Carbonate Dissolution in the Deep-Sea
277
matter buried with CaC0 3 ; 4) whether the carbonate particles have an organic coating to retard dissolution; 5) whether there are currents to stir the
layer of dissolution around the CaC0 3 -particles (Le
and Shackleton 1992). Only in the ideal situation,
where the rain rate of carbonate and non-carbonate material is constant, can the amount of calcite
lost to dissolution be calculated from the percentage CaC0 3 in the sediment (Farrell and Prell 1989;
Curry and Lohmann 1990; Bickert et al. 1997).
Another method to distinguish the three environments leads to the new approaches made here.
The continuous increase of dissolution stages of
Globigerina bulloides ultrastructure with increasing water depths complies to the concept of perpetual decrease of ~COt within the water column
(Broecker and Takahashi 1978). For all transects,
dissolution increases by about one dissolution stage
towards the calcite lysocline. Samples below the
lysocline drastically increase in BDX values. However, the investigated tests of the equatorial Atlantic and the Walvis Ridge transect above the lysocline
show an offset of about one dissolution stage less
than at comparable depths ofthe continental margin (Fig. l3a). This is due to the strong productivity of organic and inorganic carbon within the
Benguela upwelling system which yields higher
benthic respiration rates and hence a larger contribution to carbonate dissolution (Berger et al.
1987). However, each realm can be distinguished
by discrete clusters. The important point is that this
parameter does not depend on the ecology of the
surface water, i.e., ifthere are tests broken without being affected by dissolution - whatever the
reason may be - they will raise micropaleontological
parameters and additionally they even will change
the fragmentation index towards stronger dissolution (Dittert and Henrich subm).
The Calcidiscus leptoporus - Emiliania
huxleyi Dissolution Index (CEX) shows comparable results. Considering the investigated areas,
E. huxleyi and C. leptoporus show rather similar
ecological behavior in response to nutrient distribution and temperature. Consequently, the changing ratio of these two species can be attributed to
their different dissolution susceptiblity. Hence, we
are able to form clusters which separate the continental margin from the open ocean realm above
and below the lysocline. That is, CEX values rise
steadily with increasing water depths above the
lysocline and then turn to stronger dissolution below the lysocline. Due to higher productivity, CEX
values are offset by about 0.1 towards stronger
dissolution above the lysocline at the continental
margin (Fig. 13b). However, in surface waters
which are distinctly more nutrient-depleted or
cooler, CEX may fail.
The rain ratio of organic to inorganic carbon can
serve as a useful tool. Our results show, that rain
ratio forms clusters which are comparable to BDX
and CEX. Low values «0.01) above the lysocline
and strongly increasing values (>0.01) below represent the open ocean situation, whereas very high
values (»0.0 I) in the upper few thousand meters
of the water column stand for the continental margin realm. This supports the concept of Berger
(1991) who describes maximum values of organic
carbon for the upper continental margin of the
coastal ocean within the classical upwelling areas
on the basis of a combination of enormous productivity, high export and rapid sedimentation (Fig. 9,
lower). On the other hand, oxidation of such an immense amount of organic carbon deposits results
in (1) a successive CO 2 release which lowers the
pH of pore water; (2) reduced oxygen content; and
(3) supralysoclinal dissolution of carbonates. Consequently, a high rain ratio may be attributed to the
fact that a) productivity of organic carbon is intensified which leads to supralysoclinal dissolution, and
b) carbonate values decrease due to sublysoclinal
dissolution. Where the rain rate of carbonate and
non-carbonate material is constant and neither productivity nor dilution are enhanced, the rain ratio
of organic to inorganic carbon remains constant and
low. All three situations can clearly be distinguished
in separate clusters (Fig. l3c). Questions on the
origin of organic carbon may arise if the eolian
supply of terrigenous organic material related to the
trade winds was of some importance in the deep
Equatorial Atlantic 01 erardo and Ruddiman 1996);
furthermore, riverine particulate organic carbon
deposited nearshore might enforce the total organic
carbon signal (Emerson and Hedges 1988). However, extended input ofland-derived organic carbon would raise the total organic carbon content
and consequently also increase the rain ratio to-
277
matter buried with CaC0 3 ; 4) whether the carbonate particles have an organic coating to retard dissolution; 5) whether there are currents to stir the
layer of dissolution around the CaC0 3 -particles (Le
and Shackleton 1992). Only in the ideal situation,
where the rain rate of carbonate and non-carbonate material is constant, can the amount of calcite
lost to dissolution be calculated from the percentage CaC0 3 in the sediment (Farrell and Prell 1989;
Curry and Lohmann 1990; Bickert et al. 1997).
