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

Deyanira Cisneros-Lazaro,Jinming Guo,Alain Baronnet,Anders Meibom,Torsten Vennemann,Sylvain Bernard,Lukas P Baumgartner,Olivier Grauby,Damien Daval,Arthur Adams,Jarosław Stolarski,Stéphane Escrig

doi:10.1038/s41467-021-27782-8

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.

Highlights

Oxygen isotope compositions of fossil foraminifera tests are commonly used proxies for ocean paleotemperatures, with reconstructions spanning the last 112 million years
The use of the oxygen isotope paleothermometer relies on the widely accepted paradigm that fossil foraminifera tests collected from ocean sediments have retained their original elemental and isotopic compositions; at least those tests that appear pristine when observed with optical microscopy or scanning electron microscopy (SEM)[11–16]
Pristine tests of three species of recently collected modern benthic foraminifera, Ammonia sp., H. germanica, and A. lessonii, as well as abiotic Iceland spar calcite crystals, were incubated for 6 days in a highly 18O-enriched (18O/16O = 0.30) seawater analogue in chemical equilibrium with calcite at 90 °C; additional experiments lasting only 4 h were conducted on Ammonia sp. and A. lessonii tests under the same conditions

Summary

Introduction

Oxygen isotope compositions of fossil foraminifera tests are commonly used proxies for ocean paleotemperatures, with reconstructions spanning the last 112 million years. The banded distributions of these elements reflect, to different degrees, the presence and distribution of organic matter within the test wall[22,24–26] This ultrastructure creates a very large internal surface on which elemental and isotopic exchange with sediment porewater can take place, provided that this water can penetrate deep enough into the structure, which seems to be the case according to several recent studies[27,28]. Assuming solid-state diffusion as the dominant process, these authors showed that substantial bulk oxygen isotopic exchange of a fossil foraminifera tests can take place on a timescale of 10 Myr under ambient conditions in ocean sediments, resulting in paleotemperature overestimations and explaining the unrealistically flat inferred temperature gradients in the paleoocean as a result of diagenetic bias

Methods

Results

Conclusion