Past and future ocean productivity: Using ancient extinction crises to inform how we respond to today’s threats

Map showing regions at risk from eutrophication
Coastal areas experiencing eutrophication and hypoxia (1-30% dissolved oxygen saturation) including some of the world’s most fragile and economically important ecosystems (Diaz & Sellman, 2010).
The nitrogen cycle has been pushed beyond Earth's tolerable boundaries even more than its climate
This image from Johan Rockström et al., 2009, Nature, shows that Earth’s nitrogen cycle has already been pushed well beyond its tolerable limits. The inner green shading represents the proposed safe operating space for nine planetary systems. The red wedges represent an estimate of the current position for each variable. The boundaries in three systems (rate of biodiversity loss, climate change and human interference with the nitrogen cycle), have already been exceeded.

The World’s ocean ecosystems may change fundamentally in the next few centuries due to the severe threats posed by present-day human activities which together are changing natural systems like never before. The Intergovernmental Panel for Climate Change modelling of future oceans predicts that a variety of extreme environmental and ecological changes will occur if we continue on our current trajectory. These changes include an imminent “productivity crisis” due to changes in the availability of the essential nutrient nitrogen (N) discharged from agricultural practices, wastewater treatment, fossil fuel and biomass combustion. Nitrogen drives eutrophication – the over-fertilization the oceans – and changes patterns of global marine primary productivity (Fig. 1), causing proliferation of harmful algal blooms and the depletion of ocean oxygen. The dramatic increases in N discharges since pre-industrial times have altered the balance of the marine fixed-N inventory and will drive future primary productivity changes as global populations, and so demand for resources, continue to grow. Confounding factors associated with climate change are likely to fundamentally change the amount and types of primary and secondary productivity in continental-margin ecosystems (e.g. more bacteria, less fish). These synergistic stresses are predicted to dramatically reduce already diminished biodiversity with catastrophic results for marine ecosystems and the services they provide to humans (e.g., fisheries, coastal protection). Such changes will also destabilise the carbon cycle, with major, unpredictable implications for atmospheric CO2 and global warming (dead biomass is sequestered as carbon in the oceans, reducing the amount in the atmosphere). Of great concern is that we do not know how, and over what time, ecosystems will respond to change, as current models are inadequate.

Earth history records numerous examples of N-cycle perturbations that probably occurred – and had impacts persisting over timescales that were orders of magnitudes longer (millions not thousands of years) than those predicted for the present-day oceans. These ancient crises are associated with the greatest mass extinctions, suggesting a causal link between these phenomena. Models for the modern oceans might hugely underestimate the temporal, spatial, and ecological scales of the impending crisis.

There is a clear mismatch between the forecasts for the future and ancient ocean: it has been suggested that atmospheric N fixation will relieve N limitation in modern oceans, stimulating primary production that in turn sequesters carbon into the oceans, buffering rising atmospheric CO2 concentrations. In contrast, runaway greenhouse conditions have occurred multiple times in deep time (e.g. Early Triassic, 250 million years ago) and were associated with changes in ocean upwelling patterns along continental margins, a reduction in marine productivity and a deepening of the nutricline, reducing the rate of delivery of nutrients to the photic zone, suppressing biodiversity and carbon burial. In order to predict the impact of nutrient changing dynamics on the future oceans, and the global climate they regulate we need to understand the long-term interactions between global warming, productivity, and the global C and N cycles. If the events of the past do play out in the modern oceans, the impending “productivity crisis” will be the greatest threat to life on Earth.

This project will evaluate the causes and consequences of past N-driven productivity crises in the geological record, and determine their links to multiple marine mass extinction events through novel and standard geochemical and palaeontological methods. The data will inform predictions about the Earth’s future.

Aims and objectives

Recent modelling predicts an impending productivity crisis driven by anthropogenic atmospheric N discharge acting synergistically with climate change and lasting for thousands of years. The damage to global biodiversity is likely to be catastrophic. To understand if these predictions are accurate, we need to examine past examples of productivity crises to calibrate our expectations. This demands several research questions:

1)         Have marine productivity crises happened in the geological past – and if so what was their link to mass extinction events (are they e.g. a common feature of these)?

Revealed by geochemical and palaeontological studies of marine sequences deposited in palaeo-continental margin settings (sites of upwelling and flourishing life) across major, but stylistically contrasting extinctions: the Late and End Devonian, Middle and End Permian, and Early Jurassic catastrophes.

2)         How environmentally widespread and long-lasting were past productivity crises?

The above establishes a temporal framework for records of past productivity crises. Analyses from a range of marine settings will establish the spatial extent and variability of these events.

3)         What environmental and climatic factors drove past productivity crises?

Further geochemical and sedimentological analyses of the sample suite, complemented by geochemical modelling, will test for common factors in the development of productivity crises.

4)         What was the impact of past productivity crises on ecosystems?

Examined through comparison of the geochemical and sedimentological records to those of faunal losses and recovery during extinction events. Although it is difficult to disentangle the effects of global warming, acidification and declines in ocean oxygen levels from those of productivity, the project will test whether productivity crises are a common, causal or consequential factor in major mass extinction events.

5)         What do these questions mean for models and predictions of Earth’s near future?

The results will be used to verify modern models (e.g. IPCC), evaluating whether predictions of near-future productivity crises vastly underestimate their potential temporal and spatial scale.

Skills that the student will acquire

  • Field collection of new samples for geochemical and palaeontological analyses from marine palaeo-continental margin settings (e.g. western N. America, S. America, Europe) will be complemented by sampling from archive materials at the universities of Hull, Leeds and the Geological Survey of Canada.


  • Mixed geochemical methods including analysing N and Cd isotopes (by GS-IRMS and ICP-MS), trace metals (e.g. biogenic barium [Babio], by ICP-MS and XRF), phosphorus concentrations (by sequential extraction) and total organic carbon (analysed by Rock Eval™) as proxies for marine productivity. These are supported by excellent facilities in Hull, Leeds and Canada, including state of the art mass spectrometry, XRF, elemental analysers, Rock Eval™, and electron microscopy.


  • Constraining extinction losses and timing for each of the five extinctions under scrutiny relies on fossil identification in the field and thin section. The student will be trained in the taxonomic identification of fossil invertebrates. Linking together these independent changes reveals the timing and consequences of change. These techniques complement the novel geochemical analyses, and are supported by thin section laboratories at Hull and Leeds.

The approach and the scale of the project is highly ambitious, but made feasible by (1) the extensive expertise of the team; (2) the outstanding repository of samples already available; and (3) the excellent, international-standard geochemical facilities available at the institutions involved.