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Geochimica et Cosmochimica Acta 73 (2009) 5308–5319

Calibration of sclerosponge oxygen isotope records to temperatureusing high-resolution d18O data

Brad E. Rosenheim a,*, Peter K. Swart b, Philippe Willenz c

a Department of Earth and Environmental Sciences, Tulane University, Blessey 210, 6823 St. Charles Avenue, New Orleans, LA 70118, USAb Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Key Biscayne, FL, USA

c Royal Belgian Institute of Natural Sciences, Brussels, Belgium

Received 31 July 2008; accepted in revised form 12 May 2009; available online 21 May 2009

Abstract

A revised calibration is presented relating the oxygen isotope composition of the aragonite-secreting sclerosponge Cerato-

porella nicholsoni, oxygen isotope composition of seawater, and ambient water temperature. This new relationship has beenobtained using high-resolution d18O data measured in sclerosponges from the Bahamas and Jamaica compared to ambienttemperature measurements and d18O values of seawater from the two locations, both measured and published. New dataimprove an existing calibration which was determined using measurements of salinity rather than directly measured d18O val-ues of the seawater and was composed of measurements from different species of sclerosponge and other aragonite-secretingorganisms. The updated calibration (n = 12, r2 = 0.95) is:

0016-7

doi:10.

* CoE-m

T ð�CÞ ¼ 16:1ð�3:1Þ � ½6:5ð�1:1Þ�ðdarag � dswÞ;

where T is temperature in degrees Celsius, darag is the d18O value of aragonite normalized to VPDB, and dsw is the d18O valueof water normalized to VSMOW. This calibration improves accuracy and precision of Caribbean sclerosponges for recon-structions of temperature as well as d18O values of seawater.� 2009 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Sclerosponges, or coralline sponges, possess long recordsof environmental change locked in the chemistry of their ba-sal skeletons. Recently, significant efforts have focused onhigh-resolution, in situ, calibration of geochemical parame-ters in sclerosponge skeletons to ambient environmentalparameters (Lazareth et al., 2000; Swart et al., 2002;Haase-Schramm et al., 2003; Rosenheim et al., 2004; Fallonet al., 2005; Haase-Schramm et al., 2005; Rosenheim et al.,2005a). Absent from the sclerosponge literature is a high-resolution calibration of oxygen isotope values to tempera-ture and oxygen isotope composition of water in which cal-cification occurred. The slower growth rate of sclerosponges

037/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.

1016/j.gca.2009.05.047

rresponding author. Fax: +1 5048655199.ail address: brosenhe@tulane.edu (B.E. Rosenheim).

relative to zooxanthellate corals offers the potential for long,continuous records from comparably sized specimens, butcomplicates efforts to calibrate them in similar manners(Weber and Woodhead, 1970; Leder et al., 1996; Swartet al., 1996; Marshall and McCulloch, 2002; Watanabeet al., 2003; Goodkin et al., 2007). Despite these challenges,such a calibration is essential to exploiting the full potentialof sclerosponges as paleoceanographic proxies.

Paleoclimate interest in sclerosponges has spawned sev-eral high-resolution calibration studies, but these have beenlimited to proxies measureable by high-resolution tech-niques such as laser ablation ICP-MS (Rosenheim et al.,2004; Fallon et al., 2005). Laser micro-sampling techniquesare not currently viable for analysis of sclerosponge oxygenisotopes in CaCO3 because of the high accuracy and preci-sion necessary to calibrate over a 2–5 �C seasonal tempera-ture range (Sharp, 1992; Sharp and Cerling, 1996). Inaddition, laser decomposition of sclerosponge aragonite

Fig. 1. Map of sample locations. One sample was analyzed fromJamaica (PTB) and four samples were analyzed from differentdepths (67, 91, 101, 136 m) in Exuma Sound, Bahamas. Photoshows the staining procedure used to mark the beginning of thecalibration study in Jamaica.

Calibration of sclerosponge oxygen isotope records to temperature using high-resolution d18O data 5309

for stable isotope measurement may also liberate oxygenbound within siliceous spicules surrounded by the aragonitebasal skeleton, although no work has been completed spe-cifically quantifying isotope contributions from spiculesusing any technique. Intra-specimen calibrations of sclero-sponge skeletal d18O values to water composition and tem-perature are near the limits of mechanical micro-sampling(computerized milling) techniques due to their slow growthrate and the need for sub-annual data for such calibrations.

A published inter-specimen calibration of bulk sclero-sponge d18O values to temperature (Bohm et al., 2000) pro-vides a basis for the current study. The Bohm et al. (2000)calibration demonstrates that sclerosponge aragonite likelyresponds to temperature linearly with both a slope and anintercept similar to that found in mollusks, aragoniticforaminifera, and abiogenic aragonite. Although this initialcalibration has proven useful, it suffers from two funda-mental shortcomings. First, it relies on measured salinity,employing an assumed linear relationship between salinityand d18O values of seawater (d18Osw) rather than usingmeasured values of the latter variable. Second, the sclero-sponge d18O data from the Bohm study are ultimatelygrouped with data measured from organisms growing overa much larger dynamic range in both temperature and iso-tope composition for derivation of a calibration equation,masking uncertainties in the sclerosponge-only calibrationdata.

