Geology Reference
In-Depth Information
A Case of Gas
Many scientists now suspect that another positive feedback—a mechanism that is also of
significantconcerntoourmoderntimes—mayhaveexacerbatedtherapidglobalwarming.
Methane (CH 4 ), the simplest hydrocarbon fuel and what we burn in our homes as “natural
gas,” is also a greenhouse gas, but one that is, molecule for molecule, much more effect-
ive than carbon dioxide in trapping the Sun's energy. For billions of years, methane has
accumulated in ocean-floor sediments, probably by two contrasting mechanisms. The first,
which is much better documented and thus a lot less controversial, involves microbes that
release methane as part of their normal metabolic cycle. These methanogens thrive in an-
oxic ocean sediments near many known methane reserves, so large natural gas deposits are
thought to have formed by the sustained action of these microorganisms.
Recent experiments point to a possible second, much deeper source of methane—a
source that doesn't rely on biology at all. Some scientists suggest that in the deep crust and
uppermantle,atdepthstomorethanahundredmileswhereextremetemperaturesandpres-
sures prevail, water and carbon dioxide can react with common iron-bearing minerals to
produce methane. Experiments at high temperatures and pressures attempt to mimic these
suspected deep-Earth reactions. In an oft-cited 2004 study at the Geophysical Laboratory,
postdoctoral fellow Henry Scott mixed two common crustal ingredients, calcite (calcium
carbonate—the common carbon-containing mineral of limestone) and iron oxide, with wa-
ter. Scott sealed these ingredients into a diamond anvil cell and laser-heated the sample to
more than a thousand degrees—the same extreme conditions found in the upper mantle.
Remember, the neatest thing about the diamond anvil cell is that diamonds are transparent,
so you can see the sample as it reacts and changes. Henry Scott watched as tiny bubbles of
methaneformedinthesamplechamber.Thehydrogenofwaterhadreactedwiththecarbon
of calcite to form natural gas. Other experiments in Russia, Japan, and Canada have found
similar synthesis of hydrocarbons under a range of supposed deep-Earth conditions.
These experiments are potentially important for understanding Neoproterozoic global
warming, for methane may have contributed to a particularly strong positive feedback.
Much of the methane stored near the ocean floor is trapped in a fascinating compound
called methane clathrate—an icelike crystalline mixture of water and gas that forms out-
crops on the continental slopes. (This freezing-cold methane ice actually burns with a
bright flame—check out the videos on YouTube.) Vast quantities of methane—several
times all other known methane reserves combined, by some estimates—are locked into
thesemethaneices,whichformwhengasrisingfrombelowreactswithcoldseawater.Sig-
nificant additional methane ice is locked in Arctic permafrost—soils in Siberia, northern
Canada, and other regions that have been frozen for thousands of years.
An extreme positive climate feedback might occur when ocean waters warm even
slightly,whichcausestheshallowestclathratedepositstomeltandreleasecopiousamounts
Search WWH ::




Custom Search