Department of Geosciences, Smith College, Northampton, Massachusetts, USA.
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
Geobiology. 2023 Sep;21(5):629-643. doi: 10.1111/gbi.12558. Epub 2023 May 24.
Marine ooids have formed in microbially colonized environments for billions of years, but the microbial contributions to mineral formation in ooids continue to be debated. Here we provide evidence of these contributions in ooids from Carbla Beach, Shark Bay, Western Australia. Dark 100-240 μm diameter ooids from Carbla Beach contain two different carbonate minerals. These ooids have 50-100 μm-diameter dark nuclei that contain aragonite, amorphous iron sulfide, detrital aluminosilicate grains and organic matter, and 10-20 μm-thick layers of high-Mg calcite that separate nuclei from aragonitic outer cortices. Raman spectroscopy indicates organic enrichments in the nuclei and high-Mg calcite layers. Synchrotron-based microfocused X-ray fluorescence mapping reveals high-Mg calcite layers and the presence of iron sulfides and detrital grains in the peloidal nuclei. Iron sulfide grains within the nuclei indicate past sulfate reduction in the presence of iron. The preservation of organic signals in and around high-Mg calcite layers and the absence of iron sulfide suggest that organics stabilized high-Mg calcite under less sulfidic conditions. Aragonitic cortices that surround the nuclei and Mg-calcite layers do not preserve microporosity, iron sulfide minerals nor organic enrichments, indicating growth under more oxidizing conditions. These morphological, compositional, and mineralogical signals of microbial processes in dark ooids from Shark Bay, Western Australia, record the formation of ooid nuclei and the accretion of magnesium-rich cortical layers in benthic, reducing, microbially colonized areas.
海洋鲕粒已经在微生物定殖的环境中形成了数十亿年，但微生物对鲕粒中矿物形成的贡献仍存在争议。本文提供了西澳大利亚鲨鱼湾 Carbla 海滩鲕粒中这些贡献的证据。来自 Carbla 海滩的直径为 100-240μm 的深色鲕粒包含两种不同的碳酸盐矿物。这些鲕粒具有 50-100μm 直径的深色核心，包含方解石、无定形铁硫化物、碎屑铝硅酸盐颗粒和有机质，以及 10-20μm 厚的高镁方解石层，将核心与方解石外皮层分离。拉曼光谱表明核心和高镁方解石层中存在有机质富集。基于同步加速器的微聚焦 X 射线荧光映射揭示了高镁方解石层以及铁硫化物和碎屑颗粒在鲕核中的存在。核心内的铁硫化物颗粒表明在存在铁的情况下过去发生了硫酸盐还原。在高镁方解石层内外保存的有机信号以及铁硫化物的缺失表明，有机物在较少硫化物条件下稳定了高镁方解石。包围核心和 Mg-方解石层的方解石外壳没有保留微孔、铁硫化物矿物和有机质富集，表明在更氧化的条件下生长。西澳大利亚鲨鱼湾深色鲕粒中这些微生物过程的形态、组成和矿物学信号记录了鲕粒核心的形成以及富含镁的皮层在底栖、还原、微生物定殖区的堆积。
