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1.
The stable isotopic composition of the bivalve shell has been widely used to reconstruct the pa-laeo-climate and palaeo-environment. The climatic and environmental significance of carbon isotopic composition of the bivalve shell is still in dispute, and incorporation of metabolic carbon can obscure carbon isotope records of dis-solved inorganic carbon. This study deals with freshwater bivalve, Corbicula fluminea aragonite shell. The results indicated that the δ13C values of bivalve shells deposited out of equilibrium with the host water and showed an onto-genic decrease, indicating that there are metabolic effects and more metabolic carbon is incorporated into larger shells. The proportion of metabolic carbon of shells varies between 19.8% and 26.8%. However, δ13CS can still be used as qualitative indicators of δ13CDIC and environmental processes that occurred during shell growth.  相似文献   

2.
We describe a new method for the calculation of river flow that uses the oxygen isotope composition of bivalve mollusk shells that grew in the river-water/seawater mixing zone of the Colorado River estuary. Sclerochronological techniques are used to identify tidally-induced, fortnight-scale bundles of daily growth increments within shell cross-sections. These fortnightly markers are used to establish a chronology for samples taken for δ18O analysis. A composite seasonal δ18O profile derived from five shells that grew in the absence of river-water flow is used as a baseline against which profiles of river-influenced shells are compared. Because this comparison is between matched fortnights within a year, the temperature of shell growth is likely to be very similar. The difference in δ18O between the river-influenced shell and the “no-flow” composite shell therefore represents the change in the δ18O of the water due to the presence of river water in the mixing zone. The river water end-member is also determined within a fortnightly context so that the change in the δ18O of mixing-zone water can be used to calculate the relative proportions of seawater and fresh-water. The fresh-water end-member is calculated from the δ18O of bivalves alive prior to the emplacement of dams and water diversions on the Colorado River. The marine end-member is based on direct measurements of the δ18O of northern Gulf of California water during times of no Colorado River flow. The system has been calibrated to absolute flow amounts using recent releases of known volume and rate.  相似文献   

3.
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4.
Throughout much of Earth's history, marine carbonates have represented one of the most important geological archives of environmental change. Several pivotal events during the Phanerozoic, such as mass extinctions or hyperthermal events have recently been associated with ocean acidification. Nevertheless, well‐defined geological proxies for past ocean acidification events are, at best, scarce. Here, experimental work explores the response of bivalve shell ultrastructure and isotope geochemistry (δ13C, δ18O and δ26Mg) to stressful environments, in particular to sea water acidification. In this study, the common blue mussel, Mytilus edulis, was cultured (from early juvenile stages to one year of age) at four pH regimes (pHNBS 7·2 to pH 8·0). Shell growth rate and ultrastructure of mainly the calcitic portion of the shells were compared between experimental treatments. Specimens exposed to low‐pH environments show patches of disordered calcitic fibre orientation in otherwise well‐structured shells. Furthermore, the electron backscattered diffraction analyses reveal that, under acidified conditions, the c‐axis of the calcite prisms exhibits a bimodal or multi‐modal distribution pattern. Similar shell disorder patterns have been reported from mytilids kept under naturally acidified sea water conditions. In contrast, this study found no evidence that different pH regimes affect shell carbon, oxygen or magnesium isotope ratios. Based on these observations, it is proposed that: (i) stressful environments, in this case low sea water pH, predictably affect bivalve biomineralization patterns; and (ii) these findings bear potential as a novel (petrographic) proxy for ancient sea water acidification. An assessment of the applicability of these data to well‐preserved fossil shell material from selected time intervals requires additional work.  相似文献   

