Early taphonomy of benthic foraminifera in Storfjorden ‘sea-ice factory’: the agglutinated/calcareous ratio as a proxy for brine persistence

Abstract

The recurrent latent-heat polynya characterizing Storfjorden (Svalbard, Norway) triggers seasonal formation of thin first-year sea ice. This leads to the production of dense, salty, and corrosive brines that cascade towards the sea floor and mix with shelf waters. The bottom topography of the fjord is responsible for the retention of these dense waters in two central deep basins throughout the year. Recent studies show that living benthic foraminifera in Storfjorden are particularly affected by the persistence of brines on the sea floor, with a strong dominance of agglutinated (A) species and high degrees of dissolution of calcareous(C) faunas. Therefore, the A/C ratio, calculated on living faunas, was proposed as a proxy for brine persistence. In the present study we analyse the fossil faunas, found below the taphonomically active zone, to investigate the residual signal of the A/C proxy after the intense early taphonomic processes and challenge its applicability in sedimentary archives. Our results show that despite the generally high taphonomic loss inside the fjord, a high proportion of agglutinated species is still visible in fossil faunas at the stations experiencing regular and/or persistent presence of brine-enriched shelf waters. These results support the application of the A/C ratio in historical records to reconstruct the persistence of brines and indirectly the first-year sea ice formation in Storfjorden. This can be further applied to other Arctic fjords with similar settings and characterized by the production of brines during the winter–early spring season.