Planktic foraminifera of sediment core GeoB1710-3

<p>In this study, we test various parameters in deep-sea sediments (bulk sediment parameters and changes in microfossil abundances and preservation character) which are generally accepted as indicators of calcium carbonate dissolution. We investigate sediment material from station GeoB 1710-3 in the northern Cape Basin (eastern South Atlantic), 280 km away from the Namibian coast, well outside today&#39;s coastal upwelling. As northern Benguela upwelling cells were displaced westward and periodically preceded the core location during the past 245 kyr (Volbers et al., submitted), GeoB 1710-3 sediments reflect these changes in upwelling productivity. Results of the most commonly used calcium carbonate dissolution proxies do not only monitor dissolution within these calcareous sediments but also reflect changes in upwelling intensity. Accordingly, these conventional proxy parameters misrepresent, to some extent, the extent of calcium carbonate dissolution. These results were verified by an independent dissolution proxy, the Globigerina bulloides dissolution index (BDX&#39;) (Volbers and Henrich, 2002, doi:10.1016/S0025-3227(02)00333-X). The BDX&#39; is based on scanning electronic microscope ultrastructural investigation of planktonic foraminiferal tests and indicates persistent good carbonate preservation throughout the past 245 kyr, with the exception of one pronounced dissolution event at early oxygen isotopic stage (OIS) 6.
The early OIS 6 is characterized by calcium carbonate contents, sand contents, and planktonic foraminiferal concentrations all at their lowest levels for the last 245 kyr. At the same time, the ratio of radiolarian to planktonic foraminiferal abundances and the ratio of benthic to planktonic foraminiferal tests are strongly increased, as are the rain ratio, the fragmentation index, and the BDX&#39;. The sedimentary calcite lysocline rose above the core position and GeoB 1710-3 sediments were heavily altered, as attested to by the unusual accumulation of pellets, aggregates, sponge spicules, radiolaria, benthic foraminifera, and planktonic foraminiferal assemblages.
Solely the early OIS 6 dissolution event altered the coarse fraction intensely, and is therefore reflected by all conventional calcium carbonate preservation proxies and the BDX&#39;. We attribute the more than 1000 m rise of the sedimentary calcite lysocline to the combination of two processes: (a) a prominent change in the deep-water mass distribution within the South Atlantic and (b) intense degradation of organic material within the sediment (preserved as maximum total organic carbon content) creating microenvironments favorable for calcium carbonate dissolution.</p>

本研究旨在测试深海沉积物中的多种参数，包括总体沉积物参数、微化石丰度和保存特征的变迁，这些参数通常被视为碳酸钙溶解的指标。研究采集了来自北部开普盆地（东大西洋）GeoB 1710-3站点的沉积物材料，该站点位于纳米比亚海岸280公里之外，远离现今的海岸上升流区域。鉴于过去245千年前，北部本格拉上升流细胞向西迁移，并周期性地在核心位置之前发生（Volbers等，投稿中），GeoB 1710-3的沉积物反映了上升流生产力的这些变化。最常用的碳酸钙溶解代理指标不仅监测了这些钙质沉积物中的溶解作用，而且也反映了上升流强度的变化。因此，这些传统的代理参数在一定程度上未能准确反映碳酸钙溶解的程度。这些结果通过一个独立的溶解代理，即有孔虫球状体溶解指数（BDX）（Volbers和Henrich，2002，doi:10.1016/S0025-3227(02)00333-X）得到了验证。BDX基于对浮游有孔虫壳的超微结构扫描电子显微镜调查，表明在过去245千年的整个期间，碳酸保存状况一直良好，仅有一个明显的溶解事件发生在早期氧同位素阶段（OIS）6。早期OIS 6的特征是碳酸钙含量、沙含量和浮游有孔虫浓度均处于过去245千年中的最低水平。与此同时，介形虫与浮游有孔虫丰度的比率、底栖与浮游有孔虫壳的比率、雨水比、破碎指数以及BDX都显著增加。沉积物中的碳酸盐溶解度层上升至核心位置之上，GeoB 1710-3的沉积物发生了显著变化，这从异常的球状体、聚集体、海绵针状体、放射虫、底栖有孔虫和浮游有孔虫群落的积累中可以证实。仅在早期OIS 6的溶解事件中，粗颗粒物质受到了强烈的影响，因此所有传统的碳酸钙保存代理指标和BDX都有所反映。我们将沉积物碳酸盐溶解度层上升超过1000米归因于两个过程的结合：（a）南大西洋深水质量分布的显著变化；（b）沉积物中有机物质（以最大总有机碳含量保存）的强烈降解，形成了有利于碳酸钙溶解的微环境。