AVANCE DEL FRENTE DE INYECCIÓN DE AGUA DEL

Within the microfossil assemblages o f this study, variation in species abundances is observed throughout the studied section. These species abundance variations can be used to interpret changes in environmental conditions, such as water depth, current strength and organic flux to the sea floor. For species distribution, relative abundances (in percentages) can be compared between the samples. However, with many species present in the assemblages it is almost impossible to analyse variations o f every single species separately to come to a conclusion. Diversity indices offer an alternative, more generalised way to describe species abundance relationships in communities. The limitations and results will be discussed below.

Multivariate statistical techniques are another way to simplify and condense massive data sets in order to obtain possible relationships between the different species. However, due to the sample limitations (with an unknown degree of mixing, unknown total population, and unknown amount of caving) calculations will not be significant.

Diversity indices can be used to characterise species abundance relationships in communities. There are two distinct components involved in diversity. The first component is species richness, or variety, i.e. the total number of species in a community, and the second one is species evenness, or equitability, i.e. the way abundance is spread between species.

3.3.2.1 Richness Index

At its most simple, a measure o f richness would be the total number o f species (5) observed in a community. However, species richness is strongly dependent on sample size. Therefore, direct comparison between richness values is only possible in samples o f equal size. To overcome the unequal sample size problem, rarefaction may be used to allow comparisons o f species numbers between communities. This method computes the expected number o f species in a random sample (Hurlbert's index) as the sum of the probability that each species observed in the assemblage will be included in the sample. Unfortunately, use of rarefaction has its limitations. Because rarefaction assumes that studied communities do not differ in their species-individual distributions, rarefaction will predict the same species richness for all samples (Lu d w ig & Re y n o l d s, 1988). Furthermore, rarefaction leads to a great loss of information (Ma g u r r a n,

1988). This is because the number o f species and their relative abundances are known for each sample before rarefaction. After rarefaction, all that remains is the expected number o f species per sample.

Therefore, species density might be an alternative measure o f species richness. Species density is defined as the number o f species per specified collection area (MAGURRAN, 1988). In this

Study it can be easily determined for every sample by calculating the number o f species («) per gram o f sediment processed. This allows direct comparison o f species richness between samples o f unequal size. Yet another way to describe species richness, is numerical species richness. This is defined as the number o f species (S) per specified num ber o f individuals (MAGURRAN, 1988), which is calculated in this study as number o f species per num ber o f specimens present (i.e. S/n).

3.3.2.2 Diversity Indices

Both species richness and evenness are combined in the diversity index. One useful diversity index is from HiLL (1973), where his family o f diversity numbers are represented by the equation:

/=i

where S: total number o f species observed

Pi', proportion o f individuals belonging to i-th species

(= , with n total number o f individuals observed in the sample and rij the number o f individuals o f the i-th species)

A: order of diversity

H i l l (1973) shows that the 0th, 1st and 2nd order o f these diversity numbers (i.e. A=0, 1 and 2) coincide with three o f the most important measures o f diversity. These diversity numbers are defined as:

A=0 Ng = S (Eq. 3.2)

where S: total number o f species observed

A = 1 N j = e ^ (Eq. 3.3)

where H ’: Shannon’s index (see equation 3.5)

A= 2 N , = — (Eq. 3.4)

A

where A: Simpson’s index (see equation 3.7)

The diversity numbers equate the “effective number o f species” present in a sample, i.e. the degree to which proportional abundances are distributed among the species. N<, is the total number o f species in the sample, N, measures the number o f abundant species and Nj is the number o f very abundant species in the sample (where N<, > N/ > N;).

