Final Report Summary - TIBETMETH (Microbial Biomarker Records in Tibetan Peats: Monsoon Variability and its Impact on Methane Biogeochemistry)
We have achieved all of the key project objectives with respect to biomarker analyses from the peats in the Tibetan Plateau:
1) We quantified a suite of archaeal and bacterial lipids from Tibetan peats using GC-MS and HPLC-MS. The specific lipids are archaeol and hydroxyarchaeol (sn-2 and sn-3 isomers), collectively serving as a proxy for methanogen biomass (Pancost et al., 2011). These revealed a sharp but predicted decrease in methanogen biomass during a local drying event induced by changes in the Asian Monsoon. We also quantified a wide range of related archaeal membrane lipids that supplemented these interpretations.
2) We quantified a range of other lipid biomarkers that provide insight into vegetation change (sterols and n-alkyl lipids) as well as peat bog redox change (stanol/sterol proxy).
3) We determined the carbon isotopic compositon of hopanoids and n-alkanes. The former documented shifts in methanotrophy, which appears to have become more efficient during dry periods.
4) In collaboration with partners in the Department of Geographical Sciences, we interrogated our interpretations using climate models.
5) To facilitate interpretation, I have compiled records of the Asian monsoon systems, especially with respect to Tibetan Plateau precipitation, and these generally indicate a precipitation decline during the Holocene.
(2) Main Results
Holocene variations of Asian summer monsoon precipitation. The temporal pattern of decreasing precipitation during the Holocene has been recorded by humification indices (Yu et al., 2011) and δ13C values of C. muliensis cellulose (Hong et al., 2003) in the Hongyuan peat. These records resemble other Holocene precipitation records across the AM-influenced region, that is, Dongge Cave, southern China (Wang et al., 2005) and Indian summer monsoon records (Fleitmann et al., 2003). Specifically, the Holocene optimum with maximum effective precipitation for the monsoon region occurs from ca. 10.5 to 6.5 ka BP (Zhang et al., 2011). The long-term decrease in precipitation after 6 ka BP suggests that similar monsoon precipitation trends dominate over large areas. Modelled P–E for the region using the coupled ocean-atmosphere Hadley Centre climate model also indicates a decreased effective precipitation through the Holocene. However, the Hongyuan Peat δ13C values and humification records do not suggest only a monotonic decrease in Tibetan Plateau precipitation but instead document a long-term drying overprinted by a pronounced dry interval from 6 to 4 ka.
Methanogenesis associated with Asian summer monsoon. Intervals with high concentrations of archaeol and hydroxyarchaeol are indicative of enhanced methanogen biomass, and by extension methanogenic activity, occurring under anaerobic conditions, and vice versa. These are expected to occur during dry intervals and this has been invoked as an explanation for atmospheric methane variability in the Holocene. Indeed markedly lower concentrations of archaeal diether lipids coincide with the low P–E of the mid-Holocene from 6.4 to 4ka BP, recorded both elsewhere on the Tibetan Plateau and more widely in the AM region. After 4 ka BP, the methanogen biomass increases again but varies markedly, possibly at millennial scales, although these shallow records could also reflect the influence of currently living organisms. This interval corresponds to a generally weak ASM but elevated Tibetan peatland wetness. Overall, a close linkage between precipitation and methanogenesis is consistent with investigations of the nearby Zoige wetland of the Tibetan Plateau, where archaeal community (methanogen) abundances are 10-times lower during drought years (Tian et al., 2012).
Methanotrophy during the mid-Holocene. We also observed diploptene δ13C values below ~40‰ in the 6.4- to 4-ka interval indicating a relatively large methanotroph population. This was not expected as low methanogenesis could have led to a smaller methanotroph population. We have proposed that this maximum in the Hongyuan peat reflects particularly efficient oxidation of methane and that this arose from either a diffusive flux regime or rhizosphere oxidation associated with deeper roots, both of which are consistent with drier climatic conditions.
(3) Conclusions
The Hongyuan Peat, therefore, appears to record a combination of local, regional and global forcings, and we have interrogated these by comparing modelling results with local proxy records. First, these suggest that the Tibetan Plateau became less methanogenic during the mid-Holocene, presumably because of orbital forcing. This effect appears to have been even stronger elsewhere in East China. Second, this minimum in methane production was, at least in the Hongyuan peat, associated with a non-intuitive increase in methanotrophy that we attribute to more efficient methane oxidation. Third, a weakened ASM, especially in marginal regions and at the high elevation of the Tibetan Plateau, apparently brought about a mid- Holocene minimum in CH4 emission from about 6 to 4 ka. Climate impacts on wetland extent and methanogenesis were likely not limited to the Tibetan Plateau, especially given the widespread, orbitally paced decrease in monsoon intensity through the Holocene. Therefore, the Tibetan peat data provide evidence for how monsoon-driven hydrological conditions could have more widely influenced CH4 emissions during the Holocene. We propose that CH4 emissions, at least in East Asia, were indeed controlled by interactions of large-scale atmospheric circulations, but modulated by regional factors, and that future work should explore the regional variation of these responses.
