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1.
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   

2.
Diploastrea heliopora forms dense, robust, dome-shaped coral colonies throughout the reef ecosystems of the tropical Pacific and Indian Oceans. This slow-growing (2 to 6 mm/yr) coral has the potential to yield continuous paleoclimate records spanning up to 1000 yr for the warmest waters on Earth, the Indo-Pacific Warm Pool, and has a long fossil history as a single recognizable species. Despite the potential of Diploastrea to be an important new paleoclimate archive, little is known about the systematics of geochemical tracers incorporated into its skeleton. To fill this knowledge gap, we compared skeletal δ18O signatures in live Diploastrea and Porites colonies from Southwest Lagoon, near Amédée Lighthouse, New Caledonia (at the southern latitudinal limit for Diploastrea) and Alor, Indonesia (in the core area of the Indo-Pacific Warm Pool). We designed a new microsampling technique to minimize smoothing and distortion of the isotopic records due to the complex calyx architecture and slow growth of Diploastrea. High-resolution isotope profiles from the septal portion of the Diploastrea corallite are attenuated, whereas those restricted to the central columella are similar in quality to those extracted from the well-established Porites coral archive. The δ18O-temperature relationship for the columellar portion of Diploastrea (−0.18‰/°C) is in good agreement with that derived for a nearby Porites (−0.19‰/°C; Quinn et al., 1996), on the basis of comparison with an in situ sea surface temperature record from Amédée Lighthouse. There is a measurable difference of 0.3 ± 0.1‰ between the kinetic/biological disequilibrium offsets from seawater δ18O composition for Diploastrea and Porites. Despite this offset in mean δ18O, Diploastrea accurately records the regional differences in mean temperature and salinity between New Caledonia and the Indo-Pacific Warm Pool. Additional tests show that Diploastrea records El Niño-Southern Oscillation (ENSO) interannual variability in sea surface temperature and salinity across the southwestern Pacific, indicating that it should yield dependable paleo-ENSO records. Based on these results, we propose that D. heliopora has the potential to provide an important new coral archive of tropical paleoclimate.  相似文献   

3.
Coupled records of Sr/Ca and oxygen isotope ratios (δ18O) of coral skeletons have been used to produce quantitative estimates of paleo-sea surface temperature (SST) and δ18O of surface seawater that can in some cases be converted to sea surface salinity (SSS). Two fossil corals from Kikai Island in the subtropical northwestern Pacific, a location affected by East Asian summer and winter monsoons, were analyzed to investigate differences between mid-Holocene and present-day SST and SSS. At 6180 cal yr BP, SSTs were roughly the same as today, both in summer and winter; δ18Oseawater and SSS values were higher both in summer (+ 0.5‰, +1.1 psu) and in winter (+ 0.2‰, + 0.6 psu) than modern values. At 7010 cal yr BP, SSTs were slightly cooler both in summer and winter (−0.8 and −0.6 °C), whereas δ18Oseawater and SSS had higher values in summer (+ 0.3‰, + 0.6 psu) and in winter (+ 0.8‰, + 1.9 psu) than present-day values. These results are consistent with other marine records for the mid-Holocene of the low and midlatitudes in the northwestern Pacific. Such regional conditions indicate that the East Asian summer and winter monsoons were more intense in the mid-Holocene, which was likely a function of the mid-Holocene insolation regime.  相似文献   

4.
Isotopic and geochemical evidence of paleoclimates, especially for the last glaciation, has been obtained from deep confined groundwaters of southern India. The δ13C, δ18O, chloride, and deuterium analyses of groundwaters show distinct excursions inferred to be related to climatic variations. The arid climatic episode associated with the last glaciation (18,000 ± 2000 yr B.P.) is conspicuously identified by signatures of relatively enriched δ13C (−10 to −12‰ PDB) and δ18O (−5.3 to −4.8‰ SMOW) values, and high chloride concentration (80 to 160 mg/l). The transition from an arid to humid period ca. 12,000–8000 yr B.P. is shown by a decreasing trend in the δ13C (−9.5 to −17‰) and δ18O (−4.5 to −6.3‰) contents of groundwaters. The late Holocene (since 4000 yr B.P.), marked by a more humid but unstable climate, is identified by further depletion of δ13C (−13 to −20‰) and δ18O (−5.2 to −6.3‰). Similar variation between δ18O and chloride values in confined groundwaters further demonstrates two distinct climatic excursions (arid and humid) governed by the “amount effect.” This is the first time that isotopic and geochemical signatures related to changing paleoclimates have been identified in the confined groundwaters of the southern Indian landmass.  相似文献   

