25th International Meeting on Organic Geochemistry IMOG 2011

P-510
Testing the potential of bacterial branched tetraether lipids as
temperature proxy in peat and immature coal deposits
Johan Weijers 1,2 , Philipp Steinmann 3 , Ellen Hopmans 2 , Stefan Schouten 1,2 , Jaap
Sinninghe Damsté 1,2
1 Utrecht University, Utrecht, Netherlands, 2 NIOZ Royal Netherlands Institute for Sea Research, Den Burg -
Texel, Netherlands, 3 Université de Neuchâtel, Neuchâtel, Switzerland (corresponding
author:j.weijers@geo.uu.nl)
Branched glycerol dialkyl glycerol tetraether
(brGDGT) membrane lipids occur ubiquitously in peat
and soil and derive from as yet unknown bacteria. In
soils, the degree of methylation and cyclisation of
branched tetraethers (MBT index and CBT ratio,
respectively) have shown to relate to both soil pH and
annual mean air temperature (MAT). Using this
relation, past annual MATs can be reconstructed by
analysing brGDGTs in marine sediment records near
large river outflows. More recently, the potential of this
MBT/CBT proxy is also being explored in lakes.
Despite being more abundant in peat than soils,
however, the utility of the proxy has not yet been fully
explored in peat records. Present day peat records
generally extent back to the early Holocene, but if the
MBT/CBT proxy were shown to be applicable in peat
deposits, there is also potential to apply it to immature
coal deposits which could provide valuable snapshots
of continental climate back to the early Cenozoic.
To test the utility of the MBT/CBT proxy in peat,
brGDGTs have been analysed in the Etang de la
Gruère (EGr) peat bog from the Jura Mountains,
Switzerland, extending back ca. 13ka. Annual MAT
reconstructed for the top of the bog (i.e., at the water
table level) is with 13°C clearly higher than measured
annual MAT (5.5°C) and annual mean in situ
temperature of the bog (~8°C). Also in other peat
bogs reconstructed MAT seems slightly higher than
measured MAT although the magnitude differs [1,2].
Analysis of more peat bogs is needed to verify the
consistency of this offset.
Comparison of down-core MBT/CBT-derived
temperature with in situ measured pore water pH and
temperature of EGr bog indicates that the brGDGT
composition does not reflect present day in situ
conditions (Fig). Instead, the CBT-derived pH record
reflects a stratigraphic boundary between Carex and
Sphagnum dominated peat at 4 m depth that is not
present in the pore water profile, testifying to a ‗fossil‘
nature of the brGDGTs down the peat bog. Similarly,
reconstructed MAT also shows a pronounced shift at
this stratigraphic boundary that is too large to be
accounted for by natural climate change during the
Holocene. It is suggested that a change in wetness
and nutrient status accompanying the boundary had
an impact on the bacterial community composition,
including the brGDGT synthesising bacteria.
In addition to EGr bog, three immature coals from the
Argonne Premium Coal Series were analysed.
Branched GDGTs are clearly present in the Beulah
Zap lignite (Ro = 0.25%) but only just above detection
limit in the Wyodak Anderson coal (Ro = 0.32%),
which is of about the same age (Late Palaeocene)
and location (central U.S.A.). In the more mature
Illinois #6 coal (Ro = 0.46%), brGDGTs are completely
absent. Reconstructed annual MAT for the Beulah
Zap lignite is ca. 29°C, several degrees higher than
estimates based on leaf margin and oxygen isotope
analyses. It does testify, however, to the warm
continental conditions at the late Palaeocene of the
central U.S.A..
Although further validation is required, these results
do show potential for application of the MBT/CBT
temperature proxy in peat and lignite deposits.
Reconstructed (black) and measured (blue) pH and
temperature down the EGr peat core.
[1] Huguet et al. (2010) Org. Geochem. 41, 559-572.
[2] Liu et al. (2010) Org. Geochem. 41, 653-660.
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