The Early Jurassic was marked by a progressive recovery from the end-Triassic mass extinction and punctuated by recurring episodes of anoxia. These changes, associated with fluctuations in carbon isotope composition of marine carbonates, remain incompletely understood. Here we present a highresolution carbon and oxygen isotope record for the Early Jurassic based on well-preserved marine mollusks (belemnites) from Dorset, UK. Our new data show a number of d 13 C excursions, starting with a negative excursion at the Sinemurian-Pliensbachian boundary Event followed by lesser negative excursions showing in the Polymorphous, Jamesoni, and Masseanum-Valdani Subzones. The recognition of the Sinemurian-Pliensbachian boundary Event in this study and elsewhere suggests that observed carbon-isotope trends are likely to represent a supraregional perturbation of the carbon cycle. A prominent positive carbonisotope event is also seen within the Pliensbachian Ibex Zone. This event is also clearly evident in the data from belemnites from Spain. This carbon-isotope excursion is not, however, coincident with inferred peak temperatures. The oxygen isotope and Mg/Ca data allow the determination of a number of pronounced Pliensbachian cool events. From the low point in the Brevispina Subzone, oxygen isotopes become more negative coupled with an increase in Mg/Ca values culminating in an Early Pliensbachian thermal maximum during the Davoei Zone. Taken with existing data, it appears that the Pliensbachian is characterized by two major warmings, first within the Davoei Zone followed by warming beginning in the latest Pliensbachian and peaking in the Early Toarcian.
• Premise of the study: Humans frequently identify pollen grains at a taxonomic rank above species. Grass pollen is a classic case of this situation, which has led to the development of computational methods for identifying grass pollen species. This paper aims to provide context for these computational methods by quantifying the accuracy and consistency of human identification.• Methods: We measured the ability of nine human analysts to identify 12 species of grass pollen using scanning electron microscopy images. These are the same images that were used in computational identifications. We have measured the coverage, accuracy, and consistency of each analyst, and investigated their ability to recognize duplicate images.• Results: Coverage ranged from 87.5% to 100%. Mean identification accuracy ranged from 46.67% to 87.5%. The identification consistency of each analyst ranged from 32.5% to 87.5%, and each of the nine analysts produced considerably different identification schemes. The proportion of duplicate image pairs that were missed ranged from 6.25% to 58.33%.• Discussion: The identification errors made by each analyst, which result in a decline in accuracy and consistency, are likely related to psychological factors such as the limited capacity of human memory, fatigue and boredom, recency effects, and positivity bias.
Llandovery strata at Llanystumdwy are described. Ninety graptolite species have been identified enabling the establishment of the following zones: acinaces, cyphus, triangulatus, magnus, leptotheca, convolutus, ?sedgwickii, turriculatus and crispus. The name Llanystumdwy Mudstone Formation is proposed for previously undescribed mudstones cropping out on the River Dwyfor at Llanystumdwy and includes all strata referable to the acinaces to turriculatus zones. Comparison is made between Llanystumdwy and other areas, notably Northern England and Southern Scotland.
Fire regimes are changing due to both anthropogenic climatic drivers and vegetation management challenges, making it difficult to determine how climate alone might influence wildfire activity. Earth has been subject to natural-background climate variability throughout its past due to variations in Earth’s orbital parameters (Milkankovitch cycles), which provides an opportunity to assess climate-only driven variations in wildfire. Here we present a 350,000 yr long record of fossil charcoal from mid-latitude (~35°N) Jurassic sedimentary rocks. These results are coupled to estimates of variations in the hydrological cycle using clay mineral, palynofacies and elemental analyses, and lithological and biogeochemical signatures. We show that fire activity strongly increased during extreme seasonal contrast (monsoonal climate), which has been linked to maximal precessional forcing (boreal summer in perihelion) (21,000 yr cycles), and we hypothesize that long eccentricity modulation further enhances precession-forced fire activity.
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