The Baltic Sea basin (BSB) is one of the world's largest intra-continental basins, occupying 373 000 km2 and with a drainage area 4 times this size (Fig. 1). Its mean depth is ∼ 54 m, although a few relatively deep basins exist (e.g., the eastern Gotland Basin, 248 m, and the Landsort Deep, 459 m). The BSB has served as depositional sink throughout at least the last during the last glacial cycle and its sediments comprise a unique high-resolution archive of the paleoenvironmental history of the large drainage area, the basin itself, and the neighboring gateways to the North Sea. The location of the BSB in the heartland of the recurring Scandinavian ice sheet (SIS) has resulted in a complex developmental history, characteristic of many glaciated regions of the Northern Hemisphere: repeated glaciations of different magnitudes, sensitive responses to sea-level and gateway threshold changes, large shifts in sedimentation patterns, and high sedimentation rates.

The geographical position of the BSB makes it a unique link between the Eurasian and the northwest European terrestrial records and as such also serves as a link to the North Atlantic marine records and the Greenland ice cores.

The high sedimentation rates (100–500 cm/1000 years) of the BSB provide an excellent opportunity to reconstruct in some parts climate variability of global importance controlled by, for example, changes in meridional overturning circulation (MOC), the North Atlantic Oscillation (NAO), and the Arctic oscillation (AO). These paleoenvironmental reconstructions offers a unique resolution from a marine-brackish setting as some of the sediments can even be resolved on inter-annual timescales. This makes the BSB highly suitable for sediment coring from the last glacial cycle and a unique location to achieve scientific objectives of high-resolution paleoceanography and paleoclimate studies, as comparable sequences cannot be retrieved anywhere in the surrounding onshore regions.

The BSB has undergone many glaciations during the Quaternary. During the last interglacial (the Eemian, marine isotope stage (MIS) 5e), the BSB was a larger and more saline sea than the present Baltic Sea (Funder et al., 2002). 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 northwest Finland at ∼ 64∘ N (Salonen et al., 2007). The first Baltic ice lobe that advanced into Denmark is dated to ∼ 55–50 ky BP (thousand years before present; Houmark-Nielsen, 2007). From detailed correlations and dating of southwestern Baltic glacial stratigraphies (Houmark-Nielsen and Kjær, 2003), it is proposed that the southwestern Baltic experienced two major ice advances during MIS 3, at ∼ 50 and 30 ky BP. However, the latter is highly debated (e.g., Kjellström et al., 2010). This enigmatic period between ∼ 50 and 25 ky BP, with its partly incompatible records, was followed by a complex glaciation in the southern BSB (Houmark-Nielsen and Kjær, 2003) leading up to the Last Glacial Maximum.

Deglaciation of the southern BSB between 22 and 16 ky BP was complex, with major deglacial phases interrupted by some intriguing SIS still stands and re-advances (Houmark-Nielsen and Kjær, 2003; Larsen et al., 2009), possibly as surges.

Earlier studies have suggested that the Younger Dryas cold event (12.9 to 11.6 ky BP) was caused by freshwater runoff from the Laurentide (American) ice sheet slowing the Atlantic MOC (Marshall and Clark, 1999). However, state-of-the-art climate models require a continuous freshwater forcing to produce millennia-long cold events (Liu et al., 2009) and there are indications of substantial freshwater forcing from the BSB predating the final drainage from the Baltic Ice Lake (e.g., Björck, 1995; Bennicke and Jensen, 2013).

During the final drainage of the Baltic Ice Lake at ∼ 11.7 ky BP (Fig. 2a), almost 8000 km3 of freshwater was released rapidly into the North Atlantic (Jakobsson et al., 2007).

The next Baltic Sea stage, the Yoldia Sea, coincided with the onset of the Holocene epoch (Walker et al., 2009) and the associated rapid warming. In fact, the thicknesses of glacial varves in the northwestern Baltic proper and δ18O values in ice cores from the Greenland Ice Core Project (GRIP) display a noticeably similar pattern over a 150-year-long transition period from Younger Dryas to pre-Boreal (Andrén et al., 1999, 2002). These records show a distinct increase in sedimentation rate as the ice sheet began to melt and rapidly retreat. The following few hundred years were characterized by rapid deglaciation of the SIS. Relative sea-level lowering of the Yoldia Sea played an important role and were the result of a combination of rapid regression in the recently deglaciated regions and normal regression rates in southern Sweden (1.5–2 m/100 years).

