Biogeochemical Responses of the Earth System to Massive Carbon Cycle Perturbations and the Cenozoic Long-Term Evolution of Climate : A Modeling Perspective
出版項
2017
說明
1 online resource (207 pages)
文字
text
無媒介
computer
成冊
online resource
附註
Source: Dissertation Abstracts International, Volume: 79-08(E), Section: B
Thesis (Ph.D.)--University of Hawai'i at Manoa, 2017
Includes bibliographical references
Both short-term and long-term changes in climate and carbon cycling are reflected in oxygen (delta18O) and carbon (delta 13C) isotope fluctuations in the geological record, often indicating a highly dynamic nature and close connection between climate and carbon cycling through the ocean-atmosphere-biosphere system. When used in conjunction with mathematical models, stable carbon and oxygen isotopes provide a powerful tool for deciphering magnitude and rate of past environmental perturbations. In this study, we focus on two transient global warming events and a multi-million-year evolution of climate: (1) the end-Permian (̃252 Ma), (2) the Paleocene Eocene Thermal Maximum (PETM; ̃56 Ma), and (3) climatic and ocean chemistry variations across the Cenozoic. The transient events (1) and (2) are both accompanied by a massive introduction of isotopically light carbon into the ocean-atmosphere system, as indicated by prominent negative excursions of both delta 13C and delta18O. We use a combination of the well-established GEOCARB III and LOSCAR models to examine feedbacks between the calcium and carbon cycle during massive and rapid CO2 release events, and feedbacks between biological production and the cycles of carbon, oxygen and phosphorus (C-O-P feedback). The coupled GEOCARB-LOSCAR model enables simulation of marine carbonate chemistry, delta13C, the calcite compensation depth (CCD) and organic carbon burial rates across different time scales. The results of the coupled carbon-calcium model (LOSCAR only model) suggest that ocean acidification, which arises due to large and rapid carbon input, is not reflected in the calcium isotope record during the end-Permian, contrary to the claims of previous studies. The observed changes in calcium isotopes arise due to 12,000 Pg C emitted by Siberian Trap volcanism, the consequent extinction of the open ocean primary producers, and variable v calcium isotope fractionation. The results presented in Chapter 3 indicate that the C-O-P mechanism may act as a negative feedback during high CO2 emission events such as the PETM, restoring atmospheric CO2 through increased organic carbon burial as a consequence of an accelerated nutrient delivery to the surface ocean and enhanced organic carbon export. Our results indicate that the feedback was triggered by an initial carbon pulse of 3,000 Pg C followed by an additional carbon leak of 2,500 Pg C. Through the C-O-P feedback, ̃2,000 Pg C could be sequestered during the recovery phase of the PETM but only if CaCO3 export remained constant. Regarding long-term Cenozoic changes (Chapter 4), we propose that the temperature effect on metabolic rates played an important role in controlling the evolution of ocean chemistry and climate across multi-million-year time scales by altering organic carbon burial rates. Model predicted organic carbon burial rates combined with the ability to simulate the CCD changes imposes a critical constraint on the carbon cycle and aids in a better understanding carbon cycling during the Cenozoic. Our results suggest that the observed CCD trends over the past 60 million years were decoupled from the continental carbonate and silicate weathering rates. We identify two dominant mechanisms for the decoupling: (a) shelf-basin carbonate burial fractionation and (b) decreasing respiration of organic matter at intermediate water depths as the Earth transitioned from the greenhouse conditions of the Eocene to the colder temperatures of the Oligocene
Electronic reproduction. Ann Arbor, Mich. : ProQuest, 2018
