Abstract
The response of the Antarctic ice sheets to global warming is an existential challenge. While past records show that melting from warming waters beneath the ice sheets can be quantified, recent research reveals that the East Antarctic Ice Sheet is susceptible to melting from atmospheric warming, a mechanism that is not well constrained by paleoclimate data. However, the information preserved within permafrost and glacial geomorphology has the potential to reveal greater detail of past terrestrial climate, landscape evolution and environmental conditions.
To test if permafrost and glacial deposits can be used to constrain past terrestrial climate, this thesis presents a novel data set that combines surface exposure dating and depth profiles of in-situ cosmogenic 10Be and 26Al, which provide age and erosion constraints for glacial deposits and permafrost strata from the McMurdo Dry Valleys of Antarctica. Alongside microbial community analyses, these data reveal a time-constrained record of permafrost paleoecology.
In the lower Wright Valley, the lack of attenuation in the 10Be and 26Al depth profiles show that the permafrost was deposited after the Last Glacial Maximum. The relatively recent permafrost conditions (including microbial diversity) at lower Wright Valley are used as a benchmark against which we can assess older permafrost environments further inland and at high elevations. At Pearse Valley, surface exposure ages and 10Be and 26Al depth profiles reveal that Taylor Glacier retreated from Pearse Valley ~65 – 74 ka, and that cold-based glaciers advanced in the McMurdo Dry Valleys during MIS 5. Beneath the surface drift, shallow permafrost at Pearse Valley was deposited ~180 ka, and permafrost deeper than 2.09 m is >180 ka. The 26Al/10Be ratios of permafrost sediments at both lower Wright Valley and Pearse Valley, have exposure-burial histories of at least 1.2 Ma, suggesting multiple recycling episodes of exposure, deposition, burial, and deflation prior to deposition at their current locations. In contrast, at the high elevation site, Table Mountain, surfaces of Sirius Group strata appear younger than their inferred depositional age derived from stratigraphic correlation. Thus, cosmogenic nuclide measurements at Table Mountain are best interpreted as erosion rates. Calculated erosion rates show the underlying Sirius Group sediments are eroding faster than the boulders above them, forming an erosional lag deposit.
Surface exposure dating, and depth profile modelling coupled with 16S rRNA gene amplicon sequencing reveals that surface soil and subsurface permafrost from lower Wright Valley (7,000 – 25,000-year-old) and Pearse Valley (>180,000-year-old) have diverse and distinct microbial communities. The deepest branching clade in both lower Wright Valley and Pearse Valley permafrost samples is Clostridiaceae, which is absent in the surface soil. Nitrospira, Gaiella, and Methyloceanibacter, also do not occur in the surface soil, and exhibit only a small number of branching points in both valleys, indicating their distant evolutionary relationships and isolation from the surface soil. At a high elevation site in the stable upland zone, the Friis Hills, DNA was below detection limits. The inability to identify DNA using amplicon sequencing in the Friis Hills is consistent with previous efforts to analyse high elevation soils and permafrost using 16S rRNA gene amplification sequencing, suggesting microbial habitability is severely restricted in persistent cold, arid habitats.
The study shows that surface sediments and subsurface permafrost can provide a temporally constrained record of environmental conditions and subsequently Antarctic ice sheet fluctuation. The integrated approach applied here can be extended to other permafrost occurrences and regions to expand our understanding of past environmental changes in Antarctica and other regions. Moreover, a complete Pleistocene paleoecological record could be used to develop a transfer function to reconstruct past climate by identifying microbial taxa that are indicative of specific environmental conditions, such as temperature, moisture, and nutrient availability.