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1.
This study presents a sea-level curve from 9500 to 6500 cal BP for the farfield location of Singapore, on the Sunda Shelf in southeast Asia. The curve is based on more than 50 radiocarbon dates from elevations of +1.43 m to −15.09 m representing sea-level index points in intertidal mangrove and shallow marine sediments deposited by sea-level rise accompanying deglaciation. The results indicate that mean sea level rose rapidly from around −17 m at 9500 cal BP to around −3 m by 8000 cal BP. After this time, the data suggest (but do not unequivocally prove) that the rate of sea-rise slowed for a period of 300–500 years centred on 7700 cal BP, shortly after the cessation of meltwater input to the oceans from the northern hemisphere. Renewed sea-level rise amounting to 3–5 m began around 7400 cal BP and was complete by 7000 cal BP. The existence of an inflection in the rate of sea-level rise, with a slow-down centred on 7700 cal BP, is broadly consistent with other available sea-level curves over this interval and is supported by evidence of stable shorelines and delta initiation elsewhere at this time, as well as evidence of comparatively rapid retreat of the West Antarctic ice sheet beginning around 7500 cal BP. ‘Stepped’ sea-level rise occurring shortly after 7500 cal BP and also earlier during deglaciation may have served to focus significant post-glacial episodes of human maritime/coastal dispersal, into comparatively narrow time intervals.  相似文献   

2.
A multi-proxy approach based on organic (organic carbon, alkenones, and C/N) and inorganic geochemical proxies (biogenic opal, inorganic carbon, Fe, Ti, and Ca) preserved in the sediments of the Jacaf channel (CF7-PC33; 44°21′S, 72°58′W, 510 m water depth), Chilean fjords, yields evidence of major past productivity fluctuations in accordance with climatic changes over the last ∼1800 years. The downcore record clearly shows two productivity/climate modes. The first period, prior to 900 cal yr BP, is characterized by decreased marine productivity and a reduced continental signal, pointing to diminished precipitation and runoff. In contrast, the second period between ∼750 cal yr BP and the late 1800s (top of core) is illustrated by elevated productivity and an increased continental signal, suggesting higher precipitation and runoff. Both time intervals are separated by a relatively abrupt transition of ∼150 years which roughly coincides with the beginning of the Little Ice Age. The increased content of freshwater diatoms and Chrysophyte cysts that characterize the last 200 years of the latter mode coincides with a significant decrease in the carbonate content of the sediment; together they further indicate increased terrigenous contributions and decreased marine carbonate productivity at the end of the Little Ice Age. The correspondence between our record and other paleoclimate studies carried out in South America and Antarctica demonstrates that the Chilean fjord area of Northern Patagonia is not just sensitive to local climatic variability but also to regional and possibly global variability.  相似文献   

3.
0-Group sole, Solea solea (Linnaeus, 1758) were sampled in four nursery grounds: two on the Northern French coast and two on the Portuguese coast. Juvenile sole were collected at the Vilaine estuary (Northern Bay of Biscay) in 1992, in the Authie estuary (Eastern English Channel) in 1997, and in the Douro and Tagus estuary (Northern and central Portugal, respectively) in 2005. Left lapilli otoliths were used to estimate age and investigate variability in growth rates and hatch dates. In the French study areas nursery colonisation ended in early June in the Vilaine estuary and in late June in the Authie estuary. In the Portuguese estuaries nursery colonisation ended in May in the Douro estuary and in late June in the Tagus estuary. Growth rates were higher in the Portuguese estuaries, 0.767 mm d−1 in the Tagus estuary and 0.903 mm d−1 in the Douro estuary. In the French nurseries, growth rates were estimated to be 0.473 mm d−1 in the Villaine estuary and 0.460 mm d−1 in the Authie estuary. Data on growth rates from other studies shows that growth rates are higher at lower latitudes, probably due to higher water temperature. Spawning took place between early January and early April in the Villaine estuary's coastal area in 1992. In 1997, in the Authie estuary spawning started in late January and ended in early April. On the Douro estuary's adjacent coast spawning started in mid-January and ended in late March, in 2005, while on the Tagus estuary's adjacent coast spawning started in mid-February and ended in mid-April, in the same year. Literature analysis of the spawning period of sole along a latitudinal gradient ranging from 38°N to 55°N in the Northeast Atlantic indicated that there is a latitudinal trend, in that spawning starts sooner at lower latitudes. Results support that local conditions, particularly hydrodynamics, may overrule general latitudinal trends.  相似文献   

