Radiocarbon simulations for the glacial ocean: The effects of wind stress, Southern Ocean sea ice and Heinrich events

Simulations of oceanic radiocarbon for the Last Glacial Maximum are presented, using a three-dimensional global ocean circulation model forced with glacial background states according to various reconstructions. We investigate the influence of sea surface temperatures, sea ice margins, wind stress and Antarctic sea ice formation on the glacial tracer distribution and meridional overturning circulation. The aim of these sensitivity studies is to reconcile available radiocarbon data from marine sediments with reconstructed sea surface temperatures and estimated sea ice production rates. Model runs with a modified freshwater balance in the Southern Ocean, mimicking increased brine release due to enhanced divergence of Antarctic sea ice, arrive at radiocarbon values close to observations. These experiments also yield abyssal temperatures and salinities which are consistent with recent inferences. In the simulation with the best agreement with radiocarbon observations, North Atlantic Deep Water export is reduced by 40% compared to present day, while Antarctic Bottom Water flow is intensified to similar strength in the South Atlantic. Transient simulations show that glacial freshwater discharge into the North Atlantic can cause abrupt increases of atmospheric radiocarbon as observed during Heinrich event 1. However, the effect is only significant in scenarios with a massive short-time discharge at the beginning which is followed by low-level freshwater input for the rest of the event, or if it is assumed that the meridional overturning circulation was already in a modern operational mode.