healthmedicinet

The sulfur cycle in the monomictic Lake Kinneret was studied over 1 year cycle of
varying hydrographic conditions that affect irradiation, pH as well as concentrations
of dissolved oxygen, sulfate, hydrogen sulfide, nutrients and metals availability
52]–54], 57]. In this discussion, we first consider the challenges associated with the analytical
techniques for detection of zero-valent sulfur used in this work. We then review the
seasonal changes in the lake chemical composition, with an emphasis on sulfur cycle
and its effects on the isotopic composition of sulfur species. Finally, we discuss
the relevance of Lake Kinneret as an analog for the Proterozoic Ocean.

Lake mixis

Temperature and conductivity profiles are good indicators of the stratification of
the water column. The annual conductivity changes in Lake Kinneret water column are
caused by a combination of thermal stratification, evaporation, and the inflow of
both fresh and saline water. Fresh water onshore sources (i.e. Jordan River, runoff,
rain etc.) contribute approximately 89% of the total inflow while the saline springs
at the bottom of the lake contribute to the remaining 11% 1]. The less dense fresh water tends to remain in the epilimnion, thus decreasing its
conductivity, while the saline water increases the conductivity of the hypolimnion
85]. In March (LK1), the conductivity was constant through the entire water column, implying
that the water column was completely mixed (Figure 2a). During the mixed period (LK1, Figure 2b), the pH at the surface was slightly higher than at the bottom (0.41 pH units) due
to photosynthesis in the photic layer, which decreases carbon dioxide concentrations.
Indeed, pe values are consistent with a carbonate-buffered system in equilibrium with
the atmosphere 53]. The low iodide concentrations (11 nM) measured during the mixis period are consistent
with the oxidation of iodide, produced by microbial mineralization of organic matter,
to iodate by dissolved oxygen 34] or by microbial oxidation of iodide to iodate by nitrifying bacteria 32], 33]. During this time period, the water column is oxic and microbial sulfate reduction
in the water column does not occur. Hydrogen sulfide, which is produced by microbial
sulfate reduction in the chemocline of Lake Kinneret, is not detectable and is likely
only produced within the sediments. During the LK1 sampling, no sulfide oxidation
intermediates were detected, even in the bottom waters (Figures 4g, 5a). This indicates that sulfide oxidation intermediates that may be produced in the
sediments are further oxidized to sulfate fast enough to prevent their diffusion to
the overlying waters of Lake Kinneret. Indeed, sulfate accounted for 99.9% of total
analyzed sulfur species in the lake during mixis (Table 2).

Table 2. Depth integrated inventory of sulfur

Early stage of stratification

Lake Kinneret thermal stratification usually begins in April. In our campaign the
thermal stratification was first detected in the May (LK2) sampling (Figure 2c). Following the onset of stratification (LK2 sampling, Figure 4b), hypolimnetic dissolved oxygen and nitrate were depleted due to heterotrophic microbial
activity 57] and sulfate reduction took over, leading to the gradual accumulation of hydrogen
sulfide during the stratified period 54]. The pH was elevated in the epilimnion (Figure 2d) due to photosynthesis and a clearly visible redoxcline formed due to depletion
of oxidized nitrogen compounds, followed by further decrease in pe due to formation
of sulfide below a depth of 33.1 m. LK2 sampling was the only one in which the thermocline
did not coincide with the chemocline (Figure 2c). The thermocline was not sharp while the chemocline indicated the upper border
of the benthic boundary layer 86]. In the oxic part of the epilimnion, iodide was depleted to concentrations were below
the MDL (Figure 3b). In anoxic non-sulfidic waters, iodide concentration slowly increased with depth,
while in hydrogen sulfide-rich bottom layer iodide concentrations increased sharply.
This profile is typical for stratified systems characterized by diffusion of iodide
from sediments, where it is produced by decomposition of organic matter, and its consumption
in the oxygen-rich epilimnion 34]. During that period the hydrogen sulfide was detected only in the bottom layer of
the lake. The highest detected concentration of hydrogen sulfide in the water column
was only 8.0 µM and sulfate accounted for 99.9% of total analyzed sulfur (Table 2).

