25th International Meeting on Organic Geochemistry IMOG 2011

P-516
Distributions of branched tetraethers in soils and suspended
particulate matter in the Amazon basin: Implications for the
MBT/CBT palaeothermometer
Claudia Zell 1 , Jung-Hyun Kim 1 , Marie-Paule Bonnet 2 , Gwenaël Abril 3 , Jean-Michel
Mortillaro 4 , Rodrigo Sobrinho 5 , Jaap Sinninghe Damsté 1
1 NIOZ Royal Netherlands Institute for Sea Research, Den Hoorn, Netherlands, 2 Universite Toulouse,
Toulouse, France, 3 Laboratoire Environnements et Paléoenvironnements Océaniques, Talance, France,
4 CNRS, Paris, France, 5 Universidade Federal Fluminense, Niteroi, Brazil (corresponding
author:claudia.zell@nioz.nl)
Branched glycerol dialkyl glycerol tetraethers
(GDGTs) are membrane lipids of bacteria living
predominantly in soils. Their distribution reflects mean
annual air temperature (MAAT) and soil pH, which
was used to develop the Methylation index of
Branched Tetraethers (MBT) and Cyclization ratio of
Branched Tetraethers (CBT) [1]. By erosion of soil,
branched GDGTs are transported from the continent
to the ocean by rivers. Hence, branched GDGTs
found in marine sediments deposited close to the river
mouth may record an integrated climate signal of the
river drainage basin [2, 3]. In this study, we address
the processes that affect the distribution of branched
GDGTs in the Amazon basin before it enters the
ocean. We compared branched GDGT distributions in
Amazon soils and suspended particulate matter
(SPM) collected in the Amazon River, its tributaries,
and floodplain (varzea) lakes. For the conversion of
the MBT/CBT values into MAAT and soil pH, we
applied two different calibrations: the global soil
calibration [1] and a regional calibration [3]. For soils
from the Amazon low lands, instrumental data show a
mean annual air temperature of about 27ºC and a soil
pH of 4.8±0.1 was measured. The MBT/CBT-derived
average temperature of the soils using the regional
calibration is about 23.9ºC±1.0 (n=18), which is cooler
but less scattered in comparison to that calculated
with the global calibration (25.0ºC±3.0, n=18). Both
calibrations result in similar reconstructed soil pH of
4.6±1.4 (global) and 4.6±1.3 (regional) (n=18).
The SPM of the Amazon River shows MBT/CBTderived
MAAT of 25.7ºC±1.3 (n=4, global calibration)
or 22.6ºC±0.7 (n=4 regional calibration). MBT/CBTderived
MAAT obtained from SPM in tributaries
originating in the Andes (Madeira and Solimões) do
not show significantly lower MBT/CBT-derived MAAT
compared to other tributaries originating from the
Amazon low lands (Negro, Tapajos and Trombetas),
even though a lower temperature signal was expected
for the rivers originating in the Andes as soils at
higher altitude experience lower temperatures. The
CBT-derived pH signal in the Amazon River SPM is
relatively constant around 5.8±0.2 (n=4, global
calibration) or 5.4±0.2 (n=4, regional calibration). The
rivers Negro and Trombetas show a reconstructed pH
of about one pH unit lower. The pH measured in the
water column of the Amazon River and its tributaries
is 6.3±0.1 (n=5) except those of the Negro and
Trombetas river which have pH values of 4.9 and 5.3,
respectively. That the CBT-derived pH signal of
branched GDGTs from river SPM is higher than that
of most of the Amazon soil which has a pH of 4-5 may
suggest an influence of in-situ produced branched
GDGTs in the river water column. Our study implies
that the regional calibration needs to be further
developed for more precise MAAT predictions in the
Amazon basin. As MBT/ CBT-derived MAAT from
river SPM are not significantly lower than those of soil
we conclude that the MBT/CBT signals in the Amazon
River mostly reflect Amazon basin low land conditions
and thus no major influence of colder mountainous
Andes region although this has been suggested for
the Early Holocene [3].
Temperature o C
26
25
24
23
22
21
Soil
(n=18)
River Varzea
SPM
(n=8)
SPM
(n=7)
pH
6
5
4
3
Soil
(n=18)
River Varzea
SPM
(n=8)
SPM
(n=7)
Figure 1: Box plot of MBT/CBT-derived pH and
temperature, calculated with the regional formula [3].
[1] Weijers, J.W.H., et al. (2007), Geochim.
Cosmochim. Acta 71, 703–713.
[2] Weijers, J.H.W., et al. (2007), Science 315, 1701.
[3] Bendle, J.A., et al. (2010), Geochem. Geophys.
Geosyst. 11, Q12007.
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