In this paper, two efforts are presented to improve theoriginal calibration of Bohm et al. (2000). First, high-reso-lution d18O measurements on skeletal aragonite (d18Oarag)are compared with in situ temperature and d18Osw measure-ments and, second, annual average d18Oarag values are com-pared with measured average high-resolution d18Osw andtemperature data. The latter approach allows incorporationof data from other regions to expand the dynamic range ofthe original inter-specimen calibration, allowing for moreaccuracy as well as more meaningful paleo-environmentalreconstructions.

2. METHODS

2.1. Sclerosponge and coral sample recovery

Sclerosponges of species Ceratoporella nicholsoni livingin a reef enclosure on the northern coast of Jamaica(Fig. 1) were stained with calcein, a fluorochrome (Willenzand Hartman, 1985; Willenz and Hartman, 1999) by scubadivers at 25 m water depth. The exact location, Pear TreeBottom (18�27050.62N, 77�21018.13W), is located east ofDiscovery Bay Marine Laboratory, at the mouth of a smallstream known as the Pear Tree Bottom River. This locationallows access to sclerosponges by scuba near the reef crestwhere formerly vigorous coral growth has resulted in con-joined reef spurs above a groove, forming a tunnel-likeenclosure. The Pear Tree Bottom River delivers sporadicfreshwater discharges near this site, however, this waterwas only observed as a cool, freshwater lens extending toapproximately 0.2 m depth during one visit to the site. Mostmixing of this water with saltier open ocean water likely oc-curs on sections of the reef where ocean swells break; the

reef crest closest to the sclerosponge enclosure did not gen-erate breaking waves during calm sea conditions (diveableconditions), however, there were breaking waves on the reefcrests several hundred meters to the east and to the west ofthe site. Whereas cool, refractive, submarine groundwater isobservable in the nearby Discovery Bay Lagoon, none wasobserved while at depth at the Pear Tree Bottom (PTB) reefenclosure site.

Fluorochrome stain was administered to mark thebeginning of a continuous temperature record kept adjacentto individual sclerosponges at PTB. Temperatures were re-corded bi-hourly using Hobo digital thermistors (OnsetComputer Corporation, Bourne, MA) attached to the basesof the sclerosponges. After a period of 2.5 years, a piece waschipped from each sclerosponge specimen and brought tothe laboratory for subsequent analysis. Thin sections ofthe chipped pieces were analyzed for the presence of the ori-ginal calcein stain line, and measurements of skeletal exten-sion were made in order to determine the average growth

Table 1Sclerosponge sampling information. Extension rates were calculated by measuring calcein bands (PTB) or from U/Th chronology (LSIsclerosponges, Rosenheim et al., 2007). The difference between conventional and incremental depth transects is explained in the text.

Specimen Location Depth (m) Extension rate (lm year�1) Transects (conventional) Transects(incremental depth)

PTB Pear Tree Bottom, Jamaica 22 197 11 1LSI-16 Lee Stocking Island, Bahamas 67 271 3 0LSI-6 Lee Stocking Island, Bahamas 91 232 2 0LSI-13 Lee Stocking Island, Bahamas 101 241 2 0LSI-20 Lee Stocking Island, Bahamas 136 147 3 0

5310 B.E. Rosenheim et al. / Geochimica et Cosmochimica Acta 73 (2009) 5308–5319

rate over the study period (see Rosenheim et al., 2004 fordetails). The work presented below is solely from one ofthese sclerosponges because it offered the best visible stainline and therefore the least ambiguity in time calculatedfrom extension rate (Table 1). A section of this sclero-sponge chip (Fig. 2) was prepared for isotope analysis ona mirror slab of the stained sclerosponge used for Sr/Ca –temperature calibration (Rosenheim et al., 2004). The re-sults presented below are from multiple transects (Fig. 2)milled into the surface of the sclerosponge. The locationof transects towards the center of the slab (Fig. 2b) corre-sponds to the part of the sclerosponge that was analyzedusing laser ablation for Sr/Ca calibration (Rosenheimet al., 2004).

Several samples were used for comparison to the PTBspecimen. For regional comparison, sclerosponges (C. nich-

olsoni) from different depths in Exuma Sound, Bahamas(Fig. 1) were also analyzed, using U/Th dates for chronol-ogy (Rosenheim et al., 2007) because these sclerospongeswere too deep for fluorochrome staining by scuba divers.They were collected by submersible between 1992 and1995 at depths ranging between 67 and 136 m. For compar-ison of isotope records from the same site in Jamaica, aspecimen of zooxanthellate coral Montastraea faveolata

was also taken from the top of the cryptic reef enclosure

Fig. 2. Mounted section of PTB sclerosponge. Work was aimed atthe top 0.5 mm of the basal skeleton, far from the bio-erodedsections nearer the base. The space labeled a was the site of depthincremental milling and the site labeled b was the site of severalconventional milling transects; most of the latter were parallel andsome were milled on top of previous transects. This section is amirror section of that used to Sr/Ca – temperature calibration(Rosenheim et al., 2004).

(14 m depth) in which the sclerosponges grew. Tissue fromthe coral specimen was removed using high-pressure freshwater, and the basal skeleton was subsequently dried andcut into slabs for X-ray analysis and micro-milling.

