Comparison of Diatom, Total Carbon, and Grain Size Proxies for Sea-Level Reconstruction

Abstract

Sediment lithology and biological assemblages from low energy intertidal environments (tidal flat to salt-marsh then upland communities) are valuable archives of relative sea level (RSL) information. Sediment organic matter and grain size are often recorded in addition to microfossil data to aid environmental interpretation. This study aims to assess use of sediment organic matter and grain size as indicators of former tidal level to aid diatom based RSL reconstructions.
This study firstly investigates modern (top 1cm) sediments from Loch Laxford in northwest Scotland. Grain size analysis in these sediments shows local processes are overprinting the general expected pattern of decreasing grain size away from the sea. Analysis of the modern total carbon distribution shows there a linear increase in percentage total carbon with elevation (r=0.92) between the low marsh and high marsh. Comparison of the modern total carbon distribution at a contrasting site, Beluga Slough in Alaska, shows the linear relationship still exists, but with lower total carbon values for equivalent elevations. This shows the importance of location, and climate, for total carbon distribution.
Secondly, this study applies the modern Loch Laxford total carbon – elevation distribution to reconstruct paleo marsh surface elevation (PMSE) and RSL at Loch Laxford, and in an older sediment sequence from Mointeach Mhor in western Scotland. These are compared with diatom based reconstructions from the same sites. Decomposition complicates the use of organic carbon as a sea-level proxy but does appear to stabilise. At Loch Laxford, this occurs after approximately 100 years. Sediment grain size appears to influence the total carbon value and should also be investigated. Total carbon has most potential as a sea-level proxy in sediments from the last millennium, where a local modern distribution is available and it is unlikely that large changes in grain size or volume, or organic matter accumulation have occurred.