A high-resolution study of benthic foraminiferal assemblages was performed on a ca. eight metre long sediment core from Gullmar Fjord on the west coast of Sweden. The results of 210Pb- and AMS 14C-datings show that the record includes the two warmest climatic episodes of the last 1500 years: the Medieval Warm Period (MWP) and the recent warming of the 20th century. Both periods are known to be anomalously warm and associated with positive NAO winter indices. Benthic foraminiferal successions of both periods are compared in order to find faunal similarities and common denominators corresponding to past climate changes. During the MWP, Adercotryma glomerata, Cassidulina laevigata and Nonionella iridea dominated the assemblages. Judging from dominance of species sensitive to hypoxia and the highest faunal diversity for the last ca. 2400 years, the foraminiferal record of the MWP suggests an absence of severe low oxygen events. At the same time, faunas and d13C values both point to high primary productivity and/or increased input of terrestrial organic carbon into the fjord system during the Medieval Warm Period. Comparison of the MWP and recent warming revealed different trends in the faunal record. The thin-shelled foraminifer N. iridea was characteristic of the MWP, but became absent during the second half of the 20th century. The recent Skagerrak-Kattegat fauna was rare or absent during the MWP but established in Gullmar Fjord at the end of the Little Ice Age or in the early 1900s. Also, there are striking differences in the faunal diversity and absolute abundances of foraminifera between both periods. Changes in primary productivity, higher precipitation resulting in intensified land runoff, different oxygen regimes or even changes in the fjord's trophic status are discussed as possible causes of these faunal differences.

For d13C values of Cassidulina laevigata see Filippson and Nordberg (2010) datsets: doi:10.1594/PANGAEA.773806 and doi:10.1594/PANGAEA.773804. However, chronology of core Svanic90_9004 of Filipsson and Nordberg (2010) was revised for age dating in the current dataset (reservoir age of 500 yr instead of 400 yr). Foraminiferal data (except N. iridea) from core Skagerrak01_Ga113-2Aa were previously published in Filipsson and Nordberg (2004, doi:10.1007/BF02912048). Funding sources: Lamm Foundation, Swedish Research Council (VR 621-2007-4369 to Kjell Nordberg), PALEOSTUDIES and EUROPROX program, Department of Earth Sciences at GU.

Supplement to: Polovodova Asteman, Irina; Nordberg, Kjell; Filipsson, Helena L (2011): The benthic foraminiferal record of the Medieval Warm Period and the recent warming in the Gullmar Fjord, Swedish west coast. Marine Micropaleontology, 81(3-4), 95-106