The response of NW Iberian vegetation to North Atlantic climate oscillations during the last 65 kyr

Abstract

Pollen and oceanographic data from deep ocean core MD95-2039 provide a centennial to millennial scale record of conditions offshore and of the vegetation of north-west Iberia for the period 10–65 ka. The planktonic oxygen isotope record of this core, reflecting predominantly sea surface temperature (SST), shows a pattern of millennial-scale oscillations that is very similar to climatic changes recorded by the Greenland ice core records over the same interval. In turn, tree populations show a pattern of rapid expansions and contractions that follow the pronounced and abrupt isotopic shifts recorded offshore. Through Marine Isotope Stage (MIS) 3, this millennial-scale pattern of vegetation change, alternating between steppe and open woodland, is superimposed on a longer-term pattern of shrinking ericaceous heathland and decreasing size of successive interstadial tree populations. Trees persisted during the Last Glacial Maximum (LGM), at greater abundance than during many of the coldest episodes of MIS 3. This agrees with the marine data which indicate that LGM sea surface temperatures here were significantly warmer than the minima recorded in MIS 3. Our combined marine-terrestrial record, together with data from nearby sequences, provides a stepping stone between terrestrial sequences and the Greenland ice core and North Atlantic marine records. This will permit a better understanding of the behaviour of vegetation across different regions at several scales of climatic forcing.

Introduction

The abyssal depths beyond the Iberian continental shelf (Fig. 1) provide ideal conditions for the accumulation of marine pollen records. The continental shelf is narrow so deep water sedimentation takes place close to land while numerous rivers bring pollen to the area (Fig. 2). Since the first long deep ocean sediment cores were taken here in 1995 a great deal of high resolution data has been gathered, providing insight into the detailed palaeoceanography of the region (e.g. Thomson et al., 1999; Schönfeld and Zahn, 2000; de Abreu et al., 2003; Schönfeld et al., 2003), the pattern of vegetation development in Iberia and, most significantly, the precise temporal relationship between the two (e.g. Sánchez Goñi et al., 1999, Sánchez Goñi et al., 2000, Sánchez Goñi et al., 2002; Roucoux et al., 2001; Turon et al., 2003). The climatic and oceanographic changes that took place in the North Atlantic region during the last 65 kyr were dramatic and rapid, with major reorganisations taking place in a matter of years (Taylor et al., 1993; Schönfeld et al., 2003). Correlation between separate terrestrial and marine sequences must rely either on the assumption of synchronicity, which would hide the true temporal relationship, or on independent chronologies, whose error margins of hundreds of years are too large to allow the relationship to be discerned at the fine scale required. The combined terrestrial-marine sequences of the Portuguese margin provide a solution to this problem since they permit direct, in situ correlation. Avoiding the uncertainties inherent in correlation between sequences preserved in different contexts, the precision possible is limited only by the resolution of the records.

The results of palynological analysis of one of these deep ocean cores, MD95-2039 (Fig. 1), are presented here. The sequence provides a centennial to millennial scale record of the vegetation of north-west Iberia, and of conditions offshore, for the period 10 to 65 ka. Pollen records from other deep ocean cores obtained on the Iberian margin represent more southerly pollen catchments under greater Mediterranean climatic influence (Combourieu Nebout et al., 2002; Sánchez Goñi et al., 2000, Sánchez Goñi et al., 2002). Encompassing the period from the end of MIS 4 to the beginning of MIS 1, the MD95-2039 pollen sequence preserves a record of vegetation sensitivity to a wide range of conditions: the vegetation response to glacial, interglacial, stadial and interstadial climatic states is recorded. Numerous examples of the Heinrich Events (HE), Dansgaard-Oeschger (DO) cycles, including stadial and interstadial states, that characterise the last glacial period in the North Atlantic are represented. This replication allows us to explore the extent to which each type of climatic/oceanographic event differs from the others in terms of its expression in the palaeovegetation record.

The Late Glacial and early Holocene pollen record of MD95-2039 is not considered in detail here as other marine (Lézine and Denèfle, 1997; Boessenkool, 2002; Turon et al., 2003) and terrestrial (e.g. van der Knaap and van Leeuwen, 1995, van der Knaap and van Leeuwen, 1997) pollen records nearby have already provided a detailed picture of the vegetation history of this interval. However, the pollen data for this interval are shown in the figures as they provide a useful reference point for interpretation of the rest of the marine sequence, since the spatial and temporal patterns of vegetation change they represent are relatively well known.