ACTA CARSOLOGICA	33/2	14	239-248	LJUBLJANA 2004
COBISS: 1.01
COMPARISON OF 14C AND 23«Th/234U DATING OF SPELEOTHEMS FROM SUBMARINE CAVES IN THE ADRIATIC SEA (CROATIA)
PRIMERJAVA MED METODAMA 14C IN 230Th/234U NA PRIMERU DATACIJE SIG IZ PODMORSKIH JAM V JADRANU NA HRVAŠKEM
MASA SURIC1 & MLADEN JURACIC2 & NADA HORVATINCIC3
1	Department of Geography, University of Zadar, Ul. dr. F. Tudmana 24 i, 23000 Zadar, Croatia, msuric@unizd.hr
2	Department of Geology, Faculty of Science, University of Zagreb, Zvonimirova 8, 10000 Zagreb, Croatia, mjuracic@geol.pmf.hr
3	Radiocarbon and Tritium Laboratory, Ruder Boškovic Institute, Bijenička 54, P.O. Box 180, 10002 Zagreb, Croatia, nada.horvatincic@irb.hr
Abstract	UDC: 551.44(26.03)(497.5)
Maša Suric & Mladen Juračic & Nada Horvatinčic: Comparison of 14C and ^^'Th/'^^U dating of speleothems from submarine caves in the Adriatic Sea (Croatia)
Among the 16 speleothems that were collected from 7 submarine caves and pits for the purpose of 14C and U-Th dating and reconstructing sea-level changes, two speleothems were dated by both methods. Different environmental conditions during the speleothem deposition and after the submergence resulted with different appropriateness for speleothem dating by these techniques. Well preserved speleothems gave reliable results by both methods, while U-Th method showed disadvantage in the case of carbonates contaminated with detrital material, as well as in the case of carbonate from marine overgrowth that covers the speleothems. However, U-Th method using MC ICPMS technique which requires only 100-300 mg of sample per analysis (instead of ca. 30 g for 14C conventional method), offers better age resolution that is essential for speleothem dating. Key words: 14C dating, U-Th dating, submerged speleothems, submarine karst, Adriatic Sea, Croatia.
Izvleček	UDC: 551.44(26.03)(497.5)
Maša Suric & Mladen Juračic & Nada Horvatinčic: Primerjava med metodama 14C in 23"Th/234U na primeru datacije sig iz podmorskih jam v Jadranu na Hrvaškem
Z metodama 14C oziroma U-Th smo datirali 16 sig iz sedmih podmorskih jam v Jadranu. Dve sigi smo datirali z obema metodama. Primernost metod za datacijo sig je različna zaradi različnih okoljskih pogojev, ki so vladali med izločanjem sige oziroma po zalitju jame. Obe metodi sta zanesljivi na dobro ohranjenih sigah. U-Th metoda je slabša, ko so karbonati onesnaženi z detritičnim materialom oz. preraščeni s plastjo morskega izvora . Vendar metoda U-Th z uporabo MC ICPMS analize, kjer potrebujemo le 100-300mg vzorca (namesto 30 g pri metodi 14C), omogoča boljšo časovno ločljivost datacij, kar je pri datiranju sig zelo pomembno. Ključne besede: 14C datacije, U-Th datacije, potopljena siga, podmorski kras, Jadransko morje, Hrvaška.
INTRODUCTION
After the speleothems had been introduced as a relevant source of paleoclimatic proxy records (Hendy & Wilson 1968), they were also recognized as a reliable indicator of paleo sea levels (Spalding & Mathews 1972; Harmon et al. 1978; Gascoyne et al. 1979). Namely, speleothems are typical terrestrial deposits, so their findings below the present sea level indicate former lower sea levels. Dating of the last surface layer of speleothem that is typical subaerial feature, should provide the time when the speleothem was still under the vadose condition i.e. above sea level, whereas the age of the initial part of marine overgrowth that covers the submarine speleothems should indicate the time of establishment of marine conditions.