As the outlets to the west became shallower, subsequent damming forced the water level inside the Baltic Basin to rise and the next stage began, the Ancylus Lake. The sediments of this large freshwater lake generally contain little organic material, which may be explained as a result of the large meltwater inflow to the Baltic from the final deglaciation of the SIS and also a result of erosion of the young soils from the recently deglaciated drainage area. This created a lake environment with low nutrient input and hence low productivity. The freshwater environment did not enable the formation of a halocline but led to a well-mixed, oxygenated waterbody. The relatively common sulfide-banded sediments of this stage can probably be explained by later H2S diffusion from overlying, organic-rich Holocene sediments (Sohlenius et al., 2001).

The global melting of the large ice sheets over a couple of millennia caused a 30 m rise in absolute sea level (Lambeck and Chappell, 2001). A consequence of this was the flooding of the Öresund Strait, believed to be the main gateway for the onset of the marine Littorina Sea stage. The outlets/inlets through Öresund and Great Belt widened and became deeper, resulting in greater water flow and gradually increasing salinity.

The onset of the Littorina Sea stage can often be recognized as a marked lithologic change in Baltic Sea sediment cores. The onset is represented by a distinct increase in organic content and an increasing abundance of brackish-marine diatoms (e.g., Sohlenius et al., 2001).

Periods of deep-water hypoxia in the open Baltic Basin are evident in the sediment record as extended sequences of laminated sediment. The Littorina Sea (Fig. 2b) experienced a sustained period of elevated salinity and resulting hypoxia between 8 and 4 ky BP (Zillén et al., 2008). After ∼ 4 ky BP, the salinity decreased gradually and oxygen concentrations increased in the bottom waters.

The human population growth and large-scale changes in land use that occurred in the Baltic Sea watershed during the Medieval period between AD 750 and 1300 has been suggested as a triggering mechanism behind the expansion of hypoxia that again occurred at this time (Zillén et al., 2008). Alternative hypotheses suggest that the development of hypoxia in the open Baltic Sea over the past 1000 years has mainly been driven by the climate system (e.g., Kabel et al., 2012).

The objectives of expedition 347 are categorized below.

Geology and climate development objectives include the following:

to increase our understanding of the climate 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);

Microbiology and geochemistry objectives include the following:

to understand how microbial cell numbers are controlled in relation to depth, age, lithology, and other environmental parameters in relatively young and organic-rich marine sediments;

The drilling platform chosen for expedition 347 was the Greatship Manisha, an IMO (International Maritime Organization) Class II dynamically positioned vessel with geotechnical coring capability (Fig. 3a). The mobilization took place in Falmouth prior to transit to Kiel, Germany, where the ESO (ECORD (European Consortium for Ocean Research Drilling) Science Operator) expedition project managers and offshore science party participants boarded the vessel following a pre-cruise scientific meeting in Copenhagen. Operations were conducted between 12 September and 1 November, with demobilization of the vessel occurring back in Falmouth on 5 and 6 November. In total, 37.1 days of expedition 347 were spent operational on station, 8.5 days in transit between sites, 1.3 days in port, 1.3 days on standby at station because of weather, and 1.8 days on equipment-related downtime.

The vessel was equipped with a large moon pool and Geoquip GMTR 120 heave-compensated derrick, with a 120 metric ton capacity top drive. Pipe handling was carried out using a proprietary semiautomated handling system utilizing a pipe handling crane with grab, a remotely operated iron roughneck, and a proprietary catwalk system. A 4 m stroke passive heave compensation (semiactive under development) was achieved using nitrogen gas as a compensation buffer with Olmsted valve slingshot protection. The rig was used in association with a 12 metric ton seabed template, fitted with clamps and seabed transponder, to provide the reaction force for in-hole tools. Wireline operation of the core barrel was conducted through the top drive.