Shallow stable stratification

The depth of the thermocline decreased from spring to mid-summer. During our July
sampling period (LK3), the thermocline was situated in the photic zone. These conditions
are favorable for the bloom of phototrophic sulfur bacteria that oxidize hydrogen
sulfide anaerobically to sulfur and sulfate 55]. The concentration of bacterial chlorophyll in this season at the chemocline was
below detection limit (not shown). This was a rather unexpected observation as a bloom
of brown sulfur bacterium Chlorobium phaeobacteroides is usually observed during this season and the chemocline was situated in the photic
water layer 55]. A sharp increase in iodide concentrations above the chemocline coincided with sharp
decrease in dissolved oxygen concentration (Figure 4c), indicating the flux of iodide from sediments into the relatively well-mixed anoxic
hypolimnion. In July (LK3 sampling), dissolved oxygen was detectable until a depth
of 11 m while the sulfide penetration depth was 11.5 m (Figure 4c). In this sampling zero-valent sulfur concentrations (Figure 4i) were lower than the concentrations of thiosulfate and sulfite (Figure 5c). Sulfate accounted for 90.8%, sulfide for 9.0%, and S

O
₃^2?
sulfur (sum of thiosulfate and sulfite sulfur) for 0.17% of total analyzed sulfur
(Table 2). We interpret these results as demonstrating the relative instability of the anomalously
shallow chemocline. Thus relatively high concentrations of sulfur oxyanions may be
explained by a recent mixing event between oxygen and sulfide rich layers. Low concentration
of zero-valent sulfur may be explained as well by the absence of phototrophic sulfide
oxidizing bacteria bloom during that time period.

Deep stable stratification

In the summer months, the thermocline deepened under the influence of the rising epilimnion
water temperature and increasing wind shear. During our October sampling (LK4), the
thermocline was still stable, but it was situated deeper than in the summer (Figure 2g). Epilimnetic pH and pe were high due to photosynthesis, whereas hypolimnetic pH
was low due to anoxic respiration and pe was low due to the buildup of sulfide. The
chemocline was detected at 18.5 m depth. This period was characterized by the continuous
accumulation of hypolimnetic sulfide as the result of microbial sulfate reduction
in the water column and in the sediment that is supported by the influx of organic
material sinking from the photic zone 54].

The profiles of sulfate and sulfide in October differed from their shape at other
seasons. The local maximum in hydrogen sulfide concentration and local minimum in
sulfate concentrations were detected 1 m below the chemocline (Figure 4d). We explain these features by the fast microbial sulfate reduction of sulfate below
the chemocline. This explanation is supported by the unusual shape of the iodide profile.
The highest concentration of iodide was detected 1 m below the chemocline, coinciding
with the local minimum in sulfate concentrations (Figure 3d), and not in the bottom layer of the lake. High concentrations of iodide point to
fast mineralization of organic matter at this redox transition within the water column.
Additional processes that may promote the iodide maximum include the reduction of
iodate in the sulfidic-oxygenated water interface by sulfite, thiosulfate and sulfide
36], 37]. This assumption is supported by the high concentrations of sulfite, thiosulfate,
and sulfide found at the chemocline.

During the October sampling, zero-valent sulfur concentrations (Figure 4j) were higher than thiosulfate and sulfite (Figure 5d). Sulfate accounted for 83.3%, sulfide for 16.4%, S

O
₃^2?
for 0.12% of the total analyzed sulfur, and S
⁰
for 0.16% of the total sulfur (Table 2). The ratio of zero-valent sulfur to thiosulfate concentrations was higher than in
the previous sampling periods, either as a result of intense phototrophic microbial
formation of elemental sulfur, given that the chemocline was located in the lower
boundary of the photic zone, or due to the high hydrogen sulfide formation rates below
the chemocline 12]. The latter may have led to an increase in hydrogen sulfide to dissolved oxygen ratios.

