Reconstructing changes in seep activity by means of pore water and solid phase Sr/Ca and Mg/Ca ratios in pockmark sediments of the Northern Congo Fan
Reconstructing changes in seep activity by means of pore water and solid phase Sr/Ca and Mg/Ca ratios in pockmark sediments of the Northern Congo Fan
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DOI：
10.1016/j.margeo.2011.06.008
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发表时间：
2011-09
期刊：
影响因子：
2.9
通讯作者：
Kerstin Nöthen;S. Kasten
中科院分区：
文献类型：
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作者：
Kerstin Nöthen;S. Kasten
本研究的重点是垂直分布的自生碳酸盐（文石和高镁方解石），在微细浸染沉淀物的形式，以及块状碳酸盐结核和以上的天然气水合物轴承沉积物的北方刚果扇。对取自三个麻坑构造（海孔、黑洞和虫孔）的重力岩心进行了孔隙水、块状沉积物和自生碳酸盐岩中Ca、Mg、Sr和Ba的分析。此外，从一个不受流体渗漏影响的区域提取了一个背景岩心。孔隙水Sr/Ca和Mg/Ca比值被用来揭示碳酸盐岩形成的当前深度以及自生沉淀物的矿物学。根据Bayon等人提出的方法，采用块状沉积物和块状碳酸盐结核的Sr/Ca和Mg/Ca比值来推断自生文石和高镁方解石的存在和深度分布[Bayon等人（2007）。尼日尔三角洲沉积物中的Sr/Ca和Mg/Ca比值：冷渗漏环境中自生碳酸盐成因的意义Marine Geology 241（1-4），93-109].我们发现，Bayon等人（2007年）针对尼日尔三角洲冷泉沉积物开发的方法也适用于识别刚果深海扇麻点沉积物中自生碳酸盐的矿物学。我们扩展这种方法相结合的间隙与固相Sr/Ca和Mg/Ca的比例，这表明高镁方解石是主要的自生碳酸盐，目前形成的硫酸盐/甲烷反应区（SMRZ）。这是第一项研究，调查固相和孔隙水的签名典型的文石或高镁方解石沉淀相同的沉积物芯，从而能够识别活动和化石碳酸盐沉淀事件。在所有调查的麻点网站的SMRZ的化石层位推导出高镁方解石位于上方和下方的SMRZ目前的深度。此外，文石富集典型的高渗透率检测接近这些网站的沉积物表面。然而，由孔隙水特征所示的文石活性沉淀仅发生在黑洞现场。溶解和固相钡浓度被用来估计的时间SMRZ被固定在当前深度的成岩重晶石前锋。结合孔隙水和固相元素比（镁/钙，锶/钙）和钡浓度允许重建过去的变化，甲烷渗漏在调查麻点网站。在海伯利孔和虫孔站点的时间高甲烷渗漏估计已经停止至少600年BP。相比之下，一个更近的变化，从高通量到一个更休眠的阶段必须发生在黑洞网站的沉积物表面的活性文石沉淀和缺乏成岩钡富集证明。
This study focuses on the vertical distribution of authigenic carbonates (aragonite and high Mg-calcite) in the form of finely disseminated precipitates as well as massive carbonate concretions present in and above gas hydrate bearing sediments of the Northern Congo Fan. Analyses of Ca, Mg, Sr and Ba in pore water, bulk sediments and authigenic carbonates were carried out on gravity cores taken from three pockmark structures (Hydrate Hole, Black Hole and Worm Hole). In addition, a background core was retrieved from an area not influenced by fluid seepage. Pore water Sr/Ca and Mg/Ca ratios are used to reveal the current depths of carbonate formation as well as the mineralogy of the authigenic precipitates. The Sr/Ca and Mg/Ca ratios of bulk sediments and massive carbonate concretions were applied to infer the presence and depth distribution of authigenic aragonite and high Mg-calcite, based on the approach presented by Bayon et al. [Bayon et al. (2007). Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241(1–4), 93–109]. We show that the approach developed by Bayon et al. (2007) for sediments of cold seeps of the Niger Delta is also suitable to identify the mineralogy of authigenic carbonates in pockmark sediments of the Congo Deep-Sea Fan. We expand this approach by combining interstitial with solid phase Sr/Ca and Mg/Ca ratios, which demonstrate that high Mg-calcite is the predominant authigenic carbonate that currently forms at the sulfate/methane reaction zone (SMRZ). This is the first study which investigates both solid phase and pore water signatures typical for either aragonite or high Mg-calcite precipitation for the same sediment cores and thus is able to identify active and fossil carbonate precipitation events. At all investigated pockmark sites fossil horizons of the SMRZ were deduced from high Mg-calcite located above and below the current depths of the SMRZ. Additionally, aragonite enrichments typical for high seepage rates were detected close to the sediment surface at these sites. However, active precipitation of aragonite as indicated by pore water characteristics only occurs at the Black Hole site. Dissolved and solid phase Ba concentrations were used to estimate the time the SMRZ was fixed at the current depths of the diagenetic barite fronts. The combined pore water and solid phase elemental ratios (Mg/Ca, Sr/Ca) and Ba concentrations allow the reconstruction of past changes in methane seepage at the investigated pockmark sites. At the Hydrate Hole and Worm Hole sites the time of high methane seepage was estimated to have ceased at least 600yr BP. In contrast, a more recent change from a high flux to a more dormant stage must have occurred at the Black Hole site as evidenced by active aragonite precipitation at the sediment surface and a lack of diagenetic Ba enrichments.