Organic carbon accumulation and metabolic pathways in sediments of mangrove forests in southern Thailand

Rates of organic carbon accumulation, mineralization and burial in sediment s were examined during dry and wet seasons in four mangrove forests of a sh allow-water embayment (Ao Sawi) in southern Thailand. Mass sediment accumul ation (MAR), estimated from Pb-210 and Cs-137 profiles, was rapid at all fo rests, ranging from 2.9 to 7.6 kg m(-2) yr(-1); mixed layer sediment thickn esses ranged from 16 to 38 cm. Total inputs of organic carbon ranged from 2 6.4 to 40.9 mol C m(-2) yr(-1) and burial rates ranged from 15.3 to 23.4 mo l C m(-2) yr(-1). Total rates of carbon mineralization, estimated from dire ct measurements Of CO2 gas flux from exposed sediments, from DIC flux from submerged sediments, and from core incubations for Sigma -CO2, ranged from 7.0 to 16.4 Mol C m(-2) yr(-1) with few consistent seasonal differences amo ng the four forests. Rates Of O-2 gas flux into exposed sediments ranged fr om 9.4 to 91.4 mmol 0, m(-2) d(-1) and were significantly greater than rate s of dissolved O-2 flux into submerged sediments (range: 21.9-38.3 mmol O-2 m(-2) d(-1)). Oxic respiration was the largest decomposition pathway, with sulfate reduction (range: 0.6-16.9 mmol S m(-2) d(-1)) the next most impor tant pathway. Other metabolic pathways appeared to be minor, and methane wa s not detected in the porewater or in the gas flux chambers. The discrepanc y between rates of O-2 and CO2 fluxes implies involvement in geochemical pr ocesses (e.g. sulfide oxidation, authigenic mineral formation). Sediment bu dgets indicate that organic carbon preservation was greatest (71% burial ef ficiency) in the oldest forest with equivalent burial efficiency (57-59%) i n the younger forests. Mineralization efficiency ranged from 27-40% with th e lowest efficiency at the oldest forest. These tropical mangrove forests a re storage sites for sediment and, on average, retain approximately 60% of total input of organic carbon to the sediment. (C) 2001 Elsevier Science B. V. All rights reserved.