首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 992 毫秒
1.
Offsets from isotopic equilibrium in biogenic carbonates have complicated paleoclimate reconstructions for decades. A new archive of climate, deep-sea corals, is used to evaluate the calcification processes, independent of photosynthesis, that contribute to these offsets. Carbon and oxygen stable isotope data from six modern deep-sea corals show strong linear trends between δ13C and δ18O. Slopes of these trends between samples are similar and range between 1.9 to 2.6 for Δδ13C/Δδ18O. Linear trends intersect isotopic equilibrium for δ18O and are slightly depleted for δ13C. Variations in the isotopic ratios are strongly correlated with the density banding structure. Isotopically depleted aragonite is associated with light, quickly precipitating bands, whereas isotopically enriched points correspond to slowly accumulating, less dense aragonite. The densest white band at the trabecular center is furthest from isotopic equilibrium for both carbon and oxygen. Data from this region fall off the linear trend between δ18O and δ13C. This deviation, where δ13C remains constant while the δ18O continues to decrease, does not support “vital effect” mechanisms that call upon kinetic fractionation to explain offsets from isotopic equilibrium. We propose a new mechanism for vital effects in these deep-sea corals that is based on a thermodynamic response to a biologically induced pH gradient in the calcifying region.  相似文献   

2.
Previous experimental fieldwork showed that coral skeletal δ13C values decreased when solar intensity was reduced, and increased in the absence of zooplankton. However, actual seasonal changes in solar irradiance levels are typically less pronounced than those used in the previous experiment and the effect of increases in the consumption of zooplankton in the coral diet on skeletal δ13C remains relatively unknown. In the present study, the effects of four different light and heterotrophy regimes on coral skeletal δ13C values were measured. Porites compressa corals were grown in outdoor flow-through tanks under 112%, 100%, 75%, and 50% light conditions at the Hawaii Institute of Marine Biology, Hawaii. In addition, corals were fed either zero, low, medium, or high concentrations of brine shrimp. Decreases in light from 100% resulted in significant decreases in δ13C that is most likely due to a corresponding decrease in photosynthesis. Increases in light to 112% also resulted in a decrease in δ13C values. This latter response may be a consequence of photoinhibition. The overall curved response in δ13C values was described by a significant quadratic function. Increases in brine shrimp concentrations resulted in increased skeletal δ13C levels. This unexpected outcome appears to be attributable to enhanced nitrogen supply associated with the brine shrimp diet which led to increased zooxanthellae concentrations, increased photosynthesis rates, and thus increased δ13C values. This result highlights the potential influence of nutrients from heterotrophically acquired carbon in maintaining the zooxanthellae-host symbiosis in balance. In addition, evidence is presented that suggests that coral skeletal growth and δ13C are decoupled. These results increase our knowledge of how light and heterotrophy affects the δ13C of coral skeletons.  相似文献   

3.
Here we calibrate the carbonate clumped isotope thermometer in modern deep-sea corals. We examined 11 specimens of three species of deep-sea corals and one species of a surface coral spanning a total range in growth temperature of 2-25 °C. External standard errors for individual measurements ranged from 0.005‰ to 0.011‰ (average: 0.0074‰) which corresponds to ∼1-2 °C. External standard errors for replicate measurements of Δ47 in corals ranged from 0.002‰ to 0.014‰ (average: 0.0072‰) which corresponds to 0.4-2.8 °C. We find that skeletal carbonate from deep-sea corals shows the same relationship of Δ47 (the measure of 13C-18O ordering) to temperature as does inorganic calcite. In contrast, the δ13 C and δ18O values of these carbonates (measured simultaneously with Δ47 for every sample) differ markedly from equilibrium with seawater; i.e., these samples exhibit pronounced ‘vital effects’ in their bulk isotopic compositions. We explore several reasons why the clumped isotope compositions of deep-sea coral skeletons exhibit no evidence of a vital effect despite having large conventional isotopic vital effects.  相似文献   

