IS A PERIGLACIAL BIOTA RESPONSIBLE FOR ENHANCED DIELECTRIC RESPONSE IN BASAL ICE FROM THE GREENLAND ICE CORE PROJECT ICE CORE

A detailed dielectric profiling (DEP) conductivity profile (delta(infi nity)) measured in the 6 m of the basal silty ice sequence from the Gr eenland Ice Core Project (GRIP) ice core (Summit, Central Greenland) i s presented and compared to previous multiparametric studies. DEP cond uctivities span the whole glacial-interglacial range observed higher u p in the GRIP core (9-25 mu S m(-1)). Values in the bottom meter of th e sequence reach the level of some of the highest peaks from Holocene volcanic layers in the core (33 mu S m(-1)). On a steady increase of t he sigma(infinity) values down the sequence are superimposed large flu ctuations ''inphase'' with other variables measured in the core such a s delta(18)O, debris content, or gas compositions in CO2 and CH4. Anal ysis of the type and strength of intercorrelations shows that the cont rolling variable for the DEP signal must be closely related to the gas content and composition of the ice. Plausible candidates for this cau sality link are investigated. Enhancing of the sigma conductivity by C O2 and CH4 encaged in the ice lattice as gas hydrates is ruled out sin ce these are nonpolar clathrates of structure I, known as having negli gible impact on the orientational stability of the water molecules und er ac currents. NH4+ is proposed as the best candidate since it has be en shown to enhance DEP conductivities by introducing Bjerrum defects in the ice lattice and since it could have been initially present part ly as gaseous NH3 in the ice. This proposition is supported by the NH4 + profile in the basal ice sequence. Using calibration curves from hig her up in the core, it is shown that sigma is in fact fully explained by intracrystalline conductivity of pure ice solely disrupted by ammon ium impurities in the ice lattice, The origin of the NH4+ signal is di scussed in the light of organic acid profiles (formate, acetate, and o xalate). It appears that the most likely source is local degradation o f biological residues, which supports the hypothesis that part of the basal ice was formed locally, in the absence of the present-day ice sh eet.