Quaternary Dinoflagellate Cysts in the Arctic Ocean

Quaternary Science Reviews 192 (2018) 1e26

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Quaternary Science Reviews

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Invited Review Quaternary dinoﬂagellate cysts in the Arctic Ocean: Potential and limitations for stratigraphy and paleoenvironmental reconstructions

* Jens Matthiessen a, , Michael Schreck b, Stijn De Schepper c, Coralie Zorzi d, Anne de Vernal d a Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, D-27568 Bremerhaven, Germany b Department of Geosciences, University of Tromsø e The Arctic University, N-9037 Tromsø, Norway c Uni Research Climate, Bjerknes Centre for Climate Research, Jahnebakken 5, N-5007 Bergen, Norway d UniversiteduQuebec a Montreal, Centre GEOTOP, Montreal, H3C 3P8, Quebec, Canada article info abstract

Article history: The Arctic Ocean is a siliciclastic depositional environment which lacks any rock-forming biogenic Received 5 June 2017 calcareous and siliceous components during large parts of its Quaternary history. These hemipelagic Received in revised form sediments are nevertheless suitable for the study of organic-walled microfossils of which the fossil re- 20 December 2017 mains of dinoflagellates - dinoflagellate cysts - are the most important group. Dinoflagellate cysts have Accepted 22 December 2017 become an important tool in paleoceanography of the high northern latitudes, but their potential for Available online 18 May 2018 Quaternary biostratigraphy has remained largely unexplored. Dinoflagellate cysts are the dominant marine palynomorph group which is more continuously present Keywords: Arctic Ocean in the marginal seas (e.g. Barents Sea, Bering Sea) than in the Arctic Ocean itself throughout the Qua- Quaternary ternary. Most species have long stratigraphic ranges, are temporary absent and show abundance varia- Dinoflagellate cysts tions on glacial-interglacial timescales. Of the more than 30 taxa recorded, only Habibacysta tectata and Biostratigraphy Filisphaera filifera became extinct in the Pleistocene. The highest persistent occurrence of H. tectata at ca. Bioevents 2.0 Ma and the top of F. filifera acme at ca. 1.8 Ma can be used for supra-regional stratigraphic correlation Lithostratigraphy between the Arctic Ocean and adjacent basins. These events corroborate a slow sedimentation rate Composite chronostratigraphy model for the Quaternary section on the central Lomonosov Ridge, but a combination of different methods will have to be applied to provide a detailed chronostratigraphy. The occurrence of cysts of phototrophic dinoflagellates in certain stratigraphic intervals on Lomonosov Ridge supports published evidence of episodic opening of the multiyear Arctic sea ice cover during the Quaternary probably related to a stronger inflow of Atlantic water. This contradicts the hypothesis of a permanently ice covered central Arctic Ocean in the Quaternary. © 2018 Elsevier Ltd. All rights reserved.

1. Introduction seas during certain periods in the Quaternary (e.g., Svendsen et al., 2004; Ehlers and Gibbard, 2007; Knies et al., 2009; Alley et al., The Quaternary period (the last 2.58 million years; Gibbard and 2010; Laberg et al., 2010; Molodkov and Bolikhovskaya, 2010; Head, 2010) experienced the substantial expansion of the high Jakobsson et al., 2014; Bierman et al., 2016; Schaefer et al., 2016; northern latitude cryosphere as a response to a significant atmo- see Ehlers et al., 2011 and chapters therein). As a consequence of spheric and surface ocean cooling (e.g. Shackleton et al., 1984; waxing and waning ice sheets, sea level fluctuated considerably Raymo, 1994; Maslin et al., 1998 and references therein; Kleiven leading to frequently closing and opening of shallow gateways to et al., 2002; Fedorov et al., 2013). Large ice sheets recurrently the Arctic Ocean, such as the Bering Strait, resulting in the modi- formed on the circum-arctic continents and continental shelves, fication of the Arctic Ocean circulation (e.g., Backman et al., 2009; and covered Eurasia, Greenland, North America and adjacent shelf Matthiessen et al., 2009; O'Regan et al., 2010). Reconstructing the glaciation history from terrestrial records is hampered by incomplete sedimentary sequences and reworking of deposits by subsequent glaciations (e.g., Svendsen et al., 2004). * Corresponding author. However, evidence for glaciations on the circum-arctic continents E-mail address: [email protected] (J. Matthiessen). https://doi.org/10.1016/j.quascirev.2017.12.020 0277-3791/© 2018 Elsevier Ltd. All rights reserved. 2 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 may be found in the marine environment because ice sheets 1996; McNeil et al., 2001; Dixon et al., 2008), but the first scientific advancing across the shelves calve icebergs into the oceans and/or sites that cover a complete central Arctic Quaternary sequence were form extensive ice shelves that release clastic sediments when drilled on the Lomonosov Ridge only in 2004 (Fig. 1; IODP Expedition melting, and also grounding of glacial ice leaves landforms on the 302, “Arctic Coring Expedition” (ACEX), Backman et al., 2006). Sec- sea floor (e.g., Clark et al., 1980; Polyak et al., 2001; Spielhagen et al., ondly, a multitude of methods provide contradictory stratigraphic 2004; Krylov et al., 2008; Stein et al., 2010b; Niessen et al., 2013; interpretations (see Section 2 below). Among the biostratigraphic Darby, 2014; Jakobsson et al., 2014, 2016). Additionally, marine proxies, marine palynology has been utilized to establish age models sediments archive the oceanographic conditions in the past, for short sediment cores (e.g., Mudie, 1985; Matthiessen et al., 2001) including the variability of the arctic pack ice cover, providing a link but has not been applied yet on the ACEX composite record at a between ice sheets and ocean conditions (e.g., Polyak et al., 2010; higher resolution for the entire Quaternary. Cronin et al., 2010, 2013). Based on published and new data, we review the stratigraphic An important prerequisite for reconstructing Arctic paleo- occurrence of marine palynomorphs in Quaternary sediments of ceanography and glacial history is a sound chronostratigraphy, but the Arctic Ocean, and evaluate their potential for establishing a this is still lacking for the marine sedimentary archives in the central biostratigraphy. We provide an overview on the status of the Arctic Ocean. Firstly, most marine time series only comprise Middle Quaternary chronostratigraphy and the different methods applied to Upper Pleistocene sediments because gravity and piston cores, to develop age models (Section 2) followed by a review of Qua- collected over the past decades, do not extend to the base of the ternary dinoflagellate cyst biostratigraphy (Section 3). New paly- Quaternary (e.g., Weber and Roots, 1990; Thiede et al., 1990; nostratigraphic data from the Lomonosov Ridge (IODP Expedition Backman et al., 2004; Stein, 2008; Stein et al., 2010b; Polyak et al., 302), the northern Barents Sea margin (ODP Leg 151) and the 2013). Quaternary sediments have been extensively recovered at Bering Sea (IODP Expedition 323) are presented in Section 4. the margins of the Arctic Ocean (e.g., Myhre et al., 1995; Jansen et al., Possible processes that may influence the stratigraphic occurrence

Fig. 1. Modern annual sea surface temperatures in the Northern Hemisphere (base map from De Schepper et al., 2015). All sites mentioned in the text are shown (1, Kap København ^ Formation; 2, Ile de France Formation; 3, Store Koldewey Formation; 4, Lake El'gygytgyn). Inset map (Jakobsson et al., 2012) shows geographic features mentioned in the text (abbreviations: AB, Amerasian Basin; AR, Alpha Ridge; BMB, Beaufort-Mackenzie Basin; BS, Barents Sea; EB, Eurasian Basin; FS, Fram Strait; LR, Lomonosov Ridge; MR, Mendeleev Ridge; YP, Yermak Plateau; #10, GreenICE #10). J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 3 of marine palynomorphs in the Arctic Ocean are discussed before paleomagnetic polarity patterns (for reviews see Darby et al., 1989; the potential of palynostratigraphy for contributing to a high- Backman et al., 2004; Stein, 2008; Alexanderson et al., 2014). These latitude Quaternary chronostratigraphy is evaluated (Section 5). patterns were initially interpreted in terms of polarity reversals The implications of the new stratigraphic markers for the Arctic with geomagnetic excursions not being considered for alternative Ocean chronostratigraphy are discussed (Section 6), and long-term interpretation (Fig. 2; e.g., Steuerwald et al., 1968; Clark, 1970; paleoenvironmental change is considered (Section 7). Finally, Herman, 1974; Clark et al., 1980; Aksu and Mudie, 1985; Aksu, future directions of an Arctic Ocean chronostratigraphy are pro- 1985a; Witte and Kent, 1988). The paleomagnetic record thus led posed in Section 8. to propose very low sediment accumulation rates, which was supported by sedimentological evidence such as the occurrence of 2. Quaternary stratigraphy in the Arctic Ocean ferromanganese nodules (Clark et al.,1980). This approach has been widely used in combination with a standard lithostratigraphy 2.1. Contradictory interpretations of paleomagnetic polarity introduced by Clark et al. (1980) which deﬁnes 13 correlatable patterns stratigraphic units composed of silty to sandy muds and detrital carbonate-rich layers for sediments from submarine highs in the The ﬁrst age models for the relatively short sediment cores Amerasia Basin (e.g., Minicucci and Clark, 1983; Aksu and Mudie, (<10 m) from the central Arctic Ocean have been developed from 1985; Mudie and Blasco, 1985; Poore et al., 1993; Clark et al.,

Fig. 2. Alternative chronostratigraphic interpretations of Pleistocene sediments on Alpha Ridge (CESAR 83e102) in the central Arctic Ocean. The initial chronostratigraphy suggests that the change to negative inclinations between 85 and 90 cm in lithological Unit K marks the Brunhes/Matuyama boundary (Aksu, 1985a; Macko and Aksu, 1986; ca. 773 ka, Singer, 2014). Nørgaard-Pedersen et al. (2007a) placed this reversal into MIS 7 based on lithostratigraphic correlation to sediment core GreenICE #10 that has been dated by AMS14C ages, biostratigraphy, stable oxygen and carbon isotope stratigraphy on planktonic foraminifer Neogloboquadrina pachyderma. This interpretation is here slightly revised assuming that the concentration maxima of planktonic foraminifer Turborotalia quinqueloba and other subpolar species mark the top and base of MIS 5 (Nørgaard-Pedersen et al., 2007a,b; Adler et al., 2009). The lower maximum corresponds to the upper acme of Bolivina arctica (HCO: highest common occurrence). MIS 4 now corresponds to the minimum of planktonic foraminifer concentrations. MIS 3 is constrained by two tandem accelerator 14C ages (black bars) at 2e3 cm (31 ka BP ± 290) and 5e6 cm (54 ka BP ±1042) (Scott et al., 1989). The reversal may be interpreted as the Iceland Basin excursion which is the ﬁrst accepted excursion below MIS 5 in the GITS (Singer, 2014). This excursion was recently dated to the MIS6/7 boundary (Channell, 2014). Source of data: A, original magnetostratigraphic interpretation (Aksu, 1985a). The transition to the possible Iceland Basin excursion is indicated by an arrow; B, lithostratigraphic units (Mudie and Blasco, 1985; Scott et al., 1989) based on the terminology of Clark et al. (1980). Only white layers (W2, W3) and pink-white layer PW2 are recorded in this relatively short sediment core; C, stable isotope stratigraphy based on N. pachyderma (size fraction > 63e375 mm; Aksu, 1985b); D, concentrations of total planktonic and benthic foraminifers (size fraction > 63e375 mm; Aksu, 1985b); E-G, concentrations and relative abundances (size fraction >63e375 mm) of N. pachyderma, T. quinqueloba (Aksu, 1985b), and Bolivina arctica (size fraction >63 mm, Scott et al., 1989); H, age model of Nørgaard-Pedersen et al. (2007a); I, revised age model (this study). 4 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26

