XRF scanning, revisited composite depth and age model of ODP Site 159-959 (1.8-6.2 Ma)

In this study, new high-resolution XRF data from ODP Site 959 (3°37'N, 2°44'W) have been used to investigate the relationship between paleoenvironmental changes in West Africa and sedimentation in the tropical east Atlantic Ocean. Fe intensity data have been used to build a 91 meter composite depth record that has been astronomically tuned allowing the development of a detailed age model from 6.2 to 1.8 Ma. Based on this new stratigraphy we studied the variations of Ti/Al, Ti/Ca and Al/Si ratios, proxies for aeolian vs. fluvial supply, as dust indicator and fine vs. coarse grain size, respectively. We discuss sedimentation patterns at ODP Site 959 associated to the environmental changes from the late Miocene until the early Pleistocene. During the interval corresponding to the earlier stages of the Messinian Salinity Crisis our proxy records indicate enhanced run-off from the West African continent and major supply of fine material at ODP Site 959, suggesting a stronger monsoon and increased precipitation during eccentricity minima. A long-term decrease of river supply is documented after 5.4 Ma towards the early Pleistocene. From the increased values and variability of Ti/Al ratios we suggest that after 3.8 Ma dust started to reach the study site probably as a result of the southward shift of the Intertropical Convergence Zone during winter. Between 3.2 and 2.9 Ma ODP Site 959 Ti/Ca ratios exhibit three maxima corresponding to eccentricity maxima similarly to other dust records of northern Africa. This suggests continent-wide aridity or larger climate variability during that interval. Eccentricity forcing (405 kyr and 100 kyr) and precession frequencies are found in the entire studied interval. The variations of Ti/Al ratio suggest stronger seasonality between 5.8 and 5.5 Ma and after 3.2 Ma.本研究利用采自大洋钻探计划（Ocean Drilling Program, ODP）959站位（3°37'N，2°44'W）的全新高分辨率X射线荧光光谱（X-ray Fluorescence, XRF）数据，探究西非古环境变化与热带东大西洋沉积作用之间的关联。研究以铁元素强度数据构建了91米的复合深度记录，并通过天文调谐建立了6.2 Ma至1.8 Ma时段的高精度年代模型。基于这一新的地层序列，我们分析了Ti/Al、Ti/Ca及Al/Si比值的变化特征：其中Ti/Al与Ti/Ca比值分别作为风成物源与河流物源的代用指标，Al/Si比值则用于指示细粒与粗粒粒度组分的差异。我们探讨了晚中新世至早更新世期间，ODP 959站位的沉积模式与环境变化的关联。在对应墨西拿盐度危机（Messinian Salinity Crisis）早期阶段的时段内，代用指标记录显示西非大陆径流作用显著增强，且ODP 959站位接收了大量细粒物质，表明在偏心率极小值期，区域季风环流更强、降水更为充沛。5.4 Ma之后至早更新世，河流物源供给呈现长期降低的趋势。结合Ti/Al比值的升高与波动特征，我们认为3.8 Ma之后粉尘开始抵达研究站位，这可能与冬季热带辐合带（Intertropical Convergence Zone, ITCZ）向南迁移直接相关。在3.2 Ma至2.9 Ma时段内，ODP 959站位的Ti/Ca比值出现3次峰值，与偏心率极大值期高度对应，这与北非其他粉尘记录的特征一致，表明该时段内非洲大陆整体干旱化程度加剧或气候变率显著增大。整个研究时段均记录到偏心率驱动（405 kyr与100 kyr周期）与岁差频率的信号。Ti/Al比值的变化显示，5.8 Ma至5.5 Ma时段以及3.2 Ma之后，区域季节差异显著增强。