CrawZealand
& Beckett—Water
and sediment
chemistry Research,
of Sutton Salt
Lake
New
Journal of Marine
and Freshwater
2004,
Vol. 38: 315–328
0028–8330/04/3802–0315 © The Royal Society of New Zealand 2004
315
Water and sediment chemistry of Sutton Salt Lake,
east Otago, New Zealand
D. CRAW
S. BECKETT
Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
email: [email protected]
Abstract The Sutton Salt Lake is the only saline
lake in New Zealand, and has formed in a windy
cool-temperate maritime climate. Consequently, the
lake is distinctly different from most of the world’s
saline lakes that form in arid continental settings.
Sutton Salt Lake forms annually in a shallow (5 m)
bedrock-floored depression c. 50 km from the nearest coast. The site receives c. 500 mm/year rainfall
compared with coastal rainfall of near 1000 mm/year
because of a minor rain-shadow effect of coastal
hills. Surface evaporation rate is high (c. 700 mm/
year) because of frequent strong winds. Sediments
on the lake floor are derived by rain and wind erosion of the surrounding quartzofeldspathic schist
bedrock, with a contribution from organic sources,
particularly ostracods, and evaporative halite. The
sediments have a higher proportion of phyllosilicates
(muscovite, kaolinite, and chlorite) than the source
rocks because of differential transport of these minerals into the lake depression. Lake water is entirely
derived from rain, rather than groundwater, and the
lake waters have had minimal chemical interaction
with bedrock. Lake water pH is near 9 and pH of pore
waters in drying lake sediments is near 8, compared
with a pH near 7 for regional surface and ground
waters. When full, the lake has salinity about one
quarter to one third of that of sea-water, and ion
ratios are similar to sea-water. The lake salinity is
derived from marine aerosols in rainwater concentrated by c. 20 000 evaporation and refilling cycles
in the lake depression.
M03094; Online publication date 8 June 2004
Received 10 December 2003; accepted 5 April 2004
Keywords saline lake; geochemistry; marine aerosol; evaporite
INTRODUCTION
Most evaporative salt lakes form in continental areas
with arid climates, such as inland western North
America, Australia, and Antarctica (Eugster 1980;
Chivas et al. 1991; Gibson et al. 1991). Well defined
dry seasons with low humidity ensure evaporation
of sparse and rare inputs of water in these
environments. Saline lakes are widespread in these
settings, and their sediments can coat valley floors
over tens or hundreds of square kilometres. Gypsum
is one of the most common evaporite minerals in
these saline lakes (Eugster 1980; Chivas et al. 1991).
Small land masses surrounded by ocean typically
have humidity too high, and excess of rainfall over
evapotranspiration, for development of evaporative
salt lakes. However, this study describes a salt lake,
Sutton Salt Lake, which has developed in southern
New Zealand’s cool-temperate maritime climate.
The lake is small (c. 2 ha) and is the only saline lake
in New Zealand. Hence, the Sutton Salt Lake is
unique in its setting.
The lake site hosts halophytic vegetation around
the lake margin, and diverse native fauna, making
the site significant for biological and geological
reasons (Bayly 1967; Murray 1972; Patrick 1989;
Luxford 2001). Despite the biological significance
of the lake area, little information on the chemistry
of the substrate has been published even though the
site owes its uniqueness to chemical processes.
Bayly (1967) mentions some chemical characteristics in passing, and McIntosh et al. (1990) document
some basic facts on the site. This study attempts to
fill this knowledge gap by presenting descriptions of
sediments and waters, and evaluating that data in the
context of the geological and geographical setting of
the lake. In particular, we focus on lake water and
sediment chemistry, and examine these analytical
results in terms of minerals and rocks that surround
the lake, and potential external inputs to the lake.
316
New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38
Fig. 1 Digital elevation model
(from www.geographx.co.nz)
showing the topography of east
Otago, New Zealand, and the location of Sutton Salt Lake at the
southern end of the Middlemarch
Basin. Inset shows east Otago in
the South Island of New Zealand,
and the location of the inland Mackenzie Basin where rainfall chemistry was obtained (Jacobson et al.
2003).
REGIONAL SETTING
Geology and topography
The Sutton Salt Lake occurs near the southern edge
of the Middlemarch Basin (Fig. 1), a structural
depression on the downthrown side of the active
Hyde Fault (Jackson et al. 1996). The basement rock
of the area is Otago Schist, locally overlain by
remnants of a veneer of Tertiary marine and nonmarine sediments. More remnants of these sediments
underlie the Middlemarch Basin where they are
covered by Quaternary gravels of the Taieri River
(Fig. 1) and alluvial fans from rising marginal schist
ranges. The regional Waipounamu Erosion Surface
(LeMasurier & Landis 1996) is cut into the schist
basement and has resulted in widespread low relief
surfaces, which dominate the topography of the area
around the Middlemarch Basin (Fig. 1). This erosion
surface has been deformed by late Tertiary
collisional tectonics to give rise to the complex set
of ranges and basins of east and central Otago (Fig.
