Apparent preservation of primary foraminiferal Mg/Ca ratios and Mg-banding in recrystallized foraminifera

Trace element and δ18O values of foraminifera are widely used to reconstruct oceanic temperatures throughout the Cenozoic and beyond. Previous work evaluating the geochemistry of foraminifera with differing degrees of physical preservation have shown that Mg/Ca and δ18O paleothermometers give discrepant values in recrystallized tests, with planktonic oxygen isotopes often yielding significantly lower temperatures than Mg/Ca ratios. To study the mobility of elements during diagenesis, we performed microspatial trace element analyses in Eocene Morozovella. Element maps show that trace element banding is readily identifiable and preserved, to an extent, in texturally recrystallized tests. A reaction-diffusion model was used to test whether the preservation of Mg-banding and the decoupling of δ18O and Mg/Ca values could be the result of diffusively limited “closed-system” recrystallization. Results show that, in a closed system, internal features (such as Mg-banding) will dissipate prior to changes in bulk Mg/Ca composition, while the bulk δ18O value will typically change faster than Mg/Ca. This is observed regardless of what partitioning coefficient is used for Mg and demonstrates that the planktonic Mg/Ca proxy is more diagenetically robust than the δ18O proxy. Thus, this model can explain the observed decoupling of these two proxies. Furthermore, the preservation of intra-test Mg-banding shows potential for use in evaluating the preservation of primary Mg/Ca values and hence the accuracy of paleotemperature reconstructions

Revealing their true stripes: Mg/Ca banding in the Paleogene planktonic foraminifera genus Morozovella and implications for paleothermometry

The Mg/Ca ratio of foraminiferal calcite is a widely used empirical proxy for ocean temperature. Foraminiferal Mg/Ca-temperature relationships are based on extant species and are species-specific, introducing uncertainty when applying them to the fossil tests of extinct groups. Many modern species show remarkable heterogeneity in their intra-test Mg distributions, typically due to the presence of high Mg bands, which have a biological origin. Importantly, banding patterns differ between species, which could affect Mg/Ca-temperature relationships. Few studies have looked at intra-test variability in Mg/Ca ratios in extinct species of foraminifera, despite the obvious implications for paleothermometry. We used electron probe microanalysis (EPMA) to investigate intra-test Mg distributions in the fossil tests of two species of planktonic foraminifera from the extinct muricate mixed-layer-dwelling genus Morozovella, commonly used in Paleogene sea surface temperature reconstructions. Both M. aragonensis and M. crater show striking Mg banding patterns with multiple high and low Mg/Ca band pairs throughout the test wall in all chambers. The intra-test Mg variability in M. aragonensis and M. crater is similar to that in modern species widely used in paleoclimate reconstructions and banding patterns are consistent with published growth models for modern forms, albeit with subtle differences. The presence of Mg bands supports the application of Mg/Ca-palaeothermometry in extinct Morozovella species as well as the utility of EPMA for examining preservation of foraminifera tests in paleoclimatological studies. However, we emphasize the importance of rigorous assessments of inter- and intra-test Mg variability when using microanalytical techniques for foraminiferal Mg/Ca paleothermometry

Evidence for heterothermic endothermy and reptile-like eggshell mineralization in Troodon, a non-avian maniraptoran theropod

The dinosaur–bird transition involved several anatomical, biomechanical, and physiological modifications of the theropod bauplan. Non-avian maniraptoran theropods, such as Troodon, are key to better understand changes in thermophysiology and reproduction occurring during this transition. Here, we applied dual clumped isotope (Δ47 and Δ48) thermometry, a technique that resolves mineralization temperature and other nonthermal information recorded in carbonates, to eggshells from Troodon, modern reptiles, and modern birds. Troodon eggshells show variable temperatures, namely 42 and 29 ± 2 °C, supporting the hypothesis of an endothermic thermophysiology with a heterothermic strategy for this extinct taxon. Dual clumped isotope data also reveal physiological differences in the reproductive systems between Troodon, reptiles, and birds. Troodon and modern reptiles mineralize their eggshells indistinguishable from dual clumped isotope equilibrium, while birds precipitate eggshells characterized by a positive disequilibrium offset in Δ48. Analyses of inorganic calcites suggest that the observed disequilibrium pattern in birds is linked to an amorphous calcium carbonate (ACC) precursor, a carbonate phase known to accelerate eggshell formation in birds. Lack of disequilibrium patterns in reptile and Troodon eggshells implies these vertebrates had not acquired the fast, ACC-based eggshell calcification process characteristic of birds. Observation that Troodon retained a slow reptile-like calcification suggests that it possessed two functional ovaries and was limited in the number of eggs it could produce; thus its large clutches would have been laid by several females. Dual clumped isotope analysis of eggshells of extinct vertebrates sheds light on physiological information otherwise inaccessible in the fossil record

