High resolution ice core records of climate variability and forcing
Creators: name: Pedro, JB
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis exploits the high temporal resolution and precise dating of ice cores from Law Dome, coastal East Antarctica, to address questions concerning climate forcings, variability and feedbacks over a range of timescales: (i) sub-annual to decadal-scale environmental influences on the \\(^{10}\\)Be solar activity proxy; (ii) centennial to millennial-scale internal climate variability; and (iii) the phase relationship between Antarctic temperature and atmospheric CO\\(_2\\) during the last deglaciation. Cosmogenic \\(^{10}\\)Be is a primary ice core proxy for past solar activity. However, interpretation of the \\(^{10}\\)Be record is hindered by limited understanding of the physical processes governing its atmospheric transport and deposition to the ice sheets. The thesis presents a suite of monthly to annually-resolved Law Dome \\(^{10}\\)Be records, which combined span 1936-2009. The records are quantitatively assessed against observed cosmic ray intensities, instrumental and reanalysis climate data and ECHAM5-HAM General Circulation Model (GCM) simulations. The seasonal variability in \\(^{10}\\)Be is characterised by an (austral) summer to autumn concentration maximum and a winter concentration minimum. The GCM simulations, corroborated by earlier observations of \\(^{10}\\)Be:\\(^7\\)Be ratios, link the seasonal concentration maximum to direct input of \\(^{10}\\)Be from the Antarctic stratosphere to the lower levels of the Antarctic troposphere. On annual timescales, Law Dome \\(^{10}\\)Be concentrations are significantly correlated to the 11-year solar cycle modulation of cosmic ray intensity, r\\(_{xy}\\) = 0.54, with 95% confidence interval (CI) [0.31; 0.70]. A signicant correlation is also observed between annual \\(^{10}\\)Be concentrations and the zonal wave three pattern of atmospheric circulation, r\\(_{xy}\\) = -0.36, 95% CI [-0.57; -0.10]. An additional annually-resolved \\(^{10}\\)Be record, from the Das2 site in southeast Greenland spanning 1936-2002, is analysed to facilitate inter-hemispheric comparisons. Das2 \\(^{10}\\)Be concentrations are also significantly correlated to cosmic ray intensity, r\\(_{xy}\\) = 0.45, 95% CI [0.22; 0.62] and to variability in the dominant mode of atmospheric circulation in the region, the North Atlantic Oscillation, r\\(_{xy}\\) = -0.42, 95% CI [-0.64; 0.15]. The strength and spectral coherence of the solar activity signal in \\(^{10}\\)Be is enhanced, and the climate signals are reduced, when \\(^{10}\\)Be records are combined from both Antarctica and Greenland. This implies that solar reconstructions are likely to be more robust when \\(^{10}\\)Be records are included from multiple sites. The amplitudes of the 11-year solar cycles in the \\(^{10}\\)Be records are inconsistent with the view that the ice sheets receive only \\(^{10}\\)Be produced at polar latitudes, instead supporting that they sample from a globally well-mixed atmosphere. In addition, a chemical method is developed to remove the problematic \\(^{10}\\)Be isobar boron-10 from Accelerator Mass Spectrometer (AMS) targets. The last deglaciation was characterised by a `bipolar seesaw' pattern of opposing hemispheric climate variations on millennial timescales. Precise information on the timing and sequence of these climate variations can assist in identifying the mechanisms involved. The timescale of the Law Dome ice core is synchronised throughout the deglaciation (using methane ties) with four other high-resolution Antarctic and Greenland cores. The stable water isotope signal in a composite record constructed from the synchronised Antarctic cores is interpreted as a temperature proxy for the Antarctic region. The millennial warming (and cooling) trends in the Antarctic record are matched by opposing cold (and warm) periods in Greenland. There is little-to-no time lag between climate transitions in Greenland and opposing changes in Antarctica. Such rapid signal-communication between the hemispheres supports the operation of rapid bipolar ocean and/or atmospheric teleconnections. Two precisely dated ice core CO\\(_2\\) records are synchronised to the same timescale as the Antarctic temperature proxy. These records show that the deglacial CO\\(_2\\) increase lagged the Antarctic temperature increase by only 0 to 400 years. This implies a faster feedback between temperature and CO\\(_2\\) than the centennial to millennial-scale lags suggested by previous studies.