EN EL CAMAL DE SANGOLQUÍ Y PROYECCIÓN DE LA DISPONIBILIDAD DE SANGRE A FUTURO
RENTABILIDAD ANTES DEL IMPUESTO A LA RENTA
4. CONCLUSIONES Y RECOMENDACIONES
43.2.2 Calcareous Benthic Foram iniferal Differential Dissolution
Benthic foraminiferal tests are known to be more tolerant to post-mortal dissolution than planktonic foraminiferal tests: they are on average three times less susceptible to calcite dissolution than planktonic foraminifera (WiDMARK & Ma l m g r e n, 1988). As the composition of fossil calcareous benthic foraminiferal assemblages might also be affected by dissolution, its effect must be considered before any conclusions on the stratigraphical and palaeo- environmental implications are drawn.
Like the planktonic foraminifera, there are differences in susceptibility to calcite dissolution among the calcareous benthic foraminifera. WiDMARK & MALMGREN (1988) established a
dissolution ranking scheme for benthic foraminifera from the late Maastrichtian Abathomphalus mayaroensis Zone from a temperate region in the South Atlantic Ocean. The same core samples were also used by MALMGREN (1987) to study the differential dissolution o f planktonic foraminifera. Intervals of moderate and stronger dissolution in the core, based on the fragmentation o f planktonic foraminifera, are used as a framework for analysis o f dissolution of calcareous benthic foraminifera. In the study only the most abundant calcareous benthic and calcareous cemented agglutinating foraminifera are considered (WiDMARK & MALMGREN,
1988). Their dissolution ranking scheme of benthic foraminifera is adapted in this study o f wells from the Foula Sub-basin to locate possible dissolution effects on the benthic foraminiferal assemblages. Although many o f the calcareous benthic foraminiferal species included in this scheme are also found in the late Campanian - Maastrichtian sediments o f the Foula Sub-basin (see Table 4.3, p. 187), the majority o f the species observed in this study is not included, amongst which are some of the most abundant taxa. Therefore, the results o f this differential dissolution o f calcareous benthic foraminifera are only an approximation o f the dissolution effect on the assemblages.
Like in the study o f the planktonic foraminiferal dissolution, the ranking scheme o f dissolution susceptibility o f the studied taxa is subdivided into a dissolution resistant group, an unaffected group and a dissolution susceptible group (summarised in Table 4.3, p. 187), with the ranking ordered from 1 (the most susceptible species) to 8 (the most resistant species). The most
susceptible species show a significant decrease in abundance from moderate to stronger dissolution. The resistant species are relatively more abundant with increasing dissolution, while the species which are unaffected by dissolution show no significant change in response to enhanced dissolution. The huge variation in shell material and shell microstructure of the calcareous benthic foraminifera makes the ranking o f the dissolution susceptibility of these taxa very complex. Generally speaking, wall thickness is suggested to be the major factor controlling
the differential dissolution (WlDMÀRK & MALMGREN, 1988): taxa with a thick wall are found to be resistant or unaffected by dissolution. There seems to be no effect of the microhabitat of the benthic foraminifera (epifauna or infauna) on the dissolution susceptibility (WiDMARK &
Ma l m g r e n, 1988).
Like for the planktonic foraminifera, an assemblage dissolution index (see Equation 4.2, p. 176) can be calculated using the rank position as given by WiDMARK & Ma l m g r e n (1988). With
just minor dissolution effecting the calcareous benthic foraminiferal tests, this index will have low values (lower than 6), reaching higher values with increasing dissolution rates (typically
higher than 6).
Some o f the agglutinated benthic foraminiferal taxa use calcite as cement to build their tests. As the shells of these taxa will disintegrate when the cement dissolves, these taxa are also susceptible to dissolution. WiDMARK & MALMGREN (1988) included 5 agglutinating taxa in their dissolution susceptibility study, o f which only Gaudryina pyramidata is interpreted as being resistant to dissolution (see Table 4.4, p. 187). All o f the remaining most abundant taxa show no significant change in relative abundance with increased dissolution, and are therefore classed as unaffected species. In the Foula Sub-basin, these taxa are absent or are observed in very low abundances during the late Campanian - Maastrichtian interval. Therefore, no conclusions can be drawn regarding the effect o f dissolution on the composition o f the agglutinated foraminiferal assemblages.
In order to infer late Campanian to late Maastrichtian palaeo-environmental conditions in the Foula Sub-basin from the foraminiferal assemblages, the calcite dissolution susceptibility during the earlier discussed calcareous foraminiferal rich intervals is studied. The application of the dissolution susceptibility ranking scheme of calcareous benthic foraminifera (WiDMARK &
Ma l m g r e n, 1988) results in three benthic foraminiferal groups, to be plotted as the dissolution
susceptibility group, the dissolution unaffected group, and the dissolution resistant group (Figs 4.7-4.10, p. 188-191). By plotting the percentages o f these three groups in the four studied wells from the Foula Sub-basin, it is evident that only a small part o f the total calcareous benthic foraminiferal assemblages is included in the analysis. On average, the contribution o f the unaffected species is between 1 0-2 0% of the total calcareous benthic foraminiferal assemblage,
with a contribution o f about 5-10% o f the susceptible species, while the contribution o f the resistant species is negligible. Accordingly, the assemblage dissolution index shows values which are very close to value 6 (the ranking of the unaffected species; Figs 4.7-4.10, p. 188-
191), suggesting there is little effect o f dissolution. By examining the abundances o f the dissolution susceptible and unaffected groups (in specimens per gram), we see that these vary more or less coeval and also correlate well with %plankton (see Figs 4.7-4.10, p. 188-191). This
4 3 .2 .2 Calcareous Benthic Foraminiferal Differential Dissolution
is because most o f the calcareous benthic foraminiferal taxa mainly occur during the calcareous foraminiferal rich intervals. Therefore, the fluctuations in the assemblage dissolution index are related to the occurrence of the calcareous foraminiferal rich intervals.
Although only a small part of the total calcareous benthic foraminiferal assemblage is included in this dissolution susceptibility study, the results are in agreement with the conclusions drawn from the dissolution effect on the planktonic foraminiferal assemblages. When there is no dissolution effecting the planktonic foraminiferal assemblage, there will be no dissolution effect on the composition o f the calcareous benthic foraminiferal assemblage, as the tests o f planktonic foraminifera are more susceptible to dissolution than the tests of calcareous benthic foraminifera.
To summarise this study on the effect o f calcium carbonate dissolution on the composition of the calcareous foraminiferal assemblages: during the late Campanian - late Maastrichtian, there were three intervals with enhanced calcium carbonate preservation. During those intervals, no, or only minor dissolution was effecting the composition o f the calcareous benthic and planktonic foraminiferal assemblages. This is concluded from the dominance of dissolution susceptible planktonic foraminifera during those intervals. With no dissolution effecting the planktonic foraminiferal assemblages, there will consequently be no effect o f dissolution on the composition o f the calcareous benthic foraminiferal assemblage. It is impossible in this study to determine whether the calcareous foraminifera are concentrated in the reworked intervals of turbidite deposits or are autochtonous features o f the foraminiferal assemblages. However, according to the morphogroup analysis of calcareous benthic foraminifera (see Chapter 4.4.1.2, p. 215) there is no major contribution to the assemblages o f transported and reworked calcareous benthic foraminifera from shallower environments. Therefore it can be concluded that the recorded calcareous benthic and planktonic foraminiferal assemblages are good representations o f the actual standing stock.
ranking of dissolution susceptibility of calcareous benthic foraminifera (WiDMARK & M a lm g r e n , 1988) 1