Lorraine Eglinton

Holocene history of the Larsen-A Ice Shelf constrained by geomagnetic paleointensity dating

A sedimentary record collected from beneath the former Lar- sen-A Ice Shelf reveals the Holocene history of the Larsen-A re- gion. The record begins with the transition from grounded ice to a floating ice shelf, completed by 10.7 6 0.5 ka, and ends with the modern recession. The record contains several late Holocene dia- tomaceous ooze layers that suggest proximity to productive open- water events. Radiocarbon ages obtained from these sediments were complicated by the presence of detrital and reworked carbon. We have eliminated these complications and constructed a chro- nology for the Larsen-A Ice Shelf history via tuning of the geo- magnetic field paleointensity record with a reference curve. This approach provides chronological control to sediment sequences that lack appropriate material for radiocarbon dating. Geomag- netic paleointensity features with wavelengths of 2-3 k.y. can be recognized and interhemispherically correlated, illustrating the po- tential to use geomagnetic paleointensity variations as a global cor- relation and dating tool at sub-Milankovitch time scales.

Vertical and Lateral Fluid Flow Related to a Large Growth Fault, South Eugene Island Block 330 Field, Offshore Louisiana

Data from sediments in and near a large growth fault adjacent to the giant South Eugene Island Block 330 field, offshore Louisiana, indicate that the fault has acted as a conduit for fluids whose flux has varied in space and time. Core and cuttings samples from two wells that penetrated the same fault about 300 m apart show markedly different thermal histories and evidence for mass flux. Sediments within and adjacent to the fault zone in the U.S. Department of Energy-Pennzoil Pathfinder well at about 2200 m SSTVD (subsea true vertical depth) showed little paleothermal or geochemical evidence for throughgoing fluid flow. The sediments were characterized by low vitrinite reflectances (Ro), averaging 0.3% Ro, moderate to high d18O and d13C values, and little difference in major or trace element composition between deformed and undeformed sediments. In contrast, faulted sediments from the A6ST well, which intersects the A fault at 1993 m SSTVD, show evidence for a paleothermal anomaly (0.55% Ro) and depleted d18O and d13C values. Sodium is depleted and calcium is enriched in a mudstone gouge zone at the top of the fault cut in the well; this effect diminishes with distance from this gouge zone. Cuttings from other wells in South Eugene Island Block 330 show slightly elevated vitrinite reflectance in fault intercepts relative to sediments outside the fault zone. Overall, indicators of mass and heat flux indicate the main growth fault zone in South Eugene Island Block 330 has acted as a conduit for ascending fluids, although the cumulative fluxes vary along strike. This conclusion is corroborated by oil and gas distribution in downthrown sands in Blocks 330 and 331, which identify the fault system in northwestern Block 330 as a major feeder. Simple modeling of coupled heat and mass flux indicates the paleothermal anomaly in the fault zone intersected by A6ST well was short-lived, having a duration less than 150 yr. The anomaly could have been produced by a 2 ´ 106 m3 pulse of fluid ascending the fault at an actual velocity of over 1 km/yr (Darcy flux of 330 m/yr) from 3 km deeper in the basin. Simple Darcy law computation indicates a transient fault permeability on the order of 110 md during this flow. Pulsing of fluid up the fault was probably the norm, although most flow did not produce such strong thermal anomalies as the one detected in the A6ST well. Analysis of fluid pressures shows that the main fault is a profound lateral permeability barrier having up to 1800 psi of water pressure differential across it. The hydrocarbon sealing capacity of the fault depends on the pressure difference across the fault. Fault permeability is best understood in terms of effective stress. Under ambient conditions, the fault is at high pressure relative to downthrown reservoirs. A pulse of high-pressure fluid ascending End page 244---------------- the fault lowers effective stress in the fault zone sufficiently to produce a significant transient increase in permeability. If the fluid is in an area of the fault adjacent to downthrown, relatively low pressure reservoir sands, the fluid will discharge into them. Permeability in and adjacent to the fault then decreases, such that fluid cannot reenter the fault zone and escape from the reservoir.

Cytochrome P4501A expression, chemical contaminants and histopathology in roach, goby and sturgeon and chemical contaminants in sediments from the Caspian Sea, Lake Balkhash and the Ily River Delta, Kazakhstan.

Roach, goby and sturgeon were examined for cytochrome P4501A (CYP1A) expression and histopathology, in relation to contaminant burdens in fish and sediment. Gradients of induction of CYP1A were observed. Roach from the Ural and Ily River Deltas and roach and goby from the two stations nearest the Caspian Sea oil fields displayed higher levels of CYP1A expression in several organs than was observed in fish from further offshore. Great sturgeon and Russian sturgeon showed higher levels of CYP1A expression than was seen in starred sturgeon and gobies in the Ural delta. No fish showed evidence of contaminant-related histopathologies in the organs examined, despite the elevated CYP1A levels. Low levels of polychlorinated biphenyls and elevated levels of inshore and riverine petroleum hydrocarbons from these habitats suggest that this ongoing hydrocarbon exposure, and that from natural sources and long-term oil exploration on the Northeastern Caspian shore, contributed to the CYP1A induction observed.

