A Late Holocene record of landscape degradation from Heygsvatn, the Faroe Islands

Abstract

Diatom, magnetic susceptibility and loss-on-ignition (LOI) analyses of a 9-metre sediment core from Heygsvatn (Suðuroy, the Faroe Islands) were used to investigate the response of lake and catchment to mid-Late Holocene environmental change. A 30-lake training set from Faroese lakes was also used to explore the relationships between diatoms and environmental variables and assist in the interpretation of the diatom stratigraphy. Canonical Correspondence Analysis showed that lake depth, light penetration, total phosphorus (TP), pH and dissolved organic carbon (DOC) were the most significant variables affecting diatom distribution in Faroese lakes (p < 0.06). Nine ¹⁴C AMS dates indicated that the sediment record covered the last ~ 5700 years. Exceptionally high rates of sediment accumulation, an increase in LOI and magnetic susceptibility and inversion of radiocarbon dates suggested that a period of elevated soil erosion occurred at Heygsvatn ca. 1700–1200cal. yr. BP. Because this erosion pre-dates the arrival of humans the most likely cause is climate deterioration (cooler, wetter conditions). The diatom stratigraphy supports this hypothesis by showing that during the period of erosion the lake was deeper or/or more depauperate in TP, consistent with the climate becoming wetter and leading to increased lake flushing. Subsequently, with sustained sediment erosion, the lake became more shallow and pH increased as the lake infilled. In combination, the proxies support the idea that the climate became wetter ca. 1700–1200cal. yr. BP in the Faroe Islands, but that the response of the landscape was partly dependent on the increased development of peat initiated during the onset of Neoglacial cooling after 5000cal. years BP.

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.