Constraints on the application of the MBT/CBT palaeothermometer at high latitude environments (Svalbard,Norway)     

Francien Peterse  Jung-Hyun Kim  Stefan Schouten  Dorthe Klitgaard Kristensen  Nalân Koç  Jaap S. Sinninghe Damsté 《Organic Geochemistry》2009,40(6):692-699

Branched glycerol dialkyl glycerol tetraethers (GDGTs) are membrane lipids of unknown bacteria that are ubiquitous in soil and peat. Two indices based on the distribution of these lipids in soils, the Cyclization of Branched Tetraethers (CBT) and the Methylation of Branched Tetraethers (MBT) indices have been shown to correlate with soil pH, and mean annual air temperature (MAT) and soil pH, respectively, and can be used to reconstruct MAT in palaeoenvironments. To verify the extent to which branched GDGTs in marine sediments reflect the distribution pattern on land and whether these proxies are applicable for palaeoclimate reconstruction in high latitude environments with a MAT of <0 °C, we compared the branched GDGT distribution in Svalbard soils and nearby fjord sediments. Although branched GDGT concentrations in the soil are relatively low (0.02–0.95 μg/g dry weight (dw)) because of the cold climate and the short growing season, reconstructed MATs based on the MBT/CBT proxy are ca. ?4 °C, close to the measured MAT (ca. ?6 °C). Concentrations of branched GDGTs (0.01–0.20 μg/g dw) in fjord sediments increased towards the open ocean and the distribution was strikingly different from that in soil, i.e. dominated by GDGTs with one cyclopentane moiety. This resulted in MBT/CBT-reconstructed MAT values of 11–19 °C, well above measured MAT. The results suggest that at least part of the branched GDGTs in marine sediments in settings with a low soil organic matter (OM) input may be produced in situ. In these cases, the application of the MBT/CBT palaeothermometer will generate unrealistic MAT reconstructions. The MBT/CBT proxy should therefore only be used at sites with a substantial input of soil OM relative to the amount of marine OM, i.e. at sites close to the mouth of rivers with a catchment area where sufficient soil formation takes place and the soil thus contains substantial amounts of branched GDGTs.  相似文献   

Correlation of upper Bajocian-Bathonian zones in Siberia with the stage standard     

S. V. Meledina  T. I. Nal’nyaeva  B. N. Shurygin 《Stratigraphy and Geological Correlation》2009,17(3):291-297

The suggestions to transfer the Arcticoceras ishmae Zone from the middle to the lower Bathonian and the Arctocephalites arcticus Zone from the lower Bathonian to the upper Bajocian put forward by some researchers, are critically considered. These suggestions are based on paleontological records from the Sokur quarry near Saratov. Based on paleontological data from Siberia represented in a number of regional ammonite, belemnite, and retroceram zonations, we infer that the proposed zonal subdivision of the Jurassic in the Sokur quarry to be untrue and believe the revision of the correlation of the above-mentioned zones with the stage and zonal standard to be premature because of insufficient argumentation.  相似文献