Title: Sclerochronological approaches in Saint-Pierre & Miquelon: from sub-hourly to multidecadal environmental reconstructions

The panel will be composed of:

• Anne Lorrain (Examiner) IRD / LEMAR
  Claire Lazareth (Examiner) IRD / LOCEAN
  Stéphanie Thièbault (Examiner) CNRS / Director of INEE
  Philippe Archambault (Rapporteur) Univ. Laval
  Matthieu Carré (Rapporteur) CNRS / LOCEAN

Thesis supervisors:

• Laurent Chauvaud and Pascal Lazure

Summary :

Coastal ecosystems are exposed to global climate change leading to modifications of their structure and functioning. However, little is known about the variability of their physical properties before 1950, mainly because of the lack of long-term instrumental measurements. The hard parts of long-lived marine biota hold the potential to extend instrumentally derived observations, at different temporal and spatial resolutions, in order to enhance our understanding of past environmental processes.

This PhD dissertation takes place on Saint-Pierre & Miquelon (SPM), a small archipelago at the confluence of major oceanic currents marking the boundary between the North Atlantic Ocean subtropical and subpolar gyres. In addition to its global key position, the abrupt bathymetric change occurring in the North West of Miquelon Island generates the anti-cyclonic propagation of a tidal coastal trapped wave around this archipelago. This local phenomenon, leads during the stratified period to the largest (up to 11.5°C amplitude) daily (25.8 h) temperature oscillations ever observed-at any frequency-on a stratified mid latitude continental shelf.

This work is based on the analyses of local marine biota hard parts to gain insights about past environmental variability at these two scales. First, I have learned different sclerochronological methods through Spisula solidissima study. Global and multi-decadal time scales were reached using the longest lived bivalve known to date Arctica islandica, and Clathromorphum compactum a newly discovered long-lived coralline alga. The relationships observed at SPM between A. islandica and C. compactum sclerochronological records and different geographical scales environmental datasets yield details about past large-scale oceanographic variability and ecosystem dynamics. Local, high-frequency (25.8 h) temperature oscillations were tracked using sclerochronological information contained in Placopecten magellanicus a fast growing (ca. 500 µm / day) bivalve species.

This study points out the relevant position of this archipelago for studying multiple scale oceanographic variability, biological responses and ecosystem dynamics facing global changes.