Hydrothermal alteration of aragonitic biocarbonates: assessment of micro- and nanostructural dissolution–reprecipitation and constraints of diagenetic overprint from quantitative statistical grain-area analysis

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.

Temperature-induced microstructural changes in shells of laboratory-grown Arctica islandica (Bivalvia)

“…This type of information is increasingly reconstructed from bivalve shells [4][5][6], specifically stable oxygen isotope (δ 18 O shell ) values [7][8][9]. Given known limitations of this paleothermometer, i.e., vulnerability to diagenetic overprint [10] and difficulties to obtain δ 18 O water signatures of ancient and/or coastal water bodies, the search for alternative temperature proxies is in full swing (Δ 47 : [11,12]; Δ 48 : [13]; Mg, Sr: [14]). A highly promising candidate is the shell microstructure [15][16][17][18].…”

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

“…The common carbonate archive, Porites sp. corals, have been extensively documented in the literature (Pätzold, ; Allison, ; Stanley & Swart, ; Alibert & McCulloch, ; Enmar et al, ; Felis, Patzold, & Loya, ; McGregor & Gagan, ; Al‐Rousan, Felis, Manasrah, & Al‐Horani, ; Hendy, Gagan, Lough, McCulloch, & DeMenocal, ; McGregor & Abram, ; Benzerara et al, ; Motai et al, ; Lazareth et al, ; Casella et al, ). Due to the geographical range of Porites sp., it has been found to primarily record environmental information in tropical and subtropical regions (Schlager, ; Veron et al, ; Milliman, Muller, & Forstner, ).…”

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

“…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.…”

“…consist entirely of aragonite according to XRD measurements and Rietveld analysis (Fig. A2 and A4 from Casella et al, 2018) and have an organic matter content that varies significantly for the different species. Organic matter concentration in the investigated hard tissues was determined with TGA analyses (Fig.…”

“…Aiming to understand the influence of bioaragonite architecture in the kinetics of hydrothermal alteration we investigated the alteration response of three very different biocarbonate skeletons: (i) the granular aragonite that forms the shell of the bivalve Arctica islandica, (ii) the prismatic and columnar nacreous aragonite that comprises the shell of the gastropod Haliotis ovina and (iii) the acicular, fibrous aragonite that builds up the skeleton of the coral Porites sp. By comparing the hydrothermal overprint undergone by these microstructures after prolonged alteration at 80ºC (this study) and after much shorter alteration at 175 °C (Casella et al, 2018) we better identify early hydrothermal alterations steps and improve our understanding of the role of temperature and time in the progress of alteration overprint. This study demonstrates that a combination of analytical tools and evaluation techniques (TGA and XRD measurements, Rietveld analysis of XRD data, EBSD measurements and grain size statistical evaluation, laser confocal microscopy, FE-SEM and AFM imaging) provides the ideal set of data to pinpoint the structural changes caused by the hydrothermal and/or diagenetic alteration of biological hard tissues, even the subtle, difficult to address ones that mark the very first steps of biocarbonate microstructural reorganization.…”