Sinkhole expedition Luymes bank, Saba bank

The Saba Bank is a large (ca 2400 km2 ) submerged carbonate platform of 15-40m depth rising up from 800-1000m depth and fringed with coral reefs. It extends into a carbonate peninsula of ca 80m deep (Luymes Bank) which is pockmarked by sinkholes. More than twenty drowned sinkholes were distinguished in this peninsula based on available bathymetric data. Diameters of sinkholes vary from 70 to 1100 m and depths ranges between 10-300m. The area of the Luymes Bank with sinkholes is ca 66 km2 . During the NICO cruise in 2018 two sinkholes were visited in the Luymes Bank. In one of the two shallow sinkholes, which were only briefly explored with camera’s in 2018, we found peculiar pillar-like, probably calcium carbonate accretions with diameters of 40-60cm and protruding up to 1m from the sandy bottom. Pillars were found to stand neatly ordered on the bottom at a depth of ca 110 m. Based on the pink color on top, pillars look like features formed by crustose coralline algae of unusual size and density, almost in a stromatolitic fashion. In the second sinkhole such pillar-like structures were not found. Very little is known about these structures, their distribution and the conditions under which they are formed. Moreover, no information is available of the benthic communities and environmental conditions in the very deep sinkholes of more than 150m m depth. Therefore, the sinkhole expedition was completely dedicated to the sinkholes and the platform in which they occur (Luymes Bank).

The aims of the expedition were:

1. To study the distribution and environmental conditions (e.g. nutrients O2, particulate organic matter, water movement) of benthic communities on the platform between sinkholes and in the sinkholes with emphasis on areas with regularly distributed pillar-like structures in sinkholes.
2. To take high resolution pictures of the benthic communities with high-resolution camera system and NIOZ video frame in order to describe the benthic communities.
3. To collect bottom samples in order to determine the species diversity of these communities.
4. To collect pillars and assess the species consortia producing the pillars, their life history strategies, accretion rates and stratigraphic history.
5. To survey and investigate the carbonate chemistry of sinkholes of different size and depth and detect the effects of possible stratification in sinkholes.
6. To determine metagenomics and metabolomics in water samples from sinkholes of different size and depths.
7. To investigate Light-Dark shifts in metagenomics and metabolomics in near bottom water samples in relation to nutrients, O2, carbonate chemistry and POM in shallow sinkholes (20- 40m deep) with and without pillar-like structure and the platform community at approx. 80m depth.
8. To collect plankton samples for closer studies of plankton communities over the Luymes Bank