Biogenic Barium as a Proxy for Paleoproductivity
355
Finally, it is of interest that high particle sedimentation and accumulation rates do not necessarily
induce the formation of a deep sulfate reduction
zone. Niirnberg (1995) published results for the
Southern Ocean, where diatomaceous ooze with
sedimentation rates of about 100 cmlka contains
only 0.3 % TOC. Due to the high porosity and
permeability of these sediments the dominating
microbial remineralization of organic matter is limited to the surface layers resulting in low burial rates
of extensively degraded organic carbon. Hence
there is no evidence of a sulfate controlled early
diagenetic overprinting of the primary barite signal
in those areas (see below).
An additional, but relatively less important
diagenetic process in this context is related
to barium and its association to the benthic manganese cycle. Barium can co-precipitate during
formation of diagenetic Mn-oxyhydroxide
enrichments. Briefly, manganese reaches the
seafloor mainly in form of terrigenous Mn-oxides.
If the free oxygen has been consumed, Mn 4 + is
subsequently reduced to Mn2+ by microbial degradation of organic matter during burial, or by
reoxidation of other reduced species like sulfide or
Fe 2 +. After diffusing into the oxic zone, the
reoxidation ofMn2+ can locally result in Mn-crusts.
Poorly developed crystalline Mn-oxyhydrate can
contain up to 2000 ppm of barium (Dymond et al.
1981). The effects of biogeochemical Mn-cycles
are discussed in more detail by Pruysers et al.
(1991) and De Lange et al. (1994).
Case Studies from the Equatorial Atlantic
to the Southern Ocean
Surface Distribution
Knowledge of how biogenic barium is distributed
in surface sediments and the comparison with patterns of ocean productivity is a prerequisite to
evaluate barium as a productivity proxy. A dense
set of data compiled by Niirnberg (1995) from the
Southern Ocean was extended to the eastern South
Atlantic and equatorial Atlantic (Fig. 5). Large gaps
remain in the permanently ice-covered regions of
the Weddell Sea and the low-productive region of
the Subtropical Gyre. South of30° S, the calculation of Ba(bio) (equation 2) is based on a detrital
correction factor of 0.0067 as suggested by
Niirnberg (1995). The application of this factor
north of 30° S would lead to negative Ba(biO)-values at many sites. More intensive chemical weathering in lower latitudes reduces Ba contents in the
terrigenous supply. A detrital correction factor of
0.004 (Gingele and Dalunke 1994) was used for
sites north of 3 0° S. Generally, Ba(bio) contents are
higher in the southern and eastern part of the
Atlantic Ocean. However, it becomes clear that the
distribution of Ba(bio) does not necessarily mirror
patterns of recent ocean productivity (e.g. Berger
et al. 1989). Any dilution by biogenic and
terrigenous sediment components has to be considered. Low Ba(biO)-values are recorded in the
equatorial Atlantic. Only at deep sites in the equatorial upwelling zone, where carbonate has been
dissolved, Ba(bio)-values exceed 500 p.p.m. High
productivity and the association ofBa with siliceous
matter (Dehairs et al. 1980; Stroobants et al. 1991)
may be responsible for increased Ba(biO) values off
the mouth of the Congo River. High input of
terrigenous matter lowers B~bio) concentrations off
Namibia, although surface productivity is high. The
occurrence of high Ba(biO) concentrations and surface productivity coincides in the Polar Frontal
Zone. However, a calculation of accumulation rates
of Ba(biO) shifts the maximum to the south, more
towards the Antarctic zone (Niirnberg 1995).
Ba(bio) concentrations below 500 ppm occur in
coastal areas of the Antarctic continent, in the
immediate vicinity of islands and in the Weddell Sea
and Argentine Basin. Intermediate values of 1 000
-2000 ppm Ba(biO) are recorded at the Antarctic
continental slope. Calculating accumulation rates
ofBa(bio) is required to compensate diluting effects
and relate Ba(biO) concentrations to the flux ofbiogenic barium and organic matter. Unfortunately,
the set of sites where Holocene sedimentation rates
were assessed is much smaller. In the following
section paleoproductivity calculations were carried
out for selected cores from the Atlantic. Core-top
samples were compared to recent productivity
patterns and glacial-interglacial changes are discussed.
