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Biogeochemical Responses of the Earth System to Massive Carbon Cycle Perturbations and the Cenozoic Long-Term Evolution of Climate: A Modeling Perspective.

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Title:Biogeochemical Responses of the Earth System to Massive Carbon Cycle Perturbations and the Cenozoic Long-Term Evolution of Climate: A Modeling Perspective.
Authors:Komar, Nemanja B.
Contributors:Oceanography (department)
Date Issued:Dec 2017
Publisher:University of Hawaiʻi at Mānoa
Abstract:Both short-term and long-term changes in climate and carbon cycling are re
ected in oxygen
( 18O) and carbon ( 13C) isotope
uctuations 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 13C and 18O. 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,
13C, the calcite compensation depth (CCD) and organic carbon burial rates across di erent
time scales. The results of the coupled carbon-calcium model (LOSCAR only model) suggest
that ocean acidi cation, which arises due to large and rapid carbon input, is not re
ected 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
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 e ect 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.
Description:Ph.D. Thesis. University of Hawaiʻi at Mānoa 2017.
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Oceanography

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