1990, 2000; Clark, 1996; Stein et al., 2010a, 2010b). The age models paleomagnetic record by diagenetic processes in deep-sea sedi- resulted in sedimentation rates on the order of millimetres per ment from the central Arctic Ocean complicate understanding of thousand years and thus the oldest sediments recovered in the the origin of magnetic excursions (Channell and Xuan, 2009; Xuan short sediment cores from the central Arctic Ocean were consid- and Channell, 2010; Xuan et al., 2012) and therefore the use of the ered to be of latest Miocene to earliest Pliocene age (e.g., Herman, paleomagnetic records for chronostratigraphy. 1970; Clark et al., 1980, 2000; Aksu and Mudie, 1985; Spielhagen et al., 1997). 2.2. Equivocal evidence from radioisotope and geochemical The lack of independent and conclusive age control for the methods magnetostratigraphic interpretations, however, was the main problem of these initial age models and other chronological Since an unambiguous interpretation of the paleomagnetic re- schemes have been proposed. When applying stable isotope stra- cords is difficult, a number of methods were applied to obtain tigraphy and amino acid ratios using the planktonic foraminifer numerical ages for marine sediments in the Arctic Ocean (for a Neogloboquadrina pachyderma at the northern Barents Sea margin detailed review of methods see Alexanderson et al., 2014). How- an age younger than 60,000 years for alternations of normal and ever, these methods (1) are limited in their applicability to Upper reversed polarity was estimated (Løvlie et al., 1986). Therefore, Quaternary sediments, (2) are not calibrated to an independent Løvlie et al. (1986) interpreted the intervals with reversed polarity chronology, and/or (3) provide contradictory age information. as geomagnetic excursions within the normal polarity Brunhes Radiocarbon ages allowed establishment of detailed chronology for Chron (see also Darby et al., 1989). Stable oxygen isotope stratig- the past 50,000 years (e.g., Stein et al., 1994; Nørgaard-Pedersen raphy in combination with calcareous nannofossil stratigraphy and et al., 1998, 2003; Nowaczyk et al., 2003; Polyak et al., 2009; AMS 14C dates subsequently confirmed that reversed polarity ex- Hanslik et al., 2010). AMS 14C ages older than 30,000 years for cursions frequently occurred in Middle to Upper Pleistocene sedi- near-surface sediments (Fig. 2; Ku and Broecker, 1965; Clark et al., ments at the Barents Sea margin (e.g., Nowaczyk and Baumann, 1986; Scott et al., 1989; Spielhagen et al., 1997) were taken as 1992; Nowaczyk et al., 1994, 2003). However, in the central Arctic support for the initial age models interpreting the first downcore Ocean, stable oxygen isotope records of N. pachyderma are either change in the magnetic polarity as the base of the Brunhes Chron. discontinuous or difficult to compare with the global record to However, high-resolution dating revealed a condensed last glacial unequivocally define marine isotope stages (MIS) that may help to (MIS 2) in the Arctic Ocean bracketed by periods with relatively constrain ages for polarity reversals (Fig. 2; Aksu, 1985b; Polyak high sedimentation rates (e.g., Nørgaard-Pedersen et al., 1998, et al., 2004; Spielhagen et al., 2004; Nørgaard-Pedersen et al., 2003; Poore et al., 1999; Polyak et al., 2009; Hillaire-Marcel et al., 2007a, 2007b; Adler et al., 2009). Calcareous nannofossil stratig- 2017). Furthermore, radiocarbon reservoir ages in the Arctic Ocean raphy and optical stimulated luminescence ages led to suggest that are insufficiently known (e.g., Hanslik et al., 2010). Also, the ages of the first downcore change to reversed polarity on the central small samples near the limits of 14C application for old samples Lomonosov Ridge is younger than the Brunhes/Matuyama bound- should be considered with caution because of possible contami- ary (Jakobsson et al., 2000, 2003) and thus the frequent polarity nation (Hughen, 2007), notably through diagenetic processes and changes in short sediment cores (e.g., Frederichs, 1995; Jakobsson carbonate recrystallisation (Douka et al., 2010; Sivan et al., 2002). et al., 2000) rather represent geomagnetic excursions than re- Amino acid epimerization on planktonic foraminifer versals. Although this age model is strictly applicable only on N. pachyderma is potentially useful for providing numerical ages sedimentary sequences from the central Lomonosov Ridge, it has beyond the range of radiocarbon dating if these data were cali- been transferred to other regions of the Arctic Ocean suggesting brated to an independent chronology (Sejrup et al., 1984; Løvlie that the oldest sediments in most short sediment cores in the et al., 1986; Macko and Aksu, 1986; Kaufman et al., 2008, 2013). central Arctic Ocean have a Pleistocene rather than a Neogene age. Optical stimulated luminescence dating yielded reliable ages only Nevertheless, radioisotope data still support the initial magneto- for last interglacial ages, providing an independent age for the base stratigraphic interpretation that the first downcore change in the of MIS 5 on the central Lomonosov Ridge, previously defined only magnetic polarity in short sediment cores from the Mendeleev from calcareous nannofossil stratigraphy (Jakobsson et al., 2003). Ridge is the base of the Brunhes Chron, and contradictory age 10Be/9Be stratigraphy has provided useful age control in the Arctic models are published for individual sediment cores (Gusev et al., Ocean on Quaternary and Neogene time scales (Eisenhauer et al., 2013; Piskarev et al., 2013; Elkina, 2014; Krylov et al., 2014). 1994; Aldahan et al., 1997; Frank et al., 2008; Dausmann et al., Not all excursions recorded in the Arctic Ocean are accepted in 2015), but age models for short piston cores from Alpha and the Quaternary Geomagnetic Instability Time Scale (e.g. Biwa Mendeleev Ridge are in conflict with those derived from amino acid events, Singer, 2014), nor is their timing definitely resolved. Hence, racemization and calcareous nannofossil stratigraphy (Sellen et al., in the absence of unequivocal independent chronology, the first 2009). The potential of 230Th excess dating has not been fully polarity change below the long upper Middle to Upper Pleistocene evaluated yet, but calculated ages at many sites support the initial interval of normal polarity has been interpreted as an excursion magnetostratigraphic interpretations that contradict the chronol- belonging to Biwa II (Jakobsson et al., 2000, 2001; Spielhagen et al., ogy of Jakobsson et al. (2000) (e.g., Ku and Broecker, 1965; Herman 2004; Krylov et al., 2014), Pringle Falls (Stein, 2008; Stein et al., et al., 1989; Not and Hillaire-Marcel, 2010; Gusev et al., 2013; 2010b) or Iceland Basin excursions (Bazhenova, 2012). The latter Hillaire-Marcel et al., 2017). Strontium isotope ratios (87Sr/86Sr) excursion occurred at the transition to MIS 7 (mean of midpoints measured in planktonic foraminifer seem to support the initial ~190.2 ka, Channell, 2014) while the other two excursions are older magnetostratigraphic interpretations (Winter et al., 1997), but ra- by ca. 20e30 kyrs (Langereis et al., 1997; Singer, 2014). tios were interpreted based on the initial chronostratigraphy of As a consequence of contradictory magnetostratigraphic in- Clark et al. (1980). terpretations, Backman et al. (2008) did not use magnetostratigraphic data to establish the ACEX age model. O'Regan et al. 2.3. Lithostratigraphic correlation across the Arctic Ocean (2008a), using a cyclostratigraphic approach, proposed two different interpretations of some reversals and excursions for the Apart from the detailed lithostratigraphy of Clark et al. (1980),a ACEX composite section resulting in a slow and fast sedimentation common color-based lithostratigraphic approach used the cyclic rate model for the Pleistocene (Fig. 3). Finally, distortion of the alternation of brown manganese-rich and gray/yellowish J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 5

