A Phosphorus Control on Oceanic Anoxic Events under Greenhouse Climates?

Project Background:
Episodes of widespread marine oxygen depletion and enhanced organic carbon (OC) burial, which are often associated with mass extinctions, are a common feature of the geological record, as well as being a threat to the modern marine ecosystem as ‘oceanic dead zones’ progressively spread in response to global warming. These events commonly lead to the formation of extensive marine black shales, but the combination of factors that contribute to the initiation, maintainance, and cessation of oceanic anoxic events (OAEs) are poorly understood.

In many cases, enhanced OC burial was likely driven by increased primary productivity stimulated by enhanced availability of nutrients. This drove increased OC respiration, thus forcing water column de-oxygenation. As the ultimate limiting nutrient on geological timescales, the oceanic influx of phosphorus (P) from both terrestrial weathering and recycling from marine sediments is thought to play a central role in initiating and maintaining ocean-wide anoxia. However, the relative importance of P inputs from continental weathering versus internal oceanic recycling, and links to other key elements such as iron (Fe) and sulphur (S), before and during OAEs, remains unclear.

Aims, Objectives and Key Hypotheses:
This project will test the hypothesis that the continental weathering supply of P to the open ocean is modulated by recycling in shelf seas, with the processing of P on the shelf essentially acting as a ‘gatekeeper’ of P supply to the open ocean. The precise conditions on the shelf controls the behaviour of the P cycle, and hence the spread and intensity of detrimental water column conditions.

Specifically, the project will produce high resolution, multi-proxy records targeting well-preserved, strategically located marine cores from shelf and deeper margin/basin sites, covering two contrasting greenhouse scenarios of differing intensity, to test orbital effects on:
1) Redox state: A state-of-the-art multiproxy approach will be used to distinguish between key water column redox states linked to potential perturbations of the Fe and S cycles.
2) Weathering intensity: Elemental ratios will be applied to investigate whether changes in terrestrial weathering intensity drove changes in ocean redox.
3) Response of the P cycle: P speciation will be utilized in order to understand the ocean biogeochemical response to changes in weathering and ocean redox.

These novel, multi-proxy records will be evaluated via biogeochemical modelling to unravel feedbacks between weathering, redox and P cycling on the shelf and in the deep basins. We anticipate that this combined approach will lead to a step-change in our understanding of some of the most dramatic perturbations to the Earth system of the last 120 million years. This, in turn, will inform on generic processes that likely operated during other episodes of ocean anoxia under past, and potentially future, greenhouse conditions.

Methodology:
Five OAE intervals will be studied, each consisting of ~200 samples at 2 cm resolution. This will result in comprehensive data sets covering a representative range of shallow and deep water environments at the onset and termination of two contrasting OAEs. Water column redox will be obtained for all samples using Fe speciation (Poulton and Canfield, 2005) to distinguish euxinic, ferruginous, and oxic water column conditions. Trace metal ratios (e.g., Mo/Al; U/Al; V/Al; Re/Mo) will be used to provide additional detail on redox state (e.g., strongly euxinic, weakly euxinic, suboxic (e.g., Kendall et al., 2010).

The speciation of P provides critical information on P fluxes and redox-driven recycling (e.g., März et al., 2008). We will apply a revised P speciation technique to all samples (Thompson et al., 2019), which gives Fe-bound P (PFe; P associated with Fe oxides or Fe(II) phosphates), organic P (Porg), authigenic P (Paut; dominantly carbonate fluorapatite) and detrital P (Pdet). The P that was potentially reactive (Preact) is defined as PFe + Porg + Paut. This partitioning provides unprecedented insight into P cycling.

These new data will form the basis for updating an existing box model of P and O2 dynamics across the shelf, open surface ocean and deep ocean, which has previously been applied to OAEs (Tsandev and Slomp, 2009). The model will be used to provide a global context to the geochemical data, to provide a test of the processes that drove, maintained and terminated anoxic conditions in the ocean.