Garland R. Upchurch

An atmospheric pCO2 reconstruction across the Cretaceous-Tertiary boundary from leaf megafossils

The end-Cretaceous mass extinctions, 65 million years ago, profoundly influenced the course of biotic evolution. These extinctions coincided with a major extraterrestrial impact event and massive volcanism in India. Determining the relative importance of each event as a driver of environmental and biotic change across the Cretaceous-Tertiary boundary (KTB) crucially depends on constraining the mass of CO2 injected into the atmospheric carbon reservoir. Using the inverse relationship between atmospheric CO2 and the stomatal index of land plant leaves, we reconstruct Late Cretaceous-Early Tertiary atmospheric CO2 concentration (pCO2) levels with special emphasis on providing a pCO2 estimate directly above the KTB. Our record shows stable Late Cretaceous/Early Tertiary background pCO2 levels of 350–500 ppm by volume, but with a marked increase to at least 2,300 ppm by volume within 10,000 years of the KTB. Numerical simulations with a global biogeochemical carbon cycle model indicate that CO2 outgassing during the eruption of the Deccan Trap basalts fails to fully account for the inferred pCO2 increase. Instead, we calculate that the postboundary pCO2 rise is most consistent with the instantaneous transfer of ≈4,600 Gt C from the lithic to the atmospheric reservoir by a large extraterrestrial bolide impact. A resultant climatic forcing of +12 W⋅m−2 would have been sufficient to warm the Earths surface by ≈7.5°C, in the absence of counter forcing by sulfate aerosols. This finding reinforces previous evidence for major climatic warming after the KTB impact and implies that severe and abrupt global warming during the earliest Paleocene was an important factor in biotic extinction at the KTB.

Enhancement of leaf fossilization potential by bacterial biofilms

Terrestrial leaf fossils often form through authigenic preservation in which the leaf surface is coated by a variety of minerals, especially iron oxides. The mechanism of this fossilization is unclear, because the largely hydrophobic leaf surfaces do not readily bind metal ions. Previously proposed mechanisms for mineral encrustation include precipitation of minerals in sediment pore space and precipitation of iron oxides at the surface by decay-produced CO 2 . Here we demonstrate that diverse bacterial species rapidly colonize leaf surfaces and form a biofilm within days of the leaf9s entry into a stream environment. Experimental mineralization of fresh and biofilm-coated leaves indicates that leaves without biofilm do not mineralize, but leaves with biofilms rapidly adsorb metal ions such as Fe 3+ onto the anionic biofilm surface where the ions form ferrihydrite. Once these mineralized leaves are buried by the sediment, they are more likely to be converted to fossils than non-mineralized leaves. Examination of a fossil leaf surface by scanning electron microscopy shows bacteria-sized structures resembling those found in biofilms. These experimental data imply that bacterial colonization of leaves may be an essential prerequisite for authigenic preservation.

Vegetation–atmosphere interactions and their role in global warming during the latest Cretaceous

Forest vegetation has the ability to warm Recent climate by its effects on albedo and atmospheric water vapour, but the role of vegetation in warming climates of the geologic past is poorly understood. This study evaluates the role of forest vegetation in maintaining warm climates of the Late Cretaceous by (1) reconstructing global palaeovegetation for the latest Cretaceous (Maastrichtian); (2) modelling latest Cretaceous climate under unvegetated conditions and different distributions of palaeovegetation; and (3) comparing model output with a global database of palaeoclimatic indicators. Simulation of Maastrichtian climate with the land surface coded as bare soil produces high–latitude temperatures that are too cold to explain the documented palaeogeographic distribution of forest and woodland vegetation. In contrast, simulations that include forest vegetation at high latitudes show significantly warmer temperatures that are sufficient to explain the widespread geographic distribution of high–latitude deciduous forests. These warmer temperatures result from decreased albedo and feedbacks between the land surface and adjacent oceans. Prescribing a realistic distribution of palaeovegetation in model simulations produces the best agreement between simulated climate and the geologic record of palaeoclimatic indicators. Positive feedbacks between high–latitude forests, the atmosphere, and ocean contributed significantly to high–latitude warming during the latest Cretaceous, and imply that high–latitude forest vegetation was an important source of polar warmth during other warm periods of geologic history.

Evidence for the recovery of terrestrial ecosystems ahead of marine primary production following a biotic crisis at the Cretaceous–Tertiary boundary

The fossil record demonstrates that mass extinction across the Cretaceous–Tertiary (K–T) boundary is more severe in the marine than the terrestrial realm. We hypothesize that terrestrial ecosystems were able to recover faster than their marine counterparts. To test this hypothesis, we measured sedimentary δ13C as a tracer for global carbon cycle changes and compared it with palaeovegetational changes reconstructed from palynomorphs and cuticles across the K–T boundary at Sugarite, New Mexico, USA. Different patterns of perturbation and timescales of recovery of isotopic and palaeobotanical records indicate that the δ13C excursion reflects the longer recovery time of marine versus terrestrial ecosystems.

