Organic matter, isotopic composition of calcite veins in basement basalt and pore water at DSDP Leg 78A Holes

Sediments of the Barbados Ridge complex, cored on DSDP Leg 78A, contain low concentrations of acid-insoluble carbon (0.05-0.25%) and nitrogen (C/N 1.5-5) and dispersed C1-C6 hydrocarbons (100-800 ppb). The concentrations of organic carbon and 13C in organic carbon decrease with depth, whereas the concentration of dispersed hydrocarbons increases slightly with depth. These trends may reflect the slow oxidation of organic matter, with selective removal of 13C and slow conversion of the residual organic matter to hydrocarbons. Very minor indications of nitrogen gas were observed at about 250 meters sub-bottom at two of the drilling sites. Basement basalts have calcite veins with d13C values in the range of 2.0 to 3.2 per mil and d18O-SMOW values ranging from 28.5 to +30.6 per mil. Interstitial waters have d18O-SMOW of 0.2 to -3.5 per mil and dD-SMOW of -2 to -15 per mil. The oxygen isotopic composition of the calcite veins in the basement basalts gives estimated equilibrium fractionation temperatures in the range of 11 to 24°C, assuming precipitation from water with d18O-SMOW in the range of +0.1 to -1.0 per mil. This suggests that basalt alteration and precipitation of vein calcite occurred in contact either with warmer Campanian seawater or, later, with pore water, after burial to depths of 200- 300 meters. Pore waters from all three sites are depleted in deuterium and 18O, and dissolved sulfate is enriched in 34S at Sites 541 and 542, but not at Site 543.

巴巴多斯脊复合体（Barbados Ridge complex）的沉积物采自深海钻探计划（Deep Sea Drilling Project, DSDP）第78A航次的岩心，其酸不溶碳含量（0.05%~0.25%）与氮含量（碳氮比C/N为1.5~5）较低，且分散分布有C1~C6烃类（100~800 ppb）。有机碳含量及有机碳中的碳-13（¹³C）丰度随深度增加而降低，而分散态烃类的浓度则随深度略有升高。上述变化趋势或反映了有机质的缓慢氧化过程：其中碳-13（¹³C）被选择性移除，且残余有机质缓慢转化为烃类。在其中两个钻探站位的海底以下约250米处，观测到极微量的氮气信号。基底玄武岩中发育方解石脉，其碳同位素δ¹³C值介于2.0‰~3.2‰之间，氧同位素δ¹⁸O-标准平均海洋水（Standard Mean Ocean Water, SMOW）值介于28.5‰~+30.6‰之间。孔隙水的δ¹⁸O-SMOW值介于0.2‰~-3.5‰之间，δD-SMOW值介于-2‰~-15‰之间。若假设基底玄武岩中方解石脉形成于δ¹⁸O-SMOW值介于+0.1‰~-1.0‰之间的流体中，则根据其氧同位素组成可估算出平衡分馏温度介于11℃~24℃之间。这表明，在埋藏至200~300米深度后，玄武岩蚀变及方解石脉的沉淀作用要么发生于暖性的坎潘期（Campanian）海水中，要么发生于后期的孔隙水中。三个站位的孔隙水均贫化氘与氧-18（¹⁸O）；541和542站位的溶解硫酸盐富集硫-34（³⁴S），但543站位未出现该现象。