References:
Hong, Y. T. et al. Correlation between Indian Ocean summer monsoon and North Atlantic climate during the Holocene. Earth Planet. Sci. Lett. 211, 371–380 (2003).
Yu, X. F. et al Different patterns of changes in the Asian summer and winter monsoons on the eastern Tibetan Plateau during the Holocene. Holocene 21, 1031–1036 (2011).
Wang, Y. J. et al. The Holocene Asian monsoon: links to solar changes and North Atlantic climate. Science 308, 854–857 (2005).
Fleitmann, D. et al. Holocene forcing of the Indian Monsoon recorded in a stalagmite from southern Oman. Science 300, 1737–1739 (2003).
Tian, J. Q. et al. Effects of drought on the archeael community in soils of the Zoige wetlands of the Qinghai-Tibetan plateau. Eur. J. Soil Biol. 52, 84–90 (2012).
(4) Their potential impact and use and any socio-economic impact of the project
Our results have shown how changes in the Asian monsoon affected emissions of methane, a prominent greenhouse gas, from the Tibetan Plateau. During relatively dry intervals, the biomass of methane-producing microorganisms decreased while methane-consuming microorganisms apparently became more efficient. Our results provide strong evidence for previous researchers’ inferences. In modern settings, methane emissions from dryer settings are generally low. Consequently, previous researchers have speculated that as the Asian monsoon became weaker over the past six thousand years, methane emissions also decreased. Here, we show that this is exactly what happened to this peatland on the Tibetan Plateau. Our study has wider implication for how these systems work. The dry interval we studied arose from large scale changes in atmospheric circulation patterns, and just as past changes impacted methane emissions, so will future climate change: given climate projections of future monsoon intensification, our work suggests the possibility of a tropical methane feedback. Moreover, our works suggests how complex and interrelated biological, chemical and climate systems are, such that human-induced climate change will almost certainly have unexpected consequences.
1) We quantified a suite of archaeal and bacterial lipids from Tibetan peats using GC-MS and HPLC-MS. The specific lipids are archaeol and hydroxyarchaeol (sn-2 and sn-3 isomers), collectively serving as a proxy for methanogen biomass (Pancost et al., 2011). These revealed a sharp but predicted decrease in methanogen biomass during a local drying event induced by changes in the Asian Monsoon. We also quantified a wide range of related archaeal membrane lipids that supplemented these interpretations.
2) We quantified a range of other lipid biomarkers that provide insight into vegetation change (sterols and n-alkyl lipids) as well as peat bog redox change (stanol/sterol proxy).
3) We determined the carbon isotopic compositon of hopanoids and n-alkanes. The former documented shifts in methanotrophy, which appears to have become more efficient during dry periods.
4) In collaboration with partners in the Department of Geographical Sciences, we interrogated our interpretations using climate models.
5) To facilitate interpretation, I have compiled records of the Asian monsoon systems, especially with respect to Tibetan Plateau precipitation, and these generally indicate a precipitation decline during the Holocene.
(2) Main Results
Holocene variations of Asian summer monsoon precipitation. The temporal pattern of decreasing precipitation during the Holocene has been recorded by humification indices (Yu et al., 2011) and δ13C values of C. muliensis cellulose (Hong et al., 2003) in the Hongyuan peat. These records resemble other Holocene precipitation records across the AM-influenced region, that is, Dongge Cave, southern China (Wang et al., 2005) and Indian summer monsoon records (Fleitmann et al., 2003). Specifically, the Holocene optimum with maximum effective precipitation for the monsoon region occurs from ca. 10.5 to 6.5 ka BP (Zhang et al., 2011). The long-term decrease in precipitation after 6 ka BP suggests that similar monsoon precipitation trends dominate over large areas. Modelled P–E for the region using the coupled ocean-atmosphere Hadley Centre climate model also indicates a decreased effective precipitation through the Holocene. However, the Hongyuan Peat δ13C values and humification records do not suggest only a monotonic decrease in Tibetan Plateau precipitation but instead document a long-term drying overprinted by a pronounced dry interval from 6 to 4 ka.
