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

Abstract

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 (14C‐CH4) dissolved in these lake waters. All 14C‐CH4 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 CH4 increased toward coastal areas in both lakes, possibly due to higher groundwater inputs and aerobic methanogenesis associated with primary productivity. Except for one site, 14C‐CH4 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 14CH4 from nuclear power plants along the coast of Lake Michigan.

Large increases in emissions of methane and nitrous oxide from eutrophication in Lake Erie

“…Surface water CH 4 concentrations ranged from 3.5 to 60 nM in Lakes Michigan and Superior (21), much lower than in Lake Erie, where concentrations in May 2015 (the closest sampling month to June) ranged from 14 to 780 nM (Table S1). The highest CH 4 concentrations in (21) were found in coastal Lake Michigan where the highest primary production rates are found, and the highest sediment methanogenesis rates are likely (21), similar to the current study. As we also concluded in the current study, radiocarbon measurements indicated no input of CH 4 from old sources, such as fossil fuels (21), although radiocarbon measurements of CH 4 in Lake Erie would help further indicate the relative inputs of natural gas and biogenic CH 4 in active production areas.…”

“…A recent study reported CH 4 concentrations and isotopic composition of CH 4 in Lake Michigan and Lake Superior from June 2017 (21). Methane concentrations in summer in Lake Erie were more than 200 times higher than in Lake Superior (21) (Table S1).…”

“…A recent study reported CH 4 concentrations and isotopic composition of CH 4 in Lake Michigan and Lake Superior from June 2017 (21). Methane concentrations in summer in Lake Erie were more than 200 times higher than in Lake Superior (21) (Table S1). Surface water CH 4 concentrations ranged from 3.5 to 60 nM in Lakes Michigan and Superior (21), much lower than in Lake Erie, where concentrations in May 2015 (the closest sampling month to June) ranged from 14 to 780 nM (Table S1).…”

“…Methane concentrations in summer in Lake Erie were more than 200 times higher than in Lake Superior (21) (Table S1). Surface water CH 4 concentrations ranged from 3.5 to 60 nM in Lakes Michigan and Superior (21), much lower than in Lake Erie, where concentrations in May 2015 (the closest sampling month to June) ranged from 14 to 780 nM (Table S1). The highest CH 4 concentrations in (21) were found in coastal Lake Michigan where the highest primary production rates are found, and the highest sediment methanogenesis rates are likely (21), similar to the current study.…”

“…The greenhouse gases CH 4 and N 2 O are 25 and 289 times more powerful than carbon dioxide (CO 2 ) over 100 year time scales, respectively (16). Despite decades of research on nutrients and phytoplankton in the Great Lakes, there is sparse research on GHGs in Lake Erie (17)(18)(19)(20)(21). However, if increasing eutrophication in Lake Erie linked to climate change leads to increasing emissions of CH 4 and N 2 O, this represents a positive feedback that urgently needs to be both delineated and addressed (22).…”