Geoscience Reference
In-Depth Information
Figure 6.2. sedimentary rocks from Isua, greenland. note the near vertical, black, graphite-
rich layer running from top-center to lower-left. there is a similar layer just to the right. Photo
kindly provided by minik rosing.
ger than a car, were apparently deposited in deep marine waters. The
graphite-rich layers are interbedded with sediment layers known as tur-
bidites.
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These are formed as sediment from shallower water depths is
remobilized and transported rapidly downslope where it is redeposited
in deeper waters (
ig. 6.2)
. Minik interprets the graphite layers as the
background deposition of organic matter-rich particles that came from
the surface waters of the ocean. This constant, gentle flux of organic
matter was punctuated by the occasional rapid flow of turbidites. In
this way, you get a layer-cake alteration of organic-rich sediments and
turbidites. The organic matter flux to the sediments must have been
reasonably high to yield enough graphite to write with. But what form
of life did the organic matter come from? This is the million dollar ques-
tion. Could it have been produced by cyanobacteria?
The sediments are way too cooked to offer any kind of fossil evidence
of what type of organisms were present, but there is other evidence we
can use. Pick up a rock, turn it in your hand, and look long and hard at
the graphite. Might we find some clues here?
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Minik did this and de-
cided to look for evidence in the isotopes of carbon preserved in the
graphite. To understand this, we recognize that in nature carbon is
found with three different isotopes: carbon-12, carbon-13, and carbon-
14. Most have heard of carbon-14; it's radioactive, forms in the atmo-
sphere, and sticks around for only tens of thousands of years, so we
won't worry more about it. Carbon-12 has six protons and six neutrons,
whereas carbon-13 matches its six protons with seven neutrons. There-
fore, carbon-13 is about 8% percent heavier than carbon-12 (13/12 =
