Prerequisites for Geoengineering Earth's Climate
Geoengineering -- deliberate manipulation of the Earth's climate to slow or reverse global warming -- has
gained a foothold in the climate change discussion. But before effective action can be taken, the Earth's
natural biogeochemical cycles must be better understood.
Volcanoes Driving the Carbon Cycle
Two Northwestern University studies, both published online recently by Nature Geoscience, contribute new --
and related -- clues as to what drove large-scale changes to the carbon cycle nearly 100 million years ago.
Both research teams conclude that a massive amount of volcanic activity introduced carbon dioxide and sulfur
into the atmosphere, which in turn had a significant impact on the carbon cycle, oxygen levels in the oceans
and marine plants and animals.
Oxygen Level Clues from Carbon-Rich Sediments
Both teams studied organic carbon-rich sediments from the Western Interior Seaway, an ancient seabed stretching
from the Gulf of Mexico to the Arctic Ocean, to learn more about a devastating event 94.5 million years ago
when oxygen levels in the oceans dropped so low that one-third of marine life died.
The authors of the first paper, titled "Volcanic triggering of a biogeochemical cascade during Oceanic Anoxic
Event 2," reveal that before oxygen levels dropped so precipitously there was a massive increase in oceanic
sulfate levels. Their conclusion is based on analyses of the stable isotopes of sulfur in sedimentary minerals
from the central basin of the Western Interior Seaway, located in Colorado.
A Cascade of Biogeochemical Events
The researchers theorize that a massive amount of volcanic activity caused this sulfate spike, which triggered a
cascade of biogeochemical events. More sulfate led to an abundance of the nutrient phosphorous, which allowed
phytoplankton populations in the oceans to multiply. The phytoplankton thrived and then died. Their decomposing
bodies depleted oxygen levels in the oceans, leading to the widespread death of marine animals.
The sedimentary burial of marine organic carbon during this event was so large, some prior studies hypothesized that
it caused a decrease in atmospheric carbon dioxide levels. In the second Nature Geoscience paper, titled "Carbon
sequestration activated by a volcanic carbon dioxide pulse during ocean anoxic event 2," the researchers tested the
carbon dioxide drawdown prediction. By studying fossil plant cuticle material, they determined the amount of carbon
dioxide in the atmosphere at the time the plants were growing. (The cuticle samples were collected from sites
representing the western shore of the Western Interior Seaway, in present-day southwestern Utah.)
Ocean Anoxia and Atmospheric Carbon Dioxide
This work found that before the onset of ocean anoxia, the level of carbon dioxide in the atmosphere increased by
approximately 20 percent. This significant increase is consistent with the volcanic activity invoked by the first
Northwestern study (described above). The paleo-carbon dioxide reconstruction also detected two episodes of marked
decrease in carbon dioxide levels -- up to 200 parts per million -- at the time of the early phase of marine carbon
burial. This observation provides strong support for the carbon dioxide drawdown hypothesis.
"Our research highlights the previously unappreciated role that the sulfur cycle plays in regulating nutrient cycling,
the carbon cycle and climate," said Matthew Hurtgen, assistant professor of Earth and planetary sciences in the Weinberg
College of Arts and Sciences at Northwestern and lead researcher of the first study.
Accelerating the Modern Carbon Cycle
"These two complementary studies provide a much clearer picture of how the Earth's carbon cycle was dramatically affected
by catastrophic natural events long ago," said Bradley Sageman, professor and chair of Earth and planetary sciences at
Northwestern and a co-author of both papers. "Although these events played out over hundreds or thousands of years, the
magnitude of the changes, in carbon dioxide levels for example, are similar to those of the last 150 years resulting from
human influence on the carbon cycle. The evidence demonstrates that the modern carbon cycle has been accelerated by orders
The Author Team
The sulfur work reported in the paper "Volcanic triggering of a biogeochemical cascade during Oceanic Anoxic Event 2" was
conducted by Derek D. Adams, a doctoral candidate in Hurtgen's research group. Adams is first author of the paper; Hurtgen
and Sageman are the paper's other authors.
Richard S. Barclay, a doctoral candidate in Sageman's research group, is the first author of the "Carbon sequestration
activated by a volcanic carbon dioxide pulse during ocean anoxic event 2" paper. Sageman also is an author, and the third
author is Jennifer McElwain, a professor from University College Dublin who co-advises Barclay's research and is one of the
originators of the cuticle analysis method.
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|An 18 km-high volcanic plume from one of a series of explosive eruptions of Mount Pinatubo beginning on
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|The Modern Carbon Cycle: In any given year, tens of billions of tons of carbon move between the atmosphere, hydrosphere, and geosphere. Human activities add about 5.5 billion tons per year of carbon dioxide to the atmosphere. The illustration above shows total amounts of stored carbon in black, and annual carbon fluxes in purple. The human inputs have accellerated the carbon cycle to many times the pre-human rate. (NASA image.)