A 12,000-year history of vegetation and climate for Cape Cod,Massachusetts

Pollen and charcoal analysis of radiocarbon-dated sediment cores from Duck Pond in the Cape Cod National Seashore provide a continuous 12,000-yr vegetation and climate history of outer Cape Cod. A Picea-Hudsonia parkland and then a Picea-Pinus banksiana-Alnus crispa boreal forest association grew near the site between 12,000 and 10,000 yr B.P. This vegetation was replaced by a northern conifer forest of Pinus strobus-P. banksiana, and, subsequently, by a more mesophytic forest (Pinus strobus, Tsuga, Quercus, Fagus, Acer, Ulmus, Fraxinus, Ostrya) as the climate became warmer and wetter by 9500 yr B.P. By 9000 yr B.P. a Pinus rigida-Quercus association dominated the landscape. High charcoal frequencies from this and subsequent levels suggest that the pine barrens association developed during a warmer and drier climate that lasted from 9000 to about 5000 yr B.P. Increased percentages of Pinus strobus pollen indicate a return to moister and cooler conditions by about 3500 yr B.P. A doubled sedimentation rate, increased charcoal, and increased herb pollen suggest land disturbance near the pond before European settlement. These results suggest a rapid warming in the northeast in the early Holocene and support a hypothesis of a rapid sea level rise at that time. Comparison of the pollen results from Duck Pond with those from Rogers Lake, Connecticut, illustrates the importance of edaphic factors in determining the disturbance frequency and vegetation history of an area.     

The reproductive cycle of the bay scallop,Argopecten irradians irradians (Lamarck), in a small coastal embayment on Cape Cod,Massachusetts

The spawning activity of the bay scallop Argopecten irradians irradians (Lamarck) was monitored from May through September in a small embayment on Cape Cod, Massachusetts and compared with changes in hydrographic conditions. Spawning activity of bay scallops began in May and continued through July with some minor activity occurring during August and September. Activity was most closely related to changes in ambient temperature and occurred predominantly before the summer maximum temperature was recorded. As temperatures declined in the late summer and early fall, increased gametogenic activity was evident. A longer period of spawning activity was observed than was previously reported for New England waters.     

Cosmogenic-nuclide ages for New England coastal moraines, Martha's Vineyard and Cape Cod, Massachusetts, USA

Cosmogenic-nuclide exposure ages for 13 glacially transported boulders atop the Martha's Vineyard moraine, MA, USA, indicate that the southeastern margin of the Laurentide ice sheet reached its maximum extent during the last glaciation 23,200±500 yr ago. Another 10 age determinations from the younger Buzzards Bay moraine near Woods Hole, MA, indicate that this moraine complex was formed 18,800±400 yr ago. These ages correlate approximately with the terminations of cooling cycles recorded in Greenland ice cores and coeval ice-rafting events, suggesting that the marginal position of this sector of the ice sheet was tightly coupled to North Atlantic climate during the last glacial maximum.     

The role of sediment denitrification in reducing groundwater-derived nitrate inputs to Nauset Marsh estuary, Cape Cod, Massachusetts

