Karst hydrological changes during the Late-Holocene in Southwestern China

Abstract

The frequent alternation between droughts and floods in the karst regions of Southwestern China has a serious impact on the ecological environment and socio-economic development. Although some high-resolution records for this region have been published, there is a lack of multi-proxy geological records that could be used to reconstruct the relationships between the changes in the Asian summer monsoon (ASM) and the regional hydrology and ecological environment since the Late Holocene. In this study, the history of the ASM from 3109 to 694 yr BP with a mean temporal resolution of 2.5 yr is reconstructed based on 47 high-precision 230Th dating results (mean 2-sigma error of ±14 yr), 959 pairs of δ18O/δ13C data, and multiple trace element analyses of a stalagmite from Shijiangjun (SJJ) Cave in the karst area of Southwestern China. The positive δ18O and δ13C excursions accurately recorded eight interdecadal-centennial weak summer monsoon events at ∼779, 1013–911, 1282–1172, 1736–1638, 1961–1864, 2472–2375, 2931–2818, and 3050–3014 yr BP. The cross-wavelet spectrum analysis of the δ18O and δ13C of stalagmite SJJ7 indicates that they have similar periods. The 7 yr period of the δ18O record was determined to have the maximum contribution rate (36.8%) to the periods using ensemble empirical mode decomposition (EEMD) analysis. In the Late Holocene, the weak ASM events were dominated by the southward shift of the Intertropical Convergence Zone (ITCZ) and the frequent El Niño events on an interdecadal-centennial timescale. The southward migration of the ITCZ lead to frequent El Niño events, and the resultant Hadley Circulation and Walker Circulation were weakened, leading to a weak ASM and changes in the hydrological conditions in the monsoon region. The δ13C values of the stalagmite changed relatively slowly compared with the δ18O values, which may indicate that the degradation and restoration of the regional ecological environment caused by abrupt changes in the climate is a relatively slow process. When the summer monsoon decreased, the stalagmite’s Mg/Ca and Sr/Ca ratios were relatively high due to CO2 degassing, and the prior calcite precipitation (PCP)/prior aragonite precipitation (PAP) increased in the karst zone. However, the Mg/Ca ratio increased and the Sr/Ca ratio rapidly decreased during the calcite deposition due to differences in the crystal structures and partition coefficients of aragonite and calcite. The dark layers in stalagmite SJJ7 correspond to transition from heavy to light δ18O values, reflecting the transition in the ASM. The pulsed increases in the elements (Mn, Fe, Al, and Si) in the dark layers reflect the stronger mechanical transport caused by more rainfall. The multi-proxy analysis of this stalagmite may reflect the interactions between the changes in the ASM and the atmosphere-hydrosphere-pedosphere-biosphere-lithosphere in the karst critical zone during the Late Holocene.