Geochemistry of Interstitial Gases in Sedimentary Deposits of the Gulf of California, Deep Sea Drilling Project Leg 64

Abstract

We analyzed interstitial gases from holes at Sites 474, 477, 478, 479, and 481 in the Gulf of California, using gas chromatography and stable isotope mass spectrometry to evaluate their composition in terms of biogenic and thermogenic sources. The hydrocarbon gas (C1-C5) concentrations were comparable to the shipboard data, and no olefins could be detected. The δ 13 C data for the CH 4 confirmed the effects of thermal stress on the sedimentary organic matter, because the values were typically biogenic near the surface and became more depleted in 12 C versus depth in holes at Sites 474, 478, and 481. The CH 4 at Site 477 was the heaviest, and in Hole 479 it did not show a dominant hightemperature component. The CO 2 at depth in most holes was mostly thermogenic and derived from carbonates. The low concentrations of C 2 -C 5 hydrocarbons in the headspace gas of canned sediments precluded a stable carbon-isotope analysis of their genetic origin.

Molecular and Isotopic Analysis of Core Gases and Gas Hydrates, Deep Sea Drilling Project Leg 96

“…At sub-bottom depths greater than those sampled at Leg 96 sites, ethane and other C 2 -C 5 hydrocarbons were detected (Galimov et al, 1980;Schoell, 1982). In contrast, core gas-pocket studies in the Japan Trench (Whelan and Sato, 1980), California Basin (Galimov and Simoneit, 1982), and Middle America Trench (Jeffrey et al, 1985) found C 2 -C 5 hydrocarbons at most depths where gas pockets were sampled. The processes controlling the amount of C 2 -C 5 hydrocarbons in shallow sediments containing primarily biogenic gas are unclear.…”

A magmatic trigger for the Paleocene-Eocene Thermal Maximum?

“…The presence of both authigenic and detrital components was also noted in the types of organic matter found in the sediments Simoneit and Philp, 1982;Gilbert and Summerhayes, 1982;Galimov and Simoneit, 1982).…”

“…Interstitial gas contains light hydrocarbons indicative of thermogenesis with very negative δ 13 C (Whelan and Hunt, 1982;Galimov and Simoneit, 1982;Simoneit and Galimov, 1984). Bitumen fractions and the makeup of the n-alkenes similarly indicate high levels of thermogenic alteration of the sediments at DSDP Site 477 .…”

“…DSDP Leg 64 Site 477 is located in the southern area of the rift zone . Here, continental rifting causes doloritic sill and dike intrusions into unconsolidated organic-rich sediments, and the subsequent thermal alteration of these sediments causes release of carbon primarily as methane and CO 2 (Gieskes et al, 1982;Kastner et al, 1982;Galimov and Simoneit, 1982;Seewald et al, 1990).…”

“…DSDP Site 477 was chosen as an ideal location to isolate the effects of thermal alteration on the sediments due to the abundance of evidence indicating metamorphic alteration of organic matter within the sediments near the upper sill intrusion (Whelan and Hunt, 1982;Galimov and Simoneit, 1982;Simoneit and Galimov, 1984;Simoneit et al, 1986).…”