MIS 5e Southern Ocean September sea-ice concentrations and summer sea-surface temperatures reconstructed from marine sediment cores using a MAT diatom transfer function

Marine Isotope Stage (MIS) 5e is the latest period when global atmospheric temperatures were warmer than present and global sea levels were higher than present. Environmental conditions during MIS 5e therefore represent an important 'process analogue' for understanding the climatic mechanisms and responses active under future anthropogenic warming. MIS 5e diatom assemblages were reconstructed for nine Southern Ocean marine sediment cores (ANTA91-8, ELT17-9, MD03-2603, NBP9802-04, PC509, TPC287, TPC288, TPC290 & U1361A). A modern analog technique (MAT) transfer function (Crosta et al. 1998) was applied to the diatom assemblages to reconstruct the MIS 5e September sea-ice concentration (SIC) and summer (January – March) sea-surface temperature (SSST) for each sample. The MAT compares the relative abundances of 33 diatom species in each MIS 5e sample to the abundances of the same species in a modern reference dataset composed of 257 surface sediment samples (modern analogs) from the SO. Modern conditions for each surface sediment sample are interpolated on a 1o x 1o grid, with SSSTs from the World Ocean Atlas 2013 (Locarnini et al., 2013) and September SIC from the numerical atlas of Schweitzer (1995). The MAT was implemented using the “bioindic” R-package (Guiot and de Vernal, 2011), with chord distance used to select the 5 most similar modern analogs to each MIS 5e assemblage. A cut-off threshold, above which any analogs are deemed too dissimilar to the MIS 5e sample, is fixed as the first quartile of random distances determined by a Monte Carlo simulation of the reference dataset (Simpson, 2007). The reconstructed SSSTs have a Root Mean Square Error of Prediction (RMSEP) of 1.09 oC and an R2 of 0.96, and the reconstructed September SICs have a RMSEP of 9 % and an R2 of 0.93.