黄土高原西部地区末次冰期和全新世有机碳同位素变化与C3/C4植被类型转换研究

文章利用黄土高原西缘代表性的塬堡剖面有机碳同位素数据,估算了末次冰期以来地表植被中C₃/C₄植 物的相对丰度,指示出研究区域末次冰期几乎为纯粹的C₃植物,而全新世为C₃植物占优势的C₃和C₄混合植被类 型。温度是控制中国黄土高原C₄植物是否发生的关键性气候因素,末次冰期向全新世转化过程中存在的某“阈值 温度”控制了两种植被类型的存在。全新世土壤有机碳同位素偏正于末次冰期,符合前人研究得到的认识。末次 冰期间冰段(MIS3)至盛冰期,土壤有机碳同位素为偏正变化趋势,符合现代C₃植物本身随气候条件改变的碳同位 素组成变化。研究表明,利用黄土-古土壤有机碳同位素进行古气候变化研究,不能只将有机碳同位素简单的解 释为C₃/C₄植物相对丰度的变化,在单一植被类型下,还需要考虑植物本身碳同位素组成随气候条件的变化;另外, 研究还说明,我国黄土高原不同地区同时段土壤有机碳同位素值可以不同,其变化可以不具有相同的趋势,因此, 简单将有机碳同位素偏正归因于夏季风增强是值得商榷的。     

贵州石灰岩和砂岩地区C4和C3植物营养元素的化学计量对N/P比值波动的影响

陆地生态系统植物的生长受到营养元素氮(N)和磷(P)的可利用性的限制。已有的证据表明营养元素的相对丰度将控制生态系统的营养元素循环和能量流动的速度。文章提出如下假设:为了适应环境的变化,植物具有可伸缩性地调整营养元素含量的能力,也就是营养元素化学计量比值变化的能力,植物N/P比值波动的影响不仅来源于N对P的相对可利用性的变化,也来源于其他营养元素化学计量的变化,尤其是与Ca的化学计量的变化。为了验证上述假设,本研究利用3种C₄植物和11种C₃植物,研究了植物N/P化学计量比值的波动随N与Ca和P与Ca化学计量的变化模式:对C₄植物来说,N/P比值的波动主要受生物量P与Ca化学计量变化的影响;而对C₃植物来说,则同时受N与Ca和P与Ca化学计量变化的控制,它们之间的相对控制能力的大小将决定植物N/P比值波动的变化梯度,C₄植物和C₃植物的N/P比值的波动都要受土壤pH值的影响。本研究对了解物种丰度和N对P的相对可利用性、N与Ca,以及P与Ca的化学计量之间关系具有重要意义。     

湖泊沉积物C4植物含量重建方法的讨论——以新生代柴达木盆地大红沟剖面为例

当前气候变暖是一个全球面临的重大问题,它对人类赖以生存的植被生态系统造成的影响已经在全球各地逐步显现出来。为了深入了解植物生态系统对环境和气候变化的响应机制,我们需要更好地借鉴地质历史时期气候环境和植物协同演化的重要事件。C₄植物作为陆地生产力较强的植物,在植物生态演化中占举足轻重的地位。目前研究表明C₄植物可能最晚起源于始新世-渐新世之交,但从它早期起源到随后在生态系统中的大规模扩张时间间隔长达20多个百万年。是什么因素导致了C₄植物的起源和扩张是一个悬而未决的重要问题,需要开展大量的调查研究来评估和重建C₄植物在过去生态系统中的相对生物量变化。重建C₄植物的含量目前主要的方法是建立在C₃/C₄植物碳同位素和植物内部结构形态差异基础之上。最常运用的研究材料包括(古)土壤有机质、成壤碳酸盐、陆地食草动物体组织、沉积物生物标志物、孢粉、植硅体等。这些方法在重建现代以及地质历史时期C₄植物相对生物量变化的研究中发挥了重要作用,但同时也存在很多无法避免的问题。本文介绍了C₄植物起源和扩张机制的主流观点以及不同研究材料的碳同位素所推算C₄植物生物量的基本原理,并以柴达木盆地大红沟剖面为例,针对新生代湖泊沉积物中陆生高等植物的长链正构烷烃特征和单体烃碳同位素的研究结果,详细讨论C₄植物含量重建的方法与缺点,为探讨C₄植物起源、演化及控制因素提供参考。我们通过分析前人研究的大红沟剖面长链正构烷烃及单体烃 δ¹³C_alk 值特征,推测在30~24 Ma、20~17 Ma和13~7 Ma期间δ¹³C_alk值显示相对正偏的原因,可能是干旱和C₄植物在当地生态系统中出现的双重因素叠加造成的。但这一推断还需要借助于新的研究方法,即单颗粒孢粉碳同位素的方法来提供C₄植物的确凿证据。     

