Seepage Dynamics Research Articles

Using iron speciation in authigenic carbonates from hydrocarbon seeps to trace variable redox conditions

Seepage of hydrocarbon-rich fluids out of the marine sedimentary column is characterized by temporal changes of flow intensity and resultant spatially variable redox conditions. Authigenic carbonates at marine hydrocarbon seeps provide excellent geological and geochemical archives that serve to explore seepage dynamics over time. In this study, we investigated the potential of Mössbuaer spectroscopy and Fe contents of seep-related authigenic carbonates from the Congo Fan, the Gulf of Mexico, and the Black Sea for reconstructing past redox conditions and fluid seepage activity at cold seeps. The Fe speciation observed by Mössbauer spectroscopy and Fe contents suggest that (1) the Congo Fan carbonates precipitated in a sulfidic environment, (2) the formation conditions of seep carbonates were variable at the Gulf of Mexico seep site, ranging from oxic to suboxic and anoxic and even spanning into the methanogenic zone, and (3) the stratified water column of the Black Sea or suboxic condition resulted in low Fe contents of Black Sea carbonates. The study reveals that Fe speciation can provide constraints on the wide range of redox conditions that imprinted seep carbonates during the life span of seepage. Similarly, Mössbauer spectroscopy – particularly when used in combination with the analysis of redox-sensitive elements – is a promising tool to trace variable redox conditions in marine paleoenvironments other than seeps.

Time integrated variation of sources of fluids and seepage dynamics archived in authigenic carbonates from Gulf of Mexico Gas Hydrate Seafloor Observatory

Authigenic carbonate rocks recovered from the Gulf of Mexico Gas Hydrate Seafloor Observatory in Mississippi Canyon block 118 (MC118) at approximately 900m water depth were studied using mineralogical, bulk geochemical, and lipid biomarker analyses. Carbonate rocks occurred as fractured blocks and nodular masses incorporated in carbonate breccias. The carbonates were comprised mainly of high-Mg-calcite and aragonite. The stable carbon isotope composition (δ13C) of authigenic carbonate varied from −29.8‰ to −18.1‰ vs. V-PDB, suggesting a complex mixture of various carbon sources, including dissolved marine inorganic carbon (DIC), oil, as well as methane. Oxygen isotopes (δ18O) varied from +3.4‰ to +5.8‰. The observed 18O-enrichment in relation to calculated equilibrium values in the carbonates probably reflects decomposition of gas hydrates. The most abundant lipid biomarkers in the carbonates were isoprenoidal glycerol dibiphytanyl glycerol tetraethers (GDGTs), predominated by GDGT-2 and GDGT-3, which are typically indicators of anaerobic methane oxidizing archaea (ANMEs). Mono- and bicyclic biphytanes (derived after ether cleavage of GDGT-2 and GDGT-3) showed strong 13C-depletion, which is characteristic for ANMEs. Interestingly, large differences between the δ13C values of the archaeal diether archaeol and acyclic biphytane on the one hand and monocyclic biphytane on the other hand suggest the presence of archaea other than ANMEs. Archaeol and GDGT-0 (containing two acyclic biphytane moieties) are commonly assigned to various methanogenic archaea. Where methane seepage activity is intermediate or low within acoustic wipeout zones at the MC118 gas hydrate site nowadays, microbial communities must have coped with changing conditions as well as longer-term fluctuations in oil and gas seepage or the temporary cessation of hydrocarbon flux in the past. The change from methane seepage to oil seepage or vice versa in addition to flux variability apparently favors the establishment of complex prokaryotic communities dominated by archaea. In addition to anaerobic oxidation of methane, local production of methane is apparently prominent at the study site based on the occurrence of biomarkers of methanogens in the authigenic carbonate. This finding adds to the ongoing multidisciplinary effort to better constrain the environment at the MC118 observatory site and to determine the locally dominant biogeochemical processes.

