Atmospheric Methane Isotope Records from Ice Cores During the Last Glacial Period

Open Access
Author:
Mitchell, Hailey Marie
Area of Honors:
Meteorology
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Todd Anthony Sowers, Thesis Supervisor
  • Paul Markowski, Honors Advisor
Keywords:
  • methane
  • isotope
  • deuterium
  • ice cores
  • NEEM
  • paleoclimate
  • past climate change
  • MIS 5-4 transition
  • last glacial period
Abstract:
Methane (CH4) is the third most important greenhouse gas in the Earth’s atmosphere, contributing 20% of the total radiative forcing from all long-lived greenhouse gases today (IPCC 2007). Methane concentrations generally changed in sync with northern hemisphere temperature during both glacial/interglacial transitions as well as with rapid climate changes (Dansgaard-Oeschger events). Together with the CH4 interhemispheric gradient, stable isotopic studies on methane (δ13C and δD) in ice cores allow us to investigate individual CH4 source/sink changes. We measured 12 ice core samples from the NEEM (North Eemian, Greenland) ice core for δDCH4 covering DO 8 and the MIS 5-4 transition. Replicate analyses of NEEM ice from MIS 3 and DO 8 agreed with previously measured and published samples from NGRIP (North Grip core, Greenland)(Bock et al. 2010). External precision of the analyses based on replicate air standards run throughout each analytical day were ±1.8‰. These data were compared with other trace gas and climate records from the same period. The 13C/12C ratio of CH4 (denoted δ13CH4) data for the period between 70,000 years before present (ka) and 61 ka showed an increasing trend. In contrast, the D/H ratio of CH4 (denoted δDCH4) showed a decreasing trend over roughly the same interval. These two records appear to be in phase with one another but opposite in the sign of their changes. An 8-box global atmospheric methane model describing the atmospheric CH4 loading, δ13CH4, and δDCH4 data is unable to explain all three records together using constant isotope values for the various sources. Instead, it is proposed that the data are a result of a change in isotopic signature of the sources themselves. One plausible explanation for the observed decoupling involves a shift from C3 plant type to C4 plant type dominance and/or a change in the relative contribution of the two CH4 production pathways. Moreover, the shift from C3 to C4 plant dominance may be related to the observed drop in atmospheric CO2 values from 240 ppm to 195 ppm during this period.