4.
Temporarily open/closed estuaries typically open to the sea due to freshwater inflow coupled with storm surge events. In September 2008, in the absence of freshwater inflow, the mouth of the East Kleinemonde Estuary breached in response to a storm surge. The mouth of the estuary closed the following day at a high level. Marine overwash events following the breach introduced large volumes of saline water into the estuary and raised the water level by 0.07–0.33 m. Salinity was significantly higher in the 15 month closed phase after the breach (31 ± 0.9) compared to 21.9 ± 0.9 in the closed brackish phase before the breach. The historical average salinity for the estuary during a closed period is 23–25. The increase in salinity has reduced submerged macrophytes Ruppia cirrhosa and Chara vulgaris cover by 38.1%. Macroalgal cover of species such as Dictyota dichotoma, Caulacanthus ustulatus, Codium tenue and Ulva spp. have increased by 7.9%. The saline high water levels have also significantly reduced supratidal salt marsh cover by 15.2%, and reed and sedge cover by 19.7%. Loss of these habitats may result in bank destabilisation and erosion. This is the first record of an extended saline period in the 15 years the estuary has been monitored. Sea level rise in association with climate change, together with localised freshwater inflow reduction is likely to result in an increase in marine overwash events. The frequency and duration of closed saline periods are likely to increase in this type of estuary. A loss of submerged macrophytes may have significant impacts on faunal composition and abundance and on the subsequent functioning of temporarily open/closed estuaries. This has serious ecological implications since these estuaries represent 70% of the different types of estuaries found in South Africa.  相似文献   

5.
The design and operation of mathematical models of solute mixing and sediment transport in estuaries rely heavily on the provision of good-quality field data. We present some observations of salinity, suspended sediment concentration and velocity at one of the tidal limits of a semi-enclosed tidal lagoon in Southern England (Pagham Harbour, West Sussex, UK) where the natural processes of tidal incursion and solute mixing have been heavily modified as a result of the construction of sea walls dating back to the 18th Century. These observations, made immediately downstream of two parallel tidal flap gates by conductivity-temperature-depth (CTD) profiler, and also using bed-mounted sensor frames to measure velocity at 2 fixed depths, have yielded a set of results covering 11 tidal cycles over the period 2002–04. It is clear from the results obtained that over a typical tidal cycle, the greatest vertical salinity gradients occur in the 1–2 h immediately after the onset of the flood tide, and that subsequently, energetic mixing acts to rapidly break down this stratification. Under moderate-to-high fresh water flows (>0.5 m3/s), the break-down in vertical salinity gradient is more gradual, while under low fresh water flows (<0.2 m3/s), the vertical salinity gradient is generally less pronounced. Estimates of Richardson number during the early flood-tide period reveal values that vary rapidly between <1 and about 20, with lower values occurring after around 1.5–2 h after low water. Observations of suspended sediment concentration vary widely even for similar tidal and fresh water flow conditions, revealing the possible influence of wind speed, the storage effects of the water in the lagoon downstream of the observation site, and the complexity of the hydrodynamics downstream of tidal flap gates. The data also show that most of the sediment transport is landward, and occurs during flood tides, with estimated total tidal landward flood tide flux of fine sediment of the order of 50–120 kg under low fresh water flow conditions. These observations, which reinforce the results presented in Warner et al. (2004) and elsewhere, can help to provide information about the appropriate techniques for managing sediments and pollutants, including nutrients from sewage effluent waters, in estuaries where hydraulic flap gates are used to control the entry of fresh water over the tidal cycle.  相似文献   