5.
Ultramafic rocks of the Duke Island Complex in southeastern Alaska crystallized in a supra-subduction zone setting, but the serpentinization of olivine-bearing rocks involved the incursion of late-stage meteoric waters. Three textural types of serpentine (primarily lizardite) have been identified which in part reflect progress in reactions during multiple stages of fluid infiltration. The overall mesh texture of serpentine has been subdivided into a massive-type, found in dunites and wehrlites, and a dendritic-type found in wehrlites and olivine clinopyroxenites. Serpentine veins represent a late-stage in the hydrothermal alteration process. Both FeO contents and δ18O values of the three textural types of serpentine are variable at the centimeter scale. Magnetite abundance in association with serpentine is also variable with up to 5 vol% of magnetite found in samples with dendritic serpentine. Continued reaction of FeO-bearing serpentine with fluid appears to control the formation of most magnetite. Oxygen isotope ratios of the three textural types of serpentine are distinct, with the massive variety characterized by δ18O values between −3‰ and 3‰, the dendritic variety showing values between 2‰ and 6‰ and the veins having the highest values between 4‰ and 10‰. Although the δ18O values may vary by as much as 5‰ on the centimeter scale, δD values tend to show relatively less variation with over 90% of the measured values between −100‰ and −120‰. The O and H isotopic values are consistent with the involvement of meteoric water that had undergone variable degrees of isotopic exchange with country rocks prior to reacting with olivine in the Duke Island Complex. Small-scale variability in both serpentine FeO content and δ18O values suggests that chemical and isotopic equilibria may have not been attained at larger than centimeter scales. Oxygen isotopic variability in serpentine produced during relatively low-temperature hydrothermal alteration is in large part a function of exchange mediated via fluid flow through microfractures.  相似文献   

6.
The oxygen and hydrogen isotopic composition of Eocene and Miocene freshwater cherts in the western United States records regional climatic variation in the Cenozoic. Here, we present isotopic measurements of 47 freshwater cherts of Eocene and Miocene age from the Great Basin of the western United States at two different sites and interpret them in light of regional climatic and tectonic history. The large range of δ18O of terrestrial cherts measured in this study, from 11.2‰ to 31.2‰ (SMOW: Standard Mean Ocean), is shown to be primarily the result of variations in δ18O of surface water. The following trends and patterns are recognized within this range of δ18O values. First, in Cenozoic rocks of northern Nevada, chert δ18O records the same shift observed in authigenic calcite between the Eocene and Miocene that has been attributed to regional surface uplift. The consistent covariation of proxies suggests that chert reliably records and retains a signal of ancient meteoric water isotopic composition, even though our analyses show that chert formed from warmer waters (40°C) than coexisting calcite (20°C). Second, there is a strong positive correlation between δ18O and δD in Eocene age chert from Elko, Nevada and Salina, Utah that suggests large changes in lake water isotopic composition due to evaporation. Evaporative effects on lake water isotopic composition, rather than surface temperature, exert the primary control on the isotopic composition of chert, accounting for 10‰ of the 16‰ range in δ18O measured in Eocene cherts. From authigenic mineral data, we calculate a range in isotopic composition of Eocene precipitation in the north-central Great Basin of −10 to −14‰ for δ18O and −70 to −100‰ for δD, which is in agreement with previous estimates for Eocene basins of the western United States. Due to its resistance to alteration and record of variations in both δ18O and δD of water, chert has the potential to corroborate and constrain the cause of variations in isotope stratigraphies.  相似文献   

