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 531 毫秒
1.
A suite of elements (B, Na, Mg, S, K, Ca, V, Mn, Cr, Sr, and Ba) was measured in aragonitic shells of the estuarine bivalve Corbula amurensis, the Asian clam, using the Sensitive High-Resolution Ion MicroProbe with Reverse Geometry (SHRIMP RG). Our initial intent was to explore potential geochemical proxy relationships between shell chemistry and salinity (freshwater inflow) in northern San Francisco Bay (SFB). In the course of this study we observed variations in shell trace element to calcium ([M]/Ca) ratios that could only be attributed to internal biological processes. This paper discusses the nature and sources of internal trace element variability in C. amurensis shells related to the shell organic fraction and shell calcification rates. The average organic content of whole C. amurensis shells is 19%. After treating whole powdered shells with an oxidative cleaning procedure to remove organic matter, shells contained on average 33% less total Mg and 78% less total Mn. Within our analytical uncertainty, Sr and Ba contents were unchanged by the removal of organic matter. These results show that aragonitic C. amurensis shells have a large component of non-lattice-bound Mg and Mn that probably contribute to the dissimilarity of [M]/Ca profiles among five same-sized shells. Non-lattice-bound trace elements could complicate the development and application of geochemical proxy relationships in bivalve shells. Because B, Ba and Sr occur exclusively in shell aragonite, they are good candidates for external proxy relationships. [M]/Ca ratios were significantly different in prismatic and nacreous aragonite and in two valves of the same shell that had different crystal growth rates. Some part of these differences can be attributed to non-lattice-bound trace elements associated with the organic fraction. The differences in [M]/Ca ratios were also consistent with the calcification rate-dependent ion transport model developed by Carré et al. [Carré M., Bentaleb I., Bruguier O., Ordinola E., Barrett N. T. and Fontugne M. (2006) Calcification rate influence on trace element concentrations in aragonitic bivalve shells: evidences and mechanisms. Geochim. Cosmochim. Acta70, 4906-4920] which predicts that [M]/Ca ratios increase as calcification rates increase and Ca2+ channel specificity decreases. This result, in combination with the possibility that there were ontogenetic variations in growth rates among individuals younger than 2 years, underscores the need to develop an independent age model for C. amurensis shells. If growth-rate effects on lattice-bound [M]/Ca ratios can be constrained, it may yet be possible to develop high-resolution geochemical proxies for external solution chemistry in low-salinity regions of SFB.  相似文献   

2.
Continuous culture of the coccolithophorid Emiliania huxleyi reveals that coccolith Sr/Ca ratios depend on temperature and growth rate. At a constant temperature of 18°C, coccolith Sr/Ca ratios increased nearly 15% as growth rate increased from 0.1 to 1.5 divisions per day and calcification rate increased from 1.5 to 50 pg calcite per cell per day. When temperature increased from 7 to 26°C, Sr/Ca ratios increased by more than 25% (i.e., 1%/1°C), although the range in growth and calcification rates was the same as for experiments at constant temperature. The temperature dependence of Sr/Ca ratios in coccoliths is consistent with that observed in planktonic foraminifera and abiogenic calcites, suggesting that it is controlled by thermodynamic processes. However, the positive correlation of coccolith Sr/Ca with temperature contrasts with field studies in the equatorial Pacific, where Sr/Ca ratios are highest at the locus of maximum upwelling and productivity despite depressed temperatures. This paradox may reflect different calcification rate effects between E. huxleyi and the other species dominating assemblages in the equatorial Pacific sediments, which may be resolved by new techniques for separation of monospecific coccolith samples from sediments. Models of crystal growth indicate that kinetic effects on Sr partitioning in calcite due to surface enrichment could explain the Sr/Ca variations observed in constant temperature experiments but not the larger amplitude calcification rate effects observed in equatorial Pacific sediments. Despite the dual influence of temperature and growth rate on coccolith Sr/Ca, coccolith Sr/Ca correlates with “b,” the slope of the dependence of carbon isotope fractionation in biomarkers (εp) on CO2(aq) at a range of growth rates and temperatures. Consequently, using coccolith Sr/Ca in combination with alkenone εp may improve paleo-CO2 determinations.  相似文献   