6.
Juvenile Japanese flounder Paralichthys olivaceus usually inhabit high salinity inshore sandy areas. In June–August 1997, 25 individuals of juvenile Japanese flounder (33–75 mm total length) were collected in the Natori and Nanakita River estuaries in Sendai Bay, Japan. This is the first record of this species being collected in brackish estuaries in which salinities fluctuate from 0 to 30 over a spring tidal cycle. Factors of rainfall, river flow, or year class strength of Japanese flounder were unable to explain the unusual occurrence of this species in the estuaries. The collected juvenile Japanese flounder were considered to have migrated into the estuaries around the time of the passage of an unusually early typhoon that affected this area, indicating the possibility that this species utilizes estuaries for refuge from strong disturbance in its usual habitats. The juveniles fed mainly upon mysids both in the estuaries and the sandy beach area, indicating that estuaries can be a substitute nursery habitat for this species.  相似文献   

7.
Bacterial productivity (BP) and respiration (BR) were examined in relation to primary productivity (PP) for the first time in a shallow tropical ecosystem (Cochin Estuary), India. The degree of dependence of BP (6.3–199.7 μg C L−1 d−1) and BR (6.6–430.4 μg C L−1 d−1) on PP (2.1–608.0 μg C L−1 d−1) was found to be extremely weak. The BP/PP (0.05–8.5) and PP/BR (0.02–7.9) ratios widely varied in the estuary depending on the season and location. There was a seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter input through rivers which enhanced the bacterial heterotrophic activity and very high pCO2 (106–6001 μatm) levels. The heterotrophic zones were characterized by low PP but high bacterial production and respiration leading to oxygen undersaturation and exceptionally high pCO2. We propose that the CO2 supersaturation caused by increased bacterial respiration (in excess of PP) was a result of bacterial degradation of allochthonous organic matter. This indicates that sources other than planktonic compartment need to be explored to understand the C-cycling in this estuary. These results are of particular relevance to tropical ecosystems in general, where the bulk of world's river discharges occur.  相似文献   

8.
The objective of this study was to investigate the phosphorus distribution in the estuarine sediments of the Daliao river, intensively affected by municipal effluent and agricultural activity for about 50 years. Surface sediment samples were taken at 35 sites in the estuarine area and phosphorus species and contents of total P, Al, Fe, and Ca in the sediments were measured. Results showed that the content of total P in the sediments ranged from 230 to 841 mg kg−1, with an average of 549 mg kg−1. Ca–bound P, residual P, Al–bound P, reductant–soluble P, Fe–bound P, and soluble and loosely bound P were averagely 44.5, 21.6, 13.6, 11.7, 8.9 and 0.2% of total P, respectively. With the gradual increase of total P content, Al–bound P, reductant–soluble P, and Fe–bound P generally increased, while the rest species of P did not. This might indicate that anthropogenic P is bound to Al and Fe oxides. Regression analysis showed that Al–bound P and sum of Fe–bound P and reductant–soluble P were correlated to the contents of total Al and Fe, respectively. On the other hand, Ca–bound P was not correlated to the content of total Ca in the sediment, probably suggesting that Ca–P was mainly from authigenic marine origin. Whereas the content of total P in the estuarine sediments of the Daliao river was within the range of total P content for Chinese and worldwide river estuaries as well as coastal sediments, non-calcium apatite phosphorus content in the estuarine sediments of the Daliao river was relatively higher, indicating higher release risk and bioavailability of P in the sediment. On the other hand, the molar ratio of total Fe to total P was 16–34 in the estuarine sediments of the Daliao river, suggesting that iron oxides/hydroxides in the sediments might be able to sequester more phosphorus. Therefore, the accumulation or release of P in/from the estuarine sediments might be dependent on the external loading of P and the estuarine eutrophication may be sustained by the internal bioavailable P pools following the decrease of the external P loading.  相似文献   

