The origin of variations in the isotopic record of scleractinian corals: I. Oxygen

Geochimica et Cosmochimica Acta, Vol. 60, No. 15, pp. 2857-2870, 1996 Copyright © 1996 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/96 $15.00 + .00

Pergamon

P I I S0016-70370016-7037 (96) 0 0 1 1 8 - 4

The origin of variations in the isotopic record of scleractinian corals: I. Oxygen J. J. LEDER, L2 P.K. SWART, I'* A. M. SZMANT,2 and R. E. DODGE3 ~Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA 2Division of Biology and Living Resources, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA 3Nova Southeastern Institute of Oceanography, 8000 N. Ocean Drive, Dania, FL 33004, USA (Received September 13, 1995; accepted in revised form March 26, 1996)

A b s t r a c t - P r e v i o u s investigations of the 6180 of the skeletons of Florida specimens of the reef coral M o n t a s t r a e a annularis have failed to produce the full temperature range suggested by calibration studies of other corals. Explanations for this phenomenon include different relationships between temperature and the 61sO of skeletons of Floridian corals, changing 6 J*O of the water, physiological variables ( " v i t a l effects" ), and an insufficient number of samples taken per year with consequent superposition of calcium carbonate precipitated at different times within an individual sample, In this study, we investigate all of these hypotheses, by measuring the 61sO of corals grown in the field which were periodically stained with alizarin-red S and where the 6 aso of the water was measured and the temperature continuously recorded. We compare the effect of sampling the coral skeletons at different resolutions and the effect of sampling within different skeletal elements. Our study shows that discrete, high-resolution sampling of coral exotheca (fifty samples a year) is necessary to reproduce temperatures for this species in Florida waters. Coral skeletons sampled using lower resolution methods showed an artificial attenuation of the annual range in skeletal 6 ~sO, with similar 6180 minima during the skeleton represented by the summer months, but larger differences in the winter 6 ~80 maxima. Replicate isotope transects from fast and slow growing areas and different regions of the corallite were also compared. The 61sO of rapidly growing (8 m m / y ) portions of the colony was 0.1 to 0.2%0 heavier than the slowest growing ( 1.1 m m / y ) portions of the colony. This difference as well as the difference between the skeleton sampled at high and low resolutions appears to result in part from the attenuation of the 6180 signal as a result of the reduced sampling rate in slower growing sections of the coral and is not solely a result of variable kinetic effects. 1. INTRODUCTION

the basis for critical evaluation of sampling effects that can influence the interpretation of skeletal 613C and 6 ~sO variations. Specifically, sampling resolution, spacing and location (within the skeleton) and variations in growth rate were considered as factors potentially affecting 6~3C and 6~80 values. This paper deals with 6180 and its relationship to temperature and sampling method. A companion paper (Swart et al., 1996a) is concerned with 613C and its relationship to environmental and physiological variables.

Scleractinian coral skeletons provide a potentially powerful retrospective tool for the reconstruction of long-term and seasonal variations of certain climate variables (Carriquiry et al., 1988; Cole et al., 1993; Dunbar and Wellington, 1995, and others). In such studies, density bands in the skeleton are usually used to establish annual cycles of growth. Internal banding however, does not always provide absolute dating control between or within years. Time-control is necessary to fix the timing of 61~O variations and is particularly important when short-term perturbations to the environment are of interest. Accurate time-series profiles of skeletal 6~3C and 6 ~80 can provide the chronologies to estimate yearly growth rates and confirm the presence of annual temperature cycles (Dunbar and Wellington 1981; Klein et al., 1992). However, methods for sampling which average long time periods ( > 1 month) or combine various skeletal elements can create spurious results that adversely effect the establishment of annual chronologies and distort intra-annual climate variations. The present investigation is a result of our efforts to refine the techniques used for micro-sampling coral skeletons for geochemical analysis. The use of improved micro-drilling procedures enabled detailed measurements of seasonal variations in the skeletal 613C and 6 ~sO composition of Montastraea annularis from Florida to be made. Our results provide

2. SAMPLING CONSIDERATIONS A central problem underlying an understanding of 6 ~3C and 6 ~sO variations in coral skeletons is the ability to faithfully sample compositional variations that are present in the skeletal material. Coral skeletons are, in essence, continuous recorders of their growth environment, although they cannot be considered to grow at a constant rate and therefore preferentially record signals produced during optimal growth periods. Nevertheless, corals can provide high-resolution information on intra-annual climate variations which is not available from other time-averaged marine geological records. However, the information retrieved from the skeleton depends on the manner and resolution in which it is sampled. Much of the confusion surrounding the interpretation of 6~3C and 6~sO in coral skeletons results from the various methods of sampling the skeleton that have been used and the lack of recognition that corals are a three-dimensional structure whose skeletal elements are neither deposited in a laminar manner nor consistently normal to the growth axis. Many studies have used bulk sampling of the skeleton in which one or several years of skeletal growth are ground and homogenized for analysis (Keith and Weber 1965; Weber and Woodhead 1970, 1972), producing a "mean" value representative of a coral at a particular location and

*Author to whom correspondence should be addressed. 2857

2858

J.J. Leder et al.

