Carbon- and oxygen-isotope record of recent changes
NORTH ATLANTIC OSCILLATION RECORDED IN CARBONATE d18O SIGNATURE
FROM LAGUNILLO DEL TEJO (SPAIN)
*Charo Lpez-Blanco a,c,d, *, Julian Andrews b, Paul Dennis b,
Mara Rosa Miracle a and Eduardo Vicente a
aUniversity of Valencia (Spain). Department of Microbiology and
Ecology. Dr. Moliner, 50. 46100 Burjassot.
bUniversity of East Anglia (United Kingdom). School of
Environmental Sciences, Norwich, NR4 7TJ.
cEscuela Politcnica Nacional, Ladrn de Guevara E11-253, 170517
Quito, Ecuador.
dInstituto Nacional de Investigacin Geolgico, Minero, Metalrgico
(INIGEMM). De las Malvas E15-142 y los Perales. 170150 Quito,
Ecuador.
*Corresponding author: [email protected]
Abstract
Oxygen (18O) and carbon (13C) isotope compositions of authigenic
carbonates measured in Lagunillo del Tejo sediment document
precipitation variability during the last millennium in the Iberian
Range. Modern water samples show that Lagunillo 18O and D plot
below the Global Meteoric Water Line (GMWL). Sediment samples show
a covariant trend between carbonate 18O and 13C, indicating that
the precipitation/evaporation ratio has largely controlled the
isotopic composition of this lake. This covariant trend is used to
extract information about past lake level changes. Humid periods
occurred around AD 1300-1450, AD 1620-1775 and AD 1950-1980, while
the driest periods were concentrated around AD 1200, AD 1500 and
especially around the transition between the 19th and 20th
centuries. We compared these inferences with previous studies in
this lake and with NAO index and discovered strong correspondence
of signals. This sensitive site can be used to extent records of
NAO back in time, providing a framework for climatic modeling and
ecological management.
Keywords: carbon isotopes, oxygen isotopes, lake sediments, late
Holocene, climate, Iberian Range
1. INTRODUCTION
A diverse array of natural archives is being used to better
understand climate changes and atmospheric dynamics during the last
millennium. Inferences about historical climate change are made
from archives with a robust chronology, high temporal resolution
and ideally calibrated with instrumental or documentary data. In
addition, the studied system should have exceptional sensitivity to
the main climatic drivers, such as precipitation and/or temperature
(Villalba et al., 2009; Fritz, 2008)
In this paper we are exploring the hydrology of an aquatic
system in the Iberian Peninsula. In this region lake sedimentary
records are most sensitive to changes in precipitation (Luque and
Juli, 2002; Romero-Viana et al., 2011; Lpez-Blanco et al., 2012b)
rather than temperature. This is also clear in studies trying to
reconstruct past climate from documentary sources (Rodrigo and
Barriendos, 2008; Machado et al., 2011) where floods and droughts
are frequently referred to among extreme meteorological phenomena.
Iberian paleoenvironmental studies on lacustrine sediments have
shown regional differences related to the Atlantic and/or
Mediterranean modes of variability (Romero-Viana et al., 2011).
These records were derived from a variety of geological and
biological proxies. However, neither the proxies nor the lake sites
are equally sensitive to climatic changes. (e.g. Lpez-Blanco et
al., 2011; Romero-Viana et al., 2011).
Our study site, Lagunillo del Tejo (Iberian Range; Fig. 1), is a
karstic lake already known to be sensitive to hydroclimatic
variability (Lopez-Blanco et al., 2012a). Temporal variability in
precipitation that contributes to local aquifer discharge has
caused significant lake-level fluctuations (Fig. 1). Historical and
instrumental data show that during dry intervals the lake depth was
reduced to 3.5 m (e.g. 2008), whereas during periods with a
positive hydrological balance the lake has reached 12 m depth (e.g.
1982, Vicente & Miracle, 1984). Based on biological analysis of
the lake sediment, Lpez-Blanco et al. (2012b, 2013) reconstructed
lake level fluctuations during the last millennium. This
reconstruction from the sediment archive was calibrated with
instrumental rainfall and river flow data (Lpez-Blanco et al.,
2012b) and compared with hydrological signals in the varved
sediments of nearby lake La Cruz (Romero-Viana et al., 2011).
