The transition from hunter-gatherer-fisher groups to agrarian societies is arguably the most significant change in human prehistory. In the European plain there is evidence for fully developed agrarian societies by 7,500 cal. yr BP, yet a well-established agrarian society does not appear in the north until 6,000 cal. yr BP for unknown reasons. Here we show a sudden increase in summer temperature at 6,000 cal. yr BP in northern Europe using a well-dated, high resolution record of sea surface temperature (SST) from the Baltic Sea. This temperature rise resulted in hypoxic conditions across the entire Baltic sea as revealed by multiple sedimentary records and supported by marine ecosystem modeling. Comparison with summed probability distributions of radiocarbon dates from archaeological sites indicate that this temperature rise coincided with both the introduction of farming, and a dramatic population increase. The evidence supports the hypothesis that the boundary of farming rapidly extended north at 6,000 cal. yr BP because terrestrial conditions in a previously marginal region improved.

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Reconstructions of environmental changes at sub-decadal to decadal resolution based on central Baltic Sea sediments rely on accurate and precise high-resolution sediment depth/age relationships. A model chronology for Baltic Sea sediments is presented here based on established historical records of anthropogenic radionuclides ( 137 Cs/ 241 Am/bomb 14 C), polychlorinated biphenyls (PCBs), lead (Pb) and stable lead isotope ( 206/207 Pb ratios), and radionuclide 210 Pb and 14 C decay dating methods. Marker horizons consisting of chemical precipitates formed by documented Major Baltic Inflow (MBIs) events and an extended diatom bloom period were also integrated into the model. The main time markers in Baltic Sea sediments that formed during the last 120 years were the following: (i) the deepest observation of 210 Pb unsupp. (marking the 210 Pb dating horizon) and departure of Hg from natural background levels at c. AD 1900; (ii) first detectable presence of PCBs at AD 1935; (iii) radionuclide production (i.e. 241 Am) due to nuclear weapons testing between AD 1954 and AD 1975, with a peak in AD 1963; (iv) maximum heavy metal and PCB concentrations in the AD 1960s/1970s; (v) the Chernobyl nuclear accident in AD 1986 as a sharp 137 Cs increase; (vi) exceptionally strong diatom blooms with a massive diatom layer found in the Eastern Gotland Basin in AD 1988-1990 and (vii) characteristic manganese-carbonate layers in the deeper central basins formed by MBIs in AD 1993 and AD 2003. A precise and accurate sediment depth/age relationship can only be achieved in restricted areas of the Baltic Sea where continuous sedimentation has prevailed and there has been limited postdepositional disturbance. We demonstrate that parallel Hg and 137 Cs measurements can be used to assess the quality of sediment sequences for high-resolution environmental reconstructions. We show examples of sediment profiles that conform to the historical record, and examples from Western Baltic Sea areas where it appears to be impossible to establish accurate geochronologies.

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The Littorina Sea stage (past c. 7000 years) development of the northern, and its interactions with the central Baltic Sea have been influenced by spatially different but in the north very strong glacio‐isostatic uplift. Here we investigate the impact of the isostatic readjustment on the northern Baltic Sea environment and on the water exchange with the central Baltic Sea using geochemical and microfossil records from well‐dated sediment cores of the Bothnian Sea/Bay that are compared to sedimentary records of the Gotland Basin and Landsort Deep. Benthic foraminifera and manganese carbonate layers in the sediments from northern Baltic Sea sub‐basins imply oxic marine intrusions from c. 7.0 cal. ka BP. Related water‐column stratification together with enhanced primary productivity caused oxygen‐depleted bottom waters and deposition of organic‐rich laminated sediments during the Holocene Thermal Maximum. Comparable sedimentary/geochemical records in early Littorina Sea sediments of e.g. the eastern Gotland Basin therefore imply a low impact of isostatic uplift on the northern Baltic Sea environment. A climate‐controlled decline in hypoxia induced a gradual deposition of rather homogenous sediments in the central and northern Baltic Sea during the mid‐Littorina Sea stage. Progressive isostatic uplift caused a diverging environmental development in both basins during the mid‐ to late Littorina Sea stage. The Åland sill shoaled markedly allowing only strong inflows to enter the northern Baltic Sea as late as 2.3 cal. ka BP. The restricted deep‐water exchange with the central Baltic Sea probably resulted in a pronounced freshening and oxic bottom waters. This, together with an uplift‐related sediment re‐deposition induced accumulation of rather homogenous sediments throughout the remaining Littorina Sea stage. By contrast, a climate‐controlled depositional environment in the central Baltic Sea during this time caused the formation of homogeneous sediments during cold phases and laminated sediments during warm phases.