A new approach for modeling the Cenozoic oceanic lithium isotope paleo-variations : the key role of climate

Abstract:

The marine record of the ocean lithium isotope composition may provide important information constraining the factors that control continental weathering and how they have varied in the past. However, the equations establishing the links between the continental flux of Li to the ocean, its Li isotope composition and the ocean Li isotope composition are under-constrained, and their resolution are related to significant uncertainties. In order to partially reduce this uncertainty, we propose a new approach that couples the C and Li cycles, such that our proposed reconstruction of the Cenozoic Li cycle is compatible with the required stability of the exospheric carbon cycle on geological timescales. The results of this exercise show, contrary to expectations, that the Li isotope composition of rivers exerts only a minor influence on the d⁷Li of the ocean. In contrast, variations in the continental flux of Li to the ocean are demonstrated to play a major role in setting the ocean d⁷Li. We also provide evidence that Li storage in soils is an important element of the global Li cycle that cannot be neglected during the Cenozoic. Our modeling of the published foraminifera record highlight a close link between soil formation rate and indexes recording the climate evolution during the Cenozoic, such as foraminifera d¹⁸O and pCO₂ reconstructions. This leads us to propose that climate exerted a dominant control on soil production and therefore on continental weathering during the last 70 Ma.