7.
Goethite (Ax-2) from Axel Heiberg Island (∼80°N) on the margin of the Arctic Ocean is the dominant mineral in a sample of “petrified” Eocene wood, but U, Th, and He measurements suggest that the goethite (α-FeOOH) crystallized in the latest Miocene/Pliocene (ca. 5.5 to 2.8 Ma). Measured δD and δ18O values of Ax-2 are −221 (±6)‰ and −9.6 (±0.5)‰, respectively. The inferred δD and δ18O values of the ancient water were about −139‰ and −18.6‰, respectively, with a calculated temperature of crystallization of 3 (±5)°C, which compares with the modern summer (J-J-A) temperature of 3 °C and contrasts with a modern MAT of −19 °C. Published results from various biological proxies on nearby Ellesmere Island indicate a Pliocene (∼4 Ma) MAT of either −6 or −0.4 °C and corresponding seasonal amplitudes of about 18 or 13 °C. A conductive heat flow model suggests that a temperature of 3 °C could represent goethite crystallization at depths of ∼100-200 cm (for MAT = −6 °C) or ∼250-450 cm (for MAT = −0.4 °C) over seasonally restricted intervals of time.The δ18O value of the Ax-2 water (−18.6‰) is more positive than the modern J-J-A precipitation (−22‰). In combination, the paleotemperatures and δ18O values of ancient waters (from Ax-2 and published results from three Eocene or Pliocene proxy sites on Axel Heiberg and Ellesmere Islands) are consistent with a warm season bias in those isotopic proxies. The results are also consistent with higher proportions of J-J-A precipitation in the annual total. If so, this emphasizes the importance of seasonality at high latitudes even in times of warmer global climates, and suggests that the Arctic hydrologic cycle, as expressed in the seasonal distribution and isotopic composition of precipitation (perhaps modified by a warmer Arctic Ocean), differed from modern.The δ13C value of the Fe(CO3)OH component in the Ax-2 goethite is +6.6‰, which is much more positive than expected if crystallizing goethite incorporated CO2 derived primarily from oxidation of relict Eocene wood with δ13C values of about −24‰. This apparent paradox may be resolved if the goethite is a product of oxidation of 13C-rich siderite, which had previously replaced wood in an Eocene methanogenic burial environment. Thus, the goethite retains a carbon isotope “memory” of a diagenetic Eocene event, but a δD and δ18O record of the latest Miocene/Pliocene Arctic climate.  相似文献   

8.
Kaolinite, gibbsite and quartz are the dominant minerals in samples collected from two outcrops of a Cenomanian (∼95 Ma) laterite in southwestern Minnesota. A combination of measured yields and isotope ratios permitted mass balance calculations of the δD and δ18O values of the kaolinite in these samples. These calculations yielded kaolinite δD values of about −73‰ and δ18O values of about +18.7‰. The δD and δ18O values appear to preserve information on the ancient weathering system.If formed in hydrogen and oxygen isotope equilibrium with water characterized by the global meteoric water line (GMWL), the kaolinite δD and δ18O values indicate a crystallization temperature of 22 (±5) °C. A nominal paleotemperature of 22 °C implies a δ18O value for the corresponding water of −6.3‰. The combination of temperature and meteoric water δ18O values is consistent with relatively intense rainfall at that mid-paleolatitude location (∼40°N) on the eastern shore of the North American Western Interior Seaway. The inferred Cenomanian paleosol temperature of ∼22 °C is in general accord with published mid-Cretaceous continental mean annual temperatures (MAT) estimated from leaf margin analyses of fossil plants.When compared with results from a published GCM-based Cenomanian climate simulation which specifies a latitudinal sea surface temperature (SST) gradient that was either near modern or smaller-than-modern, the kaolinite paleotemperature of 22 °C is closer to the GCM-predicted MAT for a smaller equator-to-pole temperature difference in the mid-Cretaceous. Moreover, the warm, kaolinite-derived, mid-paleolatitude temperature of 22 °C is associated with proxy estimates of high concentrations of atmospheric CO2 in the Cenomanian. The overall similarity of proxy and model results suggests that the general features of Cenomanian continental climate in that North American locale are probably being revealed.  相似文献   

