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1.
Quartz microstructures and fabrics in the southeastern part of the island of Groix developed during the last stages of the Palaeozoic synmetamorphic deformation. The zonation of the quartz microstructures in map view suggests an upward positive gradient of strain. The plastic flow plane in quartz found in folds with axes aligned parallel to the stretching lineation, is parallel to the axial planes of the folds. The dominant regional sense of shear, as deduced from quartz fabrics, corresponds to the northward displacement of the upper block. This sense of displacement supports the hypothesis made by previous workers that the synmetamorphic deformation in Groix occurred in a N-directed intra-lithospheric thrust rather than in a N-dipping subduction zone. Quartz c-axis patterns argue for the distinction of two synmetamorphic phases with different transport directions. The transition between these two phases is thought to have occurred progressively during the course of the thrusting.  相似文献   

2.
《Geodinamica Acta》2013,26(4):289-307
The Hercynian Armorican belt is made of different tectonic domains separated by large-scale shear zones. One of these domains, Central Brittany, was deformed by regional pervasive dextral strike-slip during Carboniferous times. This paper examines implications that the restoration of ductile strain within Central Brittany has on the deformation history along its boundaries and within adjacent domains. Data used for the analysis include surface structural features and recent deep seismic data. A good consistency is observed between model implications and available geological and structural data. Results lead to a reappraisal of the Carboniferous history of the Armorican belt, including relationships between tectonics, plutonism and basin development. They further emphasize the usefulness of restoration for integrated tectonic analysis of crustal-scale deformation zones.  相似文献   

3.
Carbonate rocks contain prolific hydrocarbon reserves all over the world, particularly in the Middle East. For exploration and production strategies, it is essential to understand carbonate reservoirs in terms of their internal characteristics, depositional environment, relative age, diagenetic processes and impact on petrophysical properties. This study has been performed on exposed Cretaceous, Paleocene and Miocene marine carbonate sedimentary sequences in two localities (Maghdoucheh and Qennarit) near the city of Sidon (Southern Lebanon). It represents the first comprehensive study that takes into consideration the carbonate reservoir facies, diagenetic history and reservoir quality in the area. Rocks at Maghdoucheh are mainly dominated by limestone beds showing sedimentary structures and erosive bases alternating with microfossil-rich silty marls, related to a shallowing upward sequence in a restricted marine platform environment. Shells of benthic foraminifera and mollusks dominate the fossil assemblage extracted from the studied rocks. The microfossil and nannofossil assemblage detected in the Maghdoucheh sections indicates a middle Miocene age. Rocks at Qennarit are composed of mudstone/wackestone limestone beds rich in planktonic foraminifera and nannofossils related to open marine conditions. Based on the nannofossil content, rocks from Qennarit 1 and 2 are Paleocene and Cretaceous in age, respectively. Four main types of microfacies have been identified, i.e. (1) microbioclastic peloidal calcisiltite, (2) pelagic lime mudstone and wackestone with planktonic microfossils, (3) grainstone/packstone with abundant foraminifera and (4) fenestral bindstones, mudstones and packstones with porostromate microstructures. The porosity-permeability (poro-perm) analysis of representative samples reveals moderate to good porosity but very low permeability. This is mainly due to the presence of large moldic pores that are isolated in nature. The diagenetic features are dominated by micritization and dissolution (both fabric selective and non-fabric selective). Among all diagenetic features, dissolution in both localities contributes to porosity enhancement, while micritization, cementation, compaction and the filled fractures have negative impacts on permeability; hence the overall reservoir quality.  相似文献   

4.
This paper concentrates on the petrology of eclogite-faciesmetapelites and, particularly, the significance of staurolitein these rocks. A natural example of staurolite-bearing eclogitic micaschistsfrom the Champtoceaux nappe (Brittany, France) is first described.The Champtoceaux metapelites present, in addition to quartz,phengite, and rutile, two successive parageneses: (1) chloritoid+staurolite+garnetcores, and (2) garnet rims+kyanite?chloritoid. Detailed microprobe analyses show that garnet and chloritoidevolve towards more magnesian compositions and that stauroliteis more Fe-rich than coexisting garnet. A comparison of thestudied rocks with other known occurrences of eclogitic metapelitesshows that whereas staurolite is always more Fe-rich than garnetin high-pressure eclogites, the reverse is true in low- to medium-pressuremicaschists. Phase relations between garnet, staurolite, chloritoid, biotite,and chlorite are analysed in the KFMASH system (with excessquartz, phengite, rutile, and H2O). The topology of univariantreactions is depicted for a normal and a reverse Fe-Mg partitioningbetween garnet and staurolite. Mineral compositional changesare also predicted for varying bulk-rock chemistries. In the studied micaschists, the zonal arrangement of garnetinclusions and the progressive compositional changes of ferromagnesianphases record part of the prograde P–T path, before theattainment of ‘peak’ metamorphic conditions (atabout 65O–7OO?C, 18–20 kb). The retrograde path,which records the uplift of the Champtoceaux nappe, occurs underdecreasing temperatures.  相似文献   