Physical characteristics (surface area and depth), the absence of
inlets and outlet, the good chronology in its sediment and the
presence of a calibrated signal from independent proxies makes this
natural archive an obvious prospect for rainfall
reconstructions.
Among the various paleoenvironmental proxies, isotopic analysis
of sedimentary carbonates is often used to investigate climatic
conditions that prevailed in the past (Ito, 2001). These endogenic
minerals usually provide a straightforward paleoenvironmental proxy
giving a snapshot of the chemical and limnological conditions at
the time the minerals formed (e.g. Haskell et al., 1996;
Valero-Garcs & Kelts, 1995).
Our earlier work at Lagunillo del Tejo (Lpez-Blanco et al.,
2012a) shows a record of continuous oscillation in lake level.
However, to be sure this interpretation is robust (Dearing and
Foster, 1986) we here use isotopic fingerprinting (13C and 18O) of
the primary lacustrine carbonate to provide an independent analysis
of past hydrology in the Iberian Range. We present here a 1000 year
rainfall record reconstructed from 18O in lacustrine carbonate
sediment, and compare the results with previous climatic
reconstructions from the lake. Our aims are: 1) to explore the
relationship between 13C and 18O in these lacustrine carbonates; 2)
to investigate how our interpretations of the isotope data fit with
other proxies; and 3), to search for the links between the local
record and patterns of global atmospheric circulation.
2. STUDY SITE
Lagunillo del Tejo (39 59 15.56 N; 1 52 34.35 W, 1000 m a.s.l.)
is a flooded karst sinkhole of the Guadazan River (Fig.1). It is
close to the village Caada del Hoyo (about 25 km SE of the city of
Cuenca). There are 34 known sinkholes in this karstic complex,
developed in Cretaceous dolostones (Cenomanian-Turonian stages,
(Romero-Viana et al., 2009) by dissolution and fracture processes
since Pliocene times (Gutirrez-Elorza and Pena-Monne, 1998). The
presence of the Valdemoro fault and the NW-SE anticlinal folding
(Eraso, 1979) may also have been important; lateral expansion of
the fault favouring water capture with seven sinkholes intercepting
the phreatic lake level (Escudero and Regato, 1992). Water level in
the lake is maintained by an impermeable Cenomanian marl layer.
Climatically, the region has sharp daily and seasonal
temperature fluctuations.. Mean monthly temperatures range from 5-6
C for the coldest month (January) and 25 C for the warmest month
(July). The mean annual precipitation is 525 mm, with May the
rainiest month, and August the driest (means from instrumental data
series 1950-2003 from Cuenca, Agencia Estatal de Meteorologia,
Spain).
The lake is situated in the supramediterranean fringe, where the
potential vegetation is formed by a mixed forest of Pinus nigra
subsp salzmannii and Juniperus turifera (Rivas-Martnez and Loidi,
1999). Today the vegetation is impoverished woodland with a
low-density cover of dominant P. nigra salzmannii and sparce J.
thurifera individuals. However, slopes inside the sink hole have a
denser and more diverse plant cover.
The lake is monomictic thermally stratified from May to early
autumn. The waters are bicarbonate rich, with a pH around 9 in the
epilimnetic waters, with conductivity around 600 S cm-1 and
alkalinity around 5.5 meq/l. The order of major ion concentration
is HCO3>> SO4> Cl- and Mg++>> Ca++>Na+. However,
during thermal stratification, pH decreases to 7.57, and
conductivity may reach 9001000 S/ cm-1 in the anoxic hypolimnion
(Vicente and Miracle, 1984; Miracle et al., 1992; Romero-Viana et
al., 2009). In the last decade the lake has typically been 7-9 m
deep, but it has been both deeper (12 m in 1980; Vicente and
Miracle 1984; Miracle and Vicente, 1983) and shallower (3.5 m in
2008; authors observations, Fig. 1C). At the time of sediment
coring (September 2009), the lake was 5.7 m deep.
