Aretz et al_2011.pdf - ORBi - Université de Liège

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Kölner Forum Geol. Paläont., 19 (2011)
M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.)
Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, Liège, August 19-29, 2011
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Alcyonarian spiculites as possible proxy climate archives: Preliminary
results
Susanne POHLER 1 , Zulfikar BEGG 2 & John COLLEN 3
1 School of Marine Studies, Faculty of Science, Technology and Environment, University of the South
Pacific, Private Mail Bag, Suva, Fiji Islands; pohler_s@usp.ac.fj
2 Applied Geoscience and Technology Division (SOPAC), Secretariat of the Pacific Community, PMB GPO
Suva, Fiji Islands; zulfikar_begg@yahoo.com
3 School of Geography, Environment and Earth Sciences, PO Box 600, Wellington, New Zealand;
John.Collen@vuw.ac.nz
Alcyonarian spiculite (KONISHI 1981) is a carbonate rock built by a few soft coral species, notably
Sinularia minima VERSEVELDT 1971, Sinularia polydactyla (EHRENBERG 1834), Sinularia leptoclados (EHRENBERG
1834) (the latter two are common in the tropical Indo-Pacific) and possibly Lobophytum pauciflorum
(EHRENBERG 1834) (PAULAY & BENAYAHU 1999). The corals excrete 1-3 mm long sclerites (also called
spindles or spicules) of high Mg calcite from the base of the stalk which become cemented by marine
cements as the coral grows upwards (Fig. 3, A, B). In the tropical Pacific Ocean pedestals up to 1.5 m high
with living Sinularia colonies on top have been described in the literature and by eye witnesses (CAREY
1931, SCHUHMACHER 1997, R. KELLEY, written com. 2008). Spiculites were found in sediment cores as old as
7,500 years (KLEYPASS 1996, southern GBR). ACCORDI et al. (1989) reported Quaternary Alcyonarian
spiculites from the coast of Somalia. The spicules (or sclerites) are cemented soon after deposition by
several generations of aragonite and high-Mg calcite cements. X-ray analyses of sawn slabs of spiculite rock
show that the spicules are arranged in layers and that density bands are present (Fig. 3, D). These bands
may contain paleoclimate information enclosed in either cements or spicules or both, similar to hard corals.
Sinularia spiculites were investigated at three different sites in Viti Levu, Fiji: Caqalai, Nananu-i-ra and
Nukubuco. Analyses of the sclerites show that all spiculites are formed by S. polydactyla (EHRENBERG 1834).
The soft coral cover at the three different sites can be as high as 30 to 40 percent of the total coral cover. The
tallest pedestals, 90 to 100 cm high, were found on the reef flat of Caqalai Island. A 40 cm long pedestal
with a Sinularia colony on top was harvested from the Suva Barrier Reef and sectioned. The polished slab
was X-rayed and showed 10 dark and ten light bands in the lower portion (10 cm long).
The spiculite samples were sent to Core Scientific International in Canada for 210 Pb analysis. Age
calculations were completed for the 40 cm long Sinularia pedestal using the Constant Flux and Constant
Sedimentation rate (CF: CS) model. Sedimentation rate is calculated from the slope of the line derived from
the linear regression of ln 210 PbUNSUPPORTED and sample depth in the equation as shown by KRISHNASWAMY et
al. (1971) (Fig. 1). The equation is further simplified to sedimentation/accumulation rate (cm/year) =
(radioactive decay coefficient of Pb 210 : 0.03114) / slope of the linear relationship.
Using the (CF:CS) model and slope regression method, a slope was obtained from a linear curve formed
by logarithm of excess 210 Pb activity which was plotted against depth on horizontal axis. The equation from
Table 1 is y = -0.04 (x) -0.412, therefore the slope was -0.040 with a R 2 value of 0.865. Accumulation rate
(growth rate in this context) was then calculated and found to be 0.7cm/year using the aforementioned
equation. Age of the sample was derived from equation; T= m/s where m- cumulative dry weight of
sediment or sample depth and s is the sedimentation rate derived from the slope of the linear regression, T-
Age of the sample. Since the depth ranged over an interval of ten centimeters, the midpoint of the two
depth values were used for calculation. For example for the first sample, the depth was from 0-10 cm and
therefore 5 cm was used in place of m (in the equation) and divided with the growth rate of 0.7 cm/year to
give an age of 7.14 years (Fig. 2).
The results suggest that the observed banding could be annual, similar to the density bands found in
some hermatypic corals such as Porites. Although a 7.14 year old pedestal growing at 0.7 cm pa should be
only 5 cm high, the distortion probably stems from the fact that the cut through the pedestal was oblique
because the growth direction of the animal seems to change over time and often goes sideways instead of
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