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1.
《Marine and Petroleum Geology》2012,29(10):1838-1843
The hydrate-bearing sediments above the bottom simulating reflector (BSR) are associated with low attenuation or high quality factor (Q), whereas underlying gas-bearing sediments exhibit high attenuation. Hence, estimation of Q can be important for qualifying whether a BSR is related to gas hydrates and free-gas. This property is also useful for identifying gas hydrates where detection of BSR is dubious. Here, we calculate the interval Q for three submarine sedimentary layers bounded by seafloor, BSR, one reflector above and another reflector below the BSR at three locations with moderate, strong and no BSR along a seismic line in the Makran accretionary prism, Arabian Sea for studying attenuation (Q−1) characteristics of sediments. Interval Q for hydrate-bearing sediments (layer B) above the BSR are estimated as 191 ± 11, 223 ± 12, and 117 ± 5, whereas interval Q for the underlying gas-bearing sediments (layer C) are calculated as 112 ± 7, 107 ± 8 and 124 ± 11 at moderate, strong and no BSR locations, respectively. The large variation in Q is observed at strong BSR. Thus Q can be used for ascertaining whether the observed BSR is due to gas hydrates, and for identifying gas hydrates at places where detection of BSR is rather doubtful. Interval Q of 98 ± 4, 108 ± 5, and 102 ± 5, respectively, at moderate, strong and no BSR locations for the layer immediately beneath the seafloor (layer A) show almost uniform attenuation.  相似文献   

2.
The most commonly used marker for the investigation of gas-hydrates is the bottom simulating reflector (BSR), which is caused by gas-hydrate laden sediment underlain by either brine or gas-saturated sediment. A BSR has been identified by seismic experiment in the Kerala-Konkan Basin of the western continental margin of India. Here we perform AVA modeling of seismic reflection data from a BSR to investigate the seismic velocities for quantitative assessment of gas-hydrates and to understand the origin of the BSR. The result reveals a P-wave velocity of 2.245 km/s and an S-wave velocity of 0.895 km/s for the sediments above the BSR. This corresponds to a Poisson ratio of 0.406 and hydrates saturation of ∼30% in the study area. The comparison of estimated P-wave velocity (1.77 km/s) above the hydrates-bearing sediment to that (1.78 km/s) below the BSR implies that the origin of the BSR is mainly due to gas-hydrates, as the presence (even in small quantities) of free-gas reduces the P-wave velocity considerably.  相似文献   

3.
The hydrate-bearing sediments above the bottom simulating reflector (BSR) are associated with low attenuation or high quality factor (Q), whereas underlying gas-bearing sediments exhibit high attenuation. Hence, estimation of Q can be important for qualifying whether a BSR is related to gas hydrates and free-gas. This property is also useful for identifying gas hydrates where detection of BSR is dubious. Here, we calculate the interval Q for three submarine sedimentary layers bounded by seafloor, BSR, one reflector above and another reflector below the BSR at three locations with moderate, strong and no BSR along a seismic line in the Makran accretionary prism, Arabian Sea for studying attenuation (Q−1) characteristics of sediments. Interval Q for hydrate-bearing sediments (layer B) above the BSR are estimated as 191 ± 11, 223 ± 12, and 117 ± 5, whereas interval Q for the underlying gas-bearing sediments (layer C) are calculated as 112 ± 7, 107 ± 8 and 124 ± 11 at moderate, strong and no BSR locations, respectively. The large variation in Q is observed at strong BSR. Thus Q can be used for ascertaining whether the observed BSR is due to gas hydrates, and for identifying gas hydrates at places where detection of BSR is rather doubtful. Interval Q of 98 ± 4, 108 ± 5, and 102 ± 5, respectively, at moderate, strong and no BSR locations for the layer immediately beneath the seafloor (layer A) show almost uniform attenuation.  相似文献   

