Organic preservation of vase‐shaped microfossils from the late Tonian Chuar Group, Grand Canyon, Arizona, USA
Organic preservation of vase‐shaped microfossils from the late Tonian Chuar Group, Grand Canyon, Arizona, USA
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DOI：
10.1111/gbi.12544
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发表时间：
2023-01
期刊：
影响因子：
3.7
通讯作者：
Kelly E Tingle;S. Porter;M. Raven;A. Czaja;S. Webb;B. Bloeser
中科院分区：
文献类型：
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作者：
Kelly E Tingle;S. Porter;M. Raven;A. Czaja;S. Webb;B. Bloeser
瓶形微化石（VSMs）在全球新元古代中期（800-730 Ma）海相地层中被发现，是最早的有壳变形虫证据。VSM测试被假设在生命中最初是有机的，但最常见的是作为二次矿化的铸件和模具保存下来。然而，一些报告表明可能是有机保存。在这里，我们检验了一种假设，即来自亚利桑那州大峡谷Chuar组下Walcott成员页岩的vsm含有原始有机物质，正如B. Bloeser在她对Chuar vsm的开创性研究中所报道的那样。在透射光镜下，我们从Walcott Member黑色页岩的两个薄片样品中鉴定了vsm，并使用扫描电子显微镜对vsm进行了成像。碳质物质在两个样品的所有VSM测试的内腔中都被发现，并被解释为可能在白垩纪期间从Walcott页岩中动员的沥青。能量色散X射线光谱（EDS）和波长色散X射线光谱（WDS）显示，VSM测试壁主要含有碳、铁和硫，而二氧化硅仅存在于周围的基质中。利用拉曼光谱比较了样品中碳质物质的热成熟度，发现化石材料中存在黄铁矿和黄钾铁矾。X射线吸收光谱显示，在碳质测试壁、测试腔内发现的碳质物质以及沉积基质中存在还原的有机硫物质，这表明有机物硫化发生在沃尔科特页岩中。我们的光谱分析显示，沃尔科特VSM测试壁是有机的，有时是次生黄铁矿化的（黄铁矿被不同程度地氧化为黄钾铁矾）。这两种保存模式都可以发生在毫米空间尺度的样品材料中，有时甚至在单个标本中。我们认为，沃尔科特页岩内部的硫化作用促进了有机质的保存，并且铁与VSM有机质的比值控制了黄铁矿替代的程度。根据我们的证据，我们得出结论，vsm是用原始的有机测试材料保存的，并推测有机vsm可能经常不被识别，因为它们在透射光下呈浅色，半透明的外观。
Vase‐shaped microfossils (VSMs) are found globally in middle Neoproterozoic (800–730 Ma) marine strata and represent the earliest evidence for testate (shell‐forming) amoebozoans. VSM tests are hypothesized to have been originally organic in life but are most commonly preserved as secondary mineralized casts and molds. A few reports, however, suggest possible organic preservation. Here, we test the hypothesis that VSMs from shales of the lower Walcott Member of the Chuar Group, Grand Canyon, Arizona, contain original organic material, as reported by B. Bloeser in her pioneering studies of Chuar VSMs. We identified VSMs from two thin section samples of Walcott Member black shales in transmitted light microscopy and used scanning electron microscopy to image VSMs. Carbonaceous material is found within the internal cavity of all VSM tests from both samples and is interpreted as bitumen mobilized from Walcott shales likely during the Cretaceous. Energy dispersive X‐ray spectroscopy (EDS) and wavelength dispersive X‐ray spectroscopy (WDS) reveal that VSM test walls contain mostly carbon, iron, and sulfur, while silica is present only in the surrounding matrix. Raman spectroscopy was used to compare the thermal maturity of carbonaceous material within the samples and indicated the presence of pyrite and jarosite within fossil material. X‐ray absorption spectroscopy revealed the presence of reduced organic sulfur species within the carbonaceous test walls, the carbonaceous material found within test cavities, and in the sedimentary matrix, suggesting that organic matter sulfurization occurred within the Walcott shales. Our suite of spectroscopic analyses reveals that Walcott VSM test walls are organic and sometimes secondarily pyritized (with the pyrite variably oxidized to jarosite). Both preservation modes can occur at a millimeter spatial scale within sample material, and at times even within a single specimen. We propose that sulfurization within the Walcott Shales promoted organic preservation, and furthermore, the ratio of iron to labile VSM organic material controlled the extent of pyrite replacement. Based on our evidence, we conclude that the VSMs are preserved with original organic test material, and speculate that organic VSMs may often go unrecognized, given their light‐colored, translucent appearance in transmitted light.