2.2. Skeletal sub-sampling

Twelve transects were milled in the top millimeter of thebasal skeleton (not the tissue-filled pseudo-calices thatcomprise the less-dense layer in the top 0.5 mm) of onesclerosponge slab from Jamaica (Fig. 2) using two differentsub-sampling methods: (1) a conventional computerizedmicro-milling method (See Fig. 2 of Swart et al., 2002, fordetails) and (2) a modified incremental-depth milling meth-od. For method 1, each individual sample was generatedentirely from a linear milling trace parallel to concentricbanding (Fig. 2) at a depth yielding optimal sample volumeaccounting for the length of the trace (between 5 and10 mm) and the sampling increment (20 lm, or approxi-mately 9–10 samples per year (Table 1)). Thus, transectdepths between 0.5 and 1.2 mm were employed. Method 2differed by literally scratching the surface of each trace(about 5 mm in length); samples were only milled to a depthof a few 10’s of micrometers before collection of pulverizedaragonite, transmitting less drill stress to the surface of thesclerosponge slab and allowing more precision with regardsto the location of the sloped milling bur (a square-pyrami-dal tungsten-carbide bur was employed, tapering at an an-gle of approximately 35� from its vertical axis). Aftersequential collection of powder generated from each tracein the transect, the entire transect was re-milled at a depthincrement some 10’s of micrometers deeper than the previ-ous. Samples collected from subsequent transects were col-lated with those from the same trace position in theprevious iterations until adequate material was collectedfor stable isotope determination (30–50 lg). Minimizationof stress between the milling bur and the aragonite surfacedecreases the amount of flex in the bur for better constrainton drill position, and it decreases the amount frictional heatdissipation for heat-induced fractionation at the milled sur-face. The second method (incremental-depth milling), how-ever, is significantly more time-consuming than theconventional method; therefore only one transect in the Ja-maica specimen was drilled using the incremental depthmethod. Locations of the transects are shown in Fig. 2.Sclerosponges from different depths in the Bahamas weresub-sampled using only the conventional method (method1). In all sclerosponges, samples mixed both pseudo-calyx

Fig. 3. X-radiograph of zooxanthellate coral, Montastraea faveo-

lata, growing adjacent to Jamaica sclerosponges. The annualdensity bands (dashed lines) are most apparent where the coral’sgrowth axis was parallel to the cut surface (the right side). Sub-samples were drilled by hand in a continuous pattern (whitetransparent stripe) at a resolution of approximately 20 samples perdensity band, representing nearly monthly resolution. All sampleswere drilled from a single corallite.

Calibration of sclerosponge oxygen isotope records to temperature using high-resolution d18O data 5311

aragonite infill and original pseudo-calyx wall aragonite.Assuming that density differences between the surface ofthe sclerosponge and the basal skeleton in C. nicholsoni

are similar to those in Pacific Ocean species Astrosclera

willeyana (Fallon et al., 2005) and representative of calyxinfilling, approximately one third of the material in eachsample from this project is comprised of older skeletalmaterial. Averaging of a recorded signal would thereforebe similar in magnitude to the Sr/Ca results presented byHaase-Schramm et al. (2005). The Jamaican coral slabwas sub-sampled with a hand-held dental drill usingX-radiographs of annual density bands as a chronologictemplate (Fig. 3). This method differed from both sclero-sponge micro-milling techniques. Oxygen isotope minimafrom individual bands of the coral were tuned to tempera-ture maxima and ultimately interpolated to fortnightly data(using a rectangular interpolation method) anchored on thecollection date.

2.3. Isotopic analysis

Powdered aragonite comprising each sample was con-verted to CO2 with 100% phosphoric acid using an auto-mated Kiel device. Mass isotopologues of carbon dioxide(44–46) were measured using a Delta Plus ir-MS and werecorrected to d18O values with equations (Craig, 1957) mod-ified for a triple collector mass spectrometer. All measure-

ments were made with respect to a lab reference materialwith carbon and oxygen isotope ratios known relative tothe VPDB standard. Isotope exchange between phosphoricacid and carbonate powder was assumed to be identical inboth standards and samples. Isotope ratios of the labora-tory reference material were reproducible to a precision of±0.08& VPDB for oxygen and ±0.03& VPDB for carbon.

Water samples from Jamaica were intermittently col-lected at Pear Tree Bottom, close to the reef enclosure, bythe staff at Discovery Bay Marine Laboratory. Sampleswere taken both while on scuba (by filling a syringe) andby boat (using a Niskin bottle). In both cases, samples werepoisoned with mercuric chloride and stored in sealed glassbottles with no head space. Oxygen isotope analyses ofthe water samples were carried out by equilibrating500 lL of water with CO2 at 30 �C. Equilibrated CO2 wasanalyzed using a Geo 20/20 ir-MS. Masses 44–46 were usedto calculate the oxygen isotope composition of the waterthrough comparison to laboratory reference water with acomposition known relative to the VSMOW/SLAP scale.Analytical precision of the laboratory reference water was0.1& for d18O; additional uncertainty is ascribed to samplevolume differences between replicates.

3. RESULTS

3.1. Temperature

Thermistors recorded a seasonal temperature range of5 �C at Pear Tree Bottom (Fig. 4, see also Rosenheimet al., 2004). This translates into a potential seasonalDd18Oarag of nearly 1& assuming constant water isotopiccomposition and using a calibration equation,

T ð�CÞ ¼ 20:0ð�0:2Þ � ½4:42ð�0:10Þ�ðdarag � dswÞ; 3�

< T < 28 �C ð1Þ

derived empirically from a combination of sclerosponges,mollusks, abiogenic aragonite, and aragonitic foraminifera(Bohm et al., 2000), where da and dw are the isotope compo-sitions of aragonite relative to VPDB and water relative toVSMOW, respectively.

