Wet to dry climatic trend in north-western Iberia within Heinrich events

Abstract

The direct sea–land correlation applied to core MD99-2331 retrieved from the north-western Iberian margin shows a two-phase pattern within Heinrich events 4, 2 and 1 in the ocean and in the adjacent landmasses. Changes between wet/cold and dry/cool conditions in the Iberian Peninsula detected during these extreme events cannot be explained by a simple oceanographic mechanism related to changes in the strength of the Atlantic Meridional Overturning Circulation. Here we propose an additional atmospheric mechanism able to produce this scenario based on the comparison between the MD99-2331 record and other available palaeoclimate sequences from the North Atlantic region (18–75°N and 0–75°W). The climatic asymmetry observed between mid- and subtropical eastern North Atlantic latitudes (wet/dry) and the Blake Outer Ridge (dry/wet) during H4, H2 and H1 can be explained by changes in the position of the Atlantic jet-stream. During the first phase of H4, H2 and H1 the Atlantic jet-stream was located further south following the southward displacement of the oceanic thermal front as far south as 35°–37°N. On the contrary, during the second phase of H4, H2 and H1 the jet-stream was located further north following the northward displacement of this thermal front as far north as 42°N. From the atmospheric point of view, these two phases are reminiscent of the present-day negative and positive prevailing modes of the North Atlantic Oscillation (NAO), respectively, but high-resolution studies of additional North Atlantic key sites and climate simulations are needed to confirm the hypothesis of a NAO-like mechanism operating on millennial timescales.

Introduction

In recent years, many studies have been carried out to understand the sources, trigger mechanisms and global impact of the well known episodes of massive iceberg discharges into the North Atlantic that occurred during the last glacial period (see Hemming, 2004). These extreme episodes, named Heinrich events, were first documented in several North Atlantic deep-sea cores between 40 and 50°N (IRD belt) (Fig. 1) (e.g. Heinrich, 1988, Bond et al., 1992, Broecker et al., 1992, Grousset et al., 1993). They were identified by the anomalous presence of ice-rafted detritus (IRD) transported to the ocean by icebergs drifting from the Laurentide (e.g. Bond et al., 1992, Broecker et al., 1992) and other Northern Hemisphere ice sheets (e.g. Grousset et al., 1993, Elliot et al., 1998) as well as by synchronous peaks of Neogloboquadrina pachyderma (s.) (e.g. Bond and Lotti, 1995, Hemming, 2004), of magnetic susceptibility (Grousset et al., 1993) and substantial sea-surface temperature decreases (Cortijo et al., 1997). These coarse fraction intervals, representing the well known IRD layers, have also been detected beyond the Ruddiman belt, at latitudes north of 50°N (e.g. Rasmussen et al., 1996, Elliot et al., 1998, Voelker et al., 1998, Van Kreveld et al., 2000, Elliot et al., 2001) as well as below 40°N (e.g. Lebreiro et al., 1996, Zahn et al., 1997, Bard et al., 2000, Chapman et al., 2000, de Abreu et al., 2003).

However, Heinrich events have left a complex pattern imprinted along the Iberian margin deep-sea cores composed of two main phases (e.g. Bard et al., 2000). The first is marked by a substantial sea-surface cooling, low or virtual absence of IRD and peaks of magnetic susceptibility while the second records high IRD deposition (e.g. Bard et al., 2000, Schönfeld et al., 2003, Thouveny et al., 2000). Western Iberian vegetation also shows a complex pattern associated with Heinrich events (H) 2 and 1 (Naughton et al., 2007) and with H4 and H5 (Sánchez Goñi et al., 2000) revealed by the succession of wet to dry plant associations.

Different hypotheses have been proposed to explain the complex pattern observed in the mid-latitudes of the North Atlantic during Heinrich events. The first hypothesis invokes the existence of multiple pulses of the Laurentide Ice Sheet (Abrantes et al., 1998), the second suggests the persistence of multiple IRD sources (Bard et al., 2000, Sánchez Goñi et al., 2000, Thouveny et al., 2000) and finally, the third involves mechanisms related with shifts in the polar front position (Chapman et al., 2000). Although these hypotheses can partially explain the complex pattern of Heinrich events in the Iberian margin, they cannot justify the succession of wet–cold and dry–cool conditions during H4, H2 and H1 in the neighbouring landmasses. Other mechanisms have been proposed to explain the extreme dryness observed within the second phase of Heinrich events in western Iberian Peninsula as the prevailing positive mode of the North Atlantic Oscillation (NAO) (Sánchez Goñi et al., 2002). However, this mechanism is not by itself a physical process responsible for the complex atmospheric pattern detected during H4, H2 and H1 in the Iberian Peninsula.

The aim of this work is therefore to propose possible mechanisms to explain the succession of wet–cold and dry–cool conditions in the Iberian Peninsula during H4, H2 and H1, which implies explaining the oceanic and atmospheric patterns associated with these extreme events in the north-western Iberian margin and adjacent landmasses. For this, we have performed a direct correlation between marine and terrestrial climatic indicators from core MD99-2331 retrieved in the Galician margin (north-western Iberian margin) and compiled all the available palaeoclimatic records from the North Atlantic region providing a regional scenario of Heinrich events in that region (Fig. 1; Table 1).