Rates of Holocene Relative Sea-Level Rise and Differential Crustal Movements in the Mississippi Delta

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Rates of Holocene Relative Sea-level Rise and Differential Crustal Movements in the Mississippi DeltaTorbjorn TornqvistEAR 0074065Accelerated sea level rise due to global warming is expected to become a huge problem in the Mississippi Delta, where dramatic loss of coastal wetlands at rates of ~100 km2 per year is already taking place. To facilitate future wetland management strategies, a proper understanding of natural rates of sea-level rise and its spatial variability due to differential tectonic subsidence rates is indispensable. Recent studies in other major deltaic plains have shown that spatially nonuniform crustal movements can be considerable. Presently available sea-level data from the Mississippi Delta do not resolve differential tectonic subsidence rates, whereas most Holocene relative sea-level curves for the Gulf Coast exhibit a conspicuous, but commonly mutually conflicting, 'stair-step' pattern of alternating sea-level stillstands and rapid rises. The scale of these phenomena is an order of magnitude larger that the subtle (1 m), supposedly climate-related sea-level fluctuations that have been inferred elsewhere, and they are difficult to reconcile with widely accepted global sea-level data. Furthermore, recent work on the Texas Gulf Coast has proposed a middle Holocene sea-level highstand of 2 m above present sea level.This study will collect new sea-level indicators from the eastern and western margins of the Mississippi Delta for the last ~7000 years, using techniques that have so far not been applied to the Gulf Coast. The approach is based on the use of basal peats that overlie a compaction-free Pleistocene basement and can be related to mean sea level. Ecologically indicative plant macrofossils that can be related to mean sea level will be selected from basal-peat samples and 14C dated by accelerator mass spectrometry. In addition, stable carbon isotope ratios will be measured to preferentially select samples indicative of brackish or saline conditions.The premier outcome of this project will constitute new relative sea-level curves for the Mississippi Delta that will (1) assess the vertical resolution of basal peats as sea-level indicators and their potential for reconstructing subtle, climate-related sea-level rise; (2) test the validity of the inferred 'stair-step' pattern of Holocene sea-level rise; (3) determine the occurrence and magnitude of differential tectonic subsidence rates; and (4) establish whether a middle Holocene sea-level highstand is likely to have occurred on the Gulf Coast (if so, represented in this subsiding area by a distinct reduction in the rate of sea-level rise). The results will be important for future projections of coastal wetland loss in the Mississippi Delta.