Results from NSF OPP Support to WSU for Siple Dome methane record

PI: Ed Brook

With NSF support we measured methane in approximately 196 samples between 55.6 and 738.5 m (0-20 ka). In the Holocene section the methane data record the expected broad concentration minimum in the mid-Holocene, and a distinct drop in the early Holocene corresponding to the 8.2 ka cooling and methane decrease observed in GISP2 and GRIP. The rapid methane decrease at 8.2 ka in the GISP2 record, dated by layer counting, can be used to check the accuracy of the Siple Dome layer counting time scale. There is excellent agreement considering that the combined uncertainties in current layer counting and densification models are of similar magnitude.

For the deglacial section (roughly 580-710 m), where annual layers are not resolvable, we created a gas age time scale by correlation with the GISP2 methane record. D age for Siple Dome was calculated with a densification model, with temperature and accumulation estimated from dD. The Siple Dome paleotemperature record on this time scale broadly resembles the Byrd record during the deglaciation. Temperature started increasing at or prior to 18 ka, there is a prominent plateau, or Antarctic Cold Reversal (ACR) prior to the Younger Dryas, and temperatures were rising at Siple Dome during the Younger Dryas. Rapid increases in dD at 15.2 ka (674m) and 12.1 ka (624m) appear to lead the B°lling transition and Younger Dryas termination by ~ 500 years (Figure 5). However, measurements of d15N of N2 suggest that there is a hiatus in the Siple Dome record between 674 and 680 m (J. Severinghaus, personal communication), which corresponds to the section just below the rapid warming at 15.2 ka.. The evidence for the hiatus comes primarily from very low values of d15N of N2 which suggest an extremely thin firn column, possible created by very low accumulation rates or erosion of the firn. This possible hiatus is of great interest because a similar event may have occurred at Taylor Dome at about the same time but complicates interpretation of the gas age - ice age difference.

More recently we extended the Siple Dome methane record at medium resolution down to about 860m, corresponding to an age of about 45 ka. We find the rapid variations in methane we expect to see during this time period and preliminary match with the GISP2 and GRIP records is good. These results compel us to pursue high resolution methane records for the entire Siple Dome record - to establish a precise relative chronology based on the GISP2 time scale.