中东亚干旱半干旱区C3植物δ13 C值的分布 及其对气候的响应*

植物化石和土壤中的有机质碳同位素指标常用来反映古气候的变化,然而碳同位素这个指标在特定地区反映气候的定量关系缺乏检验。研究剖面选择自中国的秦岭(34°14'24″N,106°55'30″E)到蒙古人民共和国北部,接近贝加尔湖地区(51°35'08″N, 100°45'49″E)的研究剖面线,选择了3种C₃植物(Artemisia scoparia, Ajania achilleides 和 Artemisia frigida),在剖面线上沿南北方向上每隔4'到5'采取一个样点,共选取161个C₃植物茎叶样品进行了δ¹³ C值测定。同时收集了剖面线附近气象站的降水、气温等资料,用插值方法得到每个采样点的气温、降水数据。分析表明:C₃植物的δ¹³ C值分布范围为-30 ‰ ~-22 ‰ ,其平均值为-26.81 ‰ ,该平均值较全球C₃植物δ¹³ C平均值偏正。通过对比C₃植物δ¹³ C与年均温、年均降水量、生长季节的干燥度等随纬度的变化规律,发现C₃植物δ¹³ C、年均降水量、生长季节的干燥度有非常一致的变化趋势,而C₃植物δ¹³ C和年均温不具有一致性。通过一元回归分析也同样发现C₃植物δ¹³ C与年均降水量呈线性负相关关系(y=-0.0077x-24.838,n=161,R²=0.4418,p=0.01),与生长季节的干燥度呈线性正相关关系(y=0.7328x-28.806,n=161,R²=0.3685,p=0.01),而与年均温度没有明显的相关关系(y=-0.0461x-26.756,n=161,R²=0.0232,p=0.01)。在本研究区C₃植物δ¹³ C对年均降水量和生长季节的干燥度响应十分显著,而对温度的响应不明显。研究区具有明显的降水和温度的梯度分布特征,是验证植物碳同位素与气候关系的理想场所,而土壤中的有机质碳同位素与其地面上的植物碳同位素息息相关。研究也说明,在本研究区或其他气候植物组合相似的地区可以利用古土壤中的有机质碳同位素来定量或半定量地反映古气候的变化。     

新疆罗北洼地湖相沉积物有机碳同位素的变化序列及其古环境意义

测定了新疆罗布泊地区湖相沉积物CK-2钻孔样品的总有机碳含量（TOC）及其同位素组成、碳酸盐含量和C/N比值等环境代用指标,以及石膏矿物的质谱-铀系年龄。测试结果表明,20～9kaB.P.期间沉积物δ¹³C_org.在-23.4‰～-16.1‰之间波动且阶段性明显,与TOC呈现良好的相关关系,整体变化趋势同南极Dome C冰芯中记录的全球大气CO₂浓度一致;C/N比值表明有机碳来源主要是陆生高等植物。因此大气CO₂浓度变化是影响20～9kaB.P.期间罗布泊湖相沉积物δ¹³C_org.值变化的主导因素,周围山体上C₃/C₄植物相对生物量的变化则是另一重要因素。依据δ¹³C_org.的变化序列将此时间段湖区古环境的演化分成6个阶段:20.0～14.1kaB.P.期间受到末次盛冰期的影响,气温偏低,湖水丰沛;14.1～13.3kaB.P.是一个气候不稳定期,冷暖波动较频繁,但以暖为趋势;13.3～12.8kaB.P.期间经历了一段冷期,于12.8kaB.P.结束了末次冰期,随后气候开始转暖至11.8kaB.P.;其后气温再次变冷并维持到10kaB.P.;最后从10kaB.P.进入全新世暖期。δ¹³C_org.序列明显向偏负方向变化,表明该地区变暖的趋势相当明显。罗布泊地区日益干旱化是全球气候变化的结果,尤其是受到全球CO₂浓度的不断升高所制约。     