A diagenetic origin for isotopic variability of sediments deposited on the margin of Great Bahama Bank, insights from clumped isotopes

The clumped isotope temperature proxy has been used to investigate the diagenetic history of carbonate sediments in two cores recovered during ODP Leg 166 on the margin of Great Bahama Bank. While periplatform sediments constitute a tempting archive of paleo ocean chemistry as they are unlikely to be subducted, their primary limitation is a well-documented susceptibility to post-depositional diagenetic reworking. The crystallization temperatures reconstructed using the clumped isotope proxy, as well as the mineralogy and δ13C and δ18O values have been used to determine the relative effects of sediment mixing and sediment recrystallization. Results show that as sediments undergo diagenetic alteration at the seafloor, their initially “warm” clumped isotope composition is overprinted at cooler benthic temperatures. This process appears to occur in an environment with sufficient fluid exchange to overprint carbon isotopes; an observation confirmed in a separate study by analyses of calcium, a similarly rock-buffered element. This early reactive exchange between carbonates and fluids is likely driven by the conversion of metastable aragonite to calcite. This partially occurs within the “flushed zone”, where porewater compositions remain compositionally similar to seawater throughout the upper ∼40 meters of the sediment column. Sediments dominated by open system isotopic compositions correspond to a period of minimal sediment accumulation between 2 and 3 Ma. More deeply buried Miocene sediments of the more platform-proximal Site 1003 show evidence of subsequent recrystallization, incorporating the warmer geothermal TΔ47 values, and more as well as modified water δ18O values, likely driven by co-evolving porewater and carbonate oxygen isotopes. Reconstructed water δ18O values of these deeper sediments at Site 1003 are considerably more positive than the measured modern values, suggesting that porewater δ18O values were more positive during the Miocene. Sediments deposited at the platform distal Site 1006 between the early and middle Miocene did not show evidence for this deeper recrystallization. Differences in diagenetic behavior between the two sites cannot be solely accounted for by differences in sediment accumulation rate. To illustrate this, time-integrated models were constructed which simulated the burial of identically reactive material through the depositional history of each site, the sediments deposited at Site 1006 appear to “stabilize” after an initial phase of neomorphism, whereas the more platform proximal Site 1003 continues to recrystallize during deeper burial, apparently in the presence of a parent fluid with a more positive δ18O value than observed today. We conclude that despite consisting of the same end-member sediment sources, and being spatially separated by less than 30 km, the difference in clumped and oxygen isotopic composition between Sites 1003 and 1006 can be predominantly attributed to differences in the rate and duration of recrystallization during burial

An abrupt Middle-Miocene increase in fluid flow into the Leeward Margin Great Bahama Bank, constraints from δ44Ca and Δ47 values

•The history of fluid advection on the margin of Great Bahama bank was calculated.•Advection rates into the sediment column increased abruptly at 12.5 Ma.•This change in flux coincides with a significant change in platform structure. The advection of seawater into the sediments deposited on the margins of Great Bahama Bank has been demonstrated to play in important role in ventilating the uppermost sediments, as well as supplying elements for diagenetic reactions deeper within the platform. Here, we implement a numerical model to calculate the rate of fluid advection at ODP Site 1003, using calcium isotopes as a tracer for fluid advection. A key parameter for this model, the rate of recrystallization of sediments, was constrained using the clumped isotope proxy. The model was tuned to existing datasets of clumped and carbonate calcium isotope measurements for this site. Results of this modeling effort indicate that, prior to ∼15 Ma, fluids were advected into the platform at a rate substantially lower than the present day. The rate of advection abruptly increased ∼13 Ma to a value greater than the present day. The increase in advective flux during the mid-Miocene coincides with a major reorganization of the platform from a ramp-like geometry to a steeper carbonate platform, indicating a change in the relative contributions of different mechanisms governing fluid flow on the platform margin and interior. This paper aims to explore the utility of using clumped and calcium isotopes to quantitatively reconstruct past fluid advection rates using this novel technique

