Development of a quantum cascade laser absorption spectrometer for simultaneous measurement of 13C-18O and 18O-18O clumping in CO2.

RATIONALE: Dual clumped isotope paleothermometry determines carbonate formation temperatures by measuring the frequency of 13C-18O (∆638) and 18O-18O (∆828) pairs in carbonates. It resolves isotopic kinetic biases and thus enables more accurate paleotemperature reconstructions. However, high-precision measurements of 18O-18O clumping using current techniques requires large sample sizes and long acquisition times. METHODS: We developed a mid-infrared isotope ratio laser spectrometer (IRLS) for simultaneous measurement of the isotopologue ratios ∆638 and ∆828 in gas-phase carbon dioxide (CO2) at room temperature. Our IRLS uses a single laser scanning from 2290.7 to 2291.1 cm-1 and a 31 m pathlength optical cell, and it simultaneously measures the five isotopologues required for calculating ∆638 and ∆828: 16O12C16O, 16O13C16O, 16O12C18O, 16O13C18O, and 18O12C18O. In addition, our IRLS can measure 16O12C17O, enabling ∆17O analysis. RESULTS: At ~20°C and a CO2 pressure of ~2 Torr, our IRLS system achieved precisions of 0.128‰ and 0.140‰ within 20 s for abundances of the clumped isotopologues 16O13C18O and 18O12C18O, respectively, and precisions of 0.267‰, 0.245‰, and 0.128‰ for 16O12C16O, 16O13C16O, and 16O12C18O. This yielded precisions of 0.348‰ (∆638) and 0.302‰ (∆828) within 25 s. Simulated sample-reference switching highlights the potential of our system and the need for further development. CONCLUSIONS: We demonstrated simultaneous measurements of ∆638 and ∆828 in CO2 to precisions of <0.35‰ within 25 s using a room-temperature, single-laser IRLS. Future developments on better resolving 16O12C16O and 16O13C16O peaks and system temperature control could further improve the measurement precision.

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.

Biologically driven isotopic fractionations in bivalves: from palaeoenvironmental problem to palaeophysiological proxy.

Traditional bulk stable isotope (δ18 O and δ13 C) and clumped isotope (Δ47 ) records from bivalve shells provide invaluable histories of Earth's local and global climate change. However, biologically driven isotopic fractionations (BioDIFs) can overprint primary environmental signals in the shell. Here, we explore how conventional measurements of δ18 O, δ13 C, and Δ47 in bivalve shells can be re-interpreted to investigate these physiological processes deliberately. Using intrashell Δ47 and δ18 O alignment as a proxy for equilibrium state, we separately examine fractionations and/or disequilibrium occurring in the two major stages of the biomineralisation process: the secretion of the extrapallial fluid (EPF) and the precipitation of shell material from the EPF. We measured δ18 O, δ13 C, and Δ47 in fossil shells representing five genera (Lahillia, Dozyia, Eselaevitrigonia, Nordenskjoldia, and Cucullaea) from the Maastrichtian age [66-69 million years ago (Ma)] López de Bertodano Formation on Seymour Island, Antarctica. Material was sampled from both the outer and inner shell layers (OSL and ISL, respectively), which precipitate from separate EPF reservoirs. We find consistent δ18 O values across the five taxa, indicating that the composition of the OSL can be a reliable palaeoclimate proxy. However, relative to the OSL baseline, ISLs of all taxa show BioDIFs in one or more isotopic parameters. We discuss/hypothesise potential origins of these BioDIFs by synthesising isotope systematics with the physiological processes underlying shell biomineralisation. We propose a generalised analytical and interpretive framework that maximises the amount of palaeoenvironmental and palaeobiological information that can be derived from the isotopic composition of fossil shell material, even in the presence of previously confounding 'vital effects'. Applying this framework in deep time can expand the utility of δ18 O, δ13 C, and Δ47 measurements from proxies of past environments to proxies for certain biomineralisation strategies across space, time, and phylogeny among Bivalvia and other calcifying organisms.

Dual clumped isotope thermometry resolves kinetic biases in carbonate formation temperatures.

Surface temperature is a fundamental parameter of Earth's climate. Its evolution through time is commonly reconstructed using the oxygen isotope and the clumped isotope compositions of carbonate archives. However, reaction kinetics involved in the precipitation of carbonates can introduce inaccuracies in the derived temperatures. Here, we show that dual clumped isotope analyses, i.e., simultaneous ∆47 and ∆48 measurements on the single carbonate phase, can identify the origin and quantify the extent of these kinetic biases. Our results verify theoretical predictions and evidence that the isotopic disequilibrium commonly observed in speleothems and scleractinian coral skeletons is inherited from the dissolved inorganic carbon pool of their parent solutions. Further, we show that dual clumped isotope thermometry can achieve reliable palaeotemperature reconstructions, devoid of kinetic bias. Analysis of a belemnite rostrum implies that it precipitated near isotopic equilibrium and confirms the warmer-than-present temperatures during the Early Cretaceous at southern high latitudes.

