Olivier Ther

Holocene long- and short-term climate changes off Adelie Land, East Antarctica

Diatom data from a marine sediment core give insight on Holocene changes in sea-surface conditions and climate at high southern latitudes off Adelie Land, East Antarctica. The early to mid-Holocene was warmer than the late Holocene with a transition at ∼4000 calendar years B. P. Sea ice was less present and spring-summer growing season was greater during the warm period relative to the cold one, thus limiting sea ice diatom production and favoring more open ocean diatom to develop. The long-term Holocene climatic evolution in East Antarctica is explained by a combination of a delayed response to local seasonal insolation changes coupled to the long memory of the Southern Ocean. Abrupt variations of the diatom relative abundances, indicating rapid climate changes, are superimposed to the Holocene long-term trends. Spectral analyses calculate robust frequencies at ∼1600 a (where “a” is years), ∼1250 a, ∼1050 a, ∼570 a, ∼310 a, ∼230 a, ∼150–125 a, ∼110 a, ∼90 a, and ∼66 a. Such periods are very close to solar activity cyclicities, except for the periods at ∼310 a and ∼1250 a, which are close to internal climate variability cyclicities. Wavelet analyses estimate the same periods but indicate nonstationary cyclicities. Rapid climate changes at high southern latitudes may therefore be explained by a combination of external (solar) and internal (thermohaline circulation) forcings.

Centennial-scale variations in diatom productivity off Peru over the last 3000 years

The Peruvian coastal upwelling is one of the most productive systems in the global ocean, with important impacts on the carbon cycle. Primary productivity there displays strong variations at the interannual to decadal timescales. However, down-core investigations rarely reach sufficient temporal resolution to assess the response of productivity to climatic variations at these timescales beyond the instrumental and historical periods. We here analyzed diatom assemblages, sea-surface temperatures (SSTs), and nitrogen and organic carbon contents on a laminated sediment core from the Peruvian continental shelf to trace variations in regional productivity over the last 3000 years. Our record provides evidence for different climatic and oceanic conditions with more humid and less productive conditions older than 2500 cal. yr BP and drier and more productive conditions younger than 2500 cal. yr BP. The last 2500 years also present much stronger centennial-scale variability with the occurrence of six intervals with higher total diatom abundances and stronger percentages in upwelling-related diatom species, representative of intensified productivity, congruent to lower percentages in benthic diatoms, indicative of reduced rainfall. These six periods were synchronous to intervals of enhanced Walker circulation, suggesting a strong imprint of the Pacific zonal circulation on productivity variations off Peru. Our record also demonstrates that SSTs did not vary in phase with productivity, arguing against the idea of regional SSTs controlled by the upwelling intensity, but were rather in agreement to SST records off southern Chile, suggesting that Peruvian SST variations were largely controlled by oceanic currents at southern high latitudes.