Evaluating the Extent of North Atlantic Deep Water and the Mean Atlantic δ13C From Statistical Reconstructions

Abstract

AbstractBenthic δ13C is often used to infer past changes in ocean circulation, though the interpretation of this proxy is difficult due to data scarcity and uncertainties. We present two methods for reconstructing the δ13C signal of North Atlantic Deep Water (NADW) and Antarctic Bottom Water and calculating the average oceanic δ13C values for the Atlantic Ocean based on δ13C from benthic foraminifera. The two simple statistical models are described and tested for the Holocene and the Last Glacial Maximum. The first statistical model consists of regressions of the δ13C data, which vary quadratically with depth and linearly with latitude. It differentiates between two regions, one for NADW and another for Antarctic Bottom Water. The second method consists of a hyperbolic tangent regression, which is bound asymptotically by the water mass source region averages (end‐members). To test the robustness of the statistical models, two isotope‐enabled climate models, the UVic ESCM and LOVECLIM, are sampled randomly, generating “pseudoproxies.” These are then used for testing the accuracy of the statistical models against the complete climate model δ13C outputs. We quantitatively compare the average δ13C and NADW depth against the original climate model outputs. We find that both statistical approaches are robust, regardless of the spatial distribution of the pseudoproxies, with the quadratic approach better able to capture the shape of NADW δ13C signal. Hence, this method can potentially be applied to different δ13C data sets to evaluate past changes in NADW.