Earth and Planetary Science Letters 403 (2014) 446–455 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl High-resolution record of late glacial and deglacial sea ice changes in Fram Strait corroborates ice–ocean interactions during abrupt climate shifts ∗ Juliane Müller , Ruediger Stein Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Am Alten Hafen 26, 27568 Bremerhaven, Germany a r t i c l e i n f o a b s t r a c t Article history: The transition from last glacial to deglacial and subsequently to modern interglacial climate conditions Received 18 December 2013 was accompanied by abrupt shifts in the palaeoceanographic setting in the subpolar North Atlantic. Received in revised form 7 July 2014 Knowledge about the role that sea ice coverage played during these rapid climate reversals is limited Accepted 14 July 2014 since most marine sediment cores from the higher latitudes provide only a coarse temporal resolution Available online 21 August 2014 and often poorly preserved microfossils. Here we present a highly resolved reconstruction of sea ice Editor: J. Lynch-Stieglitz conditions that characterised the eastern Fram Strait – a key area for water mass exchange between Keywords: the Arctic Ocean and the North Atlantic – for the past 30 ka BP. This reconstruction is based on the sea ice distribution of the sea ice biomarker IP25 and phytoplankton derived biomarkers in a sediment core subpolar North Atlantic from the continental slope of western Svalbard. During the late glacial (30 ka to 19 ka BP), recurrent LGM advances and retreats of sea ice characterised the study area and point to a hitherto less considered Heinrich Event 1 oceanic (and/or atmospheric) variability. Along-lasting perennial sea ice coverage in eastern Fram Strait Younger Dryas persisted only at the very end of the Last Glacial Maximum (i.e. from 19.2 to 17.6 ka BP) and was AMOC abruptly reduced at the onset of Heinrich Event 1 – coincident with or possibly even inducing the collapse of the Atlantic Meridional Overturning Circulation (AMOC). Further maximum sea ice conditions prevailed during the Younger Dryas cooling event and support the assumption of an AMOC reduction due to increased formation and export of Arctic sea ice through Fram Strait. Asignificant retreat of sea ice and sea surface warming are observed for the Early Holocene. © 2014 Elsevier B.V. All rights reserved. 1. Introduction MARGO Project, 2009). Further, the rate of North Atlantic Deepwa- ter (NADW) formation in the Greenland–Iceland–Norwegian (GIN) There exists general consensus that the subpolar North At- Seas and the strength of the Atlantic Meridional Overturning Cir- lantic up to the Fram Strait – the only deepwater connection to culation (AMOC) are research targets for understanding the often the Arctic Ocean and the main outlet for sea ice (Aagaard, 1982; abrupt climate shifts that characterised the transition from last Aagaard and Carmack, 1989; Rudels and Quadfasel, 1991)– ex- glacial to interglacial conditions (Rahmstorf, 2002; McManus et perienced significant fluctuations in the oceanic circulation during al., 2004). Most of these reconstructions of AMOC strength are the late glacial and deglacial and that these fluctuations are as- based upon stable isotope data obtained from foraminifera and sociated with global climate changes. Various proxy and model provide valuable information on the ventilation or stratification of (time-slice) reconstructions of the Last Glacial Maximum (LGM; the water column (e.g. Bauch et al., 2001; Benway et al., 2010; commonly defined as the period between 23 ka and 19 ka BP) Waelbroeck et al., 2011). led to the meanwhile widely approved notion of a seasonally ice- Only very limited information is available about the variable free eastern corridor in the northern North Atlantic permitting sea ice extent and thus its considerable effect on the water column moisture release required for the last glacial build-up of Northern structure in the subpolar North Atlantic during the late glacial and Hemisphere ice-sheets (Hebbeln et al., 1994; Sarnthein et al., 2003; deglacial remains unclear. Microfossil-based sea ice reconstruc- Schulz and Paul, 2004; Meland et al., 2005; de Vernal et al., 2006; tions mainly build upon assemblage analyses of diatoms (Koç et al., 1993; Ran et al., 2006), foraminifera (Sarnthein et al., 2003; * Corresponding author. Tel.: +49 331 288 2224. Slubowska-Woldengen et al., 2008)or dinoflagellate cysts (de Ver- E-mail address: [email protected] (J. Müller). nal et al., 2013) and provide important palaeoenvironmental http://dx.doi.org/10.1016/j.epsl.2014.07.016 0012-821X/© 2014 Elsevier B.V. All rights reserved. J. Müller, R. Stein / Earth and Planetary Science Letters 403 (2014) 446–455 447 Fig. 1. Study area with core sites MSM5/5-712-2 (yellow circle) and PS2837-5 (orange circle). The major ocean currents (NC = Norwegian Current, WSC = West Spitsbergen Current; EGC = East Greenland Current) and the modern average summer sea ice extent (white dotted line) are indicated. Also included is the modelled LGM sea ice distribution by Stärz et al. (2012) with MSM5/5-712-2 and PS2837-5 core sites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) information. The often lowered preservation potential of sea ice assessment of palaeo sea ice conditions. Given that other factors associated diatoms and foraminifera (due to silica or carbonate than sea ice might control phytoplankton productivity, and this dissolution) in the Arctic Ocean and subpolar North Atlantic (e.g. has been recently shown for the Labrador Sea (Weckström et al., Scott and Vilks, 1991; Schlüter and Sauter, 2000; Zamelczyk et al., 2013), caution should be exercised when interpreting PIP25 values. 2014), however, often hamper the reconstruction of the spatial and The Arctic-wide applicability of the PIP25 index and the accuracy temporal distribution of sea ice in the past. The sea ice biomarker of the quantitative sea ice estimates derived from this approach proxy IP25, ahighly branched isoprenoid (HBI) monoene with 25 thus require further evaluation. Amore recent derivative is the carbon atoms (C25) derived from sea ice diatoms (Belt et al., 2007; so-called DIP25 index (Cabedo-Sanz et al., 2013), which builds Brown et al., 2014), has been found stable within Arctic Ocean upon the observation by Rowland et al. (2001) that the degree sediments for at least the entire Quaternary (Stein and Fahl, of unsaturation of C25-HBI molecules produced by a certain di- 2013) and thus enables the assessment of sea ice changes in atom species decreases with decreasing algal growth temperature. the High Northern Latitudes even during the distant past. De- Fahl and Stein (2012) and Stein et al. (2012) thus suggested and spite the novelty of this proxy and a certain lack of knowledge demonstrated that the ratio of sedimentary C25-HBI diene to IP25 about how environmental factors (salinity, nutrient and light avail- contents could serve as an indicator for palaeo sea surface temper- ability) might affect the productivity of IP25 synthesising sea ice atures (SSTs) in the Arctic realm. This approach has been supported diatoms, IP25 has been applied frequently for reliable sea ice re- by palaeoenvironmental reconstructions (Stein and Fahl, 2012; constructions covering different areas of the Arctic Ocean (for Cabedo-Sanz et al., 2013) and surface sediment analyses (Xiao et recent reviews see Stein et al., 2012; Belt and Müller, 2013). With al., 2013). Further investigations into the reliability of the DIP25 as the PIP25 index, a recent approach combining IP25 and phyto- well as the PIP25 index, however, are required to fully exploit the plankton biomarker data to distinguish between different sea ice capacity of these approaches (we refer to Belt and Müller, 2013 for conditions, a promising step towards semi-quantitative sea ice re- an overview of potential limitations of both indices). constructions, in particular for the Fram Strait area, has been made One of the first IP25-based sea ice reconstructions has been (Müller et al., 2011). The consideration of phytoplankton-derived conducted on a sediment core PS2837-5 from the Yermak Plateau biomarkers (e.g. brassicasterol or dinosterol; Volkman, 2006) – (Fig. 1), where the authors found significant variations in the sea indicative of open-water conditions – alongside IP25 proves very ice cover in northern Fram Strait during the past 30 ka BP (Müller helpful to avoid a misleading interpretation of the absence or et al., 2009). Unfortunately, this record does not provide a suitably minimum concentration of IP25, which may result from either high temporal resolution to document the transitions from cold to a lack of sea ice or, on the contrary, a permanent and thick warm (and vice versa) climate intervals in much detail. sea ice cover reducing the light penetration through the ice and Herein we present a new high-resolution sea ice record based thus inhibiting ice algae growth (Horner and Schrader, 1982; on biomarker data of a sediment core from the eastern Fram Müller et al., 2009). The PIP25 index (the ratio of IP25 against the Strait. By considering both IP25 and the phytoplankton derived sum of IP25 and a phytoplankton biomarker) basically constitutes biomarker lipids brassicasterol and dinosterol (Kanazawa et al., a proxy for the intensity of a sea ice cover during spring and sum- 1971; Volkman, 2006)we gain valuable insight into the palaeo- mer with high PIP25 values (> 0.7) reflecting an extended and ceanographic and environmental development in this climate sen- low PIP25 values (< 0.5) reflecting a reduced ice cover (Müller sitive area between 30 ka and 9ka BP. The core site at the western et al., 2011). Müller et al.