Shell layer variation in trace element concentration for the freshwater bivalve Elliptio complanata

While the elemental chemistry of marine mollusk shells contains valuable environmental information, extending these interpretations to freshwater bivalve shells may be problematic, given the wide range of aqueous chemistries that exist in freshwater environments. To better understand the significance of these records, 20 bivalves were collected from four freshwater streams to determine the geochemical relationships that exist between the bivalve shell and their sources streams. The concentrations of manganese (Mn), copper (Cu), strontium (Sr), barium (Ba), and calcium (Ca) were analyzed by laser ablation inductively coupled plasma mass spectrometry across the inner (INL) and outer nacreous layer (ONL) of each shell for comparison to stream data collected over the period of shell growth. Within an individual shell, the content of Mn, Sr, and Ba were significantly higher in the INL than the ONL, while Cu concentrations were similar. Strontium and Ba concentrations co-varied between the two layers in 1:1 relationships, while Mn displayed a preferential enrichment in the INL that increased from 2:1 to 5:1 as the Mn content of the shell decreased. Each elemental profile can be correlated between shell layers, except for the trace element Cu, which appeared to be more closely related to the organic content of the shell. These results suggest that the major element geochemistry of the shell layers differentially reflect the aqueous chemistry of the water in which a bivalve lived, but that these records are most likely overprinted by physiological processes specific to the mantle tissue from which a particular shell layer is secreted. Distribution coefficients (shell _{M /Ca}:water _{M /Ca}, where M?=?trace element of interest) were calculated using the median molar elemental concentration for each shell (by layer), and the appropriate water concentration. Bivalves from a contaminated site were excluded from this analysis because their shells were anomalous in size and color. For the remaining 15 shells, distribution coefficients (INL and ONL) were: Mn (0.50 and 0.21), Sr (0.26 and 0.17), and Ba (0.05: INL only). Inner nacreous layer values were comparable to the upper end of published estimates for freshwater bivalves and fish otoliths, while the ONL values were comparable to the lower range of values. Inclusion of shells from the contaminated site resulted in the calculation of distribution coefficients that fell outside the range of published estimates. These results suggest that exposure to pollutants may have a bearing on the biological factors that control the elemental concentration of bivalve shells in freshwater environments. While researchers generally avoid sampling shell material from the INL because of the potential for shell dissolution, the similarity in elemental profile trends observed here suggests that both the INL and ONL record accurate geochemical information, but in distinct forms. An obvious advantage of INL analysis is the relative increase in elemental concentration, but this is afforded at the expense of spatial resolution. Considered collectively, these results suggest that valuable information can be gleaned from the elemental concentration of freshwater bivalve shells when care is taken in the choice of material from which these records are extracted.     

Carbon isotopes in mollusk shell carbonates

Mollusk shells contain many isotopic clues about calcification physiology and environmental conditions at the time of shell formation. In this review, we use both published and unpublished data to discuss carbon isotopes in both bivalve and gastropod shell carbonates. Land snails construct their shells mainly from respired CO₂, and shell δ¹³C reflects the local mix of C3 and C4 plants consumed. Shell δ¹³C is typically >10‰ heavier than diet, probably because respiratory gas exchange discards CO₂, and retains the isotopically heavier HCO₃ ^?. Respired CO₂ contributes less to the shells of aquatic mollusks, because CO₂/O₂ ratios are usually higher in water than in air, leading to more replacement of respired CO₂ by environmental CO₂. Fluid exchange with the environment also brings additional dissolved inorganic carbon (DIC) into the calcification site. Shell δ¹³C is typically a few ‰ lower than ambient DIC, and often decreases with age. Shell δ¹³C retains clues about processes such as ecosystem metabolism and estuarine mixing. Ca²⁺ ATPase-based models of calcification physiology developed for corals and algae likely apply to mollusks, too, but lower pH and carbonic anhydrase at the calcification site probably suppress kinetic isotope effects. Carbon isotopes in biogenic carbonates are clearly complex, but cautious interpretation can provide a wealth of information, especially after vital effects are better understood.     

