Blue carbon sequestration dynamics within tropical seagrass sediments: Long-term incubations for changes over climatic scales

Abstract

Determination of blue carbon sequestration in seagrass sediments over 15 climatic time scales relies on several assumptions, such as no loss of particulate organic 16 carbon (POC) after one or two years, tight coupling between POC loss and CO 2 emissions, 17 no dissolution of carbonates and removal of the stable black carbon (BC) contribution. We 18 tested these assumptions via 500-day anoxic decomposition/mineralisation experiments to 19 capture centennial parameter decay dynamics from two sediment horizons robustly dated 20 as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described 21 for both horizons by near-identical reactivity continuum models. The models predicted 22Additional keywords: sediment geochemistry, diagenesis, carbonate, pyrogenic carbon, 31 methane, sediment isotope tomography 32 Introduction 33Seagrasses, along with mangroves, saltmarsh and seaweeds, are increasingly touted 34 as a significant global carbon sink (McLeod et al. 2011). For seagrass in particular, this 35 service is based on two separate concepts: sedimentary carbon stocks and rates of 36 sedimentary carbon sequestration. The stock or storage service concept, in the mitigation 37 of greenhouse gas emissions, is a scalar concept and conceived at the meadow scale. It has 38 traditionally been estimated by potential carbon loss to mineralisation should it be 39 disturbed over a climatic unit of time (Pendleton et al. 2012). The depth of such 40 disturbance, and the extent of its effect on the carbon stock, is dependent on the type of 41 disturbance (Siikamäki et al. 2013;Gallagher 2017) and independent of the time it took the 42 carbon to accumulate. The sediment found within seagrass beds contains a sizable organic 43 component consisting of a mix of seagrass litter, associated epiphyte and microalgal 44 detritus, and additional inputs from adjacent land activities and fluvial deposition as well 45 as saltmarsh and mangrove ecosystems (Kennedy et al. 2010). In contrast, the carbon 46 sequestration service is a vector concept. Rates of sequestration depend on the balance 47