25th International Meeting on Organic Geochemistry IMOG 2011

O-22
New insights into the sources and application of biohopanoid
molecular proxies in diverse settings
Helen Talbot
School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, United Kingdom
(corresponding author:h.m.talbot@ncl.ac.uk)
Microbially-mediated processes at the Earth‘s
surface and in the subsurface are fundamental
controls on the global carbon and climate cycle. One
way to trace these processes in the geological record
is to through the application of bacteriohopanepolyols
(BHPs) 1 , membrane lipids biosynthesised by many,
but not all bacteria. These complex molecules
consisting of a stable pentacyclic hydrocarbon
skeleton with an extended, highly functionalised side
chain containing at least 4 functional groups, are
widely recognised as being significant components in
soils and sediments, both terrestrial and marine.
Some BHPs have a diverse range of biological
source organisms 1 whilst others have more restricted
origins making them highly diagnostic markers. New
correlations to environment are emerging as BHP
inventories are obtained from an ever broader range
of settings. For example, recent studies have
covered locations from the Arctic 2 to the Tropics 3 and
from geothermal cyanobacterial mats and silica
sinters to peat bogs and anoxic lakes (Table 1).
As expected, tetrafunctionalised BHPs (with
functional groups at C32,33,34 and 35) are common
in all settings but are typically the least diagnostic
with the widest range of sources; however, other
groups have emerged with potential as tracers for
specific processes.
The prevalence of adenosylhopane (and related
structures) in soils combined with their (near) total
absence in marine samples far removed from any
significant terrestrial source, has prompted the
proposal that the relative contribution of this group of
compounds to the sedimentary BHP pool may be a
useful proxy for soil organic matter input. As a
foundation to this potential application, we have
surveyed the BHP composition of over 600 soil
samples from around the world and the comparison
of these terrestrial BHP fingerprints to those from a
range of aquatic sediments of different ages reveals
significant differences.
More recently it has been observed that
unsaturated BHPs, which are only present in trace
amounts (if at all) in most settings including soils, are
highly abundant in surface peat layers and may
reflect the activity of nitrogen-fixing bacteria.
In geothermal settings with significant
cyanobacterial biomass, a suite of novel C-2
methylated and non-methylated structures are
common at diverse locations from North America,
Chile and New Zealand. 4 However, BHPs methylated
at the C-3 position are rarely observed in any setting
in stark contrast with observations of 3methylhopanes
in older systems.
Finally, recent data have highlighted the
significance and potential of aminoBHPs (and
aminobacteriohopanepentol in particular) as tracers
for aerobic methane oxidation in both terrestrial and
marine systems with these compounds observed in
samples up to 55Ma.
Table 1. Characteristic environmental BHP
compositions
Sample type BHP groups a
No. BHPs b
Soil T> A >> P and H 18-22
Peat (oxic) UT � T >> A, P and H > P and H > A 20-22
Lakes (small) T � P � H >> A 10-12
Lakes (large) T, A >> P and H 8-10
Rivers T, A >> P and H 10-15
Estuary T, A, H > P 8-12
Deep Sea Fan T, H>> A, P and An 8-12
Open Marine T, An 0-4
Silica Sinters T > P 10-15
Mats T, MT, P, MP 6-8
a
Key: A = adenosylhopane and related structures; An
= 32,35-anhydrobacteriohopanetetrol; M= methylated;
T, P and H = tetra-, penta- and hexafunctionalised
BHPs respectively; U = unsaturated (� 6 and/or 11 )
b
Number of BHPs observed; indicated values are
typical but not exclusive
References
[1] Talbot H.M. & Farrimond P., 2007. Organic
Geochemistry 38, 1212-1225.
[2] Rethemeyer, J. et al. 2010. Organic Geochemistry
41, 1130-1145.
[3] Handley, L., et al., 2010. Organic Geochemistry
41, 910-914.
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