New Constraints on K-Pg boundary Environmental Changes with Lithium Isotopes

Abstract:

Climatic and oceanic perturbations associated with the Deccan Traps eruption and with the Chicxulub impact are still strongly debated. Because lithium isotopes significantly fractionate during low temperature processes, the composition of mineral phases formed across the K-Pg boundary can provide quantitative information concerning environmental changes at that time. In this study, we investigate in parallel the Li isotope composition of both marine authigenic smectite and benthic foraminifera over a time window of 3 Ma. Comparing both phases is important because the d⁷Li of foraminifera may be subject to vital effects during calcification, while authigenic clays are not. The objectives are: 1/ To determine the long-term variation of seawater d⁷Li due to changes in continental flux and alteration rate and 2/ To detect short-term variations of the ocean carbon chemistry, recorded by Li isotope fractionation during foraminifera growth.

Li isotopes are measured in clays by MC-ICP-MS, and in benthic foraminifera tests using the ims 1280 ion microprobe, at University of Hawaii. Initial results show a limited variation (<2‰) of clay d⁷Li across the Ir-rich layer, indicating little disturbance of the ocean Li at that time. Compared with 0-6 Ma seawater, K-Pg seawater d⁷Li was 5.5‰ lower. In contast, contemporaneous benthic foraminifera tests vary significantly on time scales much shorter than the oceanic residence time of Li (≈1.5 Ma), too short to attribute to changes in the d⁷Li of the global ocean Also, these variations precede deposition of the Ir-rich layer. These results strongly suggest changes in Li isotope fractionation through time. The d⁷Li of cultured foraminifera indicate a strong dependency on the DIC concentration, but no impact of pH and T on Li isotope fractionation during foraminifera growth. If these data are representative of natural conditions, then our results indicate significant short-term oscillations of ocean DIC before Chicxulub, possibly related to eruption events in the Deccan.