Fire across the K–T boundary: initial results from the Sugarite Coal, New Mexico, USA

Abstract It has been hypothesized that major global fires occurred following a bolide impact at the Cretaceous–Tertiary boundary. Evidence for this has been mainly from ‘soot’ or black carbon at a number of marine boundary sites as well as the occurrence of inertinites (fusinite) in coals above the boundary. In addition, the occurrence of fossil charcoal (inertinites) in potential Tsunami deposits has been used to strengthen this idea. However, fire is known to have been widespread throughout the Cretaceous based on the distribution of fossil charcoal, and it is necessary to evaluate the claims for a global fire at the K–T boundary within the context of this more extensive record of ancient fires. The occurrence of the K–T boundary within the Sugarite coal sequence in the Raton Basin, New Mexico, offers the opportunity to assess the incidence of fire in local peat-forming vegetation, in the latest Cretaceous, across the Cretaceous–Tertiary boundary interval and in the earliest Tertiary. The distribution of fire products, i.e. fossil charcoal, is assessed using a combination of inertinite group macerals in polished blocks of coal and plant particles released by chemical maceration of coal. Inertinite group macerals (fusinite, semifusinite, inertodetrinite), which represent fossil charcoal, and particles of charcoal in maceration residues are abundant throughout the Sugarite coal sequence, both before, during and after the K–T boundary event. Samples from below the boundary yield inertinites that constitute more than 20% of the coal; three horizons are inertinite rich (>50%). Fire was obviously an important element of the terrestrial environment during the latest Cretaceous. High inertinite values and abundant charcoal particles characterize the latest Cretaceous part of the coal, the carbonaceous shale just below the boundary, the boundary interval itself and Tertiary coals and carbonaceous shales above the boundary. Charcoal in mineral-rich units of the latest Cretaceous, immediately below the boundary, in the boundary interval and in the Tertiary, is dominated by small inertodetrinite particles. These might have been wind blown from regional fires or they may reflect reworking during erosion, which led to sediment input into the mire. In either case the charcoal signature in these units is the same, irrespective of their position in the sequence. Our data, combined with that from two other sites in terrestrial sequences in North America, show that fires were an integral part of mire ecosystems through the latest Cretaceous and into the early Tertiary. Therefore, evidence for distinctive global wildfires at the Cretaceous–Tertiary boundary would have to be sought from careful consideration of other aspects of charcoal deposition, including flux rates for charcoal and soot.

Rapid (10-yr) recovery of terrestrial productivity in a simulation study of the terminal Cretaceous impact event

Abstract Investigations of short-term (up to 103 yr) environmental change across the Cretaceous–Tertiary boundary provide evidence for reduced temperatures, consistent with the injection of debris and sulphate aerosols into the upper atmosphere by a large impact event. Concomitant with this was a postulated massive addition of CO2 to the atmospheric carbon reservoir by impact vaporisation of the Chicxulub carbonate platform. Taken together, a high CO2 but low irradiance environment would have created unusual conditions for the operation of the terrestrial biosphere. Here, we have evaluated this environmental influence on terrestrial ecosystems using a process-based dynamic global vegetation model forced with post-impact global climates, derived by modification of the GENESIS atmospheric climate model simulation for the latest Cretaceous. Our results suggest that terrestrial primary productivity initially collapsed and then recovered to pre-impact levels within a decade. Global terrestrial carbon storage in vegetation biomass exhibited a similar collapse but complete recovery took place on a 60–80 yr timescale. The recovery of both terrestrial net primary productivity and vegetation biomass was largely mediated by the high CO2 concentration stimulating ecosystem photosynthetic productivity in the warm low latitudes. An apparently rapid recovery of terrestrial ecosystem function stands in marked contrast to the situation for the marine realm, where the organic carbon flux to the deep ocean was suppressed for up to 3 million years.

Comparative Morphology of Fossil and Extant Leaves of Nelumbonaceae, Including a New Genus from the Late Cretaceous of Western North America

Abstract We describe in detail the foliar architecture of extant Nelumbo and propose a new genus, Exnelumbites Estrada-Ruiz, Upchurch, Wolfe & Cevallos-Ferriz, for recently discovered leaf macrofossils from the Upper Cretaceous (Campanian-Maastrichtian) Olmos Formation of Coahuila, Mexico and Jose Creek Member of the McRae Formation of south-central New Mexico, U. S. A. The fossils described here consist of centrally peltate leaves with 12–13 actinodromous primary veins that terminate in broad glandular teeth of the chloranthoid type. No secondary veins are present on the midvein, and tertiary veins are organized in an alternate percurrent to reticulate pattern. Areolation is of variable shape with four to six sides. The fossil leaves are placed within Nelumbonaceae on the basis of their orbicular shape and centrally peltate organization, the presence of a funnel-form lamina, and especially the absence of secondary venation along the midvein, but are interpreted to be more primitive than extant Nelumbo in having no central disk, a smaller number of primary veins, less highly organized tertiary venation, and predominantly non-hexagonal areolation. The presence of chloranthoid teeth in the fossils is consistent with suggestions that the chloranthoid tooth type is basal to both Proteales and eudicots as a whole. The newly described leaves add to a growing diversity of plant macrofossils from the Cretaceous that are more closely related to Nelumbo than any other extant genus, but are more primitive in their vegetative and reproductive morphology.

