Carbon isotopes and Pat/tTh response to forced circulation changes: a model perspective

Abstract. Understanding the ocean circulation changes associated with last glacial abrupt climate events is key to better assess climate variability and understand its different natural modes. Sedimentary Pa / Th, benthic δ 13 C and Δ 14 C are common proxies used to reconstruct past circulation flow rate and ventilation. To overcome the limitations of each proxy taken separately, a better approach is to produce multi-proxy measurements on a single sediment core. Yet, different proxies can provide conflicting information about past ocean circulation. Thus, modelling them in a consistent physical framework has become necessary to assess the geographical pattern, the timing and sequence of the multi-proxy response to abrupt circulation changes. We have implemented a representation of the 231 Pa and 230 Th tracers into the model of intermediate complexity iLOVECLIM, which already included δ 13 C and Δ 14 C. We have further evaluated the response of these three ocean circulation proxies to a classical abrupt circulation reduction obtained by freshwater addition in the Nordic seas under preindustrial boundary conditions. Without a priori guess, the proxy response is shown to cluster in modes that resemble the modern Atlantic water masses. The clearest and most coherent response is obtained in the deep (> 2,000 m) Northwest Atlantic, where δ 13 C and Δ 14 C significantly decrease while Pa / Th increases. This is consistent with observational data across millennial scale events of the last glacial. Interestingly, while in marine records, except in rare instances, the phase relationship between these proxies remains unclear due to large dating uncertainties, in the model the bottom water carbon isotopes (δ 13 C and Δ 14 C) response lags the sedimentary Pa / Th response by a few hundred years.