Diagenetic dolomite formation in Quaternary anoxic diatomaceous muds at DSDP Leg 64 Holes

During drilling in the Gulf of California, diagenetic carbonate rocks were recovered at 7 out of 8 sites. These are primarily dolomites which record 13C isotopic evidence of the incorporation of carbon derived from the decomposition of organic matter. In Hole 479, drilled to a sub-bottom depth of 440 meters on the Guaymas Slope, under a fertile upwelling belt, we recognized an excellent example of deep sea dolomitization in progress. This Quaternary section of organic-carbon- rich, low-carbonate, hemipelagic diatomaceous oozes contains numerous fine-grained, decimeter-thin, episodic beds of dolomite, which show sedimentologic, geochemical, and isotopic evidence of accretion by precipitation below 40 meters sub-bottom in zones of high alkalinity and low sulfate. The beds preserve original sedimentary structures. Carbon-13 varies from +3 to +14 per mil, indicating biogenic CO2 reservoirs related to active methanogenesis. In single beds, 18O values range outwardly from +5 to –7 per mil, reflecting increasing temperature with progressive accretion of dolomite with depth; the values parallel progressive trends in lithification, texture, mineralogy, and fossil preservation. We estimate slow accretion rates on the order of 0.1-0.7 mm/10**3 yr. with burial. Dolomitization does not proceed merely at the expense of nearby nannofossils. Ca and Mg ions must be derived from interstitial waters. The episodic appearance of beds in the sequence seems partly a reflection of latent climate signals. This process of deep sea dolomitization carries implications for hydrocarbon migration, as well as an interpretation of the presence of dolomite in other modern and ancient pelagic to hemipelagic sediment sequences.

在加利福尼亚湾钻探作业期间，8个钻探站位中有7个采获了成岩碳酸盐岩（diagenetic carbonate rocks）。此类岩石以白云岩（dolomite）为主，其碳同位素（¹³C）记录了源自有机质分解的碳的掺入过程。在位于富饶上升流带下方的瓜马斯斜坡（Guaymas Slope）上，钻至海底以下440米深度的479号钻孔中，我们发现了一处正在进行中的深海白云岩化作用（deep sea dolomitization）的典型实例。这套第四系富有机质、低碳酸盐的半远洋硅藻软泥沉积中，发育大量细粒、分米级厚度的间歇性白云岩层，这些岩层具备在海底以下40米以深的高碱度、低硫酸盐区域通过沉淀作用发生加积的沉积学、地球化学及同位素证据，且保留了原始沉积构造。碳-13（¹³C）数值介于+3‰至+14‰之间，表明存在与活跃甲烷生成作用（methanogenesis）相关的生物成因二氧化碳储库。单岩层内的氧-18（¹⁸O）数值范围为+5‰至-7‰，反映出随着白云岩随深度持续加积，温度逐渐升高；该同位素数值变化与岩化作用程度、结构特征、矿物组成及化石保存状态的演化趋势相一致。我们估算得出，随埋藏作用发生的白云岩加积速率较为缓慢，约为0.1~0.7毫米/千年。白云岩化作用并非仅以周边颗石藻化石（nannofossils）为消耗来源，钙（Ca）与镁（Mg）离子必须源自孔隙水（interstitial waters）。沉积序列中层状出现的间歇性特征，在一定程度上反映了潜在的气候信号。这种深海白云岩化作用过程，对油气运移（hydrocarbon migration）研究以及解释其他现代和古代远洋-半远洋沉积序列中的白云岩产出具有重要启示意义。