BLUE CARBON SEQUESTRATION IMPACTED BY A LATE HOLOCENE HIGH ENERGY SAND DEPOSIT IN A SALT MARSH, ROCKPORT MA

An anomalous sand bed within Saratoga Creek Salt Marsh (Pino and Hubeny, 2016) provides a basis for interpreting the event responsible for its deposition as well as its impact on marsh soil carbon (C) stock and C sequestration. We present data from 9 additional cores and 5 soil probes as well as chronostratigraphic control through 10 new calibrated radiocarbon dates. We hypothesize a large event caused instantaneous sand bed deposition in the marsh resulting in a sedimentation and C sequestration hiatus that persisted until sea level caught back up to marsh elevation and vegetation resumed. This study aims to define the event, timing of sedimentation hiatus, and impact of the sand deposit on marsh C sequestration. Salt marshes efficiently store C, thus their study is important in terms of climate regulation and mitigation. Sediment cores were obtained by a Livingston corer. Core subsamples were quantified for percent organic carbon (%OC) using an elemental analyzer. Soil C stock and C sequestration were determined using %OC, dry bulk density (DBD), core and subsample lengths, and marsh area. Radiocarbon dated samples of core 7 constrain sand layer age between 3,007 +/- 229 and 3,437 +/- 77 cal BP. A dated sample of core 1 yields 2,909 +/- 69 cal BP above the sand deposit. The gray sand deposit is variable thickness with a minimum observed thickness of 38cm, with multiple medium to coarse grained fining upward sequences. The thickness of the sand layer suggests rapid deposition, consistent with a high energy event such as a hurricane or tsunami. Average sedimentation rates yield 0.2 cm/yr before sand deposition and 0.05 cm/yr after. C soil stocks were calculated as 800 MgC/ha above sand deposit (0-166cm), 104 MgC/ha in the deposit (168-199cm), and 583 MgC/ha below deposit (199-305cm). The sedimentation hiatus persisted 440 +/- 310 years where up to 18,900 MgC was not sequestered in marsh, based on C Mass Accumulation Rate (MAR) 240 +/- 62 g C / m²yr. After sand deposition, sedimentation rates decrease and soil C increases. This is expected because C stored at shallow depths is younger and had less time to decompose. Decreased C in the sand deposit suggests the depositional event impacted C sequestration. High energy deposits can reduce C sequestration within salt marshes by inflicting sedimentation hiatuses, until marsh accretion resumes.