184-8 Paleoenvironmental Histories of Two Northern Icelandic Lakes for the Last Millennium

Session: Lakes of the World Through Space and Time: Archives of Climate, Paleoenvironments, Ecosystems, Geohazards, and Economic Resources (Posters)

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Abstract:

We report the results of our sediment core-based investigations of the paleoenvironment and tephrochronology of two northern Icelandic lakes, both spanning approximately the last millennium. Mývatn is a large, shallow lake that has been the subject of several previous investigations. Here, we add a new 235 cm-long record collected from a small, shallow embayment on the geothermally active northeastern side of Mývatn. Sedimentation at Mývatn is largely controlled by volcanic and geothermal activity and the record is characterized by several distinct lithologies and numerous tephras. Our investigation of core sedimentology and geochemistry (C, N, pXRF, microscopy, and magnetic susceptibility) shows three distinct facies changes between dated tephras from the Hekla 1104 and the Katla 1416 A.D. eruptions. Initially, sedimentation was dominated by eroded basalts and tephras. A sharp contact with a 15 cm-thick, organic-rich layer with high TOC (>14%) and C/N (>30) indicates a rapid transition to a marsh/wetland setting. Finally, this organic-rich layer sits in sharp contact with a thick layer of diatom-rich sediment that comprises the uppermost 127 cm of the core record and represents an abrupt transition to the geothermally active and silica-rich waters that characterize the modern lake. We also report our investigation of a 309 cm core record from Kleifarvatn, a small, shallow lake in the Breiðdalur Valley near the northeastern coast. To our knowledge, this is the first reported lake sediment core record from the Breiðdalur. A tephra at 66 cm depth was analyzed using both SEM-EDS and EPMA instruments. The chemical composition of this tephra gives a confident identification of the Veiðivotn 1717 A.D. eruption. We link a second tephra found at ~134 cm depth in the core to the Katla 1500 A.D. eruption. Prior to 1500 A.D., the Kleifarvatn core is characterized by high magnetic susceptibility and Ca and Fe content and low TOC (<5%) indicating greater input of external sedimentary inputs such as eroded basalts to the lake. Between 1500 – 1717 A.D. and perhaps due to changing climate (the Little Ice Age), in-lake sediment sources became dominant as indicated by much higher TOC (up to ~19%) and diatom content and low magnetic susceptibility. This shift to in-lake sediment sources remains apparent in the most recent part of the record which is especially rich in diatom fossils.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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