4.
Soft corals and black corals are useful proxy tools for paleoceanographic reconstructions. However, most work has focused on deep-water taxa and few studies have used these corals as proxy organisms in shallow water (<200 m). To facilitate the use of stable nitrogen and carbon isotope (δ15N and δ13C) records from shallow-water soft coral and black coral taxa for paleoceanographic reconstructions, quantification of the inherent variability in skeletal isotope values between sites, across depth, and among taxa is needed. Here, skeletal δ15N and δ13C values were measured in multiple colonies from eleven genera of soft corals and two genera of black corals from across a depth transect (5-105 m) at two sites in Palau located in the tropical western Pacific Ocean. Overall, no difference in skeletal δ15N and δ13C values between sites was present. Skeletal δ15N values significantly increased and δ13C values decreased with depth. This is consistent with changes in isotope values of suspended particulate organic matter (POM) across the photic zone, suggesting that the primary food source to these corals is suspended POM and that the stable isotopic composition of POM controls the skeletal isotopic composition of these corals. Thus, to compare the isotope records of corals collected across a depth range in the photic zone, first order depth corrections of −0.013‰ m−1 and +0.023‰ m−1 are recommended for δ15N and δ13C, respectively. Average depth-corrected δ15N values were similar between black corals and soft corals, indicating that corals in these orders feed at a similar trophic level. In contrast, average depth-corrected δ13C values of black corals were significantly lower than that of soft corals, potentially resulting from metabolic processes associated with differing skeletal compositions among the orders (i.e., gorgonin vs. chitin based). Thus, a correction of +1.0‰ is recommended for black corals when comparing their δ13C-based proxy records to soft corals. After correcting for both the depth and order effects, variability in δ15N values among corals within each genera was low (standard deviation (SD) of the mean <±0.5‰), with the exception of Acanthorgorgia. The calculated SD of <±0.5‰ provides a first order guideline for the amount of variability that could be expected in a δ15N record, and suggests that these corals may be useful for δ15N-based paleoceanographic reconstructions. Variability in δ13C values among corals within genera was also low (standard deviation of the mean <±0.5‰) with the exception of Rhipidipathes and Villogorgia. Similar to δ15N, records from the genera studied here with the exception of Rhipidipathes and Villogorgia may be useful for δ13C-based paleoceanographic reconstructions. Overall, using the recommendations developed here, stable isotope records from multiple sites, depths and taxa of these corals can be more rigorously compared.  相似文献   

5.
Coral proxy records of sea surface temperature (SST) and hydrological balance have become important tools in the field of tropical paleoclimatology. However, coral aragonite is subject to post-depositional diagenetic alteration in both the marine and vadose environments. To understand the impact of diagenesis on coral climate proxies, two mid-Holocene Porites corals from raised reefs on Muschu Island, Papua New Guinea, were analysed for Sr/Ca, δ18O, and δ13C along transects from 100% aragonite to 100% calcite. Thin-section analysis showed a characteristic vadose zone diagenetic sequence, beginning with leaching of primary aragonite and fine calcite overgrowths, transitional to calcite void filling and neomorphic, fabric selective replacement of the coral skeleton. Average calcite Sr/Ca and δ18O values were lower than those for coral aragonite, decreasing from 0.0088 to 0.0021 and −5.2 to −8.1‰, respectively. The relatively low Sr/Ca of the secondary calcite reflects the Sr/Ca of dissolving phases and the large difference between aragonite and calcite Sr/Ca partition coefficients. The decrease in δ18O of calcite relative to coral aragonite is a function of the δ18O of precipitation. Carbon-isotope ratios in secondary calcite are variable, though generally lower relative to aragonite, ranging from −2.5 to −10.4%. The variability of δ13C in secondary calcite reflects the amount of soil CO2 contributing 13C-depleted carbon to the precipitating fluids. Diagenesis has a greater impact on Sr/Ca than on δ18O; the calcite compositions reported here convert to SST anomalies of 115°C and 14°C, respectively. Based on calcite Sr/Ca compositions in this study and in the literature, the sensitivity of coral Sr/Ca-SST to vadose-zone calcite diagenesis is 1.1 to 1.5°C per percent calcite. In contrast, the rate of change in coral δ18O-SST is relatively small (−0.2 to 0.2°C per percent calcite). We show that large shifts in δ18O, reported for mid-Holocene and Last Interglacial corals with warmer than present Sr/Ca-SSTs, cannot be caused by calcite diagenesis. Low-level calcite diagenesis can be detected through X-ray diffraction techniques, thin section analysis, and high spatial resolution sampling of the coral skeleton and thus should not impede the production of accurate coral paleoclimate reconstructions.  相似文献   

