289
1988; Dehairs et al. 1991; Dymond et al. 1992;
Gingele and Dahmke 1994; Paytan et al. 1996a).
Sedimentary barite has also been used to reconstruct the strontium isotope composition of seawater over time (e.g., Paytan et al. 1993; Mearon et
al. 2003), to determine the sulfur isotope ratio of
marine sulfate (Cecile et al. 1983; Goodfellow and
Jonasson 1984; Paytan et al. 1998), and to
characterize Holocene sedimentation rates by
using excess
226
Ra decay (Paytan et al. 1996b; van
Beek and Reyss 2001; van Beek et al. 2002).
Although it is frequently stated that the
process of sulfate reduction promotes barite dissolution it is, in fact, the complete depletion of
interstitial sulfate which leads to an undersaturation of the pore water with respect to barite
and to its subsequent dissolution (e.g., von
Breymann et al. 1992; Torres et al. 1996; Gingele et
al. 1999). This process is obvious from pore water
Ba
2+
concentrations which typically increase to
micromolar concentrations with depth below the
SMT. Dissolution of barite below the SMT and
reprecipitation of diagenetic barite slightly above
the sulfate penetration depth in so-called
authigenic or diagenetic barite fronts can drastically obscure the primary barite record and thus
lead to wrong paleoceanographic interpretations.
The effect of barite dissolution in the zone of total
sulfate depletion, and the subsequent reprecipitation of diagenetic barite at higher sediment
levels is illustrated in Figure 8.10 for a sediment
core recovered from the continental margin off
Angola. The use of barite as a geochemical tracer
or archive for paleoceanographic reconstructions
is therefore limited – not to say precluded - in
sediment intervals that are either currently sulfatefree or have been strongly sulfate depleted in the
past (e.g. von Breymann et al. 1992; Gingele and
Dahmke 1994; Torres et al. 1996; Gingele et al.
1999; Dickens 2001). While the current geochemical zonation of a sediment column can be
determined from pore water sulfate data, a past
migration of the SMT – and particularly a downward strike over time as a result of a transient
decrease in methane flux from below – has also to
be considered a possible cause that may alter the
solid-phase Ba contents.
In a novel approach, Dickens (2001) used
sedimentary Ba records to assess temporal
changes during the Late Pleistocene in the upward
flux of methane within sediments of the Blake
Ridge that are rich in sub-surface gas hydrates.
Due to a lack of Ba enrichment above the present
depth of the barite front the author concluded that
the upward methane flux from the underlying gas
hydrate reservoir has not varied significantly
across major changes in sea level and hydrostatic
pressure. A possible means to identify the origin/
source of barite enrichments is the analysis of the
stable sulfur isotopic composition, δ
34
S, of the
barite particles. As the sulfur isotopic composition
of sulfate in the water column, where the productivity-related biogenic barite is assumed to be
formed, is significantly lighter (around +21 ‰;
Paytan et al. 2002) than that of pore water sulfate
at the SMT, where diagenetic barite precipitates
(around + 40 ‰; Torres et al. 1996), it is generally
assumed that the δ
34
S of barite should give insight
into its formation mechanism or origin (Paytan et
al. 2004).
8.4.4
Non-Steady State Diagenesis
A particularly pronounced alteration of the sedimentary solid phase by mineral dissolution and
authigenic mineral precipitation can take place in
connection with sulfate reduction during nonsteady-state diagenesis, i.e. during phases of
deposition by which sedimentary conditions are
not constant over time and sediment geochemistry
adjusts to the new situation. Non-steady-state
diagenesis can be initiated by any changes in the
fluxes of electron donors and acceptors and
environmental conditions, for example by changes
in type of depositing sediment, oxygen content of
the bottom water, sedimentation rate, flux of
organic matter to the seafloor, and upward flux of
methane (e.g., Kasten et al. 2003). The nonsteady-state processes that occur during the
transition from one depositional situation to
another, or at the interface between two sediment
types, typically comprise the development of geochemical reaction fronts. These fronts can either
be fixed at particular sediment horizons for a
prolonged period of time or move downwards or
upwards within the sediment column. During a
fixation or a slow migration of reaction fronts or
geochemical boundaries the diagenetic processes
acting at these fronts can produce higher concentrations of the authigenic minerals formed at
specific sediment levels than under steady-state
conditions. On the other hand, a fast upward
migration of a geochemical front may support the
burial and thus the preservation of substantial
amounts of metastable minerals (Riedinger et al.
2005, see below).
