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Meteoric and mixing-zone diagenesis can dramatically alter the geochemical signatures of shallow marine carbonates. Most preserved pre-Cretaceous carbonates were deposited in shallow marine environments and thus may have been susceptible to meteoric and mixing-zone diagenesis. However, a quantitative understanding of how the geochemical composition of carbonates changes during diagenesis still requires further development. Here, we present a new two-dimensional (2D) reactive transport model coupled with a 2D coastal hydrology model to simulate carbonate diagenesis and provide insights into its impact on the isotopic and elemental compositions of carbonates in the geological record. Using this model, we have simulated the stratigraphic trends and relationship between isotopic records (for example, δ13C and δ18O values) observed in modern (Recent-Miocene) sections where the impact of meteoric diagenesis has been clearly characterized. Our model can also reproduce anomalous Neoproterozoic carbonate geochemical profiles where the effects of meteoric diagenesis have been debated. Further, our model indicates that linear carbonate C-O isotope co-variations can either be generated in the mixing zone between freshwater and marine pore waters, or in the freshwater phreatic zone with a downward decrease in recrystallization (with no net carbonate dissolution or precipitation) rate. In addition, numerous processes were observed to decouple δ18Ocarb values from other isotopic and elemental signatures during carbonate diagenesis, indicating that a lack of linear correlation between δ18Ocarb values and other geochemical variables does not necessarily suggest limited meteoric alteration. Sensitivity analyses show that the steady-state timescale is controlled by compositional differences between fluid endmembers, the calcite-water element distribution coefficient, the recrystallization rate, porosity, and the groundwater discharge rate. Given that reactive transport models have proven to be powerful theoretical tools in many disciplines of Earth sciences, our hope is that this model will promote a more quantitative understanding of meteoric and mixing zone diagenesis of marine carbonates.

Recent studies recognized distinct stromatolite provinces in Hamelin Pool, Western Australia, each with morphologically distinct stromatolite structures paired with characteristic shelf physiography. In the present paper, we couple detailed lithofacies mapping with Hamelin Pool bathymetry and consider physiography as a control of sedimentation processes, including stromatolite development. Bathymetric transects, derived from a high-resolution bathymetry map with depths from 0 to 11 meters, allow calculation of slope gradients in the provinces. As in other settings, bathymetry is linked to energy regimes, which in turn appear to be coupled with variations in stromatolite morphologies and associated lithofacies as follows: (1) low-gradient ramps with low-energy settings are associated with sheet mats and elongate-clustered stromatolites that exhibit regular spatial patterns, possibly indicative of self-organization; (2) low gradients coupled with high-energy settings resulting from strong winds result in seif stromatolites with pronounced directional bands; (3) medium to steep gradients coupled with medium to high energy are associated with individual and merged stromatolites, often with thin basal necks; (4) headlands and promontories where the topography deflects currents are associated with elongate-nested stromatolites; and (5) medium- to high-energy slopes typically found at promontory edges and shelf margins are dominated by blocky pavement. Observations linking stromatolite morphology to physiography in a modern microbial system provide insight into the long-lived debate about biology versus environment in controlling stromatolite morphology. When physiography leads to a high-energy regime, environmental controls are the main factor determining stromatolite morphology. In contrast, when physiography promotes a low-energy environment, the response of biological communities becomes the main driver of macroscale stromatolite morphology.

Copyright © 2019 The Authors. Skeletal and non-skeletal components of marine sedimentary rocks have been analyzed for the purpose of reconstructing the rare earth element (REE) and yttrium (Y) compositions of paleo-seawater, but skeletal carbonates frequently have proven to be unreliable recorders of seawater chemistry. Here, we present a systematic multi-technique assessment of rare earth and other trace elements in ooid sands from the modern Great Bahama Bank (GBB–marine) and Great Salt Lake (GSL–continental) based on strong-acid (hydrofluoric and nitric) and weak-acid (acetic) digestions, as well as laser ablation (LA) of ooid cortices and nuclei. The results show that Bahamian ooid cortices possess shale-normalized REE + Y features nearly identical to those of shallow seawater, including limited contamination from siliciclastic REEs. An admixture of even 0.2% of detritally sourced material can modify the primary marine REE + Y patterns by, for example, increasing the light REE (LREE) content. Mean values of LA data for Bahamian ooid cortices exhibit similar REE + Y signatures to those produced by acetic acid digestion, but LA data are generally noisier, primarily as a result of low REE concentrations and the small volume of carbonate ablated in analysis. Screening out samples with ΣREE 0.9 ppm) returns a flat pattern, suggesting similar degrees of LREE depletion controlled by carbonate complexation under similar aqueous alkalinity conditions in Great Salt Lake and Bahamian waters. In summary, ooids can be a reliable proxy for REE + Y characteristics of ambient surficial waters when adopting suitable analytical methods, including laser ablation, that allow the identification and isolation of a contamination signal from siliciclastic detritus.

