Note: This program first aired on January 3, 2015.
We left off last time with the biogeochemical mechanisms by
which oil is made. Briefly, oil and gas result from primary productivity at the
ocean surface. Photosynthesis happens in the bodies of tiny phytoplankton.
Those plankton die without being eaten, and then sink to the bottom, where they
accumulate. Over a geologically relevant amount of time, they turn into oil.
Today we talk about the gas part of “oil and gas”. There are
several simple hydrocarbons that can form from dead marine organic matter, and
the shortest of these chains of carbon are gaseous. The shortest chain of all
isn’t a chain at all, it’s a single carbon atom surrounded by hydrogen atoms,
CH4, methane. Methane gets special attention in climate change circles because
it isn’t just a fossil fuel (it’s the primary component of “natural gas” when
it wears that hat), it is also a green house gas in its own right.
Methane forms when organic matter is decomposed
anaerobically, meaning without oxygen. This form of decomposition is slow, and
yields less energy than aerobic metabolism, but allows microbes exist in anoxic
environments where other things can not live. Methane occurs naturally on earth
anywhere we have anoxic conditions and organic matter. Wetlands are a major
terrestrial source, where waterlogged soils quickly become anoxic, and
methanogenic bacteria have plenty of raw material. Swamp gas is the result.
Animals are another surprising source of methane, some more so than others. Our
guts are anaerobic environments that house billions of bacteria, these bacteria
do much of the heavy lifting of digestion for us, but depending on the raw
material we provide these bacteria, they sometimes make methane as a byproduct,
a phenomenon I am sure you are all familiar with. Animals that make a living
eating difficult to digest food, like cellulose and related compounds make
methane regularly. Domestic cattle and other ruminants eat hard to digest
grasses, and as a result, give methanogenic bacteria plenty of feed stock.
Termites make a living eating wood and other high cellulose organic material,
and again, rely on gut microbes that can break down the difficult to digest
cellulose. As a result, termites are another natural source of atmospheric
methane.
Much methane resides in the permafrost areas of the high
arctic and subarctic. Productivity is slow in those regions, but the organic
matter that does form breaks down very slowly due to the cold conditions, thus
the soils there are quite peaty, and high in partially decomposed organic
matter. They also suffer poor drainage due to underlying impermeable frozen
substrate, and tend to be waterlogged as a result. These conditions lead to a
slow but steady production of soil bound methane, typically locked into
permafrost.
The largest store of methane on Earth is at the bottom of
the ocean. As a simple hydrocarbon methane forms relatively quickly from
organic matter that makes its way to the ocean floor. Over the long term these
deposits can be buried and become the oil and gas deposits we now search
continental shelves for, but in the short term the methane gas can also be
trapped in near ocean bottom sediments, held in place by the extreme pressure on
the sea floor. These deposits are called methane hydrates, because due to the
pressure, the methane molecules get trapped in a crystalline matrix of water
molecules, and form solid that looks something like ice. This methane is not only the largest store of
the gas on Earth, it is also constitutes over half of all fossil fuels on
Earth, which is why so many people are trying to figure out how to commercially
access this this odd source of natural gas, for better or for worse. Next time
we will look at methane’s role in climate change, and why it might be our best
hope for mitigating the most severe potential impacts.
References:
From the University of California San Diego: http://earthguide.ucsd.edu/virtualmuseum/climatechange1/03_3.shtml
Online text book from the University of Oxford (UK)
Environmental Change Institute: http://www.eci.ox.ac.uk/research/energy/downloads/methaneuk/chapter02.pdf
Interesting info from NOAA on non carbondioxide Green House
Gasses http://www.esrl.noaa.gov/research/themes/forcing/
Data, including details on sources and proportions from the
EPA: http://epa.gov/climatechange/ghgemissions/gases/ch4.html
About termites from the Arizona Sonora Desert Museum (be sure
to read the bit about the baby termites eating the feces of their older
siblings…) http://www.desertmuseum.org/books/nhsd_termites.html
Department of Energy take on methane hydrates: http://energy.gov/fe/science-innovation/oil-gas-research/methane-hydrate