3.
Over the last decade, sea surface temperature (SST) reconstructed from the Mg/Ca ratio of foraminiferal calcite has increasingly been used, in combination with the δ18O signal measured on the same material, to calculate the δ18Ow, a proxy for sea surface salinity (SSS). A number of studies, however, have shown that the Mg/Ca ratio is also sensitive to other parameters, such as pH or , and salinity. To increase the reliability of foraminiferal Mg/Ca ratios as temperature proxies, these effects should be quantified in isolation. Individuals of the benthic foraminifera Ammonia tepida were cultured at three different salinities (20, 33 and 40 psu) and two temperatures (10-15 °C). The Mg/Ca and Sr/Ca ratios of newly formed calcite were analyzed by Laser Ablation ICP-MS and demonstrate that the Mg concentration in A. tepida is overall relatively low (mean value per experimental condition between 0.5 and 1.3 mmol/mol) when compared to other foraminiferal species, Sr being similar to other foraminiferal species. The Mg and Sr incorporation are both enhanced with increasing temperatures. However, the temperature dependency for Sr disappears when the distribution factor DSr is plotted as a function of calcite saturation state (Ω). This suggests that a kinetic process related to Ω is responsible for the observed dependency of Sr incorporation on sea water temperature. The inferred relative increase in DMg per unit salinity is 2.8% at 10 °C and 3.3% at 15 °C, for the salinity interval 20-40 psu. This implies that a salinity increase of 2 psu results in enhanced Mg incorporation equivalent to 1 °C temperature increase. The DSr increase per unit salinity is 0.8% at 10 °C and 1.3% at 15 °C, for the salinity interval 20-40 psu.  相似文献   

4.
This study explores the potential of intertidal Protothaca staminea shells as high-resolution geochemical archives of environmental change in a coastal upwelling region. Mg/Ca and Sr/Ca ratios were analyzed by excimer laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) at sub-weekly temporal resolution in shells growing ∼1 mm per month. Growth patterns of a modern P. staminea shell from Humboldt Bay, California, collected in December 1999 made it possible to infer a lifespan from 1993 to 1998. Growth hiatuses in the shell may have excluded records of extreme events. Mg/Ca ratios appeared to be partly controlled by water temperature; the correlation coefficient between temperature and Mg/Ca was r = 0.71 in one of four growth increments. Significant year-to-year differences in the sensitivity of Mg/Ca to temperature in P. staminea could not be explained, however. Sr/Ca ratios appeared to be more closely related to shell growth rate. Oxygen isotopes, measured at 2-week temporal resolution in the same shell, did not show a clear relation to local temperature in summer, possibly because temperatures were higher and less variable at the King Salmon mudflat, where the shell was collected, than in the main channel of Humboldt Bay, where water properties were monitored. Negative shell δ13C values (<−0.5‰) marked spring and summer coastal upwelling events.The Mg contents of P. staminea midden shells dated to ∼3 ka and ∼9 ka were significantly lower than in the modern shell. This may have resulted from degradation of a Mg-rich shell organic matrix and precluded quantitative interpretation of the older high-resolution records. Elevated δ13C values in the ∼3 ka shell suggested that the individual grew in highly productive or stratified environment, such as a shallow coastal embayment or lagoon.  相似文献   

