Environmental and climatic controls since the early XVth century in the palaeoflood records of a Mediterranean arroyo (Rambla de la Viuda, Spain)

The Rambla de la Viuda arroyo is a representative ephemeral stream of the western Mediterranean area (NE Spain), both in terms of its torrential regime and land use history. A total of 10 stratigraphic profiles were described, located in areas of prevailing deposition of suspended clay, silts and fine sands, during flood events, along the canyon margins. The sedimentary comprises continuous sequences of slackwater flood deposits interbeded with episodic coluvial and edaphic horizons. The radiocarbon dating indicates that most of these deposits accumulated over the last 500 years. The stratigraphic and edaphological characterisation of the slack-water flood deposits, coluvial units and palaeosols, was supported biominerals content analysis (phytoliths), geochemical and documentary data (written historical documents). A high frequency and high magnitude palaeoflood period took place during the XV-XVII centuries (Phase I), which seem to correlate in time with general wetter conditions (higher presence of C3 photosynthetic pathway Poaceae and ligneous plants). During Phase I two well-developed palaeosols can be identified, which could correspond in time with two wetter than normal periods during ca.1550-1620 AD and 1670-1700 AD. Minimum flood discharges between 2000 m³s^-1 and 1400 m³s^-1 were estimated for this Phase. Phase II was characterised by episodic high magnitude floods (estimated minimum discharges between 1000-350 m³s^-1), interrupted by coluvial deposits. This phase seems to be correlated in time with a period of higher climatic variability and general dry conditions as it can be inferred from the analysis of the phytoliths content on fluvial and coluvial sediments. Major drier conditions have occurred during middle XVIII century, where phytoliths indicate the presence of a C4 grassland, with less than 5% of riparian and dycots plants. Phytoliths content in this maximum driest period were of 5x10³ phytoliths per gram of sediment (Acid-insoluble Fraction). Average phytolith content of sediment samples corresponding to Phase I are of ca 40x10³ per gram of sediment. Finally a third Phase can be identified after the late XVIII- early XIX century, with up to 10 flood events identified (estimated discharge between 500-2800 m³s^-1), and two palaeosols, showing nevertheless a poorer development than the soils of Phase I, which indicates shorter climatic and environmental stability during the last 100 years. During the last 50 years two large floods have occurred, in 1962 and 2000 with estimated discharges of 1500 and 640 m³s^-1 respectively.