Blas L. Valero-Garcés

A 22,000 14C year BP sediment and pollen record of climate change from Laguna Miscanti (23°S), northern Chile

Lake sediments and pollen, spores and algae from the high-elevation endorheic Laguna Miscanti (22°45′S, 67°45′W, 4140 m a.s.l., 13.5 km2 water surface, 10 m deep) in the Atacama Desert of northern Chile provide information about abrupt and high amplitude changes in effective moisture. Although the lack of terrestrial organic macrofossils and the presence of a significant 14C reservoir effect make radiocarbon dating of lake sediments very difficult, we propose the following palaeoenvironmental history. An initial shallow freshwater lake (ca. 22,000 14C years BP) disappeared during the extremely dry conditions of the Last Glacial Maximum (LGM; 18,000 14C years BP). That section is devoid of pollen. The late-glacial lake transgression started around 12,000 14C years BP, peaked in two phases between ca. 11,000 and <9000 14C years BP, and terminated around 8000 14C years BP. Effective moisture increased more than three times compared to modern conditions (∼200 mm precipitation), and a relatively dense terrestrial vegetation was established. Very shallow hypersaline lacustrine conditions prevailed during the mid-Holocene until ca. 3600 14C years BP. However, numerous drying and wetting cycles suggest frequent changes in moisture, maybe even individual storms during the mid-Holocene. After several humid spells, modern conditions were reached at ca. 3000 14C years BP. Comparison between limnogeological data and pollen of terrestrial plants suggest century-scale response lags. Relatively constant concentrations of long-distance transported pollen from lowlands east of the Andes suggest similar atmospheric circulation patterns (mainly tropical summer rainfall) throughout the entire period of time. These findings compare favorably with other regional paleoenvironmental data.

Sediment sources and siltation in mountain reservoirs : a case study from the Central Spanish Pyrenees

Abstract Based on a 137 Cs -derived chronology, we reconstruct the depositional history of the Barasona reservoir in the Esera–Isabena Basin, Central Pyrenees (Spain). Most of the sediments were delivered to the Barasona reservoir during flood events. Comparative mineralogical studies of the Esera and Isabena river channels and the Barasona reservoir sediments help to identify sediment sources and areas of high sediment yield risk, and to establish erosion and sediment transport processes in the watershed. Changes in the mineralogical composition of the sediments can be used to discriminate the relative contribution of the Esera and Isabena rivers during flood periods. Three main periods in the siltation history of the Barasona reservoir were distinguished: (1) From dam construction (1932) to the early 1950s, the reservoir was flushed every year using the lower gates, and average sediment accumulation was very low (1.5 cm/yr). (2) During the 1950s and 1960s, the sedimentation rate increased and reached the highest values: 15 cm/yr, 24.6 cm/yr and 18.2 cm/yr in the Esera river mouth, the northern and the southern areas of the reservoir, respectively. Both changes in the exploitation regime (no annual flushes) and climate variability (increased flood frequency and river inflow) caused this increase in sediment delivery and accumulation. (3) The third period was initiated by the enlargement of the dam in 1972. The new sedimentary dynamics in the reservoir caused a general decrease in the sedimentation rate (11.6 cm/yr in the southern areas; 10 cm/yr in the Esera mouth) that was especially noticeable in the northern areas, where the accumulation rate was reduced to 3.7 cm/yr. An increase in channel erosion processes during this period contributed to the transport of sediments to the inner areas of the reservoir.

Limnogeology of Laguna Miscanti: evidence for mid to late Holocene moisture changes in the Atacama Altiplano (Northern Chile)

Sedimentological, mineralogical and geochemical analyses of sediment cores from 9 m-deep, saline Laguna Miscanti, Chile (23 ° 44′S, 67 °46′W, 4140 m a.s.l.) together with high-resolution seismic profiles provide a mid to late Holocene time series of regional environmental change in the Atacama Altiplano constrained by 210Pb and conventional 14C dating. The mid Holocene was the most arid interval since the last glacial maximum, as documented by subaerial exposure and formation of hardgrounds on a playa surface. Extremely low lake levels during the mid Holocene appear consistent with lower effective moisture recorded at other sites along the Altiplano and in the Amazon Basin. Termination of this arid period represented a major shift in the regional environmental dynamics and inaugurated modern atmospheric conditions. The cores show a progressive upward increase in effective moisture interrupted by numerous century-scale drier periods of various intensities and durations that characterize a fluctuating late Holocene climate. In spite of chronological uncertainties, the major environmental changes seem to correlate with the available paleorecords from the region providing a coherent account of effective moisture variability in the tropical highlands of South America.

