Paleoenvironmental and paleoclimatic conditions in the Tagus River Estuary during the last 14 kyr

Abstract

Estuaries constitute the frontier between terrestrial, fluvial and marine environments, representing areas of high biological productivity that are particularly sensitive to global, regional and local environmental and climatic changes. The Tagus River is the longest river of the Iberian Peninsula and has a catchment area of 80 629 km 2 . In order to understand the interactions between the Tagus system (land-river) and the ocean we have studied a 52,76 m long core collected at the upper Tagus estuary off Vila Franca de Xira (VFX 38°56 24 N; 8°56 19 W, 2 meters elevation). The age model for this core was based on 6 AMS 14 C dates and covers the last 14 calendar (cal) kyr BP (1 kyr = 1000 yr). Sea surface temperature (SST), marine/estuarine productivity and river input has been reconstructed with a high resolution, multi-proxy study that includes: diatoms, phytoliths, C37 alkenones, higher plant C23 – C33 n-alkanes and C20 – C30 n-alkan-1-ols, Fe, Ti and Ca content derived from XRF analysis, Organic Carbon (Corg) and CaCO3 content and grainsize analysis. Although discontinuous, the SST record shows a decreasing trend from Early to Late Holocene, as in the Tagus prodelta SST record of Rodrigues et al. (2009). The lower SST values indicate that the river waters are colder than the coastal ocean. Important terrigenous input (revealed by higher content of terrestrial lipid biomarkers, Fe and Ti content, fresh water diatoms and phytoliths) is recorded during the Younger Dryas/Holocene transition (~11.5 – 10.5 cal kyr BP) as sea level rise allows sediment accumulation inside the estuary. Diatom (marine and brackish genera) and C37 alkenones appear between ca. 11.5 – 4 cal kyr BP supporting the existence of a marine to brackish water environment, as suggested by the previously suggested by Vis, Kasse & Vandenberghe (2008) and Vis & Kasse (2009). During this time interval, relative maxima in diatoms, C37 alkenones and Corg show a cyclic increase in local productivity that can be explained as being a response to coastal water inflow and/or fluvial supply of nutrients. SST minima are recorded at ~ 11.3 – 11.1, 10.5 – 10, 7.8, 6.6, 5.8, 5, 4.5 and 1.4 – 0.8 cal ka BP. The estimated age for these episodes coincide with the equivalent events recorded in the North Atlantic (Bond et al., 1997) and in the Tagus prodelta (Rodrigues et al., 2009). Higher terrigenous input episodes are recorded at ~10.5, 8.3, 7.7, 6.7, 6.4, 5.9, 5.4 and 4.9 cal ka BP, and could be related to flood events and/or periods of major fluvial activity. When comparing the SST minima with the markers of enhanced river discharge recorded at this site, five of these events are II Seminario Iberico IGBP – Mudanca Global Mudanca Global na Peninsula Iberica. Uma Visao Integrada. 4 e 5 | Novembro | 2010 Lisboa – Portugal synchronous: 10.5, 7.8 7.7, 6.7 6.6, 5.9 5.8 and 5 4.5 cal ka BP, corresponding to cold, wet periods and coinciding with the flood events and/or increased fluvial activity detected in the Tagus river and in the Iberian Peninsula (Vis et al., 2010, Benito et al., 2003, Benito et al., 2008). Although higher resolution and a more accurate age model are needed to improve the understanding of this estuarine record, the hydro-sedimentary dynamics, particularly during the Holocene transgression, appears to be controlling the terrigenous input and the local productivity recorded at this site. Still, this sedimentary record provides information on past climate changes occurred in the Iberian Peninsula and contributes to a better understanding of the interactions between the North Atlantic and the Mediterranean.