Palaeogeography, Palaeoclimatology, Palaeoecology 240 (2006) 389 – 404
www.elsevier.com/locate/palaeo
Charophyte growth in small temperate water bodies: Extreme
isotopic disequilibrium and implications for the
palaeoecology of shallow marl lakes
Allan Pentecost a,⁎, Julian E. Andrews b , Paul F. Dennis b ,
Alina Marca-Bell b , Sarah Dennis b
a
School of Health and Life Sciences, King's College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NH, UK
b
School of Environmental Sciences, University of East Anglia, Norwich NR4 7JJ, UK
Received 1 April 2005; received in revised form 18 January 2006; accepted 4 February 2006
Abstract
A small pond containing the charophyte Chara hispida was monitored over a one-year period for changes in growth, water
chemistry, water level and stable isotopic composition. Chara growth was found to be seasonal, with maximum growth occurring
from late April to July. During this period, pH rose to N 10 while the dissolved inorganic carbon (DIC) and calcium fell as a result
of photosynthesis and calcification. Large gradients in pH, water temperature and irradiance were found within the Chara sward
and measurements showed that most growth and photosynthesis occurred within the upper 20 cm of the water column. Chara
oospore formation was also found to be seasonal but dependent upon environmental conditions.
δ18Ow rose rapidly during summer as evaporation progressed and this was correlated with the δ18ODIC, and to some extent with
18
δ Oc of the Chara encrusted calcite. However, extreme isotopic disequilibrium was observed between the δ18Oc and the δ18Ow
and also between the δ13Cc and the δ13CDIC. This arose from the high pH allowing atmospheric CO2 to enter the water and combine
directly with OH−.
It is concluded that, within shallow eutrophic lakes containing Chara swards, inferences of climate (e.g. air temperature) cannot
be made from observations of the isotopic composition of Chara carbonates. However in combination with other geochemical data,
disequilibrium events may be identifiable in ancient lake basins and taken as evidence for lake shallowing and/or eutrophication.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Charophytes; Stable isotopes; Disequilibrium; Eutrophication; Lakes; Sedimentation; Palaeoenvironments; Chara
1. Introduction
Charophyte calcite encrustations are a major component of marl lake sediments worldwide, frequently
providing extended and continuous sequences of sedi⁎ Corresponding author.
E-mail address: [email protected] (A. Pentecost).
0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2006.02.008
mentation throughout the Holocene and sometimes
beyond (Garcia, 1994). The sediments, often composed
of N80% Chara calcite contain potentially valuable
geochemical and palaeontological archives. Minor
element variations demonstrate changes in the rate and
type of sedimentation while the stable isotope ratios of
oxygen and carbon have given useful comparative information on lake water temperature, evaporation,
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groundwater geochemistry and lake productivity (Jones
et al., 1996).
Information on the geochemistry of Chara marl lakes
has increased substantially in recent years. For example,
Coletta et al. (2001) found that Chara carbonate δ18Oc
was correlated positively to the groundwater δ18Ow with
the potential to unlock the palaeohydrology of some
freshwater ecosystems. This study showed that an ‘age
gradient’ of calcification exists along the stems of Chara,
implying that whole-plant analyses would not reflect
short-term variations in carbonate geochemistry. However, uncertainty remains over the reliability of stable
isotope ratios in charophyte encrustations as indicators of
past climates. One study has indicated strong disequilibrium effects (Huon and Mojon, 1994) while smaller
though still significant disequilibrium effects were found
in some of the UK sites investigated by Coletta et al.
(2001). In both studies, data obtained from individual
sites were limited as they either did not continue for the
entire growth season or sampled entire plants, so that
seasonal variations were liable to misinterpretation.
More recently, Andrews et al. (2004), found that
stable carbon isotopes of Chara encrustations occurring
in rapidly flowing waters were closer to isotopic equilibrium than plants growing in stagnant or low-flow
regimes, and attributed the difference to better mixing,
leading to the more rapid diffusion of dissolved carbon
dioxide. They also found that δ13C values in stem encrustations were more positive than the associated
oospore encrustations and the difference was attributed
to microenvironmental effects associated with photosynthesis. They suggested that isotopic data obtained
from Chara marls should be treated with caution until
further, more detailed information became available.
In this study, we intend to remedy this shortfall by
examining in detail the stable isotope geochemistry of a
charophyte population over its entire growth season and
investigate physical and chemical variations within the
Chara sward. The implications of this study are then
discussed with reference to the potential palaeoclimatic
archive of fossil charophytes in lake sediments.
2. Methods
2.1. Site description
Samphire Hoe is a modern land reclamation project
built to accommodate spoil resulting from the Channel
Tunnel excavations on the coast of Kent, UK (51°08′N,
1°19′ E, Nat. Grid Ref. 61/290389, altitude 10 m). The
spoil, consisting of fragmented Lower Chalk and Gault
Clay was landscaped in 1991 to provide an undulating
surface with four small ponds encouraging wildlife
(Fig. 1). The site is managed as a nature reserve and the
Fig. 1. Location of Samphire Hoe and sampling sites for Chara and water. a) Location within the UK. b) Samphire Hoe showing positions of the
sampled ponds, C, Cliff Pond; M, Main Pond. c) Enlarged plans of the two ponds showing sampling sites (marked by crosses).