Shell architecture,element composition,and stable isotope signature of the giant deep-sea oyster Neopycnodonte zibrowii sp. n. from the NE Atlantic

A conspicuous new deep-sea oyster, Neopycnodonte zibrowii sp. n., is described from the Azores Archipelago, where it thrives in 420 to >500 m water depth in high densities concealed underneath overhangs. The new species reaches a relatively large size, which may exceed 20 cm, and is characterised by a very unusual hinge line morphology, straight without a bulge of the resilium. It is compared to the extant Indo-Pacific Empressostrea kostini Huber and Lorenz, 2007 and to the cosmopolitan Neopycnodonte cochlear (Poli, 1791), which has a broadly sympatric distribution at shallower depths in the Azores and Bay of Biscay. Radiocarbon dating reveals that individuals reach an impressive lifespan of one to more than five centuries, placing them among the longest-lived molluscs known to date. They often grow on top of each other, forming stacks that resemble dish piles—an effective measure to optimise shell stability with minimal biomineralisation effort, but with the drawback of increased bioerosion ultimately leading to detachment.Three microstructure types are developed in N. zibrowii: (1) the cross-foliated, calcitic, dorsal to central endostracum and aragonitic ligostracum, (2) the porous vesicular structure of the calcitic ventral endostracum, and (3) the simple prismatic aragonitic myostracum. Foliated and vesicular shell portions show sub-millimetre-scale first-order increments delineated by conchiolin-rich growth breaks (interpreted as reproductive cyclicity), and less distinct second-order increments (interpreted as annual in nature). This pattern is clearly reflected by the elemental composition with the primary growth breaks lacking Ca and Sr but including Mg and S as organic matrix constituents. The second-order increments within the calcite are mirrored by moderately co-varying Mg/Ca and S/Ca fluctuations at stable Sr concentrations, reflecting varying proportions of organic matrix. Dorsal and central endostracum transects reveal a low inter-valve, but considerable inter-specimen variability with high Mg/Ca molar ratios and fluctuations (22.5±17.6 mmol/mol), low Sr/Ca values (0.2±0.1 mmol/mol), and a typical to high S/Ca content (6.9±2.2 mmol/mol), when compared to other calcitic bivalves.Unlike short-lived, shallow-water oysters, N. zibrowii thrives under very stable environmental conditions. Minimal temperature fluctuations and stable open marine salinity provide an optimal basis to recognise biological fractionation processes. Strong Mg/Ca fluctuations indicate a physiological control related to metabolism and biomineralisation, prohibiting the use of this ratio as a temperature or ocean chemistry proxy. Low Sr/Ca ratios indicate rather constant and low long-term accretion rates, while short-scale fluctuations may be attributed to short-term variations in growth rate and Mg incorporation. Oxygen isotopes yield a considerable spread of 1.8‰ with a mean of 2.0±0.3‰ δ¹⁸O V-PDB, and low correlation between different contemporaneous parts of the shell and between specimens. These values surprisingly exceed expected equilibrium conditions, calculated from in situ temperature data (annual mean 12.3±0.3 °C) and seawater isotopic composition (0.5±0.1‰ δ¹⁸O SMOW), by 0.5‰ on average. Such positively shifting vital effects, previously reported for limpets and barnacles, are often overlooked in high-temperature and high-amplitude settings and may be more common than is currently believed. Carbon isotopes range from 0.2 to 3.5‰ δ¹³C V-PDB (mean 1.8±0.7‰) and show an ontogenetic decrease, but may incorporate an environmental signal in adult portions, indicated by a strong correspondence of peaks between specimens. This signal is likely driven by a complex interplay of different factors, such as primary production, current-based food supply and metabolism.     

