Tephra Layers in a Sediment Core from Lake Hestvatn, Southern Iceland: Implications for Evaluating Sedimentation Processes and Environmental Impacts on a Lacustrine System Caused by Tephra Fall Deposits in the Surrounding Watershed

A 570 cm sediment core (94-HV01) from Lake Hestvatn in southern Iceland records continuous lacustrine deposition over the last 6200 yr, including numerous layers of primary fallout tephra. Significant downcore changes are observed in sediment magnetic, chemical and sedimentological characteristics of the deposit, and four zones are identified based primarily on changes in sediment magnetic properties that are supported by chemical analyses. Zone I (3830–6200 14C yr BP) is characterized by low sediment flux, low concentrations of magnetic and chemical substances, and high amounts of biogenic silica, and reflects a period of high productivity in the lake and minimal erosion in the watershed. However, an increase in magnetic and chemical concentrations in the uppermost part of the zone suggests increased erosion within the watershed that is possibly caused by a slight change in vegetation composition towards the end of this period. Zone II (2180–3830 14C yr BP) is identified by an abrupt increase in sediment flux into the lake and greatly increased concentrations of magnetic and chemical substances. This change coincides with a shift in vegetation composition within the watershed that is possibly caused by lower temperatures and/or increased precipitation, as well as in conjunction with the deposition of the H4 tephra and subsequent less voluminous tephra layers. Zone III (1180–2180 14C yr BP) is marked by low concentrations of magnetic and chemical materials and relatively low sedimentation rates as a result of less frequent deposition of fallout tephra into the watershed and less erosion, possibly caused by a slight change in composition of vegetation cover. Zone IV (present–1180 14C yr BP) is characterized by a sudden increase in sedimentation rates and high concentrations of magnetic and chemical materials, and reflects an increase in surface erosion within the watershed following the settlement of Iceland. Seventy-three tephra layers are identified in the Lake Hestvatn sediment core (94-HV01). The Katla and Hekla volcanoes are the source of most of the tephra layers in the core, although tephra layers representing the Vestmannaeyjar, Veidivötn and Grímsvötn volcanoes are also present. These tephra layers represent fallout ash deposited directly into the lake along with subsequent contributions from reworking of tephra within the lacustrine and terrestrial environment. On the basis of major and trace element concentration profiles across the Hekla 4 (H4) and Katla N (KN) tephra layers and adjacent sediment, we show that the influx of tephra particles decreased to pre-eruption values, 66 and 90 yr after the initial tephra deposition. This study indicates that in regions such as southern Iceland, where eruptions that produce great amounts of tephra are frequent, the effects of tephra deposition on lacustrine systems have to be evaluated before other environmental proxies (e.g. pollen and diatoms) in lake sediments can be interpreted accurately.