_________ 3.3.2 Diversity

3.3.2.3 Shannon’s Index

To compute Hill’s first diversity number N,, Shannon’s index H ’ is needed. This index is based on information theory (SHANNON & WEAVER, 1949) and is a measure o f the average degree of uncertainty in predicting to what species an individual chosen at random will belong. The equation for the Shannon function:

s’

(Eq. 3.5) J=1

where S*\ the total number of species in the population with known proportional abundances )

N: total individuals in the population

M,: number o f individuals belonging to the f-th species

This equation assumes that all species o f the population are represented in the sample. As the studied samples are always a limited part taken from this population in which not all the individuals o f the population occur (a population is the entire collection o f individual organisms that are potentially observable in an ecological community; L U D W iG & R E Y N O L D S , 1988), S* is normally greater than 5, the species observed in the sample. However, if n, the number of individuals in the sample, is taken large enough, this difference is small and H ’ is estimated from a sample as:

^ = - Z

/=i

(Eq. 3.6)

where n: total number o f individuals in the sample (n< N )

number o f individuals belonging to the f-th species

S: total number o f species observed

This estimator o f Shannon's index is used to calculate Hill’s first diversity number N;.

3 J.2 .4 Simpson’s Index

To compute Hill’s second diversity number N j, Simpson’s index À is needed (SiMPSON, 1949).

This index gives the probability that two individuals drawn at random from a population belong to the same species. If this probability is high, this means that the diversity o f the community in the sample is low.

The equation for Simpson’s index:

s

^ = (Eq.3.7)

1 = 1

where S’: the total number o f species in the population

Pi', proportional abundance of i-th species (p, = )

N'. total individuals in the population

number of individuals belonging to the i-th species

Again, À is to be estimated, since it is impossible to count all individuals and species from a

population in a sample. Therefore, Simpson’s index is to be estimated:

^ =

(Eq. 3.8)

where n: total number o f individuals in the sample { n<N) nf. number of individuals belonging to the f-th species

S: total number o f species observed

This estimator of Simpson's index is used to calculate H ill’s second diversity number N^.

3.3.2.5 Evenness Indices

Evenness, or equitability, refers to how the species abundances are distributed among the species. Like for the richness index, an evenness index should be independent of the number of species observed in the sample. LUDWiG & REYNOLDS (1988) found that one evenness index

remains relatively unaffected by species richness: the H ill’s ratio, E, (Equation 3.9). This ratio shows the proportions o f abundant to very abundant species in the sample.

e " " n , where N,: Hill’s first diversity number

N;: Hill’s second diversity number

This equation is a simple ratio o f Hill’s second and first diversity numbers and tends towards value 1 when one species dominates the community. Therefore, preference goes to the modified H ill’s ratio, E; (Equation 3.10).

_____________________________________________________________________________________________3.3.2 Diversity

In the case one species dominates, the modified Hill’s ratio tends towards zero, which is the most desirable property for an evenness index.

where N;i Hill’s first diversity number Nj: Hill’s second diversity number

Diversity indices combine both species richness and species evenness into one single value. Interpretation o f this value may be ambiguous. It may result from various different combinations o f species richness and evenness, resulting in similar values. For example, the same diversity value may be obtained for a community with low richness and high evenness as for a community with high richness and low evenness. By studying just the diversity index values, it is impossible to determine what the relative importance is o f the species richness and evenness. Therefore, it is necessary to study all three values (diversity, richness and evenness) for interpretative purposes.

For each separate foraminiferal group (i.e. planktonic, calcareous benthic, agglutinated and total benthic foraminifera), the various indices for richness (species density, numerical species richness and No, number o f species present), evenness (E^) and diversity (Hill's 1st and 2nd diversity numbers N/ and N^) are calculated for each sample. The results are shown in Figures 3.11-3.26 (p. 93-108). Rarefaction is not applied in this study, because o f its limiting conditions. For intervals with low abundances o f calcareous benthic and planktonic foraminifera, there are problems with the calculation o f evenness, and Hill's 1st and 2nd diversity numbers (E2, N/ and

N;). In these intervals, the values are not conform the condition No > N; > N^, or the indices can simply not be calculated. Therefore, only for the calcareous foraminiferal rich intervals (i.e. with a high %plankton), diversity and related indices are studied for calcareous benthic and planktonic foraminifera.