Early stage of lake mixing

With the decline in air temperatures in autumn, surface water temperatures decrease
and the conductive mixing increases, leading to the deepening of the thermocline until
Lake Kinneret water column finally mixes around December-January 1]. Through the late autumn, the thermocline deepens step-wise during storm events.
A criterion of a mean thermocline gradient 0.3°C m
^?1
between the depths of 10 and 35 m was defined to determine the stability of the water
column 87]. While the stabilization of the water column may be in some cases fortified by the
salinity gradient, according to this criterion, the water column stratification was
metastable during the December (LK5) sampling (Figure 2i). The gradient decrease in pH in the thermocline was less pronounced in the December
sampling than in October (Figure 2j) possibly due to a lower primary production in the epilimnion after the end of the
algal blooms. Although pe profiles were similar during both periods, the local maximum
of iodide concentrations below the chemiocline was much less pronounced than in the
previous sampling and was not accompanied by either maximum in hydrogen sulfide concentrations
or minimum in sulfate concentrations. The absence of the iodide peak is probably due
to the deepening of the chemocline. Increase in iodide concentrations in the surface
waters may be attributed either to photolytic decomposition of organic matter or to
microbial reduction of iodate. Sulfate accounted for 81.5%, sulfide for 18.2%, S

O
₃^2?
for 0.04%, and S
⁰
for 0.23% of the total analyzed sulfur (Table 2). Although this sampling was performed at the beginning of the new mixis period,
the inventory of sulfide at point A represented the annual maximum (3.65 mol S m
^?2
) detected in the lake (Table 2).

Advanced stage of lake mixing

In early winter, the chemocline deepened following the erosion of the thermocline
(LK6, Figures 2k, 4f). In January, the water column was on the verge of mixing, as the temperature difference
between epilimnion and hypolimnion was only 0.34°C, but a very sharp conductivity
gradient deep in the water column (approximately 2.0 m above bottom) marked a border
between the epilimnion and hypolimnion (Figure 2k). The pH decrease at the thermocline was much less pronounced than in the previous
sampling periods, but the pe still decreased sharply at a depth of 38 m due to the
high concentration of hydrogen sulfide in the bottom waters (Figure 2l). Simultaneously, iodide concentrations decreased in the epilimnion as well as in
the hypolimnion, likely due to the oxidation of iodide by dissolved oxygen during
the active mixing phase of the water column. Around the chemocline, dissolved oxygen
co-existed with hydrogen sulfide (Figure 4f), and the mixing of oxic and sulfidic waters led to formation of relatively high
concentrations of zero-valent sulfur (Figure 4l), thiosulfate, and sulfite (Figure 5f). Sulfate accounted for 99.1%, sulfide for 0.45%, S

O
₃^2?
for 0.16%, and S
⁰
for 0.31% of the total sulfur (Table 2). During this period, the inventory of sulfide oxidation intermediates ([S
⁰
] + [SO
₃^2?
] + 2 × [S
₂
O
₃^2?
]), 0.098 mol S m
^?2
, was slightly higher than the inventory of sulfide, 0.094 mol S m
^?2
(Table 2).

Isotope composition of sulfur species

In May, at the beginning of the stratification period, the sulfur isotope fractionation
between sulfate and sulfide was ? = 11.6 ‰ (Figure 7a). During the LK3, LK4, and LK5 samplings, which are characterized by stable chemocline
conditions, the depth profiles of ? have similar shapes (Figure 7b–d). In these profiles the highest ? values, 25.8, 34.0, and 30.2 ‰ were detected at 3.5, 3.5, and 0.5 m depths, below
the chemocline. ? values at the chemocline were 1.6–5.2 ‰ lower than the higher ? values deeper within the water column. The average values of ? from all horizons of the same sampling differ throughout the year (Figure 7, numbers in parentheses). The highest average ? (30 ± 4 ‰) was detected in the LK4
sampling performed in October. During earlier seasons and during the de-stratification
of the lake, lower sulfur isotope fractionations were measured.

Figure 7. Seasonal variations in the depth profiles of the isotopic fractionation between sulfate
and sulfide (?) in Lake Kinneret. Black dashed lines denote the depth of the chemocline. a LK2 sampling, b LK3 sampling, c LK4 sampling, d LK5 sampling, e LK6 sampling. Please notice difference in depth scale on panel e.
Numbers in parentheses represent the average isotope fractionation between sulfate
and sulfide and its standard deviation.

