Title:

Isotopic Signatures from Lake Sediments, Plants, and Methane Bubbles: A Latitudinal Study of Lake Sediment Methane and Organic Carbon

Isotopes of carbon can be measured in gases and in organic carbon material like sediments and plants. The stable carbon isotopic ratios in methane (δ13C-CH4) are used in atmospheric models to track both the location and magnitude of emissions. δ13C-CH4 is influenced not only by microbial production and oxidation processes but also by the precursor organic matter’s isotopic ratios (plants and dissolved inorganic carbon [DIC]). Lakes account for 9-24% of global methane emissions and their role is expected to increase with climate change and warming temperatures. The δ13C-CH4 signatures in bubbles retrieved from lake sediments were compared across lakes from three different latitudes (~ 20 lakes per latitude). Properties of the lakes, including the lake sediment organic carbon isotopes, plants’ carbon isotopes, the species of vegetation in the lake or on the shore, the lability of the organic matter in the porewater, lake productivity, dissolved organic carbon content (DOC), the 13DIC in sediment porewater, dissolved oxygen, pH, and temperature were analyzed to explain differences in the isotopes. Lakes of various sizes (>1 acre to 200 acres) in Florida (30°N), New Hampshire (43°N), and Sweden (68°N) were chosen for sampling. In addition to the properties of the lakes listed previously, the sediments were poked, releasing methane bubbles captured for isotopic analysis. No variations were found in the 13C-CH4 by latitude nor the 13CSOM, 13CVOM, or the methanogenesis pathways. Therefore, we conclude that at this point 13C-CH4 signatures cannot be used to distinguish lakes from different latitudes via top-down methods.

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