9.
Transplant trials of the seagrass Posidonia australis were carried out after loss of seagrasses following eutrophication and increased turbidity in two marine inlets on the south coast of Western Australia. A pilot study in Oyster Harbour measured survival and growth in situ for 4 years. Long-term survival rates were high (96–98%), providing plants were anchored into the sediment. All unanchored plants were lost in the first winter. Following the success of the pilot study, a more comprehensive program began 3 years later with over 500 transplant units collected from either actively growing edges of nearby patches (plagiotropic growth form) or within established meadows (orthotropic growth form). Transplant units from edges expanded at a faster rate compared to units from mid-meadow but increases in shoot numbers were similar. Growth rates in the first 2.5 years averaged 10–20 cm yr−1 horizontal rhizome extension, depending on the source of the transplant units, and 4–12 shoots per initial shoot yr−1, depending on the initial shoot number of the transplant unit. After 5 years, shoot numbers of individual transplants were similar to shoot densities recorded for natural meadows, >500 shoots m−2. Approximately, 10% of transplants from mid-meadow flowered in the first year, whereas transplants from edges flowered only after 5 years.  相似文献   

10.
The Little Ice Age (∼1600–1900 AD) and 20th century sediment accumulation rates in Billefjorden, a subpolar fjord on Svalbard, were reconstructed by applying 210Pb, 137Cs and AMS 14C datings. The modern sediment accumulation rate decreases from more than 0.39 cm y−1 at the fjord head to 0.08 cm y−1 close to the fjord mouth. However, during the Little Ice Age the sediments accumulated at a much lower rate of 0.02 cm y−1 in the central fjord basin. This difference is most likely related to the rapid retreat of glaciers during the 20th century, when most of them withdrew up to 2 km. The post-Little Ice Age increases in temperature and a negative glacier mass balance resulted in a larger meltwater discharge transferring substantial amounts of sediments released from the glaciers, as well as those eroded from previously stored unconsolidated glacial sediments. A comparison of data from the subpolar fjords of Svalbard suggests that the increase in the sediment accumulation rate is a common trend, and further increases might be expected if climate warming continues. The properties of the fjord sediments (grain size, IRD, coarse-fraction composition, clay mineralogy) from the Little Ice Age and the 20th century showed no distinct differences. The change in the accumulation rate may be the most evident sedimentary record of this climatic change.  相似文献   

11.
Sediment processes in estuaries are controlled by the interaction of factors that include tides, fresh water inputs, bed morphology, sediment supply, and hydrodynamics. The interaction of these factors strongly influences the pattern of sediment deposition. The ability to quantify sediment deposition on a regional scale will improve the understanding of the underlying processes, and provide valuable information for managing estuarine systems. This paper describes our approach for obtaining the deposition pattern and quantifying the amount of 20th century impacted sediments in the Haverstraw Bay section of the Hudson River Estuary. Through the combination of high-resolution seismic data and rapidly acquired geochemical information from numerous sediment cores, we estimate that our study site experiences an average sediment accumulation rate of ∼3 mm/y and that ∼75,000 t/y or ∼10% of the annual total sediment input measured at the Poughkeepsie, NY gauging station (USGS) is stored in this reach of the Hudson River on ∼100 y timescales. A detailed analysis of the depositional pattern indicates that the accumulation rate varies considerably throughout the study area ranging from non-depositional to >8 mm/y. Our data also clearly indicate that the dredged channel in Haverstraw Bay is currently the main focus of deposition in this area.  相似文献   