5.
An experimental and theoretical examination has been made of the settling, entrainment and overturning of 176 valves representing 16 common Northwest European marine bivalve species, together with a comparative study of 15 plastic models in the form of segments from cylindrical tubes. Settling behaviour in both stagnant and moving water depends on particle mass, symmetry and concavo-convexity. Separated empty bivalve shells spin and spiral while settling and, if sufficiently elongated, also pitch. At the observed Reynolds numbers, the shells and models fall concave-up, the terminal fall velocity increasing as the square root of the unit immersed mass or weight. The drag coefficient is independent of Reynolds number but increases with surface roughness and, particularly, particle elongation. Turbulence slightly lowers the critical elongation for pitching. A separation vortex lies captive on the upper side of each descending particle. Consequently, an empty bivalve shell traversing a suspension of sand traps grains on its upper side at a rate proportional to their volume concentration and terminal fall velocity. This process, increasing the effective shell mass, is limited only by the capacity of the shell and grain spillage due to the possible onset of pitching. The ratio (non-dimensional) of a quantity proportional to the applied fluid force and the particle unit immersed weight consistently describes the entrainment of concave-up and convex-up particles, and also the immediate overturning of a valve on settling concave-up to the bed. These thresholds vary in relative magnitude with bed-particle friction and particle concavo-convexity. In general, convex-up particles are the most stable; the concave-up entrainment and overturning thresholds are of a substantially lower but similar magnitude. The high frequency of concave-up bivalve attitudes in turbidites is understandable largely in terms of the ability of a settling valve to increase in effective mass by grain entrapment. Convex-up attitudes in the lower parts of turbidites may record currents stronger than the overturning threshold.  相似文献   

6.
Biodegradation of shell material is widespread in the cool-water skeletal carbonate deposits on Scott shelf, northwestern Vancouver Island, and is especially evident in the large aragonitic bivalves, Glycymeris and Humilaria, major primary sediment contributors. Ten types of endolithic microborings have been identified in the shells, including representatives of green algae (e.g., Ostreobium quekettii), blue-green algae (e.g., Plectonema terebrans, ?Scytonema sp.), fungi, bacteria and clionid sponges, as well as macroborings of phoronids, polychaetes and naticid gatropods. Microcrystalline carbonate is not precipitated in vacated bores. Boring physically weakens the shells, rendering them more prone to mechanical abrasion during sediment transport and bioturbation, and to biological abrasion by grazing benthos. Tumbling experiments demonstrate that the rate of carbonate mud production is much greater for bored as compared to fresh bivalve shells, and that mud production rates decrease with tumbling time because most endolithic microborings are confined to the periphery of grains. Boring also increases significantly the porosity and surface area of skeletal grains, and destroys their organic matrix, making them susceptible to maceration and dissolution on cool-water shelves. Fostered by the generally low rates of carbonate production and accumulation, many aragonitic bivalve shells on Scott shelf have become thoroughly degraded through a combination of endolithic microboring, maceration and dissolution within about 1000 years in ambient sea water. In geologic terms, such selective taphonomic loss of skeletal material may be considerable in ancient temperate-shelf limestones and should be evaluated when interpreting their paleoecology and paleoenvironments.  相似文献   

7.
The modern invasive bivalve Corbicula fluminea was collected in 2006 from three sites with different land uses located in a North Carolina River Basin. The primary objective was to describe the δ18O and δ13C profiles of C. fluminea shells under various land use conditions. An additional aim was to evaluate whether growth patterns of C. fluminea form seasonally. Annual shell growth patterns were measured from the umbo to the margin and co-varied with estimates of ambient water temperature, corresponding to seasonal variation. The C. fluminea growth patterns as translucent bands (slower growth) appeared to form during winter months and opaque bands (rapid growth) formed during summer. A mixed model analysis (ANOVA) showed a significant site level effect of δ18O and δ13C profiles examined among sites (F = 17.1; p = 0.003). A second model showed a borderline significant site effect among profiles with variability more pronounced at the urban site, Crabtree Creek (p = 0.085). Previous habitat assessment ratings and water chemistry measurements suggested that the urban site was more impacted by storm water runoff. Understanding δ18O and δ13CSHELL profiles and shell growth patterns of the invasive bivalve (C. fluminea) may help establish a framework for using these animals as biomonitors to record water temperature and nutrient pollution.  相似文献   

8.
The separated valves of bivalve molluscs, certain gastropods, and many brachiopods closely resemble the regular geometrical form known as the shell. The packing concentration of natural accumulations of such organic shells may be placed within bounds with the aid of packing models making use of ordered arrangements of equal geometrical shells, either conical, cylindrical or spherical. The low packing concentrations, comparable with 0·1–0·2, indicated by these models are confirmed by experiments using the shells of four common British species of bivalve or gastropod mollusc. Packings of these shells have a substantial intrinsic strength, and experimentally appear able to support without failing sedimentary overburdens equivalent to loads comparable with 1000 kg mass/m2. The observed and predicted low packing concentrations suggests that natural shell beds can hold relatively very large amounts of pore fluids or mineral cements. The resistance to compaction of the packings means that the high original porosities of natural shell beds have a good chance of being permanently preserved.  相似文献   

