Environmental constraints of foraminiferal assemblages distribution across a brackish tidal marsh (Caminha, NW Portugal)

Abstract

Foraminiferal living assemblages from a brackish tidal marsh were studied across three sampling transects, under Spring and Autumn conditions. Foraminiferal analysis was also extended to the dead assemblages of Spring samples. Altimetric control of each sampling point was acquired through the connection to the national datum by GPS differential positioning combined with a regional geoid model. The influence of the geochemical characteristics on the assemblages including chemical composition of the estuarine and marsh surface sediments, in addition to salinity, pH and calcite saturation of estuarine and marsh interstitial waters was evaluated.

A high affinity was found between Spring and Autumn living assemblages, probably related to the low number of representative species. The affinity also found between living and dead assemblage was seen as a consequence of low pH and calcite undersaturation that prevent the accumulation effect of several generations of foraminifera.

Salinity appears as a clear constraint to the foraminiferal assemblages composition that is enhanced by calcite undersaturation in brackish marshes interstitial waters and sediments. Generally abundant marsh species like Trochammina inflata, Jadammina macrescens and the calcareous foraminifera are limited to an episodic presence.

Foraminiferal assemblages distribution across brackish tidal marsh can be assumed as an elevation proxy. The high marsh IB sub-zone is dominated by Haplophragmoides sp. and Haplophragmoides manilaensis. Low marsh zone II is characterised by the dominance of Miliammina fusca but can be subdivided in an IIA upper part, where is followed by Pseudothurammina limnetis, and in a IIB lower part defined by a subsidiary presence of Psammosphaera sp.

Haplophragmoides manilaensis dominance between 1.55 m and 1.67 m, at living as well as dead assemblages, suggests this species as a potential sea-level indicator for the brackish tidal marshes.