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 (δ¹³C_TOC) 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 δ¹³C_TOC values corresponding to higher TIC and TOC. Along with TOC/TN, the correlation between δ¹³C_TOC and δ¹³C_TIC suggests that most of the organic matter in the core is from aquatic primary production within the lake. The combination of TOC, TIC, and δ¹³C_TOC is able to disentangle long-term glacial/interglacial cycles and, to a lesser extent, millennial scale climate variability. Over the longer term, δ¹³C_TOC shows modest variability, indicating that the δ¹³C of the dissolved inorganic carbon (DIC) pool is stabilised by the supply of karst spring water characterised by δ¹³C_DIC influenced by the bedrock δ¹³C value, and the long residence time of the lake water and well mixed upper water column promoting equilibration with atmospheric CO₂. However, comparison between arboreal pollen (AP%), TIC and TOC data indicates that the δ¹³C_TOC signal is modulated by the leaching of soil CO₂ through runoff and spring discharge, changes in primary productivity, and recycling of organic matter within the lake, all affecting δ¹³C_DIC. Exceptionally low δ¹³C_TOC 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.