Advances in mollusc sclerochronology and sclerochemistry: tools for understanding climate and environment

This Special Issue of Geo-Marine Letters compiles papers on marine, estuarine and freshwater mollusc shells as recorders of environmental and climatic conditions. Considering that many past studies have differentiated geochemical investigations from sclerochronological investigations, we propose that the sub-discipline term “sclerochemistry” be used when geochemical investigations are undertaken. This issue starts with two review papers that discuss the importance of physiology or vital effects on both sclerochronology and sclerochemistry. Several sclerochemical calibration studies on modern specimens of both bivalves and gastropods are presented (including δ¹⁸O, Mg/Ca, Sr/Ca and Ba/Ca), which illustrate the usefulness and difficulties associated with using these proxies. Studies on fossil mollusc shells are also provided, with one study that uses Pliocene scallop shells to understand past ocean circulation and another that addresses the problem of diagenesis. Finally, a sclerochronological study of crystal prism width across the shell is presented. This Special Issue demonstrates that many elemental and isotopic proxies contained in mollusc shells are complex. In spite of these complexities, environmental and climatic conditions can be extracted for use in palaeoclimatic and palaeoenvironmental research.     

In vivo and in vitro biomineralization in the presence of the inner-shell film of pearl oyster

The inner shell surface is the biomineralization site in shell formation and an inner-shell film covers it.This surface is composed of two regions:an outer calcitic region and an inner aragonitic region.In this study,some amalgamated calcite crystals were found in the calcitic region and some aragonitic "imprints" were found in the central part of the aragonitic region.The "imprints" are probably the trace of mantle cells that adhered to the inner shell surface when the shell was produced.Furthermore,to build a novel in vitro biomineralization system,the inner-shell film was detached from the shell and introduced to the calcitic crystallization solution.Crystallization experiments showed that nacre proteins could induce aragonite crystals in the novel system but inhibited calcite growth in the absence of the inner-shell film.These data suggested that the inner-shell film may induce aragonite growth in vivo by combining nacre proteins.     

Axinopsida serricata shell encrustation: A potential indicator of organic enrichment conditions in sediments in the southern Strait of Georgia,British Columbia,Canada

We evaluate the potential of the geographic and within-sediment distribution patterns of rust-coloured shell encrustations on an abundant subtidal bivalve, Axinopsida serricata (Carpenter, 1864) as geochemical indicators of organic enrichment from marine municipal outfalls. The progressive development of shell encrustation over the life of the animals is suggested by heavier encrustations on large shells compared to smaller shells regardless of the geochemical conditions of the habitat. Heavy encrustations decline in an exponential manner at sediment acid volatile sulphide (AVS) levels >7 μmol/g. Analyses show that the reddish shell encrustations are from an amorphous iron oxide or hydroxy-oxide likely micro-biologically mediated; the oxides appear to be embedded within the inner matrix of an organic layer, with a chemically distinct outer layer. A schematic model is proposed which shows how enrichment of labile organic carbon around outfalls affects the availability of dissolved iron in sediments and leads to less extensive encrustations on bivalve shells. Predominantly sandy sediments with low organic loading have much less potential for iron oxide deposition than silty sediments due to greater oxygen penetration into the surface sediment.Shell encrustation appears to be relatively persistent and indicative of long-term conditions, regardless of spatial and temporal fluctuations in sediment geochemistry. With more research on development over the life-cycle of the animal, the pattern of A. serricata shell encrustation has the potential to rapidly provide a map of cumulative labile organic loading and oxygen penetration of sediments around municipal outfalls on the west coast of North America where this species is common. However, caution must be used in interpreting results, since background sediment characteristics (substrate type, bottom currents and sediment transport) can affect encrustation patterns.     

Specific pathways for the incorporation of dissolved barium and molybdenum into the bivalve shell: An isotopic tracer approach in the juvenile Great Scallop (Pecten maximus)

Dissolved barium and molybdenum incorporation in the calcite shell was investigated in the Great Scallop Pecten maximus. Sixty six individuals were exposed for 16 days to two successive dissolved Ba and Mo concentrations accurately differentiated by two different isotopic enrichments (⁹⁷Mo, ⁹⁵Mo; ¹³⁵Ba, ¹³⁷Ba). Soft tissue and shell isotopic composition were determined respectively by quantitative ICP-MS (Inductively Coupled Plasma Mass Spectrometer) and laser ablation – ICP-MS. Results from Ba enrichment indicate the direct incorporation of dissolved Ba into the shell in proportion to the levels in the water in which they grew with a 6–8 day delay. The low spike contributions and the low partition coefficient (D_Mo = 0.0049 ± 0.0013), show that neither the soft tissue nor the shell were significantly sensitive to Mo enrichment. These results eliminate direct Mo shell enrichment by the dissolved phase, and favour a trophic uptake that will be investigated using the successive isotopic enrichment approach developed in this study.     

