Record of environmental change by α-cellulose δ 13 C of sphagnum peat at Shennongjia, 4000–1000 aBP

The Dajiuhu Basin at Shennongjia, located within typical East Asian Monsoon region, preserves a sub-alpine sphagnum peat deposition in its central area. The topmost 120 cm of the peat covers the last 4000 years according to AMS 14 C dating of pollen concentration. Carbon isotope of α-cellulose, extracted from sphagnum peat, provides a quantitative reconstruction of atmospheric relative humidity, based on transfer functions of C 3 plants carbon isotopic fractionation equation and the bryophyte photosynthesis CO 2 absorption rate equation. δ 13 C, TOC and C/N variations reveal that the Dajiuhu area has experienced a long-term tendency to dry during 4000–1000 aBP, with a major transition happening around 3000 aBP. Four relative dry events are identified at 3400–3200, 3000–2600, 2200–2000 and 1600–1400 aBP, respectively, corresponding to those climate events documented in many global records. Three periodicities, 664 a, 302 a and 277 a enclosed in the atmospheric humidity of Dajiuhu are correlated to the cycles of solar activities. The weakening of East Asia summer monsoon during this period registered in the Dajiuhu peat is consistent with the synchronous weakening of Indian Monsoon. This trend may be attributed to gradual decrease of Northern Hemispheric summer solar insolation and the consequently southward migration of Intertropical Convergent Zone (ITCZ).