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Tipping points of Mississippi Delta marshes due to accelerated sea-level rise

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Science Advances  22 May 2020:
Vol. 6, no. 21, eaaz5512
DOI: 10.1126/sciadv.aaz5512
  • Fig. 1 RSL curve and rate of RSL rise in the MD.

    A total of 72 sea-level index points (SLIPs) from basal marsh peat provides the Holocene RSL history with respect to the compaction-free, (mostly) Pleistocene basement. The location of the study areas is shown in Fig. 2. The gap between ~3.0 and 1.5 ka BP (before present) is merely due to the fact that this time interval was not sampled for 14C dating; it should not be interpreted as a phase with a rate of RSL rise that deviates from the long-term trend. Note also that the youngest SLIP predates the global sea-level acceleration of the past century (not shown in this record). All other (non-SLIP) data points are shown by means of the small black symbols that elucidate the shape of the function that was fitted to the SLIP data. The first derivative of the RSL curve exhibits rates of RSL rise between 8.5 and 0.15 ka BP.

  • Fig. 2 Distribution of the 355 boreholes across the MD that were used for the paleo-marsh analysis.

    Note that the distance between the two study areas (A and B) is about 100 km and that boreholes within these two regions are scattered up to 40 km apart. This circumvents sampling bias associated with local conditions that may not be representative for a wider region. The location of the 15 boreholes illustrated in Fig. 3 is shown in orange or red. Base map data: Google, SIO, NOAA, U.S. Navy, NG, GEBCO, Landsat/Copernicus, Terrametrics.

  • Fig. 3 Characteristic transgressive facies successions from the past 8500 years.

    A selection of 15 representative sedimentary logs features the transition from a wetland paleosol to basal Holocene facies and the overlying succession (details about facies units are provided in table S1). For ease of comparison, the logs have been adjusted vertically to align the top of the paleosol and arranged with increasing age (and depth) from left to right [in reality, the elevation of the top of the paleosol decreases from ~0.5 to 16 m below North American Vertical Datum of 1988 (NAVD 88); see Supplementary Spreadsheet]. The serrated pattern is used for logs that do not extend all the way to the land surface. For each log, the weighted mean calibrated age (in ka BP) of the basal facies that drape the paleosol and the rate of RSL rise (the first derivative of the equation in Fig. 1) is provided. Directly dated cores are indicated in red; for the other cores, the age of the basal Holocene facies was calculated by means of the equation in Fig. 1. Note that the two logs marked by arrows with largely similar stratigraphy are ~100 km apart (all core locations are shown in Fig. 2). Additional sedimentary logs are shown for the western and eastern study areas separately in figs. S1 and S2.

  • Fig. 4 Relationship between the rate of RSL rise and marsh persistence.

    The relative proportion of facies successions (table S2) is determined for eight increments of rates of RSL rise to identify the tipping point for marsh drowning. The abrupt increase in the proportion of drowned sites between 2.5 and 3.0 mm year−1 illustrates the nonlinear nature of the marsh response to the rate of RSL rise. Similar plots are shown for the western and eastern study areas separately (fig. S4) and for basal marsh successions ≥50 cm thick (fig. S5).

  • Table 1 Thickness and estimated life span of drowned marsh facies.

    Drowned marsh facies—all sites*Drowned marsh facies—selected sites†
    Median (mean)
    thickness (m)
    Median (mean)
    life span (years)‡
    Number of
    observations
    Median (mean)
    thickness (m)
    Median (mean)
    life span (years)‡
    Number of
    observations
    Mostly gradual drowning
    (after 8.2 ka BP)
    0.30 (0.43)212 (282)§510.50 (0.58)310 (372)§25
    Mostly rapid drowning
    (before 8.2 ka BP)
    0.10 (0.18)25 (50)170.15 (0.34)46 (98)8

    *All sites (n = 68) that feature lagoonal facies, including those where the contact between terrestrial (usually marsh) and lagoonal facies is erosional.

    †Selected sites (n = 33) where the terrestrial to lagoonal facies contact is less likely to be erosional.

    ‡Estimated by correcting the thickness for compaction (a factor of 2.5) and assuming that the accretion rate approximately tracked the rate of RSL rise at the onset of marsh formation.

    §Note that mean values always exceed median values (typically due to one or a few sites with relatively large numbers); therefore, median values are used in the interpretation.

    Supplementary Materials

    • Supplementary Materials

      Tipping points of Mississippi Delta marshes due to accelerated sea-level rise

      Torbjörn E. Törnqvist, Krista L. Jankowski, Yong-Xiang Li, Juan L. González

      Download Supplement

      The PDF file includes:

      • Figs. S1 to S5
      • Tables S1 to S4
      • Legend for supplementary spreadsheet
      • References

      Other Supplementary Material for this manuscript includes the following:

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