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1.
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   

2.
Evidence for ocean acidification in the Great Barrier Reef of Australia   总被引:1,自引:0,他引:1  
Geochemical records preserved in the long-lived carbonate skeleton of corals provide one of the few means to reconstruct changes in seawater pH since the commencement of the industrial era. This information is important in not only determining the response of the surface oceans to ocean acidification from enhanced uptake of CO2, but also to better understand the effects of ocean acidification on carbonate secreting organisms such as corals, whose ability to calcify is highly pH dependent. Here we report an ∼200 year δ11B isotopic record, extracted from a long-lived Porites coral from the central Great Barrier Reef of Australia. This record covering the period from 1800 to 2004 was sampled at yearly increments from 1940 to the present and 5-year increments prior to 1940. The δ11B isotopic compositions reflect variations in seawater pH, and the δ13C changes in the carbon composition of surface water due to fossil fuel burning over this period. In addition complementary Ba/Ca, δ18O and Mg/Ca data was obtained providing proxies for terrestrial runoff, salinity and temperature changes over the past 200 years in this region. Positive thermal ionization mass spectrometry (PTIMS) method was utilized in order to enable the highest precision and most accurate measurements of δ11B values. The internal precision and reproducibility for δ11B of our measurements are better than ±0.2‰ (2σ), which translates to a precision of better than ±0.02 pH units. Our results indicate that the long-term pre-industrial variation of seawater pH in this region is partially related to the decadal-interdecadal variability of atmospheric and oceanic anomalies in the Pacific. In the periods around 1940 and 1998 there are also rapid oscillations in δ11B compositions equivalent changes in pH of almost 0.5 U. The 1998 oscillation is co-incident with a major coral bleaching event indicating the sensitivity of skeletal δ11B compositions to loss of zooxanthellate symbionts. Importantly, from the 1940s to the present-day, there is a general overall trend of ocean acidification with pH decreasing by about 0.2-0.3 U, the range being dependent on the value assumed for the fractionation factor α(B3-B4) of the boric acid and borate species in seawater. Correlations of δ11B with δ13C during this interval indicate that the increasing trend towards ocean acidification over the past 60 years in this region is the result of enhanced dissolution of CO2 in surface waters from the rapidly increasing levels of atmospheric CO2, mainly from fossil fuel burning. This suggests that the increased levels of anthropogenic CO2 in atmosphere has already caused a significant trend towards acidification in the oceans during the past decades. Observations of surprisingly large decreases in pH across important carbonate producing regions, such as the Great Barrier Reef of Australia, raise serious concerns about the impact of Greenhouse gas emissions on coral calcification.  相似文献   

3.
Sr/Ca, B/Ca, Mg/Ca and δ11B were determined at high spatial resolution across ∼1 year of a modern Hawaiian Porites lobata coral by secondary ion mass spectrometry (SIMS). We observe significant variations in B/Ca, Mg/Ca, Sr/Ca and δ11B over short skeletal distances (nominally equivalent to periods of <20 days). This heterogeneity probably reflects variations in the composition of the extracellular calcifying fluid (ECF) from which the skeleton precipitates. Calcification site pH (total scale) was estimated from skeletal δ11B and ranged from 8.3 to 8.8 (± ∼0.1) with a mean of ∼8.6. Sr/Ca and B/Ca heterogeneity is not simply correlated with calcification site pH, as might be expected if Ca-ATPase activity increases the pH and decreases the Sr/Ca and B(OH)4/CO32− ratios of the ECF. We produced a simple model of the ECF composition and the skeleton deposited from it, over a range of calcium transport and carbonate scenarios, which can account for these observed geochemical variations. The relationship between the pH and Sr/Ca of the ECF is dependent on the concentration of DIC at the calcification site. At higher DIC concentrations the ECF has a high capacity to buffer the [H+] changes induced by Ca-ATPase pumping. Conversely, at low DIC concentrations, this buffering capacity is reduced and ECF pH changes more rapidly in response to Ca-ATPase pumping. The absence of a simple correlation between ECF pH and skeletal Sr/Ca implies that calcification occurred under a range of DIC concentrations, reflecting variations in the respiration and photosynthesis of the coral and symbiotic zooxanthellate in the overlying coral tissues. Our observations have important implications for the use of coral skeletons as indicators of palaeo-ocean pH.  相似文献   