Five methods of wireline coring were employed in addition to open-hole drilling using a non-coring assembly. The primary coring tool was a piston corer system (PCS), operated by advancing the core barrel into the formation through hydraulic pushing. However, where formation lithologies were either unconsolidated, noncohesive, friable or very hard, an extended coring system (ECS), non-rotating core barrel system (NRCB), push coring assembly (PCA), or hammer sampler (HS) were employed. The HS was the most rudimentary system used during this expedition, consisting of a built-in hammer, which was raised and lowered onto an anvil over a few meters distance. This tool was used to obtain a spot sample when open-holing through coarser-grained deposits, or when conventional coring methods had been unable to acquire a sample. The non-coring assemble (NCA) used a Tricone Rock Roller drill bit to plug the hole in the main core bit to advance without recovery.

The core collected was 62 mm in diameter. The maximum core run length was 3.3 m. However, the length of a core run was chosen to maximize core recovery and quality while maintaining hole stability, even at the expense of overall penetration speed. When attempting to capture a lithologic interface, as defined from seismic profiles, the run lengths were often shortened by raising the corer above the bottom of the borehole by a known height prior to pressuring the drill string. In some instances, the hole was advanced by open holing – drilling ahead without recovering sediments. This was done in difficult lithologies that could not be recovered conventionally to enable recovery of other lithologies beneath these intervals or when recovery (composite or from an individual hole) of an interval had already reached > 90 %, and the scientific rationale was to try and get deeper within the time constraints. The advance varied from a small offset of 0.5 m to ensure maximum core overlap between holes in some locations to a more regular spacing of 3 m through the till lithologies to monitor when or whether the lithology was changing.

At the Ångermanälven River estuary site (M0061), there were initial restrictions on coring the upper 50 cm of sediment due to potential heavy metal contamination from industry. This was relaxed on assessment on-site. At Hanö Bay, Bornholm basin, and Anholt loch sites (M0064, M0065, and M0060, respectively), downpipe camera or ROV (remotely operated vehicle) surveys were conducted prior to commencing coring, due to the potential for dumped WWII munitions on the seabed. In addition, restrictions were placed on coring the upper 2m of sediment at Bornholm Basin (M0065) due to the risk of dumped chemical munitions.

Offshore, the cores from paleo-oceanographic designated holes were carefully curated by ESO staff before ephemeral physical (multi-sensor core logger, MSCL) and geochemical properties were measured and preliminary core catcher samples taken. Initial lithological and micropaleontological descriptions were conducted by visual inspection through the liner and by using core catcher materials. No further sampling, core splitting, or analysis work was undertaken offshore.

On microbiological designated holes, samples were taken on the drill floor from the cut ends of core sections to capture ephemeral properties, including methane gas and contamination tracer quantities. The cores were then curated and run through a fast-track MSCL prior to extensive sampling in the designated laboratory containers (Fig. 3b) for microbiological research and for interstitial water analyses. In total > 5800 samples were taken offshore.

All cores, core catchers, headspace gas samples, interstitial water splits, and digital data were transferred to the Integrated Ocean Drilling Program (IODP) core repository in Bremen, Germany (BCR), at the end of the offshore phase. Prior to the start of the Onshore Science Party (OSP), additional thermal conductivity measurements and natural gamma radiation measurements were conducted on whole core sections. The complete science party, as well as ESO and BCR personnel and student helpers, met at the BCR from 22 January to 21 February to split, analyze, and sample the cores according to standard IODP procedures, and for post-cruise scientific research.

In the following we present selected preliminary results from leg 347 on lithology, biostratigraphy, dating, geochemistry, and microbiology. By these chosen examples we briefly summarize how the expedition met some of the objectives outlined above.

The OSL ages from site M0059 indicate that we reached at least some 350 ky back in time. Without having all dates at hand, we assume that at site M0060 we recovered sediments dating back to the previous interglacial. This means that the lower part of the holes from these two sites may cover the end of MIS 6, MIS 5, and MIS 4, although with partly fragmented stratigraphies.

