Radiogenic isotope trace composition of the Labrador Sea

Geological reconstructions and general circulation models suggest that the onset of both Northern Hemisphere glaciation, 2.7 Myr ago, and convection of Labrador Sea Water (LSW) were caused by the closure of the Panama Gateway ~4.5 Myr ago. Time series data that have been obtained from studies of ferromanganese crusts from the northwestern Atlantic suggest that radiogenic isotopes of intermediate ocean residence time (Pb and Nd) can serve as suitable tracers to reconstruct these events. However, it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering. In this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in LSW but can be explained in terms of increases of erosion levels due to the glaciation of Greenland and Canada. Furthermore, we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic Pb and nonradiogenic Nd from continental detritus eroding into the Labrador Sea. This can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic Pb should provide such a rich supply of radiogenic Pb to the oceans. An important general conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources. Perhaps more importantly, the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater.地质重建结果与全球环流模型均表明，270万年前启动的北半球冰期（Northern Hemisphere glaciation）与拉布拉多海海水（Labrador Sea Water, LSW）对流活动，均由约450万年前巴拿马地峡（Panama Gateway）的闭合所引发。从西北大西洋铁锰结壳（ferromanganese crusts）研究中获取的时间序列数据显示，具备海洋中等停留时间的放射成因同位素（Pb与Nd）可作为重建上述地质事件的合适示踪剂。然而迄今为止，学界仍未明确彼时观测到的同位素组成变化，究竟是温盐环流（thermohaline circulation）增强所致，还是冰川风化作用加剧的结果。本文采用箱式模型（box model）方法，证明放射成因同位素组成的偏移不太可能由拉布拉多海海水对流活动变化所引发，而可通过格陵兰与加拿大冰川化导致的侵蚀强度升高加以解释。此外，我们提供实验证据表明，侵蚀进入拉布拉多海的大陆碎屑（continental detritus）会以非一致方式释放出易活动的强放射成因铅与非放射成因钕组分。这一现象可归因于古老大陆的冰川风化作用，同时也解释了一个看似矛盾的问题：全球放射成因铅最为匮乏的区域之一，为何能为海洋提供如此丰富的放射成因铅供给。一项重要的普遍性结论为，海水中放射成因同位素的组成并非总能反映其大陆源区特征。或许更为关键的是，从化学风化向机械侵蚀（mechanical erosion）的转变，可能会导致海水中放射成因示踪剂出现显著变化。