The shape of the ? profile, characterized by increase of ? with depth below the chemocline followed by decrease toward the bottom of the lake
may be explained in two ways. The first explanation is based on the lower sulfate
concentration in the bottom waters, which may result in lower isotope fractionation
by sulfate reducing bacteria 51], 68]. Alternately, microbial sulfate reduction may be accompanied by microbial or chemical
disproportionation of sulfur and thiosulfate produced by chemical and microbially
assisted oxidation of hydrogen sulfide. Disproportionation processes are known to
increase isotope fractionation between sulfate and sulfide 40]. Two factors allow microbial disproportionation of sulfide oxidation intermediates
below the chemocline.

The first factor is an increase in bioavailability of zero-valent sulfur due to the
reaction of elemental sulfur with hydrogen sulfide, which results in the formation
of water soluble inorganic polysulfides (Eq. 9).

(9)

Although polysulfides were not detected in the water column, analyses performed by
a more sensitive, although less robust method 88], suggested that polysulfides are present in Lake Kinneret water column at the hundredth
of pM concentrations 59]. The second factor is the relatively high concentration of thiosulfate measured:
up to 0.18–5.54 µM, depending on the season.

In October (LK4), the lake stratification was stable and high rates of bacterial sulfate
reduction in the chemocline resulted in local minimum of sulfate concentration and
local maximum of sulfide concentration. These processes left their footprint on the
sulfur isotope composition (Figure 6c). A sharp increase in the ?
³⁴
S
_SO4
, followed by decrease in ?
³⁴
S
_SO4
with depth was likely due to higher fraction of sulfate being reduced in the chemocline.
In December (LK5), a similar profile of ? as a function of depth was recorded (Figure 7e). The highest value of ? = 30 ‰ was detected at 21 m depth, while ? at the chemocline
and near the bottom is 25–27 ‰. During this sampling period, chloroform-extractable
zero-valent sulfur in the chemocline was heavier than sulfide by 3.1 ‰. This effect
may be explained either by bacterial oxidation of sulfide to elemental sulfur 89] and reference therein] or by partial equilibration between hydrogen sulfide and elemental
sulfur due to formation of inorganic polysulfides (Eq. 9) 90]. Kinetic studies of this reaction (Eq. 9) under controlled conditions and in natural aquatic systems show that equilibrium
is reached in seconds in the hydrogen sulfide-polysulfide-dissolved sulfur system
91], in minutes to hours in the hydrogen sulfide-polysulfide-colloidal sulfur system
92], 93], and in hours or longer in the hydrogen sulfide-polysulfide-rhombic sulfur system
94]. A scenario, which combines these two processes, includes the formation of elemental
sulfur with unknown isotopic composition by microbial oxidation of hydrogen sulfide,
followed by a shift in the isotopic composition of elemental sulfur to ?
³⁴
S values slightly larger than those of sulfide by (at least partial) equilibration
with polysulfides (Eq. 9).

During disruption of the stratification (LK6, January 17), the isotopic fractionation
decreased to ? = 18–21 ‰. These values do not necessarily support presence of microbial sulfur disproportionation
in the lake. Although the highest concentrations of sulfide oxidation intermediates
(up to 53 µM zero-valent sulfur and up to 5.5 µM thiosulfate) were detected, the sulfur
isotope fractionation was relatively low, possibly, due to the formation of relatively
isotopically light sulfate due to massive reoxidation of isotopically light hydrogen
sulfide. During this sampling period, zero-valent sulfur in the hypolimnion was 1.1–3.3 ‰
heavier than sulfide. At the chemocline the difference between ?
³⁴
S of zero-valent sulfur and sulfide was as low as 0.6 ‰. We interpret these values
as partial equilibration between zero-valent sulfur and sulfide through polysulfide
formation in the hypolimnion and lack of such equilibration at the unstable chemocline
during the period of mixing of the lake water layers.

