Holocene variability of bulk organic C/N and δ 13 C and implications for the sugarcane cultivation

Organic geochemical proxies can effectively distinguish the sources of organic matter in sediments, and thus reflect the human impacts on delta evolution. The natural vegetation in the Pearl River basin is dominated by C 3 plants. But with the development of agriculture in the historical period, sugarcane has been cultivated in the Pearl River basin from Han dynasty, which possibly changed the composition of vegetation in the river basin and increased sediment load into the river mouth. In this study, we collected two boreholes HP(22°43'54.92″N, 113°25'41.86″E) and MZ(22°38'9.69″N, 113°32'24.23″E) along the Hongqili Channel in the Pearl River Delta, and the sediments above 18.00 m in borehole HP and 45.00 m in borehole MZ were analyzed. We carried out measurement of organic carbon,nitrogen and delta13 C values ​​for the bulk sediments and AMS 14 C dating in the two boreholes. The organic geochemical analysis was carried out on 102 samples containing no gravel in borehole HP and 160 samples containing no gravel in borehole MZ. Two charcoals and four shells were selected from borehole HP, and one charcoal, four plant fragments, two woods and six shells were selected from borehole MZ for AMS 14 C dating. The age of borehole HP at 18.00 m was about 8700 cal.a BP and that of borehole MZ at 45.00 m was about 13000 cal.a BP In order to better explain the source of organic matter, we also collected the C/N and δ 13C dataset of surficial and borehole sediments in previous studies for comparison. The results show that C/N varied from 2.09 to 28.05 and δ 13 C values ​​are in the range of -29.4 ‰~-21.6 ‰. The organic geochemical compositions in the two boreholes indicate the dominance of terrestrial organic matter in the Holocene sediments, suggesting the prevalence of fluvial function in the sheltered inner bay of palaeo-Pearl River mouth. However, the δ 13C values ​​in borehole MZ increased to mostly higher than -28.0 ‰ from about 4900 cal.a BP and the C/N decreased to about 8.00 from about 2800 cal.a BP, which indicates the increase in the contribution of marine organic matter, possibly induced by the enhancement of tidal function as a response to the topographic changes. An abrupt increase in the δ 13 C values ​​over past 2000 years was widely observed in the sediment sequences of the Pearl River Delta, for example, mostly being between -26.0 ‰ and -21.0 ‰ in boreholes HP and MZ. This reflects the significant increase in the organic carbon contribution from C 4plants and associated sugarcane cultivation in the drainage basin since the Han Dynasty. This finding also supports the increased impact of agriculture on the construction of the Pearl River Delta over past 2000 years.