Paleolimnology of Iberian Lakes: the 2004 LRC-IPE survey
Iberian lacustrine sediment records: responses to past and recent global changes in the
Mediterranean region
Blas L. Valero-Garcés and Ana Moreno
Pyrenean Institute of Ecology, Spanish National Research Council, Apdo 13034, E-50080
Zaragoza, Spain
Keywords: Paleolimnology, Iberian Peninsula, Holocene, Paleoclimate, Human impact,
Mediterranean
Introduction
In spring 2004, the Pyrenean Institute of Ecology (Spanish Research Scientific Council,
IPE- CSIC) and the Limnological Research Center (LRC), University of Minnesota, USA,
organized the first paleolimnological field expedition to the Iberian lakes. Coring was facilitated
by use of a platform and a Kullenberg coring system. Doug Schnurrenberger, Mark Shapley and
Anders Noren from the LRC were responsible for technical and logistical aspects of the coring
expedition. The IPE – CSIC selected the lakes using location and accessibility as criteria. Kerry
Kelts, the former director of the LRC, was the inspiration for this campaign. Unfortunately, Kerry
did not live to see the success of the expedition. He passed away on February 8th, 2001 at the
age of 54, after a long and courageous battle with Hodgkins Disease. Kerry played a major role
in the development of limnogeological studies in Spain during the 1980s and 1990s. He came to
Spain in 1985 to attend the 6th International Association of Sedimentologists meeting in Lleida
and again the next year to teach a short course in Barcelona. Even after becoming ill, Kerry
participated actively in the first two meetings of the Southern European Group of the European
Lake Drilling Program, in Zaragoza (1999) and Lisbon (2000). We discussed the Iberian lakes
expedition with Kerry many times. Finally, in 2004, it became a reality. This collection of papers
is a tribute to Kerry’s vision.
Most paleolimnological studies in Spain prior to the 2004 expedition had been conducted
on peat bogs, shallow water bodies, dry lakes or even outcrops. For the first time, as part of this
expedition, long sediment cores were recovered from relatively deep lakes (11-50 m water
depth) in a variety of climatic areas and geographic regions. During one month, the scientific
team traveled more than 6,000 km, surveyed eight lakes and recovered >200 m of sediment
core. The selected lakes were: (1) Estanya and Montcortés in the Pre-Pyrenean Range, (2)
Caicedo de Yuso - Arreo in the western Ebro Basin, (3) Enol in the Cantabrian Mountains, (4)
Sanabria in northwestern Spain, (5) El Tobar and Taravilla in the Iberian Range, and (6) Zoñar
in the Guadalquivir River Basin (Fig. 1). Cores were shipped to the LRC where initial core
descriptions, including magnetic susceptibility measurements, were done in fall 2004. This “core
library” represents the best terrestrial archive of climate variability on the Iberian Peninsula. A
multidisciplinary team has been working on the cores for the last few years, studying
sedimentological, geochemical and biological variables. Several papers from Estanya (Morellón
et al. 2008, 2009, 2010), Taravilla (Valero-Garcés et al. 2006; Moreno et al. 2008) Enol (Moreno
et al., 2010) and Zoñar (Martín–Puertas et al. 2008, 2009, 2010, 2011) have been published.
Three PhD theses using those lake records have been defended (Martín-Puertas 2008;
Morellón 2009; Corella 2011) and two more are in progress. The sequences have been studied
using a multidisciplinary approach and the chronologies are well constrained with AMS-14C,
210Pb
and
137Cs
dates, and in some cases (Arreo, Montcortès), varve counting. This Iberian
dataset, as a whole, provides high-resolution reconstructions of past climate and environmental
variability on the Iberian Peninsula at several timescales: millennial since the last glacial
maximum (Enol, Sanabria, Estanya), centennial during the Holocene (all records) and decadal
resolution during the last few centuries (Enol, Sanabria, Arreo). In addition, they show a
comprehensive picture of recent environmental and climate changes, i.e. information needed to
implement policies to manage and preserve these aquatic systems.
