Branched glycerol dialkyl glycerol tetraethers (GDGTs) are bacterial derived membrane
lipids found ubiquitously in soils and lacustrine sediments (Weijers et al.
2006b, Blaga et al. 2009). The degree of methylation and cyclization of these lipids
have been shown to be dependent on the temperature and pH of the growth environment
(Weijers et al. 2007a). These relationships are the basis of the MBT-CBT
proxy, which has been used to reconstruct paleotemperature from marine and lacustrine
sediment archives (Weijers et al. 2007b, Blaga et al. 2010). Here, we aim to
test the validity of the MBT proxy in terrestrial soils and lake sediments to determine
whether branched GDGT distributions do refl ect annual mean air temperature (MAT)
of the watershed, as suggested by studies, and how other environmental factors,
such as seasonality of growth and sub-environments within a watershed, might
infl uence the preserved MBT-CBT temperature signal. GDGT-derived annual MAT
was compared with instrumental temperature measurements at three sites in the
continental United States. Watershed soils were collected monthly for one year under
three different types of vegetative cover in Minnesota and Ohio. In Florida, soils
were collected twice from an open fi eld environment. Sediment cores were collected
from corresponding lakes in each of the three states. We observed no signifi cant
differences in soil proxy-derived annual MAT or soil GDGT concentration with seasonal
changes in temperature at any of the three sites. Concentrations of GDGTs in
the soil were found to have a slight positive correlation with organic carbon content.
The effects of vegetative cover on proxy estimates of annual MAT were minimal.
Only under deciduous vegetation in Minnesota and Ohio, did proxy-derived annual
MAT differ signifi cantly from instrumental measurements. Soil GDGT concentration
was unaffected by vegetation type in Minnesota, and in Ohio, pine soils had consistently
higher concentrations, usually by an order of magnitude, than other vegetation
types. The CBT proxy was found to be an accurate estimate of soil pH in some
sub-environments, but in Minnesota and Ohio deciduous soils and Ohio open fi eld
soils, CBT-pH differed signifi cantly from measured values. In the sediments of all
three lakes studied, the MBT-CBT proxy provided a good estimate of measured annual
MAT within the error of the proxy. Proxy-derived temperatures from the surface
sediments of all lakes studied were cooler than corresponding instrumental measurements.
These cooler surface temperatures could be attributed to in-situ production
of branched GDGTs, but surface sediment MBT-MAT only differs signifi cantly from
instrumental measurements in Bath Pond, Ohio.
University of Minnesota M.S. thesis. November 2010. Major: Water Resources Science. Advisor: Josef P. Werne. 1 computer file (PDF); v, 77 pages, appendices A.
Bernhardt, Beth A..
Validation of the MBT-CBT paleotemperature proxy:Effects of environmental and seasonal variability in soils and lacustrine sediments..
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