4.
《Marine and Petroleum Geology》2012,29(10):1932-1942
A dense seismic reflection survey with up to 250-m line-spacing has been conducted in a 15 × 15 km wide area offshore southwestern Taiwan where Bottom Simulating Reflector is highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between north–south trending thrust faults and northwest–southeast oblique ramps exists in this region, leading to the formation of 3 plunging anticlines arranged in a relay pattern. Landward in the slope basin, a north–south trending diapiric fold, accompanied by bright reflections and numerous diffractions on the seismic profiles, extends across the entire survey area. This fold is bounded to the west by a minor east-verging back-thrust and assumes a symmetric shape, except at the northern and southern edges of this area, where it actively overrides the anticlines along a west-verging thrust, forming a duplex structure. A clear BSR is observed along 67% of the acquired profiles. The BSR is almost continuous in the slope basin but poorly imaged near the crest of the anticlines. Local geothermal gradient values estimated from BSR sub-bottom depths are low along the western limb and crest of the anticlines ranging from 40 to 50 °C/km, increase toward 50–60 °C/km in the slope basin and 55–65 °C/km along the diapiric fold, and reach maximum values of 70 °C/km at the southern tip of the Good Weather Ridge. Furthermore, the local dips of BSR and sedimentary strata that crosscut the BSR at intersections of any 2 seismic profiles have been computed. The stratigraphic dips indicated a dominant east–west shortening in the study area, but strata near the crest of the plunging anticlines generally strike to southwest almost perpendicular to the direction of plate convergence. The intensity of the estimated bedding-guided fluid and gas flux into the hydrate stability zone is weaker than 2 in the slope basin and the south-central half of the diapiric fold, increases to 7 in the northern half of the diapiric fold and plunging anticlines, and reaches a maximum of 16 at the western frontal thrust system. Rapid sedimentation, active tectonics and fluid migration paths with significant dissolved gas content impact on the mechanism for BSR formation and gas hydrate accumulation. As we begin to integrate the results from these studies, we are able to outline the regional variations, and discuss the importance of structural controls in the mechanisms leading to the gas hydrate emplacements.  相似文献   

5.
This paper presents results of a seismic tomography experiment carried out on the accretionary margin off southwest Taiwan. In the experiment, a seismic air gun survey was recorded on an array of 30 ocean bottom seismometers (OBS) deployed in the study area. The locations of the OBSs were determined to high accuracy by an inversion based on the shot traveltimes. A three-dimensional tomographic inversion was then carried out to determine the velocity structure for the survey area. The inversion indicates a relatively high P wave velocity (Vp) beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. The bottom-simulating reflectors (BSR) closely follow the seafloor and lies at 325 ± 25 m within the well-constrained region. Mean velocities range from ~1.55 km/s at the seabed to ~1.95 km/s at the BSR. We model Vp using an equation based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ~200 m below the seafloor to 25% of pore space close to the BSR in the survey area.  相似文献   

6.
An analysis of 3D seismic data from the Zhongjiannan Basin in the western margin of the South China Sea (SCS) reveals seismic evidence of gas hydrates and associated gases, including pockmarks, a bottom simulating reflector (BSR), enhanced reflection (ER), reverse polarity reflection (RPR), and a dim amplitude zone (DAZ). The BSR mainly surrounds Zhongjian Island, covering an area of 350 km2 in this 3D survey area. The BSR area and pockmark area do not match each other; where there is a pockmark developed, there is no BSR. The gas hydrate layer builds upward from the base of the stability zone with a thickness of less than 100 m. A mature pockmark usually consists of an outside trough, a middle ridge, and one or more central pits, with a diameter of several kilometers and a depth of several hundreds of meters. The process of pockmark creation entails methane consumption. Dense faults in the study area efficiently transport fluid from large depths to the shallow layer, supporting the formation of gas hydrate and ultimately the pockmark.  相似文献   

7.
The multichannel seismic data along one long-offset survey line from Krishna-Godavari (K-G) basin in the eastern margin of India were analyzed to define the seismic character of the gas hydrate/free gas bearing sediments. The discontinuous nature of bottom simulating reflection (BSR) was carefully examined. The presence of active faults and possible upward fluid circulation explain the discontinuous nature and low amplitude of the BSR. The study reveals free gas below gas hydrates, which is also indicated by enhancement of seismic amplitudes with offsets from BSR. These findings were characterized by computing seismic attributes such as the reflection strength and instantaneous frequency along the line. Geothermal gradients were computed for 18°C and 20°C temperature at the depth of BSR to understand the geothermal anomaly that can explain the dispersed nature of BSR. The estimated geothermal gradient shows an increase from 32°C/km in the slope region to 41°C/km in the deeper part, where free gas is present. The ray-based travel time inversion of identifiable reflected phases was also carried out along the line. The result of velocity tomography delineates the high-velocity (1.85–2.0 km/s) gas hydrate bearing sediments and low-velocity (1.45–1.5 km/s) free gas bearing sediments across the BSR.  相似文献   