5.
This study was designed to investigate the effect of light and temperature on Sr/Ca and Mg/Ca ratios in the skeleton of the coral Acropora sp. for the purpose of evaluating temperature proxies for paleoceanographic applications. In the first experiment, corals were cultivated under three light levels (100, 200, 400 μmol photons m−2 s−1) and constant temperature (27 °C). In the second experiment, corals were cultivated at five temperatures (21, 23, 25, 27, 29 °C) and constant light (400 μmol photons m−2 s−1). Increasing the water temperature from 21 to 29 °C, induced a 5.7-fold increase in the rate of calcification, which induced a 30% increase in the Mg/Ca ratio. In contrast, by increasing the light level by a factor of 4, the rate of calcification was increased only by a factor of 1.7, with a corresponding 9% increase in the Mg/Ca ratio. Thus, the relative change in the calcification rate in the two experiments (5.7 vs. 1.7) scales with the corresponding relative change in Mg/Ca ratio (30% vs. 9%). We conclude that there is a strong biological control on the incorporation of Mg.For Sr/Ca, good correlations were also observed with water temperature and the calcification rate induced by temperature changes. However, in sharp contrast with the Mg/Ca ratio, a temperature-induced 5.7-fold increase in the calcification rate only induced a 4.5% change (decrease) in the Sr/Ca ratio. An important finding for paleoceanographic applications is that the Sr/Ca ratio did not appear to be sensitive to changes in the light level, or to changes in calcification rate induced by changes in the light level. Thus, in this study, water temperature was found to be the dominant parameter controlling the skeletal Sr/Ca ratio.  相似文献   

6.
A 4-year study in a central Texas cave quantifies multiple mechanisms that control dripwater composition and how these mechanisms vary at different drip sites. We monitored cave-air compositions, in situ calcite growth, dripwater composition and drip rate every 4-6 weeks. Three groups of drip sites are delineated (Groups 1-3) based on geochemical variations in dripwater composition. Quantitative modeling of mineral-solution reactions within the host carbonate rock and cave environments is used to identify mechanisms that can account for variations in dripwater compositions. The covariation of Mg/Ca (and Sr/Ca) and Sr isotopes is key in delineating whether Mg/Ca and Sr/Ca variations are dictated by water-rock interaction (i.e., calcite or dolomite recrystallization) or prior calcite precipitation (PCP). Group 1 dripwater compositions reflects a narrow range of the extent of water-rock interaction followed by varying amounts of prior calcite precipitation (PCP). Group 2 dripwater compositions are controlled by varying amounts of water-rock interaction with little to no PCP influence. Group 3 dripwater compositions are dictated by variable extents of both water-rock interaction and PCP. Group 1 drip sites show seasonal variations in dripwater Mg/Ca and Sr/Ca, whereas the other drip sites do not. In contrast to the findings of most previous dripwater Mg/Ca-Sr/Ca studies, these seasonal variations (at Group 1 drip sites) are independent of changes in water flux (i.e., rainfall and/or drip rate), and instead significantly correlate with changes in cave-air CO2 concentrations. These results are consistent with lower cave-air CO2, related to cool season ventilation of the cave atmosphere, enhancing calcite precipitation and leading to dripwater geochemical evolution via PCP. Group 1 dripwater Mg/Ca and Sr/Ca seasonality and evidence for PCP as a mechanism that can account for that seasonality, have two implications for many other regions where seasonal ventilation of caves is likely: (1) speleothem trace-element records may provide seasonal signals, and (2) such records may be biased toward recording climate conditions during the season when calcite is depositing. Additionally, we use our results to construct a forward model that illustrates the types of speleothem Mg/Ca and Sr/Ca variations that would result from varying controls on dripwater compositions. The model provides a basis for interpreting paleo-dripwater controls from high frequency Mg/Ca and Sr/Ca variations for speleothems from caves at which long term monitoring studies are not feasible.  相似文献   

7.
Three planktonic foraminiferal species Globigerina bulloides, Neogloboquadrina pachyderma (d), and Globorotalia inflata collected from core-tops spanning 35° to 65°N in the North Atlantic were used for U/Ca and Mg/Ca and foraminiferal shell weight analyses. Except for U/Ca in G. bulloides calcified under warm conditions (>∼13 °C), U/Ca ratios in all three studied species increase with decreasing latitude and show strong positive correlations with Mg/Ca ratios. A dissolution effect on planktonic U/Ca is suggested by decreased shell weight and U/Ca and Mg/Ca ratios for shells from very deep water depth (>4.4 km) along the latitudinal transect. G. bulloides from down core samples in the North Atlantic show low U/Ca ratios during the last glacial and high ratios during the Holocene, similar to the Mg/Ca evolution trend. In general, our data indicate that the U incorporation into planktonic foraminiferal carbonates is strongly influenced by calcification temperature, although U/Ca in G. bulloides may be affected by seawater carbonate ion concentration under warm conditions and/or other factors.  相似文献   