9.
Unusual 18O depletion, with δ18O values as negative as −10‰ to −4‰ relative to VSMOW, was reported in zircons from ultrahigh-pressure eclogite-facies metamorphic rocks in the Dabie-Sulu orogenic belt, China. But it is critical for the negative δ18O zircons to be distinguished between magmatic and metamorphic origins, because the 18O depletion can be acquired by high-T eclogite-facies metamorphism of meteoric-hydrothermally altered low δ18O rocks. While zircon O diffusion kinetics has placed a reasonable constraint on this, zircon trace element compositions can provide a straightforward distinction between the magmatic and metamorphic origins. This paper reports our finding of unusual 18O depletion in zircon from granitic gneiss in the northeastern end of the Sulu orogen. Zircon δ18O values vary from −7.8‰ to −3.1‰ along a profile of 50 m length at Zaobuzhen. They are close to extremely low δ18O values of −9.0‰ to −5.9‰ for metagranite at Qinglongshan and adjacent areas in the southwestern end of the Sulu orogen. CL imaging suggests that the low δ18O zircons at Zaobuzhen are primarily of magmatic origin, but underwent different degrees of metamorphic modification. Zircon U-Pb dating yields middle Neoproterozoic ages of 751 ± 27 to 779 ± 25 Ma for protolith crystallization and Triassic ages of 214 ± 10 to 241 ± 33 Ma for metamorphic resetting. However, no metamorphic modification occurs in zircon REE patterns that only indicate magmatic recrystallization and hydrothermal alteration, respectively. Thus, the negative δ18O zircons are interpreted as crystallizing from negative δ18O magmas due to melting of meteoric-hydrothermally altered negative δ18O rocks in an active rift setting at about 780 Ma. The variation in zircon δ18O values indicates considerable O isotope heterogeneity in its granitic protolith. Zircon Lu-Hf isotope analyses give positive εHf(t) values of 1.6-4.1 and Hf model ages of 1.18-1.30 Ga. This suggests that the granitic protolith was derived from the mid-Neoproterozoic reworking of late Mesoproterozoic juvenile crust. The metagranites at Zaobuzhen and Qinglongshan, about 450 km apart, are two known occurrences of the unusually low δ18O zircons below −6‰ so far reported in the Sulu orogen. They are similar to each other in both protolith and metamorphic ages, so that they share the same nature of both Neoproterozoic protolith and Triassic metamorphism. Therefore, the locally negative δ18O zircons may register centers of low δ18O magmatism during the supercontinental rifting.  相似文献   

10.
We evaluate the impact of exceptionally sparse plant cover (0-20%) and rainfall (2-114 mm/yr) on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet, the Atacama Desert in northern Chile. δ13C and δ18O values of carbonates from the Atacama are the highest of any desert in the world. δ13C (VPDB) values from soil carbonate range from −8.2‰ at the wettest sites to +7.9‰ at the driest. We measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of [Cerling (1984) Earth Planet. Sci. Lett.71, 229-240], in which we assumed an exponential form of the soil CO2 production function, and relatively shallow (20-30 cm) average production depths. Overall, we find that respiration rates are the main predictor of the δ13C value of soil carbonate in the Atacama, whereas the fraction C3 to C4 biomass at individual sites has a subordinate influence. The high average δ13C value (+4.1‰) of carbonate from the driest study sites indicates it formed—perhaps abiotically—in the presence of pure atmospheric CO2.δ18O (VPDB) values from soil carbonate range from −5.9‰ at the wettest sites to +7.3‰ at the driest and show much less regular variation with elevation change than δ13C values. δ18O values for soil carbonate predicted from local temperature and δ18O values of rainfall values suggest that extreme (>80% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. The effects of evaporation compromise the use of δ18O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.  相似文献   

