 This is Heron Island, a coral cave in the southern Great Bear Reef, Australia. Its inner reef flat is covered in 68% sediments, made up of calcium carbonate. These sediments provide habitat and important ecological functions such as photosynthesis, calcification and nitrogen fixation. Seasonal variations in these processes remain poorly studied. We came here to study the dissolution and precipitation of these sediments and how they change over time. The chamber is pictured here that were inserted into the sediments and incubated for 24 hours. Water samples were taken at dusk, dawn and the following dusk and analyzed for total alkalinity and dissolved in organic carbon. From that, we calculated rates of photosynthesis, respiration, calcification and dissolution of the sediments. We repeated these incubations for nine months of the year to cover a near annual cycle. Sediment dissolution in December was driven by high rates of nighttime dissolution, whereas in July, net dissolution was caused by depressed rates of daytime calcification. The results of this study highlight the large temper of variability of coral reef sediment metabolism and the need to include repeated measurements over time, particularly in shallow reef systems that can experience large swings in light, temperature and carbonate chemistry.