Systematic Correlation Sets for Greenland Ice Methane-Temperature During Last 50 kyr

Based on studies of Greenland and Antarctic ice cores, it is conventionally thought that the time series of atmospheric methane concentrations [CH4] extracted from occluded bubbles closely track Greenland temperatures, as calculated from δ18Oice. In this classical view, considerable debate continues regarding the lead-lag relationships of temperature, methane and carbon dioxide concentrations. In the Greenland GISP2 record, this apparent synchronous relationship between ice [CH4] and temperature is generally inferred only through visual inspection of the time series. In this study, we examined the true correlations between GISP2 [CH4] and δ18Oice for the last 50 kyr BP. We do observe the expected trend towards increasing methane mixing ratios with increasing local temperatures. The linear regression through the entire data set reveals an approximate 20 ppbv increase in [CH4] with each 1 °C rise. However, our closer inspection of the GISP2 [CH4]- δ18Oice correlation reveals clearly discernable variations in the magnitude of this response during the late Pleistocene (<50 kyr BP). Our study shows distinct temporal sets of responses that range from 10 to 30 ppbv per 1 °C rise. Our tests show that these different correlations are insensitive to 1) uncertainties in aligning the gas age scales with the ice ages, 2) factors other than temperature that affect δ18Oice, e.g., seasonality and proximity of precipitation, sea surface conditions and atmospheric circulation, 3) errors in the calculation of temperature from δ18Oice measurements (oxygen isotope temperature sensitivity (α), i.e., δδ18Oice/δT). During the Holocene, this [CH4]-δ18Oice relationship decouples, in fact. This indicates that other factors have modulated the methane budget in the past 10 kyr BP than during the Pleistocene. These warmer Holocene data (ca. -30 °C) have no apparent relationship to temperature despite a ca. 200 ppbv range in [CH4]. The change in the [CH4]-δ18Oice correlations indicate a variable teleconnection between the climate in Greenland and that of methane producing regions, over relatively short geologic time spans. These are likely governed by different climate conditions within and since the last ice age. Caution is required when reconstructing latitudinal climate connections from [CH4]-δ18Oice relationships. However, these variations may also provide clues to understanding the interdependence of North Atlantic and tropical climate.