Eos, Vol. 7 9 , No. 52, December 29, 1998
E O S , T R A N S A C T I O N S , A M E R I C A N G E O P H Y S I C A L U N I O N
VOLUME 79 NUMBE R 5 2 DECEMBER 29,199 8 PAGES 633-64 0
Sclerosponges May Hold
New Keys to Marine Paleoclimate
PAGES 633, 636
The potential for using s c l e r o s p o n g e s , ma
rine organisms that s e c r e t e a hard c a l c e r o u s skeleton, as p a l e o c l i m a t i c indicators has at
tracted the interest of a n u m b e r of scientists.
S c l e r o s p o n g e s a r e c o m p o s e d mainly of cal
c i u m c a r b o n a t e a n d they are very long lived.
Variations in their skeletal chemistry c o n t a i n proxy information regarding their environ
m e n t and that information has the potential to augment, if not supplant, data from scler- actinian corals in interpreting past water tem
perature, salinity, and productivity over periods of 100s to 1000s of years.
S c l e r o s p o n g e s , or calcified d e m o s p o n - ges, c o n t a i n aragonite or c a l c i t e and a small a m o u n t of s i l i c e o u s material. Lang et al.
[ 1975] report that these s p o n g e s grow within a reef framework, u n d e r coral talus in the shallower parts o f a reef less than 55 m d e e p a n d on steep surfaces of the fore-reef be
tween 55 and 145 m d e e p . T h e largest and most c o n s p i c u o u s of the s c l e r o s p o n g e s de
s c r i b e d by those authors is Ceratoporella nicholsoni (Figure 1), which is reported to at
tain a diameter in e x c e s s of 1 m. T h e s e s p o n g e s are similar in growth habit to many massive vanities of scleractinian corals, the live sponge inhabiting the upper portion of the skeleton, while the lower portion of the s k e l e t o n is essentially dead.
It has not yet b e e n demonstrated that s c l e r o s p o n g e s form annual bands, but s o m e s p e c i e s exhibit growth bands, of m o r e or less d e c a d a l frequency, that run approximately parallel to their surface (Figure 2 ) . Rates of skeletal a c c r e t i o n are typically only about 0.2 mm/yr for the d e e p e r dwelling sponges and up to 1 mm/yr for the shallower varieties. With these slow growth rates, a specimen 10 c m in diameter may b e 400 years old; o n e that is 1 m in diameter may b e 4000 years old. However, information on the distribution of sclerospon
ges in many locations is s c a r c e simply b e c a u s e of the inaccessibility of their habitats. Shallow- growing organisms c a n b e c o l l e c t e d using s c u b a gear; d e e p e r samples necessitate mixed gas diving gear or submersibles.
S c l e r o s p o n g e s are a d v a n t a g e o u s over cor
als as p a l e o c l i m a t i c indicators for a n u m b e r of important reasons. First, s c l e r o s p o n g e s ap
parently s e c r e t e their s k e l e t o n s in c a r b o n and oxygen isotopic equilibrium with their environments with minimal intersample or in
terspecific variations. S e c o n d , in addition to being very long lived, s c l e r o s p o n g e s c a n b e easily dated. Even relatively small s p e c i m e n s can b e several hundred years old. Third, s c l e r o s p o n g e s live in a range of different depths and therefore it is possible to recon
struct the history of the water c o l u m n from in
formation they c o n t a i n .
Isotopic E q u i l i b r i u m
T h e r e have as yet b e e n only a few studies on the stable c a r b o n and oxygen ( 81 3C and 81 80 ) isotopic c o m p o s i t i o n of s c l e r o s p o n g e s and no direct experimental investigations on
relationships b e t w e e n temperature, physi
ological variables, a n d 81 3C and 51 80 . How- ever, indications are that t h e 8i 0C a n d 5l o0 of the s k e l e t o n s are c l o s e to isotopic equilib
rium with their e n v i r o n m e n t s [Druffel and Benvides, \986;B6hm etal., 1 9 9 6 ] . If s c l e r o s p o n g e s are in equilibrium, m e a s u r e m e n t of the stable isotopic c o m p o s i t i o n of the skele
tons will allow 81 80 to b e used directly to de
termine temperature and salinity. Likewise, 81 3C c a n b e used directly to m e a s u r e the dis
solved inorganic c a r b o n of the water without any c o r r e c t i o n s for disequilibrium. In c o n trast, organisms s u c h as s c l e r a c t i n i a n corals, molluscs, and m a n y foraminifera s e c r e t e a
lO 1 Q
skeleton w h o s e 8 C a n d S O are a c k n o w ledged to b e heavily b i a s e d by various types of well c o n s t r a i n e d a n d not s o well under
stood vital effects as well as varying d e g r e e s of a n t h r o p o g e n i c influences a n d variations in the 51 3C of the l o c a l environment. T h e 81 3C and 81 80 equilibrium of the s k e l e t o n s of s c l e r o s p o n g e s may b e a result of the rela
tively simple biology of the organism, a l a c k of the algal symbionts that are present in cor
als, and a slow growth rate, w h i c h r e d u c e s or
Fig. 1. A specimen of the species Ceratoporella from top to bottom (Photograph by P. Willenz).
