Authigenic Pb isotopes from the Laurentian Fan: Changes in chemical weathering and patterns of North American freshwater runoff during the last deglaciation
Authigenic Pb isotopes from the Laurentian Fan: Changes in chemical weathering and patterns of North American freshwater runoff during the last deglaciation
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DOI：
10.1016/j.epsl.2010.09.031
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发表时间：
2010-11
影响因子：
5.3
通讯作者：
F. Kurzweil;M. Gutjahr;D. Vance;L. Keigwin
中科院分区：
文献类型：
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作者：
F. Kurzweil;M. Gutjahr;D. Vance;L. Keigwin
气候依赖的化学风化趋势对冰川终止期间大陆径流中溶解的Pb同位素组成有强烈的影响，因此这种示踪剂可以用来重建北美冰川消融对淡水径流和海洋化学方面的影响。在这里，我们从劳伦扇下部沉积物中获得了自生铁锰氢氧化物衍生的Pb同位素记录，这些记录追踪了通过圣劳伦斯湾的当地去冰大陆径流信号。我们利用这些记录来研究北美径流中铅同位素组成的变化及其与冰川消融过程的关系。新的Pb同位素记录与西北大西洋副热带深海的Pb同位素记录非常相似，但由于Laurentide冰盖近端环境，新数据分辨率更高，变化幅度更大。在全新世早期（在11.2ka时206Pb/204Pb=20.1）的高放射性成因成分转变中，非放射性成因成分（在18.5ka时206Pb/204Pb低至18.8）。晚全新世206pb /204Pb值回归到206pb /204Pb=19.6附近的中间值。这些新数据支持了先前的推断，即大西洋深处的铅同位素组成在消冰期间受到大陆化学风化模式和强度变化的控制。更详细地说，整个Pb同位素记录中最显著的变化始于12.5ka，一直持续到11ka左右。北大西洋地区在气候寒冷时期的强烈铅同位素变化表明，在这个位置，深海自生铁锰氧氢氧化物中记录的铅同位素组成不仅可以追踪化学风化强度的冰川消融变化，而且高度依赖于北美内陆主要的淡水径流路线。我们的数据表明，在新仙女木时期，向东的淡水径流路线逐渐开放。
Climate-dependent chemical weathering trends have a strong impact on the dissolved Pb isotopic composition of continental runoff during glacial terminations, so that this tracer can be used to reconstruct the impact of the North American deglaciation on aspects of freshwater runoff and ocean chemistry. Here we present authigenic Fe–Mn oxyhydroxide-derived Pb isotope records from lower Laurentian Fan sediments that trace the local deglacial continental runoff signal through the Gulf of St. Lawrence. We use these records to investigate changes in the Pb isotopic composition of the North American runoff, and their relationship to deglacial processes. The new Pb isotope records are very similar to those from the subtropical deep NW Atlantic locations, though the new data are at higher resolution and exhibit much greater amplitudes of change due to their Laurentide Ice Sheet-proximal setting. Unradiogenic compositions (206Pb/204Pb as low as 18.8 at 18.5ka) during the latest glacial change towards highly radiogenic compositions during the early Holocene (206Pb/204Pb=20.1 at 11.2ka). Late Holocene206Pb/204Pb values return to intermediate values around206Pb/204Pb=19.6. These new data support previous inferences that the Pb isotopic composition of the deep Atlantic is controlled during deglaciation by changes in the pattern and intensity of continental chemical weathering. In more detail, the most significant change in the entire Pb isotope record starts at 12.5ka and lasts until about 11ka. This observation, of strong Pb isotopic changes across a climatically cold period in the North Atlantic region, suggests that at this location Pb isotopic compositions recorded in deep marine authigenic Fe–Mn oxyhydroxides not only trace deglacial changes in chemical weathering intensity but also highly depend on major freshwater runoff routes in interior North America. Our data suggest a gradual opening of the eastward freshwater runoff route in the course of the Younger Dryas.