Holocene North Atlantic surface circulation and climatic variability: evidence from diatom records

Abstract

Diatom analyses have been performed on a 425cm long Holocene marine sediment sequence from the North Atlantic south of Iceland and a 920cm sediment core spanning the last c. 7500 years from Skalafjord, Faeroe Islands. Additional core data include stable isotopes and results from magnetic susceptibility measurements, while chronostratigraphic control has been provided by AMS 14C measurements. The diatom records reveal distinct changes in North Atlantic surface circulation and climate that have been correlated with published terrestrial and marine records. Initial Holocene warming is dated at 9900 14C years BP, and after a Preboreal cold spell enhanced North Atlantic Current activity and warming prevailed from 9600 to 8800 14C years BP. From 8800 to 8000 "4C years BP the advection of warm Atlantic water masses weakened, presumably under atmospheric circulation conditions characterized by a dominating negative NAO. Prior to the Holocene Climatic Optimum (6000-5000 14C years BP) distinct hydrographic gradients existed in the northern North Atlantic that were probably most pronounced during the reported '8200 cal. years BP cold event'. After 5000 "4C years BP both core records indicate increased climate instability and periods of cooling ('Neoglaciation'), with enhanced cyclone activity affecting the northern North Atlantic particularly at around 4700, 4200, 3200, 2000, 1500 and 1000 14C years BP.

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

“…The elevated relative abundances (~55%) of this species through this interval are particularly distinctive when viewed in the context of the downcore abundances in MD99-2251 and also exceed the values of 44% in the NEAP18B core top from further south on Gardar Drift (where SSTs are~1°C warmer than at core MD99-2251). The distinct peak in T. oestrupii abundance found in MD99-2251 also has a clear counterpart in the mid-Holocene diatom abundance records from core LO09-14 on the Reykjanes Ridge (Andersen et al, 2004b) and the adjacent DS97-2P core site (Witak et al, 2005). The very strong similarities between the records imply that this interval of enhanced NAC flow has a regional significance, and hence might be expected to correlate with warming events at sites further north.…”

“…The data treatment in this study follows the approaches applied in previous studies (e.g., Andersen et al, 2004b;Witak et al, 2005). Assemblage changes are described in terms of variations of individual species percentage abundances and where possible, linked with groups of indicator species based on previous research (Berner et al, 2008).…”

“…Significantly, the warming at the onset of the Holocene at 11.5 kyr is not marked by a widespread expansion in warm assemblage taxa, changes are driven largely by the establishment of the intermediate Subarctic flora. The early Holocene diatom record from nearby core DS97-2P (59°N, 30°W) is dominated by Arctic species, which typically signify high latitude and sea ice conditions, that are quickly replaced by floras dominated by T. gravida vegetative and R. styliformis (Witak et al, 2005). This change is interpreted as a repositioning of the East Greenland Current, but the absence of such a well-developed cold flora in any of the core MD99-2251 samples suggests that this cold water mass must have been situated close to the mid-Atlantic Ridge and didn't extend into the Iceland Basin even during the Younger Dryas stadial.…”

Quantitative Diatom-Based Reconstruction of Paleoenvironmental Conditions in Florida Bay and Biscayne Bay, U.S.A.

“…Freshwater taxa are usually better preserved compared to marine and brackish water species because they generally have thicker walls . Furthermore, siliceous valves usually dissolve fast in anoxic, organic-rich environments with neutral pH, which often explains the absence or very low abundance of diatoms in peat deposits (Bennett et al 1991 (Mcquoid and Hobson 1998;Mcquoid and Nordberg 2003;Witak et al 2005).…”

“…The majority of the diatom-based paleoenvironmental studies have been conducted in freshwater areas, but in the last few decades they have also been successfully carried out in estuaries and coastal regions to study eutrophication , water circulation (Witak et al 2005), water temperature (De Seve 1999), salinity , and sea level changes (Hamilton and Shennan 2005). Most of these investigations have been done in higher latitudes and only a few have been carried out in subtropical and tropical coastal regions (Fluin et al 2007;Taffs et al 2008) and estuaries (e.g., Zong et al 2006;Sounders et al 2007).…”