Project

Hettangian; Paleogeography; Triassic; Jurassic Boundary; Palaeoecology; Palaeogeography

Lead
Lay summary
As for most geological period boundaries, the Triassic-Jurassic (T-J) changeover, about 200 Ma ago, was a critical juncture in Earth history during which profound biotic and environmental changes occurred. The purpose of this proposal is to address key questions concerning the understanding of evolutionary turnover of influenced fossils groups and of geochemical history of marine environments, and this by investigating coral reef associations. At the T-J boundary the reefal ecosystems are strongly affected and undergo a dramatic decline considering their high sensitivity to ecological factors. Particularly the Rhaetian corals, which were the main frame-builders on all the Tethyan margins, suffer a striking reduction correlative of their diversity. The characteristic coral association restriction in palaeogeographic distribution at down of the Jurassic arouses interest. They have almost uniquely been found in the westernmost point of the Tethyan gulf, being the only ideal place liable to be influenced by the polar waters at that time. This is consistent with the hypothesis of "hothouse". These Jurassic coral associations are very little known still today, to such a point that some authors reported the first occurrence of a Jurassic reef from the Sinemurian of the Rocky Mountains. This is not a fact because at least ten occurrences of coeval reef/reefal environments exist, from the South of France to the British Isles across Luxemburg, some of which have not been published yet. The project will focus on an integrated study of the T-J boundary, involving sedimentological, palaeontological, palaeoecological and biogeochemical approaches. To achieve our goals we will:-define the coral association capacities versus their recovery phase during Early Jurassic, as well as their evolutionary steps characterising the T-J mass extinction events;-establish the palaeotemperatures to depict the ecological habitat under which coral communities developed, and understand when the occurrence and disappearance of the taxa can be linked to the ecosystem variations;-precise the palaeogeographic setting at the T-J boundary, and especially the nature of the seaway connections between the polar waters and the westernmost Tethys;-to document the quasi total disappearance of reefs at the T-J transition, caused by a possibly dramatic climatic change (cooling followed by "super-greenhouse"), would be of the greatest importance in the perspective of the global warming of the Earth.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

Biotic, environmental and climatic changes at the Triassic-Jurassic boundary: Causes and effects highlighted via the coral reef associations (TJCORA).As for most geological period boundaries, the Triassic-Jurassic (T-J) changeover, about 200 Ma ago, was a critical juncture in Earth history during which profound biotic and environmental changes occurred. At the T-J boundary, after almost 100 Ma of existence, the Pangea supercontinent began a fragmentation that has proceeded to the present day. The most obvious manifestation of this process was the formation of the CAMP, an estimated 2.5 million cubic kilometers of Central Atlantic Magmatic Province. At approximately the same time, profound changes took place in the key elements of the biosphere, most remarkably in the marine carbonate producing organisms. Many processes and causes have been evoked to explain the biotic and environmental transformations at the T-J transition: a temporary, sudden rise in temperature; a short-term cooling and acidification phase due to volcanic SO2 impact; a dramatic “super greenhouse” in the lowermost Jurassic caused by long-term accumulation of volcanic CO2. More recently, a global cooling trend through the Triassic, with a decreasing of CO2, seems to have beneficial effects to global reef growth than previously thought.At the T-J boundary the reefal ecosystems have been strongly affected by these geological modifications and undergo a dramatic decline considering their high sensitivity to ecological factors. Particularly the Rhaetian corals, which were the main frame-builders on all the Tethyan margins, suffer a striking reduction correlative of their diversity. At down of the Jurassic, coral associations have been almost uniquely confined in the westernmost point of the Tethyan gulf, being the only ideal place liable to be influenced by the polar waters at that time. These corals are very little known still today, to such a point that some authors reported their findings as the first occurrence of a Jurassic reef from the Sinemurian of the Rocky Mountains. This is a misleading (ambiguous) because at least ten occurrences of coeval reef/reefal environments have been previously mentioned from the South of France to the British Isles across Luxemburg, some of which have not been studied yet. The purposes of this proposal are numerous:-to address key questions concerning the understanding of evolutionary turnover of coral reef associations at the T-J boundary, as several aspects of it are reminiscent of the current crisis;-to identify and document new and important earliest Jurassic coral occurrences in Western Europe; -to improve the taxonomy of these corals, which are currently poorly known; -to define the coral association capacities versus their recovery phase during the Early Jurassic;-to evaluate the role of seawater composition in the evolution of biota using biogeochemical analyses, which is factor not yet explored in ancient reefs. In synergy with paleoecological study, the changing role of paleotemperatures (d18O) and salinity (i.e., d18O versus d13C) on coral communities will also be assessed;-to determine whether geochemical variations could yield reliable proxy data to assess if ocean acidification has played a role in the corals extinction; -to precise the paleogeographic setting at the T-J boundary, and especially the nature and function of the seaway connections between the polar waters and the westernmost Tethys, balancing field work and map analysis.These results will then be integrated within a well constrained sedimentological and stratigraphic framework allowing us to capture climate variability representative of a large area of the western Tethys during the T-J accelerated biotic turnover. In the Rhaetian mass extinction, rates are very high comparing to very low originating rates. The understanding of these processes may tell us a lot about the causes leading to crises and recovery of strongly calcifying marine organisms and would be of the greatest importance in the perspective of the ongoing debate about the causes behind Earth global warming.