Toward a Cenozoic history of atmospheric CO2.

The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO2) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO2 beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO2 record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO2 thresholds in biological and cryosphere evolution.

Arctic ice and the ecological rise of the dinosaurs.

Abundant lake ice-rafted debris in Late Triassic and earliest Jurassic strata of the Junggar Basin of northwestern China (paleolatitude ~71°N) indicates that freezing winter temperatures typified the forested Arctic, despite a persistence of extremely high levels of atmospheric Pco2 (partial pressure of CO2). Phylogenetic bracket analysis shows that non-avian dinosaurs were primitively insulated, enabling them to access rich deciduous and evergreen Arctic vegetation, even under freezing winter conditions. Transient but intense volcanic winters associated with massive eruptions and lowered light levels led to the end-Triassic mass extinction (201.6 Ma) on land, decimating all medium- to large-sized nondinosaurian, noninsulated continental reptiles. In contrast, insulated dinosaurs were already well adapted to cold temperatures, and not only survived but also underwent a rapid adaptive radiation and ecological expansion in the Jurassic, taking over regions formerly dominated by large noninsulated reptiles.

A new calibration technique for quantitative gas analyses of fluid inclusions using a quadrupole mass spectrometer.

RATIONALE: Quantitative gas analysis by quadrupole mass spectrometry is used widely in the study of fluid inclusion volatiles, but the currently available instrument calibration techniques have a number of limitations. We describe a new method to overcome these by employing custom-machined stainless-steel calibration volumes that fit into modified bellows valves and can be filled on the high-vacuum prep line of the instrument. This allows the calibration gas to be introduced in a pulse-like fashion, replicating a burst of sample gas from a ruptured fluid inclusion. METHODS: The modified bellows valves equipped with calibration volumes are loaded onto the high-vacuum inlet of a quadrupole mass spectrometer. Cobalt-based alloy stem tips are then used to seal calibration gases (dry air, in this case) at known pressures into the calibration volumes for analysis. RESULTS: Two volumes of different dimensions were used to produce a relationship between moles introduced and integrated beam area for m/z 14, 32, 40, and 44. A linear fit along with a 95% confidence band for the relationship of moles introduced and integrated beam area was determined for the purpose of quantitative gas analysis in fluid inclusions containing atmospheric air. The calibration curves for each m/z have R-squared (COD) values of 0.94 or better. For validation of this calibration technique, measurements were made using outside air as an unknown. CONCLUSIONS: A new calibration technique has been developed for measuring femto- to pico-mole quantities of atmospheric air that greatly reduces contamination and introduces calibration gas in a pulse similar to sample gas. This technique has reduced the error in quantifying these small aliquots of air to an uncertainty of ±1.5%.

The extraterrestrial impact evidence at the Palaeocene-Eocene boundary and sequence of environmental change on the continental shelf.

We have identified clear evidence of an extraterrestrial impact within the onset of the carbon isotope excursion (CIE) that defines the Palaeocene-Eocene (P-E) boundary hyperthermal event (approx. 56 Ma) from several sites on the eastern Atlantic Coastal Plain and offshore. We review and update the state of the evidence for an impact at the P-E boundary, including a K-Ar cooling age of the ejecta that is indistinguishable from the depositional age at the P-E, which establishes the ejecta horizon as an isochronous stratigraphic indicator at the P-E. Immediately above the ejecta peak at the base of the coastal plain Marlboro Clay unit, we identify a sharp increase in charcoal abundance coincident with the previously observed dramatic increase in magnetic nanoparticles of soil pyrogenic origin. We therefore revisit the observed sequence of events through the P-E boundary on the western Atlantic Coastal Plain, showing that an extraterrestrial impact led to wildfires, landscape denudation and deposition of the thick Marlboro Clay, whose base coincides with the spherule horizon and CIE onset. The Sr/Ca ratio of the spherules indicates that the carbon responsible for the onset may be vaporized CaCO3 target rock mixed with isotopically light carbon from the impactor or elsewhere. Crucially, we do not argue that the impact was responsible for the full manifestation of the CIE observed globally (onset to recovery approx. 170 kyr), rather that a rapid onset was triggered by the impact and followed by additional carbon from other processes such as the eruption of the North Atlantic Igneous Province. Such a scenario agrees well with recent modelling work, though it should be revisited more explicitly.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

Detecting Photosymbiosis in Fossil Scleractinian Corals.

The evolutionary success of reef-building corals is often attributed to photosymbiosis, a mutualistic relationship scleractinian corals developed with zooxanthellae; however, because zooxanthellae are not fossilized, it is difficult (and contentious) to determine whether ancient corals harbored symbionts. In this study, we analyze the δ15N of skeletal organic matrix in a suite of modern and fossil scleractinian corals (zooxanthellate- and azooxanthellate-like) with varying levels of diagenetic alteration. Significantly, we report the first analyses that distinguish shallow-water zooxanthellate and deep-water azooxanthellate fossil corals. Early Miocene (18-20 Ma) corals exhibit the same nitrogen isotopic ratio offset identified in modern corals. These results suggest that the coral organic matrix δ15N proxy can successfully be used to detect photosymbiosis in the fossil record. This proxy will significantly improve our ability to effectively define the evolutionary relationship between photosymbiosis and reef-building through space and time. For example, Late Triassic corals have symbiotic values, which tie photosymbiosis to major coral reef expansion. Furthermore, the early Miocene corals from Indonesia have low δ15N values relative to modern corals, implying that the west Pacific was a nutrient-depleted environment and that oligotrophy may have facilitated the diversification of the reef builders in the Coral Triangle.

