A multi-proxy approach to assessing isolation basin stratigraphy from the Lofoten Islands, Norway

a r t i c l e i n f o This study takes a comprehensive approach to characterizing the isolation sequence of Heimerdalsvatnet, a coastal lake in the Lofoten Islands, northern Norway. We use established methods and explore new techniques to assess changes in marine influence. Bathymetric and sub-bottom profiles were acquired to examine basin-wide sedimentation and a 5.8 m sediment core spanning the last 7800 cal yr BP was analyzed. We measured magnetic susceptibility, bulk organic matter properties, molecular biomarkers, diatom assemblages, and elemental profiles acquired by scanning X-ray fluorescence. These characteristics of the sediment reflect detailed changes in salinity and water column conditions as the lake was progressively isolated. Three distinct litho/chemo-stratigraphic units represent a restricted marine phase (7800–6500 cal yr BP), a transitional phase characterized by intermittent marine influence (6500–4900 cal yr BP), and complete isolation and freshwater sedimentation (4900 cal yr BP to present). Although there are uncertainties in the estimate of the threshold elevation of the lake, the timing of these phases generally corresponds with previous interpretations of the local relative sea-level history. This record captures sea-level regression following the Tapes transgression and supports the interpretation of a subsequent sea-level stillstand, dated in Heimerdalsvatnet from 6500 to 4900 cal yr BP. Introduction Relative sea-level histories can be reconstructed from a variety of coastal sedimentary environments: raised beaches, wave cut terraces, estuaries and isolation basins. Isolation basins are particularly useful because they provide continuous sedimentary archives of marine– lacustrine transitions, the transgression and isolation contacts can usually be well-dated, and basin thresholds provide information on former sea-level elevations. In addition, isolation basin records are analogues for processes affecting coastal environments that are being impacted by present rising or falling sea level. Reconstructing the environmental history of isolation basins requires the identification of changes in marine influence (i.e. marine, brackish, and freshwater facies), which can be difficult since sea-level fluctuations affect water column chemistry, organic matter input, and sediment source. Methods of characterizing isolation basin stratigraphy traditionally rely on microfossil assemblages and their relationship to salinity (e. identifying microfossils is time consuming and can be affected by microfossil abundance, preservation, non-analogue situations, and when species have wide salinity tolerances. Recently, different approaches have emerged to overcome limitations of microfossil records and to better characterize paleoenviron-mental conditions. Properties of bulk sedimentary organic matter, mainly δ 13 C and C/N ratios, have been used to identify salinity changes …