volume.
nicholsoni from Jamaica. Sample is about 10 cm Original color image appears at the back of this
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Eos, Vol. 79, No. 52, December 29, 1998
eliminates kinetic effects (T. M c C o n n a u g h e y , personal c o m m u n i c a t i o n , 1 9 9 8 ) .
T h e most arresting e x a m p l e of this iso
topic equilibrium is the almost precise repli
cation of the 1 3C Suess effect in all the s c l e r o s p o n g e s analyzed to date. T h e 1 3C Suess effect is the i n c r e a s e in 81 3C in the at
m o s p h e r e as a result of the addition of fossil CO2. T h e i n c r e a s e is estimated to b e a b o u t 1 to 1.25%o over the past 150 years. Data pre
s e n t e d at a r e c e n t meeting on the application of s c l e r o s p o n g e s to p a l e o c l i m a t i c p r o b l e m s indicate that without e x c e p t i o n sclerospon
ges from both the Atlantic and the Pacific O c e a n s s h o w this c h a n g e (Figure 3 ) . T h e data replicate previous work on sclerospon
ges by Druffel and B e n a v i d e s [ 1 9 8 6 ] . Future a d v a n c e s in the dating of s c l e r o s p o n g e s will e n a b l e differences to b e determined in the timing of the addition of 1 2C to the o c e a n s at different geographic and depth l o c a t i o n s and therefore allow inferences to b e m a d e a b o u t ventilation and mixing rates of the o c e a n basins.
T h e 81 80 of the s c l e r o s p o n g e skeleton also appears to b e c l o s e to equilibrium with the a m b i e n t environment. Bulk s a m p l e s from a n u m b e r of sites in the Pacific s h o w an e x c e l lent a g r e e m e n t with a m b i e n t temperature;
the relationship b e t w e e n temperature and 81 80 is similar to that determined for inor
g a n i c aragonite (Figure 4 ) .
T h e slow growth rate of s c l e r o s p o n g e s also gives them a major advantage over cor
als. At 0.25 mm/yr, a s c l e r o s p o n g e with a di
a m e t e r of 10 c m c a n b e 4 0 0 years old. But a coral with a growth rate of 1 c m / y r would n e e d to b e 4 m high in order to p r o d u c e a similar length record. Not only are corals of s u c h size extremely rare but if present, their
Fig. 2. Polished slab of a section of a sclerosponge (Ceratoporella nichol- soni) from Lee Stocking Island in the Bahamas. The variation in banding can be clearly seen. The age of this specimen is about 400 years based on preliminary U/Th dates (Photograph by Grammar). Original color image appears at the back of this volume.
utility is likely to b e r e d u c e d as a result of bio- erosion. In p r a c t i c e the useful range of c o r a l s as p a l e o e n v i r o n m e n t a l proxies is limited to a b o u t 2 0 0 to 3 0 0 years, e x c e p t in rare c i r c u m s t a n c e s . B a s e d on limited studies, s c l e r o s p o n ges have b e e n reported up to 1 m in d i a m e t e r [Lang, etal. 1975] and records of up to 7 0 0 years already exist (Figure 3 ) .
Dating S c l e r o s p o n g e s
Radiometric t e c h n i q u e s remain the most reliable methods for dating s c l e r o s p o n g e s . R a d i o c a r b o n has b e e n utilized successfully, although regional variations in s e a surface
1 4C ( a n d b o m b effects in very young sam
ples) add uncertainty to the method. At pre
sent, mass-spectrometric m e a s u r e m e n t s of U-series isotopes (2 3 8U -2 3 4U -2 3 0T h ) a p p e a r most definitive. Relatively high uranium (up to 7 ppm; J . R u b e n s t o n e , personal c o m m u n i cation, 1998) and low initial Th c o n c e n t r a tions m a k e this m e t h o d very useful; a g e uncertainties of only a few years are obtain
a b l e even for very young s p e c i m e n s . Growth rates c a l c u l a t e d from multiple U-Th ages on individual s p o n g e s agree well with b i o l o g i c a l rate estimates.
