Mollusc and brachiopod skeletal hard parts: Intricate archives of their marine environment

Abstract

The biogenic carbonate hard parts of fossil bivalves, cephalopods and brachiopods are among the most widely exploited marine archives of Phanerozoic environmental and climate dynamics research. The advent of novel analytical tools has led many workers to explore non-traditional geochemical and petrographic proxies, and work performed in neighbouring disciplines sheds light on the complex biomineralization strategies applied by these organisms. These considerations form a strong motivation to review the potential and problems related to the compilation and interpretation of proxy data from bivalve, cephalopod and brachiopod hard parts from the viewpoint of the sedimentologist and palaeoceanographer. Specific focus is on the complex biomineralization pathways of a given dissolved ion or food particle from its aquatic environment via the digestion and biomineralization apparatus in molluscs and brachiopods and its incorporation into a biomineral. Given that molluscs and brachiopods do not secrete their hard parts from seawater but rather from their mantle and periostracum, this paper evaluates differences and similarities of seawater versus that of body fluids. Cephalopods, bivalves and brachiopods exert a strong biological control on biomineralization that, to some degree, may buffer their shell geochemistry against secular changes in seawater chemistry. Disordered (amorphous) calcium carbonate precursor phases, later transformed to crystalline biominerals, may be significant in carbonate archive research due to expected geochemical offset relative to the direct precipitation of stable phases. A reasonable level of understanding of the related mechanisms is thus crucial for those who use these skeletal hard parts as archives of the palaeo-environment. The impact of what is commonly referred to as 'biological factors' on the geochemistry of mollusc and brachiopod hard parts is explored for conventional isotope systems such as carbon, oxygen, strontium and traditionally used element to calcium ratios. In particular, the often used d STATE OF THE SCIENCE the architecture of mollusc and brachiopod hard parts and their ultra-structures must guide sampling strategies for geochemical analyses. Where feasible, a detailed understanding of the diagenetic pathways and the application of multi-proxy and multi-archive approaches should form the foundation of fossil carbonate archive research. The uncritical compilation of large data sets from various carbonate-shelled organisms collected at different locations is not encouraged.

The Li isotope composition of marine biogenic carbonates: Patterns and mechanisms

“…Most skeletal organisms calcify in a reservoir isolated from external seawater. They favor calcification by either (i) increasing saturation state (by increasing their internal pH, DIC and/or internal Ca concentration), (ii) reducing their internal fluid Mg/Ca ratio (in particular for modern calcite organisms), or (iii) using a complex organic template to control orientation and distribution of crystals during nucleation (Immenhauser et al, 2016;Ries, 2010). These pathways are not mutually exclusive.…”

“…These pathways are not mutually exclusive. Mollusks have an extracellular-type process of biomineralization (Immenhauser et al, 2016;Weiner and Addadi, 2011;Weiner and Dove, 2003). Precipitation of the inner layer is inferred to take place somewhere in the extrapallial space (which contains extrapallial fluid or EPF), located between the inner shell surface and the outer mantle epithelium.…”

“…Bivalves, brachiopods, and echinoderms are present in widely distributed habitats in the modern-day ocean, are often well-preserved during diagenesis, and are abundant in the Phanerozoic record (Immenhauser et al, 2016;Veizer et al, 1999;Wilkinson, 1979 (Delaney et al, 1989;Füllenbach et al, 2015;Thébault et al, 2009;Thébault and Chauvaud, 2013), but no Li isotope data on modern bivalves and brachiopods. Here we test the influence of temperature, mineralogy and biology on Li isotopic composition and Li/Ca ratio on a set of mollusk, brachiopod and echinoderm samples from various environments, in order to evaluate the suitability of these taxa to reconstructing past δ 7 Li seawater .…”

Experimental diagenesis: insights into aragonite to calcite transformation of <i>Arctica islandica</i> shells by hydrothermal treatment

“…Despite more than a century of research on carbonate diagenesis, many of the controlling processes are still only understood in a qualitative manner Veizer, 1980, 1981;Swart, 2015). One of the main problems is that diagenetically altered carbonates occur as the product of a complex alteration pathway with an unknown number of intermediate steps and controlling factors (Immenhauser et al, 2015;Swart, 2015;Ullmann and Korte, 2015). Motivated by the lack of quantitative data on rates and products of marine, meteoric, and burial diagenesis, we performed laboratorybased alteration experiments with Arctica islandica shells with the aim to obtain time series data sets.…”

Mapping of recent brachiopod microstructure: A tool for environmental studies

“…Recent brachiopods are unparalleled archives on how microstructure and geochemistry may respond or adapt to general or specific environmental conditions (Watson et al, 2012;Cross et al, 2016). Also, they allow for the study of complex relationships between different shell microstructures and the oceanographic geochemical record (e.g., Immenhauser et al, 2016).…”