25th International Meeting on Organic Geochemistry IMOG 2011

P-499
The relationship between peatland hydrology, biogeochemistry
and biomarker assemblages
Rich Pancost 1 , Richard Evershed 1 , Edward Hornibrook 2 , Erin McClymont 1 , Elizabeth
Bingham 1 , Lidia Chaves 1 , Katie Lim 1 , Frank Chambers 3
1 Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol, United Kingdom, 2 Bristol
Biogeochemistry Research Centre, School of Earth Sciences, University of Bristol, Bristol, United Kingdom,
3 Department of Natural & Social Sciences, University of Gloucester, Cheltenham, United Kingdom
(corresponding author:r.d.pancost@bristol.ac.uk)
Lipid biomarkers are now widely applied in the study
of peatlands, both in modern biogeochemical and
ancient palaeoclimate contexts. However, little work
has attempted to bring these two lines of inquiry
together and it remains unclear exactly how, or if at
all, biomarker distributions in peat reflect microbial
populations or biogeochemical processes at the time
of peat deposition. To address that, we have
determined archaeal (archaeol and hydroxyarchaeol)
and bacterial (hopanoid) concentrations as well as
stanol/sterol ratios in four Holocene ombrotrophic
peatlands, spanning a range of European climate
zones.
Neither ether lipid was present in the aerobic acrotelm
of the peat, consistent with an origin from anaerobic
archaea, presumably methanogens. At the depth of
the maximum seasonal water table, archaeol and
hydroxyarchaeol concentrations markedly increase at
all four sites, again consistent with an anaerobic
source, but the concentrations differed strongly
among sites, apparently reflecting a combination of
vegetation and temperature influences on
methanogenesis (Fig. 1). In particular, low ether lipid
concentrations in Finland probably reflect the lack of
vascular vegetation having well developed root
systems, together with low mean annual
temperatures. Similarly low concentrations of
archaeol and sn-2-hydroxyarchaeol in the German
bog most likely result from below freezing winter
temperatures and a short growing season. The
occurrence of hydroxyarchaeol is limited to a narrow
and shallow depth range, suggesting that it is poorly
preserved, but archaeol persists throughout the peat
cores. Decoupling of archaeol and the more labile
hydroxyarcheol concentrations below ca 50 cm
suggests that the former records fossil rather than
living biomass. If so, then downcore variations in
archaeol concentration likely reflect past changes in
peatland hydrology, biogeochemistry and rates of
methanogenesis.
Figure 1. Comparison of annual (open circle) and
minimum air temperatures (closed circle) with
maximum concentrations of archaeol observed in four
European peatlands (GB= Butterburn Flow, Great
Britain; IR = Ballyduff Bog, Ireland; DM = Bissendorfer
Moor, Germany; FI = Kontolanrahka Bog, Finland).
Stanol/sterol ratios exhibit remarkably similar trends.
Ratios are low in shallow acrotelm peat but then
increase dramatically at and just below the water
table. Moreover, in deeper horizons the ratios co-vary
with archaeol concentrations and reconstructed water
table levels, with low ratios and low archaeol
concentrations occurring during times when the water
table level was deep and more oxidising conditions
presumably prevailed. This indicates that the
transformation of plant sterols to their stanol
analogues is associated with reducing conditions, as
has been suggested for marine settings.
Combined, the records could provide new insights
into past changes in peat redox conditions. Caution is
required in interpretation due to the complex controls
on, especially, methanogen biomass, however, our
data suggest that a combination of biomarker and
other proxies could be used to identify times when
shallow water tables were associated with reducing
and methanogenic conditions.
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