Researchers from the University of Granada (UGR) have compared the
disaster caused by the Aznalcóllar spillage in the Doñana National Park
in Andalusia 11 years ago with the biggest species extinction known to
date. What do these two disasters have in common? The scientists say
that carrying out comparisons of this kind will make it possible to find
out how ecosystems recover following mass extinctions.
Until now, scientists used to study the fossil record in order to
analyse how organisms responded to major environmental changes in the
past, such as the mass extinction of species during the Cretaceous
period (65 million years ago) and their subsequent recovery.
Now a team of scientists from the UGR has proposed a different
methodology: "Another way of looking at this issue is to compare present
day disasters that have also caused an abrupt ecological change, and
which have therefore also had a major impact on organisms," says
Francisco Javier Rodríguez-Tovar, lead author of the study and a
researcher at the UGR’s Department of Stratigraphy and Palaeontology.
The study, published recently in the journal Geobiology, was
based on "one of the worst environmental disasters to have happened in
Spain over recent decades."
The pyrite mine at Aznalcóllar, in the Doñana National Park, burst on
25 April 1988, spilling four million cubic metres of acidic water and
one million cubic metres of waste material containing high levels of
toxic compounds, which affected more than 4,500 hectares of the rivers
Agrio and Guadiamar and the land around them.
The researchers carried out a detailed analysis of how the pollution
from Aznalcóllar evolved, and how the local plant and animal communities
responded following the event, by studying the affected soil.
"Comparing this with what happened 65 million years ago could help to
better interpret this past event," explains Rodríguez-Tovar.
The similarities are obvious — sudden impact, high levels of toxic
compounds, and the existence of a polluted layer covering the affected
area. However, the scientist also points out some of the most
significant differences, such as recovery following the impact, which
was "much faster after the disaster at Aznalcóllar," and in terms of the
area affected, which was "global for the Cretaceous-Tertiary boundary
event," says Rodríguez-Tovar.
In search of signs of life
The scientists were able to carry out a range of experiments on the
layers of mud that have not been removed from Doñana. Geochemical
analysis showed that "there is still significant contamination, with
high concentrations of toxic elements, and high acidity levels,"
stresses the palaeontologist. However, less than 10 years after the
disaster, the scientists could identify trails and nests made by
Tapinoma nigérrima, an aggressive and opportunistic species of ant. "We
even found this ant’s larvae just below the layer of highly-contaminated
mud," explains the expert.
This ant’s opportunism, aggressiveness and high levels of
independence were compared with the organism that created Chondrites, a
trace fossil that scientists have recorded near the red layer associated
with the Chixulub crater in Mexico, generated by the impact of the
meteorite that caused the Cretaceous-Tertiary extinction. Previous
ichnological studies (on trace fossils) have shown that "the
Chondrites-generating organism was able to inhabit the substrate
immediately after the event, due to its opportunistic and independent
nature," says Rodríguez-Tovar.
Using the data on trace fossils and on comparisons with present day
disasters, the scientists were able to prove that "the community started
to recover fairly rapidly following the mass extinction caused by the
impact 65 million years ago, possibly within hundreds or thousands of
years," concludes the palaeontologist.