Strengthening Atlantic Inflow Across the Mid-Pleistocene Transition

The development of larger and longer lasting northern hemisphere ice sheets during the mid-Pleistocene Transition (MPT) coincided with global cooling. Here, we show that surface waters of the north-eastern Atlantic actually warmed across this interval (similar to 1.2-0.8 Ma), which we argue reflects an increase in the north-eastward transport of heat and moisture via the North Atlantic Current (NAC) into the Nordic Seas (the Atlantic Inflow). We suggest that simultaneous cooling and warming along the north-western and south-eastern margins (respectively) of the NAC during Marine Isotope Stage 28 (similar to 995 ka) reflected the increasing persistence of northern ice sheets as Atlantic Inflow increased. This resulted in a diachronous shift from similar to 40 to similar to 100 kyr cyclicity across the North East (NE) Atlantic as the growing influence of northern ice sheets spread southwards; to the north-west of the NAC the first "100 kyr" cycle preceded Termination 12 (similar to 960 ka), while on the south-eastern margin of the NAC the transition occurred similar to 100 kyr later. Exploratory climate model simulations suggest that increasing Atlantic Inflow at this time could have accelerated ice sheet growth because pre-existing moderately sized ice sheets allowed the positive effect of increasing precipitation to outpace melting. In addition, we propose that the dependence of post-MPT ice sheets on moisture transport via the Atlantic Inflow may ultimately contribute to their apparent vulnerability to insolation forcing once a critical size threshold is crossed and high latitude ice sheets become starved of a vital moisture source.