Impact of Natural Hazards on Oil and Gas Extraction: The South Caspian Basin

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The waveforms for this event were too small for us to confirm its mechanism by our modelling techniques, and we cannot assess the reliability of this CMT solution, which is clearly unusual in the Alborz region. The Alborz accommodates the overall motion between the southern Caspian and central Iran, and seems to involve oblique left-lateral shortening. There is abundant evidence for recent uplift in the Alborz, in the form of incised river terraces and coastal marine terraces, and it is very likely that the crust of Iran is being thrust over the South Caspian Basin Berberian However, there is no convincing evidence in the earthquake focal depths for current subduction of the Caspian Sea crust into the mantle beneath the Alborz, as there is on the Apsheron—Balkhan sill.

Hatzfeld, private communication, In this respect, the significance of the Quaternary stratovolcano of Damavand Fig. It contains many of the same rocks as the Alborz, including the thick Paleogene andesitic volcaniclastic sequence. The steep and heavily forested eastern flank of the Talesh is scored by narrow ravines and defiles formed by short, torrential streams plunging towards the Caspian, but none cross the entire range. The earthquake mechanisms in the Talesh are quite distinct from those in the Alborz: they indicate thrusting on almost flat faults rare in the Alborz at depths of 15—26 km deeper than in the Alborz , with slip vectors directed towards the Caspian Sea Figs 4 , 5 and 8.

The locations of these events are all close to the coastline and significantly west of the N—S fold axes offshore that are so important for the oil industry Fig. The centroid depths of these earthquakes place the active faults close to the base of the sedimentary section in the western Caspian Fig.

It is possible that these low-angle thrusts continue to the surface some distance offshore where the surface folds are observed, but more likely that the surface sediments are decoupled from the basement by the overpressured mud, and that the offshore folds simply reflect shortening of the sedimentary cover in response to deeper convergence further to the west.

Impact Of Natural Hazards On Oil And Gas Extraction The South Caspian Basin 1st Edition

Sliding of the offshore cover above the decollement horizon is encouraged by the uplift of the mountains in the west and the steep topographic gradient from west to east. Most large earthquakes are on the basement thrusts at 10—20 km depth beneath the Iran—Azerbaijan border, whereas the surface folding in the young sediments occurs further NE and offshore, where the thrusts reach shallower levels.

The vertical and horizontal scales are equal. There is no indication in either the location of the seismicity or in the focal mechanisms for substantial offshore faulting in the basement on the west side of the southern Caspian basin. There is consequently no support in this data set for the major N—S basement faults offshore that are such a common feature in the Russian literature e. Nor is there any sign in the earthquake mechanisms of N—S right-lateral strike-slip faulting in the Talesh, though this is drawn on some maps e.

Karakhanian ; , Nadirov Preliminary fieldwork by M. Allen and colleagues at the Geology Institute of Azerbaijan , summarized in Fig. The Greater Caucasus forms a relatively narrow — km but high range that decreases in elevation from to m near its centre to die out eastwards at the Caspian shoreline on the Apsheron peninsula. Early Jurassic extension led to the deposition of a thick, deep marine clastic succession, followed by continued marine deposition through the Cretaceous and early Cenozoic, with marginal carbonate platforms flanking a zone of deeper water.

Major exhumation and uplift of the range did not begin until the late Cenozoic, possibly as recently as the late Miocene, presumably consequently, of the Arabia—Eurasia collision e. Philip Most of the available earthquake mechanisms in the eastern Greater Caucasus show low-angle thrusting on planes dipping gently towards the mountains on both sides Figs 4 and 5. It is in this region that the range bulges north and where a series of destructive earthquakes involving such thrusting occurred in May Fig.

The NE side of the eastern Caucasus, such as the Kopeh Dag, shows a large free-air gravity low, with the characteristic shape of a flexural foreland basin and an effective elastic thickness of 16 km Maggi a. This indicates that the eastern Greater Caucasus is being thrust over the southern edge of the Russian platform, but there is no indication in the seismicity of subduction of material into the mantle.

On its south side, the eastern Caucasus is also being thrust over the sediments of the Kura depression Fig. Some earthquakes occur beneath the Kura Basin, but their significance is enigmatic. There is one poor first-motion solution in Fig. The almost complete lack of shallow earthquakes within the South Caspian Basin itself indicates that it behaves as a rigid block within the Eurasia—Iran—Arabia collision zone. We reach this conclusion in spite of abundant folding of the sedimentary cover offshore, which we do not believe indicates faulting of its underlying basement.