Another method to distinguish the three environments leads to the new approaches made here.
The continuous increase of dissolution stages of
Globigerina bulloides ultrastructure with increasing water depths complies to the concept of perpetual decrease of ~COt within the water column
(Broecker and Takahashi 1978). For all transects,
dissolution increases by about one dissolution stage
towards the calcite lysocline. Samples below the
lysocline drastically increase in BDX values. However, the investigated tests of the equatorial Atlantic and the Walvis Ridge transect above the lysocline
show an offset of about one dissolution stage less
than at comparable depths ofthe continental margin (Fig. l3a). This is due to the strong productivity of organic and inorganic carbon within the
Benguela upwelling system which yields higher
benthic respiration rates and hence a larger contribution to carbonate dissolution (Berger et al.
1987). However, each realm can be distinguished
by discrete clusters. The important point is that this
parameter does not depend on the ecology of the
surface water, i.e., ifthere are tests broken without being affected by dissolution - whatever the
reason may be - they will raise micropaleontological
parameters and additionally they even will change
the fragmentation index towards stronger dissolution (Dittert and Henrich subm).
The Calcidiscus leptoporus - Emiliania
huxleyi Dissolution Index (CEX) shows comparable results. Considering the investigated areas,
E. huxleyi and C. leptoporus show rather similar
ecological behavior in response to nutrient distribution and temperature. Consequently, the changing ratio of these two species can be attributed to
their different dissolution susceptiblity. Hence, we
are able to form clusters which separate the continental margin from the open ocean realm above
and below the lysocline. That is, CEX values rise
steadily with increasing water depths above the
lysocline and then turn to stronger dissolution below the lysocline. Due to higher productivity, CEX
values are offset by about 0.1 towards stronger
dissolution above the lysocline at the continental
margin (Fig. 13b). However, in surface waters
which are distinctly more nutrient-depleted or
cooler, CEX may fail.
The rain ratio of organic to inorganic carbon can
serve as a useful tool. Our results show, that rain
ratio forms clusters which are comparable to BDX
and CEX. Low values «0.01) above the lysocline
and strongly increasing values (>0.01) below represent the open ocean situation, whereas very high
values (»0.0 I) in the upper few thousand meters
of the water column stand for the continental margin realm. This supports the concept of Berger
(1991) who describes maximum values of organic
carbon for the upper continental margin of the
coastal ocean within the classical upwelling areas
on the basis of a combination of enormous productivity, high export and rapid sedimentation (Fig. 9,
lower). On the other hand, oxidation of such an immense amount of organic carbon deposits results
in (1) a successive CO 2 release which lowers the
pH of pore water; (2) reduced oxygen content; and
(3) supralysoclinal dissolution of carbonates. Consequently, a high rain ratio may be attributed to the
fact that a) productivity of organic carbon is intensified which leads to supralysoclinal dissolution, and
b) carbonate values decrease due to sublysoclinal
dissolution. Where the rain rate of carbonate and
non-carbonate material is constant and neither productivity nor dilution are enhanced, the rain ratio
of organic to inorganic carbon remains constant and
low. All three situations can clearly be distinguished
in separate clusters (Fig. l3c). Questions on the
origin of organic carbon may arise if the eolian
supply of terrigenous organic material related to the
trade winds was of some importance in the deep
Equatorial Atlantic 01 erardo and Ruddiman 1996);
furthermore, riverine particulate organic carbon
deposited nearshore might enforce the total organic
carbon signal (Emerson and Hedges 1988). However, extended input ofland-derived organic carbon would raise the total organic carbon content
and consequently also increase the rain ratio to-