Radionuclides in sediment cores of the Atlantic sector of the Southern Ocean

We present time series of export productivity proxy data including 230Thex-normalized deposition rates (rain rates) of 10Be, dissolution-corrected biogenic Ba, and biogenic opal as well as authigenic U concentrations which are complemented by rain rates of total (detrital) Fe and sea ice indicating diatom abundances from five sediment cores across the Atlantic sector of the Southern Ocean covering the past 150,000 years. The results suggest that 10Be rain rates and authigenic U concentration cannot serve as quantitative paleoproductivity proxies because they have also been influenced by detrital particle fluxes in the case of 10Be and bulk sedimentation rates (sediment focussing) and deep water oxygenation in the case of U. The combined results of the remaining productivity proxies of this study (rain rates of biogenic opal and biogenic Ba in those sections without authigenic U) and other previously published proxy data from the Southern Ocean (231Pa/230Th and nitrogen isotopes) suggest that a combination of sea ice cover, shallow remineralization depth, and stratification of the glacial water column south of the present position of the Antarctic Polar Front and possibly Fe fertilization north of it have been the main controlling factors of export paleoproductivity in the Southern Ocean over the last 150,000 years. An overall glacial increase of export paleoproductivity is not supported by the data, implying that bioproductivity variations in the Southern Ocean are unlikely to have contributed to the major glacial atmospheric CO2 drawdown observed in ice cores.本研究提供了输出生产力代用数据（export productivity proxy data）的时间序列，涵盖铍-10（10Be）、经溶蚀校正的生源钡（biogenic Ba）、生源蛋白石（biogenic opal）的230Th过剩（230Thex）归一化沉积速率（沉降通量速率），以及自生铀（authigenic U）浓度；补充数据还包括总（陆源碎屑）铁（detrital Fe）的沉降速率、以及指示硅藻丰度的海冰相关数据，上述数据采自南大洋大西洋扇区的5个沉积岩芯（sediment cores），时间跨度覆盖过去15万年。研究结果显示，10Be沉降速率与自生铀浓度无法作为定量古生产力代用指标（paleoproductivity proxies）：10Be的信号同时受陆源碎屑颗粒通量（detrital particle fluxes）影响，而铀浓度则受整体沉积速率（bulk sedimentation rates，即沉积物聚集作用sediment focussing）与深水氧化环境（deep water oxygenation）的调控。本研究剩余的生产力代用指标（无自生铀层位的生源蛋白石与生源钡沉降速率），结合此前已发表的南大洋（Southern Ocean）相关代用数据（231Pa/230Th比值与氮同位素）的综合分析结果表明：在过去15万年中，南极极锋（Antarctic Polar Front）现今位置以南区域的海冰覆盖、较浅的有机质再矿化深度（remineralization depth）、冰期水柱层结（stratification of the glacial water column），以及极锋以北区域可能存在的铁输入施肥效应（Fe fertilization），共同构成了南大洋输出古生产力的主要控制因素。数据并不支持冰期南大洋输出古生产力整体升高的结论，这意味着南大洋生物生产力的变化，不太可能是冰芯（ice cores）中观测到的大气CO₂大规模抽取（atmospheric CO2 drawdown）的驱动因素。