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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position of growth and it is unlikely that all corals sampled from this zone would be redeposited from
older parts of the reef. Hence, this dating anomaly may correspond with a zone of increased marine
diagenesis that appears to have affected the U-Th dating system so as to make corals appear to be too old.
Such an increase in U-series age could result from U loss or Th addition; either process would result in an
apparent increase in the 230 Th/ 238 U ratio and thus the calculated 230 Th age from the ratio would be too old.
However, in this case, the dated corals also contain high U values, making leaching of U an unlikely culprit.
As this uppermost reef zone contains abundant microbialites (WEBB & JELL 1997) and the microbialites
contain high Th concentrations rendering them unsuitable for U-series dating (Webb & Jell 2006)
microbialite contamination could be a problem. However, the dated corals contain very little 232 Th,
suggesting against incorporation of a large amount of microbialite-derived Th. Hence, against expectation
there appears to be direct evidence of Th open-system behaviour that allowed preferential 230 Th
mobilisation and enrichment in this environment. It is likely that this zone of intense and unexpected
diagenesis may reflect long-term suspension of corals immediately below the reef flat within the intertidal
zone during the relatively long ‘still-stand’ when local sea-level was maintained at or near its current
elevation since ~7 ka (LEWIS et al. 2007; YU & ZHAO 2010). Coral skeletons that occur in this ‘still-stand’
environment for long intervals of time may be affected and have unreliable dates.
Fig. 1 U-Th age-depth profiles for five cores at 5 m
intervals on the leeward margin, western end of Heron
Reef. Ages increase with depth except within upper
~1.5 m (gray band), where they increase in apparent
age towards the surface. This apparent dating anomaly
may reflect open system Th behaviour in the
diagenetically very active ‘stillstand’ zone. Age error
bars are less than the width of symbols.
Shallow reef cores are only very rarely collected from closely spaced transects that can identify
aggradation versus progradation ratios and many reef cores have only limited numbers of U-series dates in
vertical sequence. Hence, zones of anomalous dates may not be recognised in some cases and some coral
dates previously used for refining sea level curves, reef aggradation rates and for documenting reef growth
models could be affected. Similar problems could exist for older corals obtained from current active reef
flats. Hence, although the processes that might be responsible for preferential 230 Th uptake in coral
skeletons are under investigation, recognition of the potentially significant impacts for both dating and
environmental proxies of the ‘still-stand’ diagenetic zone in ancient and modern coral reefs is critical.
LEWIS, S.E., WÜST, R.A.J., WEBSTER, J.M. & SHIELDS, G.A. (2007): Mid-late holocene sea-level variability in eastern
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NOTHDURFT, L.D. & WEBB, G.E. (2009): Earliest diagenesis in scleractinian coral skeletons: implications for
palaeoclimate-sensitive geochemical archives. - Facies, 55: 161-201.
NOTHDURFT, L.D., WEBB, G.E., BOSTROM, T. & RINTOUL, L. (2007): Calcite-filled borings in the most recently deposited
skeleton in live-collected Porites (Scleractinia): Implications for trace element archives. - Geochimica et
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WEBB, G.E. & JELL, J.S. (1997): Cryptic microbialite in subtidal reef framework and intertidal solution cavities in
beachrock, Heron Reef, Great Barrier Reef, Australia: preliminary observations. - Facies, 36: 219-223.
WEBB, G.E. & JELL, J.S. (2006): Growth rate of Holocene reefal microbialites – implications for use as environmental
proxies, Heron Reef southern Great Barrier Reef. - Extended Abstracts, Australian Earth Science Convention,
Melbourne, 2-6 July, 2006, Searchable Compact Disc (ISBN 0-646-46265-2) doi: 10.1071/ASEG2006ab191.
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