11.
We investigated the Sea-Rain-Lake relation during the Last Glacial-Holocene in the East Mediterranean region by comparing the δ18O and δ13C records of authigenic aragonite deposited in Lake Lisan, the Dead Sea, Mediterranean foraminifera, and speleothems. The Lisan Formation data display long- and short-term variations of δ18O, representing steady-state conditions of the lake (e.g., 5.6‰ ± 0.5‰ and 4.5‰ ± 1‰ in the Upper and Lower Members of the Lisan Formation, respectively), and short-term excursions reflecting large floods and droughts. The long-term (steady-state) δ18O values of the Lisan aragonites show similarity to the corresponding time-equivalent records of the Eastern Mediterranean foraminifera and Judea Mountain speleothems: The Last Glacial deposits are in all of them 2‰-3‰ heavier than the Holocene ones. We interpret this similarity as reflecting the significance of the source effect on the long-term behavior of isotopic reservoirs: Speleothem δ18O is strongly influenced by the marine reservoir that contributes its vapor to rain formation; the lake δ18O is dominated by the composition of the inflowing water. Short-term variations in the isotopic composition of rainfall are dominated by the amount effect and the temperature and those of the Lake’s upper water mass by the lake’s water balance.δ13C values are more variable than δ18O in the same Lisan sequences (e.g., δ13C in the Lower Member is 1.0‰ ± 1.7‰, whereas δ18O is 4.6‰ ± 0.7‰) and are 1‰ to 1.5‰ higher in the Upper Member than in the Lower and Middle Members of the Lisan Formation. These variations reflect significant increase in primary productivity of the lake and algal bloom activity. It appears that the hypersaline-saline lakes were not as “dead” as the Dead Sea is and that algal activity had an important impact upon the geochemistry of Lake Lisan.The δ18O data combined with independent geochemical and limnologic information (e.g., level fluctuations) indicate that Lisan time was characterized by high precipitation-high lake stands-high atmospheric humidity, whereas the Holocene Dead Sea shows the opposite behavior. This paleoclimatic reconstruction is consistent with independent evidence for significantly wetter conditions in the East Mediterranean region during the Last Glacial period.  相似文献   

12.
To understand oxygen and carbon stable isotopic characteristics of aragonite stalagmites and evaluate their applicability to paleoclimate, the isotopic compositions of active and fossil aragonite speleothems and water samples from an in situ multi-year (October 2005-July 2010) monitoring program in Furong Cave located in Chongqing of China have been examined. The observations during October 2005-June 2007 show that the meteoric water is well mixed in the overlying 300-500-m bedrock aquifer, reflected by relatively constant δ18O, ±0.11-0.14‰ (1σ), of drip waters in the cave, which represents the annual status of rainfall water. Active cave aragonite speleothems are at oxygen isotopic equilibrium with drip water and their δ18O values capture the surface-water oxygen isotopic signal. Aragonite-to-calcite transformation since the last glaciation is not noticeable in Furong stalagmites. Our multi-year field experiment approves that aragonite stalagmite δ18O records in this cave are suitable for paleoclimate reconstruction. With high U, 0.5-7.2 ppm, and low Th, 20-1270 ppt, the Furong aragonite stalagmites provide very precise chronology (as good as ±20s yrs (2σ)) of the climatic variations since the last deglaciation. The synchroneity of Chinese stalagmite δ18O records at the transition into the Bølling-Allerød (t-BA) and the Younger Dryas from Furong, Hulu and Dongge Caves supports the fidelity of the reconstructed East Asian monsoon evolution. However, the Furong record shows that the cold Older Dryas (OD) occurred at 14.0 thousand years ago, agreeing with Greenland ice core δ18O records but ∼200 yrs younger than that in the Hulu record. The OD age discrepancy between Chinese caves can be attributable to different regionally climatic/environmental conditions or chronological uncertainty of stalagmite proxy records, which is limited by changes in growth rate and subsampling intervals in absolute dating. Seasonal dissolved inorganic carbon δ13C variations of 2-3‰ in the drip water and 5-7‰ in the pool and spring waters are likely attributed to variable degrees of CO2 degassing in winter and summer. The variable δ13C values of active deposits from −11‰ to 0‰ could be caused by kinetically mediated CO2 degassing processes. The complicated nature of pre-deposition kinetic isotopic fractionation processes for carbon isotopes in speleothems at Furong Cave require further study before they can be interpreted in a paleoclimatic or paleoenvironmental context.  相似文献   