8.
微量元素是岩溶洞穴沉积中非常重要的一类古气候环境替代指标,为近20年来国内外的一个研究热点。总结前人的研究,主要取得了以下一些重要认识:（1）洞穴上覆土壤 和围岩是洞穴次生碳酸盐沉积Mg、Sr的主要来源;（2）Mg/Ca与Sr/Ca能够指示气候环境变化,但需结合其它指标综合考虑。（3）洞穴次生碳酸盐沉积Mg/Ca与Sr/Ca受多种气候环境因素（包括土壤和围岩的组成和性质、水-岩相互作用、先期碳酸盐沉积、分配系数等）影响,其古气候环境指示意义具有多解性;（4）矿物结晶作用对Mg/Ca与Sr/Ca有一定的影响,特别是文石在向方解石转变的过程中容易丢失Mg、Sr,此外,杂质的混入也将抑制Mg、Sr进入方解石,从而引起洞穴次生碳酸盐沉积Mg/Ca与Sr/Ca比值的变化。今后应进一步加强对石笋中这些微量元素的影响机制研究,尤其是对一些影响因素与微量元素含量变化之间的定量关系进行探讨。   相似文献   

9.
Trace elements in calcareous organisms have been widely used for paleoclimatic studies. However, the factors controlling their incorporation into mollusc shells are still unclear. We studied here the Sr, Mg, Ba and Mn serial records in the shells of two aragonitic marine bivalve species: Mesodesma donacium and Chione subrugosa from the Peruvian Coast. The elemental concentrations were compared to local temperature and salinity records. The relationships with crystal growth rate G were investigated thanks to well defined periodic growth structures providing a precise shell chronology. Our results show that for both species, environmental parameters only have minor influence, whereas crystal growth rate strongly influences trace elements concentrations, especially for Sr (explaining up to 74% of the variance). The relationship between G and Sr/Ca exhibits variability among the shells as well as inside the shells. For a same growth rate value, Sr/Ca values are higher in more curved shell sections, and the growth rate influence is stronger as well. We show that intercellular and Ca2+-pump pathways cannot support the calcification Ca2+ flux, leading us to propose an alternative mechanism for ionic transport through the calcifying mantle, implying a major role for calcium channels on mantle epithelial cell membranes. In this new calcification model, Sr/Ca shell ratios is determined by Ca2+-channel selectivity against Sr2+, which depends (i) on the electrochemical potential imposed by the crystallisation process and (ii) on the Ca2+-channel density per surface unit on mantle epithelia.  相似文献   

10.
《Chemical Geology》2007,236(3-4):339-349
We present a new high precision analytical method for the determination of Mg/Ca and Sr/Ca ratios in carbonates using an inductively coupled plasma quadrupole mass spectrometer (ICP-QMS) with a 650-W cold plasma technique and a desolvation introduction system. Signal intensities are detected in pulse-counting mode and Mg/Ca and Sr/Ca ratios are calculated directly from intensity ratios of 24Mg/43Ca and 86Sr/43Ca using external matrix-matched standards for every 4–5 samples to correct for instrumental mass discrimination and low-frequency ratio drift. Significant matrix effect of Ca content on Mg/Ca determination (− 0.018 Mg/Ca (mmol/mol)/[Ca] (ppm)), can be overcome by diluting [Ca] to 6–8 ppm in the sample solution or using an empirical correction. The Sr/Ca ratio affects the Mg/Ca determination, with a factor of − 0.32% Mg/Ca per mmol/mol. This is mainly caused by the influence of doubly charged 86Sr, which biases the intensity measurement of the 43Ca+ ion beam. This effect results in a trivial offset of less than 0.1% on Mg/Ca measurements for Quaternary foraminiferal and coral samples. The internal precision of our method ranges from 0.1 to 0.2%. Replicate measurements made on standards and samples show long-term external uncertainties (2σ) of Mg/Ca = 0.84% and Sr/Ca = 0.49%. The minimum sample size requirement is only 3.5 μg of carbonate. The application of this newly developed technique on the planktonic foraminifera Globigerinoides ruber from a core recovered in the southern South China Sea yields a glacial–interglacial difference in sea surface temperature (SST) of 3 °C. Three-year coral Sr/Ca data suggest that the seasonal SST ranged from 22.6–23.8 °C in winter to 26.9–27.9 °C in summer in Nanwan, south Taiwan, during 2000–2002. The coral-Sr/Ca inferred SSTs in 2002 match well with instrumental records, which demonstrates the validity of this ICP-QMS method.  相似文献   

