Exploring the controls of Early Aptian Tethyan and proto-Atlantic reefal palaeoecology turnover
Final Report Abstract
This project has a long history and commenced with an early paper recognizing out-of-balance microencruster facies in OAE1a-time equivalent units of the Lower Aptian of Oman. A first tentative interpretation of the environmental drivers for the enigmatic Lithocodium aggregatum and Bacinella irregularis units in Oman was first proposed in Immenhauser et al. (2005) and quite a series of papers subsequently published by colleagues described comparable features from other sections in the Tethyan realm. A number of detailed studies followed, all dealing with various aspects of the epeiric platform top record of oceanic anoxia (here, OAE1a) and its possible relation to microencruster facies. By the time this report is written, a very significant level of detailed knowledge has been compiled, but the mechanism(s) that control a transient demise of rudist-facies and the temporary dominance of microencruster facies remain poorly understood. In this sense and figuratively speaking project should be considered a `successful failure´. Successful, in the sense of publishable output that has triggered debate and feedback from the community, successful in the sense of that the proponents have performed the science they originally proposed to the DFG, successful in the sense that we have learnt, that these fossil shoalwater settings and their extinct biota are highly complex systems that cannot be understood based on simple working hypotheses. A `failure´ in the sense that the central working hypothesis: `transient periods of epeiric seawater anoxia induce mass mortality of coral-rudist ecosystems and allow microencruster facies to fill the empty niches´ is herewith falsified, at least when generalized (verified for the case of the middle Tethyan sections). These problems are partially related to the controversy surrounding the taxonomic origin of these extinct microencruster organisms (Bacinella versus bacinellid fabrics; codiacean alga, sponge, red alga, lituolid foraminifera and ulotrichalean green algae etc.). Further, the redox proxy data compiled from these fossil coastal carbonates are clearly not easy to interpret. We probably know too little with regard to the complex interaction of the effects of contamination, diagenesis and genuine environmental (seawater dissolved oxygen (DO) level) patterns that are all reflected in data presented here. The fact that we find a rather clear pattern of a transient seawater oxygen level decline in the central Tethys (Croatia), but no clear evidence for suboxia in the Proto-Atlantic domain is interesting and intriguing but also shows that there is not a simple relation between microencruster blooms and platform top seawater oxygen levels. Most likely, seawater DO levels are but one, and perhaps not even the most important one, of the factors that control the palaeoecological patterns observed in these sections. For instance, a late Aptian platform-wide shift to (bacinelloid) microbial carbonate production was recently observed by one of the proponents. Considering a newly established integrated stratigraphic framework, the initial phase of microbial proliferation coincides with the final stage of the so-called late Aptian `cold snap´ and the subsequent temperature increase, which was paralleled by a significant sea-level rise. These results contrast with observations from the early OAE1a-equivalent platform carbonates in Croatia, where a similar shift toward microbial `bacinelloid´ carbonate production has been linked to exceptionally warm conditions and hypoxia. In our opinion, the best way forward is to study other locations/time intervals that display comparable microencruster intervals with the tools and methods proposed here. We suggest that future studies should focus on the effects of exceptionally warm SSTs (as proposed for the OAE1a) by applying clumped isotopes. We should consider investigating the role of seawater pH and carbonate carbon saturation state in controlling microbial blooms by means of boron isotopes. A better understanding of the formation of the automicrite and its diagenetic status would also help.
Publications
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(2017) Exploring the Controls of Early Aptian Tethyan and Proto-Atlantic Reefal Palaeoecology Turnover. International Meeting of Sedimentology – Toulouse (France)
Hueter, A., Bodin, S., Huck, S., Immenhauser, A.
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(2018) Anoxic platform top water masses during OAE1a – the carbonate archive. Early Career Sedimentologists Meeting – Hanover (Germany)
Hueter, A., Huck, S., Bodin, S., Heimhofer, U., Immenhauser, A., Weyer, S., Jochum, K. P.
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(2018) Anoxic platform top water masses during OAE1a – the carbonate archive. EGU General Assembly – Vienna (Austria)
Hueter, A., Immenhauser, A., Huck, S., Bodin, S.
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2019, Central Tethyan platform-top hypoxia during Oceanic Anoxic Event 1a: Climate of the Past, v. 15, no. 4, p. 1327-1344
Hueter, A., Huck, S., Bodin, S., Heimhofer, U., Weyer, S., Jochum, K. P., and Immenhauser, A.