Relative variations in the ?
³⁴
S
_SO4
and ?
¹⁸
O
_SO4
may provide further insights into the pathway of microbial sulfate reduction in Lake
Kinneret. The slopes of the ?
³⁴
S
_SO4
vs. ?
¹⁸
O
_SO4
plot (Figure 8) calculated for the LK4 (0.34 ± 0.01), LK5 (0.40 ± 0.01), and LK6 (0.33 ± 0.04) sampling
periods are moderate relative to steeper slopes that can be found in marine sediments
83]. We excluded LK3 sampling from this discussion as it was performed soon after stratification
and only a minor fraction of sulfate was reduced, resulting in a high variability
in slope (RSD = 39%) that was not reliable. The moderate slopes measured at LK4, LK5,
and LK6 suggest that the isotopic composition of both oxygen and sulfur in sulfate
varies due to large sulfate uptake by microbial cells, and usually characterize environments
with high sulfate reduction rate (higher than 1 ?mol cm
^?3
 year
^?1
). Slopes of this range have previously been found in estuaries and cold seeps 95], 96]. Calculating sulfate reduction rates in Lake Kinneret is exceptionally difficult,
due to mixing by high eddy diffusion in the lake, sulfate diffusion in and out of
the sediment, and redox reactions involving sulfide oxidation or sulfur disproportionation
that may mitigate any change in sulfate concentration. Due to the low concentration
of sulfate in the lake (less than 600 ?M), it is possible that sulfate supply limits
the rate of microbial sulfate reduction. Similar to a previous study 83], we suggest that the low slopes in the cross plot of ?
³⁴
S
_SO4
vs. ?
¹⁸
O
_SO4
in Lake Kinneret indicate a low intracellular rate of reoxidation of sulfide oxidation
intermediates back to sulfate, as is common during high rates of sulfate reduction.

Figure 8. Seasonal variations in the sulfur vs. oxygen isotope composition of sulfate in Lake
Kinneret. Formula represent equations of the corresponding linear regressions. See
“Discussion” on the observed trends in the text.

Finally, during the LK6 sampling, when active oxygenation of sulfidic waters took
place, the ?
¹⁸
O
_SO4
was shifted to lower values compared to LK4 and LK5. This may be due to the intensification
of sulfide oxidation explained by the sinking of the chemocline, decrease in the concentration
of sulfide, and simultaneous increase in the concentration of sulfate in the deep
waters (Figure 4e, f). During sulfide oxidation the oxygen atoms in the resulting sulfate derive from
water if the oxidation is purely anoxic, and partially from water and partially from
atmospheric oxygen if the oxidation happens near the surface. Given the ?
¹⁸
O of the lake waters, the lower ?
¹⁸
O
_SO4
values could result from a combination of anoxic and oxic sulfide oxidation.

Lake Kinneret as an analog of Proterozoic ocean

Sulfate concentrations, as far as they are able to be reconstructed, have undergone
significant variations throughout Earth history. Our knowledge of sulfate concentrations
is based on three approaches: measuring the magnitude and the rate of change of the
sulfur isotopic composition of sedimentary sulfur species such as pyrite, carbonate
associated sulfate (CAS), and sulfate evaporite minerals, measuring the concentration
of sulfate in fluid inclusions in evaporite minerals 97], and the more qualitative approach of assessing the presence and extent of sulfate
evaporites 98]. Sulfur isotope fractionation during bacterial sulfate reduction is ranging between
?3 and 66 ‰ 68], 99], 100]. At sub-millimolar sulfate concentrations, the absolute value of fractionation decreases
as a function of sulfate concentration 51]; this isotopically light sulfur can then be incorporated into sedimentary pyrite
and may be utilized to estimate seawater sulfate concentrations in the ancient ocean.
Similarly, the sulfur isotope composition of marine sulfate, which reflects the balance
of the sources and sinks of sulfate to the ocean, can change as a function of changes
in the sulfur biogeochemical cycle. Fast rates of change in the sulfur isotope composition
of sulfate have been used to argue that sulfate concentrations must have been correspondingly
low 101]. Sulfate concentrations can also be directly measured in fluid inclusions, but depend
on an assumed concentration of calcium and therefore are less reliable 97]. Finally, the precipitation of gypsum before halite in ancient shallow-marine evaporite
deposits requires ?2.5 mM sulfate concentration and thus may be also used for a rough
evaluation of sulfate concentrations 98].

