Climate proxy optimization - quantifying factors controlling growth, geochemistry and micro/nanostructural design of bivalve mollusk shells
Zusammenfassung der Projektergebnisse
This project aimed at a better understanding of how to extract environmental variables from shells of bivalve mollusks. To break away from the scourge of δ18Oshell values, we explored ways to make other proxies serviceably. According to the results of this study, a combined sclerochronological and geochemical approach is required that takes the bivalve’s physiology (e.g., activity patterns) as well as the distribution of organic matrices and crystal fabrics into account. The study was based on extensive laboratory and field experiments and a variety of different analytical methods. Our main focus was on Sr/Ca, Mg/Ca, Ba/Ca, Li/Ca and δ13Cshell values. This following summarizes a selection of our findings. (1) Bivalves were not constantly active, but show circadian activity patterns which are based on biological clocks and were unrelated to ontogenetic age or species. Without entrainment, freerunning cycles of 28 to 34 hours were observed, whereas a 12/12-hour light/dark-cycle entrainment caused a distinct 24-hour period. During each cycle, shells were most active over time intervals of 8-10 hours. These activity periods occurred preferably during night. (2) Even during activity periods, the shells were not constantly recording their environment, but opening and closing valves at regular time intervals of 5 to 7 min. These ultradian cycles were temperature-variant. (3) Gaping was highly sensitive to thermal stress. Although different species exhibited different thermal tolerances, temperatures above 31°C were lethal to all studied bivalves. (4) Organic matrices are heterogeneously distributed in shells; this is partly a function of variations in shell growth. A number of trace and minor elements, e.g., Mg are bound to organics rather than substituting calcium in the crystal lattice of aragonite. If the precise content of organics at the sampled position is not known, results on the element chemistry may be incorrect, specifically if in-situ analytical techniques such as LA-ICP-MS are used. A compound-specific analysis of trace and minor elements, e.g., by wet analytical techniques, may be advantageous. (5) Likewise, crystal fabrics vary significantly across the shells. The size, habit and orientation of the crystals can determine the amount of certain trace and minor elements and likely reflect processes involved in the biomineralization. For example, Sr and Mg are strongly enriched in the irregular simple prisms near annual growth lines, but depleted in the portions between adjacent growth lines (crossed-lamellar/cross-acicular fabrics). The organic content and type of crystal fabrics should be mapped prior to the analysis of trace and minor elements. (6) Sr/Ca and Mg/Ca of Arctica islandica shells increase with ontogenetic age and decreasing growth rates. If these trends are mathematically removed, Sr/Ca and Mg/Ca ratios are inversely correlated to water temperature as predicted by thermodynamics. Sr/Ca and Mg/Ca of this species can therefore be used an independent temperature proxy. (7) Stable carbon isotope values of A. islandica shells exhibit a constant negative offset from equilibrium of ~2.7‰. This offset is caused by a contribution of ca. 10% metabolic carbon to the shell carbonate and remains unchanged through lifetime. Therefore, the δ13Cshell values of this species can be used to estimate changes of the dissolved inorganic carbon in the ocean and, therefore, changes of the primary productivity and the ability of the ocean to absorb anthropogenic CO2. Results of our study will significantly improve bivalve shell-based climate proxy reconstructions and make the climate archive ‘bivalve shell’ more widely applicable in paleoclimate analyses.
Projektbezogene Publikationen (Auswahl)
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2007. Environmental controls on daily shell growth of Phacosoma japonicum (Bivalvia: Veneridae) from Japan. Mar. Ecol. – Prog. Ser. 336, 141-150
Miyaji T, Tanabe K and Schöne BR
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2008. Stable carbon and oxygen isotope fractionation in bivalve (Placopecten magellanicus) larval aragonite. Geochim. Cosmochim. Acta 72, 4687-4698
Owen EF, Wanamaker AD Jr, Feindel SC, Schöne BR and Rawson PD
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2008. The curse of physiology – Challenges and opportunities in the interpretation of geochemical data from mollusk shells. Geo-Mar. Lett. 28, 269-285
Schöne BR
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2009. Changes in gape frequency, siphon activity and thermal response in the freshwater bivalves Anodonta cygnea and Margaritifera falcata. J. Molluscan Stud. 75, 51-57
Rodland DL, Schöne BR, Baier S, Zhang Z, Dreyer W and Page NA
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2009. Investigation of Li/Ca variations in aragonitic shells of the ocean quahog Arctica islandica, northeast Iceland. Geochem., Geophys., and Geosyst. 10, Q12008
Thébault J, Schöne BR, Hallmann N, Barth M and Nunn EV
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2009. Using ocean quahog (Arctica islandica) shells to reconstruct palaeoenvironment in Öresund, Kattegat and Skaggerak, Sweden. Int. J. Earth Sci. 98, 3-17
Dunca E, Mutvei H, Göransson P, Mörth C-M, Schöne BR, Whitehouse MJ, Elfman M and Baden SP
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2010. Effect of organic matrices on the determination of the trace element chemistry (Mg, Sr, Mg/Ca, Sr/Ca) of aragonitic bivalve shells (Arctica islandica) – comparison of ICP-OES and LA-ICP-MS data. Geochem. J. 44, 23-37
Schöne BR, Zhang Z, Jacob D, Gillikin DP, Tütken T, Garbe-Schönberg D, McConnaughey T and Soldati A
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2011. Annually resolved δ13Cshell chronologies of long-lived bivalve mollusks (Arctica islandica) reveal oceanic carbon dynamics in the temperate North Atlantic during recent centuries. Palaeogeog., Palaeoclimatol., Palaeoecol. 302, 52-64
Schöne BR, Wanamaker AD Jr, Fiebig J, Thébault J and Kreutz KJ
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2011. Sr/Ca and Mg/Ca ratios of ontogenetically old, long-lived bivalve shells (Arctica islandica) and their function as paleotemperature proxies. Palaeogeog., Palaeoclimatol., Palaeoecol. 302, 31- 42
Schöne BR, Zhang Z, Radermacher P, Thébault J, Jacob D, Nunn EV and Maurer A-F
