380
23.3.2 Impacts
Impacts on density and diversity of benthic foraminifera were measured on a transect, west from the Ixtoc 1 wellhead. The reduction in density and diversity observed
in the southern GoM records during and following Ixtoc 1 was not to the same
degree as the impacts observed in the northern GoM after DWH, which is likely due
to preservation effects.
A decrease in density (42–63%) in the Ixtoc 1 layer (IL, dated using excess
210
Pb)
relative to the pre-Ixtoc down-core mean was observed at all sites (Fig. 23.4). The
largest decrease in density was in core IXW250. Density values were below pre-IL
mean at, and after, the IL in core IXW250. Density decreased in core IXW500 at the
IL, followed by a continuous increase for the next three increments, where it reached
down-core mean values. However, density did not reach pre-IL values. After the
decrease in density at the onset of the IL in core IXW750, there was an immediate
increase to above pre-IL and down-core mean values (probably due to tolerant or
opportunistic species that preyed on the available carbon). Therefore, the degree of
impact in the area seems to be correlated with distance from the wellhead.
Benthic foraminiferal diversity exhibited a different behavior in the three sites. In
the shallowest site (IXW250), there was a slight increase in species richness (from
14 to 17) and heterogeneity (from 3.0 to 3.5) at the onset of the event. These
increases were possibly caused by an increase in organic carbon that initially provided more food for tolerant or opportunistic species. After this event, species richness slowly increased to down-core mean values. All of these variations were within
the variability of the parameter in the core. There was a similar trend in diversity (S
and H) in core IXW500 to that of density and a similar behavior to that observed in
the northern GoM. Both indices decreased at the onset of the IL (H value was within
the down-core variability of the parameter, but S was the lowest of all the studied
sections) and increased immediately following the IL. Species richness was not
affected at the IL in core IXW750 but increased abruptly following. In general,
density and diversity values did not vary considerably throughout the cores.
Machain-Castillo et al. (unpublished) suggest that the southern GoM has been under
natural and/or anthropogenic oil seepage influence at least during the last 100 years
and that multiple episodes of oil contamination were probable in the area.
There were three types of δ
13
C records present in the southern GoM with respect
to preservation of the Ixtoc 1 spill: (1) no depletion in δ
13
C CaCO3 present in IL, (2) a
depletion in δ
13
C CaCO3 present in IL but also within natural variability (similar depletion present in pre-IL and post-IL), and (3) a depletion in δ
13
C CaCO3 present in the IL
but beyond natural variability (Fig. 23.5; Schwing et al. unpublished). The third
category, which is most consistent with long-term preservation of Ixtoc 1, included
cores IXW750 and IXN750.
P. T. Schwing and M. L. Machain-Castillo
23.3.2 Impacts
Impacts on density and diversity of benthic foraminifera were measured on a transect, west from the Ixtoc 1 wellhead. The reduction in density and diversity observed
in the southern GoM records during and following Ixtoc 1 was not to the same
degree as the impacts observed in the northern GoM after DWH, which is likely due
to preservation effects.
A decrease in density (42–63%) in the Ixtoc 1 layer (IL, dated using excess
210
Pb)
relative to the pre-Ixtoc down-core mean was observed at all sites (Fig. 23.4). The
largest decrease in density was in core IXW250. Density values were below pre-IL
mean at, and after, the IL in core IXW250. Density decreased in core IXW500 at the
IL, followed by a continuous increase for the next three increments, where it reached
down-core mean values. However, density did not reach pre-IL values. After the
decrease in density at the onset of the IL in core IXW750, there was an immediate
increase to above pre-IL and down-core mean values (probably due to tolerant or
opportunistic species that preyed on the available carbon). Therefore, the degree of
impact in the area seems to be correlated with distance from the wellhead.
Benthic foraminiferal diversity exhibited a different behavior in the three sites. In
the shallowest site (IXW250), there was a slight increase in species richness (from
14 to 17) and heterogeneity (from 3.0 to 3.5) at the onset of the event. These
increases were possibly caused by an increase in organic carbon that initially provided more food for tolerant or opportunistic species. After this event, species richness slowly increased to down-core mean values. All of these variations were within
the variability of the parameter in the core. There was a similar trend in diversity (S
and H) in core IXW500 to that of density and a similar behavior to that observed in
the northern GoM. Both indices decreased at the onset of the IL (H value was within
the down-core variability of the parameter, but S was the lowest of all the studied
sections) and increased immediately following the IL. Species richness was not
affected at the IL in core IXW750 but increased abruptly following. In general,
density and diversity values did not vary considerably throughout the cores.
Machain-Castillo et al. (unpublished) suggest that the southern GoM has been under
natural and/or anthropogenic oil seepage influence at least during the last 100 years
and that multiple episodes of oil contamination were probable in the area.
There were three types of δ
13
C records present in the southern GoM with respect
to preservation of the Ixtoc 1 spill: (1) no depletion in δ
13
C CaCO3 present in IL, (2) a
depletion in δ
13
C CaCO3 present in IL but also within natural variability (similar depletion present in pre-IL and post-IL), and (3) a depletion in δ
13
C CaCO3 present in the IL
but beyond natural variability (Fig. 23.5; Schwing et al. unpublished). The third
category, which is most consistent with long-term preservation of Ixtoc 1, included
cores IXW750 and IXN750.
P. T. Schwing and M. L. Machain-Castillo