Instead, the surface folding is likely to be completely decoupled, and can be spatially separated, from any basement shortening by the overpressured muds. Other folds in the centre of the South Caspian Basin could also be a response to any of these factors, aided by mud diapirism.

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If the South Caspian Basin is rigid, the deformation in the belts that surround it result from its motion relative to Iran and Eurasia. McKenzie This westward component of motion enhances the underthrusting of the south Caspian basement beneath the Talesh. Sketches to illustrate the probable active tectonics of the South Caspian Basin. Note the left-lateral strike-slip component in the eastern Alborz, and right-lateral component in the Kopeh Dag. The white arrow shows the approximate direction of the South Caspian Basin relative to Iran, and the black arrow shows its motion relative to Eurasia.

It will eventually evolve to the offset configuration shown by the white semi-circles. Both the earthquake mechanisms and the geology indicate that considerable shortening accompanies the strike-slip in both the Alborz and western Kopeh Dag and, because we do not know the relative importance of these two components, we cannot determine the overall slip vectors or how they change along the strike of the belts.

Nonetheless, some crude estimates of probable rates are possible. However, we emphasize that these rates are very uncertain. We can now compare these postulated rates and motions with the geological evidence. However, two observations suggest that the South Caspian—Eurasia shortening is both recent and relatively small in magnitude.

First, there is no sign of recent volcanism north of the Apsheron—Balkhan sill, which might be expected if the deep earthquakes represent the declining activity in a subduction zone that has been active for some time. Secondly, the Greater Caucasus, Apsheron—Balkhan sill and the Kopeh Dag are all roughly collinear, yet the polarity of the underthrusting on the sill is opposite to that on either side: both the Greater Caucasus and the Kopeh Dag are overriding shields to the north, whereas the earthquakes and the bathymetry indicate that the central Caspian overrides the South Caspian Basin to the south.

This configuration would soon evolve into a non-collinear geometry if it persisted for long Fig. Much of the deformation within the South Caspian Basin sediments is younger than about 3. It is possible that some re-arrangement in the motions happened at this time, with shortening switching from the Alborz to the Apsheron—Balkhan sill. Prior to the Pliocene we know the Alborz was elevated from the coarse Neogene clastic sediments on both of its margins.


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It would perhaps be no surprise if it later broke along the narrow gap following the Apsheron—Balkhan sill, and if the lower South Caspian Basin then started to underthrust the higher and thicker crust on the northern side. Thus the entire present-day geometry of the deformation in the Southern Caspian Basin and its surroundings may date from about 3.

The trigger for this postulated reorganization could be the initiation of shortening in the Simple Folded Belt of the Zagros Falcon , , which perhaps heralded the final accretion into the collision zone of the various tectonic blocks within Iran, and the start of truly intracontinental shortening.

On the basis of Fig. If it is true that shortening has been concentrated in the Caucasus—Talesh—Alborz—Kopeh Dag arc, then we would also expect evidence for a N—S right-lateral shear between the Caspian and NW Iran in the Talesh, of which there is no sign in the earthquake mechanisms. N—S right-lateral strike-slip faults linking the eastern Caucasus with the Talesh are shown on various maps of Russian origin e.

Karakhanian ; , Nadirov , but it is not clear on what evidence their existence is postulated. Both Trifonov and , Kopp show right-stepping en echelon folds and right-lateral strike-slip faults splaying NW—SE away from the thrusts west of Baku and entering the northern side of the Kura Basin. We are unaware of any convincing evidence that such faulting crosses the Kura Basin or enters the Talesh as N—S right-lateral strike-slip, though H.

Philip private communication, interprets an apparent right-lateral offset of the Kura river as evidence that such a N—S fault zone is active. At shallow levels, the strong curvature of the Talesh fold axes, from E—W in the north to N—S along the coast, could be related to a shear that has rotated the originally E—W folds clockwise to become parallel to the coast as the western Iranian side moves north past the Caspian. In this case, it may be that shear is distributed in the wedge of relatively weak sediment above the low-angle thrusts at 15—25 km rather than concentrated on to a strike-slip fault, while the E—W shortening component related to the westward component of the Caspian's motion is taken up by N—S thrusts.