1; Jackson et al. 1996). Broad low relief uplands that
separate the Middlemarch Basin from coastal Otago
(Fig. 1) resulted from the same tectonism.
The basement schist is coarse grained (>100 µm)
and highly segregated into micaceous and quartzofeldspathic laminae on the mm to cm scale. The
schist consists of quartz, albite, muscovite, chlorite,
epidote, titanite, and calcite, with minor pyrite.
Secondary calcite and pyrite are common along
fractures in unoxidised schist. The schist beneath the
erosion surface was intensely altered to a predominantly kaolinite-quartz assemblage, up to 20 m
vertically, by groundwater within the sedimentary
veneer (Craw 1994). Depth of alteration varied, and
was controlled by subtle lithological variations in the
schist. Alteration at the erosion surface resulted in
varying degrees of oxidation of the schist, resulting
in decomposition of chlorite and pyrite to kaolinite
and brown iron oxyhydroxide.
The kaolinitic rock was rapidly removed when the
erosion surface was exhumed, and an undulating
Craw & Beckett—Water and sediment chemistry of Sutton Salt Lake
317
Fig. 2 Contour map (m a.s.l.) of the Sutton Salt Lake catchment, east Otago, New Zealand, and immediately adjacent drainage areas, constructed using differential global positioning system. Heavy dashed line defines the drainage
divide around the salt lake catchment.
basement surface, studded by brown-stained schist
tors, resulted. The steep-sided tors are typically 5–
10 m high and are separated by broad smooth
depressions 20–100 m wide, containing sandy soils
and colluvium generally <2 m thick. Tors and
intervening depressions form a linear pattern
trending north-west (Fig. 2) reflecting the underlying
schist structural and lithological control, described
as “gefugerelief” by Turner (1952).
The Sutton Salt Lake is impounded by bedrock
in a depression between tor ridges. There are
many similar depressions that can contain
ephemeral lakes in the vicinity (Fig. 2), but none
of these others is saline. These other lakes are
dammed by colluvium in depressions between
lines of tors. Lake water apparently leaks slowly
through this colluvium and escapes along
depressions into the stream system.
Ba
895
558
697
846
Cr
160
57
394
65
V
100
80
86
106
Zn
119
112
96
123
Y
34
31
30
34
Zr
197
182
262
178
Lake sediment 1
Lake sediment 2
Lake sediment 3
Lake sediment 4
Lake sediment 1
Lake sediment 2
Lake sediment 3
Lake sediment 4
Sr
273
322
281
254
Rb
148
136
112
160
Trace elements (ppm)
Pb
Ni
Cu
31
32
37
27
32
36
22
34
42
34
33
39
As
11
8
8
13
Total
98.80
99.19
99.40
99.36
LOI
6.86
7.25
5.24
5.37
P2O5
0.24
0.30
0.18
0.19
K2O
3.89
5.17
4.66
4.70
Na2O
2.98
1.90
2.37
2.38
Major elements as oxides (wt%)
MnO
MgO
CaO
0.07
2.35
2.47
0.15
2.80
3.91
0.09
2.35
1.70
0.09
2.31
1.81
Fe2O3t
4.69
6.46
6.05
6.10
Al2O3
16.00
18.57
18.12
18.43
Wet lake sediments were sampled from a range of
sites over the lake while it was full in January 1998.
These sediments were dried in an oven at 60°C, then
powdered with an agate mortar and pestle. The
powders were analysed for major, minor, and
selected trace elements (Table 1) by X-ray
fluorescence spectrometry in the Geology
Department, University of Otago, using international
standards (Craw et al. 2000). Major and minor
elements were determined on fused discs, and trace
elements were determined with pressed powder
discs. Loss on ignition (LOI) was determined from
weight loss after heating at 1100°C for 2 h. Chlorine
(Cl) analyses were obtained for sediments sampled
in November 1998 on dry parts of the lake bed.
Samples were taken at dry surface, 10 cm deep, and
35–50 cm deep, at two localities 50 m from the lake
edge. Cl analyses were determined by titration
(oxygen-filled flask method) by the Microanalytical
Laboratory, University of Otago, with detection limit
of c. 0.2 wt%. Sulfur (S) analyses were obtained on
a representative set of surface and deep (c. 50 cm)
TiO2
0.68
0.72
0.71
0.73
METHODS
SiO2
58.57
51.96
57.93
57.25
Climate
The Sutton Salt Lake lies inland of two ranges of
hills, and rainfall is lower than at the nearby Otago
coast (Fig. 1), which receives c. 1000 mm/year. The
lake area is essentially devoid of forest and is
vegetated principally with native and exotic grasses.