A. Pentecost et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 240 (2006) 389–404
391
Fig. 2. Diagram of a Chara hispida plant (left) and enlarged view of reproductive organs (right). an, antheridium; br, bracteole; oo, oosporangium.
ponds are filled by a mixture of groundwater, surface
runoff and sea spray. Two ponds have supported swards
of the charophyte Chara hispida L. over the past few
years. The size of the populations varies from year to
year, growth beginning in spring and dying back in the
late autumn or earlier. Samphire Hoe is owned by
Eurotunnel UK and managed by the White Cliffs Countryside Project on their behalf.
2.2. Field sampling
The Main Pond contained a healthy population of
Chara, and was chosen as the major sampling location,
with some supplementary sampling from the smaller
Cliff Pond containing the same species (Fig. 1c). In
January 2003 Main Pond measured 190 × 24 m with a
landscaped margin and an area of 0.46 ha. Sampling was
undertaken at two-week intervals throughout 2003
excepting the winter when no Chara was detectable
and the sites were visited once per month. On each visit,
the water level was recorded with reference to a fixed
datum and surface water samples taken in glass-stoppered bottles for water and isotope geochemistry. Water
and air temperature was taken with a thermistor in situ
and Chara, if present, collected with a drag-line for
laboratory examination.
Fig. 3. Seasonal variation in water depth of Main Pond using a local datum coincident with the deepest point. Horizontal line labelled i indicates
period in which a small area of the pond containing Chara became isolated from the remainder of the pond. Hatched area shows period and height of
the growing Chara sward. Note displacement from the zero datum. Double-hatched area shows fertile period of Chara.
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During May and June 2003 a series of more detailed
studies was undertaken on Main Pond. Samples were
taken through periods of the day and night to obtain
physical and chemical profiles at 5 cm depth intervals.
Temperature, dissolved oxygen and irradiance were
measured with appropriate probes and pH, DIC, Ca and
conductivity measured on small samples secured with a
suction pump lowered into the pond among the Chara
sward. In order to observe the growth and reproduction
more closely, individual plants and their attached sediment were transferred to two clear plastic tubes 50 cm
long and 10 cm in diameter and filled with pond water.
They were placed in a large water-filled container and
maintained outside the laboratory where their growth
could be monitored by direct observation.
2.3. Laboratory preparation and analyses
On return to the laboratory the water was analysed
immediately for alkalinity, total dissolved inorganic
carbon (DIC), pH, conductivity/salinity, Ca content and
the calcite saturation quotient Ω using methods described
previously (Pentecost, 1992). Other minor components
were determined from samples stored and acidified to ca
pH 3 in nitric acid using atomic absorption/emission
spectrophotometry. The DIC was precipitated using ammoniacal SrCO3 (Friedman, 1970) at the collection temperature and the precipitate collected on a Whatman
Glass Fibre (GFC) paper for subsequent stable isotope
analysis. A further water sample was retained for determination of δ 18Ow. Chara samples were divided into
two. After removal of entangled filamentous algae, most
of the material was preserved dry on paper sheets for
isotope analysis. For stable isotope analysis of the Chara
encrustations, only the upper two whorls (Fig. 2) were
used to prevent complications arising from age-gradients. Several series of whorls were detached from the
dried collections to provide sufficient material for
analysis, and powdered by shaking with about 20
1 mm glass beads. This separated most of the adherent
carbonate from the stems. On the same plants the female
reproductive structures (oospores) were also removed
from the upper 2 or 3 whorls for isotope analysis. For the
determination of Chara organic δ13C, samples were
decalcified in 0.01 M HCl at 20 °C for 3 h, then washed
twice in d.w., dried and lightly ground.
For the stable isotope analysis, volatile organic matter
was removed from Chara calcites by low temperature
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(b80 °C) oxygen plasma ashing for three hours at 300 W
forward power in a Bio-Rad PT 7300 plasma barrel
etcher. For the Chara and DIC carbonates a 300 μg
sample was reacted with anhydrous H3PO4 at 90 °C in an
automated ‘common acid bath’ preparation system built
in-house, and isotope ratios were measured on a Europa
SIRA Hybrid mass spectrometer. Repeated analysis of
the laboratory standard (n = 147) gave a 1σ precision of
± 0.10‰ for carbon and ± 0.06‰ for oxygen. For lower
mass Chara oospore calcite (30–100 μg), sample CO2
pressure was balanced manually and for these samples
the 1σ precision from 26 standard analyses was ± 0.27‰
for carbon and ±0.22‰ for oxygen. The δ13C of organic
matter was measured in continuous flow mode by combusting the samples at 1000 °C using a Europa ANCA
elemental analyser on-line with a Europa 20–20 mass
spectrometer. Each sample was measured in triplicate. A
pair of internal laboratory reference materials (casein) was
analysed every third sample and repeated analysis of the
internal laboratory standard (alanine) gave a 1σ precision
of 0.24‰. The δ18O of water samples were analysed
following automated equilibration with CO2 of known
isotopic composition (Epstein and Mayeda, 1953) and
measured on a Europa SIRA Hybrid mass spectrometer
(precision ± 0.06‰).
Mean monthly surface water temperature was estimated using the measured air temperature as a proxy
through a significant (p b 0.01) linear regression. Daily
mean air temperatures were obtained from the Dover
meteorological station nearby, then reduced to sea-level
using an adiabatic lapse rate of 8 °C km− 1.
3. Results
3.1. Pond hydrology and Chara growth 2003
The growth of Chara was limited to a few small areas
of the Main Pond in 2003, in contrast to previous years.
