Interpreting the Difference in Magnitudes of PETM Carbon Isotope Excursions in Paleosol Carbonate and Paleosol Organic Matter
Interpreting the Difference in Magnitudes of PETM Carbon Isotope Excursions in Paleosol Carbonate and Paleosol Organic Matter

Author: Christopher Guy Cacciatore

Publisher:

Published: 2016

Total Pages: 78

ISBN-13:

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The Paleocene-Eocene Thermal Maximum (PETM) was a rapid global warming event at ~56 Ma that was driven by a rapid release of carbon into the ocean-atmosphere system. The most recognizable feature marking the PETM in the rock record is a negative carbon isotope excursion (CIE) recorded in organic carbon and calcium carbonates deposited in both marine and terrestrial environments. Differences among excursion magnitudes ([Delta]CIE) exist between marine and terrestrial proxies, and between carbonates and organic carbon. We evaluated the plausibility of two hypothetical mechanisms behind the observed ~ 1.9‰ difference between the magnitude of the CIE as recorded by paleosol carbonate and paleosol organic matter ([Delta]CIEpc-som). Specifically, we test whether 1) oxidation within soils of isotopically light methane or 2) increases in soil respiration rates are plausible explanations for the observed [Delta]CIE. A production-diffusion model used to simulate carbon isotope compositions of soil CO2 and paleosol carbonates is coupled with a box model that constrains methane flux from hydrates into atmosphere. The box model simulates atmospheric CO2 concentrations, the [delta]13C values of atmospheric CO2 and of plants, and the methane flux into soils, which are all used in the production-diffusion model to simulate the [delta]13C value of paleosol carbonate. Given conservative prior distributions for model inputs grounded in previous empirical studies, model output demonstrates that oxidation of atmospheric methane in soil pore space is unlikely to cause the [Delta]CIEpc-som even for rapid methane release rates. However, increased respiration rates during the PETM could explain the observed [Delta]CIE, with a minimum approximate doubling of respiration rates required to reproduce a [Delta]CIEpc-som ≥ 2‰.

Evaluation of Variability in Soil Organic Carbon Isotope Record and Implications for Organic Matter Preservation
Evaluation of Variability in Soil Organic Carbon Isotope Record and Implications for Organic Matter Preservation

Author: Angela Chung

Publisher:

Published: 2015

Total Pages:

ISBN-13:

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A large carbon isotope excursion (CIE) is one of the main signatures of the PETM (Paleocene-Eocene Thermal Maximum). Throughout the warming of the PETM, negative shifts in the carbon isotope record on land and in the sea can be observed in various magnitudes. In particular, while lipid and carbonate carbon archives on land display larger CIE magnitudes compared to marine records, the total organic carbon record exhibits a smaller and more variable CIE. This study examines the terrestrial bulk organic carbon isotope record to evaluate factors influencing [delta]13C changes and their variability. The differences in CIE magnitudes provide an opportunity to investigate the carbon cycle in soils during the PETM. Ancient floodplain sediments from the Wasatch Formation in western Colorado were examined for lithology, degree of pedogenesis and analyzed for stable isotope ratios of organic carbon (reported as [delta]13Corg) and total organic carbon content (TOC%). [delta]13Corg values are independent of lithology and degree of pedogenesis of the sample, denoting a lack of a relationship between [delta]13C of organic matter and depositional environment. No correlation exists between [delta]13Corg and TOC(%) or [delta]13Corg and one over total organic carbon (1/TOC). The absence of any relationship makes the evidence for effects of decomposition and mixing of refractory organic carbon and indigenous carbon not readily apparent. The attenuated CIE in terrestrial organic matter archives suggest decreased soil organic matter residence times in the study area during the PETM. Studies utilizing carbon isotopes of bulk organic matter would benefit from separating out different fractions of organic matter and compound specific isotope analysis. Given the extremely low TOC(%) preserved in these samples and attenuated CIE, we suggest that in the hyper-greenhouse conditions of the PETM, the residence time of carbon in soils was reduced.