Temperatures typically range from lows of –10°C in
winter to +30°C in summer, with occasional
extremes beyond this range. The barren landscape
is subject to regular strong winds, particularly in the
spring (Edwards et al. 1988). The strongest and most
common winds are from the north-west and southwest, with hourly mean speeds up to 25 m/s recorded
20 km to the west (Edwards et al. 1988). Wind may
have contributed to formation of the tor topography
(Fahey 1981), and wind erosion has been implicated
in formation of saline depressions in the area
(Raeside 1949). The east Otago uplands receive c.
500 mm/year precipitation, mainly as rain (Bayly
1967; Allen et al. 1997; Craw & Nelson 2000). Rain
events arise with winds from the east, north-west or
south-west, but the surrounding hills (Fig. 1) cause
a rain-shadow effect for all winds. The surface can
lose up to 700 mm/year moisture via evaporation
(Bayly 1967; Allen et al. 1997; Craw & Nelson
2000), and surface water loss is especially rapid in
warm dry north-west winds.
Ce
136
69
112
119
New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38
Table 1 Representative X-ray fluorescence analyses of lake sediments for Sutton Salt Lake, east Otago, New Zealand. (All iron is calculated as ferric. LOI, loss
on ignition.)
318
250*
250*
250*
250*
250*
250*
250*
250*
250*
250*
250*
250*
1000
920
850
1030
747
734
737
740
727
717
707
549
523
100*
100*
100*
505
509
476
459
–1.9
3.5
3.9
5.7
4.9
3.9
2.4
1.8
1.1
5.7
3.0
7.3
2.9
0.6
4.0
4.2
1.48
1.47
1.48
1.46
1.46
1.39
1.41
1.7
1.7
0.15
0.16
0.09
1.44
1.46
1.41
1.42
3.65
3.63
3.64
3.62
3.6
3.47
3.47
3.27
3.24
0.34
0.22
0.14
3.65
3.71
3.54
3.56
–2.07
–2.08
–2.07
–2.1
–2.09
–2.08
–2.06
–1.68
–1.66
–1.7
–1.63
–1.48
–1.39
–1.45
–1.45
–1.35
–3.16
–3.15
–3.14
–3.18
–3.11
–3.11
–3.11
–3.44
–3.42
–3.52
–3.37
–3.71
–2.29
–2.36
–2.41
–2.37
319
* Denotes analyses at or marginally above detection limit of method.
7610
8230
8310
8720
8670
8630
8540
5490
5660
5310
6280
4430
23400
21100
20600
21700
176
170
174
174
167
171
170
50
50
59
58
82
509
458
418
453
64
63
65
62
63
64
67
119
125
94
128
148
156
141
145
159
5070
4890
4880
4940
5000
5080
5180
3470
3610
2940
3660
2250
13400
12730
11550
12150
111
109
108
109
108
109
109
85
88
87
113
70
713
642
608
635
(halite)
SI
(calcite) (dolomite) (gypsum)
% error
alk.
SO42–
Cl–
Mg2+
Ca2+
Na+
K+
pH
9.1
9.1
9.1
9.1
9.1
9.0
9.0
9.1
9.1
8.2
8.1
7.9
9.0
9.1
9.0
9.0
lake
lake
lake
lake
lake
lake
lake
lake
lake
pores
pores
pores
lake
lake
lake
lake
Jan 1998
Jan 1998
Jan 1998
Jan 1998
Jan 1998
Jan 1998
Jan 1998
Nov 1998
Nov 1998
Nov 1998
Nov 1998
Nov 1998
Aug 2003
Aug 2003
Aug 2003
Aug 2003
Table 2
samples. S contents were determined by a PerkinElmer Elemental Analyser in the Microanalytical
Laboratory. About 5 mg of powdered sample was
heated to 1100°C for analysis, and detection limit
was c. 0.02 wt%.
Water samples were collected in plastic bottles
from the lake, and these waters were filtered
before analysis for principal cations and anions.
Seven samples from different points in the lake
were taken in January 1998 when water was c.
50 cm deep, and an additional two lake water
samples were obtained in November 1998. Four
more samples of water were taken from remnant
pools when the lake was almost dry in August
2003. Saturated sediments taken at three of the
sampling sites in November 1998 were
centrifuged to release pore waters, which were
also analysed (Table 2).
Initial water analyses (Table 2) were done by
University of Otago Chemistry Department, and
the August 2003 analyses were done by
Chemsearch, an internationally accredited
laboratory in the Chemistry Department. Charge
balance errors for the analyses are shown in Table
2. APHA (1995) method 3111B was used for
Ca2+, Mg2+, Na+, and K+, 4500–Cl C for chloride,
and 2320B for dissolved carbonate. Detection
limits for these common ions are c. 1 mg/litre, and
analyses for these ions are reproducible to less
than 1% error in the compositional ranges
observed at Sutton Salt Lake. Initial sulfate
analyses were done by nephelometry, with a
detection limit of c. 200 ppm, and only trace
amounts were detected in the dilute lake waters.