Terrestrial ecosystem responses to global environmental change across the Cretaceous‐Tertiary boundary

Investigations of long-term (10³–105 yr) environmental change across the Cretaceous-Tertiary (K/T) boundary resulting from the impact of a large bolide indicate increases in temperature and precipitation due to the impact-related release of CO2. We evaluate the effects of these long-term changes in the global environment on terrestrial ecosystems using a vegetation-biogeochemistry model forced with a ‘best guess’ modified latest Cretaceous climate simulation by the GENESIS atmospheric general circulation model. The imposition of long-term global environmental changes after the K/T impact resulted in spatially heterogeneous increases in canopy leaf area index, net primary productivity, and soil carbon concentrations, relative to the latest Cretaceous preimpact situation. Terrestrial carbon storage increased by circa 2000 Gt.

Charbeckia macrophylla gen. et sp. nov. from the Lower Mississippian Price (Pocono) Formation of southeastern West Virginia

A fossil plant discovery from the Price (Pocono) Formation of southeastern West Virginia provides new information on the poorly known compression floras of the Lower Mississippian of North America. The new plant described herein consists of long tapering bipinnate fronds with imbricate basal pinnae, planate apical pinnae, and unlobed, elliptical to obovate pinnules with open dichotomous venation. Veins concentrated in the medial region of the pinnule curve toward the lateral margin, suggestive of an early stage in the evolution of a midrib. Pinnules exhibit a distinctly revolute or otherwise reinforced margin. The exceptional length of some fronds (over 1m) and pinnule size (some over 3cm by 2cm) strongly contrast with the generally diffusely branched fronds and small or highly dissected pinnule morphology that typify Early Mississippian leaf taxa. A new generic assignment, Charbeckia macrophylla, is thereby justified. The rigid appearance of the tapering rachis, the imbricate pinnae that appear to have resisted compression, and the reinforced pinnule margins imply thick evergreen leaves, perhaps adapted for drought tolerance. A possible Calamopityalean affinity is indicated by the size of the fronds and the stout petioles, which fall within the expected size range of the Kalymma-type petiole bases described from the nearby New Albany Shale of Tournaisian age.

Onset of the Laramide orogeny and associated magmatism in southern New Mexico based on U-Pb geochronology

The Laramide orogeny is a classic yet controversial mountain-building event that resulted, in the southwest United States, in uplifts, sedimentation, and magmatism that can be used to constrain the onset of this event in the region and expand our knowledge of Late Cretaceous to Paleogene tectonism. The McRae Formation marks the onset of deposition in the Laramide Love Ranch Basin, which was located to the northeast of the west-northwest-trending coeval Rio Grande uplift in south-central New Mexico, but its age is not well constrained. A previously published late Maastrichtian age for the McRae Formation was based on the presence of dinosaur bones in the upper of two members of the formation. We obtained new U-Pb dates from one dacite clast and three ash-fall tuffs from the lower Jose Creek Member and from one ash-fall tuff from the lower part of the overlying Hall Lake Member of the McRae Formation. The clast yielded a date of 75.0 ± 1.1 Ma, whereas the ages of the tuffs, in ascending stratigraphic order, are 74.9 ± 0.7 Ma, 74.7 ± 0.6 Ma, 75.2 ± 1.3 Ma, and 73.2 ± 0.7 Ma. These dates indicate that the onset of Laramide deposition in the Love Ranch Basin must have occurred earlier, in late Campanian time, similar to deposition in the Laramide Ringbone Basin in southwestern New Mexico. In addition, U-Pb zircon dates of 75.7 ± 1.3 Ma and 75.0 ± 2.8 Ma were obtained on the Twin Peaks stock and on a dacite sill, respectively, in the Burro Mountains of southwestern New Mexico. These dates are similar to those of other Laramide arc magmatic centers in southern New Mexico, which have a limited range of ages from 75 to 70 Ma, including the Hidalgo Formation in the Little Hatchet Mountains, the Silver City-Pinos Altos region, and the Copper Flat porphyry system. These new and previously published dates indicate that during the onset of Laramide deformation in southwestern and south-central New Mexico, the angle of subduction of the Farallon plate may have been steep enough to allow partial melting of an asthenospheric wedge, resulting in arc magmatism far inboard of the trench.