Fig. 3. A, Age models of the ACEX composite record, ODP Holes 910A and 911A; B, The inset shows an enlargement of the different age models for the ACEX composite record. Below 10 rmcd the age offset between the models is steadily increasing. The original 10Be/9Be age model of Frank et al. (2008) has been updated by Chen et al. (2012). The ages given for samples by Chen et al. (2012: Table 1) are included to illustrate that the new 10Be/9Be ages are younger by up to 200 ka. O'Regan et al. (2008a) established a cyclostratigraphic age model for the last 1.2 Ma, and proposed two models for the Lower Pleistocene based on different interpretations of the paleomagnetic record (slow and fast sedimentation rate model). Since both initial magnetostratigraphic interpretations converge at the base of Subchron C2An.1n (37.70 rmcd, 3.032 Ma) this age marker has been included in the age/depth plot (Backman et al., 2006; O'Regan et al., 2008a). Based on these age models the thickness of the Quaternary sediments ranges from ca. 28.5e41 m (abbreviations: H., Holocene; L., Late; depth scale: Holes 910A and 911A, mbsf, meter below sea floor; ACEX composite record, rmcd, revised meter composite depth). manganese-poor beds to identify potential glacialinterglacial and 2014). More importantly, the use of color-based cyclostratigraphy stadialinterstadial cyles (e.g., Herman, 1974; Clark et al., 1980; for stratigraphical purpose is now seriously questioned because Phillips and Grantz, 1997; Jakobsson et al., 2000; Polyak et al., several studies invalidate the assignment of brown beds to given 2004; Stein et al., 2010a, 2010b; Lowemark€ et al., 2014). intervals at the scale of the Arctic Ocean due to diagenetic processes Jakobsson et al. (2000) proposed a correlation of brown beds to (e.g., Marz€ et al., 2011; Macdonald and Gobeil, 2012; Lowemark€ interglacials/interstadials in a low latitude stack of oxygen isotope et al., 2014; Sundby et al., 2015; Meinhardt et al., 2016). Site to stages resulting in an age model for a sediment core from the site correlations across the Arctic Ocean based only on the brown central Lomonosov Ridge for the past ca. 900,000 years. Later, beds should thus be avoided. Polyak et al. (2004) introduced a numbering scheme for these cy- Some coarse-grained layers, and the detrital carbonate-rich cles on Mendeleev Ridge for the Upper Pleistocene and Holocene coarse white (W1, W2, W3) and pink-white layers (PW1, PW2) as that was expanded by Stein et al. (2010a, 2010b) into older defined by Clark et al. (1980) may form useful lithostratigraphic sediments. isochrons for supra-regional correlation in the Amerasia Basin This approach is complicated by the fact that the number of (Fig. 2; Stein et al., 2010a, 2010b). Stein et al. (2010a,b) applied brown beds in specific intervals is not identical between cores and marine isotope stratigraphy on these alternations of coarse-grained the timing is not well-constrained beyond the range of radiocarbon layers (¼glacial stages) interbedded with brown beds (¼ intergla- dating (e.g., Polyak et al., 2009; Stein et al., 2010b; Lowemark€ et al., cial/interstadial stages) leading to a MIS 16 age for the oldest 6 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 coarse-grained layer. However, they may be difficult to distinguish sediments on Yermak Plateau (Sato and Kameo, 1996). visually, and the synchroneity of individual layers has still to be Application of planktonic foraminifers in the Arctic Ocean is tested. Nevertheless, the onset of glacial sedimentation in the limited by low diversity assemblages almost exclusively composed central Arctic Ocean associated with the first supply of detrital of Neogloboquadrina pachyderma (for taxonomic comments see carbonates from the Canadian Arctic (e.g. Bazhenova et al., 2017)is Darling et al., 2006) but stratigraphically useful acmes of subpolar thought to be synchronous with that in the North Atlantic, dated to species also occur during Pleistocene interglacials/ interstadials in ca. 640 ka and recognized as the first Heinrich event (Hodell et al., the Arctic (Ericson et al., 1964; Herman, 1964, 1974; Clark, 1971; 2008; Stein et al., 2010b). Although based on assumptions that need Aksu, 1985b; Herman et al., 1989; Spiegler, 1996; Nørgaard- to be tested by further studies (Stein et al., 2010b), this datum, the Pedersen et al., 2007a, 2007b; Cronin et al., 2008, 2014; Adler first downcore change to reversed polarity, and biostratigraphic et al., 2009). Thus, two abundance maxima of subpolar Turbor- events were used to develop age models for sediments as old as 1.5 otalia quinqueloba and Neogloboquadrina incompta (size fraction Ma (e.g., Cronin et al., 2013, 2014; Polyak et al., 2013; Dong et al., >63 mm) have been associated with MIS 5a and 5e (Fig. 2; Aksu, 2017). In contrast, when applying the initial magnetostratigraphic 1985b; Nørgaard-Pedersen et al., 2007a, 2007b; Adler et al., 2009). interpretation, this onset of glacial sedimentation is dated Recently, Cronin et al. (2014) showed that restricted strati- approximately to the base of the Quaternary (e.g., Clark, 1996; Clark graphic ranges of ostracods may be useful for biostratigraphy in the et al., 2000; Backman et al., 2004). Middle and Upper Pleistocene. The almost exclusive occurrence of pteropods in Upper Pleistocene sediments on submarine highs 3. Quaternary biostratigraphy in the Arctic Ocean (e.g., Ericson et al., 1964; Herman, 1974; Scott et al., 1989; Eynaud et al., 2009) indicate poor aragonite preservation in the Arctic 3.1. Calcareous and biosiliceous microfossil stratigraphy Ocean prior to the Late Pleistocene. Calcareous dinoflagellate cysts are abundant in a restricted interval possibly belonging to the This review of previous stratigraphic approaches shows that a Middle Pleistocene in the Amerasian Basin, but assemblages were consistent Quaternary chronostratigraphic framework for the composed exclusively of Caracomia arctica (as Thoracosphaera arc- Arctic Ocean has not been developed in the last decades because tica in Gilbert and Clark, 1983; Mudie, 1985; Orthopithonella sp. in age models established with different methods are contradictory in Herman et al., 1989; probably Thoracosphaera spp. in Backman many ways. This demonstrates that conventional biostratigraphy is et al., 2009; see Streng et al., 2002). still required to provide independent age control in particular for Benthic foraminifers are the most diverse microfossil group in Lower and Middle Pleistocene sediments for which radiometric age the Arctic Ocean with more than 300 species present today determinations are presently available only at a low resolution for (Wollenburg, 1995), making them particularily useful for the ACEX composite record (Frank et al., 2008). biostratigraphic analysis. Calcareous benthic foraminifers usually The routinely used calcareous and biosiliceous microfossil predominate in the central Arctic Ocean in the Middle to Upper groups have their own inherent drawbacks for biostratigraphic Pleistocene, being replaced by agglutinated species in older sedi- application in the Arctic Ocean. In contrast to lower latitudes, the ments (e.g., Herman, 1974; O'Neill, 1981; Herman et al., 1989; discontinuous and sporadic occurrence, low taxonomic diversity Ishman et al., 1996; Cronin et al., 2008; Kender and Kaminski, and restricted calibration of bioevents to an independent time scale 2013). Calcareous species are present at the margins of the Arctic restrict their applicability in high latitudes to a much lower reso- Ocean through the Quaternary enabling the definition of zonations lution of biostratigraphic schemes (Backman et al., 2004). Bio- applicable for regional stratigraphic correlation (e.g., McNeil, 1989; siliceous microfossils (radiolarians, diatoms, silicoflagellates) are Mullen and McNeil, 1995; McNeil in Dixon, 1996; Osterman, 1996; useful only in the marginal Eurasian Arctic Ocean in the Plio- Polyak et al., 2013). Pleistocene where they preserve (e.g., Herman, 1974; Thiede A number of benthic foraminifer bioevents may allow a supra- et al., 1990; Polyakova, 2001 and references therein; Bjørklund regional correlation of sequences from comparable water depths, and Kruglikova, 2003; Backman et al., 2004; Tsoy and Obrezkova, but records often lack a well-constrained chronostratigraphy and 2017). Calcareous microfossils such as foraminifers, calcareous the temporal resolution necessary to define robust bioevents (e.g., nannofossils, ostracods, pteropods, and calcareous dinoflagellate Scott et al., 1989; Jakobsson et al., 2001; Backman et al., 2004; cysts are restricted to Middle and Upper Pleistocene sediments in Polyak et al., 2004, 2013; Cronin et al., 2008; Adler et al., 2009; the central Arctic Ocean according to the age model of Jakobsson Cronin et al., 2014; Lowemark€ et al., 2016). A quantitative analysis et al. (2000) while calcareous nannofossils and foraminifers are of benthic foraminifer assemblages must include small specimens present at the margins of the Arctic Ocean through the Quaternary (size fraction > 63 mmto<125 mm) that comprise species important (e.g., Herman, 1969; Hunkins et al., 1971; Herman, 1974; Clark et al., for ecological and stratigraphic interpretations in the Arctic and 1980; Herman and Hopkins, 1980; Worsley and Herman, 1980; sub-arctic (e.g., Herman, 1974; Schroder€ et al., 1987; Scott et al., O'Neill, 1981; Aksu, 1985b; McNeil, 1990; Gard, 1993; Mullen and 1989; Polyak et al., 2004). For example, the presence of Bulimina McNeil, 1995; McNeil in Dixon, 1996; Osterman, 1996; Backman aculeata has been considered a marker for MIS 5a/5.1 in many et al., 2004; Cronin et al., 2008, 2014; Eynaud et al., 2009; Gusev studies (Polyak et al., 2004; Adler et al., 2009; Alexanderson et al., et al., 2009; Polyak et al., 2013). 2014). This species ranges through the Late and Middle Pleistocene Calcareous nannofossil bioevents that are well-suited for high- in the Arctic Ocean (Osterman, 1996; Polyak et al., 2004; resolution stratigraphic correlation in lower to middle latitudes Alexanderson et al., 2014) but its concentration (specimens/gram (e.g., Backman et al., 2012) have been calibrated to oxygen isotope dry sediment) shows a distinct acme in sediments assigned to stratigraphy in the adjacent Nordic Seas for the Middle to Late upper MIS 5 (Jakobsson et al., 2001; Polyak et al., 2004; Nørgaard- Pleistocene (Gard and Backman, 1990). These bioevents led to Pedersen et al., 2007b) according to the chronostratigraphy of identify MIS 5 to MIS 1 sediments in the area of the ACEX drill sites Jakobsson et al. (2000). (Fig. 1) proving the presence of the last interglacial in an interval The stratigraphic occurrence of the small-sized Bolivina arctica previously assigned to the early Middle Pleistocene (Gard, 1993; also illustrates the potential of benthic foraminifer bioevents in Jakobsson et al., 2000; Spielhagen et al., 2004; Backman et al., sediment core CESAR 83e102 from Alpha Ridge (Fig. 2; Scott et al., 2009). Moreover, calcareous nannofossil bioevents are consistent 1989). An acme was interpreted to occur at the base of MIS 5 (ac- with the magnetostratigraphic interpretation of Quaternary cording to the age model of Nørgaard-Pedersen et al., 2007a) above J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 7 a pronounced supply of freshwater indicated by low d18O and d13C describe the taxonomy and stratigraphic ranges in the temperate to of N. pachyderma (Aksu, 1985b). Both concentrations and relative subpolar North Atlantic, primarily based on sites drilled by the abundances of B. arctica drop across the overlying pink-white layer DSDP, ODP, and IODP programs, and supplemented by data from PW2. A comparable stratigraphic range has been observed the North Sea Basin where stratigraphic ranges were also docu- throughout the Arctic Ocean (e.g., Backman et al., 2004; Polyak mented (Fig. 1; e.g., Harland, 1988; de Vernal et al., 1992; Head, et al., 2013; Lazar and Polyak, 2016), and a highest common 1993, 1996b; Stover et al., 1996; Louwye et al., 2008; De Schepper occurrence (HCO) could be tentatively defined, located at the base and Head, 2009; Dybkjaer and Piasecki, 2010; Kothe,€ 2012; of PW2 (Fig. 2). This species is only common in younger sediments Schreck et al., 2012). These studies and early compilations of in which dissolution has altered assemblage composition (Scott and Neogene to Quaternary data from the Arctic Ocean and adjacent Vilks, 1991; Wollenburg, 1995; Ishman and Foley, 1996; Scott et al., North Atlantic and North Pacific(Bujak, 1984; Mudie et al., 1990; de 2009). Vernal et al., 1992; Harland, 1992; de Vernal and Mudie, 1992; The stratigraphic applicability of agglutinated benthic fora- Mudie, 1992; Powell, 1992; Mudie and Harland, 1996; Stover minifers may be extended beyond Middle Pleistocene sediments et al., 1996) document that several dinocysts and acritarchs have but this potential has not been explored yet (e.g., Herman, 1974; stratigraphically useful ranges. While the compilations lack a O'Neill, 1981; Cronin et al., 2008 and references therein; Kender rigorous calibration of stratigraphic occurrences to an independent and Kaminski, 2013). The basinwide change in dominance from chronostratigraphy, studies on subarctic sequences after IODP agglutinated to calcareous benthic foraminifers has been Expedition 302 (ACEX) with an excellent independent chro- considered as a stratigraphic marker for MIS 7 sediments, but nostratigraphy demonstrated that robust numerical ages may be this transition is time-transgressive across the central Arctic calculated for palynomorph events in the Neogene and Quaternary. Ocean, occurring between the late Middle Pleistocene and Ho- This allowed assessing the synchroneity and/or diachroneity of locene (Cronin et al., 2008 and references therein). bioevents on a regional and supra-regional scale (De Schepper and Head, 2008, 2013; Schreck et al., 2012; De Schepper et al., 2015), 3.2. Quaternary palynostratigraphy which indicates a response to climate variability rather than evolutionary turnover. Quaternary marine palynomorphs including dinoflagellate cysts Climate variability additionally complicates not only the (¼dinocysts) are still taxonomically and stratigraphically the least definition of bioevents but also that of biozonations. As a result, documented microfossil group in the Arctic Ocean. Based on these bioevents are rarely applicable beyond a specificregion,and grounds, Backman et al. (2004) stated in a review on Quaternary dinocyst biozonations in the high northern latitudes often have a stratigraphy of the Arctic Ocean that among other microfossil low temporal resolution comprising two to three zones in the groups dinocysts are not well-suited for stratigraphic correlation Quaternary (e.g., Mudie et al., 1990; Mudie, 1992; Mudie and because their taxonomy is still under development, stratigraphic Harland, 1996; Harrison et al., 1999; Bujak, 2009; for review ranges in the Arctic Ocean are insufficiently known, and bioevents see De Schepper and Head, 2009; Smelror et al., http:// appear to be inadequately calibrated to independent time scales. In nhm2.uio.no/norges/full/erik/dino2.php). Moreover, high- this review, we advocate that the Neogene to Quaternary high resolution studies demonstrate a pronounced variability in latitude dinocyst taxonomy has now reached a maturity that allows dinocyst assemblages in the past million years (e.g., Mudie and building a solid and detailed stratigraphy for the Arctic Ocean and Aksu, 1984; de Vernal and Mudie, 1992; de Vernal et al., 1992; adjacent basins. Harland, 1992; Mudie, 1992; Matthiessen et al., 2001; Van It was not until the 1980s that the first organic-walled dinocysts Nieuwenhove et al., 2011, 2016; Aubry et al., 2016). Hence, an and other marine palynomorphs were reported from the Quater- ecostratigraphic approach, using changes in assemblage compo- nary of the Arctic Ocean (Harland et al., 1980; Hill et al., 1985; sition during glacial-interglacial cycles appears promising for Mudie, 1985). They are generally rare in sediments on Alpha high latitude environments. Ridge recovered from a drifting ice station during the CESAR expedition (the Canadian Expedition to Study the Alpha Ridge- Aksu and Mudie, 1985; Mudie, 1985; Mudie and Blasco, 1985; Aksu 4. New palynostratigraphic data from the Arctic Ocean et al., 1988). The known stratigraphic ranges at that time supported the magnetostratigraphic interpretations through the PlioePleis- Since the initial Arctic Ocean studies on the cores from the tocene. The range charts were later used to propose a palynomorph CESAR expedition, only exploration wells from the Beaufort- zonation for the PlioePleistocene of the central Arctic Ocean but Mackenzie Basin and sites drilled during ODP Leg 151 at the their ages were based on the initial magnetostratigraphic age northern Barents Sea margin and IODP Expedition (Exp.) 302 on the models (Mudie et al., 1990; Mudie, 1992). Thus, the highest occur- Lomonosov Ridge have been studied for marine palynomorphs. In rences (HO) of Impagidinium pallidum and Filisphaera filifera at the the Beaufort-Mackenzie Basin, Pleistocene sediments represent former PlioePleistocene boundary (ca. 1.8 Ma) supported the initial coastal plain to pro-delta environments with some marine in- magnetostratigraphic interpretation of the CESAR cores on Alpha tercalations. Therefore, pollen and other terrestrial palynomorphs Ridge (Aksu and Mudie, 1985). Subsequent studies, however, often dominate the sequences, and marine palynomorphs are rare revealed that I. pallidum occurs through the Plio-/Pleistocene (e.g., (e.g., McNeil et al., 1982; Norris, 1986, 1997; White, 1989; Dixon, de Vernal et al., 1992; Matthiessen and Brenner, 1996) and is an 1996). At the northern Barents Sea margin, where fluvial input important component of modern plankton and sediment assem- does not play a major role, hemipelagic sediments comprise diverse blages in the Nordic Seas (Dale and Dale, 1992; Matthiessen, 1995), dinocyst assemblages in ODP Hole 911A both in the Quaternary while F. filifera disappeared later than previously thought, at the (Matthiessen and Brenner, 1996) and Pliocene (Grøsfjeld et al., Lower/Middle Pleistocene boundary (Mudie et al., 1990; Head, 2014). IODP Exp. 302 (ACEX) offered the first opportunity to 1993; Mudie and Harland, 1996). Therefore, the revised HOs do study oceanic palynomorph assemblages from the central Arctic not support the initial magnetostratigraphic interpretations in the Ocean, but a low-resolution shipboard study revealed only a central Arctic Ocean any longer. restricted stratigraphic occurrence of dinocysts (Backman et al., Since the pioneering palynostratigraphic studies on Quaternary 2006). Here we present new data from the Arctic Ocean and sub- sequences from the Arctic Ocean, much progress has been made to arctic basins. 8 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26