3.2. Water

The d18Osw values of the water did not show seasonalvariability. For the purposes of this study, the d18Osw areassumed to be constant (average = 0.95&, 2r = 0.37&) ex-cept where otherwise noted. The 95% confidence boundsare presented as the standard deviation of the entire dataset; they reflect variability from the mean recorded over sev-eral seasons (Fig. 4). This precludes sampling artifacts thatcould be introduced into the time series by interpolation ofa sporadic and aliased time series. For demonstration, pre-dictions of d18Oarag values using Eq. (1) and the data shownin Fig. 4 are displayed in Fig. 5a by employing the d18Osw

data both as a constant and as an interpolated time seriesderived from the quasi-continuous data. Sclerosponge datain Fig. 5 are plotted relative to the time axis using the cal-cein extension rate (Table 1).

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Aug-1999 Oct-2000 Nov-2001 Dec-2002 Jan-2004

δ18O

(%0 V

SMO

W)

Dec−1999 Mar−2000 Sep-2000 Apr-2001 Oct-2001 May−2002

26

27

28

29

30

Tem

pera

ture

(o C)

a

b

Fig. 4. (a) Oxygen isotope data from water samples at Pear Tree Bottom, Jamaica. Error bars are ±2r from the mean of each sample,calculated from triplicate analyses of the water samples. Uncertainties on replicate analyses and the irregular spacing in sampling frequencypreclude interpolation of this data set to regular monthly intervals due to the incorporation of sampling bias into the data. Thus these data areused to assess the average water oxygen isotope composition and the potential seasonal range. (b) Recorded temperature at Pear Tree Bottom,Jamaica, interpolated from bi-hourly to daily. Note different time scales (x-axes) used in (a) and (b).

5312 B.E. Rosenheim et al. / Geochimica et Cosmochimica Acta 73 (2009) 5308–5319

3.3. Sclerosponge data

Stable carbon and oxygen isotope data (n = 2126) fromthis project are shown in Fig. 6 and presented in electronicannex EA-1. Carbon isotope values are generally heavier inthe Bahamas specimens. This likely reflects different sourcesof DIC in both locations; the Bahamas are vast carbonatebanks with little terrestrial organic matter input whereasthe Jamaica site was adjacent to a small stream draining aproductive basin in the wet north shore of the island. Oxy-gen isotope values reflect warmer overall temperatures inJamaica (lower d18Oarag values) than in the Bahamas (high-er d18Oarag values) and/or different d18Osw compositions.

Sclerosponge d18Oarag data for all conventionally sub-sampled transects (method 1) in the Jamaica sclerospongeare shown in Fig. 5b. None of the transects showed annualcyclicity in the data in order to correlate and/or tune thedata to each other or to time series of temperature ord18Osw. The data were averaged for each distance alongthe transect (relative to the surface of the basal skeleton),and uncertainties associated with these values were calcu-lated from the standard deviation of measurements fromdifferent transects at the same distance from the surface

(±2r) (Fig. 5b). Distances with only one acceptable mea-surement (generating ion currents above the mass spec-trometer analytical capability as determined from internalprecision of the measurement) were plotted with only theanalytical uncertainty of the laboratory reference materialreplicates (Fig. 5b). Fig. 5c shows data analyzed by thedepth incremental milling method (method 2). Data inFig. 5c also suggest time transgressive averaging as evi-denced from the lack of variability on the seasonal timescale. With a greater certainty, however, the sub-samplesmaking up these samples were from exactly the same dis-tance in the x–y plane, and only the z-axis distance (depth)was changing. If this is accountable for any averaging effectdue to differences in the transect location along the z-axis,then samples from the conventional method will also incurthe same effect. The goal of the depth incremental methodwas to avoid such time transgressive averaging.

3.4. Coral data

The X-radiograph of the Pear Tree Bottom sample ofM. faveolata (Fig. 3) shows consistent annual banding.Interpolated d18Oarag values measured from the coral

-2.0

-1.5

-1.0

-0.5

0.0

0.5

11/93 4/95 8/96 12/97 5/99 9/00 2/02 6/03

Incremental depth milled (method 2)

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Calculated Sclerosponge(Constant δ18Ow)Calculated Sclerosponge (Interpolated δ18Ow)

-2.0

-1.5

-1.0

-0.5

0.0

0.5

δ18O

(%0 V

PDB) Conventionally milled (method 1)

Predicted sclerosponge δ18O space

Predicted sclerosponge δ18O space

a

b

c

Fig. 5. (a) Calculated sclero-sponge aragonite values using Eq. (1). Isotope values are shown using both an assumption of constant butuncertain water isotope composition (points with error bars) as well as the interpolated time series of water isotope measurements (gray line).(b) The average conventional milling isotope analyses from the PTB sclerosponge consistently display lighter values than the calculatedisotope values (gray cloud, same as data shown in A). Uncertainties are ±2r, except in cases where only one measurement was made andstandard analytical error was used. Error bars of replicate analyses are significantly larger than the standard analytical error (0.08& VPDB) insome cases, implying that exact locations in the coral slab are difficult to know at this resolution. (c) One transect was drilled using theincremental depth method (method 2) and it lies within the prediction error of calculated sclerosponge values.