Despite these rather poor proxies for sulfate concentration over Earth history, our
basic understanding is that marine sulfate concentrations follow the concentration
of atmospheric oxygen, partially because oxidative weathering of pyrite is one of
the main sources of sulfate to the ocean 102]. Based on the low absolute isotope fractionation between sulfide and sulfate in sedimentary
rocks (15 ‰), sulfur isotope systematics of volcanogenic massive sulfide ore deposits
65], and an absence of significant sulfate evaporites 98] and references therein], marine sulfate concentrations of 80–200 µM are suggested
to have existed in the Archean ocean before 2.5 billion years ago 51], 65]. In turn, the ocean water column during the Archean has been suggested to be ferruginous,
e.g. to contain abundant dissolved iron (II) and no dissolved oxygen or hydrogen sulfide
103].

Following the rise of atmospheric oxygen concentrations c.a. 2.4 Gyr, sulfate concentrations
in the ocean would have risen to ?2.5 mM by c.a. 2.22–2.06 Gyr (Lomagundi event) due
to the initial oxidation of sulfur contained in surface rocks 98], 104]. As the ocean redox state adjusted to sustained, if potentially low, oxygen production
after the Lomagundi event, sulfate concentrations in the ocean likely dropped, possibly
to sub-millimolar level, as estimated from the isotopic composition of coexisting
pyrite and sulfate 49], 104], 105]. Sulfate concentrations of 0.5–4.5 mM have been suggested for the Mesoproterozoic
ocean 101]. It is thought that hydrogen sulfide was abundant in the deep Proterozoic ocean 66] or at least in mid-depth near-shore waters 105]. During the Neoproterozoic (800–550 million years ago), sulfate concentrations in
the ocean were likely higher than during the Mesoproterozoic, perhaps between 0.8–10.1 mM
101]. Sulfur isotope fractionation between sulfate and sulfide increased at this stage
from 15 ‰ to 15 ‰ in the early Paleoproterozoic 106] and further increased to larger than 45 ‰ in the Neoproterozoic 107]. The latter increase is attributed to the disproportionation of sulfide oxidation
intermediates, which started to form in larger quantities due to an increase in oxidation
state of the planet 107], 108]. The next increase in sulfate concentrations is associated with an increase in oxygen
concentrations that occurred during the latest Neoproterozoic. At this stage (the
early Ediacaran) the sulfur isotope fractionation between sulfate and sulfide reached
?65 ‰ 109].

Lake Kinneret represents a unique natural laboratory for studying the sulfur cycle
in a stratified aquatic system with sub-millimolar concentrations of sulfate. The
concentration of aqueous sulfate in the lake during the winter mixis period (c.a.
550 µM) and the presence of free hydrogen sulfide in the hypolimnion during the stratification
period correlate well with our understanding of global ocean chemistry with respect
to the sulfur species in the late Paleoproterozoic and Mesoproterozoic. The evolution
from the ferruginous Archean ocean to the sulfidic deep-water Proterozoic ocean occurred
most likely at sulfate concentrations similar to those existing today in Lake Kinneret.
In order to estimate the impact of iron on the sulfur cycle in the water column of
the lake, we compared the sulfide and iron budgets. Total iron concentrations in the
lake are always lower than 1.2 µM 84], and iron sedimentation rates are estimated to be 200–500 ton/year which corresponds
to 9 × 10
⁶
 mol/year. The average concentration of sulfide in the hypolimnion (200 µM) may be
estimated from the data presented in Table 2, and the width of the anoxic layer in summer and autumn (at least 18 m). Together
with the volume of hypolimnion of Lake Kinneret during the stable stratification period
(10
⁹
 m
³110]), a minimum hydrogen sulfide production rate of 2 × 10
⁸
 mol/year can be estimated from these data. As part of the sulfide is re-oxidized
to intermediate valence state sulfur species and sulfate during the stratification
period, the actual rate of sulfide formation should be even higher. Thus, a ratio
between the minimum sulfide production rate and the maximum iron sedimentation rate
in Lake Kinneret is estimated to be 23 mol/mol. An excess of sulfide production over
iron sedimentation in the lake implies that precipitation of iron sulfides in the
water column should have a minor effect on both the hydrogen sulfide budget and its
isotopic composition in the water column.

The combination of sub-millimolar sulfate concentrations with relatively high concentrations
of sulfide oxidation intermediates that are prone to microbial disproportionation
(e.g. zero-valent sulfur and thiosulfate, Figures 4, 5) should result in relatively high sulfur isotope fractionation in Lake Kinneret (up
to 34 ‰), higher than commonly measured in Paleoproterozoic sedimentary rocks, yet
lower than the highest values from the Neoproterozoic sedimentary record. This apparent
discrepancy between the sulfur isotope fractionation observed in Paleoproterozoic
sedimentary rocks versus that in Lake Kinneret may be explained as follows.

