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Authors: Höche, Nils
Title: Bivalve shell microstructures - exploring a novel marine temperature proxy
Online publication date: 10-Aug-2022
Year of first publication: 2022
Language: english
Abstract: Bivalve shells are unparalleled high-resolution climate archives. Not only can the environ- mental conditions prevailing during shell formation be stored in the physical, chemical, and, as recently shown, microstructural properties of the shells, but their paleoclimate records can also excellently be temporally contextualized by recurring growth lines, which form due to periodic shell growth. Nevertheless, most paleotemperature proxies employed in bivalve shells come with limitations. Temperature reconstructions based on the oxygen stable isotope (δ18Oshell) value of the shells, for example, also depend on the δ18Owater, which is often unknown. In addition, the δ18Oshell values of the shells are prone to alteration during diagenesis. The trace elemental composition of the shells, while correlating to the water temperature, were shown to be strongly affected by physiological processes such as variations in shell growth rate. The width of growth increments of the shells simultaneously inform about food conditions and temperature and are likewise strongly physiologically controlled. The microstructural properties of the shells, in contrast, might be less affected by factors other than temperature, and be preserved in fossil shells when geochemical signal such as the 18Oshell are already lost. However, a link between the shell microstructure and the ambient water temperature was hitherto only demonstrated in afew short-lived bivalve species. Before microstructural properties can confidently be used to infer paleotemperature, these proxies need to be adequately calibrated and tested. This thesis examined whether an influence of temperature on microstructural properties such as the biomineral unit (BMU) size is a common phenomenon across long-lived bivalve taxa commonly used in sclerochronological paleoclimate reconstructions. In addition, it was evaluated whether food availability and quality, salinity, shell growth rate and ontogenetic age also affect the microstructural properties of the shells, possibly overprinting temperature signals. The results of this project comprise three manuscripts published in international, peer-reviewed scientific journals. In the 1st manuscript, it is investigated whether the microstructure of the shells of Gly- cymeris bimaculata, a long-lived bivalve species broadly distributed in temperate coastal to brackish regions, is affected by changes in water temperature. G. bimaculata forms crossed-lamellar shells, a microstructure which occurs in over 90% of all marine mollusks in some form. Novel image processing techniques applied to scanning electron microscopy images combined with stable oxygen isotope analysis revealed that the size, length and width of BMUs of G. bimaculata indeed correlate strongly with the water temperature. This link can be used to infer water temperatures with up to 2.3°C precision, introducing a promising new paleotemperature proxy that could potentially apply to a wide range of mollusk taxa which also form crossed-lamellar shells. In manuscript two, shells of Arctica islandica, a well-studied sclerochronological archive known for its extreme longevity and wide distribution across the northern North Atlantic, were analyzed. In order to study the effects of water temperature on the shells in isolation of other factors, specimens were analyzed that were raised in a lab under different temperature settings, while keeping salinity and food conditions constant. These experiments revealed a direct influence of water temperature on the size of the BMUs and the size of pores incorporated into the shells of A. islandica, suggesting that a microstructural response to temperature variations is a common phenonmenon across different bivalve taxa. In Manuscript three, it was tested whether different environmental regimes can effectively be discriminated by the analysis of microstructural properties of A. islandica shells collected across multiple habitats. Analysis were also performed in different shell portions and across different ontogentic stages in order to determine physiological influences on the shell microstruce. This way, it could be revealed that temperature changes as small as approx. 1-2 °C trigger alterations of the shell microstructure. In naturally grown shells however, temperature signals can be obstructed when unfavorable growth conditions grown such as small as low and variable or low dissolved oxygen content. In addition, the microstructural properties of the shells change strongly during ontogeny. In summary, the experiments of this thesis revealed an influence of the water tempera- ture on the microstructure of bivalve shells. However, the microstructural properties are also strongly coupled to the physiology of the bivalve and to its biomineralization processes, which complicates temperature reconstructions. The methods developed in this study strongly facilitate quantitative analysis of microstructural properties of carbonate shells and open up plenty of research applications not only in paleoclimatology but also in biomineralization research. I carried out all laboratory work (oxygen stable isotope analysis, laser ablation–inductively coupled plasma–mass spectrometry measurements, growth pattern analysis, scanning electron microscopy and image analysis) of this thesis. I also developed the image analysis methods used for BMU morphometry, analyzed and interpreted the data and produced the figures. I wrote all text for manuscripts one and three and contributed substantially to the text of manuscript two. Contributions of other authors are stated at the beginning of each manuscript.
DDC: 333.7 Natürliche Ressourcen
333.7 Natural resources
500 Naturwissenschaften
500 Natural sciences and mathematics
550 Geowissenschaften
550 Earth sciences
560 Paläontologie
560 Paleontology
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 09 Chemie, Pharmazie u. Geowissensch.
Place: Mainz
URN: urn:nbn:de:hebis:77-openscience-f56de137-7a74-49e7-9ece-d7739fdc54425
Version: Original work
Publication type: Dissertation
License: CC BY
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Extent: xxiii, 223 Seiten, Illustrationen, Diagramme
Appears in collections:JGU-Publikationen

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