Applied Geochemistry (v.17, #11)

Current Lit Survey (III-VII).

Geochemistry of fluid phases and sediments: relevance to hydrothermal circulation in Middle Valley, ODP Legs 139 and 169 by Joris M Gieskes; Bernd R.T Simoneit; Wayne C Shanks; Wayne D Goodfellow; Rachael H James; Paul A Baker; Jun-ishiro Ishibashi (1381-1399).
Geochemical and isotopic studies of pore fluids and solid phases recovered from the Dead Dog and Bent Hill hydrothermal sites in Middle Valley (Ocean Drilling Program Leg 169) have been compared with similar data obtained previously from these sites during Ocean Drilling Program Leg 139. Although generally the hydrothermal systems reflect non-steady state conditions, the data allow an assessment of the history of the hydrothermal processes. Sediment K/Al ratios as well as the distribution of anhydrite in the sediments suggest that the Dead Dog hydrothermal field has been, and still is, active. In contrast, similar data in the Bent Hill hydrothermal field indicate a waning of hydrothermal activity. Pore fluid and hydrothermal vent data in the Dead Dog hydrothermal field are similar in nature to the data collected during ODP Leg 139. In the area of the Bent Hill sulfide deposit, however, the pore water data indicate that recent wholesale flushing of the sediment column with relatively unaltered seawater has obliterated a previous record of hydrothermal activity in the pore fluids. Data from the deepest part of Hole 1035A in the Bent Hill locality show the presence of hydrothermal fluids at greater depths in this area. This suggests the origin of the hydrothermal fluids found to be emanating from Hole 1035F, which constitutes one of the first man made hydrothermal vents in the Middle Valley hydrothermal system. Similarly, CORKed Hole 858G, because of seal failures, has acted as a hydrothermal vent, with sulfide deposits forming inside the CORK.

Samples from the Ocean Drilling Program Leg 169 in Middle Valley, NE Pacific Ocean, were analyzed to study maturation by accelerated diagenesis and catagenesis of the sedimentary organic matter to hydrothermal petroleum. Some data from Site 858 in Middle Valley (Leg 139) are also discussed for completeness. Sediments at Sites 858 and 1036 in Middle Valley were altered either by the full variety of high temperature fluids, which generated hydrothermal petroleum, or by accelerated thermal diagenesis of the immature organic matter in situ without specific migration of those petroleum (bitumen) products. The hydrothermal petroleums from the deeper sediments have migrated and accumulated in discrete intervals at shallower depths within a contemporary temperature window of 60–135 °C at both of these sites. Maturation for the n-alkanes proceeded from high CPI values (immature) to <1.0 (mature), i.e., strong even C number preference. Due to their low TOC contents, these sediments did not generate high amounts of hydrocarbons from the kerogen and thus revealed this unique even C number preference. Furthermore, the significant content of terrestrial organic matter yielded petroleums low in gas and volatiles. Biomarkers were converted from the biological natural product precursors to the geological derivatives with maturities reflecting the thermal stress. High-temperature alteration also yielded polynuclear aromatic hydrocarbons.

Individual aliphatic hydrocarbons in samples of bulk hydrothermal petroleum from Middle Valley (ODP Site 858) were submitted to compound specific isotope analysis to trace their origins. The C isotope compositions of the n-alkanes and isoprenoid hydrocarbons [means n-alkanes: −30.1±1.01 (858A), −28.3±1.7 (858C) and −24.8±1.4 (858D)‰; isoprenoids: −26.0±2.7 (858C) and −27.9±0.4 (858D)‰, respectively] reflect mixed marine and terrestrial organic matter sources. Biphytane from thermophiles has a δ13C value of −21.0‰.

Hydrothermal geochemistry of sediments and pore waters in Escanaba Trough—ODP Leg 169 by Joris M Gieskes; Bernd R.T Simoneit; Wayne D Goodfellow; Paul A Baker; Chris Mahn (1435-1456).
Geochemical studies of pore fluids and solid phases in two Ocean Drilling Program (ODP) drill sites (Sites 1037 and 1038) in the Escanaba Trough off Northern California have provided further data on the hydrothermal processes associated with the spreading of the Gorda Ridge. Previous work in the area of ODP Site 1038 includes the discovery of a hydrothermal system and associated sulfide deposits centered around an uplifted sediment hill in this sedimented extensional environment. This earlier work provided some insights into the present nature of venting; however, only deep drilling investigations can provide the means to fully understand the genesis and evolution of this system and associated hydrothermal deposits. ODP Leg 169 is the third deep drilling operation to explore the magnitude, genesis, and evolution of hydrothermal systems on sedimented ridges. Previous studies centered on the Guaymas Basin in the Gulf of California and the Middle Valley in the NE Pacific Ocean. Pore water studies in the reference ODP Site 1037 and in the hydrothermally active area of ODP Site 1038 have revealed the presence of a complex system of hydrothermally originated fluids. Whereas the data in the reference site indicate recent hydrothermal activity in the basal part of the drill site, the evidence in Site 1038 suggests that fluids of hydrothermal origin spread out at shallow depths around the central hill, causing substantial sediment alteration as well as deposition of hydrothermal sulfides in the near surface zone of the sediments. A second major discovery at Site 1038 was the evidence for fluid phase separation at depth at temperatures possibly in excess of 400 °C. This conclusion is based on the presence of both low Cl and high Cl fluids. The latter appear to be advected rapidly towards the surface, presumably along cracks and faults. The low Cl fluids, however, appear to be transported laterally along sandy horizons in the sediments, thus signifying two very different migration pathways for high Cl and low Cl hydrothermally phase separated fluids. Studies of the organic geochemistry of dissolved gases and matured organic matter corroborate these findings of extensive hydrothermal alteration of the sediments.

