Stable isotope record and age model for sapropel stratigraphy of ODP Site 160-967@en

Stable oxygen isotope data from four holes drilled at the Ocean Drilling Program Site 967, which is located on the lower northern slope of the Eratosthenes Seamount, provide a continuous record of Eastern Mediterranean surface-water conditions during the last 3.2 Ma. A high-resolution stratigraphy for the Pliocene-Pleistocene sequence was established by using a combination of astronomical calibration of sedimentary cycles, nannofossil stratigraphy, and stable oxygen isotope fluctuations. Sapropels and color cycles are present throughout the last 3.2 Ma at Site 967, and their ages, as determined by calibration against the precessional component of the astronomical record, are consistent with those estimated for the sapropels of the classical land-based marine sequences of the Punta Piccola, San Nicola, Singa, and Vrica sections (southern Italy). The Site 967 oxygen isotope record shows large amplitude fluctuations mainly caused by variations in surface water salinity throughout the entire period. Spectral analysis shows that fluctuations in the d18O record were predominantly influenced by orbital obliquity and precessional forcing from 3.2 to 1 Ma, and all main orbital frequencies characterize the d18O record for the last million years. The start of sapropel formation at 3.2 Ma indicates a possible link between sapropel formation and the build up of northern hemisphere ice sheets. The dominance of the obliquity cycle in the interval from 3.2-1 Ma further points to the sensitivity of Eastern Mediterranean climate to the fluctuations in the volume of Arctic ice sheets. An intensification of negative isotope anomalies at Site 967, relative to the open ocean, supports a link between high run-off (during warm periods) and sapropel formation. freshwater input would have inhibited deep-water formation, which led to stagnation of deeper waters. Comparison with the land sections also confirms that differential preservation and diagenesis play a key role in sapropel occurrence.

采自埃拉托色尼海山（Eratosthenes Seamount）北坡下部的大洋钻探计划（Ocean Drilling Program）967站位4个钻孔的稳定氧同位素（stable oxygen isotope）数据，构建了过去3.2 Ma以来东地中海表层水体环境的连续演化记录。本研究通过结合沉积旋回天文校准、超微化石（nannofossil）地层学以及稳定氧同位素波动特征，建立了上新世-更新世（Pliocene-Pleistocene）序列的高分辨率地层框架。967站位过去3.2 Ma以来均发育腐泥层（sapropels）与颜色旋回，通过天文记录的岁差分量校准得到的这些腐泥层年代，与意大利南部经典陆相海相序列——蓬塔皮科拉（Punta Piccola）、圣尼古拉（San Nicola）、辛加（Singa）和弗里卡（Vrica）剖面——中的腐泥层估算年代一致。967站位的δ18O记录显示，整个时期内存在大幅波动，该波动主要由表层水体盐度变化引发。频谱分析结果表明，3.2 Ma至1 Ma期间，δ18O记录的波动主要受轨道黄赤交角与岁差驱动影响；而过去100万年的δ18O记录则涵盖了所有主要的轨道周期频率特征。3.2 Ma时腐泥层开始形成，这表明腐泥层形成与北半球冰盖发育之间可能存在关联。3.2 Ma至1 Ma期间黄赤交角周期占主导，进一步说明东地中海气候对北极冰盖体积变化极为敏感。相较于开阔大洋，967站位的负同位素异常强度增强，这支持了暖期高径流与腐泥层形成之间的关联。淡水输入会抑制深层水体形成，进而引发深层水体停滞。与陆相剖面的对比同样证实，差异保存与成岩作用对腐泥层的发育起着关键控制作用。