8.
2D and 3D seismic reflection and well log data from Andaman deep water basin are analyzed to investigate geophysical evidence related to gas hydrate accumulation and saturation. Analysis of seismic data reveals the presence of a bottom simulating reflector (BSR) in the area showing all the characteristics of a classical BSR associated with gas hydrate accumulation. Double BSRs are also observed on some seismic sections of area (Area B) that suggest substantial changes in pressure–temperature (P–T) conditions in the past. The manifestation of changes in P–T conditions can also be marked by the varying gas hydrate stability zone thickness (200–650 m) in the area. The 3D seismic data of Area B located in the ponded fill, west of Alcock Rise has been pre-stack depth migrated. A significant velocity inversion across the BSR (1,950–1,650 m/s) has been observed on the velocity model obtained from pre-stack depth migration. The areas with low velocity of the order of 1,450 m/s below the BSR and high amplitudes indicate presence of dissociated or free gas beneath the hydrate layer. The amplitude variation with offset analysis of BSR depicts increase in amplitude with offset, a similar trend as observed for the BSR associated with the gas hydrate accumulations. The presence of gas hydrate shown by logging results from a drilled well for hydrocarbon exploration in Area B, where gas hydrate deposit was predicted from seismic evidence, validate our findings. The base of the hydrate layer derived from the resistivity and acoustic transit-time logs is in agreement with the depth of hydrate layer interpreted from the pre-stack depth migrated seismic section. The resistivity and acoustic transit-time logs indicate 30-m-thick hydrate layer at the depth interval of 1,865–1,895 m with 30 % hydrate saturation. The total hydrate bound gas in Area B is estimated to be 1.8 × 1010 m3, which is comparable (by volume) to the reserves in major conventional gas fields.  相似文献   

9.
A dense seismic reflection survey with up to 250-m line-spacing has been conducted in a 15 × 15 km wide area offshore southwestern Taiwan where Bottom Simulating Reflector is highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between north–south trending thrust faults and northwest–southeast oblique ramps exists in this region, leading to the formation of 3 plunging anticlines arranged in a relay pattern. Landward in the slope basin, a north–south trending diapiric fold, accompanied by bright reflections and numerous diffractions on the seismic profiles, extends across the entire survey area. This fold is bounded to the west by a minor east-verging back-thrust and assumes a symmetric shape, except at the northern and southern edges of this area, where it actively overrides the anticlines along a west-verging thrust, forming a duplex structure. A clear BSR is observed along 67% of the acquired profiles. The BSR is almost continuous in the slope basin but poorly imaged near the crest of the anticlines. Local geothermal gradient values estimated from BSR sub-bottom depths are low along the western limb and crest of the anticlines ranging from 40 to 50 °C/km, increase toward 50–60 °C/km in the slope basin and 55–65 °C/km along the diapiric fold, and reach maximum values of 70 °C/km at the southern tip of the Good Weather Ridge. Furthermore, the local dips of BSR and sedimentary strata that crosscut the BSR at intersections of any 2 seismic profiles have been computed. The stratigraphic dips indicated a dominant east–west shortening in the study area, but strata near the crest of the plunging anticlines generally strike to southwest almost perpendicular to the direction of plate convergence. The intensity of the estimated bedding-guided fluid and gas flux into the hydrate stability zone is weaker than 2 in the slope basin and the south-central half of the diapiric fold, increases to 7 in the northern half of the diapiric fold and plunging anticlines, and reaches a maximum of 16 at the western frontal thrust system. Rapid sedimentation, active tectonics and fluid migration paths with significant dissolved gas content impact on the mechanism for BSR formation and gas hydrate accumulation. As we begin to integrate the results from these studies, we are able to outline the regional variations, and discuss the importance of structural controls in the mechanisms leading to the gas hydrate emplacements.  相似文献   