depth. Bulk sampling can potentially provide information about mean annual climate conditions, but is not suitable for examining seasonal variations, short-term perturbations in the physical environment, or understanding seasonal changes in coral metabolic performance. Bulk sampling of the skeleton also does not provide chronological information which can be used to establish the seasonal cycle and estimate growth rates. For the purpose of seasonal climate reconstruction from the coral isotopic record, the skeleton must be sampled at subannual intervals. There is disagreement as to the merits of the different methods used to extract carbonate samples from the skeleton, and past studies that examined seasonal variations in skeletal 6 ~3C and 6 ~ O have made use of several different techniques. These include grinding incremental samples from a skeletal section (J. P~itzold, pets. commun.; Emiliani et al., 1978), " w e t " extraction of mini-cores (Aharon 1991 ), and drilling with tiny carbide or diamond bits (Dunbar and Wellington 1981; Fairbanks and Dodge 1979; P~itzold 1984). The number and spacing of increments sampled per year has also varied widely. Typically, four to twelve evenly spaced samples are obtained along a linear transect from each year of growth. Linear growth of the skeleton throughout the year is often assmned, although it is not known how much variation in extension rate in fact occurs. Periods of slower growth (e.g., during winter months) may be underrepresented in the isotope record as a result of reduced sampling rates. The potential also exists for conversion of the skeletal material from aragonite to calcite by the sample extraction procedure and some work has indicated that drilling of carbonate shells can cause such conversion with a consequent isotopic effect (Druffel and Benavides 1986; Aharon 1991; Gill et al., 1995; Swart and Leder, 1996). Previous workers have suggested that high-resolution ( i.e., greater than twelve samples/y) oxygen isotope profiles are necessary to accurately reconstruct seasonal temperature histories (Goreau 1977; Fairbanks and Dodge 1979; Winter et al., 1991 ). Others have even suggested that six samples a year or less are necessary to reveal the full annual amplitude (Roulier and Quinn, 1995). However, t)w studies have since employed fine-scale sampling of the skeleton for isotope analysis. Lower sampling rates generally result in attenuation of the seasonal 6 ~80 amplitude, and when evenly spaced intervals are sampled along a growth transect, gaps in the skeleton (and thus in time) will not be sampled. Although the 6 ~ O of coral skeletons has been suggested to be affected by rate of formation (McConnaughey 1986, 1989: Land et al., 1975; Keith and Weber, 1965), quantitative data on this phenomenon are rare. In order to reconstruct time-dependent changes in skeletal isotope composition, most workers have sampled along the major growth axis, where growth rates are at their m a x i m u m (Fairbanks and Dodge, 1979) and isotope disequilibria are believed to be more constant (McConnaughey, 1989). McConnaughey (1989) suggested that the skeletons of corals of the genus Porites sp. and Pavona sp. growing at rates of less than 4 m m / y tended to approach isotopic equilibrium with their environment and proposed that corals growing faster than this rate were more suitable for paleoclimatic analyses as they attain a more consistent disequilibrium. Nevertheless in spite of these studies, the exact relationship between linear growth rates and skeletal composition, and how temporal changes in skeletal growth could influence long term isotope records retrieved from corals, remains obscure. Some work has shown large variations in isotopic composition within and between coral skeletons (Land et al.. 1975; Wang et al., 1995). Different skeletal elements within individual calices can differ in 6 ~C and 6 J~O by as much as 3%~. Other workers have shown large variations when sampling across similar time lines in different parts of the colony (McConnaughey 1986, 1989; Aharon 1991). These data indicate the need for discrete, consistent sampling techniques which must consider the variations described above and sample accordingly to prevent spurious results introduced by the sampling procedure itself.

3. MATERIALS AND METHODS 3.1. Field Site and Experimental Approach The study site was a small patch reef within Biscayne National Park, Florida, USA (Fig. 1). In the spring of 1988 a plexiglass

platti3rm measuring 1.5 × 1.5m was emplaced at 5.5 m depth in the seagrass adjacent to Alina's Reef. This reef (25°23.42N, 80°10.29W) is approximately 6.5 km southeast of Elliot Key and is situated between Hawk Channel and the linear bank reefs further offshore. Twelve individual nodules of M. annularis, measuring ca. 10-15 cm height by ca. 8 - 1 0 cm diameter, were collected from the adjacent reef and cemented in growth position to the platform. The corals were arranged on the platform such that individual field incubation chambers could be fitted over each nodule and sealed to the platform during experiments. For further details of the experimental procedure see Swart et al. (1996a). Every two to three months, after physiological measurements, the corals were stained with alizarin-red S. At the conclusion of the experiment, the corals were removed from the platform and returned to the laboratory at RSMAS. The tissues of the corals were removed for analysis (Swart et al., 1996) and the skeletons were examined using sclerochronological and isotopic techniques.

3.2. Temperature A Hydrolab Datasonde 1 monitoring device was installed by Biscayne National Park (BNP) during the spring of 1990 within a few meters of and at the same depth as the research platform at Alina's Reef. The Hydrolab provided a continuous record of in situ temperature readings ( 1 h intervals) during its deployment period. A second Hydrolab was situated on a patch reef 7 km south of Alina's reef (Ball Buoy). Data from this site was used to fill in gaps (days to weeks ) that occurred in the record from Alina's Reef. For the period prior to the deployment of the Hydrolabs, temperature records were obtained t'rom a C-man monitoring station located at Molasses Reef, approximately 50 km southwest of the BNP site (Fig. l ). This station, maintained by the Florida Institute of Oceanography, provided sea temperature extending back to before August 1988. During the periods when temperature records overlapped for these three stations the correlation coefficient between the hydrolab at Alina's reef and Ball Buoy and between Ball Buoy and Molasses were 0.98 and 0.99, respectively. As a result of this high degree of correlation we felt justified in using ai1 three datasets for correlation to the b ~sO of the coral skeletons. No salinity data are available from the Cman stations during the study period.

3.3. Sample Preparation At the end of the experiment, the tissues of all corals were removed by airbrushing (Szmant and Gassman, 1990), and the residual organic matter removed by soaking intact skeletons in 10% bleach for <24h, followed by a freshwater rinse. The coral skeletons were slabbed along major growth axes in preparation for standard X-radiographic analysis. The procedures used for X-radiography are similar to those described by Dodge et al. (1992). Skeletal sections varying from 1 - 6 mm were X-rayed to reveal density band patterns. Great care was taken to produce sections that were parallel to the growth axis to avoid distortion of the skeletal chronology. Sections of skeleton to be used for isotope analysis were embedded in a polyester resin to preserve the structural integrity of the skeleton during drilling and handling. The resin did not interfere with isotope measurements and was translucent to X-rays. The embedded sections were ground flat, mounted to glass, and polished under cool water to expose the skeletal surface. The desired result was skeletal sections in which an individual corallite was bisected parallel to the maior growth axis, exposing the thecal margin (or " w a l l " ) of that corallite. The stain lines were most clear in this area and the distances between ac[jacent stain lines could be measured to estimate rates of linear extension. Exothecal transects for isotope analysis followed these peaks. No further pretreatment of the carbonate was carried out prior to analysis because the skeletal sections analyzed appeared free of endolithic borings, algae, and other organic matter and because the line-scale sampling for isotopes that was used (see below) permitted microscopic inspection of each sample as it was extracted. Studies in the past have employed a variety of pre-treatment regimes to remove organic matter contamination (e.g., heating and/or soaking in H,O: of pulverized coral), as suggested by Epstein et al. ( 1951,

Oxygen isotope composition in corals

2859

Fowey Rocks

Enlarged Area ~J

• Long Reef

Ceoser

creek

.k~Study Site ~ PaeificReef Ball B u o y Reef

:i::. L~:i:i:~:~:i:i:~:i:i:i:i:i:!:i:!:~:i:i:~:~:i:

Bal°ri yF da

tg Carysfort Reef

~/J

, 10 km

, ses

NI[

FtG. 1. Location map of the Bicayne National Park study site and vicinity. The star indicates the location of the underwater platform at Alina's Reef. Sea surface temperature (SST) data (P) were recorded at Alina, Ball Buoy, and Molasses Reefs.

