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Characterization of the major ion chemistry of the saline lakes of the Vestfold Hills, Antarctica

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Stark, Scott Charles (2000) Characterization of the major ion chemistry of the saline lakes of the Vestfold Hills, Antarctica. PhD thesis, University of Tasmania.

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Abstract

The Vestfold Hills is a predominantly ice-free 'oasis' lying on the coast of Princess
Elizabeth Land, East Antarctica. The region was formed as a consequence of isostatic
readjustment of the coastline following retreat of the Pleistocene continental ice sheet.
During this event, sea water was constrained and eventually trapped by the rising
landmass. Over a period of ca. 8-10 000 yr, pockets of relict sea water have evolved to
produce several hundred lakes, scattered throughout the Vestfold Hills. While some of
the lakes have been flushed of their sea salt by glacial meltwaters, others have become
hypersaline brines with total dissolved salt contents in excess of 200 g kg- 1.
An accurate and precise characterization of the major ion chemistry of a natural
brine is fundamental to an understanding of its geochemical evolution, the current state
of mineral equilibria in solution and in the sediment, the prediction of physicochemical
properties such as density and activity coefficients, and the accurate determination of
absolute salinity. However, the accurate and reproducible analysis of the major ions in
concentrated, multicomponent brines is often problematic, and obtaining a satisfactory
ion balance between anions and cations is not a trivial task.
Methods for the determination of the major ionic components of sea water and
marine-type brines (sodium, potassium, magnesium, calcium, strontium, chloride,
sulphate, bromide, and total alkalinity) were investigated and a set of suitable methods
was adopted for the characterization of the brines of the Vestfold Hills. The methods
employed were mainly classical 'wet chemistry' techniques and include potentiometric
and photometric titration along with conventional titrimetry, gravimetry, colorimetry,
and flame atomic emission and absorption spectrometry. The reliability of the set of methods was demonstrated by determining precise major
ion composition profiles for 36 brine samples collected from 10 hypersaline lakes, with
a mean absolute ion balance error of only 0.09 ± 0.07 %. Analysis of a sample of
secondary standard sea water, and in some cases analyte recovery tests, provided a
measure of the accuracy of the methods.
Empirical composition relationships for the saline lakes of the Vestfold Hills were
derived using the major ion data obtained for the set of brine samples. Furthermore,
measurements of the density of brine solutions at 20 °C were combined with estimates
of their TDS content, calculated from the composition data, to derive relationships
between density and absolute salinity (and chlorinity). These complement existing relationships correlating the conductivity, temperature and density of Vestfold Hills brines.
Interpretation of the major ion data for the brines suggests that in general, they
conform to a simple closed-basin brine evolution model in which the freezing of sea
water was the main process directing evolution. This was likely to have begun before
the complete isolation of the relict water from the sea, in stratified marine basins within
bays and fjords (open systems with restricted circulation). Biological sulphate reduction
and the input of solutes from non-marine sources appear to have had little influence on
the present-day composition of the brines examined in this study.
All of the brines were saturated with sodium sulphate, precipitated as mirabilite, and
the most saline were saturated with sodium chloride (hydrohalite and/or halite). The
observed fractionation of magnesium, potassium, chloride and bromide, however,
suggests that at least some of the brines were considerably more saline in the past,
probably concentrated to saturation with the chlorides of potassium and magnesium
(sylvite, magnesium chloride dodecahydrate, carnallite). This would have required
climatic conditions that were more frigid and/or arid than exist today. There is also evidence for the precipitation of calcium sulphate (gypsum) in the brines, as well as other sulphate salts such as strontium sulphate (celestite) and possibly potassium-sulphate phases. This is attributed to diagenetic reactions and also to the
mixing of lake waters with sulphate-rich brines, derived from mirabilite dissolution
occurring within the lake or in deposits located in the catchment. The best evidence for
the latter mechanism, favoured by a net positive water balance, was uncovered by
examining major ion depth profiles for Deep Lake, one of the most saline brines in the
Vestfold Hills.

Item Type: Thesis (PhD)
Keywords: Salt lakes, Salinity
Copyright Holders: The Author
Copyright Information:

Copyright the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s). 2000.

Additional Information:

Includes CD-ROM in back pocket. Thesis (Ph.D.)--University of Tasmania, 2000. Includes bibliographical references

Date Deposited: 03 Feb 2015 03:25
Last Modified: 11 Mar 2016 05:54
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