The Nauset Marsh estuary is the most extensive (9.45 km²) and least disturbed salt marsh/estuarine system within the Cape Cod National Seashore, even though much of the 19 km² watershed area of the estuary is developed for residential or commercial purposes. Because all of the Nauset watershed is serviced by on-site individual sewage disposal systems, there is concern over the potential impact of groundwater-derived nutrients passing from these systems to the shallow receiving waters of the estuary. The purpose of this study was to determine whether denitrification (the bacterial conversion of nitrate to gaseous nitrogen) in estuarine sediments could effectively remove the nitrate from contaminated groundwater before it passed from the watershed to the estuary. Rates of denitrification were measured both in situ and in sediment cores, in areas of active groundwater discharge, in relatively pristine locations, and in areas situated down-gradient of moderate to heavily developed regions of the watershed. Denitrification rates for 47 sediment cores taken over an annual cycle at 5 stations ranged from non-detectable to 47 μmol N₂ m⁻² h⁻. Mean denitrification rates were positively correlated with sediment organic content, and varied seasonally due to changes in sediment organic content and to the effect of water temperatures on sediment oxygen penetration depths. There was no correlation between observed denitrification rates and corresponding nitrate concentrations in groundwater. A comparison of in situ denitrification rates (supported by groundwater nitrate) with denitrification rates observed in sediment cores (supported by remineralized nitrate) showed that groundwater-driven denitrification rates were small, and not in excess of denitrification rates supported by remineralized nitrate. Most of the denitrification in Nauset sediments was apparently fueled by remineralized nitrate through coupled nitrification/denitrification. Denitrification did not contribute significantly to the direct loss of nitrate from incoming groundwater at Nauset Marsh estuary. Groundwater flow was rapid, and much of it occurred in freshwater springs and seeps through very coarse, sandy, well-oxygenated sediments of limited organic content. There was little opportunity for denitrification to occur during groundwater passage through these sediments. These results have important management implications because they suggest that the majority of nitrogen from contaminated groundwater crosses the sediment/water interface and arrives at Nauset Estuary, where it is available to primary producers. Preliminary budget calculations suggest that while denitrification was not an effective mechanism for the direct removal of nitrate in contaminated groundwater flowing to Nauset Marsh estuary, it may contribute to significant nitrogen losses from the estuary itself.     

Vegetation Cover and Elevation in Long-Term Experimental Nutrient-Enrichment Plots in Great Sippewissett Salt Marsh,Cape Cod,Massachusetts: Implications for Eutrophication and Sea Level rise

Nitrogen inputs restructure ecosystems and can interact with other agents of ecological change and potentially intensify them. To examine the effects of nitrogen combined with those of elevation and competition, in 2005 we mapped vegetation and elevation within experimental plots that have been fertilized since 1970 in Great Sippewissett salt marsh, Cape Cod, MA, USA and compared the resulting effects on marsh vegetation. Decadal-scale chronic nutrient enrichment forced changes in cover and spatial distribution of different species. With increasing enrichment, there was a shift in species cover primarily involving loss of Spartina alterniflora and an increase in Distichlis spicata. Percent cover of near monocultures increased with nitrogen fertilization, owing mainly to the proliferation of D. spicata. The experimental fertilization prompted a shift from the short form of S. alterniflora to taller forms, hence increasing above-ground biomass, where this species managed to remain. Chronic enrichment increased upper and lower limits of the elevation range within which certain species occurred. The shift to increased cover of D. spicata was also associated with faster accretion of the marsh surface where this species was dominant, but not where S. alterniflora was dominant. Interactions among nutrient supply, elevation, and competition altered the direction of competitive success among different species of marsh plants, and forced changes in the spatial distribution and composition of the salt marsh plant communities. The results imply that there will be parallel changes in New England salt marshes owing to the widespread eutrophication of coastal waters and the increasing sea level rise. Knowing the mechanisms structuring marsh vegetative cover, and their role in modification of salt marsh accretion, may provide background with which to manage maintenance of affected coastal wetlands.     

Spawning densities,egg densities,size structure,and movement patterns of spawning horseshoe crabs,<Emphasis Type="Italic">Limulus polyphemus</Emphasis>, within four coastal embayments on Cape Cod,Massachusetts

