Dan Hammarlund

Values of deltaC-13 obtained from conventional bulk sediment radiocarbon dates encompassing the Pleistocene-Holocene boundary have been compiled and plotted against C-14 age. In all, 286 lake sediment dates from southern Sweden in the range 8,000 to 13.000 BP have been evaluated. A significant decrease in deltaC-13 values, initiated shortly before 10,000 BP and amounting to 5 parts per thousand, is distinguished. This change is accompanied by increased limnic productivity, decreased erosive input and increased organic carbon content of the sediments. A probable explanation for the deltaC-13 decline in organic material is decreased importance of dissolution of silicates at the transition to the Holocene. During the Late Weichselian, extensive weathering of exposed minerogenic material with subsequent input of bicarbonate to the lake water may have caused a relative enrichment of C-13 in dissolved inorganic carbon. Furthermore, the early Holocene increase in terrestrial vegetation cover probably led to an increased supply of C-13 depleted carbon dioxide to the lake water by root respiration. Altered limnic vegetation, presumably towards increased production of phytoplankton, could also have contributed to the observed decreasing deltaC-13 trend. The importance of these processes compared to other possible influencing factors, mainly endogenic carbonate production and changes in the global carbon cycle, is discussed.