25th International Meeting on Organic Geochemistry IMOG 2011

P-193
A new global calibration for GDGTs preserved in stalagmites
Alison Blyth 1 , Stefan Schouten 2
1 The Open University, Milton Keynes, United Kingdom, 2 NIOZ, Den Burg, Netherlands (corresponding
author:a.blyth@open.ac.uk)
Recovering reliable temperature records is central to
palaeoclimatic research, informing our understanding
of the past and our models of the future. Organic
geochemistry has provided a number of
palaeotemperature proxies including TEX86, UK‘37,
and the MBT/CBT index. However, despite good
calibrations of the latter in soils, to date these have
primarily been of use in the aquatic realm, due to the
better preservation conditions provided by the
sedimentary environment.
Here we expand the range of palaeoenvironmental
records to which organic temperature proxies can be
applied, and present global calibration data for
bacterial glycerol dialkyl glycerol tetraethers
preserved in stalagmites. Stalagmites are chemically
precipitated cave deposits which grow incrementally
where drip-waters enter a cave. As they are formed
from waters transported from the overlying soil, they
contain a wide range of environmental proxies
including stable isotope records linked to rainfall and
climate, and organic matter derived from the overlying
vegetation. Due to the nature of their calcite matrix,
stalagmites are also easy to date accurately and
precisely via U-Th dating. The development of a well
calibrated palaeothermometer in this context is
therefore of considerable importance, as, by
combining the temperature record with the other
available proxies, it will be possible to create an
integrated record of changes in temperature, rainfall,
and the associated environmental response.
Glycerol dialkyl glycerol tetraethers (GDGTs) are
microbial membrane lipids, which vary in structure
with temperature and pH [1]. Bacterially derived
GDGTs have been used to measure terrestrial
organic matter input to aquatic deposits via the BIT
index [2], and developed into a temperature related
index (MBT/CBT) in soils and near-shore marine
sediment, based upon the degree of branching and
cyclicisation of the carbon skeleton [3].
Our previous analysis of speleothem samples has
shown that GDGTs are recoverable from stalagmites,
but that the relationship with temperature is not
simple. Therefore a stalagmite specific calibration is
required. Here we present the first attempt at a global
stalagmite based calibration for bacterial GDGTs.
Thirty-eight modern or known age calcite samples
from twenty one different cave sites around the world
were cleaned and decalcified with HCl, and lipids
extracted into dichloromethane via liquid-liquid
extraction. Each extract was then prepared and
analysed via HPLC-MS following Weijers et al. [3].
The results show low but usable levels of GDGTs in
all but one sample. Peak size measurements were
made for all recognised compounds associated with
BIT and MBT/CBT.
Using MBT/CBT, significant correlation is seen
between calculated MAT and known air temperature;
following the methodology used by Weijers et al. [3]
this has a 2-D correlation of r2 = 0.68, indicating a
genuine temperature signal is present. However, the
results have a large degree of scatter, predominantly
representing an under-estimation of temperature. We
hypothesise that this is due to the potential mixture of
sources for the signal (cave bacteria, aquifer bacteria,
soil bacteria, and compounds from the degraded
organic matter pool). One of the most striking
aspects of the results is that the BIT index for the
samples is generally very low (23 samples have a BIT