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doi:10.2204/iodp.sp.347.2012 Scientific themes1. Climate and sea level dynamics of MIS 5, including onsets and terminationsLast Interglacial (Eemian, Mikulino, MIS 5e; ~130–115 ka) deposits have been described from a number of marine and terrestrial sites, but are only partly documented in the North Greenland Ice Core Project (NGRIP) ice core (Andersen et al., 2004). A land-sea correlation of European pollen zones and marine isotope stages was presented for the first time by Sánshez-Goñi et al. (1999), who demonstrated a delay between the beginning of MIS 5e and that of the European terrestrial Eemian (discussed by Kukla et al., 2002). High-resolution Eemian marine shelf records (here correlated with MIS 5e) from northern Europe are, however, very scarce and usually contain only fragmented paleoenvironmental information. The same is valid for the early Weichselian stadials and interstadials (MIS 5d to 5a), which were, however, fully recovered in the NorthGRIP ice core. Data from marine sediments in the Nordic Seas show three substantial sea-surface temperature fluctuations during MIS 5e (Fronval and Jansen, 1996). The study implies that the Last Interglacial at high northern latitudes was characterized by rapid changes in Polar Front movement, ocean circulation, and oceanic heat fluxes. This may have resulted in noticeable temperature changes in neighboring land areas, which is different from Holocene climate development, where only minor fluctuations occurred within a general cooling trend. From presently onshore Eemian (MIS 5e) records we know that the BSB Eemian began with a lacustrine phase covering ~300 y before marine conditions were established (Kristensen and Knudsen, 2006), and higher mean sea-surface and seafloor temperatures (~6°C) and salinities (~15‰) than today characterized the first ~4 k.y. of the Eemian Baltic Sea (Kristensen and Knudsen, 2006; Funder et al., 2002). During the first 2–2.5 k.y. a pathway existed between the Baltic Basin and the Barents Sea through Karelia, but to what degree it was of importance for general ocean circulation and the climate of northern Europe is debatable (Funder et al., 2002). This circulation pattern and these high salinities may have created strong salinity stratification and the development of a permanent halocline resulting in hypoxic bottom conditions during large part of the Eemian—conditions akin to the development of the Baltic Sea during the last 8000 y and today’s situation. Also, the difference between the warm and well-ventilated southwestern Eemian BSB and the cold, stagnant conditions of its easternmost parts implies that the ocean–continental climate gradient from the west to the east in northern Europe was steeper than during the Holocene (Funder et al., 2002). After ~6 k.y. into the interglacial, the Eemian Baltic Sea was characterized by a falling sea level and decreased salinity as observed in the diatom and foraminifer records (Eiríksson et al., 2006; Kristensen and Knudsen, 2006), but its further development during the subsequent MIS 5 stadials and interstadials is largely unknown. Theme-specific scientific objectivesThe following are theme-specific scientific objectives for Expedition 347:
2. The complexities of the last glacial, MIS 4–MIS 2Since the confirmation of the high climate variability during the last glacial in the MIS 4–MIS 2 interval from the Greenland Ice Core Project (GRIP) and Greenland Ice Sheet Project 2 (GISP2) Greenland ice core records (Johnsen et al., 1992; Grootes et al., 1993), paleoclimatologists have been presented with several corresponding records, both from the marine and terrestrial North Atlantic margins (Rasmussen et al., 1997; Dickson et al., 2008; Grimm et al., 2006; Wohlfarth et al., 2008). The huge ice sheets in Eurasia—of which the SIS was the largest—and North America played major roles for this high degree of variability. The impact of sea ice and icebergs (e.g., Dokken et al., 2003), as well as glacial advances and retreats, upon the North Atlantic marine system by their interaction with the MOC were most likely key players in the variable climate pattern of the last glacial. The direct effects of this variability were best registered in proximal areas to the ice sheets. It is therefore essential to gather modern and detailed stratigraphic information from the “sediment trap areas” of these two main glaciated regions, of which the Baltic Basin is the main one for the SIS, to decipher, date, and analyze the recurrent stages of ice-covered and ice-free conditions. The Baltic glacial history is only fragmentarily known, but we know that a first Baltic glacial event occurred during MIS 4 as recorded in sediments from northwest Finland at ~64°N (Salonen et al., 2007), whereas the first Baltic ice lobe advance into Denmark is dated to ~55–50 ka (Houmark-Nielsen, 2007). It is likely that freshwater lakes covered the deeper subbasins of the central and southern BSB until at least 60 ka, when sea level was >50 m lower than today (Lambeck and Chappell, 2001; Siddall et al., 2003). The Hanö Bay, Bornholm, and Landsort Deep Basins must have been infilled with sediments over several tens of millennia (40–60 ka?), through the first half of the last glacial. The BSB then experienced a more dynamic and variable glacial-interstadial