Neogene palaeoceanographic and palaeoclimatic events inferred from palynological data: Cape Basin off South Africa, ODP Leg 175

Sites 1085, 1086 and 1087 were drilled off South Africa during Ocean Drilling Program (ODP) Leg 175 to investigate the Benguela Current System. While previous studies have focused on reconstructing the Neogene palaeoceanographic and palaeoclimatic history of these sites, palynology has been largely ignored, except for the Late Pliocene and Quaternary. This study presents palynological data from the upper Middle Miocene to lower Upper Pliocene sediments in Holes 1085A, 1086A and 1087C that provide complementary information about the history of the area. Abundant and diverse marine palynomorphs (mainly dinoflagellate cysts), rare spores and pollen, and dispersed organic matter have been recovered. Multivariate statistical analysis of dispersed organic matter identified three palynofacies assemblages (A, B, C) in the most continuous hole (1085A), and they were defined primarily by amorphous organic matter (AOM), and to a lesser extent black debris, structured phytoclasts, degraded phytoclasts, and marine palynomorphs. Ecostratigraphic interpretation based on dinoflagellate cyst, spore-pollen and palynofacies data allowed us to identify several palaeoceanographic and palaeoclimatic signals. First, the late Middle Miocene was subtropical, and sediments contained the highest percentages of land-derived organic matter, even though they are rich in AOM (palynofacies assemblage A). Second, the Late Miocene was cool-temperate and characterized by periods of intensified upwelling, increase in productivity, abundant and diverse oceanic dinoflagellate cysts, and the highest percentages of AOM (palynofacies assemblage C). Third, the Early to early Late Pliocene was warm-temperate with some dry intervals (increase in grass pollen) and intensified upwelling. Fourth, the Neogene \"carbonate crash\" identified in other southern oceans was recognized in two palynofacies A samples in Hole 1085A that are nearly barren of dinoflagellate cysts: one Middle Miocene sample (590 mbsf, 13.62 Ma) and one Upper Miocene sample (355 mbsf, 6.5 Ma). Finally, the extremely low percentages of pollen suggest sparse vegetation on the adjacent landmass, and Namib desert conditions were already in existence during the late Middle Miocene.

在海洋钻探计划（Ocean Drilling Program, ODP）第175航次期间于南非近海钻探的1085、1086及1087站位，旨在研究本格拉洋流系统。尽管此前研究聚焦于重建这些站位的新近纪古海洋学（palaeoceanographic）与古气候学（palaeoclimatic）历史，但孢粉学（palynology）研究却大多被忽视，仅晚上新世与第四纪时段除外。本研究呈现了1085A、1086A及1087C孔中中新世晚期至上新世早期下部沉积物的孢粉学（palynology）数据，为该区域历史提供了补充信息。研究从中获取了丰富多样的海洋孢粉形态（主要为沟鞭藻囊肿）、罕见的孢子与花粉，以及分散有机质。对分散有机质的多元统计分析在连续性最佳的1085A孔中识别出三个孢粉相（palynofacies）组合（A、B、C），这些组合主要由无定形有机质（amorphous organic matter, AOM）界定，其次为黑色碎屑、结构化植物碎屑、降解植物碎屑及海洋孢粉形态。基于沟鞭藻囊肿、孢子-花粉及孢粉相数据的生态地层学（ecostratigraphic）解释，使我们得以识别出若干古海洋与古气候信号：其一，晚中中新世为亚热带气候，沉积物中陆源有机质占比最高，尽管其富含AOM（孢粉相组合A）；其二，晚中新世为凉温带气候，以上升流增强期、生产力提升、海洋沟鞭藻囊肿丰富多样及AOM占比最高为特征（孢粉相组合C）；其三，早至上新世早期下部为暖温带气候，存在若干干旱间隔（草花粉增多）且上升流增强；其四，在其他南大洋中识别出的新近纪“碳酸盐崩溃”现象，在1085A孔的两个孢粉相A样本中被发现——这些样本几乎不含沟鞭藻囊肿，分别为中中新世样本（海底以下590米，13.62百万年）与晚中新世样本（海底以下355米，6.5百万年）；最后，花粉占比极低表明邻近陆地植被稀疏，且纳米布沙漠环境在晚中中新世已具雏形。