Paleotemperature Equation Research Articles

Evaluating the temperature dependence of δ44/40Ca along with δ18O, Mg/Ca, and Sr/Ca in calcite tests of multiple foraminifera species

Preserved biomineral chemical compositions serve as valuable tools in constructing proxies for reconstructing past climates. A persistent effort exists to enhance and innovate temperature proxies, ensuring a comprehensive understanding of historical temperature variability—a vital Earth System parameter. Calcium isotopes (δ44/40Ca) in foraminifera have been debated to be sensitive to temperature and requires further refinement. In this study we present a comprehensive assessment of δ44/40Ca temperature sensitivity in foraminifera by utilizing concurrent δ44/40Ca, δ18O, Mg/Ca, and Sr/Ca ratios data on same samples. These coupled isotopic and elemental ratios were determined across various foraminifera species, including benthic (C. wuellerstorfi), thermocline planktic (G. truncatulinoides, G. inflata), and mixed layer planktic (G. ruber, G. bulloides, O. universa, G. siphonifera) from core-top and sediment trap samples from the Atlantic Ocean. The δ44/40Ca -T calibration included calcification temperatures derived from both δ18O and Mg/Ca, utilizing established δ18O -T and Mg/Ca-T relationships. The temperature sensitivity of δ44/40Ca in foraminifera is 0.042–0.058 ‰/°C using all species data from published work and this study (except data from <3 °C habitat temperature). We report foraminifera δ44/40Ca and temperature relationships: δ44/40Ca (‰) = 0.042 × T (°C) + 0.289 (r2 = 0.80, p < 0.0001), using temperatures derived from δ18O and Mg/Ca, respectively. Furthermore, we introduce a novel paleotemperature equation for foraminifera, employing a multi-regression approach that combines two temperature proxies, δ44/40Ca and Mg/Ca: SST (°C) = (9 ± 2.6) × δ44/40Ca + (3.28 ± 0.63) × Mg/Ca - (2.89 ± 1.09). This multi-proxy approach can mitigate the uncertainty associated with individual proxy measurements and allows cross-validation. Application of this multi-proxy relationship to an East Atlantic Site (GeoB1112) yields a glacial-interglacial temperature change of 4.2 ± 0.93 °C between MIS 1 and MIS 3 which is in agreement with published work.

A modern snapshot of the isotopic composition of lacustrine biogenic carbonates – records of seasonal water temperature variability

Abstract. Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The carbon isotopic composition (δ13C) of such carbonates depends on the δ13C of dissolved inorganic carbon (DIC). Their oxygen isotopic composition (δ18O) is controlled by the δ18O of the lake water and by water temperature during carbonate precipitation. Lake water δ18O, in turn, reflects the δ18O of atmospheric precipitation in the catchment area, water residence time and mixing, and evaporation. A paleoclimatic interpretation of carbonate isotope records requires a site-specific calibration based on an understanding of these local conditions. For this study, samples of different biogenic carbonate components and water were collected in the littoral zone of Lake Locknesjön, central Sweden (62.99∘ N, 14.85∘ E, 328 ma.s.l.) along a water depth gradient from 1 to 8 m. Carbonate samples of living organisms and subfossil remains in surface sediments were taken from the calcifying alga Chara hispida, from bivalve mollusks of the genus Pisidium, and from adult and juvenile instars of two ostracod species, Candona candida and Candona neglecta. Our results show that neither the isotopic composition of carbonates nor the δ18O of water vary significantly with water depth, indicating a well-mixed epilimnion. The mean δ13C of Chara hispida encrustations is 4 ‰ higher than the other carbonates. This is due to fractionation related to photosynthesis, which preferentially incorporates 12C into the organic matter and increases the δ13C of the encrustations. A small effect of photosynthetic 13C enrichment in DIC is seen in contemporaneously formed valves of juvenile ostracods. The largest differences in the mean carbonate δ18O between species are caused by vital offsets, i.e., the species-specific deviations from the δ18O of inorganic carbonate which would have been precipitated in isotopic equilibrium with the water. After subtraction of these offsets, the remaining differences in the mean carbonate δ18O between species can mainly be attributed to seasonal water temperature changes. The lowest δ18O values are observed in Chara hispida encrustations, which form during the summer months when photosynthesis is most intense. Adult ostracods, which calcify their valves during the cold season, display the highest δ18O values. The seasonal and interannual variability in lake water δ18O is small (∼ 0.5 ‰) due to the long water residence time in the lake. Seasonal changes in the temperature-dependent fractionation are therefore the dominant cause of carbonate δ18O differences between species when vital offsets are corrected. Temperature reconstructions based on paleotemperature equations for equilibrium carbonate precipitation using the mean δ18O of each species and the mean δ18O of lake water are well in agreement with the observed seasonal water temperature range. The high carbonate δ18O variability of samples within a species, on the other hand, leads to a large scatter in the reconstructed temperatures based on individual samples. This implies that care must be taken to obtain a representative sample size for paleotemperature reconstructions.

