Characterizing Terrestrially-Derived Organic Matter in Sediments of Laurentian Great Lakes Superior and Huron

Abstract

While lacustrine systems contain a small fraction of the world’s water, they collectively account for nearly half of the world’s annual total organic matter (OM) burial. Recent years have brought considerable efforts to fully understand the carbon cycle of lacustrine environments with increasing emphasis on anthropogenic impacts. With such an important role in the global carbon cycle, it is critical to fully understand the function of carbon influx and storage in these systems. The carbon cycle and general composition of OM in waters of the Laurentian Great Lakes (LGLs) have been widely studied, yet there remains a large knowledge gap in carbon inputs and the fate and transport of sedimentary organic matter (SOM) in each LGL. To address this, we have characterized the chemical composition and source of OM spatially in the sediments of LGLs Superior and Huron, and temporally in LGL Superior. Surface sediments were collected at 30 sites in LGL Superior and 33 sites in LGL Huron and analyzed for bulk elemental composition (%OC and %N) and δ13C signatures using elemental analysis-isotope ratio mass spectrometry (EA-IRMS), and lignin-phenol biomarkers using gas chromatography-mass spectrometry (GCMS). Additionally, sediment cores were collected at 3 sites in LGL Superior to assess temporal SOM changes. Nearshore sediments exhibited low SOM degradation extent relative to high offshore SOM degradation extent in both lakes suggesting prominent lateral transport mechanisms. High lignin-phenol proxy ratios in offshore SOM may be attributed to airborne pollen as a major contributor of terrestrially-derived SOM into both lakes, suggesting that previously overlooked aeolian influx of pollen into the LGLs may constitute a significant unquantified carbon input. Similar lignin-phenols were measured in sediment cores collected at three of the sample sites in LGL Superior. The temporal degradation extent of terrestrial OM in downcore sediments remained relatively unchanged, while total OM concentrations decreased. These results indicate that terrestrially-derived OM becomes buried and relatively well-preserved in LGL Superior, with an average overall organic carbon burial rate of 2.27 gC m⁻² yr⁻¹. There are also clear temporal changes in lignin-phenol compositions and bulk elemental composition that correspond to changes in productivity, European colonization, and anthropogenic logging activities in the surrounding watershed of LGL Superior. This study advances our understanding of carbon inputs, transport, and storage in LGLs Superior and Huron by characterizing sedimentary OM spatially and temporally.