Environmental significance of δ13Ccell in living sedges in SWGT peatland,Chengbu County,Hunan Province
The existence of either a long-term cooling or warming trend during the Holocene has been much debated over the last decade,and seasonality has been proposed as a possible cause for this so-called"Holocene temperature conundrum".A long-term trend of increasing winter insolation during the Holocene is well documented and accepted,and hence there is the need to find a reliable proxy to reconstruct the long-term trend in summer temperature during the Holocene.In this context,we measured the stable carbon isotope compositions of extracted α-cellulose(δ13Ccell)from living sedge plants in the sub-alpine Shiwangutian(SWGT)peatland in Chengbu County in Southwest Hunan Province,in the sub-tropical monsoonal humid climate zone of South China.During March 2021 to July 2022,at approximately monthly intervals,we sampled the newly-emergent leaves of sedge plants(SCNY)from three sites in relatively open(SX and ZX)and relatively closed(beneath forest;XX)environments in the SWGT peatland,which were used for δ13Ccell measurements.During this interval we also conducted in situ monitoring of air temperature,water temperature,precipitation amount,and relative humidity in this peatland.The SCNY δ13Ccell data range from-29.9‰ to-25.4‰,with an average of-27.4‰.The SCNY δ13Ccell data from the SX and ZX sites,in a relatively open environment,are generally similar and both are systematically～0.8‰ more positive than the data from the XX site beneath forest.The time series of SCNY δ13Ccell data from the SX and ZX sites consistently show seasonal characteristics,with more positive values in summer/autumn and more negative values in winter/spring.However,the SCNY δ13Ccell data from the XX site do not show seasonal characteristics.We conducted linear correlation analyses between the SCNY δ13Ccell data and the in-situ monitoring data of air temperature,water temperature,precipitation amount,and relative humidity of the month prior to the sampling day.The results show that:(1)At both the SX and ZX sites,SCNY δ13Ccell has significant positive correlations with both the air and water temperature,but there are no significant correlations with precipitation amount and relative humidity.This indicates that temperature is the major factor influencing sedge δ13Ccell in relatively open environments in the SWGT peatland.(2)At the XX site,SCNY δ13Ccell is not significantly correlated with air temperature,water temperature,precipitation amount,and relative humidity,which highlights the influence of site-specific environmental factors on sedge δ13Ccell.Our sedge δ13Ccell data from a relatively open environment in the sub-tropical monsoonal humid climate zone in South China are consistent with previously reported sedge δ13Ccell values from the Sahara sand peatland(above the tree line and therefore in an open environment)in the Altai Mountains in arid Northwest China.These observations demonstrate the potential of δ13Ccel1 in peatland sedge plants as an indicator of temperature changes.Sedges are common plants in peatlands and therefore we suggest that sedge δ13Ccell in peat cores is a potentially valuable proxy for obtaining reliable Holocene summer temperature reconstructions,especially in high northern latitude and high-altitude peatlands,because sedges in those peatlands only grow in summer.We conclude that δ13Ccell in both modem sedges and sedge residues in peat cores warrants further detailed investigations,especially in high-altitude and high-latitude peatlands.
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