2. Chlorin and Porphyrin Geochemistry of Dsdp Leg 40 Sediments

Even thick sequences of sediments in marine depositional basins contain only limited segments of the record of chlorophyll diagenesis. Extracts from Pleistocene deep-sea sediments (Baker and Smith, 1973a) yielded extensive information concerning early chlorin transformations. Investigations of marine sediments of Pliocene and Miocene age resulted in the first reports of free base porphyrins occurring with nickel porphyrins and chlorins (Baker and Smith, 1975a; Smith and Baker, 1974). Initial studies of Pliocene, Eocene, and late Cretaceous marine sediments indicated the co-existence of nickel porphyrins and chlorins (Baker and Smith, 1975b). The characterization of late diagenetic products, the petroporphyrins, has previously been reported in Baker et al., 1967. The present suite of samples from DSDP Sites 361 and 364 has yielded the most complete record of the free base-metalloporphyrin transition in an early Cretaceous setting known to date. These samples fill the gap between early diagenesis (Baker and Smith, 1973a) and late diagenesis (petroleum and bitumen). Only until data such as these are obtained for each stage of chlorophyll diagenesis can statements concerning the relationships between tetrapyrrole structure and geological history of marine basins (including expression of oxidation-reduction state of the enclosing sediments, effect of thermal treatment and age/depth of burial) be made with a high degree of reliability. The abundance of pigment in early Cretaceous samples permitted more extensive separation and purification techniques than was previously reported for DSDP samples. Separation of the pigments into nickel, vanadyl, and free base varieties and mass spectrometry of these fractions have provided important new insights into the distribution of the geoporphyrins. Compared to previous reports in which the total pigment was demetallated and subsequently subjected to mass spectrometric analysis, the present approach shows that many important differences exist among these fractions. The most extensive mass spectral data yet obtained on metalloporphyrins in deep ocean sediments are reported here. These analyses extend and/or confirm previous ideas concerning the most probable middiagenetic pathway of chlorophyll and its derivatives (Baker and Smith, 1973b). Nineteen samples ranging in age from Pleistocene to early Cretaceous from DSDP Sites 361-364 were examined for tetrapyrrole content. Sediments from Walvis Ridge (Site 362), ranging from Pleistocene to Miocene in age, contained chlorins with trace amounts of free base porphyrins present. The amount of chlorin decreased with increasing depth/geologic age consistent with previous reports. Early Cretaceous sediment samples from Site 361 (Cape Basin) and particularly Site 364 (Angola Basin) contained abundant nickel and vanadyl porphyrins in line with expectations for older organic-rich sediments. Free base porphyrins were found at Site 361 and 364 samples along with the two chelates. Site 363 samples were barren of pigment except for the oldest core (early Cretaceous) which contained a small amount of nickel porphyrin. Results from Site 356 (Leg 39) where small amounts of metalloporphyrins were found are included (see Table 1). A small amount of a previously unrecognized metalloporphyrin was found in extracts from Site 364. Electronic spectral data, chromatographic behavior, and fluorescence characteristics show definitely that this metalloporphyrin is not a typical (nickel or vanadyl) chelate. Total characterization is not complete, but the data best fit that of a zinc chelate of a highly polar porphyrin ligand.