1953). The necessity of pretreatment procedures is uncertain; while Weber et al. (1976) stress the importance of such sample preparation steps, McConnaughey (1989) compared several treatments and found results to vary only slightly, with no improvement in reproducibility. Similar conclusions were reached in studies conducted in our laboratory.

3.4. Sampling Technique An important component of the study reported in this paper is the ability to extract and analyze fine increments of skeletal growth from the coral to provide a high-resolution record of seasonal variations in skeletal b ~3C and 6 ~sO. Sections for isotopic analysis were cut to bisect an individual corallite parallel to the major growth axis and were 1 - 6 m m thick. The coral was drilled mainly along the thecal wall (0.5-1 mm) of this individual corallite (approximately 5 m m ) , where stain lines in the BNP corals indicated the best chronology was present. In addition sampling transects were made within endothecal regions and along polyp tracts in the slower growing sides of the nodules, at the side of the coral colony. Microsamples for isotope analyses were extracted from the skeleton using a low speed milling

technique with 0 . 4 - 0 . 6 m m diameter diamond drill bits and a microdrill whose speed was manually regulated with a rheostat. The milling procedure was observed under magnification and involved slowly advancing the drill bit along the linear path of the transect, stopping the drill at uniform increments and collecting the powder such that all material was removed and analyzed. This procedure results in a groove approximately 0.5 m m wide and 0.5 to 1.0 m m deep in the thecal. The number of subsamples recovered from the skeleton using high-resolution sampling ranged from twenty-five to fifty-five per annual cycle and therefore each sample represents can represent a small increment of time within the coral skeleton. In a previous study (Leder et al., 1991 ), up to fourteen samples/y were obtained using a slightly different technique. In that case, evenly spaced holes were drilled and larger areas of the corallite were sampled instead of the discrete sampling described here. Powdered samples obtained by the described method were examined by X-ray diffraction and showed no evidence of alteration to calcite. Within our ability to sample portions of the skeleton formed at similar times we concluded that the drilling had no discemable affect on either the carbon or oxygen isotopic composition of the skeleton (Swart and Leder, 1996). The stain lines acted as a guide to skeletal growth and provided

2860

J.J. Leder et al.

(a)

(b)

DATE 3l-Jui-9l

T(C)

stain# 13

-29.5--

26-Mar-91

12

-25.7--

t 5-Dec-90

lI

24.4--

23-Sep-90

24-Jul-90

l0 9

30.0-__.30.0_

16-May-90

8

14-Mar-90

7

30-Jan-90 12-Dec-89

6 5

12-Oct-89

4

20-Jul-89 03-Jun-89

3 2

19 18 17 16 15

~

14

13 12 27.3-- 11 -25.1-- ~ 10 24,8-- 9 -23.0-8 30.1 -7 31.5--

---29.0--

-24.1--

3t-Aug-88

0

29.8--

1 12-Oct-89 SI#4

4

6 E

4 l

E 2 .!

contour of stain line

5

02-Feb-89

t

4

2 1

3 2 1 0

" I/\f " "

~, ~

23.0oc t2-Dec-89 sl#5 30.1 cc 12-Oct-89 sl#4

f @-:::,:: \.

ISOTOPE SAMPLE ,'

theca

FIG. 2. Chronology of stain lines in the endothecal region of a skeleton of Montastraea annularLs. {a ) Photomicrograph of coral BNP#4, showing the distinct alizarin-red stain bands in the skeleton. The upper stain line indicates the last time this particular coral was stained (March 26, 1991 ). (b) Schematic diagram of stain lines in the BNP corals illustrates some problems associated with sampling of the coral skeleton for isotope analysis. The saw-tooth pattern of the skeleton's leading growth edge interweaves disparate time periods, which can be included in individual samples. In the example shown, two adjacent stain lines represent a change of temperature of 7°C, yet these can be easily sampled effectively as one sample using an inappropriate sampling method. The shaded areas in the right hand panel represent hypothetical portions of the skeleton which might be included in a sample depending upon the method used (see text).

Oxygen isotope composition in corals

(a)

2861

?

14-day Reef Tern ~erature 30 28

b-,

26 24

22 J. F. M A. 1~ J. J. ,~ S. q b~

(b)

. . . . . . . . . . .

2.0

1.8 1.6 1.4 O

o~

PtX~

M a r c h 91 J u n 89

1.2 1.0 0.8 0.6

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1988

1989

1990

i i i

1991

months FIG. 3. South Florida Environmental Data. (a) Composite Sea-surface temperatures (SST) for the period 19881991. The temperature data were recorded at Alina's Reef and Ball Buoy Reef, 1990-1991 (BNP Hydrolabs) and Molasses Reef, 1988-1990 (NOAA C-Man Weather Station). The correlation coefficient between the stations for the portions in which the record overlapped was better than 0.98. (b) Record of seawater 6180 values relative to standard mean ocean water (SMOW) measured at Alina's Reef during this study.

