POTENTIAL OF SEDIMENTARY SULFUR VARIABILITY IN A SUBARCTIC BACK-BARRIER FRESHWATER LAKE TO REVEAL PAST MARINE INFLUENCES

The study of limnogeology is important in the understanding of global and regional climate change since the sedimentary record can preserve past environmental conditions. Arctic and subarctic regions are especially sensitive to such climate variability, and are therefore prime locations to utilize lacustrine sedimentary records to examine past climate events. One particularly important uncertainty in our understanding of the climate system is the variability of storm frequency and strength. Studies of back-barrier depositional environments (i.e. marshes and lagoons) that are traditionally used to reconstruct past storm and overwash events are not well represented for high latitudes. In this study we utilize a back-barrier fresh water lake located near the Arctic Circle to examine the feasibility of using sedimentary sulfur concentrations and isotopic compositions to reconstruct past overwash and/or breeching events. Rekavík bak Latrar, or Rekavikurvatn (66⁰25’N;23⁰2.5’W), located on the Hornstrandir peninsula of Iceland, is confined by a sand and boulder-welded barrier that links two rocky headlands. The lake has one minor inlet, drains to the Atlantic through the interstitial space between boulders and has a maximum depth of ca. 22m. A composite sediment record was constructed using cores taken in 1996 and 2012. The cores have two distinct sediment types; an organic-rich lacustrine deposit characteristic of anoxic environments, and an organic-poor, bioturbated, mollusk-rich deposit. Sediment was sub-sampled at 5mm sampling resolution and analyzed for the concentrations and stable isotopic composition of organic carbon, nitrogen and sulfur. The average δ³⁴S value from the sedimentary record was ca. 17‰, significantly lower than average marine sulfate values of ca. 21‰. Therefore we interpret increases in δ³⁴S, paired with the decreases of OC/S in the sedimentary record as representing episodes of marine water incursion. Multiple such events are evident in the record, and suggest past overwash and/or breeching events. Intervals of δ³⁴S and OC/S anti-correlation are likely associated with more complex processes that are examined in light of organic variability in the record, as interpreted from OC/N ratios, as well as δ¹³C and δ¹⁵N.