VARIABILITY IN THE DELTA-C-13 OF LOWER PALEOZOIC PALYNOMORPHS - IMPLICATIONS FOR THE INTERPRETATION OF ANCIENT MARINE-SEDIMENTS

We have analyzed the stable carbon isotope ratios of various organic-w alled microfossils preserved in marine sediments of Silurian and Devon ian age. The aim of this study is to determine the amplitude of isotop ic fractionations among the different extinct palynomorph groups. Twen ty six samples of marine palynomorphs have delta(13)C values in the ra nge -30.0 parts per thousand to -26.3 parts per thousand whereas two s amples, likely of terrestrial origin (tracheids and woody tissue), hav e the highest delta(13)C values (-24.8 parts per thousand and -23.3 pa rts per thousand). Among six sediments that only contain marine palyno morphs, three show delta(13)C variations of 0.8, 1.0 and 2.5 parts per thousand, respectively, whereas the others do not show significant di fference. In the presence of fragments of woody tissue mixed with the marine plankton, these internal isotopic variations may reach about 4 parts per thousand. The bulk delta(13)C values of organic matter in th ese ancient marine sediments are interpreted as average values which i ntegrate, in addition to chemical characteristics of surface waters (e .g. [CO2](aq)), source- and species-dependent isotopic compositions. T he possible occurrence of such delta(13)C heterogeneities at the scale of a sediment sample could result at least from three kinds of materi al fluxes: (1) autotroph and heterotroph elements from the water colum n; (2) oceanic current and river inputs; and (3) selective molecular p reservation related to specific biochemical structures.