MODERN FORAMINIFERAL FACIES IN A SUBTROPICAL ESTUARINE CHANNEL, BERTIOGA, SAO PAULO, BRAZIL

Abstract

ABSTRACT Numerical analyses of modern foraminiferal abundanceand environmental data from the Bertioga Channel (Sa˜oPaulo, Brazil) reveal multiple biofacies within an overallparalic setting. Despite its fisheries, mariculture and attrac-tion to tourists, the environmental state of Bertioga Channelremains poorly studied. The present investigation is anattempt to partly fill this gap; the parameters examinedinclude depth, salinity, temperature, organic carbon, sulfurcontent and bottom sediment type. Muddy sediments withhigh organic carbon content derived from land drainage arefound in the inner parts of the channel, whereas sandysediment dominates the areas adjacent to the Atlantic Ocean.In the eastern entrance to the channel, sandy sediment containspecies of Rotaliida from Facies 1 (including Elphidiumdiscoidale, Elphidium poeyanum, Hanzawaia boueana, Para-rotalia cananeiaensis and Nonionella atlantica), reflectingnormal marine salinity. Sediments with high percentages ofsilt and clay in polyhaline and euryhaline environments of theeastern part and Itapanhau´ River contain Facies 2, whichincludes Ammonia beccarii and Pararotalia cananeiaensis.Inthe western entrance and central, western and eastern parts,where salinities vary from 18 to 30 psu and the sedimentscontain both low and high organic carbon, the foraminiferafrom Facies 3 are dominated by Quinqueloculina milletti,Arenoparrella mexicana, Pararotalia cananeiaensis, Ammo-nia beccarii, Buliminella elegantissima, Elphidium sp.,Elphidium excavatum, Elphidium gunteri and Elphidiumpoeyanum. In mesohaline and polyhaline waters of the centralpart, the organic-carbon-rich silt and clay contain Facies 4,which includes Ammonia beccarii, Pararotalia cananeiaensis,Elphidium excavatum and Elphidium sp. Most of organic-carbon-enriched, silty-clay substrates that are subject to thehighest fresh-water discharge and high bottom temperaturessupport two different assemblages: one of mostly Rotaliinaand the other mostly of Textulariida (Facies 5 and 6). Facies5 includes Ammonia beccarii, Elphidium excavatum, Areno-parrella mexicana, Haplophragmoides wilberti, Siphotro-chammina lobata, Trochammina inflata and Trochamminasp., all of which are typical of mesohaline sites (mainlyCrumau´ and Trindade rivers), and Facies 6 includes Bolivinasp., Ammoastuta salsa, Arenoparrella mexicana, Haplo-phragmoides wilberti and Trochammina sp., all of which aretypical of oligohaline and mesohaline mangrove fringes. Theforaminiferal species from the present study are frequentlyfound in paralic environments in Brazil, western Africa andother estuaries around the world.INTRODUCTIONIn the context of the environmental variability of theBrazilian coastline, which has many estuaries, the foraminif-eral record in the sediment has not been extensively orintensively studied. Some past studies (Tinoco, 1965, 1971,1972; Bro¨nnimann, 1979) cover the continental shelf, but themajority relate to estuarine, lagoonal and marsh foraminifera.Examples from various coastal states include (1) Rio Grandedo Sul, Patos Lagoon (Closs, 1963); (2) Parana´ (Scott andothers, 1990; Barbosa, 1995; Barbosa and others, 2005); (3)Santa Catarina, Sa˜o Paulo and Bahia (Zaninetti and others,1977, 1979; Eichler and others, 1995; Eichler, 1996; Duleba,1997; Debenay and others, 1997, 2001a; Debenay, 2000;Duleba and Debenay, 2003); and (4) Rio de Janeiro,Guanabara Bay (Eichler and others, 2001, 2003; Vilela andothers, 2003, 2004). In contrast to most of these studies, whichunveiled broad distribution patterns of Brazilian paralicforaminifera, the present investigation is focused on thedistinction of foraminiferal facies within one estuarine areaand the relationship of such facies with measured environ-mental variables. The study area is the Bertioga Channel, a 25-km-long, secondary branch of the Santos estuarine system(Fig. 1), located in the coastal plain of central Sa˜o Paulo. It isa predominantly brackish-water, partially-mixed, subtropicalestuary, directly connected with the Atlantic Ocean at theBertioga Bar (23u519S, 46u089W) and indirectly through theSantos estuary (23u569S, 46u209W; Fig. 1).Despite its fisheries, mariculture and attraction to tourists,the environmental state of Bertioga Channel remainsunsatisfactorily studied. The present investigation is anattempt to partly fill this gap; the parameters examinedinclude a census of foraminifera, depth, salinity, tempera-ture, organic carbon, sulfur content and bottom sedimenttype. Our goal is to use the foraminiferal distributions thatare related to environmental variables as a baseline study tomonitor future changes in the health of the estuary.STUDY AREABertioga Channel ranges from 200–700 m wide andusually 3–6 m deep, but depressions of up to 10 m arerecorded mainly at the most abrupt channel curves. Thechannel is widest at its central part (Largo do Candinho), atthe confluence with the Santos estuary (western entrance)and at Bertioga Bar (eastern entrance), where depths up to10–12 m can be found (Harari and Camargo, 1998). Itnarrows considerably between the Largo do Candinho andSantos estuary, inhibiting exchange of saline waters acrossthis segment (Miranda and others, 1998).