The Role of Petrogenic Carbon in Cenozoic Climate Events
Open Access
- Author:
- Lyons, Shelby
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 13, 2020
- Committee Members:
- Katherine Haines Freeman, Dissertation Advisor/Co-Advisor
Katherine Haines Freeman, Committee Chair/Co-Chair
Lee Kump, Committee Member
Timothy Bralower, Committee Member
Raymond Gabriel Najjar, Jr., Outside Member
Mark E Patzkowsky, Program Head/Chair - Keywords:
- paleoclimate
paleoceanography
Paleocene-Eocene Thermal Maximum
PETM
geochemistry
thermal maturity
carbon cycle
Cretaceous-Paleogene
K-Pg
mass extinction
polycyclic aromatic hydrocarbons
PAH
hopanoids
biomarkers
organic geochemistry
stable isotopes
alkanes
petroleum
thermal maturation - Abstract:
- Ancient, sedimentary carbon, known as petrogenic carbon, has the potential to dramatically influence Earth’s global carbon cycle. While petrogenic carbon is preserved on geologic timescales, weathering and transport processes can remobilize and oxidize it within the Earth’s ocean-atmosphere system, ultimately leaking carbon from Earth’s crust to its actively cycled surface pools. The release of petrogenic carbon to Earth’s exogenic system serves as both a driver for and a response to climate change. As a climate driver, CO2 released from petrogenic carbon oxidation can cause warming, while petrogenic carbon burning can drive soot into Earth’s upper atmosphere and cause anti-greenhouse effects and cooling. As a climate response, the rates of erosion and oxidation of petrogenic carbon are enhanced by climate warming and associated intensification of the water cycle. Predictions for Earth’s future are informed by our understanding of the drivers, responses, and feedbacks to climate change in Earth’s past. Unfortunately, little is known about geosphere-to-biosphere reduced carbon fluxes in Earth’s history. Although petrogenic carbon may have responded to or driven climatic change in Earth’s past, it is difficult to distinguish and quantitatively assess petrogenic carbon release from ancient sedimentary records. Thus, studies of climatic events in Earth’s history document many carbon cycle responses to climatic and environmental perturbations, but typically do not include the role of petrogenic carbon. Here, I present methods to identify and determine the effects of petrogenic carbon release on Earth’s past climate system and carbon cycle. The Paleocene, an epoch lasting from 66 to 56 million years ago, was bracketed by two climatic perturbations: the Cretaceous-Paleogene (K-Pg) boundary impact and the Paleocene-Eocene Thermal Maximum (PETM). Here, I determine whether petrogenic carbon release was a contributing driver for the K-Pg mass extinction and whether it was a response to the PETM hyperthermal. The PETM hyperthermal (~56 Ma) serves as the best-known ancient analogue for anthropogenic climate change due to the amount and rate of CO2 released. Using hopanoid thermal maturity assessments of sediments from the Mid-Atlantic and Tanzania, I determined petrogenic carbon delivery to coastal sediments increased 15 to 50 times during the PETM and lagged the initiation of PETM warming on the order of 104 years. The associated oxidation of petrogenic carbon released between 102 to 104 PgC of CO2 to Earth’s oceans and atmospheres over 104 to 105 years. I suggest the oxidation of petrogenic carbon extended the PETM duration for many thousands of years. While intensified erosion can remobilize petrogenic carbon and drive CO2 release to the atmosphere, it can also drive biosphere carbon burial, which draws down atmospheric CO2. I further assess the coevolution of biosphere and petrogenic carbon burial in response to the PETM hyperthermal using coastal n-alkane records from the US Atlantic coastal plain. I demonstrate enhanced burial of terrigenous biosphere carbon commenced ~4–15 kyr into the event and maintained the region’s effectiveness as a CO2 sink. Petrogenic carbon remobilization and oxidation to CO2 lagged the PETM onset by ~21–83 kyr. Organic matter transport in the Mid-Atlantic transformed from a CO2 drawdown to a CO2 release mechanism. The petrogenic carbon was likely released from the Appalachian region and oxidized during transport in response to warmer, higher-CO2 climates. If this also took place globally, then CO2 released from petrogenic carbon oxidation had the ability to transform fluvial organic matter transport processes in coastal regions from CO2 sinks into CO2 sources, and ultimately could have extended the duration of hyperthermal events for 10’s to 100’s of thousands of years. Additionally, I assessed if petrogenic carbon release was a contributing driver to the K-Pg mass extinction. The asteroid impact at the Yucatán carbonate platform ~66 million years ago vaporized a ~3 km-thick section of carbonates and evaporites and released 325 ± 130 Pg of sulfur, 425 ± 160 Pg CO2, and dust that drove the cooling and darkness. Global K-Pg boundary records contain burn markers, which were derived from wildfires and/or the ejection of petrogenic carbon from the target rock. Soot sourced from the target rock would have resided high enough in the atmosphere to block sunlight, and likely contributed to global cooling and darkness that drove the extinction. I assessed the character and potential sources for K-Pg associated petrogenic carbon using polycyclic aromatic hydrocarbon (PAH) isomer and alkylation patterns preserved in sediments from the Chicxulub crater and distal, deep ocean sites. PAH isomer patterns suggest K-Pg boundary burn markers were formed via rapid heating, while their alkylation patterns suggest a petrogenic source. I determined that K-Pg boundary PAHs were partially derived from vaporized petrogenic carbon, and we suggest petrogenic carbon was injected into the stratosphere and contributed to global darkness and cooling. In this dissertation, I present evidence that petrogenic carbon release has both initiated and served as a feedback to climatic and environmental perturbations in Earth’s history. I demonstrate fluxes of reduced carbon from Earth’s geosphere to its biosphere changed Earth’s climate in the past, and I suggest it has significant potential impacts on Earth’s future climate.