A (Not So) Brief History of Carbon on Earth

Presenter: George Shaw, Union College
When: October 24, 2006 2:30PM PDT

It is widely agreed that carbon first arrived on Earth in a reduced form,
as found in almost all meteorites, and was abiotic in origin. For more
than thirty years, the prevailing view has been that the carbon in Earth’s
early atmosphere (and near surface environment) was virtually all in the
form of carbon dioxide, the oxidized chemical state found in volcanic
gases that are thought to be the source of atmospheric carbon compounds
resulting from degassing of Earth’s interior.

For about the same period of time there has also been broad agreement that
a large fraction of the near-surface volatiles, including both water and
carbon compounds, were degassed very early in Earth’s history, implying a
carbon dioxide rich early atmosphere. This has been thought by many to be
a suitable explanation of the necessary enhanced greenhouse effect
required to compensate for the early faint sun.

On the other hand there are several lines of evidence strongly at odds
with this model for the early atmosphere:

1) Very early, Rubey pointed out the drastic geochemical and
sedimentological consequences of a large CO2-rich atmosphere, including
both severe weathering effects and consequent massive deposition of
carbonate rocks, for which there is little or no evidence in the early
Archean.

2) The delay in oxygenation of the atmosphere following the advent of
oxygenic photosynthesis in cyanobacteria, as early as 2.8 BYBP (perhaps
even earlier) is a long recognized (if often ignored) problem. Analysis of
various sinks and nutrient constraints does not eliminate this problem.

3) The record of carbon isotopes in sediments points to a longstanding (at
least since ca. 3.5 BYBP) balance between carbonate carbon and biogenic
(fixed organic) carbon at a ratio of about 4 to 1. This implies
substantial (and very early) fixation of large amounts of biogenic carbon
and release of proportional amounts of free oxygen, which is inconsistent
with geologic and isotopic evidence for an anoxic surface environment
until ca. 2.1-2.3 BYBP.

These problems could be solved if one could identify a reservoir to hold
the degassed carbon and release it into the biosphere on a geologic time
scale. The lack of residual early Archean carbonate sediments (or
metasediments) from such a hypothetical reservoir speaks against carbonate
as the reservoir substance. The likelihood of early degassing precludes a
deeper (e.g. upper mantle) reservoir. The only remaining choice is a
reduced carbon reservoir at or near the surface. This reservoir cannot be
atmospheric methane (or other gaseous hydrocarbon) because photochemical
reactions rapidly remove such compounds from the atmosphere.

An early ocean with a high concentration of photochemically (and
electrically) produced complex organic compounds solves all of these
problems, with the added attraction that it is a favorable environment for
the emergence of life. The oxidation of subducted organic rich sediments
during upper mantle magmagenesis slowly provides CO2 to the surface
environment, on a time scale consistent with the time scale for
oxygenation of the surface environment by photosynthetic cyanobacteria,
with the record of carbon isotopes in sedimentary rocks, and with the
record of carbonate sedimentation.

An early reduced carbon reservoir at/near Earth’s surface follows directly
from early degassing, under reducing conditions, of the original (and/or
hydrogenated) meteoritic carbon compounds. The largely methane atmosphere
so produced is short lived, but the photochemical products accumulate in
the ocean and are continuously recycled into the atmosphere as methane by
low temperature hydrothermal activity. This model provides a suitable
source of the early (methane) enhanced greenhouse effect.

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