12.
Investigations of multi-layer current regime, variations in sea level and wave parameters using a bottom-mounted RDCP (Recording Doppler Current Profiler) during 20 December 2006–23 May 2007 were integrated with surveys on changes of shorelines and contours of beach ridges at nearby Harilaid Peninsula (Saaremaa Island). A W-storm with a maximum average wind speed of 23 m s−1 occurred on 14–15 January with an accompanying sea level rise of at least 100 cm and a significant wave height of 3.2 m at the 14 m deep RDCP mooring site. It appeared that in practically tideless Estonian coastal waters, Doppler-based “vertical velocity” measurements reflect mainly site-dependent equilibrium between resuspension and sedimentation. The mooring site, 1.5 km off the Kelba Spit of Harilaid, was located in the accumulation zone, where downward fluxes dominated and fine sand settled. As a result of storms in January and April, the distal part of the accumulative gravel spit advanced by 50 m, whereas a 30–50 m retreat of the shoreline in the western and northern parts occurred at Cape Kiipsaare. The location of the beach ridges shows that the development of the spit occurs through relatively short-period but infrequent storm events, roughly 2–3 times each decade.  相似文献   

13.
The levels of 19 kinds of organochlorine pesticides (OCPs) in the aqueous phase, suspended particulate matter (SPM), pore water and sediments from Daliao River estuary of Liaodong Bay (Bohai Sea) in northeast China were investigated to evaluate their potential pollution risks. The total OCPs concentrations in the aqueous phase, SPM, pore water and sediments were 3.7–30.1 ng l−1, 4.6–52.6 ng l−1, 157–830 ng l−1 and 2.1–21.3 ng g−1 dry weight, respectively. The concentrations of OCPs, in the Daliao River estuary, are in the mid-range, as compared to those reported in other estuaries worldwide. The distribution of HCHs and DDTs were different indicating different contamination sources. Lindane is the main type of HCH and continuing use in northeast China of ‘pure’ HCH (lindane) rather than technical HCH accounts for the source. The ratios of (DDE + DDD)/DDT in the samples indicate no recent inputs of these chemicals to the estuary.  相似文献   

14.
Turbidity limits gas exchange in a large macrotidal estuary   总被引：1，自引：0，他引：1  
In estuaries, the gas transfer velocity (k) is driven by a combination of two major physical drivers, wind and water current. The k values for CO2 in the macrotidal Gironde Estuary were obtained from 159 simultaneous pCO2 and floating chamber flux measurements. Values of k increased with wind speed and were significantly greater when water currents and wind were in opposing directions. At low wind speeds (<1 m s−1), k increased with water current velocities (0–1.5 m s−1) following an exponential trend. The latter was a good proxy for the Y-intercept in a generic equation for k versus wind speed in estuaries. We also found that, in this turbid estuary, k was significantly lower at high turbidity. The presence of suspended material in great concentrations (TSS > 0.2 g L−1) had a significant role in attenuating turbulence and therefore gas exchange. This result has important consequences for modeling water oxygenation in estuarine turbidity maxima. For seven low turbidity estuaries previously described in the literature, the slope of the linear regression between k and wind speed correlates very well with the estuary surface area due to a fetch effect. In the Gironde Estuary, this slope follows the same trend at low turbidity (TSS < 0.2 g L−1), but is on average significantly lower than in other large estuaries and decreases linearly with the TSS concentration. A new generic equation for estuaries is proposed that gives k as a function of water current velocity, wind speed, estuarine surface area and TSS concentration.  相似文献   

15.
We used non-destructive methods to study the bi-monthly changes in standing stock, turnover, and net aerial primary productivity (NAPP) of Spartina alterniflora in the Bahía Blanca Estuary, Argentina, from 2005 to 2007. Tillers were tagged and counted bimonthly and a weight:height relationship developed for the live and dead stems in a regularly flooded zone (low marsh, LM) and an irregularly flooded one (high marsh, HM). The annual tiller natality in year one compared to year two decreased from 440 ± 68 to 220 ± 58 new individuals m–2 yr–1 in the HM and from 500 ± 103 to 280 ± 97 new individuals m−2 yr−1 in the LM (μ ± 1 SE). Tiller mortality averaged 670 ± 70 individuals m−2 yr−1.  相似文献   

