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1. Trace Element Patterns in Otoliths: The Role of Biomineralization

   https://doi.org/10.1080/23308249.2020.1760204  

   Hüssy, Karin; Limburg, Karin E.; de Pontual, Hélène; Thomas, Oliver R.; Cook, Philip K.; Heimbrand, Yvette; Blass, Martina; Sturrock, Anna M. (January 2020, Reviews in Fisheries Science & Aquaculture)

   Otolith chemistry has gained increasing attention as a tool for analyzing various aspects of fish biology, such as stock dynamics, migration patterns, hypoxia and pollution exposure, and connectivity between habitats. While these studies often assume otolith elemental concentrations reflect environmental conditions, physiological processes are increasingly recognized as a modulating and/or controlling factor. In particular, biomineralization—the complex, enzyme-regulated construction of CaCO3 crystals scaffolded by proteins—is believed to play a critical role in governing otolith chemical patterns. This review aims to summarize the knowledge on otolith composition and biophysical drivers of biomineralization, present hypotheses on how biomineralization should affect element incorporation, and test the validity thereof with selected case studies. Tracers of environmental history are assumed to be dominated by elements that substitute for Ca during crystal growth or that occur randomly trapped within the crystal lattice. Strontium (Sr) and barium (Ba) largely comply with the biomineralization-based hypotheses that otolith element patterns reflect environmental concentrations, without additional effects of salinity, but can be influenced by physiological processes, typically exhibiting decreasing incorporation with increasing growth. Conversely, tracers of physiology are assumed to be elements under physiological control and primarily occur protein-bound in the otolith’s organic matrix. Physiological tracers are hypothesized to reflect feeding rate and/or growth, decrease with fish age, and exhibit minimal influence of environmental concentration. The candidate elements phosphorus (P), copper (Cu) and zinc (Zn) confirm these hypotheses. Magnesium (Mg) is believed to be randomly trapped in the crystal structure and hence a candidate for environmental reconstruction, but the response to all examined drivers suggest Mg to be coupled to growth. Manganese (Mn) substitutes for Ca, but is also a co-factor in matrix proteins, and therefore exhibits otolith patterns reflecting both environmental (concentration and salinity) and physiological (ontogeny and growth) histories. A consistent temperature response was not evident across studies for either environmental or physiological tracers, presumably attributable to variable relationships between temperature and fish behavior and physiology (e.g., feeding rate, reproduction). Biomineralization thus has a controlling effect on otolith element concentrations for elements that are linked with somatic growth, but not for elements that substitute for Ca in the crystal lattice. Interpretation of the ecological significance of patterns from field samples therefore needs to consider the impact of the underlying biomineralization processes of the element in question as well as physiological processes regulating the availability of ions for inclusion in the growing crystal lattice. Such understanding will enhance the utility of this technique to address fisheries management questions. 

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2. Assessing the Benthic Mg/Ca- Temperature Proxy: A Uvigerina Core-Top Study from New Zealand

   Stirpe, CR; Allen, KA; Sikes, EL; Zhou, X; Rosenthal, Y; Cruz-Uribe, AM (January 2019, 2019 Fall Meeting AGU)

   The magnesium to calcium ratio (Mg/Ca) of benthic foraminiferal calcite serves as an important tool for reconstructing past deep water temperature. Application of this proxy relies upon accurate calibrations and an understanding of the factors that may influence the Mg/Ca ratios of foraminifer tests. Core-top calibrations are a method of assessing the temperature sensitivity of deep-dwelling benthic taxa which are difficult to raise in culture. This study contributes a new set of Mg/Ca core-top measurements for the infaunal species Uvigerina peregrina derived from a suite of sediment cores in the Southwest Pacific spanning water depths of 600 to 4400 m. Results agreed with previous calibrations for samples shallower than 2000 m, but unexpectedly high Mg/Ca values were found between the depths of 2400 and 3300 m, necessitating further investigation into potential non-temperature influences. Specimens of different morphotypes were analyzed separately, but variations between hispid and costate samples failed to account for the high-Mg anomaly observed. Lack of correlation between Mg/Ca and the contaminant indicators Mn/Ca, Al/Ca, Fe/Ca, and Ti/Ca suggests contaminant phases are not the source of excess Mg. Laser ablation ICP-MS analysis of chamber cross-sections revealed that the high-Mg signature is located within the interior of test walls, rather than contained in an external coating or contaminant phase. The high- Mg anomaly observed in mid-depth New Zealand waters is likely related to a secondary, non-temperature control on Mg incorporation. Samples with excess Mg are those most strongly influenced by carbon-rich (high dissolved inorganic carbon, high alkalinity) waters flowing south from the northern Pacific, suggesting that inorganic carbonate chemistry plays a role. 