8.4
Effects of Sulfate Reduction on Sedimentary Solid Phases
1988; Dehairs et al. 1991; Dymond et al. 1992;
Gingele and Dahmke 1994; Paytan et al. 1996a).
Sedimentary barite has also been used to reconstruct the strontium isotope composition of seawater over time (e.g., Paytan et al. 1993; Mearon et
al. 2003), to determine the sulfur isotope ratio of
marine sulfate (Cecile et al. 1983; Goodfellow and
Jonasson 1984; Paytan et al. 1998), and to
characterize Holocene sedimentation rates by
using excess
226
Ra decay (Paytan et al. 1996b; van
Beek and Reyss 2001; van Beek et al. 2002).
Although it is frequently stated that the
process of sulfate reduction promotes barite dissolution it is, in fact, the complete depletion of
interstitial sulfate which leads to an undersaturation of the pore water with respect to barite
and to its subsequent dissolution (e.g., von
Breymann et al. 1992; Torres et al. 1996; Gingele et
al. 1999). This process is obvious from pore water
Ba
2+
concentrations which typically increase to
micromolar concentrations with depth below the
SMT. Dissolution of barite below the SMT and
reprecipitation of diagenetic barite slightly above
the sulfate penetration depth in so-called
authigenic or diagenetic barite fronts can drastically obscure the primary barite record and thus
lead to wrong paleoceanographic interpretations.
The effect of barite dissolution in the zone of total
sulfate depletion, and the subsequent reprecipitation of diagenetic barite at higher sediment
levels is illustrated in Figure 8.10 for a sediment
core recovered from the continental margin off
Angola. The use of barite as a geochemical tracer
or archive for paleoceanographic reconstructions
is therefore limited – not to say precluded - in
sediment intervals that are either currently sulfatefree or have been strongly sulfate depleted in the
past (e.g. von Breymann et al. 1992; Gingele and
Dahmke 1994; Torres et al. 1996; Gingele et al.
1999; Dickens 2001). While the current geochemical zonation of a sediment column can be
determined from pore water sulfate data, a past
migration of the SMT – and particularly a downward strike over time as a result of a transient
decrease in methane flux from below – has also to
be considered a possible cause that may alter the
solid-phase Ba contents.
In a novel approach, Dickens (2001) used
sedimentary Ba records to assess temporal
changes during the Late Pleistocene in the upward
flux of methane within sediments of the Blake
Ridge that are rich in sub-surface gas hydrates.
Due to a lack of Ba enrichment above the present
depth of the barite front the author concluded that
the upward methane flux from the underlying gas
hydrate reservoir has not varied significantly
across major changes in sea level and hydrostatic
pressure. A possible means to identify the origin/
source of barite enrichments is the analysis of the
stable sulfur isotopic composition, δ
34
S, of the
barite particles. As the sulfur isotopic composition
of sulfate in the water column, where the productivity-related biogenic barite is assumed to be
formed, is significantly lighter (around +21 ‰;
Paytan et al. 2002) than that of pore water sulfate
at the SMT, where diagenetic barite precipitates
(around + 40 ‰; Torres et al. 1996), it is generally
assumed that the δ
34
S of barite should give insight
into its formation mechanism or origin (Paytan et
al. 2004).
8.4.4
Non-Steady State Diagenesis
A particularly pronounced alteration of the sedimentary solid phase by mineral dissolution and
authigenic mineral precipitation can take place in
connection with sulfate reduction during nonsteady-state diagenesis, i.e. during phases of
deposition by which sedimentary conditions are
not constant over time and sediment geochemistry
adjusts to the new situation. Non-steady-state
diagenesis can be initiated by any changes in the
fluxes of electron donors and acceptors and
environmental conditions, for example by changes
in type of depositing sediment, oxygen content of
the bottom water, sedimentation rate, flux of
organic matter to the seafloor, and upward flux of
methane (e.g., Kasten et al. 2003). The nonsteady-state processes that occur during the
transition from one depositional situation to
another, or at the interface between two sediment
types, typically comprise the development of geochemical reaction fronts. These fronts can either
be fixed at particular sediment horizons for a
prolonged period of time or move downwards or
upwards within the sediment column. During a
fixation or a slow migration of reaction fronts or
geochemical boundaries the diagenetic processes
acting at these fronts can produce higher concentrations of the authigenic minerals formed at
specific sediment levels than under steady-state
conditions. On the other hand, a fast upward
migration of a geochemical front may support the
burial and thus the preservation of substantial
amounts of metastable minerals (Riedinger et al.
2005, see below).
8.4
Effects of Sulfate Reduction on Sedimentary Solid Phases