Description: The shells of marine mollusks are widely used archives of past climate and ocean chemistry. Whilst the measurement of mollusk δ18O to develop records of past climate change is a commonly used approach, it has proven challenging to develop reliable independent paleothermometers that can be used to deconvolve the contributions of temperature and fluid composition on molluscan oxygen isotope compositions. Here we investigate the temperature dependence of 13C–18O bond abundance, denoted by the measured parameter Δ47, in shell carbonates of bivalve mollusks and assess its potential to be a useful paleothermometer. We report measurements on cultured specimens spanning a range in water temperatures of 5 to 25 °C, and field collected specimens spanning a range of −1 to 29 °C. In addition we investigate the potential influence of carbonate saturation state on bivalve stable isotope compositions by making measurements on both calcitic and aragonitic specimens that have been cultured in seawater that is either supersaturated or undersaturated with respect to aragonite. We find a robust relationship between Δ47 and growth temperature. We also find that the slope of a linear regression through all the Δ47 data for bivalves plotted against seawater temperature is significantly shallower than previously published inorganic and biogenic carbonate calibration studies produced in our laboratory and go on to discuss the possible sources of this difference. We find that changing seawater saturation state does not have significant effect on the Δ47 of bivalve shell carbonate in two taxa that we examined, and we do not observe significant differences between Δ47-temperature relationships between calcitic and aragonitic taxa.

Description: The East Asian monsoon is one of Earth’s most significant climatic phenomena, and numerous paleoclimate archives have revealed that it exhibits variations on orbital and suborbital time scales. Quantitative constraints on the climate changes associated with these past variations are limited, yet are needed to constrain sensitivity of the region...

Description: The clumped isotope (Δ47) proxy is a promising geochemical tool to reconstruct past ocean temperatures far back in time and in unknown settings, due to its unique thermodynamic basis that renders it independent from other environmental factors like seawater composition. Although previously hampered by large sample-size requirements, recent methodological advances have made the paleoceanographic application of Δ47 on small (〈5 mg) foraminifer samples possible. Previous studies show a reasonable match between Δ47 calibrations based on synthetic carbonate and various species of planktonic foraminifers. However, studies performed before recent methodological advances were based on relatively few species and data treatment that is now outdated. To overcome these limitations and elucidate species-specific effects, we analyzed 14 species of planktonic foraminifers in sediment surface samples from 13 sites, covering a growth temperature range of ∼0–28 °C. We selected mixed layer-dwelling and deep-dwelling species from a wide range of ocean settings to evaluate the feasibility of temperature reconstructions for different water depths. Various techniques to estimate foraminifer calcification temperatures were tested in order to assess their effects on the calibration and to find the most suitable approach. Results from this study generally confirm previous findings that there are no species-specific effects on the Δ47-temperature relationship in planktonic foraminifers, with one possible exception. Various morphotypes of Globigerinoides ruber were found to often deviate from the general trend determined for planktonic foraminifers. Our data are in excellent agreement with a recent foraminifer calibration study that was performed with a different analytical setup, as well as with a calibration based exclusively on benthic foraminifers. A combined, methodologically homogenized dataset also reveals very good agreement with an inorganic calibration based on travertines. Our findings highlight the potential of the Δ47 paleothermometer to be applied to recent and extinct species alike to study surface ocean temperatures as well as thermocline variability for a multitude of settings and time scales.

Description: Until recently, the Arctic was assumed to be characterised by relatively warm conditions throughout the Early Cenozoic. However, recent investigations on deep-sea drilling cores from northern high latitudes, including the Arctic Ocean, give apparently contradicting results. Here we report the intermittent occurrence of certain temperature indicators in the Early Tertiary sedimentary sequence on Svalbard, which may represent the first direct northern high-latitude record of cold water temperatures for the early Cenozoic. Glendonites (calcite pseudomorphs after calcium carbonate hexahydrate) and erratics in otherwise fine-grained sediments require near-freezing temperatures and the presence of at least seasonal sea ice. This succession also contains coal seams and other warm climate indicators, but not on the same stratigraphical levels. These results imply the occurrence of cooling phases episodically during the warm background climate of the Paleocene and Eocene, suggesting that Arctic temperature variability was much greater than previously recognized.

