According to a latest study in the journal Nature, The cyclic
Earth’s wobble on its axis controls the production of a nutrient extremely necessary
to the health of the ocean. The factor’s discovery that control this nutrient,
known as "fixed" nitrogen, gives researchers insight into how the
ocean regulates its own system that
supports life, turn in this cycle affects Earth's climate and the size of
marine fisheries. Researchers from Princeton University and the Swiss Institute
of Technology in Zurich (ETH) reports:
“during the past 160,000 years nitrogen fixation rose and
fell in a pattern that closely matched the changing orientation of Earth's axis
of rotation, or axial precession. Axial precession occurs on a cycle of roughly
26,000 years and arises because Earth wobbles slightly as it rotates, similar
to the wobble of a toy top. Studies from the 1980s revealed that precession
leads to a regular upwelling of deep water in the equatorial Atlantic Ocean
roughly every 23,000 years. The upwelling in turn brings nitrogen-poor water to
the surface where blue-green algae convert nitrogen drawn from the air into a
form that is biologically usable.”
Scientist had fined that nitrogen fixation is determined by
precession-driven upwelling appears to indicate that the ocean's fixed nitrogen
reservoir seems flexible a
nd that the ocean biosphere can recover from even the
most dramatic ecological changes, said second author Daniel Sigman, Princeton's
Dusenbury Professor of Geological and Geophysical Sciences.
"By studying the response of nitrogen fixation to
different environmental changes in the Earth's past, we have found connections
that may ensure that the ocean's fixed nitrogen level will always
rebound," Sigman said. "This suggests that an ocean over time has a
relatively stable nutrient reservoir, and thus stable productivity."
The rise of deep water spurs nitrogen fixation because that
water is low in nitrogen but contains an excess of another key nutrient,
phosphorus, Sigman said. The phosphorus fuels the fixing of nitrogen carried
out by blue-green algae, also known as cyan bacteria.
"The phosphorus-rich, nitrogen-poor water is a boon to
cyanobacteria that can fix their own nitrogen," Sigman said. "By
growing more rapidly, the nitrogen-fixers 'top up' the fi
xed nitrogen to the
levels needed by other phytoplankton."
Sigman collaborated on the study with Princeton graduate student
Mathis Hain; first-author Marietta Straub, Alfredo Martínez-García, A. Nele
Meckler and senior author Gerald Haug, all in the Department of Earth Sciences
at ETH; and Haojia Ren of the Columbia University Lamont-Doherty Earth
Observatory.
The researchers fined changes in nitrogen fixation in the
North Atlantic Ocean by measuring the fixed nitrogen contained in the shells of
marine animals recovered from sediment in the Caribbean Sea. Working in
Sigman's lab, the investigators measured the amount of two types of nitrogen
known as 14N and 15N contained in the shells of tiny marine animal plankton
called foraminifera. The ratio of 15N to 14N was then used to reconstruct the
rate of nitrogen fixation.
The pattern of nitrogen fixation measured in foraminifera
matched the historical record of axial precession and the resulting ocean
upwelling. The investigators also compared the fluctuations in nitrogen
fixation to historical records of water temperature and levels of iron --
another crucial nutrient -- both of which influence cyanobacteria survival and
thus nitrogen fixation. No correlation was found.
"Our findings suggest that this upwelling was the
dominant influence on nitrogen fixation," Sigman said.
Douglas Capone, a professor and chair of biological sciences
at the University of Southern California, said that the research is notable
both for understanding the nitrogen cycle and for providing a method to study
it.
"I have long pondered and hoped for ways to reconstruct
deeper historical trends in this important nitrogen-cycle process," Capone
said. "The new study by the Sigman and Haug groups is a major breakthrough
in providing a means to do this along with throwing light on the major forces
of this key process over long time scales."
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