6.
The relationship between molluscan shell growth rate and skeletal δ18O and δ13C was investigated in a detailed field study for the scallop, Pecten maximus. Seasonal variation in shell growth rate was found to be a governing factor influencing shell δ18O and δ13C. At low shell growth rates, shell δ18O were more positive (of the order +0.4‰) and δ13C more negative (up to −2‰) as compared with predicted values for precipitation of inorganic calcite in isotopic equilibrium with seawater. The deviations in δ18O were hypothesized as reflecting possible differences in solution carbonate chemistry at the site of mineralization in the extrapallial fluid as compared with that of the external seawater medium. The deviations in shell δ13C were consistent with incorporation of isotopically depleted respiratory 13C (i.e., a metabolic effect). A trend toward more depleted shell δ18O and δ13C values occurred at higher shell growth rates, with negative δ18O values as compared with predicted equilibrium at shell growth rates above 0.13 mm per day. These simultaneous negative deviations in skeletal δ18O and δ13C were interpreted as resulting from a kinetic effect. The implications for environmental reconstruction from molluscan isotopic records are discussed in light of a model of isotopic behavior based on the findings of the study.  相似文献   

7.
Skeletal cadmium-to-calcium (Cd/Ca) ratios in hermatypic stony corals have been used to reconstruct changes in upwelling over time, yet there has not been a systematic evaluation of this tracer’s natural variability within and among coral species, between depths and across environmental conditions. Here, coral skeletal Cd/Ca ratios were measured in multiple colonies of Pavona clavus, Pavona gigantea and Porites lobata reared at two depths (1 and 7 m) during both upwelling and nonupwelling intervals in the Gulf of Panama (Pacific). Overall, skeletal Cd/Ca ratios were significantly higher during upwelling than during nonupwelling, in shallow than in deep corals, and in both species of Pavona than in P. lobata. P. lobata skeletal Cd/Ca ratios were uniformly low compared to those in the other species, with no significant differences between upwelling and nonupwelling values. Among colonies of the same species, skeletal Cd/Ca ratios were always higher in all shallow P. gigantea colonies during upwelling compared to nonupwelling, though the magnitude of the increase varied among colonies. For P. lobata, P. clavus and deep P. gigantea, changes in skeletal Cd/Ca ratios were not consistent among all colonies, with some colonies having lower ratios during upwelling than during nonupwelling. No statistically significant relationships were found between skeletal Cd/Ca ratios and maximum linear skeletal extension, δ13C or δ18O, suggesting that at seasonal resolution the Cd/Ca signal was decoupled from growth rate, coral metabolism, and ocean temperature and salinity, respectively. These results led to the following conclusions, (1) coral skeletal Cd/Ca ratios are independent of skeletal extension, coral metabolism and ambient temperature/salinity, (2) shallow P. gigantea is the most reliable species for paleoupwelling reconstruction and (3) the average Cd/Ca record of several colonies, rather than of a single coral, is needed to reliably reconstruct paleoupwelling events.  相似文献   

8.
The integrity of coral-based reconstructions of past climate variability depends on a comprehensive knowledge of the effects of post-depositional alteration on coral skeletal geochemistry. Here we combine millimeter-scale and micro-scale coral Sr/Ca data, scanning electron microscopy (SEM) images, and X-ray diffraction with previously published δ18O records to investigate the effects of submarine and subaerial diagenesis on paleoclimate reconstructions in modern and young sub-fossil corals from the central tropical Pacific. In a 40-year-old modern coral, we find secondary aragonite is associated with relatively high coral δ18O and Sr/Ca, equivalent to sea-surface temperature (SST) artifacts as large as −3 and −5 °C, respectively. Secondary aragonite observed in a 350-year-old fossil coral is associated with relatively high δ18O and Sr/Ca, resulting in apparent paleo-SST offsets of up to −2 and −4 °C, respectively. Secondary Ion Mass Spectrometry (SIMS) analyses of secondary aragonite yield Sr/Ca ratios ranging from 10.78 to 12.39 mmol/mol, significantly higher compared to 9.15 ± 0.37 mmol/mol measured in more pristine sections of the same fossil coral. Widespread dissolution and secondary calcite observed in a 750-year-old fossil coral is associated with relatively low δ18O and Sr/Ca. SIMS Sr/Ca measurements of the secondary calcite (1.96-9.74 mmol/mol) are significantly lower and more variable than Sr/Ca values from more pristine portions of the same fossil coral (8.22 ± 0.13 mmol/mol). Our results indicate that while diagenesis has a much larger impact on Sr/Ca-based paleoclimate reconstructions than δ18O-based reconstructions at our site, SIMS analyses of relatively pristine skeletal elements in an altered coral may provide robust estimates of Sr/Ca which can be used to derive paleo-SSTs.  相似文献   