Investigating the influence of temperature and seawater δ18O on Donax obesulus (Reeve, 1854) shell δ18O

The coastline of Peru lacks long-lived marine organisms useful for paleoclimatic reconstructions generating a need for novel archives. Short-lived (<5 years) bivalves are commonly found in geological and archaeological deposits and thus can provide “snapshots” of past climatic variability (i.e., seasonal range), similar to data obtained by individual foraminifera analysis, rather than continuous, cross-dated time series (e.g., trees and corals). Previous studies have found success using the short-lived intertidal clam Mesodesma donacium. However, M. donacium are vulnerable to die-offs from the warmer sea surface temperatures (SST) associated with El Niño events and are functionally extinct in northern Peru thus limiting the possibility of modern analog studies for that region. Here we investigate the short-lived (1–3 years) surf clam, Donax obesulus, commonly found in northern Peru, as a paleoclimate archive. Donax obesulus populations are able to survive the warmer SSTs present during El Niño years although they are vulnerable to colder SSTs associated with La Niñas. We assessed the environmental drivers underlying subannual δ18O variability in D. obesulus from live collected shells from fish markets and coastal beaches near the Nepeña Valley, Peru in 2012 (La Niña), 2014 (ENSO-neutral), and 2016 (El Niño). Forward modeling of pseudo-shell δ18O reveals that SST variations are a dominant driver with secondary contributions from seasonally-varying seawater δ18O (δ18Osw). By accounting for varying δ18Osw, we isolated the temperature dependent variable resulting in a paleotemperature equation for D. obesulus δ18O. We verified our results with the δ18O record of a D. obesulus shell collected in 2006. Our results suggest that the paleotemperature equation we developed is useful for reconstructing El Niño-Southern Oscillation (ENSO)-related climatic variations in this region and the pseudo-shell approach may be useful for understanding shell δ18O in other locations.

Controls Over Oxygen Isotope Fractionation in the Waters and Bivalves (Arthritica helmsi) of an Estuarine Lagoon System

AbstractOxygen isotope ratios in bivalve shells have long been used as a proxy for environmental change, reflecting both temperature and the oxygen isotope composition of host water. In estuarine systems, the oxygen isotope composition of water is complicated by variable mixing between river and seawater, as well as evaporative enrichment. In addition, due to species‐specific variation in temperature‐dependent fractionation into bivalve carbonate, modern calibrations are necessary prior to applications in paleoenvironmental studies. In this study, live specimens of the micromollusc Arthritica helmsi were collected from five sites in the Coorong Lagoon, an estuarine system at the mouth of the River Murray, Australia, on six occasions from November 2016 to May 2018. Whole shell oxygen and carbon isotope compositions (n = 131) were measured alongside monthly temperature and oxygen and hydrogen isotope analyses of waters from the Coorong and neighboring Lake Alexandrina (n = 137). Oxygen and hydrogen isotope ratios in water were mostly controlled by evaporation of source waters, though a period of high river water discharge was reflected in the isotopic values of the Coorong North Lagoon. A species‐specific temperature‐dependent oxygen isotope fractionation equation was calibrated for A. helmsi: T (°C) = (21.39 ± 0.45) − (4.43 ± 0.38) × (δ18Oshell − δ18Owater). This equation is similar to other published paleotemperature equations for biogenic carbonates. These contemporary observations of the isotope hydrology of the Coorong, coupled with our contemporary calibration of oxygen isotope fractionation, lay the foundation for paleoenvironmental studies using bivalves collected from the sediments of the Coorong.

