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A glaciochemical study of the Mill Island ice core

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Inoue, M (2016) A glaciochemical study of the Mill Island ice core. PhD thesis, University of Tasmania.

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Abstract

The IPCC 5th Assessment Report states that there are insufficient Southern
Hemisphere climate records to adequately assess climate change in much
of this region. Ice cores provide excellent archives of past climate, as they contain
a rich record of past environmental tracers archived in trapped air and
precipitation. However Antarctic ice cores, especially those from East Antarctica,
are limited in quantity and spatial coverage. To help address this, a 120
m ice core was drilled on Mill Island, East Antarctica (65° 30' S, 100° 40' E).
Mill Island is one of the most northerly ice coring sites in East Antarctica, and
is located in a region with sparse ice core data.
The specific project aims were: 1) To produce a high resolution, welldated
record of water stable isotopes (δ18O, δD), and trace ion chemistry (sea
salts, sulphate, methanesulphonic acid); 2) to investigate the seasonal and
interannual variability of sea salts, in order to reveal which climate factors
influence the Mill Island record; 3) to perform a regional comparison of δ18O
and snow accumulation rate with nearby existing climate records from ice
cores, observational stations, and atmospheric models, in order to seek the
optimal method for temperature reconstruction using the Mill Island ice core
record.
Hydrogen peroxide, water stable isotopes, and trace ion chemistry were
measured at high resolution throughout the entire core. The ice core was
dated using a combination of chemical species, but primarily using water stable
isotopes. The Mill Island ice core contains 97 years of climate record (1913
{ 2009), and has a mean snow accumulation of 1.35 metres (ice-equivalent)
per year (mIE/yr). Concentrations of trace ions were generally higher than
at other Antarctic ice core sites (e.g., mean sodium levels were 254 μEq/L).
The full trace ion record contained a mix of periods with well-defined seasonal
cycles and periods with weak seasonality and a higher baseline. An abrupt
change was observed in the sea salt record in the mid-1930s. This may be
related to a significant change in the local ice-scape. Sea salts were compared
with instrumental data, including atmospheric models and satellite-derived sea
ice concentration, to investigate influences on the Mill Island ice core record.
The mean annual sea salt record did not show a correlation with wind speed.
Instead, sea ice concentration to the east of Mill Island was found to be likely
influencing the annual mean sea salt record, at least during the period of 1979
to 2009. A mechanism involving formation of frost flowers on sea ice was proposed to explain the extremely high sea salt concentration. Post-depositional
migration of magnesium and methanesulphonic acid were observed in the trace
ion record, and for the first time, migration of sodium and chloride were observed.
Snow accumulation rate was compared with snow accumulation or precipitation
record from nearby sites. The Mill Island snow accumulation was
found to be influenced by local orography, i.e., the annual snow accumulation
record is not strongly related with precipitation in nearby sites. The Zonal
Wave Three (ZW3), large scale atmospheric mode, modulates precipitation at
nearby Law Dome, and to a lesser extent, modulates Mill Island precipitation.
Snow accumulation and δ18O were compared with precipitation and temperature
data from atmospheric models. The climatology of precipitation at
Mill Island shows evidence of higher snowfall during winter, consistent with
other Antarctic sites. The linear monthly ice core dating was adjusted using
the precipitation climatology, and the adjusted δ18O record resulted in a
warmer annual signal. This finding indicates that without this adjustment,
there is a small cold bias in annual temperature reconstructions from ice cores
that share this elevated winter precipitation. This bias should be considered
when reconstructing temperatures where climate trends differ with season and
when comparing with other temperature reconstructions (e.g., terrestrial or
ocean based records).
In situ temperature data (e.g., a co-located Automatic Weather Station)
are not available at Mill Island. Instead, the annual mean δ18O record was
compared with atmospheric reanalysis model output temperature at Mill Island.
The correlation was found to be statistically insignificant. To attempt
a more accurate palaeothermometer reconstruction, the annual record was divided
into summer and winter \windows" with the maximum δ18O value set
as the summer window centre, and the minimum δ18O value set as the winter
window centre. It was found that when using narrow summer and winter windows,
the δ18O value was significantly correlated with December to April mean
model temperatures and May to July mean model temperatures, respectively.

Item Type: Thesis (PhD)
Keywords: Ice core, Mill Island, East Antarctica, Temperature reconstruction, Sea ice proxy, Climate proxy
Copyright Information:

Copyright 2015 the Author

Date Deposited: 31 Oct 2016 03:00
Last Modified: 13 Dec 2016 22:20
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