Concentration and Size Distribution of Microparticles in the NGRIP Ice Core (Central Greenland) during the Last Glacial Period

Abstract

were investigated in the deep North Greenland Ice Core Project(NGRIP) ice core continuously from 1405 m to 2930 m depth. Themeasurements were accomplished using a novel, optical detectorwhich is based on laser light attenuation by individual particles.The device works on a flow-through basis, and together with samplepreparation via continuous melting it allows for very timeefficient analyses at high depth resolution. The presentedwork also covers the partial development and the application of acontinuous ice core melting setup as well as analytical systems ofelectrolytical conductivity and acidity.In the NGRIP ice core, the concentration of microparticles wasfound to be around 70 µg / kg during Preboreal Holoceneand 8000 µg / kg during the last glacial maximum (LGM).Variations by typically a factor of 8 of the insoluble particlemass and number concentrations were encountered across the rapidDansgaard/Oeschger transitions within the last glacial period.The (Ca2+)/(insoluble microparticle) mass ratio wasinvestigated in various selected core sections. Relatively lowCa2+ contents were found concurring with high crustalconcentrations. Such systematic variations were observed on longtime scales (> 1000 years) and also on seasonal to multi-annualtime scales. Strong enhancements of the (Ca2+)/(insolublemicroparticle) ratio by up to a factor of 3 were found duringvolcanic events due to increased dissolution of CaCO3 byvolcanogenic acids. These findings limit the use of Ca2+ asan unequivocal quantitative reference species for mineral dust.Systematic variations of the size distribution were observed withthe tendency towards larger particles during colder climates. Thelognormal mode of the volume distribution was found at about 1.3µm diameter during Preboreal Holocene and 1.7 µm diameterduring peak LGM. Size changes occurred largely synchronous withconcentration changes. By use of a simple, semi-quantitative modelpicture it is inferred that (i) the observed variations mainlyreflect changes of the airborne particle size distribution overGreenland, and that (ii) these size changes probably are aconsequence of changed particle transit times from the source tothe ice sheet. Long range transport times shorter by 25% duringthe LGM with respect to Preboreal Holocene can explain theobserved size changes. Further, it is estimated that these changesin transport can account for a concentration increase of less than1 order of magnitude and clearly cannot explain the total observedconcentration increase during LGM. Therefore, sourceintensifications must have occurred synchronously to changes oflong range transport. Furthermore, a higher variability of thelognormal mode was observed during warmer climates as reflected byincreased point-to-point variability and also by increaseddistribution widths for the multi-year samples considered. It thusis inferred that atmospheric circulation was more variable duringsuch times.