Sedimentology of ODP sites in the Cape Basin, southeast Atlantic Ocean

Middle/late Miocene to early Pliocene sedimentary sequences along the continental margin of southwest Africa have changes that correspond to the carbonate crash (12-9 Ma) and biogenic bloom events (~7-4 Ma) described in the equatorial Pacific by Farrell et al. (1995, doi:10.2973/odp.proc.sr.138.143.1995). To explore the origins of these changes, we analyzed the carbon and coarse fraction contents of sediments from ODP Sites 1085, 1086, and 1087 at a time resolution of 5 to 30 kyr. Several major drops in CaCO3 concentration between 12 and 9 Ma are caused by dilution from major increases in clastic input from the Oranje River during global sea level regressions. Abundant pyrite crystals and good preservation of fish debris reflect low oxygenation of bottom/pore waters. Regional productivity was enhanced during the time equivalent to the carbonate crash period. Higher benthic/planktic foraminiferal ratios indicate that CaCO3 dissolution at Site 1085 peaked between 9 to 7 Ma, which was after the global carbonate crash. This period of enhanced dissolution suggests that Site 1085 was located within a low-oxygen water mass that dissolved CaCO3 more easily than North Atlantic Deep Water, which began to bathe this site at 7 Ma. At 7 to 6 Ma, the onset of the biogenic bloom, increases and variations in total organic carbon and benthic foraminiferal accumulation rates show that paleoproductivity increased significantly above values observed during the carbonate crash period and fluctuated widely. We attribute the late Miocene paleoproductivity increase off southwest Africa to ocean-wide increases in nutrient supply and delivery.

非洲西南部大陆边缘的中新世中晚期至上新世早期沉积序列，其演化变化与Farrell等人（1995年，doi:10.2973/odp.proc.sr.138.143.1995）在赤道太平洋报道的碳酸盐崩溃（carbonate crash）事件与生物勃发（biogenic bloom）事件（分别对应12~9 Ma和约7~4 Ma）相吻合。为探究这些变化的成因，我们以5~30千年的时间分辨率，对大洋钻探计划（Ocean Drilling Program, ODP）1085、1086及1087站位的沉积物开展了碳组分与粗组分含量分析。12~9 Ma期间碳酸钙（CaCO₃）浓度的数次显著下降，系全球海平面下降时期奥兰治河（Oranje River）碎屑输入大幅增加引发的稀释作用所致。丰富的黄铁矿晶体与保存完好的鱼残骸，反映了底层水/孔隙水的低氧环境。在对应碳酸盐崩溃事件的时段，区域初级生产力得到提升。1085站位更高的底栖/浮游有孔虫比值表明，碳酸钙溶解作用在9~7 Ma期间达到峰值，该时间晚于全球碳酸盐崩溃事件。此次增强的溶解作用提示，1085站位处于低氧水团中，该水团的碳酸钙溶解能力强于北大西洋深层水（North Atlantic Deep Water）——后者自7 Ma起开始覆盖该站位。7~6 Ma期间生物勃发事件启动，总有机碳与底栖有孔虫堆积速率的升高与波动显示，古生产力相较于碳酸盐崩溃事件时期出现显著提升，并伴随剧烈波动。我们将非洲西南部近海晚中新世古生产力的提升，归因于全球尺度下营养盐供给与输送的增强。