Early And Middle Cambrian Carbon And Strontium Isotopic Chemostratigraphy And The Influence Of Bioturbation On The Carbon Cycle PDF Download

The end-Triassic mass extinction is recognized as one of the five most severe biotic crises of the Phanerozoic. It was accompanied by an equally profound geochemical perturbation. A negative carbon isotope excursion and carbonate poor boundary-marl occur coincident with the disappearance of the Triassic biota at many sites around the globe. These are followed by a protracted positive carbon isotope excursion extending for hundreds of meters above the Triassic-Jurassic boundary. The main hypothesized driver for the carbon cycle perturbation is the release of volatiles associated with the emplacement of the Central Atlantic Magmatic Province (CAMP). However, the exact cause-and-effect relationships between the physical drivers and the geochemical responses have remained elusive. In this thesis I expand our knowledge of the end-Triassic carbon cycle perturbation by adding a large body of new carbon and oxygen stable isotope data, both in the immediate vicinity of the Triassic-Jurassic boundary and in the hundreds of meters following it. This data includes the first published extended carbon isotope curve containing measurements of both organic and carbonate carbon from the same samples. I also develop a flexible multi-element numerical box model based on a novel interpretation of the link that alkalinity provides between ocean chemistry and the cycling of elements on geologic timescales. I then utilize the model to make predictions regarding the impacts of the hypothesized carbon release on the chemistry of the ocean-atmosphere system and the resulting geochemical trends. Specifically, in Chapter 1 I present close to 1000 new data points from both organic and carbonate carbon, and detailed descriptions of the localities and geological settings in which they occur. I show that the previously observed protracted positive carbon isotope excursion that occurs above the Triassic-Jurassic boundary in many places, can be traced for tens of kilometers across sections in the southern Alps, and occurs in a correlative position hundreds of kilometers away in the southern Apennines. The spatial extent of the perturbation, and its occurrence in both organic and carbonate phases suggests that it is unlikely to be of diagenetic origin. Additionally, its stratigraphic extent suggests that the carbon cycle perturbation that began at the boundary persisted for a substantial period of geologic time (likely hundreds of thousands to millions of years) following the extinction event. In Chapter 2 I develop and apply a numerical carbon cycle model to investigate the underlying mechanism for the isotopic trends described in Chapter 1. I show that considering the role of alkalinity in the ocean leads to the conclusion that an increase in the burial of organic carbon that is unaccompanied by an increase in the input of carbon, can only lead to a reduction in the partial pressure of carbon dioxide in the atmosphere. To produce a positive excursion that is accompanied by an increase in atmospheric carbon dioxide levels, the amount of carbon brought into the exogenic carbon pool must be increased. The results of this modeling exercise elucidate the mechanism that underlies the common association between volcanic degassing and positive carbon isotope excursions in the geologic record. In Chapter 3 I focus on the Triassic-Jurassic boundary interval, and examine the relationship between the carbonate-poor interval associated with the extinction and the negative carbon isotope excursion that occurs within it. I show that the negative excursion is unique to the boundary marl, and does not occur in other stratigraphically adjacent carbonate-poor beds, arguing against a strict association between the carbon isotope value and percent carbonate. Rather, it is likely that a third factor was responsible for simultaneously driving both trends. First, I examine the possibility of anaerobic respiration of organic carbon within the marl having driven the precipitation of carbonate depleted in carbon-13 and oxygen-18. Second, I investigate the hypothesis that a rapid release of isotopically depleted carbon concomitantly reduced the carbon isotope composition and carbonate saturation state of the ocean. I conclude that if the negative excursion represents the release of depleted volatiles, the duration of the release and the associated acidification event must have been extremely short, on the order of a few thousand years, and that the isotopic composition of the released volatiles must have been far below mantle values. Together these three chapters consist of a coherent and thorough examination of the carbon isotopic record of the end-Triassic mass extinction, and represent a strong step forward in our understanding of the physical events and resulting geochemical cascades that led to the biotic crisis.