Rainfall variability and vegetation recovery in rocky desertification areas recorded in recently-deposited stalagmites from Guilin, South China

摘要

An actively growing stalagmite (sample DY-2) from Maomaotou Big Cave in Guilin, south China was dated using a combined approach of Th-230/U, Pb-210 and AMS C-14 dating techniques. The post-bomb C-14 curve shows that the DY-2 was deposited during 1964-2009 CE and a fast growth rate of similar to 2.25 mm/a was determined for this 9.3 cm-long stalagmite. The Pb-210 dating of the top 7 mm part of another stalagmite sample PL4 from Panlong Cave located in the same area as the Maomaotou Big Cave constrained a depositional interval during 1830-2009 CE with a much slower growth rate of similar to 0.04 mm/a. The delta O-18 variations of DY-2 and PL4 are highly comparable on decadal time-scales within age uncertainties, and both reveal a good coherence with local instrumental rainfall and historic dry-wet index records. These two delta O-18 records suggest that rainfall variability in the region of Guilin is modulated by Pacific Decadal Oscillation (PDO) and El Nino-Southern Oscillation (ENSO), as the latter can affect the East Asian Summer Monsoon (EASM) and Western Pacific Subtropical High (WPSH). On annual-to-decadal scales, the increase of EASM intensity and WPSH as well as the shift of negative (cold) PDO and La Nina phases would enhance the summer rainfall in the Guilin region, as revealed by the lower stalagmite delta O-18 values. While weakening EASM and WPSH with positive (warm) PDO and El Nino phases tend to lead to a dry climate in this region of south China. In addition, the stalagmite delta C-13 and Dead Carbon Proportion (DCP) records show gradually decreasing trends over the last several decades. According to the findings of synchronous increase in both soil pCO(2) and dissolved inorganic carbon (DIC) concentrations in seepage waters from the area of Guilin, the variation of stalagmite delta C-13 and DCP may reflect a history of vegetation recovery in such a rocky desertification area like Guilin, where the area of rocky desertification has been reducing since the 1990s due to afforestation policies and measures.