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High resolution ice core records of climate variability and forcing


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Pedro, JB (2012) High resolution ice core records of climate variability and forcing. PhD thesis, University of Tasmania.

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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 in°uences on the 10Be solar activity proxy; (ii) centennial to millennial- scale internal climate variability; and (iii) the phase relationship between Antarctic temperature and atmospheric CO2 during the last deglaciation. Cosmogenic 10Be is a primary ice core proxy for past solar activity. However, interpretation of the 10Be 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 10Be 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 10Be is characterised by an (austral) summer to autumn concentration maximum and a winter concentration minimum. The GCM simulations, corroborated by earlier observations of 10Be:7Be ratios, link the seasonal concentration maximum to direct input of 10Be from the Antarctic stratosphere to the lower levels of the Antarctic troposphere. On annual timescales, Law Dome 10Be concentrations are signicantly correlated to the 11-year solar cycle modulation of cosmic ray intensity, rxy = 0:54, with 95% condence interval (CI) [0:31; 0:70]. A signicant correlation is also observed between annual 10Be con centrations and the zonal wave three pattern of atmospheric circulation, rxy = ¡0:36, 95% CI [¡0:57;¡0:10]. An additional annually-resolved 10Be record, from the Das2 site in southeast Greenland spanning 1936{2002, is analysed to facilitate inter-hemispheric comparisons. Das2 10Be concentrations are also signicantly correlated to cosmic ray intensity, rxy = 0:45, 95% CI [0:22; 0:62] and to variability in the dominant mode of at-mospheric circulation in the region, the North Atlantic Oscillation, rxy = ¡0:42, 95% CI [¡0:64;¡0:15]. The strength and spectral coherence of the solar activity signal in 10Be is enhanced, and the climate signals are reduced, when 10Be records are combined from both Antarctica and Greenland. This implies that solar reconstructions are likely to be more robust when 10Be records are included from multiple sites. The amplitudes of the 11-year solar cycles in the 10Be records are inconsistent with the view that the ice sheets receive only 10Be 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 10Be isobar boron-10 from Accelerator Mass Spectrometer (AMS) targets. The last deglaciation was characterised by a `bipolar seesaw' pattern of opposing hemi- spheric 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 Green- land cores. The stable water isotope signal in a composite record constructed from the synchronised Antarctic cores is interpreted as a temperature proxy for the Antarc- tic 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 CO2 records are synchronised to the same timescale as the Antarctic temperature proxy. These records show that the deglacial CO2 increase lagged the Antarctic temperature increase by only 0 to 400 years. This implies a faster feedback between temperature and CO2 than the centennial to millennial-scale lags suggested by previous studies.

Item Type: Thesis (PhD)
Keywords: ice core, climate forcing, palaeoclimate, cosmogenic, Be-10, inter-hemispheric phasing, climate feedback, solar activity
Additional Information: Copyright the Author
Date Deposited: 17 Aug 2012 04:45
Last Modified: 18 Nov 2014 04:41
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