东海泥质区单体正构烷烃的碳同位素组成及物源分析 *

利用气相色谱(GC)和气相色谱/同位素比值质谱(GC/IRMS)对东海近岸泥质区、济州岛西南泥质区和冲绳海槽北部表层沉积物中正构烷烃的单体碳同位素组成及分布进行了分析。结果显示东海不同泥质区典型海洋藻类源正构烷烃C₁₉同位素组成基本相似,在-27.4 ‰ ~-28.0 ‰ 之间,平均为-27.7 ‰ 。典型海洋水生植物源C₂₃同位素组成在-28.5 ‰ ~-31.6 ‰ 之间,平均为-30.5 ‰ ,碳同位素组成从近岸泥质区到冲绳海槽北部逐渐变重,表明海槽区与陆架区海洋水生植物种类有所不同。陆架区长链正构烷烃(C₂₅~C₃₁)部分随着碳数的增加,其同位素组成逐渐变轻,但海槽区这一变化不大,显示陆架区的陆源高等植物蜡具有相似的物源,而冲绳海槽北部由于黑潮主干区和黑潮分支(对马暖流)对陆架沉积物进入深海的控制性阻隔作用,其物源与陆架区区别较大。现代输入东海的陆源植物以C₃植物为显著优势,C₃植物对近岸泥质区北部、近岸泥质区南部、远端济州岛西南泥质区和冲绳海槽北部陆源植物的贡献分别为83 % ,95 ％,75 % 和70 % 。     

植物——大气N2O的一个潜在排放源

N₂₂和CH₄的重要温室气体。目前,全球N₂₂₂O不仅是一个普遍存在的自然现象,而且其排放量可达到与土壤排放相比较的水平,因而植物可能是未知的大气N₂O的一个重要排放源;植物排放N₂O受植物的种类、生长发育阶段、养分供给、光照强度及CO₂浓度等因素的影响。     

中国东部CO2气地球化学特征及其气藏分布

中国东部CO₂气田(藏)发育广泛,分布复杂。本文对中国东部松辽、渤海湾、苏北、三水、东海、珠江口、莺琼、北部湾等盆地和内蒙古商都地区以及部分现代构造岩浆活动区CO₂气田(藏)和气苗中CO₂的地球化学特征进行了分析和研究,探讨了中国东部CO₂气的成因、来源及分布。中国东部CO₂气的含量主要分布区间为0~10%,其次为90%~100%,呈现典型的U字型。δ¹³C_CO2值则呈现典型的单峰式分布,峰值区间为-6‰~-4‰。CO₂含量、δ¹³C_CO2值和R/Ra值综合表明,中国东部高含CO₂气以幔源无机成因为主,混有部分有机成因气和(或)壳源无机气。中国东部已发现的36个无机成因CO₂气田(藏)在空间分布上与新近纪及第四纪北西西向玄武岩活动带展布一致,深大断裂和岩浆活动是无机成因CO₂富集、运移和分布最重要、最直接的两大主控因素。     

水射流破碎联合CO2置换开采天然气水合物新工艺及实验研究

本文对目前开采天然气水合物的5种方法进行了归纳总结,重点分析了CO₂置换开采以及固体开采法,并通过分析这2种开采方法的优劣势,提出了水射流冲蚀、破碎海洋天然气水合物储层联合CO₂置换开采天然气水合物的新思路。水射流冲蚀、破坏水合物储层后形成的采空区能为CO₂提供更好的储藏空间并提高其与储层的作用面积,提高置换效率;封存的CO₂水合物也可以提高水合物储层的稳定性,具有良好的互补效应。实验结果表明,在整个置换过程中,含采空区储层CH₄置换率为24.3%,CO₂封存率为22.1%;完整储层CH₄置换率为15.3%,CO₂封存率为20.9%,置换率提升约59%,封存率提升约5.7%。采空区的作用主要体现在提升水合物置换介质的注入量上。     