4.1. Sample processing, data evaluation and age models core PS2138-1 residues were sealed with paraffin wax. All aquatic palynomorphs were counted along non-overlapping All sites referred to in the paper are listed in Table 1 and are traverses at 400 magnification using a transmitted light micro- shown in Fig. 1. Age models from ODP Leg 151 and IODP Exp. 302 scope. Species identification was checked at 630 and 1000 sites drilled in the Arctic Ocean have been established with various magnification. The preservation of most palynomorphs ranges from stratigraphic methods (Fig. 3; Supplementary Data). Age models for poor to good making it unequivocal identification difficult in case of the IODP Exp. 323 Sites U1341 and U1343 in the Bering Sea are rare species. The relative abundance of taxa was described as taken from Takahashi et al. (2011) while the age model of sediment dominant (>30%), abundant (10e30%), common (5e10%), rare core PS2138-1 is based on Matthiessen et al. (2001) and (<5%) or present (single specimen). Matthiessen and Knies (2001). A list of all taxa recorded during this study and taxonomic The samples were treated with standard palynological prepa- comments can be found in the appendix. ration methods (e.g. de Vernal et al., 1996; Matthiessen and The slides from ODP Leg 151, IODP Exp. 302 and core PS2138-1 Brenner, 1996; De Schepper and Head, 2008). The samples from were counted by JM and are stored at AWI. The slides of Holes ODP Leg 151 Sites 910 and 911, IODP Exp. 302 Holes M0002A, U1341A and B were counted by SDS and De Clercq (2015) and are M0004A and M0004C, and sediment core PS2138-1 were prepared stored at Ghent University and Bjerknes Centre for Climate at the Alfred Wegener Institute Helmholtz Centre for Polar and Research, and those of Hole U1343E were counted by CZ and are Marine Research in Bremerhaven, Germany (AWI), whereas those stored at GEOTOP. All data can be found in the Supplementary Data. from IODP Exp. 323 U1341 Holes A and B and U1343E were pro- ODP Leg 151 Holes 910A and 911A (53 samples) have been cessed at Ghent University and the Centre GEOTOP, Universitedu sampled at a variable resolution (Supplementary Data Tables 2 and Quebec a Montreal, in Montreal, Canada, respectively. 3). Hole 910A has been analysed at approximately 20 cm intervals The dried and weighed samples were digested in hydrochloric to identify samples useful for a biostratigraphic analysis. One mi- (HCl) and hydrofluoric (HF) acids. Cold acids (HCl 10%; HF 38e40%) croscope slide from every sample was initially counted and the were used at AWI and at Ghent University whereas samples were more productive samples were then selected to count additional treated with warm HCL (10%) and warm HF (49%) at GEOTOP. No slides to reach a minimum of 100 dinocysts. Based on an initial oxidation, alkali and ultrasound treatments were used. The ODP Leg palynostratigraphic study (Matthiessen and Brenner, 1996) addi- 151 and IODP Exp. 302 samples and those of core PS2138-1 were tional samples were analysed from Hole 911A to improve strati- sieved on 6-mm polyester mesh. The samples from IODP Exp. 302 graphic resolution in the Lower Pleistocene. Previously studied were additionally sieved at 125 mm to provide additional samples samples with relatively high abundances of Filisphaera filifera sensu for foraminifer studies. The samples from IODP Exp. 323 were lato and Habibacysta tectata were counted again to apply a common sieved on 10-mm nylon mesh, and samples from Hole U1343E taxonomic concept. For this review a composite palynomorph re- additionally at 106 mm. cord excluding the barren samples in Hole 911A has been con- One to two tablets with Lycopodium spores were added to each structed based on the age models. sample to calculate dinocyst concentrations per gram dried sedi- Sediment core PS2138-1 (73 samples) was studied at variable ment according to the method of Stockmarr (1971). The palyno- intervals (3e10 cm) with a generally higher resolution in MIS 5 logical residues were mounted on microscope slides with glycerine (Supplementary Data Table 4; Matthiessen et al., 2001; Matthiessen jelly. The coverslips of slides from ODP Leg 151, IODP Exp. 302 and and Knies, 2001). IODP Exp. 302 (ACEX) Holes M0002A (cores 5X to 10X, 169 samples), M0004A (cores 1He3H, 35 samples), and M0004C (cores Table 1 1He6H, 114 samples) were sampled every 10e30 cm correspond- Geographic locations of DSDP, ODP and IODP boreholes and sediment cores. ing to a stratigraphic resolution of less than 10,000 years according Expedition Hole/Core Longitude Latitude water depth to the established age models (Fig. 3; Supplementary Data Tables 5, 6, 7). Disturbed intervals (see Backman et al., 2006: Table 24) have 93 603C 35.496 70.031 4639.5 94 607 41.001 32.957 3427 been partly analysed to improve stratigraphic resolution at the top 94 610A 53.222 18.887 2417 of Subunit 1/3. One microscope slide from each sample was 94 611A 52.841 30.310 3201 completely scanned for marine palynomorphs, and two slides of all 104 642A 67.225 2.928 1294 samples with common palynomorphs were counted. A composite 104 642B 67.225 2.928 1294 ACEX section has been constructed including Holes M0002A, 104 643A 67.715 1.033 2780 104 644A 66.678 4.577 1226 M0004A and M0004C based on the revised meter composite depth 104 644B 66.678 4.577 1226 scale (rmcd; O'Regan et al., 2008b). A reliable scale could be 105 645B 70.457 64.654 2019 developed only for the upper 20e25 m (O'Regan et al., 2008a, 105 646B 58.209 48.369 3459 2008b). In the Lower Pleistocene below 25 rmcd, low recovery 105 647A 53.331 45.262 3869 151 910A 80.265 6.590 567 and core disturbance cause some major gaps in the stratigraphic 151 911A 80.477 8.227 912 record. Data from the ACEX composite are presented here down to 162 986C 77.341 9.078 2052 ca. 45 rmcd corresponding to an age of >2.8 to >3.3 Ma depending 162 986D 77.340 9.078 2052 on the age model used (Fig. 3). 162 987E 70.496 17.936 859.4 Initial shipboard palynostratigraphic analyses revealed the HO 191 1179C 41.080 159.963 5563.9 fi 303 U1304A 53.001 33.530 3069.1 of F. lifera in the Bering Sea in the Lower Pleistocene of IODP 323 U1341A 54.033 179.008 2139.6 Expedition 323 sites (Takahashi et al., 2011). Twelve samples from 323 U1341B 54.033 179.009 2139.6 Hole U1343E cores 42X to 47H were selected to better constrain the 323 U1343E 57.556 175.817 1956 HO of F. filifera in core 44X. In addition to F. filifera and Filisphaera 302 M0002A 87.921 139.365 1209 302 M0004A 87.867 136.177 1288 microornata, H. tectata was observed during a shorebased palyno- 302 M0004C 87.868 136.190 1288 logical study of Upper Pliocene and Lower Pleistocene sediments CESAR 83e102 85.635 111.118 1495 from IODP Exp. 323 Holes U1341A and B and data from 48 samples GreenICE #10 84.800 74.283 1040 are presented in this study (Supplementary Data Tables 8 and 9). ARK-VIII/2 PS2138-1 81.538 30.876 862 The chrono-biostratigraphic terminology defined in De J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 9

Schepper and Head (2008) has been used. Bioevents were cali- Quaternary, always comprise at least 70%. Some taxa having their brated to an independent chronostratigraphy based on the lowest occurrence (LO) above the base of the Quaternary are only Geological Time Scale 2012 (Gradstein et al., 2012). The Quaternary intermittently absent because they occur in the lower Pliocene of System now includes the Gelasian Stage and its base is dated at 2.58 Hole 911A (Grøsfjeld et al., 2014). Filisphaera filifera including the Ma (Gibbard and Head, 2010). Stratigraphic data and paleoenvir- subspecies filifera and pilosa, F. microornata and H. tectata are the onmental interpretations published prior to this revision were only species becoming extinct in the Quaternary (Fig. 4; e.g., Head, adjusted accordingly. The definition of bioevents is based only on 1993, 1996b). samples for which at least 100 specimens have been counted. Rare Fluctuations in the common to dominant taxa (cysts of Proto- specimens of a particular species may be found beyond their true ceratium reticulatum, Brigantedinium spp. indet., Filisphaera spp. stratigraphic range because of reworking. The color coding used for (F. filifera and F. microornata), Nematosphaeropsis labyrinthus, the chronostratigraphic subdivisions are from Gradstein et al. Bitectatodinium tepikiense, Impagidinium pallidum) allow the defi- (2012). Fig. 1 was drawn with the Ocean Data View software nition of assemblage zones in the Quaternary (Fig. 5). Assemblage (Schlitzer, 2017). Zone I is characterized by Brigantedinium spp. indet. together with Filisphaera spp., and Habibacysta tectata. At ca. 2.1 Ma, this assem- 4.2. The Barents Sea margin record blage is replaced by the acme of Filisphaera spp. (Assemblage Zone II) while Brigantedinium spp. indet., H. tectata and cysts of Marine palynomorphs occur with variable but generally low P. reticulatum occur in variable abundances. Assemblage Zone III abundances, and are strongly diluted by black debris (partly coal) (ca. 1.85e1.2 Ma) is characterized by variable abundances of Brig- and darker coloured degraded organic matter (mainly terrestrial antedinium spp. indet. and cysts of P. reticulatum together with rare reworked organic matter such as pollen, spore, plant debris; few to common Filisphaera spp. The uppermost Assemblage Zone IV is reworked dinoflagellate cysts). Quaternary pollen (mainly bissac- marked by a distinct increase of Nematosphaeropsis labyrinthus. The cates) are rare. assemblages are usually dominated by cysts of P. reticulatum Most samples analysed in sediment core PS2138-1 and ODP accompanied by variable abundances of N. labyrinthus and Brig- Holes 910A and 911A contained dinocysts but in contrast to sub- antedinium spp. indet. polar Neogene and Quaternary records (e.g., De Schepper and Head, The assemblages recorded in sediment core PS2138-1 resemble 2008, 2013; Schreck et al., 2013) acritarchs are rare (Matthiessen those of Assemblage zone IV (Fig. 6; Matthiessen et al., 2001; and Brenner, 1996). More than 30 dinocyst taxa occur in the Matthiessen and Knies, 2001). The higher resolution compared to Barents Sea margin record but Brigantedinium spp. indet., cyst of the composite record of Holes 910A/911A clearly reveals a pro- Protoceratium reticulatum, Nematosphaeropsis labyrinthus and nounced variability since MIS 6, and concentration maxima asso- Achomosphaera/Spiniferites spp. indet., which occur throughout the ciated with higher abundances of cysts of P. reticulatum during the

Fig. 4. Stratigraphic occurrences of selected dinocysts in the Barents Sea margin record based on a composite record of ODP Holes 910A and 911A. The lithological Subunit IA is distinguished from Subunit IB by an increase of ice-rafted debris in the predominately clayey silts to silty clays (Myhre et al., 1995) (abbreviations: L., Late; Jar., Jaramillo; Old., Olduvai). 10 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26

Fig. 6. Upper Quaternary ecostratigraphy of sediment core PS2138-1 from the northern Barents Sea continental margin based on selected taxa. The stable isotope stratigraphy is from Knies et al. (1999). Cysts of Protoceratium reticulatum are often abundant to dominant in the assemblages but only the Holocene and the last interglacial (LIG) are simultaneously characterized by high dinocyst concentrations. Relative abundances are calculated only for samples with counts exceeding 50 specimens but MIS 2 samples with low counts (>30 specimens) are included to demonstrate the similar assemblage composition.

present and penultimate interglacials.

4.3. The ACEX Lomonosov Ridge composite record

Fig. 5. Quaternary ecostratigraphy of the Barents Sea margin record based on a Most samples in the upper 45 m of the ACEX composite record composite section of Holes 910A and 911A. Deﬁnition of assemblage zones is based on are barren or only contain rare palynomorphs (Supplementary Data the abundant and consistently occurring species. Tables 5, 6, 7). Barren samples often only contain opaque black debris and sporadically degraded pollen. J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 11

The stratigraphic occurrence of marine palynomorphs is Achomosphaera/Spiniferites spp. indet. constitute more than 80% of restricted to lithological Subunits 1/1 and 1/3 and the upper part of the assemblages in these intervals. Other taxa are usually rare and Subunit 1/2 (Fig. 7). Most samples contain few specimens, and occur only occasionally. Aquatic palynomorphs other than dino- samples with relatively high numbers of dinocysts are restricted to cysts (e.g., Pediastrum, Botryococcus, Sigmopollis, algal spores) are six intervals in the undisturbed sections. These intervals comprise only common at the transition of Subunit 1/2 to 1/1 variable numbers of samples, and percentage abundances have (Supplementary Data Table 7) corresponding to MIS 6 (O'Regan been calculated from the sum of all dinocysts counted in the indi- et al., 2008a). A maximum of Pediastrum spp. has been observed vidual intervals. Dinocysts are usually the dominant palynomorphs in lithological Unit L in gravity core CESAR 83e103 which is located whereas acritarchs and other aquatic palynomorphs are rare. Cysts close to CESAR 83e102 (Fig. 2; Mudie, 1985) suggesting that this of P. reticulatum, F. filifera, H. tectata, N. labyrinthus and event may be useful for stratigraphic correlation across the Arctic Ocean. Coring disturbance has considerably blurred the stratigraphic record in the uppermost Subunit 1/3. Dinocysts are scattered throughout the disturbed sections, probably because productive intervals may have been mixed with thicker intervals barren of any palynomorph (Fig. 7). The number of taxa is low in the productive intervals, and H. tectata together with F. filifera are the dominant species and disappeared at the top of Subunit 1/3. Filisphaera microornata is always rare and co-occurred with F. filifera and H. tectata. Nematosphaeropsis labyrinthus, and Achomosphaera/Spi- niferites spp. indet. dominate in a single sample in the Lower Pleistocene.

4.4. The Bering Sea record

Of the dinocysts recorded in Quaternary sediments drilled during IODP Exp. 323, only F. filifera has been recognized as stratigraphic marker (Takahashi et al., 2011). Based on the shorebased studies, F. microornata and H. tectata may be additionally useful stratigraphic markers at Site U1341 in the Quaternary. Filisphaera filifera and F. microornata were sporadically present in the uppermost Pliocene to lowest Pleistocene at Site U1341 whereas F. filifera is common to abundant in the Lower Pleistocene of Hole U1343E enabling to define a HO at 1.257 Ma (Supplementary Data Table 9). In the composite record of Site U1341, H.tectata was regularly present to common up to the Pliocene-Pleistocene boundary and had a HO at 2.527 Ma (Supplementary Data Table 8).

