Due to burning of fossil fuels, carbon dioxide is being absorbed by the ocean where its chemical conversion to carbonic acid has already caused the surface ocean to become more acidic than it has been for at least the last 2 million years. Global ocean modeling suggests that the carbonate compensation depth (CCD) has already risen by nearly 100 m on average since pre-industrial times and will likely rise further by several hundred meters more this century. Potentially millions of square kilometres of ocean floor will undergo a rapid transition in terms of the overlying water chemistry whereby calcareous sediment will become unstable causing the carbonate “snow line” to rise.

We carried out a spatial analysis of seafloor geomorphology to assess the area newly submerged below the rising CCD. We found that shoaling of the CCD since the industrial revolution has submerged 12,432,096 km2 of ocean floor (3.60% of total ocean area) below the CCD. Further hypothetical shoaling of the CCD by 100 m increments illustrated that the surface area of seafloor submerged below the CCD has risen by 14% with 300 m of shoaling, such that 51% of the ocean area will be below the CCD. All categories of geomorphic feature mapped in one global database intersect the lysocline and will be (or already are) submerged below the CCD with much regional variation since the rise in CCD depth during the last 150 years varies significantly between different ocean regions. For seamounts, the highest percentages of increase in area submerged below the CCD occurred in the Southern Indian Ocean and the South West Atlantic regions (6.3% and 5.9%, respectively). For submarine canyons we found the South West Atlantic increased from 3.9% in pre-industrial times to 8.0% at the present time, the highest percentage of canyons found below the CCD in any ocean region.

We also carried out a relative risk assessment for future submergence of ocean floor below the CCD in 17 ocean regions. In our assessment we assumed that the change in CCD from pre-industrial times to the present is an indicator of the likelihood and the change in percentage of seafloor submerged below the CCD due to a hypothetical 300 m rise in the CCD is an indicator of the consequences. We found that the western equatorial Atlantic is at high risk and 9 other Ocean Regions are at moderate risk. Overall, geomorphic features in the Atlantic Ocean and southern Indian Ocean are at greater risk of impact from a rising CCD than Pacific and other Indian Ocean regions.

A separate analysis of the Arctic Ocean points to the possible submergence of glacial troughs incised on the continental shelf within a mid-depth (400–800 m) acidified water mass. We also found that the area of national Exclusive Economic Zones submerged below the rising CCD exhibits extreme variability; with 300 m of CCD shoaling we found a > 12% increase in area submerged below the CCD for 23 national EEZs, whereas there was virtually no change for other countries.

Tags: Marine Geology Carbonate compensation depth Ocean acidification Seafloor geomorphology Calcareous sediment climate change