International Journal of Sediment Research, Vol. 29, No. 1, 2014, pp. 59–72 - 59 -      International Journal of Sediment Research 29 (2014) 59-72 Recent surface marine sediments of Cocos Island in Costa Rica Jeffrey A. SIBAJA-CORDERO 1 , Jesús S. TRONCOSO 2 , Eddy GÓMEZ-RAMÍREZ 3 Abstract Subtidal sediments of Isla del Coco (Cocos Island), Costa Rica were described in their grain size, sorting, organic matter, and carbonates from 27 dredge samples (3–75 m), collected in April 2010. The organic matter range between 1.37–3.31% and carbonates presented a mean of 74±17%. The sorting was moderately or poorly. The grain size ranged between 0.1–1.1mm. The pattern is that sediment change from inner to mouth of bays. Carbonates and gravel fractions increased offshore and organic matter have high values inner the bays. Input of vegetal debris and mud was from the rivers of this island covered with rain and cloud forest. Key Words: Sand bottom, Sorting, Organic matter, Carbonates, Isla del Coco 1 Introduction The characterizing of the subtidal marine sediments of Tropical Eastern Pacific (TEP) has been carried out in scatter zones mainly near the American continent, for example in Costa Rica. Maurer and Vargas (1984) presented data of grain type and organic matter content in several depths in an estuarine system (Golfo de Nicoya) (Fig. 1). In the Golfo Dulce (Fig. 1), Hebbeln et al. (1996) sweeping the bottom with an echosounding established that the sediments of the internal and deep zone of this gulf have a 60% turbidites and that the sediments of this region differ of the inclined walls. León- Morales and Vargas (1998) also presented the contents of silt and clay collected by means of dredge in this tropical tectonic fjord. The results of Guzmán et al. (1987) and Cortés et al. (1996), presented grain size data and content of carbonates in mud and sand bottom, close and far of the coral reefs of Isla del Caño (Fig. 1). In Panama, Mair et al. (2009) published the distribution of grain types in the bottom of the Islas Las Perlas a Panamanian archipelago (Fig. 1). In Ecuador, Soledispa and Villagres (1989) described the continental shelf of the Ecuador mainland, and point out the transport offshore of material results of the erosion of the rocky coast and the presence of mud banks in the deeper zone. Oceanic region of TEP was studied by Johnson et al. (1976) with acoustical profiles exploring the Galapagos ridge and the Galapagos Islands, and finding a thin layer of sediment near the islands (Fig. 1). Saukel et al. (2011) study surface sediment samples collected in the Tropical SE Pacific (5ºN–25ºS, 70º–100ºW) during 1969 and 2000. Four zones of mineral were found based in the silt-clay fractions according to their origin. The Cocos ridge lay in the zone of dust/hemipelagic clays. Samples close to the shelf of Isla del Coco, and within the Cocos ridge presented mid values of smectite, quartz and higher of chlorite. The smectite comes of a source located between 5° and 10°N, possibly by wet chemical weathering of volcanic rocks inland of the Cocos Ridge. The Cordillera de Talamanca in Costa Rica (Fig. 1) is probably the provenance of quartz and chlorite. Water currents are responsible for the increased chlorite contents on the ridge and this mineral is not present around the Galapagos Islands (Saukel et al., 2011). On the other hand coarse fractions have low transport, and depend of local factors as waves, tides, currents, the balance between erosion and accumulation, geological or biological sources (Cortés et al., 1996; Eleftheriou and McIntyre, 2005). The Parque Nacional Isla del Coco (PNIC) is an UNESCO World Heritage Site and the nucleus of the Área de Conservation Marina Isla del Coco (ACMIC). The island is of volcanic origin and their platform is described in Lizano (2001). Around the 183 m isobath a marked slope falls until the 1,000 m deep. The platform has their narrow region in   1 Prof. Escuela de Biología & Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), Universidad de Costa Rica, E- mail: jeffro@costarricense.cr, Phone No. +00 (506) 2511 5544, Fax No. +00 (506) 25114216, Office number: 24.5, and Dr. grad. by Departamento de Ecología y Biología Animal, Facultad de Ciencias del Mar, Universidad de Vigo, Spain. 2 Prof. Departamento de Ecología y Biología Animal, Facultad de Ciencias del Mar, Universidad de Vigo, Campus Lagoas Marcosende, 36310 Vigo, Spain 3 Assoc. Res. Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), Universidad de Costa Rica, San Pedro, 11501– 2060 San José, Costa Rica, and Dr. (Cand). Departamento de Biología, Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz, Spain. Note: The original manuscript of this paper was received in Apr. 2012. The revised version was received in Oct. 2012. Discussion open until Mar. 2015.