355
Finally, it is of interest that high particle sedimentation and accumulation rates do not necessarily
induce the formation of a deep sulfate reduction
zone. Niirnberg (1995) published results for the
Southern Ocean, where diatomaceous ooze with
sedimentation rates of about 100 cmlka contains
only 0.3 % TOC. Due to the high porosity and
permeability of these sediments the dominating
microbial remineralization of organic matter is limited to the surface layers resulting in low burial rates
of extensively degraded organic carbon. Hence
there is no evidence of a sulfate controlled early
diagenetic overprinting of the primary barite signal
in those areas (see below).
An additional, but relatively less important
diagenetic process in this context is related
to barium and its association to the benthic manganese cycle. Barium can co-precipitate during
formation of diagenetic Mn-oxyhydroxide
enrichments. Briefly, manganese reaches the
seafloor mainly in form of terrigenous Mn-oxides.
If the free oxygen has been consumed, Mn 4 + is
subsequently reduced to Mn2+ by microbial degradation of organic matter during burial, or by
reoxidation of other reduced species like sulfide or
Fe 2 +. After diffusing into the oxic zone, the
reoxidation ofMn2+ can locally result in Mn-crusts.
Poorly developed crystalline Mn-oxyhydrate can
contain up to 2000 ppm of barium (Dymond et al.
1981). The effects of biogeochemical Mn-cycles
are discussed in more detail by Pruysers et al.
(1991) and De Lange et al. (1994).
Case Studies from the Equatorial Atlantic
to the Southern Ocean
Surface Distribution
Knowledge of how biogenic barium is distributed
in surface sediments and the comparison with patterns of ocean productivity is a prerequisite to
evaluate barium as a productivity proxy. A dense
set of data compiled by Niirnberg (1995) from the
Southern Ocean was extended to the eastern South
Atlantic and equatorial Atlantic (Fig. 5). Large gaps
remain in the permanently ice-covered regions of
the Weddell Sea and the low-productive region of
the Subtropical Gyre. South of30° S, the calculation of Ba(bio) (equation 2) is based on a detrital
correction factor of 0.0067 as suggested by
Niirnberg (1995). The application of this factor
north of 30° S would lead to negative Ba(biO)-values at many sites. More intensive chemical weathering in lower latitudes reduces Ba contents in the
terrigenous supply. A detrital correction factor of
0.004 (Gingele and Dalunke 1994) was used for
sites north of 3 0° S. Generally, Ba(bio) contents are
higher in the southern and eastern part of the
Atlantic Ocean. However, it becomes clear that the
distribution of Ba(bio) does not necessarily mirror
patterns of recent ocean productivity (e.g. Berger
et al. 1989). Any dilution by biogenic and
terrigenous sediment components has to be considered. Low Ba(biO)-values are recorded in the
equatorial Atlantic. Only at deep sites in the equatorial upwelling zone, where carbonate has been
dissolved, Ba(bio)-values exceed 500 p.p.m. High
productivity and the association ofBa with siliceous
matter (Dehairs et al. 1980; Stroobants et al. 1991)
may be responsible for increased Ba(biO) values off
the mouth of the Congo River. High input of
terrigenous matter lowers B~bio) concentrations off
Namibia, although surface productivity is high. The
occurrence of high Ba(biO) concentrations and surface productivity coincides in the Polar Frontal
Zone. However, a calculation of accumulation rates
of Ba(biO) shifts the maximum to the south, more
towards the Antarctic zone (Niirnberg 1995).
Ba(bio) concentrations below 500 ppm occur in
coastal areas of the Antarctic continent, in the
immediate vicinity of islands and in the Weddell Sea
and Argentine Basin. Intermediate values of 1 000
-2000 ppm Ba(biO) are recorded at the Antarctic
continental slope. Calculating accumulation rates
ofBa(bio) is required to compensate diluting effects
and relate Ba(biO) concentrations to the flux ofbiogenic barium and organic matter. Unfortunately,
the set of sites where Holocene sedimentation rates
were assessed is much smaller. In the following
section paleoproductivity calculations were carried
out for selected cores from the Atlantic. Core-top
samples were compared to recent productivity
patterns and glacial-interglacial changes are discussed.