Disentangling shallow‐water bulk carbonate carbon isotope archives with evidence for multi‐stage diagenesis: An in‐depth component‐specific petrographic and geochemical study from Oman (mid‐Cretaceous)

Abstract

Disentangling shallow-water bulk carbonate carbon isotope archives into primary and diagenetic components is a notoriously difficult task and even diagenetically screened records often provide chemostratigraphic patterns that significantly differ from global signals. This is mainly caused by the polygenetic nature of shallow-water carbonate substrates, local carbon cycle processes causing considerable neritic-pelagic isotope gradients and the presence of hiatal surfaces resulting in extremely low carbonate preservation rates. Provided here is an in-depth petrographic and geochemical evaluation of different carbonate phases of a mid-Cretaceous (Barremian-Aptian) shallow-water limestone succession (Jabal Madar section) deposited on the tropical Arabian carbonate platform in Oman. The superposition of stable isotope signatures of identified carbonate phases causes a complex and often noisy bulk carbon isotope pattern. Blocky sparite cements filling intergranular pores and bioclastic voids evidence intermediate to (arguably) deep burial diagenetic conditions during their formation, owing to different timing or differential faulting promoting the circulation of fluids from variable sources. In contrast, sparite cements filling sub-vertical veins reveal a rock-buffered diagenetic fluid composition with an intriguing moderate enrichment in 13 C, probably due to fractionation during pressure release in the context of the Miocene exhumation of the carbonate platform under study. The presence of abundant, replacive dedolomite in mudsupported limestone samples forced negative carbon and oxygen isotope changes that are either associated with the thermal breakdown of organic matter in the deep burial realm or the expulsion of buried meteoric water in the intermediate burial realm. Notwithstanding the documented stratigraphically variable and often facies-related impact of different diagenetic fluids on the bulk-rock stable isotope signature, the identification of diagenetic end-members defined d 13 C and d 18 O threshold values that allowed the most reliable 'primary' bulk carbon isotope signatures to be extracted. Most importantly, this approach exemplifies how to place regional shallow-water stable isotope patterns with evidence for a complex multi-stage diagenetic history into a supraregional or even global context.

The potential of carbonate ramps to record C-isotope shifts: insights from the upper Miocene of the Central Mediterranean area

“…Furthermore, carbon isotopes of carbonates are almost insensitive to temperature changes (Emrich & Vogel 1970), thus a reliable proxy for the isotopic composition of the DIC (Maslin & Swann 2006). However, meteoric and deep burial diagenesis might alter the original carbon isotope signature (Burla et al 2008; Colombié et al 2011; Huck et al 2017). It is widely accepted that pelagic carbonate successions are the most reliable record for carbon isotope stratigraphy (Weissert et al 2008 and reference therein).…”

Early-Middle Frasnian (Late Devonian) carbon isotope Event in the Timan-Pechora Basin (Chernyshev Swell, Pymvashor River section, North Cis-Urals, Russia)

“…A synoptic screening diagram based on Huck et al (2017), Immenhauser et al (2003), Lohmann (1988) and Chen et al (2016) was used (Fig. 2).…”

Carbon and oxygen stable isotope record of upper Kimmeridgian shallow-marine ramp carbonates (Iberian Basin, NE Spain): the imprint of different burial and tectonic histories

“…skeletal grains, coated grains, micrite matrix) (Anderson and Arthur, 1983;Hudson, 1977;Jenkyns and Clayton, 1986;McConnaughey, 1989a, b;Nelson and Smith, 1996;Schobben et al, 2015). iii) Diagenetic processes, from early meteoric eogenesis due to subaerial exposure to burial mesogenesis and meteoric telogenesis, can alter the primary marine isotopic signature, thus complicating the use of these isotopic records as palaeoenvironmental proxies (Brand and Veizer, 1981;Coimbra et al, 2014;Glumac and Walker, 1998;Huck et al, 2017;Immenhauser et al, 2002;Marshall, 1992;Moore, 1989;Oehlert and Swart, 2019).…”

“…On the other hand, the magnitude of the diagenetic alteration also depends on the microstructure of the grains and on the degree of the primary porosity and permeability of the sediment. Coarse-grained grain-supported fabrics, in the absence of mud between grains, can be more favourable for infiltration of diagenetic fluids, and may result in a higher degree of diagenetic alteration and resetting of stable isotope signatures (Al-Mojel et al, 2018;Coimbra et al, 2014;Huck et al, 2017;Immenhauser et al, 2002;Marshall, 1992;Vincent et al, 2004). Different diagenetic processes and products result from the interaction between carbonate sediments and rocks with diagenetic fluids of a different nature (e.g.…”

“…Therefore, diagenetic processes usually are associated with chemical, mineralogical and/or textural changes of carbonates. These must be taken into account when interpreting the carbon and oxygen isotope signature of bulk rocks and specific diagenetic phases Veizer, 1980, 1981;Coimbra et al, 2014;Colombié et al, 2011;Huck et al, 2017;Lavoie and Bourque, 1993).…”