10.
This article provides new constraints on gas hydrate and free gas concentrations in the sediments at the margin off Nova Scotia. Two-dimensional (2-D) velocity models were constructed through simultaneous travel-time inversion of ocean-bottom seismometer (OBS) data and 2-D single-channel seismic (SCS) data acquired in two surveys, in 2004 and 2006. The surveys, separated by ∼5 km, were carried out in regions where the bottom-simulating reflection (BSR) was identified in seismic reflection datasets from earlier studies and address the question of whether the BSR is a good indicator of significant gas hydrate on the Scotian margin. For both datasets, velocity increases by 200–300 m/s at a depth of approximately 220 m below seafloor (mbsf), but the results of the 2006 survey show a smaller velocity decrease (50–80 m/s) at the base of this high-velocity layer (310–330 mbsf) than the results of the 2004 survey (130 m/s). When converted to gas hydrate concentrations using effective medium theory, the 2-D velocity models for both datasets show a gas hydrate layer of ∼100 m thickness above the identified BSR. Gas hydrate concentrations are estimated at approximately 2–10% for the 2006 data and 8–18% for the 2004 survey. The reduction in gas hydrate concentration relative to the distance from the Mohican Channel structure is most likely related to the low porosity within the mud-dominant sediment at the depth of the BSR. Free gas concentrations were calculated to be 1–2% of the sediment pore space for both datasets.  相似文献   

11.
The presence of gas hydrates, one of the new alternative energy resources for the future, along the Indian continental margins has been inferred mainly from bottom simulating reflectors (BSR) and the gas stability zone thickness mapping. Gas hydrate reserves in Krishna Godawari Basin have been established with the help of gas-hydrate related proxies inferred from multidisciplinary investigations. In the present study, an analysis of 3D seismic data of nearly 3,420 km2 area of Mahanadi deep water basin was performed in search of seismic proxies related with the existence of natural gas hydrate in the region. Analysis depicts the presence of BSR-like features over a large areal extent of nearly 250 km2 in the central western part of the basin, which exhibit all characteristics of a classical BSR associated with gas hydrate accumulation in a region. The observed BSR is present in a specific area restricted to a structural low at the Neogene level. The coherency inversion of pre-stack time migration (PSTM) gathers shows definite inversion of interval velocity across the BSR interface which indicates hydrate bearing sediments overlying the free gas bearing sediments. The amplitude versus offset analysis of PSTM gathers shows increase of amplitude with offset, a common trend as observed in BSR associated with gas hydrate accumulation. Results suggest the possibility of gas hydrate accumulation in the central part of the basin specifically in the area of structural low at the Neogene level. These results would serve as preliminary information for selecting prospective gas hydrate accumulation areas for further integrated or individual study from geophysical, geological, geochemical and microbiological perspectives for confirmation of gas hydrate reserves in the area. Further, on the basis of these results it is envisaged that biogenic gas might have been generated in the region which under suitable temperature and pressure conditions might have been transformed into the gas hydrates, and therefore, an integrated study comprising geophysical, geological, geochemical and microbiological data is suggested to establish the gas hydrate reserves in Mahanadi deep water basin.  相似文献   

12.
《Marine Geology》2001,172(1-2):1-21
In this paper we present and discuss the frequency-dependent behaviour of the acoustic characteristics of methane hydrate-bearing sediments in Lake Baikal, Siberia. Five different types of seismic sources (airgun-array, two types of single airguns, watergun and sparker) are used, encompassing a frequency bandwidth from 10 up to 1000 Hz. On low-frequency airgun-array data, the base of the hydrate stability zone (HSZ) is observed as a high-amplitude bottom-simulating reflection (BSR) with reversed polarity. The amplitude and continuity of the BSR decrease or even disappear on medium- to high-frequency data, a feature explained in terms of vertical and horizontal resolution. The increasing reflection amplitude of the BSR with increasing offset, the calculated reflection coefficient of the BSR and the occurrence of enhanced reflections below the BSR suggest the presence of free gas below the HSZ. The observation of some enhanced reflections extending above the BSR may be interpreted as an indication for free gas co-existing with hydrates within the HSZ. Amplitude blanking above the BSR is highly variable while the BSR itself appears to act as a low-pass frequency filter for medium- to high-frequency data.New single-channel airgun profiles provide the first seismic information across the Baikal Drilling Project (BDP-97) deep drilling site, at which hydrate-bearing sediments were retrieved at about 200 m above the base of the local HSZ. At the drilling site there are no seismic characteristics indicative of the presence of hydrates. Combination of the drilling and seismic information has allowed us to make a rough estimation of the volume of hydrates and carbon stored in the sediments of Lake Baikal, which lead us to conclude that the Lake Baikal gas hydrate reservoirs do not form a prospective energy resource.  相似文献   

