Evidence for carbon cycling in a large freshwater lake in the Balkans over the last 0.5 million years using the isotopic composition of bulk organic matter

In the DEEP core from the Lake Ohrid ICDP drilling project, the carbon isotope composition of bulk organic matter (? C ) over the last 516 ka shows a negative correlation with total organic carbon (TOC) and total inorganic carbon (TIC). This relationship is marked by periods of lower ? C values corresponding to higher TIC and TOC. Along with TOC/TN, the correlation between ? C and ? C suggests that most of the organic matter in the core is from aquatic primary production within the lake. The combination of TOC, TIC, and ? C is able to disentangle long-term glacial/interglacial cycles and, to a lesser extent, millennial scale climate variability. Over the longer term, ? C shows modest variability, indicating that the ? C of the dissolved inorganic carbon (DIC) pool is stabilised by the supply of karst spring water characterised by ? C influenced by the bedrock ? C value, and the long residence time of the lake water and well mixed upper water column promoting equilibration with atmospheric CO . However, comparison between arboreal pollen (AP%), TIC and TOC data indicates that the ? C signal is modulated by the leaching of soil CO through runoff and spring discharge, changes in primary productivity, and recycling of organic matter within the lake, all affecting ? C . Exceptionally low ? C during some interglacial periods (e.g. MIS7 and MIS9) possibly indicate rapid intensification of organic matter recycling and/or increasing stratification and enhanced methanogenesis, even if the latter process is not supported by the sedimentological data.