Characterization of organic matter from the Cretaceous sedimentary and volcano-sedimentary strata from Livingston Island, Antarctic Peninsula: Insights from organic petrology, molecular proxies and carbon and hydrogen isotopes

Rock samples from the Early to Late Cretaceous marine and non-marine strata of Livingston Island, South Shetland Islands, Antarctica, were studied. Organic matter (OM) originates predominantly from terrestrially-derived organic particles. Mostly highly fragmented plant remains, and locally vitrinite/semifusinite as part of plant compressions/imprints or thin vitrain/fusain lenses, are present. Bulk geochemical results indicate Type III kerogen with poor hydrocarbon generation potential. Tmax and molecular proxies (i.e hopane and sterane isomerization ratios, MPI-1, etc.), together with the local presence of oily droplets, argue for an early oil window maturity, probably caused by regional volcanic activity. In contrast to the results of petrography and Rock-Eval pyrolysis, molecular composition of extractable OM is dominated by short-chain n-alkanes, saturated fatty acids (FAs) and n-alkanols implying a major contribution of autochthonous OM. This contradiction is most likely caused by decomposition of OM from plant tissues, the predominance of wood remains (i.e., xylites) and the low input of FAs from cuticular waxes. The presence of terrestrial OM is indicated by plant wax-derived lipids and resinous compounds. Prominent even carbon number predominance in the short-chain n-alkane range argue for deposition/decomposition under reducing environmental settings, most probably caused by high sedimentation rates and quick burial. Terpenoid biomarker compositions indicate contribution from conifers, whereas input from phytoplankton/algal organic matter is evidenced by C27 steroids, the presence of 4-methyl steranes and dinosterol. Low concentrations of hopanoids and C15 and C17 branched n-FAs argue for limited bacterial input. Only in one sample from Hannah Point, a difference between the δ13C values of the short-chain versus long-chain n-alkanes was obtained, arguing for mixed OM sources (i.e., autochthonous input, land plants). The presence of charred organic matter in most of the studied samples, together with the considerable concentrations of combustion-derived PAHs, argue for the common occurrence of wildfires within the terrestrial environments. Despite the active volcanic arc settings, the compound specific carbon and hydrogen isotopes do not record significant changes in climatic/hydrological conditions, although some short-term events cannot be ruled out. Instead, the δ13C and δ2H isotopic fractionation is most likely caused by the specific biosynthetic mechanisms of the plants and the global carbon and hydrogen cycles.