Silvia Watanabe

Atlas of Benthic Shelf Foraminifera of the Southwest Atlantic

Area of Study.- Water masses.- Cape Horn current.- Malvin current.- Coastal Argentine waters.- Area influenced by the Rio de la Plata.- Coastal waters of Uruguay and Southern Brazil.- Benthic foraminifera provinces.- North Patagonian subprovince.- South Patagonian subprovince.- Malvin subprovince.- Summary of Benthic Foraminiferal Investigations in the Area.- Principal Factors Influencing the Benthic Foraminiferal Distribution.- Salinity.- Euhaline foraminifera.- Hypohaline foraminifera.- Fresh water foraminifera.- Salt marsh foraminifera.- Temperature.- Depth.- Water mass.- Systematics and Distribution.- Euhaline species.- Hypohaline and fresh water species.- References.- Plates.- Maps.

Biostratigraphy of Tertiary and Quaternary benthic bathyal foraminifers of DSDP Site 317 (Tropical Pacific)

Abstract A study is made of the benthic foraminifers (size fraction > 63 μm) recovered from 59 upper Eocene through Quaternary sediment samples at DSDP Site 317 (Leg 33), located at a depth of 2598 m in the central part of the Manihiki Plateau (South Pacific). The sediments cored are disturbed in only two samples. The stratigraphic assignements used are based on previous studies of planktic foraminifers and other microfossils. In total, 216 taxa are identified. A cluster analysis based on the 77 species which comprised 5% or more of the entire foraminiferal assemblage in at least one sample suggests the presence of 3 major biostratigraphic zones corresponding approximately to the following ages, zone A: middle Miocene-Quaternary; zones B-C: early Miocene-Oligocene; and zone D: Eocene. The most important faunal turnover occurred between the Eocene and the Oligocene; a less pronounced break took place between the early and the middle Miocene, and an additional minor turnover between the Oligocene and the early Miocene. Eighteen taxa are long-ranging, being recorded from the middle Eocene through the Pliocene-Quaternary. It is concluded that, in general, benthic foraminifers of the bathyal zone are poor worldwide stratigraphic guide fossils; the following taxa are conditionally considered as the most suitable in the Eocene-Quaternary sequence:Aragonia aragonensis, Quadrimorphina profunda, Nuttallides truempyi, Abyssamina poagi, Buliminella grata, Bulimina jarvisi, B. macilenta, Turrilina alsatica, Cibicides notocenicus, C. wuellerstorfi, Pyrgo murrhina. However, most of these species are relatively rare.

Area of Study

This study is based on those benthic foraminifera of the southwest Atlantic which occupy the broad continental shelf off the Argentine, Uruguayan and south Brazilian coasts.

Chapter 6 The Argentine continental shelf: morphology, sediments, processes and evolution since the Last Glacial Maximum

Abstract The Argentine continental shelf is one of the largest and smoothest siliciclastic shelves in the world. Although it is largely emplaced in a passive continental margin, the southernmost regions are related to transcurrent and active margins respectively associated with the Malvinas Plateau and Scotia Arc. Sea-level fluctuations, sediment dynamics and climatic/oceanographic processes were the most important conditioning factors in the modelling of the shelf, with a minor influence from isostatic and tectonic factors that are more relevant in the southernmost regions. The shelf is shaped by diverse geomorphic features, among which the most significant are four sets of terraces genetically associated to sea-level stillstands during the post-glacial transgression; the final one occurred at around 11 ka and is associated with the Younger Dryas event. The Last Glacial Maximum (LGM) sedimentary sequence is composed of, on average, 5–15 m-thick terrigenous, siliciclastic, relict–palimpsest sands mainly sourced from the Andean region, with minor amounts of bioclast and gravels, resulting from the reworking of pre-transgressive coastal environments.

