Flooded mangroves as a record of sediment accretion and carbon burial rates during Holocene sea-level rise in Rio Lagartos, northern Yucatan Peninsula, Mexico

Coastal zones are dynamic regions that change over time because of both natural processes and human activities. Such changes occur over different time scales, from days (e.g., those caused by hurricanes) to millennia (e.g., those caused by sea level rise). Sediments in these zones preserve information about past environmental changes, which can be inferred from shifts in the composition and accumulation rate of accrued deposits. We investigated the transformation of an intertidal zone dominated by mangroves into an underwater system characterized by carbonate accumulation. We used stratigraphic records of sediment cores to estimate organic carbon burial rates during the intertidal phase (flooded mangrove), which existed more than 3000 years ago. We collected sediment sequences using a piston corer in the eastern sector of the coastal Río Lagartos Lagoon, northern Yucatán Peninsula, Mexico, which, because of its shallow depth and morphology, is very sensitive to drivers of change. Organic matter was relatively higher at depth in the core (15-50 cm), but upper sediments (0-15 cm) were dominated by carbonate. AMS ¹⁴C ages on total organic matter were obtained at two depths. The ¹⁴C age at 40-45 (3498 ± 35 years BP) was only slightly older than the age at 25-30 cm (3312 ± 35 years BP). The mean organic carbon (OC) burial rate in the interval 25-40 cm was 53 g m^-2 yr^-1, a value in the range reported for recent OC accumulation in mangroves of Yucatán. Based on the age of the 25-30 cm section and the current water depth in the area (~1 m), we estimate an increase in the sea-level rise of 0.4 mm yr^-1 over the last 3000 years, lower than values reported for the area using estimates of recent accretion of sediments. This work highlights the high OC burial rates in ancient mangroves, which today are flooded and are areas that are accumulating carbonate. Our findings illustrate the importance of conserving mangrove-dominated lagoon systems on the Yucatán Peninsula, which are important carbon sinks.