青藏高原现生禾本科植物的δ13C与海拔高度的关系

文章通过分析青藏高原4种C3禾本科植物碳同位素值随海拔高度的变化,发现穗三毛(Trisetumspicatum),垂穗鹅观草(Roegnerianutans),紫花针茅(Stipapurpurea),垂穗披碱草(Elymusnutans)的碳同位素值随海拔高度增加而变重的趋势明显,平均每增高1km变重1.37‰,其中Trisetumspicatum和Roegnerianutans的碳同位素值随海拔高度增加而变重的趋势更为显著。研究认为温度和大气CO₂分压是引起C3植物碳同位素值随海拔高度变化的主要因素。另外,研究发现一些C4植物的生长高度可以达到海拔4000m以上,最高可达海拔4520m。     

Influence of physiology and climate on δD of leaf wax n-alkanes from C3 and C4 grasses

We measured hydrogen isotope compositions (δD) of high-molecular-weight n-alkanes (C₂₇-C₃₃) from grasses grown in greenhouses and collected from the US Great Plains. In both cases, n-alkanes from C₄ grasses are enriched in D by more than 20‰ relative to those from C₃ grasses. The apparent enrichment factor (ε_C29-GW) between C₂₉n-alkane and greenhouse water is −165 ± 12‰ for C₃ grasses and −140 ± 15‰ for C₄ grasses. For samples from the Great Plains, δD values of C₂₉n-alkanes range from −280 to −136‰, with values for C₄ grasses ca. 21‰ more positive than those for C₃ grasses from the same site. Differences in C₃ and C₄ grass n-alkane δD values are consistent with the shorter interveinal distance in C₄ grass leaves, and greater back-diffusion of enriched water from stomata to veins, than in C₃ grass leaves. Great Plains’ grass n-alkane isotopic ratios largely reflect precipitation δD values. However, the offset or apparent fractionation between n-alkanes and precipitation is not uniform and varies with annual precipitation and relative humidity, suggesting climatic controls on lipid δD values. The dryer sites exhibit smaller absolute apparent fractionation indicative of D-enrichment of source waters through transpiration and/or soil evaporation. To explore the relationship between climate and n-alkane δD values, we develop three models. (1) The ‘direct analog’ model estimates δD_C29 values simply by applying the apparent enrichment factors, ε_C29-GW, observed in greenhouse grasses to precipitation δD values from the Great Plains. (2) The ‘leaf-water’ model uses a Craig-Gordon model to estimate transpirational D-enrichment for both greenhouse and field sites. The transpiration-corrected enrichment factors between C₂₉ and bulk leaf-water, ε_C29-GW, calculated from the greenhouse samples (−181‰ for C₃ and −157‰ for C₄) are applied to estimate δD_C29 values relative to modeled bulk leaf-water δD values. (3) The ‘soil- and leaf-water’ model estimates the combined effects of soil evaporation, modeled by analogy with a flow-through lake, and transpiration on δD_C29 values. Predictions improve with the addition of the explicit consideration of transpiration and soil evaporation, indicating that they are both important processes in determining plant lipid δD values. D-enrichment caused by these evaporative processes is controlled by relative humidity, suggesting that important climatic information is recorded in leaf wax n-alkane δD values. Calibration studies such as this one provide a baseline for future studies of plant-water-deuterium systematics and form the foundation for interpretation of plant wax hydrogen isotope ratios as a paleo-aridity proxy.     

Carboxylic acid distribution patterns of temperate C3 and C4 crops

Agricultural grasses cover a major part of the land surface in temperate agro-ecosystems and contribute significantly to the formation of soil organic matter. Crop-derived lipids are assumed to be responsible for fast carbon turnover in soils. Differences in lipid distribution patterns between crops following C₃ and C₄ photosynthesis pathways have rarely been described, but could be useful for source apportionment of crop-derived input into soils or sediments. The distribution of long chain n-carboxylic acids (C₂₂, C₂₄, C₂₆) reveals significant differences between crop plants following either the C₃ or the C₄ photosynthetic carbon fixation pathway. The plant compartments leaves, stems and roots of C₄ plants contain relatively large proportions (> 40%) of n-C₂₄ carboxylic acid when compared to C₃ plants. These reveal larger relative proportions of n-C₂₂ and n-C₂₆ acids, whose relative abundance is subject to change between different plant compartments and during the growing season. The carboxylic acid ratio [CAR = n-C₂₄/(n-C₂₂ + n-C₂₆) carboxylic acids] provides distinct ratios for C₄ (> 0.67) and C₃ crops (< 0.67) and can thus be used as a molecular marker for the differentiation of crop plant biomass. In combination with the bulk stable carbon isotopic composition (δ¹³C) the CAR can be used as a tool for the estimation of the C₄ derived carbon proportion in soils or sediments.     

