| Summary: | New measurements of the carbon and hydrogen isotopic ratios of methane (δ13C of CH4 and δD of CH4) over the last millennium are presented from the WAIS Divide, Antarctica ice core (WDC05A), showing significant changes that likely were influenced by humans even before the industrial revolution. The δ13C of CH4 data corroborate the record from Law Dome, Antarctica with high delity. The new δD of CH4 data set covaries with the δ13CH4 record. Both δ13C of CH4 and δD of CH4 were relatively stable and close to the present-day values from ~1000 to ~1500 CE. Both isotopic ratios decreased to minima around 1700 CE, remained low until the late 18th century, and then rose exponentially to present-day values. Our new δD of CH4 data provide an additional independent constraint for evaluating possible CH4 source histories. We searched a broad range of source scenarios using a simple box model to identify histories consistent with the constraints of the CH4 concentration and isotope data from 990-1730 CE. Results typically show a decrease over time in the biomass-burning source (found in 85% of acceptable scenarios) and an increase in the agricultural source (found in 77% of acceptable scenarios), indicating preindustrial human influence on atmospheric methane.
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