North Atlantic Current variability through marine isotope stage M2 (circa 3.3 Ma) during the mid‐Pliocene

Abstract

[1] The mid-Pliocene was an episode of prolonged global warmth and strong North Atlantic thermohaline circulation, interrupted briefly at circa 3.30 Ma by a global cooling event corresponding to marine isotope stage (MIS) M2. Paleoceanographic changes in the eastern North Atlantic have been reconstructed between circa 3.35 and 3.24 Ma at Deep Sea Drilling Project Site 610 and Integrated Ocean Drilling Program Site 1308. Mg/Ca ratios and d 18O from Globigerina bulloides are used to reconstruct the temperature and relative salinity of surface waters, and dinoflagellate cyst assemblages are used to assess variability in the North Atlantic Current (NAC). Our sea surface temperature data indicate warm waters at both sites before and after MIS M2 but a cooling of $2-3°C during MIS M2. A dinoflagellate cyst assemblage overturn marked by a decline in Operculodinium centrocarpum reflects a southward shift or slowdown of the NAC between circa 3.330 and 3.283 Ma, reducing northward heat transport 23-35 ka before the global ice volume maximum of MIS M2. This will have established conditions that ultimately allowed the Greenland ice sheet to expand, leading to the global cooling event at MIS M2. Comparison with an ice-rafted debris record excludes fresh water input via icebergs in the northeast Atlantic as a cause of NAC decline. The mechanism causing the temporary disruption of the NAC may be related to a brief reopening of the Panamanian Gateway at about this time.

Late Pliocene changes in the North Atlantic Current

“…In agreement with our interpretation, the palynological records at Sites 610 and 1308 suggest that the AF moved southward during MIS M2, but still remained to the north of 53°N (De Schepper et al, 2009). Our results show that a long-term weakening in the influence of the NAC is preceding MIS M2 and the development of a more glacial-like surface circulation in the North Atlantic.…”

“…Changes in surface water characteristics in the North Atlantic led changes in ice volume (benthic foraminiferal δ 18 O) by a few ka during the late Pliocene and early Pleistocene (De Schepper et al, 2009;Lawrence et al, 2009;Versteegh et al, 1996). This suggests that changes in the NAC and subsequent decrease in northward heat transport were a cause for the intensification of NHG rather than a result.…”

“…Intense Atlantic deep-water circulation during the Pliocene has been proposed as a cause for increased northward heat transport (Cronin, 1991;Dowsett et al, 1992;Robinson, 2009). However, changes in surface water characteristics lead the changes in deep-water formation and ice volume by a few ka (De Schepper et al, 2009;Lawrence et al, 2009;Versteegh et al, 1996). Thus, changes in deep-water formation could only have acted as positive feedbacks for the intensity of the NAC.…”

“…So far, none of these have given a complete satisfactory explanation and the ultimate cause remains an enigma. Nevertheless, recent studies suggest that cooling of the higher latitudes and increase in meridional SST gradient were crucial for the intensification of NHG (Berger and Wefer, 1996;Brierley and Fedorov, 2010;De Schepper et al, 2009;Lunt et al, 2008). This means that the NAC, by which the excess in heat from the tropics was transported northwards during the Pliocene, had to weaken and/or change its path during the intensification of the NHG in order to allow the higher latitudes to cool and the meridional SST gradient to increase.…”

“…Although covering only a small time interval around MIS M2, Mg/Ca based SST and dinoflagellate cyst assemblages from the northern North Atlantic (DSDP Site 610 and IODP Site U1308) also show a reduction in northward heat transport during this time period (De Schepper et al, 2009). In agreement with our interpretation, the palynological records at Sites 610 and 1308 suggest that the AF moved southward during MIS M2, but still remained to the north of 53°N (De Schepper et al, 2009).…”