time control. A schematic representation of the stain lines is shown in Fig. 2. In the theca and exotheca, the stain lines outlined the serrated outer edge of the costae and septa at the time of staining. The peaks and valleys of the serrations remain in approximate vertical alignment as the skeletal structures grow. Linear growth measurements were made from peak to peak or valley to valley between consecutive stain lines (Fig. 2). Dates were assigned to each of the sample positions by interpolating between the known, fixed stain dates. A sample was considered to fall on the date of staining when it was taken from the peak of the stain line (representing the prevailing growing edge of one costa). The time interval included in each sample depends on the vertical extent of the skeleton sampled. The carbonate samples were processed by an automated carbonate device (Common Acid Bath @ 90°C) attached to a Finnigan-MAT 251 gas ratio mass spectrometer. The external precision was calculated from replicate analyses of the internal laboratory calcite standard and was 0.02%0 for 6~3C and 0.03 for 6~80. Data were corrected for the usual isobaric interferences and are expressed relative to PDB. The 6 tsO of waters collected during each of the field trips was determined by equilibrating 1 cm 3 of sample with CO2, following a method described by Epstein and Mayeda (1953). Data were corrected for the usual isobaric interferences and are expressed relative to SMOW.

generated using data from all three monitoring stations (Fig. 3). Seawater temperatures during the study period ( 1 9 8 9 1991 ) averaged 26.6°C and showed a strong seasonal signal ( T a b l e 1). The average monthly temperatures ranged between about 21.5°C during February to about 3 0 - 3 1 ° C in August and September. The average annual range in daily temperatures measured during the study period was 9.5°C. Winter temperatures were m u c h more variable than those during the s u m m e r months, with seasonal cold fronts dropping sea-surface temperatures down to as low as 18°C. Biweekly averages of the daily temperatures were calculated and this 14-day average temperature record for the B N P site was used for comparison with coral skeletal 6180 time-series.

4.2. Salinity Salinities at the B N P study site vary between about 35 to 37%0, although no significant seasonal trend was detected in these data.

4. RESULTS

4.1. Reef Seawater Temperature Records

4.3. Seawater 5~sO

A single, composite record of dally mean, m a x i m u m , and m i n i m u m reef temperatures for the period 1 9 8 8 - 1 9 9 1 was

The 61so of the seawater, although exhibiting variations related to the tidal cycle remained relatively constant

2862

J.J. Leder et al. Table 1. Weekl t Interpolated ox[~en ,soto~ic and temlerature data.

04-Sep-89 18-Sep-89 _ 02-Oct-89 _ 16-Oct-89 . 30-Oct-89 13-Nov-89 I 27-Nov-89~ L 11-Dec-89~ m 25-De~ 08-Jan-90] 22-Jan-90 l 05-Feb-90 ~ 19-Feb-90~ 05-Mar-901 19-Mar-90 02-Apr'90 16-Apr-90~ 30-Apr-90 L 14-May-9_0_. 28-May-90 11-Jun-90 25-Jun-90 09-Jul-90 23-Jul-90 06-Au9-90 20-Aug-90 03-Sep-90 i 17-Sep-90 01-Oct-90 15-Oct-90 i 29-Oct'9012-Nov-90 26-Nov-90 ~_ 10-Dec-90_. 24-Dec-90 07-Jan-91 21-Jan-91 04-Feb-91 [ [ 18-Feb-91[ 04-Mar-91~ T

/

18-Mar-91~

Ol-A~-91_

/

15-Apr-91 " [ __ 29-Apr-9~l~

/

13-May~91

F

27-aay-91~ 11-Jun-91[ i 25-Jun-91 09-Jul-91 232~Jul-91

_4,:46

:1:44

30.24 29.96 29.74T 29.16 26.74 26.68 25.94~ 24.641 23.21 23.86~ 24.02] 24.69 L 24 .53 24.46 24.97 25.29~ 25.85 25.77 26.74 26.96 28.54 28.87 29.29 29.80 30.39 30.51 30.49 30.10 29.54 28.48 27.45 26.4626.05 25.22 24.94 25,11 24.50 25.13 22.48[ 24.24 ] 23.381

-4.67 -4.43 -4.10~ -&85 -3.85J -3.78 -3.63 -3.41 -3.20 -3.20 -3.261 -3.35 -3.52 -3.42 -3.21 -3.32 -3.51 -3.63. -3.65 -3.80 -4.03 -4.08 -4.15 -4.45 -4.40 -4.27 -4.28 -4.17[ -4.00 -3.67 -3.55 -3.52 [ -3.48 -3.16 -3.00 T -2.97 -3.01 -3.16" -3.21 -3.25 -3.29

-4.60 -4.60 -4.60 -4.43 -4.25 -4.16 -4.01 -3.67 -3.17 -3.14 -3.20 -3.20 -3.23 -3.40 -3.46 -3.50 -3.54 -3.66 -3.91 -4.20 -4.29 -4.30 -4.39 -4.50 -4.51 -4.60 -4.60 -4.41 -4.49 -4.24 -3.90 -3.67 -3.60 -3.60 -3.41 -3.39 -3.30 -3.33 -3.40 -3.50 -3.60"

-4.49 -4.30 -4.41 -4.49 -4.30 -4.30 -3.99 -3.79 -3.46 -3.30 -3.177 -3.19" -3.25 [ -3.20 -3.31 -3.31 -3.37 -3.43 -3.60 -3.79 -3.89 -3.93 -4.17 -4.30 -4.30 -4.30 -4.40 -4.50 -4.50 -4.50 -4.33 -4.16 -3.87 -3.75 -3.64 -3.52 -3.50" -3.40 -3.40 -3,40 -3.3" -3.25 -3.49

249~

-333

-3.53

-367

-3.56-3.66 -3.94 -4.13 -4.17 -437 -4,56 -4.64-

-3.50 -4.301 -4.36, -4.50 -4.17 -4.371 -4.56 -4.64

-3.86 -4.09 -4.3" -4.37 -4.29 -4.54 -4.56 -4.44

25.73 ~ - 27_22~ 27.261 27.37 i 2&53 ~ 29.41~ 30.18 30.71 -

throughout the study period (m = +1.1%o: range = 0.5%c) (Fig. 3b).

