Research paperResponse of marine palynomorphs to Neogene climate cooling in the Iceland Sea (ODP Hole 907A)
Introduction
The Neogene is a crucial epoch for the evolution of Earth's climate as it went through the fundamental transition from a relatively warm Early Miocene to the colder conditions at the end of the Pliocene (Zachos et al., 2008). After the Miocene Climate Optimum (MCO, 17–15 Ma), when Earth experienced warmest temperatures since the Middle Eocene (Zachos et al., 2008), global surface and deep ocean temperatures cooled significantly (Wright et al., 1992, Billups and Schrag, 2002, Shevenell et al., 2004, Kuhnert et al., 2009) across the Middle Miocene Climate Transition (MMCT, 14.2–13.7 Ma), and the East Antarctic Ice Sheet experienced major expansion (e.g. Flower and Kennett, 1994, John et al., 2011, Paschier et al., 2011). This transition marked the onset of progressive long-term cooling both on land (Pound et al., 2012) and in sea (Zachos et al., 2008) that ultimately pushed the climate system into the bipolar glaciated mode of today. Superimposed, distinct short-term climate variability has been observed in marine records (Miller et al., 1991, Turco et al., 2001, Andersson and Jansen, 2003, Westerhold et al., 2005, John et al., 2011) and partly attributed to ice sheet growth on Antarctica and/or bottom-water cooling (Mi events sensu Miller et al., 1991). In this context, however, the high northern latitude oceans are of eminent relevance to decipher causes and consequences of the Neogene climate variability (e.g. Thiede et al., 1998) since they influence global climate through feedback mechanisms related to the formation of perennial/seasonal sea ice cover (Miller et al., 2010, Serreze and Barry, 2011), and production of northern-sourced deep-water (Flower and Kennett, 1994, Rahmstorf, 2006).
The timing of the onset of glaciations in the Northern Hemisphere is still debated but ice-rafted debris (IRD) has been recorded as early as the Middle Eocene in the Arctic Ocean (Stickley et al., 2009) and the Greenland Sea (Eldrett et al., 2007, Tripati et al., 2008). Although it has been speculated that a Neogene Greenland Ice Sheet has existed since at least 18 Ma (Thiede et al., 2011), published records suggest that small scale glaciations large enough to reach sea-level might have occurred on Greenland not before the early to middle Late Miocene (Schaeffer and Spiegler, 1986, Wolf and Thiede, 1991, Fronval and Jansen, 1996, Wolf-Welling et al., 1996, Helland and Holmes, 1997, Winkler et al., 2002). In contrast, ice sheets probably developed in the northern Barents Sea and on Scandinavia already in the Middle Miocene (Fronval and Jansen, 1996, Knies and Gaina, 2008).
The formation of glacial ice on the circum-Arctic continents might have been linked to the fundamental reorganization of the circulation in the Nordic Seas. The opening of Fram Strait and the subsidence of the Greenland–Scotland Ridge (GSR) have led to an enhanced exchange of water masses between the Arctic Ocean and the North Atlantic via the Nordic Seas (Bohrmann et al., 1990, Wright and Miller, 1996, Poore et al., 2006, Jakobsson et al., 2007, Knies and Gaina, 2008). Jakobsson et al. (2007) assumed that Fram Strait reached sufficient width (40–50 km; present-day width 400 km) to efficiently ventilate the Arctic Ocean at ~ 17.5 Ma, and began to open at greater depth by ~ 14 Ma, which is supported by benthic foraminiferal evidence from the Lomonosov Ridge and Fram Strait (Kaminski et al., 2006, Kaminski, 2007). Simultaneously, a prominent shift from biosiliceous to calcareous-rich sediments in the Norwegian Sea (Bohrmann et al., 1990, Cortese et al., 2004) was likely coupled to the subsidence of the Greenland–Scotland Ridge, which consequently modulated exchange of North Atlantic and Arctic water masses. Reduced fluxes in Northern Component Water (NCW) have been correlated with uplifts of the GSR and vice versa (Wright and Miller, 1996, Poore et al., 2006, Abelson et al., 2008), and Bohrmann et al. (1990) related periodic changes in biogenic carbonate and opal accumulation in the Norwegian Sea to variable exchange of surface waters between the Nordic Seas and the North Atlantic. The initiation of a proto-East Greenland Current (EGC) may have coincided with the establishment of a modern-like ice drift pattern through Fram Strait at around 14 Ma (Knies and Gaina, 2008), but Wei (1998) suggest an onset around 12 Ma based on calcareous nannofossils from the Irminger Basin. An even younger onset at around 10.5 Ma is proposed by Wolf-Welling et al. (1996) based on changes in bulk accumulation rates in the Fram Strait. The modern EGC was probably first established during the intensification of Northern Hemisphere glaciations with the thermal isolation of Greenland due to the final closure of the Isthmus of Panama and the opening of Bering Strait at the Pliocene–Quaternary transition (Sarnthein et al., 2009).
