Significance of fluid chemistry throughout diagenesis of aragonitic Porites corals – An experimental approach

Abstract

Marine carbonates are among the most important archives of environmental information in both modern and past environments. Widely used but particularly sensitive archives are the aragonitic skeletons of scleractinian corals. However, due to the metastable nature of aragonite, a multitude of chemical, mineralogical, and (micro)biological processes can lead to diagenetic alteration of these archives and their proxy information can be altered or lost. Here, hydrothermal alteration experiments were performed to create a better understanding of the diagenesis of an often‐utilized genus, Porites. Same‐specimen subsamples were heated in two fluid types and at two temperatures and durations, and their resulting alteration features were assessed to allow insight to the mechanisms and drivers of diagenetic modification. Experiments with fluid temperatures of 130°C induced remobilization and darkening of organic matrices, with no other evidence for alteration observed. In contrast, specimens exposed to a temperature of 160°C underwent significant diagenetic alteration dependent on fluid type. Fluid chemistry, particularly Mg/Ca ratios, was found to regulate the type of alteration (e.g. neomorphism or reprecipitation). Reaction with meteoric water resulted in almost complete neomorphism of the aragonite skeleton to blocky calcite, as well as significant exchange with the experimental fluid. In comparison, samples altered in the experimental burial fluids record no mineralogic or significant isotopic change, but instead, small‐scale dissolution–reprecipitation of the primary skeleton, along with the precipitation of pore‐filling aragonitic needle cements was observed. The δ18Ocarbonate data indicate transformation via dissolution–reprecipitation mechanisms depending on the degree and mechanism of diagenesis, and are strongly dependent on fluid chemistry. The main outcome of this work is that a multi‐proxy approach has the best potential to shed light on the interpretation of processes and pathways of aragonitic coral alteration. This study has implications for early alteration of carbonate archives within varying diagenetic fluids, with results aiding in the identification of past burial conditions.

Long-term experimental diagenesis of aragonitic biocarbonates: from organic matter loss to abiogenic calcite formation

“…In these composites mineral and organic matrices are intricately interrelated at all scale levels. This study, as well as previous ones, (Casella et al, 2018, Pederson et al, 2019a, 2019b, 2020 deciphered the main intermediate steps undergone by aragonitic microstructures during hydrothermal alteration. The alteration usually starts with the degradation of the biopolymers which are incorporated into the biocarbonate material.…”

“…The used fluid composition (100 mM NaCl + 10 mM MgCl2) simulates the fluid present at a burial diagenetic realm. The composition of the fluid was identical to that previously used by Casella et al, (2017Casella et al, ( , 2018 and Pederson et al, (2019a;2019b;2020) in their hydrothermal alteration experiments.…”

“…The effect of laboratory-based hydrothermal alteration of several aragonitic biominerals has recently been studied in great detail (Ritter et al, 2017;Casella 2018;Pederson et al, 2019aPederson et al, , 2019bPederson et al, , 2020. A multi-analytical approach was used to characterize the hard tissues, both pristine and after their alteration at temperatures above 100 ºC with meteoric (Mgfree) and burial-mimicking (Mg-bearing) fluids.…”

Internal Water Facilitates Thermal Resetting of Clumped Isotopes in Biogenic Aragonite

“…The exchange of isotopes between carbonate and ambient fluids during the aragonite to calcite transition commonly results in an isotopic shift toward the equilibrium δ 18 O values of the newly formed calcite. In the presence of an experimental “diagenetic fluid,” it was shown that calcite formation is accelerated and initiates at 160°C–200°C on experimental timescales (Casella et al., 2017; Milano et al., 2016; Pederson, Mavromatis, et al., 2019; Pederson, Weiss, et al., 2019; Pederson et al., 2020). This reaction rate is considerably accelerated and occurs at lower temperatures if a powdered aragonite is used in the alteration experiment (Guo et al., 2019).…”

“…Dry heating (or “roasting”) experiments revealed that the transition to calcite requires more time and/or higher temperatures, but is nevertheless accompanied by an oxygen isotopic shift in the carbonate (Milano et al., 2018; Müller, Staudigel, et al., 2017; Staudigel & Swart, 2016). This isotopic shift, which occurs in absence of interaction with an outside medium, is believed to be caused by a set of exchange reactions that occur with internal fluids (Moon et al., 2020; Pederson, Mavromatis, et al., 2019; Pederson, Weiss, et al., 2019; Uemura et al., 2019), and/or internal organic material (Li et al., 2020).…”