It does not, however, explain the lack of any seismological evidence for N—S right-lateral shear at deeper levels. The E—W folds and thrusts in the northern Talesh are an important indication that on the west side of the Caspian some of the Iran—Eurasia shortening is taken up south of the Kura Basin, accounting for the relatively minor shortening in the eastern Greater Caucasus, represented by the decreasing elevation of that range as it approaches the Apsheron—Balkhan sill.

These folds are in regions of generally low seismicity, most likely because they are decoupled from their underlying basement by thick mud sequences. In summary, it seems clear that the South Caspian Basin has a westward component of motion relative to both Eurasia and Iran, and that this enhances the underthrusting in the Talesh. This is a common phenomenon in oceanic island arcs e. Fitch ; , McCaffrey and makes it very difficult to estimate overall convergence directions in such zones e.

DeMets Slip partitioning is thus an important mechanism by which oblique convergence is achieved on the continents. This evolutionary process is perhaps occurring in the Kopeh Dag today Figs 4 and 5. Within the range are NW—SE striking right-lateral faults that are oblique to the regional strike and presumably accommodate shortening by rotating anticlockwise, while on its NE margin the strike-slip faulting becomes range-parallel. The reasons as to why these blocks are rigid and presumably stronger than their surroundings are not understood and may be different in each example.

If the Southern Caspian lithosphere was originally oceanic, it may be relatively strong because of its composition e. Alternatively, if it is thinned continental crust it may be more analogous to the southern Aegean Sea, another low, thin and rigid continental block, which some have suggested may be strong because of its temperature structure e. However, the South Caspian Basin has an additional feature that is important: its very low elevation, which is related to its thin and dense possibly oceanic crust.

It is this feature that causes it to be easily overridden by thrust faults, which is occurring on all sides, and will ultimately cause its complete destruction. Oceanic crust can, in principle, be subducted into the mantle, but this is not an easy process to initiate as it is resisted by forces associated with bending, friction on the subduction interface, and the buoyancy of the crustal part of the subducting lithosphere e. There are indications of subduction beneath the Apsheron—Balkhan sill, but whether it is still active at the surface is not clear.

The low-angle thrusts of the Talesh, so characteristic of oceanic subduction zones, also make this an additional likely site of future subduction. It is easy to see how the South Caspian basement could eventually be consumed by these processes. It is also easy to see how a piece of oceanic lithosphere can become isolated and surrounded by continental lithosphere in a collision zone. An example is provided by the eastern Mediterranean Fig.

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In this region, the shortening between Africa and Eurasia is achieved by the westward motion of Turkey away from the collision zone similar to the westward motion of the South Caspian away from the eastern Alborz and western Kopeh Dag. This westward motion is, in turn, accommodated by thrusting the continental lithosphere of the Aegean over the Mediterranean sea floor in the Hellenic Trench. When the two finally meet, they will have isolated the eastern Mediterranean sea floor as a low oceanic? Continued shortening will cause the low-lying block to be overridden by thrusts and eventually consumed.

The underlying oceanic basement is likely to be subducted, though the overlying sediments may well be scraped off and attached to the continents. The final result may be an isolated slab with deep earthquakes in an entirely continental mountainous region, with only once-oceanic sediment remaining at the surface to record its origin. The Hindu Kush deep seismic zone is an example of such a slab Fig. Furthermore, if subduction occurs, or is attempted, in more than one different direction, as it may be under the Talesh and the Apsheron—Balkhan sill in the Southern Caspian, the final result will be a distorted or twisted slab, such as that seen in the Hindu Kush e.

Thus we see the isolation and consumption of small oceanic basins in collision zones as a natural consequence of the very 3-D nature of continental tectonics, as exemplified in the eastern Mediterranean. Bathymetry deeper than m is in grey. Note how the continued anticlockwise rotation of Turkey and SW motion of Greece will eventually isolate the eastern Mediterranean basin and surround it by continental material.

Data are from the catalogue of Engdahl in the period — The South Caspian Basin is an important element in the Arabia—Eurasia collision, being a low, apparently rigid, block being overthrust by continental material on all sides. Its origin remains uncertain: it is either a remnant piece of oceanic lithosphere with unusually thick crust or stretched continental crust with unusually high velocity. The margins of the Caspian produce a bewildering variety of earthquake focal mechanisms and depths, revealing strike-slip, high- and low-angle thrusting, and normal faulting. It is an important lesson of this study that the simple tectonic patterns contained within this variety could not have been revealed by examination of the routinely published catalogues of locations and CMT solutions, mainly because the depths in those catalogues are too inaccurate.