High-resolution trace element profiles in shells of the mangrove bivalve Isognomon ephippium: a record of environmental spatio-temporal variations?

The shell chemistry of Isognomon ephippium from three Kenyan sites (Tudor, Gazi and Mida) has been investigated to determine whether these bivalves record environmental parameters. The Mg, Sr, Ba and Mn distributions in the calcite shell layer were determined by using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). In addition, whole-shell analyses were made to evaluate inter-site differences. While some variability is observed for mean Mg concentrations, the mean Sr concentrations were similar for the three sites. The decreasing mean Ba and Mn concentrations, following the order Tudor > Gazi > Mida, are related to distinct regimes of freshwater and nutrient supply. The Mg profiles, determined by LA-ICP-MS, displayed a close to regular sinusoidal pattern, depending on specimen and sample site. For the Tudor shells, an arbitrary fitting of the Mg profiles to sea-surface temperature (SST) variations emphasised the good relationship between these two parameters and allowed for the calculation of mean annual growth rates. In most of the shells, Sr partly co-varied with Mg and Ba, highlighting the complexity of Sr incorporation. The Ba and Mn profiles of the Tudor shells displayed several sharp maxima. With a time scale deduced from the Mg–SST relationship, the Ba and Mn maxima of the Tudor shells closely followed periods of maximal rainfall associated with the southeast monsoon. These Ba and Mn maxima were tentatively associated with algal bloom events known to succeed these periods of high rainfall. The less clearly marked seasonality of the Ba and Mn maxima for the Gazi and Mida specimens is thought to result from weaker seasonal variations in nutrient supply and reduced nutrients inputs. This study highlights the potential of I. ephippium as a recorder of spatio-temporal environmental variations in tropical coastal ecosystems.     

Shells of Paphia undulata (Bivalvia) from the South China Sea as potential proxy archives of the East Asian summer monsoon: a sclerochronological calibration study

The climate of the South China Sea is dominated by the East Asian monsoon (EAM) system. Existing paleoclimate reconstructions offered an excellent insight into longer-term EAM variations. However, due to a lack of appropriate high-resolution paleoclimate data, relatively little is known about the frequency and strength of EAM extremes during the Holocene. To evaluate and establish a potential proxy archive for past variations of the EAM on shorter time-scales, we have carried out a calibration study on shells of the bivalve mollusk, Paphia undulata (Born 1778) from Daya Bay, China. This species has a short lifespan (3 years). Shells grow uninterruptedly between February/March and mid-November and are formed near oxygen isotopic (δ¹⁸O) equilibrium with the ambient environment. Shell growth patterns, δ¹⁸O_shell and δ¹³C_shell values, can be used to estimate the relative amount of precipitation and terrestrial runoff. Therefore, shells of this species can provide reliable, sub-seasonally resolved data on past East Asian summer monsoon strengths. The feasibility of this method has been tested with two Holocene shells from sediment cores taken from the nearby Beibu Gulf. A rather peculiar finding is that shell growth of P. undulata seems to be largely uncoupled to measured local environmental variables. Growth rates are negatively correlated to seawater temperature and chlorophyll a levels and positively to salinity. It is hypothesized here that extraordinary fast shell growth in early spring (February/March; low temperature and primary productivity) are facilitated by preserved energy resources, ensuring that the bivalve quickly reaches the predation window and the required size for reproduction.     

Effects of salinity on mineralogy and chemical composition of Cerastoderma edule and Monodonta articulata shells

Shells of the mollusks Cerastoderma (Cerastoderma) edule (Linnaeus 1758) and Monodonta (Osilinus) articulata Lamark 1822 from salt evaporate basins in western Sicily are studied and the effect of environmental conditions (particularly salinity and water chemistry) on mineralogy and chemical composition is investigated and discussed.

