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Twenty-six core samples from Leg 64, Holes 474, 474A, 477, 478, 479, and 481A in the Gulf of California, were provided by the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES) Advisory Panel on Organic Geochemistry for analysis. The high heat flow characteristic of the basin provides an opportunity to study the effect of temperature on the diagenesis of organic matter. The contents and carbon isotope compositions of the organic matter and bitumen fractions of different polarity, isoprenoid and normal alkane distributions, and the nature of tetrapyrrole pigments were studied. Relative contents of hydrocarbons and bitumens depend on the thermal history of the deposits. Among other criteria, the nature and content of tetrapyrrole pigments appear to be most sensitive to thermal stress. Whereas only chlorins are present in the immature samples, porphyrins, including VO-porphyrins, appear in the thermally altered deposits, despite the shallow burial depth. Alkane distributions in thermally changed samples are characterized by low values of phytane to Λ C 1 8 ratios and an odd/even carbon preference index close to unity. The thermally altered samples show unusual carbon isotope distributions of the bitumen fractions. The data also provide some evidence concerning the source of the organic matter and the degree of diagenesis. INTRODUCTION Core samples were studied from Holes 474, 474A, 477, 478, 479, and 481A in the Gulf of California (for site locations, see the introduction to the organic geochemistry section, this volume). The samples provide a unique opportunity to study the effect of temperature on the diagenesis of sedimentary organic matter of Recent geologic age. The activities of recent tectonic processes generate extremely diversified thermal conditions within the sedimentary basin. Holes 474 and 474A were drilled at the base of the continental slope, 55 km southeast of the tip of Baja California (water depth = 3022-3023 m). Site 474 is on ocean crust overlain by about 500 meters of mainly diatomaceous mud and mudstone turbidites. Hole 474 was cored to 182.5 meters and Hole 474A to 626 meters. Sediments are penetrated by two dolerite sills at the bottom of the hole. The heat flow is 3.1 units (μcal cms~)Hole 477 is in the Guaymas Basin, an actively spreading ocean basin that includes two spreading rifts. Hole 477 was drilled to 191 meters in the southern rift (water depth = 2003 m). The sediments are late Quaternary diatom ooze and turbidites intruded by a dolerite sill from 58-105.5 meters. Dolomite, anhydrite, and pyrite occur below the sill. Mineral paragenesis includes epidotes, quartz, albite, and chlorite. These hydrothermal minerals indicate temperatures up to 300 °C (Einsele et al., 1980). The heat flow in the hole has a value of about 20 units. A major heat source appears to be below the drilled section. Hole 478 is 12 km from Site 477 and outside the present rift zones in the transform fault zone (water depth = Curray, J. R., Moore, D. G., et al., Init. Repts. DSDP, 64: Washington (U.S. Govt. Printing Office). 1889 m). The sub-bottom depth is 464 meters. Several sill intrusions occur in the sediments, but the heat flow is lower (3.65 units) than in Hole 477. The oldest sediments are late Pleistocene. Hole 481A was drilled in the southwestern part of the northern rift of Guaymas Basin at a water depth of 1998 meters; drilling terminated at 364 meters sub-bottom. The heat flow value is about 4 units. The oldest sediments are late Quaternary diatomaceous ooze and mudstones. Several sills penetrated the sediments at 169197, 328-333, 350-363, and 365-366 meters. The temperature in this hole should not have exceeded 150°C (Einsele et al., 1980). Hole 479 lies on the northwest continental slope of the Guaymas Basin at a water depth of 747 meters. The hole penetrated 440 meters into a sequence of diatomaceous ooze to laminated mudstones. This site lies in the oxygen-minimum zone and has a high sedimentation rate. The heat flow was measured at 2.36 units. METHODS Core samples (70-300 g; average = 150 g) were obtained unfrozen in airtight plastic packets, which precluded cleaning the samples by removing the outer surface layers. Samples were placed in glass dishes, air-dried, ground in an agate mortar, sifted through a 60-mesh sieve, and then used for preparation and analysis. The method of separation, fractionation, and analysis of the organic matter is described in Galimov et al. (1980). Separation and Fractionation of Bitumen Dry core samples were exhaustively extracted in a Soxhlet apparatus with benzene-methanol (9:1). Coextracted inorganic salts were removed by washing the dry residues with distilled water; elemental sulfur was removed with activated Cu foil. The bitumen extracts were then evaporated, air-dried, and weighed. The extracts were colorless (Sections 481A-13-6, 477-16-5, 477-71), light yellow (Section 474A-40-3), or bright green (Sections 478-2-5, 478-13-2) to dark brown (Sections 481A-30-5, 478-28-4). Asphaltenes