Catchment environmental change over the 20th Century recorded by sedimentary leaf wax n-alkane δ ¹³C off the Pearl River estuary

Abstract

The compound-specific stable carbon isotope compositions (δ ¹³C) of leaf wax n-alkanes from two short sediment cores recovered off the Pearl River estuary (PRE) were analyzed to check for their capability of indicating decadal scale catchment environmental change. Sedimentary long-chain n-alkanes exhibited an odd-over-even predominance, with a maximum at n-C₂₉ or n-C₃₁, indicating their leaf wax origin was from vascular plants. The δ ¹³C values of C₂₉ and C₃₁ n-alkane in all the sediment samples were in the range of -28.8‰ to -31.2‰, consistent with the C₃ plant-dominated vegetation in the Pearl River catchments. The time series of δ ¹³C records from the two cores were comparable and displayed a decreasing trend from the early 20th century to the end of the 1970s, followed by a reversal in that change leading to continued increase for ca. 15 years. After being corrected for the effect of atmospheric CO₂ rise and δ ¹³C_atm decline, the δ ¹³C₂₉ records largely retained their raw changing pattern; the post-1980 increase being more conspicuous. The slightly decreasing trend in corrected δ ¹³C records before around 1980 may have been caused by an increase in precipitation, whereas the subsequent increase of δ ¹³C is likely associated with the observed dry climate and/or intensive anthropogenic deforestation. Our results thus demonstrate that leaf wax n-alkanes buried in the sediments off the PRE may well reflect change in the regional climate and/or human activity in the river catchments over the past century.