Holocene Climate Variability in Southern Greenland: Results from the Galathea 3 Expedition
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ROSA_2008:ROSA-2008 01/07/09 15:49 Side 77 Holocene climate variability in southern Greenland: results from the Galathea 3 expedition Niels Nørgaard-Pedersen, Naja Mikkelsen, Majken Djurhuus Poulsen and Aaju S. Simonsen The third Galathea expedition (Galathea 3) left Copenhagen The present study investigates climate changes in South in August 2006 for a circumnavigation of the globe with the Greenland in relation to the previously established pattern of aim of conducting more than 70 scientific programmes en Holocene palaeoceanographic and atmospheric changes as route. The first geological programme took place in South known from other North Atlantic palaeoclimatic studies. Greenland and included sampling of sediment cores and seis- mic profiling. The aim of the study is to obtain detailed knowledge about Holocene climate changes and the glacio - Study area and methods marine history. Glacially eroded and over-deepened fjords reaching depths of A number of cores with high sedimentation rates were col- 600–700 m dissect South Greenland around Qaqortoq and lected near Narsaq in South Greenland (Fig. 1). Analyses of Narsaq (Fig. 1). Sediment cores were collected in Bredefjord the cores elucidate the mid- to late Holocene climatic and and Narsaq Sund (Fig. 1) using a gravity corer and a box corer environmental evolution of the area, which is highly influ- at water depths of 270–670 m. A gravity corer with a 750 kg enced by the dynamic nature of the Greenland ice sheet and lead weight was used to collect sediment cores up to 6 m long changes of the North Atlantic climate. Two major ocean cur- with a diameter of 12 cm. Surface sediments were sampled rents, the cold East Greenland Current and the warmer with a cylindrical 30 cm diameter box corer. All cores were (Atlantic Water) Irminger Current influence the deep fjords subsampled for analyses of microfossils and age determina- of the region, and the two currents are in turn influenced by tion using accelerator mass spectrometry 14C dating. The both polar and lower latitude climate changes. cores were split lengthwise, and magnetic susceptibility was Evidence from ice cores and marine and lacustrine records in the North Atlantic region show a general climatic Greenland 46°W cooling from the mid- to late Holocene EGC Iceland 2000 as a response to decreasing summer insolation. About 5000 years ago, a transition occurred from the Holocene IC thermal maximum to the Neoglacial 1250 (Dahl-Jensen et al. 1998). The cooling Qassimiut lobe trend shows different timing and ampli- tudes in different parts of the North Narsarsuaq Atlantic region (Kaufman et al. 2004), Bredefjord Ga3-2 and an apparent counterphase between Ga3-10 Narsaq Ga3-11 the climatic conditions of western Narsaq Sund Ga3-7 Greenland and north-western Europe Qaqortoq during certain time periods has recently been suggested (Seidenkrantz et al. 2008). above 200 m 60°N Fig. 1. Map of South Greenland showing 200–800 m the main current systems and locations of 800–2500 East Greenland Current investigated cores. The waters of the East m Greenland Current and the warmer and more Irminger Current saline Irminger Current (deeper than 200 m) form a stratified water column along the coast 60°N 50 km 44°W of southern Greenland. © GEUS, 2009. Geological Survey of Denmark and Greenland Bulletin 17, 77–80. Available at: www.geus.dk/publications/bull 77 ROSA_2008:ROSA-2008 01/07/09 15:49 Side 78 measured in high resolution using a Bartington MS2E1 benthic foraminifera were picked for analyses. In sediment probe. Nine samples of benthic foraminifera were used for cores with a fairly uniform sedimentation rate and limited 14C dating at the Leibniz Laboratory for Radiometric Dating calciumcarbonate dissolution, the abundance of calcareous and Isotope Research in Kiel, Germany, and the box cores benthic foraminifera mainly reflects productivity related to were analysed for 210Pb and 137Cs content at the Gamma food supply. Dating Center, Department of Geography and Geology, Uni - versity of Copenhagen. A number of basic sediment parame- ters were determined including sediment wet-bulk density, Fjord environment and sedimentation water content, dry-bulk density and grain-size distribution. records Magnetic susceptibility measured at high resolution in general The records from the three gravity cores from Bredefjord are parallels the records of coarse-fraction content, and can there- dominated by episodic turbidite sedimentation with very fore be used for correlation and as a high-resolution measure high sedimentation rates of up to 1 cm/year (based on a cal- of coarse-fraction influx. In order to estimate the