Helium and carbon gas geochemistry of pore fluids from the sediment-rich hydrothermal system in Escanaba Trough by Jun-ichiro Ishibashi; Masanori Sato; Yuji Sano; Hiroshi Wakita; Toshitaka Gamo; Wayne C Shanks (1457-1466).
Ocean Drilling Program (ODP) Leg 169, which was conducted in 1996 provided an opportunity to study the gas geochemistry in the deeper part of the sediment-rich hydrothermal system in Escanaba Trough. Gas void samples obtained from the core liner were analyzed and their results were compared with analytical data of vent fluid samples collected by a submersible dive program in 1988. The gas geochemistry of the pore fluids consisted mostly of a hydrothermal component and was basically the same as that of the vent fluids. The He isotope ratios (R/RA=5.6–6.6) indicated a significant mantle He contribution and the C isotopic compositions of the hydrocarbons [δ13C(CH4)=−43‰, δ13C(C2H6)=−20‰] were characterized as a thermogenic origin caused by hydrothermal activity. On the other hand, the pore fluids in sedimentary layers away from the hydrothermal fields showed profiles which reflected lateral migration of the hydrothermal hydrocarbons and abundant biogenic CH4. Helium and C isotope systematics were shown to represent a hydrothermal component and useful as indicators for their distribution beneath the seafloor. Similarities in He and hydrocarbon signatures to that of the Escanaba Trough hydrothermal system were found in some terrestrial natural gases, which suggested that seafloor hydrothermal activity in sediment-rich environments would be one of the possible petroleum hydrocarbon generation scenarios in unconventional geological settings.

Samples from the Ocean Drilling Program Leg 169 in Escanaba Trough, Gorda Ridge, NE Pacific Ocean, were analyzed to study maturation by accelerated diagenesis and/or by catagenesis of the sedimentary organic matter to hydrothermal petroleum. At Site 1038 the hydrothermal petroleums have migrated after generation to shallower horizons. The n-alkane maturation was indicated by the strong even C number preference (i.e., CPI <1.0) as in the case of Middle Valley. All samples contained admixed organic matter of terrigenous and marine components as indicated by the distributions of the biomarkers. The biological precursors were catagenetically altered to their equivalent mature compounds. The presence of high molecular weight PAHs in some sediment sections at Site 1038 reflected the high temperature alteration and reworking of organic matter into mature hydrothermal petroleum. At Site 1037, the reference hole in the Escanaba Trough, the n-alkanes with a strong predominance of odd C number homologs reflected immature non-marine lipid components essentially throughout the hole. Maturation of organic matter was only observed below 450 mbsf with n-alkanes showing a CPI <1.0. The strong even C number predominance in those intervals was attributed to initial maturation by high heat flow during the early rifting process.

Oxidation measurements of nonvolatile dissolved organic carbon (DOC) have been determined by high temperature combustion for pore waters from sediments of Middle Valley and Escanaba Trough, Northeastern Pacific, sampled by the Ocean Drilling Program (ODP) Legs 139 and 169, as well as for overlying and near bottom seawater. The DOC values in the interstitial waters are generally greater than those in the overlying water column, ranging from 0.1 to 158 mg of C per liter (mg C/L). Some of the profiles of DOC in the pore waters are similar to total organic C and total inorganic C profiles. DOC maxima at shallow depths of Site 858 are probably enriched due to higher temperature alteration of organic matter. At Sites 856 and 1035, the DOC increases at greather depths due to the hydrothermal activity. The DOC values correlate with MnO and MgO profiles in Hole 856B. The changes below 30 mbsf in the DOC depth profiles of Sites 858 and 1036 parallel those of the volatile hydrocarbon gases. The reference Site 1037 in Escanaba Trough has DOC values that increase with depth according to the increase in thermal stress.