Carbon-isotopic analysis of individual pollen grains from C3 and C4 grasses using a spooling-wire microcombustion interface

Pollen grains from grasses using the C₃ and C₄ photosynthetic pathways have distinct ranges of δ¹³C values that may be used to estimate their relative abundance in paleorecords. We evaluated a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer (SWiM-IRMS) for δ¹³C analysis of individual grass-pollen grains. Pollen from four C₃ and four C₄ grass species was isolated through micromanipulation and analyzed as single grains suspended in water. A carbon yield greater than the 2σ range of the carbon content of blanks containing only water was used to distinguish samples containing pollen (“pollen present”) from those not containing pollen. This criterion resulted in the exclusion of ∼45% of the 946 samples applied to the wire. The average δ¹³C values (±1σ) of the remaining samples were −26.9‰ (±6.3‰) and −11.5‰ (±9.6‰) for C₃ grasses and C₄ grasses, respectively, after blank-correcting the δ¹³C data. These results suggest that the SWiM-IRMS system can be used to distinguish C₃ from C₄ grass pollen. The high variability in measured δ¹³C values is likely caused by a combination of factors. These include natural isotopic variability among individual pollen grains; the relatively poor precision that can be obtained when determining δ¹³C values of such small samples; and the uncertainty in the magnitude, isotopic composition, and stability of the analytical blank. Nonetheless, high percentages of individual pollen grains were correctly classified as being of either C₃ or C₄ origin. On average, 90% (range = 78-100%) of pollen grains from C₃ grasses had δ¹³C values more negative than the cutoff threshold of −19.2‰; while 84% (range = 77-90%) of pollen grains from C₄ grasses had δ¹³C values more positive than −19.2‰. Compared with analysis using an elemental analyzer interfaced with an IRMS (EA-IRMS), the number of pollen grains required for δ¹³C-based evaluation of C₃/C₄ grass composition is many times lower with the SWiM-IRMS. Additionally, δ¹³C data from the SWiM-IRMS does not need to be incorporated into a mixing model to derive estimates of the abundance of C₃ and C₄ grass pollen. Carbon-isotopic analysis of individual grass-pollen grains using the SWiM-IRMS system may help improve our understanding of the evolutionary and ecological significance of grass taxa in the paleorecord.     

Late Quaternary vegetation changes around Lake Rutundu,Mount Kenya,East Africa: evidence from grass cuticles,pollen and stable carbon isotopes

Woody, subalpine shrubs and grasses currently surround Lake Rutundu, Mount Kenya. Multiple proxies, including carbon isotopes, pollen and grass cuticles, from a 755‐cm‐long core were used to reconstruct the vegetation over the past 38 300 calendar years. Stable carbon‐isotope ratios of total organic carbon and terrestrial biomarkers from the lake sediments imply that the proportion of terrestrial plants using the C₄ photosynthetic pathway was greater during the Late Pleistocene than in the Holocene. Pollen data show that grasses were a major constituent of the vegetation throughout the Late Pleistocene and Holocene. The proportion of grass pollen relative to the pollen from other plants was greatest at the last glacial maximum (LGM). Grass cuticles confirm evidence that C₄ grass taxa were present at the LGM and that the majority followed the cold‐tolerant NADP‐MEC₄ subpathway. Copyright © 2003 John Wiley & Sons, Ltd.     

