Chemical Composition of Carbonate Hardground Cements as Reconstructive Tools for Phanerozoic Pore Fluids

Abstract

In this study, we report the chemical composition of early carbonate cement precipitates in carbonate hardgrounds to understand the geochemical signature of near‐surface carbonate mineral precipitation. As carbonate hardgrounds lithify at or near the sediment‐water interface, they acquire cements that may be minimally evolved from paleoseawater. Using a suite of chemical measurements, we explore the potential of carbonate hardground cements as paleoenvironmental proxies. Trace metal and isotopic ratios, including some rare earth elements, Mg/Ca, manganese, and strontium concentrations, δ18O, δ13C, and 87Sr/86Sr, were analyzed in the carbonate cements from 17 Phanerozoic carbonate hardgrounds. The sensitivity of the geochemical signal to alteration depends on the geochemical analysis in question and the environmental water‐rock ratio. Of these samples, only our modern sample has measurements consistent with primary precipitation from seawater; all other samples precipitated from chemically evolved seawater or were influenced by meteoric water, even if only minimally changed. The more recent samples from the Cenozoic had seawater 87Sr/86Sr. The Mesozoic samples, in contrast, did not preserve seawater 87Sr/86Sr, even though the Mg/Ca, δ18O, and δ13C values were consistent with precipitation from seawater. Finally, the Paleozoic samples preserved expected seawater 87Sr/86Sr, though rare earth element and δ18O suggest primary precipitation was from evolved seawater. Additionally, we place our results in the context of open versus closed system precipitation using transects of the Mg/Ca ratios across individual cements. Overall, we stress that one geochemical measurement provides only a partial record of fluid composition, but multiple measurements allow a potential understanding of the seawater geochemical signal.

Shallow-marine serpentinization-derived fluid seepage in the Upper Cretaceous Qahlah Formation, United Arab Emirates

“…The δ 13 C values of the hardground cements are similar to those of the JHM late cement, although one of the values (−9.1 ‰) is the most negative of all the carbonates we measured from the Qahlah Formation (Fig. 4), and also more negative than any marine hardgrounds in the geological record documented by Erhardt et al (2020), with the lowest value of −5.15 ‰ from the Lower Cretaceous of Oman. From this we infer, again, that the fluid from which the hardground cements precipitated was not pure seawater.…”

The record of sea water chemistry evolution during the Ediacaran–Cambrian from early marine cements

“…Inferred variation in sea water Mg/Ca molar ratio from Ediacaran (550 Ma) to Ordovician (480 Ma). Dru., Drumian stage; Guz., Guzhangian stage; Pai., Paibian stage (Erhardt et al, 2020; Ries, 2010). …”

“…The Ce anom data have been combined here with published data from different components from Ediacaran to early Cambrian successions (Cui et al, 2016;Frank et al, 2019;Li et al, 2015;Ling et al, 2013;Wallace et al, 2017; F I G U R E 1 1 Inferred variation in sea water Mg/Ca molar ratio from Ediacaran (550 Ma) to Ordovician (480 Ma). Dru., Drumian stage; Guz., Guzhangian stage; Pai., Paibian stage (Erhardt et al, 2020;Ries, 2010). Ward et al, 2019;Wei et al, 2018;Yang et al, 2022) in order to compare how oxygenation in distinct basins developed and to determine whether oxygenation events correlate globally (Figure 12).…”

Effects of early marine diagenesis and site-specific depositional controls on carbonate-associated sulfate: Insights from paired S and O isotopic analyses

“…Sr, Fe vs. Sr and δ 13 Ccarb vs. δ 18 Ocarb, are generally used to screen samples for evidence of meteoric, burial, brine and marine diagenesis (Banner and Hanson, 1990;Bartley et al, 2001;Erhardt et al, 2020;Halverson et al, 2007;Turchyn et al, 2009). Sr abundance is considered an approximate indication of the degree of carbonate recrystallization, as most fluids distinct from seawater will remove Sr from carbonate minerals with progressive recrystallization.…”