Those patterns are, however, clear when the depths and mechanisms are improved by long-period waveform modelling. From an examination of the active faulting around its margins we estimate that the South Caspian Basin has a westward component of motion relative to both Eurasia and Iran and that this enhances its underthrusting beneath the Talesh mountains in the west. Our attempts to reconcile the present-day motions with the geology and geomorphology suggest to us that the current tectonic pattern in the region is young: possibly dating from about 3.

The South Caspian Basin is now being destroyed by underthrusting on its western and northern margins and may ultimately become a twisted slab in the upper mantle, similar to that beneath the Hindu Kush today. We thank M. Khorehie and M. Qorashi of the Geological Survey of Iran for their constant support of our research in Iran over many years. We thank T. Mamedov for providing the Azerbaijan seismic network's locations of the earthquakes near Baku in November.

Cambridge Earth Sciences contribution ES There are two formats of figure used in this appendix. The format of Fig. Station positions on the P top and SH bottom focal spheres are identified by capital letters and arranged clockwise starting from north. STF is the source time function. Vertical ticks on the seismograms indicate the inversion window. Numbers beneath the header line are strike, dip, rake, centroid depth km , and moment N m. Stations were weighted according to azimuthal density and then the S seismogram weights were halved, to compensate for their larger amplitudes.

Comments appropriate to individual earthquakes are given in the figure captions. The second type of format is that of Fig. The purpose of this kind of figure is to demonstrate the sensitivity of the waveforms to a particular source parameter usually depth , in cases where there were insufficient waveforms to carry out a full inversion for all source parameters of the sort displayed in Fig. An explanation of each line of waveforms is given in the individual figure captions.

The depth of the interface between the layer and half-space is given in the figure captions. For the two earthquakes at 40 and 33 km near Baku in Fig. For many of the earthquakes the purpose of the inversion was to constrain or confirm the centroid depth, which is not well determined by the Harvard CMT solutions, especially for shallow events.

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The simple patterns of seismicity discussed in the main text cannot be seen unless the centroid depths are known with confidence. The corresponding Harvard best-double-couple CMT solutions are given in the figure captions and are often similar in orientation to those determined here. The difference in depth between the solutions is often accompanied by a difference in moment, which might arise from a combination of factors including: 1 a trade-off between depth and M 0 for shallow earthquakes; 2 differences in the velocity models used; 3 a contribution from a non-double-couple component in the Harvard solution; 4 a contribution from poorly resolved M xz and M yz components for shallow earthquakes in the Harvard solution see Wright Waveforms for the January 20 earthquake M,, 5.

The interest in this earthquake is in its depth fixed at 33 km by Harvard and its location near a prominent ENE-WNW left-lateral strike-slip fault Figs 7 and 9c. There were insufficient clear waveforms for a full inversion. Line 1 shows two P and three SH observed solid and synthetic dashed waveforms corresponding to the Harvard best-double-couple solution at its fixed depth of 33 km and with its E-W left-lateral orientation.

The fit of the synthetic P waves to the observed P seismograms is not good, mainly because, at 33 km, the centroid is too deep. A better fit to the P waves is achieved when the centroid is moved to 13 km line 2 , but the fit of the SH waveforms remains bad. Minimum misfit solution for the earthquake of June 21 M,,. Minimum misfit solution for the earthquake of 28 November M,,.

A2 , one that involved N-S reverse faulting Fig. Minimum misfit solution for 31 August Mn,, 5. This event has the deepest confirmed 0 centroid in the region, at 76 km. Minimum misfit solution for the first at of two earthquakes on 1 July near Krasnovodsk in Turkmenistan at - 40 km depth Fig. The depth is well resolved by the separated direct P, S and reflected pP, sP, sS phases at many stations.

Synthetics were calculated with the interface separating the layer of V,, 6. Minimum misfit solution for the second earthquake at on 1 July near Krasnovodsk, also at km. As in Fig. A5, the depth is well resolved by the separated direct and reflected phases. Synthetics were calculated in the same velocity structure as for Fig. Minimum misfit solution for a deep 61 km strike-slip event near the Apsheron-Balkhan sill Fig. The earthquake was only Mw 5. Nonetheless, the clear surface reflections n the P waves at eastern stations e.