Regarding these facts, submerged speleothems have been studied in order to reconstruct eustatic (global) and relative (local/regional) sea-level changes worldwide (Li et al. 1989; Allesio et al. 1992; Richards et al. 1994; Antonioli et al. 2001; Vrhovec 2001; Bard et al. 2002; Suric 2002; Fornos et
Fig. ': Study area with sampling sites.
al. 2002). The most frequently used dating methods are '4C and 230Th/234U. For both methods the good quality of sample is very important, e.g. sample taken from chemically-closed system since the time of deposition, with no sign of any recrystallization, no contamination with detrital material, etc. (Richards & Dorale 2003).
With the intention of reconstructing relative sea-level changes on the Eastern Adriatic coast, 16 samples of speleothems were taken from seven submarine caves (from the depths of 1.5 m to 41.5 m below mean sea level - m.s.l.) located along the mainland and Adriatic islands' coasts. Speleothem samples, as well as marine overgrowth, were dated by conventional 14C method (Suric 2002, Suric et al. 2004, in press).
Here we compare and discuss the results of 14C and U-Th dating of two speleothems from two submerged karst features in the Adriatic Sea: Cave in Tihovac Bay (Pag Island) and submarine spring Vrulja Zečica (Fig. 1).
STUDY AREA - GEOLOGICAL AND ENVIRONMENTAL SETTINGS AND
SAMPLING
Both of the studied caves were formed within formerly tectonically disturbed limestones during the periods of lower sea levels, whereas their present positions and hydrological functions were subsequently defined by the last, Late Pleistocene-Holocene transgression (Fig. 2).
Cave in Tihovac Bay, formed in Upper Cretaceous limestones, situated ca. 100 m off-shore has no hydrological function and it is entirely within marine environment. Stalactite P-23 was collected from the SE channel, from the depth of 23 m below m.s.l. and it was completely covered with 0.5-1.0 cm thick marine overgrowth (biocenosis of caves and ducts in complete darkness). For 14C measurements, carbonate samples were drilled from overgrowth (layer P), the youngest speleothem layer (A) and from the oldest, central part of the speleothems (B) (Figs. 3a and 3c). U-Th measurements were performed on wafers from marine overgrowth (P1) and from the first two layers beneath the
Fig. 2: (a) Cave in Tihovac Bay; (b) submarine spring Vrulja Zečica (after Bakran-Petricioli & Petricioli 1999).
overgrowth (T1 and T2) (Fig. 3b). Both of these samples, T1 and T2, measured by U-Th method overlap with sample A measured by 14C method.
Submarine spring Vrulja Zečica was formed in Tertiary limestones and its outlet is situated 20 m off the coast at the depth of -9 m. Although it is presently periodical freshwater spring, numerous speleothems (stalagmites, stalactites, draperies, etc) at the depth of ca. -40 m are evidence of its former vadose phases. Stalagmite Z-41 was taken from the depth of 41.5 m below m.s.l. Scarce, eroded vermetid tubes on its surface and destroyed side of the stalagmite oriented toward main channel (Fig. 4), suggest very strong periodical freshwater outflow resulting in mechanical erosion. Inner part also shows discontinuities and inhomogeneous clayey layers, probably originating from periodical flooding of the cave during the speleothem growth.
Sample for the 14C measurement (S) was taken after removing 5 mm of speleothem surface, whereas the samples for U-Th dating were drilled from the layers 5 mm (B-40) and 17 mm (B-28) below the stalagmite surface (Fig. 4).
Fig. 3: Stalactite P-23 with marked sampling sites; well preserved upper part and lower part destroyed by boring organisms (scale bar '0 cm): a) longitudinal section; b) slice with wafers from longitudinal section; c) perpendicular section; d) microscopic photo of perpendicular slice. Samples A, B and P were dated by '"C and P', T' and T2 by U-Th.