Spawning densities, spawning indices, egg densities, size distributions, and movement patterns of horseshoe crabs (Limulus polyphemus) were quantified for four coastal embayments (Monomoy National Wildlife Refuge, Pleasant Bay, Nauset Estuary, and Cape Cod Bay) on Cape Cod, Massachusetts from 2000 to 2002. Spawning activity was highest from mid May through mid June, but densities varied throughout the Cape Cod region. Average spawning densities (male and female crabs combined), measured using 25-m² quadrats, were lower than 1 crab 25 m^?2, although certain locations had consistently higher densities averaging 2 to 3 crabs 25 m^?2 with individual survey densities recorded as high as 17 crabs 25 m^?2. Spawning densities during night surveys were either similar or slightly higher than day surveys, except at a few sites within Pleasant Bay. Spawning indices were considerably lower ranging from 0 to 1.3 females 25 m^?2 throughout the Cape Cod region. Spawning sex ratios varied from 1∶1.6 to 1∶3.1 (females:males) throughout the region, except within Pleasant Bay where highly male skewed ratios were observed (e.g., 1∶5.8, 3-yr average). Egg densities were low overall (<1 egg cm^?2) throughout Cape Cod and egg densities tended to be higher in deeper sediments (5–20 cm deep) compared to shallow sediments (0–5 cm deep) at most locations. Over 7,800 horseshoe crabs were tagged on Cape Cod from 2000 to 2002. Average size and size frequency distributions of tagged crabs varied among regions. Larger individuals were observed at Monomoy National Wildlife Refuge while the smallest individuals were from Cape Cod Bay. We documented an overall recapture rate of 6.7% and our tag-recapture data indicated that 62% of crabs were recaptured at the original tagging location and 70% of recaptures traveled less than 2 km from the original tagging location, providing evidence for localized populations on Cape Cod. We have observed that horseshoe crabs differ among embayments within a regional area, suggesting the potential need for management plans specific to embayments or subregions depending on the characteristics of a population.     

Mineral dissolution in the Cape Cod aquifer, Massachusetts, USA: I . Reaction stoichiometry and impact of accessory feldspar and glauconite on strontium isotopes, solute concentrations, and REY distribution

To compare relative reaction rates of mineral dissolution in a mineralogically simple groundwater aquifer, we studied the controls on solute concentrations, Sr isotopes, and rare earth element and yttrium (REY) systematics in the Cape Cod aquifer. This aquifer comprises mostly carbonate-free Pleistocene sediments that are about 90% quartz with minor K-feldspar, plagioclase, glauconite, and Fe-oxides. Silica concentrations and pH in the groundwater increase systematically with increasing depth, while Sr isotopic ratios decrease. No clear relationship between ⁸⁷Sr/⁸⁶Sr and Sr concentration is observed. At all depths, the ⁸⁷Sr/⁸⁶Sr ratio of the groundwater is considerably lower than the Sr isotopic ratio of the bulk sediment or its K-feldspar component, but similar to that of a plagioclase-rich accessory separate obtained from the sediment. The Si-⁸⁷Sr/⁸⁶Sr-depth relationships are consistent with dissolution of accessory plagioclase. In addition, solutes such as Sr, Ca, and particularly K show concentration spikes superimposed on their respective general trends. The K-Sr-⁸⁷Sr/⁸⁶Sr systematics suggests that accessory glauconite is another major solute source to Cape Cod groundwater. Although the authigenic glauconite in the Cape Cod sediment is rich in Rb, it is low in in-grown radiogenic ⁸⁷Sr because of its young Pleistocene age. The low ⁸⁷Sr/⁸⁶Sr ratios are consistent with equilibration of glauconite with seawater. The impact of glauconite is inferred to vary due to its variable abundance in the sediments. In the Cape Cod groundwater, the variation of REY concentrations with sampling depth resembles that of K and Rb, but differs from that of Ca and Sr. Shale-normalized REY patterns are light REY depleted, show negative Ce anomalies and super-chondritic Y/Ho ratios, but no Eu anomalies. REY input from feldspar, therefore, is insignificant compared to input from a K-Rb-bearing phase, inferred to be glauconite. These results emphasize that interpretation of groundwater chemistry, even in relatively simple aquifers, may be complicated by solute contributions from “exotic” accessory minerals such as glauconite. To detect such peculiarities, groundwater studies should combine the study of elemental concentration and isotopic composition of several solutes that show different geochemical behavior.     