5. Dinocyst stratigraphy synthesis for the Quaternary (sub) arctic oceans

5.1. Rarity of palynomorphs in the Quaternary as a combination of biology, taphonomy and sampling strategy

The dinocyst fossil record is strongly controlled by the life cycle and ecology of dinoflagellates. Since the vegetative stages are not preserved, the planktonic dinoflagellates are only represented by their cysts in the fossil record. Dinoflagellates may form fossilizable cysts during their life cycle (e.g., de Vernal et al., 2007) and today, cyst-forming species amount only to approximately 10e20% of all living species (e.g., Head, 1996a). In the Arctic Ocean, only a small number of dinoflagellates form fossilizable cysts (Okolodkov, 1998, 1999). Furthermore, only few dinoflagellate and dinocyst species are in fact adapted to polar environments, and many dinoflagellates are advected with warmer waters into the Arctic Ocean (Okolodkov and Dodge, 1996; de Vernal et al., 2007; for review see Matthiessen et al., 2005). In conjunction with the preference of both dinoflagellates and their cysts for neritic environments the exten-

Fig. 7. Stratigraphic occurrences of selected dinocyst taxa in the ACEX composite re- sively ice covered central Arctic Ocean has a lower number of cord based on Holes M0002A, M0004A, and M0004C. The six productive intervals species than the marginal shelf seas. The ecological preference of (yellow bars) are located in undisturbed core sections. Extensive core disturbance (blue dinocysts for warmer water environments complicates their bars) in the upper Subunit 1/3 limits the detailed interpretation of the dinocyst record. stratigraphic application in the central Arctic Ocean. A seasonal to The different age models are shown. Lithological Unit I is composed of silty clay, and perennial sea ice cover might have prevailed over long periods in Subunits 1/1 and 1/3 consist of coarser sediments. Biogenic carbonate is absent below Subunit 1/2 which has a characteristic brownish color in contrast to the olive colors of the Quaternary, and the intervals with enhanced warmer water Subunit 1/3. inﬂow might be detected in the fossil record only by chance. 12 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26

Moreover, significant reworking of pre-Quaternary microfossils, in (24.49 rmcd) from the lower Pleistocene contains abundant dino- particular at sites proximal to ice sheets at the continental margins, cysts, whereas samples a few cm above and below this layer in the may dilute sparse in situ palynomorphs (e.g. Smelror, 1999). composite record are barren of palynomorphs (Supplementary Various abiotic factors also determine the stratigraphic occur- Data Table 6). For obtaining high-resolution stratigraphic records rence of dinocysts. Firstly, selective preservation can reduce the in the central Arctic Ocean, a continuous sampling is required, as number of dinocyst species in the fossil record (e.g., Zonneveld demonstrated by the CESAR 83e102 record, where interglacials/ et al., 2008, 2010), since cyst walls are composed of various bio- interstadials are less than 5 cm thick (Fig. 2; Aksu, 1985b). macromolecules that are partly susceptible to chemical degradation in the sediment (Bogus et al., 2014). This is especially the case 5.2. Quaternary dinocyst bioevents across the Arctic Ocean for cysts of heterotrophic species, which may lose their color or disintegrate completely when exposed for longer time to corrosive 5.2.1. Calibration of dinocyst events bottom waters (i.e. due to low sedimentation rates). The sporadic The majority of dinocyst species observed in the Quaternary of occurrence of robust dinocysts resistant to degradation in the ACEX the Arctic Ocean are extant and stratigraphically long-ranging. Such composite record may indicate selective preservation or alterna- species rather reflect environmental variability than evolutionary tively harsh environmental conditions (low productivity linked to turnover. The low number of productive samples in the ACEX perennial sea ice cover). The almost complete absence of dinocysts composite record and the predominance of extant species pre- from Subunit 1/2 may be explained by a combination of poor cludes the definition of a detailed dinocyst zonation based on preservation during enhanced formation of manganese-rich layers stratigraphic ranges. This differs from the Barents Sea margin re- (e.g., Marz€ et al., 2011) and a closed pack ice cover during certain cord, where assemblage zones based on the stratigraphic occur- periods. rence and variable abundance of extant taxa can be defined. Secondly, the Arctic Ocean is a siliciclastic depositional envi- Filisphaera filifera and Habibacysta tectata, and possibly Filisphaera ronment, where the accumulation of sediments is rather discon- microornata are the only species that have their HOs in the Qua- tinuous, and controlled by the variable input of erosional products ternary and are potentially useful for stratigraphic correlation from the surrounding continents. Sediment transport to the distal across the Arctic Ocean. The different age models for the ACEX Arctic basin is variable depending upon the amount of terrigenous composite record result in a considerable age range for the HOs of material incorporated into sea ice on the shallow shelf seas, thus F. filifera and H. tectata (Fig. 8; Supplementary Data Table 10). These leading to decrease in sedimentation rates with the distance from bioevents are better calibrated in the subarctic domain where an the coasts and the shelf break (e.g., Polyak et al., 2009; Stein et al., independent and robust chronostratigraphy is available. 2010b; Levitan, 2015). The low sedimentation rates are only out- paced in the deep basins where gravitational processes lead to thick 5.2.1.1. Filisphaera filifera. Filisphaera filifera has a LO in the Lower accumulations of mass flow deposits (e.g., Grantz et al., 1996). to Upper Oligocene of the North Sea Basin (Head, 1993; Van Perennial sea ice cover and/or extensive ice shelves may almost Simaeys et al., 2004, 2005; Schiøler, 2005), and the western completely inhibit any accumulation leading to condensed sections North Atlantic (Egger et al., 2016). In mid-latitudes, Williams et al. in the central Arctic Ocean (e.g., Mudie and Blasco, 1985; Nørgaard- (2004) define a first appearance at 24.62 Ma. The species is Pedersen et al., 1998, 2003; Poore et al., 1999; Polyak et al., 2009; frequently recorded in Miocene to Pleistocene sediments of the Hanslik et al., 2010; Jang et al., 2013; Dausmann et al., 2015). The North Pacific, North Atlantic, Nordic Seas, North Sea Basin and variable supply of terrigenous matter to the oceanic environment is marginal Arctic Ocean, and disappeared in the Lower to Middle not compensated by accumulation of biogenic calcite and opal at Pleistocene (Bujak, 1984; Matsuoka et al., 1987; Matsuoka and the sea floor due to the restricted production in surface waters and Bujak, 1988; Manum et al., 1989; Head, 1996b, 1998; Matthiessen dissolution in the water column and sediments. Thus, the irregular and Brenner, 1996; Mudie and Harland, 1996; Poulsen et al., 1996; accumulation leads to a variable thickness of lithological units in Louwye and Laga, 1998; Smelror, 1999; Piasecki, 2003; Louwye the Amerasian basin (Clark et al., 1980). Moreover, intensive glacial et al., 2004, 2008; Kuhlmann et al., 2006; Eidvin et al., 2007; erosion as indicated by submarine landforms down to 1200 m Kothe€ and Piesker, 2007; De Schepper et al., 2009, 2011, 2013, water depth may cause long stratigraphic gaps in the sedimentary 2017; De Schepper and Head, 2009; Schreck et al., 2013; Grøsfjeld archives (e.g., Polyak et al., 2001; Kristoffersen et al., 2004; Knies et al., 2014; Panitz et al., 2017). et al., 2007; Frank et al., 2008; Gebhardt et al., 2011; Niessen The stratigraphic ranges of the subspecies Filisphaera filifera fil- et al., 2013; Jakobsson et al., 2016; Stein et al., 2016). Scouring by ifera and F. filifera pilosa are less known because these have been icebergs (oldest evidence is dated to 1.5 Ma; Mattingsdal et al., rarely distinguished on a routine basis. Filisphaera filifera filifera has 2014) may occur more frequently than erosion by marine based been recorded from the Middle Miocene to Lower Pleistocene in the ice sheets, ice shelves or large coherently moving fields of icebergs Bering Sea, North Atlantic, Nordic Seas, North Sea Basin and Lab- (e.g., Vogt et al., 1994; Dowdeswell et al., 2010; Gebhardt et al., rador Sea (Bujak, 1984; Mudie, 1987, 1989; de Vernal and Mudie, 2011) causing locally restricted hiatuses. These sedimentary pro- 1989a, 1989b; de Vernal et al., 1992; Head, 1993; Bennike et al., cesses will result in incomplete Quaternary sections, which may be 2002; Head et al., 2004; Louwye et al., 2008; De Schepper et al., biased to periods with high sediment input. 2009; De Schepper and Head, 2009; Verhoeven et al., 2011; De Thirdly, the sample spacing affects the completeness of the Schepper et al., 2017). Filisphaera filifera pilosa has been reported stratigraphic record. The fixed-interval sampling, as it has been only from the uppermost Miocene to Lower Pleistocene of the done at 10e30 cm intervals in the ACEX holes, does not provide a Bering Sea, North Atlantic, North Sea Basin, and Nordic Seas complete record. Brief intervals recording warmer Atlantic water (Matsuoka and Bujak, 1988; Bennike et al., 2002; Head et al., 2004; inflow via specific dinocyst assemblages may be missed, possibly De Schepper et al., 2009, 2017; De Schepper and Head, 2009). leading to under-representation of potentially biostratigraphically Despite its long stratigraphic range, F. filifera and its subspecies, useful warm-adapted species (e.g. Schreck et al., 2012). This is occur rather sporadically and are only rare to common in Oligocene illustrated in the ACEX composite record where the Holocene is to Upper Miocene sediments where percentages rarely exceed 10% represented by a single sample from Hole M0004C (Supplementary (e.g., Matsuoka and Bujak, 1988). In the Pliocene, this species is Data Table 7, Sample 1H1, 8e10 cm, ca. 10.9 ka BP based on Cronin more continuously present with fluctuating abundances in the et al., 2008). Comparably, Sample 302-M0004A-3H1, 58e60 cm marginal Arctic Ocean, North Atlantic, Nordic Seas, and the J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 13

Fig. 8. Calibrated stratigraphic occurrences of Filisphaera ﬁlifera and Habibacysta tectata in Quaternary sediments of the high northern latitudes. Details are given in the Supplementary Data-Table 10. The red bar marks the most reliable age for the highest occurrences of F. ﬁlifera and H. tectata in the ACEX composite record. The orange bars indicate possible bioevents at other sites (stippled lines: possible stratigraphic occurrence; black square: single occurrence of H. textata in Hole 910A; abbreviations: HO, highest occurrence; HPO, highest persistent occurrence; Jar., Jaramillo; Old., Olduvai; L., Late).

southern and central North Sea Basin (e.g. Bujak, 1984; Mudie, earlier at the top of Subunit 1/3 between 1.4 and 2.1 Ma depending 1989; Piasecki et al., 2002; Louwye et al., 2004; De Schepper and on the selected age model (Fig. 8). Head, 2009; Verhoeven et al., 2011; Grøsfjeld et al., 2014; De Instead of using the HO of F. filifera, we suggest the top of the Schepper et al., 2013, 2017; Panitz et al., 2017). Distinct acmes acme to be a more reliable stratigraphic event. At the northern occur frequently since the initiation of the Northern Hemisphere Barents Sea margin it corresponds to the top of the Olduvai Sub- Glaciation from 3.6 Ma and culminate at the Plio-Pleistocene chron (Matthiessen and Brenner, 1996; this study). Anthonissen transition (de Vernal and Mudie, 1989a; Mudie, 1989; (2008) gave in a compilation of stratigraphic ranges in the Nordic Matthiessen and Brenner, 1996; Smelror, 1999; Bennike et al., Seas and North Sea an age of 1.8e2 Ma for the top of an acme but 2002; Kuhlmann et al., 2006; De Schepper and Head, 2009; this is based on questionable interpretations of published data. Mudelsee and Raymo, 2005). Kuhlmann (2004) probably observed in North Sea well A15-3 only For defining bioevents in this study, both subspecies of F. filifera part of the acme at the base of the Olduvai Subchron. This bioevent are combined since the subspecies are distinguished only in few may be useful for biostratigraphic correlations in the North Sea and studies. Thus, F. filifera disappeared in the Lower Pleistocene at Nordic Seas but must be tested by further studies as already noted most sites but HOs range from Subchron (C2r.1r) into the Brunhes by Anthonissen (2008). Chron (Fig. 8). Head et al. (2004) suggested that reworking may have extended the stratigraphic range into the Middle Pleistocene 5.2.1.2. Filisphaera microornata. Filisphaera microornata has a at Site 986, and that its actual HO is in the Upper Matuyama Sub- stratigraphic range from the Middle Miocene to Lower Pleistocene chron at the Lower/Middle Pleistocene transition. In the North in the North Atlantic, Nordic Seas, Labrador Sea and North Sea Basin Atlantic, the highest persistent occurrences (HPO) and HOs cluster (Head, 1993, 1998; de Verteuil, 1996, 1997; Louwye and Laga, 1998; around 1.4 to 1.5 Ma, which is comparable to the HO in the Nordic Bennike et al., 2002; Louwye, 2002; Head et al., 2004; Louwye et al., Seas zonation at 1.4 Ma (Smelror et al., http://nhm2.uio.no/norges/ 2004; Kuhlmann et al., 2006; De Schepper et al., 2009, 2017; full/erik/dino2.php). In the northern North Pacific, we recorded a Wijnker et al., 2008; De Schepper and Head, 2009; Grøsfjeld comparable age (1.3 Ma). et al., 2014; Hennissen et al., 2017). Since F. microornata forms a The stratigraphic occurrence of F. filifera at the northern Barents morphological series with the extant Bitectatodinium tepikiense Sea margin confirms that the definition of a HO is problematic. The (vermiculate morphotype) (Hennissen et al., 2017; Mudie et al., HO is probably in the Middle Pleistocene in ODP Hole 911A at ca. 1.0 2017), it is difficult to assess a stratigraphic range unequivocally. Ma, but well-preserved specimens occur in the Brunhes Chron in Filisphaera microornata may be assigned to Bitectatodinium because ODP Hole 910A (Figs. 4, 8). In the ACEX composite record the HO is of an archeopyle consisting of two precingular paraplates 14 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26

