Hafnium isotope evidence for enhanced weatherability at high southern latitudes during Oceanic Anoxic Event 2

The Oceanic Anoxic Event 2 (OAE 2; ca. -94 Ma) represents one of the most extreme carbon cycle perturbations of the Phanerozoic, which coincided with major environmental and climate reorganization in both terrestrial and marine realms. Chemical weathering of continental silicate rocks is thought to have played a crucial role during OAE 2, through enhanced release of bio-essential nutrients to the ocean, promoting high rates of marine primary production and organic carbon burial, but also due to its effect on atmospheric CO2 drawdown, which altogether possibly drove the OAE 2 termination. Yet, the evolution of continental chemical weathering during OAE 2 remains poorly defined, especially in high-latitude regions. In this study, we present a combined hafnium-neodymium isotope investigation of the clay-size detrital fraction (A epsilon Hfclay) of late Cenomanian to early Turonian sediments from the southwest Australian margin, at a site (International Ocean Discovery Program U1516) located in the southern high latitudes (-62 degrees S) during the late Cretaceous. The reliability of A epsilon Hfclay as a proxy for continental chemical weathering in ancient anoxic marine sediments was assessed by analyzing a suite of samples retrieved from methanogenic sediments experiencing marine silicate weathering at ocean margins, suggesting negligible effect of reverse weathering on hafnium-neodymium isotope compositions. At Site U1516, the early stage of OAE 2 was characterized by relatively low A epsilon Hfclay values (-5.9 +/- 2), typical of reduced chemical weathering in nearby continental regions. At the onset of the most prominent carbon isotope excursion, an abrupt decrease in A epsilon Hfclay points towards accelerated export of poorly weathered sediments resulting from the abrupt reactivation of river systems in southwest Australia. This period was followed by a pronounced A epsilon Hfclay shift towards positive values, indicative of intensifying chemical weathering conditions during the OAE 2 interval showing the highest delta 13C anomaly. Based on these results, we posit that enhanced hydrological cycle, most likely caused by a southward shift of the westerlies, led to a large increase in weatherability at southern high latitudes during peak OAE 2 warmth. This finding provides empirical support for the potential role played by high-latitude weathering systems in driving the termination of OAE 2, via weathering-driven consumption of atmospheric CO2 and accelerated riverine fluxes of nutrients leading to enhanced organic carbon burial in marine sediments.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

The Oceanic Anoxic Event 2 (OAE 2) during the late Cretaceous was characterized by extreme carbon cycle perturbations and major environmental and climate reorganization in terrestrial and marine realms. Chemical weathering of continental rocks played a crucial role in OAE 2, promoting marine primary production and organic carbon burial, and potentially driving the termination of OAE 2 through atmospheric CO2 drawdown. This study focuses on the evolution of continental chemical weathering in high-latitude regions during OAE 2.