SOURCE AND MATURITY OF ORGANIC MATTER IN GLACIAL AND CRETACEOUS SEDIMENTS FROM PRYDZ BAY , ANTARCTICA , ODP HOLES 739 C AND 741 A 1

Organic matter in Miocene glacial sediments in Hole 739C on the Antarctic Shelf represents erosional recycled continental material. Various indications of maturity in bulk organic matter, kerogens, and extracts imply that an exposed section of mature organic carbon-rich material was present during the Miocene. Based on biomarker, n-alkane, and kerogen analysis, a massive diamictite of early Eocene/Oligocene age at Hole 739C contains immature organic matter. Visual and pyrolysis analyses of the kerogens suggest a predominance of terrestrial organic matter in all samples from Hole 739C. A reversal of thermal maturities, i.e., more-mature overlying less-mature sections, may be related to redeposition generated from glacial erosion. Siliciclastic fluviatile sediments of Lower Cretaceous age from Hole 741A were analyzed. The organic matter from this hole contains immature aliphatic and aromatic biomarkers as well as a suite of odd carbon number-dominated nalkanes. Visual examination and pyrolysis analysis of the kerogen suggests that predominantly immature terrestrial organic matter is present at Hole 741A. The similarities between Hole 739C Unit V and Hole 741A suggest that the source of the organic matter in the glacial sediments in Unit V at Hole 739C could be Cretaceous in age and similar to sediments sampled at Hole 741A in Prydz Bay. INTRODUCTION The molecular and bulk characteristics of organic matter can be useful in defining paleodepositional environments, determining original biological inputs, and inferring sedimenfs thermal history (e.g., Barker, 1979). For example, long-chain n-alkanes (>21 carbon atoms) with an odd over even carbon number predominance (Eglinton et al., 1962) and abundant C29 steranes are believed to be primarily associated with terrestrial or land-derived inputs (Huang and Meinschein, 1976). A second example is that the presence of a complex mixture of cyclic, branched, and normal alkanes and polynuclear aromatic hydrocarbons (PAH) indicates a petroleum or mature organic matter origin for hydrocarbons (Farrington and Meyers, 1975). Biologically derived hydrocarbons are relatively simple mixtures containing only a few structurally specific alkanes, alkenes, and aromatics derived from the remains of plankton, bacteria, and/or terrestrial debris (Farrington et al., 1973). Ratios between biological precursors and their transformed products, such as the 22S/22R isomers of the extended hopanes and ratios between methyl phenanthrene isomers, reflect the thermal history of a sedimentary sequence (see Philp, 1985 for an overview). The chemical and optical characteristics of kerogen also define organic matter sources and thermal history (i.e., thermal indexes, van Krevelen diagrams, etc.; Tissot and Welte, 1978). In this report the molecular, bulk, and visual characteristics of sedimentary organic matter are used to understand the source and maturity of sediments from Ocean Drilling Program (ODP) Prydz Bay, Holes 739C and 741 A. 1 Barron, J., Larsen, B. , et al., 1991. Proc. ODP, Sci. Results, 119: College Station, TX (Ocean Drilling Program). 2 Geochemical and Environmental Research Group, College of Geosciences, Texas A&M University. 833 Graham Road, College Station, TX 77845, U.S.A. 3 Present address: Conoco, Inc., 1000 South Pine, RW 6457, P.O. Box 1267, Ponca City, OK 74603, U.S.A. 4 PTI Environmental Services, 15375 S.E. 30th Place, Suite 250, Bellevue, WA 98007, U.S.A. 5 Present address: Great Lakes National Program Office, U.S.E.P.A. , 230 South Dearborn St., Chicago, IL 60604, U.S.A. GEOLOGIC SETTING Prydz Bay lies oceanward of a graben occupied by the Lambert Glacier and Amery Ice Shelf in east Antarctica. The Lambert Glacier drains a large part of the east Antarctica ice sheet including the subglacial Gamburstev Mountains. The glacier follows the line of the Lambert Graben, which extends 700 km inland and is of Permian or Early Cretaceous age. The present configuration of the ice drainage basin is thought to have remained relatively constant during the Cenozoic; therefore, Prydz Bay sediments should reflect all stages of Antarctic glaciation and preglacial continental climate (Barron, Larsen, et al., 1989). Hole 739C lies in central Prydz Bay and is approximately 30 km from the shelf edge, 200 km from land, and 140 km from the Amery Ice Shelf. Hole 741A was drilled in eastern Prydz Bay and is approximately 50 km southeast of Hole 739C.