Evolution Of C₃/C₄ Vegetation In Eastern China Since The Late Pliocene: Evidence From Black Carbon Record In The South Yellow Sea

Terrestrial vegetation is important part of global ecosystem.The C₃/C₄ evolution has great implication for understanding the response and adaptation mechanism of ecosystem to global changes.The time,process,global synchronicity and mechanism of C₄ plant expansion in East Asia since the Late Pliocene are still controversial.Vegetation evolution varies in different latitudes at different time scales.Summarizing the spatiotemporal evolution law can help understand the mechanism of C₃/C₄vegetation evolution.The content,MAR and carbon isotopic composition of black carbon as well as the chronology,grain size,clay minerals and trace elements of sediments obtained from Continental Shelf Drilling Program core CSDP-1 in the South Yellow Sea,were used to reconstruct the C₃/C₄ vegetation evolution in eastern China since the Late Pliocene.The plant evolution history and related climatic and human activity mechanisms were then involved.Since the Late Pliocene,the black carbon obtained from core CSDP-1 in the South Yellow Sea was mainly derived from the Yangtze River and the Yellow River.During 3.5-0.8 Ma,the provenance was mainly the Yangtze River,and then changed to the Yellow River.In the Late Pliocene（3.0-2.6 Ma）,the carbon isotope composition of black carbon(δ¹³C_BC)showed a distinct increase,indicating that the relative abundane of C₄ vegetation in the Yangtze River Basin obviously increased.Combined with the records from different latitudes,it is suggested that enhanced aridity and the decline of atmospheric CO₂ concentration were the main driving forces in the middle-low latitude.Since the Middle Pleistocene（～1.0 Ma）,the carbon isotope composition of black carbon showed more negative,suggesting that the relative content of C₄ vegetation in the Yellow River Basin declined.We propose that temperature has become the most important factor affecting the evolution of C₃/C₄vegetation since the Middle Pleistocene due to the global cooling and t he change of provenance.The sediment,including black carbon,since the last glacial period in core CSDP-1 was mainly transported by the Yellow River.The carbon isotope composition of black carbon showed more positive from the last glacial period to the Holocene,as well as theδ¹³C value of organic matter in the loess profile,suggesting the C₄vegetation significant expansion in the Yellow River Basin.Combined with the records about modern vegetation distribution and plant evolution in different latitudes,we propose that low temperature constrained the C₄ plant growth,and warm and humid interglacial period is more suitable for C₄ vegetation in the middle-high latitude.Since the Middle Holocene（7 ka B.P.）,the sea level and ocean current system in the South Yellow Sea had been relatively stable,and the vegetation types in the source area indicated by black carbon records were mainly controlled by climate and human activity.During 7-2.5 ka B.P.,theδ¹³C value of black carbon of core CSDP-1gradually shifted negatively.The decline of climatic factors,including temperature and precipitation,may cause the decrease of C₄ vegetation content.However,after～2.5 ka B.P.,the relationship between climate and plant growth was decoupled,δ¹³C_BC value suddenly increased while temperature and precipitation continued to decrease.～2.5 ka B.P.,the human beings felled and burned forests,mined and smelted ores,and the population grew rapidly as well as the agriculture in the Yellow River Basin,which had increasing significant impact on the environment.Human impact had become the primary mechanism to affect the distribution of plants.The carbon isotope composition of black carbon in core CSDP-1 since～2.5 ka B.P.showed more positive,which was caused by the massive cultivation of C₄ crops by human.This study reconstructs the evolution history of C₄ vegetation in eastern China at the tectonic to millennial scale.Combined with the C₄ vegetation evolution at different latitudes and the distribution of modern C₃/C₄ plant,we infer that in the low-middle latitude,when the temperature meets the growth needs of C₄ plant,the arid climate is more conducive to the growth advantages of C₄ vegetation,while the lower temperature in the middle-high latitude will become a limiting factor for the growth of C₄ vegetation.The effects of temperature and precipitation on vegetation are consistent at all time scales.Since～2.5 ka B.P.,the human impact overwhelmed the natural environment as the primary factor affecting vegetation,which may supply evidence for the stratigraphic definition of the Anthropocene.