Grain size analysis and sedimentation rates of ODP Hole 175-1085A off the Namibian coast

Sediments from the ODP Site 1085A were studied to investigate the impacts of global cooling in the Middle and Late Miocene on the climate in Southwestern Africa. The size composition of the sediment was analysed emphasising the silt fraction. A comparison with the modern grain size distribution and suitable transport processes made it possible to assign specific transport processes to the grain size composition. Three processes are considered for transport of terrigeneous silt: while there was no evidence found for (1) transport by ocean currents, the analyses showed signals of (2) wind transport indicating dry conditions associated with a cool climate and (3) fluvial transport that points to humid and warm conditions. Three climatic phases were defined. The first phase from 13.8 to 11.8 Myr reveals a stable humid climate in Southwest Africa independent of the Antarctic glaciations. During the second phase from 11.8 to 10.4 Myr the regional climate cooled considerably but was not drier. Additionally, the climate during this phase reacted to the Antarctic glaciations. This cooling-trend continued during phase 3 from 10.4 to 9.0 Myr with a significant increase in dust input, pointing to overall drier conditions. However, fluvial transport still remained as the main source.

本研究以大洋钻探计划（Ocean Drilling Program）1085A站位的沉积物为研究对象，旨在探究中中新世至晚中新世的全球变冷对非洲西南部气候的影响。研究对沉积物的粒度组成开展分析，重点关注粉砂级组分。通过与现代粒度分布及合理搬运过程进行对比，可将特定搬运过程与沉积物粒度组成建立对应关联。
针对陆源粉砂的搬运机制，本次研究考虑了三类过程：尽管未发现(1)洋流搬运的相关证据，但分析结果显示(2)风成搬运信号，该信号对应气候变冷伴随的干旱环境，以及(3)河流搬运信号，其指示湿润温暖的气候条件。
研究划定了三个气候阶段：第一阶段为13.8至11.8百万年，该时期非洲西南部气候稳定湿润，且不受南极冰川作用影响；第二阶段为11.8至10.4百万年，区域气候出现显著降温，但并未趋于干旱，同时该阶段气候对南极冰川活动产生响应；第三阶段为10.4至9.0百万年，降温趋势持续，且粉尘输入量显著升高，表明整体环境愈发干旱，但河流搬运仍为主要的物质来源。