MGPV EARLY GEOLOGIC CAREER AWARD LECTURE: ARE LITHIUM ISOTOPES GOOD TRACERS OF CONTINENTAL WEATHERING?

Chemical weathering plays an important role on continental crust evolution, as it preferentially removes soluble elements such as Li and Mg and can shift the crust composition towards more felsic compositions, thus helping to solve the crustal composition paradox. In addition, chemical weathering is critical in making Earth’s surface habitable by regulating the global carbon and oxygen cycles and by transporting nutrients into the oceans. Lithium isotopes have been developed to trace continental weathering, especially continental silicate weathering due to its low content in carbonate rocks. Using Li isotopic compositions in planktonic foraminifera as a seawater proxy, Misra and Froelich (2012) demonstrated that the δ⁷Li of seawater increased by 9‰ during the Cenozoic. They suggested this change was due to an increase in incongruent weathering, resulting in increased clay formation, which could elevate δ⁷Li in the riverine input into the ocean associated with uplift of the Himalaya around 40 Ma. However, the interpretation of this δ⁷Li curve in Cenozoic and other geological time is complicated by the many variables that influence δ⁷Li in seawater.

Here we presented coupled Li and Sr isotope data in marine carbonates to understand how ocean biogeochemical cycles may impacted most severe mass extinction in Earth history – End-Permian mass extinction. Previous studies have argued that increasing continental weathering was linked to End-Permian mass extinction. However, we observed the lithium isotopic composition of seawater remained constant until a sharp decrease, before the End-Permian Mass Extinction (~ 252 Ma). This massive drop cannot be explained by conventional idea like changing continental weathering rate based on our mass balance modeling. Rather, increased reverse weathering in the ocean was probably responsible for the quick drawdown and extremely low Li isotopic values in the End-Permian to early Triassic ocean. Importantly, this rapid increase in reverse weathering, and its associated changes in ocean chemistry, such as ocean acidification, might have contributed to the End-Permian mass extinction and a protracted biotic recovery in the Early Triassic.