Wet to dry climatic trend in north-western Iberia within Heinrich events
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).
Section snippets
Area of study and environmental setting
The MD99-2331 deep-sea core was recovered from the Galician margin (42° 09′ 00 N, 09° 40′ 90 W) at ~ 100 km from the coast and at 2110 m water depth (Fig. 1). This region is at present-day dominated by the surface Portugal Current System (PCS) which is composed of a slow equatorward current in the open sea and a fast, seasonally reversing, coastal current (Ambar and Fiúza, 1994). At intermediate depths, the Mediterranean Outflow Water (MOW), characterised by high salinity and relatively warm
Materials and methods
Core MD99-2331 was retrieved using a giant CALYPSO corer during the GINNA (IMAGES V) oceanographic cruise on board the R/V Marion Dufresne (Fig. 1). This sedimentary record, mainly composed of hemi-pelagic clay, is 37.2 m long and covers the Marine Isotopic Stages (MIS) 7 to 1. In this study, we will focus on the core section covering the 40–15 ka interval, with 43 cm kyr− 1 to 30 cm kyr− 1 sedimentary rates providing a high-resolution palaeoclimatic record for this time period off north-western
Delimitation of Heinrich events
Previous studies performed on several deep-sea cores from western Iberian margin have shown that Heinrich events are characterised by a synchronous substantial drop of sea-surface temperatures (SSTs) and expansion of polar planktic foraminifera N. pachyderma (s.) which precede the maximal deposition of ice-rafted detritus (IRD) in this region (e.g. Bard et al., 2000, de Abreu et al., 2003). Although, IRD were virtually absent in this region at the beginning of Heinrich events, the presence of
Mapping patterns of Heinrich events (H4, H2 and H1) in the North Atlantic region
The comparison of MD99-2331 palaeoclimatic record (Fig. 3, Fig. 4) with other North Atlantic deep-sea core data (18–75°N and 0–75°W) (Fig. 1; Table 1) provides insights into the spatial distribution of patterns left by Heinrich events (H4, H2 and H1) in the North Atlantic region (Fig. 5).
Conclusions
The comparison between the Iberian margin multi-proxy record (MD99-2331) and other available palaeoclimate sequences from the North Atlantic region (18–75°N and 0–75°W) allows us to recognise a two-phase pattern within Heinrich events 4, 2 and 1 in the mid- and subtropical latitudes outside the IRD belt and to propose potential mechanisms explaining this complex scenario. H4, H2 and H1 events are characterised by two main phases in the regions outside of the IRD belt between 45 and 18°N. The
Acknowledgements
This study is a contribution to ARTEMIS, RESOLuTION (ESF-EUROCORES Programme EUROCLIMATE), IDEGLACE (ANR-05-BLAN-0310-01) and ECLIPSE projects and has been partially supported by the FCT, ICCTI, INSU (PNEDC) and CEA.
We would like to thank the Institut Polaire Francais Paul Emile Victor (IPEV) and the R/V Marion Dufresne for the MD99-2331 deep-sea core recovering. Furthermore we thank Fátima Abrantes, Heikki Seppä, Constancia Lopéz-Martinez and the two anonymous reviewers for their valuable
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