9.
The bioaccumulation of trace metals in the carbonate shells of mussel and clams was investigated at seven hydrothermal vent fields of the Mid-Atlantic Ridge (Menez Gwen, Snake Pit, Rainbow, and Broken Spur) and the Eastern Pacific (9°N and 21°N at the East Pacific Rise and the southern trough of Guaymas Basin). Mineralogical analysis showed that the carbonate skeletons of the mytilid mussel Bathymodiolus sp. and the vesicomyid clam Calyptogena m. are composed mainly of calcite and aragonite, respectively. The first data were obtained for the content of a variety of elements in the bivalve carbonate shells from various hydrothermal vent sites. The analysis of the chemical compositions (including Fe, Mn, Zn, Cu, Cd, Pb, Ag, Ni, Cr, Co, As, Se, Sb, and Hg) of 35 shell samples and 14 water samples from the mollusk biotopes revealed the influences of environmental conditions and some biological parameters on the bioaccumulation of metals. Bivalve shells from hydrothermal fields with black smokers are enriched in Fe and Mn by a factor of 20–30 relative to the same species from the Menez Gwen low-temperature vent site. It was shown that the essential elements Fe, Mn, Ni, and Cu were more actively accumulated during the early ontogeny of the shells. The high concentration factors of most metals (n × 102−n × 104) indicate an efficient accumulation function of bivalve carbonate shells. Passive metal accumulation owing to adsorption on the shell surface was estimated to be no higher than 50% of the total amount, varying from 14% for Fe to 46% for Mn.  相似文献   

10.
Collections of the molluscan species Arctica islandica from seven sites in Norway, Scotland and the North Sea, ranging in age from recent to Early Pleistocene, have been sampled for amino acid analyses. The shells were sampled in profiles through the valves at different distances from the umbo. In the fossil material a general trend of increasing degree of isoleucine (lle) epimerisation and decreasing amino acid concentrations were found going from the inner part of the valve to the outer. Although less pronounced, there is a similar trend from the central part of the valve to the margin. As the concentration of alloisoleucine (alle) remains nearly constant, the observed changes in alle/lle ratios are a result of variations in the amount of isoleucine. The amino acid composition is fairly uniform in recent shells and the observed gradients are established in mid-Holocene samples. Therefore, it is suggested that the gradients are established during rapid early degradation of the protein, possibly due to microbiological activity. Differences in alle/lle ratios observed in molluscs from sites of the same age and same thermal history could be explained by differences in the exposure to amino-acid-consuming microorganisms during the very early stage of diagenesis.  相似文献   

11.
Four new specimens of Euscalpellum? coatesi Donovan and Davis-Strickland from the Maastrichtian of Jamaica are described; two of these occur in association with the right valve of the rudistid bivalve Chiapasella radiolitiformis (Trechmann). This demonstrates that E.? coatesi is not a barnacle, but rather represents an unusual preservational state of the left valve of C. radiolitiformis retaining only the cellular inner part of the shell.  相似文献   