In all four wells, the top parts o f the sections are characterised by almost barren samples (indicated with grey shadings in Figs 3.11-3.26, p. 93-108), with only few species present. Only the lower part o f the Palaeocene interval in wells 205/10-2B and 208/22-1 yielded a significant benthic foraminiferal richness (depth intervals 7100 - 8060 ft. and 5400 - 7140 ft. respectively, see Figs 3.11 and 3,14, p. 93 and 96). Just below the almost barren interval, these two wells show a very distinct interval o f high foraminiferal species richness (N^ and species density), mainly contributed by agglutinated foraminifera (7100 - 7310 ft. in well 205/10-2B and 5880 - 6120 ft. in well 208/22-1, see Figs 3.11 and 3.14, p. 93 and 96). This high species richness event

is coeval with high abundances of agglutinated foraminifera (see Figs 3.7 and 3.10, p. 82 and 85). However, there is no change in the numerical species richness and evenness index with respect to the interval below (7310 - 8060 ft. in well 205/10-2B and 5400 - 7140 ft. in well 208/22-1, see Figures 3.11 and 3.14, p. 93 and 96). This indicates that the abundant interval with high species richness (N^) is not different in its species-individual distribution compared to the succession below, producing an equivalent number o f species per sample with similar evenness. The higher values of the diversity indices Ny and are the result o f the high number o f species (N^) present, which in its turn is the result from the high number o f specimens present in the interval. By comparing the plots of agglutinated foraminiferal species richness and species density with the equivalent agglutinated foraminiferal abundance plots, it is noticed at once that the trends are similar throughout the studied intervals (Figs 3.7-3.14, p. 82-96). This means that in this study the number of species in the sample is directly related to the number of specimens in the sample. Therefore, the numerical species richness is considered to be the best richness index, although this is only valid for intervals with abundant specimens present. For example, the numerical species richness for a sample with two specimens divided over two species is much higher than for a sample with 100 species per 300 specimens (1 and 0.33 respectively).

A drop in agglutinated foraminiferal diversity in wells 205/10-2B and 208/22-1 is noticed across the Base Tertiary unconformity (at 8223 ft. and 7140 ft. respectively; Figs 3.7 and 3.14, p. 82 and 96). Fewer agglutinated species and fewer abundant species are present (lower N^, N/ and N; values), while evenness is comparable to the Palaeocene samples. In the calcareous foraminiferal rich intervals (indicated by high %plankton; Figs 3.15-3.18 and 3.23-3.26, p. 97- 100 and 105-108), both calcareous benthic and planktonic species richness is high. Despite the comparatively low abundance o f calcareous benthic foraminifera, numerous species are present. Planktonic foraminiferal richness is comparatively low. During the uppermost calcareous foraminiferal rich interval, there are more calcareous benthic and planktonic species present than in the two lower calcareous foraminiferal rich intervals, while evenness values are comparable.

There is an increase in the number of agglutinated foraminiferal species (N^) fi-om the base of the second calcareous flux downhole (from 9680 ft., 6520/6540 ft. and 5550 ft. for respectively wells 205/10-2B, 206/3-1 and 206/5-1; Figs 3.11-3.14, p. 93-96). This increase coincides with a decrease in numerical species richness towards low values. Subsequently, there is a decrease in number o f agglutinated species (N@) towards the mid-Campanian unconformity (at 7240 ft. in well 206/3-1 and at 6250 ft. in well 206/5-1), followed by a downhole increase (Figs 3.12-3.13, p. 94-95). The pattern observed in the number o f agglutinated species (N@) is coeval with the fluctuation in agglutinated foraminiferal abundance (Figs 3.7-3.10, p. 82-85), suggesting they

_____________________________________________________________________________________________3.3.2 Diversity

are closely related. The numerical species richness however, indicates a constant low species richness (Figs 3.11-3.14, p. 93-96). Also from the base o f the second calcareous foraminiferal flux downhole (from 9680 ft., 6520/6540 ft. and 5550 ft. for respectively wells 205/10-2B, 206/3-1 and 206/5-1, there is a decrease in equitability (lower evenness values) and a decrease in number of abundant and very abundant agglutinated species (N/ and N^) (Figs 3.11-3.13, p. 93-95). This indicates that few species dominate the samples in the lower part of the sections, while only a small number o f other species are present with comparable abundances.