Oxygen isotope records of the Australian flat oyster (Ostrea angasi) as a potential temperature archive

The response of the oxygen isotope ratio ( δ 18 O) in shells of the Australian native flat oyster Ostrea angasi to changes in water temperature has been assessed using growth experiments conducted for one year at two locations on the east coast of Australia. The analysed δ 18 O of the oyster shells ( δ 18 O shell ) closely follows the predicted oyster shell δ 18 O constructed from measured δ 18 O in the water ( δ 18 O water ) and water temperature. Influxes of freshwater that occur in the estuarine habitats of O. angasi can modify the δ 18 O water , and consequently δ 18 O shell . Salinity fluctuations can also cause interruptions to shell growth in this species. This can cause overestimated temperatures in O. angasi δ 18 O shell temperature reconstructions. A δ 18 O shell –temperature calibration was established for O. angasi , yielding the relationship: T°C = 13.97 − (3.57 δ 18 O shell − δ 18 O water ) + (0.17 δ 18 O shell − δ 18 O water ) 2 , n = 79, R 2 = 0.79, which, within the experimental uncertainties of the data, is similar to other published biogenic carbonate paleotemperature equations. • The δ 18 O shell of the Australian oyster Ostrea angasi records the ambient water temperature. • Changes in the δ 18 O water in the oysters’ estuarine habitat will influence the δ 18 O shell . • A δ 18 O shell –temperature calibration specific to Ostrea angasi -specific was developed.

Improved oxygen isotope temperature calibrations for cosmopolitan benthic foraminifera

Despite decades of use as a paleoceanographic proxy, considerable uncertainty still surrounds the temperature dependence of benthic foraminiferal δ18O. Widely applied paleotemperature equations may mix non-equilibrium foraminifera with equilibrium synthetic calcite, resulting in temperature sensitivities that are too large. Warm-water foraminiferal calibrations may give temperature sensitivities that are too small for very cold waters. Here we combine new core top measurements from the Florida Straits and the Arctic Ocean with published data to derive new δ18O:temperature relationships for three groups of benthic foraminifera. We derive a quadratic equation for Cibicidoides and Planulina that agrees well with equilibrium synthetic calcite, and that should be applicable over all oceanographic temperatures. We find that Uvigerina is not at equilibrium and is isotopically heavier than Cibicidoides and Planulina by 0.47‰, in contrast to the historically used 0.64‰. Hoeglundina elegans is further enriched and appears to be slightly heavier than equilibrium aragonite. Finally we discuss the implications of the Florida Straits observations for the hypothesis that benthic foraminifera precipitate their shells from a pH-dependent mixture of bicarbonate and carbonate ions.

Oxygen isotopes and MgCO3 in brachiopod calcite and a new paleotemperature equation

Modern brachiopods and their ambient seawater, compiled from 34 localities, covering shallow-waters from the poles to the tropics were analyzed for their oxygen isotopic compositions, and SrCO3 and MgCO3 contents. If the routine seawater δ18O composition and the ‘MgCO3 effect’ on oxygen isotopes are considered in calculations, calcification temperatures are concordant with measured ambient seawater temperatures. The SrCO3 contents of modern brachiopods appear to be unrelated to ambient growth temperatures, while their MgCO3 contents exhibit some relationship with temperature and growth rate, controlled, in part, by ambient productivity or clearance rates or local environment, and by taxonomic affinity such as presence or absence of caeca.Linear least-squares regression analysis yields a new oxygen isotope paleotemperature equation for brachiopod calcite, which makes adjustment for shell MgCO3 contents, and covers habitats with water temperatures ranging from −2° to +32°C:T°C=16.192–3.468δc–δSW–MgcN=319,R2=0.98.The Mg-effect adjustment is based on the change in shell-calcite δ18O by +0.17‰ per mol% MgCO3 (Jiménez-López et al., 2004). This adjustment is critical to the determination of calcification temperatures for modern articulated brachiopods, especially the ones from cold and warm water habitats, and/or the ones with exceptionally low or high MgCO3 contents, which otherwise may be offset (lower or higher) by as much as 7°C. Furthermore, this adjustment may be critical for fossil brachiopods and other marine calcitic invertebrates with variable MgCO3 contents, and when using their δ18O and ∆47 for determining ancient seawater δ18O compositions and temperatures.