3. MATERIAL AND METHODS
Surface water samples for hydrogen and oxygen isotope analysis
were collected in October 2010 in 30 ml polyethylene bottles
(refilling three times and capping the bottle underwater to remove
any trapped air). After collection, all samples were immediately
stored in a cooler and kept refrigerated until analysis at the
University of East Anglia (UK). Filtered water samples were
analysed for 18O and D using a Picarro cavity ring-down
spectroscopy (CDRS) laser instrument; each 2.2 l sample was
measured six times to overcome memory effects. Repeat analyses of
the standards, Norwich Tap Water (NTW), Greenland Ice Sheet
Precipitation (GISP), USGS67400 and USGS64444 calibrated the data
and gave a measurement precision for of 0.16 for 18O and 1.05 for
D. Water isotopic data are reported in per mil notation on the
Vienna Standard Mean Ocean Water (VSMOW) scale.
For sediment samples, a 40 cm sediment core (CN-4) was recovered
from Lagunillo del Tejo in September 2009 using a 6 cm diameter
UWITEC core. The core was subsampled into 2 cm thick slices.
Samples of 0.5 cm3 of each slice were treated with a 3% solution of
sodium-hypochlorite for 24 h to oxidise organic material and to
disaggregate the sediment (Ito, 2001; Eastwood et al., 2007).
Samples were then passed through a 90 m sieve and the collected
fraction centrifuged (10 min, 3,500 rpm) to concentrate the fine
carbonates. The sample was then rinsed and dried for analysis.
Carbonates were analysed on the UEA SIRA mass spectrometer
interfaced to a common acid bath sample preparation system run at a
reaction temperature of 90 degrees. Data were normalized to the
VPDB scale using NBS 19 and UEACMST. Repeat analysis of UEACMST
gave a precision of better than 0.1 for both 18O and 13C.
Water content, density, organic matter and carbonate content
were calculated for each sediment sample. Water content was
measured by oven-drying aliquots of wet sediment for 2h at 105 C.
Density was calculated as wet sediment mass normalized by the known
volume of wet sediment aliquots. Organic matter content was
determined from dried samples by loss-on-ignition (LOI) for 6 h at
460 C and is expressed as the percentage of dry matter. The
resulting fraction after organic matter treatment was used to
calculate the carbonate content by loss-on-ignition for 4h at 950
C(APHA-AWWA-WEF, 1992).
Bulk samples for each main lithological zone were used for SEM
(Scanning Electronic Microscopy) analysis, where qualitative
elemental compositional analysis was also done. The main aim was to
confirm whether predominant content was non-biogenic, clastic or
biogenic carbonate. Additionally, gyrogonites from Chara sp. were
counted in each layer of the sediment core.
Sediment accumulation rates in the lake sediments during the
last millennium were established by Lpez-Blanco et al. (2012b)
using a chronology based on 210Pb and 137Cs dating, augmented with
five 14C AMS dates. Three periods of differing sedimentation rate
were identified; 0.87mm/a between 1850 AD and 2010 AD; 0.44 mm/a
between ca. 1500 AD and 1850 AD and 0.26mm/a between 1500 AD and
ca. 1150 AD.
4. RESULTS
Surface water samples taken around the lake perimeter show small
variations in 18Owater and Dwater (Table 1). These values fall
below (Fig.2) both the Global Meteoric Water Line (GMWL) (Craig,
1961) and the Mediterranean Meteoric Water Line (MMWL) (Gat and
Carmi, 1987) (Fig.2).
Sediment core (CN-4) showed an alternation of oxidized (O) and
reduced zones (Fig. 2), as described in Lpez-Blanco et al. (2012b),
which were characterized by different organic matter content and
density. Total carbonate content (Fig.3) ranged from ca. 15% in
zone R3 to 25% in zone O3, its profile being essentially the same
as sediment density and opposite to organic matter content. SEM
analysis of bulk samples from the main lithological zones confirmed
that the carbonates are mainly authigenic water column
precipitates, composed of calcite aggregates with characteristic
rhombohedral habit (Fig.4). Any detrital fraction, if present
during climatically dry phases, should have been largely removed by
the pre-treatment. However, skeletal carbonates precipitated around
Chara sp. stems were found in zones R5, R4 and R3. Chara
gyrogonites were found in several parts of the core reaching a
maximum value in the upper part (17 and 21 gyrogonites per 20 cm3
respectively at 2-3 and 3-4 cm of depth; Fig. 3).
