The overall goal of the task was describe the Holocene evolution of the Baltic Sea ecosystem. To achieve this goal, we participated in a number of different sampling campaigns throughout the 3 years of the project and collected 33 long sediment gravity cores with accompanying surface cores (Fig. 1).

Figure 1: Gravity cores recovered during 2009-2011. Cores discussed in the deliverable have larger text and green symbols, the rest are white.

Briefly, cores were split into two sections, described for sediment characteristics and photographed. A number of analysis were preformed, especially different methodologies for dating sediments using 14C and lead concentrations/stable istopes, and mineral magnetic measurements. These measurements provided a general overview of the stratigraphy and magnetic susceptibility of sediments throughout the Baltic Sea.

The Littorina Sea sediment sequences have a special feature, the laminations, in most of the cores (Fig. 2). In the Baltic Proper they are present during three general time periods corresponding to the Holocene Thermal Maxim - HTM (lowermost laminations), the Medieval Warm Period -MWP (middle laminations) and the industrial revolution (top laminations), although the extent of laminations vary depending upon location in the Baltic Sea (Zillén et al., 2008).

Figure 2: Selected magnetic susceptibility and sediment stratigraphy from gravity cores in the Baltic Proper, from the shallowest to the deepest, a) LZGB2 (110,5 m depth), b) LL19 (169 m), c) F80 (181 m), and d) LZLD (400 m).

A precise determination of reservoir ages is one of the most problematic parts of establishing an accurate chronology of sedimentation in the Baltic Sea. Reservoir ages in the Baltic Sea vary due both to changes in salinity, e.g. due to salt water input from the Kattegat, and due to older carbon entering the Baltic Sea from freshwater sources. The dates presented are preliminary results with no ∆R applied (=400 reservoir years). The onset of the lower laminated section in the Gotland Basin is between ~8800-8100 cal years BP and the end occurs ~5000-5800 cal years BP. The onset in the Gulf of Bothnia is at ~9800-7200 cal years BP while the end occurs at ~5700-6700 cal years BP. The upper laminated section is only present in Gotland Basin cores and starts between ~1600-3900 cal years BP and ends ~1200-1900 cal years BP. Due to the feature mentioned earlier, that the laminated sections are thicker at deeper sites, will also affect the dates with longer periods of laminations forming in the deeper sites (Fig. 3).

Figure 3: Preliminary 14C dates from LL19 (169 m depth), F80 (181 m), MSM16/1-073-13 (193,5 m), LZGB2 (110,5 m), MSM16/1-082-03 (128 m), and MSM16/1-095-03 (107,2 m). The dates are calibrated with a ∆R-value of 0 years (i.e. no reservoir age).

In addition, we successfully applied a methodology new to the Baltic Sea regarding the use of lead concentration profiles and isotopes (Zillén et al., Accepted) to better determine time markers for the last 1000 years of Baltic Sea history (Fig. 4).