16.
The temporal and spatial variability of dissolved inorganic phosphate (DIP), nitrogen (DIN), carbon (DIC) and dissolved organic carbon (DOC) were studied in order to determine the net ecosystem metabolism (NEM) of San Diego Bay (SDB), a Mediterranean-climate lagoon. A series of four sampling campaigns were carried out during the rainy (January 2000) and the dry (August 2000 and May and September 2001) seasons. During the dry season, temperature, salinity and DIP, DIC and DOC concentrations increased from oceanic values in the outer bay to higher values at the innermost end of the bay. DIP, DIC and DOC concentrations showed a clear offset from conservative mixing implying production of these dissolved materials inside the bay. During the rainy season, DIP and DOC increased to the head, whereas salinity decreased toward the mouth due to land runoff and river discharges. The distributions of DIP and DOC also showed a deviation from conservative mixing in this season, implying a net addition of these dissolved materials during estuarine mixing within the bay. Mass balance calculations showed that SDB consistently exported DIP (2.8–9.8 × 103 mol P d−1), DIC (263–352 × 103 mol C d−1) and DOC (198–1233 × 103 mol C d−1), whereas DIN (5.5–18.2 × 103 mol N d−1) was exported in all samplings except in May 2001 when it was imported (8.6 × 103 mol N d−1). The DIP, DIC and DOC export rates along with the strong relationship between DIP, DIC or DOC and salinity suggest that intense tidal mixing plays an important role in controlling their distributions and that SDB is a source of nutrients and DOC to the Southern California Bight. Furthermore, NEM ranged from −8.1 ± 1.8 mmol C m−2 d−1 in September to −13.5 ± 5.8 mmol C m−2 d−1 in January, highlighting the heterotrophic character of SDB. In order to explain the net heterotrophy of this system, we postulate that phytoplankton-derived particulate organic matter, stimulated by upwelling processes in the adjacent coastal waters, is transported into the bay, retained and then remineralized within the system. Our results were compared with those reported for the heterotrophic hypersaline coastal lagoons located in the semi-arid coast of California–Baja California, and with those autotrophic hypersaline systems found in the semi-arid areas of Australia. We point out that the balance between autotrophy and heterotrophy in inverse estuaries is dependent on net external inputs of either inorganic nutrients or organic matter as it has been indicated for positive estuaries.  相似文献   

17.
Modern (last 100 yr) accumulation rates of shelf mud deposits in the Yellow and East China Seas were investigated using the distribution of excess 210Pb (210Pbex) in sediment core samples. Compilation and merger of new and previously published data helped clarify sediment accumulation in these seas. The estimated accumulation rates, together with data of suspended sediment concentrations, provided findings on the sediment budget, origin, and transport pathway of the mud deposits. The overall accumulation distribution in the Yellow and East China Sea shelf revealed a general, cross-shelf decreasing trend along the sediment dispersal system away from the rivers, except for the South Sea (SSM) and southeastern Yellow Sea (SEYSM) mud patches found along the Korean coast. Notably, 210Pbex activity profiles within the SSM and the SEYSM yielded a relatively high accumulation rate of 2-5 mm/yr, implying a sedimentation rate of 4-15 × 107 tons per year in this coastal zone. Such an annual accumulation rate is about one order of magnitude greater than the total sediment discharge (6-20 × 106 tons/yr) from Korean rivers, suggesting an additional offshore source. The distribution pattern of the well-defined suspended plume clearly showed the possible transport and exchange of fine-grained sediments between the ECS shelf and the coastal area of Korea, especially during winter. Such a high accumulation in Korean coastal areas is attributable to the sediments supplied from the mud deposit of the ECS (i.e., SWCIM), with origins in Chinese rivers. Therefore, the Korean coastal area may be an important sink for some of Chinese river sediments being transported from the south by the Yellow Sea Warm Current.  相似文献   

