Calcareous nannoplankton as a tracer of the marine influence on the NW coast of Portugal over the last 14 000 years.

Abstract

A calcareous nannoplankton taphonomic and palaeoecological study was performed on cores from the Minho and Douro Estuaries (NW Portugal) and from the NW Portuguese middle continental shelf. Because certain nannoplankton species are taphonomically resistant to the highly energetic and aggressive conditions between the estuaries and the shelf, their sedimentary record acts as an oceanic tracer in such coastal domains. For the Late Quaternary, this allows interpretation of eustatic variation from the induced sequence of palaeoenvironmental changes. Palaeogeographically, the time-series pattern of nannofossil abundances in these coastal sediments is interpreted as follows: (1) persistently high amounts of nannofossils (2x107 nannofossils/g) are related to marine environmental conditions, produced by post-glacial flooding of pre-existing valleys; (2) the gradual increase in intermittency of nannofossil abundances through time indicates a regressive trend, first due to recovery of the estuarine conditions, followed by a total absence of nannofossils, indicating evolution towards complete emersion either by fluvial (Core CPF1) or barrier-beach sediments (Cores M1, M2, D1A and D1B). During the Late Holocene, the nannoplankton assemblages were similar to those found in Present Day neritic conditions. The main taxa comprise Gephyrocapsa spp., Helicosphaera carteri, Emiliania huxleyi and Coccolithus pelagicus sensu lato. An unexpected and extremely high percentage of H. carteri in the Douro Estuary’s mouth was likely related to local confinement, which probably provided adequate and particular nutrient concentrations that prompted opportunistic behaviour in