Rhodolith facies are characterized by an abundance of free-living coralline red algae, and are a common feature of modern and ancient carbonate shelves worldwide. Rhodolith communities contribute significantly to the global calcium carbonate budget, and fossil rhodoliths are commonly used to obtain paleoecologic and paleoclimatic information. Few attempts have been made to quantify the spatial extent of modern rhodolith facies, despite their importance and common occurrence. Combining sediment sampling with acoustic facies mapping, this study provides the first set of quantitative data on rhodolith facies distribution in the warm-temperate southwestern Gulf of California, Mexico.

Though rhodoliths were the main carbonate producers, other important calcareous biota were bivalves (19%), bryozoans (13%), and corals (6%). Based on cluster analysis, biota were grouped into a rhodolith, a rhodolith–bivalve–coral, and a bivalve–bryozoa biofacies. The acoustic seafloor classification distinguished four acoustic facies, which were confirmed by ground-truthing: an outer-shelf silt facies, a biogenic sand facies, a peripheral rhodolith facies, and a central rhodolith facies. Rhodolith facies covered approximately 40% of the surveyed seafloor (45 km2). Results from quantitative analysis of sediment samples and acoustic mapping showed a significant correlation between grain size, biofacies, and acoustic seafloor facies. We successfully applied an acoustic device to provide highly resolved continuous coverage of the seafloor and discriminate modern rhodolith facies from surrounding sediment. This has important implications for quantifying rhodolith carbonate production in other regions, as well as for ecological and conservation studies.

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