RECONSTRUCTION OF THE LATE QUATERNARY PALEOENVIRONMENT UTILIZING SEISMIC INTERPRETATION AND GEOCHEMICAL ANALYSES, VATNSDALSVATN, ICELAND

Seismic surveys and stable isotopic proxies preserve the depositional history and reconstruct the general paleoenvironment of the Vatnsdalsvatn basin, Iceland. Sub-bottom profiles provide quantitative stratigraphic data to assist in interpretation of late Quaternary lacustrine sediment packages. Two piston cores (2002) and one short gravity core (2012) allow for a correlation between seismic- and litho-facies. Grain size analyses serve to ground-truth a prominent erosional reflector in the seismic data. Grain size data show bimodal distributions with a consistent mode centered at 6.2Φ and a second mode of 2.7Φ-3.9Φ , from 6.5-12.4cm. GEOTEK data on the 2002 cores provide impedance and magnetic susceptibility (MS) data. These are applied to the seismic-facies and suggest three primary units. Unit 1, the acoustic basement, is identified as Miocene basalt overlain by till. Unit 2 is interpreted as a proglacial lake unit, typified by high MS (959.9 ± 426.3 SI) and dense sediment (1.9 ± 0.5 g/cm³). Reflectors within Unit 2 show a consistent, lake-wide erosional truncation into a high amplitude reflector. This reflector, ground-truthed as a gravelly sand unit (d_max = 1cm) in the 2012 gravity core, is indicative of a regression followed by modern lacustrine sedimentation. Unit 3 has lower MS (245.2 ± 43.1 SI) and density (1.4 ± 0.1 g/cm³) than Unit 2. Internal reflectors display baselap, and the unit pinches out towards the lake axis suggesting prograding fan deposits with thicknesses ranging from <1.0-5.5m. We correlate 2002 cores using MS data to well-dated sediment cores taken in 1979 (Thompson & Turner, 1985). The 1979 cores lack the elevated MS values observed in Unit 2 of the 2002 cores and so we infer that they contain only Unit 3 sediments. This interpretation places the start of lacustrine deposition as starting >9,300 ± 300 cal BP, consistent with reconstructions of the Gláma ice cap retreat after 10,100 cal BP (Ingolfsson et al., 2010). A prominent reflector observed in the upper part of Unit 3 indicates accelerated sedimentation rates. Radiocarbon dates suggest this reflector is associated with Icelandic Landnám. Ongoing analysis of δ¹³C, δ¹⁵N, δ³⁴S will constrain environmental conditions, such as productivity, nutrient flux and local vegetation throughout the late Quaternary.