The content and stable isotopic composition of carbon in individual micrometeorites from Greenland and Antarctica

The C contents and delta(13)C values of eleven individual micrometeorites have been determined using a combination of stepped combustion and static mass spectrometry. A new low-blank procedure, involving pretreatment of the samples with a solvent to remove surficial contaminants, has enabled samples of 6-84 mu g to be analysed successfully. The eleven samples (seven separated from Greenland cryoconite and four from Antarctic ice) were each split prior to C determination and a fragment taken for study using analytical electron microscopy. In this way, the chemical compositions were obtained thereby allowing comparison with other investigations. As with previous studies of micrometeorites collected at the Earth's surface, the major difficulty with interpreting the results involves distinguishing indigenous components from terrestrial contaminants. Overall C contents were typically <0.2 wt%, although one of the Greenland samples contained 1.5 wt% C, considered to arise mainly from algal contamination. For the other samples, around 0.05-0.15 wt% Of the total C in each micrometeorite was considered to be organic in nature with at least some of this (if not all) being terrestrial in origin; the remainder was probably indigenous, being analogous to the macromolecular organic material found in primitive carbonaceous chondrites. The generally low content of this indigenous organic material, compared to conventional meteorites, is presumably a reflection of C loss from the micrometeorites either during atmospheric heating, or subsequent weathering. For that C combusting between 500 and 600 degrees C, ten of the samples appeared to show a simple two-component system (i.e., a mixture of blank and an isotopically fight component; delta(13)C less than or equal to -32 parts per thousand). It is possible that the light component is C-delta, a fine-grained form of presolar diamond which is known to be prevalent in primitive chondritic meteorites. If so, then it is present in the micrometeorites at concentrations of similar to 30-600 ppm (typically 200 ppm), which is a similar level to that in meteorites. An analysis of algae separated from Greenland cryoconite shows tentative evidence for the presence of extraterrestrial silicon carbide; however, further work will be needed to substantiate this.