Comparison of species-specific oxygen isotope paleotemperature equations: Sensitivity analysis using planktonic foraminifera from the Cariaco Basin, Venezuela

Abstract The oxygen isotope compositions of seven species of planktonic foraminifera ( Globigerinoides ruber (pink), Globigerina bulloides , Globorotalia menardii , Globigerinoides sacculifer , Orbulina universa , Globorotalia crassaformis , and Neogloboquadrina dutertrei ) were measured from biweekly sediment trap samples collected in Cariaco Basin, Venezuela between 2003 and 2008. Sixteen published paleotemperature equations were used to estimate calcification temperatures from this δ 18 O data. As part of the CARIACO time series, routine in situ water column chemistry measurements and hydrographic monitoring has occurred since 1995. The calcification temperature estimates were compared with these monthly water column temperature measurements to ascertain species depth habitats and to determine which equations yielded the most realistic temperature estimates. No single equation provided accurate temperature estimates for all species. Rather, the equations yielded temperature estimates that varied by 2–4 °C for each species. Overall species-specific paleotemperature equations based on plankton tow samples yield the coldest temperature estimates, while generic equations produce the warmest temperatures. Our results indicate that when using planktonic foraminiferal δ 18 O to make paleotemperature estimates, equations must be carefully selected on a species by species basis. Here we provide general guidelines for selecting paleotemperature equations. We also evaluated how well the δ 18 O difference between the surface dwelling G. ruber and the deeper dwelling N. dutertrei estimates the surface to thermocline temperature gradient. The Δδ 18 O is small (0.61‰) during winter due to upwelling and increases (1.37‰) during summer as upwelling ceases and surface waters become thermally stratified.

Structural analysis and paleoenvironmental potential of dog cockle shells (Glycymeris glycymeris) in Brittany, northwest France

To compensate for the lack of high resolution data on variation in environmental parameters before the establishment of monitoring networks, we evaluated dog cockle shells (Glycymeris glycymeris; Mollusca: Bivalvia) as archives for temperature and climate perturbations such as the North Atlantic Oscillation (NAO) over the last 50years. Oxygen isotope compositions were measured from annual increments of shells collected from the Bay of Brest and the Chausey Islands in northwestern France. Simultaneously, sclerochronological analyses were performed on the same shells. Oxygen isotopic ratios measured in shells collected from the Bay of Brest were compared to in situ temperature and salinity data from monitoring stations and buoys in order to build a specific paleotemperature equation (r2=0.87; p<0.001) for the dog cockle. The specific equation was used to estimate sea surface temperatures around the Chausey Islands between 1966 and 1994. Standardized growth indices (SGI) calculated through sclerochronological methods were strongly correlated to oxygen isotope-derived temperatures. The SGI however did not capture the area's predominant climate pattern, the NAO. This study demonstrates that dog cockle shells can be used for long-term high-resolution reconstructions of past sea surface temperatures.

DECIPHERING THE MODERN CALCIFICATION DEPTH OF GLOBIGERINA BULLOIDES IN THE SOUTHWESTERN INDIAN OCEAN FROM ITS OXYGEN ISOTOPIC COMPOSITION

