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Global redox state of the ocean during Cretaceous oceanic anoxic events: new insights from molybdenum isotopes

Final Report Summary - MOLY-OAES (Global redox state of the ocean during Cretaceous oceanic anoxic events: new insights from molybdenum isotopes)

The chemistry of the ocean is closely linked to the global carbon cycle by several feedback loops. Understanding changes through Earth’s history is therefore of fundamental importance in trying to understand how the Earth reacts to climate forcing. Though the oceans have maintained generally high levels of oxygen in the last past 540 Ma, the rock record is punctuated by episodes of widespread deposition of black shales (laminated and organic-rich sedimentary rocks). These deposits are related to rapid perturbations in the carbon cycle and environmental changes, defining Oceanic Anoxic Events (OAEs, Schlanger and Jenkyns, 1976). Even though each OAE may have its own triggering mechanism, a general scheme involving eutrophication of seawater, and a slowdown of carbonate production relative to organic carbon production can be put forward (e.g. Meyers, 2006). One long-running controversy surrounding OAEs is whether these events are truly global in extent or are restricted to poorly-ventilated marginal deep basins. This is the main target of this research project, with particular reference to the relationship between the development of oceanic euxinia (anoxic and sulfidic conditions), the significantly increased burial of organic carbon that results, and the link to global perturbations of the carbon cycle recorded in carbon isotopes. The geochemistry of molybdenum (Mo), and its isotopes, has emerged as a key new paleoceanographic tool, with potential for probing the redox state of the global ocean in the past and its relation with the global carbon cycle (Barling et al., 2001; Anbar and Rouxel, 2007; Archer and Vance, 2008).

We investigated four positive carbon-isotope excursions recorded worldwide during the Early to early Late Cretaceous (i.e. the Valanginian, early Aptian, early Albian and Cenomanian/Turonian positive excursions in stable carbon isotopes). In the following, we present and discuss the main results, obtained during this two-year project.

The Valanginian Weissert event (about 136 Ma)
The Valanginian stage is considered to include the first carbon perturbation of the Cretaceous, whose onset is marked by a pronounced increase in the carbon-isotope record, and defines the so-called “Weissert event”. Organic-rich layers from two sites (one from the Tethys Ocean, one from the Pacific), that are usually cited in the literature as examples for strong anoxic conditions during the Weissert event, have been investigated. At both sites, the organic-rich samples from the Weissert Event show evidence for deposition in a weakly reducing environment (suboxic to anoxic, but non-sulfidic conditions). Under such conditions, the Mo isotope proxy cannot be used as a global tracer for the oceanic oxygenation state. This suggests that the Valanginian Ocean was characterized by relatively well-oxygenated conditions and the simplest interpretation would be is that organic-rich sediments are limited to only a few localities worldwide. This also indicates that the conceptual weathering-productivity feedback model usually invoked to explain Cretaceous oceanic anoxic events is not appropriate for this particular event.

The early Aptian OAE 1a (about 125-124 Ma)
The early Aptian corresponds to one of the most studied anoxic events. The sample set studied, from an Italian section representing western Tethys, provided evidence for variations in the oxygenation state of the water column reaching to anoxic/euxinic conditions during the OAE 1a interval. The Mo isotope compositions of these samples are surprisingly light, close to the Mo signature of modern sediments deposited in well-oxygenated marine settings. This suggests that these sediments are recording a strongly reducing local environment, but one with only minor dissolved sulphide.

The early Albian OAE 1b (about 111 Ma)
The early Albian OAE is distinguished from other Cretaceous OAEs by its short duration, the occurrence of a highly specialized biota, and the perturbation of the hydrological regime of tropical regions. For this time interval, the selected set of samples (site from North-Atlantic) provided evidence for their deposition under dysoxic to anoxic conditions in an unrestricted marine system. The Mo isotope signatures obtained shows relatively high values, close to the signature of modern sediments deposited in suboxic/anoxic environments. This suggests that during the early Albian, the oxygenation state of the world ocean might have been similar to the present day. This argues for different mechanisms and feedbacks in the atmosphere–land–ocean system to explain the widespread deposition of black shale layers compared to other Cretaceous OAEs. In order to confirm this interpretation, further studies from other parts of the ocean will be needed.

The Cenomanian/Turonian OAE 2 (about 94 Ma)
Amongst the Cretaceous OAEs, the Cenomanian/Turonian boundary interval (OAE 2) is probably the most extensive event. It is characterized by a globally recognized, stratigraphically distinct perturbation of the carbon cycle. Four sites (located in the western Tethys and the North Atlantic, respectively) were investigated. The redox-sensitive trace metal composition suggests contrasting depositional conditions and paleoceanographic processes in the western Tethys compared to the North Atlantic. Nevertheless, both the western Tethys and the Northern Atlantic sites show redox variations, reaching anoxic/euxinic conditions during OAE 2. In the western Tethys, Mo isotopes show rather low values suggesting that the redox conditions may not have been fully euxinic. In the North Atlantic, Mo isotopes are generally heavier and are consistent with fully euxinic conditions. Our data show how seawater molybdenum isotope ratios may have evolved during OAE 2 and oxygen deficient conditions expanded. Our data also place constraints on the areal extent of marine euxinia during the event.

Despite their intermittent occurrence over geological history, OAEs have an important contemporary relevance. This is because the absolute magnitude and high rates of environmental change during these periods of time were broadly similar to those occurring at the present day. An important finding of this research lies in the observation of rapid variations in seawater oxygenation state during OAEs, at least at a regional scale, suggesting a dynamic response of the ocean to environmental change, especially in terms of the oxygenation state of the water column.

A second point of the work is that, for the Cenomanian/Turonian interval, our data indicate that some samples may have recorded the seawater signature during OAE 2, and suggest an expansion of strongly reducing conditions during this OAE interval. We also provide some constraints on the areal extent of marine euxinia and show that some parts of the ocean were less affected by these perturbations of the carbon cycle.

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Archer, C. and Vance, D. (2008). Nature Geoscience 1, 597-600.
Barling, J., et al. (2001). Earth and Planetary Science Letters 193, 447-457.
Meyers, P. A. (2006). Palaeogeography, Palaeoclimatology, Palaeoecology 235, 305-320.
Schlanger, S. O. and Jenkyns, H. C. (1976). Geologie en Mijnbouw 55, 179-188.