11.
Trace element ratios in the Bt2 stalagmite from Botuverá cave, Southern Brazil, are explored as a proxy for changes in the local rainfall recharge during the last 116 ky. BP Mg/Ca and Sr/Ca ratios, measured with an electron microprobe, are significantly positively correlated with one another throughout the entire record, and vary in a way that is very consistent with variations of δ18O in the same speleothem during the last glacial period. We suggest that prior calcite precipitation in the vadose zone of the cave system is the main factor affecting the incorporation of Mg and Sr into calcite of the stalagmite. This interpretation is supported by trace element correlation patterns and by results from a hydrochemistry study performed in a cave located in the same region and in a similar environmental setting. Therefore, we conclude that higher (lower) Mg/Ca and Sr/Ca values are associated with lower (higher) levels of recharge into the karstic aquifer, as such conditions lead to an increase (decrease) in the volume of calcite precipitated in the unsaturated zone above the cave during dry (wet) climate periods.Trace element variations point to generally dryer (wetter) conditions during lower (high) phases of summer insolation in the southern hemisphere. These periods coincide with decreased (increased) activity of the South American summer monsoon, as revealed by δ18O stalagmite records. In addition trace element variations show that rather wet conditions persisted throughout most of the last glacial period from approximately 70 to 17 ky BP. We suggest that during this period the glacial boundary conditions, especially ice volume buildup in the northern hemisphere, played an important role for monsoon rainfall intensification in the region.  相似文献   

12.
Magnesium/calcium, Sr/Ca, and Na/Ca atom ratios were determined in the calcite and aragonite regions of Mytilus edulis shells which were grown in semi-artificial ‘seawater’ solutions having varying Mg/Ca, Sr/Ca, and Na/Ca ratios. These ratios were measured by instrumental neutron activation, atomic absorption, and electron microprobe analytical techniques. Strontium/calcium ratios in both calcite and aragonite were linearly proportional to solution Sr/Ca ratios. Magnesium/calcium ratios in calcite increased exponentially when solution Mg/Ca ratios were raised above the normal seawater ratio; whereas in aragonite, Mg/Ca ratios increased linearly with increases in solution Mg/Ca ratios. Sodium/calcium and sulfur/calcium ratios in calcite covaried with Mg/Ga solution ratios. Conversely, in aragonite, Na/Ca ratios varied linearly with solution Na/Ca ratios.Magnesium is known to inhibit calcite precipitation at its normal seawater concentration. We infer from the results of the work reported here that Mytilus edulis controls the Mg activity of the outer extrapallial fluid, thus facilitating the precipitation of calcitic shell. Increases in sulfur content suggest that changes in shell organic matrix content occur as a result of environmental stress. Certain increases in Mg content may also be correlated to stress. Sodium/calcium variations, and their absolute amounts in calcite and aragonite, are best explained by assuming that a substantial amount of Na is adsorbed on the calcium carbonate crystal surface. Strontium/calcium ratios show more promise than either Mg/Ca or Na/Ca ratios as seawater paleochemistry indicators, because the Sr/Ca distribution coefficients for both aragonite and calcite are independent of seawater Ca and Sr concentrations.  相似文献   

