Geochemical evidence from the Kioto Carbonate Platform (Tibet) reveals enhanced terrigenous input and deoxygenation during the early Toarcian
Geochemical evidence from the Kioto Carbonate Platform (Tibet) reveals enhanced terrigenous input and deoxygenation during the early Toarcian
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DOI：
10.1016/j.gloplacha.2022.103887
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发表时间：
2022-07
影响因子：
3.9
通讯作者：
Zhong Han;Xiumian Hu;Zhongya Hu;H. Jenkyns;Tianhao Su
中科院分区：
文献类型：
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作者：
Zhong Han;Xiumian Hu;Zhongya Hu;H. Jenkyns;Tianhao Su
各种沉积资料表明，早陶拉纪具有突发性负碳同位素偏移（CIE）叠加在长期正趋势上的特征，并伴有显著的气候环境变化。然而，大陆风化流和浅水海洋脱氧的变化及其在低纬度地区引起碳酸盐岩台地危机的可能作用仍然知之甚少。大部分水不溶性元素（如Ti、Sc、Th和总稀土元素）在Pliensbachian-Toarcian边界处开始明显增加，随后在T-OAE CIE负相位出现微弱增加或相对较高的值，表明陆源输入的增加可能与这一时间间隔内全球快速变暖有关。随着水不溶性元素丰度的增加，Mn、Ce和Ce异常立即开始增加，随后在负CIE区间内增加值。这些观测结果表明，在这段时间内，浅水地区发生了锰（缺氧）条件的脱氧过程和发展，可能与大陆风化和养分输入的增强有关，有利于初级生产力和氧气消耗。在地层较高的位置，水不溶性元素在CIE恢复阶段呈逐渐减少的趋势，与氧化还原指标值升高平行，表明大陆风化强度下降，浅水发生了相应的二次脱氧。在这种情况下，脱氧可能是由于海洋环流的减缓和/或生物必需营养物质的加强再循环造成的。生物变化、碳酸盐含量和地球化学数据的耦合关系表明：(1)在Pliensbachian-Toarcian边界时间前后，浅海陆源内流和脱氧的增强可能导致KCP的轻微恶化；(2)在CIE负期，陆源内流和脱氧的急剧增强可能对底栖碳酸盐生产体更严重的危机起关键作用。
The early Toarcian, as registered in a variety of sedimentary archives, was characterized by an abrupt negative carbon-isotope excursion (CIE) typically superimposed on a long-term positive trend, and was accompanied by significant climatic and environmental changes. However, the changes in continental weathering influx and oceanic deoxygenation in shallow waters and their possible role in causing carbonate-platform crises in low latitudes remains poorly constrained. Here, we present carbonate content and carbonate-hosted elements for the Pliensbachian–Toarcian transitional interval from the Kioto Carbonate Platform (KCP) in the Tibetan Himalaya. The most water-insoluble elements (e.g. Ti, Sc, Th and total rare earth elements) show an obvious increase starting at the Pliensbachian–Toarcian boundary, followed by a weak increase or relatively high-level values during the negative phase of the T-OAE CIE, suggesting that the enhanced terrigenous input can be linked to rapid global warming during this time interval. The Mn, Ce and Ce anomaly start to increase immediately following the rise in abundance of the water-insoluble elements, followed in turn by enhanced values over the interval of the negative CIE. These observations indicate that the deoxygenation process and development of manganous (suboxic) conditions occurred in shallow water during this time interval and were likely linked to enhanced continental weathering and nutrient input, favoring both primary productivity and oxygen consumption. Stratigraphically higher, the water-insoluble elements show a gradual decreasing trend parallel with elevated values of redox proxies during the recovery phase of the CIE, suggesting decline in continental weathering intensity and a corresponding second deoxygenation in shallow waters. In this instance, deoxygenation might have been caused by a slackening of ocean circulation and/or enhanced recycling of bioessential nutrients. The coupled relationship between biotic changes, carbonate content and geochemical data suggest that: (1) the onset of enhanced terrigenous influx and deoxygenation in shallow waters likely led to slight deterioration to the KCP around Pliensbachian–Toarcian boundary time, and (2) the drastically enhanced terrigenous flux and deoxygenation likely played a pivotal role in the more severe crisis for benthic carbonate producers during the negative phase of the CIE.