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Development of biomolecular climate proxies

The development of biomarkers from peat has been progressed as follows. High-resolution proxy-climate records based on biomarker distributions have been generated for four ombrotrophic bogs located on a latitudinal transect across Europe, namely: Ballyduff Bog (Ireland), Butterburn Flow (Britain), Bissendorfer Moor (Germany) and Kontolanrahka (Finland). All the biomarker records have been generated at a resolution of 2 cm, and focus on the time intervals of 2950 - 2450 BP, which spans the Sub Boreal/Sub Atlantic transition, and AD800-1800, which includes the transition from the Medieval Warm Period to the Little Ice Age, as well as the modern interval of AD 1800-2000. The biomarker distributions of modern bog-forming vegetation have also been analysed to aid further interpretation of the biomarker records. The suite of new and established biomarkers measured largely reflected changes in the macrofossil proxy data, but were particularly useful where macrofossils were scarce or highly decayed in the peat record. Biomarkers recorded changes in environmental conditions, as well as vegetation, as certain biomarker transformations appear indicative of changes in redox conditions.

The method used was developed as follows. Peat samples were freeze-dried, ground and small aliquots of powdered peat submitted to elemental analysis for nitrogen, organic carbon, hydrogen, and sulphur determinations (percentage dry weight). Lipids were extracted from approximately 0.5 g peat using repeated ultrasonication with 5 ml of dichloromethane/methanol (1:1, v/v). Total lipid extracts were hydrolysed with a 0.5 M methanolic sodium hydroxide solution for 1 h at 70oC and the neutral lipids were removed using hexane. Neutral lipids were further separated into apolar and polar fractions using alumina columns and eluting with hexane/dichloromethane (9:1, v/v) for apolar fractions and dichloromethane/methanol (2:1, v/v) for polar fractions. All fractions were analysed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). Lipid extracted peat residues were also analysed by pyrolysis-GC/MS (pyrolysis-GC/MS).

Compound-specific stable isotope work has been initiated and a small number of analyses have been performed. Gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) was used to provide d13C values on individual lipid biomarkers. The stable carbon isotope composition (d13C) of n-alkanes and hopanes were measured at Butterburn Flow, GB site, to confirm the origin of hopanoid biomarkers. Results show the two C31-hopanes lie in the range -25.9 0 to -26.8 0 and are enriched relative to the C23 and C31 n-alkanes (-31.9 0 and -31.5 0, respectively). This implies a heterotrophic bacterial origin for the hopanes at Butterburn Flow, consistent with observations for other peats. Work on the dD records of n-alkane biomarkers has been initiated. All the n-alkanes have been extracted and quantified from fossil and modern peat samples. dD values obtained are consistent with values obtained from other bogs but further analyses need to be performed to complete the records. This is part of on-going research at the University of Bristol, building on our published work which has shown dD values can yield a temperature signal from peat bogs.

The above work involved the extraction, analysis and interpretation of biomarker records from ~400 individual samples which equates to 800 apolar and polar fractions each of which contain numerous biomarkers (10 to 100). This has generated an unprecedented volume of geochemical data and a huge biomarker archive, which will underpin important future research into understanding the biogeochemical and palaeoclimate signals preserved in ombrotrophic bogs, and is to be taken further involving collaboration between the University of Bristol and the ACCROTELM scientific co-ordinator.

Related information

Reported by

Organic Geochemistry Unit
University of Bristol
BS8 1TH Bristol
United Kingdom
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