A positive correlation between the δ13C and δ18O values of carbonate rocks is a screening tool widely used to identify the overprint of meteoric diagenesis on the original isotopic composition of a sample. In particular, it has been suggested that systematic change from negative to positive δ13C and δ18O values with increasing depth in the core is an indicator of alteration within the zone of mixing between meteoric and marine waters. In this paper, we propose that such covariance is not generated within the traditionally defined mixing zone, and that positive correlations between δ13C and δ18O values in marine carbonates are not necessarily indicators of meteoric alteration. This new interpretation is based on data collected from the shallow sub-surface of the Bahamas, a region unequivocally influenced by meteoric waters to depths of at least 200 m below the current sediment-water interface. The classic interpretation of the diagenetic environments, based on changes in the δ13C and δ18O values, would suggest the maximum penetration of freshwater occurs between 65 and 100 m below seafloor. Below these depths, a strong positive covariation between the δ13C and δ18O values exists, and would traditionally be defined as the mixing zone. However, based upon known changes in sea level, the penetration of the freshwater lens extends significantly below this limit. We contend that the zone showing covariance of δ13C and δ18O values is actually altered within the freshwater lens, and not the mixing zone as previously proposed. The co-varying trend in δ13C and δ18O values is the result of diagenetic processes occurring at the interface between vadose and phreatic zones. Significantly greater rates of recrystallization and neomorphism are driven by the increased rates of oxidation of organic matter at this transition with progressively less alteration occurring with increasing depth. As sea level oscillates, the position of this interface moves through the deposit, causing cumulative alteration throughout the section. Hence, we propose that the covariation between δ13C and δ18O values is a consequence of varying degrees of alteration, rather than the result of diagenesis occurring within the zone where marine and freshwater fluids mix. Furthermore, within the pervasively altered vadose zone, there is little correlation between δ13C and δ18O values, and therefore covariation between δ13C and δ18O values is not an unequivocal indicator of meteoric diagenesis.

The Ediacaran was a period of intense environmental change in Earth history. Transient pulses of oxygenation and shifts in ocean chemistry have been interpreted from a range of geochemical proxies, including rare earth elements and yttrium (REY), but it is still a challenge to reconstruct paleoceanographic conditions due to the scarcity of reliable recorder. In this study, we explore the possibility that a newly-discovered deposit of giant ooids in the Nanshanping outcrop in South China contains a relatively intact record of the REE composition of Ediacaran seawater, providing an important window into the redox state of the surface ocean at this time. These ooids are relatively large in size (~1–6 mm), and exhibit densely banded laminae. A regional chemostratigraphic correlation shows that the giant-ooid-bearing succession in South China is in the upper part of the second Ediacaran positive carbon isotopic shift (EP2), and thus its occurrence was not associated with the Shuram Excursion. Additionally, the occurrences of giant-ooid-bearing oolites in different blocks were not synchronous during the middle Ediacaran. In-situ laser ablation analysis of giant-ooid cortices reveals that the shale-normalized REY patterns are similar to those of modern seawater, including depleted light REEs, superchondritic Y/Ho ratios, and positive La anomalies. Yet, no Ce anomalies (1.0 ± 0.15) in numerous ooid cortices seems to indicate a suboxidized condition of surface ocean in South China at that time. The petrographic and geochemical evidence support the weak diagenetic alteration of the pure giant ooids during burial. They formed in open-ocean settings with energetic hydrodynamic environments and were affected by early dolomitization that might preserve mostly original petrographic fabrics and REY signatures from further diagenetic overprinting. The characteristics of Ce anomalies and REY compositions acquired from the giant ooids indicate that the surface ocean was suboxidized in South China during the late stage of EP2, implying the very low atmospheric O2 level at that time. Our results suggest that giant ooids may be a reliable analytical target that provide a snapshot into palaeoceanographic conditions during this crucial period of dynamic changes in early Earth history.