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The application of the MBT’/CBT proxy in Palaeogene lignites. GDGT-based proxies: State of the art and future directions.

Inglis, G. N. and Collinson, M. E. and Zhu, C. and Hinrichs, K. U. and Wilde, V. and Riegel, W. and Lenz, O. K. and Pancost, R. D. (2014):
The application of the MBT’/CBT proxy in Palaeogene lignites. GDGT-based proxies: State of the art and future directions.
In: Program GDGT 2014 workshop, Texel, Netherlands, In: GDGT 2014 Workshop, Texel, Netherlands, [Online-Edition: https://www.nioz.nl/files/afdelingen/BGC/documenten/GDGT%202...],
[Conference or Workshop Item]

Abstract

Early Palaeogene (65-34Ma) wetl ands are estimated to have been ~3 times more extensive than today. Despite this, lithified peat sequences (i.e. lignites) have received little attention in terms of application of the MBT’/CBT proxy. The MBT’/CBT proxy, based upon the distribution of soil-derived br anched GDGTs, can be used to reconstruct mean annual air temperature (MAAT) and pH, but its application in peat deposits – and by extension lignites – has been argued to be problematic. We examin e GDGT distributions in a series of lignite seams inferred to represent ancient ombrotrophic bog deposits based upon the presence of Sphagnum spores. Samples, which range in age from late Palaeocene to middle Eocene, are of relatively low thermal maturity, contain ab undant GDGTs and allow a comparison of MBT’/CBT estimates with other environmental proxies. In a typical lig nite seam, branched GDGTs comprise ~60% of the total GDGT assemb lage. Unlike modern wetlands, we detect substantial quantities of regular, methylated and H-shaped isoprenoida l GDGTs bearing 0 to 4 cyclopentane moieties. Applicat ion of the MBT’/CBT proxy to a late Palaeocene seam indicates stable MAAT and pH estimates. Th is contrasts with modern peatlands where MBT’/CBT values can fluctuate significantly; we tentatively s uggest that this reflects the compressed character of lignites such that shorter-term variability has been removed. MBT’/CBT-derived MAAT estimates are warm (24-26 ° C), in agreement with floral evidence and modelled estimates, whereas CBT-derived pH estimates are higher than expected for an ombrotrophic bog. However, a systematic MAAT offset is also observed between lignite, sand and clay facies; th is likely reflects different sources of GDGTs but also suggests that lignites are recording a different climate signal than the tota l catchment. We observe no long- term changes in MBT’/CBT-derived MAAT estim ates between the late Palaeocene and early- middle Eocene. This may reflect a stable climate or ‘maxing-out’ of the MBT’/CBT palaeothermometer as MBT values approach 1.

Item Type: Conference or Workshop Item
Erschienen: 2014
Creators: Inglis, G. N. and Collinson, M. E. and Zhu, C. and Hinrichs, K. U. and Wilde, V. and Riegel, W. and Lenz, O. K. and Pancost, R. D.
Title: The application of the MBT’/CBT proxy in Palaeogene lignites. GDGT-based proxies: State of the art and future directions.
Language: English
Abstract:

Early Palaeogene (65-34Ma) wetl ands are estimated to have been ~3 times more extensive than today. Despite this, lithified peat sequences (i.e. lignites) have received little attention in terms of application of the MBT’/CBT proxy. The MBT’/CBT proxy, based upon the distribution of soil-derived br anched GDGTs, can be used to reconstruct mean annual air temperature (MAAT) and pH, but its application in peat deposits – and by extension lignites – has been argued to be problematic. We examin e GDGT distributions in a series of lignite seams inferred to represent ancient ombrotrophic bog deposits based upon the presence of Sphagnum spores. Samples, which range in age from late Palaeocene to middle Eocene, are of relatively low thermal maturity, contain ab undant GDGTs and allow a comparison of MBT’/CBT estimates with other environmental proxies. In a typical lig nite seam, branched GDGTs comprise ~60% of the total GDGT assemb lage. Unlike modern wetlands, we detect substantial quantities of regular, methylated and H-shaped isoprenoida l GDGTs bearing 0 to 4 cyclopentane moieties. Applicat ion of the MBT’/CBT proxy to a late Palaeocene seam indicates stable MAAT and pH estimates. Th is contrasts with modern peatlands where MBT’/CBT values can fluctuate significantly; we tentatively s uggest that this reflects the compressed character of lignites such that shorter-term variability has been removed. MBT’/CBT-derived MAAT estimates are warm (24-26 ° C), in agreement with floral evidence and modelled estimates, whereas CBT-derived pH estimates are higher than expected for an ombrotrophic bog. However, a systematic MAAT offset is also observed between lignite, sand and clay facies; th is likely reflects different sources of GDGTs but also suggests that lignites are recording a different climate signal than the tota l catchment. We observe no long- term changes in MBT’/CBT-derived MAAT estim ates between the late Palaeocene and early- middle Eocene. This may reflect a stable climate or ‘maxing-out’ of the MBT’/CBT palaeothermometer as MBT values approach 1.

Title of Book: Program GDGT 2014 workshop
Place of Publication: Texel, Netherlands
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Earth Science
11 Department of Materials and Earth Sciences > Earth Science > Applied Sedimentary Geology
Event Title: GDGT 2014 Workshop
Event Location: Texel, Netherlands
Date Deposited: 26 Jan 2016 19:29
Official URL: https://www.nioz.nl/files/afdelingen/BGC/documenten/GDGT%202...
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