Evolution of the oceanic circulation in the subtropical Atlantic across the Mid-Pleistocene Transition
Final Report Abstract
The aim of this project was to investigate if and how changes in the North Atlantic Subtropical Gyre (STG) circulation contributed to increased ice-sheet growth over the course of the Mid-Pleistocene Transition (MPT), ca. 1200-500 ka ago. For this purpose, thermocline temperature and salinity records have been generated on IODP Site U1313 (at the northern boundary of the STG) and ODP Site 1058 (western STG, within the Gulf Stream pathway), across the MPT. Thermocline temperatures and salinities were reconstructed by means of Mg/Ca analyses on the deep dwelling foraminifera G. crassaformis (U1313) and G. truncatulinoides (dextral). Comparison with existing surface SST and δ18O records, respectively, allowed to investigate upper-ocean stratification, which is an ample measure of STG circulation strength. These records were complemented by δ13C records of the thermocline dwellers, which in the case of Site U1313 provided novel insights into the impact of changes in the meridional heat and salt transport on deep- and intermediate-water formation and global ocean circulation. Contrary to the long-held perception that heat and salinity are transported northward at the surface via Gulfstream and North Atlantic Current, our data support novel insights from oceanographic data that point at the dominance of a subsurface trajectory along isopynals. This is particularly evident during Marine Isotope Stage (MIS) 22, a glacial that is characterized by massive ice-sheet growth and cold surface conditions in the mid-latitude North Atlantic. Our new data shows that the subsurface at Site U1313 (41°N) experienced anomalously warm conditions, providing a source of warm water that is released at the high-latitude North Atlantic, as evidence from warming at ODP Site 982 at 57°N. Proxy data further suggest that a driver of the thermocline warming and salinification might have been enhanced production of Mediterranean Outflow Water (MOW), dominating the intermediate water depths in the mid-latitudes today. Backed by numerical modelling, we further propose that the high-latitude warming generated excess atmospheric moisture that contributed to ice-sheet growth during the MPT. The dominant role of hydrographic changes in the subsurface of the mid-latitude eastern North Atlantic for the northward heat advection is further supported by the thermocline temperature record of Site 1058. This record is markedly different from that of Site U1313 and rather reflects western boundary current dynamics during the MPT, which apparently did not substantially contribute to the northward heat advection into high latitudes. This observation is rather unexpected and stresses the need for new conceptual models acknowledging isopycnal heat transport independent of Gulf Stream dynamics. Our MPT data from Site U1313 further indicate that changes in the heat and salt advection towards the loci of deep-water formation modified the buoyancy of the water masses in the North Atlantic and was a main driver of generating Glacial North Atlantic Deep Water (GNAIW). Comparison of the thermocline δ13C and δ18O records from Site U1313 with those from the northeastern and northwestern North Atlantic for the first time demonstrates that GNAIW became a regular constituent of the glacial hydrography not before MIS 22 and that it is primarily driven by deep convection in the Labrador Sea. In addition to the MPT records, we generated thermocline temperature and salinity data from Site U1313 for the last glacial and deglaciation (30–6 ka). First intended to serve as a reference for the new MPT record, the data for the first time revealed that Heinrich Events were accompanied by a drastic (~10°C during HE 1) increase in thermocline temperature in the mid-latitude central North Atlantic. The relation of this warming to proxies for ice-sheet stability and AMOC strength suggest that this subsurface warming is a response to AMOC weakening, and at the same time helped to suppress AMOC strength. The negative feedback on AMOC strength is a remarkable aspect as even state-of-the-art numerical models fail to simulate the prolonged AMOC weakening or even shutdown during Heinrich Stadials. The proxy data generated within the framework of this project thus demonstrate the need to more reliably simulate subsurface heat transport and its influence on AMOC strength.
Publications
- (2021): Oceanic subsurface heat pathway fueled ice-sheet growth across the Mid-Pleistocene Transition. Geophysical Research Letters, 28, e2020GL091899
Catunda, M.C.A., Bahr, A., Kaboth-Bahr, S., Foukal, N., Zhang, X., Friedrich, O.
(See online at https://doi.org/10.1029/2020GL091899)
