The Mediterranean coral Cladocora caespitosa: a proxy for past climate fluctuations?

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Abstract

Sclerochronology was applied to recent, Holocene and Pleistocene samples of Cladocora caespitosa. Late Pliocene samples were recrystallised and thus unsuitable for sclerochronology. Quaternary samples showed a clear, alternating banding pattern as in the living coral, confirming a marked seasonality of past climate. The computed mean annual growth rates ranged from 2.1 to 6.9 mm year−1, with highest growth rates during the warmer phase (isotope stage 5e) of the first climate cycle. It is hypothesised that the largest fossil banks of C. caespitosa grew in a coastal environment with considerable alluvial inputs and warmer temperatures than today.

Introduction

Cladocora caespitosa (L.) is the main native zooxantellate and constructional coral of the Mediterranean Sea, abundant both in the geological past and in recent times. Large fossil Cladocora formations were found as old as Late Pliocene, when this coral formed true reefs both in the eastern and western Mediterranean Sea Aguirre and Jiménez, 1998, Dornbos and Wilson, 1999. Large fossils banks of C. caespitosa are also known from the Early Pleistocene (Bernasconi et al., 1997), from the Middle and Late Pleistocene and from the Holocene (for a review, see Peirano et al., 1998).

Today, C. caespitosa is a major carbonate producer among Mediterranean organisms (Peirano et al., 2001). It occurs on shallow bottoms (4–10 m) forming ‘beds’ of numerous, distinct, hemispherical colonies. From 10 m down to 40 m of water depth, the coral becomes more rare. But, where present, it can form build-ups rising up to 1 m above the surrounding seafloor and covers several square metres in surface area.

Recent C. caespitosa growth has been studied by sclerochronology, i.e., the study of seasonal growth marks in the skeleton. X-radiographs of these skeletons show a clear banding pattern, indicating that the coral deposits two bands per year: a high-density band during the cold and rainy winter season (November–March) and a low-density band in the warm and dry summer season (July–September). No differences in banding were found between colonies collected from shallow and those from deeper water. Timing of band formation proved to be correlated with monthly sea temperature and irradiance levels (Peirano et al., 1999).

Sclerochronology has been largely used in the tropics to assess the response of corals to environmental changes and to provide a proxy for seawater temperature of past centuries Hudson, 1981, Lough and Barnes, 1997. Here, we present preliminary data on the sclerochronology of fossil C. caespitosa. The present work aims to study the presence and preservation of the annual banding pattern in fossil Cladocora skeletons. In case of positive results, we want to compare annual growth rates of recent and fossil samples with climate parameters.

Section snippets

Material and methods

Recent coral skeletons were samples isolated from the Ligurian Sea (NLS). Fossil samples come from Tuscany (PV), Sardinia (C1 and C2), Calabria (LB95), Apulia (TaSV, TaSR and TaST) and SE Spain (AL).

NLS samples were collected in 1994–1997 in the Ligurian Sea at 4–30 m of depth from Bocca di Magra to Bonassola (La Spezia) (Peirano et al., 1999). Sclerochronology allowed measuring annual growth rates of 114 colonies back to 1930.

All the Holocene and Pleistocene samples were single colonies; they

Results

All the skeletons from the Quaternary showed a clear banding pattern (Fig. 1). Corallites of the Pliocene from Spain (AL), recrystallised, did not show growth bands. Mean annual growth rates of Quaternary samples ranged from 2.1 to 6.9 mm year−1 (Table 1).

Notwithstanding the low number of fossil corals analysed and the fact that samples came only from one colony per site, mean growth rates were in good accordance with Shackleton's (2000) isotopic reconstruction of climate cycles (Fig. 2). In

Discussion and conclusions

An identical banding pattern in fossil and recent C. caespitosa indicates that also the past Mediterranean climate was characterised by a marked seasonality, with cold winters and warm summers. However, fossil corals formed larger and higher banks than those of today, hence the environmental conditions of past Mediterranean Sea, in terms of both temperature and water quality should not have been exactly the same. The descriptions of fossil coral banks, their stratigraphic sequences and their

Acknowledgments

Thanks to J. Geister (Bern) and to Kl. Oekentorp (Muenster) for the critical reading of the manuscript. This work has been done within the framework of the research projects SINAPSI and Ambiente Mediterraneo (Accordo di programma MURST-CNR “Ecosistemi marini”).

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