(Table 1) Th and U concentrations and activity ratios in deep-sea sediments at ODP Hole 105-646B

The Labrador Sea is a particularly suitable high-latitude basin for investigating U and Th behavior in deep-sea sediments. During the late Quaternary, the cyclic development and decay of huge ice sheets on adjacent land masses resulted in large-amplitude changes in sedimentation rates and organic paleoproductivities. The resulting magnification of U and Th response is well illustrated by high-resolution studies on piston-cored sediments from the Greenland continental rise at Ocean Drilling Program Leg 105 Site 646 spanning isotopic stages 8 to 1. Our results show a clear positive correlation of 238U/232Th ratios with organic paleoproductivity indicators (e.g., dinocyst) due to U uptake in the water column and/or during the early early diagenesis of organic matter responding to carbon fluxes and to their climate forcing. 230Th excesses over 234U exceed the theoretical value of the 230Th rain from the overlying water column, indicating lateral input possibly from the Greenland slope and shelf. Because these horizontal fluxes of 230Th may be partly controlled by physical parameters, 230Th excesses cannot be unequivocally correlated with sedimentation rates and/or productivity as reported elsewhere. In this subarctic basin characterized by low overall organic carbon burial, the 238U/232Th ratio appears to be a sensible geochemical indicator of organic activity and paleoproductivity.

拉布拉多海（Labrador Sea）是研究深海沉积物中铀（U, Uranium）与钍（Th, Thorium）地球化学行为的尤为适宜的高纬度盆地。晚第四纪时期，邻近陆块上巨型冰盖的周期性发育与消融，致使沉积速率与古有机生产力发生大幅波动。借助大洋钻探计划（Ocean Drilling Program, ODP）第105航次646站位格陵兰陆隆的活塞取芯沉积物的高分辨率研究（其沉积序列覆盖同位素阶段8至1），铀钍响应的这种放大效应得到了清晰印证。我们的研究结果显示，受碳通量及其气候强迫作用驱动，有机质在水柱中及有机质早期成岩阶段发生铀富集，使得238U/232Th比值与古有机生产力指标（如沟鞭藻孢囊（dinocyst））呈显著正相关。230Th相对于234U的过剩活度超过了上覆水柱沉降的230Th的理论值，表明存在来自格陵兰陆坡与陆架的侧向输入。由于这类230Th水平通量可能部分受物理参数调控，因此无法如其他已有研究那般，将230Th过剩活度与沉积速率及/或生产力进行明确关联。在这个整体有机碳埋藏水平较低的副极地盆地中，238U/232Th比值可作为灵敏的地球化学指标，用于指示有机活动与古生产力。