Fire History In The Yangtze River And Pearl River Basins: Black Carbon Records From The China Seas

Fire is an important ecological factor of the Earth's system as it exerts a significant impact on both biosphere and atmosphere by regulating the composition of terrestrial ecosystem and atmosphere.Changes in fire is closely related with climate,vegetation and human activity.A better understanding of the forcing factors of fire activity and the anthropogenic impact on natural fire history can be achieved by reconstructing fire history during geological periods.In this study,the regional fire history since the last glacial period especially during the Holocene in the Yangtze River and Pearl River Basins was reconstructed based on the proxies of abundance and carbon isotope compositions of black carbon,chronology,grain size,element and mineralogy of sediments from core ECMZ on the continental shelf of the East China Sea,core T1 on the subaqueous Yangtze River delta,core K6-2 on the northern continental shelf of the South China Sea and core ZK20 on the subaqueous Pearl River delta,in order to investigate the relationships between fire,climate,vegetation and human activity.Provenance analysis suggests that black carbon in core ECMZ located on the continental shelf of the East China Sea was mainly derived from the Yangtze River.Therefore,the content of black carbon in ECMZ revealed a high-resolution fire history in the Yangtze River Basin since ?7.0 ka BP.The result shows fire was positively correlated with temperature and precipitation on both millennial-and centennial-timescales from 7.0?3.0 ka BP,with more fire occurred during warm and wet periods but less fire occurred under cold and dry climate conditions,suggesting climatic control on regional fire activity.In contrast,there has been a clear divergence between fire and climate change on multi-timescales since ?3.0 ka BP,implying anthropogenic impact overwhelmed natural control of regional fire activity in the Yangtze River Basin.On millennial-timescale,the temperature and precipitation have remained relatively constant from 3.0?1.0 ka BP,and then slightly increased since?1.0 ka BP.But the regional fire activity showed a rapid decline since ?3.0 ka BP,which is coincided with an abrupt increase in the abundance of heavy metals such as Pb and Cu,indicating that fire was suppressed by decline in forest cover(fuel load)associated with increasing human activity such as agriculture and metal smelting and thus altered the inherent relationships previously formed between fire and climate.On centennial-timescale,six fire peaks identified from fire history all occurred during relatively cold and dry periods,which were contemporaneous with periods characterized by increased human activity associated with population migration from the Yellow River Basin to the Yangtze River Basin or technological advance in Chinese history,indicating increasing anthropogenic impact on regional fire.Evidence from archaeology and climate suggests that episodes of famine,war and population migration usually occurred during cold and dry periods.A short-term enhancement in human activity associated with population migration from the Northern China to the Yangtze River Basin or technological advance could promote more biomass burning during specific time interval.Provenance analysis suggests that black carbon in core K6-2 located on the northern continental shelf of the South China Sea was mainly derived from the Pearl River,implying that the regional fire history in the Pearl River Basin since ?50 ka BP could be reconstructed based on the black carbon.The result shows relatively high but decrease trend of regional fire activity at 50.0?27.0 ka BP,generally high regional fire activity at 13.0?7.0 ka BP,and weak regional fire activity at 7.0?4.0 ka BP.A comparison of black carbon with climatic factors indicates that the response of regional fire to climate was distinct during 50.0?27.0 ka BP and 13.0?4.0 ka BP.Regional fire activity was suppressed at 50.0?27.0 ka BP but encouraged at 13.0?4.0ka BP when climate became warmer and wetter.Regardless of the human impact on fire before ?4.0 ka BP,the divergence of the relationships between fire and climate might arise from deference in the amplitude of variation of temperature and precipitation during this two phases.The black carbon in samples of upper 9 m at core ZK20 located in the Pearl River Estuary is used to reconstruct the high-resolution regional fire history in the Pearl River Basin since ?7.0 ka BP.The result shows generally high regional fire activity at 7.0?3.9 ka BP,the highest regional fire activity at 3.9?2.9 ka BP and rapid weakening of regional fire activity since ?2.9 ka BP.A comparison of black carbon record with proxies of climate,pollen and human activity suggests that the highest regional fire activity at 3.9?2.9 ka BP was probably linked to the initial development of agriculture,while the abrupt decline of regional fire activity since ?2.9 ka BP coincided with wide spread of agriculture and the clearance of forest.In this study,we investigate the fire history and its relationships with climate in the Yangtze River and Pearl River Basins on regional scales,and reveal that higher natural fire activity occurred under warmer and wetter climatic conditions without evident human disturbance in the Middle Holocene,suggesting an increase in temperature,precipitation and vegetation abundance could promote regional fire activity.In contrast,we reveal an abrupt decline in regional fire activity in the Yangtze River and Pearl River Basins since 3.0?2.9 ka BP because of increasing human disturbance in the Late Holocene,which supplies evidence for the stratigraphic definition of the Anthropocene.Such findings will also provide insights into fire predictability and fire management policy in Asian monsoon region.