However, to date, most information is derived from a few Neogene sections located along the path of the inflowing North Atlantic waters in the Norwegian Sea (Fig. 1) while the Neogene paleoceanography of the Greenland and Iceland seas is almost unknown due to the discontinuous occurrence of calcareous microfossils in these sediments (e.g. Fronval and Jansen, 1996). Here we present a comparatively high-resolution record of organic-walled microfossils (e.g. dinoflagellate cysts = dinocysts, prasinophytes, acritarchs) from the almost continuous middle Miocene to upper Pliocene sequence of ODP Hole 907A in the Iceland Sea, an area close to the growing Greenland and Iceland ice sheets, which experienced the effects of sea ice cover, migrating wind fronts and ocean currents. The pristine paleomagnetic record of Hole 907A provides the unique opportunity for detailed investigations on the response of palynomorph assemblages to the MMCT and subsequent long-term cooling, and thus to derive complementary information on the Neogene of the Nordic Seas cold-water domain. Palynomorph-based interpretations have been supplemented with a low-resolution alkenone SST record that provides new constraints on the thermal evolution of the Iceland Sea.
Section snippets
Material and methods
ODP Hole 907A is located on the eastern Iceland Plateau (69°14.989′ N, 12°41.894′ W; 2035.7 m water depth; Fig. 1), and was drilled in an undisturbed hemipelagic sequence that mainly consists of unlithified silty clays and clayey silts (Shipboard Scientific Party, 1995). Based on the revised magnetostratigraphy (Channell et al., 1999), which has been adjusted to the latest astronomically-tuned Neogene timescale (ATNTS) 2004 (Lourens et al., 2005), a total of 120 samples spanning the early Middle
Palynomorph assemblages
Hole 907A yielded a diverse and moderate to well preserved palynomorph assemblage, where 84 samples contained enough material to enumerate a statistically relevant quantity of 350 dinocysts. Counts in 36 samples ranged between 0 and 196 dinocysts of which 21 samples are virtually barren (< 10 cysts/slide). The majority of these samples cluster in two distinctive intervals between c. 8.1–7.4 Ma (Late Miocene, 100–103 mbsf) and c. 4.1–2.6 Ma (Middle to Late Pliocene, 49–62 mbsf; Fig. 2). Both intervals
Discussion
Superimposed on the general long-term trend from a highly diverse Middle Miocene palynomorph assemblage towards the impoverished assemblages of the Late Pliocene significant fluctuations in abundance and composition have been observed (Figs. 2 and 3). Some of these fluctuations are apparently linked to short-term Mi-events and other paleoenvironmental events as shown in Fig. 4. The palynological data are here complemented with alkenone SSTs and TOC data (Fig. 2) to characterize and discuss the
Conclusions
For the interval between 14.5 and 2.5 Ma, ODP Hole 907A has been palynologically investigated on ~ 100 kyr resolution in order to provide new constraints on the Neogene paleoenvironmental evolution of the Iceland Sea. In addition, an alkenone based SST record has been constructed to portray its long-term thermal development. In general, our findings are in good accordance with the few Neogene high northern latitude proxy records available highlighting the potential for palynomorph based
Acknowledgments
This research uses samples and data provided by the Ocean Drilling Program (ODP), and Walter Hale and Alex Wülbers are thanked for technical support while sampling at the IODP Core Repository Bremen. J.M. and M.S. acknowledge support by the German Research Foundation (DFG MA 3913/2). We are very grateful to the Editor-in-Chief and two anonymous reviewers for constructive comments. Data supplement is available online at http://doi.pangaea.de/10.1594/PANGAEA.807163.
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2018, Global and Planetary ChangeCitation Excerpt :However, it should be noted that the full SST variability at Site 982 is likely not recorded due to the low temporal resolution (see also Lawrence et al., 2009). At ODP Site 907 in the Iceland Sea, the gradual disappearance of dinocyst species between 4.50 and 4.30 Ma likely reflects decreasing water temperatures and salinity due to the establishment of a proto-EGC (Schreck et al., 2013). The increased export of cool Arctic waters into the Nordic Seas via a modern-like EGC has been linked to the prolonged establishment of northward water flow through the Bering Strait, possibly as a result of the shoaling of the CAS (De Schepper et al., 2015; Schreck et al., 2013; Verhoeven et al., 2011).