MOVING FORWARD WITH MARINE ISOTOPIC RECORDS GOING BACK IN TIME - WHAT INSIGHTS INTO CENOZOIC CONTINENTAL WEATHERING?

Marine isotopic records shed light on continental inputs to the oceans and on the associated chemical weathering processes that regulate the evolution of Earth’s surface environment over geologic time. Recent additions to the isotopic library for the late Cenozoic include the record of seawater stable Li isotopes and ¹⁰Be/⁹Be ratios. Prior interpretations of these two records provide contrasting pictures about how global weathering changed during the Cenozoic cooling, which is one of the remarkable transitions of recent geologic time. This submission will consider some of the complexities and some of the potential insights from the Cenozoic Li and late Neogene Be isotope records, along with records from radiogenic isotope systems including strontium and osmium isotopes.

In particular, we have re-analyzed the global oceanic Li cycle and identified the potential importance of variable fractionation during removal of Li from seawater as an important control on the past isotopic composition of the oceans. Considering a range of scenarios for Li supply and removal, we conclude that plausible explanations for the ~9‰ rise in seawater δ⁷Li most likely require significant increase in the δ⁷Li of continental inputs. However linking changes in the Li isotope composition of continental inputs to changes in global weathering is not straightforward.

We assess the meaning of Li isotope variations in the context of other isotopic records and dynamics of the long-term global carbon cycle. Is decreasing weathering intensity consistent with the ¹⁰Be/⁹Be record, which implies relative stability in long-term weathering flux at least for the Neogene? If radiogenic Os reflects the weathering of shales that contain abundant pyrite and organic carbon, does the increasingly radiogenic seawater Os reflect enhanced release of CO₂ from the sedimentary reservoir? How important might such C release be in reconciling the Cenozoic records? We consider current knowledge that helps address these questions and speculate on how further work might answer them.