/

-4.24 -4 16 -4.05 -4.23 -4.08 -3.81 -3.651 -3.24 -3.39! -3.28 -3.06 -2.69 -2.93 -3.23 -3.25 -3.35 -3.29 -3.05 -3.31 -3.55 -366 -3.96 -4.30 -4.23 -4.31 -4.54 -4.60 -4,60 -4.47 -4.60 -4.54 -4.30 -4.09 -3.79 -3.75 -3.60 -3.54 -3.16 -3.19 -3.03 ~ -2.92 -3.06 -3.20"

1.12 1.17 1.22 1.27 1.32 1.37 1.36 1.34 1.31 1.29 1.26 1.24 1.21 1.19 1.17 1.15 1.13 1.11 1.09 ".07 ~.05 1.03 1.02 1.01 1.00 0.98 0.97 0.96 0.95 0.93 0.92 0.91 6.89 0.88 0.87 0.86 0.89 0.97 1.04 1.08 1.09 1.10 1.11

-3.201 -3.40 -3.69 [

1.12

-3.91 -4.00 -4.09 -4.10 -4.27 -4.46

and closely matches the timing of seasonal temperature variations at this site. The ~ O data calculated at biweekly intervals are shown in Table 1.

4.4. Skeletal ~$1sO Composition The time-dependent variations in skeletal 6 ~ O for tour experimental corals examined are shown in Fig. 4. The average oxygen isotopic composition of the four corals ranges - 3 . 7 4 to -3.96%0, approximately 2% more negative compared to inorganic aragonite precipitated from similar water compositions at 25°C using the equation of Grossman and Ku (1986). Within each coral the 6 tsO of the skeleton varies between approximately - 2 . 7 and -4.8%c and the annual range varies between 1.4 and 1.8~:. Time-dependent 6JsO profiles show a clear and regular annual periodicity, with annual 6 ~sO maxima occurring in the winter months of December through February, while the annual 6tsO minima typically occur in August-September. The pattern of 5 ~sO variation is very similar for each of the experimental corals,

4.5. Range of Skeletal 6180 The seasonal amplitude of the b ~ O in the skeleton is related to the number of samples taken within a given year. Skeletons sampled at lower resolutions (20 samples/y) showed a reduced seasonal amplitude compared to skeletons sampled at high resolutions (50 samples/y) (Fig. 5). The annual amplitude of the b ~O was also reduced in the tram sects made in the endothecal portion of the skeleton (Fig. 6).

4.6. Growth Rate The corals used in this study exhibited clear ammal density bands prior to the commencement of the study. After the

Oxygen isotope composition in corals

/ BNP#3

A

[ BN~4 .

.

.

samples/yr .

30-40

.

'20-25samples/~

-

-3.5 C" ~o@

2863

day intervals using a rectangular interpolation method (Davis, 1973). The stain lines were used as a time control. Values for a , the fractionation between water and carbonate ( E q n . 1 ) , were calculated for each interval and correlated to the temperature over the same period. In this equation 6c = the isotopic composition of the coral skeleton relative to S M O W and 6w = the isotopic composition of the seawater relative to S M O W . The results from these cross correlations are s h o w n in Fig. 8 and indicate a negative statistically significant regression coefficient between

-3 -4.5

a -

BNP#9

-4 -3.5 -3 |

-4.5

I

i

BNP#10

-4 -3.5 -3 ,

.

|

i

|

I

i

,

i

|

30 28

~

26

["

24 22 1988

1989

1990

1991

FIG. 4. Time-series of 6~O from the skeleton of Montastraea annularis compared with mean weekly reef temperatures at Alina's Reef. The timing and pattern of both skeletal 6 ~SO and reef temperature are similar. The coral skeletons were sampled at the following rates: BNP 10-45 to 55 samples/y, BNP 3 - 3 0 to 40/y; BNP 4, 20 to 25/y, and BNP 9, 25 to 30/y. Growth rates in the fastest portion of the coral are presented in the companion paper (Swart et al., 1996) and vary between 5 and 9 mm/y.

corals were transplanted to the platform, the density banding patterns b e c a m e less clear and the density of the corals tended to increase (Swart et al., 1996a). The annual density pattern of the coral was nevertheless still visible and growth rates as measured by skeletal extension were similar prior to and during the experimental period. The amplitude of the annual 6~so signal was reduced in a transect made along theca at the side of the coral colony when c o m p a r e d to the top of the coral colony. The side transect showed similar annual m i n i m a in oxygen, but not as heavy m a x i m a w h e n compared to the top of the coral. These differences resulted in the slower growing side portion of the coral possessing a slightly more depleted oxygen isotopic composition than the top (Fig. 7). 5. DISCUSSION

5.1. Absolute Calibration of ~3180 versus Temperature As all the four corals used in the study were sampled at resolutions of greater than twenty samples a year, we interpolated the water temperature and the 6 ksO data to 14

6~ + 1000 6,~ + 1000

(1)

the two variables in all cases. The most significant correlation coefficient was obtained in the case of B N P 10, the coral which was sampled at a rate of between fifty and fiftyfive samples a year. Greater scatter was obtained in other corals, a fact which we attribute to slight misalignment between the 61sO and the time chronology of the coral. For this reason we feel that the relationship between temperature and 6 ~80 s h o w n by B N P 10 is the most accurate. The relationship is shown in more conventional terms in Eqn. 2. In this equation 6c and 6w are the same as in Eqn. 1, with the exception that 6c is expressed relative to PDB rather than S M O W . Note that as the isotopic composition of water is expressed relative to S M O W and the carbonate relative to PDB, one is not able to simply subtract the water values from carbonate values. The correct m a n n e r in which to do this is to convert the carbonate values to S M O W units, subtract the appropriate water values, and convert the answer back to PDB. Without performing this adjustment the adjusted carbonate values are too light by approximately 0.02%c. T°C = 5.33 - 4.519 (_+.19)*(6c - 6w)

(2)

The parameters in this equation are in the same range as the relationships between temperature and 6~sO previously reported in corals by W e b e r and W o o d h e a d ( 1 9 7 2 ) , M c C o n naughey ( 1 9 8 9 ) , and Weil et al. ( 1 9 8 1 ) , but Eqn. 2 has a slightly greater slope (4.5 vs. 4.2) than the paleotemperature equation of Epstein et al. ( 1 9 5 3 ) and significantly greater than the equation of G r o s s m a n and Ku ( 1981 ) for aragonite (4.5 vs. 3.5). In spite of the fact that we had in this experiment significantly better time control than in other calibration experiments reported in the literature, we still had a range of slopes and intercepts between temperature and 6 ~O. Such differences could lead to speculation that there is variation between temperature and 6~sO in different corals and therefore that the use of the 6 ~so to predict temperatures is open to question. However, it is our contention that the differences which we observe in this careful study are still a result of sampling uncertainties. W e believe that similar problems may also affect other geochemical proxies such as strontium (de Villiers et al., 1996) and probably explains the variations in relationship shown by these workers.

