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: charo.lopez-blanco@uv.es

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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