4.
The boron isotope composition of marine carbonates is considered to be a seawater pH proxy. Nevertheless, the use of δ11B has some limitations such as the knowledge of the fractionation factor (α4-3) between boric acid and the borate ion and the amplitude of “vital effects” on this proxy that are not well constrained. Using secondary ion mass spectrometry (SIMS) we have examined the internal variability of the boron isotope ratio in the shallow water, symbionts bearing foraminiferan Amphistegina lobifera. Specimens were cultured at constant temperature (24 ± 0.1 °C) in seawater with pH ranging between 7.90 and 8.45. Intra-shell boron isotopes showed large variability with an upper limit value of ≈30‰. Our results suggest that the fractionation factor α4-3 of 0.97352 (Klochko et al., 2006) is in better agreement with our experiments and with direct pH measurements in seawater vacuoles associated with the biomineralization process in these foraminifera. Despite the large variability of the skeletal pH values in each cultured specimen, it is possible to link the lowest calculated pH values to the experimental culture pH values while the upper pH limit is slightly below 9. This variability can be interpreted as follows: foraminifera variably increase the pH at the biomineralization site to about 9. This increase above ambient seawater pH leads to a range in δ11B (Δ11B) for each seawater pH. This Δ11B is linearly correlated with the culture seawater pH with a slope of −13.1 per pH unit, and is independent of the fractionation factor α4-3, or the δ11Bsw through time. It may also be independent of the pKB (the dissociation constant of boric acid) value. Therefore, Δ11B in foraminifera can potentially reconstruct paleo-pH of seawater.  相似文献   

5.
Laboratory experiments on the branching, symbiont-bearing coral genus Porites and Acropora have been carried out to determine the dependence of the skeletal boron isotopic composition (δ11B) on the pH of seawater. The results show a clear relationship similar to previously established empirical calibrations for planktonic foraminifera and inorganic calcite. A −0.6‰ offset exists between P. cylindrica and A. nobilis which is systematic over the pH range of 7.7-8.2. To test whether the δ11B of coral skeletons changes with physiological processes such as photosynthesis and respiration, corals were grown along a depth transect in their natural environment and under controlled conditions in the laboratory at varying light intensities and food supply. Although we also observe an isotopic offset between P. compressa and Montipora verrucosa, neither experimental treatment systematically changed the δ11B of the two species. These findings are encouraging for using the boron isotope paleo-pH proxy in corals, because it appears that seawater pH is the dominant control on the boron isotopic composition in corals.  相似文献   

6.
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   

7.
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   

8.
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   

9.
As paleoceanographic archives, deep sea coral skeletons offer the potential for high temporal resolution and precise absolute dating, but have not been fully investigated for geochemical reconstructions of past ocean conditions. Here we assess the utility of skeletal P/Ca, Ba/Ca and U/Ca in the deep sea coral D. dianthus as proxies of dissolved phosphate (remineralized at shallow depths), dissolved barium (trace element with silicate-type distribution) and carbonate ion concentrations, respectively. Measurements of these proxies in globally distributed D. dianthus specimens show clear dependence on corresponding seawater properties. Linear regression fits of mean coral Element/Ca ratios against seawater properties yield the equations: P/Cacoral (μmol/mol) = (0.6 ± 0.1) P/Casw(μmol/mol) - (23 ± 18), R2 = 0.6, n = 16 and Ba/Cacoral(μmol/mol) = (1.4 ± 0.3) Ba/Casw(μmol/mol) + (0 ± 2), R2 = 0.6, n = 17; no significant relationship is observed between the residuals of each regression and seawater temperature, salinity, pressure, pH or carbonate ion concentrations, suggesting that these variables were not significant secondary dependencies of these proxies. Four D. dianthus specimens growing at locations with Ωarag ? 0.6 displayed markedly depleted P/Ca compared to the regression based on the remaining samples, a behavior attributed to an undersaturation effect. These corals were excluded from the calibration. Coral U/Ca correlates with seawater carbonate ion: U/Cacoral(μmol/mol) = (−0.016 ± 0.003) (μmol/kg) + (3.2 ± 0.3), R2 = 0.6, n = 17. The residuals of the U/Ca calibration are not significantly related to temperature, salinity, or pressure. Scatter about the linear calibration lines is attributed to imperfect spatial-temporal matches between the selected globally distributed specimens and available water column chemical data, and potentially to unresolved additional effects. The uncertainties of these initial proxy calibration regressions predict that dissolved phosphate could be reconstructed to ±0.4 μmol/kg (for 1.3-1.9 μmol/kg phosphate), and dissolved Ba to ±19 nmol/kg (for 41-82 nmol/kg Basw). Carbonate ion concentration derived from U/Ca has an uncertainty of ±31μmol/kg (for ). The effect of microskeletal variability on P/Ca, Ba/Ca, and U/Ca was also assessed, with emphasis on centers of calcification, Fe-Mn phases, and external contaminants. Overall, the results show strong potential for reconstructing aspects of water mass mixing and biogeochemical processes in intermediate and deep waters using fossil deep-sea corals.  相似文献   

