Proc. Natl. Acad. Sci. USA Vol. 94, pp. 8354–8361, August 1997 Colloquium Paper This paper was presented at a colloquium entitled ‘‘Carbon Dioxide and Climate Change,’’ organized by Charles D. Keeling, held November 13–15, 1995, at the National Academy of Sciences, Irvine, CA. Geochemistry of corals: Proxies of past ocean chemistry, ocean circulation, and climate ELLEN R. M. DRUFFEL Department of Earth System Science, University of California, Irvine, CA 92697 ABSTRACT This paper presents a discussion of the status Second, a synthesis is presented of the available proxy of the field of coral geochemistry as it relates to the recovery of records from corals as they relate to past climate, ocean past records of ocean chemistry, ocean circulation, and climate. circulation changes, and anthropogenic input of excess CO2. The first part is a brief review of coral biology, density banding, For some databases, the anthropogenic CO2 appears clear and and other important factors involved in understanding corals as globally distributed. For other databases it appears that vari- proxies of environmental variables. The second part is a syn- ability on interannual to decadal time scales complicates a thesis of the information available to date on extracting records straightforward attempt to separate natural perturbations of the carbon cycle and climate change. It is clear from these from the anthropogenic CO2 signal on the Earth’s environ- proxy records that decade time-scale variability of mixing pro- ment. Shen (4) presents a review of the types of coral archives cesses in the oceans is a dominant signal. That Western and available from the perspective of historical El Nin˜o-Southern Eastern tropical Pacific El Nin˜o-Southern Oscillation (ENSO) Oscillation (ENSO) influences on the tropical Pacific Ocean. records differ is an important piece of the puzzle for under- Also, Druffel et al. (5) present a review of coral growth and the standing regional and global climate change. Input of anthro- factors that affect it. The present paper attempts to compli- 13 14 pogenic CO2 to the oceans as observed by C and C isotopes ment the information contained in these two publications. in corals is partially obscured by natural variability. Nonethe- Most of the records available to date are from surface corals less, the general trend over time toward lower d18O values at that are restricted to the upper 50–75 m of the ocean’s surface. numerous sites in the world’s tropical oceans suggests a gradual However, there is a growing effort devoted to the study of warming andyor freshening of the surface ocean over the past deep-sea corals and gorgonians found at all depths of the century. world’s oceans (#10,000 m). Most of the discussion in this review centers on surface corals, though new and exciting The biogeochemical cycling of carbon on Earth has undergone research involving the use of deep species as paleoclimatologic marked changes over the past glacial-to-interglacial period, tools are mentioned. and nearly all of these changes have occurred prior to scientific Review of Corals as Geochemical Proxies observation. The key to understanding present and future changes in climate and the cycling of biogeochemically impor- Biology and Microstructure. Corals are from the order Scler- tant elements (i.e., C, N, S, P, O, Cd, Ba, Si, etc.) is to actinia, a group in the subclass Zoantharia. Scleractinians include adequately account for the past changes in their cycling. To solitary and colonial species of corals, many of which secrete enable retrospective studies, a number of substrates on Earth external skeletons of aragonite. The oldest known scleractinians have acted as integrators of these changes. Within the bands of are shallow water corals from the Middle Triassic (6). tree rings lie records of relative seasonal rainfall and 14C 12C y The polyp portion of the coral secretes calcium carbonate ratios (1) of the atmospheric CO used during photosynthesis. 2 (CaCO ) as the mineral aragonite. The cylindrical outer portion Another recorder of climate is oceanic sediment, which con- 3 of the polyp is called the polyp wall and is terminated above by tains a layered time history of faunal shells that lived in the horizontal oral disc and below by the basal disc (see Fig. 1). overlying surface and deep waters. Ice cores have been used to The oral disc is bound by one or more ringlets of tentacles that reveal past air temperatures (2) and atmospheric CO2 con- are covered with stinging nematocysts, which aid in the collection centrations (3) during the past 100,000 years. of food and in defense. The mouth is located in the center of the The skeletons of corals provide an unaltered record of the oral disc and leads to the interior gastrovascular cavity where the chemical and physical conditions that existed in the surround- organs of digestion, absorption, and excretion lie. Within the ing seawater at the time of accretion