Fishes and decapod crustaceans of Cape Cod eelgrass meadows: Species composition,seasonal abundance patterns and comparison with unvegetated substrates

Bimonthly trawl samples from eelgrass and nearby unvegetated areas on Cape Cod, Massachusetts, showed greater species richness in eelgrass meadows relative to unvegetated areas, and greater summer abundance in vegetation for decapod crustaceans and fishes. The composition of eelgrass-associated decapods and fishes was dominated by cold-water taxa and was strikingly different from that of the better studied eelgrass meadows of the mid-Atlantic coast. Four of the eight decapod species collected, including the second and third most abundant taxa, do not even appear in collections reported from Chesapeake Bay eelgrass meadows. Similarly, 10 of the 22 fish species taken, including the first and sixth most abundant species, are not reported from Chesapeake Bay eelgrass samples. Cape Cod eelgrass beds seem to play a nursery role for several commercially important fish species, although the nursery function is less obvious than in previously studied mid-Atlantic eelgrass meadows.     

Plant Cover,Herbivory, and Resiliency in a Cape Cod Salt Marsh: Multi-year Responses and Recovery Following Manipulation of Nutrients and Competition

To assess responses and recovery of salt marsh grass species, we manipulated external nutrient supply and removed shoots of Spartina alterniflora or Spartina patens from fertilized and control plots in a Massachusetts marsh. The pulse-and-recovery experiment included treatments for 5 years, and recovery for an additional 7 years. Responses of these potential competitors sensitively depended on local contingencies of elevation and nutrient dose, and, moreover, the differences in outcomes were emphasized at longer time scales. Local contingencies became manifest as differences in responsiveness and resiliency of the different grass species, with differing time courses and longer-term outcomes as to what species were present. These features might explain the diverse results obtained in shorter-term salt marsh experiments, and provide a view of the complex suite of species-specific and external environmental variables that may control competitive interactions in general.     

Food of juvenile American shad,Alosa sapidissima,juvenile blueback herring,Alosa aestivalis,and pumpkinseed,Lepomis gibbosus,in the Connecticut River below Holyoke Dam,Massachusetts

Despite·a paucity of available prey, little feeding competition occurred between juvenile American shad,Alosa sapidissima, juvenile blueback herring,Alosa aestivalis, and pumpkinseed,Lepomis gibbosus, in the Connecticut River below Holyoke Dam. Pumpkinseed differed from clupeids in pronounced selection for chironomid larvae, and bottom feeding. Feeding conflicts between the twoAlosa species were reduced by: 1) more opportunistic feeding by shad; 2) differential selection for cladoceran prey; 3) higher utilization of copepods by herring.     

The history of Cu,Pb, and Zn inputs to Narragansett Bay,Rhode Island as recorded by salt-marsh sediments

The distribution of metals with depth in sediment cores sampled from industrialized estuaries can reveal long-term trends in loadings to the waterbody. Salt marsh cores were sampled from five locations from the head to the mouth of Narragansett Bay and from one location from a marsh inside a lagoon on the coast of Block Island Sound with the intent of reconstructing historical loadings of Pb, Cu, and Zn to this estuary. Concentrations of Fe and Mn were measured as indicators of redox conditions of the sediment column. Chronologies were developed using accretion rates determined previsly from²¹⁰Pb analyses of the same cores. Excess metal inventories and enrichment over pre-industrial concentrations were greater in upper bay cores reflecting the location of sources at the head of the estuary. The bay cores were similar with respect to the distributions of Cu, Pb, and Zn. Concentrations of all metals began to increase over background levels at depths corredponding approximately to the year 1900. Most of the cores showed peak concentrations of Cu and Pb in the early 1950s and 1970s. Distributions of Zn were more variable among cores, showing peaks in the early 1920s in some cores and in the 1950s and 1970s in others. In general, the observed distributions in the bay cores are consistent with estimated long-term trends in loadings from atmospheric and sewage sources. The metal distributions in the lagoon core appear to reflect atmospheric loadings. However, there are features in some cores that are not explainable using the estimated trends in source inputs. There also is not a 1∶1 correspondence between changes in sediment metal concentrations and changes in loadings. It is likely that this method of reconstruction would benefit from a more detailed characterization of sources, but comparison of sediment and historical records do show that attempts to reduce loadings to the bay have been successful.     

