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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“Still-stand” Diagenetic Zone, Heron Reef, Great Barrier Reef:
Implications for Dating, Reef models, Sea Level Reconstruction and
Environmental Records
Gregory E. WEBB 1 , Luke D. NOTHDURFT 2 , Jian-xin ZHAO 3 , Gilbert PRICE 1 , & Bradley
OPDYKE 4
1 School of Earth Sciences, The University of Queensland, Brisbane QLD 4072, Australia; g.webb@uq.edu.au,
g.price@uq.edu.au
2 Biogeosciences, Queensland University of Technology, GPO Box 2434, Brisbane QLD 4001, Australia;
l.nothdurft@qut.edu.au
3 Radiogenic Isotope Laboratory, Centre for Microscopy and Microanalysis, University of Queensland, St.
Lucia 4072, Qld, Australia; j.zhao@uq.edu.au
4 Earth Environment, Research School of Earth Sciences, The Australian National University, Canberra ACT
0200, Australia; Bradley.Opdyke@anu.edu.au
Data from reef flats and shallow reef cores provide the basis for many Quaternary sea level curves,
Holocene palaeoclimate reconstructions and reef growth models generally. Scleractinian coral skeletons are
ideal for such studies as they are ecologically constrained to specific bathymetric ranges and some coral
growth morphologies, such as microatolls, potentially provide high-resolution sea level data. Additionally,
coral skeletons can be dated using both radiocarbon and U-series techniques, they have annual growth
banding, and they preserve isotopic and trace element proxies for environmental parameters such as sea
surface temperature, salinity, and productivity. Hence, coral skeletons provide a wealth of data for charting
and understanding late Pleistocene and Holocene palaeoclimate. However, it has long been appreciated
that geochemical data obtained from corals are subject to the quality of preservation of the skeletal
aragonite and that meteoric diagenesis typically disrupts original geochemistry. Meteoric diagenesis
involving the stabilisation of original skeletal aragonite to calcite in particular has been shown to disrupt
trace element and isotope geochemistry so as to corrupt environmental proxies and make dates obtained
from such corals unreliable. Recent investigations of living coral skeletons (NOTHDURFT & WEBB 2009) and
shallow coring beneath the surface on the leeward margin of Heron Reef, southern Great Barrier Reef
suggest that a zone of relatively intense marine diagenesis may exist immediately below the reef flat and
marine diagenesis within this zone may have implications in particular for U-series dating.
A transect of five shallow(~4-10 m) cores was obtained at 5 m intervals from the same area of the
leeward reef margin of Heron Reef as NOTHDURFT & WEBB (2009) demonstrated a variety of diagenetic
processes in living coral skeletons on the reef flat. The skeletons of living corals were found to contain
abundant aragonite, high Mg-calcite, low Mg--calcite and brucite cements suggesting a relatively extreme
diagenetic environment. The high degree of diagenetic alteration was attributed to the intertidal position
with abundant and frequent wave and tidal pumping of water masses, temperature extremes, evaporation,
CO2 degassing and, significantly, biological activity. In some cases, observed diagenetic effects could
seriously compromise environmental proxies, such as calcite-filled borings, which could severely impact
Sr/Ca coral palaethermometry (NOTHDURFT et al. 2007). The zone immediately below the reef flat is also
characterised by high degrees of diagenetic alteration as generally evidenced by enhanced lithification by
cryptic microbialites and a variety of marine cements.
Corals were selected from the shallow cores for dating on the basis that they were apparently in growth
position and were well preserved with little cement and free of obvious microbialite. Corals were dated
using the U-Th technique with elemental measurements by thermal ionisation mass spectrometry (TIMS) at
the University of Queensland. Corals were found to have relatively high U concentrations and dates ranged
from ~4 to 7.5 ka with most dates increasing with depth as expected. However, above depths of ~1.5 m
coral dates show a significant deflection in the wrong direction (i.e., above 1.5 m depth corals appear to
increase in age towards the surface; Fig. 1). Although such a pattern could be produced by accidental
dating of redeposited, older coral debris, which can occur on reef flats, all samples appear to be in their