Synthetics were calculated with the interface between the top layer V, 6. Gazprom, the Russian gas monopoly, bought up potential supplies of natural gas in the Caspian region. In this contest, Russia also dragged out talks on the status of the sea and east-west trans-Caspian pipelines for 22 years, the length of time the convention was under negotiation before the signing in August. Russia and the West have also been in a tug of war over influence in Ukraine, which, like Georgia, is an important transit country for energy. Central Asia trade had been diverted not to Russia, but to Iran, with Chinese backing.

And some Central Asian energy exports have not gone to Russia, but instead east to China because of difficulties exporting west over the Caspian Sea. Eurasian pipeline politics, not unlike the web of pipes themselves, is an interconnected game. Russia also offered the Caspian agreement as a concession, said Ilya Ponomarev, a former member of the Russian Parliament, to ease acceptance of something the Russians value more: the Nord Stream 2 pipeline to Germany.

The Caspian agreement, in contrast, could help diversify European energy supplies away from Russia. Unlike marine oil spills, where petroleum enters through the oxygenated water column undergoing powerful breakdown by aerobic respiration Head et al. Hence, a different succession of microbial steps is expected in seeps compared to spills. Despite the increase in the number of studies on anaerobic degradation of hydrocarbons, there is still a lack of understanding how hydrocarbon-degraders act as a community in the environment and how petroleum is successively degraded under anoxic conditions Head et al.

So far, selective utilization of hydrocarbons has been classically studied in enrichment cultures and isolates for example, Ehrenreich et al. However, the use of batch cultures is insufficient to know the fate of petroleum in a natural ecosystem Horowitz and Atlas, Because it is impossible to mimic all environmental determinants in the laboratory, Horowitz and Atlas suggested that the best chance to predict the fate of petroleum in a natural ecosystem is through chemostats, which maintain a constant influx and efflux of nutrients and products, respectively.

There are few studies in the literature that are based on continuous flow-through systems to study petroleum hydrocarbon degradation Bertrand et al. Investigations of hydrocarbon seeps often capture only snapshots of biogeochemical features for example, Bauer et al. The system enables the monitoring of biogeochemical alterations in the sediment core during petroleum seepage over time. Offshore drilling and land-based activities such as oil refineries, petrochemical plants, pipeline constructions have led to pollution and contamination of the Caspian Sea Karpinsky, ; Dumont, , ; Abilov et al.

As the Caspian Sea is an enclosed basin, pollutants discharged into it accumulate and are partly trapped, e. However, so far only a few studies have focused on the microbial community and petroleum degradation in sediments from the Caspian Sea Hassanshahian et al. A Schematic diagram of the SOFT system for the simulation of petroleum seepage through whole round sediment cores.

Petroleum was supplied in by a pump P3 at 3. Vertically aligned rhizons 2. Silicon-sealed holes 4 mm diameter on the opposite side were used for microsensor measurements. From the oxic supernatant electron acceptors e. Characteristic features like on- and offshore mud volcanoes green dots , abandoned offshore wells and infrastructures white spots and lines in the image , and a central oil slick area dark gray area in the image are indicated.

FC1 and FC2 are nearby sites where geochemical analyzes were done by Jost Map was produced by using ArcGIS The aim of the present study was to investigate the development of biogeochemical gradients related to microbial petroleum degradation and the successive consumption of hydrocarbons in Caspian Sea sediment during a simulated seepage through a whole round sediment core.

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Impact of Natural Hazards on Oil and Gas Extraction: The South Caspian Basin

We hypothesize that petroleum seepage through the Caspian Sea sediment will affect the vertical i zonation of redox processes, ii distribution and activity of petroleum-degrading microbial communities, and iii composition of seeping petroleum. We used the SOFT system to identify the above processes as a function of petroleum seepage to address the aforementioned hypotheses.


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  • This study presents detailed datasets on the successive biogeochemical response of the sediment to petroleum seepage and the alteration of the petroleum hydrocarbons. The biogeochemical data obtained here, were then correlated with microbial community analysis in a study by Stagars et al. The Caspian Sea is the largest continental water body and the rivers Volga, Kura, and Ural are the three biggest contributors of its inflow and nutrients Dumont, It has an area exceeding , km 2 with a water volume of around 78, km 3 Kosarev, The water depth at the sampling site was around 60 cm.

    Sediment cores were collected in November by directly walking into the water and pushing core liners into the sediment by hand. Additionally, two replicate mini push cores polycarbonate liner, 30 cm long, 2. The sediment cores were sealed air-free filled with seawater to the brim with rubber stoppers.

