The development of an ultrasonic sediment probe for in situ blue carbon estimation: Toward an improved mapping tool

Ten percent of the total organic carbon (TOC) absorbed by the ocean each year is stored in seagrass-bearing sediments. The preservation of these carbon stores is considered a vital method to mitigate climate change. Currently, an in situ method for rapidly estimating sediment-sequestered TOC in seagrass meadows does not exist. Such a tool could allow for finer-resolution spatiotemporal TOC mapping to aid in reducing the uncertainty of global blue carbon estimates and better understanding how carbon stores react to naturogenic and anthropogenic stressors. Previous work using cores collected from a Thalassia testudinum meadow in South Texas established a highly correlated relationship between sediment primary wave (p-wave) modulus and TOC. In this meadow, the sediment-sequestered TOC ranged from 0.5% to 6% by dry mass, and the correlation with p-wave modulus exceeded that of other sediment properties including mud content, dry density, and porosity. Building on these results, a proof-of-concept in situ ultrasonic p-wave modulus sediment probe was developed. The probe was designed to be inserted into the sediment to rapidly measure the p-wave modulus without the need for the collection and post-processing of sediment samples. The probe was tested in the same T. testudinum meadow described above along a track with spatially varying TOC content. Core samples were collected at each deployment, analyzed in the laboratory with conventional methods, and compared with in situ measurements. Using in situ p-wave modulus measurements and the previously derived relationship between p-wave modulus and TOC, TOC depth profiles were inferred along the measured track. [Work supported by ARL:UT IR&D]