A multi-proxy modern training set for reconstructing Holocene relative sea level using salt-marsh sediment (Prince Edward Island, Canada)

• Published in: Canadian journal of earth sciences Vol. 62; no. 6; pp. 1172 - 1190
• Main Authors: Kemp, Andrew C., Edwards, Robin J., Alvarez-Agoues, Fermin, Bustamante, Emmanuel, Roseby, Zoë A., Hawkes, Andrea D., Woodworth, Philip L.
• Format: Journal Article
• Language: English
• Published: Ottawa Canadian Science Publishing NRC Research Press 01.06.2025
• Subjects: Anthropogenic factors; Carbon 13; Datasets; Diurnal; Foraminifera; Holocene; Plants (botany); Salt marshes; Saltmarshes; Sea level; Sea level changes; Sediment; Sediment samplers; Sediment samples; Sediments; Soil erosion; Standardization; Tidal range; Tidal regimes; Training; Vegetation; Zonation; Canada; Prince Edward Island Canada
• ISSN: 0008-4077; 1480-3313
• DOI: 10.1139/cjes-2024-0166

Reconstructing Holocene relative sea-level change from salt-marsh sediment requires a modern training set that captures the observable relationship between proxies and local tidal elevation. We collected 143 surface sediment samples from four salt marshes in Prince Edward Island (PEI, Canada) to develop a modern training set of foraminifera and bulk-sediment δ 13 C and δ 15 N values. Two sites have semi-diurnal tidal regimes, one has a mixed regime, and one has a diurnal regime. Combining and standardizing datasets from different tidal regimes fails to adequately characterize the relationship between elevation and inundation. Our principal results are from the semi-diurnal sites. Salt-marsh foraminifera in PEI form low-diversity, elevation-dependent zones that are replicated within sites, among sites, and between two studies conducted 44 years apart. Botanical zonation of PEI salt marshes often includes a platform with C 4 plants and an adjacent, higher transitional community of C 3 plants. Bulk-sediment δ 13 C values mirror the dominant vegetation. Isotopic results from PEI are combined with datasets from Delaware to Massachusetts to explore how elevation thresholds and δ 13 C cut-off values can recognize environments of deposition. We propose that δ 13 C values more negative than –20.0‰ indicate formation above an elevation threshold placed slightly below mean higher high water, while δ 13 C values less negative than –17.0‰ characterize environments lower than an elevation threshold slightly above mean higher high water. Bulk-sediment δ 15 N values display a correlation with tidal elevation and botanical zone, but may be subject to anthropogenic and post-depositional modification limiting their utility as a sea-level proxy. The dataset of modern foraminifera is suitable for reconstructing relative sea level and could be combined with informative δ 13 C values in a multi-proxy approach.