10.
A generalized physicochemical model of the response of marine organisms’ calcifying fluids to CO2-induced ocean acidification is proposed. The model is based upon the hypothesis that some marine calcifiers induce calcification by elevating pH, and thus ΩA, of their calcifying fluid by removing protons (H+). The model is explored through two end-member scenarios: one in which a fixed number of H+ is removed from the calcifying fluid, regardless of atmospheric pCO2, and another in which a fixed external-internal H+ ratio ([H+]E/[H+]I) is maintained. The model is able to generate the full range of calcification response patterns observed in prior ocean acidification experiments and is consistent with the assertion that organisms’ calcification response to ocean acidification is more negative for marine calcifiers that exert weaker control over their calcifying fluid pH. The model is empirically evaluated for the temperate scleractinian coral Astrangia poculata with in situ pH microelectrode measurements of the coral’s calcifying fluid under control and acidified conditions. These measurements reveal that (1) the pH of the coral’s calcifying fluid is substantially elevated relative to its external seawater under both control and acidified conditions, (2) the coral’s [H+]E/[H+]I is approximately the same under control and acidified conditions, and (3) the coral removes fewer H+ from its calcifying fluid under acidified conditions than under control conditions. Thus, the carbonate system dynamics of A. poculata’s calcifying fluid appear to be most consistent with the fixed [H+]E/[H+]I end-member scenario. Similar microelectrode experiments performed on additional taxa are required to assess the model’s general applicability.  相似文献   

11.
We used positive thermal ionization mass spectrometry (PTIMS) to generate high precision δ11B records in Porites corals of the mid-late Holocene from the South China Sea (SCS). The δ11B values of the Holocene corals vary significantly, ranging from 22.2‰ to 25.5‰. The paleo-pH records of the SCS, reconstructed from the δ11B data, were not stable as previously thought but show a gradual increase from the Holocene thermal optimal and a sharp decrease to modern values. The latter is likely caused by the large amount of anthropogenic CO2 emissions since the Industrial Revolution but variations of atmospheric pCO2 cannot explain the pH change of the SCS before the Industrial Revolution. We suggest that variations of monsoon intensity during the mid-late Holocene may have driven the sea surface pH increase from the mid to late Holocene. Results of this study indicate that the impact of anthropogenic atmospheric CO2 emissions may have reversed the natural pH trend in the SCS since the mid-Holocene. Such ocean pH records in the current interglacial period can help us better understand the physical and biological controls on ocean pH and possibly predict the long-term impact of climate change on future ocean acidification.  相似文献   

