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

  1. James D. L. White5 and
  2. Nancy R. Riggs6
  1. J. Hardardóttir1,†,
  2. Á. Geirsdóttir2 and
  3. T. Thórdarson3

Published Online: 24 MAR 2009

DOI: 10.1002/9781444304251.ch11

Volcaniclastic Sedimentation in Lacustrine Settings

Volcaniclastic Sedimentation in Lacustrine Settings

How to Cite

Hardardóttir, J., Geirsdóttir, Á. and Thórdarson, T. (2009) 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, in Volcaniclastic Sedimentation in Lacustrine Settings (eds J. D. L. White and N. R. Riggs), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304251.ch11

Editor Information

  1. 5

    Department of Geology, University of Otago, PO Box 56, Dunedin, 9015, New Zealand

  2. 6

    Geology Department, Northern Arizona University, Box 4099, Flagstaff, AZ 86011, USA

Author Information

  1. 1

    Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, CB 450, Boulder, CO 80309-0450, USA

  2. 2

    Department of Geosciences, University of Iceland, 101 Reykjavík, Iceland

  3. 3

    CSIRO, Magmatic Ore Deposit Group, Division of Exploration and Mining, Private Bag, PO Wembley, W.A. 6014, Australia

  1. National Energy Authority, Grensasvegur 9, 108 Reykjavik, Iceland

Publication History

  1. Published Online: 24 MAR 2009
  2. Published Print: 18 APR 2001

ISBN Information

Print ISBN: 9780632058471

Online ISBN: 9781444304251

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Keywords:

  • lakes - sensitive recorders of eruptions and response of distal landscapes;
  • tephra layers in a sediment core from Lake Hestvatn, southern Iceland;
  • Lake Hestvatn - geological and geographical setting;
  • core collection, core description and sediment sampling;
  • identification of tephra layers and chronology;
  • sedimentological, magnetic and chemical analyses;
  • tephra layers;
  • depositional processes and impact of tephra;
  • downcore changes - magnetic zone I, II, III, IV;
  • volcanic activity of Nevado de Toluca during late Pleistocene times

Summary

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.