Vertical Saltmarsh Accretion and its Relationship to Sea-level in the Severn Estuary, U.K.: An Investigation using Foraminifera as Tidal Indicators

Abstract

Saltmarsh vertical accretion has been used as a proxy for sea-level rise in previous studies on the assumption that accretion is in quasi-equilibrium with sea-level rise. This assumption requires verification in each case, as accretion may lag or exceed sea-level rise, leading to under- or over-estimates of sea-level rise, respectively. Verification may be provided by biostratigraphic analysis, where biological remains in saltmarsh sediments may be related to former tide levels. Foraminifera are well-suited, as studies of modern saltmarshes have demonstrated distinct tidal relationships which may be used to calibrate downcore/temporal foraminifera sequences. It is hypothesized that, depending on the relationship between sea-level rise and accretion, quasi-equilibrium, submergence, and emergence foraminifera sequences may be recognized. These are explored here in a Severn Estuary (U.K.) saltmarsh, where an emergence foraminifera sequence is identified, suggesting that previous sea-level rise rates for the Severn Estuary, based on saltmarsh accretion rates, may be over-estimates. Wider consequences of this may include over-estimation of regional crustal subsidence rates, required to accommodate artificially high sea-level rise rates. Saltmarsh accretion rates are clearly not a suitable proxy for sea-level rise in all cases, and as such each saltmarsh requires biostratigraphic evaluation to establish sea-level rise/accretion relationships. Preferably, independent Sea-Level Index Points (SLIPs) should be used over saltmarsh accretion rates in constructing sea-level histories.