12.
Boron isotope compositions (δ11B) and B concentrations of rains and snows were studied in order to characterize the sources and fractionation processes during the boron atmospheric cycle. The 11B/10B ratios of instantaneous and cumulative rains and snows from coastal and continental sites show a large range of variations, from −1.5 ± 0.4 to +26.0 ± 0.5‰ and from −10.2 ± 0.5 to +34.4 ± 0.2‰, respectively. Boron concentrations in rains and snows vary between 0.1 and 3.0 ppb. All these precipitation samples are enriched in 10B compared to the ocean value (δ11B = +39.5‰). An empirical rain-vapour isotopic fractionation of +31‰ is estimated from three largely independent methods. The deduced seawater-vapour fractionation is +25.5‰, with the difference between the rain and seawater fractionations principally reflecting changes in the speciation of boron in the liquid with ∼100% B(OH)3 present in precipitations. A boron meteoric water line, δD = 2.6δ11B − 133, is proposed which describes the relationship between δD and δ11B in many, but not all, precipitations. Boron isotopic compositions of precipitations can be related to that of the seawater reservoir by the seawater-vapour fractionation and one or more of (1) the rain-vapour isotopic fractionation, (2) evolution of the δ11B value of the atmospheric vapour reservoir via condensation-precipitation processes (Rayleigh distillation process), (3) any contribution of vapour from the evaporation of seawater aerosols, and (4) any contribution from particulate matter, principally sea salt, continental dust and, perhaps more regionally, anthropogenic sources (burning of biomass and fossil fuels). From the δ11B values of continental precipitations, a sea salt contribution cannot be more than a percent or so of the total B in precipitation over these areas.  相似文献   

13.
We examined δ13C values of shallow and deep-water scallop shells as well as δ13C of dissolved inorganic carbon (DIC) from the Bay of Brest in western Brittany. Time series of shell calcite δ13C do not reflect seasonal variation in seawater δ13C, but rather show a consistent pattern of decreasing δ13C with age, suggesting a metabolic effect rather than direct environmental control. This δ13C trend reflects an increasing contribution of metabolic CO2 to skeletal carbonate throughout ontogeny, although this respired CO2 does not seem to be the major source of skeletal carbon (contribution of only 10% over the first year of life). We propose a model of this effect that depends on the availability of metabolic carbon relative to the carbon requirements for calcification. A ratio of “respired to precipitated carbon” is calculated, and represents the amount of metabolic carbon available for calcification. Interestingly, this ratio increases throughout ontogeny suggesting a real increase of metabolic carbon utilization into the skeleton relative to carbon from seawater DIC. This ratio allows us to separate two different populations of Pecten maximus of different water depth. It is therefore suggested that shell δ13C might be used to track differences in the metabolic activity of scallops from different populations.  相似文献   

14.
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   

15.
Low temperature vent fluids (<91 °C) issuing from the ultramafic-hosted hydrothermal system at Lost City, 30°N Mid-Atlantic Ridge, are enriched in dissolved volatiles (H2,CH4) while attaining elevated pH values, indicative of the serpentization processes that govern water/rock interactions deep in the oceanic crust. Here, we present a series of theoretical models to evaluate the extent of hydrothermal alteration and assess the effect of cooling on the systematics of pH-controlled B aqueous species. Peridotite-seawater equilibria calculations indicate that the mineral assemblage composed of diopside, brucite and chrysotile likely dictates fluid pH at moderate temperature serpentinization processes (<300 °C), by imposing constraints on the aCa++/a2H+ ratios and the activity of dissolved SiO2. Based on Sr abundances and the 87Sr/86Sr isotope ratios of vent fluids reported from Lost City, estimated water/rock mass ratios (w/r = 2-4) are consistent with published models involving dissolved CO2 and alkane concentrations. Combining the reported δ18O values of vent fluids (0.7‰) with such w/r mass ratios, allows us to bracket subseafloor reaction temperatures in the vicinity of 250 °C. These estimates are in agreement with previous theoretical studies supporting extensive conductive heat loss within the upflow zones. Experimental studies on peridotite-seawater alteration suggest that fluid pH increases during cooling which then rapidly enhances boron removal from solution and incorporation into secondary phases, providing an explanation for the highly depleted dissolved boron concentrations measured in the low temperature but alkaline Lost City vent fluids. Finally, to account for the depleted 11B composition (δ11B ∼25-30‰) of vent fluids relative to seawater, isotopic fractionation between tetrahedrally coordinated aqueous boron species with BO3-bearing mineral sites (e.g. in calcite, brucite) is proposed.  相似文献   

