Variations In Sphagnum Carbon And Oxygen Isotopic Compositions In Hani Peatland And Their Significance

Peatlands can document past climatic and environmental conditions through their peat accumulation and are important archives for paleoenvironmental studies.Northeast China has developed abundant peatlands that are represented by the extensively studied Hani peatland in Changbai Mountains.Stable carbon and oxygen isotopic compositions are important paleoenvironmental proxies for peatlands and have been used as indicators of long-term precipitation/moisture and temperature changes in the past.However,the modern process studies that explore the paleoenvironmental significance of carbon and oxygen isotopic compositions are insufficient.Also,existing studies of paleoenvironmental reconstruction often selected bulk peat materials for stable isotope analyses and did not separate different botanical parts,resulting in difficulties in proxy interpretations.This study conducts a modern process investigation of the paleoenvironmental significance of Sphagnum carbon and oxygen isotope proxies.Surface Sphagnum samples were collected along a local hummock-hollow moisture gradient and a local surface moisture source gradient（with peatland surface water samples collected）.Also,a long-strand Sphagnum sample（about 28 cm）was collected from the side of the hummock that likely has been continuously growing for years.Laboratory work mainly includes separating the newest stem parts beneath the capitula for surface Sphagnum samples.For the long-strand Sphagnum sample,it was sectioned and cut to derive many stem and branch samples.These samples were extracted for cellulose and then were analyzed for carbon and oxygen isotopic compositions.The results show that under the moisture gradient,surface Sphagnumδ¹³C has a significantly positive correlation with water content,supporting the“water film”effect,but the surface Sphagnum samples growing in waterlogged,extremely wet conditions have extremely negativeδ¹³C value indicative of methane recycling.Surface Sphagnumδ¹⁸O has a significantly negative correlation with water content,supporting the“leaf water evaporation”effect.Under the surface moisture source gradient,there is no significant correlation between Sphagnumδ¹⁸O and peatland surface waterδ¹⁸O,likely partly supporting the importance of the“leaf water evaporation”effect.The long-strand Sphagnumδ¹³C andδ¹⁸O show four smooth annual cycles with the distance from the capitulum.Their values vary at an amplitude of 4‰and 6‰,respectively,with roughly the same phase relationship in between,both becoming higher in summer and lower in winter.These match with the local conditions of rainy and wet summer(highδ¹³C)with high precipitationδ¹⁸O and dry winter(lowδ¹³C)with low precipitationδ¹⁸O.It is inferred that the growth rate of the Sphagnum strand can be 7 cm per year.Sphagnumδ¹³C andδ¹⁸O cycles also show inter-annual variations.The highestδ¹³C point in an annual cycle is higher in very rainy years（Year 2018,2020）than in less rainy years（Year 2019,2021）and they also continuously affect the subsequent winterδ¹³C values.The highestδ¹⁸O point in an annual cycle does not obviously differ in different years,but the lowestδ¹⁸O point in the warm winter of 2010–2011 is 2‰lower than that in the cold winter of 2021–2022and also lasts a longer period of time.It is opposite to the common“temperature effect”of precipitation isotopes.For this finding,this study proposes that during warm winters,snow accumulation decreases and the snowmelt timing and the growing season becomes earlier.Then,Sphagnum mosses can utilize the snowmelt water with extremely lowδ¹⁸O and enhance their growth.Though a systematic modern process investigation,this study demonstrates the spatiotemporal changes and mechanisms for Sphagnum carbon and oxygen isotopic compositions in Hani peatland.It shows that Sphagnumδ¹³C is a reliable proxy for wetness or precipitation amount,but Sphagnumδ¹⁸O might be affected by winter temperature and snowmelt with complexity.This study further supports the idea about the internal link between Sphagnumδ¹³C andδ¹⁸O,suggesting that we should consider the coupling between Sphagnumδ¹³C andδ¹⁸O proxies in paleoenvironmental studies.Also,this study also suggests that stable isotope proxies of Sphagnum mosses have the potential to reconstruct past extreme ecological environments in peatlands.