1 Carbon Dioxide and Methane Emissions from Red Sea Mangrove 1 Sediments 2 3 Mallory A. Sea 1 , Neus Garcias-Bonet 1 , Vincent Saderne 1 * and Carlos M. Duarte 1 4 5 [1] {King Abdullah University of Science and Technology (KAUST), Red Sea Research Center 6 (RSRC), Thuwal, 23955-6900, Saudi Arabia} 7 8 *Correspondence to: V. Saderne (vincent.saderne@kaust.edu.sa) 9 10 Abstract 11 Mangrove forests are highly productive tropical and subtropical coastal systems that provide a 12 variety of ecosystem services, including the sequestration of carbon. While mangroves are 13 reported to be the most intense carbon sinks among all forests, their sediments can also support 14 large emissions of greenhouse gases (GHG), such as carbon dioxide (CO2) and methane (CH4), 15 to the atmosphere. However, data derived from arid mangrove systems like the Red Sea are 16 lacking. Here, we report emission rates of CO2 and CH4 from mangrove sediments along the 17 Saudi Arabian coast of the Red Sea, and assess the relative role of these two gases in supporting 18 total GHG emissions. Diel CO2 and CH4 emission rates in Red Sea mangrove sediments ranged 19 from -3452 to 7500 µmol CO2 m -2 d -1 and from 0.9 to 13.3 µmol CH4 m -2 d -1 , respectively. The 20 rates reported here fall within previously reported ranges for both CO2 and CH4, but maximum 21 CO2 and CH4 flux rates in the Red Sea are 10 to 100-fold below those previously reported for 22 mangroves elsewhere. Based on the isotopic composition of the CO2 and CH4 produced by 23 mangrove sediments, we identified the origin of the organic matter that supports GHG emissions. 24 In most of the mangrove stands, GHG emissions were supported by organic matter from mixed 25 Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-37 Manuscript under review for journal Biogeosciences Discussion started: 31 January 2018 c  Author(s) 2018. CC BY 4.0 License.