16.
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   

17.
南海全新世大暖期海表水的高酸性证据   总被引:1,自引:1,他引:0       下载免费PDF全文
利用正热电离质谱高精度测定了南海化石珊瑚样品的δ11B 比值,定量地重建了中晚全新世以来南海海表水的pH值。结果显示南海海表水pH值并不像预先设想的那样稳定。其中古海水pH值最低为6100aB.P.前的7.91;   最高为4300aB.P.前和1200aB.P.前的8.29。南海海表水pH值从全新世大暖期开始,整体上呈缓慢增加趋势,而到现代以后明显下降。珊瑚δ11B记录表明南海海表水在中晚全新世有两个高酸性的时期,一个发生在全新世大暖期,一个为现代。全新世大暖期时南海出现的高海平面、偏强的东亚夏季风和偏弱的冬季风可能是导致其海表水出现高酸性的原因。现代南海海表水pH值显著偏低,背离了中晚全新世以来南海海表水pH值逐渐增加的趋势,这很可能说明人类大量排放的CO2确实改变了南海海表水自然变化的规律,南海在变酸。  相似文献   

18.
The reef corals Pocillopora damicornis and Montipora verrucosa were cultured under various controlled temperature and light conditions. The corals were analyzed for growth rate, tissue pigment content and skeletal 13C and 18O. Coral skeletal δ13C values varied with light dose and correlated with changes in zooxanthellar pigment. The δ13C values of skeletal aragonite seem to be modified by oxidation of photosynthetically produced organic matter.Functionally significant relationships between coral skeletal δ18O values and temperature have been determined. The temperature coefficients of the δ18O values [?4.4°C (%.)?1] are similar to the first order coefficient in the equilibrium paleotemperature equation, but the δ18O values have taxonomically consistent offsets from equilibrium. The offsets may be attributed to the coral metabolism with slight but statistically significant differences between the two genera. Environmental and metabolic variables other than temperature have little or no effect on skeletal δ18O.  相似文献   

19.
Pagani et al. [Pagani M., Lemarchand D., Spivack A., and Gaillardet J. (2005). A critical evaluation of the boron isotope-pH proxy: the accuracy of ancient ocean pH estimates. Geochim. Cosmochim. Acta69(4), 953-961] use data from previous boron isotope studies to suggest that the fractionation between boric acid and borate in seawater as well as the history of δ11B in seawater are poorly understood, thus limiting our ability to capture realistic ocean pH with this proxy. Although we agree with the authors that the long recognized uncertainty in the secular variation of δ11Bseawater imposes a temporal limit on paleo-pH reconstructions, their evaluation of the δ11B/pH relationship in carbonates is flawed. Potential complications from vital, temperature and dissolution effects reported in that paper are based on studies that are experimentally and/or analytically poorly constrained. Using published validation studies we will demonstrate that many of the problems outlined by Pagani et al. have already been addressed, or are based on misinterpretations of previous work. Most importantly, statistical evaluation suggests empirical data are best described by a fractionation of ∼20‰. Recent paleoreconstructions confirm that the boron isotope proxy can be used with confidence, if sample selection and analyses are done carefully.  相似文献   

20.
The boron isotope-pH technique is founded on a theoretical model of carbonate δ11B variation with pH that assumes that the boron isotopic composition of carbonates mirrors the boron isotopic composition of borate in solution (δ11Bcarb = δ11Bborate). Knowledge of the fractionation factor for isotope exchange between boric acid and borate in solution (α4-3), the equilibrium constant for the dissociation of boric acid (pKB*), as well as the isotopic composition of boron in seawater (δ11Bsw) are required parameters of the model.The available data suggests that both the value of α4-3 and the history of δ11Bsw are poorly constrained. However, if one assumes that δ11Bcarb = δ11Bborate, an empirical value for α4-3 can be estimated from the results of inorganic carbonate precipitation experiments. This exercise yields an α4-3 value of ∼0.974 in accordance with recent theoretical estimates, but substantially deviates from the theoretical value of 0.981 often used to estimate paleo-ocean pH. Re-evaluation of ocean pH using an α4-3 value of 0.974 and published foraminiferal δ11B values for the Cenozoic yield pH estimates that are relatively invariant, but unrealistically high (∼8.4-8.6). Uncertainty increases as foraminiferal ‘vital effects’ are considered and different models for secular changes in seawater δ11B are applied.The inability to capture realistic ocean pH possibly reflects on our understanding of the isotopic relationship between carbonate and borate, as well as the mechanism of boron incorporation in carbonates. Given the current understanding of boron systematics, pH values estimated using this technique have considerable uncertainty, particularly when reconstructions exceed the residence time of boron in the ocean.  相似文献   

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