5.
Sensitive multi-element analysis techniques were applied to water samples collected in the vicinity of a small Zn-Pb-Cu sulfide mineralization in the region of Menez-Albot (Finistère, France).The variation of the trace-element content along a local stream shows the presence of the mineralization, mainly through a simultaneous positive anomaly in solution for a group of about 10 elements (e.g. Ni, Cu, Zn, As, Sb) which are connected with this type of mineralization. The anomaly decreases steeply due to the influx of swamp water rich in Fe, Mn and organic matter. The precipitation barrier is reflected in the stream sediment composition. Contamination from fertilizers was observed in some samples.  相似文献   

6.
The Ordovician volcano-sedimentary succession of Erquy (northern Brittany) is made of immature sediments thermally metamorphosed at the contact of intruding basic sills. Pillow lavas constitute the upper part of the sequence. The trace element geochemistry of sills and pillow lavas suggests that they were derived from a tholeiitic source located beneath a passive margin. This volcanic sequence was metamorphosed under low to moderate greenschist facies conditions. In this study the direction and amplitude of chemical and isotopic fluxes in the basalt-sediment-water system were established and the oxygen and hydrogen isotope compositions of the aqueous fluid that reacted with the volcanic rocks were characterized. Cationic thermometry on chlorites and isotopic thermometry on plagioclase-chlorite pairs indicate closure metamorphic temperatures in the range 200–250°C for the basaltic sills. Stable isotope compositions of iron-rich chlorites (18O-5.5; D from-60 to-50) and plagioclases (18O from +9 to +10) reveal that the source of the fluid was certainly seawater. The 18O variations within the sills are strongly correlated with the rate of progress of the main metamorphic reaction:clinopyroxene+plagioclase+ilmenite chlorite+albite+epidote+quartz+sphene that produced major element mobility at the scale of the volcanosedimentary sequence. Calculation of elemental fluxes by mass balance combined with oxygen isotopic compositions of basalts shows that the highest water-rock ratios (1) were at sill-sediment boundaries and within pillow lavas at the top of the pile. The volcanosedimentary sequence of Erquy was a net sink for Na and a source for Ca. No Mg uptake could be detected whereas the hydrothermal alteration of the sediments released Fe, Si, and K trapped by the volcanic rocks. The 18O value of the fluid reacting with sills appears to have shifted no more than +4 after percolation at low temperature through immature sediments (18O12). The Erquy volcano-sedimentary sequence represents a marine hydrothermal system dominated by low-temperature exchange which allowed a general 18O-enrichment of the volcanic rocks and a 18O-depletion of sediments.  相似文献   

7.
This paper combines geological knowledge and geophysical imagery at the crustal scale to model the 3D geometry of a segment of the Hercynian suture zone of western Europe in the Champtoceaux area (Brittany, France). The Champtoceaux complex consists of a stack of metamorphic nappes of gneisses and micaschists, with eclogite-bearing units. The exhumation of the complex, during early Carboniferous times, was accompanied by deformation during regional dextral strike–slip associated with a major Hercynian shear zone (the South Armorican Shear Zone, SASZ). Dextral shearing produced a km-scale antiformal structure with a steeply dipping axial plane and a steeply eastward plunging axis. Armor 2 deep seismic profile shows that the regional structure was cut by a set of faults with northward thrusting components. Based on the seismic constraint, direct 2D crustal-scale modelling was performed throughout the Champtoceaux fold on seven radial gravity profiles, also using geological data, and density measurements from field and drill-hole samples. The 3D integration of the cross-sections, the digitised geological map, and the structural information (foliation dips) insure the geometrical and topological consistency of all sources of data. The 2D information is interpolated to the whole 3D space using a geostatistical analysis. Finally, the 3D gravity contribution of the resulting model is computed taking into account densities for each modelled geological body and compared to the Bouguer anomaly. The final 3D model is thus compatible with the seismic and gravity data, as well as with geological data. Main geological results derived from the modelling are (i) the overall 3D geometry of the south dipping thrust system interpreted on the seismic profile emphasises northward thrusting and folding of the Champtoceaux complex which was coeval with strike–slip along the South Armorican Shear Zone; (ii) the gravity modelling suggests the presence of a relatively dense body below the Champtoceaux complex that could be interpreted as a result of relative uplift of midcrustal material during thrusting along the E–W trending wrench–thrust system; (iii) the northern limb of the Champtoceaux anticline is a relatively shallow feature; and (iv) Vigneux synkinematic granitic body is a laccolith sheared and rooted along the southern branch of the SASZ and spreads away from the strike–slip zone within weak country-rocks.  相似文献   