The elements determined both in seawater and shell carbonate are: Ca, Mg, Mn, Fe, Ni, Co, Cu, Zn, Sr and Pb.

Both species studied discriminate against all the minor and trace elements determined; the degree of such discrimination, for some elements, is inversely related to salinity.

The mineralogical composition of the shells (Monodonta has a mixed mineralogy while Cerastoderma is entirely aragonitic) appears to be controlled neither by temperature nor by salinity.

In Monodonta shells direct correlations have been found between the following parameters: calcite-Sr (in younger shells only), calcite-Fe, calcite-Pb (in older shells only), calcite-Zn, salinity-Mg and salinity-Pb, whereas inverse correlations were observed between calcite and Sr (in older shells only) and calcite and Pb (in younger shells only). In Cerastoderma shells a direct correlation was found between salinity and Fe and Mg.

The thickness of the shells appears, in both species, to be an inverse linear function of salinity.     

Experimentally determined Mg/Ca and Sr/Ca ratios in juvenile bivalve calcite for Mytilus edulis: implications for paleotemperature reconstructions

To further evaluate the potential use of Mg/Ca and Sr/Ca ratios as a paleothermometer in the shell carbonate of the blue mussel Mytilus edulis, we grew juvenile mussels (～15 mm shell height; <2 years old) collected from Maine, USA, in controlled environments for 4 months. The four-by-three factorial design consisted of four circulating temperature baths (7, 11, 15 and 19°C), and three salinity ranges (23, 28, and 32). During the experiment, water Mg/Ca and Sr/Ca molar ratios were monitored weekly, and showed little variation across all salinity and temperature ranges. Data from sampled shells including all salinity treatments yielded relatively poor relationships between shell elemental chemistry and water temperatures. However, if only the low salinity treatment data (23) are used, the relationships between shell elemental chemistry and water temperature improve moderately. Based on the data presented here, it may be possible to use Mg/Ca and Sr/Ca ratios from the shell carbonate of juvenile M. edulis to reconstruct paleotemperatures in estuarine settings (salinity below 24) with a corresponding RMSE (root mean squared error; 95% confidence interval) of ±2.4°C and ±2.8°C, respectively. In order for this methodology to be statistically meaningful, water temperature changes must be rather large, as the errors associated with using Mg/Ca and Sr/Ca ratios from the shell material of M. edulis are substantial. Further work is required to determine if the findings presented here can be duplicated, and if the potential salinity effect is pervasive.     

Size-differential feeding in Pinna nobilis L. (Mollusca: Bivalvia): Exploitation of detritus,phytoplankton and zooplankton

The endangered fan shell Pinna nobilis is a large bivalve mollusc (<120 cm shell length) endemic to the Mediterranean that lives one-third buried in soft substrata, generally in shallow coastal waters. We hypothesised that P. nobilis of different sizes would ingest different food sources, because small fan shells will inhale material from closer to the substratum than do large fan shells. We studied stomach contents and faeces of 18 fan shells, 6 small (mean 23.0 cm length), 6 medium-sized (mean 41.5 cm length) and 6 large (mean 62.7 cm length) living in a small area of a low-energy coastal detritic bottom characterised by mud, sand and macroalgae at Mali Ston Bay, Croatia. We found that all P. nobilis ingested copious quantities of undetermined detritus (probably at least 95% of ingested material), phytoplankton, micro and mesozooplankton and pollen grains. Large P. nobilis stomach contents showed a preponderance of water column calanoid copepods, while small fan shells had higher numbers of bivalve larvae. All fan shells took in high numbers of harpacticoid copepods that are benthonic, feeding on microbial communities of detritus and benthic vegetation. There was also a significant selection of phytoplankton species, some apparently occurring between inhalation and ingestion. The stomach contents of small P. nobilis had a higher organic matter content than either medium-sized or large fan shells; this indicated that small fan shells ingested detritus of higher organic content than did larger P. nobilis. As the faeces of all P. nobilis had similar organic matter content, this also indicates higher assimilation efficiencies in small fan shells. The demonstration of differential dietary selectivity by different sized animals has implications for future trophic studies of this endangered species. This study also provides the first demonstration of predation on zooplankton by P. nobilis.     

