Organic carbon accumulation in the South Atlantic Ocean

A compilation of 1118 surface sediment samples from the South Atlantic was used to map modern seafloor distribution of organic carbon content in this ocean basin. Using new data on Holocene sedimentation rates, we estimated the annual organic carbon accumulation in the pelagic realm (>3000 m water depth) to be approximately 1.8*10**12 g C/year. In the sediments underlying the divergence zone in the Eastern Equatorial Atlantic (EEA), only small amounts of organic carbon accumulate in spite of the high surface water productivity observed in that area. This implies that in the Eastern Equatorial Atlantic, organic carbon accumulation is strongly reduced by efficient degradation of organic matter prior to its burial.
During the Last Glacial Maximum (LGM), accumulation of organic carbon was higher than during the mid-Holocene along the continental margins of Africa and South America (Brazil) as well as in the equatorial region. In the Eastern Equatorial Atlantic in particular, large relative differences between LGM and mid-Holocene accumulation rates are found. This is probably to a great extent due to better preservation of organic matter related to changes in bottom water circulation and not just a result of strongly enhanced export productivity during the glacial period. On average, a two- to three-fold increase in organic carbon accumulation during the LGM compared to mid-Holocene conditions can be deduced from our cores. However, for the deep-sea sediments this cannot be solely attributed to a glacial productivity increase, as changes in South Atlantic deep-water circulation seem to result in better organic carbon preservation during the LGM.

本研究采用南大西洋海域1118个表层沉积物样品的汇编数据集，绘制了该海盆现代海底有机碳含量的空间分布格局。借助全新世沉积速率的新数据，本研究估算得到远洋域（pelagic realm，水深>3000m）内的年有机碳埋藏通量约为1.8×10¹² g C/年。在东赤道大西洋（Eastern Equatorial Atlantic, EEA）辐散区下伏沉积物中，尽管该区域表层水体生产力较高，但有机碳的埋藏量却极低。这表明东赤道大西洋海域内，有机碳在埋藏前即被高效降解，进而大幅抑制了其埋藏积累过程。

在末次盛冰期（Last Glacial Maximum, LGM），非洲、南美（巴西）大陆边缘以及赤道海域的有机碳埋藏量均高于全新世中期。其中东赤道大西洋海域的LGM与全新世中期有机碳埋藏速率的相对差异尤为显著。该差异在很大程度上可归因于底层水环流变化带来的有机碳保存储存条件改善，而非仅由冰期时期输出生产力的大幅提升所致。基于所分析的岩芯数据，本研究推断：相较于全新世中期，LGM时期有机碳埋藏速率平均提升2~3倍。但深海沉积物中的有机碳埋藏量提升并非完全由冰期生产力增长所致，因为南大西洋深水环流的变化似乎提升了LGM时期有机碳的保存效率。