There was in addition, much growth of the aquatic
macrophyte Myriophyllum spicatum which may have
out-competed the Chara in some areas. Filamentous
green algae (Rhizoclonium, Spirogyra, Zygnema) also
became abundant in summer, smothering much of the
charophyte sward. The development of Chara in 2003
and its reproductive period is shown in Fig. 3. Measurement of area, depth and water level in the Main
Pond allowed us to monitor the change in volume in
response to evaporation and infiltration. From January
Fig. 4. Seasonal variation in some physical and chemical determinands of Main Pond, 2003. a) Water temperature °C. b) Specific conductivity mS cm− 1
at 20 °C. c) pH. d) Dissolved inorganic carbon (DIC) mmol L− 1. Broken line indicates period of high sulphide. e) Dissolved calcium mmol L− 1.
f) Calcite saturation index Ω. Dotted line represents calcite saturation (Ω = 1.0).
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until September there was an almost linear fall of water
depth from a maximum of 143 cm above datum to a
minimum of 3 cm. In January the estimated water volume was 3400 m3. By mid-June the area containing
Chara became isolated from the main body of water by
a bar of exposed sediment and its volume declined
rapidly and by late September it was almost dry. The dry
summer and autumn of that year led to only partial
replacement of the water to its former winter level by
December 2003.
The growth period and position of the Chara sward
in relation to the water level is also shown on Fig. 3.
Chara was not found in the January sampling but from
February to March isolated patches were encountered by
dredging. This was followed by a rapid phase of growth
during April and early May when the plants grew to
lengths of 35–40 cm and met the falling water surface.
By mid-May growth slowed and the declining water
level left the plants in a prostrate condition. By late June
the plants were yellowish and moribund and in a state of
partial decay and in late July Chara was left stranded on
mud. By the end of September, decay was almost complete and intact plants could no longer be identified in
the pond.
Measurable growth of C. hispida was confined to the
upper three internodes corresponding approximately to
the upper 6 cm of the plants. In the isolated individuals
growth was clearly evident as well as elongation of the
whorl-branches. Growth rates, measured as whole-plant
extension, appeared to be greatest in early May (Fig. 3).
Growth rate measurements in the sward approached
10 mm/day at this time, and in the experimental tubes,
where growth of individuals was monitored with more
precision, maximum growth was 11 mm/day. Mean
growth over the entire growth period from March–May
was 2.5 mm/day. In the tubes, growth rate was significantly correlated with water temperature (r = 0.61,
p = 0.026).
The reproductive organs (Fig. 2) were first observed
in the upper whorls on 10th May. They were immature
and in an early state of development. By May 21st, the
antheridia and oosporangia on whorls 2–4 were maturing and by June 5th, fertilisation of the matured
oosporangia had begun, evidenced by their darkening
via wall pigmentation on the lower whorls. Development and fertilisation of oosporangia continued until the
end of June. In July the entire plants began to decay as
noted above although the fertilised oogonia remained
attached to the Chara but presumably detached as the
plants decayed further. The surface sediment of the Main
Pond contained small numbers of detached oospores.
3.2. Physical and chemical changes
Relevant data are provided in Fig. 4 and a summary
of the determinands provided in Table 1. Surface water
temperature showed a seasonal pattern with a range of
2.2–27.9 °C. During the period of Chara growth it
averaged 20.1 °C. Water temperature was usually close
to the overlying air temperature due to the shallowness
of the pond. Specific conductivity (Fig. 4b) changed
little during the period January–June and was close to
1 mS cm− 1. A significant contribution to the conductivity is sodium and chloride owing to proximity of the
sea, but the waters are predominantly fresh and the
seawater contribution is estimated at no more than 1%.
Once the area containing Chara became isolated from
the main part of the pond, conductivity rose rapidly as
Table 1
Time-weighted means and ranges for some chemical measurements for Main Pond, Samphire Hoe, 2003
Determinand
Annual mean
Growth period mean (16.4–18.7)
Annual range
Air temperature °C
Surface water temperature °C
Specific conductivity mS cm− 1
pH
DIC mmol L− 1
CO2 (aq) μmol L− 1 a
pCO2 atm. × 10− 2 a
Ω
Ca mmol L− 1
Mg mmol L− 1
Na mmol L− 1
K mmol L− 1
SO4 mmol L− 1
Cl mmol L− 1
13.6
14.0
1.17
8.37
3.71 a
247 a
46.5 a
4.90
1.33
0.90
3.98
0.74
0.52
4.72
17.6
20.1
0.93
9.53
1.40
7.2
1.84
11.3
0.86
0.65
3.63
0.59
0.47
4.23
5.7–23.5
2.2–27.9
0.74–2.45
7.28–10.6
0.38–12.1 a
0.001–1720 a
0.001–297 a
0.72–19.1
0.65–2.36
0.5–1.84
1.5–9.7
0.13–1.84
0.36–1.09
1.75–11.3
a
Upper values probably overestimated (see text).
A. Pentecost et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 240 (2006) 389–404
water evaporated. Conductivity peaked in October when
the pond was almost dry but was also variable owing to
sensitivity to incoming rainfall and runoff. With the
autumn rains, conductivity fell close to its previous
winter value. During the main period of Chara growth
the conductivity showed little change overall.
The pH showed a marked seasonal variation linked
directly to Chara growth (Fig. 4c). During January the
pH was 7.7 then rose steadily to a maximum of 10.6 in
early June. This rise coincided with the Chara growth
period, but also appeared to extend beyond that period
into June where no increments in Chara length were
recorded. Additional growth may have occurred by
plants producing side-branches during this time. The
highest pH values recorded (9–10.7) coincided with the
period of Chara fertility. After June, the pH declined to
about 7.5 in September then rose slightly in the autumn
to around 8.