12.
A suite of elements (B, Na, Mg, S, K, Ca, V, Mn, Cr, Sr, and Ba) was measured in aragonitic shells of the estuarine bivalve Corbula amurensis, the Asian clam, using the Sensitive High-Resolution Ion MicroProbe with Reverse Geometry (SHRIMP RG). Our initial intent was to explore potential geochemical proxy relationships between shell chemistry and salinity (freshwater inflow) in northern San Francisco Bay (SFB). In the course of this study we observed variations in shell trace element to calcium ([M]/Ca) ratios that could only be attributed to internal biological processes. This paper discusses the nature and sources of internal trace element variability in C. amurensis shells related to the shell organic fraction and shell calcification rates. The average organic content of whole C. amurensis shells is 19%. After treating whole powdered shells with an oxidative cleaning procedure to remove organic matter, shells contained on average 33% less total Mg and 78% less total Mn. Within our analytical uncertainty, Sr and Ba contents were unchanged by the removal of organic matter. These results show that aragonitic C. amurensis shells have a large component of non-lattice-bound Mg and Mn that probably contribute to the dissimilarity of [M]/Ca profiles among five same-sized shells. Non-lattice-bound trace elements could complicate the development and application of geochemical proxy relationships in bivalve shells. Because B, Ba and Sr occur exclusively in shell aragonite, they are good candidates for external proxy relationships. [M]/Ca ratios were significantly different in prismatic and nacreous aragonite and in two valves of the same shell that had different crystal growth rates. Some part of these differences can be attributed to non-lattice-bound trace elements associated with the organic fraction. The differences in [M]/Ca ratios were also consistent with the calcification rate-dependent ion transport model developed by Carré et al. [Carré M., Bentaleb I., Bruguier O., Ordinola E., Barrett N. T. and Fontugne M. (2006) Calcification rate influence on trace element concentrations in aragonitic bivalve shells: evidences and mechanisms. Geochim. Cosmochim. Acta70, 4906-4920] which predicts that [M]/Ca ratios increase as calcification rates increase and Ca2+ channel specificity decreases. This result, in combination with the possibility that there were ontogenetic variations in growth rates among individuals younger than 2 years, underscores the need to develop an independent age model for C. amurensis shells. If growth-rate effects on lattice-bound [M]/Ca ratios can be constrained, it may yet be possible to develop high-resolution geochemical proxies for external solution chemistry in low-salinity regions of SFB.  相似文献   

13.
九种现代双壳类壳体物相组成的对比研究   总被引：2，自引：0，他引：2  
利用X射线衍射分析技术,对黄东海常见的9种现代双壳类壳体进行了物相分析,结果表明这些双壳类壳体分别属于三种类型,即文石质壳、方解石质壳、文石+方解石混合质壳。其中,菲律宾蛤仔(Ruditapesphilippinarum)、中国蛤蜊(Mactrachinensis)、四角蛤蜊(Mactraveneriformis)、薄片镜蛤(Dosinialaminata)、毛蚶(Scapharcasubcrenata)属于文石质壳,长牡蛎(Crassostreagigas)、中国不等蛤(Anomiachinensis)、海湾扇贝(Argopectenirradieus)属于方解石质壳,紫贻贝(Mytilusedulis)属于文石+方解石质的混合质壳;双壳类壳体物相组成与其生活方式有着一定的联系,营底栖埋入式生活者趋于形成文石质壳,底栖固着生活方式者趋于形成方解石质壳或混合质壳;不同大小的菲律宾蛤仔(Ruditapesphilippinarum)的物相组成上几乎没有差别,表明其壳体物相组成不受生命效应的影响。  相似文献   

14.
The relationship between potential elemental proxies (Mg/Ca, Sr/Ca and Mn/Ca ratios) and environmental factors was investigated for the bivalve Pecten maximus in a detailed field study undertaken in the Menai Strait, Wales, U.K. An age model constructed for each shell by comparison of measured and predicted oxygen-isotope ratios allowed comparison on a calendar time scale of shell elemental data with environmental variables, as well as estimation of shell growth rates. The seasonal variation of shell Mn/Ca ratios followed a similar pattern to one previously described for dissolved Mn2+ in the Menai Strait, although further calibration work is needed to validate such a relationship. Shell Sr/Ca ratios unexpectedly were found to co-vary most significantly with calcification temperature, whilst shell Mg/Ca ratios were the next most significant control. The temporal variation in the factors that control shell Sr/Ca ratios strongly suggest the former observation most likely to be the result of a secondary influence on shell Sr/Ca ratios by kinetic effects, the latter driven by seasonal variation in shell growth rate that is in turn influenced in part by seawater temperature. P. maximus shell Mg/Ca ratio to calcification temperature relationships exhibit an inverse correlation during autumn to early spring (October to March-April) and a positive correlation from late spring through summer (May-June to September). No clear explanation is evident for the former trend, but the similarity of the records from the three shells analysed indicate that it is a real signal and not a spurious observation. These observations confirm that application of the Mg/Ca proxy in P. maximus shells remains problematic, even for seasonal or absolute temperature reconstructions. For the range of calcification temperatures of 5-19 °C, our shell Mg/Ca ratios in P. maximus are approximately one-fourth those in inorganic calcite, half those in the bivalve Pinna nobilis, twice those in the bivalve Mytilus trossulus, and four to five times higher than Mg/Ca ratios in planktonic and benthonic foraminifera. Our findings further support observations that Mg/Ca ratios in bivalve shell calcite are an unreliable temperature proxy, as well as substantial taxon- and species-specific variation in Mg incorporation into bivalves and other calcifying organisms, with profound implications for the application of this geochemical proxy to the bivalve fossil record.  相似文献   

