Abstract

Abstract. The assessment of diagenetic overprint on microstructural and geochemical data gained from fossil archives is of fundamental importance for understanding palaeoenvironments. The correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies show (i) that replacement of biogenic carbonate by inorganic calcite occurs via an interface-coupled dissolution–reprecipitation mechanism. (ii) A comprehensive understanding of alteration of the biogenic skeleton is only given when structural changes are assessed on both, the micrometre as well as on the nanometre scale.In the present contribution we investigate experimental hydrothermal alteration of six different modern biogenic carbonate materials to (i) assess their potential for withstanding diagenetic overprint and to (ii) find characteristics for the preservation of their microstructure in the fossil record. Experiments were performed at 175 °C with a 100 mM NaCl + 10 mM MgCl2 alteration solution and lasted for up to 35 days. For each type of microstructure we (i) examine the evolution of biogenic carbonate replacement by inorganic calcite, (ii) highlight different stages of inorganic carbonate formation, (iii) explore microstructural changes at different degrees of alteration, and (iv) perform a statistical evaluation of microstructural data to highlight changes in crystallite size between the pristine and the altered skeletons.We find that alteration from biogenic aragonite to inorganic calcite proceeds along pathways where the fluid enters the material. It is fastest in hard tissues with an existing primary porosity and a biopolymer fabric within the skeleton that consists of a network of fibrils. The slowest alteration kinetics occurs when biogenic nacreous aragonite is replaced by inorganic calcite, irrespective of the mode of assembly of nacre tablets. For all investigated biogenic carbonates we distinguish the following intermediate stages of alteration: (i) decomposition of biopolymers and the associated formation of secondary porosity, (ii) homoepitactic overgrowth with preservation of the original phase leading to amalgamation of neighbouring mineral units (i.e. recrystallization by grain growth eliminating grain boundaries), (iii) deletion of the original microstructure, however, at first, under retention of the original mineralogical phase, and (iv) replacement of both, the pristine microstructure and original phase with the newly formed abiogenic product.At the alteration front we find between newly formed calcite and reworked biogenic aragonite the formation of metastable Mg-rich carbonates with a calcite-type structure and compositions ranging from dolomitic to about 80 mol % magnesite. This high-Mg calcite seam shifts with the alteration front when the latter is displaced within the unaltered biogenic aragonite. For all investigated biocarbonate hard tissues we observe the destruction of the microstructure first, and, in a second step, the replacement of the original with the newly formed phase.

Highlights

  • Biomineralized hard parts composed of calcium carbonate form the basis of studies of past climate dynamics and environmental change

  • For a more comprehensive understanding of microstructural and chemical controls at diagenesis we extend our studies to hard tissues of other modern marine carbonate biomineralizers such as the bivalve Mytilus edulis, the coral Porites sp., and the gastropod Haliotis ovina

  • field-emission scanning electron microscope (FE-SEM) images shown in Figs. 1, A1, and A2 highlight characteristic mineral units and their assembly within the skeletons of the investigated species: the modern bivalves Arctica islandica and Mytilus edulis, the modern coral Porites sp., and the modern gastropod Haliotis ovina

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Summary

Introduction

Biomineralized hard parts composed of calcium carbonate form the basis of studies of past climate dynamics and environmental change. Our previous study on the shell of the modern bivalve Arctica islandica has shown that laboratory-based, simulated diagenetic alteration discloses microstructural and geochemical features that are comparable to those found in fossil specimens (Casella et al, 2017; Ritter et al, 2017). For a more comprehensive understanding of microstructural and chemical controls at diagenesis we extend our studies to hard tissues of other modern marine carbonate biomineralizers such as the bivalve Mytilus edulis, the coral Porites sp., and the gastropod Haliotis ovina. With these we cover the major calcium carbonate phases, and with the inclusion of the shell of A. islandica, six distinct microstructures. When selecting organisms for this study, strict care was taken to investigate those taxa where fossil counterparts are often used for palaeoclimate and palaeoenvironmental reconstruction

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