    Two of the large push cores were used for immediate sediment and porewater analyzes, respectively, upon arrival in Kiel. The cores were 18 cm and 16 cm long, respectively. The two replicate mini push cores 12 and 16 cm long were used for SRR determination immediately upon arrival in Kiel. One large push core 16 cm long was selected for the SOFT experiment. Untreated cores are here defined as sediment cores that were not subjected to petroleum addition in this study and hence, would serve as a reference for sediment conditions prior to the applied petroleum seepage.

    No significant difference between the cores was observed Stagars et al. The upper end of the core liner was provided with a PVC cap with a central opening diameter 4 cm and three small openings diameter 3. The three small openings were used to feed through tubing Iso-versinic, LLG; inner diameter 1 mm and outer diameter 3 mm from an air pump and a seawater reservoir inflowing artificial seawater to the SOFT core , and into a wastewater reservoir outflowing seawater from the SOFT core. The cap was semi open and wrapped with permeable laboratory film Parafilm, Pechiney Plastic Packaging , to allow gas exchange with the atmosphere.

    Aeration by air pump in the supernatant water was applied to facilitate a natural redox zonation from oxic to anoxic in the sediment core. Care was taken that the air flow was not too strong to avoid disturbance of the sediment surface layer. All tubing connections sediment core, crude oil reservoir, seawater reservoir, collection bottle and air pump were established with gas tight and autoclavable Iso-versinic tubes LLG , polypropylene tube connectors and fast couplers.

    Petroleum pumping was switched on and off at frequent intervals 2—3 days of no-flow vs. A layer of petroleum oil slick formed at the water—air interface from the petroleum that seeped out of the sediment core.

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    The oil slick was periodically removed with sterile syringes to avoid overflow. Light, live crude oil, i. The crude oil was not directly analyzed for the presence of bacteria. Laufer, unpublished data. We therefore assume that the undiluted crude oil was unable to sustain a viable microbial community. Cotton plugs were applied to the artificial seawater reservoir to maintain sterile and oxic conditions. The outlet tube connecting the artificial seawater reservoir with the SOFT liner was integrated into the cotton plug. Artificial seawater was prepared by mixing 12 g of sea salts Sigma—Aldrich, product number S in ml of sterile deionized water to achieve a salinity of 12 psu, according to the salinity found at the study site see methods of porewater analyzes below.

    No specific nutrients or vitamins were added to avoid unnatural enrichments of microorganisms. Additional sulfate was added in the form of magnesium sulfate 5. The 2. The temperature represents the average temperature of Caspian Sea surface water in Baku in November , i. Microelectrode measurements for dissolved oxygen and sulfide were done exclusively for the SOFT core, periodically every 30 to 40 days after the start of the SOFT experiment.

    Hence, it has to be kept in mind that total sulfide data were not corrected for individual pH data points. The microsensor calibration was done prior to measurements using the calibration software offered by Unisense SensorTrace PRO , which provided signals in millivolt and the corresponding concentration for each data point. The data were corrected for the shift in the electronic signal at the end of the measurement. A Temporal development of sediment microprofiles of dissolved oxygen after the start of the SOFT experiment.

    The dashed horizontal line represents the sediment-water interface. B Temporal development of the oxygen penetration depth PD and the diffusive oxygen uptake DOU generated from the mean oxygen profiles. Around 1. Out of the 2 ml extracted porewater, around 0. Porewater concentration of sulfate was determined by ion-chromatography for details, see Steeb et al. Total dissolved sulfide was determined photometrically at nm after Cline, Sediment samples of both the untreated and the SOFT core were collected from sediment cores by slicing for subsequent solid phase analyzes.

    One of the large push cores was sampled upon arrival in Kiel. The sediment core was sliced vertically from top to bottom every 1 0—5 cm to 2 cm 5 cm until end of core. The SOFT core was sampled at the end of the experiment, i. The sediment core was sliced vertically from top to bottom every 1 0—3 cm to 2 cm 3 cm until end of core.

    During removal of the supernatant with a syringe, an oil slick settled on the sediment surface. The top 0—1 cm sediment layer of the core is therefore considered non-representative for some of the sediment parameters e. In addition to the parameters mentioned for the untreated core above, the SOFT core was also subsampled for the analyzes of volatile C2—C6 n -alkanes, heavier C10—C38 n -alkanes, SRRs, and enrichment culturing of methanogens and sulfate reducers. Dissolved volatile hydrocarbons C1—C6 in sediment subsamples were determined by the headspace technique.