METHODS
Samples were dated at the Ruder Boškovic Institute, Zagreb by conventional 14C method, using gas proportional counter (GPC). For each sample ~30 g of carbonate (speleothem or marine overgrowth) was separated, treated with dilute HCl to obtain CO2, which was subsequently converted to methane. The conventional 14C ages were corrected for initial 14C activity of 85% and 95% for speleothem and marine overgrowth, respectively. Additional calculation of the starting time of marine overgrowth was made according to mathematical model developed by Allesio et al. (1992) that assumes continuous deposition and simultaneous decay of 14C isotopes within the overgrowth. 14C ages are also expressed in calibrated 14C ages.
Five samples were dated by 230Th/234U method using Multi Collector Inductively Coupled Plasma Mass Spectrometry (MC ICPMS) at the Isotope Geochemistry Laboratory and Mass Spectrometry Laboratory, University of Bristol. For each measurement ~300 mg of carbonate was separated, either in the form of powder (from stalagmite Z-41) or in the form of wafer (from stalactite P-23). After the separation of uranium and thorium in ion exchange columns, and plasma ionisation, their amounts were measured by mass spectrometry.


Fig. 4: Longitudinal section of stalagmite Z-41 with marked sampling sites (scale bar 10 cm). Reconstructed part oriented toward recent periodical fresh-water inflow destroyed by mechanical erosion.
RESULTS AND DISCUSSION
Results of '4C and U-Th dating of the submerged stalactite P-23 from the Cave in Tihovac Bay (-23 m) and of stalagmite Z-41 from submarine spring Vrulja Zečica (-41 m) are shown in the Table 1. '4C results are expressed in '4C age corrected for A0 and in calibrated age as a mean of the calibrated range obtained by OxCal calibration software (Bronk & Ramsey 2003) (with 1 sigma error) for the'4C ages <22000 yr BP, and for '4C ages >22000 yr BP calibrated ages were determined by the proposed extension of the calibration curve (Bard et al. 2004). Results obtained by U-Th dating were corrected for bulk earth value of (230Th/232Th )jnjtjal that is 0.8±0.8 (activity ratio) (Wedepohl 1995).
Stalactite P-23
Stalactite P-23 is an example of typical submerged speleothem covered with marine organisms, partially destroyed (Fig. 3a, lower part). However, as indicated in Fig. 3c, upper part of the speleothem was well preserved from boring organisms. ö'3C values measured from the overgrowth (1.8 %o) and from speleothem samples (-7.5 and -8.5 %o) showed that there was no contamination of terrestrial carbon with marine one or vice versa (Suric 2002). Microscopic photo of perpendicular slice (Fig 3d) suggests that there was no dissolution on contacts between calcite crystals. It supports the conclusion that there was no dissolution on the surface of the speleothem.
'4C age of starting time of P-23 marine overgrowth, calculated by model of Alessio et al (1992) and calibrated (7,920 cal BP), is in good correlation with existing Holocene sea-level curves (Antonioli et al. 2001). Additionally, an attempt was made by U-Th dating of marine overgrowth (sample P1) by U-Th method, despite the likelihood of post-depositional alteration. Uncorrected U-Th age of 52.6 ka was lowered to 28.2 ka by correction for a bulk earth value of (230Th/232Th )initial. Anomalously high age could be explained either with uranium loss from the system, or by addition of thorium with 230Th/232Th ratio greater then bulk earth. It is interesting that the measured 234U/238U activity ratio (1.1511) is close to the modern ocean value of 1.145. It suggests recent U exchange with the ocean reservoir.
As far as the terrestrial part of the stalactite P-23 is concerned, '4C age (30,300 cal BP) is comparable to the U-Th ages of the same part of the speleothem (37 and 33 ka). Obtained U-Th ages are reversed, so apparently, outer part (T1) is older than inner (T2). It indicates that either uranium loss or thorium addition occurred. The latter is more likely because the 238U concentration is actually greater for outer part, and 232Th concentration is also much greater (57 ng g-1 compared with 0.84 ng g-1 of inner part). Correction for bulk earth value of (230Th/232Th )initial reduces age of T1 from 46.9 ka to 37.3 ka, but it is likely that the true age of cessation prior to submergence is closer to 33 ka (based on T2). Dating of subsequent layer below T2 (not labelled on Fig. 3b) should enable extrapolation and age determination of the outer contaminated layer T1.