The water depth at which the planktic foraminiferal species Globigerina bulloides calcifies its shell has been deciphered based on the oxygen isotopic composition of its shells in surface sediment samples collected from the southwestern Indian Ocean. The seawater temperature for water depths ranging from 0−200 m was estimated from G. bulloides δ 18 O by using various paleotemperature equations. Out of several paleotemperature equations, the one given by Epstein and others (1953) provided the best-fit results. Similarly, the estimated seawater salinity was calculated using the seawater δ 18 O calculated from G. bulloides δ 18 O, and measured seawater temperature taken from the Levitus database. A comparison of seawater salinity and temperature estimated from G. bulloides δ 18 O, with the measured seawater salinity and temperature at different water depths, shows that the calcification depth of G. bulloides varies latitudinally. From the equator to ~15°S, the calcification depth of G. bulloides was inferred to be ~75 m. Between 15°S and ~43°S, the estimated seawater temperature matched well with the seawater temperature at ~200 m water depth, indicating that the G. bulloides calcification depth was comparatively deeper in this region. Farther south of ~43°S, however, the calcification depth appears to be considerably shallower than in the northern part of the study area. The comparison also showed that irrespective of latitudinal region, the estimated seawater temperature matches well with the seawater temperature during the austral spring season suggesting that G. bulloides is abundant at that time. The findings will help in paleoclimatic reconstruction studies based on characteristics of G. bulloides.

Fossil dripwater in stalagmites reveals Holocene temperature and rainfall variation in Amazonia

Most proxy records used for reconstruction of Holocene climate of Amazonia are unable to quantitatively distinguish between the effect of temperature and rainfall amounts. We present a new isotope technique applied to a ∼ 13,500 yr stalagmite archive from Peruvian Amazonia. By analysing the coupled isotope composition of fossil dripwater trapped in stalagmite fluid inclusions, and that of the calcite hosting the fluid inclusions, we were able to calculate independent paleotemperatures and rainfall amounts. This stalagmite record shows that Holocene climate variation was controlled by orbitally-forced Southward migration of the Inter Tropical Convergence Zone. While temperature remained constant, isotope variation of rainwater, reflected in fluid inclusion water δ 18O composition, suggests a ∼ 15–30% increase in convective rainfall through the Holocene. A comparison of the low-land Peruvian fluid inclusion record with the high Andean Huascaran ice core record shows a constant ∼ 12‰ offset of δ 18O curves for the Holocene, suggesting that Andean vertical temperature gradients (lapse rates) did not vary much over the last 9000 years. During the Younger Dryas interval, however, the offset of δ 18O values was much higher than in the Holocene. This may be attributed to a relative drop in air temperatures in the highlands (higher lapse rate), caused by long distance teleconnections to climate perturbations in the North Atlantic. In a wider perspective, fluid inclusion isotope analysis drastically improves paleotemperature reconstructions based on speleothem calcite δ 18O data, because it provides the δ 18O value of drip water through time, which is usually the most important unknown in paleotemperature equations.

An expression for the overall oxygen isotope fractionation between the sum of dissolved inorganic carbon and water

Stable oxygen isotope fractionation factors (α's) between individual dissolved carbonate species (CO2(aq.), H2CO3, HCO3−, CO32−) and water, and their sum ( = [CO2(aq.)] + [H2CO3] + [HCO3−] + [CO32−]) and water are fundamental geochemical parameters. While values of , α, and have recently been examined, an overall mass balance for stable oxygen isotopes in the carbonate system still needs to be derived. The theoretical treatment of this mass balance is not trivial (in contrast to, e.g., stable carbon isotopes) because (1) oxygen is exchanged with H2O and (2) complications arise from the nature of the oxygen mole fractions. I use the recently reported individual α's to derive an expression for the overall oxygen isotope fractionation between and water. This allows the calculation of α(−H2O) at any given pH of the solution. The result differs from a relationship published earlier, and implications for previous studies are discussed. Numerical routines in MATLAB and Excel for the present calculation have been developed and are available as auxiliary material (Software S1). Applications of the expression include, for example, examination of biomineralization mechanisms and corrections to isotopic paleotemperature equations. The latter application can significantly alter reconstructions of Phanerozoic climate change.

