Studies Of Methods For Measurement Of Black Carbon In Geological Samples And Their Preliminary Application In The Cretaceous Songliao Basin

Black Carbon(BC) refers to the carbonious material mainly from incomplete combustion of fossil fuels and biomass.It plays a significant role in many biogeochemial processes, and may represent a significant sink in the global carbon cycle. Due to its low chemical reactivity, BC is difficult for microbial degradation and being oxidized at normal temperature. Ancient sediments of black carbon can record fire activities and volcanic eruption events, and can also reflect the concentration of oxygen in the past atmosphere. In this paper, first of all, the methods of the determination of total organic carbon and the separation of black carbon will be compared and optimized. And then black carbon in Cretaceous sedimentary sequence of the Nenjiang formation and Qingshankou formation in Northeast China will be recorded. Major conclusions are present as follows:Total organic carbon(TOC) is important properties of soils and sediments. In fact, the pretreatment procedures may have certain impacts on the measured results of TOC and d13C. In this study, the influences of pretreatment procedures on the determination of TOC and d13C in different samples were investigated by two ways: acid treatments and dry methods. The results shown that the acid treatments have important effects on the determination of TOC in different samples. The TOC values determined using methods of hydrochloric acidification/separation method but reserving residual acid and in-situ Ag cup acidification are significantly higher than that of other groups. These differences can be explained by the fact that dissolved organic carbons(DOC) were removed during the acid treatment process. The d13C values determined with these two methods are slightly positive than that of other groups. However the drying methods of of heat drying and vacuum freeze drying didn’t have observable effects on the determination of TOC and d13C in different samples, which may indicated that influences of volatile organic carbon(VOC) on the determination of TOC and d13C in these tested samples are ignorable. In summary, owing to the low DOC and VOC content in ancient sediments, the pretreatment procedures may have little influence on the measured results of TOC and d13C. Therefore any method fitting experimental conditions can be choose.Chemical oxidation and thermal oxidation are used commonly in separating BC from soil and sediments. However, they also have some disadvantages. For instance,high maturity kerogen will make the results overestimating in chemical oxidation, and organic carbon may coke in thermal oxidation. In this study, two kinds of low maturity kerogen are chosed and prepared to different maturity by thermal simulation. For both sets of samples, the traditional method of acid dichromate oxidation and CTO-375 are compared and optimized.The results shown that:(1) The traditional method of acid dichromate oxidation is applicable for separating BC from sediment samples containingⅠkerogen which maturity is under 1.25%. For samples maturity in 0.38-0.89%, it will take 120 h to remove the organic matter. However, when samples’ maturity is increased above 1.25%, the kerogen cann’t be removed at all. Besides, concentration of dichromate has a great influence in removing the organic matter. The method can be optimized by increasing concentration of dichromate.(2) Traditonal acid dichromate oxidation method is also not suitable in sediment samples containing Ⅲ kerogen.(3) This study also indicates that high maturity has no impacts on separating BC using the method of CTO-375.This study records the black carbon in the Member 1, 2 of Nenjiang Formation and the Upper Member 1, 2 of Qingshankou Formation, reconstructs the paleoclimate in that depositional sequence, and represent the fire activities and the volcanic events. The sediments are investigated by the optimized method of acid dichromate oxidation method and CTO- 375 ℃.Results show that:(1) Formation of Nenjiang: The lower Member 1 of Nenjiang Formation has frequent volcanic activity. Before this stage some small fire activities also occur. Biomass increased greatly after this stage. The reason may be that the animal and plant residues and ash after volcanic provides rich nutrients to soils. But on the Upper Member 1 of Nenjiang Formation there is low ancient productivity. At the same time the lake is difficult to preserve organic matter. On the Member 2 of Nenjiang the content of TOC and the BC have a good correlation. The volcanic events are not recorded in this stage. Due to the large lake transgression event happened, a lot of BC from soil was scoured and deposited in the lake. What’s more, because of the warm humid climate, biological yield increased.(2) Formation of Qingshankou: Owing to the frequent fire activites and volcanic events TOC and the BC content the Member 2 of Qingshankou Formation have opposite tendency. But there is a special period in which there are frequent volcanic activities. Before this period the biomass has greatly increased. On the Upper Member 1 of Qingshankou Formation fire activites and volcanic events don’toccur frequently or the size of these events are not very small. The climate from the Member 1 to 2 of Qingshankou Formation may dry gradually.