Foraminíferos bentônicos na plataforma continental da Bacia de Campos

Visando caracterizar a plataforma continental da Bacia Sedimentar de Campos com base em foraminiferos bentonicos no periodo seco (ressurgencias menos frequentes) de 2008 e chuvoso (ressurgencias mais frequentes) de 2009, foram estudadas amostras distribuidas em transectos perpendiculares a costa, provenientes de cinco isobatas (25, 50, 75, 100 e 150 m). Os sedimentos (0 a 2 cm) foram coletados em triplicata, fixados em formol a 4% tamponado com borax, corados com Rosa Bengala e lavados em peneira de 63 μm. Em laboratorio realizou-se a triagem sob lupa para retirar todas as carapacas com celula corada. Analises de agrupamento e de escalonamento multidimensional nao metrico (NMDS) permitiram reconhecer diferentes ambientes e com analises fatoriais investigou-se as relacoes entre biota e dados ambientais. As variaveis-resposta quantitativas foram biomassa e abundância. No total foram identificados 519 taxons de foraminiferos e a estrutura das associacoes (densidade, diversidade, dominância, equitabilidade). O padrao de distribuicao espacial dos foraminiferos esta fortemente associado a profundidade, aos sedimentos, a disponibilidade e qualidade de materia orgânica. Analises integradas dos periodos identificaram tres areas principais; a Analise de Especies Indicadoras (ISA) permitiu reconhecer as principais especies de cada area: (i) Plataforma interna – 25 e 50 m, sedimentos predominantemente siliciclasticos, predominio de especies epifaunais positivamente correlacionadas com areia, maior temperatura e maior concentracao de clorofila a ; (ii) Area de ressurgencias – predominio de especies infaunais positivamente correlacionadas com lama, feofitina a e carbono orgânico total; (iii) Plataforma media-externa – 75 a 150 m, caracteristicas predominantemente oligotroficas, sedimentos (areia, cascalho e lama) bem selecionados e com alto teor de carbonato; especies positivamente correlacionadas com o carbono orgânico total. Foraminiferos fitodetritivoros indicaram outras areas de ressurgencia associadas a intrusoes da Agua Central do Atlântico Sul (ACAS), proximo ao Cabo de Sao Tome e ao norte da area estudada, provavelmente influenciada pelo vortice de Vitoria. A biomassa na plataforma interna foi similar a de outras areas investigadas, mas valores nas plataformas media e externa foram mais altos que o esperado para uma regiao oligotrofica sazonalmente mesotrofica. Curvas de abundância e biomassa indicaram que as areas de ressurgencia mostram perturbacao moderada, assim como a area de plataforma interna que apresenta indicios de perturbacao moderada a forte, dependendo do periodo avaliado.

Principal Factors Influencing the Benthic Foraminiferal Distribution

The specific ecology of benthic foraminifera is treated in Boltovskoy & Wright (1976) and data on environmental parameters from the area can be found in the various publications cited in the bibliography. Consequently we are going to discuss here only some general aspects of ecology and environmental relationships of the foraminifera with special reference to conditions in the southwest Atlantic shelf.

Summary of Benthic Foraminiferal Investigations in the Area

The initial study of foraminifera from the southwest Atlantic shelf was published in 1839 by d’Orbigny as part of his classic work “Voyage dans l’Amerique Meridionale”. Among the samples analyzed by d’Orbigny were littoral sands from the Patagonian coast between Bahia San Blas and the Golfo San Jorge, bottom samples from the near-shore waters around the Islas Malvinas, and a sample taken “en vue de terre au Cap Horn” at a depth of 160m. D’Orbigny described 52 species from the two sites mentioned above on the Argentine inner shelf (19 from the Patagonian coast, 38 from the Islas Malvines, with 5 common to both sites). From Cape Horn he described 5 species, 4 of which have proven to be common on the Argentine shelf and the fifth to be common on the western coast of South America.

Systematics and Distribution

The classification used in this work is essentially that of Loeblich & Tappan (1964, 1974) as it is the best known and most easily used. Nevertheless we often find ourselves in favor of a generic concept which is somewhat more conservative than that proposed by Loeblich and Tappan. We think that the proliferation of taxa (even at the generic level) as reflected in Loeblich & Tappan (op. cit.) and followed by many other workers, is prejudicial to the practical interests of the science (Boltovskoy, 1965d). Consequently, we will not always rigidly follow the Loeblich and Tappan classification, but will sometimes use other generic names, either to maintain a somewhat broader interpretation, or to follow the principle of conservation rather than that of priority (Todd, 1963; Le Calvez, 1969; Boltovskoy & Wright, 1976). Insofar as species identification is concerned we have expended a great deal of energy, not only at the microscope, but also in searching the literature and examining original type material, to be as precise and faithful to the original descriptions as possible.