variation in ibrated 14C age from Ga3-7 (435 cm depth) of c. 465 years bottom-current velocity, detailed grain-size measurements BP) and is under influence from meltwater supply and gla cier (range 0.01–1000 µm) were carried out on bulk-sediment calving from the adjacent margin of the Greenland ice sheet samples using a Malvern Mastersizer 2000 laser particle-size (Fig. 1). For this study, we selected core Ga3-2 from the adja- analyser at the Department of Geography and Geology, cent Narsaq Sund (Fig. 1), which represents a continuous and University of Copenhagen. The weighted grain-size means of more slowly accumulating (c. 70 cm/1000 years) sedimenta- the sortable silt fraction (10–63 µm) were calculated and tion record covering the last c. 8000 years. The chronology is used as a proxy for bottom current. The number of lithogenic based on eight 14C ages (Fig. 2). The record is characterised grains >1 mm (coarse ice-rafted debris, IRD) was counted by a grain-size spectrum of dominantly suspension-settled using a dissecting microscope. The rationale for using grains mud modulated by variable bottom-current in fluence and >1 mm is a compromise between having an adequate num- IRD rain (Fig. 3). The sedimentation in this turbidite-shel- ber of grains and using grains so large that aeolian transport tered part of the fjord system has been controlled by a num- can be considered negligible. The IRD content is thus used as ber of related processes including influx of sediment-laden a proxy for rock fragments from melting of icebergs or sea ice meltwater plumes with fine-grained sediment, iceberg calv- deposited at the site. ing and coarse-sediment IRD transport and strength and The sediment-size fraction 125–1000 µm was separated changes of fjord circulation/water-mass stratification by by dry sieving, and from this fraction 300–400 calcareous Irminger Current and East Greenland Current water masses 0 100 Age model: 200 calibrated 14C dates ) m (2 sigma age range) 300 400 Depth (c 500 Neoglaciation Holocene thermal maximum 600 LIA MWPDA RWP 600 12 Fig. 2. The Narsaq Sund record (core Ga3-2). ) 500 10 g 14 s/ The chronology is based on C age determina- SI) n -6 400 8 rai tions of eight benthic foraminifera samples. g 300 6 ( sc. (10 sc. Sedi ment parameters shown: magnetic sus- u mm . S 200 4 ceptibility with five-point running mean, ice- gn Ma 100 2 IRD >1 rafted debris content (IRD >1 mm), weighted 0 0 mean grain size of sortable silt (bottom current 600 th ) proxy), and abundance of calcareous benthic ng m 30 500 g (µ s/ foraminifera/g dry sediment. The grey columns n 28 t stre m n 400 ea indicate periods of increased bottom currents m 26 rre u 300 C during the Neo gla ci ation. European historical 24 200 thic fora n climate periods (cf. Lamb 1995) are indicated: 22 100 Be LIA, Little Ice Age; MWP, Medieval Warm Sortable silt 20 0 0 1000 2000 3000 4000 5000 6000 7000 8000 Period; DA, ‘Dark Ages’; RWP, Roman Warm Age (years BP) Period. 78 ROSA_2008:ROSA-2008 01/07/09 15:49 Side 79 penetrating into the fjord system (Ribergaard 2007). During two following periods, at c. 2.3–1.7 ka and c. 1.3–0.8 ka, Modified Irminger water is found below c. 200 m water marked increases in bottom-water flow also occurred, and it depths and East Greenland waters above. During the summer is suggested that the fjord circulation during these time inter- a local brackish meltwater-rich layer characterises the upper- vals was enhanced by an increased outflow of meltwater. The most 10–15 m of the water column. timing of the latter two events may imply a link to climate changes during the European Roman Warm Period and the Medieval Warm Period (cf. Lamb 1995). The Narsaq Sund record Radiocarbon dating and palaeoecological studies show The lower part of the Narsaq Sund record, deposited from that the fjord system became ice-free at 13 to 9 ka and the about 8.0 to 4.8 ka (ka = thousand years before present), rep- East Greenland Current and the Irminger Current entered resents the Holocene thermal maximum and shows a pro- the deep and glacially eroded fjords (Weidick et al. 2004). gressive reduction in the supply of coarse-grained material, During the recession of the Greenland ice-sheet margin punctuated by increased influx of IRD at about 7.5–6.8 and from the region, large amounts of icebergs characterised the 6.5–5.7 ka (Fig. 2). The interval 5.7–4.8 ka appears to have fjords, but with the onset of the Holocene thermal maxi- been characterised by markedly lower amounts of IRD, and mum at 8–5 ka, the temperature increased and became a few it is suggested that this could reflect a situation in which sev- degrees higher than today. During the Holocene thermal eral of the present tidewater glaciers terminated on land. The maximum the Greenland ice sheet reached a minimum size.