8.
On Ile de Groix, Variscan metamorphic former tholeiitic and alkaline basalts occur as glaucophane-eclogites, blueschists and greenschists in isolated lenses and layers within metapelites. Whole-rock '18OSMOW values of the metabasites show limited variations (10.4-12.0‰) and no systematic differences among rock types and metamorphic grades. This provides no argument for large-scale blueschist-to-greenschist transformation driven by infiltration of externally derived fluids. Metamorphic mineralogical changes should have been triggered by internal fluids. Element variations in interlayered blue- and greenschists can be attributed to magmatic fractionation. Assemblages with garnet, clinopyroxene and glaucophane of a high-pressure/low-temperature (HP-LT) metamorphism M1, and NaCa-amphiboles (barroisite, magnesiohornblende, actinolite) of a medium-pressure/medium-temperature metamorphism M2 crystallized during deformation D1. Detailed core-rim zonation profiles display increasing and then decreasing AlIV in glaucophane of M1. NaCa-amphiboles of M2, mantling glaucophane and crystallized in porphyroblasts, show first increasing, then decreasing, AlIV and AlVI. Empirically calibrated thermobarometers allowed P-T path reconstructions. In glaucophane-eclogites of a metamorphic zone I, a prograde evolution to M1 peak conditions at 400-500°C/10-12 kbar was followed by a retrograde P-T path within the glaucophane stability field. The subsequent M2 evolution was again prograde up to >600°C at 8 kbar and then retrograde. Similarly, in metamorphic zones II and III, prograde and retrograde paths of M1 and M2 at lower maximal temperatures and pressures exist. The almost complete metamorphic cycle during M2 signalizes that the HP-LT rocks escaped from an early erosion by a moderate second burial event and explains the long-lasting slow uplift with low average cooling rates.  相似文献   

9.
In the southern part of the French Armorican massif, the Ligerian domain is located along the boundary between Gondwana and Armorica. Lithological, geochemical and structural data on the Saint-Georges-sur-Loire Unit, which is the northern part of the Ligerian domain, allow us to distinguish two sub-units. A southern sub-unit, formed by various blocks (chert, limestone, sandstone, rhyolite, mafic rocks) of Silurian to Middle Devonian age included as olistoliths in a Middle-Late Devonian terrigeneous matrix, overthrusts a sandstone-pelite northern sub-unit. Both units experienced two deformation events. The first one is a top-to-the-NW thrusting and the second one is a left-lateral wrenching. The Saint-Georges-sur-Loire Unit is an accretionary prism formed during the Late Devonian closure of the Layon rift, coeval with the main phase of the Variscan orogeny. The Layon rift, which according to the mafic olistoliths was partly floored by oceanic crust, appears as a buffer structural zone that accounts for the lack in Central Brittany of any tectonic or sedimentary echo of the closure of the Medio-European Ocean. The tectonic evolution of the Saint-Georges-sur-Loire Unit supports a polyorogenic model for this part of the Variscan Belt.  相似文献   

10.
Quaternary deposits are described from well‐exposed sections in cliffs at Plage de Mezpeurleuch, Finistère, Brittany (France). The general lithostratigraphic sequence comprises Holocene perched dune sand, calcrete and palaeosol overlying Weichselian head and coarse clastic palaeobeach units attributed to the last interglacial (Eemian–isotope substage 5e). A lower and an upper palaeobeach, separated by a thin head deposit, are reported resting on an irregular palaeorock platform eroded in Hercynian granite. From a comparison with modern coastal environments a palaeoenvironmental interpretation of the palaeobeach deposits is made. Erosionally bounded facies of the lower palaeobeach, are considered to be a transgressive sequence preserved by drowning associated with a rapid rise in relative sea‐level. Its subsequent abandonment is attributed to a relative sea‐level fall, which allowed a terrestrial environment to develop briefly. Facies of the upper palaeobeach however indicate a prograding shoreline developed during a prolonged period of relatively stable sea‐level. These sea‐level observations are in good agreement with other evidence for a brief widespread intra‐Eemian cold event. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