5.2. Calibration of the Range of 6~sO in Coral Skeletons Corals growing on the Florida Reef Tract normally experience a range in seawater temperatures of ca. 9°C (Fig. 2).

2864

J.J. Leder et al.

-2.8

It

-3.0 -3.2 -3.4 ~o~

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1.60 %° , 18-24 samEles/yr i =ca 7.50C i (coral IqNP#4"I

195 %; 50-55 samples/yr =ca 9.0°C ! (coral BNP# 10]

FIG. 5. Comparison of the annual range in skeletal b~sO for M o n t a s t r a e a a n n u l a r i s sampled at high and low resolution. The line with filled circles is the 6 ~sO profile recorded from corals at Carysfort Reef, Florida, USA (Leder et al., 1991 ). The other lines are profiles for corals BNP 4 and BNP 10 (this study). The Carysfort coral record is for the period 1985-1988, and the BNP coral for 1989 1991, although the temperature ranges were similar for all years (ca. 8 - 9 ° C ) . Reduced sampling rates tend to underestimate the true range in reef temperature due to attenuation of the 6 ~80 signal, an effect which is apparently greater during periods of lower water temperatures.

Using published paleotemperature equations for corals (Weber and Woodhead, 1972), this temperature range should translate into an approximate 2%c range in skeletal 6~80. Previous studies (Emiliani et al., 1978; Leder et al., 1991), however, only observed approximately 50% of the expected range. Three possibilities have been suggested to explain these discrepancies: ( 1 ) Florida reef corals exhibited different 6 ~80 vs. temperature responses; (2) There are large seasonal changes in the 6~80 of the water; (3) These corals were simply being sampled at too low a resolution to detect the full seasonal variation (Fairbanks and Dodge, 1979; Leder et al., 1991). In this investigation we have studied all three of these hypotheses. ( 1 ) The new relationship reported here is essentially similar to previous coral skeleton 6180 vs. temperature equations (Weber and Woodhead, 1972; Dunbar and Wellington, 1981) and does not explain the less than 50% variation in coral 6180 as measured by Leder et al. ( 1991 ) and Emiliani et al. (1978). (2) It had been suggested that large variations in the 6 ~80 of the water are taking place seasonally on the Florida reef tract, thereby masking the 6 ~80 of the coral skeleton (Leder et al., (1991). However, while such changes may take place during other years, during this study period only small changes in the water 6 ~80 were noted. Although the period of study ( 1 9 8 8 - 1 9 9 1 ) was a significantly drier period than normal, precipitation was similar to the time period studied by Leder et al. (1991) and the measured changes in water 6 ~80 were insufficient to cause the measured discrepancies

in the range of the skeletal ~ ~O noted by previous workers. We therefore discount this idea. (3) It is obvious that depending on the manner in which the temperature data are calculated, that different expected ranges in skeletal 6180 will result. For example, the annual range in mean monthly sea temperatures at the site was 6.5°C, 8.7°C when calculated from mean weekly temperature data, or 9.5°C from daily mean temperatures. Using Eqn. 2, these ranges will produce different expected ranges in ~5180; 1.44%c using monthly temperature data, 1.92%c from the weekly temperature data, and 2. l%v from the daily temperature data. In the case of the coral (BNP-10), which was sampled at a rate of fifty samples a year, we retrieved an annual range of 1.8%~, close to the expected range of 1.92%~ for weekly samples. Other corals sampled from this site that were sampled at lower resolutions (fewer samples / year) showed lower annual 6 JsO ranges, but close to what would be expected based on the number of samples taken and an averaging of the temperature data (Fig. 6). Reduced sensitivity of the coral thermometer can also result when less than 100% of the material in a given transect is sampled, for example when evenly spaced samples are drilled (e.g., Leder et al., 1991; Klein et al., 1992). In a previous study (Leder et al., 1991 ), we obtained twelve to fourteen evenly spaced samples/year from skeletons of M. annularis in Florida, but were unable to detect more than ca. 1%~ of the 1.5%c range in 6~80 predicted from monthly temperatures (Fig. 5 ). Klein et al. (1992) sampled skeletons of Porites lobata at a rate of eight evenly spaced samples/

Oxygen isotope composition in corals

2865

-3.0

l\ -3.5

! "IL endothecal

\ O O

"4

-4.0

"th~

\

it "~"°°'tL

-4.5

-5.0 1989

1990

1991

FIG. 6. Coral skeletal 6t80 time series from thecal and endothecal transects in coral BNP#4, (Montastraea annular is). Note that the endothecal transect exhibits a much reduced range in 6~80 despite being sampled at a similar rate. This is a result of the fact that each endothecal sample contains material from many different time periods and hence temperatures.

year and measured a range in 6 ~so that was about ca. 70% of the range predicted from annual temperatures. They sampled only about 60% of the skeletal material in each year along the linear transect. It is likely that, in both studies, some of the carbonate produced during the seasonal highs and lows in temperature was not included in the isotope samples. Winter et al. ( 1991 ) noticed this problem and suggested that the coral skeleton would have to be sampled in a continuous transect to accurately detect the full range in seasonal temperature variations. An additional benefit to sampling the skeleton continuously is that a more reliable measure of the yearly mean temperature can be obtained, regardless of the sampling resolution. This requires knowledge of the timeperiod encompassed by each isotope sample. In contrast to the work presented here, other workers have presented data which suggests that full seasonality can be obtained with lower sampling resolutions. For example the study of Fairbanks and Dodge (1979) examined isotopic variability in a coral from Bermuda. This coral was only sampled at a resolution of twelve samples/y yet appeared to show the full seasonal range in 6180. Work by Roulier and Quinn (1995) also apparently showed that increasing sampling rate did not

appear to increase the isotopic range recovered from the skeletons. In a companion study related to this project (Swart et al., 1996b) we sampled a 250 year old coral from the same site at frequencies of between five and twenty-five samples a year. We found very little relationship between the sampling frequency and annual oxygen isotopic range. However, this result we concluded was not a consequence of the absence of a relationship between these two parameters, but a result of a high degree of heterogeneity in the coral skeleton.

