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doi:10.2204/iodp.pr.347.2014 Scientific objectivesThe scientific objectives of Expedition 347, Baltic Sea Paleoenvironment, can be summarized with four overarching themes. Theme 1: climate and sea level dynamics of MIS 5Deposits from the Last Interglacial (Eemian, MIS 5e; ~130–115 k.y. BP) have been described from a number of marine and terrestrial sites, but the interglacial is only partly documented in the NGRIP ice core (North Greenland Ice Core Project Members, 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 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–MIS 5a), which were, however, captured 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 appreciable temperature changes in neighboring land areas, which differs from Holocene climate development, with only minor fluctuations on a general cooling trend. From 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). Higher mean sea-surface and sea bottom 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 the degree to which this was important for the general ocean circulation and the climate of northern Europe is debatable (Funder et al., 2002). The circulation pattern and high salinities may have created strong salinity stratification and the development of a permanent halocline, resulting in hypoxic bottom conditions during a large part of the Eemian. This hypoxia would simulate conditions similar to those developed in the Baltic Sea during the last 8000 y. 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 west to east in northern Europe was steeper than during the Holocene (Funder et al., 2002). After ~6 k.y. of the interglacial, the Eemian Baltic Sea was characterized by a falling sea level and decreased salinity, as observed in 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 objectivesScientific objectives specific to this theme are as follows:
Theme 2: complexities of the last glacial, MIS 4–MIS 2Since the confirmation of high climate variability during the last glacial in the MIS 4–MIS 2 interval from the GRIP and Greenland Ice Sheet Project 2 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 in this high degree of climatic 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 because of their interaction with MOC suggests they were key influences on the variable climate pattern of the last glacial. The direct effects of this variability were best registered in areas proximal 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. Only fragments of the Baltic glacial history are known. However, we do know that a Baltic glacial event occurred during MIS 4 as recorded in sediments from northwestern Finland at ~64°N (Salonen et al., 2007), whereas the first Baltic ice lobe advance into Denmark is dated to ~55–50 k.y. BP (MIS 3) (Houmark-Nielsen, 2007). It is likely that freshwater lakes covered the deeper subbasins of the central and southern BSB until at least 60 k.y. BP when the sea level was >50 m lower than today (Lambeck and Chappell, 2001; Siddall et al., 2003). The Hanö Bay, the Bornholm Basin, and the Landsort Deep acted as depocenters for several tens of millennia (~40–60 ka) through the first half of the last glacial. The BSB also experienced a dynamic and variable glacial-interstadial development during the remaining parts of the glacial. We have several indications that the southern Baltic may contain relatively complete stratigraphies of MIS 3. From detailed correlations and dating of southwestern Baltic glacial stratigraphies (Houmark-Nielsen and Kjær, 2003; Houmark-Nielsen, 2007, 2008), it is concluded that the southwestern Baltic experienced two major ice advances during MIS 3, at ~50 and 30 k.y. BP. However, the latter is highly debated (Kjellström et al., 2010), as well as the general asynchroneity of MIS 3 ice advances at the western margin of the SIS (Mangerud, 2004) compared to the ice margins in the south (Houmark-Nielsen et al., 2005). This enigmatic period between ~50 and 25 k.y. BP, with its partly incompatible records, is followed by a complex glaciation in the southern BSB (Houmark-Nielsen and Kjær, 2003) leading up to the 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. F1) and two varved clay sequences—the upper one dating from the last deglaciation—separated by an organic-rich layer dated to >35 k.y. 14C 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 with a variety of lithologic units, including gravel-sand-silt, clays of glaciolacustrine and brackish origin, interstadial lacustrine gyttja (lake mud), and peat with ages of 39 and 41 k.y. BP, sandwiched between several glacial diamicts (Anjar et al., 2010). A complicating and key factor for 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 whether and how much oceanic water can enter the BSB. Today the two main thresholds are the Öresund Strait (–7 m) and the Store Belt (approximately –20 m). However, the bedrock threshold of the BSB is today situated 60 m below sea level and is located in the buried Alnarp-Esrum bedrock valley, which runs through southwestern Skåne in Sweden and northernmost Själland in Denmark, 120 km long and 6 km wide (Fig. F1). From deep cores taken in the 1970s from this main aquifer, fluvial and lacustrine sediment units were 14C dated. The ages summarized by Anjar et al. (2010) imply that the valley was sediment filled during the later part of MIS 3. The valley thus served as an outlet river for the complete BSB until it was filled up by sediments. Clearly, the age of this last infilling is crucial for the younger sedimentary history of the basin, including the relationship between sea level and the BSB during MIS 3. Analysis of 128 cores acquired for planning of the Kriegers Flak windmill park indicates that complex stratigraphies occur in this shallow part of the BSB, but the records remain incomplete (Anjar et