Precise age dating of s c l e r o s p o n g e s in
itially presented a c h a l l e n g e , but r e c e n t find
ings offer c o n s i d e r a b l e h o p e that dating c a n b e a c c o m p l i s h e d a c c u r a t e l y ( ± 1 y e a r ) a n d relatively cheaply. Unlike scleractinian cor
als used in p a l e o c l i m a t e studies, sclerospon
ges do not have any annual density b a n d s . Work presented at the r e c e n t meeting, how
ever, revealed that the slight c o l o r variations visible in s o m e s p e c i e s of s c l e r o s p o n g e s may record annual cyclicity. If this observation is correct, then dating of s c l e r o s p o n g e s would
b e immensely sim
plified and essen
tially would b e c o m e a matter of counting bands.
Additional an
nual periodicity may also b e pre
sent in the c a r b o n and oxygen iso
topic c o m p o s i t i o n if the skeleton is sampled at suffi
ciently high resolu
tion. R e s e a r c h e r s at the University of Miami's Rosenstiel S c h o o l of Marine and Atmospheric S c i e n c e s have b e e n a b l e to sam
ple the s k e l e t o n of t h e s c l e r o - s p o n g e at increments as small as 3 0 u.m,
roughly equivalent to 10 s a m p l e s per year.
Spectral analysis of these data, using radio
metric m e t h o d s to establish the chronology, shows clearly a signal from 1.5 to 0.7 years.
S u c h analysis not only supports the radiomet
ric dating, but suggests that the growth rate of the s c l e r o s p o n g e s c a n b e tuned to maximize the annual 81 80 signal and that this itself c a n b e used for dating purposes.
S c l e r o s p o n g e s o c c u r over a wide range of water depths from shallow reefs to as d e e p as
150 m. Over all ranges of depths the sponges inhabit c a v e s , ledges, and a r e a s under over
hangs a n d are c o n s i d e r e d to b e cryptofauna.
T h e wide range of depths offers the exciting possibility of using the s c l e r o s p o n g e s to re
construct histories of the water c o l u m n . For e x a m p l e , e n h a n c e d wind stress c a n have the influence of increasing the thickness of the mixed layer which will b e translated as an in
c r e a s e d annual cyclicity in the s e a s o n a l sig
nal of the oxygen isotopic composition.
Therefore, for the first time, a t e c h n i q u e might b e available with w h i c h to e x a m i n e d e c a d a l to c e n t e n n i a l variability of wind stress.
Studies N e e d e d
R e s e a r c h into the use of s c l e r o s p o n g e s as p a l e o c l i m a t i c proxies is still in its infancy and n e e d s support from funding a g e n c i e s so that critical a d v a n c e s c a n b e a c h i e v e d . Con
sider the a m o u n t of effort w h i c h has b e e n ex
p e n d e d over the past d e c a d e in calibrating various proxy indicators in c o r a l s and foraminifera. At least ten studies have b e e n d o n e on the calibration of the Sr/Ca ratio with temperature in corals a l o n e and a simi
lar n u m b e r investigating 81 3C and 81 80 . In contrast, there have b e e n no such studies on s c l e r o s p o n g e s .
In view of the promise of s c l e r o s p o n g e s f o r p a l e o c e a n o g r a p h i c and climatological in
terpretation, the participants at the recent meeting proposed a series of r e c o m m e n d a tions to establish s c l e r o s p o n g e s as recog
nized proxy indicators. For o n e , participants proposed a series of calibration studies to b e initiated involving trace and minor elements as well as s t a b l e c a r b o n a n d oxygen isotopes.
T h e calibration studies would test the ability of s c l e r o s p o n g e s to record variations in salin
ity and temperature and in wind stress and upwelling, the latter s o that c h a n g e s in the mixed layer c a n b e reconstructed. T h e stud
ies would also determine w h e t h e r g e o c h e m i - cal records c a n b e r e p r o d u c e d from scleros
ponge skeletons.
Also r e c o m m e n d e d was e x a m i n a t i o n of the g e o c h e m i c a l response of s c l e r o s p o n g e s to naturally occurring climatic a n o m a l i e s such as the 1997-1998 El Nino. Measurements should i n c l u d e population level responses ( s u c h as mortality and new settling), physi-
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Eos, Vol. 79, No. 52, December 29, 1998
2000 -a 1900 - |
1800 -I = Le e Stocking Islan d
CD
Q
1700 1600 1500 1400 1300 - = 1200
Jamaica
I I I I I I I I I I I I I I I I I I I I I I
4.00 5.0 0
Carbon Isotopi c Compositio n
Fig. 3. Comparison of carbon isotopic records from specimens of Ceratoporella nicholsoni col- lected from Lee Stocking Island, Bahamas, and Jamaica (F. Bohm and P. Swart, unpublished data, 1998).
ological responses ( s u c h as growth rates, par
tial die-backs, a n d r e g e n e r a t i o n ) , and the be
havior of skeletal climatic proxies. A study also was proposed to determine the c a u s e of visible banding in s c l e r o s p o n g e skeletons.
