Constraining calcium isotope fractionation (δ44/40Ca) in modern and fossil scleractinian coral skeleton

The present study investigates the influence of environmental (temperature, salinity) and biological (growth rate, inter-generic variations) parameters on calcium isotope fractionation (δ44/40Ca) in scleractinian coral skeleton to better constrain this record. Previous studies focused on the δ44/40Ca record in different marine organisms to reconstruct seawater composition or temperature, but only few studies investigated corals. This study presents measurements performed on modern corals from natural environments (from the Maldives for modern and from Tahiti for fossil corals) as well as from laboratory cultures (Centre Scientifique de Monaco). Measurements on Porites sp., Acropora sp., Montipora verrucosa and Stylophora pistillata allow constraining inter-generic variability. Our results show that the fractionation of δ44/40Ca ranges from 0.6 to 0.1‰, independent of the genus or the environmental conditions. No significant relationship between the rate of calcification and δ44/40Ca was found. The weak temperature dependence reported in earlier studies is most probably not the only parameter that is responsible for the fractionation. Indeed, sub-seasonal temperature variations reconstructed by δ18O and Sr/Ca ratio using a multi-proxy approach, are not mirrored in the coral's δ44/40Ca variations. The intergeneric variability and intrageneric variability among the studied samples are weak except for S. pistillata, which shows calcium isotopic values increasing with salinity. The variability between samples cultured at a salinity of 40 is higher than those cultured at a salinity of 36 for this species. The present study reveals a strong biological control of the skeletal calcium isotope composition by the polyp and a weak influence of environmental factors, specifically temperature and salinity (except for S. pistillata). Vital effects have to be investigated in situ to better constrain their influence on the calcium isotopic signal. If vital effects could be extracted from the isotopic signal, the calcium isotopic composition of coral skeletons could provide reliable information on the calcium composition and budget in ocean. Highlights ► Corals cultured in aquaria or from natural environment show the same Ca isotopic composition. ► δ44/40Ca of coral skeleton is independent of depositional setting environment. ► Strong influence of vital effects on coral skeleton δ44/40Ca composition and calcification mechanism

CONTROLE ENVIRONNEMENTAL DE LA PHYSIOLOGIE ET DE LA COMPOSITION ISOTOPIQUE DU SQUELETTE DES SCLERACTINIAIRES A ZOOXANTHELLES (APPROCHE EXPERIMENTALE)

NICE-BU Sciences (060882101) / SudocSudocFranceF

Effect of light on skeletal δ13C and δ18O, and interaction with photosynthesis, respiration and calcification in two zooxanthellate scleractinian corals

International audienc

A novel culture technique for scleractinian corals:application to investigate changes in skeletal δ18O as a function of temperature

International audiencenovel experimental protocol is described that assists investigations of the effect of environmental parameters on records from the carbonate skeletons of scleractinian corals. It involves the culture of coral colonies on glass slides so as to time the skeletal deposition and environmental records precisely. The value of the technique is demonstrated via calibration of the relationship between skeletal δ18O and seawater temperature in 2 species of coral obtained from the Gulf of Aqaba. Colonies were grown at 5 temperatures between 21 and 29°C. For Acropora sp. this relationship gave a slope of -0.27o/oo °C-1, a value close to previous estimates. The δ18O signature of Stylophora pistillata displayed a high variability between colonies and gave an average slope much lower than previous estimates (-0.13o/oo °C-1). These data may indicate a taxonomic difference and the need to re-examine the systematics of this genus. Nevertheless, such variability in colonies of a single species or of a set of closely related species may have implications for the use of coral skeleton as proxy records

Recent Advances in Coral Biomineralization with Implications for Paleo-Climatology: A Brief Overview

The tropical oceans drive climatic phenomena such as the El Niño-southern oscillation (ENSO) and the Asian–Australian monsoon, which have global scale impacts. In order to understand future climatic developments, it is essential to understand how the tropical climate has developed in the past, on both short and longer timescales. However, good instrumental records are limited to the last few decades. The oxygen isotopic (δ18O) composition and strontium/calcium (Sr/Ca) ratio of massive corals have been widely used as proxies for past changes in sea surface temperature (SST) of the tropical and subtropical oceans, because the geochemistry of the skeleton is believed to vary as a function of several environmental parameters (such as seawater temperature, salinity, light, …). However, recent microanalytical studies have revealed large amplitude variations in Sr/Ca and oxygen isotopic composition in coral skeletons; variations that cannot be ascribed to changes in SST or in salinity. Such micro- and nanometer scale studies of geochemical variations in coral skeletons are still few and somewhat scattered in terms of the species studied and the problems addressed. But collectively they show the great potential for determining chemical variations at length scales of direct relevance to the biomineralization process. For example, it is now possible to measure geochemical variations within the two basic, micrometer-sized building blocks of the coral skeleton: Early mineralization zones (EMZ) and aragonite fibres. Such micro- and nanometer scale observations, in combination with controlled laboratory culturing of corals, hold the promise of yielding important new insights into the various biomineralization processes that may affect the chemical and isotopic composition of the skeletons. One aim of these efforts is to better understand the elemental and isotopic fractionation mechanisms in order to improve the conversion of the geochemical variability into environmental changes

Calcification does not stimulate photosynthesis in the zooxanthellate scleractinian coral Stylophora pistillata

The interaction between photosynthesis and calcification remains poorly known in zooxanthellate scleractinian corals. We tested whether calcification is a significant source of CO2 for photosynthesis in Stylophora pistillata. Rates of net photosynthesis, respiration, add calcification were measured on colonies incubated in synthetic seawater (SSW) controlled with respect to the inorganic carbon system and containing standard (11.40 mmol kg(-1)) and low (2.85 mmol kg(-1)) calcium concentrations. Net photosynthesis and respiration are not significantly different in standard and low-Ca2+ SSW despite a rate of calcification 2.0-2.4 times lower in Ca2+-depleted SSW. Additional experiments carried out on the noncalcifying zooxanthellate Anthozoa Anemonia viridis demonstrate that a low calcium concentration has no direct effect on rates of photosynthesis and respiration. It is suggested that calcification is not a significant source of photosynthetic CO2 and that photosynthesis stimulates calcification rather than the opposite