al., 2010). These results imply that the nearby deep basins of Bornholm Basin and Hanö Bay may hold more complete stratigraphies. The >35 k.y. 14C BP old organic mud between two units of undisturbed varved clay in Hanö Bay shows that this basin contains long records of sedimentation history, which is also supported by seismic data. These subbasins are controlled by bedrock topography and may potentially contain better preserved pre-LGM sediment records in comparison with other deep subbasins carved out by the youngest glacial ice streams. In these cases, older sediments have been stripped off and redeposited as arcuate end moraines along the Baltic coasts. Theme-specific scientific objectivesWe hypothesize that deeper bedrock-controlled basins in the southern BSB were left largely untouched by the erosive powers of surging glaciers, a hypothesis supported by the Kriegers Flak and Hanö Bay records. By combining shallow offshore drilling and 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 the following crucial scientific objectives:
Theme 3: deglacial and Holocene (MIS 2–MIS 1) climate forcingDeglaciation of the southern BSB between 22 and 16 k.y. BP was complex, with major deglacial phases interrupted by some intriguing stillstands and re-advances (Houmark-Nielsen and Kjær, 2003), possibly as surges. Near-complete varved sedimentary records of this dynamic period may be found in the basin of Hanö Bay and Bornholm Basin. Low-lying areas were dammed in front of the retreating ice margin, forming the BIL, and huge amounts of freshwater were released into the North Atlantic during this dynamic phase of the deglaciation between ~16 and 11.7 k.y. BP (Gyldenholm et al., 1993; Björck, 1995; Majoran and Nordberg, 1997; Jiang et al., 1998). During the final drainage of the BIL at ~11.7 k.y. BP, almost 8000 km3 of freshwater was released rapidly into the North Atlantic (Jakobsson et al., 2007). However, the effects on North Atlantic thermohaline circulation may only have been minor (Andrén et al., 2002). It has also been suggested that this freshwater release may have triggered the Preboreal oscillation (Björck et al., 1996), as well as ice advances in northern Norway (Hald and Hagen, 1998). 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. The oceanographic effects of the drainage should be detectable in varved sediments at the Anholt Loch site. At ~10 k.y. BP, the complete BSB was deglaciated. During the course of deglaciation, varved glacial clay was deposited in front of the retreating ice sheet. This record has 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 varve series should contain at least 10,000 varve years 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 years in the Ångermanälven River (Sander et al., 2002). Because of the region’s geographic location, it is 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) related variability. In addition, the site reflects the precipitation records over a wide area and helps to integrate the abundant lake record data available. Laminated sediments are used as a proxy for hypoxic bottom water 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., 2000b; Zillén et al., 2008) indicate that the open Baltic Sea ecosystem has experienced several regime shifts during its environmental history. These high-resolution sediment records provide an excellent archive to study long-term patterns such as atmospheric circulation (similar to the variability of the NAO, its trend and strength) during the Holocene. Precipitation and evaporation are critical parameters for understanding the function of the ecosystems of the Baltic Sea, as they affect salinity through dilution, vertical mixing, and river inflow. These parameters are closely related to changes in atmospheric circulation, which modify the inflow of saline waters. Salinity (together with temperature) determines stratification and thus controls the extent of hypoxia and the “health” of the ecosystem in this enclosed sea. The sensitivity of the BSB system to changes in sea level/salinity, productivity, hydrology/river discharge, and atmospheric circulation is a well-known fact (Zillén et al., 2008). The south–north transect of Baltic Sea sites proposed in Expedition 347 will allow detailed reconstruction of Holocene hydrologic and climatic changes influencing the whole BSB, as well as a detailed exploration of the underlying mechanisms of change. Theme-specific scientific objectivesScientific objectives specific to this theme are as follows:
Theme 4: The deep biosphere 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). Although concealed in the subsurface, the deep biosphere is an important global component in the long-term biological cycling of carbon and nutrients and for the chemistry of the oceans and atmosphere. The current database on prokaryotic cells in deep sediment cores indicates that the marine deep biosphere may comprise half of all microorganisms in the ocean (Kallmeyer et al., 2012). The population densities in the oceanic seabed are 104–107 cells/cm3 to >1500 m sediment depth (Roussel et al., 2008), densities that 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 these deeply buried communities (cf. Hoehler, 2004; Jackson and McInerney, 2002; Hoehler and Jørgensen, 2013). 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). However, they maintain complex functions at an energy flux that barely allows cell growth over many years. It has been proposed that these deeply buried communities are relicts of a time when the sediment was originally deposited (Inagaki et al., 2005). If this is the case, then they may still reflect the past oceanographic conditions, in particular 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 objectivesScientific objectives specific to this theme are as follows:
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