13.
A wide-spread bottom simulating reflector (BSR), interpreted to mark the thermally controlled base of the gas hydrate stability zone, is observed over a close grid of multichannel seismic profiles in the Krishna Godavari Basin of the eastern continental margin of India. The seismic data reveal that gas hydrate occurs in the Krishna Godavari Basin at places where water depths exceed 850 m. The thickness of the gas hydrate stability zone inferred from the BSR ranges up to 250 m. A conductive model was used to determine geothermal gradients and heat flow. Ground truth for the assessment and constraints on the model were provided by downhole measurements obtained during the National Gas Hydrate Program Expedition 01 of India at various sites in the Krishna Godavari Basin. Measured downhole temperature gradients and seafloor-temperatures, sediment thermal conductivities, and seismic velocity are utilized to generate regression functions for these parameters as function of overall water depth. In the first approach the base of gas hydrate stability is predicted from seafloor bathymetry using these regression functions and heat flow and geothermal gradient are calculated. In a second approach the observed BSR depth from the seismic profiles (measured in two-way travel time) is converted into heat flow and geothermal gradient using the same ground-truth data. The geothermal gradient estimated from the BSR varies from 27 to 67°C/km. Corresponding heat flow values range from 24 to 60 mW/m2. The geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed BSR depths.  相似文献   

14.
It is the intent of this paper to explore a significant extent of an entire passive continental margin for hydrate occurrence to understand hydrate modes of occurrence, preferred geologic settings and estimate potential volumes of methane. The presence of gas hydrates offshore of eastern Canada has long been inferred from estimated stability zone calculations, but little physical evidence has been offered. An extensive set of 2-D and 3-D, single and multi-channel seismic reflection data comprising in excess of 140,000 line-km was analyzed. Bottom simulating reflections (BSR) were unequivocally identified at seven sites, ranging between 250 and 445 m below the seafloor and in water depths of 620-2850 m. The combined area of the BSRs is 9311 km2, which comprises a small proportion of the entire theoretical stability zone along the Canadian Atlantic margin (∼715,165 km2). The BSR within at least six of these sites lies in a sedimentary drift deposit or sediment wave field, indicating the likelihood of grain sorting and potential porosity and permeability (reservoir) development. Although there are a variety of conditions required to generate and recognize a BSR, one might assume that these sites offer the most potential for highest hydrate concentration and exploitation. Total hydrate in formation at the sites of recognized BSR’s is estimated at 17 to 190 × 109 m3 or 0.28 to 3.12 × 1013 m3 of methane gas at STP. Although it has been shown that hydrate can exist without a BSR, the results from this regional study argue that conservative estimates of the global reserve of hydrate along continental margins are necessary.  相似文献   