9.
Uptake of anthropogenic CO2 by the oceans is altering seawater chemistry with potentially serious consequences for coral reef ecosystems due to the reduction of seawater pH and aragonite saturation state (Ωarag). The objectives of this long-term study were to investigate the viability of two ecologically important reef-building coral species, massive Porites sp. and Stylophora pistillata, exposed to high pCO2 (or low pH) conditions and to observe possible changes in physiologically related parameters as well as skeletal isotopic composition. Fragments of Porites sp. and S. pistillata were kept for 6-14 months under controlled aquarium conditions characterized by normal and elevated pCO2 conditions, corresponding to pHT values of 8.09, 7.49, and 7.19, respectively. In contrast with shorter, and therefore more transient experiments, the long experimental timescale achieved in this study ensures complete equilibration and steady state with the experimental environment and guarantees that the data provide insights into viable and stably growing corals. During the experiments, all coral fragments survived and added new skeleton, even at seawater Ωarag < 1, implying that the coral skeleton is formed by mechanisms under strong biological control. Measurements of boron (B), carbon (C), and oxygen (O) isotopic composition of skeleton, C isotopic composition of coral tissue and symbiont zooxanthellae, along with physiological data (such as skeletal growth, tissue biomass, zooxanthellae cell density, and chlorophyll concentration) allow for a direct comparison with corals living under normal conditions and sampled simultaneously. Skeletal growth and zooxanthellae density were found to decrease, whereas coral tissue biomass (measured as protein concentration) and zooxanthellae chlorophyll concentrations increased under high pCO2 (low pH) conditions. Both species showed similar trends of δ11B depletion and δ18O enrichment under reduced pH, whereas the δ13C results imply species-specific metabolic response to high pCO2 conditions. The skeletal δ11B values plot above seawater δ11B vs. pH borate fractionation curves calculated using either the theoretically derived αB value of 1.0194 (Kakihana et al. (1977) Bull. Chem. Soc. Jpn.50, 158) or the empirical αB value of 1.0272 (Klochko et al. (2006) EPSL248, 261). However, the effective αB must be greater than 1.0200 in order to yield calculated coral skeletal δ11B values for pH conditions where Ωarag ? 1. The δ11B vs. pH offset from the seawater δ11B vs. pH fractionation curves suggests a change in the ratio of skeletal material laid down during dark and light calcification and/or an internal pH regulation, presumably controlled by ion-transport enzymes. Finally, seawater pH significantly influences skeletal δ13C and δ18O. This must be taken into consideration when reconstructing paleo-environmental conditions from coral skeletons.  相似文献   

10.
This paper will discuss the use of chemical proxies in coral skeletons to reconstruct the history of salinity (from the δ18O of the skeleton) and nutrients in the water (from the δ13C) in Florida Bay between 1824 and 1994. Monthly salinity and water temperature data collected since 1989 were used to establish a correlation between salinity, temperature, and the δ18O of the skeleton of the coralSolenastrea bournoni from Lignumvitae Basin in Florida Bay. This relationship explains over 50% of the variance in the δ18O of the skeleton. Assuming that interannual variations in the temperature of the water are small, we have applied this relationship to the δ18O measured in the coral skeleton collected from Lignumvitae Basin which has a record between 1824 and 1993. These data provide a revised estimate of salinity variation in Lignumvitae Basin for the period when historical records for salinity were not available, and show that the highest salinity events occurred in the past 30 yr. Using the relationships between the salinity in Lignumvitate Basin and other basins, obtained using a modern dataset, we are able to estimate ranges in salinity for other portions of Florida Bay. Skeletons of specimens of the coral speciesSiderastrea radians collected from other areas of Florida Bay show similar patterns in the δ18O over the past 10 yr, indicating that corals in most portions of Florida Bay are recording salinity variations in their skeletons and therefore support the idea that salinity variations in different portions of Florida Bay can be related. Fluorescence analysis of the coral from Lignumvitae Basin shows a large change in the magnitude of the 10-yr signal coincident with the construction of the railway, confirming that this event had a significant impact upon Florida Bay. The δ13C of the coral skeletons reveals a long-term history of the oxidation of organic material, fixation of carbon by photosynthesis (algal blooms), and the intrusion of marine water into the bay. Since the construction of the railway from Miami to Key West there has been a long-term decrease in the δ13C of the coral skeleton from Lignumvitae Basin, suggesting the increased oxidation of organic material in this area. This decrease in δ13C appears to have reached a minimum value around 1984 and has increased since this time in the western portions of Florida Bay. The increase may be related to the algal blooms prevalent in the area or alternatively could result from intrusion of more marine water. In the eastern areas, a small increase in the δ18C between 1984 and 1988 was followed by further decline suggesting more oxidation of organic material. We have also attempted to use the concentration of barium in the coral skeleton as a proxy indicator of the nutrient status in Florida Bay.  相似文献   

