Holocene vegetation history of the Mississippi River Delta: A regional synthesis

The modern Mississippi River Delta (MRD) contains extensive areas of wetlands with a distinct vegetational zonation determined by a north-to-south salinity gradient. This paper presents the first reconstruction of Holocene ecological history of the MRD based on a review and regional synthesis of new multi-proxy records from four wetland sites across these vegetation zones. During the past 6 ka, ecological development in the MRD was primarily driven by the geological processes of delta lobe switching, as the Mississippi River formed six delta lobes along the coast. The progradation of the St. Bernard delta lobe during 4.6-2.6 ka delivered abundant freshwater and sediments to the northern sites, rapidly changing the estuarine or marine-influenced interdistributary environment to a freshwater lake, swamp, or marsh. Freshwater wetlands were widespread in the MRD during 2.3-0.7 ka, before they were progressively transformed into the modern saline and intermediate marshes in the southern sites over the past 700 years due to relative sea level rise. The arrival of black mangroves and its proliferation during the past century marked the latest chapter in the Holocene ecological history of the MRD. The modern vegetation zones—as governed by the salinity gradient—are relatively young ecosystems, having developed at their present locations for only a few hundred years. Human activity has played an increasingly important role in affecting the ecosystem and environment during the past 3000 years. • Mississippi River delta lobe switching drove Holocene vegetation changes in MRD. • Freshwater wetlands were widespread in MRD during 2300-700 cal yr BP. • Modern salinity gradient and vegetation zonation were formed only centuries ago. • Wetland ecosystems can respond to changes in water and sediment supply quickly.