15.
The cruise conducted in the spring inter-monsoon (pre-monsoon) of April 2015 in the frame of a funded project epitomized an open ocean survey that allowed procuring a quasi-synoptic picture of cyclopoid copepod community structure in 18 open ocean stations of the Minicoy Island, Lakshadweep archipelago, off the southwestern Indian coast, based on the analysis of standing stock and composition in the integrated 0–10 m water column. The main objective was to explore the community structure of cyclopoid copepods prevailing here and which environmental variable influences the same. It is witnessed that sea surface temperature (SST) (30.42 ± 0.19°C), sea surface salinity (SSS) (33.56 ± 1.15 psu), and dissolved oxygen (DO) (4.32 ± 0.06 ml/L) with SSS and DO are the best matching variables diversifying cyclopoid copepod species (av. abundance 700 ± 386 no. of individuals/100 m3). Average zooplankton biomass (0.03 ml/m3) and abundance (8,989 ± 3,866 no. of individuals/100 m3) were also observed. Fifty-one cyclopoid copepod species belonging to four families and seven genera were identified, with the dominance of high saline species such as Sapphirina, Copilia, Farranula, and Oncaea. An abundance biomass curve (ABC) plot analysis indicated an undisturbed community with normal environmental conditions. TAXDTEST (taxonomic distinctness) depicted a diverse condition where all points clustered together within a 95% confidence level. Our results indicate that the cyclopoid copepod assemblage near Minicoy Island is a diverse, undisturbed community and hypothesize that the stable environmental conditions during pre-monsoon (spring inter-monsoon) preferred the diversification of cyclopoid copepods with the prevalence of high saline species. The data evolved could be used as a standard reference/benchmark to detect any deviation from an undisturbed/diverse community status of Minicoy Island in the looming scenario of climate change in and around the Indian Ocean.  相似文献   

16.
The bottom simulating reflector (BSR), the boundary between the gas hydrate and the free gas zone, is considered to be the most common evidence in seismic data analysis for gas hydrate exploration. Multiple seismic attribute analyses of reflectivity and acoustic impedance from the post-stack deconvolution and complex analysis of instantaneous attribute properties including the amplitude envelope, instantaneous frequency, phase, and first derivative of the amplitude of seismic data have been used to effectively confirm the existence of a BSR as the base of gas hydrate stability zone. In this paper, we consider individual seismic attribute analysis and integrate the results of those attributes to locate the position of the BSR. The outputs from conventional seismic data processing of the gas hydrate data set in the Ulleung Basin were used as inputs for multiple analyses. Applying multiple attribute analyses to the individual seismic traces showed that the identical anomalies found in two-way travel time (TWT) between 3.1 and 3.2 s from the results of complex analyses and l 1 norm deconvolution indicated the location of the BSR.  相似文献   

17.
In this paper we focused on understanding the isostatic compensation of the Ninetyeast Ridge in the overall context of the Bay of Bengal oceanic lithosphere and the interaction of the ridge system with the north Andaman subduction zone from north of 7–18°N. This region is characterized by the initial interaction of the Kerguelen hotspot with the Bay of Bengal oceanic lithosphere. We used satellite altimeter-derived marine geoid, as it should comprehensively reflect the compensations caused by large spatial wavelength dominated deeper anomaly sources in a hotspot affected lithospheric load like the Ninetyeast Ridge. Our analyses of the geoid-to-topography ratio (GTR), residual geoid, gravity-to-topographic kernel and upward continuation of anomalies show the existence of two different types of source compensation bodies beneath the northern (12–18°N) and southern (7–12°N) Ninetyeast Ridge. In the northern region, the geoid to topography ratio varies from 0.63 ± 0.05 to 0.44 ± 0.03, while in the southern region it ranges from 1.34 ± 0.09 to 1.31 ± 0.07 which resulted in a north to south increase in the apparent compensation depth from ~9 to 28 km. The presence of a shallow Moho, low GTR, broader gravity to topography kernel and the absence of a ridge anomaly from the mantle density dominated upward continued anomaly at z = 300 km indicates that at the northern segment the underplated low density crustal melt is the dominant isostatic compensating body. However, at the southern ridge segment the high GTR, strong gravity-to-topography kernel and the subsistence of the anomaly at long wavelengths, even at z = 300 km represents the existence of large volumes of hotspot related underplated dense material as the source of compensation. The proximity of the dense source compensating body of the southern Ninetyeast Ridge to the Andaman subduction zone affected the regional mantle driven density gradient flow, as observed from the z = 300 km continued gravity anomaly. The existence of a southern Ninetyeast Ridge in such a transpressional regime has caused the formation of a forearc sliver at its eastern flank, which is a major crustal deformational structure developed as a result of ridge-trench collision.  相似文献   

