Geology topics

T. Lorenson

Publications and source records attributed to T. Lorenson.

7 recordsLinked to original sources

USGS2010

The tail of the Storegga Slide: Insights from the geochemistry and sedimentology of the Norwegian Basin deposits

Deposits within the floor of the Norwegian Basin were sampled to characterize the deposition from the Storegga Slide, the largest known Holocene-aged continental margin slope failure complex. A 29 to 67 cm thick veneer of variable-coloured, finely layered Holocene sediment caps a homogeneous, extremely well-sorted, poorly consolidated, very fine-grained, grey-coloured sediment section that is >20 m thick on the basin floor. This homogeneous unit is interpreted to represent the uppermost deposits generated by a gravity flow associated with the last major Storegga Slide event. Sediments analogous to the inferred source material of the slide deposits were collected from upslope on the Norwegian Margin. Sediments sampled within the basin are distinguishable from the purported source sediments, suggesting that size sorting has significantly altered this material along its flow path. Moreover, the very fine grain size (3·1 ± 0·3 μm) suggests that the >20 m thick homogeneous unit which was sampled settled from suspension after the turbulent flow was over. Although the turbulent phase of the gravity flow that moved material out into the basin may have been brief (days), significantly more time (years) is required for turbid sediments to settle and dewater and for the new sea floor to be colonized with a normal benthonic fauna. Pore water sulphate concentrations within the uppermost 20 m of the event deposit are higher than those normally found in sea water. Apparently the impact of microbial sulphate reduction over the last ca 8·1 cal ka bp since the re-deposition of these sediments has not been adequate to regenerate a typical sulphate gradient of decreasing concentration with sub-bottom depth. This observation suggests low rates of microbial reactions, which may be attributed to the refractory carbon composition in these re-deposited sediments.

Storegga Slide↗

USGS2007

Understanding shallow gas occurrences in the Gulf of Lions

New coring data have been acquired along the western Gulf of Lions showing anomalous concentrations of methane (up to 95,700 ppm) off the Rhône prodelta and the head of the southern canyons Lacaze-Duthiers and Cap de Creus. Sediment cores were acquired with box and kasten cores during 2004–2005 on several EuroSTRATAFORM cruises. Anomalous methane concentrations are discussed and integrated with organic carbon data. Sampled sites include locations where previous surveys identified acoustic anomalies in high-resolution seismic profiles, which may be related to the presence of gas. Interpretation of the collected data has enabled us to discuss the nature of shallow gas along the Gulf of Lions, and its association with recent sedimentary dynamics. The Rhône prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake, and leading to the potential for methanogenesis with burial. Away from the flood-related sediments off the Rhône delta, the organic matter is being reworked and remineralized on its way along the western coast of the Gulf of Lions, with the result that the recent deposits in the canyon contain little reactive carbon. In the southernmost canyons, Lacaze-Duthiers and Cap de Creus, the gas analyses show relatively little shallow gas in the core samples. Samples with anomalous gas (up to 5,000 ppm methane) are limited to local areas where the samples also show higher amounts of organic matter. The anomalous samples at the head of the southern canyons may be related to methanogenesis of recent drape or of older sidewall canyon infills.

Gulf of Lions↗

USGS2007

Methane-derived authigenic carbonates from the northern Gulf of Mexico - MD02 Cruise

Authigenic carbonates were sampled in piston cores collected from both the Tunica Mound and the Mississippi Canyon area on the continental slope of the northern Gulf of Mexico during a Marion Dufresne cruise in July 2002. The carbonates are present as hardgrounds, porous crusts, concretions or nodules and shell fragments with or without carbonate cements. Carbonates occurred at gas venting sites which are likely to overlie gas hydrates bearing sediments. Electron microprobe, X-ray diffraction (XRD) and thinsection investigations show that these carbonates are high-Mg calcite (6–21 mol% MgCO 3 ), with significant presence of framboidal pyrite. All carbonates are depleted in 13 C ( δ 13 C = − 61.9 to − 31.5‰ PDB) indicating that the carbon is derived mainly from anaerobic methane oxidation (AMO). Age estimates based on 14 C dating of shell fragments and on regional sedimentation rates indicate that these authigenic carbonates formed within the last 1000 yr in the Mississippi Canyon and within 5500 yr at the Tunica Mound. The oxygen isotopic composition of carbonates ranges from + 3.4 to + 5.9‰ PDB. Oxygen isotopic compositions and Mg 2+ contents of carbonates, and present in-situ temperatures of bottom seawater/sediments, show that some of these carbonates, especially from a core associated with underlying massive gas hydrates precipitated in or near equilibrium with bottom-water. On the other hand, those carbonates more enriched in 18 O are interpreted to have precipitated from 18 O-rich fluids which are thought to have been derived from the dissociation of gas hydrates. The dissociation of gas hydrates in the northern Gulf of Mexico within the last 5500 yr may be caused by nearby salt movement and related brines.

