Stable isotope geochemistry of pore waters and marine sediments from the New Jersey shelf: Methane formation and fluid origin
Stable isotope geochemistry of pore waters and marine sediments from the New Jersey shelf: Methane formation and fluid origin
复制标题
新泽西陆架孔隙水和海洋沉积物的稳定同位素地球化学：甲烷形成和流体起源
DOI：
10.1130/ges00859.1
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发表时间：
2013
期刊：
影响因子：
2.5
通讯作者：
S. Stadler
中科院分区：
文献类型：
--
作者：
R. Geldern;Takeshi Hayashi;M. Böttcher;M. Mottl;J. Barth;S. Stadler
对美国综合海洋钻探计划（IODP）第313远征队（新泽西州浅陆架）的间隙水和沉积物样品进行了化学成分和稳定同位素比值分析。分析表明，下面的淡水透镜体具有以前未知的复杂几何形状，其中淡水-盐水交替间隔由岩心上部的清晰边界划分。确定了三种流体来源：(1)大气淡水，(2)海水，(3)盐水。孔隙流体稳定同位素值定义了一条混合线，其端元δ18O和δ2H值在淡水中分别为-7.0‰和-41‰，在盐水中分别为-0.8‰和-6‰。这与新泽西州降水的现代平均值和今天的新泽西州陆架水相似。对于淡水来说，这要么表明现代的大气补给是通过新泽西大陆上的含水层产生的，要么表明在气候和水文条件与现代相似的时候形成的。本研究的稳定同位素数据未证实其来源为贫同位素的更新世冰川融水。盐水也代表了现代同位素值，表明沿可渗透的粗粒度砂质单元渗透。下部岩心部分的特点是混合了来自地下深处蒸发岩的盐水流体。
Interstitial water and sediment samples of Integrated Ocean Drilling Program (IODP) Expedition 313 (New Jersey Shallow Shelf) were analyzed for chemical composition and stable isotope ratios. The analyses indicate a previously unknown complex geometry of the underlying fresh-water lens with alternating fresh-water–salt-water intervals divided by sharp boundaries in the upper part of the cores. Three fluid sources were identified: (1) meteoric fresh water, (2) marine seawater, and (3) brine. The pore-fluid stable isotope values define a mixing line with end members that have δ18O and δ2H values of –7.0‰ and –41‰ for fresh water, and –0.8‰ and –6‰ for salt water, respectively. This is similar to the modern mean value of New Jersey precipitation and today’s New Jersey shelf water. For fresh water, this either indicates modern meteoric recharge via aquifers that crop out on mainland New Jersey or emplacement at a time with climatic and hydrologic conditions similar to modern. An origin from Pleistocene glacial meltwaters with depleted isotope values is not confirmed by stable isotope data of this study. Salt water also represents modern isotope values suggesting an infiltration along permeable, coarse-grained sandy units. The lower core parts are characterized by mixing with brine fluids that originate from evaporites in the deep underground.
Stable carbon isotope analyses of gas and fluids prove the existence of methane formation from degradation of marine organic matter and CO2 reduction in the lower core parts below ∼350 m below seafloor. Methane concentrations above 10000 ppm and δ13Cmethane values of ∼–80‰ were measured. Methane formation is also indicated by authigenic carbonates with low δ13Ccarbonate values. Although not reaching the surface at present conditions, the venting out of variable fluxes of methane from passive continental margins due to sea-level fluctuations is significant for the long-term carbon cycle. Authigenic carbonates indicate the precipitation from pore fluids with marine oxygen stable isotope ratios at low temperatures. The geochemical data and interpretations presented in this study supply the missing link between existing onshore and offshore data and may provide the basis for an integrated approach to construct a geochemical transect across the New Jersey shallow shelf.