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.

Past climates inform our future.

As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation-a practice that we argue should be widely adopted.

Biogenic carbonate mercury and marine temperature records reveal global influence of Late Cretaceous Deccan Traps.

The climate and environmental significance of the Deccan Traps large igneous province of west-central India has been the subject of debate in paleontological communities. Nearly one million years of semi-continuous Deccan eruptive activity spanned the Cretaceous-Paleogene boundary, which is renowned for the extinction of most dinosaur groups. Whereas the Chicxulub impactor is acknowledged as the principal cause of these extinctions, the Deccan Traps eruptions are believed to have contributed to extinction patterns and/or enhanced ecological pressures on biota during this interval of geologic time. We present the first coupled records of biogenic carbonate clumped isotope paleothermometry and mercury concentrations as measured from a broad geographic distribution of marine mollusk fossils. These fossils preserve evidence of simultaneous increases in coastal marine temperatures and mercury concentrations at a global scale, which appear attributable to volcanic CO2 and mercury emissions. These early findings warrant further investigation with additional records of combined Late Cretaceous temperatures and mercury concentrations of biogenic carbonate.

Heinrich events triggered by ocean forcing and modulated by isostatic adjustment.

During the last glacial period, the Laurentide Ice Sheet sporadically discharged huge numbers of icebergs through the Hudson Strait into the North Atlantic Ocean, leaving behind distinct layers of ice-rafted debris in the ocean sediments. Perplexingly, these massive discharge events-Heinrich events-occurred during the cold portion of millennial-scale climate oscillations called Dansgaard-Oeschger cycles. This is in contrast to the expectation that ice sheets expand in colder climates and shrink in warmer climates. Here we use an ice sheet model to show that the magnitude and timing of Heinrich events can be explained by the same processes that drive the retreat of modern marine-terminating glaciers. In our model, subsurface ocean warming associated with variations in the overturning circulation increases underwater melt along the calving face, triggering rapid margin retreat and increased iceberg discharge. On millennial timescales, isostatic adjustment causes the bed to uplift, isolating the terminus from subsurface warming and allowing the ice sheet to advance again until, at its most advanced position, it is poised for another Heinrich event. This mechanism not only explains the timing and magnitude of observed Heinrich events, but also suggests that ice sheets in contact with warming oceans may be vulnerable to catastrophic collapse even with little atmospheric warming.

End-Cretaceous extinction in Antarctica linked to both Deccan volcanism and meteorite impact via climate change.

The cause of the end-Cretaceous (KPg) mass extinction is still debated due to difficulty separating the influences of two closely timed potential causal events: eruption of the Deccan Traps volcanic province and impact of the Chicxulub meteorite. Here we combine published extinction patterns with a new clumped isotope temperature record from a hiatus-free, expanded KPg boundary section from Seymour Island, Antarctica. We document a 7.8±3.3 °C warming synchronous with the onset of Deccan Traps volcanism and a second, smaller warming at the time of meteorite impact. Local warming may have been amplified due to simultaneous disappearance of continental or sea ice. Intra-shell variability indicates a possible reduction in seasonality after Deccan eruptions began, continuing through the meteorite event. Species extinction at Seymour Island occurred in two pulses that coincide with the two observed warming events, directly linking the end-Cretaceous extinction at this site to both volcanic and meteorite events via climate change.

The effects of Porapak™ trap temperature on δ(18)O, δ(13)C, and Δ47 values in preparing samples for clumped isotope analysis.

RATIONALE: The clumped isotope paleothermometer, a new proxy widely applicable in studies of paleoclimate, tectonics, and paleontology, relates the abundance of doubly substituted isotopologues of carbonate-derived CO2 to the temperature of formation of the carbonate phase. As this technique becomes more widely used, more is discovered about the effects of everyday laboratory procedures on the clumped isotopic composition of CO2 gas. METHODS: Preparation of CO2 for clumped isotope analysis requires the removal of isobaric contaminants prior to measurement, achieved dynamically by passing the CO2 through a gas chromatography column using a helium carrier gas or cryogenically pumping CO2 through a static trap filled with Porapak™ Q (PPQ) material. The stable and clumped isotopic compositions of carbonate standards prepared at PPQ trap temperatures between -40°C and -10°C were measured by isotope ratio mass spectrometry to evaluate potential artifacts introduced by the static PPQ trap method. RESULTS: The stable isotopic composition of carbonates run at temperatures below -20°C was fractionated, despite achieving >99% retrieval of gas at temperatures as cold as -30°C. The δ(13)C and δ(18)O values decreased by ~0.01 and ~0.03 ‰/(°C below -20°C). The raw Δ47 values decreased by 0.003-0.005 ‰/(°C below -20°C), but the final reference-frame-corrected values (Δ47-RFAC ) were unaffected as long as the carbonate samples and standard gases were prepared identically. CONCLUSIONS: Preparing carbonate samples for clumped isotope analysis using a PPQ trap that is too cold can result in erroneous stable isotopic compositions. New and existing labs using the static PPQ trap cleaning procedure should determine the ideal PPQ trap temperature for their particular system through monitoring not only yield through the PPQ trap, but also stable isotopic composition at various PPQ trap temperatures.

Clumped isotope measurements of small carbonate samples using a high-efficiency dual-reservoir technique.

RATIONALE: The measurement of multiply substituted isotopologues of CO2 derived from carbonate has allowed the reconstruction of paleotemperatures from a single phase (CaCO3 ), circumventing uncertainty inherent in other isotopic paleothermometers. Traditional analytical techniques require relatively large amounts of carbonate (3-8 mg per replicate), which limits the applicability of the clumped isotope proxy to certain geological materials such as marine microfossils, commonly used for paleoclimate reconstructions. METHODS: Clumped isotope ratio measurements of small samples were made on a new, high-efficiency, dual-reservoir sample-preparation inlet system attached to a Thermo-Finnigan MAT 253 mass spectrometer. Sample gas produced on the inlet is introduced from a 10 mL reservoir directly into the source via a capillary. Reference gas fills an identical 10 mL reservoir installed between the reference bellows and the capillary. The gas pressures in the two reservoirs are initially balanced, and are allowed to decrease together over the run. RESULTS: Carbonate samples from 1 mg to 2.6 mg produced Δ47 values equivalent to those from the traditional two-bellows method with identical single-sample precision (1 SE = 0.005-0.015‰) and external standard error (SE = 0.006-0.015‰, n = 4-6). The size of sample needed to achieve good precision is controlled by the sensitivity of the mass spectrometer and the size of the fixed reservoirs and adjacent U-trap installed on our inlet. CONCLUSIONS: The high-precision clumped isotope measurements of small aliquots of carbonate obtained in this method allows for the application of this proxy to a wider range of geological sample materials, such as marine microfossils, that until now have been nearly impossible to analyze given sample size limitation.