CARBON CYCLE DYNAMICS AND ECOLOGY REVEALED BY THE CARBON ISOTOPIC COMPOSITION OF SINGLE ORGANIC MICROFOSSILS DURING THE DEVONIAN FRASNIAN-FAMENNIAN BIOTIC CRISIS

Acritarchs, or close-walled organic microfossils, are abundant in many siliciclastic strata in the Proterozoic and Paleozoic. However, their taxonomic affinities and paleoecology remains enigmatic. Studies that employ direct analysis of acritarch organic matter have been limited, but this approach can provide an addition dimension of data against which hypotheses about their affinities and paleoecology can be tested. We employ a relatively simple nano-EA-IRMS method for the analysis of single organic microfossils and physically separable components of sedimentary organic matter (e.g. kerogen particles) for their carbon isotopic composition. We have applied this technique to the Frasnian-Famennian strata of Appalachian Basin through the Late Devonian Kellwasser Events and mass extinction.

We analyzed several hundred leiospherid and acanthomorphic (ornamented) acritarchs and kerogens from the Late Devonian of the Appalachian Basin. The δ¹³C of kerogen grains were isotopically similar between individual grains and to bulk δ¹³C_org. However, the δ¹³C of individual microfossils varied between individuals and was significantly different from co-occcuring bulk organic matter. For example, the δ¹³C of individual leiospherids from the Lower Kellwasser black shale range from -23.8 to -25.4‰ compared to -30.4‰ for kerogens. The δ¹³C of leiospherid an acanthomorphic organic matter show no ecological difference and are consistent with algal origin. Additionally, we observe no relationship between cell size and δ¹³C. We speculate that the isotopic difference between kerogen and the microfossils is result of (1) a surface water ecology where the DIC is ¹³C-enriched because of a strong biological pump, (2) that these organisms may have had a high rate of growth under eutrophic conditions that depressed ε_p, or (3) they employed an alternative means of carbon fixation that resulted in the observed ¹³C-enrichment. Alternatively, the ¹³C-depletion of bulk organic matter could signal that a significant proportion of the sedimentary organic is derived from another source such as chemoautotrophic organisms living within the chemocline or a terrestrial organic matter, though macerates and organic geochemical investigations reveal little terrestrial organic matter in these strata. The δ¹³C range between individual microfossils likely represents environmental variability captured by a relatively short duration of life compared to time range of a single sample.