11.
The South Asian Summer Monsoon induces vertical mixing in the surface Arabian Sea, leading to a reduction in the Sea Surface Temperature (SST) of the order of 3–4°C. This reduction in temperature is recorded by modern corals (Porites) that grow in the Lakshadweep Islands (coralline islands located at about 350 km off the south-west coast of India) in their stable oxygen isotope ratios (denoted by δ18O). As large coral colonies of this genus are available, our results show that palaeomonsoon records for a few centuries back in time, a crucial input for climatic models aimed at predicting the Asian Monsoon, can be obtained from these corals. We also show that two corals separated by ? 60 km show similar (518O variations as does a giant clam (Tridacna maximus) that grew near one end of the coral colonies. As this clam is known to precipitate CaCO3 in isotopic equilibrium with the ambient sea-water, it is possible to estimate the isotopic offset of coralline δ18O from that of the CaCO3 precipitated in isotopic equilibrium. This ‘disequilibrium effect’ appears to be constant around 4.5±0.2%0. Our calculations show that SST (t, °C) is related to the coral δ18O (δc) and the sea-water δ18O (δw) by the equation t = 3.0–4.68 (δc - δw), which is in good agreement with such relationships for corals from the Pacific and Atlantic.  相似文献   

12.
Understanding the influence of climatic and non-climatic factors on geochemical signals in corals is critical for assessing coral-derived records of tropical climate variability. Porites microatolls form large, disk-shaped colonies constrained in their upward growth by exposure at or close to mean spring low water level, and occur on Indo-Pacific reefs. Microatolls appear suitable for paleoclimate reconstruction, however the systematics of the microatoll chemistry-climate relationship are yet to be characterized. In this study, the δ18O signal in Porites microatolls from well-flushed reef flats on Kiritimati (Christmas) Island, central Pacific was investigated for intra-coral (growth aspect and extension rate effects) and between-coral effects, and to explore the climate signal contained within their skeletons. Samples for δ18O analysis were taken from six individual transects from different positions within Porites microatoll XM22. The results show that: (1) the average standard deviation for the mean δ18O values of transects that represent the same time periods is 0.03‰, and is within measurement error for a single analysis (0.04‰); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07‰ and; (3) comparison of the average δ18O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04‰ and 0.11‰ for different time periods. These differences in mean δ18O are within the range for intra- and inter-colony differences in seasonal and interannual δ18O reported for dome-shaped Porites. Based on these results, a stacked microatoll δ18O record was constructed for the period 1978-2007 for comparison with published coral δ18O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in δ18O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of δ18O, and there is a strong correlation (r = −0.71) between microatoll δ18O and local sea surface temperature (SST). The δ18O-SST slope relationship for microatolls is −0.15‰/°C, very similar to that reported for fast-growing domed corals (−0.18‰ to −0.22‰/°C). Statistical analysis of the stacked microatoll δ18O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, annual and interannual timescales. Our results show that the signal reproducibility and fidelity of skeletal δ18O in coral microatolls is comparable to that observed for more conventional coral growth forms. Longer-lived, and fossil, Porites microatolls, where they have grown in suitably flushed environments, are likely to contain δ18O signals that can significantly extend instrumental records of tropical climate variability.  相似文献   

