Carbon isotopes and PaĝTh response to forced circulation changes: a model perspective

Understanding the ocean circulation changes associated with abrupt climate events is key to better assessing climate variability and understanding its different natural modes. Sedimentary Pa=Th, benthic δ¹³C and Δ¹¹⁴C 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 multiproxy 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 and the timing and sequence of the multiproxy response to abrupt circulation changes. We have implemented a representation of the ²³¹Pa and ²³⁰Th tracers into the model of intermediate complexity iLOVECLIM, which already included δ¹³C and Δ¹¹⁴C. 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. 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 (> 2000 m) northwest Atlantic, where δ¹³C and Δ¹¹⁴C 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 isotope (δ¹³C and Δ¹¹⁴C) response lags behind the sedimentary Pa=Th response by a few hundred years.