Stable isotope analysis and biomarkers of carbonates from the Columbia River in southwestern Washington, USA

Exotic limestone masses with silicified fossils, enclosed within deep-water marine siliciclastic sediments of the Early to Middle Miocene Astoria Formation, are exposed along the north shore of the Columbia River in southwestern Washington, USA. Samples from four localities were studied to clarify the origin and diagenesis of these limestone deposits. The bioturbated and reworked limestones contain a faunal assemblage resembling that of modern and Cenozoic deep-water methane-seeps. Five phases make up the paragenetic sequence: (1) micrite and microspar; (2) fibrous, banded and botryoidal aragonite cement, partially replaced by silica or recrystallized to calcite; (3) yellow calcite; (4) quartz replacing carbonate phases and quartz cement; and (5) equant calcite spar and pseudospar. Layers of pyrite frequently separate different carbonate phases and generations, indicating periods of corrosion. Negative d13Ccarbonate values as low as -37.6 per mill V-PDB reveal an uptake of methane-derived carbon. In other cases, d13Ccarbonate values as high as 7.1 per mill point to a residual, 13C-enriched carbon pool affected by methanogenesis. Lipid biomarkers include 13C-depleted, archaeal 2,6,10,15,19-pentamethylicosane (PMI; d13C: -128 per mill), crocetane and phytane, as well as various iso- and anteiso-carbon chains, most likely derived from sulphate-reducing bacteria. The biomarker inventory proves that the majority of the carbonates formed as a consequence of sulphate-dependent anaerobic oxidation of methane. Silicification of fossils and early diagenetic carbonate cements as well as the precipitation of quartz cement - also observed in other methane-seep limestones enclosed in sediments with abundant diatoms or radiolarians - is a consequence of a preceding increase of alkalinity due to anaerobic oxidation of methane, inducing the dissolution of silica skeletons. Once anaerobic oxidation of methane has ceased, the pH drops again and silica phases can precipitate.美国华盛顿州西南部哥伦比亚河北岸，裸露着赋存于中新世早-中期阿斯托里亚组（Astoria Formation）深水海相硅质碎屑沉积物中的、带有硅化化石的特殊灰岩块体。研究人员对4个采样点的样品展开分析，以阐明这类灰岩沉积的成因与成岩作用过程。经生物扰动与再改造的灰岩中产出的生物组合，与现代及新生代深水甲烷渗漏（methane-seeps）沉积的生物组合相似。该成岩共生序列包含5个阶段：（1）微晶灰岩与微亮晶；（2）纤维状、条带状及葡萄状文石胶结物，部分被硅质交代或重结晶为方解石；（3）黄色方解石；（4）交代碳酸盐相的石英及石英胶结物；（5）等粒方解石亮晶与假亮晶。黄铁矿层常分隔不同的碳酸盐相及其世代，指示曾发生溶蚀作用。碳酸盐碳同位素（δ¹³C_carbonate）值最低可达-37.6‰（V-PDB标准），表明其碳源源自甲烷成因碳；部分样品中该同位素值最高可达7.1‰，指示存在受甲烷生成作用影响的富集¹³C的残余碳库。脂质生物标志物包含¹³C亏损的古菌源2,6,10,15,19-五甲基二十烷（PMI；δ¹³C：-128‰）、番茄红素（crocetane）、植烷（phytane）以及多种异构与反异构碳链化合物，其来源大概率为硫酸盐还原菌。该生物标志物组合证实，绝大多数碳酸盐岩的形成依赖于硫酸盐还原型甲烷厌氧氧化作用。化石与早期成岩碳酸盐胶结物的硅化作用，以及石英胶结物的沉淀作用——这一现象同样见于其他赋存于富硅藻或放射虫沉积物中的甲烷渗漏灰岩中——均源于甲烷厌氧氧化作用先期引发的碱度升高，该过程可导致硅质骨骼的溶解。当甲烷厌氧氧化作用停止后，水体pH值再次下降，硅质相便可发生沉淀。