Purpose <p>Insight into the distribution and sedimentation patterns of organic and inorganic carbon (OC and IC) in urban lake sediments is essential for understanding their role in the carbon (C) cycling of inland waters and supporting effective ecosystem management.</p> Methods <p>This study investigated the spatial variability of sediment OC and IC accumulation in a mesotrophic human-made urban lake (Lake Berendonck; 45&#xa0;ha) by combining high-resolution hydroacoustic sub-bottom profiling surveys and sediment coring.</p> Results <p>The results revealed strong spatial variations in sediment C accumulation rates. Deep central and southeastern areas of the lake exhibited relatively high C deposition, even though deep areas with low C content were also found. Lake Berendonck had a mean ± standard deviation sediment accumulation rate of 0.7 ± 0.5&#xa0;cm year^− 1, with areal OC and IC accumulation rates ranging between 24 and 557 and 3–37&#xa0;g m^− 2 year^− 1, respectively. Lake Berendonck’s mean sediment OC accumulation rate (155&#xa0;g m^− 2 year^− 1) was approximately four times higher than the mean OC accumulation rate of global lakes (37&#xa0;g m^− 2 year^− 1), while Lake Berendonck’s mean IC accumulation rate (12&#xa0;g m^− 2 year^− 1) falls in the mid-range for global lakes and seas.</p> Conclusions <p>Our findings indicate that C accumulation is highly variable in space and that spatially integrated data are needed to estimate C stocks and unravel within-lake C processes reliably. Furthermore, this study highlights that the OC accumulation in Lake Berendonck ranks among the highest rates observed in global lakes with similar surface areas (0.4–0.5 km²). This underscores the global importance of small urban water bodies in C cycling, particularly as key C storage systems.</p>