The more concentrated August 2003 waters were
analysed for sulfate by ion chromatography
(Metrohm column) at Chemsearch, and again by
ion chromatography (Dionex column) at Hill
Laboratories, Hamilton, New Zealand. The
August waters were also analysed for total S by
ICP at Hill Laboratories and results converted to
sulfate assuming all dissolved S is sulfate. Results
for the two latter methods of sulfate analyses
agree within 10% of the Chemsearch values, and
the Chemsearch results (1 mg/litre detection limit)
are presented in Table 2. Analytical error lies
within the size of the symbols in all diagrams in
this paper. Water pH was determined with an
Oakton portable pH meter calibrated with
standard pH solutions. Additional waters were
collected from small streams between the east
Otago coast and the Middlemarch Basin, for
comparison with the lake waters. These waters
Analyses of lake waters and pore waters. (SI, modelled saturation index, log scale; alk., alkalinity as HCO3– ; % error is charge balance error in analyses.)
Craw & Beckett—Water and sediment chemistry of Sutton Salt Lake
320
New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38
Fig. 3 Composite section (centre) through the sediments on the floor of the Sutton Salt Lake, east Otago, New
Zealand, as measured in pits dug in the dry lake bed. Chlorine concentrations in two profiles through the sediments are
depicted on the left. Pie diagrams on right depict the principal constituents of the surface layer: upper pie shows the
grain size distribution, and lower pie shows the proportions of constituents in the sand fraction.
were analysed for Na+ and Cl– in the Chemsearch
laboratory by the above methods.
The mineralogical nature of a representative
sample of lake sediment was determined by dry sieve
separation (63 µm) of sand and finer material. The
calcareous content of the sand fraction (containing
abundant ostracods) was calculated from weight loss
after addition of hydrochloric acid, and the organic
content then determined by weight loss after
treatment with hydrogen peroxide. The remnant
terrigenous sand was examined optically and by Xray diffraction. The fine component (lacking
ostracods) was characterised by X-ray diffraction in
the samples taken from the lake bed in January 1998
(above). Surface salt encrustations and associated
dry surface sediments were also characterised by Xray diffraction.
SUTTON SALT LAKE
The Sutton Salt Lake is an ephemeral lake that lies
in a c. 5-m-deep depression between two lines of
tors, c. 300 m from the edge of the Middlemarch
Basin (Fig. 2) and c. 250 m a.s.l. (Fig. 3). The salt
lake is 150 m wide and 340 m long, and is elongate
parallel to the tor lines (Fig. 2, 3). Schist crops out
at or near the lake margins, and the lake is apparently
dammed by schist all round. The lake dries up during
most summers, apart from pools after rainstorms,
and refills during the following winter. However, the
lake can remain full over wet summers, and this was
so for the first season of this study (summer of 1998–
99). When full, the lake is 40–50 cm deep, with little
variation in depth over the whole depression. Waves
>40 cm high build up on the lake on windy days, and
the water readily becomes turbid and discoloured as
bottom sediment is stirred.
When empty, the lake depression becomes a flatbottomed muddy plain with extensive formation of
polygonal desiccation cracks on the 10–50 cm scale.
Desiccation cracks extend c. 10 cm below the
surface. A halite crust develops on the dry surface,
and halite films float on thin (cm scale) water
remnants. Gypsum is a trace component of some Xray diffractograms of crusts. The upper 10 cm of
sediment contains abundant ostracods, and the
ostracod component decreases to low levels below
10 cm. The lake depression contains c. 50 cm of
sediment (Fig. 3), and sediment thickness varies little
Craw & Beckett—Water and sediment chemistry of Sutton Salt Lake
321
Fig. 4A–D Whole rock major element geochemical comparisons between lake sediments below the surface layer
(black diamonds), and fresh basement schist (open squares) nearby at Barewood (Fig. 1).
over the whole depression. The sediments are
unstratified and massive because of bioturbation,
wave disturbance, and historic farm animal
incursion. Angular schist blocks up to 8 cm across
are irregularly distributed at and near the base of the
lake sediment column, and larger boulders (m scale)
occur near the lake margin.
Lake sediments are dominated by silt and clay
grains (80–90%; Fig. 4), with the remainder being
fine sand. The silt and clay is predominantly
muscovite and kaolinite, with minor chlorite, quartz,
and feldspar. The sand-size fraction (Fig. 4) is c. 70%
terrigenous, consisting of quartz, albite, muscovite,
and kaolinite, with minor chlorite, derived from the
schist basement. The remaining 30% is made up of
calcareous material and organic material in subequal
proportions (Fig. 3), all of which is derived from
ostracods.
