The first appearance of Glacial North Atlantic Intermediate Water (GNAIW) during the Mid-Pleistocene transition*

The Mid-Pleistocene Transition (MPT) is arguably the most prominent shift in the Earth's climate system during the entire Pleistocene. It is characterized by a shift from a -40 kyr to a -100 kyr glacial cyclicity, going in hand with considerable growth of glacial continental ice-sheets. In search for potential drivers of these changes in the cryosphere, a slow-down of deep ocean circulation has been proposed as an important factor that might have reduced the atmospheric pCO2 level and thus created boundary conditions favourable for pertaining large ice -sheets. Additionally, as argued for the Last Glacial Maximum, enhanced production of intermediate waters might enhance intermediate to deep oceanic stratification and thus suppress CO2 release from the deep ocean and thereby reduce atmospheric greenhouse gas concentrations. To investigate potential changes in the North Atlantic intermediate water-mass configuration across the MPT we utilize 818O and 813C records obtained on the deep thermocline-dwelling foraminifera Globorotalia crassaformis from mid-latitude International Ocean Dis-covery Program (IODP) Site U1313. In conjunction with benthic and planktic 818O and 813C data from reference sites for Labrador Sea Water (IODP Site U1305) and intermediate water formation in the north-eastern North Atlantic (Ocean Drilling Program Site 982) our data show that major changes in the production and dispersal of intermediate water masses took place across the MPT. In particular, we note a shoaling of glacial intermediate waters commencing with Marine Isotope Stage (MIS) 22, which we interpret as the first occurrence of Glacial North Atlantic Intermediate Water, a prominent constituent of Late Pleistocene glacial water masses in the Atlantic Ocean. We further infer that the amount and vertical position of the warm and saline Mediterranean Outflow Water (MOW) had a major impact on the spatial configuration of North Atlantic intermediate water masses during the MPT, due to its capacity to modulate subsurface heat transport and isopycnal configuration. Our new findings thus provide evidence for a so far underestimated role of intermediate water circulation in shaping oceanic and potentially atmospheric changes during the MPT.