Sedimentos laminados de la bahía mejillones como registro de cambios temporales en la productividad fitoplanctónica de los últimos ∼200 años

Abstract

We analyzed the contents of total organic carbon, biogenic opal, chlorins, and alkenones (as productivity proxies) from a laminated sediment core recovered within Bahía Mejillones, northern Chile, in order to reconstruct recent temporal changes in phytoplankton productivity. Physical parameters (water content, dry bulk density, magnetic susceptibility, X-radiography, and grey scale intensity) were also used to characterize the sediments. A sedimentation rate of 0.18 cm year -1 (based on 210Pb measurements) was determined for the core (Core 3; 23°03' S, 70°27' W), which was recovered with a box-corer (water column depth: 80 m). By extrapolating this sedimentation rate to the base of the core, it yielded a total age of ∼223 years. The sediments were characterized by a succession of light and dark millimetre-thick laminae, which differed in both physical properties and composition; the denser dark laminae had less water content and represented periods of increased phytoplankton productivity, whereas the less dense, light laminae had higher water content and reflected less productive periods. The good preservation of the laminae, the absence of structures associated with processes of bioturbation, and the high sedimentary δ 15N values (X̄ = 11.37) suggest low dissolved oxygen levels in the bay's bottom water during the two centuries of sedimentary deposition reported herein. Our record is divided into two large productivity intervals: one prior to AD 1820, with lower phytoplankton productivity and relatively warmer conditions, and another from ∼AD 1877 to the present, characterized by higher productivity, high variability, and relatively colder conditions, and accompanied by intensified winds favouring coastal upwelling. We suggest that the productivity of Mejillones Bay since AD 1820 has been strongly dependent on siliceous production and that calcareous primary production has increased since AD 1877.