Effects of a photosynthesis inhibitor,atrazine, on the salt-marsh fiddler crab,Uca pugnax (Smith)

Atrazine (2-chloro-4-ethylamino-6-isopropylamine-s-triazine) concentrations of 1,000 ppm either killed or eliminated the escape response (considered to be analogous to death) ofUca pugnax in laboratory experiments in August 1977. Adverse effects were observed at concentrations as low as 100 ppm and the severity was dependent on size and sex of the crab. However, in subsequent experiments, each with a new group of crabs, the effects of atrazine became smaller and smaller until a November experiment when no deleterious effects were observed even when exposed to 1,000 ppm. Experiments in August, 1978 confirmed the data obtained 1 year earlier. Ecological and physiological considerations of this seasonal variation in response are discussed. Crabs fed for 20 days with detritus wetted with 10^?4 M atrazine were not adversely affected. Crabs exposed to a single application of 0, 100, 1,000, and 10,000 ppm atrazine in the field and in microecosystems were adversely affected only by the 10,000-ppm rate. Toxicity of atrazine to the larval stage of the crab was not investigated.     

Sampling the Cape Verde Mantle Plume: Evolution of Melt Compositions on Santo Antao, Cape Verde Islands

The volcanic history of Santo Antão, NW Cape Verde Islands,includes the eruption of basanite–phonolite series magmasbetween 7·5 and 0·3 Ma and (melilite) nephelinite–phonoliteseries magmas from 0·7 to 0·1 Ma. The most primitivevolcanic rocks are olivine ± clinopyroxene-phyric, whereasthe more evolved rocks have phenocrysts of clinopyroxene ±Fe–Tioxide ± kaersutite ± haüyne ± titanite± sanidine; plagioclase occurs in some intermediate rocks.The analysed samples span a range of 19–0·03% MgO;the most primitive have 37–46% SiO₂, 2·5–7%TiO₂ and are enriched 50–200 x primitive mantle in highlyincompatible elements; the basanitic series is less enrichedthan the nephelinitic series. Geochemical trends in each seriescan be modelled by fractional crystallization of phenocrystassemblages from basanitic and nephelinitic parental magmas.There is little evidence for mineral–melt disequilibrium,and thus magma mixing is not of major importance in controllingbulk-rock compositions. Mantle melting processes are modelledusing fractionation-corrected magma compositions; the modelssuggest 1–4% partial melting of a heterogeneous mantleperidotite source at depths of 90–125 km. Incompatibleelement enrichment among the most primitive magma types is typicalof HIMU OIB. The Sr, Nd and Pb isotopic compositions of theSanto Antão volcanic sequence and geochemical characterchange systematically with time. The older volcanic rocks (7·5–2Ma) vary between two main mantle source components, one of whichis a young HIMU type with ²⁰⁶Pb/²⁰⁴Pb = 19·88, 7/4 =–5, 8/4 0, ⁸⁷Sr/⁸⁶Sr = 0·7033 and ¹⁴³Nd/¹⁴⁴Nd= 0·51288, whereas the other has somewhat less radiogenicSr and Pb and more radiogenic Nd. The intermediate age volcanicrocks (2–0·3 Ma) show a change of sources to two-componentmixing between a carbonatite-related young HIMU-type source(²⁰⁶Pb/²⁰⁴Pb = 19·93, 7/4 = –5, 8/4 = –38,⁸⁷Sr/⁸⁶Sr = 0·70304) and a DM-like source. A more incompatibleelement-enriched component with 7/4 > 0 (old HIMU type) isprominent in the young volcanic rocks (0·3–0·1Ma). The EM1 component that is important in the southern CapeVerde Islands appears to have played no role in the petrogenesisof the Santo Antão magmas. The primary magmas are arguedto be derived by partial melting in the Cape Verde mantle plume;temporal changes in composition are suggested to reflect layeringin the plume conduit. KEY WORDS: radiogenic isotopes; geochemistry; mantle melting; Cape Verde     

