Silicate and carbonate weathering perturbation at the Eocene‐Oligocene Transition recorded by Mg isotopes

During the Eocene-Oligocene Transition (ca. 34 Ma), the Earth underwent a dramatic decline in atmospheric CO2, global cooling, a deepening of the carbonate compensation depth (CCD), and the formation of a permanent ice sheet on Antarctica. The expansion of Antarctic glaciers eroded the underlying bedrock and increased the weathering flux to the ocean. However, the role silicate and carbonate weathering play in atmospheric CO2 removal and the CCD through Ca2+ and alkalinity production is poorly understood. Magnesium isotopes (δ26Mg) are fractionated during carbonate and clay mineral formation and can be used to quantify the relative flux from silicate and carbonate weathering. Here, we report the δ26Mg composition of the carbonate, reactive (ferromanganese coatings), and residual (silicate) fraction of marine sediments from the Kerguelen Plateau (Ocean Drilling Program Site 738), near a major drainage system of the East Antarctic Ice Sheet, to explore the response of subglacial and shelf weathering to ice sheet expansion. The δ26Mg of the carbonate fraction (−2.29‰ to −0.95‰), reactive fraction (−0.36‰ to 0.10‰), and residual fraction (−0.05‰ to 0.55‰) display similar values to surface-dwelling calcareous nannofossils, deep-water ferromanganese nodules, and Antarctic bedrock, respectively. Isotope fluctuations in all three phases suggest that the formation of the Antarctic ice sheet drove efficient chemical weathering of underlying silicate bedrock, which was rapidly transported to the Southern Ocean, resulting in further CO2 drawdown, while a local sea-level low stand exposed carbonates on the Antarctic continental shelf to weathering, contributing to a deepening of the CCD.