Methanogenesis associated with Asian summer monsoon. Intervals with high concentrations of archaeol and hydroxyarchaeol are indicative of enhanced methanogen biomass, and by extension methanogenic activity, occurring under anaerobic conditions, and vice versa. These are expected to occur during dry intervals and this has been invoked as an explanation for atmospheric methane variability in the Holocene. Indeed markedly lower concentrations of archaeal diether lipids coincide with the low P–E of the mid-Holocene from 6.4 to 4ka BP, recorded both elsewhere on the Tibetan Plateau and more widely in the AM region. After 4 ka BP, the methanogen biomass increases again but varies markedly, possibly at millennial scales, although these shallow records could also reflect the influence of currently living organisms. This interval corresponds to a generally weak ASM but elevated Tibetan peatland wetness. Overall, a close linkage between precipitation and methanogenesis is consistent with investigations of the nearby Zoige wetland of the Tibetan Plateau, where archaeal community (methanogen) abundances are 10-times lower during drought years (Tian et al., 2012).
Methanotrophy during the mid-Holocene. We also observed diploptene δ13C values below ~40‰ in the 6.4- to 4-ka interval indicating a relatively large methanotroph population. This was not expected as low methanogenesis could have led to a smaller methanotroph population. We have proposed that this maximum in the Hongyuan peat reflects particularly efficient oxidation of methane and that this arose from either a diffusive flux regime or rhizosphere oxidation associated with deeper roots, both of which are consistent with drier climatic conditions.
(3) Conclusions
The Hongyuan Peat, therefore, appears to record a combination of local, regional and global forcings, and we have interrogated these by comparing modelling results with local proxy records. First, these suggest that the Tibetan Plateau became less methanogenic during the mid-Holocene, presumably because of orbital forcing. This effect appears to have been even stronger elsewhere in East China. Second, this minimum in methane production was, at least in the Hongyuan peat, associated with a non-intuitive increase in methanotrophy that we attribute to more efficient methane oxidation. Third, a weakened ASM, especially in marginal regions and at the high elevation of the Tibetan Plateau, apparently brought about a mid- Holocene minimum in CH4 emission from about 6 to 4 ka. Climate impacts on wetland extent and methanogenesis were likely not limited to the Tibetan Plateau, especially given the widespread, orbitally paced decrease in monsoon intensity through the Holocene. Therefore, the Tibetan peat data provide evidence for how monsoon-driven hydrological conditions could have more widely influenced CH4 emissions during the Holocene. We propose that CH4 emissions, at least in East Asia, were indeed controlled by interactions of large-scale atmospheric circulations, but modulated by regional factors, and that future work should explore the regional variation of these responses.
References:
Hong, Y. T. et al. Correlation between Indian Ocean summer monsoon and North Atlantic climate during the Holocene. Earth Planet. Sci. Lett. 211, 371–380 (2003).
Yu, X. F. et al Different patterns of changes in the Asian summer and winter monsoons on the eastern Tibetan Plateau during the Holocene. Holocene 21, 1031–1036 (2011).
Wang, Y. J. et al. The Holocene Asian monsoon: links to solar changes and North Atlantic climate. Science 308, 854–857 (2005).
Fleitmann, D. et al. Holocene forcing of the Indian Monsoon recorded in a stalagmite from southern Oman. Science 300, 1737–1739 (2003).
Tian, J. Q. et al. Effects of drought on the archeael community in soils of the Zoige wetlands of the Qinghai-Tibetan plateau. Eur. J. Soil Biol. 52, 84–90 (2012).
(4) Their potential impact and use and any socio-economic impact of the project
Our results have shown how changes in the Asian monsoon affected emissions of methane, a prominent greenhouse gas, from the Tibetan Plateau. During relatively dry intervals, the biomass of methane-producing microorganisms decreased while methane-consuming microorganisms apparently became more efficient. Our results provide strong evidence for previous researchers’ inferences. In modern settings, methane emissions from dryer settings are generally low. Consequently, previous researchers have speculated that as the Asian monsoon became weaker over the past six thousand years, methane emissions also decreased. Here, we show that this is exactly what happened to this peatland on the Tibetan Plateau. Our study has wider implication for how these systems work. The dry interval we studied arose from large scale changes in atmospheric circulation patterns, and just as past changes impacted methane emissions, so will future climate change: given climate projections of future monsoon intensification, our work suggests the possibility of a tropical methane feedback. Moreover, our works suggests how complex and interrelated biological, chemical and climate systems are, such that human-induced climate change will almost certainly have unexpected consequences.