Luc Ortlieb

Coastal cooling and increased productivity in the main upwelling zone off Peru since the mid-twentieth century

[1]xa0We reconstructed a high-resolution, alkenone-based sea surface temperature (SST) record spanning the last ca. 150 years, from a sediment core retrieved within the main upwelling zone off Peru. A conspicuous SST decline is evidenced since the 1950s despite interdecadal SST variability. Instrumental SST data and reanalysis of ECMWF ERA 40 winds suggest that the recent coastal cooling corresponds mainly to an intensification of alongshore winds and associated increase of upwelling in spring. Consistently, both proxy and instrumental data evidence increased productivity in phase with the SST cooling. Our data expand on previous reports on recent SST cooling in other Eastern Boundary upwelling systems and support scenarios that relate coastal upwelling intensification to global warming. Yet, further investigations are needed to assess the role of different mechanisms and forcings (enhanced local winds vs. spin-up of the South Pacific High Pressure cell).

Supratidal Evaporitic Dolomite at Ojo De Liebre Lagoon: Mineralogical and Isotopic Arguments for Primary Crystallization

ABSTRACT This pair deals with a new site of dolomite formation in supratidal evaporite flats, at the southernmost edge of the Ojo de Liebre evaporitic complex (Baja California, Mexico). The sedimentary context is mostly siliciclastic, except for calcareous outcrops around the flats. Both chemical and isotopic data on the interstitial solutions demonstrate a mixing between Mg2+-rich marine-derived brines and calcium bicarbonate-bearing continental waters, which are subjected to concentration by capillary evaporation. The lack of any in situ solid CaCO3 in the sedimentary matrix argues for direct crystallization of dolomite from aqueous solutions. This is documented by the oxygen isotope compositions of the dolomite which are consistent with deposition under equilibrium conditions of ambient temperature and 18O of water of the interstitial solutions. Furthermore, the low 13C values of the dolomites and the absence of living plants or organisms in the basin indicate that a fraction of the total dissolved inorganic carbon reservoir is biogenic and is mostly provided by continental groundwaters. Therefore, crystallization of dolomite occurs within the sands infiltrated by solutions whose state of saturation relative to dolomite is mainly governed by the chemistry of continental groundwaters mixing with the inflowing marine brines.