LAKE SEDIMENT COMPOSITION
Throughout the following descriptions, Sutton Salt
Lake sediment compositions are compared to the
basement schist from which most of the sediment
was derived (above). A set of analyses of schist from
the Barewood area (Fig. 1; MacKenzie 1990) is used
for this purpose. The Barewood schist is
lithologically similar to that at Sutton Salt Lake, and
lies directly along strike from the lake. The
Barewood samples were taken from fresh outcrops
and drill cores, and therefore represent unweathered
schist.
The lake sediments contain only 50–65% silica,
which is generally lower than Barewood schists, and
relatively high titanium (Fig. 4A). As such, the lake
sediments chemically resemble the more micaceous
parts of the basement schist schists (Mortimer &
322
New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38
Roser 1992), rather than the more quartzofeldspathic
schists that the Barewood samples represent. There
is minor enrichment of iron (Fe) and magnesium
(Mg) in lake sediments compared to most samples
of schist bedrock, but the range in Fe2O3 and MgO
analyses is similar for sediments and schist (Fig. 4B).
Lake sediments as a group have generally higher
potassium (K) content than Barewood schists, but
there is considerable overlap of sodium (Na) contents
(Fig. 4C). There is a distinct antithetic relationship
between K2O and Na2O in lake sediments (Fig. 4C).
The basement schists have a wide range of calcium
(Ca) contents and the lake sediments show similar
high and variable CaO (Fig. 4D). Likewise, the “loss
on ignition” that mainly includes CO2 and H2O is
variable (2–4 wt%) in both bedrock schists and the
lake sediments. Most trace element concentrations
are similar in Barewood schists and the lake
sediments (Table 1), although both chromium (Cr)
and lead (Pb) contents are slightly elevated in the
lake sediments compared to the bedrock schist.
Sulfur contents of six lake sediments are nearly
all at the detection limit of the analytical method:
0.02 wt%. The highest measured value is 0.17 wt%.
There was no systematic difference between samples
taken from the surface or from up to 50 cm depth.
The observed S contents of the lake sediments are
similar to, or lower than, those of nearby
unmineralised Barewood schists (typically 0.1–0.2
wt% S).
WATER CHEMISTRY
Sutton Salt Lake waters have distinctive chemistry
(Table 2), and in this section these waters are
compared with other waters from the region to
emphasis the key points of the lake water chemistry.
The sea is the largest water reservoir in the area, so
this is used as one point of comparison (Wilson
1975). Sea-water components migrate inland via
marine aerosols in the atmosphere, deposited as rain,
so rainwater composition is relevant for comparison.
An analysis of rainwater in an inland rain-shadow
area (Jacobson et al. 2003) is used for this
comparison. Likewise, we compare the lake waters
with surface waters having a substantial rainwater
component from the Taieri Basin, immediately south
of the Sutton Salt Lake (Fig. 1). The data set used,
from Litchfield et al. (2002), includes waters with a
wide range of dissolved loads, including two samples
that may have a tidal influence. We also compare
lake waters with groundwater in the schist basement,
Fig. 5 Spatial and temporal variation of pH in Sutton
Salt Lake (east Otago, New Zealand) wet sediments in
November 1998, measured in a traverse from the shore
(m scale, open squares) and in sections through the
sediments in pits (cm scale, black diamonds). Water pH
in January 1998 is indicated with an open rectangle. Typical regional surface and groundwater pH range is indicated with black bar (lower left).
using a set of analyses from near Macraes Flat (Fig.
1; Craw & Nelson 2000).
The pH of the water in the lake when it is full is
consistent at 9.0 to 9.1 across the lake (Fig. 5). When
the lake was nearly dry, pH of the saturated
sediments was measured at sites progressively
farther from the shore, and at various depths in pits
dug in the sediments. These data (Fig. 5) show that
the pH ranged from 7.7 to 8.4, with no systematic
variation with position in the lake. All pH
measurements are at or above the highest levels
normally observed in surface waters and groundwaters in the region (Fig. 5).
The analysed lake waters are dominated by Na+
and Cl– (Table 2), and these two ions are present in
approximately equal molar concentrations (Fig. 6A).
