Research ArticleENVIRONMENTAL STUDIES

Anatomy of Mississippi Delta growth and its implications for coastal restoration

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Science Advances  11 Apr 2018:
Vol. 4, no. 4, eaar4740
DOI: 10.1126/sciadv.aar4740
  • Fig. 1 Major past and present paths of the Mississippi River.

    (A) Channel belts and avulsion sites in the Lower Mississippi Valley and Mississippi Delta (for location, see inset), after the study of Saucier (43). (B) Mississippi Delta, including the Lafourche subdelta, the Modern (Balize) subdelta with the birdfoot delta, and the Atchafalaya subdelta with the Wax Lake and Atchafalaya deltas. Trunk channels that feed these subdeltas branch into multiple distributaries at polyfurcation points, which define the landward limit of bayhead deltas. The two most recent deltaic avulsion sites are the Lafourche-Modern (L-M) and Modern-Atchafalaya (M-A) avulsions. Previous work (18) was conducted at Paincourtville (PV) and Napoleonville (NV). (C) Location of cross sections, with distance in river kilometers from the Lafourche subdelta polyfurcation point shown in parentheses.

  • Fig. 2 Schematic illustration of the stratigraphy associated with bayhead delta progradation and aggradation.

    Red, sand (S); yellow, silt (Si); green, clay (Cl); OK, overbank deposits; MB, mouth bar deposits; DF, delta front deposits; BF, bay floor deposits.

  • Fig. 3 Cross sections illustrating the stratigraphy adjacent to Bayou Lafourche.

    (A) Example of a cross section perpendicular to the main distributary at Galliano; additional cross sections are shown in the Supplementary Materials (fig. S1). Location and orientation of cross sections are shown in Fig. 1 (B and C). (B) Cross section parallel to the main distributary of the Lafourche subdelta. Deposits underlying the Lafourche bayhead delta that formed in a subaerial setting are referred to as “Pre-Lafourche.” In (B), weighted mean OSL ages and average sample depths are shown for locations seaward of the polyfurcation point, and the chronology for the PV I and NV II boreholes is from the previous studies (18, 20). Note that the uppermost portion of the overbank unit is highly generalized; for details, see the study of Shen et al. (18). All ages are presented as thousands of years (ka) ago, relative to 2010.

  • Fig. 4 Downstream trend in the thickness of lithogenetic units.

    The average thickness of the MB and DF deposits for each transect is plotted against distance with reference to the Lafourche polyfurcation point (Fig. 1C). The thickness of the bayhead delta (BD) and foundation (FN) strata is also shown. The colored horizontal lines show the average thickness of the MB, DF, and FN deposits, and the orange line indicates the trend in thickness of the BD deposits. See the Supplementary Materials (fig. S2 and table S2) for uncertainties on the average thickness.

  • Fig. 5 Growth history of the Lafourche bayhead delta.

    (A) Weighted mean OSL ages determine the timing of new land creation through progradation of the bayhead delta of the Lafourche subdelta (inset). The bayhead delta is bounded to the north and west by the paleo-shoreline, to the south by transgressive Lafourche barrier islands, and to the east by open water (interdistributary lakes). (B) The progradation history of MB deposits associated with the main channel (filled symbols) and lesser distributaries (open symbols) is fit with a linear regression (r2 = 0.89).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/4/eaar4740/DC1

    Stratigraphic data for all cross sections

    Lithogenetic unit thickness calculation

    OSL dating approach

    Sample exclusions and additions to analyses

    Cleaning of outlying aliquots

    Sample rejection

    Comparison with previous OSL approach

    fig. S1. Cross sections illustrating the stratigraphy and OSL ages for all study sites.

    fig. S2. Thickness of lithogenetic units at main and lesser distributary cross sections.

    fig. S3. Comparison of mouth bar sand ages estimated using two approaches.

    table S1. Characterization of lithogenetic units.

    table S2. Lithogenetic unit thickness.

    table S3. Details of the SAR protocol.

    table S4. Overdispersion details, laboratory code, and OSL sample collection year, location, and depth.

    table S5. Dose rate details and paleodose.

    table S6. Experimental details of the OSL approach used in the present study versus the approach used by previous studies.

    table S7. Comparison of OSL ages estimated with two approaches.

    References (64, 65)

  • Supplementary Materials

    This PDF file includes:

    • Stratigraphic data for all cross sections
    • Lithogenetic unit thickness calculation
    • OSL dating approach
    • Sample exclusions and additions to analyses
    • Cleaning of outlying aliquots
    • Sample rejection
    • Comparison with previous OSL approach
    • fig. S1. Cross sections illustrating the stratigraphy and OSL ages for all study sites.
    • fig. S2. Thickness of lithogenetic units at main and lesser distributary cross sections.
    • fig. S3. Comparison of mouth bar sand ages estimated using two approaches.
    • table S1. Characterization of lithogenetic units.
    • table S2. Lithogenetic unit thickness.
    • table S3. Details of the SAR protocol.
    • table S4. Overdispersion details, laboratory code, and OSL sample collection year, location, and depth.
    • table S5. Dose rate details and paleodose.
    • table S6. Experimental details of the OSL approach used in the present study versus the approach used by previous studies.
    • table S7. Comparison of OSL ages estimated with two approaches.
    • References (64, 65)

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