Paleoenvironmental reconstruction of the Mar Chiquita coastal plain (Buenos Aires Province, Argentina) during the Holocene

FAYÓ ROCÍO; BEDMAR JOSE M.; ISLA FEDERICO; ESPINOSA MARCELA A.

Mendoza

Congreso; 4th International Paleontological Congress; 2014

Asociación paleontológica Argentina/ CONICET

Diatom assemblages of a core from Mar Chiquita coastal lagoon area, Buenos Aires Province, Argentina, were analyzed in order to recognize environmental changes in term of salinity and depth. The core contains sediments that range in age from 5.802±70 14C years BP (6.751-6.440 cal years BP) to present times, and it was obtained using vibracoring techniques, in a site north of the lagoon (37º 23`39? S, 57º 9`33?W). Four sedimentary facies were defined according to different characteristics such as grain size distribution, macrofossil content, sedimentary structures (when present) and general aspect. From bottom to top they were: greenish-grey silt, brownish-grey sandy silt, heterolitic facies (showing flasser bedding) and brown sandy silt. Two diatom zones were characterized by cluster analysis (single linkage, Euclidean distance) in the fossil sequence. Fossil and modern diatom assemblages were compared with multidimensional scaling (MDS). Two hundred diatom species were identified and grouped based on their salinity tolerances and life form characteristics. The lower section of the sequence (Zone 1) was dominated by Paralia sulcata (Ehrenberg) Cleve and Psammococconeis cf. disculoides (Hustedt) García, both tychoplanktonic and marine/brackish species. They were accompanied by marine and marine/ brackish plankton, epipsammon and epiphyte diatoms. This assemblage indicates a coastal environment with significant marine influence. To the top, the marine influence and the depth decrease. Salinity decrease was showed by the abrupt reduction of P. sulcata and the presence of brackish species like Planothidium delicatulum (Kützing) Round and Bukht (epiphytic) and brackish/freshwater species like Hippodonta hungarica Grunow, Staurosirella pinnata (Ehrenberg) Williams and Round, and Staurosira venter (Ehrenberg) Kobayasi. This assemblage is typical of tidal marshes. The shallower environment was inferred by the increase of benthic and epiphytic species. Furthermore, the results of MDS showed that fossil diatom assemblages were similar to the modern diatom assemblages located at the inlet of the Mar Chiquita coastal lagoon. The new diatom and sedimentological data presented here support the regional model for the Holocene sea-level fluctuation in this area. These analyzes allowed to infer the existence of a marginal coastal system in the area during the regressive phase close to the maximum sea-level after 6.500-6.000 years BP. This coastal environment evolved gradually into a brackish/freshwater marsh from the final regressive state until today.