Radionuclides and accumulation rates of sediments from the equatorial Atlantic

Variations in carbonate flux and dissolution, which occurred in the equatorial Atlantic during the last 24,000 years, have been estimated by a new approach that allows the point-by-point determination of paleofluxes to the seafloor. An unprecedented time resolution can thus be obtained which allows sequencing of the relatively rapid events occurring during deglaciation. The method is based on observations that the flux of unsupported 230Th into deep-sea sediments is nearly independent of the total mass flux and is close to the production rate. Thus excess 230Th activity in sediments can be used as a reference against which fluxes of other sedimentary components can be estimated. The study was conducted at two sites (Ceará Rise; western equatorial Atlantic, and Sierra Leone Rise; eastern equatorial Atlantic) in cores raised from three different depths at each site. From measurements of 230Th and CaCO3, changes in carbonate flux with time and depth were obtained. A rapid increase in carbonate production, starting at the onset of deglaciation, was found in both areas. This event may have important implications for the postglacial increase in atmospheric CO2 by increasing the global carbonate carbon to organic carbon rain ratio and decreasing the alkalinity of surface waters (and possibly the North Atlantic Deep Water). Increased carbonate dissolution occurred in the two regions during deglaciation, followed by a minimum during mid-Holocene and renewed intensification of dissolution in late Holocene. During the last 16,000 years, carbonate dissolution was consistently more pronounced in the western than in the eastern basin, reflecting the influence of Antarctic Bottom Water in the west. This trend was reversed during stage 2, possibly due to the accumulation of metabolic CO2 below the level of the Romanche Fracture Zone in the eastern basin.

过去24000年间赤道大西洋海域内的碳酸盐通量（carbonate flux）与碳酸盐溶解（dissolution）变化，可通过一种可逐点测定海底沉积古通量（paleofluxes）的全新方法进行定量估算。该方法可实现前所未有的时间分辨率，进而能够解析冰消期（deglaciation）内发生的相对快速地质事件。该方法的理论依据为下述观测结果：未受陆源输入干扰的未受支撑230Th（unsupported 230Th）向深海沉积物的通量，几乎不受总物质通量的制约，且其通量值接近自身的生成速率。因此，沉积物中的过剩230Th（excess 230Th）放射性活度可作为参照标准，用于估算其余沉积组分的沉积通量。本研究在两个站位开展：西赤道大西洋的塞阿拉海隆（Ceará Rise）与东赤道大西洋的塞拉利昂海隆（Sierra Leone Rise），每个站位均采集了3个不同水深的沉积岩芯。通过对230Th与碳酸钙（CaCO3）的实验室测定，研究得到了碳酸盐通量随时间与水深的变化特征。研究在两个区域均发现，冰消期伊始碳酸盐生成速率便出现快速升高。该事件可通过提升全球碳酸盐碳与有机碳的沉降比值、降低表层海水（或包括北大西洋深层水（North Atlantic Deep Water）在内）的碱度，对冰后期大气二氧化碳浓度升高产生重要调控作用。冰消期期间两个海域均出现碳酸盐溶解作用增强的现象，随后在中全新世（Holocene）溶解强度降至最低，晚全新世时溶解作用再度加剧。过去16000年间，西盆地的碳酸盐溶解强度始终高于东盆地，这反映出南极底层水（Antarctic Bottom Water）对西赤道大西洋的影响。这一趋势在第二沉积阶段（stage 2）发生逆转，其原因可能是东盆地罗曼奇断裂带（Romanche Fracture Zone）下方海域聚集了代谢产生的二氧化碳。