Saltmarsh and Estuary evolution since 8000 yr BP in the northeastern coastal plain of Argentina (35.5°S)

I. VILANOVA; B. LYNN INGRAM; FRANCES MALAMUD-ROAM; WENBO YANG; ALDO R. PRIETO

Viña del Mar

Congreso; International Geological Congress on the Southern Hemisphere; 2013

Universidad Andrés Bello

Reconstructing Holocene coastal environments occupies a central role in the effort to predict future responses to accelerated sea-level rise and climate change. One tool widely used for reconstructing both vegetation and sedimentary system dynamic in coastal research has been pollen analysis, which along with carbon, nitrogen and oxygen stable isotopes can account for detailed information about marine input and its influence on estuaries and saltmarsh vegetation. Here we reconstruct the saltmarsh zones and environmental conditions since ca. 8800 cal yr BP through the combination of palynology, C/N, delta 13C and delta 18O data from different biogenic fractions preserved in sediments collected near Río Salado in the coastal plain of Argentina (35°S), 30 km upstream from the central Bahía Samborombón shoreline. Our aim is to decipher past-estuary and vegetation dynamics and the identification of carbon sources before, during and after the Holocene sea-level maximum transgression (6500 - 6000 years BP).Our results show alternating dominance between Poaceae and Chenopodiaceae pollen, variations in delta13C and C/N from organic matter in sediments as well as in delta 13C and delta 18O values from mollusk shells, ostracod valves and foraminifera tests over time. We recognize the following sequence of events:From ca. 8800 to 7800 cal yr BP dominance of Chenopodiaceae pollen (72-93 %), along with light to heavier (-22.84 to -17.25 ppm) delta13C values and increasing C/N between 9 and 15 imply middle to high saltmarsh environments and organic source mostly from C3/CAM and C4 plant detritus. We infer the onset of marine tidal influence and the development of estuarine conditions based on the appearance of mollusks, ostracods and foraminifera, all of which have delta 18O values showing fluctuating trends that we interpret as salinity variations in a mixohaline estuary.From ca. 7800 to 6200 cal yr BP Poaceae increases steadily up to 70 % suggesting the establishment of a low saltmarsh and intertidal estuarine environments, also evinced by the marine tidal deposit having mollusk shells and valves (e.g. Ostrea sp., Mactra isabelleana, Tagelus plebeius). The delta 13C values between -24 and -20 ppm and C/N values between 8 and 12 suggest marine particulate and dissolved organic carbon (POC and DOC) sources. Most of the delta 13C and delta18O values from mollusks, ostracods and foraminifera increase to heavier values than previously reflecting the higher salinity and marine input in an intertidal mixohaline estuary associated to a sea-level highstand.From ca. 6230 to 5050 cal yr BP Chenopodiaceae values up to 60 % reflect a transition toward middle-high marsh, with marine algae and suspended organic matter (POC and DOC) as carbon sources indicated by delta 13C values between  -22 and -23 ppm, and C/N between 8 and 12. The fluctuating delta 13C and delta 18O values from mollusks, ostracods and foraminifera indicate variable salinity related to marine tidal influence and evaporation processesFrom ca. 5050 to 3100 cal yr BP the minimum percentages of Poaceae (2-9 %) along with Chenopodiaceae up to 93 % represent the middle marsh probably developed on the Río Salado floodplain. Delta 13C and C/N suggest mixed C3 and C4 plant detritus along with marine POC as carbon sources. The amount of records of the estuarine mollusk H. australis is lower than the records of the freshwater-brackish H. parchappii which suggest less marine tidal influence than in the previous periods, which can be associated with sea-level fall. H. parchappi delta 18O values up to 2 ppm indicate that evaporation processes may be responsible for salinity variations. Both ostracod species show higher fluctuations than before in their delta 18O values related to salinity variations under predominantly brackish conditions in semi-aquatic to terrestrial high marsh, whereas foraminifera values may indicate unfavorable decrease in salinity and temperature for these organisms.From ca. 3100 to 1080 cal yr BP Chenopodiaceae and Poaceae codominance suggests grassland and halophytic vegetation the latter gradually becoming dominant over the river floodplain. Decreasing trend of delta 13C values from -19 to -21 ppm and C/N above 10 suggest semiaquatic to terrestrial environment and carbon source mostly composed by mixed C3/CAM and C4 plant detritus. The absence of marine influence can be inferred by absence of H. australis whereas brackish conditions are suggested by isotope values from ostracod valves and foraminifera tests.Overall, the estuary evolution demonstrate the strong marine impact associated to sea-level highstand during most of the Holocene that ended with the atrophy of estuary at ca. 3000 cal yr BP in relation to the sea-level regressive phase. Saltmarsh zone dynamic is coherent with the estuary evolution, going from middle-high marsh to low marsh during the highstand and expansion of the estuary, followed by the return of the former zone as the estuary degenerated, evolving into high marsh halophytic vegetation on the Río Salado floodplain surrounded by Pampa grasslandsThe C/N ratio was a useful tool not only for establishing the onset of estuary but also for estimating the input of marine carbon sources over time. Data showed coherence between interpretations from palynology and those arising from the delta 13C and C/N values. On the other hand, delta 18O values from biogenic carbonates allow proposing that evaporation process acted simultaneously with marine influence resulting in water salinity increase and possible warmer waters under less humid and variable climatic conditions than today.These interpretations show the useful combination of palynology with carbon nitrogen and oxygen stable isotope for providing detailed information concerning the evolution and dynamic of an ancient estuary as well as the migrations and replacement of saltmarsh zones related to sea-level change and climatic conditions.This investigation was supported by Fulbright fellowship and FONCYT-PICT 1338 grant to I. Vilanova and by LESIG, UC Berkeley.