Experimental determination of salinity, temperature, growth, and metabolic effects on shell isotope chemistry ofMytilus eduliscollected from Maine and Greenland

To study the effects of temperature, salinity, and life processes (growth rates, size, metabolic effects, and physiological/genetic effects) on newly precipitated bivalve carbonate, we quantified shell isotopic chemistry of adult and juvenile animals of the intertidal bivalveMytilus edulis(Blue mussel) collected alive from western Greenland and the central Gulf of Maine and cultured them under controlled conditions. Data for juvenile and adultM. edulisbivalves cultured in this study, and previously by Wanamaker et al. (2006), yielded statistically identical paleotemperature relationships. On the basis of these experiments we have developed a species‐specific paleotemperature equation for the bivalveM. edulis[T °C = 16.28 (±0.10) − 4.57 (±0.15) {δ18OcVPBD −δ18OwVSMOW} + 0.06 (±0.06) {δ18OcVPBD −δ18OwVSMOW}2; r2= 0.99; N = 323; p &lt; 0.0001]. Compared to the Kim and O'Neil (1997) inorganic calcite equation,M. edulisdeposits its shell in isotope equilibrium (δ18Ocalcite) with ambient water. Carbon isotopes (δ13Ccalcite) from sampled shells were substantially more negative than predicted values, indicating an uptake of metabolic carbon into shell carbonate, andδ13Ccalcitedisequilibrium increased with increasing salinity. Sampled shells ofM. edulisshowed no significant trends inδ18Ocalcitebased on size, cultured growth rates, or geographic collection location, suggesting that vital effects do not affectδ18OcalciteinM. edulis. The broad modern and paleogeographic distribution of this bivalve, its abundance during the Holocene, and the lack of an intraspecies physiologic isotope effect demonstrated here make it an ideal nearshore paleoceanographic proxy throughout much of the North Atlantic Ocean.

Reconstruction of seasonal temperature variability in the tropical Pacific Ocean from the shell of the scallop, Comptopallium radula

We investigated the oxygen isotope composition (δ 18O) of shell striae from juvenile Comptopallium radula (Mollusca; Pectinidae) specimens collected live in New Caledonia. Bottom-water temperature and salinity were monitored in-situ throughout the study period. External shell striae form with a 2-day periodicity in this scallop, making it possible to estimate the date of precipitation for each calcite sample collected along a growth transect. The oxygen isotope composition of shell calcite (δ 18O shell calcite) measured at almost weekly resolution on calcite accreted between August 2002 and July 2003 accurately tracks bottom-water temperatures. A new empirical paleotemperature equation for this scallop species relates temperature and δ 18O shell calcite: t ( ° C ) = 20.00 ( ± 0.61 ) - 3.66 ( ± 0.39 ) × ( δ 18 O shell calcite VPDB - δ 18 O water VSMOW ) The mean absolute accuracy of temperature estimated using this equation is 1.0 °C at temperatures between 20 and 30 °C. Uncertainties regarding the precise timing of CaCO 3 deposition and the actual variations in δ 18O water at our study sites probably contribute to this error. Comparison with a previously published empirical paleotemperature equation indicates that C. radula calcite is enriched in 18O by ∼0.7‰ relative to equilibrium. Given the direction of this offset and the lack of correlation between shell growth rate and δ 18O shell calcite, this disequilibrium is unlikely to be related to kinetic isotope effects. We suggest that this enrichment reflects (1) a relatively low pH in the scallop’s marginal extrapallial fluid (EPF), (2) an isotopic signature of the EPF different from that of seawater, or (3) Rayleigh fractionation during the biocalcification process. Relative changes in δ 18O shell calcite reflect seawater temperature variability at this location and we suggest that the shell of C. radula may be useful as an archive of past seawater temperatures.

The oxygen isotope composition of planktonic foraminifera from the Cariaco Basin, Venezuela: Seasonal and interannual variations

