Stable isotopes from peat profiles from the western Himalayas (India as indicators of westerlies and monsoon variability during the Holocene
Final Report Abstract
Present-day conditions and calibration. This project was funded as a feasibility study. The 32 peat, stream and rainwater samples, and 28 samples representing the modern Din Gad valley plant community were collected in 2012. The stable isotope signatures of summer rainwater samples correspond with GNIP data from Indian stations, the stream samples are depleted, whereas the peat-water samples are intermediate and could indicate evaporation. It remains unresolved if, and to what extent, evaporation from the mire surface causes changes of the meteoric isotope input signal. Modern plant samples cover a range of several per mil in carbon and nitrogen stable-isotope values, carbon isotope values of cellulose are enriched by more than one per mil compared to the bulk plant material. The respective cellulose oxygen-isotope values cover an even broader range of more than 10 per mil (samples from a single site). Overall, carbon and oxygen isotope values of plant cellulose were found to be negatively correlated, probably related to plant-life form. Palaeoclimate reconstruction. Himalayan peat deposits have the potential to help disentangle Holocene monsoon variability. Since the variability in the isotopic composition of parent material in the mires and their vicinity was shown to be very pronounced, detailed taxonomic and isotopic studies of modern plants and macrophytic remains are indispensable for further studies. Peat composition has to be closely tracked in time to obtain reliable knowledge from isotope proxies. Derivation of the isotopic composition of meteoric water meets two challenges, evaporation effects through transpiration and open-water evaporation, and an isotopic offset induced by early decomposition of dead plant material. For those phases where the archive is consolidated our data indicate the potential to mirror relative changes in site hydrology and climate conditions. Additional measures are needed, however, to perform a calibration between the meteoric input and the time integrating the response of peat.