Contribution of “Old” carbon from natural marine hydrocarbon seeps to sedimentary and dissolved organic carbon pools in the Gulf of Mexico

Natural radiocarbon (14C) abundances and stable carbon isotope (δ13C) compositions were measured for sediment total organic carbon (TOC), and total lipid fractions of sediments, bottom water, and hydrate-water collected from two hydrocarbon seepage sites in Green Canyon, Northern Gulf of Mexico to determine the contribution of “old” carbon from seeps to sediment TOC and dissolved organic carbon (DOC) pools. Our results indicate that 40–60% of the organic carbon preserved in the sediments and 30% of the DOC in the deep water above the seeps were seep-derived 14C-depleted organic carbon. This new evidence along with our previous studies suggest that natural marine hydrocarbon seepage could be a significant source contributing “old” carbon to the marine environment. Our findings suggest that the global importance and the long-term impact of this contribution to biogeochemical carbon cycling in the ocean need to be more thoroughly investigated.

An experimental study of organic-inorganic interactions during vitrinite maturation

Abstract Vitrinite reflectance is a parameter widely used to determine the extent of thermal maturation in sedimentary basins. Laboratory heating experiments were conducted to examine the role of aqueous fluid and sediment composition, temperature, and time on the development of vitrinite reflectance at elevated temperatures and pressures. Samples of organic-lean Middle Valley sediment containing dispersed vitrinite and Wyodak Coal composed almost entirely of vitrinite were heated in the presence of compositionally variable aqueous fluids at 225° to 375°C and 350 bars. Inorganic fluid composition was monitored as a function of time, and vitrinite reflectance was measured at the termination of each experiment. The rate of vitrinite maturation was influenced by inorganic fluid and vitrinite composition, in addition to time and temperature during the experiments. In particular, the rate at which vitrinite reflectance increased for a given temperature correlated positively with in situ H + activity. This observation suggests that acid catalyzed ionic reaction mechanisms and/or catalytically active transition metals and sulfur species play an important role in organic transformations responsible for increased vitrinite reflectance. The activity of other inorganic ionic species such as Mg ++ , Ca ++ , Na + , K + , and SO 4 = , and the redox state of the fluid did not influence the rate of vitrinite maturation during these experiments. Results of this study demonstrate that the geochemical environment surrounding vitrinite influences the rate of maturation. The presence of water may facilitate reaction pathways that are not readily available in dry systems. Vitrinite maturation profiles that deviate from what are considered to be “normal” trends may reflect subsurface variations in the composition of pore fluids or the presence or absence of an aqueous phase in contact with vitrinite particles. Accordingly, kinetic models that consider organic reactions responsible for increased vitrinite reflectance in terms of time and temperature alone may not accurately account for geochemical processes occurring in natural systems.

Short-time-scale (year) variations of petroleum fluids from the U.S. Gulf Coast

Abstract Evolving short-term (less than 5 yr) compositional changes in hydrocarbon charge from some Eugene Island Block 330 (EI-330) wells are demonstrated. Storage, analytical, and production artifacts are shown to be minimal. In some wells, compositions remain constant from 1985 to 1993, whereas in others in the same reservoir, significant changes are observed. In some cases, temporal variability is greater than spatial variability. Maximum temporal change is strongest for specific compounds: toluene and C 6 to C 9 normal alkanes, but is also observed to a lesser extent for higher-molecular-weight components (up to n -C 32 ). Principal coordinate analysis shows the highest degree of overall temporal compositional change over an 8-yr period in the shallowest wells where there is also evidence of biodegradation. Small temporal compositional changes are also observed in two deeper wells that are below the thermal window favorable for biodegradation. An exception is an unusual oil, where a very large increase in toluene, as well as smaller changes in a number of n -alkanes, was observed in 1993. The δ 13 C compound-specific isotopic signature of toluene, in addition to several other C 7 -C 8 compounds in this oil, yields convincing evidence that it is related to the same family as other EI-330 oils and unlikely to be due to a drilling or laboratory contaminant. Minor isotopic differences in other C 7 compounds (1.5‰) are consistent with extensive gas washing of this oil. The short-term compositional changes in EI-330 oils are attributed to gas washing, which causes overprinting of biodegraded oils with light n -alkanes in shallower GA and HB reservoirs where oils are currently being biodegraded in situ. Patterns of smaller changes in heavier compounds in both shallower and deeper wells are also consistent with this interpretation.