean Drilling Program Site 1148 in the northern South China Sea and construct an astronomically tuned timescale (TJ08) for these records based on natural gamma radiation and color reflectance data at this site. Our results show that a 41 ka cycle has dominated sediment records at this location over the Neogene, displaying a linear response to orbital forcing. A 100 ka cycle has also been significant. However, it is correlated nonlinearly with Earth's orbital variations at the 100 ka band. The sediment records also display a prominent 405 ka cycle. Although this cycle was coherent with orbital forcing during the Oligocene and the early Miocene, it was not coherent with Earth's orbital variations at the 405 ka band over the whole Neogene. Amplification of Northern Hemisphere and Southern Hemisphere glaciation since the middle Miocene may be responsible for this change in sedimentary response. Our benthic foraminifera d18O and d13C records further exhibit amplitude variations with longer periods of 600, 1000, 1200, and 2400 ka. Apparently, these cycles are nonlinear responses to insolation forcing.新近纪以来的全球性普遍降温过程，受到地球轨道参数变化的调控。对深海沉积序列的研究表明，不同纬度或不同时间区间的气候记录，可被不同的轨道周期主导。然而，由于普遍缺乏延伸至上新世之外的长连续沉积记录，学界至今尚未对整个新近纪时期的天文印记形成全面认知。本文以南海北部大洋钻探计划（Ocean Drilling Program，ODP）1148站位为研究对象，重建了过去23 Ma以来的底栖有孔虫（benthic foraminiferal）δ¹⁸O与δ¹³C记录，并依托该站位的自然伽马辐射及颜色反射率数据，为上述记录构建了天文调谐时间标尺TJ08。研究结果显示，新近纪以来该站位的沉积记录以41 ka周期为主导，表现出对轨道强迫的线性响应；100 ka周期同样显著，但该周期在100 ka频段内与地球轨道变化呈非线性相关。沉积记录还呈现出显著的405 ka周期：尽管该周期在渐新世与中新世早期与轨道强迫保持一致，但在整个新近纪的405 ka频段内，其与地球轨道变化并不相干。中中新世以来南北半球冰川作用的增强，或可解释这种沉积响应的转变。本研究的底栖有孔虫δ¹⁸O与δ¹³C记录还显示出周期更长的600、1000、1200及2400 ka的振幅变化，这些周期显然是对日射强迫的非线性响应。