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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Fig. 1: Plot of natural logarithm of 210 Pb with depth for the
four samples from the Sinularia pedestal collected at
Nukubuco.
132
#
Sample
Depth
(CM)
Age
(years)
1 0-10 7.14
2 10-20 14.29
3 20-30 21.43
4 30-40 28.6
Fig. 2: Age of 40 cm long Sinularia pedestal
using 210Pb activity.
straight up. Alternatively, later cementation has increased the thickness of the column which contains 30-
40% of cements that formed after excretion of the spindles. Because bulk samples were used for the age
dating it could also be that the lower portion of the pedestal contains younger cements which distort the
results slightly. SCHUHMACHER (1997) calculated a growth rate of 3-8 mm per year from the banding pattern
in a spiculite from the Red Sea. The species was S. minima rather than S. polydactyla but the growth rate is in
good agreement with our specimen.
Polished thin sections were used to unravel the sequence of cementation which enables the build up of
spiculitic rock. Petrographic studies showed that the first cement formed on the spindles was an aragonitic
botryoidal cement. Later followed fibrous needle cements (aragonite) and high Mg blocky cements. The
blocky cements are less abundant in the upper part of the pedestal and more abundant in the lower part,
suggesting that occlusion of the remaining pore spaces is a continuous process.
Electron Probe Microanalyses (EPMA) of a polished slab of Sinularia spiculite were made using a JEOL
733 Superprobe. The selected area of spiculite was a segment of a spindle with overgrowing botryoidal
cement 80 µm 2 in size (Fig. 3, C). The distribution of the 6 elements Ca, Mg, Na, Si, Sr and Zr was mapped.
The analysis confirmed that the Sinularia spindles are composed of high Mg calcite. The presence of Si and
Zr is very low and the elements are evenly distributed throughout the sample with no apparent trend. The
Mg shows a concentric zoning pattern with increasing concentrations from the interior of the spindle to the
surface. Mg is practically absent from the extra-spicular area of the analysed segment which consists of a
botryoidal cement. This cement is high in Ca and also Sr. Mg shows negative correlation with Ca (R 2 >0.99)
suggesting Ca/Mg substitution in the carbonate phase. Sr is very low in the spindles and higher in the
cements, suggesting that the cement is aragonitic. For the zoning observed in the image of the spindle, the
dark bands correlate to a higher Mg content with the highest Mg content in the dark outermost layer of the
sclerite. The botryoidal cements are also zoned (Fig. 3, B) but the Sr content does not conform with the
banding but instead is evenly distributed.
Distribution of Mg suggests that spindles are composed of two different calcite types: a thin, high-Mg
outer layer, and primary inner low-Mg layers. The data could suggest that two biomineralization pathways
are employed in the formation of the different calcite types. However, in foraminifera the strong
concentration in Mg is associated with organic matter or primary organic membranes (POM) (e.g.
HEMLEBEN et al. 1986; BENTOV & EREZ 2005). Magnesium is known to exert a significant control on calcium
carbonate precipitation and, when present in sufficient amount, the precipitation of aragonite rather than
calcite results (REDDY & WANG 1980; MUCCI & MORSE 1983). This might explain the presence of botryoidal
aragonite cements as the first phase of cementation following excretion of the spindles.
COHEN et al. (2002) showed that significant distortion of the Sr/Ca temperature record in coral skeletons
occurs in the presence of algal symbionts. In Porites skeletons only the cements formed during night time