There is a slight excess of Cl over Na, similar to that
observed in sea-water (Fig. 6A). The Na+/Cl– ratio
of the lake water is different from the distinct Na+
enrichment observed in groundwater in the schist
bedrock (Craw & Nelson 2000; Fig. 6A). K+ and
Mg2+ are approximately constant at c. 110 and 170
ppm respectively in the January 1998 (full) lake
waters, but lake and pore waters sampled at other
times are different from the earlier samples and more
variable (lower in November 1998 and higher in
August 2003; Table 2, Fig. 6B). The Ca2+ content
of all analysed waters is 60–150 ppm, in contrast to
highly variable alkalinity which ranges from c. 100
to 740 ppm (Fig. 6C). With respect to sulfate and
Craw & Beckett—Water and sediment chemistry of Sutton Salt Lake
323
Fig. 6A–D Lake water chemistry (black diamonds) compared to sea-water (black square) and groundwater in the
basement schist (black triangles). Large open circles in D are waters from the Taieri Basin (Fig. 1).
sodium, Taieri Basin groundwaters form a trend of
varying degrees of dilution towards sea-water
because of marine aerosol deposition (Fig. 6D;
Litchfield et al. 2002). The Sutton Salt Lake waters
lie slightly on the sodium-enriched or sulfatedepleted side of the concentrated end of this trend
(Fig. 6D).
DISCUSSION
Source of lake sediment
The composition of the lake sediments is generally
similar to that of the basement schist (Table 1; Fig.
4). These similarities in composition, combined with
observations of schist minerals in the sediment,
confirm that most of the sediment has been derived
by erosion of the surrounding schist. However, there
are some minor differences between basement schist
and lake sediments that indicate that there has been
some fractionation of schist debris during erosion
and transport.
The trend from high silica, low titanium to low
silica, high titanium (Fig. 4A) is characteristic of
decreasing proportions of quartzofeldspathic
component and increasing proportions of micaceous
component of the schist (Mortimer & Roser 1992).
The enrichment in titanium in micaceous material
arises because titanite, the main titanium mineral,
occurs principally as micron-scale grains dispersed
through the micaceous segregations. We infer from
these data that erosion and transport of bedrock
material into the lake has resulted in minor
enrichment in micas, and relative depletion in the
324
New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38
Elevated CaO and LOI in the lake sediments and
some schists arises from the variable component of
calcite: calcareous and organic ostracod material in
the sediments (Fig. 3), and calcite veins in the
basement schists. Calcite veins are readily removed
from schist outcrops by weathering, and outcrops at
Sutton Salt Lake have fewer remaining calcite veins
than the fresh rocks at Barewood. Hence, the
variability of CaO and LOI in the lake sediments
reflects the variability of ostracod component, rather
than the schist source material.
Fig. 7 Sketch map of east Otago, New Zealand, showing molar Na/Cl ratios of surface waters in streams and
small lakes, and some shallow groundwaters. Most sites
have Na/Cl near 1, reflecting rainout of marine aerosols.
High Na/Cl reflects water-rock interaction with basement
schist.
quartzofeldspathic component. This is in accord with
direct observations of the sedimentary material that
confirm a high mica component (above).
The general elevation of K2O content in
sediments (Fig. 4C) results from the same increase
in micaceous component during sedimentation
inferred above. The inverse relationship between
K2O and Na2O in the sediments (Fig. 4C) directly
reflects relative enrichment in muscovite and
depletion of albite in the sediments compared to the
schist basement. Chlorite is the principal Fe-Mg
mica, but muscovite also contains significant Fe and
Mg in these schists (Brown 1968). Enrichment in
these micas therefore accounts for the minor and
inconsistent elevation in Fe2O3 and MgO in the
sediments (Fig. 4B). Cr occurs principally in solid
solution in chlorite in the Otago Schist, and the
slightly elevated Cr in the lake sediments may reflect
decomposition of chlorite in the weathering
environment. Pb enrichment may arise from similar
processes.
Rainwater and schist water-rock interaction
The Sutton Salt Lake has strongly elevated levels of
dissolved constituents, even when full, compared
with nearby water bodies. The two most plausible
potential sources of dissolved constituents in saline
lakes are from rainwater that has chemically
interacted with the underlying rocks (e.g., Eugster
1980), or from direct input of rainwater containing
dissolved salts (e.g., Chivas et al. 1991). Both of
these sources of water involve rainwater input. The
dissolved constituents in inland South Island
rainwater are dominated by marine aerosols, with a
Na+/Cl– ratio of near 1 (Ahlers & Hunter 1989;
Jacobson et al. 2003), similar to that seen nearer the
coast (Litchfield et al. 2002; above). Na+/Cl– ratios
in streams in the hills between Sutton Salt Lake and
the sea reflect the marine aerosol input from
rainwater (Fig. 7). Hence, we can evaluate the two
potential sources of dissolved constituents with
reference to the composition of sea-water and marine
aerosols in rain.
Chemical interaction between water and schist
bedrock results in progressively increasing Na+
content of the water from dissolved albite, whereas
Cl– remains essentially constant, so the Na+/Cl– ratio
rises (Fig. 6A,D; Craw 2000; Craw & Nelson 2000).