Calibration of sclerosponge oxygen isotope records to temperature using high-resolution d18O data 5313

skeleton are converted to temperature using the calibrationof aragonite from M. faveolata (Leder et al., 1996),

T ð�CÞ ¼ 5:33� 4:19ðdarag � dswÞ ð2Þ

where variables are as in Eq. (1). Better time control in cor-al d18Oarag values allows the use of interpolated rather thanaveraged d18Osw data to reconstruct temperature, but aver-age values were used for points prior to the d18Osw measure-ment period. The seasonal temperature range is captured

reasonably well (Fig. 7), however, the coral reconstructiongenerally overestimates summer temperatures by recordinglighter than expected d18O values. The X-radiograph(Fig. 3) of this coral does not indicate a considerably fasterannual growth rate (3 mm year�1) than the corals on whichthe original calibration of M. faveolata oxygen isotope val-ues to temperature was based (Leder et al., 1996; Swartet al., 1996), nor is there evidence of intra-annual growthrate variability.

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

3.00 3.50 4.00 4.50 5.00 5.50

δ18O

arag

(% V

PDB)

δ13Carag (%0 VPDB)

Fig. 6. Oxygen and carbon isotope data for Jamaica (triangles, n = 123) and Bahamas (squares, n = 2003) sclerosponges.

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26

28

30

32

May-98 May-99 May-00 May-01 May-02

Calculated Temperature

Tem

pera

ture

(oC

)

Tcalc(oC) = 5.33 - 4.19( a - w)Measured Temperature

-3.8

-4.2

-5.0

-4.6

δ18O

arag

(%0 V

PDB)

Fig. 7. Oxygen isotope ratios from Jamaica coral (top) and reconstructed coral temperature compared to measured temperature (bottom).Prior to August, 1999, measured aragonite isotope ratios (open circles, top) are corrected with the average water oxygen isotope ratio; the restof the record reflects aragonite isotope ratios corrected for interpolated oxygen isotope values of the water. Reconstructed temperature (graytriangles, bottom) was calculated from measured oxygen isotope values after the calibration of Leder et al. (1996). Both the measuredaragonite isotope values (a) and the temperature reconstruction (b) were tuned to yearly minima and maxima in temperature. Reconstructedtemperature was consistently warmer by nearly 1 �C, however, the seasonal range in measured temperature is reproduced to the first order bythe coral reconstruction.

5314 B.E. Rosenheim et al. / Geochimica et Cosmochimica Acta 73 (2009) 5308–5319

Calibration of sclerosponge oxygen isotope records to temperature using high-resolution d18O data 5315

4. DISCUSSION

4.1. High-resolution calibration of sclerosponge oxygen

isotope records to temperature

It is evident from Fig. 5 that high-resolution d18Oarag

analyses do not replicate the modeled seasonal cycle. Signif-icant interannual cycles have been analyzed in minor ele-ment ratios of sclerosponges (Swart et al., 2002;Rosenheim et al. 2004; Rosenheim et al., 2005b), however,this was the first attempt to recuperate small oxygen isotopesamples uniquely from the top of the sclerosponge basalskeleton adjacent to the porous tissue-supporting layer ford18Oarag analysis. Aside from the depth of each transect,the size and location of the d18Oarag transects on the slabsurface were similar to the transect used to calibrate Sr/Ca to temperature by laser ablation in the other mirror im-age slab of that shown in Fig. 2. It is understood that,although performed at the same desired resolution, millingis far less precise than laser micro-sampling and can resultin smoothing of variability (Haase-Schramm et al., 2005).In order to attribute the lack of seasonal variability to skel-etal milling imprecision (milling bur flexure, sub-orthogonalslabbing of the sclerosponge, angular disposition of themilling bur, etc.), destructive interference between tempera-ture signatures and d18Osw signatures within d18Oarag valuesmust be ruled out. Modeling of anticipated isotope trends(Fig. 5) is not adequate because of the aliased nature ofthe d18Osw data set.

Replication of the seasonal cycle by the coral sampledemonstrates that interference of seasonal salinity andd18Osw variability with temperature variability does not re-sult in constant d18Oarag values. This differs from previousanalyses of Jamaican corals (Goreau, 1977) which couldnot discern seasonal isotope changes in the d18Oarag values.However, in that study not only was d18Osw not measured,but the coral was not sampled at adequately high-resolutionin order to capture the seasonal temperature amplitude. Inanother study Fairbanks and Dodge (1979) measured a sep-arate specimen of Montastraea spp. from Jamaica and wereable to capture a range in d18Oarag values comparable tothis work. In both cases, carbon and oxygen isotopes werestrongly positively correlated. In no case was the oxygenisotope variability as low as that measured in sclerosponges(Fig. 5); this suggests that sampling imprecision using mi-cro-milling techniques (relative to laser micro-sampling)caused by the increased depth of d18Oarag transects com-pared to laser ablation transects (Rosenheim et al., 2004)as well as the angle of the milling bur may mask the sea-sonal variability in high-resolution oxygen isotope analysisof sclerosponge aragonite. Higher growth rates in coralsprevent this issue.

