ASSESSMENT OF A HOLOCENE HIGH ENERGY DEPOSIT AND ITS IMPACT ON CARBON SEQUESTRATION WITHIN A NEW ENGLAND SALT MARSH

Covering around 22,000 km² of global temperate coastline, salt marsh ecosystems are powerful at sequestering and storing organic carbon (C) in plant biomasses and marsh soil, playing a significant role in climate regulation and mitigation. Their sediments provide archives of past high energy events and environmental changes. This study examines a previously defined anomalous sand deposit (Pino & Hubeny, 2016) seen in stratigraphy of back-barrier Saratoga Creek Salt Marsh, Rockport, MA. The sediment archival record helps interpret a high energy event responsible for deposition of this bed and its impact on marsh soil C stock and C sequestration. We present data from 10 sediment cores with chronostratigraphic constraints through 18 calibrated radiocarbon dates. We hypothesize a large event abruptly deposited the sand bed in the marsh causing a sedimentation and C sequestration hiatus persisting until sea level rose to marsh elevation and marsh vegetation resumed. A sedimentation hiatus persisted an estimated 440 ± 310 years based on core 7 dates and deposit thickness. Radiocarbon dates constrain sand layer between 3007 ± 229 and 3437 ± 77 cal BP in core 7. Preliminary results of core 7 yield C sequestration rate of 114 g C/m²yr and C stock of 583 MgC/ha prior to 3440 cal BP, and sequestration rate of 24 g C/m²yr with C stock of 800 MgC/ha after 3007 cal BP based on sedimentation rates (SR) of 0.2 cm/yr to 0.05 cm/yr, respectively. C/N and δ¹³C in sandy peat beneath sand deposit suggest organic matter sourced from C₃ plants of high marsh and peat beds above sand deposit indicate C₄ low marsh plants. The relatively thick deposit has at least 4 fining upward sequences and has been observed 200m from present mean shoreline, extending 1km across marsh, suggesting a possible tsunami signature. At 3500 cal BP sea level was about 2.8m below present therefore the sand was deposited about 310m inland from the paleo-shoreline. Based on annual SR of 0.2 cm/yr the 38cm thick deposit would cause a sedimentation hiatus of about 190 years where up to 817 MgC was not sequestered in this marsh, fitting within the sedimentation hiatus error margin. Ongoing work includes grain size analyses to quantitatively describe deposition and transport dynamics of the sand deposit and additional carbon isotope analyses to calculate more C sequestration and stock values.