5.3. Comparisons with Theoretical Sampling Limits It is a relatively easy exercise to construct a simple model of seasonal temperature and calculate the expected amplitude in the 61sO signal that would be achieved by sampling the skeleton at different resolutions. An example of such an approach is shown in Fig. 9. In this model the effect of continuous rather than discrete sampling was simulated by averaging successive samples together and then calculating the minimum and maximum isotopic value within a year with the same range of temperatures as 1989. The results are summarized in Fig. 9 and suggest that contrary to the results from this study

2866

J.J. Leder et al.

-2.0 Top(8.1mm/yr) ,

,,

.,.,

~ -4.0 ~ -4"5 f, 4.0

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.

.

1990

.

.

.

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.

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.

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1991

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FIG. 7. Comparison of a ~SO time series lor transects sampled from the top (8.1 mlrdy) and side ( 1.4 mm/y) of coral BNP10 (Montastraea annularis). There is no appreciable difference in (5~sO between these two areas. The reduction in range of annual 6J'~O in the side samples can be explained by the lower sampling density in this region as a result of very slow growth rates of this portion of the coral skeleton.

that true seasonal amplitudes can be achieved with relatively coarse sampling regimes. For example, a reduction from fiftytwo to twelve samples a year in the theoretical model is only accompanied by a reduction in the measured isotopic range of 0.03%c. However, the situation is different if real rather than theoritical temperature data are used. In this case, a reduction in the sampling rate, from fifty-two to twelve samples a year, produces a difference in the calculated temperature range of 1.9°C or from 1.94 to 1.52%c. A reduction in the sampling rate to six and four within a year causes a further decrease in the expected annual range to 1.3%c. These annual ranges are still higher than those actually measured by Leder et al. ( 1991 ) and Emiliani et al. ( 1 9 7 8 ) which can be ascribed to various different reasons. First, the mathematical approach used does not accurately reflect the true temporal variability of the temperature and assumes a constant rate of change throughout the year. Further problems arise in the sampling of the coral because the coral does not grow at a uniform rate throughout the year. In particular, growth rates decrease during the winter and summer months (Swart et al., 1996a) and therefore these portions of the temperature record are under represented in any uniform sampling regime. Furthermore, even at the finest resolution that can be achieved, an individual sample may include material deposited over a greater time period than is indicated by the sampling resolution. This is because corals possess an intricate and convoluted architecture in which the growth surface is not laminar or necessarily parallel to the present growth surface. A particularly good example of the spatial resolution over which temperatures are recorded in

coral skeletons is shown in Fig. 2. Here the peaks of two adjacent stain lines, one representing a temperature of 30°C and the other 23°C are vertically only I mm apart. However, in the horizontal direction they are much closer, perhaps only 200 #m. In such a situation one could easily include portions of the skeleton produced both during the warmest and coldest times of the year in one sample. With a few exceptions most previous studies of stable oxygen isotopic variation in corals have used corals which only experience small annual ranges in temperature variation (3 to 4 "C). In such corals the problems of sampling as described so far are reduced as not only is the magnitude of the temperature range smaller, but the coral skeleton in all probability grows at a more uniform rate throughout the year. These corals are therefore less likely to experience some of the problems described in this study. However, it should be recognized that the goal of paleoclimatic reconstruction using coral skeletons is usually designed to discover anomalous temperature or salinity patterns. Such anomalous conditions are more likely to cause reductions in growth rate in the skeleton (e.g., Leder et al., 1991 ) and hence will be under represented in a scheme in which only a few samples a year are taken. In studies which have examined corals growing under large temperature ranges the results have been confusing. As mentioned all studies on Floridan corals (Emiliani et al., 1978; Leder et al., 1991 ) have failed to show the theoritical range of oxygen isotopes in their skeletons. Workers at other localities such as Bermuda, however, have shown the full temperature range of almost

Oxygen isotope composition in corals

2867

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9°C with only twelve samples a year (Fairbanks and Dodge, 1979). Other researchers sampling specimens of Montrastraea cavernosa from Bermuda (J. Patzold, pers. commun.) with a finer sampling resolution did not find the correct temperature range. This discrepancy remains unresolved. 5.4. Sampling Resolution, Sample Spacing, and Attenuation Effects The sampling technique for this study was chosen because it allowed for discrete sampling of only the thecal portion of the skeleton, while still maximizing the sampling resolution. Following the peaks in consecutive stain lines in this fashion (Fig. 2) minimized the distortion of the isotopic signal as a result of time-averaging and thus reduced attenuation. Another common technique used to sample coral skeletons involves grinding a small block or strip encompassing ca. 12 polyp diameters (e.g., P~itzold, 1984, for Porites lutea). Sample material is "shaved" downward from the growth

surface sequentially removing a small cross-sectional area of the slab. This technique can yield finer sampling intervals than other techniques available. However, considering the skeletal structure of M. annularis (Fig. 2), we realize that this is not the best sampling technique for this species. No matter how thin the "shaved'"plane is, there will be an unavoidable cross-sampling of skeleton produced as much as 2 - 3 months apart, as shown in Fig. 2. In fact using this method it would not matter how may samples were taken from within an annual band, each sample would still be an average of a broad time interval. As a result of this signal attenuation, it would not be possible to record the true annual 6 ~80 maxima and minima, and this effect would be magnified in slower growing corals or if the coral's growth rate is not constant throughout the year. Furthermore, the substantial variation observed in the isotopic composition of individual calices ofM. annularis (Land et al., 1975) suggests that cross-sampling of these different skeletal elements (endothecal and exothecal regions of skeleton) will introduce an additional level of variability whose magnitude is not known.

2868

J.J. Leder et al. -3.2

-3.6

-4 O O

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...... i. . . . . . ~ ..... . . . .

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FIG. 9. Comparison of b ~SO generated from a theoritical coral experiencing an annual temperature range of 9.5°C (a), compared to actual data generated from temperature taken from the year 1989 at Alina's Reef (b). The ranges in 6 nso expected from sampling at different resolutions are shown.