13.
The origin of δ13C variations within the skeletons of zooxanthellate scleractinian corals is still a matter of considerable debate. In particular, the role respired CO2 plays in controlling the eventual δ13C of the skeleton remains unclear. In this study, the temporal variability of the δ13C of respired CO2 produced by Montastraea faveolata has been measured at approximately monthly intervals over a 1-year period. In these experiments, three corals maintained on a platform at 8 m depth near Molasses Reef in the Florida Keys were incubated in closed chambers for 24-h periods and samples of the incubation water analyzed for the δ13C of the dissolved inorganic carbon (ΣCO2) at ∼3-h intervals. Throughout the incubation, the concentration of O2 was measured continuously within the chamber. Our results show that during daylight, the δ13C of the ΣCO2 in the incubation water becomes enriched in 13C as a result of fractionation during the fixation of C by photosynthesis, whereas at night the δ13C of the ΣCO2 becomes more negative. The δ13C of the respiratory CO2 ranges from −9‰ in the late spring to values as low as −17‰ in the autumn. The lighter values are significantly more negative than those reported by previous workers for coral tissue and zooxanthellae. An explanation for this discrepancy may be that the corals respire a significant proportion of isotopically negative substances, such as lipids, which are known to have values up to 10‰ lighter compared to the bulk δ13C of the tissue. The clear seasonal cycle in the δ13C of the respiratory CO2 suggests that there is also seasonal variability in either the δ13C of the coral tissue or the type and/or amount of organic material being respired. A similar temporal pattern and magnitude of change was observed in the δ13C of the coral tissue samples collected from a nearby reef at monthly intervals between 1995 and 1997. These patterns are similar in timing to the δ13C measured in the coral skeletons. We have also calculated an annual mean value for the fractionation factor between dissolved CO2− in the external environment and photosynthate fixed by the zooxanthellae of 1.0121 (±0.003). This value is inversely correlated with the ratio of photosynthesis to respiration (P/R) of the entire organism and shows the highest values during the summer months.  相似文献   

14.
The build up of the isotopic signal in corals was followed by sampling the newly formed skeleton at a monthly resolution for a period of two years in order to establish the interrelations between the calcification processes and the skeletal isotopic composition. We deployed two underwater sampling schemes, which provide a monitor of the changes in water temperature and δ18O and in the corresponding newly accreted skeleton of undisturbed Porites lutea colonies under natural conditions and four transplanted colonies, which maintained the genetic identity throughout the experiment. The results indicate that δ18O of the newly accreted skeleton does not correlate with ambient temperature although the seasonal temperature variability at the site (winter to summer) is in the order of 6 °C and δ18O of seawater is constant throughout the year. In contrast to the newly formed surface skeleton, the isotopic compositions of the deep and older parts of the skeleton show the predicted annual isotopic pattern with highly significant correlation between δ18Os and SST. The transformation between temperature-independent to temperature-dependent isotopic signal occurs several months after the skeleton was formed at the surface. The position of the skeleton in relation to the open sea may generate the difference between δ18Os of the surface skeleton and that of the skeleton previously accreted further down the tissue layer. Our data support the general model of a multi-step skeletogenesis process, where the temperature independent skeleton is entails the first step, the production of skeletal scaffold, and the environmental temperature signature is captured by the next two other steps: the thickening and the periodic abrupt uplift occurring at the depth of the tissue layer. However, re-examination and development of the current isotopic models for coral calcification are required in order to explain the observed different temperature dependency during the growth’s sequence.  相似文献   

15.
We conducted a year-long, intensive monitoring program of live aquatic gastropods (Helisoma duryi, Melanoides tuberculata, Physa virgata, Pyrgulopsis sp., and Tyronia sp.) and their host springs in the Ash Meadows National Wildlife Refuge of southern Nevada. Our purpose was to constrain the degree of natural variation in the isotopic values of shell aragonite for gastropods living in near-constant conditions. Inter- and intraspecies variations, as well as within-shell variations, of δ18O and δ13C values for all taxa were larger than predicted based on variations in environmental conditions alone. This result suggests that different organisms growing in identical or nearly identical environmental conditions may not produce shells with equilibrium isotopic compositions and that these offsets from equilibrium may differ by small, but statistically significant amounts. For the gill-breathing, fully aquatic gastropods M. tuberculata, Pyrgulopsis sp., and Tyronia sp., the deviation of measured isotopic values compared to predicted values based on average environmental conditions were consistent with differences between taxa in the seasonal timing of shell growth. Measured values for the lung-breathing gastropods H. duryi and P. virgata were higher for δ18O and lower for δ13C than predicted at isotopic equilibrium, even when accounting for seasonality effects. We suggest that explaining the differences between the shell isotopic composition of lung- and gill-breathing snails requires a combination of both behavioral and physiologic factors. Our results illustrate the potential complexities of interpreting stable isotopic data from fossil gastropod shells even when environmental conditions are nearly constant, and place limitations on the paleoenvironmental deductions that can be made from the isotopic measurements on fossil gastropods.  相似文献   