南黄海牡蛎壳元素组成的原位微区分析及环境指示

碳酸盐生物壳体的周期性生长纹层是记录气候环境变化的天然材料。随着原位微区测试技术的快速发展,高分辨率的同位素和化学元素组成的快速分析显著推动了古气候环境变化及生物地球化学研究。应用激光剥蚀等离子体质谱仪(LAICP-MS)对南黄海现代牡蛎Crassostrea gigas壳体韧带部的元素组成进行原位微区测试,利用内标元素43Ca进行元素比值的校正,探讨了牡蛎壳中化学元素特征及其环境意义。研究的长牡蛎壳体中Mg/Ca、Sr/Ca、Na/Ca比值具有显著的季节性周期变化。环境水体物理化学性质的变化对壳体生长速率影响较大,壳中白垩质方解石和叶片方解石生长层分别对应较高和较低的环境温度,白垩方解石层具有较高的Mg/Ca、Sr/Ca和较低的Na/Ca比值。Mg/Ca比值可指示牡蛎壳体生长环境水体温度,利用前人Mg/Ca比值公式计算可以恢复与器测资料相符的近岸海水温度结果。本研究对应用LA-ICP-MS分析技术开展高分辨率的生物壳体元素组成和环境示踪研究具有借鉴意义。     

Blooms of <Emphasis Type="Italic">Trichodesmium erythraeum</Emphasis> in the South Eastern Arabian Sea during the onset of 2009 summer monsoon

This study presents in situ evidence for the blooms of Trichodesmium erythraeum observed in the shelf waters of the South Eastern Arabian Sea (SEAS) during the onset of the southwest monsoon in June 2009. Evidence showed that water surface discoloration was caused by the accumulation of T. erythraeum, and that the water column contained a colony of T. thiebautii. The surface water color in the bloom region varied from pale brown to pinkish red. Pale brown indicated healthy algae at the peak of its photosynthetic activity, while pinkish red indicated the presence of photosynthetically less active filaments. Zooplankton abundance, especially copepodites, in the bloom area substantiated the theory that Trichodesmium filaments are excellent epiphytes to which the copepodites cling. The bloom area was very fertile with copious quantities of dissolved oxygen (6.85 ml L^?1), PO₄-P (0.108 μmol L^?1) and SiO₄ (1.29 μmol L^?1). Lower NO₃-N (0.028 μmol L^?1) values in the bloom area did not appear to affect Trichodesmium growth from molecular nitrogen fixation. However, lower NO₃-N values altered the normal phytoplankton composition of this area.     

Occurrence and turnover of DMSP and DMS in deep waters of the Ross Sea,Antarctica

High concentrations of the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) and its degradation product dimethylsulfide (DMS) are associated with blooms of Phaeocystis antarctica in the Ross Sea, Antarctica. Episodic and rapid vertical export of Phaeocystis biomass to deep water has been reported for the Ross Sea, therefore we examined the distribution and microbial consumption rates of DMSP and DMS throughout the sub-euphotic water column. Total DMSP (dissolved+particulate; DMSPt) was present at 0.5–22 nM at depths between 70 and 690 m during both the early bloom (November) and the late bloom (January). Sub-euphotic peaks of DMSP were sometimes associated with mid-water temperature maxima, and elevated DMSP below 70 m was found mainly in water masses characterized as Modified Circumpolar Deep Water or Antarctic Shelf Water. Overall, 50–94% of the integrated water-column DMSPt was found below the euphotic zone. At one station during the early bloom, local maxima of DMSPt (14 nM) and DMS (20 nM) were observed between 113 and 240 m and these maxima corresponded with high chlorophyll a concentrations, P. antarctica cell numbers, and Fv/Fm (the quantum yield of photosystem II). During the late bloom, a sub-euphotic maximum of DMSPt (15.8 nM) at 250 m cooccurred with peaks of chlorophyll a concentration, DMSP lyase activity, bacterial production and dissolved DMSP consumption rates. DMSP turnover contributed ～12% of the bacterial carbon demand between 200 and 400 m. DMS concentrations peaked at 286 m but the maximum concentration (0.42 nM) was far lower than observed during the early bloom, probably because of relatively rapid biological consumption of DMS (1–3 turnovers per day) which, in turn, contributed to elevated dissolved dimethylsulfoxide (DMSO) concentrations. Relatively stable DMSPt distributions at some sites suggest that rapid sinking of Phaeocystis biomass is probably not the major mechanism responsible for mesopelagic DMSP accumulations. Rather, subduction of near-surface water masses, lateral advective transport or trapping of slowly sinking P. antarctica biomass in intermediate water masses are more likely mechanisms. We found that a culture of P. antarctica maintained cellular integrity during 34 days of darkness, therefore the presence of intact cells (and DMSP) at depth can be explained even under a slow sinking/advection scenario. Whatever the mechanism, the large pools of DMSP and DMS below the euphotic zone suggest that export exerts a control on potential DMS emission from the surface waters of the Ross Sea.     