11.
Abstract The high-grade metamorphic rocks of southern Brittany underwent a complex tectonic evolution under various P-T conditions (high-P, high-T), related to stacking of nappes during Palaeozoic continentcontinent collision. The east to west thrusting observed in the whole belt is strongly perturbed by vertical movements attributed to the ascent of anatectic granites in the high-T area. The field reconstruction of subvertical, closed elliptical structures in gneisses and migmatites, associated with the subhorizontal, doubly radial pattern of stretching lineation in the mica schists, suggests the existence of an elliptical diapiric body buried at depth beneath the present erosion level. Deformation is associated with a complex P-T evolution partly recorded in aluminous gneisses (kinzigites, e.g. morbihanites). A chronology of successive episodes of mineral growth at different compositions is established by detailed studies of the mineral-microstructure relationships in X-Z sections, using the deformation-partitioning concept (low- and high-strain zones). Several thermometric and barometric calibrations are applied to mineral pairs either in contact or not in contact but in equivalent microstructiiral positions with respect to the deformation history. This methodology provides a continuous microstructural control of P-T variations through time and leads to three P-T-t-d paths constructed from numerous successive P-T estimations. Path 1 is a clockwise retrograde path preserved in low-strain zones, which records general exhumation movements after crustal thickening. Paths 2 and 3 are clockwise prograde/retrograde paths from high-strain zones; they are interpreted and discussed in the light of models of crustal anatexis and upward movement of magma (diapirism). Deformation and P-T effects induced by diapirism can be distinguished from the general deformation-metamorphic history of a belt, and would seem to be produced during a late stage of its history. The present microstructural-petrological approach to defining successive mineral equilibria in relation to progressive deformation steps provides a far more accurate evaluation of the metamorphic evolution than is possible by ‘standard’thermobarometry.  相似文献   

12.
Late Variscan granites intruded Brioverian (Upper Proterozoic) and Lower Paleozoic pelitic sequences to the north of the South Armorican shear zone. In the vicinity of the granites, Brioverian garnet micaschists contain pre/syn-S2 assemblages with garnet + staurolite and post-S2 assemblages with staurolite ± andalusite. Andalusite appeared pre/syn- and post-S2 in garnet-free micaschists. The garnets in the Brioverian micaschists are zoned with increasing Mg and decreasing Mn and Ca from core to inner rim. Only poor garnet zonations occur in Paleozoic hornfelses of enclaves in the Rostrenen granite. The results of a microstructurally controlled application of garnet–biotite geothermometers and garnet–plagioclase geobarometers are similar to P–T trends obtained by the Gibbs method of garnet zonation modelling in the system NCFMnMASH. The P–T paths of a pre/syn-S2 regional metamorphism are clockwise between 500–550°C/8 kbar and 700°C/5 kbar, followed by cooling decompression. They contrast with isobaric contact metamorphism between 500 and 700°C at 2.5–3 kbar in Paleozoic hornfelses. This points to a two-stage Variscan metamorphism with a pre-granitic pressure-dominated event in the Brioverian micaschists, followed by Late Variscan contact metamorphism, and suggests the existence of a pre-granitic tectonic boundary between the micaschists and overlying low-grade sequences.  相似文献   

13.
In the framework of the European project 'Climate change and coastal evolution in Europe', the microscale (< 1 year) morphological evolution of beaches in Brittany (W France) has been monitored for some years and linked to immediate forcings, the first one being tides (with a 13-m tidal range), and the second storm intensity. For the same sites, a mesoscale (decades) evolution was reconstructed from air photographs and historical data. The evolution is forced first of all by human activities, and secondly by climatic shift: wind direction changes and average wind velocity evolution. These results were used to build and calibrate a model of coastal behaviour. The model is based on a simple sediment cell (input – transit – output) and simulates the movements of volumes of sands under successive morphodynamic processes. Each of these processes is modelled (in terms of volume exchanges) and parameterized by the relevant atmospheric element. Thus, the model is not deterministic but simulates different probabilities for atmospheric events and morphological response. The model runs for one year and produces realistic sediment accumulation values and coastal retreat rates. When run for half a century, results are also quite comparable to the field data. Storm frequency changes appear to be an important element for the evolution, but the main and first forcing is the availability of sediment.  相似文献   