(Versteegh, 1997; De Schepper and Head, 2009). In the Arctic support a HPO for sites along the path of North Atlantic waters close Ocean, it usually co-occurred with F. filifera (Figs. 4 and 5), and on to the base of the Olduvai Subchron C2n (~2.0 Ma) (Fig. 8). Yermak Plateau this species has a HO at the same time as F. filifera.A thorough taxonomic revision is required before its potential as 5.2.2. Quaternary ecostratigraphy stratigraphic marker can be evaluated. The LOs and HOs of most marine palynomorphs in the Quater- nary are likely diachronous along meridional and zonal gradients 5.2.1.3. Habibacysta tectata. The first appearance of H. tectata in the because of climate and oceanographic conditions changing with early Middle Miocene (for an overview see Schreck et al., 2012 and time (e.g., De Schepper and Head, 2008, 2009; De Schepper et al., references therein) is probably linked to global cooling after the 2015, 2017). Therefore, an ecostratigraphic approach using vari- Miocene Climate Optimum (1715 Ma; Zachos et al., 2008). This able species abundance and concentrations appears appropriate for species has a widespread distribution across the North Pacific stratigraphic correlation on a regional scale. In the Atlantic sector of Ocean, North Atlantic Ocean, North Sea, Nordic Seas, and marginal the high northern latitudes, cysts of Protoceratium reticulatum Arctic Ocean in the Middle Miocene to lower Pleistocene (e.g., usually dominate during interglacials but this species is opportu- Head, 1994 and references therein; de Verteuil, 1996, 1997; nistic and may also be abundant during glacials such as MIS 4 and 6 Matthiessen and Brenner, 1996; Head, 1998; Smelror, 1999; (Fig. 6; de Vernal and Mudie, 1992; de Vernal et al., 1992; Mudie, Kanazawa et al., 2001; Piasecki, 2003; Munsterman and 1992; Matthiessen et al., 2001; Van Nieuwenhove et al., 2011, Brinkhuis, 2004; Louwye and Laga, 2008; Backman et al., 2006; 2016). Concentrations of dinocysts including those of De Schepper and Head, 2008; Dybkjaer and Piasecki, 2010; Kothe,€ P. reticulatum usually peak during interglacials such as MIS 1 and 5e 2012; Schreck et al., 2012; Grøsfjeld et al., 2014; Quaijtaal et al., (e.g. de Vernal and Mudie, 1992; Matthiessen and Baumann, 1997; 2014). Habibacysta tectata is only sporadically abundant to domi- Matthiessen and Knies, 2001; Van Nieuwenhove et al., 2011, 2016). nant at the western and eastern Greenland margin and in the Concentration maxima apparently allow interglacials to be pin- eastern Atlantic Ocean from the Middle to Upper Miocene (Louwye, pointed more reliably if these were distinctly higher than during 2002; Piasecki, 2003; Louwye et al., 2008; Schreck et al., 2012, glacials because a variable lithology may also affect concentrations. 2013), and in Belgium and the southern Nordic Seas (Norwegian Higher concentrations are usually found in sediments rich in clay Sea) in the Pliocene (Louwye et al., 2004; De Schepper et al., 2009, and silt rather than sand (Wall et al., 1977). Although concentration 2017; Panitz et al., 2017). In the Lower Pleistocene (Gelasian), maxima characterise interglacials, they are not necessarily syn- H. tectata is common to dominant in the Mediterranean (Versteegh, chronous on a regional scale. 1997), and from the eastern North Atlantic and North Sea to the Coeval changes in assemblage composition may provide addi- eastern Arctic Ocean (Head, 1996b, 1998; Matthiessen and Brenner, tional bioevents for stratigraphic correlation in the Nordic Seas 1996; Head et al., 2004; Kuhlmann, 2004; De Schepper and Head, during the last deglaciation and Holocene (e.g., Matthiessen and 2008). The ACEX composite record shows abundant H. tectata at Baumann, 1997; de Vernal et al., 2005a; Van Nieuwenhove et al., the top of Subunit 1/3 dated to between 1.4 and 2.1 Ma (Fig. 8). 2016). In the Lower Holocene the dominant dinocyst Nem- Assessing the stratigraphic occurrence is complicated by the atosphaeropsis labyrinthus is replaced by cysts of P. reticulatum uncertain taxonomic assignments in earlier studies. Different spe- almost synchronously throughout the Nordic Seas. Moreover, the cies or various informally described taxa may be attributed to occurrence and abundance of accessory species may allow different H. tectata (for overview see Head, 1994; De Schepper et al., 2017). interglacials to be distinguished (e.g., de Vernal and Mudie,1992)as Head (1994) tentatively assigned Tectodinium pellitum and Dinocyst illustrated by Bitectatodinium tepikiense which is common in the sp. 1 of Mudie (1989) to H. tectata but including these records may Nordic Seas in MIS 5e sediments and can dominate the assemblages lead to considerable uncertainties in the stratigraphic interpreta- of MIS 32(Fig. 6; e.g., Eynaud et al., 2002; de Vernal et al., 2005a) tion (Supplementary Data Table 8). The HO is apparently not a but is usually rare in MIS 1 (Van Nieuwenhove et al., 2008, 2011, reliable stratigraphic event as H. tectata disappeared diachronously 2016). Such features, however, may only be useful for strati- in the Atlantic sector of the high northern latitudes in the Lower to graphic correlation within a particular area of comparable climate Middle Pleistocene. In the subpolar North Pacific, H. tectata dis- and oceanographic conditions. appeared in IODP Hole U1341B at ca. 2.5 Ma but it occurs sporad- These general patterns may be useful to identify interglacials in ically at ODP Site 1179 up to the lower Matuyama Subchron (ca. 1.5 the high northern latitudes if the dinocysts are studied at millenial- Ma; Kanazawa et al., 2001). scale resolution. The Barents Sea margin record may give the Head et al. (2004) considered a HCO at the base of the Olduvai impression that a variability on glacial/interglacial time scale is Subchron in DSDP Hole 603C in the western North Atlantic, and at recorded in the Arctic Ocean over the past 1.2 million years (Fig. 5; the Reunion Subchron both in DSDP Hole 610A in the eastern North Matthiessen and Brenner, 1996) but sample spacing is too large to Atlantic and ODP Hole 644A from the Norwegian Sea (as dinocyst even superficially resolve climate cyles. Nevertheless, the low- sp. 1 in Mudie, 1989), in Labrador Sea ODP Hole 646B in the Pia- resolution data set reflects long-term changes in the assemblage cenzian (as Tectatodinium sp. I in de Vernal and Mudie, 1989b), and composition during the Quaternary, and a higher resolution will near the Plio-Pleistocene boundary in ODP Site 645 in Baffin Bay (as likely resolve climate variability on Milankovitch time scales. Tectatodinum sp. I in de Vernal and Mudie, 1989a) as more reliable stratigraphic events. Based on this compilation, Head et al. (2004: 6. Implications of dinocyst events for Arctic Ocean p.285) suggested that “H. tectata has a confirmed HO in the up- chronostratigraphy permost Pliocene (Gelasian, now Lower Pleistocene), with the sporadic and rare higher occurrences considered to represent The age models for the ACEX composite record are based on two possible reworking”. De Schepper and Head (2008) confirmed separate approaches that result in considerable offsets in the age/ these observations and defined a HPO in DSDP Hole 610A from the depth plot (Fig. 3B; Frank et al., 2008; O'Regan et al., 2008a, 2010; northeastern Atlantic Ocean at the top of Subchron C2r.1r. In ODP Chen et al., 2012). Firstly, Frank et al. (2008) calculated 10Be/9Be Hole 911A, a HPO occurs below the base of the Olduvai Subchron estimates for the past 12.3 Ma including 10 samples from the (Matthiessen and Brenner, 1996), while the HPO in DSDP Hole 603C Quaternary that were later updated by Chen et al. (2012) after is younger at the base of Subchron C1r.3r (Fig. 6; Head in De revision in the half life of 10Be. Secondly, O'Regan et al. (2008a) Schepper and Head, 2008). Our new data from ODP Hole 911A applied a cyclostratigraphic approach for the past 1.2 Ma, and J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 15 two age models, based on geomagnetic polarity changes, were Backman et al., 2008; O'Regan et al., 2008a) based on a correlation proposed for the ACEX composite record down to 26.69 and 26.79 to ODP Hole 911A (Matthiessen and Brenner, 1996). The absence of rmcd, corresponding to an age of 2.485 and 2 Ma, respectively. dinocysts in younger sediments was interpreted as an indication of These age models were supported only by two poorly constrained an age older than the HO (>1.1 Ma) below 22.69 rmcd (Backman and somewhat contradictory bioevents (Backman et al., 2008; et al., 2008; O'Regan et al., 2008a). O'Regan et al., 2008a). The first bioevent, the common occurrence Our new palynostratigraphic data revise the HO of H. tectata to of Neogloboquadrina pachyderma in a single sample at 19.33 rmcd 22.61 rmcd based on Hole M0002A, approximately a meter below has been interpreted to indicate a maximum age of about 1.8 Ma the top of Subunit 1/3 (Fig. 7). In contrast to the shipboard data, (Backman et al., 2008; O'Regan et al., 2008a) based on its LO in the H. tectata is abundant to dominant in two intervals at 22.61 and Nordic Seas (Spiegler, 1996). However, this species is present in 24.50 rmcd indicating an age older than the HPO at 2.0 Ma (Fig. 9A). older sediments in ODP Holes 910C and 911A and sites in the Fram Filisphaera filifera was not recorded during the shipboard Strait, but it occurs more frequently in ODP Hole 911A only after the biostratigraphic analysis (Backman et al., 2006), but we document a Olduvai Subchron from ca. 1.7 Ma (Fig. 9B; Spiegler, 1996). common occurrence of F. filifera at 22.61 and 24.50 rmcd, sug- Anthonissen (2008) noted a lowest common occurrence in North gesting that this stratigraphic interval is older than the top of the Atlantic DSDP and ODP sites approximately at the top of the Olduvai acme in ODP Hole 911A at 1.8 Ma (Figs. 6 and 9B). By comparison to Subchron (1.778 Ma). The second bioevent was the HO of Hab- ODP Hole 911A, the co-occurrence of abundant H. tectata and ibacysta tectata being a rare occurrence in a single sample from F. filifera indicates an age slightly older than the HPO of H. tectata Hole M0004C at 22.69 rmcd (Backman et al., 2006). This HO was (ca. 2.0 Ma), coeval with the base of the F. filifera acme. dated to near the base of the Jaramillo Subchron (ca. 1.1 Ma, The common occurrence of N. pachyderma at 19.33 rmcd

Fig. 9. Comparison of the ACEX dinocyst assemblages with paleoenvironmental proxies, and possible stratigraphic correlation between different records. A, age/depth plot based on selected age tie points for the stage boundaries (from O'Regan et al., 2008a; this study). Bars depict species abundances (%) in the productive intervals. Because of relatively low abundances the counts of all samples from a productive interval were used to calculate percentages. The stratigraphic occurrence of Neogloboquadrina pachyderma is shown (Cronin et al., 2008; Eynaud et al., 2009). The seasonal extent of the sea ice cover varied during the Quaternary and only the presence of more than 5 iron grains with a Victoria and Banks Island provenance are interpreted to indicate perennial ice (Darby, 2014). The ε-Nd is a proxy for either brine formation in the Arctic Ocean (low values) or intermediate water inflow from the Atlantic (high values) (Haley et al., 2008). B, stratigraphic occurrence of Filisphaera filifera and Habibacysta tectata, and N. pachyderma ^ in the Barents Sea composition record; C, Concentrations of F. filifera in the Ile de France Formation on East Greenland (Bennike et al., 2002). (m asl: meter above sea level). The youngest grayish bar denotes the F. filifera acme in East Greenland and at the northern Barents Sea margin which is below the lowest occurrence of frequent N. pachyderma. The older grayish bar is located at the youngest abundance maximum of H. tectata at the northern Barents Sea margin and in the ACEX composite record. The stippled line indicates a speculative correlation between peaks of H. tectata close to 2.7 Ma. 16 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26