development during the remaining parts of the glacial. There are several indications that the southern Baltic may contain rich and more or less complete stratigraphies of MIS 3. From detailed correlations and dating of the southwestern Baltic glacial stratigraphies, Houmark-Nielsen and Kjær (2003) and Houmark-Nielsen (2007, 2008) conclude that the southwest Baltic may have experienced two major ice advances during MIS 3, at ~50 and 30 ka. The latter advance is being vividly discussed (Wohlfarth, 2009), as well as the general asynchroneity of MIS 3 ice advances at the western margin of the SIS (Mangerud, 2004) compared to ice margins in the south (Houmark-Nielsen et al., 2005). This partly enigmatic period between ~50 and 25 ka with its partly incompatible records is followed by complex glaciation in the southern BSB (Houmark-Nielsen and Kjær, 2003) leading up to the Last Glacial Maximum (LGM). Furthermore, previous offshore studies in the southern Baltic have documented the presence of marine-brackish sediments, dated to MIS 3 or older, that were overridden by a glacier (Klingberg, 1998) at Kriegers Flak (Fig. F2), and two varved clay sequences—the upper one dates from the last deglaciation—separated by an organic-rich layer dated to >35 ka 14C before present (BP) (bulk date) in Hanö Bay (Björck et al., 1990). Also, and maybe even more significant, an ongoing study of the shallow Kriegers Flak area shows a surprisingly complex stratigraphy (Fig. F2) with a variety of lithologic units, gravel-sand-silt, clays of glaciolacustrine and brackish origin, interstadial lacustrine gyttja (lake mud), and peat with ages of 39 and 41 ka BP, sandwiched between several glacial diamicts (Anjar et al., 2010). A complicating and key factor for the BSB history is the geographic location and altitude (in relation to sea level) of the critical threshold, or “gateway,” between the open ocean and the BSB, which determines if and how much marine water can enter the BSB. Today the two main thresholds are the Öresund Strait (7 m below present sea level) and the Store Belt (~20 m below present sea level). However, the bedrock threshold of the BSB is situated 60 meters below sea level (mbsl), with the buried Alnarp-Esrum bedrock valley (SWECO VIAK, 2004) running through southwest Skåne in Sweden and northernmost Själland in Denmark, 120 km long and 6 km wide (Fig. F2). From deep coring in the 1970s of this main aquifer, fluvial and lacustrine sediment units were more or less reliably 14C dated (ages summarized by Anjar et al., in press), implying that the valley was sediment filled during the latter part of MIS 3. The valley thus served as the outlet river route for the complete BSB until it was filled with sediment. Clearly, the age of this last infilling is crucial for the younger sedimentation history of the basin, including relationships between sea level and the BSB during MIS 3. The 128 cores from drilling undertaken during planning of the Kriegers Flak windmill park, of which Figure F2 shows 9, indicate that complex yet incomplete stratigraphies occur in this shallow part of the BSB. These incomplete stratigraphies imply that the nearby deep basins, Bornholm Basin and Hanö Bay, hold more complete stratigraphies; the old organic mud between two units of undisturbed varved clay in Hanö Bay shows that this basin contains long records, which is also supported by seismic data. These subbasins are controlled by bedrock topography and should potentially have better preserved pre-LGM sediment records compared to other deep subbasins carved out by the last deglaciation ice streams, where older sediments have been stripped off and piled up in arch-shaped end moraines along the Baltic coastlands. Theme-specific scientific questions and objectivesWe hypothesize that deeper bedrock-controlled basins in the southern BSB were left more or less untouched by the erosive powers of surging glaciers, supported by the Kriegers Flak and Hanö Bay records. By combining shallow offshore drilling and the land-based stratigraphy around the southern Baltic with more complete sedimentary archives in the two deep basins of the southern BSB, we will be able to address crucial scientific issues, including the following:
3. Deglacial and Holocene (MIS 2–MIS 1) climate forcingThe deglaciation of the southern BSB between 22 and 16 ka was complex, with major deglacial phases interrupted by some intriguing still-stands and even re-advances (Houmark-Nielsen and Kjær, 2003), possibly as surges. More or less complete varved sedimentary records of this dynamic period may be found in the basins of Hanö Bay and Bornholm Basin. Low-lying areas were dammed up in front of the retreating ice front comprising the Baltic Ice Lake (BIL), and huge amounts of freshwater were released into the North Atlantic during this dynamic phase of the deglaciation between ~16 and 11.7 ka (Gyldenholm et al., 1993; Björck, 1995; Majoran and Nordberg, 1997; Jiang et al., 1998). During the final drainage of the BIL at ~11.7 ka almost 8000 km3 of freshwater was released rapidly into the North Atlantic (Jakobsson et al., 2007), but the effects on North Atlantic thermohaline circulation may only have been minor (Andrén et al., 2002). However, it has also been suggested that it may have triggered the Preboreal Oscillation (Björck et al., 1996), as well as ice advances in