Federico Ignacio Isla

The Evolution of the Bío Bío Delta and the Coastal Plains of the Arauco Gulf, Bío Bío Region: the Holocene Sea-Level Curve of Chile*

Abstract ISLA, F. I.; QUEZADA, J. F.; MARTÍNEZ, C.; FERNÁNDEZ, A.; and JAQUE, E. 2012. The evolution of the Bío Bío delta and the coastal plains of the Arauco Gulf, Bío Bío region: The Holocene sea-level curve of Chile. Mid-Holocene highstands are characteristic of the Southern Hemisphere. The Chilean coast extends from 17°S to 56°S in a dominant microtidal regime; thus, it is an ideal place to test ages and altitudes of this highstand with minimal errors. However, coseismic events, the dynamic phenomena they triggered (tsunamis), and the behaviour of land in relation to the overriding of the South American Plate over the oceanic Nazca Plate, make it necessary to distinguish these effects from purely eustatic changes. To the south, the glacioisostatic uplift has been approximately measured. At 37°S, the Coronel coastal plain extends several kilometres inland. Its sediment availability has been related to the supplies of the Bío Bío River. From this beach-ridge plain, shell remains gave a radiocarbon age of 4370 ± 90 years before present (YBP), indicating a highstand not higher than 5 m. Further south, at the Carampangue coastal plain, southern coast of the Arauco Gulf, a radiocarbon age of 8010 ± 90 YBP marks the oldest age of this transgression. Some consequences of the earthquake and tsunami of February 27, 2010, are reported here. The radiocarbon ages of these plains permit completion of a Holocene sea-level curve. These Holocene sea-level data were compared to other regions of South America.

Macrobenthic Distribution Patterns at a Sewage Disposal Site in the Inner Shelf off Mar del Plata (SW Atlantic)

Abstract The inner shelf (5–15 m depth) off Mar del Plata (Argentina, Southwestern Atlantic) was studied to assess the influence of an intertidal sewage effluent on macrobenthic populations and to provide baseline information for a future monitoring program. Bottom samples were obtained with van Veen grabs at 49 stations. Macrobenthic organisms retained in a 0.5 mm mesh screen were used for biological analysis. Very fine sand-bottoms with a Natica–Owenia association occur in shallower sites, while patchy coarse sand bottoms plus gravel-sized shell debris inhabited by an association of patchy populations of Melita–Prionospio–Kalliapseudes–Caulleriella characterize the rest of the area. Low organic matter content in the area (from 0.78 to 2.05 %), and absence of any known indicator species (such as Prionospio spp.) in relation to distance to effluent, suggest that local benthic populations remain relatively undisturbed in spite of the heavy input of organic matter. The major factor contributing to this situation is the high hydrodynamics of the area. SUMMARY La plataforma somera frente a la ciudad de Mar del Plata (38° S, 57° W, Atlántico Sudoccidental) recibe la descaga del efluente intermareal cloacal de la ciudad. El estudio se centra en la influencia de dicho impacto en las comunidades bentónicas someras y en el establecimiento de la situación de pre-impacto en el área más profunda para futuros estudios de monitoreo y evaluación ambiental cuando el proyectado emisario submarino sea funcional. Una asociación de Natica y Owenia domina en las estaciones someras (5–7 m), caracterizadas por arenas finas y muy finas, mientras que el resto del área se halla dominada por la asociación constituida por Melita–Prionospio–Kalliapseudes–Caulleriella en sedimentos de arenas más gruesas con fragmentos de conchillas de tamaño grava. Estos organismos se distribuyen alternativamente en parches entre los 7 y los 15 m de profundidad. Las comunidades bentónicas no reflejan un gradiente de contaminación orgánica, aunque la presencia de especies indicadoras (como Prionospio y Caulleriella) evidencian un enriquecimiento de los sedimentos. La distribución en parches de sedimentos y organismos, así como los bajos tenores de materia orgánica en el sedimento se deben a la gran hidrodinámica del área, debida a corrientes de marea y tormentas periódicas, lo cual también explicaría la relativamente buena salud del ambiente.