352
Gingele et al.
The tentative relation of barium accumulation
and export production - based on a few studies with
sediment trap data - has to be confirmed by additional data from suspended matter and surface
sediments. Two major topics await further investigation. Firstly, the preservation factor of barite
during burial, which relates barium accumulation in
the sediment to barium flux in the water column,
was assessed only a few locations by Dymond et
a!. (1992), McManus et a!. (1994) and Paytan and
Kastner (1996). Corroboration requires dated cores
from which Holocene mass accumulation rates can
be calculated. The resulting barium flux or TOC/
Ba(h;O) ratios at the given water depth can be compared to the theoretical values derived from the
power function of Dymond et a!. (1992) or Francois
eta!. (1995), (Fig. 3). Secondly, the linkage ofB~h;O)
flux and export production needs further investigation. Looking at areas of similar surface productivity, higher concentrations ofBa(b;o) are recorded
in the Southern Ocean sediments as compared to
those oflow latitudes (Fig. 5) and in the Pacific as
compared to Atlantic sediments. This may suggest
that barite formation and preservation in the water
column is either not completely independant of the
concentrations of dissolved barium or that other
unknown processes in the sediment might affect
this relationship.
Postdepositional Influence of Early
Diagenesis on the Primary Biogenic
Barium Signal
In principle there is only one important diagenetic
process which prevents the use of barium as a
proxy for paleoproductivity changes. The dissolution of biogenic barite in sulfate-reducing sediments
and the corresponding precipitation of authigenic
barium sulfate can obscure the original Ba(b;O)
content. The corresponding combination of
geochemical and microbial catalyzed reactions is
of special importance in regions with high accumulation rates of organic matter, the common prerequisite for anoxic environmental conditions.
Therefore, great care must be taken upon interpreting the sedimentary Ba(b;O) signal used as
paleoproductivity indicator on continental shelves,
slopes and coastal upwelling regimes (e.g. Dymond
et al. 1992). In the following we will give a brief
description of the relevant biogeochemical mechanism.
Since the solubility of barite is low under oxic
and suboxic conditions, these microcrystals generally have a high stability in seawater and surface
sediments (Church and Wolgemuth 1972). This
situation changes ifthe increasing supply with organic matter made available through the action of
microorganisms produced at least in anoxic conditions. As a consequence, authigenic barite (fronts)
precipitates at the oxic/anoxic boundary in such
sulfate-reducing sediments (Brumsack and Gieskes
1983, Brumsack 1989, Von Breymann et a!. 1990,
1992). Fig. 6 gives an example for this early
diagenetic process.
The sediments in this core recovered off south
Angola are terrigenous muds with minor amounts
of biogenic silica and carbonate and show very high
mass accumulation rates. Based on 1 4 C-datings, an
age of 18.000 years at 10 mbsf was documented
by Kolling (1991). The calculated terrigenous
barium constitutes 50-70 % oftotal barium (Gingele
and Dahmke 1994). The pore water gradient of
sulfate below the suboxic sediment layers indicates
an anoxic sulfate reduction zone (starting approximately between 6 and 8 mbsf). Dissolved barium
diffuses upward into this zone from deeper parts
of the sediment column where complete sulfate
exhaustion results in an undersaturation of barite
and, therefore, in dissolution of barite crystals. Both
interstitial constituents were removed from the
solution by the formation of an authigenic barite
front terminated just above the sulfate-depleted
zone where the pore water is supersaturated with
barite. Although the scawater equilibrium concentration, relative to the solubility of barite, amounts
to 0.36 flM Ba (Church and Wolgemuth 1972),
40flM Ba was determined at the end of the core.
However the maximum asymptotic concentration
was not obtained. The anoxic Ba-release produces
in extremely high concentrations, more than 2000
flM at about 300 mbsf (ODP-Ieg studies in the
Japan Sea: Von Breymann et al. (1992)). Using
Fick's first law of diffusion and assuming steady
state conditions, Kolling (1991) estimated a time
period of8300 years for the precipitation ofthe Baspike as shown in example of Fig. 6. The calcula-
Gingele et al.