The number of extant lakes in Spain is relatively small, compared with other countries,
but they occur in a variety of geographic, climatic and ecologic settings (Alonso 1998). There
are three main lake types, distinguished by their mode of origin: 1) lakes in the mountains
(Cantabrian, Pyrenees, Central System, Iberian Range) originated by glacial activity, 2) karstic
lakes that owe their origin to exokarstic activity (travertine damming as in Taravilla, Ruidera
Lakes in the Guadiana River Basin) or to dissolution of evaporites or carbonates (many lake
systems such as Banyoles, Montcortès, Arreo, El Tobar, Cañada del Hoyo, Zoñar), and 3)
ephemeral, saline lakes in the Ebro, Duero and Tajo River Basin, that arose from a combination
of wind erosion and karstic processes. Paleolimnological studies in Spain started in the late 19th
century in some small ephemeral lakes (Martín-Donayre 1873; Calderón 1888; Hernández
Pacheco 1900). Martín-Donayre (1873) had described the extensive mudflats of Gallocanta
Lake (“the terrain surrounding the lake is extremely muddy”) as well as the correlation between
the amount of salt extracted from La Salineta Lake (Zaragoza Province) and annual rainfall. In
the early 20th century, the two largest lakes in Spain, Sanabria in Zamora Province (Ciria and
Vinent 1908; Taboada 1913), and Banyoles in Girona Province (see detailed references in Julià
1977), were surveyed. In spite of these early works, a limnological tradition was never
established. Only later, with the studies of Ramón Margalef, including his classic book
“Limnología” published in 1983, did pioneering limnological work in Spain really begin.
For decades, the Spanish scientific community did not have the technical capability to
retrieve cores from relatively deep lakes, and most paleolimnological studies in the region were
restricted to short cores. The exception was the IPE floating platform, developed during the
1980s (Montserrat Martí, 1992) and used successfully in glacial Lake Tramacastilla (Jalut et al.
1992) and karstic Lakes Estanya (Wansard et al. 1998) and La Cruz (Julià et al. 1998). An
integrated approach to Quaternary lake basin studies, including multiple cores, seismic surveys
and multidisciplinary techniques, was not undertaken until the last decade. Recently,
paleolimnological research on Iberian lakes has increased, as is evident in the number of
presentations at the last few Paleolimnology Symposia and Limnogeological Congresses. A
country with a tradition in recent lake ecology and pre-Quaternary limnogeology, has seen a
new synergy, with expanded paleolimnological study of extant lakes. During the 2004
expedition, IPE-CSIC researchers provided the scientific basis for selecting coring sites, and
scientists involved in the study of the lakes participated in the coring operations. Members of the
community who joined the expedition were involved in the study of the cores, and came from
many disciplines. The idea of a special JOPL issue on the Iberian lakes started with the IV
Limnogeology Meeting, held in Barcelona in 2007. We now have a collection of papers that
showcase the results of the 2004 expedition.
The new records presented here enabled us to address some fundamental questions
about paleoenvironmental and paleoclimate evolution on the Iberian Peninsula, and to identify
similarities and differences between Iberian records and those in northern Europe. In particular,
these Iberian sequences demonstrate:
i) Maximum extent of alpine glaciers occurred prior to the global Last Glacial Maximum
(LGM). Long cores from glacial lakes (Enol, Sanabria) have proglacial sediments older than 20
ka (the global LGM), demonstrating that the maximum local glacier extent in the Cantabrian and
NW mountains occurred earlier than the LGM. After several decades of debate about possible
early deglaciation in the Pyrenees and the mountains of southern Europe, these data support a
distinctive history for Mediterranean alpine glaciers all over the Iberian Peninsula (IP).
ii) There were large hydrological changes in the IP associated with glacial, late glacial and
Holocene climate phases. The Iberian region is key to answering some questions about climate
variability and abrupt changes because of its location at middle latitudes, with strong Atlantic
and subtropical influences. In particular, sediment sequences from karstic lakes show great
hydrological sensitivity to past climate changes. Alpine and coastal lakes fail to record such
large hydrological variations during the late glacial and Holocene. For example, the long record
from Estanya (Morellón et al. 2008) shows desiccation phases at the onset of the Holocene, an
arid “Mystery Interval,” and relatively humid phases during the LGM. All the records show large
hydrological variability during the last 4,000 years. The closed-basin nature of some of these
lakes probably amplifies the response to changes in moisture balance, but this enabled us to
identify long, arid periods during the late glacial and short-term drought episodes during the late
Holocene, events for which we have detailed and robust chronologies.
