Marsh development and sea level changes in the Gernika Estuary (southern Bay of Biscay): foraminifers as tidal indicators

Abstract

SUMMARY:The ecological distribution of some species of Foraminifera living in estuaries can be used in the interpretation of former coastal lines. There is a close relationship between tide level and the distribution of particular benthic foraminifer species. A characterisation of tide levels based on the benthic foraminifers in four cores from this estuary allowed us to interpret the changes over time in transitional environments during the modern Holocene. The estuary was filled during the last 6500 years following a general accretional model. Marshes have existed in this area for about 4600 years. This model is described for the first time in the area, based on benthic foraminiferal zones. These zones define a general emergence sequence, interrupted by a transgressive marine pulsation. We describe the evolution of marsh sub-environments in relation to the sea-level changes, materialised by a transgressive pulsation which shows an interruption in the accretional process dated 1910 ± 50 BP.Keywords: foraminifers, palaeoenvironmental evolution, Holocene, Gernika estuary, southern Bay of Biscay. RESUMEN: EVOLUCIÓN DE UNA MARISMA Y CAMBIOS DEL NIVEL DEL MAR EN EL ESTUARIO DE GERNIKA (SUR DEL GOLFO DE VIZCAYA): LOS FORAMINÍFEROS COMO INDICADORES DE MAREAS.-La distribución ecológica de algunas especies de foraminí-feros que viven en estuarios pueden ser utilizadas en la interpretación de antiguas líneas de costa. Existe una clara relación entre el nivel mareal y la distribución de especies concretas de foraminíferos bentónicos. La caracterización de estos niveles de mareas basados en los foraminíferos bentónicos en cuatro sondeos de este estuario, permite interpretar los cambios en estos ambientes transicionales durante el Holoceno. El estuario ha sido rellenado durante los últimos 6500 años, siguiendo un modelo general acrecional. Las marismas han existido en esta área desde hace unos 4600 años. Este modelo describe la evolución de los subambientes de marisma, en relación con cambios en el nivel del mar, materializado por una pulsación transgresiva que evidencia una interrupción en el proceso acrecional, datada de 1910 ± 50 BP.

Natural and anthropic pollution episodes during the Late Holocene evolution of the Tinto River estuary (SW Spain)

“…In the last few decades, numerous investigations have focused on the Holocene paleoenvironmental evolution of coastal areas through multidisciplinary analysis of sediment cores and seismic stratigraphy. In most cases, studying paleoenvironmental evolution involves combining sedimentological, paleontological, geochemical, isotopic and radionuclide techniques (Pascual and Rodríguez Lázaro 2006, Ercilla et al 2010, Blázquez-Morilla et al 2018. These studies are used to determine local geochemical backgrounds, with a subsequent application to detecting environmental pollution episodes caused by mining, industrial waste, urban discharges and agricultural activities (Cearreta et al 2002, Beck et al 2020).…”