25th International Meeting on Organic Geochemistry IMOG 2011

P-209
A comparison of methane emission and oxidation histories on
the Amazon and Congo fans over the last 200 kyr
Luke Handley, Helen Talbot, Kate Osborne, Thomas Wagner
Newcastle University, Newcastle upon Tyne, United Kingdom (corresponding
author:luke.handley@gmail.com)
The Congo and Amazon fans are important
regions of methane (CH4) storage and seepage, with
gas hydrates abounding in the shallow subsurface
alongside more deeply-buried thermogenic methane
reservoirs linked with hydrocarbon source rocks.
However, past changes in CH4 emission and
oxidation at these localities remain unconstrained
primarily due to the lack of adequate geochemical
tracers for such processes.
Bacteriohopanepolyols (BHPs) are lipid
membrane constituents and occur with a wide range
of structural and functional variability in many
bacteria. AminoBHPs are produced by methaneoxidising
bacteria and 35-aminobacteriohopane-
30,31,32,33,34-pentol (aminopentol) is highly specific
to Type I aerobic methane oxidising bacteria. Here,
we present new BHP data from the Amazon fan (ODP
Site 942) and compare aminopentol and other paleorecords
to similar data from the Congo fan (ODP Site
1075) to investigate how aerobic methane oxidation,
and inferred emission, varied over the last 200 kyr on
opposite sides of the tropical Atlantic.
BHP concentrations and diversity in the Amazon
are generally greater than in the Congo, with a
number of uncommon compounds such as a
methanotroph-derived unsaturated aminopentol,
bacteriohopanehexol and others occurring throughout
the core. Indeed, the BHP assemblage from the
Amazon fan is to date by far the most complex BHP
signature reported from marine sediments. This may
be due to the unique setting in a region characterised
by a large terrigenous input and high primary
productivity, leading to a diverse array of precursor
bacteria.
These new biomarker records from the Amazon
fan also reveal dramatic variability in aminopentol,
and other aminoBHP, concentrations over the last
200 kyr. We have previously reported that in the
Congo core, changes in aminopentol concentration,
and hence fluctuations in the intensity of aerobic
methane oxidation (AMO) and inferred CH4 emission,
follow a distinct pattern that appears to correlate with
late Quaternary glacial-interglacial climate cycles, with
generally higher emissions during interglacials, and
we hypothesised that our record reflects orbitallydriven
shifts in the gas hydrate stability zone within
the shallow subsurface. In the Amazon, although
concentrations do vary within each climate mode,
aminoBHP concentrations are also generally higher
during interglacials, with a particularly strong increase
in AMO intensity during marine isotope stages (MIS) 5
and 3, suggesting a possible link between Quaternary
climate and methane emission on the Amazon fan, as
is the case in the Congo. However, on the Congo Fan
MIS 3 is the only interglacial period over the last ca.
900 kyr that is not characterised by a notable increase
in sedimentary aminopentol concentrations. This
could reflect local differences in the susceptibility of
the respective methane reservoirs to climate
variability. Also, aminopentol concentrations are much
higher in the Amazon core, with maximum values
more than double those reported from the Congo,
suggesting that AMO intensity and methane emission
into the water column was generally greater off the
Amazon.
To summarise, although there are some
differences between the BHP paleo-records from the
Congo and the Amazon fan over the last 200 kyr,
there appears to be a link between aerobic methane
oxidation (and inferred emissions) and Quaternary
climate variability at both sites of the tropical Atlantic,
with greater emissions during interglacials. This work
continues to highlight the biomarker potential of BHPs
and in particular aminopentol as a sedimentary tracer
for the aerobic oxidation of methane.
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