Carbon and hydrogen isotopic composition of methane over the last 1000 years.

New measurements of the carbon and hydrogen isotopic ratios of methane (ö¹³C 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 the result of human influences prior to the industrial revolution (at least as early as the 16th century of the common era (CE)). The δ¹³C of CH₄ data corroborate the record from Law Dome, Antarctica, with high fidelity. The new SD of CH₄ data set covaries with the δ¹³C of CH₄ record. Both ö¹³C of CH₄ and SD 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 as proposed in previous studies. [ABSTRACT FROM AUTHOR]