northern Norway (Hald and Hagen, 1998). The oceanographic effects of the drainages should be registered in varved sediments at the Anholt Loch site. Earlier studies have suggested that the Younger Dryas cold period was caused by freshwater runoff from the Laurentide (American) Ice Sheet (Marshall and Clark, 1999), but the influence of the Baltic region has not been studied in detail. At ~10 ka the complete BSB was deglaciated, and during the entire course of deglaciation, varved glacial clay was deposited in front of the retreating ice sheet. These deposits have been used to date the ice recession and postglacial events; varves are still being deposited in the Ångermanälven River estuary, providing a link to present time (Cato, 1985). Although we know that this long series of varved sediments should contain at least 10,000 y of historical deposition records in the estuary, it has not been possible to explore the potential of this record. We know that correlation exists between maximum daily discharge and mean varve thickness during the last 2000 y in the Ångermanälven River (Sander et al., 2002) (Fig. F4). The region’s geographic location makes it possible to use these unique laminated sediment archives to reconstruct atmospheric (e.g., shifts in NAO and AO variability) and oceanic (e.g., effects of changes in the strength of MOC) circulation patterns during the Holocene. These patterns might also reflect changes in Equator-to-pole teleconnections (e.g., in El Niño Southern Oscillation [ENSO]–related variability). In addition, the estuarine site reflects the precipitation records over a wide area and helps to integrate the many lake record data available. Precipitation and evaporation are critical parameters for understanding the ecosystem functioning of the Baltic Sea, affecting salinity through dilution, vertical mixing, and river inflow. These parameters are closely related to changes in atmospheric circulation, which modify also the inflow of saline waters through the thresholds. Salinity determines stratification (together with temperature) and thus controls the extent of hypoxia and the health of the ecosystem in this enclosed sea. The south–north transect of Baltic Sea sites will allow detailed reconstructions of Holocene hydrologic and climatic changes influencing the whole BSB. The sensitivity of the BSB system with respect to changes in sea level/salinity, productivity, hydrology/river discharge, and atmospheric circulation is a well-known fact (Zillén et al., 2008). With the planned IODP sites this sensitivity could be explored in much more detail regarding underlying mechanisms. Laminated sediments are used as a proxy for hypoxic bottom conditions and are formed during periods with strong permanent salinity stratification, probably as a result of increased evaporation/decreased precipitation and possibly also by enhanced primary productivity. Laminated sediments in the deep basins of the Baltic Sea (e.g., Andrén et al., 2000a; Zillén et al. 2008) indicate that the open Baltic Sea ecosystem has passed certain thresholds and experienced several regime shifts during its environmental history. These high-resolution sediment records provide an excellent archive to study the long-term patterns of, for example, atmospheric circulation (such as the variability of the NAO and its trends and strength) during the Holocene. Theme-specific scientific questions
4. Deep biosphere responses to glacial–interglacial cycles in Baltic Sea Basin sedimentsThe discovery of microorganisms in deep subsurface sedimentary deposits, and even in basement rock, has profoundly changed our perspective on the limits of living organisms on our planet (Parkes et al., 1994, 2005; D’Hondt et al., 2004; Jørgensen et al., 2006). The current database of prokaryotic cells in deep sediment cores indicates that the marine deep biosphere may comprise 10% of all living biomass and more than half of all microorganisms on Earth (Whitman et al., 1998). The population densities, 104–107 cells per cubic centimeter to >1500 m sediment depth (Roussel et al., 2008), are greater than those found in ocean water. Understanding the minimum energy requirements for growth and survival may offer a means of interpreting the distribution, composition, and activity of deeply buried communities (cf. Schink, 1997; Hoehler, 2004; Jackson and McInerney, 2002; Jørgensen and D’Hondt, 2006). With increasing depth and age of marine sediments, microbial cells become increasingly energy limited (D’Hondt et al., 2002, 2004; Schippers et al., 2005; Biddle et al., 2006). How is it possible to maintain complex functions in prokaryotic cells at an energy flux that barely allows cell growth over many years? Are the deeply buried communities relicts of a time when the sediment was originally deposited (Inagaki et al., 2005)? If this is the case, do they then reflect past oceanographic conditions and whether the sediment was deposited under marine or limnic conditions, high or low burial rates of organic material, or high or low concentrations of oxygen? Theme-specific scientific questionsScientific drilling in the Baltic Sea provides unique possibilities to study several basic questions concerning the deep biosphere and the Baltic Sea. General deep biosphere questions
Specific Baltic Sea objectives
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