Geochemical evidence of Milankovitch cycles in Atlantic Ocean ferromanganese crusts
Geochemical evidence of Milankovitch cycles in Atlantic Ocean ferromanganese crusts
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DOI：
10.1016/j.epsl.2020.116651
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发表时间：
2021-01
影响因子：
5.3
通讯作者：
P. Josso;T. Peer;M. Horstwood;P. Lusty;B. Murton
中科院分区：
文献类型：
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作者：
P. Josso;T. Peer;M. Horstwood;P. Lusty;B. Murton
氢成因锰铁结壳被认为是受大陆块体风化作用影响的海水同位素组成的忠实记录。鉴于它们在地球上400-7000米深处的所有海洋中无处不在，它们形成了分布最均匀、最容易获取的水团混合和气候记录之一。然而，它们的积累速度慢，年龄约束差，迄今为止限制了它们用于探索100 ka古气候现象的应用。本文显示了东北大西洋铁锰地壳的Pb同位素特征和主要元素含量如何响应古新世气候岁差控制下西非季风降雨强度和地理范围的变化。所研究的高空间分辨率（4 μm）激光烧蚀多收集器电感耦合等离子体质谱（LA-MC-ICP-MS） Pb同位素数据与微钻亚样品相比，在空间和时间分辨率上提高了近2个数量级。记录显示了单Fe-Mn氧化层尺度下206Pb/204Pb、208,207pb /206Pb比值的周期性，以及Fe/Mn比值、Al、Si和Ti含量的变化。Pb同位素数据的时频分析和天文校正表明，气候岁差（~ 20 ka）的印记由偏心率（~ 100和405 ka）调制，产生1.5-3.5 mm/Ma的增长率，与以前的化学地层年龄模型一致。在这种情况下，近日点的北纬夏季导致西非更强的日照，导致比远日点北纬夏季条件更强烈和地理范围更广的季风降雨。这反过来又影响了滋养东北大西洋盆地的风化端部之间的平衡。这些结果提供了将铁锰地壳记录校准为天文解的新方法，并使其同位素和化学档案能够以提高的1000-5000年的时间分辨率进行利用。
Hydrogenetic ferromanganese crusts are considered a faithful record of the isotopic composition of seawater influenced by weathering processes of continental masses. Given their ubiquitous presence in all oceans of the planet at depths of 400–7000 meters, they form one of the most well-distributed and accessible records of water-mass mixing and climate. However, their slow accumulation rate and poor age constraints have to date limited their use to explore 100 ka paleoclimatic phenomena. Here it is shown how the Pb isotope signature and major element content of a Fe-Mn crust from the north-east Atlantic responded to changes in the intensity and geographic extent of monsoonal rainfall over West Africa, as controlled by climatic precession during the Paleocene. The studied high-spatial resolution (4 μm) laser-ablation multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS) Pb isotope data is a nearly 2 order of magnitude improvement in spatial and temporal resolution compared to micro-drill subsamples. The record demonstrates cyclicity of the206Pb/204Pb and208, 207Pb/206Pb ratios at the scale of single Fe-Mn oxide laminae, in conjunction with variations in the Fe/Mn ratio, Al, Si and Ti content. Time-frequency analysis and astronomical tuning of the Pb isotope data demonstrates the imprint of climatic precession (∼20 ka) modulated by eccentricity (∼100 and 405 ka), yielding growth rates of 1.5–3.5 mm/Ma consistent with previous chemostratigraphic age models. In this context, boreal summer at the perihelion causes stronger insolation over West Africa, resulting in more intense and geographically extended monsoonal rainfalls compared to aphelion boreal summer conditions. This, in turn, influences the balance between the weathering endmembers feeding the north-east Atlantic basin. These results provide a new approach for calibrating Fe-Mn crust records to astronomical solutions, and allow their isotopic and chemical archive to be exploited with an improved temporal resolution of 1000–5000 years.