Evolution of drainage patterns on Cape York Peninsula,northeast Queensland

Drainage patterns along passive continental margins are often hypothesised to be the result of drainage disruption following highland uplift and downwarping of the highland flank. Several studies of stream catchments throughout southeast Australia have demonstrated that the opposite tends to be the case in this region because the field evidence favours stream and continental drainage‐divide stability. While significant advances have been made towards understanding this phenomenon in the southeastern corner of the continent, little is known of the evolution of streams and highlands in northeast Australia. Our study examines palaeochannels and fluvial sedimentary units close to the continental drainage divide in six stream catchments along the length of Cape York Peninsula. The results show that four of the catchments (Barron‐Mitchell and Stewart‐Holroyd) have experienced continental divide and drainage stability, whereas the Pascoe‐Wenlock system appears to have experienced westward migration of the continental drainage divide and diversion of the Pascoe River. River diversion here is likely to be a result of the raising of base‐level and flooding of stream channels during the Cretaceous marine transgression and subsequent stream incision by the Pascoe River along structural weaknesses in the underlying strata, following cessation of marine conditions.     

Composition of larval, juvenile, and small adult fishes relative to changes in environmental conditions in Florida Bay

We compared (1) ichthyoplankton composition and (2) basin and channel habitat ichthyofauna and seagrass densities between 1984–1985 and 1994–1996 in Florida Bay. Stations and sampling techniques employed in 1984–1985 were duplicated in the 1994–1996 study.Thalassia testudinum, Halodule wrightii, andSyringodium filiforme densities within most of the basin and channel strata sampled in 1994–1996 had decreased by as much as 100%. We did not observe changes in the total density of juvenile and small adult fishes coincident with the reductions in seagrass densities except in the deep-water channel habitats. There was an increase in the proportion of the total ichthyofauna represented by pelagic atherinids, engraulids, and clupeids, particularly the engraulidAnchoa mitchilli, and a concomitant decrease in the proportion represented by canopy-dwelling and morebenthic-dwelling seagrass inhabitants. This suggested a shift toward a planktonic-feeding community. We observed an increase in the density and frequency of engraulid larvae, particularly in the western and Gulf of Mexico portions of Florida Bay, but no significant changes in densities of the commonly collected ichthyoplankton that are demersal as adults (i.e., Gobiidae, Callionymidae, and Blennioidei).Lucania parva, Eucinostomus spp.,Lagodon rhomboides, Floridichthys carpio, Haemulon plumieri, andSyngnathus floridae represented nearly 86% of the juvenile and small adult fish collected in 1984–1985 but represented only 29% of the ichthyofauna a decade later. The distribution of juvenile spotted seatrout had expanded into the central and northeastern basins of our sampling area, perhaps in response to reduced salinities or to the availability of food resources.     

渤海湾西北岸全新世牡蛎礁研究概述

环太平洋西岸,特别是渤海湾西北岸沿海平原全新世地层中广泛分布着由Crassostrea gigas牡蛎形成的礁体。总结了渤海湾西北岸30余年来众多研究者的主要成果。论述了C秘的定名沿革、生态习性、礁体特征、礁体与海面的关系。测定了C萌秘壳体的^18O和^13C同位素组成,发现了壳体生物碳酸钙的年际变化律,结合可靠的^14C数据,建立了正常建礁与个体分泌壳体之间的线性函数关系（约1cm／a）、水平夹层的平均历时（约200a,／层）等定量参数,进而提出“礁体历时假说”,并将该地区时间跨度约7750-950calBP的化石牡蛎礁划分为8道礁群,论证了礁体内部水平夹层和礁体之上泥质“转换层”可能记录的百年尺度的“缓变型”环境变化。     

Burial of reefs by shallow-water carbonates,Silurian Gower formation,Iowa, U.S.A.