Selected contributions from the 11th Gas in Marine Sediments International Conference of September 2012, Nice: an introduction

This Introduction presents an overview of selected contributions from the 11th Gas in Marine Sediments International Conference held on the 4–7 September 2012 in Nice, France, and published in this special issue of Geo-Marine Letters under the guest editorship of Catherine Pierre, Patrice Imbert and Jean Mascle. These cover fluid seepage dynamics at widely varying spatiotemporal scales in a giant buried caldera of the Caspian Sea, mud volcanoes and pockmarks in the Mediterranean and adjoining Gulf of Cadiz, as well as Lake Baikal, pockmarks of shallower waters along the Atlantic French coast and in Baltic Sea lagoons, deepwater pockmarks and cold seeps on the Norwegian margin and the Hikurangi Margin of New Zealand, asphalt seepage sites offshore southern California, and the tectonically controlled southern Chile forearc. We look forward to meeting all again at the 12th Gas in Marine Sediments conference scheduled for 1–6 September 2014 in Taipei, Taiwan.

PASSIVE MARGIN EVOLUTION AND CONTROLS ON NATURAL GAS LEAKAGE IN THE ORANGE BASIN, SOUTH AFRICA

Throughout exploration Block 2 of the Orange Basin offshore the South African continental margin, different natural gas leakage features and the relationship between natural gas leakage with structural and stratigraphic elements were studied. This study also quantifies liquid/gas hydrocarbon generation, migration and seepage dynamics through the post-rift history of the basin. The interpretation of seismic data reveals two mega-sequences: Cretaceous and Cenozoic that are subdivided by major stratigraphic unconformities into 5 and 2 sub-units, respectively. The basin is also divided into 2 structural domains: 1. an extensional domain characterized by basinward dipping listric normal faults rooted at the Cenomanian/Turonian level identified between 500 to 1500 m of present-day depth, 2. a compressional domain that accommodates the up-dip extension on the lower slope, and which is characterized by landward dipping thrust faults. One hundred and thirteen observed gas chimneys are identified and classified into stratigraphically-controlled (sa-c) and structurally-controlled (s-c) chimneys. The ratio of s-c versus s-ac chimneys is estimated as 2:5, which suggest a strong stratigraphic control on natural gas leakage. The chimneys either terminate at the seafloor where active leaking gas is manifested by pockmarks, or are sealed within the Miocene (14 Ma) sequence as paleo-pockmarks. The s-c chimneys are located along the normal faults in the extensional domain, and terminate as seafloor mounds up to 1500 m in diameter and with heights between 10 to 50 m. The sa-c pockmarks range between 100 to 400 m in diameter, and are linked to stratigraphic onlaps and pinch-outs within the Aptian sequence. Several giant chimneys, with diameters of more than 7 km, are also identified. At least one of these displays apparent internal gravitational collapse structures. Bright spots indicative of gas presence within these large chimneys were identified, but there is no evidence of acoustic turbidity or seismic pull-downs within these large structures. This suggests the giant chimneys are inactive paleo-gas-escape structures. Modelling suggests that gas from the lower Aptian and the Barremian source rocks migrates laterally-updip to the proximal parts of the basin where it accumulates beneath the Cenomanian/Turonian sequence that acts as a regional seal. Across the shelf-break and the upper slope, chimneys and pockmarks are fed from younger Cenomanian/Turonian source rocks. The migration model also indicates that fluids are about 24 times more likely to flow out of the study area than to be preserved within it. Since methane gas escaping across the sea floor into the exosphere (combined hydrosphere and atmosphere) may contribute to Earth’s climate fluctuations, and because escaping gas must have been cut off when at least half of identified s-c chimneys were sealed within the Miocene sequence, decrease of gas escape along the southern African continental margin may have to be factored into global Neogene cooling models.

