Award: OCE-1760556

Wetlands cover about 5% of global land area yet they are estimated to store more than 20% of sediment carbon based previous research. These wetlands store CO2 as biomass and bury organic matter in sediments. Thus, salt marsh habitats mitigate long-term atmospheric CO2. Perturbations to salt marsh environments, including via extratropical storms, have the potential to reduce their C sequestration rate and overall impact on the global carbon cycle. We studied the effects of extratropical storms of varying intensity on salt marsh ability to store carbon by quantifying the changes in carbon burial rates and the quality of the carbon pre- and post-storm (2014, 2018) in two Texas marshes. Sediment cores were collected at both locations in 2014 and 2018 and analyzed for % organic matter (OM), porosity, TOC, TN, δ13C, δ15N, 210Pbex, 137Cs, D14C, and ramped pyrolysis oxidation. The observed increases in sediment porosity averaged about 20% at both sites between 2014 and 2018. Given the length of inundation time at Anahuac, higher sediment porosities were expected. Conversely, %OM decreased between pre- and post-storm sediments at Aransas and Anahuac. Changes in % OM, ranging from 5 to 20 %, were most noticeable in the top 20 cm at both sites and are attributed to the sediment saturation from the storm water inundation. Total organic carbon (TOC) in sediments dated after 2000 in the pre-storm samples are similar at both sites, but post-storm sediments showed more variability between stations. Differences in TOC between sampling periods and between post-storm stations beginning around 2005 are likely due to the spatial heterogeneity of the marsh across stations and changes in vegetation over time at Aransas. At Anahuac, silts and clays comprise between 40 and 60% of the marsh sediments and are likely stabilizing the existing sediment carbon storage at this site. Pre- and post-storm sediment thermograms at Aransas show that post-storm sediments were comprised of a more heterogeneous mixture of organic material. Before Hurricane Harvey arrived in August 2017, the Aransas marsh was struggling to maintain elevation because of decreasing sediment delivery, shifts in dominant vegetation species, and an increase in tidal height, all of which contributed to this marsh being unstable. Storm-induced sedimentation was not observed on the Aransas marsh platform after Hurricane Harvey and vegetation density decreased by 15% after the storm. The Aransas marsh has continued to destabilize and become more vulnerable to drowning and eventual loss of carbon storage between 2014 and 2018. At tropical storm strength, Harvey did not appear to impact the physical stability of the older and deeper Anahuac marsh. Thermochemical and carbon isotope observations show that organic material content in the sediments was altered by physical flushing and removal and not through enhanced remineralization. Intellectual Merit: Coastal marsh sediments store carbon, thereby mitigating atmospheric CO2. These marsh environments are vulnerable to sea level rise, storm surge, erosion, and drought, which may impact their ability to effectively sequester carbon. Event-driven physical forcing appears to be diminish sediment carbon content and significantly increases marsh carbon vulnerability over the more subtle effects of remineralization. Broader Impacts: Three students completed research while working on this grant. A PhD student completed her dissertation and is now a postdoc at Penn State. Two undergraduate students completed senior honors theses prior to graduating from UNC-Chapel Hill. One is now finishing his second year of graduate school at VIMS and the other student graduated in May 2020. Last Modified: 05/23/2020 Submitted by: Jaye E Cable