A 13 million-year record of Li isotope compositions in island carbonates: constraints on bulk inorganic carbonate as a global seawater Li isotope archive
A 13 million-year record of Li isotope compositions in island carbonates: constraints on bulk inorganic carbonate as a global seawater Li isotope archive
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DOI：
10.1016/j.gca.2023.01.013
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发表时间：
2023-01
影响因子：
5
通讯作者：
Guangyi Wei;Feifei Zhang;Yi-Sheng Yin;Yi-bo Lin;Philip A. E. Pogge von Strandmann;Mengchun Cao;Na Li;Guolin Xiong;Xinran Chen;Caiwei Fan;Changgui Xu;Fei Tan;Xiyang Zhang;Hongqiang Yang;H. Ling;S. Shen
中科院分区：
文献类型：
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作者：
Guangyi Wei;Feifei Zhang;Yi-Sheng Yin;Yi-bo Lin;Philip A. E. Pogge von Strandmann;Mengchun Cao;Na Li;Guolin Xiong;Xinran Chen;Caiwei Fan;Changgui Xu;Fei Tan;Xiyang Zhang;Hongqiang Yang;H. Ling;S. Shen
海洋碳酸盐沉积物和岩石中锂同位素（δ 7 Li）的变化已被广泛用于重建全球海水Li同位素组成，进而追踪古风化过程。然而，由于浅水碳酸盐岩的δ 7 Li值与碳酸盐岩矿物学和成岩蚀变密切相关，因此，古碳酸盐岩是否能忠实地记录原始海水δ 7 Li信号仍存在争议。本文介绍了南海浅水碳酸盐岩沉积物的高分辨率δ 7 Li、微量元素和碳酸盐矿物学数据，这些数据来自两个浅部钻孔（旧章A和B）和一个深部钻孔（XK-1）。我们将这些新的δ 7 Li数据与ca.为了更好地控制碳酸盐矿物学和早期成岩作用对块状无机碳酸盐δ 7 Li值的影响，对中中新世至更新世1300万年海水δ 7 Li演化史进行了研究。我们观察到九张A、B原生碳酸盐矿床和最上面的XK-1钻孔岩芯的δ 7 Li值（24.6% ± 1.6%，n = 37，1σ）明显低于现代海水δ 7 Li值（Δ 7 Liprimary-海水=-6.0%）。而XK-1岩芯中海相成岩碳酸盐岩的δ 7 Li值为28.7% ± 0.7%（n = 54，1σ），接近于同时代海水的δ 7 Li值（Δ 7 Limarine成岩-海水= 0.0%-2.0%），远高于原生碳酸盐岩沉积的δ 7 Li值。XK-1岩芯中的大气降水成岩碳酸盐δ 7 Li值为22.4% ± 1.6%（n = 46，1σ），与旧张A、B和最上部XK-1岩芯中的原生碳酸盐沉积的δ 7 Li值接近，但明显低于同时代海水δ 7 Li值（现代和中新世海水Δ 7大气降水成岩-海水= -5.0%~-9‰）。浅水碳酸盐中的这种δ 7 Li变化归因于碳酸盐矿物学的变化（即，文石、高镁方解石和低镁方解石）和成岩机制（海洋和大气成岩作用的流体或沉积物缓冲条件）。通过对比中国南海和巴哈马的δ 7 Li数据，我们认为在海相成岩作用期间Li同位素存在强烈的流体缓冲条件，导致海相石灰岩和白云岩的δ 7 Li值接近环境海水δ 7 Li信号。相比之下，可能以沉积物缓冲条件为标志的陨石成岩碳酸盐可能继承了原生碳酸盐沉积物的原始δ 7 Li信号。因此，在充分考虑碳酸盐矿物学和成岩作用的基础上，可以更好地利用块状无机碳酸盐δ 7 Li来重建缺乏骨骼化石记录的深部古风化作用演化。
Lithium isotope (δ7Li) variations in marine carbonate sediments and rocks have been widely used to reconstruct global seawater Li isotope compositions and then trace the paleo-weathering processes. However, there are still debates on whether ancient carbonates can faithfully document original seawater δ7Li signals, as δ7Li values of shallow-water carbonates are tightly related to carbonate mineralogy and diagenetic alteration. In this study, we present high-resolution δ7Li, trace element and carbonate mineralogy data of shallow-water carbonate deposits from two shallow drillcores (Jiuzhang A and B) and a deep drillcore (XK-1) in the South China Sea. We compare these new δ7Li data to the ca. 13 Myr history of seawater δ7Li evolution from the middle Miocene to Pleistocene, in order to better constrain the effects of carbonate mineralogy and early diagenesis on δ7Li values of bulk inorganic carbonates. We observe that the δ7Li values of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores (24.6% ± 1.6%, n = 37, 1σ) are significantly lower than modern seawater δ7Li values (Δ7Liprimary-seawater= ∼–6.0%). In contrast, marine diagenetic carbonates in the XK-1 drillcore exhibit δ7Li values of 28.7% ± 0.7% (n = 54, 1σ), approaching coeval seawater values (Δ7Limarine diagenetic-seawater= ∼–2.0%) and much higher than those of primary carbonate deposits. Meteoric diagenetic carbonates in the XK-1 drillcore show δ7Li values of 22.4% ± 1.6% (n = 46, 1σ), close to that of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores, but significantly lower than coeval seawater δ7Li values (Δ7Limeteoric diagenetic-seawater= –5.0% to –9‰ for modern and Miocene seawater). Such δ7Li variations in shallow-water carbonates are attributed to variations in carbonate mineralogy (i.e., aragonite, high-Mg calcite and low-Mg calcite) and diagenetic regimes (fluid- or sediment-buffered conditions of marine and meteoric diagenesis). By comparing δ7Li data from the South China Sea to those from the Bahamas, we suggest strongly fluid-buffered conditions for Li isotopes during marine diagenesis, resulting in δ7Li values of marine limestones and dolostones approaching ambient seawater δ7Li signals. In contrast, meteoric diagenetic carbonates, likely marked by sediment-buffered conditions, may inherit the original δ7Li signals of primary carbonate deposits. Hence, full considerations of the carbonate mineralogy and diagenesis facilitate a better use of bulk inorganic carbonate-archived δ7Li to reconstruct paleo-weathering evolution in deep time lacking the skeletal fossil records.