Regional climatic and environmental changes in the Baltic Sea from 7,100 to 3,000 cal. yr BP. (A and C) Loss on ignition (LOI, black) and organic carbon content (purple) data from two key-sites P435-2 and 303600 vs. age. Sediments with high organic carbon content are dark and laminated, corresponding to periods of bottom water anoxia. Comparison of the two records shows that they are highly correlated, allowing the age-depth model for P435-2 to be adapted to provide chronology for 303600. (B) Summer SST record for Gotland Basin site 303600 based on TEX86 palaeothermometry. The solid red line plots the 5-point rolling mean. June insolation at 60°N (in W m−2) is plotted as a grey dashed line. Horizontal grey bar marks the 16 °C threshold temperature for the occurrence of cyanobacterial blooms. Note, anoxic conditions of bottom waters (i.e. laminated sediments and elevated LOI values) occurred when temperatures were above this threshold temperature. The rapid warming with the sudden appearance/spread of anoxic conditions occurred at 6,000 cal. yr BP and is followed by a regional Thermal Optimum. The general temperature decline/cooling follows the summer insolation decrease finally leading to stable oxic bottom water conditions. A strong cold phase between 6,300 and 6,000 cal. yr BP is also evident from an increase in coarse mineral fraction (ice rafted debris) in central Baltic Sea sediments.