Holocene cooling culminates in sea ice oscillations in Fram Strait
Highlights
► Biomarker and IRD data give insight into Holocene sea ice conditions in Fram Strait. ► We find increasing sea ice coverage off West Spitsbergen throughout the Holocene. ► Oceanic/atmospheric variability caused Neoglacial sea ice fluctuations. ► Ice conditions along East Greenland shelf remain rather stable until 1000 years BP.
Introduction
The extent of sea ice coverage in Fram Strait, the major gateway connecting the Arctic with the Atlantic Ocean, is intrinsically tied to the advection of warm Atlantic Water along the continental margin of West Spitsbergen. As these temperate waters head to the north, they encounter polar water (and air) and sea ice from the Arctic Ocean, which causes cooling, freezing and thus brine rejection, and subsequent descent of Atlantic Water into the Nordic Sea's deep sea basins via the Greenland Sea Gyre (Fig. 1; Aagaard, 1982; Rudels and Quadfasel, 1991; Watson et al., 1999). These processes are of crucial importance for the so-called Nordic heat pump, which bestows a comparatively temperate climate upon Europe (e.g. Broecker, 1992). The climate-shaping impact of sea ice that exits Fram Strait became particularly evident during the “Great Salinity Anomaly” in the 1970s, when an enormous discharge of Arctic sea ice hampered the thermohaline convective overturn in the North Atlantic (in terms of a vast low salinity freshwater lense), which resulted in a significant cooling in the North Atlantic area (Dickson et al., 1988; Belkin et al., 1998; Dima and Lohmann, 2007; Sundby and Drinkwater, 2007). Recently, Spielhagen et al. (2011) identified and linked natural fluctuations in the advection of Atlantic Water towards Fram Strait with shifting warm and cold climate intervals like the Medieval Climate Anomaly or the Little Ice Age. Furthermore, an unprecedented warming of North Atlantic Water throughout the past 120 years is reconstructed that highlights the importance of the direct feedback mechanisms between the atmospheric (global) warming, the oceanic heat transport through Fram Strait and the sea ice decline in the Arctic realm (Spielhagen et al., 2011). These dynamic interactions account for the Arctic amplification, which impacts not only on the Arctic Ocean but also on adjacent terrestrial (permafrost) areas and finally the global climate system (Lawrence et al., 2008; Serreze and Barry, 2011). Overland and Wang (2010), for example, put emphasis on the loss of Arctic sea ice and the resulting changes in large-scale atmospheric circulation patterns and the consequences for mid-latitude weather (wind) regimes.
The finding of past variations in the sea ice distribution in Fram Strait thus supports the identification of palaeo-fluctuations in the intensity of Atlantic Water inflow and may reveal periods of a strengthened or weakened thermohaline circulation and/or atmospheric (North Atlantic Oscillation; NAO-like) forcing. The influence of the NAO on climate and sea ice conditions in the (sub)Arctic realm frequently has been appraised as fundamental, though hardly assessable or predictable due to its highly variable temporal evolution (e.g. Dickson et al., 2000; Hurrell and Deser, 2010). In short, positive NAO phases are accompanied by stronger westerlies carrying moist air over Europe and Siberia, an increased Atlantic Water inflow through Fram Strait, and warmer temperatures in the Arctic, which lead to a reduction in sea ice formation. During intervals of a negative NAO these phenomena occur to be reversed (Dickson et al., 2000; Kwok, 2000; Hurrell and Deser, 2010). Within their thorough review about Arctic sea ice and its interaction with the atmosphere Bader et al. (2011) illustrate comprehensively how the current sea ice reduction leads to a poleward shift and an intensification of storm tracks, while the immediate impact on the NAO itself remains undetermined. Vice versa, the distinct impact of the NAO on the sea ice extent (e.g. in the Nordic Seas) has been acknowledged and documented more often (Deser et al., 2000; Dickson et al., 2000; Vinje, 2001). Given the lack of instrumental records of the NAO variability prior to 1932 (the first calculation of the NAO index dates back to 1932; Walker and Bliss, 1932) the attempts to link atmospheric fluctuations with climate changes are restricted to proxy reconstructions or numerical model experiments (e.g. Rimbu et al., 2004; Lorenz et al., 2006; Trouet et al., 2012). For instance, an Early to Late Holocene decrease in North Atlantic SSTs is interpreted to reflect a general long-term weakening of the NAO-like atmospheric circulation pattern (Rimbu et al., 2003). On shorter time scales, however, it seems essential to distinguish between the intensity and the frequency of cyclones in the North Atlantic to reasonably relate proxy data to Late Holocene NAO shifts (Trouet et al., 2012).