18.
We report the first record of vertical rhizome growth in the temperate seagrass Zostera marina. In a population of Z. marina occurring on subtidal sand in the Novigrad Sea (Croatia), an area subject to episodic high sediment transport, collected plants of Z. marina showed vertical rhizomes with shorter and narrower inter‐nodes (mean length = 3.4 ± 1.5 SD mm, mean width = 1.9 ± 0.3 SD mm) than those recorded for horizontal rhizomes (mean length = 9.0 ± 3.5 SD mm, mean width = 2.8 ± 0.4 SD mm). Out of a sample of 1130 rhizome fragments, 288 (25.5%) were vertical. Repeated moderate burial events may have stimulated the production of vertical rhizomes, and the ability of Z. marina to produce vertical rhizomes may enable it to withstand moderate burial in this highly dynamic environment.  相似文献   

19.
Benthic structure of coral reefs determines the availability of refuges and food sources. Therefore, structural changes caused by natural and anthropogenic disturbances can have negative impacts on reef‐associated communities. During the 1990s, coral reefs from Bahía Culebra were considered among the most diverse ecosystems along the Pacific coast of Costa Rica; however, recently they have undergone severe deterioration as consequence of chronic stressors such as El Niño‐Southern Oscillation and harmful algal blooms. Reef fish populations in this area have also been intensely exploited. This study compared reef fish assemblages during two periods (1995–1996 and 2014–2016), to determine whether they have experienced changes as a result of natural and anthropogenic disturbances. For both periods, benthic composition and reef fish abundance were recorded using underwater visual censuses. Live coral cover (LCC) decreased from 43.09 ± 18.65% in 1995–1996 to 1.25 ± 2.42% in 2014–2016 (U = 36, p < 0.05). Macroalgal cover (%) in 2014–2016 was sixfold higher than mean values reported for the Eastern Tropical Pacific region. Mean (±SD) fish species richness in 1995–1996 (36.67 ± 14.20) was higher than in 2014–2016 (23.00 ± 9.14; U = 20, p < 0.05). Over 40% of reef fish orders observed in 1995–1996 were not detected in the 2014–2016 surveys, including large‐bodied predators. Reduction in abundance of fish predators such as sharks, grunts, and snappers is likely attributed to changes in habitat structure. Herbivorous such as parrotfishes and pufferfishes increased their abundance at sites with low LCC, probably in response to predators decline and increased algal cover. These findings revealed significant degradation and drastic loss of structural complexity in coral reefs from Bahía Culebra, which now are dominated by macroalgae. The large reduction in structural complexity of coral reefs has resulted in the loss of diversity and key ecological roles (e.g., predation and herbivory), thus potentially reducing the resilience of the entire ecosystem.  相似文献   

20.
Bottom simulating reflectors (BSRs), known as the base of gas hydrate stability zone, have been recognized and mapped using good quality three-dimensional (3D) pre-stack migration seismic data in Shenhu Area of northern South China Sea. Additionally, seismic attribute technique has been applied to better constrain on the distribution of gas hydrate. The results demonstrate that gas hydrate is characterized by “blank” zone (low amplitude) in instantaneous amplitude attribute. The thickness of gas hydrate stability zone inferred from BSR ranges from 125 to 355 m with an average of 240 m at sea water depth from 950 to 1,600 m in this new gas hydrate province. The volume of gas in-place bound in hydrate is estimated from 1.7 × 109 to 4.8 × 109 m3, with the most likely value of around 3.3 × 109 m3, using Monte Carlo simulation. Furthermore, geothermal gradient and heat flow are derived from the depths of BSRs using a conductive heat transfer model. The geothermal gradient varies from 35 to 95°C km−1 with an average of 54°C km−1. Corresponding heat flow values range from 43 to 105 mW m−2 with an average of 64 mW m−2. By comparison with geological characteristics, we suggest that the distribution of gas hydrate and heat flow are largely associated with gas chimneys and faults, which are extensively distributed in Shenhu Area, providing easy pathways for fluids migrating into the gas hydrate stability zone for the formation of gas hydrate. This study can place useful constraints for modeling gas hydrate stability zone from measured heat flow data and understanding the mechanism of gas hydrate formation in Shenhu Area.  相似文献   

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