The meridional temperature gradient in the eastern North Atlantic during MIS 11 and its link to the ocean–atmosphere system
Highlights
► A ocean–atmosphere interaction in the North Atlantic during MIS 11 is investigated. ► We reconstructed temporal and geographical SST patterns. ► We found a cold SST anomaly in the north and a warm SST anomaly in the subtropics. ► Our data evidence prolonged humid conditions in Sahel and in the Mediterranean. ► We conclude that negative North Atlantic Oscillation index dominated during MIS 11.
Introduction
While it is widely accepted in paleoclimate research that variations in orbital forcing and atmospheric greenhouse gas concentrations are actively involved in glacial–interglacial climate changes, an interglacial climate progression results from the combination of various feedback mechanisms of the land–ocean-atmospheric interaction initiated by some primary factors such as insolation (Berger, 1978, Bonelli et al., 2009).
In recent years, paleoceanographical investigations have been focused on climate dynamics during Marine Isotope Stage 11c (MIS 11c), because the orbital forcing background of this interglacial period shares certain similarities with the Holocene. Both MIS 11c and the Holocene are characterized by low eccentricity, and resulting from this, also by low precessional variations (Berger and Loutre, 2002, Loutre and Berger, 2003). Furthermore, ice cores indicate that CO2 concentrations during MIS 11c were similar to the Holocene, preindustrial levels (Petit et al., 1999, EPICA, Community, and Members, 2004, Raynaud et al., 2005). However, a comparison of the interglacial climate progress of these two periods in the North Atlantic reveals a number of distinct regional differences in geographical SST patterns and hydrological cycles. For instance, planktic foraminiferal data show that MIS 11c was characterized by warmer SSTs in the middle latitudes (Kandiano and Bauch, 2003, Kandiano and Bauch, 2007, de Abreu et al., 2005), whereas farther north in the adjacent Nordic seas, interglacial conditions appear to have been rather cold (Bauch et al., 2000, Bauch and Erlenkeuser, 2003, Helmke and Bauch, 2003). By analogy with the modern situation, one can assume that these cold conditions in the Nordic seas may implicate extended sea-ice cover in this region which in turn further influenced ocean–atmosphere interactions. In the subtropical NE Atlantic MIS 11c is recognized as a long-lasting period characterized by enhanced vegetation cover over northern Africa (Helmke et al., 2008). Although these climate conditions were probably comparable to the early Holocene Wet African Period, the cause for a rather prolonged duration during MIS 11c remains obscure considering those concepts that suggest the dynamics of precessional changes be the main modulator of monsoonal intensity (Prell and Kutzbach, 1987, McIntyre et al., 1989, Schefuß et al., 2003).
Because geographical SST patterns and evaporation/precipitation processes are strongly related to atmospheric circulation through changes in atmospheric pressure gradients, we intend to use detailed SST reconstructions in combination with precipitation-related records in order to investigate the ocean-atmospheric interaction during MIS 11c. As known from modern conditions, atmospheric circulation in the North Atlantic region is dominated by the North Atlantic Oscillation (NAO), a seesaw of sea level pressure between subtropical anticyclone and subpolar low pressure system, and is characterized by strong interannual to multidecadal variability (Barnston and Livezey, 1987, Hurrell, 1995). Although meteorological observational data allow investigation of NAO regularities only for the last 150 years, computer simulations have already indicated the existance of changes of NAO indices at least on multi-centennial time scales (Semenov et al., 2008). Moreover, paleoceanographical studies postulated a significant influence of the NAO on regional climate conditions for longer, suborbital to orbital timescales maintaining the analogy to the present-day conditions (Keigwin and Pickart, 1999, Rimbu et al., 2003, Felis et al., 2004, Kim et al., 2004, Bout-Roumazeilles et al., 2007). Following these ideas, we also refer to the modern general scheme of North Atlantic atmospheric circulation, assuming an existence of NAO-like oscillations during MIS 11c and using the modern scheme as a template in order to make some assumptions about ocean-atmospheric interaction during this period. To enable this, we reconstructed interglacial climate dynamics in polar, middle, and low latitudes of the eastern North Atlantic during MIS 11c, calculated meridional SST gradients and reconstructed moisture regime in the Mediterranean region. Further, we extend our reconstructions to atmospheric circulation, because the NAO variability is strongly related to geographic SST patterns and impacts hydrological cycles in European and West African sector (Hurrell, 1995, Rodwell et al., 1999, Bader and Latif, 2003).
