Determining the causes of atmospheric CO2 changes during the last glacial-interglacial cycle: a model-data study
Note this function does not work properly and only enabled about 600 words to be uploaded. Please ignore the below refer to the thesis for the complete abstract. Thank you. The Earth's surface carbon cycle naturally redistributes carbon among its main reservoirs: the ocean, atmosphere, terrestrial biosphere, marine and continental sediments. This natural cycling of carbon regulates atmospheric CO2, both on long and short timeframes. The recent and distal geological past, provide useful...[Show more]
|dc.description.abstract||Note this function does not work properly and only enabled about 600 words to be uploaded. Please ignore the below refer to the thesis for the complete abstract. Thank you. The Earth's surface carbon cycle naturally redistributes carbon among its main reservoirs: the ocean, atmosphere, terrestrial biosphere, marine and continental sediments. This natural cycling of carbon regulates atmospheric CO2, both on long and short timeframes. The recent and distal geological past, provide useful learning on how the carbon cycle responds to natural environmental fluctuations, as a guide to how the carbon cycle will respond in future. The glacial-interglacial (G-IG) cycles of CO2 in particular, exhibit multiple changes in the carbon cycle on short and long time scales, and are littered with clues preserved in the geological record. Chief among these changes are ~80-90 ppm oscillations in atmospheric CO2 during the G-IG progressions. The definitive causes of these large changes in atmospheric CO2 have been keenly debated for the last 40 years, yet remain unresolved, despite substantial progress in thought, analytical and modelling techniques, and the growing bodies of proxy data as evidence. This thesis describes the development of a simple modelling tool to analyse the carbon cycle, that uses proxy data directly to constrain the modelling results, and explains its use in a model-data study of the last G-IG cycle of CO2, spanning 130 thousand years ago to the present. The thesis aims to address what caused the G-IG variation in atmospheric CO2, using the combined model-data approach. The "Simple Carbon Project Model" (SCP-M) box model, was constructed in this thesis to incorporate elements and proxies that are relatively data-rich, and also to represent the processes which influence their concentrations in the carbon cycle. Furthermore, SCP-M was built to flexibly enable exhaustive model-data experiments to explore a wide range of possible parameter values and combinations. SCP-M incorporates phosphate, alkalinity, dissolved inorganic carbon (DIC), alkalinity, isotopes of carbon (d13C and D14C) and the carbonate ion. SCP-M is documented and tested against the modern ocean and atmosphere geochemical data, in this thesis. It was tested against model predictions to the year 2100 for atmospheric CO2, and air-sea gas fluxes of carbon, from the Coupled Model Intercomparison Project (CMIP) models, for the IPCC's representative concentration pathways (RCPs). SCP-M is shown to provide a reasonable approximation to the CMIP RCP scenarios, with best matching for the lower CO2 emissions pathways. A second model-data study is undertaken in this thesis, using an enhanced version of SCP-M with 12 ocean boxes plus atmosphere. The analysis is expanded from the LGM-Holocene to cover the last G-IG cycle from 130 thousand years to the present, with model-data experiments undertaken in time-slices at each marine isotope stage (MIS). The model is forced with proxy data for SST, salinity, RSL, Antarctic sea ice cover, coral reef carbonate production and dissolution, and atmospheric 14C production. The experiment results show that sequential changes in ocean circulation and biological export productivity took place over the last glacial cycle. Early in the glacial cycle, during MIS 5a-e, with cooling SST and increased sea ice cover, the global ocean circulation began to slow, with reduced upwelling of abyssal waters in the Southern Ocean, and slower AABW formation. Later in the glacial cycle, at MIS 4, Atlantic Meridional overturning circulation also slowed, leading to a further drop in atmospheric CO2. The LGM model-data experiment showed that both ocean circulation mechanisms remained subdued, and were accompanied by an increase in Southern Ocean biological export productivity, to achieve the LGM CO2 drawdown. By the Holocene period, both ocean circulation and marine biological productivity returned to modern-type values.|
|dc.title||Determining the causes of atmospheric CO2 changes during the last glacial-interglacial cycle: a model-data study|
|local.contributor.affiliation||Research School of Earth Science, ANU College of Science, The Australian National University|
|Collections||Open Access Theses|
|O'Neill Thesis 2021.pdf||Thesis Material||14.2 MB||Adobe PDF|
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