Also plotted is the number o f genera o f each separate foraminiferal group present in the samples (Figs 3.11-3.26, p. 93-108). They follow the same pattern as for the number o f species present

(No). This indicates that the species are more or less evenly distributed amongst the genera, without a significant dominance o f one genus. Between the four studied wells there are no major differences observed in richness, eveimess or diversity for each foraminiferal group.

To conclude: the number o f species and genera present in a sample is strongly related to the number o f specimens present. The high abundance interval during the Palaeocene appears to have comparable diversity with respect to other Palaeocene samples below. In the Upper Cretaceous part o f the succession, samples in the three calcareous foraminiferal rich intervals contain a highly diverse calcareous benthic foraminiferal fauna with a moderately diverse planktonic foraminiferal assemblage. From the base o f the second calcareous foraminiferal rich interval downhole towards the bottom of the studied sections, agglutinated foraminiferal diversity is decreasing.

6500 - O ^ 8500 11500 7000 — C 9000 0 0 9500

^

10000 10500 11000

Figure 3.11 Distribution o f richness, evenness and diversity o f agglutinated foraminifera in well 205/10-2B, plotted versus depth (in ft.). Plotted are respectively species density, numerical species richness, Hill's diversity numbers N@, N, and eveimess, genus richness and %plankton. Grey shadings indicate interval which is almost barren (6260 - 7100 ft), where no samples are available (8060-8480 ft), or dolerite dykes occur (10820-11090 ft). Depth o f Base Tertiary unconformity (8223 ft.) is taken ftom the lithostratigraphy.

species density num. richness No, N„ N. evenness genus richness %plankton

0.0 0.5 1.0 1.5 0.0 0.1 0.2 0.3 0.4 0 10 20 30 40 0.0 0.2 0.4 0.6 0.8 0 10 20 30 0 20 40 60 80

5000 -

c 6500

1

Figure 3.12 Distribution of richness, evenness and diversity of agglutinated foraminifera in well 206/3-1, plotted versus depth (in ft.). Plotted are respectively species density, numerical species richness, Hill's diversity numbers N^, N, and N^, evenness, genus richness and %plankton. Grey shading indicates interval which is almost barren (4970 - 5850 ft.). Depths o f Base Tertiary unconformity (5890 ft.) and mid-Campanian unconformity (7240 ft.) are taken from the lithostratigraphy.

4500 -

5500 -

E 6000

6500 -

Figure 3.13 Distribution of richness, evenness and diversity of agglutinated foraminifera in well 206/5-1, plotted versus depth (in ft.). Plotted are respectively species density, numerical species richness, Hill's diversity numbers N„, N, and N^, evenness, genus richness and %plankton. Grey shading indicates interval which is almost barren (4400-4950 ft). Depths of Base Tertiary unconformity (4895 ft.) and mid-Campanian unconformity (6250 ft.) are taken from the lithostratigraphy.

species density num. richness N^, N„ evenness genus richness %plankton

0.0 0.5 1.0 1.52.0 0.0 0.1 0.2 0.3 0.40 20 40 600.0 0.4 0.8 1.20 10 20 30 40 0 10 20 30 40

5000 -

6500

7500

Figure 3.14 Distribution of richness, evenness and diversity of agglutinated foraminifera in well 208/22-1, plotted versus depth (in ft.). Plotted are respectively species density, numerical species richness, Hill's diversity numbers N®, N, andN^, evenness, genus richness and %plankton. Grey shading indicates interval which is almost barren (4800 - 5400 ft.). Depth of Base Tertiary unconformity (7140 ft.) is taken from the hthostratigraphy.

