Palaeolimnology and holocene environmental change from endoreic lakes in the Ebro Basin, North-East Spain

Abstract

Lake sediments from inland endoreic (saline) lakes in the semi-arid Ebro Basin, NE Spain have been analysed to provide a history of lake level, vegetation, catchment erosion and anthropogenic activity over the last 10,000 years. Analysis was undertaken for pollen, macrofossils (seeds, Cladocera ephippia, Chironomid head capsules etc), charcoal, geochemistry (total cations/trace metals, sulphate, carbonate & LOI) and sediment composition. Fourteen AMS radiocarbon dates provide dating control. Seven cores were investigated from 4 seasonal playa lakes, I shallow (<1.5m) semi-permanent salt lake, 1 deep (5.0m) permanent salt lake and 1 Medieval-age reservoir. Over 40 surface samples were also taken to investigate modern analogue environments. A review of the use and interpretation of saline lake macrophytes (seeds & pollen), Cladocera and Chironomids in palaeolimnology is provided, with special emphasis on those found in Spanish salt lakes. Taphonomic problems and nearshore-offshore facies were also investigated using a surface sample transect across a small playa lake. A surface sample pollen data set from 30 lakes in the Ebro Basin is presented and the implications for palaeo-interpretation discussed. The sensitivity of the pollen record as a climate indicator is investigated using 6 sites across a climatic gradient from sub-humid to semi-arid. Lake level reconstruction is based on an 8 stage semi-quantitative palaeohydrological model, developed from a surface sample data set from 32 lakes ranging from temporary to permanent, and hypersaline to freshwater. A simple hydrological model for groundwater fed lakes is also discussed which can be used to quantify these palaeohydrological changes. The early Holocene (<9.3-8.6Kyr BP) was characterised by semi-arid extreme continental conditions in the Ebro Basin, with an extensive Juniper thurifera woodland. Lake levels rose to their highest point in the Holocene between 8.6-7.2Kyr BP when evergreen oak and pine forest dominated. This is interrupted by a short recession in lake level between 8.2-7.6Kyr BP. A distinct regional early-Neolithic fire and clearance event occurs between 7.7-7.3Kyr BP. A drop in lake level and development of a monospecific pine forest (P. halepensis) indicates warmer and drier conditions in the mid Holocene (7.2-5.4Kyr BP). Evergreen oak reappears as forest cover declines after 5.4Kyr BP, although this is not marked by any increase in charcoal or cultivation indicators. Low groundwater but moderate lake levels (4.0-2.7Kyr BP) may be linked to high summer storm frequency and low winter rainfall. This coincides with catchment erosion and valley floor alluviation. Lake level rises again significantly between 2.7-1.8Kyr BP during Iberian and Roman times when archaeological evidence indicates a peak in population. Agriculture changes from small scale pastoral to small scale arable without any further decline in woodland cover. A rapid fall in lake level at ca. 1.8Kyr BP is followed by complete forest recovery (P. halepensis) and depopulation, until major deforestation around 1.4Kyr BP marks the arrival of the Visigoths/Arabs and extensive nomadic pastoralism. Intensive grazing pressure or lower temperatures resulted in Juniperus increasing again between 1.4-0.4Kyr BP. This also coincides with a second period of catchment erosion and valley floor alluviation. After ca. 0.4Kyr BP, lake levels have increased along with extensive olive cultivation and the development of modern (irrigated) arable agriculture.