The Hirnantian carbon isotopic excursion (HICE) was characterized by a positive δC excursion (up to ∼ + 7‰) of oceanic dissolved inorganic carbon (DIC). However its mechanism remains controversial, impeding our understanding of this global event and its environmental significance. Here, we conducted a comprehensive analysis of global carbon-isotope records, based on 65 paleogeographically-widely distributed sections (4612 carbon-isotope data), documenting a steep oceanic DIC δC water-depth gradient (e.g., a 4.6‰ offset in the HICE excursion magnitude within Laurentia) during the Hirnantian Stage. We propose a hybrid hypothesis that the HICE records a shift toward more positive values owing to a large glacio-eustatic fall (>100 m) under the herein-identified steep δC water-depth gradient, with potential contributions from global-ocean δC changes and authigenic carbonates. This study provides the first large-data evidence for a widely density-stratified ocean during the Hirnantian Glaciation, with implications for reconstructions of ocean circulation and the carbon cycle during the Early Paleozoic.

中文翻译：

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赫南蒂安冰川期陡峭的海洋 DIC δ13C 深度梯度


赫南蒂安碳同位素偏移 (HICE) 的特征是海洋溶解无机碳 (DIC) 的 δ13C 正偏移（高达 ∼ + 7‰）。然而，其机制仍然存在争议，阻碍了我们对这一全球事件及其环境意义的理解。在这里，我们基于65个古地理广泛分布的剖面（4612个碳同位素数据）对全球碳同位素记录进行了全面分析，记录了陡峭的海洋DIC δ13C水深梯度（例如，HICE中的4.6‰偏移）赫南阶阶段期间劳伦大陆内的偏移幅度）。我们提出了一个混合假设，即 HICE 记录了由于在本文确定的陡峭 δ13C 水深梯度下出现较大的冰川海平面下降（>100 m）而向更正值的转变，其中全球海洋 δ13C 变化和自生碳酸盐。这项研究为赫南蒂安冰川时期存在广泛密度分层的海洋提供了第一个大数据证据，对重建早古生代的海洋环流和碳循环具有重要意义。