Late Holocene evolution of a coupled, mud-dominated delta plain-chenier plain system, coastal Louisiana, USA

Abstract. Major deltas and their adjacent coastal plains are commonly linked by means of coast-parallel fluxes of water, sediment, and nutrients. Observations of the evolution of these interlinked systems over centennial to millennial timescales are essential to understand the interaction between point sources of sediment discharge (i.e. deltaic distributaries) and adjacent coastal plains across large spatial (i.e. hundreds of kilometres) scales. This information is needed to constrain future generations of numerical models to predict coastal evolution in relation to climate change and other human activities. Here we examine the coastal plain (Chenier Plain, CP) adjacent to the Mississippi River delta, one of the world's largest deltas. We use a refined chronology based on 22 new optically stimulated luminescence and 22 new radiocarbon ages to test the hypothesis that cyclic Mississippi subdelta shifting has influenced the evolution of the adjacent CP. We show that over the past 3 kyr, accumulation rates in the CP were generally 0–1 Mt yr−1. However, between 1.2 and 0.5 ka, when the Mississippi River shifted to a position more proximal to the CP, these rates increased to 2.9 ±1.1 Mt yr−1 or 0.5–1.5 % of the total sediment load of the Mississippi River. We conclude that CP evolution during the past 3 kyr was partly a direct consequence of shifting subdeltas, in addition to changing regional sediment sources and modest rates of relative sea-level (RSL) rise. The RSL history of the CP during this time period was constrained by new limiting data points from the base of overwash deposits associated with the cheniers. These findings have implications for Mississippi River sediment diversions that are currently being planned to restore portions of this vulnerable coast. Only if such diversions are located in the western portion of the Mississippi Delta plain could they potentially contribute to sustaining the CP shoreline. Our findings highlight the importance of a better understanding of mud-dominated shorelines that are often associated with major deltas, in light of the enormous investments in coastal management and restoration that will likely be made around the globe, now and especially later during this century. </jats:p

Connecting the backwater hydraulics of coastal rivers to fluvio-deltaic sedimentology and stratigraphy

Fluvial channels encounter a backwater reach when they approach a standing body of water, and recent studies have shown that the transition from normal flow to backwater-influenced flow is associated with sediment mass extraction through deposition. Here we test the hypothesis that systematic changes in the geometry of channel-belt deposits and sedimentary architecture occur across this transition, using data from the late Holocene Mississippi (southern USA) and Rhine (The Netherlands) fluvio-deltaic systems. We use the estimated backwater length and average channel width as characteristic length scales to non-dimensionalize the downstream trends in channel-belt width for these systems. The collapsed data follow similar trends, suggesting that the observed variations in channel-belt geometry and fluvio-deltaic stratigraphy are tied to the location of the backwater transition zone. These findings suggest a unifying hydraulic control on fluvio-deltaic channel belts and provide a new framework for predicting and understanding the properties of ancient rivers in the coastal zone

Supporting dataset for the paper: Does load-induced shallow subsidence inhibit delta growth?

The dataset includes stratigraphic information for boreholes of ten cross sections that were hand drilled from 2013-2015 CE in the bayhead region of the Lafourche subdelta, Mississippi Delta, USA. The study area spans ~6000 km2 and the cross sections are located near distributary channels. Data herein describe the location, surface elevation, and depth of each borehole as well as the depth relative to the surface of the mouth-bar to overbank (M-O) boundary and OSL ages for the mouth-bar deposits. We used these data to estimate centennial- to millennial-timescale cumulative subsidence and subsidence rates of a buried stratigraphic horizon, the M-O boundary, as detailed in the associated publication. Subsidence calculation methods are presented in the dataset, and subsidence data obtained from peat-top elevations at a relatively inland site (Paincourtville, LA, USA) are presented for comparison

Anatomy of Mississippi Delta growth and its implications for coastal restoration

Prehistoric rates of land gain in a large portion of the Mississippi Delta are significantly outpaced by present-day rates of land loss.</jats:p

Supporting dataset for the paper: Does load-induced shallow subsidence inhibit delta growth?

This is the supporting dataset for “Chamberlain, E.L., Shen, Z., Kim, W., Törnqvist, T.E., McKinley, S., & Anderson, S. Does load-induced shallow subsidence inhibit delta growth? In prep for Journal of Geophysical Research - Earth Surface”. Estimated submission date: March, 2021.The dataset includes stratigraphic information for boreholes of ten cross sections that were hand drilled from 2013-2015 CE in the bayhead region of the Lafourche subdelta, Mississippi Delta, USA. The study area spans ~6000 km2 and the cross sections are located near distributary channels. Data herein describe the location, surface elevation, and depth of each borehole as well as the depth relative to the surface of the mouth-bar to overbank (M-O) boundary and OSL ages for the mouth-bar deposits.We used these data to estimate centennial- to millennial-timescale cumulative subsidence and subsidence rates of a buried stratigraphic horizon, the M-O boundary, as detailed in the associated publication. Subsidence calculation methods are presented in the dataset, and subsidence data obtained from peat-top elevations at a relatively inland site (Paincourtville, LA, USA) are presented for comparison.</div

Supporting dataset for the paper: Does load-induced shallow subsidence inhibit delta growth?

This is the supporting dataset for “Chamberlain, E.L., Shen, Z., Kim, W., Törnqvist, T.E., McKinley, S., & Anderson, S. Does load-induced shallow subsidence inhibit delta growth? In prep for Journal of Geophysical Research - Earth Surface”. Estimated submission date: March, 2021.The dataset includes stratigraphic information for boreholes of ten cross sections that were hand drilled from 2013-2015 CE in the bayhead region of the Lafourche subdelta, Mississippi Delta, USA. The study area spans ~6000 km2 and the cross sections are located near distributary channels. Data herein describe the location, surface elevation, and depth of each borehole as well as the depth relative to the surface of the mouth-bar to overbank (M-O) boundary and OSL ages for the mouth-bar deposits.We used these data to estimate centennial- to millennial-timescale cumulative subsidence and subsidence rates of a buried stratigraphic horizon, the M-O boundary, as detailed in the associated publication. Subsidence calculation methods are presented in the dataset, and subsidence data obtained from peat-top elevations at a relatively inland site (Paincourtville, LA, USA) are presented for comparison.</div