The tentative relation of barium accumulation
and export production - based on a few studies with
sediment trap data - has to be confirmed by additional data from suspended matter and surface
sediments. Two major topics await further investigation. Firstly, the preservation factor of barite
during burial, which relates barium accumulation in
the sediment to barium flux in the water column,
was assessed only a few locations by Dymond et
a!. (1992), McManus et a!. (1994) and Paytan and
Kastner (1996). Corroboration requires dated cores
from which Holocene mass accumulation rates can
be calculated. The resulting barium flux or TOC/
Ba(h;O) ratios at the given water depth can be compared to the theoretical values derived from the
power function of Dymond et a!. (1992) or Francois
eta!. (1995), (Fig. 3). Secondly, the linkage ofB~h;O)
flux and export production needs further investigation. Looking at areas of similar surface productivity, higher concentrations ofBa(b;o) are recorded
in the Southern Ocean sediments as compared to
those oflow latitudes (Fig. 5) and in the Pacific as
compared to Atlantic sediments. This may suggest
that barite formation and preservation in the water
column is either not completely independant of the
concentrations of dissolved barium or that other
unknown processes in the sediment might affect
this relationship.
Postdepositional Influence of Early
Diagenesis on the Primary Biogenic
Barium Signal
In principle there is only one important diagenetic
process which prevents the use of barium as a
proxy for paleoproductivity changes. The dissolution of biogenic barite in sulfate-reducing sediments
and the corresponding precipitation of authigenic
barium sulfate can obscure the original Ba(b;O)
content. The corresponding combination of
geochemical and microbial catalyzed reactions is
of special importance in regions with high accumulation rates of organic matter, the common prerequisite for anoxic environmental conditions.
Therefore, great care must be taken upon interpreting the sedimentary Ba(b;O) signal used as
paleoproductivity indicator on continental shelves,
slopes and coastal upwelling regimes (e.g. Dymond
et al. 1992). In the following we will give a brief
description of the relevant biogeochemical mechanism.
Since the solubility of barite is low under oxic
and suboxic conditions, these microcrystals generally have a high stability in seawater and surface
sediments (Church and Wolgemuth 1972). This
situation changes ifthe increasing supply with organic matter made available through the action of
microorganisms produced at least in anoxic conditions. As a consequence, authigenic barite (fronts)
precipitates at the oxic/anoxic boundary in such
sulfate-reducing sediments (Brumsack and Gieskes
1983, Brumsack 1989, Von Breymann et a!. 1990,
1992). Fig. 6 gives an example for this early
diagenetic process.
The sediments in this core recovered off south
Angola are terrigenous muds with minor amounts
of biogenic silica and carbonate and show very high
mass accumulation rates. Based on 1 4 C-datings, an
age of 18.000 years at 10 mbsf was documented
by Kolling (1991). The calculated terrigenous
barium constitutes 50-70 % oftotal barium (Gingele
and Dahmke 1994). The pore water gradient of
sulfate below the suboxic sediment layers indicates
an anoxic sulfate reduction zone (starting approximately between 6 and 8 mbsf). Dissolved barium
diffuses upward into this zone from deeper parts
of the sediment column where complete sulfate
exhaustion results in an undersaturation of barite
and, therefore, in dissolution of barite crystals. Both
interstitial constituents were removed from the
solution by the formation of an authigenic barite
front terminated just above the sulfate-depleted
zone where the pore water is supersaturated with
barite. Although the scawater equilibrium concentration, relative to the solubility of barite, amounts
to 0.36 flM Ba (Church and Wolgemuth 1972),
40flM Ba was determined at the end of the core.
However the maximum asymptotic concentration
was not obtained. The anoxic Ba-release produces
in extremely high concentrations, more than 2000
flM at about 300 mbsf (ODP-Ieg studies in the
Japan Sea: Von Breymann et al. (1992)). Using
Fick's first law of diffusion and assuming steady
state conditions, Kolling (1991) estimated a time
period of8300 years for the precipitation ofthe Baspike as shown in example of Fig. 6. The calcula-