Chapter 10 Recent Advances in Coral Biomineralization with Implications for Paleo-Climatology: A Brief Overview (original) (raw)
Abstract
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.
Introduction
Accurate estimation of human impacts on climate variability is an important subject, not only for scientists and environmentalists, but also for lawmakers, politicians, and for society in general. In order to predict the effects of anthropogenic activities on the future climate, it is necessary to document how the climate has fluctuated in the past. Instrumental records can extend up to about 150 years (Kaplan et al., 1998), but have limited geographical distribution. Satellite-based temperature surveys of the oceans cover large regions, but high-quality data are limited to the last two decades. On the other hand, the global climate of the last few centuries is known to have been variable and complex, spatially as well as temporally (e.g., Jones and Mann, 2004).
Massive, hermatypic corals are present today in the tropical and subtropical oceans and can be found in a significant fraction of the geologic record dates back to the middle Triassic. These corals potentially provide high-resolution records of climate variability in the tropical oceans that, through ocean-atmosphere interactions, greatly affect the Earth's climate system (Dunbar and Cole, 1999). Paleo-environmental records based on modern corals can go back in time as far as 300–400 years (Quinn et al., 1998; Cobb et al., 2004; Linsley et al., 2004) and the annual density banding of coral skeletons provides an accurate chronology (Knutson et al., 1972; Barnes and Lough, 1993). Thus, corals offer many advantages for climate reconstructions, provided that precise proxies for environmental changes, including sea surface temperature (SST), can be developed. In this paper, we discuss, briefly, recent advances in the use of corals as proxies for past SST variations with special attention to progress made in understanding the effects of biomineralization processes on the chemical and isotopic composition of the coral skeletons.
Section snippets
Development and Problems Associated with Coral Paleo-Temperature Proxies
The oxygen isotopic (δ18O) and strontium/calcium (Sr/Ca) ratios of massive coral skeletons have been widely used as proxies for past changes in SST of the tropical and subtropical oceans, because both the geochemical parameters are believed to depend on the temperature of the ambient seawater.
The temperature dependency of the oxygen isotopic composition in inorganically precipitated carbonates was established by Urey (1947), but it was realized early on that the δ18O of biogenic carbonates
Biological Sources of Discrepancies in Modern Calibrations
Although biological or “vital” effects are broadly recognized as a source of uncertainty or discrepancy in coral paleo-climatic records relatively little attention has been given to the study of how corals form their skeletons and how, in detail, these biologically induced processes can affect the geochemical composition of the skeletons. For example, the isotopic effect of seawater temperature on skeletal δ18O has been tested in laboratory by using cultures of corals (Reynaud-Vaganay et al,
Conclusions
Taken together, the observations described in this brief overview suggest that the distribution of trace elements and stable isotopic composition of hermatypic coral skeletons are strongly affected, if not completely controlled, by biological processes. From a paleo-climatic point of view, the main challenge for the future is to establish the degree to which the SST affects these biological processes; i.e., how much of the trace element and stable isotope variation can be ascribed to seasonal
Acknowledgments
We thank Anders Meibom and two anonymous reviewers for valuable comments on our manuscript.
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However, our ICP data show that 86Sr does not dissolve into seawater when simply added as solid 86SrCO3 as reported by Houlbreque (unpublished data), questioning the method of Sr uptake into the tissues and skeleton in that study. Understanding the mechanisms of coral skeletal deposition and growth, and the factors affecting these, is also fundamental to the field of coral geochemistry, where Sr/Ca, Mg/Ca and O isotopic ratios are intensively used as past seawater temperature proxies (see reviews by Gagan et al., 2000; Lough, 2004; Correge, 2006; Watanabe et al., 2007). Microanalytical studies have indeed revealed that there are significant compositional heterogeneities that cannot be explained by temperature alone (Cohen et al., 2001; Meibom et al., 2004, 2006, 2008; Allison and Finch, 2004, 2007; Shirai et al., 2005, 2008; Gaetani and Cohen, 2006; Holcomb et al., 2009; Allison et al., 2010; Gaetani et al., 2011).
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