Burial of Terrestrial Organic Carbon in a Predominantly Anoxic Swedish Fjord

Anthropogenic activity has significantly altered our planet and exacerbated global climate change. In particular, increases in the abundance of atmospheric greenhouse gases (e.g., CH4 and CO2), largely responsible for global warming, has disrupted the balance (e.g., sources, sinks and exchange rates) of the global carbon cycle. The coastal margin receives large amounts of both terrestrial and marine organic matter and is able to store the organic carbon (OC) long term in the sediments, thus playing an important role in the global carbon cycle. Recent research has identified fjords as sites with efficient organic OC burial rates, burying 11% of all oceanic OC though only covering 0.1% of the ocean area. Fjords are estuaries meaning they receive and bury autochthonous and allochthonous OC, or marine OC (OCmar) and terrestrial OC (OCterr). OCterr accounts for 55-62% of OC buried in fjord sediments, highlighting the importance of fjords as an OCterr sink. Here, we present data on recent and historical changes of OCterr inputs and burial in the By fjord, a predominantly anoxic Swedish fjord. This fjord was also the site of a 2.5-year long engineering experiment (2010-2013) in which oxygenated surface waters were pumped into deep waters. Sediment cores collected from 2009 and 2012, when the fjord was naturally anoxic and forced oxic, respectively, were examined for bulk elemental (C, 13C, N, 15N), radionuclide (137Cs), and lignin biomarker analyses. The OC burial rates for the By fjord ranged from 24-38 g OC m-2 yr-1, which agrees with estimates for regional fjords in NW Europe. Bulk elemental analyses show rapid OC degradation in the top 1.5 cm, a mixed layer between 2-4 cm, and a deeper sub-surface peak in OC at 8-12 cm. The range of 13C values across both years (-22% to -26%) is indicative of a mixture of both OCterr and OCmar with pelagic diatoms as a dominant source of OC to the sediments. Carbon to nitrogen molar ratios (C/N) ranged from 10 to 14 and suggest inputs of both OCterr and OCmar. Inputs and burial of OCterr, as reflected by lignin concentration/composition have yet to be determined and will be included in this presentation. Preliminarily, these diagenetic profiles of OC reflect changing anaerobic and aerobic decay processes.