Dissolved inorganic carbon (DIC) also showed a
seasonal trend (Fig. 4d). In early winter, the DIC was
around 3 mmol L− 1 then declined to reach minimum
values of 0.5 mmol L− 1 in April and May, coinciding
with the period of maximum Chara growth. In June a
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steady rise was initiated until September where values
exceeded 10 mmol L− 1. At the DIC maximum, water
levels were low and there was a strong smell of hydrogen sulphide. Since the DIC was calculated from alkalinity to which sulphides contribute, these high values
may be overestimates, and on Fig. 4d they are indicated
by a broken linel. As temperature fell in autumn, DIC
values also fell but did not attain the levels of the
previous winter.
Dissolved calcium (Fig. 4e) reflects to a large degree
the changes in the DIC. A marked decline appeared
during the period of maximum Chara growth followed
by a rise in late summer. This rise is not so extreme as that
of the DIC further suggesting an influence of sulphide on
alkalinity, but also indicating that a real increase in DIC
took place at this time. Calcium concentrations then
began to fall in late autumn. The calcite saturation index
Ω in January (Fig. 4f) was close to equilibrium but soon
began to rise and this was sustained until late June where
a maximum value of 19.1 was obtained. This represents a
water highly supersaturated with respect to calcite and
corresponds to the period of maximum biomass at the
end of the fertile period of Chara. During the growth
Fig. 5. Daytime variation of some physical and chemical determinands within the Chara sward on 15th May, 2003. a) Irradiance at water sub-surface.
b) Temperature. Open circles, water temperature; closed circles, air temperature. c) pH at water surface.
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period, the saturation index averaged 11.3. It fell in
August and remained slightly above unity in autumn and
winter.
3.3. Daily variation within the Chara sward
On May 15th and on June 5th, covering the period
when Chara had attained its maximum biomass, measurements were undertaken at half-hourly or longer
intervals to investigate chemical variations within the
sward and with time of day. Measurements of subsurface irradiance, temperature and pH are provided in
Fig. 5. From 9:30 until 15:00 h irradiance remained high
(Fig. 5a). Air temperature showed slight variations with
cloud cover but remained close to 15 °C while surface
water temperature gradually rose to a maximum of
25 °C at 14:30 h. Water pH was high throughout the day
and close to 10.
Profiles within the sward are shown in Fig. 6.
Irradiance within the sward fell rapidly and at 20 cm
depth had dropped to an average of 9% of the subsurface
value (Fig. 6a). At 30 cm, it had declined to about 2% so
little light penetrated to the benthos. The decline was
approximately exponential with a mean extinction coefficient within the sward of 11.1 m− 1. Temperature also
varied with depth (Fig. 6b). As the day progressed, water
surface temperature increased by more than 8 °C in 4.5 h
but temperature at the sediment level rose only by 1 °C.
In all cases, the lowest temperature, close to 11 °C was
measured at the sediment surface, while temperatures at
the water surface ranged from 14–24 °C. Thus, strong
temperature gradients existed in the water column, especially in the upper 20 cm.
Water pH showed an interesting pattern with depth
(Fig. 6c). During the period 13:00–14:45 h, surface pH
varied little, and ranged between 10.15 and 10.33.
However, all pH profiles showed an increase of pH to
values of 10.32–10.52 at depths of 15–20 cm and thereafter declined to values between 9 and 10 as the sediment
was approached. A similar set of measurements (not
Fig. 6. Spatial and temporal variation recorded on 15th May, 2003 within the Chara sward at Main Pond. a) Irradiance. Profiles 1–5 taken at
15 minute intervals between 11:30 and 12:30 h. b) Water temperature: 1, 9:30 h; 2, 11:00 h; 3, 12:30 h; 4, 14:00 h. c) pH: 1, 13:00 h; 2, 13:30 h; 3,
14:15 h; 4, 14:45 h.
A. Pentecost et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 240 (2006) 389–404
figured) was taken on the evening of June 5th and gave
the same pattern with depth, with pH values reaching a
maximum of 10.78. In this set, the pattern persisted until
after dark. Measurements were also taken of DIC, Ca and
conductivity with depth. They did not reveal any trends
but were based on a single profile. The DIC ranged from
0.4–0.53 mmol L− 1, Ca 0.67–0.71 mmol L− 1 and
conductivity ranged from 832–853 μS cm− 1.
3.4. δ18O of water and δ13C of the DIC
The surface water of the Main Pond became progressively enriched in 18O as the year progressed (Fig. 7a).
In January 2003, δ18Ow was − 5.39‰ (VSMOW) but
climbed almost continuously until late September to
+ 4.20‰. Thereafter a rapid decline set in as autumn
rains refilled the pond to give values approaching those
of the previous winter.
The δ13CDIC is shown in Fig. 7b. In January, the
13
δ CDIC was − 4.75‰ (VPDB) and rose steadily to
− 0.64‰ in late March. In April, through to late June,
few analyses were obtained because the high pH and
low DIC of the water interfered with the precipitation of
strontium carbonate. The measurements that were obtained on small samples gave low values, below − 14‰
397
in April and June. Once water pH fell to below 8.5 in
July good precipitates were again obtained and the δ13C
of the DIC remained between − 1‰ and − 5‰ until
winter.
3.5. Chara isotopic composition
3.5.1. Calcite encrustations
The δ13Cc and δ18Oc values of adhering calcite from
the upper two whorls of Chara are shown in Fig. 8.
