Date of Award

Spring 2024

Project Type

Thesis

Program or Major

Earth Sciences

Degree Name

Master of Science

First Advisor

Ruth K Varner

Second Advisor

Katharine Duderstadt

Third Advisor

McKenzie Kuhn

Abstract

Stable isotopes of carbon can be measured in gases (methane (CH4) and carbon dioxide (CO2)) and in organic carbon material like sediments and plants. The stable carbon isotopic ratios in methane (δ13C-CH4) are used in atmospheric inversion models to determine ground sources and calculate the magnitude of emissions. δ13C-CH4 from natural ecosystems 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 emissions are expected to increase with climate change and warming temperatures. The δ13C-CH4 signatures in methane 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 and vegetation organic carbon isotopes (13CSOM and 13CVOM), vegetation characteristics in the lake or on the shore, lability of the organic matter in the porewater, lake productivity, dissolved organic carbon content (DOC), 13DIC in sediment porewater, dissolved oxygen, pH, and water temperature were analyzed to explain differences in δ13C-CH4 isotopes. Lakes of various sizes (>1 acre to 200 acres) in sub-tropical (Florida; 30°N), temperate (New Hampshire; 43°N), and sub-arctic (Sweden;68°N) biomes were studied. 13C-CH4 did not vary by latitude, as hypothesized, nor did 13CSOM, 13CVOM,,but 13C-CH4 was positively correlated with percent cover of vegetation. Thus, my results suggest that13C-CH4 signatures from bubble samples collected from sediment disturbance in the lakes in this study cannot be used to distinguish lakes from different latitudes via top-down methods, but vegetation could be an important control. However, more studies are needed to better understand what controls 13C-CH4 signatures in lake methane bubbles and improve modeling approaches.

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