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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
Authors/Creators:Pedro, JB
Keywords: ice core, climate forcing, palaeoclimate, cosmogenic, Be-10, inter-hemispheric phasing, climate feedback, solar activity
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