Exploring the impact of diagenesis on (isotope) geochemical and microstructural alteration features in biogenic aragonite

Abstract

For the Quaternary and Neogene, aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment. Conversely, pre‐Neogene aragonite is often diagenetically altered and calcite archives are studied instead. Nevertheless, the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material. Here, experiments were performed to understand biogenic aragonite alteration processes and products. Shell subsamples of the bivalve Arctica islandica were exposed to hydrothermal alteration. Thermal boundary conditions were set at 100°C, 175°C and 200°C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different 18O‐depleted fluids for durations between two and twenty weeks. Alteration was documented using X‐ray diffraction, cathodoluminescence, fluorescence and scanning electron microscopy, as well as conventional and clumped isotope analyses. Experiments performed at 100°C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Δ47 which show the effects of annealing processes. At 175°C, valves undergo significant aragonite to calcite transformation and neomorphism. The δ18O signature supports transformation via dissolution and reprecipitation, but isotopic exchange is limited by fluid migration through the subsamples. Individual growth increments in these subsamples exhibit bright orange luminescence. At 200°C, valves are fully transformed to calcite and exhibit purple‐blue luminescence with orange bands. The δ18O and Δ47 signatures reveal exchange with the aqueous fluid, whereas δ13C remains unaltered in all experiments, indicating a carbonate‐buffered system. Clumped isotope temperatures in high‐temperature experiments show compositions in broad agreement with the measured temperature. Experimentally induced alteration patterns are comparable with individual features present in Pleistocene shells. This study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits. The limitations of a study restricted to a single test organism are acknowledged and call for refined follow‐up experiments.

What do we really know about early diagenesis of non-marine carbonates?

“…Clumped isotope thermometry can help in constraining conditions of diagenesis in carbonates at temperatures higher or lower than temperature of formation (Swart, 2015) and to reconstruct burial depths and fluid flow history (Huntington et al, 2006;Eiler, 2011;Bergman et al, 2013;Ritter et al, 2017;Sample et al, 2017). According to Ferry et al (2011) clumped isotope thermometry should record the depositional temperature, diagenesis, and recrystallization of carbonate minerals up to at least 100-200°C, though caution is needed (Ritter et al, 2017).…”

“…Focused case studies, where observations and monitoring of active carbonate deposition can be integrated with exposed or subsurface fossil deposits, prove very insightful. In addition, an experimental approach, using in-vitro (Pedley, 2014;Mercedes-Martín et al, 2016) and in-situ (Melim and Spilde, 2011;Boch et al, 2015) setups could help detangle the exact diagenetic pathways (Ritter et al, 2017), controlling parameters and rates of individual processes, across the continuum of non-marine carbonate depositional contexts. A better understanding of the short-lived, intermediate steps and diagenetic products, and the effects on individual geochemical proxies Ritter et al, 2015) is needed to bridge a growing gap between field and lab-based studies at the nanoscale, mostly focussed on the very first steps of carbonate nucleation and precipitation (Benzerara et al, 2006;Sánchez-Román et al, 2011;Krause et al, 2012;Roberts et al, 2013;Burne et al, 2014;Brauchli et al, 2016;Pace et al, 2016), and the rock record of possibly diagenetically modified carbonate deposits, used -or not -in paleoclimatic and -environmental studies.…”

“…According to Ferry et al (2011) clumped isotope thermometry should record the depositional temperature, diagenesis, and recrystallization of carbonate minerals up to at least 100-200°C, though caution is needed (Ritter et al, 2017). With the calculated Δ 47 -based temperature of formation and using the measured δ 18 O carbonate value and the temperature dependence of oxygen isotope fractionation between carbonate and water, the δ…”

“…O values which are more easily reset (Ritter et al, 2017). Andrews (2006) points out that in studied riverine tufas, it is hard to distinguish when stable carbon and oxygen isotope values may have been affected by a range of diagenetic processes (including cementation, micritisation and aggrading neomorphism).…”

“…The effect of early diagenetic processes on stable and radiogenic isotope and trace element geochemical signals in non-marine paleoclimatic carbonate records remains a critical, though delicate, point (Andrews, 2006;Fairchild et al, 2006;Martín-García et al, 2009;Milliere et al, 2011;Millière et al, 2011;Wassenburg et al, 2012;Zhang et al, 2014). Only seldom the effect of a single diagenetic process on different stable and radiogenic isotope and trace elemental proxies is investigated in field samples or lab precipitates (Martín-García et al, 2009;Domínguez-Villar et al, 2017), nor are the cumulative effects of successive diagenetic transformation processes (Scholz et al, 2014;Ritter et al, 2017).…”