of its calcium carbonate polyp are three layers of tissue: ectoderm, mesogloea, and skeleton. The advantages of corals as an oceanic recorder are endoderm. Parts of the ectoderm that are in direct contact with the enhanced time resolution (biweekly to seasonal) available the skeleton are known as the calcioblast and are responsible for from the high growth rate, and the absence of mixing processes secretion of the aragonite. Within the endoderm of most herma- that are present in all oxic sediments (i.e., bioturbation). typic (reef-building) corals are found symbiotic dinoflagellate There are two aspects of this synthesis paper. The first is a algae, called zooxanthellae. brief review of the field of coral geochemistry. A brief discus- Scleractinians reproduce both sexually and asexually. In sion of coral biology, taxonomy, and microstructure is pre- most species, ova and sperm are located in the same polyp. sented. Factors affecting coral skeletal growth and isotopic Fertilization takes place inside the gastrovascular cavity, and and chemical records within the coralline aragonite are re- the larvae are ejected through the mouth. The young larvae or viewed, and a discussion of the annual and subannual density planulae settle on a hard, solid surface to form a coral colony. banding within the skeletal matrices is included. A few days after fixation, the first complete set of mesenteries © 1997 by The National Academy of Sciences 0027-8424y97y948354-8$2.00y0 Abbreviations: ENSO, El Nin˜o-Southern Oscillation; SST, sea surface PNAS is available online at http:yywww.pnas.org. temperature(s); DIC, dissolved inorganic carbon. 8354 Downloaded by guest on September 24, 2021 Colloquium Paper: Druffel Proc. Natl. Acad. Sci. USA 94 (1997) 8355 the surface of the coral epitheca (see Fig. 1) to be daily growth bands. He found about 400 ridges (days) per annum, which agreed with astronomical expectations of the deceleration of the Earth’s period of rotation since the Devonian. The most significant record contained in most surface coral skeletons are annual density bands. They are primary skeletal characteristics that consist of a high and low density portion per year discernible by x-ray of a thin slab cut along the axis of upward corallite growth (15, 16). Annual variations in density represent changes in both the rate of linear skeletal extension and calcification. These growth bands were first conclusively demonstrated as annual by Knutson et al. (17). They found excellent agreement between autoradiographs of Enewetak coral, which depict nuclear bomb products incor- porated during nuclear blasts (mostly 90Sr), and x-radiographs, which reveal density structure. At a few locations, multiple FIG. 1. Anatomical structure of a coral polyp and its association banding within an annual growth period has been identified, with the skeleton (redrawn from ref. 7). though this is not common (18, 19). Also, Buddemeier and Kinzie (18) have identified lunar bands in P. lobata from Kahe and septa are formed. Also, asexual reproduction takes place Point, Oahu, Hawaii. Using x-radiography and scanning den- in an existing coral colony, thereby rendering the distance sitometry of Montastrea annularis from St. Croix, U.S. Virgin between coral polyps roughly constant as the surface area of Islands, Dodge and Brass (20) found that skeletal extension the colony increases. (linear growth) is negatively correlated with density and The microstructure of a coral skeleton reveals an intricate positively correlated with calcification (mass addition). matrix that is reflective of the coral’s habits. The skeleton Hudson et al. (21) used alizarin-staining techniques and consists of minute, orthorhombic crystalline needles of ara- x-radiography to observe annual density bands in M. annularis gonite, about 2 m in diameter. When a planulae first settles to from the Florida Straits and the Gulf of Honduras. They form a coral colony, it accretes a horizontal layer of aragonite noticed that dense aragonitic skeleton accreted during the called the basal disk (Fig. 1). As the polyp grows upward, the warm summer months of July through September and that margin of the base turns upward, forming a cup, called the thicker, less dense bands accreted during the cooler months of epitheca. The epitheca is believed to contain daily growth October through June. At some locations, such as the Gulf of bands (8) that are formed as a result of daily changes in the Eilat in the Red Sea, corals (Porites lobata) accrete high density shape of the calcioblastic layer (9). Also extending from this bands during the season of cold sea surface temperatures basal disk are vertical sheets of septa that support and separate (SST). In Florida, thicker, dense layers, known as stress bands, the mesenteries in the polyps (Fig. 1). Each septum consists of also formed during the unusually severe winters of late 1969, numerous crystal clusters called sclerodermites.