Though Northern Hemisphere climate (boundary) conditions throughout the Holocene are generally considered as fairly stable (Grootes and Stuiver, 1997), variations in sea surface temperatures (SSTs), glacier growth or terrestrial vegetation communities are increasingly substantiated within marine and terrestrial Arctic palaeoclimate studies (Birks, 1991; Svendsen and Mangerud, 1997; Andersen et al., 2004; for recent review see; Miller et al., 2010). Recently, Risebrobakken et al. (2011) demonstrated reasonably that, when interpreting marine proxy derived climate information (e.g. SSTs) in the Nordic Seas, the individual impacts of orbital forcing (mainly affecting sea surface conditions) and oceanic heat advection (affecting deeper parts of the ocean and convective processes) requires careful consideration as these are different mechanisms of climate change. Thus, the partly contradictory Holocene SST reconstructions in the Nordic Seas, which are based on coccolithophore-derived alkenone or foraminifer data (Calvo et al., 2002; Risebrobakken et al., 2003) can be explained by the simple fact that different proxies may respond to different mechanisms (Risebrobakken et al., 2011).
Concerning sea ice conditions, Holocene changes in the ice coverage in the Nordic Seas, however, have been deduced mainly indirectly from microfossil or geochemical data (Andrews et al., 2001; Jennings et al., 2002; Bonnet et al., 2010). A quantitative approach using diatom transfer functions in the Nordic Seas has been presented by Justwan and Koç (2008). By means of a sediment core north off Iceland, they reconstruct relatively constant sea ice concentrations of ca 5%–10% for the Early Holocene and slightly higher sea ice concentrations of about 10%–20% during the Late Holocene (Justwan and Koç, 2008). The application of this promising approach, however, may be limited by the comparatively high silica dissolution rate in the High Northern Latitudes (Kohly, 1998; Schlüter and Sauter, 2000).
The molecular sea ice proxy IP25 – a biomarker lipid associated with sea ice diatoms – seems to be a direct and thus valuable tool for the reconstruction of a previous spring sea ice cover in the Arctic (Belt et al., 2007; Brown, 2011). Besides the identification of highly branched C25 and C30 isoprenoids as diatom specific biomarkers (e.g. Rowland and Robson, 1990; Volkman et al., 1994; Massé et al., 2004) that even may be found in Cretaceous sediments (Damsté et al., 2004), the derivation of the monounsaturated C25 highly branched isoprenoid (i.e. the IP25 alkene) from diatoms that live within the Arctic sea ice has been strengthened in various studies (Belt et al., 2008; Brown et al., 2011; Brown and Belt, 2012). With regard to this distinct association of IP25 with sea ice, the detection even of trace abundances of this molecule in a sediment sample, which indeed is a question of instrumental sensitivity, may directly serve as an indication of a previous ice cover. The increasing use of IP25 for palaeo sea ice assessments and its agreement with other proxy (Massé et al., 2008; Müller et al., 2009; Vare et al., 2009, Vare et al., 2010; Belt et al., 2010) and instrumental data (Müller et al., 2011) on sea ice occurrences hence supports the applicability of this biomarker.
In 2009, Vare et al. and Müller et al. presented reconstructions of sea ice conditions based on the IP25 content in sediment cores from the central Canadian Archipelago and northern Fram Strait, respectively, which cover the entire Holocene. Both studies suggest gradually increasing (spring) sea ice occurrences from the Mid to the Late Holocene, presumably as a response to the Neoglacial cooling (Müller et al., 2009), but do not provide an in-depth analysis of the palaeoenvironmental and palaeoceanographic setting. The Neoglaciation – the general use of this term was first suggested by Porter and Denton (1967) – covers the period characterised by glacier advances, southward migration of the northern treeline and colder sea surface conditions in different regions of the Northern Hemisphere that followed the warm Early to Mid Holocene (for further review see Wanner et al., 2008; Miller et al., 2010 and references therein).
