1. The varved record of Lake Montcortès (southern Pyrenees, NE Spain): climate variability and human activities in a Mediterranean mountain since the Roman Period
Juan Pablo Corella1(*), Achim Brauer2, Clara Mangili3, Valentí Rull4, Teresa Vegas-Vilarrúbia5, Mario Morellón6 and Blas L. Valero-Garcés1
1Pyrenean Institute of Ecology-CSIC, Apdo 202, E Zaragoza, Spain
2Deutsches GeoForschungsZentrum Potsdam, Sektion 5.2 Klimadynamik und
Landschaftsentwicklung, D Potsdam, Germany
3Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000,
61 Route 9W, Palisades, NY , USA
4Botanical Institute of Barcelona (CSIC), Passeig del Migdia s/n, Barcelona, Spain
5Dept. Ecology, Fac. Biology. University of Barcelona. Barcelona, Spain
6Eawag (Swiss Federal Institute of Aquatic Science & Technology). Überlandstrasse 133,
8600 Dübendorf (Switzerland)
INTRODUCTION:
High resolution studies for climate reconstruction over the last 1500 yr are the best tool to examine temporal and spatial patterns of natural climate variability under boundary conditions similar to those of present day. The 1548-yr-long varve record from Montcortès Lake constitutes the first high resolution (annual to seasonal) study in the Iberian Peninsula for the last two millennia.
METHODS:
The studied cores (MON04-3A-1K and MON07-1A-1M) were retrieved from the deepest part of the lake (Fig. 2) in two different coring surveys (2004 and 2007) using Kullenberg and gravity corers. The uppermost 528 cm were sampled for large thin sections (100x15x35mm). A Zeiss Axioplan 2 imaging optical microscope was used for microfacies studies and varve counting. In addition, 10 samples were studied with a Scanning Electron Microscope (Ultra Plus Zeiss SEM, coupled with an Edex (Baltec) analyzer working with 20kV and an analysis time of 30´´). The elemental geochemical composition was analyzed in core MON04-1A-1K by X-Ray Fluorescence (XRF) using an ITRAX XRF core scanner from the Large Lakes Observatory (Duluth, University of Minnesota)
GEOGRAPHICAL SETTING
Lake Montcortés (42º 19´ 50´´N, 0º 59´ 46´´E, 1027m s.n.m) is a karstic and meromictic lake (30 m depth) located in the PrePyrenean Range (Iberian Peninsula). The lake watershed is dominated by Triassic bedrock (mainly carbonates, evaporates claystones and shales). Montcortés Lake hidrology is mainly controlled by groundwater inputs. Evaporation and a small stream in the northern shore are the main outputs (Fig. 1). The lake is oligothrophic, with lake water of bicarbonate-sulphate-calcium type (pH is 8.4 and electrical conductivity is 528 µS/c). The permanent anoxic conditions at the bottom of the lake allows the preservation of annually laminated sediments. The lake is in a submediterranean bioclimatic domain with a strong rainfall gradient.
CHRONOLOGY:
An independent varve chronology starting at 2007 has been established and complemented by 4 radiocarbon dates (Fig. 3A). Since organic and detrital layers are not always present, each annual varve has been identified by the presence of the calcite layer. The good correlation of the varve counting with the 14C AMS dates underlines the annual nature of the lamination. The 528 cm studied interval covers the last 1548 years. From the total of 1548 varves, 79 have been interpolated using average sedimentation rates in intervals of poor varve preservation (5.1%). The sedimentation rates (SR) fluctuate strongly from 5 mm/yr ( varve yr AD; cm) to mm/yr ( varve yr AD; cm). (Fig. 3A). These abrupt changes in the SR are due to the variable detrital input (DL1 and DL2) along the record. The original studied interval thickness (528 cm) is reduced to 202 cm if the event layers (61.7 % of the total thickness) are removed from the sedimentary sequence. Thus, if detrital layers are not considered, the sequence displays a more constant SR (0.13 mm/yr) except for the lower part which displays a lower SR (0.037 mm/yr) (Fig. 3B). 1
Outlet
3A-1K
1A-1M 2
Bathymetric map showing the
location of the studied sediment cores
Mean rainfall map in the Iberian Peninsula; b) Geological map
of Montcortés Lake catchment; c) Ombrothermic diagram of the study area
SEDIMENTARY MICROFACIES:
Three different microfacies (MF) can be distinguished in Lake Montcortès sedimentary sequence (Fig 4); varve MF 1.1 and 1.2 and graded coarse detrital layers MF 2: 3 4
-Microfacies 1.1 (MF1.1) are biogenic varves composed of a couplet of white (calcite) and brownish (organic) layers with no discrete detrital layers. The calcite layer is the result of endogenic precipitation in the epilimnion during spring/summer while the organic-rich layer represents deposition in Lake Montcortés after the period of calcite precipitation, which likely includes summer and winter. Calcite layers display different sublayering types reflecting different settling velocities.
