Fossil leaves show high atmospheric carbon spurred ancient 'global
greening'
A unique New Zealand deposit opens insights into how modern climate
change may proceed

Date:
August 20, 2020
Source:
Earth Institute at Columbia University
Summary:
Scientists studying leaves from a 23-million-year-old forest have
for the first time linked high levels of atmospheric carbon dioxide
with increased plant growth, and the hot climate off the time.



FULL STORY ========================================================================== Scientists studying leaves from a 23-million-year-old forest have for
the first time linked high levels of atmospheric carbon dioxide with
increased plant growth, and the hot climate off the time. The finding
adds to the understanding of how rising CO2 heats the Earth, and how
the dynamics of plant life could shift within decades, when CO2 levels
may closely mirror those of the distant past.


========================================================================== Scientists retrieved the leaves from a unique onetime New Zealand lake
bed that holds the remains of plants, algae, spiders, beetle, flies,
fungi and other living things from a warm period known as the early
Miocene. Scientists have long postulated that CO2 was high then, and some plants could harvest it more efficiently for photosynthesis. This is the
first study to show that those things actually happened in tandem. The
findings were published this week in the journal Climate of the Past.

"The amazing thing is that these leaves are basically mummified, so
we have their original chemical compositions, and can see all their
fine features under a microscope," said lead author Tammo Reichgelt,
an adjunct scientist at Columbia University's Lamont-Doherty Earth
Observatory and assistant professor of geosciences at the University
of Connecticut. "Evidence has been building that CO2 was high then, but
there have been paradoxes." The so-called "carbon fertilization effect"
has vast implications. Lab and field experiments have shown that when CO2 levels rise, many plants increase their rate of photosynthesis, because
they can more efficiently remove carbon from the air, and conserve water
while doing so. Indeed, a 2016 study based on NASA satellite data shows
a "global greening" effect mainly due to rising levels of human-made
CO2 over recent decades; a quarter to a half of the planet's vegetated
lands have seen increases in leaf volume on trees and plants since about
1980. The effect is expected to continue as CO2 levels rise.

This might seem like good news, but the reality is more complex. Increased
CO2 absorption will not come close to compensating for what humans are
pouring into the air. Not all plants can take advantage, and among those
who do, the results can vary depending on temperature and availability
of water or nutrients. And, there is evidence that when some major
crops photosynthesize more rapidly, they absorb relatively less calcium,
iron, zinc and other minerals vital for human nutrition. Because much of today's plant life evolved in a temperate, low-CO2 world, some natural
and agricultural ecosystems could be upended by higher CO2 levels,
along with the rising temperatures and shifts in precipitation they
bring. "How it plays out is anyone's guess," said Reichgelt. "It's
another layer of stress for plants. It might be great for some, and
horrible for others." The deposit is located in a small, long-extinct
volcanic crater now located on a farm near the southern New Zealand city
of Dunedin. The crater, about a kilometer across, once held an isolated
lake where successive layers of sediments built up from the surrounding environment. The feature was recognized only within about the last 15
years; scientists dubbed it Foulden Maar.

Recognizing it as a scientific gold mine, they have been studying it
ever since. Some have also been fighting an actual mining company that
wants to strip the deposit for livestock feed.



==========================================================================
In the new study, the researchers took samples from a 2009 drill core
that penetrated 100 meters to near the bottom of the now-dry lake
bed. Larded in between whitish annual layers of silica-rich algae that
bloomed each spring for 120,000 years are alternating blackish layers
of organic matter that fell in during other seasons. These include
countless leaves from a subtropical evergreen forest. They are preserved
so perfectly that scientists can see microscopic veins and stomata,
the pores by which leaves take in air and concurrently release water
during photosynthesis. Unlike most fossils, the leaves also retain their original chemical compositions. It is the only such known deposit in the Southern Hemisphere, and far better preserved than the few similar ones
known from the north.

The Miocene has long been a source of confusion for paleoclimate
researchers.

Average global temperatures are thought to have been 3 to 7 degrees C
hotter than today, and ice largely disappeared at the poles. Yet many
proxies, mainly derived from marine organisms, have suggested CO2 levels
were only about 300 parts per million-similar to those of preindustrial
human times, and not enough to account for such warming. With evidence
of high CO2 elusive, scientists have speculated that previous proxy measurements must be off.

Based on the new study and a related previous one also at Foulden Maar,
the researchers were able to get at this conundrum. They analyzed the
carbon isotopes within leaves from a half-dozen tree species found at
various levels in the deposit. This helped them zero in on the carbon
content of the atmosphere at the time. They also analyzed the geometry
of the leaves' stomata and other anatomical features, and compared these
with modern leaves. By combining all the data into a model, they found
that atmospheric CO2 was not 300ppm, but about 450-a good match for the temperature data. Second, they showed that the trees were super-efficient
at sucking in carbon through the stomata, without leaking much water
through the same route-a factor that all plants must account for. This
allowed them to grow in marginal areas that otherwise would have been
too dry for forests. The researchers say this higher efficiency was
very likely mirrored in forests across the northern temperate latitudes,
with their far greater landmasses.

Human emissions have now pushed CO2 levels to about 415 parts per million,
and they will almost certainly reach 450 by about 2040-identical to
those experienced by the Foulden Maar forest. Estimates of the resulting temperature increases over decades and centuries vary, but the new study suggests that most are in the ballpark.

"It all fits together, it all makes sense," said study coauthor William D'Andrea, a paleoclimate scientist at Lamont-Doherty. In addition to
showing how plants might react directly to CO2, "this should give us more confidence about how temperatures will change with CO2 levels," he said.

Study coauthor Daphne Lee, a paleontologist at New Zealand's University of Otago, led the charge to study Foulden Maar's rich ecosystem after it came
to light. More recently, she became an unexpected defender of the maar,
when a company with owners in Malaysia and the United Kingdom announced
plans to strip-mine the deposit for use as a feed additive for for pigs,
ducks and other intensively farmed animals. With many more discoveries
probably to be made, scientists were horrified, and allied themselves
with locals who feared noise and dust. The Dunedin city council is now
looking into buying the land to protect it.

The study was also coauthored by Aili'n del Valdivia-McCarthy, a former
intern at Lamont-Doherty; Bethany Fox of the University of Huddersfield; Jennifer Bannister of the University of Otago; John Conran of the
University of Adelaide; and William Lee of the University of Auckland.


========================================================================== Story Source: Materials provided by
Earth_Institute_at_Columbia_University. Original written by Kevin
Krajick. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Tammo Reichgelt, William J. D'Andrea, Aili'n del
C. Valdivia-McCarthy,
Bethany R. S. Fox, Jennifer M. Bannister, John G. Conran,
William G. Lee, Daphne E. Lee. Elevated CO2, increased
leaf-level productivity, and water-use efficiency during the
early Miocene. Climate of the Past, 2020; 16 (4): 1509 DOI:
10.5194/cp-16-1509-2020 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200820102444.htm

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