Journal
BIOGEOSCIENCES
Volume 12, Issue 11, Pages 3141-3151Publisher
COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/bg-12-3141-2015
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Funding
- National Basic Research Program of China [2014CB954001]
- National Science Foundation of China [41176164, 41476062]
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The methylation index of branched tetraethers (MBT) and cyclization ratio of branched tetraethers (CBT) based on the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGT) are useful proxies for the reconstruction of mean annual air temperature (MAT) and soil pH. Recently, a series of 6-methyl brGDGTs were identified which were previously co-eluted with 5-methyl brGDGTs. However, little is known about 6-methyl brGDGTs in the Qinghai-Tibetan Plateau (QTP), a critical region of the global climate system. Here, we analyze 30 surface soils covering a large area of the QTP, among which 6-methyl brGDGTs were the most abundant components (average 53 +/- 17% of total brGDGT). The fractional abundance of 6-methyl brGDGTs showed a good correlation with soil pH, while the global MBT'(5ME) calibration overestimates MAT in the QTP. We therefore proposed a MBT5/6 index including both 5- and 6-methyl brGDGTs, presenting a strong correlation with MAT in QTP: MAT = -20.14 + 39.51 x MBT5/6 (n = 27; r(2) = 0.82; RMSE=1.3 degrees C). Another index, namely IBT (isomerization of branched tetraether), based on carbon skeleton isomerization of the 5-methyl to 6-methyl brGDGTs, is dependent on soil pH: pH = 6 .77-1.56 x IBT (n = 27; r(2) = 0.74; RMSE = 0.32). Our study suggests that changing the position of methyl group of brGDGTs may be another mechanism for some soil bacteria to adapt to the ambient pH change in addition to the well-known cyclization.
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