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

Abstract

The marine record of 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 links between the continental flux of Li to the ocean, the continental 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 Cenozoic evolution of the Li isotope composition of rivers did not necessarily mimic the oceanic δ7Li rise. In contrast, variations in the continental flux of Li to the ocean are demonstrated to play a major role in setting the ocean δ7Li. We also provide evidence that Li storage in secondary phases is an important element of the global Li cycle that cannot be neglected, in particular during the early Cenozoic. Our modeling of the published foraminifera record highlights a close link between soil formation rate and indexes recording the climate evolution during the Cenozoic, such as foraminifera δ18O and pCO2 reconstructions. This leads us to conclude that the Li isotope record does not provide persuasive, unique evidence for erosional forcing of Cenozoic change because it could alternatively be consistent with a climatic control on soil production rates.