In light of the results presented in this study it is perhaps remarkable that certain studies have published results which have indicated that significantly lower sample resolutions can apparently yield the correct temperature ranges (e.g., Fairbanks and Dodge, 1979) in coral skeletons. We have no explanation for this other than that the temperature range in some of these areas is considerably reduced compared to southern Florida. The greater the range of environmental variables, such as temperature which affect coral growth, the less likely it will be that the coral grows in an uniform

manner and therefore that the coral growth is uniform. Isotopic analyses therefore will be less likely to reveal the true range o f variability. 5.5. C o m p a r i s o n O f E x o t h e c a l A n d E n d o t h e c a l I s o t o p e Transects The fine-scale sampling used in this study is susceptible to errors introduced by isotope variations that may exist between different skeletal elements. As seen from the stain

Oxygen isotope composition in corals lines in the coral skeletons, time horizons do not parallel the skeleton external surface (Fig. 2). Therefore, cross sampling between different types of skeleton may contribute to reduced sensitivity of the isotope record as a result of timeaveraging. In addition, there may be a further effect due to the isotopic heterogeneity between individual skeletal elements. Previous workers have suggested that different regions of coral skeletons can have widely varying isotope compositions, even when deposited under apparently similar conditions (Land et al., 1975; Patzold, 1992). In order to compare the time-series isotopic compositions of transects drilled in different skeletal elements, linear transects were drilled from both the exothecal margin (normal sampling, see Fig. 2), and from endothecal skeleton (dissepiments and columella) in coral sample BNP4. The records from both skeletal regions are plotted in Fig. 6. The endothecal transect from BNP4 shows a significantly reduced seasonal amplitude of skeletal 6~80 and a more irregular interannual pattern. The endothecal region is characteristically less dense and "spongy," which necessitates the drilling of larger sample sizes to yield the same amount of carbonate material compared to the denser thecal region. Therefore, it is possible that, while the mean values are similar between the two skeletal transects, the reduced sampling rate creates an artificial attenuation of the skeletal 6 ~80 signal due to increased time-averaging.

5.6. Growth Rate and Skeletal Isotopic Composition It has been suggested that changes in the 6J3C and 6180 of coral skeletons was related to the rate of linear extension of the skeleton (Land et al., 1975; McConnaughey 1986, 1989,; Aharon 1991 ; others). McConnaughey (1989) proposed that as corals secrete their skeletons at slower rates they approach the isotopic composition expected from equilibrium precipitation. In the case of the corals from this study, the b~sO of a precipitate at equilibrium would be between - 1 and -3%~ using the equations of Grossman and Ku (1986). The phenomenon of changing skeletal 6 JsO relative to growth rate was investigated in the present study by comparing the 61sO from fast growing tops of the coral compared to the slower growing side. The relationship between growth rate and carbon isotope composition of the skeletons of the BNP corals is discussed in accompanying paper. Mean annual extension rates in the side transect were ca. 16% of the top; or 1.4 vs. 8.l mm/y in coral BNP10. The high-resolution, time-series profiles obtained from the top and side of coral BNP10 show a similar annual periodicity in skeletal 6 ~80, with slight differences in the annual amplitudes. The 6 ~80 records show a similar attenuation effect as described previously, with reduced ranges measured in the side transects (Fig. 7). Given the large difference in growth rates between tops and sides (ca. 7.0 vs. 1.0 ram/y), the isotope records are remarkably similar, considering the work of McConnaughey (1989) who described large growth rate affects between corals growing at 8 and 4 mm/y. He proposed that slow growing corals would not be suitable for isotopic analyses as their signal would be dominated by kinetic affects rather than effects dictated by their environ-

2869

ment. In fact our data reveal significantly heavier oxygen isotopic compositions in the faster growing tops of the colonies during the winter months. This phenomenon we believe is a result of the slow growth side of the coral during the colder months. As a consequence of the slow growth, we are unable to sample the coral with the spatial resolution necessary to detect heavy oxygen isotopic compositions related to the colder temperatures. 6. CONCLUSIONS The results of this study underline the importance of selecting a sampling regime that is appropriate for the timescales of interest to a particular question. Sampling techniques which average long periods of time ( > 1 month) (Emiliani et al., 1978) or sample at lower resolutions using evenly spaced samples (e.g., Winter et al., 1991; Klein et al., 1992) cannot be expected to provide 6180 records that resolve short-term SST anomalies or accurately predict the full range in seasonal temperature. False conclusions can be reached when this attenuation effect is not recognized. The reduced range (ca. 1.0%o) of 6~80 in Florida M. annularis led Emiliani et al. (1978) to suggest that "the temperature effect is being masked by oxygen isotopic changes caused by variations in physiological activity," specifically activity of the zooxanthellae. Similarly, Leder et al. (1991) sug-. gested that large seasonal salinity changes were masking the 6 JsO signal in the coral. Our study has further emphasized the need to sample in the correct portions of the skeleton in order to avoid areas of the skeleton in which the chronology is not clear. Despite this apparent serious problem in sampling coral skeletons, many workers have been successful in reconstructing paleoclimatic signals with corals using inappropriate sampling regimes. The success of such studies results from at least two factors. First it must be recognized that corals growing in geographical areas which experience relatively small annual variations in temperature, will suffer less from the problems discussed in this paper. In particular intra-annual variations in growth rate will be less and ranges in 6~80 suitably reduced. Second, large variations in the climatic signal often overwhelm problems of sampling. However, the very perturbations which are of interest to paleoclimatologists are also likely to have an adverse affect on coral growth and the consequent retention of an associated isotopic signal. A coarse sampling scheme would probably be unlikely to record such signals. Acknowledgments--This project would not have been possible without the help of many persons. Richard Curry of Biscayne National Park kindly provided the Hydrolabs which supplied the temperature data. Amy Forrester, Phil Kramer, and Luigi Ferret helped with field analyses. This project was funded by NSF grant OCE-8900005 to PKS and AMS, and OCE-8907101 to RED. This paper benefitted from comments by T. McConnaughey, J. Erez, G. Wellington, and T. Guilderson. Editorial handling: H.P. Schwarcz

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