Five sedimentary cores were collected on the ancient sedimentary clay deposits of Itupanema Beach, Barcarena town, Pará State, Amazon region, where diatom analysis was performed to infer the history of the paleoecological and paleohydrological changes during the last millennium. The results revealed 58 diatoms species among benthic and planktonic life forms. Correspondence analysis applied to relative abundance of diatoms and associated with sedimentary texture, OM contents and 14C dating, defined three ecozones. The lower sandy ecozone 1 dated at 930 ± 40 14C yr BP (Beta 217590) was dominated by the benthic/epipsammic species Staurosira obtusa, Staurosirella pinnata and Staurosirella crassa (nov. comb.), that indicated more hydrodynamic energy than in present days, originating a shallow sedimentary environment, under erosive conditions, unfavorable to the colonization of vegetation. The intermediate grey clay ecozone 2 that was observed only in the IT1 core, dated at 520 ± 40 14C yr BP (Beta 217591) showed the prevalence of planktonic diatoms, providing evidence of a deeper and calmer environment, located in a probably protected area, with intense sedimentation of clay particles with abundant plant remains, colonized by mangrove forest and alluvial palm forest. The upper organic-clay to organic-sand ecozone 3 much younger than 520 ± 40 14C yr BP, showed more agitated and erosive sedimentary conditions, however, with less energy than that in lower ecozones, indicated by higher abundance of A. granulata, strongly silicified. This new sedimentary environment allowed colonization by aquatic macrophyte vegetation which persists on the beach today
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