In 1980, Luis Alvarez and his collaborators stunned the world with their
discovery that an asteroid impact 65 million years ago probably killed
off the dinosaurs and much of the the world’s living organisms. But ever
since, there has been an ongoing debate about how long it took for life
to return to the devastated planet and for ecosystems to bounce back.
Now, researchers from MIT and their collaborators have found that at
least some forms of microscopic marine life — the so called "primary
producers," or photosynthetic organisms such as algae and cyanobacteria
in the ocean — recovered within about a century after the mass
extinction. Previous research had indicated the process might have taken
millions of years.
It has taken so long to uncover the quick recovery because previous
studies looked mostly at fossils in the layers of sediment from that
period, and apparently the initial recovery was dominated by tiny,
soft-bodied organisms such as cyanobacteria, which do not have shells or
other hard body parts that leave fossil traces. The new research looked
instead for "chemical fossils" — traces of organic molecules (compounds
composed of mostly carbon and hydrogen) that can reveal the presence of
specific types of organisms, even though all other parts of the
organisms themselves are long gone.
The new research, published in the Oct. 2 issue of Science,
was led by Julio Sepúlveda, an MIT postdoc who carried out part of the
work while still a graduate student at the University of Bremen,
Germany, and MIT Professor of Geobiology Roger Summons, among others.
The team had two major advantages that helped to make the new
findings possible. One was a section of the well-known cliff face at
Stevns Klint, Denmark, that happens to have an unusually thick layer of
sediment from the period of the mass extinction — about 40 centimeters
thick, compared to the few cm thickness of the layers that Alvarez
originally studied from that period at Gubbio (Italy) and Stevns Klint
(Denmark). And team members tapped one of the most powerful Gas
Chromatograph-Mass Spectrometers (GC-MS) in the world, a device that can
measure minute quantities of different molecules in the rock. MIT’s
advanced GC-MS is one of only a few such powerful instruments currently
available at U.S. universities.
When people look at microfossils in the sediments from the period but
are unable to detect the chemical biomarkers with the level of
sensitivity the MIT team was able to achieve, they "miss a big part of
the picture," Sepúlveda says. "Many of these microorganisms" that were
detected through molecular signatures "are at the base of the food
chain, but if you don’t look with biochemical techniques you miss them."
The analysis clarified the sequence of events after the big impact.
Immediately after the impact, certain areas of the ocean were devoid of
oxygen and hostile to most algae, but close to the continent, microbial
life was inhibited for only a relatively short period: in probably less
than 100 years, algal productivity showed the first signs of recovery.
In the open ocean, however, this recovery took much longer: previous
studies have estimated that the global ocean ecosystem did not return to
its former state until 1 to 3 million years following the impact.
Because of the rebound of primary producers, Sepúlveda says "very
soon after the impact, the food supply was not likely a limitation" for
other organisms, and yet "the whole ecology of the system remained
disrupted" and took much longer to recover.
The findings provide observational evidence supporting models
suggesting that global darkness after the impact was rather short.
"Primary productivity came back quickly, at least in the environment we
were studying," says Summons, referring to the near-shore environment
represented by the Danish sediments.
"The atmosphere must have cleared up rapidly," he says. "People will
have to rethink the recovery of the ecosystems. It can’t be just the
lack of food supply" that made it take so long to recover.
The team hopes to be able to study other locations with relatively
thick deposits from the extinction aftermath, to determine whether the
quick recovery really was a widespread phenomenon after the mass
extinction.
These findings seem to rule out one theory about how the global
ecosystem responded to the impact, which held that for more than a
million years there was a "Strangelove ocean" — a reference to the
post-apocalyptic scenario in the movie Dr. Strangelove — in which all
the primary producers remained absent for a prolonged period, Summons
says.
In addition to Sepúlveda and Summons, the work was carried out by
Jens Wendler of the Friedrich-Schiller University in Jena, Germany, and
Kai-Uwe Hinrichs of the University of Bremen. The work was funded by the
DFG, European Graduate College Europrox and the NASA Astrobiology and
Exobiology Programs.
Source : Massachusetts
Institute of Technology