Riddle of the Nautilus: Specific Structural Features of Its Shell

The shells of the chambered nautilus (Nautilus pompilius) are studied by X-ray structure analysis (XRD) and electron microscopy (SEM and TEM). The shell structure is examined in a nanometer scale using XRD and synchrotron-based small angle X-ray scattering (SAXS). The mineral composition of shells includes 98–99 wt % aragonite, ~1 wt % organic substance (conchiolin), 1 wt % strontianite, and 0.5 wt % calcite. Trace elements comprise Na, Mg, Al, K, Si, Fe, P, S, Zn, Sr, Ba, and Cl. The laminated layers are composed of aragonite crystals with a width of 5–10 μm and a thickness of 0.5–1 μm; the prismatic layers, with a length of 7–12 μm long and a width of 1–3 μm. According to XRD and SAXS assays, the aragonite layers are formed of flawless monocrystals lacking any internal structuring. The shell conchiolin is a polymer formed of a system of parallel fibers folded with a step of approximately 100 nm without longitudinal structuring. Presumably, the unique structure of the nautilus shells has determined the existence of Nautiloidea over 500 Ma and their preservation as fossils for over 100 Ma.     

Shell growth and oxygen isotopes in the topshell Osilinus turbinatus: resolving past inshore sea surface temperatures

Shells of the rocky shore intertidal gastropod Osilinus turbinatus (von Born), often abundant in archaeological deposits in the Mediterranean region, are a potential source of data on palaeotemperature, palaeoseasonality and archaeological seasonality. To evaluate this species as a climate archive, investigations of annual patterns of shell growth and of monthly variations in oxygen isotopes in shell carbonates were made on different populations in NW Sicily. Mark-recapture experiments at San Vito lo Capo and Mazzaforno show that O. turbinatus grows almost continuously throughout the year but at different rates in different seasons. Around 75% of the yearly shell growth occurs in the autumn and winter. On average, larger/older shells produce narrower annual growth increments than smaller/younger ones. Conspicuous growth lines in larger/older shells show that growth stops during the hottest part of the summer. Oxygen isotope analyses on monthly collected shells of O. turbinatus from three shores (Cala Grande, Monte Cofano and Mazzaforno) show that the isotope values record temperature variations through the year. In all the datasets, surface seawater temperatures (SSTs) calculated from δ¹⁸O_SHELL mostly underestimate measured SSTs, offsets being generally greater in summer. Minimum annual offsets range from 0.0°C to 0.7°C and maximum annual offsets from 3.1°C to 8.7°C. δ¹⁸O_SHELL values fail to record temperatures higher than 25°C. Careful selection of shells to be analysed can reduce offsets between δ¹⁸O_SHELL temperature estimates and measured SSTs for many parts of the year, except the hottest. Allowing for this, shells of O. turbinatus offer good potential as climate archives and for archaeological studies of seasonal patterns of human foraging for shellfish.     

