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Skeletal density bands in scleractinian corals have been extensively used in the last 2 decades for environmental and palaeoenvironmental studies. Despite the great importance of coral banding in such studies, the environmental cues that control the timing of band deposition are poorly understood. Here we present a study based on stable isotopic composition and X-ray chronology of Porites spp. corals from shallow (3 m), intermediate (30 m) and deep (51 m) water in the Gulf of Eilat, Red Sea. The results indicate a depth-related timing of density band formation, with opposite patterns in shallow and deep water corals. These observations may play an important role in understanding environmental controls of density banding in corals. The skeletal aragonite deposited by massive scleractinian reef corals is characterized by alternating bands of high and low density calcium carbonate (Knutson et al. 1972, Buddemeier et al. 1974, Buddemeier & Kinzie 1976). This banding pattern is generally annual (Knutson et al. 1972) and has been used in the reconstruction of past and modern environments (Dodge & Vaisnys 1975, Weber et al. 1975, Hudson et al. 1976, Isdale 1984, Klein et al. 1990, Leder et al. 1991). Nevertheless, environmental factors controlling the banding pattern are still uncertain (Buddemeier & Kinzie 1975, Highsmith 1979, Wellington & Glynn 1983). A comparison of the chronology of density bands, detected from X-radiographs of coral slabs with their seasonal pattern of stable oxygen isotopic composition, provides a precise tool for timing their formation (Weber et al. 1975), due to the relationship between 8 " 0 and temperature (Weber & Woodhead 1972). A study based on such analysis was carried out on Porites spp, corals from the Gulf of Eilat, Red Sea. In most cases, determination of the intra-annual timing of high (HD) or low (LD) density band formation was based on analysis of corals from shallow and intermediate water depths (down to 30 m). In the present study, Pon'tes spp. heads were collected from 3 depths: 3 (shallow), 30 (intermediate) and 51 m (deep) in the Red Sea, Eilat, Israel. The study site (29O31'29" N, 34O56'34" E) is characterized by a fringing reef in shallow water, a steep slope covered with patch reefs down to 40 m, and a moderate slope down to 70 m, covered by a dense stand of soft and stony corals (Reiss & Hottinger 1984). The depths of 3, 30 and 51 m represent 3 different habitats, especially in terms of light availability for photosynthesis by zooxanthellae, the symbiotic algae living in association with the corals. That is, photosynthesis progressively decreases over the depth range 3 to 51 m (McCloskey & Muscatine 1984, Muscatine et al. 1989). The dried corals were sectioned to ca 6 mm thick slabs parallel to their growth axis and X-rayed to reveal their annual growth bands (Knutson et al. 1972). Vertical growth profiles exhibiting the most distinct growth bands, free of bioerosion, were detected from the X-ray images and selected for stable isotope measurements. In order to attain high resolution, carbonate samples for the stable isotope measurements were prepared in 2 different ways: (1) corals collected from the shallow and intermediate depths (high coral growth rate) were drilled along a vertical growth profile using a 0.5 mm diameter drill (Fig. l a , b), and (2) corals from the deep reef (slow coral growth rate) were microsampled by removing (polishing) skeletal carbonate powder from the upper section, oriented in the direction of upward growth, using 200 pm step increments (Fig. lc). The number of sub-samples taken ranged from 6 to 12 per annual band (for details see also Fig. 2). O Inter-Research 1993 100 Mar. Ecol. Prog. Ser. 97: 9 9 1 0 4 , 1993 Fig. 1. Porites spp. X-radiographs of the 3 coral slabs from which subsamples for stable-isotope analysis were taken. (a) Sample S4, 3 m depth; (b) Sample MB30, 30 m depth; (c) Sample IS51, 51 m depth. The sampling scheme is presented ' in each image (see text for further details). The profiles for stable-isotope measurements were selected in accordance with their quality (i.e. distinct bands, avoiding traces of boring organisms or other disturbances), hence they 1. represent different time series (specified in Fig. 2) b .:b.& Fig. 2 summarizes optical density profiles (panels a to c) and stable isotope profiles (d to f ) performed on the corals. Profiles in Fig. 2 (panels d to f ) show the stable carbon and oxygen isotopic compositions of the corals at 3. 