From February to the end of May, the δ13Cc ranged from
− 2.59‰ to − 0.02‰ in Main Pond and fell to a
minimum of − 7.93‰ in June (Fig. 8a). From late June
to August the δ13Cc values rose to + 0.10‰. The measurements suggest a seasonal trend with the lowest
values occurring during the main growth period for
Chara, but there was considerable variation from sample to sample. The mean δ13Cc was − 2.14‰. The May
15th sample was an average taken from the measurements of six separate plants collected within an area of
4 m2 of the pond to provide data on the variability of the
Chara population. The range from these individuals was
− 1.42‰ to − 4.65‰ with a standard deviation of
1.15‰. Confidence limits for this set of measurements
are plotted on Fig. 8a.
Fig. 7. a) δ18Ow (VSMOW ‰) of Main Pond water during 2003. b) δ13CDIC (VPDB ‰) of Main Pond dissolved inorganic carbon, 2003. The broken
line connects values obtained when the pond water pH exceeded 8.5.
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Fig. 8. Chara stem carbonate stable carbon and oxygen isotope ratios (VPDB‰). Closed circles show stem encrustation (upper 5 cm), open circles
calcified oogonia (upper 5 cm), May 15th sample in a), b) with 95% confidence limit to the mean. a) Chara carbonate δ13C Main Pond, 2003. b)
Chara carbonate δ18O Main Pond, 2003. c) Chara carbonate δ13C Cliff Pond, 2003. d) Chara carbonate δ18O Cliff Pond, 2003.
The carbonate δ18Oc values showed a similar seasonal trend to the δ13Cc values in Main Pond (Fig. 8b).
In January the δ18Oc was − 4.22‰ and fell irregularly to
a minimum of − 8.07‰ in June. Thereafter it rose
rapidly to a maximum of − 1.04‰ in late July. The
δ18Oc was only weakly correlated with δ18Ow (r = 0.46,
p = 0.11). Seasonal trends of both δ13Cc and δ18Oc are
apparent through their significant mutual correlation
(r = 0.66, p = 0.01). The overall δ 18 O mean was
− 4.81‰. The standard deviation of the May 15th set
of six samples was higher than the δ13C at 1.92‰ giving
wider confidence limits to the mean when compared
with the δ13Cc.
3.5.2. Oospores
During the development of the oosporangia in May, the
level of carbonate encrustation was too low to obtain
reliable stable isotopic data in Main Pond. Only after
fertilisation did calcification become sufficiently intense to
obtain measurable samples. Subsequently, only two sets of
measurements were obtained (Fig. 8a,b, open circles). The
δ13Cc of the oospores was significantly lower than that of
the stems. The lowest value was recorded on June 12th
(−14.0‰), 9.61‰ lower than the stem calcite. The δ18Oc
however, gave anomalous results. On June 5th the oospore
calcite was lower than the stem calcite by 3.1‰, while the
June 12th sample was higher by 4.04‰.
A. Pentecost et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 240 (2006) 389–404
Further data on oospore stable isotopes were
obtained from the smaller Cliff Pond together with
Chara stem calcite (Fig. 8c,d). In this pond, the
oospores were more heavily calcified, but data were
only available for a limited period since a dense
phytoplankton bloom during spring obscured the
earlier growths. Again, oospore δ13Cc was lower
than Chara stem δ13C though the difference was much
smaller than in Main Pond and in late August this
situation was reversed (Fig. 8c). Most δ13C values
ranged between − 1‰ and − 5‰ but the Chara stem
calcite δ13C fell to − 6‰ in late August. The δ18Oc of
the oospores was consistently higher than the Chara
stem δ18Oc (Fig. 8d). Most δ18O values ranged from
− 2‰ to − 5‰ but again fell in late August to
− 10.2‰.
3.5.3. Chara organic fraction
The stable carbon isotopic composition of the upper
three whorls of decalcified C. hispida from Main Pond
is shown in Fig. 9. The data show considerable
variability with a range of 9.9‰ and mean of
− 19.7‰. Excepting the first two measurements on
winter material, there is a trend of falling δ13 Corg
during the growth period until fruiting and then a rise in
plants that were moribund and decaying later in
summer. The six individual plants sampled on May
15th showed less variation with a standard deviation of
1.07‰. If the degree of variation is similar at the other
sampling times, then the seasonal trend will be
statistically significant. A weak positive correlation
(r = 0.66, p = 0.079) was found between δ 13 Corg and
δ13 Ccalcite and a weak non-significant correlation was
found between δ13 Corg and δ13 CDIC (r = 0.55, p = 0.16).
3.6. Deposition temperatures and isotopic equilibrium
The deposition temperature of the Chara encrustations can be estimated from the Hays and Grossman
399
(1991) (Eq. (1), below), and the results are presented,
together
T -C ¼ 15:7−4:36ðd18 OcalciteðVPDBÞ
−d18 Owater ðVSMOWÞ Þ þ 0:12ðd18 OcalciteðVPDBÞ
−d18 Owater ðVSMOWÞ Þ2
with the measured surface water temperature and the
estimated mean monthly surface water temperature in Fig.
10. For all Chara samplings in Main Pond between
February and August 2003, the Hays and Grossman
estimate exceeds both the measured surface water temperature and the mean monthly estimate. In both cases the
minimum difference was found for the March 8th
sampling while the maximum difference for the measured
water temperature was 30.4 °C on 5th June, and 39.4 °C on
15th May. The mean difference was 17.6 and 22.0 °C for
measured and estimated monthly differences, respectively.
