Coastal marshes are threatened by relative sea-level (RSL) rise, yet recent studies predict marsh survival even under the high rates of RSL rise expected later in this century. However, because these studies are mostly based on short-term records, uncertainty persists about the longer-term vulnerability of coastal marshes. We present an 8500-year-long marsh record from the Mississippi Delta, showing that at rates of RSL rise exceeding 6 to 9 mm year−1, marsh conversion into open water occurs in about 50 years. At rates of RSL rise exceeding ~3 mm year−1, marsh drowning occurs within a few centuries. Because present-day rates of global sea-level rise already surpass this rate, submergence of the remaining ~15,000 km2 of marshland in coastal Louisiana is probably inevitable. RSL-driven tipping points for marsh drowning vary geographically, and those for the Mississippi Delta may be lower than elsewhere. Nevertheless, our findings highlight the need for consideration of longer time windows in determining the vulnerability of coastal marshes worldwide.

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Millennial-scale climate variability has been increasingly recognized as one of the most prominent features of the Holocene. However, regional responses, especially in terms of moisture conditions, are poorly documented and understood. Here we present lithologic and magnetic evidence from White Lake in northern New Jersey, USA, to show that low lake levels occurred at about 1.3, 3.0, 4.4 and 6.1 ka (1 ka0/1000 cal. yr BP). The low lake levels are indicated by heterogeneous coarse calcareous sediment layers showing strong magnetic intensities. These detrital layers were likely formed during low stands when shallow-water marls were exposed, oxidized, transported and redeposited. This model is supported by laboratory experiments showing that oxidation of marls can enhance magnetic intensities. The dry periods inferred from the low lake levels of White Lake appear to occur concurrently with the cold periods recorded in the North Atlantic sediments. The correlation between millennial-scale dry/wet cycles inferred from lake-level fluctuations of White Lake and cold/warm cycles in North Atlantic sediments suggests sensitive moisture responses to Holocene millennial-scale climate variability. The dry-cold (or wetwarm) association is supported by instrumental records of the last century showing that the Mid-Atlantic region was dominated by wet conditions, while most parts of the conterminous USA experienced droughts, when the North Atlantic Ocean was warm. The consistent moisture responses of the Mid-Atlantic region to temperature changes of the North Atlantic Ocean may have persisted for the past 6000 years.

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