An attempt to reconcile subsidence rates determined from various techniques in southern Louisiana
Introduction
Our need to quantify the geologic and anthropogenic contributions to coastal subsidence has never been greater. Subsidence accelerates relative sea level rise and increases the vulnerability of coastal communities and natural resources to storm surge flooding. The remarkably destructive 2005 hurricane season caused over $100 billion dollars of damage in the northern Gulf of Mexico alone. The proportion of the population living in coastal counties now exceeds 50% in the United States (>150 million; Crossett et al., 2004). Many global population centers (e.g. New Orleans, Bangkok, Semarang, Jakarta, Seoul, Venice) are in low-lying coastal plains and have experienced notable subsidence (Abidin et al., 2001; Lee et al., 2005; Dixon et al., 2006; Phien-wej et al., 2006; Marfai and King, 2007). New datasets are becoming available (e.g. geodetic releveling, GPS, InSAR), often highlighting the spatial and temporal variability of subsidence rates from different measurement techniques (Teatini et al., 2007).
Stratigraphic, structural, and radiocarbon data leave no doubt that the Mississippi River delta in southern Louisiana subsided throughout the Quaternary. Geologic processes such as sediment compaction, faulting, salt movement, and lithospheric flexure have been active over those time scales, although their relative contributions during those timescales are poorly constrained. The relative contributions of various geologic and anthropogenic processes to historic and recent/current subsidence are vigorously debated, primarily because few data exist which quantify their contributions to total subsidence rates.
Data reviewed here quantify contributions from two processes considered influential to subsidence: natural sediment compaction and shallow groundwater withdrawal. Little work has investigated the impact of regional groundwater use on elevation change in the Louisiana delta plain, where much controversy over subsidence contributions persists. The range of subsidence rates that result from these two processes differ by at least an order of magnitude. Radiocarbon-based subsidence rates averaged over geologic time scales in the Louisiana coastal plain are similar to numerically modeled natural sediment compaction processes at similar time scales. Geodetic releveling subsidence rates since 1920 are consistently higher than both radiocarbon-based subsidence rates and modeled sediment compaction rates. A reconciliation of these disparate rates is desirable for understanding deltaic morphology evolution, relative sea level change, and for considering the magnitude and variability of the different processes that contribute to subsidence today and throughout the Quaternary in many coastal/deltaic settings.
Section snippets
Groundwater withdrawal data
Potential subsidence impacts from groundwater withdrawal are well known. Groundwater pumping at rates greater than aquifer recharge rates over extended periods of time can result in land surface subsidence due to aquifer compaction (Wilson and Grace, 1942). Subsidence from groundwater use is documented in central Louisiana (Nunn, 2003), with modeled subsidence rates of 1–18 mm/yr near Baton Rouge. Yet direct observations of subsidence rates associated only with this process throughout the delta
Results
Fig. 2 compares the subsidence rate distributions for the four different datasets reviewed above. Timescales over which these data apply range from geologic timescales (1000s of years; radiocarbon and modeled sediment compaction) to recent decades (groundwater withdrawal and geodetic rates). Radiocarbon-based subsidence rates are in the range of modeled compaction rates, suggesting that compaction processes have significantly influenced subsidence during time scales recorded by radiocarbon data
Discussion
Data presented here emphasize that recently observed geodetic leveling subsidence rates in southern Louisiana are regionally higher than historic (radiocarbon) subsidence rates from geologic processes. Unfortunately, data presented do not allow the dominant process contributing to recent regional geodetic subsidence rates in coastal Louisiana to be directly identified. This manuscript emphasizes the difficulty in reconciling geodetic releveling rates with documented processes and available
Conclusions
Geodetic subsidence rates observed since 1920 are an order of magnitude higher than both radiocarbon-based subsidence rates and modeled shallow compaction rates. Differences between historic subsidence rates at geologic time scales and recent subsidence rates appear to require explanations beyond typical geologic processes. Such explanations must occur over a broad region (1000s of km2) and have initiated relatively recently (decades to 100s of years). Radiometric data may incorporate short
Acknowledgments
M. Kasmarek (USGS, Houston) provided the raw extensometer data for the Houston area. Helpful reviews were provided by J. Gibeaut and reviewers. The author wishes to acknowledge the USGS Mendenhall program, and especially S. J. Williams, U. ten Brink, and R. Kotra for their support during this research.
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