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Oceanic anoxic event conundrums: reconciling palaeontology and geochemistry

Dr Crispin Little (SEE), Professor Simon Poulton (SEE), Dr. Christian März (SEE), Dr. Fiona Gill (SEE)

Project partner(s): British Geological Survey (CASE)

Contact email: earctsl@leeds.ac.uk

One of the many detrimental effects of future climate warming will be the expansion of oxygen minimum ‘‘dead zones’’ in shallow marine areas, leading to the loss of commercially important fish and invertebrate stocks (1). General circulation models predict that climate change will directly deplete oceanic dissolved oxygen levels by increasing stratification and warming, as well as indirectly by causing changes in rainfall patterns, nutrient run-off and shelf eutrophication; all of which will increase marine areas affected by hypoxia and anoxia. Hypoxia can occur at a variety of temporal and spatial scales. Only the smallest temporal and spatial scales may be readily observed or recreated experimentally, and it is unclear whether these results are applicable at larger scales. Data from the largest temporal, spatial and ecological scales can, however, be sourced from the fossil record, which provides an archive of natural data from a number of past episodes of climatic and environmental change. A detailed fossil record with good temporal resolution that spans past climate change events can help in forecasting future ecosystem changes, especially if predicted climate changes move outside the parameters experienced by modern ecosystems and into regimes known only from the deeper geological record. Rocks spanning the Pliensbachian-Toarcian interval of the Early Jurassic (185-181 Ma) are an archive of natural data from one of these past episodes of global warming and anoxia (2). Temperatures are estimated to have increased by 2–3.5 degrees C in subtropical areas and 6–8 degrees C at higher latitudes, values which are similar to the increases forecast for the end of the 21st century. In many early Toarcian shallow, epicontinental basins worldwide, laminated, organic-rich, black shales (Fig. 1), which formed under reduced oxygen conditions, were deposited, and so this event is widely referred to as the Toarcian Oceanic Anoxic Event (TOAE)(3). In some localities, there is evidence that anoxia temporarily spread into the lower photic zone, together with euxinia, as indicated by the presence of biomarkers of green sulphur bacteria in some black shales. Marine ecosystems were adversely affected by these climate driven environmental changes, and early Toarcian strata record a major extinction of marine organisms, particularly amongst the infaunal benthos. However, for the TOAE (as for many Oceanic Anoxic Events) there are significant discrepancies in the estimation of water column and sediment oxygenation using palaeontological and geochemical data (4). For example, prior to the main TOAE laminated black shale event there are several shorter intervals of laminated black shale deposition (Fig. 2), which are not associated with basin-wide extinction events, and within the main thickness of Toarcian black shales containing the geochemical evidence for greatest oxygen reduction, including photic zone euxinia, are numerous shell beds of epifaunal benthic bivalves (usually mono-specific).

Figure 1. Cliff exposure of lower Toarcian bioturbated grey shales (recessed) and laminated black shales (projecting), Kettleness, N. Yorkshire. Figure 2. Transition from laminated black shales to bioturbated grey shales, ‘Sulfur Band’, Kettleness, N. Yorkshire.

Aims and outcomes:

The project will investigate the sometimes contradictory interpretations of sediment and water column redox states obtained from palaeontological and geochemical analysis of black shale sequences during the TOAE, where the geochemical evidence suggests greater oxygen restriction than the fossil evidence. It will do so by studying rapid redox transitions from massive or bioturbated (oxygenated) shales into laminated black shale (oxygen-depleted) facies, using benthic macrofauna combined with modern geochemical redox proxies, including Fe abundance (Fe/Al) and Fe-S speciation, trace metal (e.g. U, Mo, Re and V) enrichments, and the biomarkers for green and purple sulfur bacteria (e.g. okenone and isorenieratane), as well as applying this methodology to ‘event layers’ of benthic macrofauna within laminated black shale facies. The issues may relate to the temporal resolution that the proxies (palaeontological and geochemical) are recording in the sediment, which can likely be resolved by sampling at high resolution (cm to mm). The focus of the study will be Toarcian material from outcrops on the Yorkshire Coast (Figs. 1&2), supplemented by samples from existing (e.g. Schandelah-1, N. Germany) and soon-to-be-drilled Pliensbachian-Toarcian core material (e.g. Mochras 2, N. Wales).

The expected results from the project will be: (i) improved understanding of the response of benthic animals to rapid redox transitions; (ii) resolution of the common mismatch between palaeontological and geochemical interpretations of redox in laminated black shale facies in Mesozoic Oceanic Anoxic Events; (iii) better knowledge of what ‘event layers’ of benthos mean in a basinal content.

Potential for high impact outcome

The project addresses the NERC societal challenge of ‘managing environmental change’ as the triggers, temporal and geographic extent of future oxygen minimum ‘‘dead zones’’ in shallow marine areas are poorly understood.

Training

The project is interdisciplinary and the student will work within the Earth Surface Science Institute (ESSI) under the supervision of Dr. Crispin Little (macrofossil palaeontology), Professor Simon Poulton (Fe-S speciation), Dr. Christian März (trace metals) and Dr. Fiona Gill (organic geochemistry). As a member of two research groups within ESSI (Palaeo@Leeds and Cohen Geochemistry), the student will have access to a broad spectrum of relevant expertise, which will be supplemented by an extensive range of research and personal development workshops delivered by the University of Leeds, from numerical modelling, through to managing your degree, and preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).

References

(1)  UNEP, United Nations Environment Programme website (2004) GEO Year Book 2003. GEO Section/UNEP, Nairobi. Available: http://www.unep.org/yearbook/2003/.

(2)  Danise, S., Twitchett, R.J., and Little, C.T.S. (2015) Environmental controls on Jurassic marine ecosystems during global warming. Geology 43:263-266.

(3)  Wignall, P.B., Newton, R.J. and Little, C.T.S. (2005) The timing of paleoenvironmental change and cause-and-effect relationships during the early Jurassic mass extinction in Europe. Am. J. Sci. 305: 1014-1032.

(4)  Friedrich, O. (2010) Benthic foraminifera and their role to decipher paleoenvironment during mid-Cretaceous Oceanic Anoxic Events – the ‘anoxic benthic foraminifera’ paradox. Rev. Micropal. 53: 175–192. 

Related undergraduate subjects:

  • Geoscience