So far, direct high-resolution SST records in the North Atlantic during MIS 11c are still very scarce (de Abreu et al., 2005, Kandiano and Bauch, 2007, Martrat et al., 2007, Stein et al., 2009, Voelker et al., 2010). In order to examine SST gradients in the North Atlantic a submeridional transect of five core sites was selected for the current investigations. Our SST reconstructions are based on planktic foraminiferal census data and alkenones. These data are further supported by benthic and planktic O- and C-isotope records and, at subpolar and polar sites, by ice-rafted debris (IRD) content and iron concentrations off NW Africa. Additionally, abundances of planktic foraminiferal species representing certain water masses were analyzed separately from conventional foraminiferal census counts following the strategy already applied during previous investigations (Kandiano and Bauch, 2002, Bauch and Kandiano, 2007).
Section snippets
Regional atmospheric–oceanographic conditions
The main axes of wind and storm track directions in the North Atlantic range between a subzonal and a submeridional orientation. The changes in between these two atmospheric “endmembers” are depending on the strength of atmospheric pressure gradients between the Iceland-low and the Azores-high which is calculated as NAO indices. A positive NAO mode is characterized by increased pressure gradients between Iceland-low and Azores-high (Hurrell, 1995). This results in stronger westerlies which
M23414, MD992277/PS1243, M23063
Core sites M23414, MD992277/PS1243 and M23063 underlie the northward continuation of the Gulf Stream (Fig. 1, Table 1). Core site M23414 is situated in the middle latitudes under the western edge of the North Atlantic Current (NAC), southward from where the Irminger Current (IC) branches off the NAC. The position of core site M23414 enables us to record changes in warm water input of the NAC and its possible deflections from the modern position. Moreover, core M23414 is highly suitable for SST
Planktic foraminiferal census counts and foraminiferal SST calculations
A minimum of 300 planktic foraminiferal test was identified in every considered sample. Following the strategy of Pflaumann et al. (1996) Neogloboquadrina pachyderma dextral (d) and intergrades between N. pachyderma (d) and N. dutertrei as well as Globorotala menardii and G. tumida were grouped together to avoid taxonomic controversies. The left and right-coiling forms of Globorotalia truncatulinoides were counted separately due to their different environmental preferences (Herman, 1972,
Records of oxygen isotopes and IRD
In cores from Sites 958 and 975, sharp changes in planktic and benthic δ18O towards lighter values are registered between 434 and 420 ka, a time interval which defines the period of Termination V. Records from our other cores presumably start within Termination V (Fig. 2). In core MD992277 from the Nordic Seas, benthic δ18O produced on Oridorsalis umbonatus show large-amplitude fluctuations of up to 1.07‰ during Termination V (Fig. 2). Such strong fluctuations in benthic δ18O are a distinct
Discussion
Orbital similarities between MIS 11c and the Holocene are based on a certain configuration of parameters with a cyclicity of about 400 ky. A dampened eccentricity characteristic for both intervals resulted in diminished precessional variations and, therefore, a decreased insolation in the high latitudes (Berger, 1978). According to modelling experiments, insolation impulse during Termination V was somewhat weak to drive glacial climate quickly towards full-interglacial conditions (Imbrie et al.,
Summary and conclusions
Employing sediment cores along a meridional transect within the northeastern Atlantic sector, ranging from polar to subtropical latitudes (~ 70 to 20° N), we have investigated the regional SST dynamics across interglacial stage MIS 11c. According to our results on the basis of various foraminiferal data and SST estimates, two phases are clearly recognizable during the period when full-interglacial conditions prevailed, i.e. the interval after the main deglaciation of Termination V and before
Acknowledgments
The authors are very grateful to Viktor Gouretski for a computation of the modern SST values. The manuscript has benefited from fruitful comments of Graziella Bozzano, Vladimir Semenov. The study was supported by Deutsche Forschung Gemeinschaft (DFG; Ba1367/8-1,2 PAINTER). All data presented in this paper are available at www.pangaea.de.
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