Improving irrigation water operation in the lower reaches of the Amu Darya River – current status and suggestions

AbstractIrrigated agriculture is widespread in the Central Asian drylands and important for food security of the region. However irrigation practices based on rules made for cotton production on large units do not provide adequate guidance for the now widespread small farms that produce cotton wheat and rice. Excessive unsustainable water use is the consequence. Land and water resource management practices were analysed in 2006 for the irrigated area (approx. 1885 ha) of a water users' association (WUA) as a case study. The Shomakhulum WUA is situated in the Khorezm region Uzbekistan in the Aral Sea Basin. The designed water allowance of 0.40 l s−1 ha−1 was 46% higher than the 0.21 l s−1 ha−1 required in 2006. A ponding experiment conducted at two locations under typically shallow groundwater conditions showed that actual seepage losses in the major distributing canal (Pakhtakiyar) in the WUA were about 2% at one site and negative at the other. This does not support the present blanket recommendation of an additional 29% of water to compensate for anticipated seepage losses. Considering the effects of shallow groundwater the seasonal leaching requirements were estimated at 200 mm versus the 400 mm based on the blanket local recommendations for a 14 ha cotton field. However the actually applied water amounted to 670 mm which exceeds the calculated requirements by 235% and the local recommendation by 68%. The findings showed (i) excessive irrigation water supply and in turn high wastage owing to an underestimation of the potential shallow groundwater contribution (ii) overestimation of conveyance losses and (iii) excessive leaching due to outdated knowledge on leaching requirements when compared to standards. We suggest that the outdated water allowance the inaccurate estimation of the conveyance efficiency and excessive water applications during leaching cause a waste of precious fresh water and a rise in the shallow groundwater levels which consequently increases secondary soil salinity. The revision of irrigation and leaching norms combined with an improved knowledge on seepage dynamics under shallow groundwater levels is needed before any further recommendations can be made to the farmers in order to reach the objective of sustainable agriculture in the area. Copyright © 2011 John Wiley & Sons Ltd.

Fabric and formation of grapestone concretions within an unusual ancient methane seep system (Eocene, Bulgaria)

Terra Nova, 23, 56–61, 2011 Abstract Authigenic carbonates at methane seeps provide valuable information on the evolution of seepage dynamics. In this study, we report on an unusual type of 13C-depleted [δ13C: −44.8‰ to −8.3‰V-PDB (Vienna PeeDee Belemnite)] calcite-cemented grapestone concretion composed of small spheroidal nodules, <0.1–15 cm across, associated with an ancient seep setting in Eocene marine sediments (Pobiti Kamani area, NE Bulgaria). Field observations suggest that the nodules started to grow close to the ancient sea floor and apparently were agitated by fluid flow, promoting their spheroidal shape. The precipitation of calcite between sand grains started at multiple closely spaced nucleation sites, induced by locally elevated calcite supersaturation in response to anaerobic microbial consumption of ascending methane. Consecutive nodule growth continued deeper in the sediment, with the formation of dark and light concentric bands surrounding one or more nodule cores. The occurrence of similar grapestone concretions at seeps might indicate changing redox conditions at the seep site.

A long-term hydrological soil study on the effects of soil and land use on deep seepage dynamics in northeast Germany

Starting in 1993, interactions between land use and precipitation have been investigated in three research areas of northeast Germany. The research based on continuous measurements of soil water tension and water content down to a depth of 5 m at 36 plots situated at diverse soils under different land use (arable land, forest, grass fallow). At arable sites, the main period of deep seepage occurred between February and April during the study period. With 175 mm a−1, the mean annual deep seepage rate under arable land was highest at sandy soils. At loamy soils, the mean seepage rate amounting to 122 mm a−1 appeared significantly less. Smallest seepage rates, however, were determined for forested plots (pine 15 mm a−1, beech 18 mm a−1). Differences were significant as compared to arable land at sandy and loamy soils and grass fallow. It is concluded that deep seepage and groundwater recharge under forest will tend to wane at annual precipitation rates of <550 mm a−1. This should be taken into account in decision making on land use planning and conversion of arable areas to forestry in regions characterised by negative values of the climatic water balance.

