Mid-Pliocene upwelling and nutrient (Si, N) conditions in the Benguela upwelling system and their relationship to the Southern Ocean paleoceanography

在约4-2.6 Ma BP之间，巴拿马海道因地表水交换而关闭，导致海洋温盐环流的深刻重组，大陆冰盖的提前生长，并可能导致更活跃的大气环流。与这些过程相伴而来的是硅藻生产力的极大提高和本格拉上升流系统中南极硅藻的优势。这一现象被称为Matuyama硅藻极大(MDM)，这是基于纳米比亚附近的ODP175个钻探地点的结果，并被推断为沿海上升流加剧和亚南极模式水域更大的夹带的结果。另一方面，这些地点的烷酮SST重建和有机质的δ15N信号强烈表明，在MDM仍然很好的层化期间，温暖的表层水域占优势，局部固氮促进了营养物质的供应。这反对沿海上升流条件的强烈加剧，因为这将导致表层海洋冷却和次表层营养物质供应。为了解决这一争议，建议将海洋表面温度、浮游生物生产力以及表层海洋分层或上升流强度变化的详细替代记录与记录过去对本格拉系统的硅和氮供应变化的记录进行比较。这将使人们能够更好地了解导致东部边界流环境中的硅质浮游生物生产力非常高的主要因素，这些环境的平均状况比目前普遍的地表水条件更温暖和更分层。上新世条件可以作为未来全球变暖条件下沿海上升流环境发展的类比。

The closure of the Panama seaway for surface water exchange between about 4 and 2.6 Ma BP led to a profound reorganisation of the ocean thermohaline circulation, the advanced growth of continental ice-sheets, and probably to a more vigorous atmospheric circulation. Concomitant with these processes an extreme increase in diatom productivity and a dominance of Antarctic diatom species in the Benguela upwelling system occurred. This phenomenon is known as the Matuyama Diatom Maximum (MDM) based on results of ODP Leg 175 drill sites off Namibia, and was inferred to be a result of intensification of coastal upwelling and larger entrainment of subAntarctic mode waters. On the other hand, alkenone SST reconstructions and the δ15N signal of organic matter from these sites strongly imply that during the MDM still well stratified, warm surface waters prevailed and nutrient supply was fostered by local nitrogen fixation. This argues against a strong intensification of coastal upwelling conditions, which would have led to surface ocean cooling and subsurface nutrient supply. To resolve this controversy it is proposed to compare detailed proxy records for changes in sea surface temperatures, plankton productivity, as well as for surface ocean stratification or upwelling intensity with records documenting past changes of silicon and nitrogen supply to the Benguela system. This will allow a better understanding of the major factors leading to very high siliceous plankton productivity in eastern boundary current settings under mean states of warmer and more stratified surface water conditions than those prevailing today. The Pliocene conditions may serve as an analogue for the development of coastal upwelling settings under future global warming conditions.