Bathymetric zonation of modern microborers in dead coral substrates from New Caledonia—Implications for paleodepth reconstructions in Holocene corals

Microboring endoliths were studied in dead modern corals collected at various depths and in growth position on La Surprise atoll in New Caledonia. Eight taxa of microborers were regularly observed: three cyanobacteria (Hyella caespitosa, Mastigocoleus testarum, and Plectonema terebrans), two chlorophytes (Ostreobium quekettii and Phaeophila dendroides) and three fungal forms (Dodgella priscus, and two unidentified fungi). The relative abundance of each taxon was determined in the modern corals throughout the studied sampling interval (from 0 down to 40 m water depth). This allowed us to propose for the first time a bathymetric model based on microendolith assemblages colonizing in situ hard-substrates of corals. In this model, the occurrence of the cyanobacteria Hyella and Mastigocoleus indicates a bathymetric range from 0 down to a water depth of 3 and 6 m respectively. The chlorophyta O. quekettii, associated with the cyanobacterium P. terebrans and fungi, is dominant from 10 to 40 m depth, which is the lower limit of our sampling. The distribution ranges of individual taxa observed in the dead coral substrates from New Caledonia are much narrower than those reported in the literature for other substrates.The bathymetric model based on distribution of microborers in modern corals was then applied to interpret the trace assemblages of microborers observed in Holocene corals collected by drilling on two reef sites of New Caledonia : La Surprise islet, in the vicinity of the sampling sites of the modern corals, and the barrier reef of Bayes, on the NE coast of Grande Terre. The paleodepth interpretations performed at various levels along the cores allow identification of several shallowing-upward sequences during the last 7000 years of reef growth in New Caledonia. Differences in the thickness of these sequences between La Surprise and Bayes sites were interpreted as differences in the space available for accommodation between the two sites, in turn related to the differential subsidence that affected New Caledonia during Holocene times. The use of the microendolith trace assemblages in paleodepth reconstructions of Holocene reefs from New Caledonia allowed detection of relative sea level variations finer than those deduced from changes in coralgal assemblages.