25th International Meeting on Organic Geochemistry IMOG 2011

O-64
Simplification and recalibration of the MBT/CBT
paleothermometer based on branched tetraether lipids in
globally distributed soils
Francien Peterse 1 , Jaap van der Meer 1 , Johan Weijers 2 , Noah Fierer 3 , Robert Jackson 4 ,
Jung-Hyun Kim 1 , Stefan Schouten 1 , Jaap Sinninghe Damsté 1
1 Royal NIOZ Netherlands Institute for Sea Research, Texel, Netherlands, 2 Department of Earth Sciences,
Utrecht University, Utrecht, Netherlands, 3 Department of Ecology and Evolutionary Biology, University of
Colorado, Boulder, United States of America, 4 Department of Biology, Duke University, Durham, United
States of America (corresponding author:francien.peterse@nioz.nl)
The recently developed MBT/CBT proxy for the
reconstruction of continental air temperature and past
soil pH is based on the distribution of branched
glycerol dialkyl glycerol tetraether (GDGT) membrane
lipids in soils. An empirical study of soils from over 90
locations worldwide, showed that the relative
distributions of branched GDGTs correlate well with
mean annual air temperature (MAAT) and soil pH [1].
To quantify these changes, the Methylation of
Branched Tetraether (MBT) and the Cyclisation of
Branched Tetraether (CBT) indices were developed.
Combination of the indices resulted in the MBT/CBT
proxy, which has subsequently been used to
reconstruct climatic changes in several areas of
different geological age [2-5].
In this study, we extended the initial soil calibration
data set, now including 175 globally distributed
surface soils (Fig.), and reassessed the relation of the
different branched GDGTs with MAAT and soil pH.
Statistical analyses showed that only five of the nine
branched GDGTs are needed to describe the
relations with the environmental parameters.
Subsequently, we statistically correlated all possible
indices based on these five GDGTs, which lead to the
new MBT‘ and CBT‘ indices, and transfer functions to
estimate MAAT and soil pH. MAAT can now be
estimated with an accuracy of 4.8˚C (original
error=5.2˚C), solely using the MBT‘ index (r 2 =0.62).
The relation with pH is described by the CBT‘ index
(r 2 =0.74) with an accuracy of 0.7 pH unit (originally
0.8).
We tested these new indices on previously published
MBT/CBT-derived records. The records represent
large temperature shifts of different geological eras;
atmospheric warming of tropical Africa [2] and
southeast Asia [3] over the last deglaciation, the onset
of long-term cooling near the Eocene-Oligocene (E-O)
boundary [4], and the period of extreme warmth
during the Paleocene-Eocene thermal maximum
(PETM) at the Arctic [5].
For all tested records, the newly derived temperature
records follow the same warming and cooling trends
as those based on the original MBT/CBT proxy,
indicating that the MBT‘ index reflects changes in
temperature as expected. Regarding the absolute
values of reconstructed MAAT, the Eocene-Oligocene
boundary record remained practically unchanged.
However, the new temperature estimates for the other
records, i.e. deglacial tropical Africa and southeast
Asia, and the Arctic PETM are all substantially lower,
varying between 4-5˚C in Africa to 4-11˚C in Asia,
than the original MBT/CBT-derived temperatures.
The pH record for the Congo River basin [2],
representing the hydrologic changes in this region,
was again reconstructed, now using the CBT‘ index.
The obtained pH record follows the exact same trends
as the original CBT-derived record, but absolute
values are offset by 0.2 pH unit. However, this is well
within the error of both the CBT and the CBT‘ index.
The MBT‘ and CBT‘ indices thus result in MAAT and
pH estimates within the expected range, and can now
be calculated based on only five branched GDGTs.
The widespread occurrence and generally high
abundance of the selected branched GDGTs should
make quantification more straight forward and hence
improve the accuracy of the proxy.
Fig. Extended global soil calibration set. Soil locations
included in the original calibration are indicated in
black, and locations of the additional soils in red.
References:
[1] Weijers et al., 2007, GCA 71, 703-713
[2] Weijers et al., 2007, Science 315, 1701-1704
[3] Peterse et al., 2011, EPSL 301, 256-264
[4] Schouten et al., 2008, Geology 36, 147-150
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