Palaeoceanographic changes in the North Atlantic during the Mid-Pleistocene Transition (MIS 31-19) as inferred from planktonic foraminiferal and calcium carbonate records

Marine sediments from the Integrated Ocean Drilling Project (IODP) Site U1314 (56.36°N, 27.88°W), in the subpolar North Atlantic, were studied for their planktonic foraminifera, calcium carbonate content, and Neogloboqudrina pachyderma sinistral (sin.) δ13C records in order to reconstruct surface and intermediate conditions in this region during the Mid-Pleistocene Transition (MPT). Variations in the palaeoceanography and regional dynamics of the Arctic Front were estimated by comparing CaCO3 content, planktonic foraminiferal species abundances, carbon isotopes and ice-rafted debris (IRD) data from Site U1314 with published data from other North Atlantic sites. Site U1314 exhibited high abundances of the polar planktonic foraminifera N. pachyderma sin. and low CaCO3 content until Marine Isotope Stage (MIS) 26, indicating a relatively southeastward position of the Arctic Front (AF) and penetration of colder and low-salinity surface arctic water-masses. Changing conditions after MIS 25, with oscillations in the position of the AF, caused an increase in the northward export of the warmer North Atlantic Current (NAC), indicated by greater abundances of non-polar planktonic foraminifera and higher CaCO3. The N. pachyderma sin. δ13C data indicate good ventilation of the upper part of the intermediate water layer in the eastern North Atlantic during both glacial and interglacial stages, except during Terminations 24/23, 22/21 and 20/1. In addition, for N. pachyderma (sin.) we distinguished two morphotypes: non-encrusted and heavily encrusted test. Results indicate that increases in the encrusted morphotype and lower planktonic foraminiferal diversity are related to the intensification of glacial conditions (lower sea-surface temperatures, sea-ice formation) during MIS 22 and 20.

Solar forcing of North Atlantic surface temperature and salinity over the past millennium

There were several centennial-scale fluctuations in the climate and oceanography of the North Atlantic region over the past 1,000 years, including a period of relative cooling from about AD 1450 to 1850 known as the Little Ice Age1. These variations may be linked to changes in solar irradiance, amplified through feedbacks including the Atlantic meridional overturning circulation2. Changes in the return limb of the Atlantic meridional overturning circulation are reflected in water properties at the base of the mixed layer south of Iceland. Here we reconstruct thermocline temperature and salinity in this region from AD 818 to 1780 using paired δ18O and Mg/Ca ratio measurements of foraminifer shells from a subdecadally resolved marine sediment core. The reconstructed centennial-scale variations in hydrography correlate with variability in total solar irradiance. We find a similar correlation in a simulation of climate over the past 1,000 years. We infer that the hydrographic changes probably reflect variability in the strength of the subpolar gyre associated with changes in atmospheric circulation. Specifically, in the simulation, low solar irradiance promotes the development of frequent and persistent atmospheric blocking events, in which a quasi-stationary high-pressure system in the eastern North Atlantic modifies the flow of the westerly winds. We conclude that this process could have contributed to the consecutive cold winters documented in Europe during the Little Ice Age.