INVESTIGATING THE UTILITY OF FORAMINIFERAL ISOTOPE PROXIES FOR SEA-LEVEL VARIABILITY IN THE BUENOS AIRES PROVINCE, ARGENTINA

The utility of sediment and foraminiferal isotopic proxies are tested for developing relative sea-level (RSL) records in coastal central Argentina. Single and multi- proxy reconstructions were investigated based on foraminiferal δ¹³C and δ¹⁸O signatures and bulk sedimentary δ¹³C compositions with respect to modern marsh elevations. An absence of agglutinated foraminifera commonly used for RSL reconstructions necessitated alternative proxy development. Ammonia parkinsoniana δ¹³C and δ¹⁸O values produced the strongest linear correlations with modern marsh elevation compared to other species or sediment isotopic composition. There was positive covariance between the δ¹³C and δ¹⁸O signatures defining the parameters of four marsh zones with elevation (tidal flat, low marsh, mid-marsh, and high marsh). However, initial tests show that the isotopic range of each of the four zones varies by site making the proxy site specific. This site specificity is likely due to changes in vital effects (respiration), microhabitat effects related to pore water composition (pH, OM degradation), or a combination of both.

When applied to sediment cores, the proxy reconstruction approaches (δ¹³C, δ¹⁸O, and multi) record a sea-level regression <3m over the last 4,000 years, agreeing with and refining previously published RSL records in the region. An abrupt marsh elevation increase in the last ~40 years is hypothesized to be a result of increased human activities in the Bahía Blanca estuary, such as dredging, maritime and urban development, and river divergence. The exposure of eroded or barren mudflats facilitated the expansion of S. alterniflora marshes and produced an apparent relative sea-level drop in the recent sedimentary record contrary to tide gauge data.