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.