DIATOMS AS INDICATORS OF SALINITY VARIATIONS DURING THE LATE HOLOCENE IN THE RIO NEGRO, NORTHERN PATAGONIA, ARGENTINA

DIATOMS AS INDICATORS OF SALINITY VARIATIONS DURING THE LATE HOLOCENE IN THE Rio NEGRO, NORTHERN PATAGONIA, ARGENTINA. Diatom assemblages from a sediment core and a sequence outcropping 6 km from the Rio Negro mesotidal inlet were studied in order to reconstruct salinity and depth changes. The sequence comprises the interval between 2027 +/- 34 C-14 years BP to the present. Diatom data from 44 samples were analyzed according to ecological characteristics of each taxon. The basal section was dominated by the oligohalobous indifferent taxa: Navicula viridula var. germainii Kutz. (benthos), Epithemia adnata (Ehrenb.) Katz. (epiphyte) and Surirella brebissonii Kramm. and L.-Bert. (oligohalobous halophilous and benthos). From 2027 yr BP a tidal channel characterized by the dominance of the marine plankton and tychoplankton Thalassionema nitzschioides Grun., Delphineis surirella (Ehrenb.) Andrews, Delphineis minutissima Hustedt. Simonsen and Rhaphoneis amphiceros Ehrenb. was installed. Paralia sulcata (Ehrenb.) Cleve and Cymatosira belgica Grun. were present to the top of the sequence, where brackish-freshwater aerophilous taxa, such as Diploneis ovalis (Hilse) Cleve and Pinnularia viridis (Nitz.) Her., were dominant. The shallow and vegetated brackish-freshwater environment from the bottom of the core, evolved towards a tidal channel, declining gradually in depth and salinity, to a marsh subject to tides and floods at the top of the sequence.