Salt Marsh Elevation Limit Determined after Subsidence from Hydrologic Change and Hydrocarbon Extraction

Abstract

Levee construction aboveground and hydrocarbon removal from belowground in coastal wetlands can create hydrologic changes that increase plant stress through flooding. But the significance of the subsidence they cause individually or in combination is contested. This study untangled them to demonstrate elevational limits of salt marshes by studying dredged and natural waterways in two salt marshes in Louisiana, USA. The areas had a homogenous plant cover before drilling for oil and gas extraction peaked in the 1960s, and now are a mixed network of natural waterways and dredged canals used to drill wells with an average drill date of 1965.8 ± 2.7 (µ ± 1 SEM; n = 18) and well depth of 4661.0 m ± 56.6 (µ ± 1 SEM; n = 18). Aerial imagery was used to document how canals widened to become 2 to 4 times larger than their original construction width at the high production site and 50% larger at the low production site, whereas increases at the nearby natural channels were much less. Light detection and ranging (LIDAR) measurements at the high production site from 2002 showed that the marsh surface near wells subsided by 34 cm compared to undredged sites. Elevation in marshes at producing and dry wells were equal at the low production site, but high production well locations were even lower than at dry wells. An elevation vs. percent open water curve developed from these data overlapped with an independent analysis of a brackish marsh. A relative subsidence rate between 7.4 to 10.4 mm y−1 transformed these salt marshes to an open water habitat within a few decades. The local creation of accommodation space through hydrocarbon removal and leveed wetlands is a parsimonious explanation for the spatial and temporal land loss rates on this deltaic coast over the last 80 years of oil and gas exploration. Substantial losses from the accelerating rates of sea level rise are indicated to occur before 2050.

Wetland Restoration Progress 39 Years After Canal Backfilling

“…Migratory fish do not have access to the canal, but grassbeds may be more abundant there that birds may be attracted to Neill and Turner, 1987a,b. A plug, however, blocks boat access which may help reduce erosion because of boat wakes at the canal's entrance (Turner and Mo, 2021).…”

“…Restoring wetlands is focused on mitigating these blockages to the water flow above-and belowground because they create both longer flooding periods and longer drying cycles; as a result a dose-response relationship between canals and land losses occurs over time and space (Turner and McClenachan, 2018). The volume of oil and gas extracted thousands of meters beneath the canal can be high enough to cause soil subsidence at the surface that results in unhealthy flooding of emergent plants (Stagg et al, 2019;Turner and Mo, 2021). A logical case can be made, based on these cause-andeffect relationships and others, that canals and their levees, and the fluid extraction made possible beneath them are a major cause of Louisiana's coastal wetlands from the 1930s to the present (Turner and McClenachan, 2018).…”

Geological evolution of the Mississippi River into the Anthropocene

“…Subsidence in Louisiana’s wetlands is rapid at 9 ± 1 mm per year (Jankowski et al, 2017; Nienhuis et al, 2017), which is expected as a natural part of the delta lobe cycle (Yuill et al, 2009). However, continued hydrocarbon extraction, and the position of shipping canals in the marshes, has changed the hydrology of the modern delta, resulting in prolonged lower ground water level, which contributes to more rapid shallow subsidence than would be natural (Nienhuis et al, 2020; Turner and Mo, 2021).…”

Satellite-Derived Trends in Inundation Frequency Reveal the Fate of Saltmarshes

“…Nevertheless, some studies raise concerns that saltmarshes may experience habitat loss due to high rates of sea level rise (SLR) in combination with decreasing sediment availability [(for example due to damming of rivers; see Grill et al (2019)], so that vertical sediment accretion may not be sufficient to keep pace with global SLR (Reed, 1995;Weston, 2014). In some areas, subsidence caused by water, oil, or gas extraction adds to drowning of the saltmarsh (Turner and Mo, 2020). On the other hand, Kirwan et al (2016) showed in their meta-analysis that saltmarshes are more resilient to SLR than widely assumed.…”