13.
The Sr/Ca of aragonitic coral skeletons is a commonly used palaeothermometer. However skeletal Sr/Ca is typically dominated by weekly-monthly oscillations which do not reflect temperature or seawater composition and the origins of which are currently unknown. To test the impact of transcellular Ca2+ transport processes on skeletal Sr/Ca, colonies of the branching coral, Pocillopora damicornis, were cultured in the presence of inhibitors of Ca-ATPase (ruthenium red) and Ca channels (verapamil hydrochloride). The photosynthesis, respiration and calcification rates of the colonies were monitored throughout the experiment. The skeleton deposited in the presence of the inhibitors was identified (by 42Ca spike) and analysed for Sr/Ca and Mg/Ca by secondary ion mass spectrometry. The Sr/Ca of the aragonite deposited in the presence of either of the inhibitors was not significantly different from that of the solvent (dimethyl sulfoxide) control, although the coral calcification rate was reduced by up to 66% and 73% in the ruthenium red and verapamil treatments, respectively. The typical precision (95% confidence limits) of mean Sr/Ca determinations within any treatment was <±1% and differences in skeletal Sr/Ca between treatments were correspondingly small. Either Ca-ATPase and Ca channels transport Sr2+ and Ca2+ in virtually the same ratio in which they are present in seawater or transcellular processes contribute little Ca2+ to the skeleton and most Ca is derived from seawater transported directly to the calcification site. Variations in the activities of Ca-ATPase and Ca-channels are not responsible for the weekly-monthly Sr/Ca oscillations observed in skeletal chronologies, assuming that the specificities of Ca transcellular transport processes are similar between coral genera.  相似文献   

14.
Calcite Mg/Ca is usually assumed to vary linearly with solution Mg/Ca, that a constant partition coefficient describes the relationship between these two ratios. Numerous published empirical datasets suggests that this relationship is better described by a power function. We provide a compilation of these literature data for biotic and abiotic calcite in the form of Calcite Mg/Ca = F(Solution Mg/Ca)H, where F and H are empirically determined fitting parameters describing the slope and deviation from linearity, respectively, of the function. This is equivalent to Freundlich sorption behavior controlling Mg incorporation in calcite. Using a power function, instead of a partition coefficient, lowers Phanerozoic seawater Mg/Ca estimates based on echinoderm skeletal material by, on average, 0.5 mol/mol from previous estimates.These functions can also be used to model the primary skeletal calcite Mg/Ca of numerous calcite phases through geologic time. Such modeling suggests that the Mg/Ca of all calcite precipitated from seawater has varied through the Phanerozoic in response to changing seawater Mg/Ca and that the overall range in Mg/Ca measured among various calcite phases would be greatest when seawater Mg/Ca was also high (e.g., “aragonite seas”) and lowest when seawater Mg/Ca was low (e.g., “calcite seas”). It follows that, during times of “calcite seas” when the seawater Mg/Ca is presumed to have been lower, deposition of calcite with low Mg contents would have resulted in a depressed drive for diagenetic stabilization of shelfal carbonate and, in turn, lead to greater preservation of crystal and skeletal microfabrics and primary chemistries in biotic and abiotic calcites.  相似文献   

15.
In order to investigate the interindividual and ontogenetic effects on Mg and Sr incorporation, magnesium/calcium (Mg/Ca) and strontium/calcium (Sr/Ca) ratios of cultured planktonic foraminifera have been determined. Specimens of Globigerinoides sacculifer were grown under controlled physical and chemical seawater conditions in the laboratory. By using this approach, we minimised the effect of potential environmental variability on Mg/Ca and Sr/Ca ratios. Whereas temperature is the overriding control of Mg/Ca ratios, the interindividual variability observed in the Mg/Ca values contributes 2-3 °C to the apparent temperature variance. Interindividual variability in Sr/Ca ratios is much smaller than that observed in Mg/Ca values. The variability due to ontogeny corresponds to −0.43 mmol/mol of Mg/Ca ratio per chamber added. This translates into an apparent decrease of ∼1 °C in Mg/Ca-based temperature per ontogenetic (chamber) stage. No significant ontogenetic effect is observed on Sr incorporation. We conclude that the presence of a significant ontogenetic effect on Mg incorporation can potentially offset Mg/Ca-based temperature reconstructions. We propose two new empirical Mg/Ca-temperature equation based on Mg/Ca measurements of the last four ontogenetic (chamber) stages and whole foraminiferal test: Mg/Ca = (0.55(±0.03) − 0.0002(±4 × 10−5) MSD) e0.089T and, Mg/Ca = (0.55(±0.03) − 0.0001(±2 × 10−5) MSD) e0.089T, respectively, where MSD corresponds to the maximum shell diameter of the individual.  相似文献   