30 and 51 m depth, respectively. A pattern of annual periodicity was observed in both 6180 and SI3C values for the 3 colonies. Samples were 1 8 0 depleted in the period of high water temperature (summer) and 1 8 0 enriched in the period of low water temperature (winter), as expected from the known oxygen-isotope to temperature relationship (Epstein et al. 1953). The stable carbon isotopic composition responds to variations in light intensity through the photosynthetic activity of the symbiotic algae, where photosynthesis selectively removes 12C02, leading to higher '3C/'2C ratios in DIC (dissolved inorganic carbon) and therefore also enriches the carbon isotope ratio of the coral skeleton (Weber & Woodhead 1970, Fairbanks & Dodge 1979, Swart 1983). The negative S'" maxima (lightest values) usually preceded the S180 minima (Fig. 2d to f ) . In the Red Sea, there is a time lag between light intensity and seawater temperature, where sun irradi.ance maxima and minima precede temperature maxima and minima. Sun irradiance P ro fi le le n g th (m m ) P ro fi le le n g th (m m ) P ro fi le le n g th (m m ) l P ro fi le le n g th (m m ) P ro f~ le le n g th (m m ) P ro fi le le n g th (m m ) F ig . 2 . P or it es s p p . (a to c ) O pt ic al d en si ty p ro fi le s ru n o n ea ch o f th e X -r ad io gr ap hs o f th e 3 co ra l sl ab s; ( d to f ) st ab le o xy ge n an d c ar bo n is ot op ic c om po si ti on s of t h e sa m e co ra l sl ab s. (a a n d d ) S am p le S 4, 3 m d ep th ; (b a n d e ) S am p le M B 30 , 3 0 m d ep th ; (c a n d f ) S am p le IS 51 , 5 1 m d ep th . S h ad ed a re as = h ig h de ns it y b an d s; w hi te a re as = lo w de ns it y ba nd s. T h e co ra l s am pl es w er e re ac te d in 1 00 O/ o or th op ho sp ho ri c ac id a t 7 5 'C to p ro du ce c ar bo n di ox id e. T h e is ot op ic m ea su re m en ts w er e pe rf or m ed o n a F in ni ga n M A T 2 51 m as s sp ec tr om et er . R es ul ts a re g iv en i n th e co nv en ti on al S n ot at io n re la ti ve t o PD B ( B el em ni te s fr om t h e P ee d ee F or m at io n of S ou th C ar ol in a) is ot op ic s ta n d ar d , ca li br at ed b y m ea ns o f th e N B S 19 s ta n d ar d : 6" O (% o) = [ [ ( '8 0 /' 6 0 )s am p le (1 8 0 /' 6 0 )s ls n d ar d l / (' 8 0 /' 6 0 )s ta n d ar d t X 10 °0 an d s im il ar ly f or 6 I3 C . T h e pr ec is io n of th e m ea su re m en ts w as b as ed o n re pl ic at e m ea su re m en ts o f th e in te rn al l ab or at or y st an d ar d ( S ol nh of en l im es to ne o f 63 to 8 0 p m ) an d w as * 0. 10 % f or 6 1 8 0 a n d * 0. 05 "/ 00 fo r s 3 C 102 Mar. Ecol. Prog. Ser values reach their minima between December and January, while minimum seawater temperatures are recorded between February and March. The time lag between Si3C and 6180 reflects the time lag between these 2 environmental parameters in the Gulf of Eilat (Klein et al. 1992). The stable oxygen isotope composition is of particular importance in this study as it is used to determine the intra-annual timing of band formation. The average intra-annual variations in S180 for corals from 3, 30 and 51 m were 0.8, 1.1 and 0.8%0, respectively. Based on the seasonal differences in seawater temperature (ca 5"C), the differences in S180 represent 70 to l00 % of the expected difference, using Epstein's S180 to temperature relationship (Epstein et al. 1953). The position of high density (HD) and low density (LD) bands was detected from optical density profiles run on each of the X-radiographs. The borderlines between HD bands and LD bands were marked as the density =idway between maximum and minimum optical density values (Dodge & Brass 1984) (Fig. 2a to c), and were then superimposed on the stable oxygen isotope curves (Fig. 2d to f ) for determination of the intraannual timing of band deposition. Alternating shaded and white areas in Fig. 2 (a to f ) represent the periods of HD and LD band deposition, respectively. The corals from the 3 water depths showed different patterns of band deposition times. These patterns gradually shifted from deposition of HD bands during winter and LD bands during summer at 3 m depth (Fig. 2d, see also Klein & Loya 1991), through deposition of HD bands from late winter to early summer and LD bands from early summer to late winter at 30 m depth (Fig. 2e), to a deposition pattern of HD bands during summer and LD bands during winter at 51 m depth (Fig. 2f). The pattern of density banding has been reported previously for shallow water colonies (ca 3 m depth) (Klein & Loya 1991). Colonies that were collected during summer showed complete HD outermost bands or a transition to incipient LD bands, and vice versa for colonies that were collected during winter. In order to verify the major finding of opposite patterns of density band formation between shallow and deep water corals as obtained from the stable-isotope analysis (Fig. 2d, f ) , we examined 4 additional X-radiographs of coral samples from deep water (ca 50 m depth). Two of the deep water corals were collected during summer (June 1990) and 2 during winter (February 1993). Colonies that were collected during summer showed thin HD outermost bands (probably incipient) and colonies that were collected during winter showed incomplete LD outermost bands (compared to the previous year). Although an exceptional pattern of banding variation in corals within one reef environment was reported (Lough & Barnes 1990), most studies indicate that a uniform band

Depth-related timing of density band formation in Pontes spp. corals from the Red Sea inferred from X-ray chronology and stable isotope composition