Geochemical analyses reveal the importance of environmental history for blue carbon sequestration

J J Kelleway,G Jacobsen,H Heijnis,A Zawadzki,P Gadd,P I Macreadie,N Saintilan,P J Ralph,J A Baldock

doi:10.1002/2017jg003775

Abstract

Coastal habitats including saltmarshes and mangrove forests can accumulate and store significant blue carbon stocks, which may persist for millennia. Despite this implied stability, the distribution and structure of intertidal-supratidal wetlands is known to respond to changes imposed by geomorphic evolution, climatic, sea level and anthropogenic influences. In this study, we reconstruct environmental histories and biogeochemical conditions in four wetlands of similar contemporary vegetation in SE Australia. The objective is to assess the importance of historic factors to contemporary organic carbon (C) stocks and accumulation rates. Results from the four cores – two collected from marine influenced saltmarshes (WAP-M and POR-M) and two from fluvial influenced saltmarshes (WAP-F and POR-F) – highlight different environmental histories and preservation conditions. High C stocks are associated with the presence of a mangrove phase below the contemporary saltmarsh sediments in the POR-M and POR-F cores. 13C NMR analyses show this historic mangrove root C to be remarkably stable in its molecular composition despite its age, consistent with its position in deep sediments. WAP-M and WAP-F cores did not contain mangrove root C, however, significant preservation of char C (up to 46% of C in some depths) in WAP-F reveals the importance of historic catchment processes to this site. Together, these results highlight the importance of integrating historic ecosystem and catchment factors into attempts to upscale C accounting to broader spatial scales.

Highlights

IntroductionCoastal vegetated habitats (saltmarshes, mangrove forests, and seagrass beds) provide an important ecosystem service through the capture and long-term storage of organic carbon (C)
High C stocks are associated with the presence of a mangrove phase below the contemporary saltmarsh sediments in the Port Stephens marine site (POR-M) and POR-F cores. 13C nuclear magnetic resonance analyses show this historic mangrove root C to be remarkably stable in its molecular composition despite its age, consistent with its position in deep sediments
Radiocarbon dating of deposited materials—tree bark at 30 cm dated to 1785 ± 164 cal years AD and an oyster shell fragment at 91 cm dated to 1212 ± 150 cal years AD (Table 2)—suggests maximum long-term surface accretion rates of 1.8 ± 1.8 mm yrÀ1 and 1.2 ± 0.2 mm yrÀ1, respectively

Summary

Introduction

Coastal vegetated habitats (saltmarshes, mangrove forests, and seagrass beds) provide an important ecosystem service through the capture and long-term storage of organic carbon (C). This capacity to sequester “blue carbon” is dependent upon three broad ecosystem factors: (1) high productivity in converting CO2 into plant biomass C [Alongi, 2002; Nixon, 1980], (2) effective trapping of particulate organic C originating from within the ecosystem (autochthonous C) and/or from external sources (allochthonous C) [Kennedy et al, 2010], and (3) biogeochemical conditions within sediments which slow the decay of organic material [Fourqurean et al, 2012; Kristensen et al, 2008; McLeod et al, 2011]. Such variability may have implications for long-term C storage because the molecular composition (i.e., recalcitrance) of C inputs is KELLEWAY ET AL

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