Organized patches of bioherm growth where the Strait of Dardanelles enters the Marmara Sea, Turkey.

High-resolution multibeam mosaics show that the seafloor across the southwestern Marmara Sea is host to remarkably organized near-circular bioherm mounds, which commonly are arranged into large, tightly packed clusters. Grab samples and gravity cores reveal that the bioherms are predominantly composed of very fine-grained, calcareous, silty mud with abundant bioclasts, including centimetre-scale masses of coralline red algae and intact disarticulated mollusc shells (mainly the genera Modiolus and Mytilus ). Geometric analysis of multibeam images reveals that the average bioherm is 15.6 m in diameter, occupies ~ 190 m 2 of seabed, stands 113 cm above the adjacent seafloor, and its crest is 20.6 m from the crests of neighbouring bioherms. In regions of tightly packed bioherm clusters (referred to as ‘bioherm colonies’) the inter-mound depressions are on average 4.4 m wide and 33 cm deep. Although each bioherm mound is nearly circular, the surrounding inter-mound channels form a more rectilinear mesh of linked pentagonal and hexagonal polygons suggesting densest possible spatial packing of the mounds. Near-neighbour statistics of R = 2.11–2.14 indicate an essentially uniform spacing between the bioherms, which is the expected result for close packing on a plane and full utilization of the available space. The bioherms occur at depths between − 30 m and − 60 m. They are absent above and below these depths. In this part of the southwestern Marmara Sea at the eastern exit of the Strait of Dardanelles, water depth controls important water mass properties such as salinity, nutrient supply and availability of light. Other inferred critical controls are the availability of a suitable hard substrate where Holocene muds are thin or absent, and nutrient supply, potentially including a component from escaping methane-rich pore fluids. Evidence here and elsewhere in the Marmara Sea provisionally dates the onset of bioherm growth and development to the latest Pleistocene–earliest Holocene, after global sealevel rose to the breach depth of the Strait of Dardanelles at ~ 13.8 cal ka. [ABSTRACT FROM AUTHOR]