Journal of Geochemical Exploration↗

USGS2006

Shallow gas off the Rhône prodelta, Gulf of Lions

Sediment cores acquired in 2004 off the Rhône prodelta show consistent anomalous methane concentrations of up to 87,440 ppm. Methane compositional and isotopic data support a biogenic origin, although there are a few sites that show strongly depleted δ 13 C values (− 53&permil; PDB) suggesting a mixed source for the gas (biogenic and thermogenic). Anomalous methane concentrations (samples with more than 90 ppm) are discussed and integrated with organic carbon data, sedimentary rates and ADCP profiles. Highest gas concentrations were found directly off the river mouth (20–40 m water depth) and where the IFREMER models point to the thickest accumulation (> 2 m) in response to the Rhône flood event. In areas unaffected by the high flux of organic matter and rapid/thick flood deposition, or in between flood events, the conditions for methanogenesis and gas accumulation have not been met; in these areas, the physical and biological reworking of the surficial sediment may effectively oxidize and mineralize organic matter and limit bacterial methanogenesis in the sub-surface. We propose that in the Rhône prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake and leading to the potential for methanogenesis with burial.

Marine Geology↗

USGS2005

Geochemical constraints on the distribution of gas hydrates in the Gulf of Mexico

Gas hydrates are common within near-seafloor sediments immediately surrounding fluid and gas venting sites on the continental slope of the northern Gulf of Mexico. However, the distribution of gas hydrates within sediments away from the vents is poorly documented, yet critical for gas hydrate assessments. Porewater chloride and sulfate concentrations, hydrocarbon gas compositions, and geothermal gradients obtained during a porewater geochemical survey of the northern Gulf of Mexico suggest that the lack of bottom simulating reflectors in gas-rich areas of the gulf may be the consequence of elevated porewater salinity, geothermal gradients, and microbial gas compositions in sediments away from fault conduits.

Geo-Marine Letters↗

USGS2005

High-resolution surveys for geohazards and shallow gas: NW Adriatic (Italy) and Iskenderun Bay (Turkey)

The need for quantifying and understanding the distribution of shallow gas is both of academic interest and of relevance to offshore facilities. The combination of seafloor mapping, subbottom profiling, and multi-channel seismic data can provide information on regions of possible shallow gas, where the gas impacts the acoustic properties of the host material and the seafloor. In this paper, we present two case studies - one academic and one industry - that evaluate the distribution of shallow gas in two field areas in the Mediterranean. In the first case study, geophysical data from Iskenderun Bay, southeastern Turkey, indicate the presence and distribution of shallow gas. Pockmarks on the seafloor are associated with acoustic wipeout in the shallow subbottom data. Although deeper seismic data do not show bright spots or other indicators of possible gas, instantaneous frequency analysis clearly shows laterally restricted anomalies indicating gas-rich zones. The interpretation of possible shallow gas resulted in moving a proposed drilling location to a nearby area characterized by fewer (but still present) shallow gas signatures. In the second case study, cores acquired in the Po Delta, Adriatic Sea, provide quantitative ground-truthing of shallow gas - as suggested by geophysical data - and provide minimum estimates of the percentage of gas in the subsurface. Cores targeted on anomalous subbottom data yielded up to 41,000 ppm methane; cores with anomalous gas content are associated with thick recent flood deposits which may effectively isolate reactive terrigenous organic matter from biologic and physical re-working. ?? Springer 2005.

Marine Geophysical Research↗

USGS2002

Pockmarks off Big Sur, California

A pockmark field was discovered during EM-300 multi-beam bathymetric surveys on the lower continental slope off the Big Sur coast of California. The field contains ??? 1500 pockmarks which are between 130 and 260 m in diameter, and typically are 8-12 m deep located within a 560 km2 area. To investigate the origin of these features, piston cores were collected from both the interior and the flanks of the pockmarks, and remotely operated vehicle observation (ROV) video and sampling transects were conducted which passed through 19 of the pockmarks. The water column within and above the pockmarks was sampled for methane concentration. Piston cores and ROV collected push cores show that the pockmark field is composed of monotonous fine silts and clays and the cores within the pockmarks are indistinguishable from those outside the pockmarks. No evidence for either sediment winnowing or diagenetic alteration suggestive of fluid venting was obtained. 14C measurements of the organic carbon in the sediments indicate continuous sedimentation throughout the time resolution of the radiocarbon technique ( ??? 45000 yr BP), with a sedimentation rate of ??? 10 cm per 1000 yr both within and between the pockmarks. Concentrations of methane, dissolved inorganic carbon, sulfate, chloride, and ammonium in pore water extracted from within the cores are generally similar in composition to seawater and show little change with depth, suggesting low biogeochemical activity. These pore water chemical gradients indicate that neither significant accumulations of gas are likely to exist in the shallow subsurface ( ??? 100 m) nor is active fluid advection occurring within the sampled sediments. Taken together the data indicate that these pockmarks are more than 45000 yr old, are presently inactive, and contain no indications of earlier fluid or gas venting events. ?? 2002 Elsevier Science B.V. All rights reserved.

Marine Geology↗