Detection of Early Aragonite to Calcite Diagenesis in a Modern Inner Carbonate Ramp

Abstract

Early diagenetic replacement of aragonite by the more stable calcite mineral is not well documented in modern carbonate environments. Understanding the timing of these early diagenetic transitions is critical to our reliance on geochemical signatures in carbonates as paleoenvironmental proxies. Depositional features and early diagenetic alterations are often overprinted and challenging to recognize once sediments are lithified and buried, where pressure, temperature, and subsurface fluids impart further diagenesis. Whole-rock x-ray diffraction (XRD) is traditionally used to distinguish between calcite and aragonite minerals, but this technique cannot map these transitions on a grain-by-grain scale. Raman spectroscopy is also practical for distinguishing between carbonate minerals, and it is non-destructive. Though similar, the Raman spectra of aragonite and calcite display subtle differences in peak positions, which are ideal for differentiating these minerals within a single grain that may have experienced early diagenetic alteration. The main objective of this study was to use Raman spectroscopy to identify early diagenesis, in the form of aragonite-to-calcite transition, in inner ramp carbonate sediments from Abu Dhabi, United Arab Emirates. Additional XRD, stable isotope δ18O and δ13C, and petrographic data were used to document changes through the 26 cm section of cored sediment. XRD of unconsolidated sediment revealed a downward decrease in aragonite to calcite content (80:4 top 12 cm and 61:8 bottom 12 cm). Polished thin sections from the sediment core were used to create 2D Raman maps of grains subjected to early diagenesis. The Raman 2D maps depict an explicit transition between aragonite and calcite as the corresponding Raman spectra contain both calcite (280 cm-1; 714 cm-1) and aragonite (203 cm-1; 704 cm-1) peaks. In some of the mapped grains, laser induced heating is causing the aragonite to transform to calcite and serves as a cautionary note for conducting Raman analysis on carbonates. Optical microscopy confirmed aragonite-to-calcite alteration along the edges of grains that have experienced mechanical boring. Additionally stable isotope δ13C and δ18O data is typical of the arid climate of Abu Dhabi and δ18O is decreasing with the decreasing amount of aragonite down the core. These findings illustrate the need for further investigation into the timing and controls on early diagenesis. Aragonite that has been altered to calcite in the upper 25 cm of the sedimentary column may still record primary marine signatures. However, without further geochemical characterization of altered sediments, we cannot be confident in paleoenvironmental proxy data extracted from the carbonate record.

Department: Physical Sciences

Faculty Mentor: Dr. Hilary Corlett