These large differences are evidence for extreme isotopic
disequilibrium, especially during summer. The differences
are greater for the estimated monthly values since water
surface temperatures at the time of sampling were measured
in early afternoon when they approached their daily maximum. Similar large discrepancies were obtained for Cliff
Pond (not figured). Here, the mean difference between the
estimated (Hays and Grossman) temperatures and measured
temperatures were 43.2 and 24.5 °C for stems and oospores,
respectively. Only two oospore analyses were available for
Main Pond, and the deposition temperatures obtained also
indicated strong disequilibrium, especially with the June 5th
sample which gave an estimated temperature of 72 °C.
Carbon isotopic equilibrium can be assessed by
comparing the δ13CDIC with the δ13Cc. According to
Emrich et al. (1970) δ13CDIC should be between 1‰ and
2‰ heavier than δ13Cc. In the case of our Chara
encrustations, there was some tendency for the Main
Pond Chara δ13Cc to follow the trends of the δ13CDIC but
the predicted difference was only observed at the earliest
samplings in February and large differences were
Fig. 9. Stable carbon isotope composition (δ13C ‰VPDB) of the organic fraction of Chara hispida, Main Pond, Samphire Hoe, 2003. 95%
confidence limits are shown for the May 15th sample based upon 7 separate plants.
400
A. Pentecost et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 240 (2006) 389–404
Fig. 10. Surface water temperature in Main Pond estimated by the Hays and Grossman equation using the Chara encrustations (open circles),
temperature measured on site (closed circles), and estimated mean monthly surface water temperature (squares).
observed during the summer growth period. Coupled
with the significant correlation between the δ18Oc and
the δ13Cc this indicates a state of isotopic disequilibrium
throughout the growth period. The two oospore δ13Cc
values for Main Pond were closer in value to the δ13CDIC
and the δ13CDIC at this time was difficult to measure
owing to the high water pH. In Cliff Pond the δ13CDIC
ranged from − 9.1‰ to −13.5‰ during the period of
Chara measurement and large differences (− 5‰ to
− 10‰) were again obtained between the DIC and both
the δ13C of the stem encrustations and the oospores.
4. Discussion
4.1. Chara growth and photosynthesis
Charophytes appear to have a range of growth strategies (Westlake, 1980; Blindow, 1992; Titus et al., 2004).
Perennial growths have been reported in deep lakes,
while in temporary water bodies, growth may be erratic or
seasonal. The Samphire population clearly belongs to the
latter, where growth is terminated by falling water levels,
leaving plants exposed to the air. Exposure to air appears
to lead to senescence and although this was observed at
Samphire, plants transferred to experimental growth
cylinders and covered with water continued to grow
throughout the year. Individuals, however, did not persist
throughout the period but died off in late autumn or at
irregular intervals, to be succeeded by others growing up
through the sediment, though growth was much slower
over winter. These new individuals may have developed
from the internodes of the decaying Chara beneath the
sediment surface or anew from germinating oospores. It
is possible that senescence of individuals in this case
followed reproduction, though few of the individuals
monitored in the tubes appeared to develop oospores, but
most produced oogonia during early summer. The growth
rates appear to be the first detailed observations of this
kind, and compare favourably with previous annual
estimates of around 40 cm/year (Andrews et al., 1984;
Coletta et al., 2001) where it is assumed that most growth
occurred in early summer. A positive correlation of
growth with water temperature has also been observed
with Chara globularis (Pentecost, 1984).
Oogonium production by C. hispida was also found to
be seasonal, starting in early May and ending in July
when the plants began to die, but again, reproduction was
prolonged in the experimental tubes where the water level
was maintained. This suggests that the timing of reproduction is at least in part environmentally driven. Temperature and light may stimulate reproduction as it does
growth, since little activity was observed over winter.
Rapid photosynthesis rates in small water bodies
often result in high pH levels (Portiele and Lijklema,
1995). The uptake of carbon dioxide and/or bicarbonate
in charophytes results from the conversion of HCO3− to
CO2 via periplasmic carbonic anhydrase and subsequent
CO2 uptake (McConnaughey, 1991). It also may involve
the active uptake of HCO3− (Ray et al., 2003). Both
result in the generation of external OH− so that pH
levels exceeding 10 may be observed. The utilisation of
bicarbonate by Chara permits photosynthesis at high pH
values although optimal photosynthesis has been reported to occur at pH less than about 9.5 (Lucas, 1977).
A. Pentecost et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 240 (2006) 389–404
Where the chemical composition of the water is known, a
theoretical upper limit to pH may be calculated in the
absence of calcium carbonate precipitation, where N99%
of the dissolved carbon dioxide exists as carbonate. In
the case of the Samphire waters this limit is set at around
10.6–10.8 during summer. At this limit, both dissolved
CO2 and HCO3− concentrations are negligible, and photosynthesis presumably stops since CO32− is not utilised
by plants. Observed pH values at Samphire indicate that
the waters approach this value at times, despite the fact
that some CaCO3 is being precipitated. At high pH, the
pond water remains out of equilibrium with the atmospheric carbon dioxide owing to the slow rate of diffusion of CO2 into the water (Portiele and Lijklema,
1995). During the rapid growth phase of Chara, the
calcite saturation index, Ω was consistently greater than
unity also indicating calcite supersaturation in the waters. Calcite precipitation is frequently spontaneous
where Ω N 5 (Kelts and Hsu, 1978) and it is likely
therefore that the encrustation of calcite on charophytes
was occurring throughout the period of growth, irrespective of any additional biological process.
