Tracking ice-sheet dynamics by detrital feldspar Pb-isotope and ⁸⁷Rb/⁸⁷Sr dating during the Middle Miocene Climatic Transition, Weddell Sea, Antarctica

Antarctic ice-sheet instability is recorded by ice-rafted debris (IRD) in mid- to high-latitude marine sediment, especially throughout climate transitions. The middle Miocene climatic transition (MMCT), 14.2 to 13.8 Ma, which marks the end of a significant warm period during the mid-Miocene, saw a rapid cooling of ca. 6-7 °C in the high-latitude Southern Ocean. This climatic shift was also accompanied by a global δ¹⁸O excursion of ca. 1‰, indicating a time of global cooling and significant Antarctic ice expansion (Shevenell et al., 2004). The MMCT is recorded by numerous IRD-rich sediment horizons in deep-sea sediment cores around the Antarctic margin, reflecting iceberg calving during times of ice-sheet instability. Resolving the locations of iceberg calving sites by detrital provenance analysis during the MMCT is also an important tool for forecasting effects of anthropogenic climate change.

Here we present results of a multi-proxy provenance study by using K- and plagioclase feldspar, selected due to their relative abundance in clastic sediment, and tendency to incorporate Rb (K-feldspar only), Pb, and Sr at analytically useful concentrations, thus enabling source-terrane fingerprinting. While Pb-isotope fingerprinting is an established method for provenance analysis of glaciogenic sediment (Flowerdew et al., 2012), the combination with in-situ Sr-isotope fingerprinting and ⁸⁷Rb/⁸⁷Sr dating is a novel approach. These techniques are applied to deep-sea core ODP113-694, recovered from the Weddell Sea, ca. 750 km from the continental rise in 4671 m of water. This location is ideal, as it acts as a major iceberg graveyard making it a key IRD depocenter (Barker, Kennett et al., 1988). Within the core, several IRD layers were identified and analysed with preliminary depositional ages of 14.09 to 14.26 Ma.

Our findings are consistent with predictions made by recent palaeo-ice sheet models (eg., Gasson et al. 2016), which predict the development of sizeable and discrete embayments around the continent, including the Weddell Sea. We argue that the IRD derived from the unstable sector associated with this embayment formation at the time.