iii) Large changes in lake water balance occurred during historic times, i.e. since the
Roman Period, with marked regional variability. In southern Spain, the Iberian–Roman period is
the most humid phase during the last 4,000 years, but the northern sequences do not record
this period as the most humid. Every record shows evidence for increased aridity during a
period roughly synchronous with the Medieval Climate Anomaly (MCA), and colder and more
humid conditions after the 15th century, coinciding with the Little Ice Age. The record from the
LIA shows a complex internal paleohydrological structure and a strong link with solar irradiance
is suggested by the coherence between periods of more positive water balance and phases of
reduced solar activity in some records (Estanya, Taravilla). Changes in winter precipitation and
dominance of NAO negative phases would account for wet LIA conditions in western
Mediterranean regions. This well-established pattern in the IP has an antiphase relation with
similar records in the eastern Mediterranean, which has led to the hypothesis of a
“Mediterranean Oscillation” and illustrates the complexity of regional variability in the
Mediterranean during the last few millennia (Roberts et al. 2011).
iv) There has been profound human impact on the lakes for the last millennium. Most
lakes and wetlands in Spain have been exploited for water and other natural resources since
the Neolithic. Almost all records show a large increase in sedimentation rate and sediment
delivery to the lake, synchronous with medieval settlement, deforestation and farming of the
watersheds. Sedimentation rates of 3 mm yr-1 during the last 2,000 yrs are common, and values
of ~10 mm yr-1 over the last century were recorded in several lakes. Study of cores from the
2004 expedition show three main recent impacts on Spanish karstic lakes: i) sedimentological,
as a consequence of increased sedimentation rates, reflecting an increase of farming activities
and also increased flood events during the LIA; ii) chemical, a result of eutrophication caused by
fertilizers; and iii) hydrological, caused by human water consumption.
v) paleolimnological studies are key to ecological restoration efforts in Mediterranean
lakes. All of the studied aquatic ecosystems receive some protection and government agencies
have defined plans for conservation and restoration. Little, however, was known of the lake
dynamics at timescales longer than a few years. Monitoring data are very recent and historical
or documentary records are scarce. Long-term data on limnological variables in Spanish lakes
are rare. Short cores from several lakes, dated with
210Pb
and 137Cs have allowed comparison of
geochemical proxies with monitoring data in Enol, Arreo and Sanabria. The results, although
promising, underline the complexity of such an approach and the need to conduct further
research on how the climate signal is transferred to and preserved in the sediments.
Paleolimnological studies demonstrate that lakes in the Mediterranean region display large
natural variability that is not captured by monitoring. They also illustrate that the limnological
systems have been strongly affected by humans and that modern, and even historical
environments, do not reflect pristine conditions. These studies provide the tools to evaluate the
relative significance of human versus climatic factors in lake hydrology and watershed changes
during historic times, and help us model and predict the effects of the current period of rapid
climate change in Mediterranean lakes.
The papers
In this special issue, we present papers that came out of study of cores retrieved during the
2004 LRC-IPE survey in six lakes: Enol, Sanabria, Arreo, Estanya, Montcortès and Zoñar (Fig.
1).
Moreno et al. (2011) describe the Holocene sequence from Lago Enol, a high-altitude, karstic
and glacial lake in the Cantabrian Mountains. In this record, the Younger Dryas event (13,50011,600 cal yr BP) is well characterized as a cold and arid phase. The Holocene displays three
phases: a warm early Holocene (11,600-8,700 cal yr BP), a more arid middle Holocene (8,7004,650 cal yr BP) and a return to humid conditions during the late Holocene (4,650-2,200 cal yr
BP).
Three papers investigate the Montcortés sequence (Eastern Pre-Pyrenean range). Corella et al.
(2011a) identify relatively shallower lake levels during the middle Holocene (6,000-3,500 cal
years BP) and deeper environments, with deposition of varves, since then. Increased carbonate
production and lower clastic input occurred during the Iberian-Roman Period, the Little Ice Age,
and the middle 20th century. Episodes of higher clastic input to the lake correlate with the MCA
(690-1460 AD), the last phase of the LIA, and the maximum human occupation in the late 19th
and early 20th centuries (1870-1950 AD). Large gravitational deposits during the Holocene and
historic times are likely related to earthquake activity. Scussolini et al. (2011) analyze the diatom
record for the last 5,340 years. The record shows a conspicuous alternation between Cyclotella
comta and Cyclotella cyclopuncta, reflecting changes in trophic state, and a succession of
centric and pennate species indicative of the hydrologic balance of the lake. The diatom
assemblages identify a period of increased productivity and likely lower lake levels prior to
3,850 BP. Lake level recovers thereafter, and remains high during the Iberian and Roman
Epochs. Lower lake levels are recorded during Medieval times and relatively higher lake levels
during the LIA and afterwards. Lastly, Rull et al. (2011) report on vegetation changes of the last
millennium, identifying the influence of climate and human activities. Conifer forests were
intensely burned at the beginning of feudal times (AD 1000) and replaced by farm fields,
meadows and pastures. Changes in the Mediterranean vegetation characterized the warm
MWA and the LIA cooling in the 15th century. Forest recovery began around AD 1500,
coinciding with wetter climate, but fluctuated in association with human pressure in the
watershed.
