Millennial‐scale fluctuations in subtropical northwestern Atlantic surface ocean hydrography during the mid‐Pleistocene

Abstract

[1] We use planktonic foraminiferal (Globigerinoides sacculifer and Neogloboquadrina dutertrei) d 18O records from subtropical North Atlantic Site 1056 to reconstruct millennial-scale changes in sea surface hydrography during marine isotope stages (MIS) 10 through 12 ($340-440 ka). The difference in the d18 O records provides a measure of the thermal gradient from the mixed layer to the seasonal thermocline (DT ml-tc ), which we use to constrain upper ocean stratification and infer northward ocean heat transport. Late Holocene G. sacculifer and N. dutertrei d18 O values place controls on how the foraminifera record recent ocean conditions at Site 1056. In the down core record we find extreme excursions toward G. sacculifer d 18O maxima and DT ml-tc minima throughout the entire time interval. These cold events have a similar amplitude throughout but are more frequent during the glacial than the interglacial intervals. During interglacial MIS 11 the excursions provide evidence for repeated cooling of the sea surface contributing to existing evidence for relative climate instability during interglacial intervals. During MIS 10, millennial-scale variability in the subtropical record is very similar to that observed in the subpolar North Atlantic at Site 980, perhaps related to a common forcing such as a large Northern Hemisphere ice sheet. Within the limitation of the age models the cold events of MIS 11 are almost synchronous with benthic foraminiferal d 13 C minima at subtropical North Atlantic Site 1063, which suggests a general surface to deep ocean link via thermohaline circulation during this interval of time.INDEX TERMS: 3344

The meridional temperature gradient in the eastern North Atlantic during MIS 11 and its link to the ocean–atmosphere system

“…According to modelling experiments, insolation impulse during Termination V was somewhat weak to drive glacial climate quickly towards full-interglacial conditions (Imbrie et al, 1993;Li et al, 1998). Additionally, European ice-sheet expansion was very large during MIS 12 (Donner, 1995), a fact which might help to explain not only the exceptional length of Termination V (Oppo et al, 1998;Bauch et al, 2000;Billups et al, 2004;Healey and Thunell, 2004;de Abreu et al, 2005) but also the difference in SST dynamics between polar and middle/low latitudes as revealed by us for the early interglacial phase. Our foraminiferal and alkenone SST data clearly demonstrate that the subtropical and the temperate climate zones experienced abrupt warming in accordance with insolation changes directly after Termination V. Such an early response to warming agrees with other investigations from the N. Atlantic (Oppo et al, 1998;McManus et al, 1999;de Abreu et al, 2005;Stein et al, 2009;Voelker et al, 2010;Rodrigues et al, 2011) and continental surroundings (Desprat et al, 2005;Nitychoruk et al, 2005;de Vernal and Hillaire-Marcel, 2008).…”

Variations in mid-latitude North Atlantic surface water properties during the mid-Brunhes (MIS 9–14) and their implications for the thermohaline circulation

“…On the western boundary of the subtropical gyre at ODP Site 1056, on the other hand, peak interglacial conditions were more variable in the surface water as evidenced by short-term incursions of colder surface waters, while thermocline conditions were relative stable and comparable to the Holocene (Chaisson et al, 2002;Billups et al, 2004). During the transition to MIS 10 the Gulf Stream waters at ODP Site 1056 experienced higher temperature variability linked to cooling episodes in the surface and thermocline waters (Chaisson et al, 2002;Billups et al, 2004), episodes that are contemporary with those recorded at ODP Site 980.…”

“…Its stable isotope values reflect hydrographic conditions at the base of the seasonal thermocline (Cléroux et al, 2007); conditions that are close to those in the winter mixed layer to which Ganssen (1983) relates G. inflata in the NW-African upwelling region. Details on stable isotope measurements for ODP Site 980 are given by Oppo et al (1998) and McManus et al (1999), for ODP Site 1056 by Chaisson et al (2002) and Billups et al (2004) and for ODP Site 1058 by Billups et al (2006) (Shackleton, 1974). Benthic and planktic stable isotope samples were measured in a Finnigan MAT 252 mass spectrometer at Marum (University Bremen, Germany).…”

“…In the subpolar North Atlantic at ODP Site 980 MIS 11c is recorded as a long, stable interval with relatively small SST variations McManus et al, 2003), while the later phase of MIS 11, contemporary with the built-up of continental ice sheets during the inception of glacial MIS 10, is marked by millennial-scale variability linked to ice-rafting events and southward incursions of arctic surface waters McManus et al, 1999). On the western boundary of the subtropical gyre at ODP Site 1056, on the other hand, peak interglacial conditions were more variable in the surface water as evidenced by short-term incursions of colder surface waters, while thermocline conditions were relative stable and comparable to the Holocene (Chaisson et al, 2002;Billups et al, 2004). During the transition to MIS 10 the Gulf Stream waters at ODP Site 1056 experienced higher temperature variability linked to cooling episodes in the surface and thermocline waters (Chaisson et al, 2002;Billups et al, 2004), episodes that are contemporary with those recorded at ODP Site 980.…”

“…As soon as this ice volume threshold was passed the Atlantic meridional overturning circulation (AMOC) became less stable resulting in oscillations between weaker and stronger AMOC modes. Most of the existing evidence for millennial-scale AMOC variability during the mid-Brunhes and its impacts on surface and deep waters is linked to the inception of MIS 10 (Poli et al, 2000;Billups et al, 2004;de Abreu et al, 2005;Hall and Becker, 2007;Martrat et al, 2007;Dickson et al, 2008;Stein et al, 2009). Only records from ODP Sites 980 and 1058 cover the older glacials with sufficient resolution (McManus et al, 1999;Flower et al, 2000;Billups et al, 2006;Weirauch et al, 2008) and here we present the previously unpublished planktic δ 13 C records for these sites.…”