In addition to the absence of seasonal variability ind18Oarag values of the sclerosponge (Fig. 5), a negative iso-tope bias is apparent in the measurements (relative to calcu-lated values) and is less pronounced in incremental depthmilled (method 2) samples. The isotopic offset betweenthe depth incremental transect (method 2) and a conven-tional transect (method 1) milled over the same growthhorizon on the sclerosponge slab is �0.2& (Fig. 5b–c),

which is within the range of isotopic variability attributedto sampling effects in coral aragonite noted by previousworkers (Aharon, 1991; Gill et al., 1995). Fractionationhas been attributed to drill stress-induced thermal re-crys-tallization in corals (Epstein et al., 1953; Aharon, 1991; Gillet al., 1995). This offset has also been reported in sclero-sponges but attributed to a combination of baking the sam-ple, storage time of the powdered samples, and drill stressconversion of aragonite to calcite (Druffel and Benavides,1986). The incremental-depth milling procedure (sclero-sponge method 2) was designed to minimize the possibilityof aragonite transformation, but still shows a bias towardlighter isotopes values (Fig. 5c); the mineralogy of thesearagonite samples was not determined after milling due tothe small size of the samples. However, the persistent offsetin measured versus expected isotope values suggests inaccu-racy of the calibration (Bohm et al., 2000) if it is assumedthat the data in Fig. 5c as well as those in Bohm et al(2000) succeeded in minimizing milling effects.

4.2. Bulk calibration of sclerosponge oxygen isotope records

to temperature

High-resolution d18Oarag values from sclerosponges,averaged over the experimental interval between the stainline and the surface of the basal skeleton, can be calibratedto temperature in order to evaluate the original calibrationof Bohm et al (2000). The combination of mean d18Oarag

data from Jamaica as well as bulk d18Oarag values fromthe Exuma sclerosponges at different depths (temperatures)can be used to refine the previously-established inter-speci-men sclerosponge calibration (Bohm et al., 2000). Temper-ature records (Fig. 4) can furnish an average over therespective periods of the isotope record (Jamaica between1999 and 2002 and Exuma between 1988 and 1993, seeRosenheim et al. (2004) for detailed records) and can beused for calibration with average isotope values over thesame periods as in previous high-resolution calibrationwork (Rosenheim et al., 2004). In addition, the d18Osw val-ues are known for Jamaica, thus bulk isotope data from theJamaican sclerosponge can be corrected with the averagemeasured d18Osw values and directly compared to the pub-lished inter-specimen calibration (Bohm et al., 2000). Thisapproach provides regional significance to the calibrationand also increases the dynamic range over which sclero-sponges are calibrated by virtue of the lower average watertemperature in the Bahamas than in Jamaica.

The compilation of Eq. (1) (Bohm et al., 2000) assumedthat d18Osw values could be accurately calculated from mea-surements of salinity and the relationship between d18Osw

values and salinity from the Gulf of Mexico (Fairbankset al., 1992, 2r prediction uncertainty of 0.12& VSMOW).The inherent assumption that water in the central Carib-bean behaves isotopically similarly to Gulf of Mexico wateris not robust as a result of the large error inherited by usingsalinity to predict isotope values of the water (Fairbankset al., 1992; Schmidt, 1999; Bigg and Rohling, 2000) as wellas the spatial heterogeneity of this relationship in the Carib-bean (Fairbanks et al. 1992). Thus, Eq. (1) (Bohm et al.,2000) cannot be rigorously applied to regions where isotope

20

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30

-2 -1.8 -1.6 -1.4 -1.2 -1

T(ºC) = 16.1(3.1) - [6.5(1.1)](δa - δw )

T(ºC) = 19.6(8.0) - [4.9(2.3)](δa - δw )

PTB

Exuma Sound

Bohm et al. (2000) (corrected, uncorrected)

(δa-δw)

Tem

pera

ture

(o C)

Fig. 8. At the 95% confidence level, the updated calibration (black)is not statistically different from the original calibration (gray,Bohm et al., 2000). The updated calibration extends the temper-ature range and increases the degrees of freedom, thus increasingoverall confidence in the model parameters. This calibrationprovides a better basis for paleoceanographic reconstructions suchas paleosalinity because less error will be propagated through thevarious calibrations. Even without the Exuma Sound points(triangles), the corrected equation represents an incrementalimprovement by inclusion of the PTB datum.

5316 B.E. Rosenheim et al. / Geochimica et Cosmochimica Acta 73 (2009) 5308–5319

composition of the water and salinity are related in a man-ner different than in the Gulf of Mexico. Averaged high-res-olution data from Jamaica corrected with measured d18Osw

(Fig. 8, circle) represents a more accurate estimate of thed18Oarag/temperature relationship of C. nicholsoni than theoriginal Bohm et al. (2000) data (Fig. 8, gray points) be-cause it is supported by d18Osw values rather than estima-tions. The average measured d18Osw values (0.95&

SMOW) are approximately 0.1–0.15& heavier than the

Table 2Measured isotope values from sclerosponge aragonite and oxygen isotopwere analyzed in this project and the sample names correspond to Table 1(2000) and the names correspond to Table 2 of that manuscript.

