NOAA/WDS Paleoclimatology - South China Sea ODP1148 Neogene Benthic Foraminiferal Isotope Data

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.

新近纪时期的全球整体降温过程，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，可由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识。本研究基于南海北部大洋钻探计划（Ocean Drilling Program, ODP）1148站位的相关数据，重建了过去23 Ma以来的底栖有孔虫δ¹⁸O与δ¹³C序列，并依据该站位的自然伽马辐射及颜色反射率数据，为上述序列构建了天文调谐时间标尺（TJ08）。研究结果显示，新近纪时期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。沉积记录还呈现出清晰的405 ka周期：尽管该周期在渐新世与早中新世时期与轨道强迫具有一致性，但在整个新近纪时段内，其并未与405 ka频段的地球轨道变化保持相干性。中中新世以来南北半球冰川作用的增强，可能是沉积响应发生这一转变的原因。本研究的底栖有孔虫δ¹⁸O与δ¹³C序列还展现出周期长达600、1000、1200及2400 ka的长周期振幅变化，这类周期显然是对日射强迫的非线性响应。