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The early Matuyama Diatom Maximum off SW Africa: a conceptual model

机译：非洲西南部的早期Matuyama硅藻极值：概念模型

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An important discovery during Ocean Drilling Program Leg 175, when investigating the record of upwelling off Namibia, was the finding of a distinct Late Pliocene diatom maximum spanning the lower half of the Matuyama reversed polarity chron (MDM, Matuyama Diatom Maximum) and centered around 2.6-2.0 Ma. This maximum was observed at all sites off southwestern Africa between 20 deg S and 30 deg S, and is most strongly represented in sediments of Site 1084, off Luderitz, Namibia. The MDM is characterized by high biogenic opal content, high numbers of diatom valves, and a diatom flora rich in Southern Ocean representatives (with Thalassiothrix antarctica forming diatom mats) as well as coastal upwelling components. Before MDM time, diatoms are rare until ca. 3.6 Ma. After the MDM, in the Pleistocene, the composition of the diatom flora points to increased importance of coastal upwelling toward the present, but is accompanied by a general decrease in opal and diatom deposition. Here we present a simple conceptual model as a first step in formalizing a possible forcing mechanism responsible for the record of opal deposition in the upwelling system off Namibia. The model takes into account Southern Ocean oceanography, and a link with deepwater circulation and deepwater nutrient chemistry which, in turn, are coupled to the evolution of North Atlantic Deep Water (NADW). The model proposes that between the MDM and the Mid-Pleistocene climate revolution, opal deposition off Namibia is not directly tied to glacial-interglacial fluctuations (as seen in the global delta~(18) O record), but that, instead, a strong deepwater link exists with increased NADW production (as seen in the deepwater delta~(18) O record) accounting for higher supply of silicate to the thermocline waters that feed the upwelling process. The opal record of Site 1084 shows affinity to eccentricity on the 400-kyr scale but not for the 100-kyr scale. This points toward long-term geologic processes for delivery of silica to the ocean.
机译：海洋钻探计划第175条腿在调查纳米比亚上涌的记录时的一个重要发现是发现了一个明显的上新世硅藻最大值，该最大值跨越了Matuyama反转极性年代的下半部分（MDM，Matuyama Diatom Maximum），并以2.6为中心-2.0马在非洲西南部20摄氏度到30摄氏度之间的所有地点都观测到了这个最大值，并且在纳米比亚卢德里茨附近的1084地点的沉积物中最明显。 MDM的特征是高生物蛋白石含量，大量硅藻瓣膜以及富含南洋代表（南极Thalassiothrix南极形成硅藻垫）的硅藻菌群以及沿海上升流组件。在MDM时间之前，硅藻很少见，直到大约20分钟。 3.6马在MDM之后，在更新世，硅藻菌群的组成表明沿海近来上升流的重要性增加，但伴随着蛋白石和硅藻沉积的普遍减少。在这里，我们提出一个简单的概念模型，作为正式确定可能的强迫机制的第一步，该机制负责记录纳米比亚外上升流系统中的蛋白石沉积记录。该模型考虑了南洋海洋学以及与深水循环和深水养分化学的联系，而这些联系又与北大西洋深水（NADW）的演化联系在一起。该模型提出，在MDM和中更新世气候革命之间，纳米比亚海域的蛋白石沉积并不直接与冰川-冰川间的波动联系在一起（如全球δ〜（18）O记录所示），相反，深水联系与NADW产量增加有关（如在深水三角洲（18）O记录中所见），这说明为上升流过程提供动力的温跃层水更多的硅酸盐供应。 Site 1084的蛋白石记录显示了对偏心率的亲和力，范围为400公斤，但不适用于100公斤。这表明了将二氧化硅输送到海洋的长期地质过程。

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