-Microfacies 1.2 (MF1.2) is characterized by the presence of an additional clastic layer (DL1) besides the calcite and organic laminae which represent higher run-off and sediment delivery from the catchment.
-Microfacies 2 (MF2) occurs as 0.8 mm to 12 cm thick grey detrital layers (DL2) characterized by a normal grading and abrupt lower boundaries. They show a discrete coarser basal sublayer. These layers would be deposited during shortlived events (days to weeks) in which the sediment accumulated in the catchment would be transported into the lake by ephemeral streams and run – off. The coarse basal layer and the erosional surface may be the result of underflow current processes.
Phosphorous content has predominantly higher values in clastic dominated intervals where MF 2 is present.
Biogenic varves form Montcortès Lake: (A) Stratigraphic column from MON04-3A-1K and MON07-1A-1M cores (B) Thin section photos and SEM images of the varves; (C) Microfacies sub-types; (D) Coarse-fine (top) and fine-coarse (bottom) sublayering succession example within calcite layers 5
(a) Montcortés Lake age-depth model based on varve counting and AMS 14C dates (b) Corrected age depth model after the extraction of the detrital layers (DL2)
DISCUSSION:
Proxies for climate and human impact
Environmental (human and climate induced) fluctuations in Lake Montcortés during the last 1548 years have been recorded by three main proxies (Fig 5 ):
-Calcite layers thickness-> Rapid oscillations in the thickness of calcite layers may respond to two possible factors: i) changes in water temperature and; ii) changes in the nutrient supply to the lake.
-Calcite internal sublayering-> Factors controlling changes in the internal structure of the calcite layers seems to respond to changes in the seasonality under colder conditions (fine->coarse trend) or in the trophic state of the lake (coarse-fine trend). There is a close relation between different algal blooms and the different sublayering types (Fig 5)
-Detrital layers thickness and frequency-> The increase in clastic input respond to arid conditions, loss of vegetation cover and more intense land use, and increase in precipitation events per year.
Layer thickness measurements and calcite sub-layering trends in the different layers from the sedimentary record
Paleoenvironmental evolution
The Montcortés Lake sequence reflects significant climate oscillations (temperature and precipitation ) and land uses changes during the last 1548 years (Fig 5):
The end of the Roman Period and the Early Middle Ages ( varve yr AD): An abrupt increase in clastic input to the lake starts in the 6th century (533 AD) coinciding with an increase in population due to the pressure of the northernward Muslim expansion. The increase in calcite layer thickness and the onset of the coarse-fine calcite sublayering occurred between varve yr AD and might indicate warmer water temperatures
The High Midlde Ages ( varve yr AD): The highest clastic input observed in Montcortés Lake during this period is coherent with more arid during the Medieval Climate Anomaly and an increase in the amount of pastures in the area. The persistence of the coarse, homogeneous crystals within the calcite layers and the increase in the thickness of these layers at 1000 varve yr AD may reflect more eutrophic conditions triggered by higher nutrient load and also higher water temperatures.
The Little Ice Age (LIA) ( varve yr AD): The abrupt decrease in the presence of detrital layers during the beginning of the LIA is coincident with a well documented depopulation in this area during the low-medieval crisis and the Black Death pandemic that caused an abrupt decrease in farming activities in the area, and thus reduced erosion in the catchment. The F-C sublayering and the decrease of the calcite layer thickness suggest colder and wetter conditions during the LIA.
The end of the LIA and the 20th century (1820 varve yr AD- present): The high detrital input to the lake and the increase in the pastures coincides with the maximum expansion of agriculture in the mountain areas of the Pyrenees during the XIXth century. The decreased clastic input after 1870 AD is linked to the depopulation of the Pyrenees due to the massive emigration from rural zones to urban industrialized areas.
Climate forcings in NE Iberian Peninsula during the last millennium
The increase in calcite layer thickness and detrital input to the lake-Ti profile (Fig. 5)- suggests warmer and arid conditions during the MCA, synchronously to the persistent positive phase of the NAO, which seems to affect continental run-off over Europe. The F-C sub-layering, indicative of longer winter conditions, is particularly more frequent between AD , , and , and coincides with the Spörer (AD ), Maunder (AD ) and Dalton (AD ) sunspot minima, observed during the LIA. Therefore, Lake Montcortés limnological and hydrological dynamics during the last millennium might have been modulated by a combination of external -solar irradiance- and internal –i.e., NAO- forcings.
Comparison of Montcrotés Lake geochemical, sedimentological and biological proxies and with lhe main historical events in the area and solar sunspot (Steinhilber et al., 2009 ) and NAO index reconstructions (Trouet et al., 2009)
AKNOWLEDGEMENTS: Juan Pablo Corella has a CONAI+D (DGA) fellowship and this study is funded by CALIBRE National Project (CGL C04/CLI) (CICYT, Spanish Government)