Seawater temperature and buffering capacity modulate coral calcifying pH.

Scleractinian corals promote the precipitation of their carbonate skeleton by elevating the pH and dissolved inorganic carbon (DIC) concentration of their calcifying fluid above that of seawater. The fact corals actively regulate their calcifying fluid chemistry implies the potential for acclimation to ocean acidification. However, the extent to which corals can adjust their regulation mechanism in the face of decreasing ocean pH has not been rigorously tested. Here I present a numerical model simulating pH and DIC up-regulation by corals, and use it to determine the relative importance of physiological regulation versus seawater conditions in controlling coral calcifying fluid chemistry. I show that external seawater temperature and buffering capacity exert the first-order control on the extent of pH elevation in the calcifying fluid and explain most of the observed inter- and intra-species variability. Conversely, physiological regulation, represented by the interplay between enzymatic proton pumping, carbon influx and the exchange of calcifying fluid with external seawater, contributes to some variability but remain relatively constant as seawater conditions change. The model quantitatively reproduces variations of calcifying fluid pH in natural Porites colonies, and predicts an average 0.16 unit decrease in Porites calcifying fluid pH, i.e., ~43% increase in H+ concentration, by the end of this century as a combined result of projected ocean warming and acidification, highlighting the susceptibility of coral calcification to future changes in ocean conditions. In addition, my findings support the development of coral-based seawater pH proxies, but suggest the influences of physicochemical and biological factors other than seawater pH must be considered.

Ocean acidification affects coral growth by reducing skeletal density.

Ocean acidification (OA) is considered an important threat to coral reef ecosystems, because it reduces the availability of carbonate ions that reef-building corals need to produce their skeletons. However, while theory predicts that coral calcification rates decline as carbonate ion concentrations decrease, this prediction is not consistently borne out in laboratory manipulation experiments or in studies of corals inhabiting naturally low-pH reefs today. The skeletal growth of corals consists of two distinct processes: extension (upward growth) and densification (lateral thickening). Here, we show that skeletal density is directly sensitive to changes in seawater carbonate ion concentration and thus, to OA, whereas extension is not. We present a numerical model of Porites skeletal growth that links skeletal density with the external seawater environment via its influence on the chemistry of coral calcifying fluid. We validate the model using existing coral skeletal datasets from six Porites species collected across five reef sites and use this framework to project the impact of 21st century OA on Porites skeletal density across the global tropics. Our model predicts that OA alone will drive up to 20.3 ± 5.4% decline in the skeletal density of reef-building Porites corals.

Fluid mixing and the deep biosphere of a fossil Lost City-type hydrothermal system at the Iberia Margin.

Subseafloor mixing of reduced hydrothermal fluids with seawater is believed to provide the energy and substrates needed to support deep chemolithoautotrophic life in the hydrated oceanic mantle (i.e., serpentinite). However, geosphere-biosphere interactions in serpentinite-hosted subseafloor mixing zones remain poorly constrained. Here we examine fossil microbial communities and fluid mixing processes in the subseafloor of a Cretaceous Lost City-type hydrothermal system at the magma-poor passive Iberia Margin (Ocean Drilling Program Leg 149, Hole 897D). Brucite-calcite mineral assemblages precipitated from mixed fluids ca. 65 m below the Cretaceous paleo-seafloor at temperatures of 31.7 ± 4.3 °C within steep chemical gradients between weathered, carbonate-rich serpentinite breccia and serpentinite. Mixing of oxidized seawater and strongly reducing hydrothermal fluid at moderate temperatures created conditions capable of supporting microbial activity. Dense microbial colonies are fossilized in brucite-calcite veins that are strongly enriched in organic carbon (up to 0.5 wt.% of the total carbon) but depleted in (13)C (δ(13)C(TOC) = -19.4‰). We detected a combination of bacterial diether lipid biomarkers, archaeol, and archaeal tetraethers analogous to those found in carbonate chimneys at the active Lost City hydrothermal field. The exposure of mantle rocks to seawater during the breakup of Pangaea fueled chemolithoautotrophic microbial communities at the Iberia Margin, possibly before the onset of seafloor spreading. Lost City-type serpentinization systems have been discovered at midocean ridges, in forearc settings of subduction zones, and at continental margins. It appears that, wherever they occur, they can support microbial life, even in deep subseafloor environments.

Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation.

The stratospheric CO(2) oxygen isotope budget is thought to be governed primarily by the O((1)D)+CO(2) isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO(2) isotopologue (16)O(13)C(18)O, in concert with (18)O and (17)O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric (16)O(13)C(18)O, observed as proportions in the polar vortex that are higher than in any naturally derived CO(2) sample to date. We show, through photochemical experiments, that lower (16)O(13)C(18)O proportions observed in the midlatitudes are determined primarily by the O((1)D)+CO(2) isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher (16)O(13)C(18)O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O((1)D)+CO(2). We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO(2) or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric (16)O(13)C(18)O enrichments may impose additional isotopic constraints on biosphere-atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.