16.
《Quaternary Science Reviews》2007,26(11-12):1670-1689
Given the growing interest in carbonate deposits from polar regions as paleoclimatic proxies, this review paper first provides a classification of the various types of cold-climate carbonate precipitates followed by a summary of the 13C and 18O composition of the carbonate deposits and parent water from which the carbonates precipitated. The cold-climate carbonate precipitates were classified into three broad categories: powders, crusts and speleothem. The carbonate powders include those that precipitated in relation to aufeis aggradation (cryogenic aufeis calcite) and in relation to the growth of various annual/perennial ice formations in freezing caves (cryptocrystalline calcite and calcite pearls). The carbonate crusts can be further subdivided based on their lithic environment; those that precipitated on the upper surface of bedrock/clasts (i.e. subglacially precipitated calcite and evaporative calcite crusts); those that are located on the underside of clasts (i.e. pedogenic carbonates); and those that precipitated in rock outcrop fissures (i.e. endostromatolites). The cold-climate carbonate precipitates have a highly variable isotopic composition with δ18O values ranging between −6.5‰ and 28‰ VSMOW and δ13C values in the −10–20‰ VPDB range. However, each type of carbonate precipitates has a specific δ13C and δ18O range, suggesting that their environmental setting and the mechanism by which they formed controls their 13C and 18O signature. It was found that carbonate deposits that precipitated under equilibrium physico-chemical conditions had a δ13C value that is in equilibrium with that of the parent water, while its δ18O composition was more variable, as it is in part controlled by the temperature of reaction and by the δ18O and calcite saturation state of the parent water. By contrast, the δ18O composition of biologically precipitated carbonate deposits (endostromatolites) reflect that of the parent water, while its δ13C composition was enriched over that of the parent water due to bacterial methanogenesis. In the case of kinetically precipitated carbonate deposits, the δ18O and δ13C values are out-of-equilibrium relative to that of the parent water due to the faster rate of reaction.  相似文献   

17.
We tested the effectiveness of stable isotopes as recorders of physiological changes that occur during coral bleaching and recovery. Montipora capitata and Porites compressa fragments were bleached in outdoor tanks with seawater temperature raised to 30 °C (treatment corals) for one month. Additional fragments were maintained at 27 °C in separate tanks (control corals). After one month, (0 months recovery), buoyant weight was measured and a subset of fragments was frozen. Remaining fragments were returned to the reef for recovery. After 1.5, 4, and 8 months, fragments were collected, measured for buoyant weight, and frozen. Fragments were analyzed for stable carbon and oxygen isotopic compositions of the skeleton (δ13Cs; δ18Os) and nitrogen and carbon isotopic compositions of the host tissue (δ15Nh; δ13Ch) and zooxanthellae (δ15Nz; δ13Cz). δ13Cs decreased immediately after bleaching in M. capitata, but not in P. compressa. δ18Os of both species failed to record the warming event. During the remaining months of recovery, δ13Cs and δ18Os were more enriched in treatment than control corals due to decreases in calcification and metabolic fractionation during that time. Increased δ15Nh of treatment P. compressa may be due to expelled zooxanthellae during bleaching and recovery. Increased δ15Nz at 1.5 months in treatment fragments of both species reflects the increased incorporation of dissolved inorganic nitrogen to facilitate mitotic cell division and/or chl a/cell recovery. Changes in δ13Ch and δ13Cz at 1.5 months in treatment M. capitata indicated a large increase in heterotrophically acquired carbon relative to photosynthetically fixed carbon. We experimentally show that isotopes in coral skeleton, host tissue and zooxanthellae can be used to verify physiological changes during bleaching and recovery, but their use as a proxy for past bleaching events in the skeletal record is limited.  相似文献   

