A New Subsidence Map for Coastal Louisiana

Nienhuis, Jaap H.; Törnqvist, Torbjörn E.; Jankowski, K. L.; Fernandes, Anjali M.; Keogh, M.

doi:10.1130/gsatg337gw.1

Cited by 31 publications

(31 citation statements)

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“…We concur with Blum and Roberts (), Shen et al. (), Nienhuis, Törnqvist, Jankowski, Fernandes, and Keogh (), and Jankowski et al. () that, over the century‐scale time period of relevance to Mississippi Delta survival, land‐surface subsidence is dominated by shallow‐seated processes (sediment compaction and dewatering) and that deep‐seated processes (such as growth fault motion) generally play a secondary role.…”

Section: Discussionsupporting

confidence: 90%

Resolving the contributing factors to Mississippi Delta subsidence: Past and Present

et al. 2018

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To date, quantification of individual components that contribute to shallow and deep‐seated subsidence in passive margin deltas worldwide has proven problematic. A new, regional gridded chronostratigraphic dataset for the Lower Mississippi Delta region, derived from 80,928 well reports across the northern Gulf of Mexico (GOM), has bridged the disparity between geodetic mean rates measuring total land surface subsidence across annual‐to‐decadal timescales and the deep‐seated stratigraphic subsidence rates that record isostatic response over timescales of >104 years. Through a quantitative assessment of gridded chronostratigraphic surfaces, sections, and subsidence rates extending from the Middle Pleistocene (0.58 Ma) to the Late Pliocene (3.85 Ma), we identify both temporal and spatial variability in deep‐seated subsidence across the northern GOM. Targeted deep‐seated subsidence data extracted across prior GOM Holocene sea‐level sample locations have revealed more than an order of magnitude greater rates of isostatic compensation in the Mississippi depocentre versus similar GOM sea‐level control sites in Florida and Alabama, casting doubt on efforts towards a representative Holocene sea‐level curve. Spatial variability in subsidence was also assessed locally in both the strike and dip directions to assess the contributions of growth faults. Fault throw displacement magnitude was discovered to decrease with depth, accounting for less than half of the total deep‐seated subsidence record of the Middle Pleistocene. Temporal subsidence complexities were also revealed including a direct, inverse logarithmic relationship between subsidence rate and time indicating variable subsidence component controls across different timescales. Despite the spatial and temporal complexities, this dataset serves as the first regional baseline for deep‐seated subsidence rates across the northern GOM.

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Section: Discussionsupporting

confidence: 90%

Resolving the contributing factors to Mississippi Delta subsidence: Past and Present

et al. 2018

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“…In addition, local land movements, caused by processes such as shallow subsidence or fluid withdrawal, can lead to more rapid submergence of the land surface in some geographic locations. For example, high rates of subsidence (9 mm/year) in the Mississippi River Delta (MRD) (Nienhuis, Tornqvist, Jankowski, Fernandez & Keogh, ), combined with a change in ocean height, yield submergence rates greater than 1 cm/year. To counterbalance the combined effects of SLR and local land movement (i.e., relative SLR), soil surfaces in coastal wetlands must build vertically at an equivalent rate.…”

Section: Introductionmentioning

confidence: 99%

Will fluctuations in salt marsh–mangrove dominance alter vulnerability of a subtropical wetland to sea‐level rise?