The Lagunillo del Tejo sediment core shows marked oscillations
in its isotopic composition and a strong correspondence between 13C
and 18O (Figs. 5 and 6). Six zones are differentiated (ISO 1-6;
Fig. 5) alternating between periods of high and low isotopic ratios
for both elements. In zone ISO 1 (40-34 cm), the isotopic
composition of the sediment shows steady values around -1.5 for 18O
and -1.2 for 13C. Zone ISO 2 (34-29 cm) is characterized by lower
isotopic compositions, 18O less than -1.5 and 13C mean values ~
1.2, respectively. In zone ISO 3 (29-23 cm) values are higher than
in ISO 1, reaching mean values of -1.3 for 18O and -1.3 for 13C .
Zone ISO 4 (23-15 cm) is marked by a decrease of 18O and 13C, with
mean values of -1.8 and -1.9, respectively. Zone ISO 5 (15-8 cm)
has the most enriched isotopic compositions, reaching values around
-0.4 for 18O and 0.0 for 13C. The upper part of the sediment core,
zone ISO 6 (8-0 cm), is characterized by decreasing values from ISO
5 with minima at the surface.
5. DISCUSSION
The isotopic composition of lake waters are controlled by the
hydrological balance between inputs (groundwater, direct
precipitation, surface and stream inflows) and outputs (groundwater
loss, evaporation, surface and stream outflows) (Leng et al.,
2005). As a consequence different types of lake physiography mean
that water isotopic composition can respond differently to the same
forcing (Jonsson et al., 2009). As a closed lake in a karstic
catchment, topographically isolated from fluvial inflows and
outflows, regional influence of surface water can be excluded from
Lagunillos mass balance. The surface waters of Lagunillo del Tejo
plot below the Global Meteoric Water Line (GMWL; Fig. 2) indicating
that evaporative processes control the lake's isotopic mass
balance.
The fundamental controls on the isotope signature in sediment
are quite well understood and documented (e.g. Leng et al., 2005;
Ito, 2001). In the specific case of sediment that shows a covariant
trend between 18O and 13C, the isotopic signal can be used as a
strong archive for climatic reconstruction (e.g. Becker et al.,
2002). In arid climatic settings like Lagunillo del Tejo
significant covariance between 18O and 13C is primarily controlled
by the precipitation/evaporation balance (Deocampo, 2011) and has
been used to reconstruct the hydrological past of these systems
(e.g. Abell et al., 1982; Gasse and Fontes, 1989; Currey,
1990).
Odd numbered ISO zones (1, 3 and 5; Fig. 5) with higher isotopic
ratios are interpreted as arid periods while ISO 2, 4 and 6 have
low isotopic ratios corresponding to wetter periods. During arid
conditions evaporation is higher and the authigenic lake carbonates
are enriched in 18O. During wetter periods the situation reverses,
and the 18O of the lake water reflects the lighter isotopic
composition of the recharge. Isotopic fractionation of 13C in
dissolved inorganic carbon (DIC) can be important in hard water
lakes, caused by surface water CO2 exchange with the atmosphere.
CO2 evasion to the atmosphere may enrich 13CDIC (Polsenaere and
Abril, 2012) an effect that can intensify in shallow water, when
residence times are longest; in Lagunillo del Tejo this would
correspond to arid periods. 13CDIC also depends on the balance
between photosynthesis and organic decay (Henderson et al., 2003).
However, alkaline lakes are generally less susceptible to
biological impacts because the reservoir of DIC is very large.
