Scientific themes

1. Climate and sea level dynamics of MIS 5, including onsets and terminations

Last 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 objectives

The following are theme-specific scientific objectives for Expedition 347:

  • To increase our understanding of the climatic system and the sea level dynamics of the Last Interglacial, including the climatic oscillations at the transition between MIS 6 and MIS 5e and in the initial, climatically oscillating part of the last glacial (MIS 5d–5a).

  • To contribute to an increased understanding of links between oceanic and terrestrial climate systems, including circulation patterns, during the highly variable MIS 5.

  • To analyze environmental conditions during the warmest interval of MIS 5e to elucidate possible future scenarios during warmer climate and higher sea level stands.

  • To improve our understanding of the response of the anthropogenically untouched Eemian (MIS 5e) Baltic Sea ecosystem to different environmental forcing factors to enhance the possibility of distinguishing anthropogenic factors from natural driving mechanisms behind presently threatened semienclosed basin environments and ecosystems.

2. The complexities of the last glacial, MIS 4–MIS 2

Since 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 objectives

We 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:

  • To what degree did the SIS respond, in time and space, to North Atlantic climate forcing during the last glacial, and to what extent did its dynamics have an impact on the North Atlantic climate system?

  • What were the feedbacks between the water body of the BSB, the SIS, and North Atlantic circulation?

  • To what degree are the glacier oscillations of the SIS margin synchronous on either side of the main ice divide, centered along the Scandinavian mountain chain, and can the advances into the southern BSB be recognized as large-scale surges? Can the substantial ice advances into the southernmost BSB be verified, perhaps triggered by rapid North Atlantic warming, paving the way for Heinrich events, or were large parts of the BSB ice free (Fig. F3) during most of MIS 3?

  • More specifically, how well is the highly oscillating climate pattern of MIS 3 recorded at the northeast margin of the North Atlantic as long continuous buried sediment sequences in the BSB? How did this huge drainage area react to these large-scale circulation changes? The southern BSB may hold unique archives of this time period; many large rivers drain into the basin and make the archives relevant on a semicontinental basis.

  • We need better constrained modeling of the SIS with its global and regional isostatic and eustatic impacts (Lambeck et al., 1998a, 1998b). This modeling requires better knowledge of the glacial chronology and dynamics, as well as of the BSB threshold history; long archives of this development are believed to exist at some of the proposed drill sites.

3. Deglacial and Holocene (MIS 2–MIS 1) climate forcing

The 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

  • What are the details of climate evolution in northwestern Europe during the deglacial phase and the Holocene (MIS 2–MIS 1), and what are the forcing mechanisms as deduced from a north–south transect of Baltic subbasins with ultra-high resolution?

  • What are the main mechanisms behind hypoxia driving processes in the Baltic, and to what degree have past and recent human activities played a role?

  • How has Baltic in- and outflow varied over time, and how is this related to changes in large-scale atmospheric circulation and sea levels (threshold depths)?

  • More specifically: is there a solar forcing signal in the melting record of a shrinking ice sheet (glacial varves at proposed Site BSB-9) or in the precipitation related fluvial system (postglacial varves at proposed Sites BSB-10 and BSB-11)? Can we reconstruct river discharge (and thereby also precipitation) with annual resolution, several millennia back in time? How did the general precipitation pattern, which is linked to the dominating AO/NAO system over the northern circum-Atlantic and circum-Arctic region, change over the Holocene (proposed Sites BSB-10 and BSB-11)? Are there periodicities in these changes, or are they linked to the large-scale insolation trend?

4. Deep biosphere responses to glacial–interglacial cycles in Baltic Sea Basin sediments

The 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 questions

Scientific 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
  • How has the alternation between (1) limnic, brackish, and marine conditions, (2) oxic and suboxic/anoxic conditions, (3) low and temperate temperature, or (4) low and high organic sedimentation controlled the prokaryotic communities and the biogeochemical processes in the seabed?

  • Are microorganisms that presently live in the deep sediments remnants of these limnic and marine populations, or are they selected by the modern sedimentary environment?

  • Do chemical and genetic fossils (i.e., biomarkers and DNA) of the original prokaryotic organisms persist today, and are they useful as paleoceanographic indicators?

  • Which biogeochemical processes predominate today in the glacial and interglacial deposits, what are their rates, and which microorganisms are carrying them out?

  • How does the phylogenetic diversity of the deep biosphere in this intracontinental sea differ from that of deep open-ocean communities?

Specific Baltic Sea objectives
  • To understand how the environmental and depositional history of the Baltic Sea system through transitions between the Saalian, Eemian, Weichselian, and Holocene (MIS 6–MIS 1) have affected the phylogenetic diversity of the microbial communities.

  • To analyze microbiological and biogeochemical responses to major shifts: (1) between limnic, brackish, and marine phases and (2) between high and low deposition of terrestrial versus marine organic and clastic material.

  • To understand how the postglacial diffusive penetration of conservative seawater ions may alter the chemical composition and microbial physiology in the subseafloor biosphere.