Organic geochemistry of sediments from sapropel 94 of ODP Hole 161-974C

Analysis of the molecular composition of the organic matter (OM) from whole sediment samples can avoid analytical bias that might result from isolation of components from the sediment matrix, but has its own analytical challenges. We evaluated the use of GC * GC-ToFMS to analyze the pyrolysis products of six whole sediment samples obtained from above, within and below a 1 million year old OM-rich Mediterranean sapropel layer. We found differences in pyrolysis products <n-C22 between the OM-rich sapropel samples and the OM-poor background marls. The presence of alkyl pyrroles, probably derived from chlorophyll, in pyrolysates of the sapropels but not in those of the marls suggests that higher marine productivity and greater OM preservation accompanied deposition of the sapropels. Detection of tetramethyl benzenes considered to be pyrolysis products of isorenieratene in the sapropel samples is evidence that nitrogen-fixing green sulfur bacteria contributed to the high productivity. Greater abundances of shorter chain aliphatic hydrocarbons, pyrroles, furans and alkyl aromatics in the pyrolysates of sapropel samples relative to the marls confirm better preservation of marine OM in the sapropels. In addition, the presence of greater amounts of thiophenes in the sapropels than in the marls is consistent with the existence of euxinic conditions during sapropel deposition. The combination of whole sediment pyrolysis and GC × GC-ToFMS is promising, but the procedure requires careful selection of its multiple analytical variables, particularly the pyrolysis temperature and the operational features of the GC columns.

对全沉积物样品中的有机质（organic matter, OM）进行分子组成分析，可规避因从沉积物基质中分离组分而可能引入的分析偏差，但该方法自身亦存在分析挑战。我们评估了全二维气相色谱-飞行时间质谱（GC × GC-ToFMS）的应用效果，以分析采自百万年富有机质地中海腐泥层上方、层内及层下方的6个全沉积物样品的热解产物。研究发现，富有机质腐泥样品与贫有机质背景泥灰岩样品的热解产物中，碳数低于n-C22的组分存在显著差异。腐泥热解产物中检出了烷基吡咯（alkyl pyrroles），其大概率源自叶绿素；而泥灰岩热解产物中未发现此类物质，这表明腐泥沉积时期伴随了更高的海洋初级生产力与更优良的有机质保存条件。在腐泥样品中检出了被认为是异瑞尼烯（isorenieratene）热解产物的四甲基苯，该证据证实固氮绿硫细菌曾对该区域的高初级生产力作出贡献。相较于泥灰岩样品，腐泥样品热解产物中的短链脂肪烃、吡咯类、呋喃类及烷基芳烃丰度更高，这进一步印证了腐泥中海洋有机质的保存状况更佳。此外，腐泥样品中的噻吩类（thiophenes）含量显著高于泥灰岩，这与腐泥沉积时期存在硫化缺氧水体环境（euxinic conditions）的结论相吻合。全沉积物热解与全二维气相色谱-飞行时间质谱联用的分析方法颇具应用前景，但该流程需谨慎选择多项分析变量，尤以热解温度与气相色谱柱的操作参数为重。