Posted by: oikosasa | August 14, 2013

Lessons from Late Jurassic

What can we learn from the Jurassic when it comes to modern Climate changes? read more in the Early View paper in Oikos Learning from the past: functional ecology of marine benthos during 8 million years of aperiodic hypoxia, lessons from the Late Jurassic by Bryony Caswell and Chris Frid.

Below is Bryony’s background story and summary:

“A few years ago whilst on a field trip Chris and I began discussing the ideas that form the basis for this paper.  To him Jurassic marine systems initially appeared to be very different from those we see today being dominated by exotic large marine reptiles, ammonites, belemnites and fish.  The seafloor however was more familiar in its composition of clams, snails, echinoids and so on.  Modern marine systems depend upon key functions delivered by sea-floor communities such as these.  The ecological functions support and regulate multiple processes in the marine ecosystem such as the regeneration of nutrients, absorption and treatment of wastes, and the provision of food. Our discussions led us to ask how will the functioning of marine systems respond to the rapidly expanding footprint of human pressures, such as climatic change and nutrient runoff, in the longer term? The effects that these pressures exert on the seafloor, and the wider marine system, are not unique to modern marine systems. The Mesozoic oceans suffered from similar, albeit natural, pressures the effects of which manifest as remarkably similar patterns of change.  This observation inspired us to explore the potential changes in the delivery of key ecological processes within the Late Jurassic oceans (~150 million years ago) as an analogue for the changes that we see today.

Our study is the first to quantify changes in ecological functioning of the ancient seafloor. The data we use comes from the Late Jurassic and covers ~8 million years of fluctuating regional ocean de-oxygenation, and with it we investigate changes in the biological attributes that supported the palaeoecological functioning in the Wessex Basin, Dorset, UK. The fossilised remains of the Late Jurassic seafloor contain gastropods, brachiopods, scaphopods, bryozoans, echinoids, serpulids, hydroids and crustaceans, but it was dominated by bivalve molluscs.

In the oceans today we are witnessing the rapid expansion of areas of low dissolved oxygen that is caused by a combination of warming and elevated nutrient/organic enrichment of the oceans. The Jurassic was a period of ‘greenhouse’ conditions and de-oxygenation was common in its shallow continental seas within restricted basins such as the Wessex Basin. The results of our analyses show that the species composition of the Late Jurassic seafloor communities changed in the face of the environmental stress caused by the decreased oxygen levels, but that ecological functioning was initially maintained – lowered oxygen levels did not trigger a switch to a seafloor ecosystem that worked in a fundamentally different way. However, as oxygen levels continued to decrease the system underwent a marked change in the way it functioned. We have been able to identify this threshold relative to geochemical proxies for environmental change.

The results of our study suggest that we may be able to identify the thresholds that will trigger this change in modern systems.  The modern seas and oceans support multiple ecosystem services and the collapse of ecological functioning has serious implications for coastal economies. Collapse of functioning is therefore a state that environmental managers should seek to avoid. The ecological changes we observe in the Jurassic are consistent with the patterns emerging from studies of modern systems. Functional collapse occurs rapidly once critical thresholds are exceeded and recovery from this often takes decades and follows a unique and unpredictable return path.

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The cliffs near Kimmeridge showing clear metre scale alternation between organic-poor and organic-rich layers.  These variations reflect changes in oxygen levels at the seafloor during the Late Jurassic. 

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Exploring the Jurassic seafloor as it is exposed, in the Kimmeridge Clay Formation, today on the foreshore.

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A fossil rich bedding plane representing one of the hypoxic palaeocommunities (E2c).  It contains several of the dominant bivalve species (Protocardia morinica, Palaeonucula menkii, and Isocyprina spp.) and the limpet Pseudorhytidopilus latissima.”

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