Annual and seasonal distribution of intertidal foraminifera and stable carbon isotope geochemistry, Bandon Marsh, Oregon, USA

Abstract

We investigated the influence of inter-annual and seasonal differences on the distribution of live and dead foraminifera, and the inter-annual variability of stable carbon isotopes (δ13C), total organic carbon (TOC) values and carbon to nitrogen (C/N) ratios in bulk sediments from intertidal environments of Bandon Marsh (Oregon, USA). Living and dead foraminiferal species from 10 stations were analyzed over two successive years in the summer (dry) and fall (wet) seasons. There were insignificant inter-annual and seasonal variations in the distribution of live and dead species. But there was a noticeable decrease in calcareous assemblages (Haynesina sp.) between live populations and dead assemblages, indicating that most of the calcareous tests were dissolved after burial; the agglutinated assemblages were comparable between constituents. The live populations and dead assemblages were dominated by Miliammina fusca in the tidal flat and low marsh, Jadammina macrescens, Trochammina in flat a and M. fusca in the high marsh, and Trochamminita irregularis and Balticammina pseudomacres-cens in the highest marsh to upland. Geochemical analyses (δ13C, TOC and C/N of bulk sedimentary organic matter) show no significant influence of inter-annual variations but a significant correlation of δ13C values (R = -0.820, p < 0.001), TOC values (R = 0.849, p < 0.001) and C/N ratios (R = 0.885, p < 0.001) to elevation with respect to the tidal frame. Our results suggest that foraminiferal assemblages and δ13C and TOC values, as well as C/N ratios, in Bandon Marsh are useful in reconstructing paleo sea-levels on the North American Pacific coast.