14.
We used the local slope/drainage area relationship to derive the basic erosion and tectonic parameters from a topography. Assuming a dynamic equilibrium between uplift and erosion, this relationship is expected to depend quite simply on the rock erodibility, and on the tectonic uplift. This relationship may then be used to quantify independently the effect of lithological variation on the erodibility, and the uplift rate. We tested the method on a computer simulated topography and showed that the uplift information can be precisely calculated from the topographic analysis alone. We then analysed the topography of Brittany (France), and obtained a good agreement with uplift data from comparative levelling studies and river incision analysis.  相似文献   

15.
Contributions to Mineralogy and Petrology - New data were obtained from mineralogical and petrological studies of the two metamorphic episodes affecting the detrital series between Lorient and...  相似文献   

16.
This study is based on the morphosedimentary analysis of the cliff-top storm deposits accumulated on the coast of Banneg Island located in the archipelago of Molène (Brittany, France). These CTSDs comprise large, tabular clasts quarried from the upper part of the cliff and the backing scoured platform by giant oceanic storm waves impacting directly the western coast of the island. An analysis of the distribution and the geomorphology of these accumulations were carried out using the DGPS topographic surveys. Most of the clasts are organised into imbricate boulder clusters or ridges deposited between 7.5 and 14.5 masl. 52 accumulations were inventoried from the north to the south of the island, representing a global volume of 1000 m3. The median size of the clasts calculated is equivalent to 0.8 × 0.6 × 0.4 m and a weight of 0.6 t. The largest one measuring 5.3 × 3.9 × 0.5 m (≈ 32 t) is located in the centre of the island (ridge #28). It has been deposited 14 m inland from the edge of the cliff at the elevation of 9 m. Sediment analysis shows that clast sizes become smaller with increasing distance from the shoreline, but there is no relationship between the sorting and the distance inland. A study of the hydrodynamic conditions inducing clast transport was undertaken by an analysis of the wave data from the 1989 to the 1990 winter storms. Models of wave runup indicate that their highest water levels may have reached up to 19 masl, 5 to 10 m higher than the top of the cliff. Submersion by giant storm waves has been more important and frequent in the centre and the south of the island. Wave data over the 1979 to the 2007 period shows that no events as powerful as those of the 1989 to the 1990 winter were recorded during the last 30 years. Yet, it appears that the 1979–1990 decade was characterized by important morphogeneous events while the following period (1990–2007) has experienced a sharp decrease in storm events. These variations could be attributed to the inversion from a negative towards a positive phase in the North Atlantic Oscillation index.  相似文献   

17.
Cadmium (Cd) variations were investigated over an annual cycle (12 surveys between February 1998–January 1999) in the Morlaix estuary (Brittany, France) in both the water column and the benthic compartment in relation to hydrological conditions. The drainage basin of the Morlaix River estuary is predominantly agricultural in character. Dissolved Cd concentrations in the water column varied from 0.04 to 0.48 nM. Particulate Cd concentrations ranged from 1 to 64 nmol g−1. These concentrations reach levels commonly observed in estuaries affected by heavy industrial activities. Extensive agricultural activities in the drainage basin may be responsible for Cd levels above pristine conditions. Metal concentrations varied significantly over the seasonal cycle and the dissolved fraction exhibited high values in summer months. Particulate concentrations were always lowest during this season. In the benthic compartment, Cd concentrations in surface sediment varied from 0.4 to 5.0 nmol g−1 and from 0.2 to 5.0 nM in porewaters. Concentrations in sediment were slightly affected by Cd contamination and temporal changes were important over the seasonal cycle. The variations seem to be controlled by the succession of sedimentation and erosion processes, which are tightly linked to seasonal changes in river discharge. A box model was constructed based on known Cd sources and sinks in the estuary. Cd is chiefly brought into the estuary by the Morlaix River and accumulates within the estuary. The accumulation within the estuary represents from 6.3 to 7.2 kg yr−1.  相似文献   

