Diatom phytoplankton response to Holocene climate change in the Subpolar North Atlantic

A detailed investigation of marine core MD99-2251 from the Gardar Drift has been undertaken to examine the extent of Holocene climate variability reflected by changes in diatom floral abundances in the subpolar North Atlantic. The results from this study provide both an overview of climate variability for the entire Holocene and a decadal-scale study focussed around the 8.2 kyr event, where ice rafted debris also has been quantified. The changing composition of diatom assemblages indicates a highly unstable early Holocene from 11.5 to 9 kyr with switches in the dominance of cool Subarctic floras and warmer North Atlantic Current floras. The presence of high productivity events in the diatom floras during this interval suggests that the core location was, at times, in close proximity of the Subarctic Front. An expansion in the importance of cold Arctic/Greenland Current floras occurred from 9 to 7 kyr, followed by a switch to a well developed warm North Atlantic Current flora between 7 and 5 kyr and then more stable conditions during the Late Holocene. Changes in sea surface hydrography, especially the relative strength of the warm North Atlantic Current, are considered to have had the greatest influence on the composition of diatom floral assemblages. The 8.2 kyr event is not recognised as a discrete climate perturbation in the diatom assemblage data, but this event occurred within a broad cooling from 9 to 7 kyr, where the presence of sea-ice and cold water species indicate an increased incursion of the Arctic and Subarctic water masses between 8.8 and 7.8 kyr. The 8.2 kyr event also was not marked by any increase in the delivery of ice rafted debris over the Gardar Drift.

Research Highlights
► Diatom floral communities of the subpolar North Atlantic change through the Holocene.
► Early Holocene is relatively unstable with cool indicator species present.
► The 8.2 kyr cooling is not a stand out event in either diatom or ice rafted debris records.
► Warm North Atlantic Current flora peaks between 7 and 5 kyr.
► This marks the major transition in Holocene diatom assemblages.