Specimen Measured d18Oarag d18Osw (Eq. (3))

PTB �0.8 0.95a

LSI-16 0.06 1.5b

LSI-6 0.25 1.5b

LSI-13 0.14 1.5b

LSI-20 0.46 1.5b

Montego Bay 961 �0.83 0.95a

Montego Bay 952 �0.85 0.95a

Montego Bay 951 �0.79 0.95a

Rio Bueno �0.71 0.95a

Discovery Bay 232 �0.48 1.06c

Discovery Bay 171 �0.47 1.06c

Pedro Bank �0.38 1.06c

a Average measured value, Pear Tree Bottom, Jamaica (2r = 0.37& Vb Atlas value assuming mixture of 60% subtropical underwater (+0.5&

details.c Atlas value for sub-thermocline water in the central Caribbean neard Interpreted values derived from salinity measurements using the relat

imposes an uncertainty of 0.12& VSMOW (2r) on dsw calculated from m

estimates in the Bohm et al. (2000) calibration, however,this difference is smaller than seasonal variations in surfacewaters. Three of seven of the sclerosponges analyzed byBohm et al. (2000) were sampled from deep waters belowthe halocline. Measurements available from the NASAGoddard Institute of Space Studies (GISS) Global Seawa-ter Oxygen-18 Database (http://www.giss.nasa.gov/data/o18data/) from near Pedro Bank (Schmidt, 1999; Biggand Rohling, 2000), a location south of Jamaica whereBohm et al. (2000) sampled one sclerosponge for their cal-ibration, suggest that subsurface d18Osw values (1.06&

VSMOW) are on the magnitude of 0.1& higher than thosemeasured at PTB in this project. If these data had beenavailable at the time of the publication of the first bulk in-ter-specimen calibration of sclerosponges, a more accuratecalibration with d18Osw values on average 0.2& higher forsclerosponges taken from z > 25 m (Fig. 8, black dia-monds), would have been possible.

It is also possible to estimate the average d18Osw value ofBahamas surface and subsurface waters from publishedmeasurements of d18Osw compiled in the GISS oxygen iso-tope database. Sclerosponges growing in the deeper chan-nels of the Bahamas islands (Exuma Sound and TOTO)are likely to receive a mixture of banktop water and high-salinity subsurface water ventilated in the central AtlanticOcean near the latitude of the Bahamas (Zhang et al.,2003). An appropriate estimate of the d18Osw value isapproximately +1.5&, although this value can range aslow as +0.5& (open ocean ventilation) and as high as+3& (bank top) (Lowenstam and Epstein, 1957; Craigand Gordon, 1965; Ostlund et al., 1987; Pierre et al.,1994). The use of +1.5& accounts for competing theoriesand data regarding the depth of density compensationand extent of admixing of density plumes sourcing fromthe banktop and cascading into deep reaches of the plat-form sounds (Wilson and Roberts, 1992, 1993; Wilber

e values of water used in Eqs. (3)–(5). The first five sclerosponges. The following seven sclerosponges were measured by Bohm et al.

d18Osw (Eq. (4)) d18Osw (Eq. (5)) Temperature (�C)

— 0.95a 27.6— — 25.4— — 24.6— — 24.2— — 23.00.81d 0.95a 27.50.81d 0.95a 27.50.81d 0.95a 27.50.81d 0.95a 27.50.84d 1.06c 26.20.83d 1.06c 26.40.86d 1.06c 25.1

SMOW).VSMOW) with 40% banktop water (+3.0& VSMOW). See text for

Pedro Bank.ionship dsw = 0.11S–3.15 (Fairbanks et al, 1992). This relationship

easured salinity.

Calibration of sclerosponge oxygen isotope records to temperature using high-resolution d18O data 5317

et al., 1993; Hickey et al. 2000; Robinson et al. 2004). Thisvalue (+1.5&) was determined assuming 60% open oceansubsurface water and 40% banktop water as an averagecondition to correct Exuma Sound bulk aragonite isotopemeasurements compiled from high-resolution analyses be-tween the years 1988 and 1993 (determined by U/Th exten-sion rates). It is possible that values could range betweenextremes of +0.5 and +3& VSMOW if measured at a sim-ilar frequency to the PTB water samples (Lowenstam andEpstein, 1957; Craig and Gordon, 1965; Ostlund et al.,1987; Wilson and Roberts, 1992, 1993; Wilber et al.,1993; Pierre et al., 1994; Hickey et al., 2000), but it is morelikely that this range would be similar in magnitude to therange in measured values at PTB (Fig. 4) if it is assumedthat Exuma Sound sclerosponges are constantly bathedby a mixture of banktop and open ocean subsurface waters,rather than episodic influence from pure endmembers.

We propose a correction of the calibration of Bohmet al. (2000) to d18Osw values reflective of our measurements(above 25 meters) and previously published subsurfacemeasurements below 25 meters (Table 2). Re-correctionof the isotope data used in the original inter-specimen cali-bration (Bohm et al., 2000) and incorporation of the PTBdatum (n = 1) and Exuma Sound data (n = 4) increases sta-tistical confidence in slope and intercept as well as a higherpercentage of variance explained by the model (n = 12,r2 = 0.95),

T ð�CÞ ¼ 16:1ð�3:1Þ � ½6:5ð�1:1Þ�ðdarag � dswÞ ð3Þ

where the variables are identical to those in Eq. (1). Thisequation yields significantly better confidence in slope andintercept, however, it represents an insignificant shift inthe original calibration line of Bohm et al. (2000, sclero-sponge data only, n = 7, r2 = 0.85)

T ð�CÞ ¼ 19:6ð�8:4Þ � ½4:9ð�2:4Þ�ðdarag � dswÞ ð4Þ

largely due to greater uncertainty on slope and intercept inEq. (4) (Fig. 8). It is suggested that Eq. (3) be used as animproved estimation of the relationship between isotopiccomposition of sclerosponge aragonite, temperature, and