16.
Li/Ca ratios were measured in planktonic and benthic foraminifera from a variety of hydrographic settings to investigate the factors influencing lithium incorporation into foraminiferal tests including temperature, dissolution, pressure, and interspecies differences. Down-core measurements of planktonic (Orbulina universa, Globigerinoides ruber, and Globigerinoides sacculifer) and benthic foraminifera (calcitic Cibicides wuellerstorfi and aragonitic Hoeglandina elegans) show a systematic variation in Li/Ca with δ18O through the last glacial-interglacial transition. All species examined exhibit an increase in Li/Ca between 14 to 50% from the Holocene to the last glacial maximum. Li/Ca generally increases with decreasing temperature as seen in a latitudinal transect of planktonic O. universa and down-slope benthic species along the Bahama Bank margins. Postdepositional dissolution possibly causes a decrease in planktonic foraminiferal Li/Ca along the Sierra Leone Rise, and increased water depth causes a decrease in benthic foraminiferal Li/Ca in the deep Caribbean. However, none of these effects are sufficient to account for the observed glacial-interglacial changes. Physiological factors such as calcification rate may affect the Li/Ca content of foraminiferal calcite. The calcification rate in turn may be a function of carbonate ion concentration of ambient ocean water. This work shows that incorporation of lithium by foraminifera appears to be influenced by factors other than seawater composition and does not appear to be dominated by changes in temperature, dissolution, or pressure. We hypothesize that the consistent increase in foraminiferal Li/Ca during the last glacial maximum may be linked to changes in seawater carbonate ion concentration. Important parameters to be tested include calcification rate and foraminiferal test size and weight. If foraminiferal Li/Ca is dominantly controlled by calcification rate as a function of seawater carbonate ion concentration, then Li/Ca may act as a proxy of past atmospheric CO2.  相似文献   

17.
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   

18.
We test for and calibrate a proxy for ocean temperature based on the skeletal composition of the widely distributed, deep-sea gorgonians in the family Isididae (bamboo corals), through use of three complementary methods: a short-term comparison of element/Ca ratios to a four-year temperature record, a long-term comparison with oceanographic records spanning forty years, and a geographic comparison of Isidids collected at sites ranging from the tropics to Antarctica. The assays consistently support a temperature-dependency for Mg/Ca ratios and suggest S/Ca is indirectly affected by temperature, but indicate little or no effect of temperature on P/Ca and Sr/Ca. The consensus relationship between Mg/Ca and temperature for Isidid calcite from the comparisons with the temperature time-series is T = −0.505 + 0.048 Mg/Ca, where T is in °C, Mg/Ca is in mmol/mol, and the applicable range is 3-6 °C. The results of the geographic assay, though imprecise, suggest the applicable range extends to temperatures below freezing. The scatter of data points around the regression of temperature and Mg/Ca is wide in all assays. This could reflect the effect of factors other than temperature on Mg/Ca ratios, but is also likely to reflect limitations of the field data, the effects of assumed constant growth rates in the corals and instrumental analytical error. The combined effects of micro-scale variability in growth rates and wide confidence intervals for each data point suggests that environmental reconstruction from Isidid internode calcite from sparse data or at time scales less than decades be done with caution. Comparisons within and among colonies do not indicate strong vital effects on ontogenetic variability in the corals, other than possibly close to the central pore of the coral. However, similar Mg/Ca ratios for Isidids from Antarctic and more temperate regions suggest adaptation to local conditions and hence a role for physiology at higher taxonomic levels, at least. Taxonomically higher level vital effects are also suggested by large differences between gorgonian families in their regressions between Mg/Ca and temperature, by Mg/Ca ratios that overlap over a wide temperature and habitat range, and for a non-linear relationship between temperature and the slope of the Mg/Ca-temperature relationship across the order.  相似文献   