13.
Speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods over the last 330,000 yr. Periods of rapid speleothem deposition occurred from 6000 to 10,500, 78,000 to 82,000, 120,000 to 135,000, 180,000 to 200,000, and 300,000 to 330,000 yr ago, with little or no growth during the intervening periods. During each of these five pluvial periods, δD values of water extracted from speleothem fluid inclusions (δDFI) are between −60 and −20‰ (VSMOW) and δ18O values of speleothem calcite (δ18OC) are between −12 and −4‰ to (VPDB). These values are much more negative than modern rainfall (for δD) or modern stalagmites (for δ18O). Previous work on the isotopic composition of rainfall in Oman has shown that northern and southern moisture sources are isotopically distinct. Combined measurements of the δD values of fluid-inclusion water with calculated δ18O values from peak interglacial speleothems indicate that groundwater was predominantly recharged by the southern (Indian Ocean) moisture source, when the monsoon rainfall belt moved northward and reached Northern Oman during each of these periods.  相似文献   

14.
Understanding past climate change is critical to the interpretation of earth history. Even though relative temperature change has been readily assessed in the marine record, it has been more difficult in the terrestrial record due to restricted taxonomic distribution and isotopic fractionation. This problem could be overcome by the use of multiple paleoproxies. Therefore, the δ18O isotopic composition of five paleoproxies (rodent tooth enamel, δ18OPhosphate = +17.7 ± 2.0‰ n = 74 (VSMOW); fish scale ganoine δ18OPhosphate = +19.7 ± 0.7‰ n = 20 (VSMOW); gastropod shell δ18OCalcite = −1.7 ± 1.3‰ n = 50 (VPDB); charophyte gyrogonite δ18OCalcite = −2.4 ± 0.5‰ n = 20 (VPDB); fish otolith δ18OAragonite = δ18O = −3.6 ± 0.6‰ n = 20 (VPDB)) from the Late Eocene (Priabonian) Osborne Member (Headon Hill Formation, Solent Group, Hampshire Basin, UK) were determined. Because diagenetic alteration was shown to be minimal the phosphate oxygen component of rodent tooth enamel (as opposed to enamel carbonate oxygen) was used to calculate an initial δ18OLocal water value of 0.0 ± 3.4‰. However, a skewed distribution, most likely as a result of the ingestion of evaporating water, necessitated the calculation of a corrected δ18OLocal water value of −1.3 ± 1.7‰ (n = 62). This δ18OLocal water value corresponds to an approximate mean annual temperature of 18 ± 1°C. Four other mean paleotemperatures can also be calculated by combining the δ18OLocal water value with four independent freshwater paleoproxies. The calculated paleotemperature using the fish scale thermometry equations most likely represents the mean temperature (21 ± 2°C) of the entire length of the growing season. This should be concordant with the paleotemperature calculated using the Lymnaea shell thermometry equation (23 ± 2°C). The lack of concordance is interpreted to be the result of diagenetic alteration of the originally aragonitic Lymnaea shell to calcite. The mean paleotemperature calculated using the charophyte gyrogonite thermometry equation (21 ± 2°C), on the other hand, most likely represents the mean temperature of a single month toward the end of the growing season. The fish otolith mean paleotemperature (28 ± 2°C) most likely represents the mean temperature of the warmest months of the growing season. An approximate mean annual temperature of 18 ± 1°C, in addition to a mean growing season paleotemperature of 21 ± 2°C (using fish scale only) with a warmest month temperature of 28 ± 2°C, and high associated standard deviations suggest that a subtropical to warm temperate seasonal climate existed during the deposition of the Late Eocene Osborne Member.  相似文献   

15.
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   

16.
Well-preserved aragonitic land snail shells (Vallonia) from late Pleistocene Eolian sediment in the Folsom archaeological site in New Mexico exhibit an overall decrease of δ18OPDB from maximum values of +2.7‰ (more positive than modern) to younger samples with lower average values of about −3.6‰ (within the modern range). The age of the samples (approximately 10,500 14C yr B.P.) suggests that the decrease in δ18O may manifest climatic changes associated with the Younger Dryas. Some combination of increased relative humidity and cooler temperatures with decreased δ18O of precipitation during the times of snail activity can explain the decrease in shell δ18O. A well-known Paleoindian bison kill occurred at the Folsom site during this inferred environmental transition.Average δ13C values of the aragonite shells of the fossil Vallonia range from −7.3 to −6.0‰ among different archaeological levels and are not as negative as modern values. This suggests that the proportion of C4 vegetation at the Folsom site approximately 10,500 14C yr B.P. was greater than at present; a result which is consistent with other evidence for higher proportions of C4 plants in the region at that time.  相似文献   