6500 - is 10000 10500 11000 11500 - VO

Figure 3.15 Distribution o f richness, evenness and diversity o f calcareous benthic foraminifera in well 205/10-2B, plotted versus depth (in ft.). Plotted are respectively species density, numerical species richness, Hill's diversity numbers N^, N, and N„ evenness, genus richness and %plankton. Grey shadings indicate interval which is almost barren (6260 - 7100 ft.), where no samples are available (8060-8480 ft), or dolerite dykes occur (10820-11090 ft). Depth o f Base Tertiary unconformity (8223 ft.) is taken from the lithostratigraphy.

species density num. richness No, N„ N. evenness genus richness %plankton 0.0 0.5 1.0 1.52.0 0.0 0.4 0.8 0 10 20 30 40 50 0.0 0.4 0.8 1.2 0 10 20 30 0 20 40 60 80

i

(D .0) & ■o 5000 — 5500 — 6000 6500 7000 7500 - 8000 00

Figure 3.16

D istribution o f richness, evenness and diversity o f calcareous benthic foram inifera in well 206/3-1, plotted versus depth (in ft.). Plotted

are respectively species density, num erical species richness. H ill's diversity numbers N^, N; and Nj, evenness, genus richness and % plankton. Grey shading indicates interval w hich is alm ost barren (4970 - 5850 ft.). D epths o f Base Tertiary unconform ity (5890 ft.) and m id-Cam panian unconform ity (7240 ft.) are taken from the lithostratigraphy.

4500 -

^ 5500

“ 6000

I

Figure 3.17 Distribution o f richness, evenness and diversity o f calcareous benthic foraminifera in well 206/5-1, plotted versus depth (in ft.). Plotted are respectively species density, numerical species richness. Hill's diversity numbers Nj and Nj, evenness, genus richness and %plankton. Grey shading indicates interval which is almost barren (4400 - 4950 ft). Depths of Base Tertiary unconformity (4895 ft.) and mid-Campanian unconformity (6250 ft.) are taken from the lithostratigraphy.

species density num. richness No, N„ evenness genus richness %plankton 0.0 0.2 0.4 0.6 0.0 0.4 0.8 0 5 10 15 20 0 1 2 3 0 2 4 6 8 10 0 10 20 30 40 5000 — 6000 - 7500 8

Figure 3.18 Distribution of richness, evenness and diversity of calcareous benthic foraminifera in well 208/22-1, plotted versus depth (in ft.). Plotted are respectively species density, numerical species richness. Hill's diversity numbers N^, N, andN„ evenness, genus richness and %plankton. Grey shading indicates interval which is almost barren (4800 - 5400 ft.). Depth of Base Tertiary unconformity (7140 ft.) is taken from the lithostratigraphy.

CO

3

s CN

1

6500 - 7000 - 7500 8000 8500 9000 9500

£

10000 0 ID 10500 11000 11500

i i i i n

4

Figure 3.19 Distribution o f richness, evenness and diversity o f total benthic foraminifera in well 205/10-2B, plotted versus depth (in ft.). Plotted are respectively species density, numerical species richness. Hill's diversity numbers N^, N, and N^, evenness and %plankton. Grey shadings indicate interval which is almost barren (6260 - 7100 ft.), where no samples are available (8060-8480 ft), or dolerite dykes occur (10820-11090 ft). Depth o f Base Tertiary unconformity (8223 ft.) is taken from the lithostratigr^hy.

species density num. richness N^, N„ evenness %plankton 3 0 .0 0 .2 0 .4 0 2 5 5 0 7 5 0 . 0 0 .4 0 .8 0 2 0 4 0 6 0 8 0

i

CM

I

c 0) 5 0 0 0 - 5 5 0 0 - 6 0 0 0 - 6 5 0 0 - 7 0 0 0 - 7 5 0 0 8 0 0 0

Figure 3.20 Distribution of richness, evenness and diversity of total benthic foraminifera in well 206/3-1, plotted versus depth (in ft.). Plotted are respectively species density, numerical species richness. Hill's diversity numbers N, and N^, evenness and %plankton. Grey shading indicates interval which is almost barren

In document Estudio de los efectos de la inyección de agua en los yacimientos U y T de la formación Napo del Campo Sacha (página 169-177)