18.
Dissolved and particulate lead were measured over an annual cycle (12 surveys between February 1998 and January 1999) in the Morlaix River estuary (Brittany, France). The concentrations were investigated in both the water column and the sediment of the river bottom in relation to hydrological conditions. In the water column, dissolved and particulate lead concentrations ranged from 0.1 to 4.4 nM and from 0.04 to 1.9 μmol g− 1, respectively. Lead concentrations in surface sediment varied from 0.04 to 0.19 μmol g− 1 and concentrations in the sediment pore water of the estuary were below the detection limit. Compared with the ranges known for pristine estuaries, concentrations of Pb in the water column of the Morlaix River estuary were found to be much higher. Concentrations of Pb also exceeded the lower range of those known for industrialized estuaries. Extensive agricultural activities in the drainage basin may be responsible for Pb levels above pristine conditions. Furthermore, the sediment appeared not to be contaminated. A mass balance was constructed quantifying all known sources and sinks for the Pb in the estuary. Riverine input accounts for most of the total annual metal flux. Burial in sediments was the major sink within the estuary, which acts as a trap especially for the particulate lead. The mass balance shows that the metal accumulation ranged between 414.6 and 446.0 kg year− 1.  相似文献   

19.
This study correlates Holocene sedimentation with regional environmental changes for the Guadiana middle shelf, SW Iberia, based on a detailed sedimentological analysis combined with radiocarbon and amino acid racemization dating. The sedimentary record of vibrocore CRIDA16 (307 cm) from the mud patch off the Guadiana River is characterized by a transgressive signature. Terrigenous sand–gravel dominates the core base up to ca. 5,000 cal. years b.p., enriched in schist/greywackes and non-reworked quartz. The shelf area was under the direct influence of Guadiana bed load discharges, as sea level was lower than today but rising, favouring the entrapment of fines during the Guadiana estuary infilling in the early Holocene. Core upwards, fines dominate the record. Sea-level stabilization at ca. 5,700–3,700 cal. years b.p. led to estuary infilling by coarser sediments and export of suspended sediment to the middle shelf. After ca. 900–700 cal. years b.p., the formation of the Guadiana Mud Patch signifies deposition in a marine environment dominated by suspended material from the Guadiana River. Comparison with earlier publications showed that a similar evolutionary mechanism of middle shelf mud deposition occurred in other parts of the Iberian Margin, pointing to a generalized establishment of this type of sedimentary body in Iberia since the mid to late Holocene. This suggests that the formation of mud belts on the north and south Iberian Margin was to a large extent synchronous. It is likely that the evolution of the Guadiana Mud Patch on the shelf was controlled foremost by the overall climate trends identified for several parts of Iberia during the Holocene (i.e. increased aridity punctuated by significant humidity stages), and less by human occupation of the region. Enhanced humidity stages led to higher rainfall and intensified floods, with a consequent increase of discharges to the shelf which ultimately were responsible for the Guadiana Mud Patch formation. It is expected that this pattern of sedimentary dynamics has been strongly disturbed since 2002, when the closing of the Alqueva Dam (the biggest on the Iberian Peninsula) was completed in the Guadiana River basin.  相似文献   

20.
To estimate the influence of mercury emitted from submarine fumaroles, the horizontal and vertical distribution of mercury in sediment of Kagoshima Bay was studied. The fumaroles are located in the northern bay head area, and the sediment samples had been taken from 52 points throughout the bay with a gravity core sampler. The core samples obtained were cut at a thickness of 1–2 cm and used for measurements. The total concentration of mercury in surface sediment in the northern and central areas of the bay was 51–679 μg kg− 1 (average 199 μg kg− 1, n = 22) and 23–100 μg kg− 1 (average 55 μg kg− 1, n = 30), respectively. The highest value was obtained in the vicinity of the fumaroles. The mercury concentration in sediment near the fumaroles varied with depth, which may reflect the variation in fumarolic activity. A successive extraction method was applied to the speciation of mercury in the sediment. The results showed that sediment taken in the vicinity of submarine fumaroles contained a higher percentage of mercury bound with organic matter.  相似文献   

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