MOLY-OAES

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

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 Obiettivo del progetto (Objective)

'The main goal of this proposal is in the investigation of the relationship between major perturbations in the carbon cycle and the global extent of euxinia during three Cretaceous oceanic anoxic events (OAEs), with important implications for the temporal evolution of ocean chemistry and for the better understanding of interactions between environmental change and the physical state of the oceans. By providing detailed analyses of both Tethyan key sections and ODP legs, we seek to better constrain the onset and the duration of reducing conditions and relate these changes to the perturbations of the global carbon cycle during the Valanginian, the early Aptian and the Cenomanian/Turonian OAE. We propose to investigate the behaviour of molybdenum (Mo) isotopes during these three periods of time. Mo isotopes provide a method to assess whole ocean chemistry from a limited numbers of sites. In the modern ocean, the preferential removal of the lighter isotopes under oxic conditions dominates, leading to a heavy Mo isotope signature for residual Mo in seawater. During OAEs, the emplacement of a larger completely anoxic sink diminished the dominance of oxic sinks. As a consequence the Mo isotope signature of seawater decreased towards that of the input. Thus, marine sediments deposited under euxinic conditions are thought to record the Mo isotopic composition of seawater at that time. From this, it is possible to estimate the proportion of euxinic areas in ancient oceans using a quantitative model of Mo isotopic balance. The importance of this project lies in the observation that Cretaceous OAEs are related to phases of rapid global environmental change. This is all the more important since most of the processes to explain the origin of OAEs during the Cretaceous - such as global warming, increased continental weathering, increased oceanic productivity and the disappearance of reef systems are observed today as the consequence of anthropogenic change.'

Introduzione (Teaser)

Researchers devised a new method that uses molybdenum in ocean floor sediments to estimate global oxygen levels in the ocean during periods of global climate change.

Descrizione progetto (Article)

Oceanic anoxic events (OAEs) are historic periods of low oxygen in the ocean, which scientists can detect in specific types of sedimentary rock. While this method can identify local hotspots of euxinia (complete lack of dissolved oxygen), there has been no reliable way to test for ocean-wide euxinia.

The EU-funded MOLY-OAES project developed a method to test ocean-wide euxinia using isotopes of the element molybdenum (Mo). The first site sampled was not suitable for testing this process because conditions were not fully euxinic. The other three sites, however, were suitable for testing the molybdenum proxy method.

By comparing molybdenum isotopic composition with that of the modern ocean, researchers showed that the method was suitable to evaluate climate conditions. The new method allowed researchers to better understand climate conditions associated with these Cretaceous OAEs.

A major finding was that ocean oxygenation fluctuated rapidly during the OAEs, suggesting that the global ocean was resilient to certain amounts of environmental change. The other important finding was that for one of the OAEs there is evidence for a euxinic environment that expanded and moved over time.

The work of MOLY-OAES has expanded knowledge of OAEs in Cretaceous oceans. It has also provided a tool to better understand how oceans are influenced by climate change.