Holocene climate variability in northernmost Europe

Abstract

The sediments of a small lake on Nordkinnhalvøya, Finnmark, Norway, were investigated in order to test the hypothesis that this region was sensitive to centennial–millennial climatic fluctuations during the Holocene related to changes in ocean circulation. Sedimentation at the site began during the Younger Dryas, although the site chronology, developed using a series of ¹⁴C age measurements, reveals an early Holocene hiatus in accumulation. Pollen analysis confirmed that the regional vegetation responded to Holocene climatic variability at centennial–millennial time scales and provided data used to make quantitative palaeoclimate reconstructions. The latter indicate that marked changes in seasonality characterised Holocene climatic fluctuations. Intervals with warmer summers, higher temperature sums and higher precipitation, but cooler winters and generally reduced moisture availability, alternated with intervals with cooler summers, lower temperature sums, lower precipitation, warmer winters and greater moisture availability. The former conditions were more prevalent between ca 8950 and 3950 cal BP, whereas the latter were predominant before ca 8950 and since ca 3950 cal BP. Sediment geochemistry indicates minerogenic material deposited in the lake was probably derived from two or more distinct sources or transport pathways that differed in their responses to palaeoclimatic conditions. A series of cryptotephras were located, although the small size of the shards rendered them unsuitable for electron microprobe analyses. Time-series analysis of pollen analytical and sediment geochemical data indicates that each exhibits statistically significant periodic behaviour (at periods of ca 190, 410, 1050, 1650 and 1810 yr). The periods detected suggest this behaviour may reflect regional expression of climate system responses to solar variability and/or of effects upon tides and ocean circulation of periodic lunar orbital variation. Comparison with records of fluctuations in ocean thermohaline circulation strength indicate some concordance with respect to timing of warmer and cooler intervals, but also some differences. The 8.2 ka event, that is evident in marine records from the Barents Sea, is clearly expressed by both the palaeovegetation and geochemical records. Distinctive temporal behaviour of the palaeovegetation and of different geochemical components indicates complexity in the underlying causes and mechanisms of regional climatic variability; ocean circulation variability alone cannot account for the complex climatic variability observed.

Introduction

Outer coastal Finnmark has an anomalously mild climate for its latitude as a result of proximity to one of the ultimate branches of the North Atlantic Current (Fig. 1(a)). This current carries warm Atlantic water north to the Arctic basin and is a key component of global thermohaline circulation (THC) (Broecker, 1992). Recent studies of north-east Atlantic and Barents Sea marine sediments provide evidence of millennial variations in strength of the THC, and associated surface currents, during the Holocene. It has been hypothesised that these variations relate to solar forcing (Bianchi and McCave, 1999; Bond et al., 2001; Duplessy et al., 2001; Sarnthein et al., 2003). Given the influence of the warm surface current in the southern Barents Sea on the present climate of northern Finnmark, past variations in this current are likely to have led to regional climatic fluctuations. We hypothesise that when THC is relatively strong, this surface current penetrates further east, importing more thermal energy to the region and leading to relatively mild winter temperatures and warm summer conditions, with the converse expected when THC is relatively weak. It has been shown that sea ice extended further south and west across the Barents Sea than at present during periods of the Holocene when THC was relatively weak (Voronina et al., 2001), extreme southward extensions of sea ice perhaps occurring when little or no North Atlantic Current water penetrated east around North Cape.

Holocene sea-surface temperature variations of 1–3 °C have been reconstructed for the Barents Sea (Voronina et al., 2001; Sarnthein et al., 2003). The associated regional climatic variation is likely to have affected the vegetation of northern Finnmark; we hypothesise that the legacy of such climatic variation can be seen in the region's present vegetation. The predominant present vegetation is tundra (Fig. 1(b)). However, isolated Betula pubescens ssp. czerepanovii (Mountain Birch) (nomenclature for vascular plants follows Mossberg and Stenberg, 2003) stands occur as far north as Oksefjorden on Nordkinnhalvøya (70°57′41″N, 27°34′12″E) in situations likely to experience local climatic conditions warmer than those typical of the region (BH personal observations). Further south in Finnmark, isolated Pinus sylvestris (Scots Pine) stands, such as that near Börselv on the east side of Porsangen (70°18′45″N, 25°34′48″E), occur well north of the regional ecotone between Pinus and Betula dominated woodlands (Fig. 1(b)). These stands also occupy positions in the landscape likely to experience local climatic conditions warmer than those typical of the region (BH personal observations). We thus hypothesise that these outlying Pinus and Betula stands are relicts from past intervals with warmer than present regional climate. We further hypothesise that for these stands to become established, the latitudes of the ecotones between Betula woodland and tundra and between Pinus and Betula woodlands, and thus the extent of the two woodland types, varied during the Holocene in response to regional climatic variations. We expect the two ecotones to have occupied more northerly positions, with more extensive Betula woodlands in northernmost Finnmark where tundra predominates at present, when THC was strong and the converse when THC was weak. Given the strong climatic gradient at present between the relatively oceanic coastal fringes of northernmost Finnmark and the extremely continental interior of Finnmarksvidda, as well as the hypothesised underlying mechanism of Holocene climatic fluctuations in the region, we further hypothesise that coastal localities were most sensitive to regional climatic variations.

Previous studies of Lateglacial and Holocene vegetation history in Finnmark (see e.g. Hyvärinen, 1975, Hyvärinen, 1976; Prentice, 1981, Prentice, 1982; Seppä, 1996) have shown the ecotones between Pinus and Betula woodlands and between Betula woodlands and tundra occupied more northerly positions during the early and mid-Holocene. None of these studies, however, examined either the extent to which these ecotones may have varied in position in response to millennial climatic fluctuations, or documented the palaeovegetation and palaeoenvironment of the northernmost coastal fringes of the region where the most sensitive response to such millennial fluctuations might be expected.

In order to address our hypotheses, we have undertaken high-resolution investigations of the Holocene palaeovegetation and palaeoenvironment record preserved in the sediments of a small lake on Nordkinnhalvøya, close to the Barents Sea coast of central northern Finnmark. Palynological data were used to infer both local vegetation changes and changes in the major vegetation patterns in Finnmark; they also provided the basis for quantitative palaeoclimate reconstruction. Sediment geochemistry provided further palaeoenvironmental evidence. Systematic analyses located cryptotephras in the sediments. AMS ¹⁴C age determinations of terrestrial plant macrofossils enabled a chronology to be derived for the record. Time-series analyses were used to investigate whether periodic millennial or sub-millennial variability seen in THC strength and in other factors potentially driving regional climatic fluctuations were apparent in the palaeovegetation and palaeoenvironment records.