18.
High-resolution δ18O and δ13C records obtained from seven cores were drilled from ledges of the reef builder gastropod Dendropomapetreaum and used to reconstruct variations in the Levantine basin sea surface temperature, hydrology and productivity during the past 500 years. The δ18O of the aragonite shell of living D. petreaum indicate that skeletal deposition occurs under isotopic equilibrium and faithfully record the temperature and surface water δ18O during summer and autumn. The mean down core δ18O record clearly captures global and local climatic events, such as the Little Ice Age (LIA) and the recent warming of surface waters in the Eastern Mediterranean. Comparison to the Western Mediterranean vermetid δ18O record reveals changes in the freshwater/evaporation budgets of the two basins during cold and warm periods. The Eastern basin had lower surface temperatures and excess evaporation during the LIA and experienced a relatively larger warming and/or a decrease in freshwater/evaporation during the past 70 years. The D. petraeum δ13C is strongly related to δ13C of dissolved inorganic carbon and to the primary productivity of the surface water. The mean down core δ13C record exhibits enrichment during the LIA maximum and a strong depletion trend during the last century. The LIA δ13C enrichment is attributed to an increase in primary production and high nutrient levels which resulted from increased vertical mixing and upwelling. The last century δ13C depletion is mostly related to the increased anthropogenic emissions of 13C depleted carbon dioxide and to a certain decrease in primary production. The data indicate that D. petraeum isotopic signatures are unique proxies for last 500 years high-resolution reconstruction of paleo-oceanographic environments in the Mediterranean and potentially in the sub-tropical Atlantic regions.  相似文献   

19.
Stable carbon and oxygen isotope measurements of biogenic carbonate provide information for reconstructing past oceanic environments. In particular, 18O/16O ratios correlate with the temperature and salinity of seawater and 13C/12C is a proxy for dissolved inorganic carbon in seawater and symbiont photosynthesis. Here, we report 13C/12C and 18O/16O values for skeletons of corals (genus Porites) with various growth rates. In faster-growing corals, 13C/12C and 18O/16O showed out-of-phase annual fluctuations. In slower-growing corals, the isotopes fluctuated in phase. We developed a simple vector notation to show two patterns of 13C/12C annual fluctuation, each with a different offset in relation to 18O/16O annual fluctuation. The phase offset between 13C/12C and 18O/16O annual fluctuations depends on the relative intensities of kinetic isotope effects on calcification and metabolic isotope effects such as photosynthesis. This model might improve our ability to infer past climate and oceanographic conditions from coral skeletons.  相似文献   

20.
δ18O was determined at high spatial resolution (beam diameter ∼30 μm) by secondary ion mass spectrometry (SIMS) across 1-2 year sections of 2 modern Porites lobata coral skeletons from Hawaii. We observe large (>2‰) cyclical δ18O variations that typically cover skeletal distances equivalent to periods of ∼20-30 days. These variations do not reflect seawater temperature or composition and we conclude that skeletal δ18O is principally controlled by other processes. Calcification site pH in one coral record was estimated from previous SIMS measurements of skeletal δ11B. We model predicted skeletal δ18O as a function of calcification site pH, DIC residence time at the site and DIC source (reflecting the inputs of seawater and molecular CO2 to the site). We assume that oxygen isotopic equilibration proceeds at the rates observed in seawater and that only the aqueous carbonate ion is incorporated into the precipitating aragonite. We reproduce successfully the observed skeletal δ18O range by assuming that DIC is rapidly utilised at the calcification site (within 1 h) and that ∼80% of the skeletal carbonate is derived from seawater. If carbonic anhydrase catalyses the reversible hydration of CO2 at the calcification site, then oxygen isotopic equilibration times may be substantially reduced and a larger proportion of the skeletal carbonate could be derived from molecular CO2. Seasonal skeletal δ18O variations are most pronounced in the skeleton deposited from late autumn to winter (and coincide with the high density skeletal bands) and are dampened in skeleton deposited from spring to summer. We observed no annual pattern in sea surface temperature or photosynthetically active radiation variability which could potentially correlate with the coral δ18O. At present we are unable to resolve an environmental cue to drive seasonal patterns of short term skeletal δ18O heterogeneity.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号