Banding is important both for determining c h r o n o l o g y within a skeleton and for cross- correlating different s k e l e t o n s and develop
ing s t a c k e d c h r o n o l o g i e s . Participants said research also should b e d o n e to quantify the vertical distribution of calcification within s c l e r o s p o n g e skeletons, using dyes a n d iso
topic tracers. It is especially important to de
termine w h e r e "time zero" lies within the skeleton and to d e v e l o p m a t h e m a t i c a l mod
els to d e s c r i b e the slurring of environmental signals as r e c o r d e d by the s c l e r o s p o n g e . This will allow the development of sampling strate
gies that will minimize vertical smearing of signals detected with microsampling proto
cols for stable isotopes and trace elements.
If s c l e r o s p o n g e s have a w e a k n e s s for pa
l e o c l i m a t e interpretative purposes, it is that they often o c c u r in habitats in w h i c h they are difficult to c o l l e c t . In d e e p e r areas, s a m p l e s have b e e n c o l l e c t e d using s u b m e r s i b l e s as ships of opportunity rather than in projects specifically a i m e d at s c l e r o s p o n g e s . Future d e v e l o p m e n t of this exciting proxy will de
p e n d on obtaining funding from enlightened program m a n a g e r s w h o will take a c h a n c e on investing in n e w p a l e o c l i m a t i c a n d o c e a n o g r a p h i c proxies.
A workshop report and abstracts of pa
pers presented at the r e c e n t s c l e r o s p o n g e meeting are available on the s c l e r o s p o n g e
-1.5 - 1 -0. 5 o P r e d . Equilibrium 81 sO (% o P D B )
F KA P (20m ) H BU N (20m ) J KP T (10m ) B NA R (15m ) I PA L (15m ) P SO L (20m ) Z SA M (20m )
= CAL(8m ) F JA M (20m ) H JA M (25m ) J JA M (84m ) B JA M (91 m) P LS I (138m)
Fig. 4. Observed and predicted oxygen iso- topic equilibrium for sclerosponges from a number of locations throughout the world (M.
Moore and C Charles, unpublished data, 1998). Sites are Kapoposang Island (KAP), Bu- naken Island (BUN), and Kapota Island (KPT) in the Indonesian seaway; Nam (NAR), Patau (PAL), and the Solomon Islands (SOL) in the western Pacific; Lee Stocking Island in the Ba- hamas (LSI), and Jamaica (JAM). (Data from Jamaica are from Druffel and Benavides [1986] and Bohm etal. [1996]). Estimates of mean annual temperatures and 51 80 are based on field observations and atlas data.
W e b site (http://mgg.rsmas.miami.edu/mgg.
htg/groups/sil/workshop.htm). T h e meeting, s p o n s o r e d by the National S c i e n c e Founda
tion and the National O c e a n i c and Atmos
pheric Administration, was held in Miami, Florida, in March 1998.
A u t h o r s
Peter K. Swart, Michael Moore, Chris Charles, and Florian Bohm
For m o r e information, c o n t a c t Peter K. Swart, Marine Geology and Geophysics, Rosenstiel S c h o o l of Marine and Atmospheric S c i e n c e s , University of Miami, 4 6 0 0 R i c k e n b a c k e r Causeway, Miami FL 3 3 1 4 9 USA R e f e r e n c e s
B o h m , F., M. M. J o a c h i m s k i , H. Lehnert, G.
Morgenroth, W. K r e t s c h m e r , J . V a c e l e t , a n d W.C. Dullo, C a r b o n i s o t o p e r e c o r d s from e x tant C a r i b b e a n a n d South P a c i f i c s p o n g e s : Evolution of 1 3C in s u r f a c e w a t e r DIC, Earth Planet. Sci. Lett., 139, pp. 291-303,
1996.
Druffel, E. R. ML, a n d L. M. B e n a v i d e s , Input of e x c e s s CO2 to the s u r f a c e o c e a n b a s e d
o n 1 3C /1 2C ratios in a b a n d e d J a m a i c a n
s c l e r o s p o n g e , Nature, 321, pp. 5 8 - 6 1 , 1 9 8 6 . Lang, J . C , W. D. Hartman, a n d L. S. Land, S c l e r o s p o n g e s : Primary framework c o n s t r u c tors o n the J a m a i c a n fore-reef, Mar. Res., 33, pp. 2 2 3 - 2 3 1 , 1 9 7 5 .
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