The highest pH values in the water column were
shown to occur at around 20 cm depth (Fig. 5c). This
presumably corresponds with the region of most rapid
photosynthesis, but it does not correlate with the zone of
maximum growth as observed in the experimental chambers. During this period, Chara grew up to the water
surface, so the growth zone occupied approximately the
upper 6 cm of the water layer. Light extinction values in
the sward can be used to estimate the depth to which
photosynthesis can be expected to occur. Light limitation
in C. globularis sets in at about 10% of the surface light
intensity on a bright day (Pentecost, 1984). If C. hispida
responds in a similar manner, then light-saturated photosynthesis will be confined to the upper 20 cm of the sward
(upper 20 cm of water column in Main Pond). This
apparent anomaly may be the result of light inhibition at
the surface, a phenomenon observed in some charophyte
taxa (Kuster et al., 2004) or from the diffusion of CO2
from the atmosphere into a stagnant surface water layer,
reducing pH slightly at the surface. It is also possible that
fixed carbon is translocated through the cells to the growth
region from below, although there is no evidence for this
in charophytes.
4.2. Seasonal variations — isotopes
The seasonal trend in the Chara crust δ13Cc appears to
reflect the seasonal variation of the δ13CDIC. Since agegradients have been previously demonstrated in Chara
stem calcite as a result of calcification occurring mainly
401
in the apical region (Coletta et al., 2001) and only apical
regions were sampled here, this is to be expected, with
the precipitated carbonate reflecting, to some extent, the
DIC composition. Unfortunately, owing to the high pH
in summer, it was not possible to obtain many δ13CDIC
values during the maximum growth period. During this
time, the photosynthetic uptake of CO2 would be
expected to enrich the DIC with 13C since this isotope
is discriminated against (McConnaughey, 1989) but this
was not observed, owing to a stronger kinetic fractionation of CO2 entering the water from the atmosphere.
The organic matter fraction of C. hispida was depleted in 13C, consistent with discrimination against this
isotope in photosynthesis. However, much stronger depletions have been observed in some lake sediments
known to have supported Chara. δ13Corg values in the
range − 30‰ to − 38‰ have been reported from marl
lakes in southern Sweden (Hammarlund et al., 2005) and
northern England (Nuñez et al., 2002). These large
depletions have been hypothesised as having resulted
either from increased kinetic fractionation during Chara
photosynthesis (Hammarlund et al., 1997) or from
assimilation of CO2 originating from bacterial methane
metabolism (Hammarlund et al., 2005). The results from
Samphire Hoe are somewhat surprising in this context.
The Samphire pond sediments were highly anoxic,
owing to the presence of free sulphate from sea spray and
gypsum in the underlying clay. This resulted in bacterial
sulphate-reduction as evidenced by the copious evolution of hydrogen sulphide. However, methane gas
production was not observed so it is possible that methanogenesis was depressed in the Samphire sediments.
Winter δ18Ow values are consistent with rainout δ18Ow
values for the southern UK (Darling, 2004) but the
δ13CDIC values are higher than most groundwater values
in the UK, the latter being around −10‰ (Coletta et al.,
2001). The δ13CDIC of groundwater is largely determined
by soil biological activity in the UK. Samphire Hoe was
landscaped from excavated clays, so it is probable that the
immature soils surrounding Main Pond are impoverished
in soil zone CO2 that is then diluted by atmospheric CO2,
enriching the DIC of the input waters in 13C.
4.3. Stable isotopes and isotopic equilibrium
Recent work on the calcite stable carbon and oxygen
isotopes of charophytes has suggested that they may be
of palaeolimnological value. Jones et al. (1996)
suggested that the δ18Oc of the oospores of C. globularis
was in equilibrium with the δ18Ow at one UK site, at
least for part of the year. Coletta et al. (2001) in a
regional survey of UK charophytes, also observed that at
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A. Pentecost et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 240 (2006) 389–404
some sites, oxygen isotopic equilibrium was attained.
However, in several studies, isotopic disequilibrium has
been observed. For example, Huon and Mojon (1994) in
a brief survey of C. globularis encrustations in a small
pond found extreme oxygen isotopic disequilibrium and
it has also been demonstrated in a small Danish lake
(Fronval et al., 1995). A further investigation by us
(Andrews et al., 2004) demonstrated that charophyte
disequilibrium was dependent upon water mixing. In
well-mixed environments, such as shallow streams, the
isotope composition was closer to equilibrium than
samples taken from lentic zones such as shallow lakes.
Our Samphire C. hispida encrustations mean values
were slightly higher than the means of a range of British
samples analysed by Coletta et al. (2001) for both δ13C
and δ18O but otherwise similar to many of the dried
samples previously taken from a range of UK sites.
In our work at Samphire Hoe, there is clear evidence of
extreme disequilibrium between δ18Oc of the Chara and
the δ18Ow. Several factors may be involved, but it is likely
that the highly alkaline pH resulting from photosynthesis
is the most important. Under these conditions, dissolved
molecular CO2 exists in low concentrations, far below
those in equilibrium with the atmosphere. As a consequence, atmospheric CO2 diffuses into the pond water to
replace that utilised in photosynthesis. Carbon isotopes in
carbonates formed at high pH are often depleted in 13C as
a result of this owing to differences in the gaseous
diffusion of 12CO2 and 13CO2 (Barnes and O'Neil, 1971).
