Uranium radionuclides and organic biomarkers in sediment core RC27-42

Uranium series radionuclides and organic biomarkers, which represent major groups of planktonic organisms, were measured in western Arabian Sea sediments that span the past 28 ka. Variability in the past strength of the southwest and northeast monsoons and its influence on primary productivity, sea surface temperature (SST), and planktonic community structure were investigated. The average alkenone-derived SST for the last glacial period was ~3°C lower than that measured for the Holocene. Prior to the deglacial, the lowest SSTs coincide with the highest measured fluxes of organic biomarkers, which represent primarily a planktonic suite of diatoms, coccolithophorids, dinoflagellates, and zooplankton. We propose that intensification of winter northeast monsoon winds during the last glacial period resulted in deep convective mixing, cold SSTs and enhanced primary productivity. In contrast, postdeglacial (<17 ka) SSTs are warmer during times in which biomarker fluxes are high. Associated with this transition is a planktonic community structure change, in which the ratio of the average cumulative flux of diatom biomarkers to the cumulative flux of coccolithophorid biomarkers is twice as high during the deglacial and Holocene than the average ratio during the last glacial period. We suggest that this temporal transition represents a shift from a winter northeast monsoon-dominated (pre-17 ka) to a summer southwest monsoon-dominated (post-17 ka) wind system.

本次研究对覆盖过去28千年（ka）的阿拉伯海西部沉积物中的铀系放射性核素（uranium series radionuclides）以及代表主要浮游生物（planktonic organisms）类群的有机生物标志物（organic biomarkers）进行了测定，探讨了西南季风（southwest monsoon）与东北季风（northeast monsoon）在地质历史时期的强度变化及其对初级生产力（primary productivity）、海表温度（sea surface temperature, SST）和浮游生物群落结构的影响。末次冰期（last glacial period）由烯酮（alkenone）衍生得到的海表温度平均值较全新世（Holocene）低约3℃。在冰消期（deglacial）之前，最低海表温度与测得的最高有机生物标志物通量相吻合，这些生物标志物主要涵盖由硅藻（diatoms）、颗石藻（coccolithophorids）、甲藻（dinoflagellates）以及浮游动物（zooplankton）组成的浮游生物群落。我们提出，末次冰期期间冬季东北季风的增强引发了深层对流混合，进而导致海表温度降低与初级生产力提升。与之相反，冰消期后（<17 ka），当生物标志物通量较高时海表温度更为温暖，伴随这一转变的是浮游生物群落结构的变化：冰消期与全新世期间，硅藻生物标志物的平均累积通量与颗石藻生物标志物累积通量的比值较末次冰期期间的平均比值高出一倍。我们认为，这一时间尺度上的转变代表了风场系统从以冬季东北季风为主（17 ka之前）向以夏季西南季风为主（17 ka之后）的转换。