were precipitated from the bitumen solutions in benzene by a 50-fold volume excess of pentane. The precipitates were separated by filtering them through an N4 sintered glass filter. The 819 E. M. GALIMOV, L. A. KODINA, M. P. BOGACHEVA, V. G. SHIRINSKY pentane solutions were fractionated on silica gel columns by using the solvent mixtures for elution in the following sequence: hexane-benzene (1:1), benzene, and benzene-methanol (1:1), producing corresponding bitumen fractions. The hexane fractions contain hydrocarbons, which are brightly luminescent and light yellow. The fractions were evaporated and then air-dried at 40°C. The next three fractions contain hexane-benzene, benzene, and benzene-methanol soluble resins, which differ from each other by their polarity. The hexane-benzene fractions are often pinkish or orange owing to the presence of Niporphyrins. The benzene fractions are the smallest and are light yellow or sometimes reddish because of the presence of other metalloporphyrin complexes. The benzene-methanol resins—the predominant fractions of the pentane solubles—are dark brown or greenish brown. The resin fractions were rotoevaporated at 40 °C and air-dried at room temperature. Gas Liquid Chromatography The hexane fractions were analyzed by gas chromatography. A copper capillary column containing Apiezon L was used. The distribution data on /?-alkanes and the isoprenoid alkanes pristane and phytane were determined. For details of the chromatographic procedure see Galimov et al. (1980). Visible Light Spectroscopy To trace the different forms of tetrapyrrole pigments, absorption spectra of some bitumen fractions (especially hexane-benzene) in the visible region from 320 to 700 nm were recorded on an SF-8 recording spectrophotometer. The electron spin resonance (ESR) spectra of hexane-benzene and benzene fractions for several core samples were also recorded. Aliquots of bitumen without Cu treatment were used for recording spectra. Isotopic Analysis of Carbon The isotopic compositions of carbon for total bitumen, all the bitumen fractions, and the total organic matter after treatment with hydrochloric acid were determined as carbon dioxide. The measurements were made on a Varian-MAT-230 mass spectrometer. The reproducibility of the experimental results, including the full sequence of separation and fractionation of organic matter and preparation of samples for isotopic analysis and measurements, does not exceed an average of ±0.2‰. RESULTS The Content and Components of Organic Matter Total Organic Matter Organic carbon content data are presented in Table 1 and Figure 1. The sediments are characterized by the rather high organic carbon content—from 0.16 to 2.7%, with an average of about 1.5%. The highest value occurred in the sediments of Hole 479 (average 1.84% and varying from 1.04 to 2.17%). Although the organic carbon (Corg) content throughout the profiles varies to some extent, it is rather uniform, except for a distinct drop in the samples near sills, interpreted to be due to their thermal effect. Bitumen The variations of bitumen content in the sequence of Hole 474 are 165 to 1786 ppm; in Hole 477, from 87 to 1840 ppm; in Hole 478, from 192 to 3163 ppm; and in Hole 481 A, from 97 to 2550 ppm (Fig. 1, Table 1). When normalized to the organic carbon content, the amount of bitumen varies from 1.22% to 36.36% (Fig. 2). The highest bitumen percentage (Fig. 1) as well as the amount of bitumen normalized to Corg content (Fig. 2) was found in Section 477-7-1, 8 meters above the sill. There were very low bitumen contents in two Sections (477-16-5, 477-20-1) affected by strong thermal stress and in Section 481-13-6, about 5 meters from a sill and dramatically influenced by that intrusive body. Similar effects were reported previously by Simoneit et al. (1978), Perregaard and Schiener (1979), and Dow (1978). Bitumen Components The concentrations of bitumen components are given in Figure 3 and Table 1. Asphaltenes are clearly predominant. Asphaltene contents for the majority of samples vary from 42 to 73% of the whole extract, except for four sections: 477-16-5, 477-20-1, 474A-40-3, and 481 A-13-6. Asphaltene concentrations in these sections are only 9 to 28% of the bitumen. Hexane fractions (hydrocarbon) contents vary from 2 to 50% of the bitumen (Fig. 3) and from 0.18 to 4.77% when normalized to the Corg content (Fig. 2). In most sections the hexane fraction amounts to 3 to 7% of the bitumen, and reaches 10 to 20% for some sections (474A-40-3, 477-71, 478-35-5, 479-43-1) because of the proximity of sills. Strong thermal stress causes hydrocarbons to dominate the bitumen fraction (up to 50% in Sections 477-16-5, 477-20-1, and 481A-13-6; Fig. 3). When normalized to Corg content, the hydrocarbon content in these thermally stressed sections, however, is lower than in samples not severely heated by the sills (in Sections 474A-40-3, 477-7-1, 478-35-5, and 479-43-1).

Organic Geochemical Studies of Samples from Deep Sea Drilling Project Leg 64, Gulf of California, Sites 474, 477, 478, 479, and 481