Publication Date:
2015-02-10
Description:
An important share of paleoclimatic information is buried within the lowermost layers of
deep ice cores. Because improving our records further back in time is one of the main
challenges in the near future, it is essential to judge how deep these records remain
unaltered, since the proximity of the bedrock is likely to interfere both with the recorded
temporal sequence and the ice properties. In this paper, we present a multiparametric
study (δD-δ18Oice, δ18Oatm, total air content, CO2, CH4, N2O, dust, high resolution
chemistry, ice texture) of the bottom 60m of the EPICA Dome C ice core from central
Antarctica. These bottom layers have been subdivided in two sections: the lower 12m
showing visible solid inclusions (basal ice) and the 48m above which we refer to as
“deep ice”. Some of the data are consistent with a pristine paleoclimatic signal, others
show clear anomalies. It is demonstrated that neither large scale bottom refreezing of
subglacial water, nor mixing (be it internal or with a local basal end-term from a previous/
initial ice sheet configuration) can explain the observed bottom ice properties.
We focus on the high-resolution chemical profiles and on the available remote sensing
data on the subglacial topography of the site to propose a mechanism by which relative
stretching of the bottom ice sheet layers is made possible, due to the progressively
confining effect of subglacial valley sides. This stress field change, combined with bottom
ice temperature close to the pressure melting point, induces accelerated migration
recrystallization, which results in spatial chemical sorting of the impurities, depending
on their state (dissolved vs. solid) and if they are involved or not in salt formation.
This chemical sorting effect is responsible for the progressive build-up of the visible
solid aggregates that therefore mainly originate “from within”, and not from incorporation
processes of allochtone material at the ice–bedrock interface. We also discuss
how the proposed mechanism is compatible with the other variables described. We
conclude that the paleoclimatic signal is only marginally affected in terms of global ice
properties at the bottom of EPICA Dome C, but that the time scale has been considerably distorted by mechanical stretching of MIS20 due to the increasing influence of the subglacial topography, a process that might have started well above the bottom ice.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
notRev
Format:
application/pdf
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