A Late Holocene record of landscape degradation from Heygsvatn, the Faroe Islands
Introduction
The Faroe Islands are ideally placed to study the natural variability of Late Holocene climate (Humlum and Christiansen, 1998a). The islands are located in the middle of the northward flowing warm, Atlantic surface water and the cold Norwegian Sea overflow water that flows southward through the Faroese channel (Fig. 1) (Kuijpers et al., 1998). Thermohaline circulation generates cold bottom water flow and is, therefore, responsible for release of heat to the atmosphere (Rasmussen et al., 2002a). Changing intensity of the North Atlantic thermohaline circulation, and the consequent north–south shifting of the polar front, is believed to have occurred throughout the Holocene and is thought to be a key factor regulating global climate change (Ruddiman and McIntyre, 1981, Koç et al., 1993, Björck et al., 1996, Rasmussen et al., 1997, Rasmussen et al., 2002b, Andersen et al., 2004). During cooler periods in the Holocene, a decline in North Atlantic deep water running through the channels near the Faroe Islands results in less warm surface water being carried north (Kuijpers et al., 1998) and has a near instantaneous impact on climate in the Faroe Islands (Humlum and Christiansen, 1998a). In the absence of continental effects, the islands should be particularly responsive to changes in atmospheric circulation and ocean current restructuring, and therefore changes recorded by lake sediments and peat sequences in the Faroe Island region are likely to be of global significance.
There are numerous small lakes on the Faroe Islands (Christoffersen et al., 2002), which have the potential to record palaeoclimate information at high resolution (decadal to centennial timescales). Importantly, cultural impacts in the Arctic and more remote islands of the North Atlantic tend to be considerably reduced with respect to NW Europe (e.g. Rasmussen and Bradshaw, 2005, but see Fredskild, 1992). The first cultural disturbances (landnám) in the Faroe Islands date to between 600 and 800 years ago (Hannon et al., 1998, Hannon and Bradshaw, 2000, Dugmore et al., 2005) and this late date of human disturbance is a considerable advantage in recovering largely undisturbed palaeoclimate records. Accordingly, lakes in the Faroe Islands are increasingly being used for palaeoclimate studies (Björck et al., 2001, Hannon et al., 2003, Andresen et al., 2006).
Accurate interpretation of palaeolimnological climate records requires an understanding of the interaction between lake and catchment processes. Aquatic systems may respond directly to climate change via water mass balance (Fritz et al., 2000), water temperature (Livingstone and Dokulil, 2001) or thermal stratification (George and Taylor, 1995). However, aquatic response to disturbance is also modified by catchment processes connected with geology, soils, vegetation and relief. These catchment characteristics interact to determine landscape response to disturbance, which is inherently variable over a range of temporal and spatial scales (Thomas, 2001). Interactions between climate, catchment hydrology, soils, vegetation and aquatic systems are therefore important processes to acknowledge in order to accurately identify climate signals from lake sediments, particularly when the magnitude of such change may be more subtle than that experienced during glacial–interglacial transitions. In such situations, a combination of physical and biological proxies can be useful in interpreting lake sediment records.
In this study we used palaeolimnological techniques to determine mid-Late Holocene environmental change on the island of Suðuroy. We used a combination of physical, (loss-on-ignition (LOI), magnetic susceptibility), chronological (radiocarbon dates) and biological (diatoms) analyses of a 9-metre sediment core from a small lake-catchment system (Heygsvatn). A training set of 30 lakes was also analysed to relate diatom assemblages to physical and limnological environmental variables using multivariate analysis and to interpret changes in the diatom assemblages from the sediment core.
Section snippets
Study area
The Faroes are a group of 18 islands situated in the Norwegian Sea between Norway, Scotland, and Iceland, around 62° 00′ N and 7° 00′ W (Fig. 1). Climate in the Faroe Islands is largely a product of its position beside the warm North Atlantic current where warm, moisture-rich air from the south and cold air from the Arctic converge. The North Atlantic depression track passes directly over the islands, creating windy but mild conditions and consequently the climate is damp and windy, with
Training set lakes
The majority of lakes in the training set were shallow, unproductive (60% had TP concentrations < 30µg L− 1; mean Chl a was 2.2µg L− 1) and transparent (Secchi depth reached the bottom in 65% of lakes) (Table 1). However, lakes ranged widely in depth (0.3–52m with four lakes > 25m deep) and TP concentration (six lakes had TP concentrations > 50µg L− 1). Chl a, on the other hand, tended to be consistently low in all lakes (all except two lakes < 3µg L− 1). Lakes had a mean specific conductivity
Catchment instability and sediment erosion
Radiocarbon analyses revealed exceptionally high rates of sediment accumulation in Heygsvatn. The deposition of ~ 7m of sediment since 1714 ± 51cal. years BP is comparable in magnitude to infilling rates of lakes in heavily culturally impacted areas (Edwards and Whittington, 2001, Rasmussen and Anderson, 2005). Although the inversion of dates makes calculation of precise sedimentation rates difficult, the estimated sedimentation rate of 9mm yr− 1 during diatom zone 2 exceeds that observed in
Acknowledgements
This work was funded by the Danish Natural Science Research Council (SNF). We are grateful to Richard Bradshaw and Gina Hannon for assistance with the fieldwork and to Susanne Amsinck and Erik Jeppesen for the use of their water chemistry data. Thanks also to Dave Ryves for assistance with diatom taxonomy and multivariate analyses and for the helpful insights of three anonymous reviewers which greatly improved the original manuscript.
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