Characterization of environmental conditions during microbial Mg-carbonate precipitation and early diagenetic dolomite crust formation: Brejo do Espinho, Rio de Janeiro, Brazil
Characterization of environmental conditions during microbial Mg-carbonate precipitation and early diagenetic dolomite crust formation: Brejo do Espinho, Rio de Janeiro, Brazil
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微生物碳酸镁沉淀和早期成岩白云石地壳形成过程中的环境条件特征：Brejo do Espinho，里约热内卢，巴西
DOI：
10.1144/sp418.11
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发表时间：
2015
影响因子：
2.7
通讯作者：
C. Vasconcelos
中科院分区：
文献类型：
--
作者：
A. Bahniuk;J. Mckenzie;E. Perri;T. Bontognali;N. Vögeli;C. E. Rezende;T. P. Rangel;C. Vasconcelos
多年来，沉积白云岩一直被认为主要是组成原始石灰岩的碳酸钙成分的替代产物，这一过程被称为次生交代白云石化。尽管地质记录中的许多白云岩建造都是由泥晶白云石的细晶组成，但由于缺乏可见的或支持原生降水的地球化学指标，另一种替代过程，即直接沉淀，往往被排除在外。在这项研究中，我们介绍了一个现代沿海高盐度泻湖的研究，它位于巴西里约热内卢州的Brejo do Epinho，它处于一个特殊的气候条件下，出现了明确的干湿季节循环。在低温高盐度条件下，现代高镁方解石和钙白云石泥浆直接从泻湖水中沉淀出来，与生活在这种受限环境中的微生物的活动有关。泥浆经历了早期的成岩作用，在泻湖边缘形成了100%的白云岩结壳。生物成矿作用以该环境中物理化学条件在年度水文循环期间的变化为特征，与同位素分析相结合，确定了导致白云质泥浆和地壳形成的早期成岩过程。碳同位素值反映了呼吸有机碳的贡献，对地壳的贡献(δ13C=−9.5‰维也纳Pee Dee Belemnite(VPDB))大于泥浆(δ13C=−1.2‰VPDB)。氧同位素值反映了泥质建造过程中的中等蒸发程度(δ18O=1.1‰VPDB)，而在早期成岩地壳形成过程中显著增强(δ18O=4.2‰VPDB)。由Brejo do Epinho泥浆测得的块状同位素形成温度为34℃，而地壳为32℃。这些温度与泻湖经历最高盐分条件的旱季测量值的上限范围一致。
Abstract For many years, sedimentary dolomite rocks have been considered to be primarily a replacement product of the calcium carbonate components comprising the original limestone, a process known as secondary replacement dolomitization. Although numerous dolomite formations in the geological record are composed of fine-grained crystals of micritic dolomite, an alternative process, that is, direct precipitation, is often excluded because of the absence of visible or geochemical indicators supporting primary precipitation. In this research, we present a study of a modern coastal hypersaline lagoon, Brejo do Espinho, Rio de Janeiro State, Brazil, which is located in a special climatic regime where a well-defined seasonal cycle of wet and dry conditions occur. The direct precipitation of modern high-Mg calcite and Ca-dolomite mud from the lagoonal waters under low-temperature hypersaline conditions is associated with the activity of microbial organisms living in this restricted environment. The mud undergoes an early diagenetic transformation into a 100% dolomite crust on the margins of the lagoon. The biomineralization process, characterized by the variations of the physico-chemical conditions in this environment during the annual hydrological cycle, is integrated with isotopic analysis to define the early diagenetic processes responsible for the formation of both dolomitic mud and crust. The carbon isotope values indicate a contribution of respired organic carbon, which is greater for the crust (δ13C=−9.5‰ Vienna Pee Dee Belemnite (VPDB)) than mud (δ13C=−1.2‰ VPDB). The oxygen isotope values reflect a moderate degree of evaporation during mud formation (δ18O=1.1‰ VPDB), whereas it is greatly enhanced during early diagenetic crust formation (δ18O=4.2‰ VPDB). The clumped isotope formation temperature derived for the Brejo do Espinho mud is 34 °C, whereas it is 32 °C for the crust. These temperatures are consistent with the upper range of measured values during the dry season when the lagoon experiences the most hypersaline conditions.
影响因子：
2.3
作者：
Huntington, K. W.;Eiler, J. M.;Came, R.
通讯作者：
Came, R.