15.
 Dreissena polymorpha is an exotic freshwater bivalve species which was introduced into the Great Lakes system in the fall of 1985 through the release of ballast water from European freighters. Utilizing individual growth rings of the shells, the stable isotope distribution (δ18O and δ13C) was determined for the life history of selected samples which were collected from the western basin of Lake Erie. These bivalves deposit their shell in near equilibrium with the ambient water and thus reflect any annual variation of the system in the isotopic records held within their shells. Observed values for δ18O range from -6.64 to –9.46‰ with an average value of –7.69‰ PDB, while carbon values ranged from –0.80 to –4.67‰ with an average value of –1.76‰ PDB. Dreissena polymorpha shells incorporate metals into their shells during growth. Individual shell growth increments were analyzed for Pb, Fe, Mg, Mn, Cd, Cu, and V concentrations. The shells show increased uptake of certain metals during periods of isotopic enrichment which correspond with warmer water temperatures. Since metals are incorporated into the shells, the organism may be useful as a biomonitor of metal pollution within aquatic environments. Received: 31 October 1996 · Accepted: 21 May 1997  相似文献   

16.
An annular laboratory flume was used to investigate the effect of mobile cockle shells on the erosion of a cohesive sediment bed. A standard clay bed was created and shells of differing sizes placed upon it. Flow in the flume was increased in increments and the onset of motion and the transport patterns of the cockles were monitored. The release of bed material to the water column was monitored and compared to controls made in the absence of shells (due only to the flow). The shells moved as bedload; first as surface creep (sliding) and then by rolling. The onset velocity of motion (Uc) of the shells was found to be directly related to the settling rate (Ws) in still water. The fluid-induced stresses did not cause any detectable erosion of the bed. The addition of even a single shell induced significant erosion rates (E). The erosion was found to be the result of abrasion rather than corrasion, as the shells never entered into saltation. There was a linear increase in erosion rate with increasing shell size, and an exponential increase in the suspended sediment concentration with time. The drag coefficients (Cd) for settling in traction were calculated. The ratio of the drag forces acting on the shells when settling and moving as traction was found to equal to 1/tan(ф) where ф is the friction angle.  相似文献   

17.
Secondary soft-bottom dwellers evolved several times in the Arcoida. These include semi-infaunal endobyssate taxa and free-burrowing species. The Jurassic arcoid bivalve Grammatodon inaequivalvis (Goldfuss, 1837) is remarkable for showing different sculptures on its left and right valve. A functional analysis of the shell points to a reclining mode of life on soft bottoms. This interpretation is corroborated by the occurrence of this species in very fine-grained sediments, and by taphonomic patterns such as “butterflied” preservation. It is proposed here that this species was resting on its left valve on the sediment, probably without byssal attachment. This reclining mode of life of G. inaequivalvis was hitherto undescribed from soft-bottom dweller arcoid bivalves.  相似文献   

18.
Tridacnidae shells, a valuable archive of past environments, are common in the Balobok Rockshelter archaeological site on Sanga‐Sanga Island in the south Philippines. This site was occupied during the mid‐Holocene (ca. 5000–8800 14C yr B.P.), a period of Neolithic cultural expansion in the Philippines. This paper focuses on the preservation of two shell specimens, Hippopus hippopus and Tridacna maxima, unearthed from two mid‐Holocene layers within the rockshelter. The shells' mineralogy and microstructure (prismatic and crossed‐lamellar) were studied using micro‐Fourier transform infrared spectroscopy and scanning electron microscopy to determine if the samples were suitable as paleoenvironmental records. Both shells are still aragonitic but aragonite crystals of both microstructure types are partly dissolved. This dissolution, characteristic of meteoric water alteration, precludes their utility in paleoenvironmental geochemical studies. Nevertheless, these shells are abundant in archaeological sites in the region and may be better preserved in other depositional contexts; more studies on Philippine Tridacnidae shell diagenesis are needed. © 2011 Wiley Periodicals, Inc.  相似文献   