Identification of C24 and C25 lanostanes in Tertiary sulfur rich crude oils from the Jinxian Sag, Bohai Bay Basin, Northern China

Unusual short chain lanostanes (C₂₄ and C₂₅) and C₃₀ lanostane were identified in sulfur rich crude oils from the Jinxian Sag, Bohai Bay Basin, northern China. Besides the regular steranes (C_27-30), a series of 4-methyl steranes (C₂₂₋₂₃, C₂₇₋₃₀), 4,4-dimethyl steranes (C₂₂₋₂₄, C₂₈₋₃₀), short chain steranes (C₂₃₋₂₆), abundant pregnanes (C₂₁₋₂₂) and androstanes (C₁₉₋₂₀), together with sulfur containing steroids (20-thienylpregnanes and thienylandrostanes) were detected in the aliphatic and branched-cyclic hydrocarbon fraction of these crude oils. A literature survey of some long chain sterane analogues (e.g., A-nor-steranes, norcholestanes, C₃₀ steranes, lanostanes) and pregnanes seems to point to a sponge and/or dinoflagellate source. 4-Methyl, 4,4-dimethyl steroids and lanosterols (4,4,14-trimethyl steroids as the basic skeleton of lanostanes) can be derived from methanotrophic bacteria. Thus, a biological origin from a prokaryotic methylotroph can be used to explain the common source of abundant short chain steranes (C_23-26), 4-methyl (C_22-23) and 4,4-dimethyl steranes (C_22-24), as well as lanostanes (C_24-25 and C₃₀ analogues) in our oil samples. Generally, the steroids appear to have been extensively sulfurized with sulfur substitution at the C-22 position in the side chain during the early stage of diagenesis, which was readily subject to attack by bacterial degradation (enzymatic cleavage) and/or abiotic oxidation. As a consequence, short chain sterane analogues (e.g., abundant pregnanes and androstanes in this study) and short chain lanostanes (C₂₄−C₂₅) might later be released through cleavage of weak C-S bonds at the C-22 carbon in the sulfurized steroids and lanostane sulfides. Finally, the formation of the short chain C₂₄−C₂₅ lanostanes and distinctive occurrence of short chain steranes in this study can be well explained by microbial biodegradation of sulfurized lanostanoids and steroids in the reservoir.     

Reconstructing the response of C3 and C4 plants to decadal-scale climate change during the late Pleistocene in southern Illinois using isotopic analyses of calcified rootlets

The δ¹³C and δ¹⁸O values of well-preserved carbonate rhizoliths (CRs) provide detailed insights into changes in the abundance of C₃ and C₄ plants in response to approximately decadal-scale changes in growing-season climate. We performed stable isotope analyses on 35-40 CRs sampled at 1-cm intervals from an 18-cm-thick paleosol formed in southern Illinois during Wisconsin interstadial 2. Minimum δ¹³C values show little variation with depth, whereas maximum values vary dramatically, and average values show noticeable variability; maximum δ¹⁸O values vary less than the minimum δ¹⁸O values. These findings indicate that a diverse and stable C₃ flora with a limited number of C₄ grass species prevailed during this interval, and suggest that the maximum growing-season temperatures were relatively stable, but minimum growing-season temperatures varied considerably. Two general patterns characterize the relationships between the δ¹³C and δ¹⁸O values obtained from the 1-cm samples. In some cases, low δ¹³C values correspond to low δ¹⁸O values and high δ¹³C values correspond to high δ¹⁸O values, suggesting that cooler growing-season temperatures favored C₃ and warmer growing-season temperatures favored C₄ plants. In other cases, low δ¹³C values correspond to high δ¹⁸O values, likely suggesting that wetter growing-season conditions were favorable to C₃ plants. The high density of well-preserved CRs in this paleosol provides a unique opportunity to study detailed ecological responses to high-resolution variability in growing-season climate.     

Primary cracking of kerogens. Experimenting and modelling C1, C2–C5, C6–C15 and C15+ classes of hydrocarbons formed

Mathematical models of hydrocarbon formation can be used to simulate the natural evolution of different types of organic matter and to make an overall calculation of the amounts of oil and/or gas produced during this evolution. However, such models do not provide any information on the composition of the hydrocarbons formed or on how they evolve during catagenesis.From the kinetic standpoint, the composition of the hydrocarbons formed can be considered to result from the effect of “primary cracking” reactions having a direct effect on kerogen during its evolution as well as from the effect of “secondary cracking” acting on the hydrocarbons formed.This report gives experimental results concerning the “primary cracking” of Types II and III kerogens and their modelling. For this, the hydrocarbons produced have been grouped into four classes (C₁, C₂–C₅, C₆–C₁₅ and C₁₅₊). Experimental data corresponding to these different classes were obtained by the pyrolysis of kerogens with temperature programming of 4°C/min with continuous analysis, during heating, of the amount of hydrocarbons corresponding to each of these classes.The kinetic parameters of the model were optimized on the basis of the results obtained. This model represents the first step in the creation of a more sophisticated mathematical model to be capable of simulating the formation of different hydrocarbon classes during the thermal history of sediments. The second step being the adjustment of the kinetic parameters of “secondary cracking”.     