The main objective of this study is to estimate to what extent this Holocene cooling affected the sea ice distribution in the Fram Strait and the East Greenland shelf. For this purpose, organic geochemical and IRD analyses were performed on sediment cores from the western continental margin of Spitsbergen and the continental shelf of East Greenland. This provides for a reconstruction of the spatial and temporal evolution of the sea ice coverage within the two most important oceanic (in and outlet) pathways that characterise the Fram Strait and influence the Arctic Ocean heat budget. The findings are compared and contextualised with previous palaeoenvironmental reconstructions for the study area.
Section snippets
Regional setting
The environmental setting in Fram Strait is controlled by a dynamic ocean current system and, owing to the high latitude, a distinct seasonality. Warm and saline Atlantic Water is directed northwards towards Fram Strait by the Norwegian Current (NC) and the West Spitsbergen Current (WSC), thus constituting the northernmost area of open (ice-free) water in the Arctic during winter (Fig. 1; Vinje, 1977; Aagaard, 1982). South of Spitsbergen these temperate waters encounter cold water and sea ice,
Sediment material and methodology
The sediment cores MSM5/5-712-2 and MSM5/5-723-2 were obtained from the western continental margin of Spitsbergen (at 78°54.94 N, 6°46.03 E; 1487 m water depth, and at 79°09.66 N, 5°20.27 E; 1349 m water depth, respectively) during a Maria S. Merian cruise in 2007 (Budéus, 2007). The core sites are both located in close vicinity to the modern winter sea ice margin (Fig. 1). Sediment cores were stored at −30 °C until further treatment. For organic geochemical analyses subsamples were taken each
Core chronologies
The chronology of the sediment cores MSM5/5-712-2 and MSM5/5-723-2 is based upon AMS 14C ages obtained from tests of the polar planktic foraminifer Neogloboquadrina pachyderma (sin.), whereas the age model of the sediment core PS2641-4 is based upon AMS 14C ages that were obtained from tests of benthic foraminifera. Additionally, AMS 14C ages were derived from shells of the bivalve Bathyarca glacialis (Evans et al., 2002). For the Maria S. Merian cores a marine reservoir correction of 408 years
West Spitsbergen continental margin (cores MSM5/5-712-2 and MSM5/5-723-2)
On the base of our organic geochemical and IRD records the sedimentary sequence of core MSM5/5-712-2 can be separated into three intervals covering the past 8500 years BP (Fig. 3). Results obtained on core MSM5/5-723-2 cover the past 7000 years BP (Fig. 4).
In core MSM5/5-712-2 the late Early Holocene (8500–7000 years BP) is characterised by lowest IRD counts (<20 grains per gram sediment), reduced TOC (0.8–1 wt%) and moderate to maximum CaCO3 contents (10–16 wt%). Accumulation of
Discussion
With the identification of the sea ice biomarker IP25 in the sediment cores MSM5/5-712-2, MSM5/5-723-2, and PS2641-4 we yield novel and direct information about the development of the sea ice conditions along the West Spitsbergen continental margin and the continental shelf of East Greenland throughout the Holocene (Fig. 6). Coincident with the sustained cooling, which is inferred from decreasing δ18O values in the NGRIP Greenland ice core (NGRIP-Members, 2004) and a decline in Northern
Conclusions
The Holocene sea ice evolution in eastern and western Fram Strait is reconstructed by means of the sea ice proxy IP25, IRD data and the phytoplankton-derived biomarkers brassicasterol and dinosterol. In line with a lowered Northern Hemisphere insolation and decreasing temperatures, the (spring) sea ice coverage along the western continental margin of Spitsbergen increased between 8500 and 1000 years BP. In contrast, sea ice conditions at the inner shelf of East Greenland probably remained
Acknowledgements
Robert Spielhagen (IFM-GEOMAR, Kiel, Germany), Christian Hass (AWI-Sylt, Germany) and Jacques Giraudeau (EPOC, Université Bordeaux 1/CNRS, France) are kindly acknowledged for providing AMS 14C datings of the Maria S. Merian cores. Financial support was provided by the Deutsche Forschungsgemeinschaft through SPP INTERDYNAMIK (STE 412/24-1). We wish to thank the two anonymous reviewers for providing comments and suggestions that improved an earlier version of this manuscript.
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