19.
The uppermost portion of the Taciba Formation, Itararé Group, Paraná Basin, Brazil, records a succession of depositional environments tied to the demise of late Paleozoic glaciation. In the study area, Teixeira Soares county, state of Paraná, the unit is dominated by massive to laminated diamictites with inclusions of sandstones and other coarse-grained lithotypes, representing re-sedimented material in proximal areas. These are succeeded by fine to medium-grained sandstones with tabular cross-stratification and pectinid-rich shell pavements, interpreted as nearshore deposits. Above this, laminated and intensely bioturbated siltstones with closed articulated bivalve shells are recorded, probably deposited in inner shelf settings. Fine to very fine sandstones/siltstones with hummocky cross-stratification and intercalated mudstones, including infaunal in situ shells, are interpreted as stacked storm deposits, generated in distal shoreface environments. These are succeeded by fossil-poor, massive to laminated siltstones/mudstones or gray shales (=Passinho shale) that are inferred to be outer shelf deposits, generated in organic-rich, oxygen-deficient muddy bottoms. In this sedimentary succession dropstones or ice-rafted debris are missing and locally the Passinho shale marks the maximum flooding surface of the Itararé succession. These are capped by the fluvio-deltaic deposits of the Rio Bonito Formation (Sakmarian–Artinskian). Six facies-controlled, bivalve-dominated assemblages are recognized, representing faunal associations that thrived in aerobic to extreme dysaerobic bottoms along a nearshore–offshore trend. Within these assemblages, nineteen bivalve species (three of them new) were recorded and described in detail. The presence of Myonia argentinensis (Harrington), Atomodesma (Aphanaia) orbirugata (Harrington) and Heteropecten paranaensis Neves et al. suggests correlation with bivalve assemblages of the Eurydesma-bearing Bonete Formation, Pillahuincó Group, Sauce Grande-Colorado Basin (Buenos Aires Province), Argentina, indicating a possible Asselian age for this diverse post-glacial bivalve fauna. Despite that, typical members of the icehouse-style Eurydesma–Trigonotreta biota (stricto sensu) have not yet been found in the studied bivalve assemblages.  相似文献   

20.
Mussel shells have been used in a number of paleoecological and environmental studies. The interpretation of stable carbon isotopic composition of shell material is still controversial. The carbon for shell carbonate precipitation can either be derived from ambient dissolved inorganic carbon (DIC), with shells recording environmental signals, or from metabolic CO2, with the potential to disguise environmental signals. To gain insight into this question, we investigated four nearly 100-yr long-term records of aragonite shells from an extant freshwater bivalve species, the endangered freshwater pearl mussel (Margaritifera margaritifera L.). Single growth increments of the outer prismatic and the inner nacreous zones were successfully and easily separated with a simple heat treatment for chronological analyses of δ13C in single layers of each zone. Autocorrelation and semivariance statistical methods reveal that mussels show distinct individual signal patterns, which extend up to 25 yr. Signal patterns are reliably reproduced with replicate samples from defined layers within one shell and show similar patterns with a slight offset for inner nacreous and outer prismatic layers for individual animals. Mussels exposed to the same environmental conditions exhibit distinct and contradictory signature patterns, which do not match between individuals. This observation can only be explained by strong metabolic influences on shell precipitation. Environmental changes in pH, temperature, electric conductivity and atmospheric carbon signature had no or little (<5%) influence, whereas body tissue protein and body tissue δ13C signatures negatively correlated with the youngest produced shell δ13C signatures, indicating that respiration causes a preferential loss of light isotopes from body mass and an inverse enrichment in shell aragonite. Hence, the shells of the freshwater pearl mussel yield a long-term record of metabolic activity, whereas the use of δ13C in these shells as recorder for environmental signals is questionable. This may also be true for shells from other species, for which metabolic carbon incorporation has been acknowledged.  相似文献   

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