Microborings from different North Atlantic shelf areas-variability of the euphotic zone extension and implications for paleodepth reconstructions

The distribution of microendolithic boring was investigated along a bathymetrical gradient in North Atlantic shelf areas off Mauritania (19°N), W-Scotland (56°N), and Tromsø (Norway, 70°N). For demonstration of possible paleoecological applications, studies included samples from the Miocene of Alaska. The boreholes were left by cyanobacteria, algae, and fungi living in carbonate substrates, here mainly molluscan shells and barnacles. Morphologically distinct boring traces were ichnotaxonomically evaluated and grouped to trace-communities (ichnocoenoses). The resulting ichnocoenoses were integrated in a bathymetrical scheme which was then compared with data deriving from experimental studies near Lee Stocking Island (Bahamas, 25°N). The experimental studies at Lee Stocking Island are based on taxonomical studies of the endolithic organisms themselves and serve as a direct comparison between the morphology of the boring organism and its trace and furthermore as an example for a bathymetrical gradient of high transparency. Initially the method of bathymetric integration of trace-communities was developed in modern and ancient tropical and sub-tropical environments. The method uses the spatial distribution of photoautotrophic microendoliths (cyanobacteria, chlorophyta, and rhodophyta) for the identification of different photic zones. In our modern as well as in our fossil non-tropical sites the characteristic ichnotaxa of the bathymetric index ichnocoenoses are also present. They show a clear pattern in the distribution along the bathymetric gradient: The lower limit of the euphotic zone was identified off Mauritania in about 25-40 m water depth, off Scotland in about 20 m, and off Norway in about 16 m. For comparison, the euphotic zone extends down to about 100 or 150 m water depth at the Bahamas. The ichnocoenose of the Miocene of Alaska displays characteristics of the deep euphotic zone. Considering its latitudinal position, the lower boundary of the euphotic zone is estimate in ca. 20 m water depth. This value is in good accordance with the conventional bathymetric reconstruction. We conclude that our method is applicable in non-tropical settings, too.