Relative sea‐level change regulates organic carbon accumulation in coastal habitats

Abstract

Because coastal habitats store large amounts of organic carbon (Corg), the conservation and restoration of these habitats are considered to be important measures for mitigating global climate change. Although future sea‐level rise is predicted to change the characteristics of these habitats, its impact on their rate of Corg sequestration is highly uncertain. Here we used historical depositional records to show that relative sea‐level (RSL) changes regulated Corg accumulation rates in boreal contiguous seagrass–saltmarsh habitats. Age–depth modeling and geological and biogeochemical approaches indicated that Corg accumulation rates varied as a function of changes in depositional environments and habitat relocations. In particular, Corg accumulation rates were enhanced in subtidal seagrass meadows during times of RSL rise, which were caused by postseismic land subsidence and climate change. Our findings identify historical analogs for the future impact of RSL rise driven by global climate change on rates of Corg sequestration in coastal habitats.

Increasing Rates of Carbon Burial in Southwest Florida Coastal Wetlands

“…Change in relative SLR is the most likely large-scale driver of increased OC burial rates in the region (Gonneea et al, 2019;Rogers et al, 2019;Watanabe et al, 2019;). Accumulation of the large soil C stocks in this region (Jerath et al, 2016) coincides with late Holocene rates of regional SLR that were largely stable over the past seven millennia, followed by mild acceleration in recent centuries (Gerlach et al, 2017;Khan et al, 2017;Scholl, 1964).…”

Unrecognized controls on microbial functioning in Blue Carbon ecosystems: The role of mineral enzyme stabilization and allochthonous substrate supply

“…C sequestration in tidal wetlands depends on the balance of organic matter input and output, processes that are primarily controlled by net primary production (NPP) and microbial decomposition, respectively (Chmura et al, ; Mcleod et al, ). However, the important ecosystem service of C sequestration is expected to be strongly influenced by several global‐change factors, such as accelerated rates of sea‐level rise (Kirwan & Megonigal, ; Osland et al, ; Watanabe, Seike, Kajihara, Montani, & Kuwae, ).…”

Variable Impacts of Climate Change on Blue Carbon

“…Past sea level fluctuations and coastal geomorphology are important factors that give rise to patterns of global blue carbon stocks and are critical factors that will affect their future. 37,[53][54][55][56][57] Most of the mangroves, saltmarshes, seagrass, sabhka (saltflats), swamp forests, and mudflats are located on coastlines with gently sloping topography, protected from high wave energy within estuaries, embayments, deltas, and lagoons (behind reefs or islands), and may also have relatively high (past or present) sediment supplies. These macro-scale patterns in the distribution and development of coastal blue carbon stocks reflect the relatively low wave energy conditions needed to support recruitment and growth of vegetation, 58,59 as well as the conditions that favor carbon burial (low energy, anoxia) over that of erosion and oxidation.…”

“…54 Rising sea levels are important as they result in increases in sediment accommodation space, which describes the landscape's capacity to accumulate blue carbon through deposition of sediment and the landward movement of the tidal boundary. 55,56 Thus, regions where the sea level has risen steadily over the Holocene (e.g., Caribbean basin) have large organic carbon deposits (e.g., in Belize, mangrove carbon deposits are up to 10 m thick 7 ) accumulated by those ecosystems for thousands of years. 7,35,60 In other regions, where sea level has risen to its highest level and then declined in the last 1,000-2,000 years of the late Holocene, sediment carbon deposits formed by mangroves and saltmarshes are found at higher elevations than the current highest astronomical tide, often underlying freshwater coastal vegetation.…”

“…55,61 These past locations and conditions of carbon burial provide insights into the future of carbon burial by coastal wetlands. 62 They demonstrate that rising sea levels may provide opportunities for increased carbon burial, 55,56 although clearly high rates of sea level rise, changes in wind and wave energy, extreme storms and associated erosion, 63 rising temperature, varying rainfall, and other interacting factors may limit potential gains (Figure 3).…”