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3. Fossil Corals With Various Degrees of Preservation Can Retain Information About Biomineralization-Related Organic Material

   https://doi.org/10.3389/feart.2021.643864  

   Drake, Jeana L.; Guillermic, Maxence; Eagle, Robert A.; Jacobs, David K. (June 2021, Frontiers in Earth Science)

   Scleractinian corals typically form a robust calcium carbonate skeleton beneath their living tissue. This skeleton, through its trace element composition and isotope ratios, may record environmental conditions of water surrounding the coral animal. While bulk unrecrystallized aragonite coral skeletons can be used to reconstruct past ocean conditions, corals that have undergone significant diagenesis have altered geochemical signatures and are typically assumed to retain insufficient meaningful information for bulk or macrostructural analysis. However, partially recrystallized skeletons may retain organic molecular components of the skeletal organic matrix (SOM), which is secreted by the animal and directs aspects of the biomineralization process. Some SOM proteins can be retained in fossil corals and can potentially provide past oceanographic, ecological, and indirect genetic information. Here, we describe a dataset of scleractinian coral skeletons, aged from modern to Cretaceous plus a Carboniferous rugosan, characterized for their crystallography, trace element composition, and amino acid compositions. We show that some specimens that are partially recrystallized to calcite yield potentially useful biochemical information whereas complete recrystalization or silicification leads to significant alteration or loss of the SOM fraction. Our analysis is informative to biochemical-paleoceanographers as it suggests that previously discounted partially recrystallized coral skeletons may indeed still be useful at the microstructural level. 

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4. Proxy Calibrations in the Cold-Water Coral Desmophyllum dianthus: Nutrient Concentrations in Antarctic Intermediate Water

   Ko, S.: Sherrell; Pelejero, C.; Martínez-Dios, A.; Lucas, A.: Calvo; Kozdon, R.; Bostock, H.; Tracey, D.M. (December 2020, AGU Fall Meeting 2020)

   null (Ed.)

   Element-calcium ratios in the skeleton of cold-water coral Desmophyllum dianthus represent potential archives for paleo-reconstruction of several ocean properties including temperature and nutrient concentrations. However, relatively large uncertainties in these proxy calibrations and heterogeneity in the skeletal composition have limited its application to date. We address these issues by analyzing corals cultured under systematically varied seawater conditions (phosphate, barium, temperature, pH, feeding frequency) over a two-year period, and refine the calibration of P/Ca, Ba/Ca, U/Ca, and Li/Mg proxies for seawater phosphate, barium, carbonate ion concentration, and temperature, respectively. Composition of the corals is determined using laser-ablation ICPMS, with robust plasma conditions established using the Normalized Argon Index [1], and proxy element incorporation is evaluated for influences of temperature, pH, and feeding frequency. The aragonite precipitated during the stages of the culturing experiment is identified using fluorescent and geochemical labelling of the skeleton through calcein and lead isotopes, respectively. This approach allows us to resolve monthly and annual increments in these slow growing (1-2mm/year) organisms, and also to evaluate the influence of calcification rate on the composition. We address the issue of heterogeneity by adapting methods for LA-ICPMS imaging to create macroscale images to reveal the full pattern of skeletogenesis and related compositional variability of D. dianthus. Preliminary images suggest that heterogeneity stems from the asymmetric precipitation of aragonite, and from centers of calcification (also known as early mineralization zones) that complicate the interpretation of elemental signals throughout the skeleton, but also help to identify new skeletal regions suitable for proxy measurement. Finally, we also discuss the role of endolithic organisms in some of these specimens. [1] Fietzke, J. & Frische, M. (2016), J. Anal. At. Spectrom. 31, 234–244. 

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5. Impact of Intra‐Skeletal Calcite on the Preservation of Coral Geochemistry and Implications for Paleoclimate Reconstruction

   https://doi.org/10.1029/2023PA004730  

   Weerabaddana, Mudith M; Thompson, Diane M; Reed, Emma V; Farfan, Gabriela A; Kirk, Jason D; Kojima, Alice C; Dettman, David L; de_Brum, Kalena; Kabua, Emma; Edwards, Florence (May 2024, Paleoceanography and Paleoclimatology)

   Abstract The geochemistry of tropical coral skeletons is widely used in paleoclimate reconstructions. However, sub‐aerially exposed corals may be affected by diagenesis, altering the aragonite skeleton through partial dissolution, or infilling of secondary minerals like calcite. We analyzed the impact of intra‐skeletal calcite on the geochemistry (δ¹⁸O, Sr/Ca, Mg/Ca, Li/Mg, Li/Ca, U/Ca, B/Ca, Ba/Ca, and Mn/Ca) of a sub‐aerially exposedPoritessp. coral. Each micro‐milled coral sample was split into two aliquots for geochemistry and X‐ray diffraction (XRD) analysis to quantify the direct impact of calcite on geochemistry. We modified the sample loading technique for XRD to detect low calcite levels (1%–2%; total uncertainty = 0.33%, 2σ) in small samples (∼7.5 mg). Calcite content ranged from 0% to 12.5%, with higher percentages coinciding with larger geochemical offsets. Sr/Ca, Li/Mg, Li/Ca, and δ¹⁸O‐derived sea‐surface temperature (SST) anomalies per 1% calcite were +0.43°C, +0.24°C, +0.11°C, and +0.008°C, respectively. A 3.6% calcite produces a Sr/Ca‐SST signal commensurate with local SST seasonality (∼1.5°C), which we propose as the cut‐off level for screening calcite diagenesis in paleo‐temperature reconstructions. Inclusion of intra‐skeletal calcite decreases B/Ca, Ba/Ca, and U/Ca values, and increases Mg/Ca values, and can therefore impact reconstructions of paleoclimate and the carbonate chemistry of the semi‐isolated calcifying fluid in corals. This study emphasizes the importance of quantifying fine‐scale calcite diagenesis to identify coral preservation levels and assure robust paleoclimate reconstructions. 

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