Oxygen Isotopic Exchange Between CO 2

Experiments have been conducted in which CO2 gases with varying C and O isotopic compositions and with stochastic and nonstochastic Δ47 values have been allowed to equilibrate with phosphoric acid of two concentrations in reaction vessels of varying dimensions at temperatures of 25 and 90 °C. Rates of 13C18O and 18O exchange between the CO2 and the phosphoric acid varied as a function of the length of exposure, volume of reaction vessel, acid strength, and difference of the initial Δ47 and δ18O values of the CO2 from theoretical equilibrium values. The Δ47 values were also altered by heated stainless steel surfaces such as those found within the Kiel device and other preparation systems. These results have been used to explain variations in the differences in the fractionation between 25 and 90 °C reported for calcite by different workers as well as differences in the slopes between temperature and Δ47 values produced by reacting samples at different temperatures (25 and 90 °C). Key Points Isotopic exchanges take place between CO2 and H2PO4 affected the Δ47 signal during normal reactions The extend of exchange affecting Δ47 is dependent upon the difference between the Δ47 of the CO2 and the theoretical equilibrium value Differences of the temperature‐dependent acid fractionation between 25 and 90 °C are dependent upon the absolute Δ47 value of the sampl

Boiled or roasted? Bivalve cooking methods of early Puerto Ricans elucidated using clumped isotopes

Cooking technique reflects a combination of cultural and technological factors; here, we attempt to constrain bivalve cooking temperatures for a pre-Columbian Puerto Rican native population using carbonate clumped isotopes. Analyses of 24 bivalve specimens ( ) from a shell midden in Cabo Rojo, Puerto Rico, suggest that samples were heated up to 200°C, indicating that roasting rather than boiling may have been the preferred cooking technique. More than half of analyzed samples exhibited a distinct change from modern uncooked shells, possibly reflecting different cooking techniques or the use of a single method wherein shells are unevenly heated, such as when placed on a heated surface. Roasting bivalves would not necessitate the use of ceramic technologies, an observation concurrent with the absence of such artifacts at this site

Prehistoric cooking versus accurate palaeotemperature records in shell midden constituents

The reconstruction of pre-depositional cooking treatments used by prehistoric coastal populations for processing aquatic faunal resources is often difficult in archaeological shell midden assemblages. Besides limiting our knowledge of various social, cultural, economic and technological aspects of shell midden formation, unknown pre-depositional cooking techniques can also introduce large errors in palaeoclimate reconstructions as they can considerably alter the geochemical proxy signatures in calcareous skeletal structures such as bivalve shells or fish otoliths. Based on experimental and archaeological data, we show that carbonate clumped-isotope thermometry can be used to detect and reconstruct prehistoric processing methods in skeletal aragonite from archaeological shell midden assemblages. Given the temperature-dependent re-equilibration of clumped isotopes in aragonitic carbonates, this allows specific processing, cooking or trash dispersal strategies such as boiling, roasting, or burning to be differentiated. Besides permitting the detailed reconstruction of cultural or technological aspects of shell midden formation, this also allows erroneous palaeoclimate reconstructions to be avoided as all aragonitic shells subjected to pre-historic cooking methods show a clear alteration of their initial oxygen isotopic composition

A calibration equation between Δ48 values of carbonate and temperature

RationaleInformation on the temperature of formation or alteration of carbonate minerals can be obtained by measuring the abundance of the isotopologues 47 and 48 (Δ47 and Δ48 values) of CO2 released during acid dissolution. The combination of these two proxies can potentially provide a greater insight into the temperature of formation, particularly if the carbonate minerals form by non‐equilibrium processes.MethodsWe have precipitated calcium carbonates at seven temperatures between 5 and 65°C and measured their δ48 values using a Thermo‐253 plus isotope ratio mass spectrometer. The values were transformed to Δ48 values in the conventional manner and then converted to the carbon dioxide equilibrium scale.ResultsUsing the Δ48 values, we have established an empirical calibration between temperature and Δ48 values:Δ48=0.0142±0.0012×106/T2+0.088±0.014R2=0.96.ConclusionsThe calibration line produced allows the determination of the temperature of natural carbonates using the Δ48 values and agrees with the measurements of the Δ47 and Δ48 values of some carbonates assumed to have formed under equilibrium conditions