Morgan F. Schaller

Atmospheric Pco2 Perturbations Associated with the Central Atlantic Magmatic Province

Emplacement of the Central Atlantic Magmatic Province 200 million years ago greatly elevated atmospheric carbon dioxide concentrations. 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.

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

Significance Calcium carbonate and carbon isotope records from the rhythmically bedded Marlboro Clay, deposited during the onset of the PETM CIE, show that the massive release of isotopically light carbon was instantaneous, providing important constraints for the magnitude of carbon released and potential mechanisms. 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 δ18O cycles that imply a climatic origin. Seasonal insolation is the only regular climate cycle that can plausibly account for δ18O amplitudes and layer counts. High-resolution stable isotope records show 3.5‰ δ13C decrease over 13 couplets defining the CIE onset, which requires a large, instantaneous release of 13C-depleted carbon. During the CIE, a clear δ13C gradient developed on the shelf with the largest excursions in shallowest waters, indicating atmospheric δ13C decreased by ∼20‰. Our observations and revised release rate are consistent with an atmospheric perturbation of 3,000-gigatons of carbon (GtC).

Continental erosion and the Cenozoic rise of marine diatoms

Significance Diatoms are silica-precipitating microalgae responsible for roughly one-fifth of global primary production. The mechanisms that led these microorganisms to become one of the most prominent primary producers on Earth remain unclear. We explore the linkage between the erosion of continental silicates and the ecological success of marine diatoms over the last 40 My. We show that the diversification and geographic expansion of diatoms coincide with periods of increased continental weathering fluxes and silicic acid input to the oceans. On geological time scales, the ocean’s biologically driven sequestration of organic carbon (the biological pump) is proportional to the input flux of inorganic nutrients to the oceans. Our results suggest that the strength and efficiency of the biological pump increased over geological time. 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.

Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years

Significance This is, to our knowledge, the first multiproxy study of climate and associated faunal change for an early Mesozoic terrestrial ecosystem containing an extensive vertebrate fossil record, including early dinosaurs. Our detailed and coupled high-resolution records allow us to sensitively examine the interplay between climate change and ecosystem evolution at low paleolatitudes during this critical interval of Earths history when modern terrestrial ecosystems first evolved against a backdrop of high CO2 in a hothouse world. We demonstrate that these terrestrial ecosystems evolved within a generally arid but strongly fluctuating paleoclimate that was subject to pervasive wildfires, and that these environmental conditions in the early Mesozoic prevented large active warm-blooded herbivorous dinosaurs from becoming established in subtropical low latitudes until later in the Mesozoic. 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 δ13Corg 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.

A 30 Myr record of Late Triassic atmospheric pCO2 variation reflects a fundamental control of the carbon cycle by changes in continental weathering.

We generate a detailed ~30 Myr record of pCO2 spanning most of the Late Triassic (Carnian-Norian-Rhaetian) to earliest Jurassic (Hettangian), based on stable carbon isotope ratios of soil carbonate and preserved organic matter from paleosols in the eastern North American Newark rift basin. Atmospheric pCO2 was near 4500 ppm in the late Carnian, decreasing to below ~2000 ppm by the late Rhaetian just before the earliest Jurassic eruption of the Central Atlantic Magmatic Province, which triggered measurable pulses of CO2 outgassing. These data are consistent with published modeling results using the GEOCLIM model, which predict a decrease in pCO2 over the Late Triassic as a result of the progressive increase in continental area subject to the intense weathering regime of the tropical humid belt due to Pangea’s northward motion. The fi ner-scale pCO2 changes we observe may be dependent on the lithology introduced to the tropics, such as the dip to ~2000 ppm around 212 Ma and its rebound to ~4000 ppm at 209 Ma, which can be accomplished by introducing a more weatherable subaerial basaltic terrain. These observations indicate that the consumption of CO2 by continental silicate weathering can force long-term changes in pCO2 comparable to those driven by presumed changes in mantle degassing.

Impact ejecta at the Paleocene-Eocene boundary

An impactful event Glassy silica spherules have been found in marine sediments from three sites across a wide area off the Atlantic coast of the United States, near the stratigraphic level of the Paleocene-Eocene boundary. The characteristics of these specimens are consistent with those of microtektites associated with extraterrestrial impact events. This discovery by Schaller et al. is evidence of an impact event at the time of the Paleocene-Eocene Thermal Maximum, a period during which global temperatures increased rapidly and the carbon cycle was substantially perturbed. Science, this issue p. 225 An extraterrestrial impact had a profound effect on the carbon cycle and global warming 56 million years ago. 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.

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.

Reply to Pearson and Nicholas, Stassen et al., and Zeebe et al.: Teasing out the missing piece of the PETM puzzle

Understanding the Paleocene-Eocene Thermal Maximum (PETM) critically depends on knowing the rate at which the perturbation carbon was released. In our report (1) we argue that the layered Marlboro Clay may provide this important constraint. Our strongest evidence in support of the rapid release of carbon at the onset of the PETM is the differential response of the %CaCO3 and δ13C in the Millville core. The sharp %CaCO3 decrease occurred over 4 mm, compared with the δ13C decrease over an interval of 25 cm (figure 3 in ref. 1). Temporal differences are predicted by a rapid (instantaneous) release of light carbon, which would lower the surface ocean in a matter of months, in contrast to the carbon isotopic equilibrium exchange, which occurs on the scale of a decade (2) and can only be recorded in a core with a high sedimentation rates (1). Based on the rhythmic bedding in the Marlboro Clay, we argue that the drop in %CaCO3 occurred in less than a year and the δ13C equilibrium was on the order of a decade.

A tight coupling between atmospheric pCO2 and sea-surface temperature in the Late Triassic

Conodont samples from the Tethys basin used in Trotter et al. (2015), were originally assigned stratigraphic ages based on the identification of stage-defining conodont biozones. However, the conodont ages assigned by Trotter et al. (2015) do not agree with established magnetostratigraphy (Maron et al., 2017; 2015; Muttoni et al., 2010; 2014) and a known U/Pb date (Furin et al., 2006) for the Lagonegro, Lombardian, and Sicani basins. We revised the original age assignments for samples from the Lagonegro, Lombardian, and Sicani basins to agree with the established magnetoand biostratigraphy, which are described here (Figure DR1, Table DR1). Stratigraphic sections used in correlations from the Lagonegro basin were Pignola-Abriola, Gianni Grieco, Pignola 2, Lagonegro, and Sasso di Castalda. Brumano and Costa Imagna are from the Lombardian basin. Pizzo Mondello and samples labelled as Portella Gebbia are from the Sicani basin. Original age estimates from Trotter et al. (2015) for conodont samples from Brumano and Gianni Grieco were not revised due to insufficient sample information available in the literature, and are therefore not used in the ESS calculation.