Fast and pervasive diagenetic isotope exchange in foraminifera tests is species-dependent

Abstract

Oxygen isotope compositions of fossil foraminifera tests are commonly used proxies for ocean paleotemperatures, with reconstructions spanning the last 112 million years. However, the isotopic composition of these calcitic tests can be substantially altered during diagenesis without discernible textural changes. Here, we investigate fluid-mediated isotopic exchange in pristine tests of three modern benthic foraminifera species (Ammonia sp., Haynesina germanica, and Amphistegina lessonii) following immersion into an 18O-enriched artificial seawater at 90 °C for hours to days. Reacted tests remain texturally pristine but their bulk oxygen isotope compositions reveal rapid and species-dependent isotopic exchange with the water. NanoSIMS imaging reveals the 3-dimensional intra-test distributions of 18O-enrichment that correlates with test ultra-structure and associated organic matter. Image analysis is used to quantify species level differences in test ultrastructure, which explains the observed species-dependent rates of isotopic exchange. Consequently, even tests considered texturally pristine for paleo-climatic reconstruction purposes may have experienced substantial isotopic exchange; critical paleo-temperature record re-examination is warranted.

Rates of carbon and oxygen isotope exchange between calcite and fluid at chemical equilibrium

“…The extent of isotopic exchange by both dissolution-precipitation and diffusion is expected to depend on calcite surface area. However, the observation of similar surface-area-normalized exchange rates between natural and synthetic calcite is inconsistent with previous studies that have reported differences in isotope exchange rates due to chemical or crystallographic differences between synthetic and natural calcite, and between abiotic and biogenic calcite (Chanda et al, 2019;Cisneros-Lazaro et al, 2022;Kronenberg et al, 1984;Lahav and Bolt, 1964). A difference in Ca isotope exchange rates between biogenic versus abiotic calcite in the study of Chanda et al (2019) could be attributed to the higher amount of chemical impurities in the biogenic calcite, facilitating greater dissolution and reprecipitation due to compositional differences between solid and fluid.…”

“…Compared to solid-state diffusion in single crystals, grain boundary diffusion of O is described by a diffusion coefficient that is ~20 orders of magnitude larger at 25°C (Farver and Yund, 1998), similar to those measured in low temperature studies. Similarly, the complex, interconnected porosity in foraminifera tests facilitate greater diffusive exchange between 18 Oenriched fluid and the solid compared to abiotic single calcite crystals (Cisneros-Lazaro et al, 2022). The lack of agreement between diffusion coefficients from experiments conducted at 25°C versus those at high temperatures for single crystals suggests that the higher diffusion coefficients at low temperatures may be attributable to the presence of fractures and 25 microporosity in the calcite and aqueous-mediated diffusion, possibly with some contribution of dissolution-precipitation, rather than a solid-state process alone.…”

“…For example, calcitic foraminifera tests can be excluded from analysis when they appear "frosty" under light microscopy due to recrystallization (Sexton et al, 2006). However, experimental studies have revealed that isotopic compositions of calcite [CaCO3] can be altered substantially without visual evidence of recrystallization if isotopic disequilibrium between fluids and calcite occurs (Bernard et al, 2017;Chanda et al, 2019;Cisneros-Lazaro et al, 2022;Géhin et al, 2021). The mechanism and rate of this isotopic alteration will determine whether and under what conditions carbonate archives preserve their isotopic compositions.…”

“…In isotope diffusion (self-diffusion), controlled by substitutional mechanisms, diffusion often depends on the availability of point vacancies throughout the crystal lattice (Cole and Chakraborty, 2001). Solid-state diffusion has been invoked to explain both C and O isotope alteration of biogenic calcite in the form of foraminifera tests in experiments at 90°C and above, and abiotic nano-crystalline calcite at 25°C (Bernard et al, 2017;Cisneros-Lazaro et al, 2022;Géhin et al, 2021).…”

Experimental burial diagenesis of aragonitic biocarbonates: from organic matter loss to abiogenic calcite formation

“…can overlook the effects of early burial diagenesis in bioaragonite hard tissues. This means that many of the bioaragonite skeletons considered pristine and used in the reconstruction of the Earth past may have been altered despite not showing significant microstructural evidence of this alteration (Bernard et al, 2017;Cisneros-Lázaro et al, 2022). At this point, the question that arises is whether bioaragonite recrystallization into abiogenic aragonite impacts the geochemical signatures, particularly isotopic, and if so, to what extent.…”