Alkalisation of lakes caused by the photosynthesis of
planktonic cyanobacteria has also been shown to result in
strongly depleted δ13CDIC by the process of chemical
enhancement, where atmospheric CO2 combines directly
with lakewater OH− (Herczeg and Fairbanks, 1987).
These processes probably account for the low δ13Cc of
the Chara crusts during early June. Thus, the δ18Oc of
Chara encrustations at both Main Pond and Cliff Pond
cannot be used to estimate deposition temperatures and
infer lake temperatures. Even if equilibrium had been
attained, this estimate would be difficult to apply considering the large temperature gradients (up to 10 °C, Fig.
6b) within the Chara sward leading to considerable
uncertainty in air temperature.
Andrews et al. (2004) found that Chara encrustations
were more 13C-enriched and less 18O-enriched than the
associated oospores and suggested that this resulted
from different calcification processes. The Samphire
oozspores contained too little carbonate for us to undertake a comprehensive study, but, with the exception of
two measurements, the same pattern was repeated here.
We suggested in our previous paper that more information was needed to establish the calcification period
of the oospores. Here we were more successful and we
have established that oospore production may indeed be
seasonal, and occur over only a few months of the year.
However, it was also clear that the period of
reproduction may be extended throughout the summer
if water levels are maintained, and in this case, isotopic
data from oospores is likely to reflect a longer period of
water chemistry conditions. The stable isotope measurements also indicate non-equilibrium conditions throughout the period for the δ18Oc, reflecting to some extent at
least, a similar process to stem encrustation.
4.4. Relevance of charophyte isotope geochemistry to
palaeoclimate interpretation
Charophyte fossils have proved valuable in the
interpretation of sedimentary records throughout the
Quaternary, Tertiary, and for parts of the Mesozoic and
Palaeozoic. The mere presence of charophytes has
been used to demonstrate lacustrine as opposed to
marine sedimentation (Reichenbacher et al., 2004). A
knowledge of charophyte identity (genus, species) and
geochemistry has been used to infer changes in lake
level, salinity, productivity and lake water temperature.
For example, the occurrence of Lamprothamnion spp.
has been used to demonstrate brackish water conditions in Pleistocene sediments (Garcia and Chivas,
2004; Elkhiati et al., 2004) and evaporite environments in both the Quaternary and Mesozoic (Burne
et al., 1980). Other charophyte taxa are indicators of
cool deep lake systems (e.g. Nitellopsis obtusa, SouliéMarsche, 1991). Several taxa show tolerance of
desiccation and have been used to infer lake level
change and evaporation (Van Geest et al., 2005). Periods of high productivity during lake shallowing have
also been inferred through charophyte abundance
proxies (Hallett et al., 2003). Stable isotopes of charophyte carbonate have been used extensively to infer
changes in both lake water temperature and productivity during the Holocene (Drummond et al., 1995;
Yu and Eicher, 1998; Anderson et al., 2001). These
studies, on low to moderately productive temperate
lakes appear to provide information consistent with
known climatic oscillations. Our work however shows
that this is not the case in shallow and highly
productive lakes.
While it is apparent that deposition temperatures
based upon charophyte calcite will be unreliable in small
productive water-bodies, the associated disequilibrium
events would provide contrasts in lake basins with long
histories of deposition where there are repeated lakelevel oscillations from deep unproductive episodes to
A. Pentecost et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 240 (2006) 389–404
shallow productive ones. Charophyte calcite geochemistry should provide supporting evidence of lake
shallowing and/or eutrophication through δ18OCaCO3
profiling. In particular this may prove useful if
combined with Mg:Ca and Sr:Ca ratios in the carbonate
giving an independent measure of the changing lake
water hydrology (Anadón et al., 2000).
5. Conclusions
1. The growth of C. hispida in a small shallow pond is
shown to be seasonal and limited by falling water
levels in summer.
2. During the maximum period for growth, the pond Ca
and DIC levels fell while pH rose to 9–10.7 and the
calcite saturation index Ω rose to 19 as a result of
intense photosynthesis.
3. Large gradients of water temperature, pH and
irradiance occurred in the Chara sward during the
period of maximum growth, while most of the growth
and photosynthesis of Chara occurred in the upper
20 cm of the water column.
4. Within the pond, a seasonal trend in Chara oospore
formation and mineralisation was observed, beginning
in May and ending in August. However, in experimental chambers where water-level was maintained,
oospores continued to form throughout the summer
into the autumn, indicating that oospore formation is
under environmental control.
5. Although the δ13Cc and δ18Oc of the Chara were
correlated to some extent with δ13CDIC and δ18ODIC,
respectively, strong kinetic disequilibrium was observed throughout the growth period resulting from
photosynthesis and chemical enhancement of the CO2.
6. It is clear that shallow eutrophic water bodies
containing Chara cannot be used to obtain reliable
air temperature estimates for two reasons: chemical
enhancement as a result of intense photosynthesis and
strong temperature gradients within the Chara sward.
However by combining stable isotopic with other
geochemical data from lake basins it may be possible
to demonstrate lake-shallowing/high productivity
events in the past.
Acknowledgements
We thank the management team at Samphire Hoe
Nature Reserve, especially the warden, Paul Holt, and the
assistance of Geoff Howitt and Tony Christian and
Eurotunnel UK for permission to sample the ponds and
take water analyses. This project was support by NERC
Grant award NER/B/S/2002/00276. The meteorological
403
office is acknowledged for permission to analyse the
Dover air temperature and rainfall data. We are also
grateful to three anonymous reviewers for their helpful
comments and suggestions.
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