Factors other than evaporation/precipitation balance are likely
to have marginal influence on isotopic signals. While seasonal
variation in temperature will influence both the isotopic
composition of the recharging rainfall and the isotopic
fractionation between water and precipitating carbonate, these
effects are generally masked by evaporation/residence time effects
in closed lakes (Talbot, 1990). Seasonal changes in the 18O of
inflowing water to a topographically closed-basin lakes will
influence 18Olakewater to a small degree, due to the isotopic
differences between, for example, spring runoff, compared to
growing season conditions when evapotranspiration is significant,
compared to evaporative concentration in summer. Overall the
isotopic signature in Lagunillo del Tejo shows a dominant
inter-annual variability (see below) without any obvious seasonal
signal due to sample resolution.
The 13C values in these lacustrine carbonates, while dominated
by residence time effects, may show marginal influence of organic
matter processing. These organic effects are, however, complex and
difficult to identify with certainty. In Lagunillo del Tejo, under
lowstand conditions the hypolimnetic volume is small relative to
epilimnia, where photosynthesis occurs and 13CDIC is typically
higher. The small volume hypolimnion and its underlying sediments
may be anoxic enough to cause some redissolution of sinking
carbonate particles, while methanogenesis could also enrich 13CDIC
in the near bottom waters (Myrbo, 2006). Conversely, during
highstand conditions, water column stratification occurs, along
with an increase in volume and permanence of the hypolimnion. This
could increase decomposition of sinking organic matter, which
reduces 13CDIC in the hypolimnion. Upon mixing in autumn, this
would result in an overall decrease in 13CDIC. Highstand late
summer hypolimnion anoxia is also frequent (Miracle and Vicente,
1983), but higher in a central area and near bottom water 13CDIC
due to dissolution of precipitated carbonates and methanogenesis,
could be counterbalanced by methane oxidation and mineralization of
organic matter in the broader water column (Wetzel, 2001).
When the isotopic data are plotted using the chronological
models of Romero-Viana et al., (2009) and Lpez-Blanco et al.,
(2012a), there is reason to suggest that isotopic variability in
sedimentary carbonate is related to lake levels changes, in turn
corresponding to precipitation variability in the last millennium
(Fig. 7). The data also agree with documentary sources cited in
Lpez-Blanco et al., (2012a, 2012b), with cladoceran and plant
macrofossil records (Lpez-Blanco et al. (2010, 2012b) and with
inferences made using R-O (reducing/oxidizing) zones (Romero-Viana
et al. 2009; Fig. 7).
R zones were interpreted by Romero-Viana et al. (2009) as
periods of high lake levels, when the stratification and
hypolimnion anoxia lead to reducing sediments rich in organic
matter. O zones occurred when the lake is shallow and
non-stratified. Similarities between the isotope signal and R-O
zones are strongest in the upper-middle part of the record where
odd ISO layers (Fig. 5) are coincident with O zones and even ISO
layers correspond with R zones (Fig. 7). This relationship is less
clear in the lower part of the core, where some R zones correspond
with low lake level periods (e.g. R3 and ISO 2/ISO 3). Inferred
lake levels from cladocerans and plant macrofossil records were
based on observations of the modern behavior of the lake. At higher
lake levels, the lake has two rings of macrophytes; the sedimentary
signal is then dominated by Chara spp. and cladocerans associated
with macrophytes, such as Graptoleberis testudinaria. When the lake
level lowers, the outer ring of macrophytes dries out, decreasing
the input of macrophyte-associated cladocerans to the sediment
(Lpez-Blanco et al., 2012a; Lpez-Blanco et al., 2012b; Lpez-Blanco
et al., 2013). Although the level of concordance between all
proxies is good for the most pronounced changes of lake level
(i.e., arid period between 19-20th century and wet period during
the Little Ice Age; Fig.7), the smaller changes of lake level are
not registered equally by all proxies. According to isotopes, plant
macrofossils and R-O zones, an arid period developed from ca. AD
1500 to AD 1600; however based only on the cladoceran data, this
arid period occurred earlier and was longer. This discrepancy could
be explained by a drop in lake level around AD 1400, prompting a
decrease in plant mass in the outer ring of macrophytes. The
cladoceran community probably did not recover again from this
event, resulting in an artificially enhanced signal of this arid
period. Alternatively, these discrepancies may simply highlight the
different climatic sensitivities of the various proxies (Magny et
al., 2008). For example, the R-O layering approach, and the
isotopic record, captures major lake level change, but does not
always identify smaller events. The phytophilous cladoceran record
of lake level lowering, however, shows a time lag in its response
(Lpez-Blanco et al., 2012b), suggesting that the plant macrofossil
record from the outer ring is the best at capturing short-term and
gradual changes in lake level.