The effect of minor to moderate biodegradation on C5 to C9 hydrocarbons in crude oils

A suite of 18 oils from the Barrow Island oilfield, Australia, and a non-biodegraded reference oil have been analysed compositionally in order to detail the effect of minor to moderate biodegradation on C₅ to C₉ hydrocarbons. Carbon isotopic data for individual low molecular weight hydrocarbons were also obtained for six of the oils. The Barrow Island oils came from different production wells, reservoir horizons, and compartments, but have a common source (the Upper Jurassic Dingo Claystone Formation), with some organo-facies differences. Hydrocarbon ratios based on hopanes, steranes, alkylnaphthalenes and alkylphenanthrenes indicate thermal maturities of about 0.8% R_c for most of the oils. The co-occurrence in all the oils of relatively high amounts of 25-norhopanes with C₅ to C₉ hydrocarbons, aromatic hydrocarbons and cyclic alkanes implies that the oils are the result of multiple charging, with a heavily biodegraded charge being overprinted by fresher and more pristine oil. The later oil charge was itself variably biodegraded, leading to significant compositional variations across the oilfield, which help delineate compartmentalisation. Biodegradation resulted in strong depletion of n-alkanes (>95%) from most of the oils. Benzene and toluene were partially or completely removed from the Barrow Island oils by water washing. However, hydrocarbons with lower water solubility were either not affected by water washing, or water washing had only a minor effect. There are three main controls on the susceptibility to biodegradation of cyclic, branched and aromatic low molecular weight hydrocarbons: carbon skeleton, degree of alkylation, and position of alkylation. Firstly, ring preference ratios at C₆ and C₇ show that isoalkanes are retained preferentially relative to alkylcyclohexanes, and to some extent alkylcyclopentanes. Dimethylpentanes are substantially more resistant to biodegradation than most dimethylcyclopentanes, but methylhexanes are depleted faster than methylpentanes and dimethylcyclopentanes. For C₈ and C₉ hydrocarbons, alkylcyclohexanes are more resistant to biodegradation than linear alkanes. Secondly, there is a trend of lower susceptibility to biodegradation with greater alkyl substitution for isoalkanes, alkylcyclohexanes, alkylcyclopentanes and alkylbenzenes. Thirdly, the position of alkylation has a strong control, with adjacent methyl groups reducing the susceptibility of an isomer to biodegradation. 1,2,3-Trimethylbenzene is the most resistant of the C₃ alkylbenzene isomers during moderate biodegradation. 2-Methylalkanes are the most susceptible branched alkanes to biodegradation, 3-methylalkanes are the most resistant and 4-methylalkanes have intermediate resistance. Therefore, terminal methyl groups are more prone to bacterial attack compared to mid-chain isomers, and C₃ carbon chains are more readily utilised than C₂ carbon chains. 1,1-Dimethylcyclopentane and 1,1-dimethylcyclohexane are the most resistant of the alkylcyclohexanes and alkylcyclopentanes to biodegradation. The straight-chained and branched C₅–C₉ alkanes are isotopically light (depleted in ¹³C) relative to cycloalkanes and aromatic hydrocarbons. The effects of biodegradation consistently lead to enrichment in ¹³C for each remaining hydrocarbon, due to preferential removal of ¹²C. Differences in the rates of biodegradation of low molecular weight hydrocarbons shown by compositional data are also reflected in the level of enrichment in ¹³C. The carbon isotopic effects of biodegradation show a decreasing level of isotopic enrichments in ¹³C with increasing molecular weight. This suggests that the kinetic isotope effect associated with biodegradation is site-specific and often related to a terminal carbon, where its impact on the isotopic composition becomes progressively ‘diluted’ with increasing carbon number.