Geochemistry of Late Ordovician dalmanelloid brachiopods from Laurentia: testing the effects of paleolatitudinal gradient
Geochemistry of Late Ordovician dalmanelloid brachiopods from Laurentia: testing the effects of paleolatitudinal gradient
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DOI：
10.1139/cjes-2018-0181
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发表时间：
2019-03-01
影响因子：
1.4
通讯作者：
Jin, Jisuo
中科院分区：
文献类型：
--
作者：
Azmy, Karem;Jin, Jisuo
大马氏腕足类贝壳采集自肯塔基州上奥陶统列克星敦组（下卡蒂安）、安大略省马尼图林岛的 Sheguindah 页岩（中卡蒂安）和马尼托巴省温尼伯地区的斯托尼山组（上卡蒂安）。对它们进行研究是为了检验贝壳底栖动物古纬度分带的假设。采用多种技术方法来评估贝壳次层的岩石学和地球化学（同位素和元素）保存情况。初步的常规显微镜、阴极发光（CL）和扫描电子显微镜（SEM）证实了初生壳超微结构（棱柱状低镁方解石）的保留。地球化学成岩作用代理（例如 Sr、Mn、Fe 和 Sigma REE）与 δ O-18 和 δ C-13 值显示出不显着的相关性，因此支持至少近原生地球化学成分的保存。在这三批贝壳中，平均 Delta O-18 值在列克星敦组 (-4.5% +/- 0.3% VPDB) 中最高，在 Stony Mountain 组中最低 (-6.8% +/- 0.4% VPDB)，在 Sheguindah 页岩中处于中间值 (-6.0% +/- 0.8% VPDB)。三角洲O-18增加的相对梯度与古纬度梯度一致，肯塔基州位于亚热带，安大略省南部位于中热带，曼尼托巴省南部位于赤道以下纬度。列克星敦组贝壳还具有最高的平均 Delta C-13 值（0.8% +/- 0.2% VPDB）和相对较高的 P 含量（170 +/- 27 ppm），表明有机生产力较高，这与之前对卡蒂安期间劳伦大陆东南缘营养丰富的冷水频繁上涌的解释一致。列克星敦贝壳的平均 Th/U (0.6 +/- 0.6) 也较低，这与有机生产力的蓬勃发展相一致，这可能导致水体中更多的氧气消耗。
Dalmanelloid brachiopod shells were collected from the Upper Ordovician Lexington Formation (lower Katian) of Kentucky, Sheguindah Shale (middle Katian) on Manitoulin Island, Ontario, and the Stony Mountain Formation (upper Katian) in the Winnipeg area, Manitoba. They were investigated to test the hypothesis of paleo-latitudinal zonation of the shelly benthos. A multi-technique approach was applied to evaluate the petrographic and geochemical (isotopic and elemental) preservation of the secondary layer of shells. Preliminary conventional microscopy, cathodoluminescence (CL), and scanning electron microscopy (SEM) confirmed the retention of primary shell ultrastructure (prismatic low-Mg calcite). The geochemical diagenesis proxies (e.g., Sr, Mn, Fe, and Sigma REE) show insignificant correlations with the delta O-18 and delta C-13 values, thus supporting the preservation of at least near-primary geochemical compositions. Among the three lots of shells, the mean delta O-18 value is the highest in those from the Lexington Formation (-4.5% +/- 0.3% VPDB), lowest from the Stony Mountain Formation (-6.8% +/- 0.4% VPDB), and intermediate from the Sheguindah Shale (-6.0% +/- 0.8% VPDB). The relative gradient in delta O-18 increase is in agreement with the paleo-latitudinal gradient, with Kentucky in subtropical, southern Ontario in mid-tropical, and southern Manitoba in subequatorial latitudes. The Lexington Formation shells also have the highest mean delta C-13 value (0.8% +/- 0.2% VPDB) and relatively high P contents (170 +/- 27 ppm), suggesting higher organic productivity, which is consistent with previous interpretation of frequent upwelling of nutrient-rich cool waters along the southeastern margin of Laurentia during the Katian. The Lexington shells also have a lower mean Th/U (0.6 +/- 0.6), which is consistent with blooming organic productivity that likely led to more consumption of oxygen in the water column.