Journal of Sedimentary Research, 2006, v. 76, 670–682 Research Article DOI: 10.2110/jsr.2006.053 SEDIMENTOLOGY AND ACOUSTIC MAPPING OF MODERN RHODOLITH FACIES ON A NON-TROPICAL CARBONATE SHELF (GULF OF CALIFORNIA, MEXICO) STEFFEN HETZINGER, 1 JOCHEN HALFAR, 2 BERNHARD RIEGL, 3 AND LUCIO GODINEZ-ORTA 4 1 Leibniz Institut fu ¨r Meereswissenschaften, IFM-GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany, 2 Institut fu ¨r Geologie und Pala ¨ontologie, Universita ¨t Stuttgart, Herdweg 51, 70174 Stuttgart, Germany, 3 National Coral Reef Institute, Nova Southeastern University Oceanographic Center, 8000 North Ocean Drive, Dania Beach, Florida 33004, U.S.A. 4 Centro Interdisciplinario de Ciencias Marinas, Av. IPN s/n Apartado, Postal 592, La Paz, B.C.S., 23096, Mexico e-mail: shetzinger@ifm-geomar.de ABSTRACT: 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 km 2 ). 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. INTRODUCTION Shallow-marine, non-tropical carbonates may contribute about one- third to global present-day shelf carbonate production (Nelson 1988), and a significant amount of non-tropical carbonate is composed of both free- living and attached coralline red algae (Carannante et al. 1988; Nelson 1988). Studies on modern non-tropical carbonate shelves in the Mediterranean Sea, tropical shelves in Brazil, and their Miocene analogues led Carannante et al. (1988) to define a distinct rhodalgal lithofacies, dominated by encrusting coralline red algae, often occurring as rhodoliths. Even though rhodalgal sediments are found worldwide (Foster 2001), they are most commonly described from shelf areas located in a transitional position between the typical cold-water realm (the heterozoan assemblage of James 1997) and the tropical realm (the photozoan assemblage of James 1997) (Carannante et al. 1988). Well- studied modern rhodalgal carbonates are located on the New Zealand continental shelf (Gillespie and Nelson 1997), the Brazilian shelf (Carannante et al. 1988), along the Atlantic coastlines (Freiwald and Henrich 1994; Hall-Spencer 1998; Bosence and Wilson 2003; Hall- Spencer et al. 2003), in the Mediterranean Sea (Henrich et al. 1995; Barbera et al. 2003), and in the Gulf of California (Steller and Foster 1995; Foster et al. 1997; Halfar et al. 2000; Foster 2001). Even though fossil (mainly Pleistocene and Tertiary) rhodalgal carbonate sediments are encountered globally (e.g., Manker and Carter 1987; Bourrouilh-Le Jan and Hottinger 1988; Nelson 1988; Aguirre et al. 2000; Halfar and Mutti 2005), they are most widespread in the Paleotethys region (e.g., Bosence and Pedley 1979; Carannante et al. 1988; Esteban 1996; Betzler et al. 1997; Fornos and Ahr 1997; James et al. 1999; Pomar et al. 2004). A variety of aspects have been studied in fossil and modern rhodalgal systems, including paleoecology, paleoclimate, conservation, and rhodo- lith mining (Barbera et al. 2003; Bosence and Wilson 2003; Grall and Hall-Spencer 2003; Hall-Spencer et al. 2003; Steller et al. 2003; Hinojosa- Arango and Riosmena-Rodriguez 2004; Wilson et al. 2004). Despite this, only few attempts have been made to quantify spatial extent and map large-scale facies distribution of modern rhodolith-dominated areas (Davies and Hall-Spencer 1996; de Grave 1999; de Grave et al. 2000). Since rhodoliths often occur in water depth beyond resolution for passive optical sensors or SCUBA surveys, mapping is restricted to grab- sampling or acoustic methods. Acoustic ground discrimination systems (AGDS) are capable of detecting differences in sediment types (Hamilton et al. 1999; Preston et al. 2000; Morrison et al. 2001; Anderson et al. 2002; Ellingsen et al. 2002; Maushake and Collins 2002; Freitas et al. 2003a; Freitas et al. 2003b; White et al. 2003), and differentiating artifacts from sediments (Lawrence and Bates 2001), and have received increasing attention by geologists wishing to map facies (Moyer et al. 2005; Riegl and Purkis 2005). The goals of the present study are to (1) conduct a first quantitative acoustic mapping survey of a rhodolith-dominated shelf using an Copyright E 2006, SEPM (Society for Sedimentary Geology) 1527-1404/06/076-670/$03.00