Climatic-hydrologic influence on redox condition in the Cryogenian interglacial Nanhua Basin: Insights from the Datangpo Formation in the northwestern Yangtze Block, South China

IF 3.2 2区 地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY Precambrian Research Pub Date : 2024-09-04 DOI:10.1016/j.precamres.2024.107557
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Abstract

Tracking climatic and oceanic redox changes throughout the Neoproterozoic Cryogenian is crucial to a better understanding of the coevolution of life and environment in geological history. However, the processes and driven mechanisms of redox evolution in the Cryogenian interglacial ocean still remain uncertain. Here, we present a combined study of geochemical proxies, such as paleoclimatic proxies (CIA and Zr/Al) and paleosalinity proxies (B/Ga and Sr/Ba), from the interglacial Datangpo Formation in the shallow-water Zuojiawan section exposed in the northwestern Yangtze Block, to constrain climatic and hydrologic changes on redox conditions. Our data indicate that two apparently climatic cycles from warming to cooling occurred during the deposition of the Datangpo Formation, supplementing the monotonous climatic warming in previous research during the Cryogenian interglaciation. The low B/Ga and Sr/Ba ratios throughout Datangpo Formation suggest a continuous freshwater condition, which is in contrast to a transition of marine condition to brackish condition in relatively deep-water sections. Compiled salinity reconstructions at different paleo-depths imply the salinity gradient in the Nanhua Basin with a density stratification like modern Baltic Sea, at least during the early Cryogenian interglacial period. We find that the climate-driven local riverine freshwater input may be a significant driver for maintaining long-term freshwater condition in the Zuojiawan section. Accompanied by synchronously climatic warming, the progressive desalinization with declining B/Ga and Sr/Ba ratios in the middle to upper deposition of the Datangpo Formation are probably attributed to the persistent freshwater input to a nearshore shallow-water paleogeographic setting. Combined the improving oceanic oxygenation backdrop with the elevating seawater sulfate concentration in the Nanhua Basin in this period, we conclude that the continuous dilution of seawater in the restrict (semi-) Nanhua Basin could attenuate the density stratification and enhance the vertical mixing of watermass, favoring the ventilation of deepwater and penetration of sulfate. This contribution provides a new insight for the study of salinity changes on redox conditions in the Yangtze Block during the Cryogenian interglacial period, and provides new data for supporting a systematic understanding of the evolutional mechanism of oceanic redox state.

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气候-水文对冰期南华盆地氧化还原条件的影响:华南西北部扬子地块大塘坡地层的启示
追踪整个新近纪冰期的气候和海洋氧化还原变化,对于更好地理解地质历史上生命与环境的共同演化至关重要。然而,冰期间冰期海洋氧化还原演化的过程和驱动机制仍不确定。在此,我们对出露于长江区块西北部左家湾浅水层段冰期大塘坡地层的古气候代用指标(CIA和Zr/Al)和古盐度代用指标(B/Ga和Sr/Ba)等地球化学代用指标进行了综合研究,以解释气候和水文变化对氧化还原条件的影响。我们的数据表明,在大塘坡地层沉积过程中出现了两个明显的由暖变冷的气候循环,补充了以往研究中冰期间冰期气候变暖的单调性。整个大唐坡地层的B/Ga和Sr/Ba比值较低,表明地层处于持续的淡水状态,这与相对深水区段的海相状态向咸水状态过渡形成鲜明对比。不同古深度的盐度重建结果表明,至少在早冰期,南华盆地的盐度梯度与现代波罗的海的密度分层相似。我们发现,气候驱动的本地河流淡水输入可能是左家湾断面淡水条件长期维持的重要驱动力。伴随着气候的同步变暖,大塘坡地层中上沉积的B/Ga和Sr/Ba比值逐渐下降,这可能是由于淡水持续输入到近岸浅水古地理环境所致。结合这一时期南华盆地海水含氧量提高和海水硫酸盐浓度升高的背景,我们认为限制(半)南华盆地海水的持续稀释可以减弱密度分层,增强水团的垂直混合,有利于深水的通气和硫酸盐的渗透。这为研究冰期间冰期盐度变化对长江流域氧化还原条件的影响提供了新的见解,为系统认识海洋氧化还原态的演化机制提供了新的数据支持。
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来源期刊
Precambrian Research
Precambrian Research 地学-地球科学综合
CiteScore
7.20
自引率
28.90%
发文量
325
审稿时长
12 months
期刊介绍: Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as: (1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology; (2) Geochronology and isotope and elemental geochemistry; (3) Precambrian mineral deposits; (4) Geophysical aspects of the early Earth and Precambrian terrains; (5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes. In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes. Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.
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