The Gower Formation was deposited in two main stages. In the first, crinoid-coelenterate reef complexes, consisting of numerous closely spaced mounds, developed to maximum relief in water about 30 metres deep initially. Complexes are asymmetrical: the well defined windward Marginal Zone has pronounced local relief; the flatter, more extensive Leeward Zone fans out behind a high central area. Equivalent off-reef beds, where correlation is clear, consist of relatively fine skeletal dolomite.The second-stage deposits record a fall in sea level and the filling of first-stage topographic lows by progressively shallower-water sediments. These were mainly laminated carbonate muds and fine sands (Brady/Anamosa facies group). In the Marginal Zone these were deposited at first as steep wedge-beds on earlier mound flanks, together with a distinctive fauna and scattered stromatolites (Brady facies). Mounds expanded laterally while developing a platform on top. Poorly fossiliferous, flatter-lying beds (Laminate-Anamosa) subsequently filled remaining inter-mound hollows. Equivalent deposits in the Leeward Zone consist almost entirely of Laminate-Anamosa. Off-reef equivalents are probably generally similar.The second stage of Gower deposition ended with the accumulation across inter-complex basins of extensive uniform sheets of stromatolites and subordinate laminated muds (Crenulate-Anamosa). Near the reefs Crenulate-Anamosa overlies Laminate-Anamosa; in basin centres it lies directly on beds of the first depositional stage. The stromatolites are believed to have originated in very shallow, perhaps tideless, water with restricted circulation and high salinity.The shallow-water origin of these sediments suggests that reef tops were exposed during second-stage deposition, and consequently accumulated little or no sediment. Filled caves and a slightly earlier erosion surface tend to support this interpretation.

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Zusammenfassung Die Sedimente der silurischen Gower-Formation im Staate Iowa wurden in zwei Phasen abgelagert. Während der ersten entwickelte sich in Krinoiden-Coelenteraten Riffkomplexe aus vielen nah benachbarten Kuppen ein Relief in ursprünglich 30 m Wassertiefe. Die Riffkomplexe sind asymmetrisch: die gut definierte Randzone in Windrichtung hat ein ausgeprägtes Detailrelief; hinter einer zentralen Erhebung erstrecken sich die weniger steilen und ausgedehnteren Fächer der Lee-Zone. Soweit korrelierbar, bestehen die riffernen Sedimente aus relativ feinem skeletalem Dolomit.Die Ablagerungen der zweiten Phase repräsentieren eine Senkung des Meeresspiegels; die topographisch tieferen Teile des vorhandenen Reliefs werden mit Sedimenten aus immer geringerer Wassertiefe gefüllt, hauptsächlich mit laminierten Karbonatschlammen und Feinsanden (Brady/Anamosa-Fazies-Gruppe). In der Randzone wurden sie zunächst als steile keilförmige Schichten auf Kuppenhängen abgelagert (Brady/Fazies); diese enthalten eine charakteristische Fauna und einzelne Stromatolithe. Die Kuppen verbreiterten sich entsprechen zu Plateaus. Fossilarme, flacher liegende Schichten (laminierte Anamosa-Fazies) füllten dann die Restsenken zwischen den Kuppen. Die entsprechenden Ablagerungen der Lee-Zone gehören fast ausschließlich zur letzteren Fazies, und rifferne Sedimente sind wahrscheinlich im allgemeinen ähnlich.Am Ende der zweiten Phase der Gower-Formation wurden ausgedehnte einförmige Schichten von Stromatolithen und — untergeordnet — laminierten Schlammen (gewellte Anamosa-Fazies) in den Becken zwischen den Riffkomplexen abgelagert. In Riffnähe wird die laminierte von der gewellten Anamosa-Fazies überdeckt, im Beckenzentrum folgt die letztere direkt auf die Ablagerungen der ersten Phase. Für die Stromatolith-Bildung wird sehr flaches, vielleicht gezeiten-loses Wasser mit beschränkter Zirkulation und hoher Salinität angenommen.Bei Ablagerung dieser Sedimente im Flachwasser müssen die Riffoberflächen während der zweiten Phase trocken gelegen haben, weshalb sie mit wenig oder keinem Sediment überdeckt wurden. Hohlraumfüllungen und eine wenig ältere Erosionsfläche scheinen diese Interpretation zu unterstützen.