The whole core sedimentary sequence shows an overwhelming
agreement between all the indicators for the major arid and humid
phases and reinforces the robustness of past lake level
reconstruction. Moreover the inferred lake levels based on isotopic
signal show a clear correlation with NAOms (North Atlantic
Oscilation, Morocco-Scotland) (Fig.7) as proposed by Trouet et al.
(2009) and calculated as the difference between the 30-year
smoothed and normalized Scotland and Morocco records (the northern
and southern nodes of the NAO dipole). Both signals share a similar
pattern from 1100 AD to ca. 1975 AD (Fig.7), recording the behavior
of NAO during the most recent warmth (Medieval Climatic Anomaly)
and cooling (Little Ice Age) over the North Atlantic/European
sector. However, from ca.1975 AD there is a decoupling of isotope
and NAO signals; 18O decreases while NAOms increases. While this
decoupling could be associated with anthropogenic alterations of
the water cycle through water capture or damming, we have no
evidence of such modifications in recent times. The NAO,
precipitation amount, intensity and frequency also vary naturally
with climate change. From 1900 to 2005 AD pronounced long term
trends have been observed in the North Atlantic/Europe,
significantly wetter in northern Europe but drier in the Sahel,
southern Africa and the Mediterranean (Trenberth et al., 2007).
Garcia-Herrera et al. (2007) describes blocking events during
negative phases of NAO in 2005 inhibiting the occurrence of
precipitation over Iberia and leading to a negative NAO index
anomalously associated with low precipitation records in that year.
Thus, recent changes in temperature and in past patterns of air
circulation may explain this decoupled signal.
The strong agreement between the carbonate 18O of Lagunillo del
Tejo and the longest NAO reconstruction (1050-2000AD; Trouet et
al., 2009) has potential to extend the NAO record further back in
time using longer cores from this lake. Negative phases of NAO lead
winter storms crossing on west-east pathway which track southwards
toward the Mediterranean Sea and bring moist air into the
Mediterranean and cold air to Northern Europe whereas positive
phases of NAO result in more and stronger winter storms crossing
the Atlantic Ocean on a more northerly track and it is associated
with low precipitation in the Mediterranean (Visbeck et al., 2001).
Given the important influence of this synoptic mode of climatic
variation on marine/terrestrial ecosystems and regional
socioeconomic activity, this reconstruction has important
implications for climatic modeling and ecological management.
CONCLUSION
Quantitative global and regional climate reconstructions for
past millennia are fundamental to place the modern trends of
observed climate variables into a long-term context. The 18O and
13C in Lagunillo del Tejo are positively correlated with each other
and mirror the instrumental, historical and other proxy records of
hydroclimate. Furthermore, the carbonate 18O record correlates with
the NAO index, giving us confidence that carbonate 18O can be used
to infer changes in precipitation back in time. This sensitive site
offers an exceptional opportunity to calibrate regional rainfall,
lake level and isotopic composition of lake water back into the
Holocene. Future studies should strive to obtain longer and higher
resolution sediment records to extend this promising
calibration.
ACKNOWLEDGMENTS
CLB acknowledges Lidia Romero-Viana for providing data about
rainfall reconstruction in Lake La Cruz. CLB was supported by a FPU
scholarship from the MICINN and the research was financed by the
MICINN: CGL2005- 04040/BOS project and the CGL2009-06772-E/BOS
grant to EV. CLB acknowledges School of Environmental Sciences at
the University of East Anglia (UEA) and especially to Alina D.
Marca for her welcome and help while performing isotope analysis.
CLB acknowledges her grant to the Prometeo Proyect from the
Secretara de Educacin Superior, Ciencia, Tecnologa e Innovacin de
la Repblica del Ecuador (SENESCYT).
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