Shallow burial alteration of dolomite and limestone clumped isotope geochemistry

Abstract

Geology of the Sample Site Andros Island, the largest island on the Great Bahama Bank, sits atop more than six kilometers of nearly 100% carbonate rock. The 4442-meter-long Andros Number 1 well was drilled into north-central Andros Island by the Stafford and Bahamas Oil companies in 1947. The core is now housed by the Florida State Geological Survey in Tallahassee, FL. The core spans the Pleistocene through the Early Cretaceous according to approximate microfossil dating and correlation with the stratigraphy of Florida (Spencer, 1967). The core material was discussed by Illing (1954) and Spencer (1967) and petrographically and geochemically described by Goodell and Garman (1969; Fig. DR1). The foraminifera and bivalve fossils, occasional reefal debris, and common pelletal muds found in the core support the interpretation of marine deposition for the entire sedimentary sequence. Some faulting and uplift occurred within the Great Bahama Bank during the Early Cretaceous, however, it is likely that the burial history of all samples from the Andros core follows an essentially linear increasing depth and temperature trend (Elberli and Ginsburg 1987). While it is true that periods of erosion are observed in core sediments, their magnitude is likely minor and no tectonic uplifts are known to have occurred (Goodell and Garman, 1969; Elberli and Ginsburg, 1987). Today, deposits on the interior of the Great Bahama Bank are generally oolitic and coralgal sands in higher-energy areas that grade into pelletal muds (Illing 1954). Most of this modern material is aragonitic, but early diagenesis has resulted in no aragonite present below ~35 m depth (Goodell and Garman, 1969; Beach, 1993). All material studied from this core is 100% carbonate, but the mineralogy varies from calcite to dolomite. Offshore cores analyzed by Melim et al. (2002) indicate a shift from aragonite-dominated mud to mature microsparitic limestone and complete alteration by marine pore fluids within 100-150 m of sea level. Recrystallization of material at ~250 m depth indicates complete dolomitization and a degree of diagenetic stabilization relatively soon after deposition, consistent with diagenetic models (Beach 1993; Whitaker et al. 1994; Melim et al. 2001; Melim et al. 2002). Marine waters have been shown to flow consistently through the upper ~400 m of the Great Bahama Bank (from the center of the platform outward) and are likely agents of lithification as well as dolomitization (e.g. Whitaker et al. 1994). Sample Characterization Samples taken from the core's uppermost layers are entirely micritic dolomite mud with no visible grains. By 250m depth, dolomite is entirely recrystallized with microspar, which is the likely result of near-surface diagenesis and lithification as described above. Foraminifera, bivalve, and coral fragments are still visible within the entirely recrystallized micritic matrix. From ~450 m to 3500 m depth, limestone beds are interbedded with dolomites, with both commonly preserving pellet traces and fossil outlines. Specifically...

Modeling the effects of diagenesis on carbonate clumped-isotope values in deep- and shallow-water settings

“…This exercise demonstrates that the trend of our measured  47 values vs. depth are predicted by previous models of diagenesis providing confidence in both the model and the external boundary conditions. Following this, we explore the model's validity in shallow-water continental shelf settings via comparison of model predictions to previously published data from the Bahamas (Winkelstern and Lohmann, 2016) and show the model can explain measurements made on samples from these settings.…”

“…Such temperatures are often considered too warm to represent Earth-surface conditions (e.g., Came et al, 2007;Eiler, 2011;Finnegan et al, 2011;Stolper and Eiler, 2016;Winkelstern and Lohmann, 2016), at least over the Phanerozoic. A common explanation for some of these elevated temperatures is that they are the result of dissolutionreprecipitation reactions occurring during sedimentary burial at elevated temperatures.…”

“…Thus when discussing our data, we use a temperature calibration generated at the Caltech laboratory. Likewise, when discussing the data of from Winkelstern and Lohmann (2016), we use the  47 vs. temperature calibration of , as both the that and calibration were generated in the same laboratory.…”

“…To study this, we compared the model to a dataset from Winkelstern and Lohmann (2016), which reports  47 values from calcite and dolomite samples taken from a drill core through carbonate shelf sediments from Andros Island, Bahamas spanning a depth range of 0 to 4500 meters We note that models of diagenesis similar to those used here to describe the exchange of oxygen isotopes between carbonate and pore fluids from 0 to 1100 mbsf in Bahama Bank ODP cores have been applied previously (Swart, 2000), supporting our model formulation for this system, but are not widely used in the interpretation of diagenesis in shallow-water systems.…”

“…Based on information given in Winkelstern and Lohmann (2016), we took an average depositional rate of the system to be 33 m/myr and ran the model for 142 million years. Due to a lack of constraints, we assumed the same porosity vs. depth relationship as site 807.…”