中侏罗世泥炭地净初级生产力及其对全球碳循环的响应——以吐哈盆地沙尔湖煤田为例

Net primary productivity of Middle Jurassic peatlands and its response to global carbon cycle:An example from Shaerhu coalfield in Tuha Basin

泥炭地具有明显的碳汇能力,对减缓全球气候变化有着至关重要的作用,煤作为泥炭地的最终产物,保存有丰富的泥炭地发育时期的古环境信息,成煤泥炭地碳聚集速率和净初级生产力(NPP)可以作为恢复古气候的替代指标。吐哈盆地南缘沙尔湖煤田ZK9E02钻孔中侏罗统西山窑组发育视厚度为121.97 m的煤层(C8),本研究使用频谱分析对其进行米兰科维奇轨道周期的识别及成煤泥炭地发育时限的估算,然后利用该时限,结合煤的碳含量以及煤化作用阶段的碳损,计算出成煤泥炭地的NPP,进而探究成煤泥炭地的碳汇能力与碳循环关系。研究发现,所研究煤层的自然伽马测井曲线中蕴含着约405 ka长偏心率、约95 ka短偏心率、约37.6 ka斜率、约18.2 ka岁差的米兰科维奇旋回天文周期,计算出该成煤泥炭地发育的煤层沉积约2218.54~2347.03 ka。该煤层碳含量74.54%,考虑到煤化过程中的碳损失约为26.75%,恢复出成煤期泥炭地碳聚集速率为66.11~69.86 g C/m^(2)·a,NPP为265.2~280.2 g C/m^(2)·a。控制泥炭地NPP的主要因素有CO_(2)含量、古纬度、古大气温度等,研究区较低的泥炭地NPP水平与当时CO_(2)含量过低或古纬度较低有一定关系。泥炭地NPP在一定程度上反映着大气中CO_(2)的变化,进一步用来揭示全球碳循环对气候变化的动态响应过程,这对"深时"高分辨率的古环境研究有重要意义。因此,预测泥炭地NPP的水平,研究生态系统中碳元素的最终去向,亦有利于更好地认识到全球碳循环过程中泥炭地的"碳汇"角色,助力实现"碳达峰、碳中和"目标。

Peatlands have obvious carbon storage capacity and play a crucial role in mitigating global climate change.Coal,as the end product of peatland,has preserved a large amount of paleoenvironmental information.The carbon accumulation rate and the net primary productivity(NPP)of coal-forming peatlands can be used as a proxy for recovering paleoenvironmental information.A extremely thick coal seam in the ZK9E02 borehole(42°34'N,91°24'E)has been developed in the Middle Jurassic Xishanyao Formation in the Shaerhu coalfield in the southern margin of the Tuha Basin,Xinjiang.In this study,we use the spectral analysis to identify the periods of Milankovitch orbital cycles in the C8 huge-thick(121.97 m,from 227.75 m to 349.72 m in depth)in the Gamma Ray curve of the coal seam,and then use the obtained cycle periods to constrain the timeframe of palaeo-peatlands.In combination with the carbon content of the coal and the carbon loss during coalification,the NPP of the Middle Jurassic peatlands are estimated.By means of the NPP,the carbon storage capacity of the coal-forming peatlands and its relationship with the carbon cycle are investigated.The results have revealed the targeted coal seam has been preserved with signals of different frequencies,including 0.04 cycle/m,0.19 cycle/m,0.48 cycle/m and 1.00 cycle/m,and these frequencies are corresponding to the Milankovitch orbital cycle of 405 ka(long eccentricity),95 ka(short eccentricity),37.6 ka(obliquity)and 18.2 ka(precession).With these cycles,the timeframe of the coal-forming peatland is constrained to be in the range from 2218.54 ka to 2347.03 ka.Considering that the carbon content of the coal seam is 74.54%and the carbon loss during the coalification is about 26.75%,the carbon accumulation rate of the targeted coal seam is estimated to be 66.11~69.86 g C/m^(2)·a,and the NPP is estimated to be 265.2~280.2 g C/m^(2)·a.The main factors to control the NPP of peatlands are CO_(2) content,paleolatitude,and paleotemperature.The reduced NPP values of the palaeo-peatlands in the Shaerhu coalfield can be attributed to the low paleolatitude and/or too low atmospheric CO_(2) content.To a certain extent,the NPP reflects the changes of atmospheric CO_(2),which can further reveal the dynamic response of the global carbon cycle to climate changes,thus the NPP of palaeo-peatlands is important for studying the high resolution paleoenvironmental evolution in the deep-time.Therefore,predicting the level of NPP in peatlands and studying the final destination of carbon in the ecosystem are also beneficial to understanding the role of peatlands as a carbon sink in the global carbon cycle,and then help achieve the goal of carbon peaking and carbon neutrality.