Assessing Diagenesis and Reliability of Multi-Method Analysis of Paleolake Carbonate Systems in the Al-Azraq Basin, Jordan: A Case Study

Abstract:

Diagenetic alteration can significantly alter oxygen and carbon isotopic composition signals of authigenic carbonate, thereby rendering their paleoenvironmental interpretations invalid. Differentiating altered and unaltered isotopic signals has important implications for paleoenvironmental interpretation. Diagenesis is particularly challenging in lacustrine carbonate systems where diagenesis occurs through several processes including: methanogenesis, sulfate reduction, and dolomitization. A robust investigation of 30m of cored sediments from the Al-Azraq basin, Jordan provides a case study for examining lacustrine carbonate diagenesis. Shifts in δ¹³C_carb values identify diagenesis processes. Additional methods support identification of diagenesis types, illustrating which are reliably used for interpretation. Occurrence of celestite (Sr SO₄) and secondary gypsum (CaSO₄•2(H₂O)) indicates the presence of sulfur, suggesting sulfate carbonate diagenesis occurred in the upper sediments. SEM data reveal the presence of dolomite throughout sediments, further identified by the percentages of Ca% vs. Mg%. Deviation from the molar value of dolomite identifies low Mg-calcite, considered an early diagenesis stage. Samples at 11m revealed Mg/Ca ratios from 10 to 12 revealing the presence of high Mg-calcite. Hydrothermal fluids also play an important role in the diagenesis of carbonate indicated by decreasing δ¹⁸O_carb values and increasing Mg/(Mg+Ca) ratios. The δ¹⁸O_carb vs. Mg (Mg+Ca) of cored sediments produced a very low correlation (R²= 0.2376) due to increased δ¹⁸O_carb, but with a decrease in Mg/(Mg+Ca) indicating the absence of hydrothermal diagenesis. The covariance of δ¹³C_carb and δ¹⁸O_carb of calcium carbonate from the Al-Azraq demonstrates the basin has been a closed system throughout its history. Multiple analyses illustrate lacustrine carbonate formation is not monolithic, and provides a refined paleoenvironmental interpretation.