First, the difference in hydrographic settings between the global Proterozoic ocean
and Lake Kinneret may generate different sulfur isotope fractionation between sulfate
and hydrogen sulfide as a result of variations in chemical and biological processes
at the redox interfaces. The availability of light represents one of the factors which
may affect sulfur isotope fractionation. Production of significant amounts of elemental
sulfur by phototrophic oxidation of hydrogen sulfide, may, for example, fuel sulfur
disproportionation (Eq. 6) and increase the sulfur isotope fractionation between sulfate and hydrogen sulfide
40]. The water column of Lake Kinneret is relatively turbid however, and the photic layer
extends only to c.a. 15 m depth 111]. Thus, the chemocline was situated in the photic zone only during one of sampling
periods, in July (LK3). Indeed, our results show that the concentration of zero-valent
sulfur (Figure 4) and the isotope fractionation between sulfate and sulfide (Figure 7) are not elevated in the chemocline during this sampling, suggesting that disproportionation
is not significant in this system. The higher concentration of dissolved oxygen in
the epilimnion of Lake Kinneret compared to the surface waters of the Proterozoic
ocean represents another difference which may result in formation of relatively high
concentrations of sulfide oxidation intermediates. During autumn and early winter,
a combination of high concentrations of dissolved oxygen in the epilimnion and of
hydrogen sulfide in the hypolimnion results in a high inventory of elemental sulfur
in the chemocline and hypolimnion (Figure 3). The role of microbial disproportionation of intermediate sulfur species in the
generation of relatively high isotope fractionation between sulfate and hydrogen sulfide
was not resolved unequivocally by this study. Measurements of triple sulfur isotope
composition (e.g. ?
³³
S and ?
³⁴
S) of sulfate, hydrogen sulfide and zero-valent sulfur may clarify the relative contribution
of microbial sulfate reduction and sulfur disproportionation to sulfur isotope fractionation
in the Lake Kinneret water column. Application of such analysis may as well reveal
if in natural aquatic systems with sub-millimolar sulfate concentrations sulfur isotope
fractionation between sulfate and sulfide of 30 ‰ may be produced by microbial sulfate
reduction alone.

Second, translation of the sulfur isotopic signal from the water column to the sediment
may affect the isotopic composition of pyrite preserved in the sedimentary record
63]. We can envision two scenarios. In the first scenario, significant amounts of hydrogen
sulfide are produced in sedimentary pore-waters that are depleted in sulfate relatively
to the water column, and no significant precipitation of iron sulfide or pyrite occurs
in the water column. Hydrogen sulfide diffuses from the sediment into the water column,
and sulfate diffuses from the water column into the sediment. In this case hydrogen
sulfide in the pore-waters may be enriched with
³⁴
S, as it is produced from sulfate, which is itself enriched with
³⁴
S due to Rayleigh distillation as well as the low sulfate-sulfide sulfur isotope fractionation
at low sulfate concentration. As this
³⁴
S-enriched hydrogen sulfide is preserved in the sediment, the sulfur isotope fractionation
between sulfate and sulfide should be lower than in the water column. In turn, hydrogen
sulfide produced in the water column should be mixed with hydrogen sulfide which diffuses
from the sediment. This sulfide pool should be eventually completely reoxidized to
sulfate, such that its isotopic composition should not be preserved in the sediment.
In the second scenario, hydrogen sulfide as well as sulfate diffuses from the water
column into the sediment. Therefore, a mixed isotopic signal of
³⁴
S-enriched sulfide produced in the sediment and relatively light water column sulfide
should be preserved in the sediment. This scenario is not relevant for Lake Kinneret,
however, as hydrogen sulfide concentrations increase with depth in its uppermost sediment
layer 112]. In both scenarios, sedimentary pyrite sulfur is to be expected to be isotopically
heavier than water column hydrogen sulfide. Measurement of the sulfur isotope fractionation
between sulfate and sedimentary pyrite is a promising future direction of this research.