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Résumé Les sédiments de la formation silurienne Gower dans l'état Iowa furent déposés en deux phases. Pendant la première phase, il se forma un relief, à partier de nombreux pitons sous-marins voisins, dans les complexes de récifs de crinoïdes-coelentérés à l'intérieur d'une profondeur d'eau originelle de 30 m. Les complexes de récifs sont asymétriques. La zone de bordure, bien définie dans la direction du vent, possède un relief de détails marqué; derrière une élévation centrale s'étendent les surfaces moins escarpées et allongées de la zone »Lee«. S'il est possible de les comparer, les sédiments des récifs se composent de dolomites relativement fines.Les dépôts de la deuxième phase représentent un affaissement du niveau de la mer. Les parties topographiques profondes du relief présent seront comblées par des sédiments provenant de profondeurs d'eau toujours plus petites, principalement avec des vases de carbonates en lamelles et des sables fins (groupe de faciès »Brady/Anamosa«). Dans la zone de bordure, ces sédiments furent d'abord déposés sous forme de strates abruptes et cunéiformes sur les versants des coupoles (faciès Brady); ils contiennent une faune caractéristique et des »Stromatolites« particulières. Les coupoles s'étendirent pour se transformer en plateau. Puis, les couches plus basses, pauvres en fossiles (faciès en lamelles Anamosa), remplirent les depressions restantes entre les coupoles. Les dépôts correspondants de la zone »Lee« appartiennent, d'une manière presque exclusive, au dernier faciès et les sédiments loins des récifs sont en général, vraisemblablement, similaires.A la fin de la deuxième phase de la formation Gower, des couches de »Stromatolites« étendués et uniformes ainsi que des vases en lamelles subordonnées (faciès ondulés Anamosa) furent déposées dans les bassins entre les complexes de récifs. A proximité des récifs, le faciès en lamelles sera recouvert par le faciés ondulé Anamosa; dans le centre du bassin, le dernier faciès suit immédiatement les dépôts de la première phase. Pour la formation de »Stromatolites« on présume l'action d'eau à bas niveau, peut être sans marées, mais avec des courants limités et de la salinités élevée.Pendant la sédimentation de ces couches dans l'eau basse, les surfaces des récifs doivent avoir été à sec durant la deuxième phase. C'est pourquoi elles ne sont couvertes que de peu de sédiment ou pas du tout. Le remplisage de cavernes et une surface d'érosion un peu plus ancienne semblent supporter cette interprétation.

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黔南宾夕法尼亚亚纪礁环境中腕足动物古生态研究

黔南地区宾夕法尼亚亚纪逍遥阶礁相地层中腕足动物普遍发育,主要产长身贝类和石燕贝类。本文主要研究该区腕足动物与造礁生物叶状藻和Fomitchevella 珊瑚之间的生态关系。在叶状藻礁内,小个体腕足动物常附着在藻叶上营假漂浮生活; 但藻叶密集处,小个体腕足动物也难见,可能藻丛造成水流循环不畅使腕足动物难以获得充足的食料; 藻叶稀疏处,腕足动物个体加大,数量增加。在大型Fomitchevella 珊瑚格架礁内,腕足动物与Fomitchevella 之间不存在食物上的竞争。腕足动物介壳层为Fomitchevella 幼虫定殖提供了生物成因的硬质基底,在此基础之上发育了大型珊瑚礁。腕足动物在 Fomitchevella 进入统殖阶段由于生态空间受到局限而退居次要地位。