Role of stratigraphy in governing pore water seepage from salt marsh sediments

Numerical modeling experiments have been used to study the effects of marsh stratigraphy on the seepage of pore water into tidal channels. If the mud‐sand contact lies at the elevation of the channel bottom and the hydraulic conductivity of the basal sand is at least an order of magnitude greater than the overlying mud, the locus of maximum seepage shifts from the tide line, as in a purely mud marsh, to the toe of the creek bank. The ratio of creek bank seepage to bottom seepage also shifts from greater than to less than one. Furthermore, as the ratio of the hydraulic conductivities of sand to mud increases, the ratio of the total seepage from the sand‐mud marsh to that from a pure mud marsh increases in a nonlinear fashion, reaching an asymptotic limit of about 20 at a conductivity ratio of about 1000. If the mud‐sand boundary is progressively lowered so that mud also lies beneath the channel, the seepage dynamics gradually revert toward those of a purely mud marsh. The ecological implications of these results for regularly flooded, low marshes are discussed.

The impact of seepage influx on cation content of a Central Amazonian floodplain lake

Summary We give the first report on seepage dynamics in an Amazonian floodplain lake, by directly quantifying seepage influx, and qualifying its impact on the hydrochemical seasonality of Lake Camaleao, an island channel lake, which is subject to the 10 m floodpulse of the lower Solimoes/Amazon River. The lake shows a distinct seasonal pattern in cation concentrations, with a rise and maximum during its low-water (isolation) phase. To quantitatively and qualitatively clarify the significance of seepage influx, a series of hydrological and hydrochemical variables was recorded in 1998. The observed seasonal pattern in the content of solute cations was similar to results reported for 1995 [G.E. Weber, Causes of hydrochemical seasonality of major cations in Lago Camaleao, a Central Amazonian floodplain lake, Ver. Int. Verein. Limnol. 26 (2) (1997) 408–411.]; when first qualitative evidence for the largely seepage driven process of cation loading was found. Concentration of the monitored Ca, Na, Mg and K, in Lake Camaleao and Solimoes were similar during high water. Solute cation content in the groundwater around Lake Camaleao was up to 20 times higher than in lake and river water. With the beginning of falling water, cation concentrations started to rise slightly in Lake Camaleao. Starting with isolation from Solimoes River the content of solute cations in the lake rose by approximately 150% within 14 days, then the concentrations had reached their maximum values. This falls in with 14 days of cation rich seepage influx into lake Camaleao. Mean seepage influx rate during this period was 1.45 mm/h, which equals a total influx into Lake Camaleao of 9.7 m 3 /min. After 14 days seepage stopped to enter the lake, lake water began to seep into the sediment, (seepage flux inversion), and the rise in cation concentrations in Lake Camaleao came to an end. Based on the water budget and measured cation concentrations of the contributing water fluxes we predicted cation concentrations at the end of the 14 days period of seepage influx into the isolated Camaleao. Observed and expected cation concentrations are fairly close. The coincidence of seepage influx and the solute cation increase in the isolated Camaleao, observed twice in 1998 and 1995, gives us strong evidence that cation loading by seepage is the main cause for the lake’s characteristic peak of cation concentration during falling water. Our field measurements correspond to values calculated by [K. Furch, Evaluation of groundwater input as major source of solutes in an Amazon floodplain lake during the low-water period, In: Verh. Int. Verein. Limnol. 27, Stuttgart, Germany, 1999.] considering changes in ion concentrations which also point to groundwater as major ion source for Camaleao Lake at low-water period.

Numerical modelling of tide-induced beach water table fluctuations

Field studies have shown that the elevation of the beach groundwater table varies with the tide and such variations affect significantly beach erosion or accretion. In this paper, we present a BEM (Boundary Element Method) model for simulating the tidal fluctuation of the beach groundwater table. The model solves the two-dimensional flow equation subject to free and moving boundary conditions, including the seepage dynamics at the beach face. The simulated seepage faces were found to agree with the predictions of a simple model (Turner, 1993). The advantage of the present model is, however, that it can be used with little modification to simulate more complicated cases, e.g., surface recharge from rainfall and drainage in the aquifer may be included (the latter is related to beach dewatering technique). The model also simulated well the field data of Nielsen (1990). In particular, the model replicated three distinct features of local water table fluctuations: steep rising phase versus flat falling phase, amplitude attenuation and phase lagging.