Material collected during a three-year sediment trapping experiment in the Cariaco Basin, Venezuela (January 1997 to December 1999) is used to examine both temporal and inter-species variability in the oxygen isotope composition of planktonic foraminifera. Specifically, this study compares the oxygen isotope composition of six species of planktonic foraminifera ( Globigerinoides ruber (pink), Globigerina bulloides, Neogloboquadrina dutertrei, Orbulina universa, Globorotalia menardii and Globorotalia crassaformis) with the climatology and hydrography of the region, and evaluates the application of each species for use in paleoceanographic reconstructions. The isotope results are consistent with known depth habitats for all six species. The lowest δ 18O values (− 1 to − 2‰) were measured on G. ruber (pink) and G. bulloides, two species that live in the surface mixed layer. Values for deeper-dwelling species such as N. dutertrei, G. menardii and G. crassaformis are higher, predominantly ranging from 0 to − 0.5‰. Temperature estimates derived using species-specific paleotemperature equations indicate that G. ruber (pink) accurately estimates sea surface temperatures (SSTs) throughout the year, while G. bulloides temperature estimates are similar to measured surface temperatures only during the upwelling season (January–April). For the remainder of the year, the δ 18O-derived temperatures for G. bulloides typically are lower than the measured SST. Although the maximum flux of all species occurs during upwelling, the flux-weighted annual mean isotopic composition of the six species indicates that only G. bulloides is biased towards this season. Therefore, we conclude that the sediment δ 18O record of G. ruber (pink) is most suitable for estimating past values of mean annual SST, while G. bulloides provides information on conditions during spring upwelling. The depth of calcification of N. dutertrei varies seasonally in response to changes in the depths of the thermocline and chlorophyll maximum. As a result, the δ 18O difference between G. ruber (pink) and N. dutertrei provides an estimate of the annual surface to thermocline temperature gradient in the basin.

An aquaculture‐based method for calibrated bivalve isotope paleothermometry

To quantify species‐specific relationships between bivalve carbonate isotope geochemistry (δ18Oc) and water conditions (temperature and salinity, related to water isotopic composition [δ18Ow]), an aquaculture‐based methodology was developed and applied to Mytilus edulis (blue mussel). The four‐by‐three factorial design consisted of four circulating temperature baths (7, 11, 15, and 19°C) and three salinity ranges (23, 28, and 32 parts per thousand (ppt); monitored for δ18Ow weekly). In mid‐July of 2003, 4800 juvenile mussels were collected in Salt Bay, Damariscotta, Maine, and were placed in each configuration. The size distribution of harvested mussels, based on 105 specimens, ranged from 10.9 mm to 29.5 mm with a mean size of 19.8 mm. The mussels were grown in controlled conditions for up to 8.5 months, and a paleotemperature relationship based on juvenile M. edulis from Maine was developed from animals harvested at months 4, 5, and 8.5. This relationship [T°C = 16.19 (±0.14) − 4.69 (±0.21) {δ18Oc VPBD − δ18Ow VSMOW} + 0.17 (±0.13) {δ18Oc VPBD − δ18Ow VSMOW}2; r2 = 0.99; N = 105; P &lt; 0.0001] is nearly identical to the Kim and O'Neil (1997) abiogenic calcite equation over the entire temperature range (7–19°C), and it closely resembles the commonly used paleotemperature equations of Epstein et al. (1953) and Horibe and Oba (1972). Further, the comparison of the M. edulis paleotemperature equation with the Kim and O'Neil (1997) equilibrium‐based equation indicates that M. edulis specimens used in this study precipitated their shell in isotopic equilibrium with ambient water within the experimental uncertainties of both studies. The aquaculture‐based methodology described here allows similar species‐specific isotope paleothermometer calibrations to be performed with other bivalve species and thus provides improved quantitative paleoenvironmental reconstructions.

The relationship between the hydrogen and oxygen isotopes of freshwater bivalve shells and their home streams

Recent technical advances have facilitated the analysis of microgram-size samples of mollusk shell for the hydrogen isotope composition of the organic matrix. Our results suggest that these data are meaningful as the δ D shell values of samples of freshwater bivalve shells from different streams preserve the relative distribution of δ D values from the water they inhabited over their lifetime (δ D shell − δ D water = − 145‰) after correcting for the contribution of exchangeable hydrogen (∼ 36%) in organic matrix. When δ D shell values are compared to the δ 18O values of the mineral fraction of the shell (CaCO 3 as aragonite), they plot along a line that reflects the isotopic composition of the waters in which the bivalves lived. This relationship is potentially useful for paleoenvironmental reconstructions because the δ D value of the organic matrix may serve as an independent proxy for the δ 18O value of water in paleotemperature equations.