18.
Marine magnetic data extracted from the geophysical database of the SHOM (the French Hydrographic and Oceanographic Service) offer a first overview of the magnetic offshore environment in the Gulf of Saint-Malo (Brittany, France). Their cross-interpretation with available geological and geophysical knowledge provides a new land and sea model of the western part of the Late Proterozoic North Armorican Cadomian belt. In particular, marine data exhibit relatively intense and heterogeneous magnetic signatures, mainly interpreted as the offshore prolongation of plutonic, volcanic, and metamorphic geologic formations recognized onshore. Imprints of major faults well known on land can be inferred at sea from discontinuities and shifting of magnetic anomalies or followed by high-resolution bathymetry. An impressive and dense dolerite dyke swarm propagated on more than 50 km from the coast seaward. Dykes are characterized by both 1-km-deep and less than 50-m-deep superficial magnetic responses, suggesting the existence of deeply rooted super dykes to which individual small-scale structures are connected.  相似文献   

19.
Ninety-seven mineral phases consisting of ten chloritoids, fifteen epidotes, sixteen garnets, four sphenes, seven rutiles, seven pyroxenes, thirteen blue amphiboles, two green amphiboles, eleven phengites, two paragonites, a mariposite, seven chlorites, and two specimens of albite were obtained from the metamorphic rocks of Île de Groix, and their chemical, physical, optical and X-ray properties determined. The chloritoids are all optically positive, monoclinic polymorphs with large 2V, moderate refractive indices and characterized by high densities. Their fluorine contents have been used to propose a new upper limit for OHF substitution in the chloritoid structure, suggesting that partial pressure of fluorine might modify the stability of chloritoids from that determined in pure H2O. The epidotes belong to the Al-Fe epidote series and are epidote sensu stricto. The almandine-rich garnets and the chloromelanites are metastable relics in the glaucophane schists. The grossular contents of the calcareous schist garnets are believed to have become depressed under high CO2 pressure and the low Tschermak's contents of the pyroxenes are to be explained by equilibria involving epidote at high and low temperature when the Tschermak's components will break down to epidote group minerals. The sphenes contain appreciable amounts of combined water, fluorine substituting for oxygen and aluminium substituting for silicon and titanium. The presence of H3O+ is suspected in a specimen of blue amphibole. The barroisite has a composition between glaucophane and hornblende. On account of its high Fe3+ content it is believed to have formed under higher P O 2 than the blue amphiboles. The paragonites which occur in the ohloritoid veins are unstable in the potassium-rich aluminous schists. The phengites show a tendency towards sericitic composition due to post-glaucophanisation readjustments under the lower pressure conditions of the greenschist facies. Some of the Fe3+ contents of the chlorites are interpreted as due to oxidation of ferrous iron, e.g. 2 [Fe(OH)2]2FeOOH + H2. The minerals show strong chemical control of the host rock and their Mn contents are directly related to those of the minerals from which they have evolved through retrogression.Chloritoids and epidotes that are not associated with garnets contain higher amounts of manganese; similarly, the two blue amphiboles with the highest FeMg ratios were obtained from rocks in which garnet has not appeared. It is therefore believed that ottrelite and piemontite would be stable only at the lowest subfacies of the greenschist facies. Also, the ironrich amphiboles must have evolved from low-grade iron-aluminium chlorites, since on the appearance of garnet in a schist iron-aluminium chlorites react with quartz to give almandine and Mg-rich chlorites. The Fe2+Mg ratios of the blue amphiboles therefore reflect the grade of the original schist in which the minerals formed.  相似文献   

20.
The oyster habitat in the USA is a valuable resource that has suffered significant declines over the past century. While this loss of habitat is well documented, the loss of associated ecosystem services remains poorly quantified. Meanwhile, ecosystem service recovery has become a major impetus for restoration. Here we propose a model for estimating the volume of water filtered by oyster populations under field conditions and make estimates of the contribution of past (c. 1880–1910) and present (c. 2000–2010) oyster populations to improving water quality in 13 US estuaries. We find that filtration capacity of oysters has declined almost universally (12 of the 13 estuaries examined) by a median of 85 %. Whereas historically, oyster populations achieved full estuary filtration (filtering a volume equivalent or larger than the entire estuary volume within the residence time of the water) in six of the eight estuaries in the Gulf of Mexico during summer months, this is now the case for only one estuary: Apalachicola Bay, Florida. By contrast, while all five estuaries on the North Atlantic coast showed large decreases in filtration capacity, none were achieving full estuary filtration at the time of our c. 1900 historic baseline. This apparent difference from the Gulf of Mexico is explained at least in part by our North Atlantic baseline representing a shifted baseline, as surveyed populations were already much reduced by exploitation in this region.  相似文献   

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