Oxygen-Isotope Records for Local Processes and Regional Hydroclimate Over the Last 2500 Years from a Permafrost-Affected Slope Peatland on the Central Tibetan Plateau

Abstract

The Tibetan Plateau (TP) has experienced pronounced climate change, resulting in a multitude of responses in land surface and ecosystems. Resultant permafrost dynamics and degradation would profoundly alter soil moisture conditions, shifting plant water use strategy as a result.Plants in the genus Kobresia—being dominant and highly successful sedge plants in alpine meadows and peatlands on the TP—have well-developed and deep root systems, often accounting for >90% of total biomass. However, the usefulness of isotopic records from Kobresia-dominated peatlands is unknown. Here we used multi-proxy records of peat properties, plant macrofossils, and stable oxygen isotopes (δ18Ocellulose) from two peat cores (~100 cm in length) of a Kobresia-dominated alpine peatland (32.960°N, 94.247°E; 4790-4870 m asl) to understand isotope behaviors and to reconstruct hydroclimate change during the last 2500 years. Core C1 is located near upper boundary of the slope peatland near upslope mineral soil, while core C4 is in the peatland center. The plant macrofossils were mainly composed of black leaf sheath and roots of Kobresia. C1 shows high δ18Ocellulose values of ~16.5‰ and a slight decreasing trend from 2200 to 900cal yr BP, a pronounced decrease to the lowest values of -4.5‰ at 500-200 cal yr BP, then a major increase to 18.5‰ until the present. In contrast, C4 shows a long-term decreasing trend (from 16.5‰ to 4.5‰) from 2500 to 900 cal yr BP with large-magnitude fluctuations between 2300 and 1900 cal yr BP, then rapid increased to 18‰ afterward until the present. The large difference in δ18Ocellulose between two cores suggests that groundwater sources used by plants were different. As C1 is at the edge of the peatland near upslope mineral coarse-grained soils, it has limited water storage capacity, and as a result plants might have received great proportion of contemporary meteoric water. In contrast, C4 is located in the center of the slope peatland, and the ultra-low hydraulic conductivity of dense peat would limit groundwater turnover, dampening signals from meteoric water. Furthermore, water-rich peat surrounding C4 would likely contribute permafrost meltwater with changing climate and active layer thickness over time, further dampening precipitation signals. Therefore, we argue that isotopes from C1 more sensitively reflect hydroclimate and atmospheric circulation changes. The high δ18Ocellulose values at 2200-900 cal yr BP in C1 reflected a weakening South Asian Summer Monsoon (SASM) and dry climate roughly during the Rome Warm Period and Medieval Climate Anomaly (MCA), while low δ18Ocellulose values afterward during the “Little Ice Age” (LIA) suggested a strong SASM and wet climate. The large difference of 20‰ in δ18Ocellulose between the MCA and LIA suggests that both high-elevation setting and local processes amplify δ18Ocellulose change; such a large shift in oxygen isotopes has been documented elsewhere from peatlands in the southeastern TP. Our results indicate the complex local hydrological processes and plant water utilization by deep-root vascular plants in this permafrost-affected peatland, potentially offering water isotopic proxies with amplifying hydroclimate signals from suitable site locations.