Dissolution of calcite during water-rock interaction
results in elevated Ca2+ and HCO3– in the
groundwater (Craw 2000; Craw & Nelson 2000). At
the same time, decomposition of chlorite increases
the dissolved Mg content, and shallow groundwaters
commonly have a Ca2+/Mg2+ ratio near 2 (Craw
2000; Litchfield et al. 2002). Hence, water that has
had significant interaction with schist becomes
dominated by Ca2+ and HCO3–, with subordinate Na+
and Mg2+, and only minor Cl–. Evaporative deposits
associated with near-surface schist groundwater
seepages are dominated by calcite, with minor iron
oxyhydroxide. Compositions of typical schist-hosted
groundwaters from the Macraes Flat area (Fig. 1)
reflect the above reactions, as shown in a Piper
Craw & Beckett—Water and sediment chemistry of Sutton Salt Lake
SO42-80
+Cl- 60
Fig. 8 Piper diagram comparing
groundwater that has interacted
with schist (black dots) with Sutton
Salt Lake (east Otago, New Zealand) waters (black squares), and
sea-water (open circles with
crosses).
40
ground
20 water
CC
C
C
CC
2+ C CC
C CC
C
C C
C
CC
CC
C
C
C
C
C
C
C
C
C
C
C
C
Mg
80
60
40
20
C C
C
C
C
CC
C
C
C
CC C
C
C
CC
C
C
C C
C
C
C
C
C
C
C
CC
C CC
Ca2+
diagram (Fig. 8). Stream waters in the Macraes Flat
area generally have a higher Na+/Cl– ratio than
coastal streams (Fig. 7) because Macraes Flat
streams are fed mainly by groundwater, except
during storms.
In contrast, the most prominent feature of the
Sutton Salt Lake geochemistry is the high Na+ and
Cl– of the site, manifested as a halite crust when the
lake is dry, and Na-Cl saline water when the lake is
full. Sutton Salt Lake waters show no evidence for
the water-rock interaction reactions described above.
The observed water compositions, including pore
waters, plot in tight clusters on the Piper diagram
(Fig. 8). These tight clusters of data plot closely to
sea-water compositions (Fig. 8). Hence, we conclude
that the Sutton Salt Lake derives it salts from seawater via marine aerosols (Jacobson et al. 2003), and
that the waters have had negligible chemical
interaction with schist bedrock.
Mineral saturation indexes
Saturation indexes for common evaporite minerals
in Sutton Salt Lake waters were calculated using
modelling program PHREEQC (Parkhurst et al.
1980), and results are listed in Table 2. These model
calculations show that the lake waters are typically
strongly oversaturated with respect to calcite (>10
times) and dolomite (>1000 times). The sampled
pore waters were not so strongly oversaturated with
respect to these minerals, and approached
UDD
D
325
2+
80 Ca
+Mg2+
60
40
sea-water
D Sutton Salt Lake
U
D
D
SO4280
60
40
C
C
C
C
C
C
C
C
C
C
CC
C
C
C
C
Na++K+ HCO3-
U
D
D
D
Cl-
equilibrium saturation levels (Table 2). Growth of
ostrocods in the lake undoubtedly facilitates
temporary removal of some dissolved Ca and Mg
carbonate into the sediments. It is probably this
biological activity that prevents precipitation of
carbonate minerals in the evaporate crusts.
Ephemeral carbonate crusts on rocks on the lake
margin have been noted in the past (C. A. Landis
pers. comm.), but were not observed during the
present study.
Halite and gypsum, the two minerals that have
been observed in evaporite crusts on the dry lake bed,
are distinctly undersaturated in even the most
concentrated brines sampled in this study (Table 2).
Differences of Sutton Salt Lake
waters from sea-water
The K and Mg contents of full lake water are similar
to those of dilute sea-water, when corrected for
differing Cl contents (Fig. 9A). K is relatively
enriched in some lake sediment pore waters (Table
2), probably from localised dissolution of muscovite
in the wet sediments. High K in rainwater from
inland South Island (Fig. 9B; Jacobson et al. 2003)
may be a result of dissolution of terrigenous
particulate micaceous dust, and this phenomenon
may contribute to elevated K in lake waters when
rain events occur on a near-dry lake bed.
Calcium is slightly depleted in Sutton Salt Lake
waters compared with sea-water (Fig. 9A), yet
326
New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38
marine aerosols are typically enriched in Ca2+
compared with sea-water (Warneck 2000). The
inland rainwater Ca2+ content (Jacobson et al. 2003)
reflects this enrichment of Ca, and the Sutton Salt
Lake waters are strongly depleted compared with
that rainwater (Fig. 9B). However, lake sediment
pore waters have higher Ca2+ than the lake waters
(Table 2; Fig. 9C). This variation in Ca2+ is probably
related to variable degrees of dissolution and uptake
of calcium carbonate from ostracods.
Most marine aerosols contain sulfate
concentrations similar to, or greater than, sea-water
(Warneck 2000), so molar SO42–/Cl– ratio of >0.1
should be expected in subsequent rainout (Fig. 9B).
