Geochemical fingerprints of dolomitization in Bahamian carbonates: Evidence from sulphur, calcium, magnesium and clumped isotopes

Abstract

In an effort to constrain the mechanism of dolomitization in Neogene dolomites in the Bahamas and improve understanding of the use of chemostratigraphic tracers in shallow-water carbonate sediments the δ 34 S, Δ 47 , δ 13 C, δ 18 O, δ 44/40 Ca and δ 26 Mg values and Sr concentrations have been measured in dolomitized intervals from the Clino core, drilled on the margin of Great Bahama Bank and two other cores (Unda and San Salvador) in the Bahamas. The Unda and San Salvador cores have massively dolomitized intervals that have carbonate associated sulphate δ 34 S values similar to those found in contemporaneous seawater and δ 44/40 Ca, δ 26 Mg values, Sr contents and Δ 47 temperatures (25 to 30°C) indicating relatively shallow dolomitization in a fluidbuffered system. In contrast, dolomitized intervals in the Clino core have elevated values of carbonate associated sulphate δ 34 S values indicating dolomitization in a more sediment-buffered diagenetic system where bacterial sulphate reduction enriches the residual SO 2À 4 in 34 S, consistent with high sediment Sr concentrations and low δ 44/40 Ca and high δ 26 Mg values. Only dolomites associated with hardgrounds in the Clino core have carbonate associated δ 34 S values similar to seawater, indicating continuous flushing of the upper layers of the sediment by seawater during sedimentary hiatuses. This interpretation is supported by changes to more positive δ 44/40 Ca values at hardground surfaces. All dolomites, whether they formed in an open fluid-buffered or closed sediment-buffered diagenetic system have similar δ 26 Mg values suggesting that the HMC transformed to dolomite. The clumped isotope derived temperatures in the dolomitized intervals in Clino yield temperatures that are higher than normal, possibly indicating a kinetic isotope effect on dolomite Δ 47 values associated with carbonate formation through bacterial sulphate reduction. The findings of this study highlight the utility of applying multiple geochemical proxies to disentangle the diagenetic history of shallow-water carbonate sediments and caution against simple interpretations of stratigraphic variability in these geochemical proxies as indicating changes in the global geochemical cycling of these elements in seawater.

Biogeochemical reappraisal of the freshwater–seawater mixing‐zone diagenetic model

“…Because of this, in order to successfully fingerprint dolomite without the potential bias of later‐stage recrystallization expected from using either Δ 47 and/or δ 18 O, the identification of the conditions in which dolomite forms must rely on the informed application of multiple isotopic systematics capable of constraining rock versus fluid buffering systems (e.g. Ahm et al ., 2018; Murray et al., 2021). In addition, newer understanding of rock buffering of porewater composition, especially in the zone marked by dissolution of aragonite and VHMC, such as in the phreatic meteoric zone, could also bias the stable isotopic signal (Banner & Hanson, 1990; Higgins et al ., 2018).…”

“…A more extended use of the isotopic composition of carbonate associated sulphate (CAS) can shed an additional light onto mixing‐zone diagenesis. This latter proxy has been applied to great effect in constraining conditions during the early diagenetic history of marine sediments (Rennie & Turchyn, 2014; Present et al ., 2015; Murray et al., 2021), and recently to the analysis of diagenetic porewaters in the Permian carbonates of the Guadalupian Mountains (Present et al ., 2019). Iron isotope systematics provide another redox sensitive proxy exhibiting a non‐conservative behaviour across mixing zones (Rouxel et al ., 2008).…”

“…With δ 26 Mg changing predominantly as a result of dolomite/VHMC formation, and Ca being incorporated into all carbonate phases, significant changes in δ 26 Mg not matched with δ 44 Ca would imply a more efficient incorporation of Mg into the solid phase products of a given diagenetic zone. Multi‐proxy approaches, such as those published for Neogene Bahamian sediments (Higgins et al ., 2018; Murray et al., 2021) and Devonian dolomite in the Williston sub‐basin, within the Western Canada Sedimentary Basin (Nadeau et al ., 2019), therefore have the distinct advantage of being based on proxies more resistant to alteration during later stages of diagenesis. At present, little to no δ 26 Mg and δ 44 Ca studies of likely freshwater–seawater mixing zones or calcareous aquifers exist.…”

Variability in sulfur isotope records of Phanerozoic seawater sulfate

“…Diagenetic processes may exchange sulfate with the primary carbonate and alter its isotopic composition (Fichtner et al, 2017;Murray et al, 2020;Present et al, 2015Present et al, , 2019. Kampschulte & Strauss (2004) suggested that the variability of multiple δ 34 S analyses from contemporaneous stratigraphic successions could be used to quantify the effect of diagenesis on the CAS record.…”

“…Carbonates recrystallizing during burial may also be prone to diagenetic modification of the δ 34 S of CAS if the burial fluids were sulfate rich (Fichtner et al, 2017(Fichtner et al, , 2018Present et al, 2015). The δ 34 S in burial fluids may be highly variable, and include sulfate from hydrocarbon or organic matter degradation, dissolved evaporites, groundwater modified by MSR, or sulfate released by dissolution of CAS (Dogramaci et al, 2001;Fichtner et al, 2018;Murray et al, 2020;Present et al, 2019;Thode & Monster, 1965, 1970.…”