17.
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   

18.
We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ56Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ26Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ56Fe; ±0.10‰ for δ26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ56Feol-cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ56Feol-opx = −0.05 ± 0.11‰; Δ26Mgol-opx = −0.09 ± 0.12‰; Δ26Mgol-cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ56Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ26Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ56Fe and δ26Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ56Fe, the larger deviations from average δ56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ56Fe and slightly heavy δ26Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt.  相似文献   

19.
13C/12C and 18O/16O ratios of aragonite shells of modern land snails from the southern Great Plains of North America were measured for samples from twelve localities in a narrow east-west corridor that extended from the Flint Hills in North Central Oklahoma to the foothills of the Sangre de Cristo Mountains in Northern New Mexico, USA. Across the study area, shell δ18O values (PDB scale) ranged from −4.1‰ to 1.2‰, while δ13C values ranged from −13.2‰ to 0.0‰. δ18O values of the shell aragonite were predicted with a published, steady state, evaporative flux balance model. The predicted values differed (with one exception) by less than 1‰ from locality averages of measured δ18O values. This similarity suggests that relative humidity at the time of snail activity is an important control on the δ18O values of the aragonite and emphasizes the seasonal nature of the climatic information preserved in the shells. Correlated δ13C values of coexisting Vallonia and Gastrocopta suggest similar feeding habits and imply that these genera can provide information on variations in southern Great Plains plant ecology. Although there is considerable scatter, multispecies, transect average δ13C values of the modern aragonite shells are related to variations in the type of photosynthesis (i.e., C3, C4) in the local plant communities. The results of this study emphasize the desirability of obtaining isotope ratios representing averages of many shells in a locale to reduce possible biases associated with local variations among individuals, species, etc., and thus better represent the “neighborhood” scale temporal and/or spatial environmental variations of interest in studies of modern and ancient systems.  相似文献   

20.
Here we present Sr, C, and O isotope curves for Ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide. These are the first Ordovician-wide isotope curves that can be placed within the newly emerging global biostratigraphic framework. A total of 182 brachiopods were selected for C and O isotope analysis, and 122 were selected for Sr isotope analysis. Seawater 87Sr/86Sr decreased from 0.7090 to 0.7078 during the Ordovician, with a major, quite rapid fall around the Middle-Late Ordovician transition, most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates. Mean δ18O values increase from −10‰ to −3‰ through the Ordovician with an additional short-lived increase of 2 to 3‰ during the latest Ordovician due to glaciation. Although diagenetic alteration may have lowered δ18O in some samples, particularly those from the Lower Ordovician, maximum δ18O values, which are less likely to be altered, increase by more than 3‰ through the Ordovician in both our data and literature data. We consider that this long-term rise in calcite δ18O records the effect of decreasing tropical seawater temperatures across the Middle-Late Ordovician transition superimposed on seawater δ18O that was steadily increasing from ≤−3‰ standard mean ocean water (SMOW). By contrast, δ13C variation seems to have been relatively modest during most of the Ordovician with the exception of the globally documented, but short-lived, latest Ordovician δ13C excursion up to +7‰. Nevertheless, an underlying trend in mean δ13C can be discerned, changing from moderately negative values in the Early Ordovician to moderately positive values by the latest Ordovician. These new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 Ma. The juxtaposition of the greatest recorded swings in Phanerozoic seawater 87Sr/86Sr and δ18O at the same time as one of the largest marine transgressions in Phanerozoic Earth history suggests a causal link between tectonic and climatic change, and emphasizes an endogenic control on the O isotope budget during the Early Paleozoic. Better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes. We recommend that future work on Ordovician isotope stratigraphy focus on this outstanding Middle-Late Ordovician event.  相似文献   

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