Reconstructing Holocene sea-level change from coastal freshwater peat: A combined empirical and model-based approach

• Small freshwater coastal wetlands provide new archives of Holocene sea-level change.
• Modelling shows that the marsh water table is linked to sea level.
• Marsh water table is controlled by stratigraphy, peat permeability and recharge rates.
• Five sea-level index points are added to the Holocene sea-level curve for SW England.

This paper presents a novel method to reconstruct sea-level change in coastal freshwater back-barrier marshes. Freshwater environments have long been considered to be unsuitable for the reconstruction of Holocene sea-level changes as they provide limiting, rather than precise, sea-level index points. We recorded the stratigraphy of a small beach and back-barrier coastal Phragmites (reed) marsh at Hallsands, south Devon, southwest England, using hand-drilled cores and ground-penetrating radar, and collected five new sea-level index points from the base of a Holocene peat sequence to refine the regional Holocene relative sea-level curve. We demonstrate that the samples, despite their freshwater origin, yield accurate sea-level index points as determined from the quantifiable relationship between tide levels and groundwater. By means of water-table monitoring and groundwater modelling we show that the primary controls on the water table in the marsh are: (1) stratigraphy; (2) peat permeability; and (3) recharge rates in the back-barrier marsh. The five index points document relative sea-level positions between 7200 and 2400 cal yr BP. Three points are in good agreement with previously collected regional data from intertidal deposits and two points usefully fill gaps in the existing reconstruction. An amended Holocene relative sea-level curve for south Devon, based on 30 data points, is presented. We conclude that the combined approach of data collection and modelling used in this paper can be applied to similar coastal settings around the world and allows the collection of sea-level index points from locations not previously thought suitable for this purpose.