Lessons from a cooling climate
Geologists glean insights about how Earth went from a hothouse to an ice
age, and what that may mean for the future

Date:
September 29, 2020
Source:
University of California - Santa Barbara
Summary:
Usually, talk of carbon sequestration focuses on plants: forests
storing carbon in the trunks of massive trees, algae blooming
and sinking to the seabed, or perhaps peatlands locking carbon
away for tens of thousands of years. While it's true that plants
take up large amounts of carbon from the atmosphere, the rocks
themselves mediate a great deal of the carbon cycle over geological
timescales. Processes like volcano eruptions, mountain building
and erosion are responsible for moving carbon through Earth's
atmosphere, surface and mantle.



FULL STORY ========================================================================== Usually, talk of carbon sequestration focuses on plants: forests storing
carbon in the trunks of massive trees, algae blooming and sinking to the seabed, or perhaps peatlands locking carbon away for tens of thousands
of years.


========================================================================== While it's true that plants take up large amounts of carbon from the atmosphere, the rocks themselves mediate a great deal of the carbon
cycle over geological timescales. Processes like volcano eruptions,
mountain building and erosion are responsible for moving carbon through
Earth's atmosphere, surface and mantle.

In March 2019, a team led by UC Santa Barbara's Francis Macdonald
published a study proposing that tectonic activity in the tropics, and subsequent chemical weathering by the abundant rainfall, could account
for the majority of carbon capture over million-year timeframes.

Now, Macdonald, doctoral student Eliel Anttila and their collaborators
have applied their new model to the emergence of the Southeast
Asian archipelago - - comprising New Guinea, Indonesia, Malaysia,
the Philippines and other nearby islands -- over the past 15 million
years. Using data from the paleo-record, they determined that the islands
are a modern hotspot of carbon dioxide consumption. Their results,
published in the Proceedings of the National Academy of Sciences, deepen
our understanding of past climatic transitions and shed light on our
current climate crisis.

The primary means by which carbon is recycled into the planet's
interior is through the breakdown of silicate rocks, especially rocks
high in calcium and magnesium. Raindrops absorb carbon dioxide from
the atmosphere and bring it to the surface. As the droplets patter
against the stone, the dissolved carbon dioxide reacts with the rocks, releasing the calcium and magnesium into rivers and the ocean. These
ions then react with dissolved carbon in the ocean and form carbonate
compounds like calcite, which consolidates on the sea floor, trapping
the atmospheric carbon for tens of millions of years or longer.

Given the right conditions, and enough time, the deep carbon cycle can
lock away enough carbon to plunge Earth into an ice age. "Last year we
found that there was a nice correlation between when we make a bunch of mountains in the tropical rain belt and when we have cooling events,"
said Macdonald, a professor in the Department of Earth Science.



========================================================================== Carbon dioxide levels in the atmosphere spiked in the mid-Miocene
climatic maximum, around 15 million years ago. Although there is still
some uncertainty, scientists believe that atmospheric CO2 levels were
between 500 and 750 parts per million (ppm), compared to pre-industrial
levels of around 280 ppm. During the mid-Miocene, warmer conditions
stretched across the globe, the Antarctic ice was meager, and the Arctic
was completely ice free.

Today we are around 411 ppm, and climbing, Macdonald pointed out.

Around that time, the Eurasian and Australian plates began colliding
and creating the Southeast Asian archipelago and few of the present
islands were emergent above sea-level. "This is the most recent example
of an arc-continent collision in the tropics," Macdonald noted, "and
throughout this period we actually have proxy data for the change
in CO2 levels and temperatures." The team was curious how large an
effect the emergence of the islands may have had on the climate. Based
on their previous hypothesis, the formation of these largely volcanic
rock provinces in the tropics should be a major factor in determining
CO2 levels in the atmosphere.

They applied geological data of ancient shorelines and lithology to
a joint weathering and climate model, which accounted for four major
variables: latitude, topography, total area and rock type. In the
tropics, a more mountainous region will experience more rain, and have
a greater surface area for weathering to occur. Once the surface rocks
are weathered, the combination of erosion and uplift exposes fresh rock.



========================================================================== "What you need to do is just keep removing that soil, keep getting
fresh rock there, and keep dissolving it," explained Macdonald. "So
having active tectonic topography is key. All of Southeast Asia has
active topography, and this is a big reason why it's just so much more effective at breaking rocks down into their constituent ions so they
can join into the geochemical cycles." The team's analysis bore this
out. They found that weathering, uplift, and erosion just in the Southeast Asian islands could have accounted for most of the drop in CO2 levels
between the mid-Miocene climate maximum and the Pleistocene ice ages,
when carbon dioxide was around 200 ppm.

These findings could provide insights on our current climate crisis. "The reason scientists are so interested in understanding the Miocene is
because we think of this as perhaps the best natural analogue to what the
world may look like at a CO2 level over 500 ppm," said Macdonald. "It
was the most recent time where we had significantly less ice on Earth,
and we had CO2 levels that are in the range of where we're going in
our current anthropogenic experiment." "People should be worried
not necessarily about the amplitude of the increase, but the slope,"
added Anttila. "That's that real problem right now." Humans have moved a comparable amount of carbon into the atmosphere in just a few generations
as it took the Earth to pull out of the atmosphere over millions of years.

"You realize that we are more effective than any geological processes
at geoengineering," Macdonald said.

The team is currently developing a model and looking at the rocks
themselves to reevaluate previous hypotheses for the initial cooling. By
a stroke of good fortune, the original specimens used to develop these hypotheses are from the Monterey Formation, a layer of rock that crops
up throughout the Santa Barbara basin. These rocks dominate cliff faces
from Santa Barbara to Goleta Pier and from Coal Oil Point to Gaviota.

"We've got this amazing opportunity right here to reconstruct this time
period, right in our backyard," said Macdonald.

"These records of going from a warmer climate in the Miocene to the cooler climate of today are recorded right here in the cliffs," he added. "So
further tests of the hypotheses -- especially in quarantine times,
when we can't travel -- may just involve going out to the beach."

========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Barbara. Original written by Harrison
Tasoff. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Thomas A. Laakso, Anna Waldeck, Francis A. Macdonald, David
Johnston.

Volcanic controls on seawater sulfate over the past 120 million
years.

Proceedings of the National Academy of Sciences, 2020; 117 (35):
21118 DOI: 10.1073/pnas.1921308117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200929123344.htm

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