McKee

Vervaeke

2017

Global Change Biology

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To avoid submergence during sea-level rise, coastal wetlands build soil surfaces vertically through accumulation of inorganic sediment and organic matter. At climatic boundaries where mangroves are expanding and replacing salt marsh, wetland capacity to respond to sea-level rise may change. To compare how well mangroves and salt marshes accommodate sea-level rise, we conducted a manipulative field experiment in a subtropical plant community in the subsiding Mississippi River Delta. Experimental plots were established in spatially equivalent positions along creek banks in monospecific stands of Spartina alterniflora (smooth cordgrass) or Avicennia germinans (black mangrove) and in mixed stands containing both species. To examine the effect of disturbance on elevation dynamics, vegetation in half of the plots was subjected to freezing (mangrove) or wrack burial (salt marsh), which caused shoot mortality. Vertical soil development was monitored for 6 years with the surface elevation table-marker horizon system. Comparison of land movement with relative sea-level rise showed that this plant community was experiencing an elevation deficit (i.e., sea level was rising faster than the wetland was building vertically) and was relying on elevation capital (i.e., relative position in the tidal frame) to survive. Although Avicennia plots had more elevation capital, suggesting longer survival, than Spartina or mixed plots, vegetation type had no effect on rates of accretion, vertical movement in root and sub-root zones, or net elevation change. Thus, these salt marsh and mangrove assemblages were accreting sediment and building vertically at equivalent rates. Small-scale disturbance of the plant canopy also had no effect on elevation trajectories-contrary to work in peat-forming wetlands showing elevation responses to changes in plant productivity. The findings indicate that in this deltaic setting with strong physical influences controlling elevation (sediment accretion, subsidence), mangrove replacement of salt marsh, with or without disturbance, will not necessarily alter vulnerability to sea-level rise.

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“…All data to reproduce these findings are available online. The subsidence map can be found at: https://osf.io/ m83z4/ (last access: 1 August 2019; Nienhuis et al, 2017). CRMS measurement station data of coastal Louisiana wetland hydrology and soils can be retrieved from: https://lacoast.gov/crms/ (last access: 1 August 2019; Steyer et al, 2003).…”

Section: Discussionmentioning

confidence: 99%

“…Many low-lying coastal areas including coastal Louisiana experience elevated rates of relative sea-level rise because of subsidence Minderhoud et al, 2018;Nienhuis et al, 2017;Teatini et al, 2011). Recent data collection through the Coastal Reference Monitoring System (Steyer et al, 2003; https://lacoast.gov/crms/, last access: 1 August 2019) ( Fig.…”

Section: Introductionmentioning

confidence: 99%

“…Recent data collection through the Coastal Reference Monitoring System (Steyer et al, 2003; https://lacoast.gov/crms/, last access: 1 August 2019) ( Fig. 1a) has allowed for unprecedented analyses of subsidence rates and patterns in coastal Louisiana (e.g., Jankowski et al, 2017;Nienhuis et al, 2017;Sanks et al, 2019), but subsidence mechanisms are still poorly understood. Subsidence at depth is likely a combination of isostasy and fault movement (Frederick et al, 2019), but tends to be much slower compared to subsidence observed at the surface (Frederick et al, 2019;Wolstencroft et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

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Altered surface hydrology as a potential mechanism for subsidence in coastal Louisiana

Nienhuis

Törnqvist²,

Erkens

2020

Proc. IAHS

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The natural wetlands of coastal Louisiana are experiencing rapid subsidence rates averaging 9 ± 1 mm yr −1 . Recent measurements based on GPS data and CRMS surface elevation tables (SETs) have shown that most of the subsidence is shallow and occurs in the uppermost 5 meters. Sources of subsidence and the origin of their spatial variability are strongly debated. Here we use CRMS SETs together with historic maps of coastal Louisiana to explore two hypotheses: (i) shallow subsidence is a result of accommodation created by (long-term) deep subsidence processes and self-weight consolidation, and (ii) changes in marsh hydrology (groundwater and surface water flows) have led to a recent increase in shallow subsidence.First, we find that, although self-weight consolidation would result in generally high observed shallow subsidence rates, it does not explain the rates nor the spatial variability of the CRMS SET data. Second, based on historic maps, we find that shallow subsidence rates are significantly higher for CRMS sites where shipping canals have reduced their distance to the marsh edge. This is potentially a result from increased sediment deposition, but CRMS data also show altered groundwater levels near the marsh edge. We find some indication that prolonged periods of low water could have led to increases in effective stresses that explain some of the rapid rates of shallow subsidence observed along Louisiana's coastline.

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A New Subsidence Map for Coastal Louisiana

Cited by 31 publications

References 10 publications

Resolving the contributing factors to Mississippi Delta subsidence: Past and Present

Resolving the contributing factors to Mississippi Delta subsidence: Past and Present

Will fluctuations in salt marsh–mangrove dominance alter vulnerability of a subtropical wetland to sea‐level rise?

Altered surface hydrology as a potential mechanism for subsidence in coastal Louisiana

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