Impact ejecta at the Paleocene-Eocene boundary.

Extraterrestrial impacts have left a substantial imprint on the climate and evolutionary history of Earth. A rapid carbon cycle perturbation and global warming event about 56 million years ago at the Paleocene-Eocene (P-E) boundary (the Paleocene-Eocene Thermal Maximum) was accompanied by rapid expansions of mammals and terrestrial plants and extinctions of deep-sea benthic organisms. Here, we report the discovery of silicate glass spherules in a discrete stratigraphic layer from three marine P-E boundary sections on the Atlantic margin. Distinct characteristics identify the spherules as microtektites and microkrystites, indicating that an extraterrestrial impact occurred during the carbon isotope excursion at the P-E boundary.

Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years.

A major unresolved aspect of the rise of dinosaurs is why early dinosaurs and their relatives were rare and species-poor at low paleolatitudes throughout the Late Triassic Period, a pattern persisting 30 million years after their origin and 10-15 million years after they became abundant and speciose at higher latitudes. New palynological, wildfire, organic carbon isotope, and atmospheric pCO2 data from early dinosaur-bearing strata of low paleolatitudes in western North America show that large, high-frequency, tightly correlated variations in δ(13)Corg and palynomorph ecotypes occurred within a context of elevated and increasing pCO2 and pervasive wildfires. Whereas pseudosuchian archosaur-dominated communities were able to persist in these same regions under rapidly fluctuating extreme climatic conditions until the end-Triassic, large-bodied, fast-growing tachymetabolic dinosaurian herbivores requiring greater resources were unable to adapt to unstable high CO2 environmental conditions of the Late Triassic.

Continental erosion and the Cenozoic rise of marine diatoms.

Marine diatoms are silica-precipitating microalgae that account for over half of organic carbon burial in marine sediments and thus they play a key role in the global carbon cycle. Their evolutionary expansion during the Cenozoic era (66 Ma to present) has been associated with a superior competitive ability for silicic acid relative to other siliceous plankton such as radiolarians, which evolved by reducing the weight of their silica test. Here we use a mathematical model in which diatoms and radiolarians compete for silicic acid to show that the observed reduction in the weight of radiolarian tests is insufficient to explain the rise of diatoms. Using the lithium isotope record of seawater as a proxy of silicate rock weathering and erosion, we calculate changes in the input flux of silicic acid to the oceans. Our results indicate that the long-term massive erosion of continental silicates was critical to the subsequent success of diatoms in marine ecosystems over the last 40 My and suggest an increase in the strength and efficiency of the oceanic biological pump over this period.

Evidence for a rapid release of carbon at the Paleocene-Eocene thermal maximum.

The Paleocene/Eocene thermal maximum (PETM) and associated carbon isotope excursion (CIE) are often touted as the best geologic analog for the current anthropogenic rise in pCO2. However, a causal mechanism for the PETM CIE remains unidentified because of large uncertainties in the duration of the CIE's onset. Here, we report on a sequence of rhythmic sedimentary couplets comprising the Paleocene/Eocene Marlboro Clay (Salisbury Embayment). These couplets have corresponding δ(18)O cycles that imply a climatic origin. Seasonal insolation is the only regular climate cycle that can plausibly account for δ(18)O amplitudes and layer counts. High-resolution stable isotope records show 3.5‰ δ(13)C decrease over 13 couplets defining the CIE onset, which requires a large, instantaneous release of (13)C-depleted carbon. During the CIE, a clear δ(13)C gradient developed on the shelf with the largest excursions in shallowest waters, indicating atmospheric δ(13)C decreased by ~20‰. Our observations and revised release rate are consistent with an atmospheric perturbation of 3,000-gigatons of carbon (GtC).

Atmospheric PCO2 perturbations associated with the Central Atlantic Magmatic Province.

The effects of a large igneous province on the concentration of atmospheric carbon dioxide (PCO2) are mostly unknown. In this study, we estimate PCO2 from stable isotopic values of pedogenic carbonates interbedded with volcanics of the Central Atlantic Magmatic Province (CAMP) in the Newark Basin, eastern North America. We find pre-CAMP PCO2 values of ~2000 parts per million (ppm), increasing to ~4400 ppm immediately after the first volcanic unit, followed by a steady decrease toward pre-eruptive levels over the subsequent 300 thousand years, a pattern that is repeated after the second and third flow units. We interpret each PCO2 increase as a direct response to magmatic activity (primary outgassing or contact metamorphism). The systematic decreases in PCO2 after each magmatic episode probably reflect consumption of atmospheric CO2 by weathering of silicates, stimulated by fresh CAMP volcanics.