25th International Meeting on Organic Geochemistry IMOG 2011

P-443
Hopanoids in silica sinters: identification of an unusual pathway
in hopanoid diagenesis with implications for the sedimentary
biomarker record
Robert Gibson 1,2 , Gurpreet Kaur 3 , Bruce Mountain 4 , Richard Pancost 3 , Helen Talbot 2
1 NIOZ Royal Netherlands Institute for Sea Research, 't Horntje, Netherlands, 2 Newcastle University,
Newcastle, United Kingdom, 3 Bristol University, Bristol, United Kingdom, 4 GNS Science, Taupo, New
Zealand (corresponding author:Robert.gibson@nioz.nl)
Geohopanoids, i.e. geohopanols, hopanes,
hopenes and hopanoic acids, are the molecular
fossils of bacteriohopanepolyols (BHPs) that are
produced by a diverse array of bacteria and are
widespread throughout modern and ancient
sedimentary depositions. Previously, we have shown
that BHPs are produced by bacteria that colonise
terrestrial geothermal vents and BHPs are well
preserved in silica sinters collected from the Taupo
Volcanic Zone, North Island, New Zealand (TVZ)[1].
In this study we have analysed the geohopanoid
composition of geothermal silica sinters from the TVZ
with a view to understanding mechanisms of
diagenesis affecting preserved BHPs and to assess
the potential of geohopanoids as biomarkers in
ancient geothermal siliceous deposits.
Throughout our investigations, a high proportion of
C31 �� hopanol relative to C32 or C30 �� hopanols was
observed. This is in stark contrast to studies of the
distribution of geohopanoids from other depositional
settings where C32 hopanol tends to predominate.
Previously, it has been suggested that the formation
of homohopanols is directly related to the degree of
functionalisation in precursor BHPs [2]. However, in
our study this is not the case as C31 �� hopanol,
which would be expected to derive from
pentafunctionalised BHPs, is the most abundant
geohopanol but pentafunctionalised BHPs do not
dominate the BHP distribution to the same extent,
(Figure 1).
Hopanoic acids dominate distributions in older
sinters and were found to closely-mirror BHP
distributions. However, hopanoic acids were less
prevalent in sinters deposited under acidic conditions.
This may reflect reduction of the acid functionality to a
terminal hydroxyl group leading to the observed
predominance of geohopanols in sinters from acidic
vents.
This investigation has highlighted a unique
mechanism of BHP diagenesis that is significantly
controlled by environmental setting and not, as
previously assumed, entirely based on biological
input. Furthermore, geohopanoids appear to be well
preserved in older sinters suggesting that hopanoids
may be useful biomarkers in ancient siliceous sinter
material.
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
‘Active’ C 31/(C31+C32) hopanol index
P/(T+P) BHP index
‘Non-active, older’
Figure 1. Relative distributions of bio- and
geohopanoids observed in active sinters (above) and
non-active sinters (below). Sample codes: CP -
Champagne Pool; LR - Loop Road; OP - Opaheke
Pool. C31/ C31+C32 hopanol index = ∑C31 ��+��+��
hopanol/(∑ C31 ��+��+�� hopanol + ∑ C32 ��+��+��
hopanol); P/(P+T) BHP index =[∑ pentafunctionalised
BHPs/(∑ tetrafunctionalised BHPs + ∑
pentafunctionalised BHPs)].
[1] Gibson R.A., Talbot, H.M., Kaur, G., Pancost,
R.D., Mountain, B.W., 2008. Organic geochemistry
39, 1020, 1023
[2] Farrimond, P., Head, I.M., Innes, H.E., 2000.
Geochimica et Cosmochimica Acta 64, 2985 – 2992.
569