Description: : Carbonate- 18 O paleothermometry is used in many diagenetic studies to unravel the thermal history of basins. However, this approach generally requires an assumed pore-water 18 O ( 18 O pw ) value, a parameter that is difficult to quantify in past regimes. In addition, many processes can change the original isotopic composition of pore water, which further complicates the assignment of an initial 18 O pw and can lead to erroneous temperature estimates. Here, we use clumped-isotope thermometry, a proxy based on the 13 C– 18 O bond abundance in carbonate minerals, to evaluate the temperatures of concretion formation in the Miocene Monterey Formation and the Cretaceous Holz Shale, California. These temperatures are combined with established carbonate–water fractionation factors to calculate the associated 18 O pw . Results demonstrate that diagenetic processes can modify the 18 O of ancient pore water, confounding attempts to estimate diagenetic temperatures using standard approaches. Clumped-isotope-based temperature estimates for Monterey Formation concretions range from ~ 17 to 35°C, up to ~ 12°C higher than traditional 18 O carbonate–water paleothermometry when 18 O pw values are assumed to equal Miocene seawater values. Calculated 18 O pw values range from +0.3 to +2.5 (VSMOW)—higher than coeval Miocene seawater, likely due to 18 O pw modification accompanying diagenesis of sedimentary siliceous phases. Clumped-isotope temperatures for the Holz Shale concretions range from ~ 33 to 44°C, about 15 to 30°C lower than temperatures derived using the traditional method. Calculated 18 O pw values range from –5.0 to –2.9 and likely reflect the influx of meteoric fluids. We conclude that the use of clumped isotopes both improves the accuracy of temperature reconstructions and provides insight into the evolution of 18 O pw during diagenesis, addressing a longstanding conundrum in basin-evolution research.

Description: A bstract :  Septarian concretions exhibit multiple generations of cements that include body, fringe, and spar phases. Classic paragenetic interpretations include initial precipitation of the body followed by fringe(s) and then by spar in more or less discrete events. Traditional approaches (e.g., carbon and oxygen isotope analyses) are generally unable to distinguish paragenetic trends as they relate to specific formation environments (e.g., precipitation during burial or with meteoric influx). Here we present carbonate clumped-isotope, 13 C ( 13 C carb ), and 18 O ( 18 O carb ) values for septarian concretions taken from four host units in order to assess cement paragenesis and overcome traditional shortcomings. Clumped-isotope and 18 O fluid data exhibit a wide range of values, with carbonate precipitation temperatures between ~ 20 and 50°C and 18 O fluid compositions of ~ –14 to +4 (VSMOW). In stable-isotope cross-plots, specific cement phases group together and confirm the paragenesis indicated by superposition. In some cases, samples analyzed from concretion bodies yield temperature and 18 O fluid values that indicate formation at shallow depths, consistent with independent data (e.g., high minus-cement porosity, external laminae deflection). In contrast, other concretion-body analyses indicate relatively high body temperatures that conflict with shallow-formation indices. Petrographic and backscatter scanning electron microscopy (SEM) reveal that concretion bodies partially consist of a secondary, replacement phase, which could explain the higher temperatures expressed in bulk body samples. Based on data for different phases in these septarian concretions, we suggest that initial body-cement precipitation occurred at relatively shallow depths from unmodified seawater, followed by fringe formation at elevated temperatures that likely coincided with the emplacement of the secondary body phase. When considered together, late-stage spar phases exhibit temperatures and 18 O fluid values supportive of spar precipitation from fluids with a significant meteoric component, possibly during uplift.