TY - JOUR T1 - Clumped Isotopes Link Older Carbon Substrates With Slower Rates of Methanogenesis in Northern Lakes JF - Geophysical Research Letters Y1 - 2020 A1 - Douglas, Peter M. J. A1 - Gonzalez Moguel, Regina A1 - Walter Anthony, Katey M. A1 - Wik, Martin A1 - Crill, Patrick M. A1 - Dawson, Katherine S. A1 - Smith, Derek A. A1 - Yanay, Ella A1 - Lloyd, Max K. A1 - Stolper, Daniel A. A1 - Eiler, John M. A1 - Sessions, Alex L. KW - clumped isotopes KW - ebullition flux KW - lakes KW - methane KW - permafrost KW - radiocarbon AB - The release of long‐stored carbon from thawed permafrost could fuel increased methanogenesis in northern lakes, but it remains unclear whether old carbon substrates released from permafrost are metabolized as rapidly by methanogenic microbial communities as recently produced organic carbon. Here, we apply methane (CH4) clumped isotope (Δ18) and 14C measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Δ18 values are negatively correlated with CH4 production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CH4 Δ18 and the fraction modern carbon. These correlations imply that CH4 derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Δ18 values, are not positively correlated with CH4 flux estimates, highlighting the likely importance of environmental variables other than CH4 production rates in controlling ebullition fluxes. VL - 47 UR - https://onlinelibrary.wiley.com/doi/abs/10.1029/2019GL086756 IS - 6 ER - TY - JOUR T1 - D/H variation in terrestrial lipids from Santa Barbara Basin over the past 1400 years: A preliminary assessment of paleoclimatic relevance RID C-2752-2008 JF - Organic Geochemistry Y1 - 2011 A1 - Li, Chao A1 - Sessions, Alex L. A1 - Valentine, David L. A1 - Thiagarajan, Nivedita AB - We analyzed D/H ratios of common terrestrial leaf wax lipids in a 1400 year sediment core from the Santa Barbara Basin (SBB) to test whether they accurately record terrestrial climate in Southern California. The D/H ratios of long chain n-alkanes vary substantially with depth, but are poorly correlated with other terrestrial climate proxies. Interference from fossil hydrocarbons may be at least partly responsible. Long chain n-alkanoic acids exhibit nearly constant downcore D/H ratio values. This constancy in the face of known climatic shifts presumably reflects a substantial residence time for leaf wax compounds in terrestrial soil and/or on the basin flanks. Alternatively, the isotopic composition of meteoric waters in Southern California may not covary with climate, particularly aridity. However, the delta D values of n-C(22) and n-C(24) fatty acids, commonly attributed to terrestrial aquatic sources, are partially correlated with Southern California winter Palmer Drought Severity Index, a tree ring-based climatic proxy (R(2) 0.25; p VL - 42 IS - 1 N1 - id: 2050; PT: J; UT: WOS:000286995900002 JO - D/H variation in terrestrial lipids from Santa Barbara Basin over the past 1400 years: A preliminary assessment of paleoclimatic relevance RID C-2752-2008 ER - TY - JOUR T1 - Phylogenetically specific spearation of rRNA from prokaryotes for isotopic analysis JF - Marine Chemistry Y1 - 2004 A1 - Pearson, Ann A1 - Sessions, Alex L. A1 - Edwards, Katrina J. A1 - Hayes, John M. VL - 92 N1 - id: 1805 ER -