Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering and Frontiers Science Center for Critical Earth Material Cycling, Nanjing University, Nanjing 210023, China;
Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10946.
Proc Natl Acad Sci U S A. 2021 Oct 19;118(42). doi: 10.1073/pnas.2026456118.
Reconstructing Cenozoic history of continental silicate weathering is crucial for understanding Earth's carbon cycle and greenhouse history. The question of whether continental silicate weathering increased during the late Cenozoic, setting the stage for glacial cycles, has remained controversial for decades. Whereas numerous independent proxies of weathering in ocean sediments (e.g., Li, Sr, and Os isotopes) have been interpreted to indicate that the continental silicate weathering rate increased in the late Cenozoic, beryllium isotopes in seawater have stood out as an important exception. Beryllium isotopes have been interpreted to indicate stable continental weathering and/or denudation rates over the last 12 Myr. Here we present a Be cycle model whose results show that variations in the Be weathering flux are counterbalanced by near-coastal scavenging while the cosmogenic Be flux from the upper atmosphere stays constant. As a result, predicted seawater Be/Be ratios remain nearly constant even when global denudation and Be weathering rates increase by three orders of magnitude. Moreover, Be/Be records allow for up to an 11-fold increase in Be weathering and denudation rates over the late Cenozoic, consistent with estimates from other proxies. The large increase in continental weathering indicated by multiple proxies further suggests that the increased CO consumption by continental weathering, driven by mountain-building events, was counterbalanced by other geological processes to prevent a runaway icehouse condition during the late Cenozoic. These processes could include enhanced carbonate dissolution via pyrite weathering, accelerated oxidation of fossil organic carbon, and/or reduced basalt weathering as the climate cooled.
重建新生代大陆硅酸盐风化的历史对于理解地球的碳循环和温室历史至关重要。几十年来，大陆硅酸盐风化是否在新生代晚期增加，从而为冰川循环奠定了基础，一直存在争议。尽管海洋沉积物中许多风化的独立示踪剂（如 Li、Sr 和 Os 同位素）被解释为表明大陆硅酸盐风化率在新生代晚期增加，但海水的铍同位素一直是一个重要的例外。铍同位素被解释为表明过去 1200 万年大陆风化和/或剥蚀率稳定。在这里，我们提出了一个 Be 循环模型，其结果表明，Be 风化通量的变化被近海清除所抵消，而来自高层大气的宇宙成因 Be 通量保持不变。因此，即使全球剥蚀和 Be 风化率增加三个数量级，预测的海水 Be/Be 比值仍几乎保持不变。此外，Be/Be 记录允许新生代晚期 Be 风化和剥蚀率增加 11 倍，与其他示踪剂的估计值一致。多个示踪剂表明大陆风化的大量增加进一步表明，由造山事件驱动的大陆风化对 CO 的消耗增加，被其他地质过程所抵消，以防止新生代晚期出现失控的冰室条件。这些过程可能包括通过黄铁矿风化增强碳酸盐溶解、加速化石有机碳氧化，以及/或随着气候变冷减少玄武岩风化。