19.
Specimens of two species of planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera, were grown under controlled laboratory conditions at a range of temperatures (18-31 °C), salinities (32-44 psu) and pH levels (7.9-8.4). The shells were examined for their calcium isotope compositions (δ44/40Ca) and strontium to calcium ratios (Sr/Ca) using Thermal Ionization Mass Spectrometry and Inductively Coupled Plasma Mass Spectrometry. Although the total variation in δ44/40Ca (∼0.3‰) in the studied species is on the same order as the external reproducibility, the data set reveals some apparent trends that are controlled by more than one environmental parameter. There is a well-defined inverse linear relationship between δ44/40Ca and Sr/Ca in all experiments, suggesting similar controls on these proxies in foraminiferal calcite independent of species. Analogous to recent results from inorganically precipitated calcite, we suggest that Ca isotope fractionation and Sr partitioning in planktic foraminifera are mainly controlled by precipitation kinetics. This postulation provides us with a unique tool to calculate precipitation rates and draws support from the observation that Sr/Ca ratios are positively correlated with average growth rates. At 25 °C water temperature, precipitation rates in G. siphonifera and G. ruber are calculated to be on the order of 2000 and 3000 μmol/m2/h, respectively. The lower δ44/40Ca observed at ?29 °C in both species is consistent with increased precipitation rates at high water temperatures. Salinity response of δ44/40Ca (and Sr/Ca) in G. siphonifera implies that this species has the highest precipitation rates at the salinity of its natural habitat, whereas increasing salinities appear to trigger higher precipitation rates in G. ruber. Isotope effects that cannot be explained by precipitation rate in planktic foraminifera can be explained by a biological control, related to a vacuolar pathway for supply of ions during biomineralization and a pH regulation mechanism in these vacuoles. In case of an additional pathway via cross-membrane transport, supplying light Ca for calcification, the δ44/40Ca of the reservoir is constrained as −0.2‰ relative to seawater. Using a Rayleigh distillation model, we calculate that calcification occurs in a semi-open system, where less than half of the Ca supplied by vacuolization is utilized for calcite precipitation. Our findings are relevant for interpreting paleo-proxy data on δ44/40Ca and Sr/Ca in foraminifera as well as understanding their biomineralization processes.  相似文献   

20.
Combined stable isotope (δ18O and δ13C) and trace element (Mg, Sr) geochemistry from bulk tufa calcite and ostracod shell calcite from an early Holocene British tufa reveal clear records of Holocene palaeoclimatic change. Variation in δ18O is caused principally by change in the isotopic composition of Holocene rainfall (recharge), itself caused mainly by change in air temperature. The δ13C variability through much of the deposit reflects increasing influence of soil‐zone CO2, owing to progressive woodland soil development. Bulk tufa Mg/Ca and Sr/Ca are controlled by their concentrations in the spring water. Importantly, Mg/Ca ratios are not related to δ18O values and thus show no temperature dependence. First‐order sympathetic relationships between δ13C values and Mg/Ca and Sr/Ca are controlled by aquifer processes (residence times, CO2 degassing and calcite dissolution/reprecipitation) and probably record intensity of palaeorainfall (recharge) effects. Stable isotope records from ostracod shells show evidence of vital effects relative to bulk tufa data. The ostracod isotopic records are markedly ‘spiky’ because the ostracods record ‘snapshots’ of relatively short duration (years), whereas the bulk tufa samples record averages of longer time periods, probably decades. The δ18O record appears to show early Holocene warming, a thermal maximum at ca. 8900 cal. yr BP and the global 8200 yr BP cold event. Combined δ13C, Mg/Ca and Sr/Ca data suggest that early Holocene warming was accompanied by decreasing rainfall intensity. The Mg/Ca data suggest that the 8200 yr BP cold event was also dry. Warmer and wetter conditions were re‐established after the 8200 yr BP cold event until the top of the preserved tufa sequence at ca. 7100 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.  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