Vertical zonations of marsh foraminifera as accurate indicators of former sea-levels

Abstract

MANY attempts to reconstruct the pattern of apparent sea-level rise that took place during the Holocene have used salt-marsh deposits as indicators of former sea level^1,2. Contemporary mature salt marshes occupy a vertical range covering the upper half of the tidal range, from mean sea level to higher high water^3,4, and geologists have arbitrarily equated ancient marsh deposits with various fixed elevations within this range. This method, disregarding any other source of inaccuracy, can introduce an error of 1–5 m depending on the tidal range. Apparent sea level movements during the late Holocene (2,000–3,000 yr BP) have been of the order of 1–2 m; hence such a method introduces errors larger than the movements being measured. Even less adequate sea-level estimates are derived from other commonly used indicators such as raised beaches, oyster beds, and submerged or emerged terraces and deltas. We describe here how marsh foraminifera can be used to delimit small-scale vertical zones along modern marsh surfaces generally corresponding to the floral zones in vertical range. This relationship was first established in the marshes of southern California⁵, and has recently been shown to be valid in other areas⁶.