-2.00

-1.50

-1.00

-0.50

0.00

0.50

11/1993 4/1995 8/1996 12/1997

18O

(%VP

DB) Calculated Sclerosponge

Calculated Sclerosponge with Interpolated Water Data

Measured Sclerosponge with D

Fig. 9. Corrected sclerosponge isotope model as per Eq. (3) compared tothat were made to minimize the milling effects noted in Fig. 3 (heavy X’s) rEq. (5). Using the improved calibration and minimizing milling effects re

isotopic composition of the water. Use of this equationnot only reflects improved accuracy for incorporation ofmeasured d18Osw values, it also involves a reduction in pre-diction uncertainties of the relationship from 0.18 to0.08 �C, assuming precise knowledge of dsw. Using Eq. (3)to re-model the darag values expected from the water tem-perature and dsw measurements made at PTB (Fig. 9) elim-inates the lighter isotope bias that was apparent using theexisting calibration.

The derivation of Eq. (3) assumes an intermediate waterisotope value between bank top and open ocean water forExuma Sound Waters. As there are no measurements ofd18Osw from the sclerosponge collection site in the Baha-mas, the value (+1.5& VSMOW) reflects an assumptionof 40% bank top water and 60% open ocean subsurfacewater at all sampled sclerosponge depths based on previ-ously measured end-member values from the BahamasIslands region (Lowenstam and Epstein, 1957; Craig andGordon, 1965; Ostlund et al., 1987; Pierre et al., 1994). Pre-vious research suggests that this proportion is highly vari-able throughout any year (Wilson and Roberts, 1992,1993; Wilber et al., 1993; Hickey et al., 2000), but it hasnot been measured directly. Furthermore, residence timeof these plumes at their respective compensation depths isnot known. As such, it is likely that this proportion is notthe same for all of the depths because of the variability ofcompensation depths for banktop plumes as inferred fromthe highly variable bank top water density throughout theyear. The proportion of water masses used to correctd18Oarag in Eq. (3) does not rely on maximization of linearfit. If Eq. (3) were derived from a regression based on uncer-tainties of the (darag�dsw) term approaching infinity (abso-lutely no constraint on these values, therefore no weightin the regression algorithm), the effect would be similar toomission of the four Exuma Sound data points and the fol-lowing equation (r2 = 0.87, n = 8) would result:

T ð�CÞ ¼ 16:4ð�11:3Þ � ½6:3ð�2:4Þ�ðdarag � dswÞ ð5Þ

Here, variables are identical to Eqs. (1)–(4) and the overallequation is similar to Eq. (3) except for larger uncertainty

5/1999 9/2000 2/2002 6/2003

epth Increment Method (method 2)

incremental depth milled measurements (method 2). Measurementsun through the mean of modeled isotope estimates formulated usingduces discrepancy between modeled and measured results.

5318 B.E. Rosenheim et al. / Geochimica et Cosmochimica Acta 73 (2009) 5308–5319

on the model parameters. Eq. (5) (prediction uncertainty of0.13 �C) can be used if subsequent measurements contradictour assumption regarding the average mixture of banktopand open ocean subsurface waters. Eq. (5) still representsan incremental improvement of the Bohm et al. (2000)equation.

5. CONCLUSIONS

1. Calibration of average high-resolution sclerosponged18Oarag data from multiple specimens to climatic tem-perature and d18Osw results in more accuracy andapplicability.

2. Averaging of high-resolution data for an inter-specimencalibration is warranted because, even at the highest res-olution, samples were mechanically homogenized bymicro-milling as evidenced by the difference in d18Oarag

amplitudes between corals and sclerosponges at the samelocation. This challenge in high-resolution intra-specimen calibration over such a small distance wasunexpected. But, short of such a calibration, the high-resolution data suggest shortcomings in the previous cal-ibration and warrant an improved calibration using acombination of old and new data.

3. The improved calibration is regional; it extends previ-ous work to the Bahamas and illustrates that samplesfrom different parts of the Atlantic basin display thesame relationship between oxygen isotopes and tem-perature with some knowledge of d18Osw values. Thisis different than similar calibrations of zooxanthellatecorals. Additionally, the revised calibration extendsto deeper depths at higher latitudes, subsequentlyincreasing the dynamic range over which C. nicholsoni

has been calibrated.4. The results presented herein illustrate an improvement

over the original inter-specimen calibration whichoffered a worthy approximation and served as the basisof this study. The improved calibration allows a morevalid application of d18Oarag values to reconstruct tem-perature and d18Osw values without dependence onsalinity-d18Osw relationships that impart significantuncertainty.

ACKNOWLEDGMENTS

This project was funded by NSF grants OCE-9819147 andOCE-0136941. The analyses described in this manuscript wouldnot have been possible without the expertise of the Universityof Miami Stable Isotope Laboratory personnel, including A.Saied, C. Schroeder, and G. Mackenzie. Sampling of sclerospon-ges and periodic sampling of water in Jamaica were carried outunder the leadership of Norman Quinn and Peter Gayle at Dis-covery Bay Marine Laboratory. PhW received travel supportfrom the Belgian Science Policy Office and the Leopold IIIFunds for Nature Exploration and Conservation. The authorssincerely thank F. Bohm and C. Romanek for very thorough re-views that greatly improved the submitted version of thismanuscript.

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Associate editor: Juske Horita