A 31,000-year high-resolution record from two sediment cores off northwest Africa

Benthic foraminiferal assemblage compositions and sedimentary geochemical parameters were analyzed in two radiocarbon dated sediment cores from the upwelling area off NW Africa at 12°N, to reconstruct productivity changes during the last 31 kyr. High-latitude cold events and variations in low-latitude summer insolation influenced humidity, wind systems, and the position of the tropical rain belt over this time period. This in turn caused changes in intensity and seasonality of primary productivity off the southern Northwest African continental margin. High accumulation rates of benthic foraminifera, carbonate, and organic carbon during times of north Atlantic melt water events Heinrich 2 (25.4 to 24.3 kyr BP) and 1 (16.8 to 15.8 kyr BP) indicate high productivity. Dominance of infaunal benthic foraminiferal species and high numbers of deep infaunal specimens during that time indicate a strong and sustained supply of refractory organic matter reworked from the upper slope and shelf. A more southerly position of the tropical rainbelt and the Northeast trade wind belt during Heinrich 2 and 1 may have enhanced wind intensity and almost permanent upwelling, driving this scenario. A phytodetritus-related benthic fauna indicates seasonally pulsed input of labile organic matter but generally low year-round productivity during the Last Glacial Maximum (23 to 18 kyr BP). The tropical rainbelt is more expanded to the North than during Heinrich Events, and relatively weak NE trade winds resulted in seasonal and weak upwelling, thus lower productivity. High productivity characterized by a seasonally high input of labile organic matter, is indicated for times of orbital forced warming, such as the African Humid Period (9.8 to 7 kyr BP). An intensified African monsoon during boreal summer and the northernmost position of the tropical rainbelt within the last 31 kyr resulted in enhanced river discharge from the northward-extended drainage area (or river basin) initiating intense phytoplankton blooms. In the late Holocene (4 to 0 kyr BP) strong carbonate dissolution may have been caused by even more enhanced organic matter fluxes to the sea floor. Increasing aridity on the continent and stronger NE trade winds induced intensive, seasonal coastal upwelling.本研究对西非西北部12°N附近上升流区的2根经放射性碳定年的沉积物岩芯开展分析，测定了底栖有孔虫（benthic foraminifera）组合组成与沉积地球化学参数，以期重建过去31 kyr以来的海洋生产力变化。此时期内，高纬度冷事件与低纬度夏季日照辐射变化共同影响了区域湿度、风系格局以及热带雨带的位置，进而改变了西北非南部大陆边缘海域的初级生产力强度与季节分布特征。 北大西洋融水事件海因里希2（Heinrich 2，25.4~24.3 kyr BP）与海因里希1（Heinrich 1，16.8~15.8 kyr BP）期间，底栖有孔虫、碳酸盐与有机碳的堆积速率较高，指示当时海洋生产力处于高位。此时期沉积物中内栖底栖有孔虫物种占优，且深内栖个体数量较多，表明有大量经上陆坡与陆架再搬运的难降解有机质持续输入。海因里希2与1事件期间，热带雨带与东北信风带位置更偏南，可能增强了风力强度并形成近乎持续的上升流，从而驱动了上述高生产力格局。 末次冰盛期（Last Glacial Maximum，23~18 kyr BP）的底栖动物群以植物碎屑相关类群为主，指示存在季节性脉冲式输入的易降解有机质，但全年整体生产力水平偏低。相较于海因里希事件时期，该阶段热带雨带北界更偏北，且东北信风强度较弱，仅形成季节性且较弱的上升流，因此生产力水平较低。 以易降解有机质季节性大量输入为特征的高生产力时段，对应轨道驱动升温时期，例如非洲湿润期（African Humid Period，9.8~7 kyr BP）。北半球夏季增强的非洲季风与过去31 kyr以来最偏北的热带雨带位置，使得向北扩张的流域（或河盆）的河流径流量增加，引发了强烈的浮游植物水华。全新世晚期（4~0 kyr BP），更强的海底有机质输入通量可能导致了强烈的碳酸盐溶解作用；同时陆地干旱化加剧与东北信风增强驱动了强烈的季节性沿岸上升流。