A Colour Guide to Paleosols
A Colour Guide to Paleosols

Author: Greg John Retallack

Publisher:

Published: 1997-07-11

Total Pages: 200

ISBN-13:

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Buried soils (paleosols) are becoming widely recognized in sedimentary rock sequences and are generating much interest among Earth scientists. One reason why paleosols have not been widely recognized until recently is that soil scientists have had little opportunity to consider paleosols in sedimentary and metamorphic rocks. Another reason may be the profound alteration of paleosols after burial, so that few paleosols look exactly like surface soils. Colour is an obvious and important feature of paleosols, which are among the most colourful of all rocks, as can be seen from Petrified Forest and Badlands National Parks, USA. With its spectacular colour plates this book serves as an introduction to the colourful world of paleosols. This volume is also a handbook. With concise text and supporting illustrations, it introduces the basic problems of recognizing paleosols in the field and through laboratory studies, unravelling alteration of paleosols after burial, and interpreting ancient environments and life from paleosols. With its extensive index and glossary it is also a gateway to the terminology of geology and soil science. This book will provide an excellent reference for advanced students, researchers and professionals in the areas of sedimentology, stratigraphy, soil science, palaeontology, oil geology, and other related disciplines.

Organic Matter in Holocene Paleosols in the US Mid-Continent
Organic Matter in Holocene Paleosols in the US Mid-Continent

Author:

Publisher:

Published: 2016

Total Pages:

ISBN-13:

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Paleosols are ancient soils that possess information about the earth's environmental and climatic conditions in the past. Buried paleosols are formed by the burial of surface horizon soils by younger layers of sediments over time. Holocene buried paleosols in Nebraska at the Bignell Hill, Old Wauneta Roadcut (OWR), and Farwell Locality were studied to better understand the organic carbon storage for over a thousand years. All three sites are located in Nebraska. The Bignell Hill and the OWR sites occur in an upland setting, and the Farwell Locality is in an alluvial setting. The stratigraphic sequence (top to bottom) of the upland sites consists of the Bignell Loess, which buries Peoria Loess, and a dark colored soil called Brady Soil that is developed in the upper part of the Peoria Loess. The Brady Soil is easily recognized and identified by a dark horizon in the soil profile that is thoroughly burrowed by insects, including cicada nymphs. The Farwell Locality is located near the Nebraska-Kansas border, along the South Fork of the Big Nemaha River. The Farwell Locality (site 65-2) occurs in the Late Gunder Member of the Deforest Formation, in an alluvial landscape. At the Bignell Hill and OWR sites, the organic C content of the Brady Soil was greater than that of the overlying Bignell Loess. The organic C content ranged between 0.32% and 0.74%, and 0.43% and 0.81% for the Bignell Hill and OWR sites, respectively. In contrast to the typical depth distribution of organic C in most soils, which decreases with increasing depth, the organic C concentration of the Brady Soil was the least at the top horizon of the soil. The clay content at both sites was relatively low, with an average content between 20-25% at both sites. Smectite was the dominant layer silicate mineral, and there appeared to be no relationship between organic C content and smectite content of the paleosols. Stable carbon isotope data at both the Bignell Hill and OWR sites suggest a mixed C3/C4 plant c

Local and Global Controls on Carbon Isotope Chemostratigraphy
Local and Global Controls on Carbon Isotope Chemostratigraphy

Author: Anne-Sofie Ahm

Publisher: Cambridge University Press

Published: 2022-03-31

Total Pages: 48

ISBN-13: 100903362X

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Over million-year timescales, the geologic cycling of carbon controls long-term climate and the oxidation of Earth's surface. Inferences about the carbon cycle can be made from time series of carbon isotopic ratios measured from sedimentary rocks. The foundational assumption for carbon isotope chemostratigraphy is that carbon isotope values reflect dissolved inorganic carbon in a well-mixed ocean in equilibrium with the atmosphere. However, when applied to shallow-water platform environments, where most ancient carbonates preserved in the geological record formed, recent research has documented the importance of considering both local variability in surface water chemistry and diagenesis. These findings demonstrate that carbon isotope chemostratigraphy of platform carbonate rarely represent the average carbonate sink or directly records changes in the composition of global seawater. Understanding what causes local variability in shallow-water settings, and what this variability might reveal about global boundary conditions, are vital questions for the next generation of carbon isotope chemostratigraphers.