Saturday, August 18, 2018

Technology: Carbonate Sedimentology

Just a quick note to point out the range of instrumentation now available to sedimentologists for analysis of samples. This is from a study of Ordovician age sediments by Yihang Fang and Huifang Xu, published in the June 2018 issue of the Journal of Sedimentary Research.

An excerpt:

 A micro-laminated carbonate with alternating dolomite–calcite layers from the mid-Lower Ordovician St. Paul Group from the Central Appalachians in southern Pennsylvania was examined using optical microscopes, X-ray diffraction (XRD), scanning electron microscopy (SEM) with X-ray energy-dispersive spectroscopy, electron microprobe analysis (EPMA), scanning transmission electron microscopy (STEM), laser-induced fluorescence (LIF) imaging, short-wave infrared (SWIR) imaging, and X-ray fluorescence (XRF) imaging. The sample is composed mainly of two types of layers. Dolomite-dominated layers are darker in color, generally thinner, and contain detrital minerals such as quartz and feldspar. In contrast, calcite-dominated layers are lighter in color, thicker, and contain less detrital minerals supported by microcrystalline calcite matrix. In situ XRD, LIF, XRF, and SWIR results show that organic remnants are enriched in the dolomite layers. The coincided spatial distribution confirmed a positive correlation between dolomite and organic matter, and hence provide evidence for microbial-EPS-catalyzed formation of sedimentary dolomite.

That is a lot of heavy weight toys!!

The researchers were trying to come up with an answer to one of the long lasting problems in sedimentary geology; that of the origin of the mineral dolomite.

I am not going into this paper in detail. I have not read it.  Let me just say that it has been suspected that microbial communities living on the ocean floor and in the sediment column may catalyze the precipitation of dolomite and this study confirms that dolomite rich layers contain organic matter. The dolomite may form at the sediment sea water interface by precipitation directly out of sea water or more commonly by replacement of the aragonite or calcite sediments during shallow burial.

On a broader time scale, dolomite abundance through the Phanerozoic does seem to correlate well with episodes of lower atmospheric oxygen concentrations and consequently less oxygenated sea water. See this figure spanning the Phanerozoic Era ( McKenzie J.A. and Vasconcelos, C.  2009). This relationship between ocean anoxia and dolomite abundance may well hold for the Precambrian too.

Anaerobic bacteria can thrive under such anoxic conditions.  Sulfate ions in sea water are thought to be a hindrance for dolomite formation. Anaerobic bacteria  remove these ions from sea water and pore fluids by sulfate reducing respiration, thus creating conditions favorable for precipitation of the mineral. They may also present specific types of organic substrates which enable easy nucleation of dolomite crystals.

For more details on the origin of sedimentary dolomite, do see my post The Dolomite Problem- Peeking Under the Hood

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