This is apparently true for surface waters on the
Taieri Basin, where relatively high SO42–/Cl– ratios
occur (Fig. 6D). Conversely, Sutton Salt Lake waters
are slightly depleted in sulfate compared with seawater (Fig. 9A). Groundwater in schist bedrock
readily dissolves S from dispersed pyrite and related
decomposition products to maintain a low but
significant dissolved sulfate content (Fig. 8, 9B;
Craw 2000; Craw & Nelson 2000). Recharging
surface waters in inland areas have low dissolved ion
contents (Kim & Hunter 2001) and these waters
rapidly develop a rock-controlled S content, as they
do a rock-controlled Na content (above). This sulfate
content is similar to that of many surface waters
nearer the coast, and higher than Sutton Salt Lake
waters (Fig. 9B).
Since all of the likely sources of input water to
the Sutton Salt Lake have significant dissolved
sulfate, the minor depletion of S at the lake site
implies that the S has preferentially left the site. This
cannot have occurred via drainage of liquid water as
other dissolved ions, especially Na+ and Cl–, would
have left the site at the same time. A more plausible,
although speculative, alternative is that the S was
volatilised and left with evaporating water. The
enrichment of marine aerosols in S compared to seawater (above) is a result of biological formation of
dimethyl sulfide in the sea, and similar processes
occur on land (Gibson et al. 1991; Warneck 2000).
Saline mudflats can be particularly productive of
volatilised S, mainly as H2S and dimethyl sulfide
(Goldberg et al. 1981; Warneck 2000), and this
environment may be a reasonable analogue for the
biologically active surface of the Sutton Salt Lake.
Timing of lake formation
The Sutton Salt Lake depression formed during late
Tertiary–Recent uplift of the schist basement and
associated stripping of the Tertiary sediment veneer.
Fig. 9 A, Comparison between sea-water and water from
Sutton Salt Lake (east Otago, New Zealand) compositions,
presented as molar ratios normalised with chloride. Black
squares, lake full; open squares, lake almost dry. Diagonal line represents 1:1 correspondence in molar ratios between sea-water and lake water. B, Similar diagram to A,
comparing sea-water and Mackenzie Basin (Fig. 1) rainwater (Jacobson et al. 1993).
The Sutton area has been progressively rising since
the late Miocene (Youngson & Craw 1996). The cool
semi-arid inland climate developed as high (2–3 km)
mountains rose along the western side of the South
Island (Fig. 1) in the Pliocene (Chamberlain et al.
1999). Hence, the lake depression may be as old as
Pliocene.
Full lake water (e.g., January 1998, Table 2) could
have formed from repeated evaporation and
accumulation of rain containing marine aerosols of
the concentration described by Jacobson et al.
(2003). Mass balance calculations suggest that c.
20 000 cycles of complete evaporation followed by
Craw & Beckett—Water and sediment chemistry of Sutton Salt Lake
lake filling and redissolution of the evaporite crusts
would be required to produce the observed salinity.
The lake does not dry out every year, so this
calculation indicates that the lake is more than
20 000 years old. Because of major climate changes
in the late Quaternary and possible wide variations
in aerosol contents of rain, no more detailed
speculation using water compositions is warranted.
CONCLUSIONS
The Sutton Salt Lake is impounded by schist
bedrock, and loses negligible amounts of water by
drainage. Instead, water is lost by evaporation,
encouraged by low rainfall and frequent dry winds.
The lake depression is floored by sediment derived
from the underlying schist and washed in by
rainwater, with a minor halite crust when the lake is
dry. Water chemistry data (Fig. 6, 8) show that there
has been negligible input into the Sutton Salt Lake
site of water that has chemically interacted with the
schist bedrock. Comparisons to sea-water and
rainout of marine aerosols (Fig. 8, 9A,B) suggest that
rainwater is the dominant water source for the lake.
Na, Cl, K, and Mg are contributed in approximately
sea-water ratios, albeit highly diluted by rainwater.
Formation of the lake required c. 20 000 cycles of
evaporation to dryness followed by refilling of the
lake and redissolution of the evaporite crusts. This
is different from the mechanism for saline soil
formation elsewhere in Otago postulated by Raeside
(1942), in which groundwater plays a prominent
role.
ACKNOWLEDGMENTS
This study was funded by the University of Otago. Discussions with C. A. Landis initiated the project, and his
enthusiasm helped keep it going. Jon Kim contributed
ideas and comments that helped with interpretations, and
Sean Barker provided some Na and Cl analyses of surface waters. Chris Pearson assisted with GPS mapping
of the lake site. Field assistance by Debra Chappell, Brian
Jones, Mark Walrond, and John Becker, and laboratory
assistance by D. Walls, greatly facilitated technical aspects of the study. Department of Conservation provided
permission for access and sampling in the Sutton Salt
Lake reserve.
327
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