Good question! Here is a paper worth looking at Article Addressing calcium carbonate cycling in blue carbon accounting

So the main mechanism for carbonate burial being an offset to organic carbon burial is where blue carbon systems enhance carbonate precipitation, which is subsequently buried. Carbonate precipitation increases the pCO2 of the water (by reducing alkalinity) and therefore there is a flux CO2 to the atmosphere. Carbonate dissolution has the opposite effect, But it isnt as simple as carbonate burial = an offset to organic carbon burial. Much of the carbonate buried may be allochthonous (externally supplied), carbonate burial can enhance organic matter sequestration (through binding of organics in mineral matrix), and the increase in pCO2 (through precipitation) may enhance primary productivity (e.g. for seagrasses), thereby increasing organic matter sequestration.

So a complex question that requires some more work to tease out the role of carbonate burial as an offset.

Cheers

Damien

Efforts are now underway to include blue carbon in global carbon offset schemes by managing mangroves, marshes and seagrasses ecosystems to enhance carbon sequestration by focusing on their effect on organic carbon processing. However, it is unclear what role inorganic carbon processing in blue carbon ecosystems plays in their overall carbon sequestration.

Considering the rapid rise of interest in blue carbon as a climate change mitigation and adaptation strategies, we argue that calcium carbonate cycling needs to be included when assessing the importance of these globally significant carbon stores as CO2 sinks. Addressing calcium carbonate cycling in blue carbon accounting is timely given that anthropogenic activities and climate change may alter calcium carbonate cycling within coastal ecosystems with unknown feedbacks for climate change. For example, with increasing atmospheric CO2 and ocean acidification, the ratio of released CO2 to precipitated calcium carbonate is expected to rise (Frankingnoulle et al. 1995). Yet increased calcium carbonate dissolution , and decreased calcification rates are also coupled to higher atmospheric CO2, with both processes lowering the rate of CO2 fluxed to the atmosphere. Moreover, partial dissolution of the large calcium carbonate stock deposited in blue carbon ecosystems may provide a buffer to ocean acidification while acting as a CO2 sink. Clearly accounting for calcium carbonates in blue carbon systems is a tricky business.