海洋地质与环境重点实验室知识库

  陆地植被是全球生态系统的重要组成部分，其C₃/C₄组成类型及演化进程对于我们理解过去生态系统对全球变化的响应及适应机制具有重要科学意义。晚上新世以来东亚C₄植被扩张的时间、过程、全球同步性和机制等仍存很大争议。不同纬度的植被演化在各时间尺度上具有差异性，研究其时空演化规律，可望对C₄植被演化的机制有新的认识。本研究利用大陆架科学钻探计划在南黄海钻取的CSDP-1孔岩芯沉积物的黑碳含量、通量及碳同位素组成以及沉积物年代、粒度、粘土矿物和微量元素等，重建了晚上新世以来中国东部地区C₃/C₄植被演化历史，探讨了植被演化与气候、人类活动之间的关系。

  晚上新世以来，南黄海CSDP-1孔沉积黑碳主要来自长江和黄河。3.5-0.8 Ma之间，沉积物源主要为古长江，自~0.8 Ma开始，物源由古长江转变为古黄河。晚上新世（3.0-2.6 Ma），CSDP-1孔黑碳碳同位素组成（δ¹³C_BC）发生较大幅度正偏，指示源区长江流域C₄植被的相对丰度增加，综合不同纬度地区记录推测干旱化加剧可能对中低纬度地区C₃/C₄植被演化有重要影响，同时，大气CO₂浓度降低也可能是C₃/C₄植被变化的原因之一。自中更新世（~1.0 Ma）以来，黑碳碳同位素组成整体负偏，表明源区黄河流域C₄植被相对含量减少。研究认为，全球气候变冷及源区的改变，使得温度成为了影响中更新世以来C₄植被演化的重要因素。

  末次冰期以来，CSDP-1孔的沉积物（包括黑碳）主要由黄河输入。末次冰期-全新世，黑碳碳同位素比值正偏，与黄土剖面有机质δ¹³C变化趋势类似，暗示黄河流域C₄植被相对丰度在全新世明显增大。综合现代植被分布、不同纬度地区的记录，推测冰期过低的温度成为了中高纬度地区C₄植被生长的限制因素，温暖湿润的间冰期更适宜C₄植被生长。

  中全新世(7 ka B.P.)以来，南黄海海平面和洋流系统相对稳定，黑碳记录指示的源区植被类型主要受气候和人类活动共同制约。其中7-2.5 ka B.P.，CSDP-1孔岩芯沉积黑碳δ¹³C值逐渐负偏，气候因子（温度和降水）的降低可能是C₄植被相对含量减少的原因。但在~2.5 ka B.P.以后，气候与植被生长之间的关系发生解耦，温度、降水持续降低，但黑碳δ¹³C值突然正偏。黄河流域人类大肆砍伐燃烧森林、开采冶炼矿石以及人口大量增长、农业迅速发展，对环境的影响越来越显著。~2.5 ka B.P.，人类活动成为了影响植被生长的首要原因。人类大量种植C₄作物，导致了CSDP-1孔黑碳碳同位素组成~2.5 ka B.P.以来的正偏。

  本研究重建了构造-千年时间尺度中国东部地区C₄植被的演化历史。结合分析不同纬度地区C₄植被的演化以及现代C₃/C₄植被的分布，我们认为在中低纬度地区，当温度满足C₄植被生长的需要，干旱的气候更有利于C₄植被获得生长优势，而中高纬度地区较低的温度会成为限制C₄植被生长的因素，温度和降水对植被的作用在各时间尺度上都成立。~2.5 ka B.P.以来，人类活动的增强取代了自然环境成为了影响植被的首要因素，或许可对人类世的划分提供科学依据。

  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/C₄ vegetation 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/C₄ vegetation since the Middle Pleistocene due to the global cooling and the 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 the δ¹³C value of organic matter in the loess profile, suggesting the C₄ vegetation 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., the δ¹³C value of black carbon of core CSDP-1 gradually 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.