Stalagmite Z-41
In case of stalagmite Z-41, results obtained by two methods are in very large discrepancy. '4C age of 11,350 cal BP is expected regarding cessation of stalagmite growth due to Holocene sea-level rise. But, U-Th measurements indicate that this speleothem was contaminated by detrital clays, giving useless results. Namely, sample B-40 has indeterminable age because of too high 230Th/238U ratio. That was probably gross error, unless the detrital clay component has a very high initial 230Th/232Th. However, this is not very likely because the sample B-28 gives a determinable age (although very
d
m
n
Z-41			P-23						Speleothem	
5									Sea depth (m)	
	CC 2	2	cc	CC	cc s	-3 2 2		- 33	š. cc	
--oo	CD	S	T 2	1	P1	ro		P	Sample	
•cj ff a C e aa era g cC	Cc r 3 a C "it	lg JS t B p e cc o	■-sT r e s	T lü O	t o CTO O «	l d cc p d O a 0 t	t " n O	o C r CTT o t	'-C3 B S	
11			3	2 2	2	3		41	) s	
		d				2	2 1_, ,	1	T 0> ra o	
								2	) c J- ü ^	
		0				3 0	00	-J 0	r CD t a ^ c	
	Ol			4 OO					cro 8 CfO, G	-=r
2 to	1		4		o^				c 22 g, h	
OO			3	8					C -T" cf	
S				3 OO					■.y- 88	
.S	n #			4	2				U	
0	n #		4	3	2				D"" rr	
old, >280 ka) with similar detrital component (230Th/232Th concentration ratio of B-40 and B-28 are 7.89 x10-6 and 7.82 x10-6, respectively). In order to improve obtained results, beside the correction for bulk earth value of (230Th/232Th , isochron method should be employed.
Apart from the dating limitations, sampling of speleothems from submarine caves along the Eastern Adriatic coast also has some limiting factors comparing with sampling of speleothem from terrestrial caves: (i) stalactites are more frequent since the stalagmites are usually buried by fine grained clastic sediment deposited within marine environment, or by the material from collapsed roofs; (ii) marine overgrowth, where abundant, conceals the real shape of speleothem that is crucial for choosing the samples; (iii) the speleothems could partially be destroyed by boring organisms and/or in the case of submerged springs, by mechanical erosion and corrosion (Juračic et al. 2002)
CONCLUSIONS
Our study showed that both dating methods, conventional 14C and U-Th MC-ICPMS, give reliable results for well preserved submarine speleothem (terrestrial carbonate). 14C age of marine overgrowth, and then model calculation, give reliable result of overgrowth starting time, while U-Th result is not acceptable.
However, due to a small amount of carbonate samples for U-Th MC-ICPMS method (100-300 mg per analysis) dating by this method provided much better age resolution of single speleothem layer than conventional 14C method (requiring ca. 30 g per analysis). In the same time, U-Th MC-ICPMS method is much more sensitive to the contamination by detrital material, whereas 14C dating method has a problem of determination of the initial 14C activity of carbonates. Combination of both methods and, if possible, using 14C AMS method could give most reliable results.
ACKNOWLEDGMENT
We thank speleodivers Tonci Rada, Petronije Tasic, Branko Jalžic, Andelko Novosel and Mladen Kuhta for providing us samples from submarine caves. We also thank David Richards and Dirk Hoffmann (School of Geographical Sciences, University of Bristol) for U-Th analyses and helpful suggestions.
REFERENCES
Alessio, M., Allegri, L., Antonioli, F., Belluomini, G., Ferranti, L., Importa, S., Manfra, L., Proposito, A., 1992: Risultati preliminari relativi alla datazione di speleotemi sommersi nelle fasce costiere del Tirreno centrale.- Giornale di Geologia, ser. 3., 54/2, 165-193. Antonioli, F., Silenzi, S., Frisia, S., 2001: Tyrrhenian holocene paleoclimate trends from spelean
serpulids.- Quaternary Science Reviews, 20/15, 1661-1670. Bakran-Petricioli, T., Petricioli, D., 1999: Život u moru - Podvelebitske vrulje.- Ekološki glasnik, 4, 17-23.