(Fig. 9A; O'Regan et al., 2008a), above the HOs of H. tectata and average ice drift rate in his conceptual model than in his previous F. filifera (22.61 rmcd), supports an age older than the top of the study (Darby, 2008) and concluded that a “… return to any Olduvai Subchron (1.778 Ma). A comparable sequence has been persistent seasonal ice occurs during the last 1.5 Ma … ”. Calcula- observed at ODP Site 911, where N. pachyderma occurs more tions of drift rates based on historical observations (Hakkinen et al., frequently only from the top of the Olduvai Subchron above the 2008) show that rates were higher during a considerable part of the acme of F. filifera (Fig. 9B; Spiegler, 1996; Matthiessen and Brenner, past 50 years than the average rate of 5 cm/s used by Darby (2014). 1996; this study). Thus, the upper part of the F. filifera acme may be This may imply that seasonal sea ice cover may have been more missing in the ACEX composite section but this does not affect the frequent during the Quaternary than what Darby (2014) proposed. stratigraphic interpretation. Recently, Cronin et al. (2013) and Polyak et al. (2013) suggested The interpretation of the magnetic polarity pattern in the low- from ostracod and foraminifer data a rather variable sea ice cover in sedimentation-rate age hypothesis which proposed an age of the central Arctic Ocean in the past ca. 1.5 myrs, providing their 2.485 Ma at 26.69 rmcd (O'Regan et al., 2008a) supports the above chronology is correct. biostratigraphic interpretation. The interval with normal polarity The occasionally common occurrence of dinocysts during below 19.73 rmcd may represent the Olduvai Subchron while the certain periods in the central Arctic Ocean also supports the hy- negative polarity at the stratigraphic level of the HOs of H. tectata pothesis of a more variable ice cover and seasonally open waters and F. filifera supports an age older than the base of the Olduvai areas during the Quaternary, including the Early Holocene (Fig. 9A; Subchron (>1.945 Ma). Both the foraminifer and dinocyst events see Cronin et al., 2010, 2017; Marzen et al., 2016; Stein et al., 2017). bracket the boundary of Subunit 1/3 to 1/2 in Hole M0002A at 21.30 These cysts of probably phototrophic dinoflagellates need light and rmcd that probably marks a distinct change in the paleoenviron- thus open water conditions during the growth season. It must be ment. This lithological boundary is correlated to the basal part of kept in mind that the almost complete absence of dinocysts in the Olduvai Subchron. Subunit 1/2 does not necessarily imply a permanent ice cover (see The assemblage composition in ODP Hole 911A at the base of the Section 5.1). F. filifera acme and at the top of lithological Subunit 1/3 in the ACEX The low-resolution composite record of ODP Holes 910A and composite record (Fig. 9A and B) resembles those in biozone RT7c 911A does not fully capture the climate and sea ice variability in the (ca. 2.08e2.24 Ma) from DSDP Hole 610A in the northeastern Quaternary but demonstrates a variable sea ice limit at the north- Atlantic Ocean (De Schepper and Head, 2009). The assemblages in ern Barents margin during the Quaternary (Figs. 5 and 6; Hole 610A are characterized by variable abundances of H. tectata Matthiessen and Brenner, 1996). However, this does not exclude a and F. filifera, co-occurring with the cool-water species Impagidi- coeval perennial sea ice cover in the central Arctic Ocean. The nium pallidum and Bitectatodinium tepikiense, and a number of common to dominant species Bitectatodinium tepikiense, Nem- warmer water species absent in the Arctic Ocean. In Hole 610A, atosphaeropsis labyrinthus and cysts of Protoceratium reticulatum Biozone RT7c comprises the Reunion Subchron (C2r.1n, may occur in modern polar to subpolar environments with a 2.128e2.148) whose top has been tentatively identified at 23.25 seasonally variable extent of sea ice (e.g. Rochon et al., 1999; de rmcd in the slow sedimentation rate model (O'Regan et al., 2008a). Vernal et al., 2001; Matthiessen et al., 2005). Brigantedinium spp. The restricted occurrence of dinocysts in the few undisturbed indet. as cysts of heterotrophic dinoflagellates suggest a higher intervals down to 37 rmcd does not provide additional age markers production at the ice margin. Sediment core PS2138-1 studied at a in the Lower Pleistocene and Upper Pliocene. Between 35.80 and higher resolution than ODP Holes 910A and 911A indicates that 36.00 rmcd, three samples comprise dominant H. tectata together during interglacial optima with substantially higher dinocyst con- with F. filifera, similar to those at 21.61 and 24.50 rmcd that might centrations and the predominance of cysts of P. reticulatum, the ice correlate with a comparable assemblage around 2.7 Ma in ODP Hole margin might have periodically retreated far to the north during 911A (Fig. 9). the summer. Moreover, the alternating occurrence of dinocysts The base of the Quaternary is tentatively placed at approxi- during glacials reflects variable climate conditions and sea ice mately 31 rmcd in the ACEX composite record based on linear extent even when the Barents Sea ice sheets have reached a interpolation between the HO of H. tectata and F. filifera at 21.61 maximum extent (e.g. MIS 6, MIS 2; Svendsen et al., 2004). rmcd (ca. 2.0 Ma) and the base of Subchron C2An.1n (3.032 Ma) at Extensive leads or even large areas with open waters (e.g. polynyas, 37.7 rmcd (Backman et al., 2006; O'Regan et al., 2008a). Including Knies et al., 1999), possibly linked to increased (sub-)surface inflow the tie points for the base of the Late and Middle Pleistocene from of relatively warmer waters from the Nordic Seas, may have occa- the low sedimentation rate model (O'Regan et al., 2008a), a low- sionally formed during a short summer season in MIS 2 and 6 resolution age model is proposed for the ACEX composite record allowing phototrophic dinoflagellates to flourish and form cysts (Fig. 9A). (e.g. cysts of P. recticulatum, B. tepikiense). Ice cover was still extensive as indicated by high abundances of cold-adapted species 7. Paleoenvironmental implications which occur today in regions characterized by only few weeks of open water (Islandinium minutum and related taxa, Impagidinium 7.1. Arctic Ocean sea ice cover pallidum). A strongly variable ice margin in the marginal Arctic Ocean The history of sea ice cover and its natural variability is a major during the entire Quaternary is supported by climate records from topic in Arctic research due to the recent reductions in the sea ice exposed shallow marine deposits at the circum-arctic coasts (e.g. extent and thickness, and the recurrent records of minimum Brigham-Grette and Carter, 1992; Repenning and Brouwers, 1992; summer ice extent (e.g., Polyak et al., 2010; Stroeve et al., 2012; Funder et al., 2001; Bennike et al., 2002, 2010) and low- Parkinson and Comiso, 2013). A permanent pack ice cover in the resolution records of the biomarker IP25 reflecting spring sea ice central Arctic Ocean in the past 12 myrs has been reconstructed cover (Stein and Fahl, 2013; Knies et al., 2014). from mineralogical studies (Darby, 2008; Krylov et al., 2008), but Cronin et al. (2008) already suggested from benthic foraminiferal 7.2. The Filisphaera filifera acme in the Olduvai Subchron analysis in the upper 18 m of IODP Hole M0004C that seasonally ice-free conditions might have prevailed during interglacial stages The acme of F. filifera in Assemblage Zone II during the upper in the Matuyama Chron. Darby (2014) applied a slightly higher Gelasian clearly stands out in the dinocyst record at the Barents Sea J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 17 margin (Fig. 6). An acme of Filisphaera spp. and/or H. tectata has occurring during MIS 77, which is dated to around 2 Ma (Melles been observed at several sites in the North Atlantic, Nordic Seas and et al., 2012). This interglacial might be a possible equivalent of North Sea broadly correlating with the Olduvai Subchron (Head, the Kap København Formation and the productive interval in the 1996b, 1998; Smelror, 1999; Bennike et al., 2002, 2010; Head ACEX composite record at ca. 2.1 to 2.2 Ma. et al., 2004; Kuhlmann, 2004; Kuhlmann et al., 2006; Meijer The circulation and exchange of water masses with the North et al., 2006; De Schepper and Head, 2009). The composition of Atlantic has probably also changed across the Olduvai Subchron. ^ the assemblages in the Ile de France Formation at the East Prior to the Olduvai Subchron, planktonic foraminifers are absent Greenland Coast are comparable to those of the upper part of the from ODP Hole 911A (Spiegler, 1996) and ACEX Hole M0004C acme in ODP Hole 911A comprising abundant to dominant F. filifera (Cronin et al., 2008; O'Regan et al., 2008a; Eynaud et al., 2009), bracketed by abundant to dominant Brigantedinium spp. indet. while thereafter carbonate preservation has improved and assem- (Fig. 6; Bennike et al., 2002; Matthiessen and Brenner, 1996). On the blages comprise variable abundances of N. pachyderma indicating Lomonosov Ridge, only the lower part of the acme, comprising predominantely cold conditions and rare inflow of temperate to relatively high abundances of H. tectata, is preserved below the warm surface waters (Fig. 9A and B). From a low-resolution neo- transition of Subunit 1/2 to 1/3 (Fig. 9A). This transition roughly dymium isotope record, Haley et al. (2008) interpreted that the correlates with a seismic reflector (Backman et al., 2008) that might long-term circulation pattern in the Arctic Ocean was generally mark an unconformity. At the northern Barents Sea margin, stable before 2 Ma, with Arctic Intermediate Water (today at Mattingsdal et al. (2014) identified seismic reflectors both at the top 200e1500 m water depth) mainly derived from brine formation in and base of the Olduvai Subchron (1.78e1.95 Ma) that may include the Eurasian shelf regions (Fig. 9B). Intermediate water exchange the same reflector as on Lomonosov Ridge. between the North Atlantic and Arctic Ocean was reduced to about The co-occurrence of abundant F. filifera and H. tectata along the half of the modern flux. The North Atlantic sourced intermediate path of the Atlantic water inflow to the high northern latitude re- water input was even more reduced at approximately 2 Ma (Fig. 9A; flects a distinct change in oceanographic conditions. Both species between 22.40 and 24.90 rmcd) and glacials thereafter related to are considered cool-tolerant (e.g. Mudie, 1987; Head, 1996b; De enhanced brine formation due to increased sea ice formation at the Schepper et al., 2011; Hennissen et al., 2017; Schreck et al., 2017). edges of the circum-Arctic ice sheets. In contrast, interglacials were The temperature range of F. filifera is estimated 10.7e25.2 C with associated with negligible brine formation, and a modern-like highest abundances usually associated with 14e17 C(De Schepper Atlantic Intermediate Water inflow. This conforms with Henrich et al., 2011). H. tectata is particularily abundant in horizons having et al. (2002) who reconstructed a much better carbonate preser- temperature estimates between 10 and 25 C, but optimum tem- vation since the base of Olduvai Subchron after 1.9 Ma probably due peratures are around 15 C(De Schepper et al., 2011; Hennissen to the first intrusions of the Proto-Norwegian Current into a narrow et al., 2017; Schreck et al., 2017). Hennissen et al. (2017) proposed corridor in the southeastern Nordic Seas, leading to North Atlantic that besides sea-surface temperature elevated salinity may explain Deep Water production. The time resolution of both the neodym- these higher abundances. ium isotope and dinocyst record is too low for detailed correlation The H. tectata and F. filifera acme may reflect a period of but there might have been a pronounced oceanographic variability considerably warmer conditions with relatively warm and saline in the circum-Arctic during the Olduvai Subchron. Atlantic waters reaching the Arctic Ocean and the East Greenland The consequence of atmospheric and surface ocean warming coast. During such intervals, enhanced northward heat and mois- might have been a substantially smaller or even absent Greenland ture transport would have resulted in a reduced meridional ther- and Scandinavian ice sheet during the Olduvai Subchron as indi- mal gradient in the Atlantic sector of the high northern latitudes. cated by background levels of ice-rafted debris (IRD) at ODP Sites Seasonal sea ice cover may have been strongly reduced at the 644 and 907, bracketed by abundance maxima at the base and top northern Barents Sea margin, and open water conditions may have of the Olduvai Subchron (Jansen et al., 1988, 2000; Henrich et al., occurred in the central Arctic Ocean (Fig. 9A; Darby, 2014). The 1989; Fronval and Jansen, 1996). The low-resolution IRD records acme at the northern Barents margin is bracketed by assemblages of ODP Sites 908, 909 and 911 from Fram Strait and the northern comprising extant species indicating cooler conditions with sea- Barents margin do not reflect a comparable trend for northern sonal sea ice (Fig. 5; e.g. Rochon et al., 1999; de Vernal et al., 2001). Greenland and the Barents Sea ice sheets (Knies et al., 2009). Interglacial shallow marine deposits are exposed at some lo- Bierman et al. (2016) assume a generally smaller Greenland Ice calities of the Arctic margins including East Greenland (see Bennike Sheet, and Schaefer et al. (2016) concluded that Greenland was et al., 2010: Fig. 9), where palynological and faunal data docu- nearly ice free at that time. Knowledge of possible ice sheets at the mented a period of increased warmth around the Olduvai Sub- margins of the Arctic Ocean is still limited, and Haley et al. (2008) ^ chron. The boreal forest or forest-tundra vegetation in the Ile de inferred from the significant excursion in the neodymium isotope France and Store Koldeway formations (Fig. 1) indicates a regional record the growth of the first major North Eurasian ice sheets mean summer air temperature higher by at least 6 C than today before the Olduvai Subchron (Fig. 9B) which grounded on large during the Olduvai Subchron (Bennike et al., 2002, 2010). Bottom areas of the Arctic shelves including the Kara Sea. Interestingly, water temperature on the inner shelf was considerably higher than Rohling et al. (2014) proposed from a Mediterranean Sea level re- today and relatively high ocean temperatures exclude a perennial cord that the first major glaciation in the Northern Hemisphere sea ice cover. The northernmost terrestrial site in northern occurred at 2.15 Ma based on a sea-level 70 m lower than for any Greenland, the Kap København Formation, records an expansion of preceding lowstand. The acmes of F. filifera and H. tectata occurred the treeline to 82300N, indicating that summer air and ocean at the end of a gradual increase in pervasive glacial episodes at Lake temperatures were much higher than today and inconceivable with El'gygytgyn, leading to full establishment of glacial/interglacial a Greenland ice sheet (Funder et al., 2001). However, the estimated cycles from 1.8 Ma (Melles et al., 2012). duration of sedimentation of about 20,000 years suggests that these deposits just represent a single interglacial (Funder et al., 8. Future directions: towards a composite Quaternary 2001). High-resolution terrestrial paleoclimate records across the stratigraphy upper Gelasian are still lacking, but the high-resolution lithofacies record from Lake El'Gygytgyn in eastern Siberia (Fig. 1) shows a This study has shown that dinocyst events are useful strati- number of super-interglacials in the Quaternary, one of these graphic tie points in the central Arctic Ocean for a composite 18 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 chronostratigraphy. However, like other microfossil groups, dino- 2012). Moreover, an undisturbed composite record may comprise cysts cannot provide a comprehensive and continuous set of age more dinocyst events than the present ACEX record that is based on control points for establishing a chronostratigraphy. A multi-proxy a single hole with core disturbance and coring gaps in the Lower biostratigraphic approach can be developed from a combination of Pleistocene. An ecostratigraphic approach may be applicable in the calcareous microfossil (calcareous nannofossils, foraminifers, os- Middle to Late Pleistocene, when changes in the assemblage tracods), agglutinated foraminifer and dinocyst events, comparable composition and concentrations of microfossils enable strati- to the composite biostratigraphy for the northern North Atlantic graphic correlation on a supra-regional scale (e.g., Backman et al., (e.g., Gradstein and Backstr€ om,€ 1996; Anthonissen, 2008; Fensome 2004). The northern Barents Sea margin might be an ideal region et al., 2008). Both biostratigraphy and magnetic events may be to calibrate the various methods to an independent chro- useful for transarctic correlation although the application of pale- nostratigraphy based on stable oxygen isotope stratigraphy, mag- omagnetostratigraphy is principally questioned (e.g., Xuan et al., netostratigraphy, foraminifer and calcareous nannofossil events. In

