33. OXIC VS. ANOXIC DIAGENETIC ALTERATION OF TURBIDITIC SEDIMENTS IN THE MADEIRA ABYSSAL PLAIN, EASTERN NORTH ATLANTIC 1Gert J. De Lange2 |
ABSTRACTUpon deposition, downward diffusing oxygen results in
the "burn-down" of the upper part of
organic-rich turbidites in the Madeira Abyssal Plain.
This oxidation front is terminated after a few thousand
years, on average, at the moment of deposition of the
next turbidite. These initially homogeneous sediments
form an excellent setting for studying long-term organic
matter remineralization processes to establish the extent
and possible selectivity of oxic vs. anoxic degradation
of organic matter in a natural environment. Samples for
this study have been taken from the oxidized upper part
and the unoxidized lower part (from 42 to 280 meters
below seafloor) of 13 organic-rich turbidites 1.2–14
m.y. old. The organic fraction in the lower part of
organic-rich turbidites has remained virtually unchanged
for periods as long as 14 m.y. under suboxic to anoxic
conditions. In contrast, dramatic changes have occurred
during the post-depositional oxidation of sediments in
the tops of the same turbidites by oxygen. This has
resulted in a large decrease in the organic carbon (-80%)
and total nitrogen content (-50%) of the upper, relative
to the lower, parts of these turbidites. In addition,
this oxidation process has resulted in a 13 C org
that is, on average, 1.5‰ more negative and a 15
N tot that is 1.3‰ more positive in the
upper oxidized samples than it is in the lower unoxidized
ones from within the same turbidite. The seemingly
selective preservation of organic matter that is low in
C/N and has a relatively negative 13C org
is usually interpreted as being indicative of better
preservation of terrestrial organic matter relative to
marine organic matter. However, it could equally be
explained by the different preservation of certain marine
compounds with, on average, distinctly different |
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