Abstract:

Coal accumulation is a crucial component of the global carbon cycle,facilitating the large-scale and long-term burial of atmospheric CO₂ on land. It is one of the primary factors leading to a decrease in atmospheric CO₂ concentration,which in turn causes a drop in global temperatures and can trigger the onset of an ice age. At the end of the Carboniferous period,a basin-wide coal accumulation event(Coal #8+9)occurred in the North China Basin,after which the globe entered the Early Permian P1 ice age. This suggests a potential causal link between the widespread coal accumulation in the NCB and the onset of the P1 ice age. To investigate this relationship,this study focuses on Coal #8+9 in the Ordos Basin. Spectral analysis was performed on its natural gamma ray logging curves to reconstruct the accumulation duration of Coal #8+9. Based on coal thickness,carbon content,and apparent density data,the carbon burial rate of the peatland was reconstructed. Using this rate and the estimated area of the Late Carboniferous peatland in the NCB,the annual carbon burial amount in the basin during that time was calculated. Subsequently,based on existing atmospheric data,the annual reduction in atmospheric CO₂ concentration resulting from this carbon burial was estimated,ultimately revealing the relationship between coal accumulation in the Late Carboniferous NCB and the onset of the P1 ice age. The results indicate that Milankovitch cycles of obliquity(35.5 kyr)and precession(21.7 kyr)were identified in Coal #8+9 of the North China Basin. The depositional duration is estimated to be between 98.31 kyr and 277.66 kyr,with an average of 188.20 kyr. The carbon burial rate of the peatland ranged from 42.41 to 78.87 g/·a, with an average of 59.21 g/m²·a. According to estimates of total atmospheric mass and CO₂ concentration,removing approximately 2.13 GtC from the atmosphere is required to lower the atmospheric CO₂ concentration by 1 μL/L. The annual carbon burial amount in the Late Carboniferous NCB was calculated to be between 3.82×10⁷ t and 7.01×10⁷ t,averaging 5.33×10⁷ t. This corresponds to an annual reduction in atmospheric CO₂ concentration of 1.79×10^-2~3.33×10^-2 μL/L,with an average of 2.51×10^-2 μL/L. The atmospheric CO₂ concentration before the end-Carboniferous coal accumulation was approximately 400 μL/L,while it was about 200 μL/L before the onset of the P1 ice age. Based on the average annual carbon burial rate of the NCB,it would take only about 8000 years to reduce atmospheric CO₂ by 200 μL/L,reaching the level associated with the P1 ice age inception. However,the average depositional duration of Coal #8+9 is 188.20 kyr,far exceeding the time required to reach the ice age threshold. Therefore,the massive carbon burial in the NCB could have led to a rapid decrease in atmospheric CO₂ concentration just before the onset of the P1 glaciation. The decoupling of ρCO₂ from continental silicate weathering,coupled with the rapid expansion of tropical rainforests and their repeated reorganization,indicates that the extensive coal accumulation in the NCB during the latest Carboniferous was the primary cause of the atmospheric CO₂ decline preceding the P1 ice age.

Key words: North China Basin, Gzhelian, peatland carbon burial rates, Milankovitch cycles, coal accumulation, P1 glaciation

摘要：

聚煤作用是全球碳循环重要环节,实现了大气CO₂在陆地的大量长期埋藏,是导致大气CO₂浓度降低的主要因素之一。大气CO₂浓度降低会引起全球气温降低,进而触发冰期。石炭纪末华北盆地发生了全盆地范围的聚煤作用(#8+9煤),随后全球进入早二叠世初P1冰期,这表明P1冰期的开始和华北盆地的广泛的聚煤作用可能存在潜在成因联系。为了揭示两者之间的关系,本研究以鄂尔多斯盆地#8+9煤层为研究对象,对其自然伽马曲线进行了频谱分析,恢复了#8+9煤层聚集时间,并根据煤层厚度、碳含量和视密度数据恢复了泥炭地碳埋藏速率,并以此为依据,根据华北盆地晚石炭世泥炭地面积得出当时华北盆地每年的碳埋藏量,根据现有大气数据计算出晚石炭世末华北盆地每年碳埋藏量减少的大气CO₂浓度,最终揭示晚石炭世末华北盆地的聚煤作用和P1冰期开始的关系。结果表明,华北盆地#8+9煤中发现地轴斜率35.5 kyr和岁差21.7 kyr周期,沉积时限为98.31~277.66 kyr,平均为188.20 kyr;泥炭地碳埋藏速率为42.41~78.87 g/m²·a,平均值为59.21 g/m²·a。 据现有大气总质量和二氧化碳浓度估计,每降低大气CO₂浓度 1 μL/L,需从大气中移除约2.13 GtC(10亿吨碳)。华北盆地晚石炭世末每年碳埋藏量为3.82×10⁷~7.01×10⁷吨,平均值为5.33×10⁷吨,每年降低大气CO₂浓度为1.79×10^-2~3.33×10^-2 μL/L,平均2.51×10^-2 μL/L。晚石炭世末聚煤作用前大气CO₂浓度约为400 μL/L,P1冰期开始前大气CO₂浓度约为200 μL/L,按照华北盆地年平均碳埋藏速率,仅8000年就可以使大气CO₂浓度降低200 μL/L,达到P1冰期形成时的大气CO₂浓度,而#8+9煤平均沉积年限为188.20 kyr,远超达到冰期所需要的时间,因此华北盆地的巨量碳埋藏可以导致P1冰期开始前大气CO₂浓度快速降低。ρCO₂与大陆硅酸盐风化的解耦、与古雨林面积快速增加和热带森林反复重组耦合,表明华北盆地晚石炭世末的聚煤作用是导致P1冰期开始前大气CO₂浓度降低和P1冰期开始的主要原因。

关键词: 华北盆地, 格舍尔阶, 泥炭地碳埋藏速率, 米兰科维奇旋回, 聚煤作用, P1冰期

CLC Number: 

• P512.2

• P612