Growth increments and stable isotope variation in shells of the deep-sea hydrothermal vent bivalve mollusk Bathymodiolus brevior from the North Fiji Basin,Pacific Ocean

Bathymodiolus brevior [von Cosel, R., Métivier, B., Hashimoto, J., 1994. Three new species of Bathymodiolus (Bivalvia: Mytilidae) from hydrothermal vents in the Lau Basin and the North Fiji Basin, western Pacific, and the Snake Pit Area, mid-Atlantic ridge. Veliger 37, 374–392] a bivalve mollusk living at deep-sea hydrothermal vents, exhibits daily microgrowth structures in its shell. This interpretation is substantiated by various lines of evidence: (1) similar shell portions of contemporaneous specimens from the same locality contain almost the same number of microincrements; (2) the number of microincrements coincides with the expected number of days in which shell portions of Bathymodiolus spp. form; (3) the width of such microincrements compares well with daily growth rates estimated for the close relative B. thermophilus [Kenk, V.C., Wilson, B.R., 1985. A new mussel (Bivalvia, Mytilidae) from hydrothermal vents in the Galapagos rift-zone. Malacologia 26, 253–271]; (4) different specimens from the same site show similar microgrowth curves. In addition, we found support for tide-controlled shell growth. Daily shell growth rates fluctuate on a fortnightly basis. Some shell portions also revealed the typical tide-controlled microgrowth pattern commonly observed in intertidal bivalves. Based on the analyses of lunar daily growth increments, a growth curve for B. brevior was computed: X_t=14 cm−(14–0.04 cm) e^−0.26t. This curve enables estimation of ontogenetic age from shell length. According to this equation, B. brevior reaches its maximum shell length of 14 cm at about age 18. Shell isotope analyses suggest that some major shell growth interruptions or retardations are related to extremely active hydrothermal venting activity. However, shell growth also stopped during periods of low venting implying physiological controls on shell formation. Results of the present study demonstrate that shells of B. brevior provide calendars and environmental data loggers that can complement or partly substitute for long-term observations of venting systems.     

The relative importance of buffering and brine inputs in controlling the abundance of Na and Ca in sedimentary formation waters

The concentration of Ca in the formation waters of petroleum reservoirs can play a major role in influencing the outcome of a number of processes that are of great significance to the oil industry. For example, formation water Ca concentration affects the risk of carbonate scale formation during production. In order to better understand the concentrations of Ca in formation waters, we have investigated the chemistries of formation waters from a range of onshore and offshore basins worldwide, using published sources, as well as unpublished data held by BP. Although calcium and sodium are the principal cations in almost all formation waters they vary enormously in their relative proportions. We have identified three distinct trends on a plot of X_Ca (Ca/(Na + Ca)) against Cl. Most data lie on a high-Ca trend, here termed Trend 1, and show an increase in X_Ca with salinity. We interpret this as tracking equilibration with Ca and Na-bearing minerals, with the ratio (mol Ca/mol Na²) remaining approximately constant irrespective of salinity for chloride-dominated fluids. At very high salinities, Br-enriched bittern brines that have taken part in dolomitisation lie at the Cl-rich end of this trend. Some brines remain Na-dominated up to very high salinities and define a distinct low-Ca trend, Trend 2. These are associated with dissolution of halite beds and are interpreted to arise when the amount of Na in the pore fluid greatly exceeds the amount of Ca available in minerals. We refer to such brines as mass-limited; the sparsity of Ca in the rock-fluid system constrains X_Ca to a low value. Remarkably few brines lie between these trends. Finally, dilute formation waters show very large variations in X_Ca and may have bicarbonate as the dominant anion. They define a distinct low-Cl trend, Trend 3. We conclude that the behaviour of Na and Ca in most formation waters reflects equilibration with minerals, and concentrations of Ca in solution are sensitive to pH and P_CO2 as well as to chloride concentration. For some brines however, the amount of salts in solution is sufficient to overwhelm the buffering capacity of the wallrocks.     

Loggerhead turtle movements reconstructed from 18O and 13C profiles from commensal barnacle shells

Commensal barnacles, Chelonibia testudinaria, from logger-head turtles have ¹⁸O and ¹³C variations in their calcitic shells that record the environments in which the turtles live. Isotopic profiles from the barnacle shells can thus be interpreted to reconstruct movements of the host turtle between open ocean and brackish-water regimes.