Bard, E., Antonioli, F., Silenzi, S., 2002: Sea-level during the penultimate interglacial period based on a submerged stalagmite from Argentarola Cave (Italy).- Earth and Planetary Science Letters, 196, 135-146.
Bard, E., Rostek, F., Menot-Comber, G., 2004: A better radiocarbon clock.- Science 303, 178-179.
Bronk Ramsey, C., 2003: The OxCal Program Manual, v.3.9, submitted on WWW http:// www.rlaha.ox.ac.uk/oxcal/oxcal.htm
Fornos, J. J., Gelabert, B., Gines, A., Gines, J., Tuccimei, P., Vesica, P., 2002: Phreatic overgrowth on speleothems: a useful tool in structural geology in littoral karstic landscaps. The example of Eastern Mallorca (Balearic Islands).- Geodinamica Acta, 15, 113-125.
Gascoyne, M., Benjamin, G. J., Schwarcz, H. P., Ford, D. C., 1979: Sea-Level Lowering During the Illinoian Glaciation: Evidence from a Bahama "Blue Hole".- Science, 205, 806-808.
Harmon, R. S., Schwarcz, H. P., Ford, D. C., 1978: Late Pleistocene Sea Level History of Bermuda.-Quaternary Research, 9, 205-218.
Hendy, C. H., Wilson, A. T., 1968: Palaeoclimatic data from speleothems.- Nature, 219: 48-51.
Juračic, M., Bakran-Petricioli, T., Petricioli, D., 2002: Cessation of Karstification Due to the Sea-level Rise? Case Study of the Y-Cave, Dugi otok, Croatia. In: Evolution of Karst: From Prekarst to Cessation, ed.: F. Grabovšek, Založba ZRC, Postojna, Ljubljana, 319-326.
Li, W.-X, Lundberg, J., Dickin, A. P., Ford, D.C., Shwarcz, H. P., McNutt, R., Williams, D., 1989: High Precision mass-spectrometric uranium-series dating of cave deposits and implications for paleoclimate studies.- Nature, 339, 534-536.
Richards, D. A., Smart, P L., Edwards, R. L., 1994: Maximum sea levels for the last glacial period from U-series ages of submerged speleothems.- Nature, 367, 357-360.
Richards, D. A., Dorale, J. A., 2003: Uranium-series Chronology and Environmental Applications of Speleothems. In: Uranium Series Geochemistry, ed.: B. Bourdon, G, M. Henderson, C. C. Lundstrom, S. P. Turner, Reviews in Mineralogy & Geochemistry, Vol. 52, Geochemical Society, Mineralogical Society of America, 407-460.
Spalding, R. F., Mathews, T. D., 1972: Stalagmites from Caves in the Bahamas: Indicators of Low Sea Level Stand.- Quaternary Research, 2, 470-472.
Suric, M., 2002: Gornjopleistocensko-holocensko kolebanje morske razine na istočnoj obali Jadrana, Master of Science Thesis, Faculty of Science, University of Zagreb, p.102, Zagreb
Suric, M., Juračic, M., Horvatinčic, N., Krajcar Bronic, I., 2004: Late Pleistocene - Holocene sea-level rise and the pattern of coastal karst inundation - records from submerged speleothems along the Eastern Adriatic Coast (Croatia).- Marine Geology, (in press).
Vrhovec, T., Mihevc, A., Lauritzen, S. E., Lundberg, J., 2001: O starosti potopljenih stalaktitov v jami pri otočku Galiola, Dalmacija, Hrvatska.- Naše jame, 43, 31-36.
Wedepohl, K. H., 1995: The composition of the continental crust.- Geochimica et Cosmochimica Acta, 59, 1217-1239.