Geochemistry of middle Eocene-lower Oligocene pelagic sediments of ODP Hole 105-647A

Geochemical analyses of the middle Eocene through lower Oligocene lithologic Unit IIIC (260-518 meters below seafloor [mbsf]) indicate a relatively constant geochemical composition of the detrital fraction throughout this depositional interval at Ocean Drilling Program (ODP) Site 647 in the southern Labrador Sea. The main variability occurs in redox-sensitive elements (e.g., iron, manganese, and phosphorus), which may be related to early diagenetic mobility in anaerobic pore waters during bacterial decomposition of organic matter. Initial preservation of organic matter was mediated by high sedimentation rates (36 m/m.y.). High iron (Fe) and manganese (Mn) contents are associated with carbonate concretions of siderite, manganosiderite, and rhodochrosite. These concretions probably formed in response to elevated pore-water alkalinity and total dissolved carbon dioxide (CO2) concentrations resulting from bacterial sulfate reduction, as indicated by nodule stable-isotope compositions and pore-water geochemistry. These nodules differ from those found in upper Cenozoic hemipelagic sequences in that they are not associated with methanogenesis. Phosphate minerals (carbonate-fluorapatite) precipitated in some intervals, probably as the result of desorption of phosphorus from iron and manganese during reduction. The bulk chemical composition of the sediments differs little from that of North Atlantic Quaternary abyssal red clays, but may contain a minor hydrothermal component. The silicon/ aluminum (Si/Al) ratio, however, is high and variable and probably reflects original variations in biogenic opal, much of which is now altered to smectite and/or opal CT. An increase in the sodium/potassium (Na/K) ratio in the upper Eocene corresponds to the beginning of coarsergrained feldspar flux to the site, possibly marking the onset of more vigorous deep currents. Although the Site 647 cores provide a nearly complete high-resolution, high-latitude Eocene-Oligocene record, the high sedimentation rate and somewhat unusual diagenetic conditions have led to variable alteration of benthic foraminifers and fine-fraction carbonate and have overprinted the original stable-isotope records. Planktonic foraminifers are less altered, but on the whole, there is little chance of sorting out the nature and timing of environmental change on the basis of our stable-isotope analyses.对拉布拉多海南部大洋钻探计划（Ocean Drilling Program, ODP）647站位中始新世至渐新世早期岩性单元IIIC（海底以下260~518米，meters below seafloor, mbsf）开展的地球化学分析显示，该沉积层序内碎屑组分的地球化学组成相对稳定。该序列的主要地球化学变异性体现在氧化还原敏感元素（如铁、锰与磷）上，其可能与有机质细菌分解过程中厌氧孔隙水内的早期成岩活动性有关。有机质的初始保存作用由较高的沉积速率（36米/百万年）所调控。高铁（Fe）与锰（Mn）含量与菱铁矿、锰菱铁矿及菱锰矿碳酸盐结核伴生。结核的稳定同位素组成与孔隙水地球化学特征表明，此类结核的形成可能响应于细菌硫酸盐还原作用所引发的孔隙水碱度与总溶解二氧化碳（CO₂）浓度升高。该类结核与晚新生代半远洋序列中的结核存在差异，其并不与甲烷生成作用相关。部分层位中沉淀有磷酸盐矿物（碳酸盐氟磷灰石），其成因可能为还原过程中磷从铁、锰矿物表面解吸。 沉积物的整体化学成分与北大西洋第四纪深海红黏土差异极小，但可能含有少量热液组分。然而，其硅铝比（Si/Al）较高且存在显著波动，该特征大概率反映了生物成因蛋白石的原始含量变化——现今其中大部分已蚀变为蒙脱石及/或CT型蛋白石。始新世上部的钠钾比（Na/K）升高对应着粗粒长石输入通量首次抵达该站位，这可能标志着更为强劲的深层洋流活动开始。 尽管647站位岩心提供了近乎完整的高分辨率高纬度始新世-渐新世沉积记录，但高沉积速率与较为特殊的成岩环境导致底栖有孔虫与细粒级碳酸盐发生了不均一蚀变，并对原始稳定同位素记录产生了叠加改造。浮游有孔虫的蚀变程度相对较低，但总体而言，依托本次稳定同位素分析难以厘清环境变化的本质与发生时序。