Krause Alexander J, Sluijs Appy, van der Ploeg Robin, Lenton Timothy M, Pogge von Strandmann Philip A E
University College London, Earth Sciences, London, UK.
Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands.
Nat Geosci. 2023;16(8):730-738. doi: 10.1038/s41561-023-01234-y. Epub 2023 Jul 31.
The Middle Eocene Climatic Optimum (around 40 million years ago) was a roughly 400,000-year-long global warming phase associated with an increase in atmospheric CO concentrations and deep-ocean acidification that interrupted the Eocene's long-term cooling trend. The unusually long duration, compared with early Eocene global warming phases, is puzzling as temperature-dependent silicate weathering should have provided a negative feedback, drawing down CO over this timescale. Here we investigate silicate weathering during this climate warming event by measuring lithium isotope ratios (reported as δLi), which are a tracer for silicate weathering processes, from a suite of open-ocean carbonate-rich sediments. We find a positive δLi excursion-the only one identified for a warming event so far -of ~3‰. Box model simulations support this signal to reflect a global shift from congruent weathering, with secondary mineral dissolution, to incongruent weathering, with secondary mineral formation. We surmise that, before the climatic optimum, there was considerable soil shielding of the continents. An increase in continental volcanism initiated the warming event, but it was sustained by an increase in clay formation, which sequestered carbonate-forming cations, short-circuiting the carbonate-silicate cycle. Clay mineral dynamics may play an important role in the carbon cycle for climatic events occurring over intermediate (i.e., 100,000 year) timeframes.
始新世中期气候适宜期（约4000万年前）是一个持续约40万年的全球变暖阶段，与大气中二氧化碳浓度增加和深海酸化有关，中断了始新世的长期降温趋势。与始新世早期的全球变暖阶段相比，其异常长的持续时间令人费解，因为温度依赖的硅酸盐风化作用本应提供一个负反馈，在这个时间尺度上降低二氧化碳含量。在这里，我们通过测量一组富含海洋碳酸盐沉积物中的锂同位素比值（以δLi表示）来研究这一气候变暖事件期间的硅酸盐风化作用，锂同位素比值是硅酸盐风化过程的一个示踪剂。我们发现了一个约3‰的正δLi偏移——这是迄今为止在变暖事件中唯一确定的一次偏移。箱式模型模拟支持这一信号反映了全球从伴随次生矿物溶解的全等风化向伴随次生矿物形成的非全等风化的转变。我们推测，在气候适宜期之前，大陆有大量土壤覆盖。大陆火山活动的增加引发了变暖事件，但它是由粘土形成的增加维持的，粘土形成会隔离形成碳酸盐的阳离子，使碳酸盐 - 硅酸盐循环短路。粘土矿物动态变化可能在中间（即10万年）时间尺度上发生的气候事件的碳循环中发挥重要作用。
