The input of foraminiferal infaunal populations to sub-fossil assemblages along an elevational gradient in a salt marsh: application to sea-level studies in the mid-Atlantic coast of North America

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The suitability of marsh sites for sea-level studies was examined based on a field study along a transect from high to low marsh. Living foraminifera at Bombay Hook (Delaware, USA) are considered to be shallow infaunal (i.e., uppermost 10 cm). Peak concentrations were found at 1–10 cm in the high marsh, 1–5 cm in the intermediate marsh, and 3–5 cm in the low marsh. However, sporadic deep infaunal inputs in the low marsh could significantly contribute to the sub-fossil assemblage. In the upper 10 cm buried (death + sub-fossil) and living assemblages showed a strong correlation, and the seasonal pattern of the buried assemblage paralleled the living one, suggesting that the buried assemblage reflected the most recent reproductive inputs. The cumulative standing crop of each dominant species was used to estimate their contribution to the buried assemblage in order to assess if the community is vertically homogeneous and, therefore, if the infaunal production causes differential sub-surface enrichment. The results showed that a “shallow” (0–5 cm) infaunal contribution is able to explain much of the sub-fossil assemblage beneath the surface in the high and intermediate marsh plots. However, in the low marsh plot, the deep infaunal contribution was greatest and significantly affected the sub-fossil assemblage. Therefore, modern analogues for sea-level studies in mid-Atlantic North American marshes should include the uppermost 5 cm. The interval proposed for the high and intermediate marsh is thin enough that epifaunal species are not underrepresented and encompasses only ~8 years (based on burial rates) providing a high temporal resolution.