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doi:10.2204/iodp.proc.310.201.2009

Summary

The present results and specifically the lack of shallow euphotic indications in the bioerosion ichnocoenoses through the middle and upper deglacial reef succession can be attributed to three possible causes or their combination:

1. The microbioerosion patterns are in good agreement with the notion of a rapid deglacial sea level rise when considering that only the ichnocoenoses developed in corals at the base of the succession indicate shallow euphotic Zone II to deep euphotic conditions present during relative sea level lowstand. The deepening-up effect is demonstrated by the ichnocoenoses found in corals, coralline algae, and microbial crusts from the middle and top ranges of the deglacial succession, which reflect dysphotic conditions due to reef growth progressively lagging behind sea level rise (Fig. F16). Furthermore, encrustation of the coral framework by microbialites soon after the demise of the corals is indicated by the similar photic conditions in corals and microbialites as indicated by ichnocoenoses.
2. Most of the recorded ichnocoenoses were not established in deeper waters but partly in cryptic and shaded areas of the otherwise photic reef (dead underside of corals, small cavities, shaded crevices, etc.) and thus are better labeled cryptophotic. This particularly applies to the cases of algal crusts and microbialites that—at least at the base of the reef succession—show ichnotaxa, indicating a lower illumination state if compared to encrusted coral skeletons. However, the implications on the timing of microbialite versus coral growth is not entirely conclusive because the contemporaneous development in the cryptic scenario can not be distinguished from the delayed microbialite growth after sea level rise, based solely on the ichnocoenosis composition.
3. Another potential factor capable of causing low illumination states in relatively shallow water is a decrease in water transparency due to an increase in nutrient level and sediment input by local river runoff. This model would explain the co-occurrence of mesotrophic/​eutrophic microbialites on the one hand and zooxanthellate corals (weakened by rapid sea level rise?) on the other.

The relative importance of these possible causes will be evaluated when integrating other data such as reliable radiocarbon dating and sedimentological proxies in further studies (cf., Heindel et al., in press). Nevertheless, the sea level rise scenario combined with elevated nutrient levels are tentatively considered the primary factors because entirely cryptophotic conditions of the sediment cores are less probable.

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