Two papers infer past climate and environmental changes in karstic lakes Zoñar and Estanya
during the last few millennia, using geochemical and sedimentological variables. Martín-Puertas
et al. (2011) used high-resolution XRF scanner and geochemical data from Zoñar Lake to
reconstruct the hydrological and depositional evolution of the lake. Chemical ratios identify
periods of endogenic carbonate formation (higher Ca/Al, Sr/Al and Ba/Al ratios), evaporite
precipitation (higher S/Al, Sr/Al ratios), anoxic conditions (higher Mo/Al, U/Th ratios and Eu
anomaly), lower lake levels (Sr and S enrichment) and elevated lake levels (higher Al, K, Ti, Fe,
trace and rare earth elements). Geochemical indicators also mark periods of higher detrital input
to the lake related to human activity in the watershed during the Iberian Roman Humid Period,
Medieval times, at the onset of the Little Ice Age and over the last 50 years. Morellón et al.
(2011) describe the complex environmental, hydrological and anthropogenic interactions that
occurred in the southern Pyrenees since Medieval times, using short cores from Estanya Lake.
Shallower and more saline conditions prevailed during medieval times (1150-1300 AD) and
generally deeper, more dilute environments occurred during the LIA (1300-1850 AD). Maximum
lake levels and flooding of the current littoral shelf occurred during the 19th century, coinciding
with the maximum expansion of agriculture in the area and prior to the last cold phase of the
LIA. Finally, declining lake levels during the 20th century, coinciding with a decrease in human
pressure, are associated with warmer climate conditions.
Three papers investigate the interactions between climate, landscape and lake dynamics in the
19th and 20th centuries in Enol, Arreo and Sanabria. López-Merino et al. (2011) describe the
climate and environmental changes in the Cantabrian Mountains using short cores from Enol.
The region was characterized by an open landscape and warmer temperatures at the end of the
Little Ice Age (~1800-1875 AD). Socioeconomic transformation during the 20th century caused
profound changes in the catchment and in the lake ecology. Lower runoff and increasing lake
productivity during the last several decades (~1970-2007 AD) suggest substantial impact of
tourism on this ecosystem.
Corella et al. (2011) conducted sedimentological, geochemical and diatom analyses on
short cores from Arreo Lake and made a detailed comparison with regional instrumental climate
data (1952-2007), limnological monitoring data from the lake (1992-2008) and recent land use
changes. The inferred lake evolution is in agreement with monitoring data that suggest a
transition from dominantly meromictic conditions prior to 1993-1994 to a predominantly
monomictic pattern since then, particularly after 2000. The synergistic effects of intensive water
extraction for irrigation and lower rainfall since 1979, and particularly since 1994, brought the
long period of meromictic conditions in Lake Arreo to an end. Water balance and sediment
delivery to the lake are dominant factors that control the circulation pattern and hence
limnological conditions in Lake Arreo. The authors advise that these factors be considered in
management and restoration plans.
Giralt et al. (2011) used meteorological, limnological, and hydrological data sets as well
as XRF core scanner measurements carried out on short cores from Lake Sanabria to better
understand the transmission of the climate signal to the lake sediments. Sediment input and
nutrient delivery from the Tera River are the main factors controlling sediment deposition in the
lake. The authors developed a quantitative reconstruction of precipitation for the period 19592005 AD that captures the regional variability and stresses the possibility of obtaining climate
reconstructions using non-laminated sediments.
Acknowledgements
The 2004 field expedition would not have been possible without the cooperation of the
Department of Geology and Geophysics, and the Limnological Research Center, University of
Minnesota. We are forever grateful to Doug Schnurrenberger, Mark Shapley and Anders Noren
for making the expedition possible. Numerous Spanish scientists participated in the coring of
the lakes and their help and enthusiasm are greatly appreciated. Financial support for the
fieldwork and subsequent analyses came from different projects of the Spanish Ministry of
Science and Innovation (CLIVAR, LIMNOCLIBER, CALIBRE, GLOBALKARST, GRACCIECONSOLIDER). Finally, we thank the referees who reviewed the papers and JOPL editor Mark
Brenner.
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Figure Captions
Fig. 1. Location of the lakes surveyed in the 2004 IPE-LRC coring expedition