Establishing carbon retention and sequestration in a submerged mangrove using isotopic analysis

Abstract

Carbon sinks are recognised as an important environmental commodity, however are often assumed to be stable. Climate change, and in particular sea-level rise may alter the stability of coastal carbon sinks. Mangrove and saltmarsh are amongst the most efficient ecosystems at sequestering carbon; however the effect of sealevel rise on sediment accretion and carbon accumulation, and its subsequent effect on carbon sequestration and carbon retention remain unknown. This study uses a range of isotopic techniques, including 210 Pb to determine a sediment core chronology, and stable carbon and nitrogen isotope analyses, to determine the effect of rapid sea-level rise on a coastal carbon sink. The study site, located at Chain Valley Bay, Lake Macquarie, underwent rapid submergence following the collapse of a long wall mine in the mid-1980s; and this submergence was used as a surrogate for exploring the effects of rapid sea-level rise on a coastal carbon sink. Temporal mapping of vegetation distribution highlighted the dieback of vegetation and subsequent recovery of vegetation following submergence; however areas in the lower intertidal zone remained permanently inundated. The permanently submerged wetland area had a higher accretion rate following submergence than the recovery area that is now vegetated with mangrove. Both zones had an increase in sediment mass, indicating a shift in sediment sources. Carbon and nitrogen isotope analyses reflected the changes in sediment sources in each zone and in some areas showed a transition from terrestrial to marine sources. These results suggest that where hydrodynamic conditions are suitable and sediment supply is sufficient, coastal carbon sinks may recover from rapid submergence associated with sea-level rise.