Shell of the Great Scallop Pecten maximus as a high‐frequency archive of paleoenvironmental changes

We investigate the environmental and biological controls on oxygen isotope ratios in the shells of modern Pecten maximus bivalves collected alive in the Bay of Brest (France). Seasonal δ18O profiles, obtained by drilling the daily calcite ridges deposited at the surface of the left valve, were compared with in situ measurements of temperature and salinity. We show that P. maximus δ18O values accurately track seasonal variations in bottom water temperature. Shell growth rate has no significant effect on shell δ18O values. Our study demonstrates that daily variations in water temperature can be reconstructed within about 0.5°C. Temperatures estimated with the paleotemperature equation established in this study were compared with temperatures derived from previously published equations. The comparison indicates that the most commonly used paleotemperature equation for biogenic calcite (Epstein et al., 1953) provides inaccurate temperature estimates, but the Kim and O'Neil (1997) equation, established from abiogenic calcite precipitation, provides results very similar to ours and should therefore be used for scallop individuals coming from populations where proper empirical calibration cannot be done. Pecten maximus bivalves precipitate calcite in isotopic equilibrium with seawater, produce large daily growth striae, are stenohaline, and are well preserved in archeological and geological deposits, making them an excellent high‐frequency archive of paleoenvironmental change.

Determining terrestrial paleotemperatures using the oxygen isotopic composition of pedogenic carbonate

Quantifying terrestrial paleotemperatures has proven to be a challenging endeavor. This paper explores the use of the oxygen isotopic composition of pedogenic calcite for terrestrial paleotemperature reconstructions. We generate three paleotemperature equations, two of them derived by simultaneous solution of equations that describe the fractionation of oxygen isotopes in the exogenic cycle, while the third equation is derived using a linear regression through modern δ18Occ/MAT (oxygen isotopic composition of pedogenic calcite versus mean annual temperature) data.We use the paleotemperature equations to evaluate climate change across the K–T boundary by analyzing the oxygen isotopic composition of pedogenic carbonates from a series of stacked paleosols in west Texas. The fabrics in the calcite nodules were evaluated in order to identify soil carbonate components that have the best chance of containing isotopic information that represents ancient pedogenic conditions. The average temperature for West Texas during the time period studied is 18+/−0.5 °C. Two temperature excursions are recognized prior to the K–T boundary in which terrestrial temperatures increase by about 4 °C for relatively short periods of time. These short greenhouse events documented in west Texas correlate with changes in marine temperatures as well as terrestrial temperatures in North Dakota during the same time period. Latitudinal temperature gradients generated using the West Texas and North Dakota data indicate that the spatial simultaneous solution yields the most reliable temperature reconstructions of the three equations considered in this study.

Modern systematics and environmental significance of stable isotopic variations in Wanxiang Cave, Wudu, Gansu, China

This paper presents the stable isotopic compositions from the cave dripwater and actively forming soda straw stalactites collected from Wanxiang Cave, Wudu, Gansu, located on the Qinghai-Tibetan Plateau and Loess Plateau transition zone, China. The δ18Odw and δDdw of dripwater samples in the cave plot directly on the local MWL, constructed by using GNIP data from 3 sites surrounding the cave regions (Lanzhou, Xi’an, and Chengdu), the nearest site to the cave, suggesting that there is a close relationship between the δ18Odw of the cave water and the δ18O of the precipitations. Using the measured δ18Odw and δ18Omc values from the mid-farthest parts from the cave entrance and the carbonate paleotemperature equation, the calculated tem peratures range from 8.9 to 12.4°C, with the mean value of 10.7°C and the temperature calculated at 8 locations in the farthest part of the cave is in the range of 10.1–12.4°C with the mean value of 11.5°C, being consistent with the survey value(10.99°C)in the cave, slightly lower than the mean annual temperature (14.4°C) in Wudu. This suggests that modern speleothems are forming under isotopic equilibrium and their isotopic composition accurately reflects the mean annual temperature at the surface, indicating that the isotopic composition of the modern speleothems records local temperature change with credibility.