    Two milliliters of sediment and 5 ml of 2. Final concentrations of C1—C6 n -alkanes are presented per volume porewater after porosity correction. For the analyzes of volatile hydrocarbons in the original crude oil, 2 ml of the oil and 5 ml of 2. It should be noted, however, that we cannot completely exclude losses of volatiles from the original crude oil as it was analyzed after the SOFT experiment, during which it was kept stored at room temperature in its original sealed container. Stable carbon isotope ratios of methane C1 were determined by continuous flow GC combustion — Isotope Ratio Mass Spectrometer combination.

    The extract was dried over sodium sulfate and was passed through a glass chromatographic column Eydam, length 25 cm, inner diameter 1 cm. The column was then washed portion wise with 20 ml of n -hexane and the resulting extract was collected and dried in a conical flask. The conical flask was then washed with 2 ml of n -hexane and this final solution was measured with GC-MS or dilution depending on the sample. Before the extraction process, deuterated tetracosane C 24 D 50 was used as an internal standard in the extraction cell.

    The ratio of the two chromatogram peak areas sample extract and reference of the internal standard was used to calculate the recovery of individual n -alkanes in our samples. Helium Alphagaz-1 Air Liquide was used as the carrier gas with a flow rate of 0. The original crude oil was extracted in the same procedure by mixing it with inert diatomaceous earth prior extraction by ASE. To get a precision of the methodology, the original crude oil was extracted four times from the first step of extraction to the final measurement step.

    A method precision range for each n -alkane in the original crude oil is provided in Supplement 2. Porosity was determined by weighing the wet and freeze-dried weights of sediment from both the untreated and the SOFT cores. Porosity was then calculated from the water content assuming a dry solid density of 2. As the bulk volume of petroleum was not removed by the freeze-drying process, porosity values might appear lower than they actually were in samples that contained oil.

    Total organic carbon TOC was then determined by the difference in carbon content after removing the total inorganic carbon TIC through acidification addition of 0. Measurements were done in duplicates. The mini cores were incubated for 6. Three to 4 ml sediment were sampled every 2 cm into 5 ml glass tubes and immediately sealed with butyl rubber stoppers Treude et al. Controls for all incubations untreated and SOFT core were prepared by adding tracer to killed samples.

    An aliquot was filtered on a 0. Microscopy was done with a Nikon eclipse 50i epifluorescence microscope. Anaerobic incubations with sediment samples from the SOFT core were set up in an anaerobic chamber to determine methanogenic rates and the potential of indigenous microorganisms to degrade selected hydrocarbons. One gram of sediment from both the sulfate-reducing and methanogenic zone of the SOFT core was each transferred into two separate autoclaved ml glass bottles containing 20 ml of sulfate-free seawater medium Widdel and Bak, The salinity of the medium was adapted to the respective original seawater conditions 12 psu by adding varying amounts of NaCl Merck, CAS-No: The glass vials were sealed with sterile butyl rubber stoppers and aluminum crimp caps.

    All tubes were flushed with N 2 to remove traces of H 2 from the anaerobic chamber. Zero and 20 mM sulfate were added to the methanogenic microcosms and sulfate-reducing microcosms, respectively. Cultures were amended with the single substrates n -hexadecane, ethylbenzene both 0. Controls without any added carbon source were incubated in parallel. Replicate cultures with 2-bromoethanesulfonate BES; 10 mM , a specific inhibitor for methanogenic microorganisms Scholten et al.

    In sulfate-reducing microcosms, sodium azide NaN 3 , 50 mM , a strong microbial toxin, was used to prepare metabolically inactive controls. The experimental set up of the SOFT system was too complicated for replication; however, in the following we will consider neighboring sediment layers as technical replicates, as they provide information about the plausibility of the individual data points along vertical profiles. Collected sediment cores were sandy with a porosity of 0. Sea-grass-like plants were growing at the sediment surface. Enhanced benthic rates of sulfate reduction and sulfide production are frequently found associated with the presence of sea grass, as the protruding plants serve as a trap for organic matter Holmer and Nielsen, ; Holmer et al.

    Temporal development of biogeochemical profiles in the Caspian Sea sediment during simulated petroleum seepage. Sulfate black line with triangles , total sulfide black line with squares , sulfate reduction rates SRR, black checkered bars and methane black line with circles.

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