Sediments of MIS 3 age have been recovered and dated at site M0059 and site M0064 and are most probably also present in the sediments from site M0065 and perhaps also M0066. The recovered sediments, of pro-glacial sandur at site M0059 and organic-bearing clay at site M0064 and M0065, indicate an ice-free event in the middle of the Weichselian glacial with an age of 44±4 ky BP. Ongoing analyses of sediments from these sites may shed new light on the history of the BSB and the behavior of the SIS during the enigmatic MIS 3.

Varved glacial clays deposited from the retreating SIS have been recovered at all drilled sites, which makes environmental reconstructions possible with annual resolution. Especially notable are the 52 m long varved sequences from site M0063, possibly containing as much as 2000 varves and probably covering the entire Younger Dryas. This unique varved sequence will give us new insights into how the SIS responded to climate fluctuations, including the slow cooling at the beginning and the rapid warming at the end of the stadial. It will also give new information on the position of the ice margin and thereby improve the correlation between ice margin positions in eastern Sweden and southwestern Finland during the Younger Dryas.

It should be noted that glacial varves have traditionally been assigned to the latest deglaciation but the two units of varved glacial clay separated by an organic-bearing clay with an age of 45 700 ± 1500 cal yr BP indicates a different scenario at site M0064. It is obvious that only the upper varved sequence was deposited during the latest deglaciation whereas the lower sequence represents an earlier deglaciation.

Sediments of Holocene age have been recovered at most drilled sites but at sites M0059 and M0063 the thickness of these sediments is spectacular, ca. 52 m at site M0059 and ca. 45 m at site M0063. The extremely high sedimentation rates at these two sites presented the possibility to reconstruct the environmental response to climate warming leading up to the Holocene thermal maximum, occurring some 8000 to 5000 years ago, with a resolution never reached before in the Baltic Sea area. It will, for example, be possible to detect small variations in, e.g., salinity, nutrient status, temperature, ice cover, and periods of hypoxia/anoxia, possibly with an annual resolution.

Mission-specific platforms routinely work in shallow waters, and so protocols are already in place to maximize the quality and amount of core recovered when working in these environments, building on the knowledge gained from four previous expeditions. However, the scope of the offshore microbiological program was something not undertaken before on an MSP (mission-specific platforms). Thanks to the early and extensive input and support from the microbiological community, we were able to achieve a successful sampling campaign. However, a detailed post-cruise review has shown areas in which additional protocols need to be established in order to improve results in this discipline, e.g., the addition of the contamination tracer to the drilling fluid at higher concentrations.

During expedition 347, four sites were selected for detailed studies of geochemistry and microbiology. Nearly 6000 samples were taken offshore for interstitial water analyses and microbiological studies. The many chemical analyses done on board the Greatship Manisha and during the onshore phase have generated data of outstanding quality and detail. The results provide novel insights into the geochemical consequences of glacial to interglacial and fresh-water to marine transitions combined with shifts from low-organic late-glacial clay to high organic Holocene clay with extremely high sedimentation rates. These data will be supplemented by a large diversity of inorganic, organic, and isotope geochemical analyses in laboratories around the world.

Detailed contamination tracer measurements were made during the expedition with improved methods relative to previous IODP standards. Off-loading of microbiology samples shortly after drilling enabled cultivation experiments and studies of microbial activity to be done in land-based laboratories and thereby overcome the limited laboratory facilities on the drillship. A comparison of microbial cell counting techniques showed that flow cytometry can provide data of equally good quality as microscopic counts and thereby may enhance the capacity for cell quantification on future drilling expeditions. A new CAS technology enables the freezing of microbiology samples for sensitive post-cruise microbiology studies that require intact DNA/RNA and cell membranes. A large number of samples are currently being analyzed in many different laboratories and will provide metagenomic, metatranscriptomic, and meta-metabolomic data as well as specific data on microbial process rates, cultivation-based microbial diversity, and other information for archaea, bacteria, and eukaryotic microorganisms as well as viruses.

Based on the preliminary lithological, biostratigraphic, paleomagnetic, dating, geochemical, and microbiological data, we conclude that expedition 347 accomplished most of the objectives that we had outlined in IODP proposal 672. This was indeed a successful expedition.

The full expedition report is available for download from http://publications.iodp.org/.