Table 2 Compilation of potentially useful stratigraphic markers and their occurrence in selected regions of the Arctic Ocean. Only selected references are given: 1, Poore et al., 1993;2, Phillips and Grantz,1997;3,Jakobsson et al., 2000;4,Polyak et al., 2004;5,Nørgaard-Pedersen et al., 2007a;6,Adler et al., 2009;7,Stein et al., 2010b;8,Gard,1993;9,Backman et al., 2009;10,Aksu, 1985b;11,Matthiessen et al., 2001;12,de Vernal et al., 2005b;13,Dokken and Hald, 1996;14,Knies et al., 1999;15,Hald et al., 2001;16,Vogt et al., 2001; 17, Nowaczyk et al., 1994;18,Dowdeswell et al., 2010;19,Jessen et al., 2010; 20, Andersen et al., 1996;21,Løvlie et al., 1986; 22, Nowaczyk et al., 2003; 23, O'Regan et al., 2008a; 24, Bazhenova, 2012; 25, Clark et al., 1980; 26, Mudie and Blasco, 1985;27,Kaparulina et al., 2016; 28, Lowemark€ et al., 2012; 29, Schubert and Stein, 1996; 30, Jakobsson et al., 2001;31,Spielhagen et al., 2004; 32; Backman et al., 2004;33,Wollenburg et al., 2001; 34, Scott et al., 1989; 35, Van Nieuwenhove et al., 2011; 36, Jakobsson et al., 2003;37, Xuan et al., 2012; 38, Mudie, 1985; 39, Cronin et al., 2008; 40, Gilbert and Clark, 1983;41,Cronin et al., 2013; 42, Mattingsdal et al., 2014; 43, Knies et al., 2009; 44, this study; 45, Spiegler, 1996. abbreviations: LP, Late Pleistocene; MP, Middle Pleistocene; EP, Early Pleistocene; B, biostratigraphy; L, lithostratigraphy; P, paleomagnetostratigraphy; PP, physical properties; FS, Fram Strait; YP, Yermak Plateau; LR, Lomonosov Ridge; AR, Alpha Ridge; MR, Mendeleev Ridge; NR, Northwind Ridge; CP, Chuckhi Plateau).

Event Age (approx.) Type Stratigraphic Marker FS YP LR AR MR NR/CP References

Holocene L brown bed B1 ? ? x x 1,2,3,4,5,6,7 B coccolith bioevents x x x 3,8,9 B planktic foraminifer maximum x x x x 1,2,5,6,10 B dinocyst concentration maximum x x 11,12

Late Pleistocene LP1 MIS2 L,B carbonate and planktic foraminifer maximum x x 13,14,15,16 PP wet bulk density maximum x x 17,18,19 LP2 MIS2/3 PP magnetic suceptibility minimum x x 17,18,19 L TOC maximum x x 14,16,20 LP3 MIS3 P Mono Lake excursion x x ? ? 21,22,23,24 LP4 MIS3 P Laschamp excursion x x ? ? ? 1,21,22,23,24 LP5 MIS3 L white layer W3 x ? x x 1,4,6,7,25,26 LP6 MIS3 L brown bed B2 ? ? x x 1,2,4,5,6,7 LP7 MIS3/4 PP magnetic suceptibility minimum x x ? ? ? 17,18 L gray layer x x 28 L TOC maximum x x 16,29 LP8 MIS4 P Norwegian-Greenland Sea excursion x x ? ? ? 1,22,23 LP9 top MIS5 B acme of small subpolar planktic foraminifers ? x 5,6,8 L brown bed B3 x ? x 2,3,4,6,7 LP10 MIS5 L white layer W2 x ? x x 1,4,6,7,25,26,27 L brown bed B4 x ? x 2,4,6,7 LP11 MIS5a/5.1 B coccolith bioevents x 3,8,9,30,31 B acme of benthic foraminifer Bulimina aculeata x x 4,30,32 LP12 top MIS 5 B acme of benthic foraminifer Pullenia bulloides x x 33 LP13 MIS5c/5.3 B coccolith bioevents x 3,8,9,30,31 LP14 ?MIC5c L brown bed B5 ? ? x x 2,4,6,7 LP15 ?MIS5c L brown bed B6 x ? x x 2,4,6,7 LP16 MIS 5.4 L pink-white layer PW2 x ? x ? 1,4,6,7,25,26,27 LP17 TOP MIS5e B HCO Bolivina arctica at top brown bed B7 ? 32,34 L brown bed B7 x ? x x 2,4,6,7,30 B acme of Bolivina arctica x ? x x 34 LP18 MIS5e/5.5 B acme of small subpolar planktic foraminifers x ? x x 5,6,8 B coccolith bioevents x 3,5,8,9,31 B dinocyst concentration maximum x x 11,35 OSL age x 36 LP19 ?MIS5e P Blake excursion x x 17 LP20 MIS5/6 P increase ARM/magnetic susceptibility x ? 37

Middle Pleistocene MP1 MIS6 B concentration maximum of Pediastrum spp. x ? 38,39 MP2 MIS6/7 P Iceland Basin excursion ? ? x ? x x 1,3,24,31 MP3 ?MIS6/7 L white layer W1 ? x x 25,26 MP4 ?MIS6-8 B faunal transition from calcareous to agglutinated benthic forams. x ? x x 40 MP5 ?MIS 8 L pink-white layer PW1 x ? x x 1,25,26 MP6 ?MIS7-9 B acme of calcareous dinocysts ? x 38,40 MP7 ?MIS 11 B acme of Turborotalita eglida x41 MP8 P base Brunhes (C1n) x x ? 23,42,43

Early Pleistocene EP1 ?1.3-1.4 ma B HO F. filifera x44 EP2 ?1.6-1.8 ma B LCO N. pachyderma x x 23,45 EP3 1.8 ma B HCO F. filifera x44 EP4 2.0 ma B HPO H. tectata xx 44 EP5 2.581 ma P top Subchron C2An.1n x x 42,43 J. Matthiessen et al. / Quaternary Science Reviews 192 (2018) 1e26 19 addition, stratigraphic intercomparison experiments may be con- formation of manganese-rich layers and a temporarily closed pack ducted. The temporal resolution of a composite bio- and ecos- ice cover. tratigraphy can be improved by adding tie points derived from Stratigraphically useful dinocysts are restricted to the Lower other stratigraphic proxies, allowing the application of a cylos- Pleistocene at the northern Barents Sea margin and on Lomonosov tratigraphic approach as already proposed for the ACEX composite Ridge. The top of the Filisphaera filifera acme and the highest record (O'Regan et al., 2008a). persistence occurrence of Habibacysta tectata are important age tie Table 2 comprises potentially useful stratigraphic events in points in the Atlantic sector of the high northern latitudes at the top different regions of the Arctic Ocean compiled from published re- of the Gelasian. The co-occurrence of both species at the top of cords. The majority of events are in the upper Middle to Late Subunit 1/3 in the ACEX composite record indicate an age slightly Pleistocene, primarily due to the predominantely available strati- older than the HPO of H. tectata at the Barents Sea margin at 2.0 Ma. graphic records from short (<10 m) sediment cores. The bioevents These are the first biostratigraphic age control points in the Lower are stratigraphically relatively well-constrained in the Late Pleis- Pleistocene of the central Arctic Ocean, improving our ability to tocene, but a rigorous calibration to an independent time scale is constrain the age of Quaternary deposits. The ages of the bioevents still required because numerical ages are available only for a few rather support the slow sedimentation rate model of O'Regan et al. stratigraphic events. Beyond the domain of application of the 14C, (2008a) which results in a thickness of about 31 m for the Qua- most ages are rather approximate due to lacking independent ternary at the ACEX Site in the central Arctic Ocean. To establish a chronostratigraphy, in particular in the Amerasian Basin. chronostratigraphic framework for the central Arctic Ocean, a Continuous downcore logging of physical properties (wet bulk combination of different methods must be applied. density, magnetic susceptibility) is routinely used for correlation From a paleoceanographic point of view, the acme of F. filifera in between cores on a regional scale (e.g., Stein et al., 2001; O'Regan the upper Gelasian probably marked inflow of North Atlantic water et al., 2008a, 2008b; Dowdeswell et al., 2010; Jessen et al., 2010; into the Arctic Ocean. The lower part of this acme comprising Sellen et al., 2010). The variable lithology in the central Arctic higher abundances of H. tectata may be recorded in the central Ocean expressed in an alternation of major sedimentation events Arctic Ocean as well. Paleoecological preferences of both species such as the white and pink-white diamictons with characteristic suggest a substantial warming of surface waters, associated with a mineralogy, geochemistry and physical properties may supplement reduction of the pack ice cover in the Arctic Ocean. these correlations. Some specific sedimentation events such as diamictons and dark gray beds spreading from a regional source Acknowledgements across the Arctic Ocean may be useful for supra-regional correlation. Sites on Lomonosov Ridge may record stratigraphic events in This research used samples and data provided by the Alfred both the Eurasian and the Amerasian parts of the Arctic Ocean Wegener Institute in Bremerhaven, and the Ocean Drilling Program (Table 2), and studying the ACEX holes with a high temporal res- (ODP) and Integrated Ocean Drilling Program (IODP) sponsored by olution and drilling new sedimentary sequences close to the shelf the US National Science Foundation (NSF) and participating coun- edge, where productivity is usually higher, might help to improve tries. Anja Bartels is thanked for assistance in the laboratory. We the chronostratigraphy of the Quaternary. sincerely thank Stephen Louwye and Jens De Clercq for sharing The Quaternary in the central Arctic Ocean can be subdivided with us their unpublished stratigraphic data from Site U1341. Ali tentatively by a combination of different stratigraphic markers Aksu is acknowledged for providing the original planktonic fora- (Table 2). The base of the Holocene is easily distinguished from the minifer and stable isotope data of sediment core CESAR 83e102. Upper Pleistocene by AMS14C ages, and a concomitant increase in The constructive comments and suggestions of Martin Head and an foraminifer and dinoflagellate cyst abundances. The base of the anonymous reviewer helped to improve the manuscript. J.M. ac- Upper Pleistocene can be constrained by numerous methods, and a knowledges financial support from the Deutsche For- few events such as the distinct peak of light stable oxygen isotopes schungsgemeinschaft DFG (Ma3913/1, Ma3913/3), and S.D.S from of Neogloboquadrina pachyderma that might be an excellent tie the Research Council of Norway (Project 229819). A.d.V and C.Z. are point on a basin-wide scale. The base of the Middle Pleistocene is grateful for support from the Natural Science and Engineering difficult to define as frequent polarity reversals hamper unambig- Research Council (NSERC) of Canada. uous identification of the Brunhes/Matuyama transition. Dinocysts may allow approximate definition of the base of the Calabrian Appendix A. Supplementary data stage, this being tentatively placed above the base of Subunit 1/2 in the ACEX composite record. Identifying the base of the Quaternary Supplementary data related to this article can be found at in the central Arctic Ocean on Lomonosov Ridge is only possible by https://doi.org/10.1016/j.quascirev.2017.12.020. interpolation between stratigraphic tie points in the Lower Pleis- tocene and the Upper Pliocene, partly because of the intense coring References disturbance in the ACEX composite record in this interval. Adler, R.E., Polyak, L., Ortiz, J.D., Kaufman, D.S., Channell, J.E.T., Xuan, C., Grottoli, A.G., et al., 2009. 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