Geological cycle causes biodiversity booms and busts every 60 million years, research suggests
A
mysterious cycle of booms and busts in marine biodiversity over the
past 500 million years could be tied to a periodic uplifting of the
world’s continents, scientists report in the March issue of The Journal of Geology.
The
researchers discovered periodic increases in the amount of the isotope
strontium-87 found in marine fossils. The timing of these increases
corresponds to previously discovered low points in marine biodiversity
that occur in the fossil record roughly every 60 million years. Adrian
Melott, a Professor of Physics and Astronomy at the University of Kansas
and the study’s lead author, thinks these periodic extinctions and the
increased amounts Sr-87 are linked.
“Strontium-87
is produced by radioactive decay of another element, rubidium, which is
common in igneous rocks in continental crust,” Melott said. “So, when a
lot of this type of rock erodes, a lot more Sr-87 is dumped into the
ocean, and its fraction rises compared with another strontium isotope,
Sr-86.”
An uplifting of the continents, Melott explains, is the most likely explanation for this type of massive erosion event.
“Continental
uplift increases erosion in several ways,” he said. “First, it pushes
the continental basement rocks containing rubidium up to where they are
exposed to erosive forces. Uplift also creates highlands and mountains
where glaciers and freeze-thaw cycles erode rock. The steep slopes cause
faster water flow in streams and sheet-wash from rains, which strips
off the soil and exposes bedrock. Uplift also elevates the deeper-seated
igneous rocks where the Sr-87 is sequestered, permitting it to be
exposed, eroded, and put into the ocean.”
The
massive continental uplift suggested by the strontium data would also
reduce sea depth along the continental shelf where most sea animals
live. That loss of habitat due to shallow water, Melott and
collaborators say, could be the reason for the periodic mass extinctions
and periodic decline in diversity found in the marine fossil record.
“What
we’re seeing could be evidence of a ‘pulse of the earth’ phenomenon,”
Melott said. “There are some theoretical works which suggest that
convection of mantle plumes, rather like a lava lamp, should be
coordinated in periodic waves.” The result of this convection deep
inside the earth could be a rhythmic throbbing—almost like a cartoon
thumb smacked with a hammer—that pushes the continents up and down.
Melott’s
data suggest that such pulses likely affected the North American
continent. The same phenomenon may have affected other continents as
well, but more research would be needed to show that, he says.
The
coauthors on the study were Richard Bambach of the National Museum of
Natural History, Kenni Petersen of Aarhus University, Denmark, and John
McArthur of University College London.
Citation:
Adrian L. Melott, Richard K. Bambach, Kenni D. Petersen, and John M.
McArthur, “A ~60 Myr periodicity is common to marine-87Sr/86Sr, fossil
biodiversity, and large-scale sedimentation: what does the periodicity
reflect?” The Journal of Geology 120:2 (March 2012, forthcoming).
