Methane Sources in the Waters of Lake Michigan and Lake Superior as Revealed by Natural Radiocarbon Measurements

The methane dynamics in the waters of Lakes Michigan and Superior, components of the North American Great Lake system, were investigated using measurements of methane concentration and natural radiocarbon ( 14 C‐CH 4 ) dissolved in these lake waters. All 14 C‐CH 4 measurements were above modern levels regardless of location and depth with a range of 117‐145% modern carbon (pMC). Methane concentrations in the deep basin of both lakes were low, ranging from 3.3 to 4.3 nM, with minimal vertical variation. However, the concentrations of CH 4 increased toward coastal areas in both lakes, possibly due to higher groundwater inputs and aerobic methanogenesis associated with primary productivity. Except for one site, 14 C‐CH 4 dissolved in the waters of Lake Michigan was greater than in Lake Superior by ~12 pMC, a difference that was likely due to inputs of excess 14 CH 4 from nuclear power plants along the coast of Lake Michigan.