A Review of 2000 Years of Paleoclimatic Evidence in the Mediterranean

The integration of climate information from instrumental data and documentary and natural archives; evidence of past human activity derived from historical, paleoecological, and archaeological records; and new climate modeling techniques promises major breakthroughs for our understanding of climate sensitivity, ecological processes, environmental response, and human impact. In this chapter, we review the availability and potential of instrumental data, less well-known written records, and terrestrial and marine natural proxy archives for climate in the Mediterranean region over the last 2000 years. We highlight the need to integrate these different proxy archives and the importance for multiproxy studies of disentangling complex relationships among climate, sea-level changes, fire, vegetation, and forests, as well as land use and other human impacts. Focusing on dating uncertainties, we address seasonality effects and other uncertainties in the different proxy records. We describe known and anticipated challenges posed by integrating multiple diverse proxies in high-resolution climate-variation reconstructions, including proxy limitations to robust reconstruction of the natural range of climate variability and problems specific to temporal scales from interannual to multicentennial. Finally, we highlight the potential of paleo models to contribute to climate reconstructions in the Mediterranean, by narrowing the range of climate-sensitivity estimates and by assimilating multiple proxies.

Moisture changes and fluctuations of the Westerlies in Mediterranean Central Chile during the last 2000 years: The Laguna Aculeo record (33°50′S)

Abstract The multi-proxy study of lacustrine sediments from Laguna Aculeo (33°50′S) provides detailed information about moisture changes in the lowlands of Mediterranean Central Chile during the last 2000 years. The lake lies just at the northern border of the strong Westerlies influence with dry summers and humid winters. Geochemical, sedimentological and diatom analyses provide evidence for an arid period between cal 200 BC and AD 200 and a subsequent increase in moisture after cal AD 200. Abundant clastic layers in the core represent flood events. Periods with a higher frequency of flood events indicate an increased intensity of the Westerlies, more winter frontal system activity and possibly ENSO-related variability, probably comparable to modern conditions in Mediterranean Central Chile. Periods of high clastic input occur around cal AD 200–400, 500–700, especially around cal AD 1300–1700, and around AD 1850–1998. During the last 50 years, at least eight flood events were detected, correlating mainly with El Nino years. A very short drier period also occurred in the late 1960s. In recent decades, human impact has resulted in a eutrophication of the lake.

Lateglacial and Late Holocene environmental and vegetational change in Salada Mediana, central Ebro Basin, Spain

The Salada Mediana lacustrine sequence, central Ebro Basin, Spain (41330@10AN, 0344@W, 350 m a.s.l.) provides an example of the potential and limitations of saline lake records as palaeoclimate proxies in the semi-arid Mediterranean region. Sedimentary facies analyses, chemical stratigraphy, stable isotopes (d18O and d13C) of authigenic carbonates, d13C values of bulk organic matter and pollen analyses from sediment cores provide paleohydrological and vegetation change reconstructions for the Lateglacial and Late Holocene in the central Ebro basin. A preliminary chronology is based on 210Pb and 14C AMS dates. The lacustrine sequence is composed of three sedimentary Sections. The Lower Section was deposited in a permanent saline to brackish lake. This stage represents the most humid period in the record and it was accompanied by the expansion of temperate trees (particularly Corylus). The Middle Section was deposited in an ephemeral playa-lake complex. Frequent subaerial exposure conditions favour the colonisation of the playa lake #oor by Chenopodiaceae during a low water table period. This interval re#ects the most arid conditions in the Salada Mediana record, including the current environment. A secondary temperate tree expansion occurred after the maximum aridity period. Aquatic plants and cyanobacterial mats spread in the lake during periods of raised water tables.This paleohydrological and vegetational evolution attests to large changes in e!ective moisture during the Lateglacial in the semi-arid northeastern Spain. The abundance of Corylus during the Lateglacial indicates that refugia for temperate trees were located along the Ebro valley during the Last Glacial Maximum. The Holocene sediments in the Salada Mediana records have been eroded, and the Upper Section represents deposition during the last few centuries. ( 2000 Elsevier Science Ltd and INQUA. All rights reserved.

Large 13C enrichment in primary carbonates from Andean Altiplano lakes, northwest Argentina

Abstract We report here extreme 13 C enrichments up to +13‰ PDB in primary calcite and aragonite precipitates in saline, well oxygenated waters from high-altitude lakes in the southern Andean Altiplano, northwestern Argentina. Biological effects, as well as variations in carbon source inputs, and in the exchange rate with atmospheric CO 2 , are commonly considered the main controls on the carbon isotope values of authigenic lacustrine carbonate. We present sedimentological and geochemical evidence that favors physical processes — evaporation effects and CO 2 -degassing — as major controls on 13 C enrichment. We propose that large enrichments may result from the non-equilibrium gas-transfer isotope fractionation during CO 2 -degassing from thermal springs and evaporation effects in arid environments. The dilution effect by large quantities of 14 C-free CO 2 hinders accurate 14 C chronology of these lake records based on lacustrine organic matter and aquatic plants. Our results indicate that geothermal and volcanic CO 2 sources in lake basins located in volcanic settings, and physical fractionation may have a greater significance than commonly accepted to explain lacustrine carbon isotope records.

Responses of a saline lake ecosystem in a semiarid region to irrigation and climate variability- The history of Salada Chiprana, central Ebro basin, Spain

Abstract To adequately manage the fragile and changing environments of semiarid regions it is essential to disentangle human from climate or other environmental impacts over longer timescales than human memory. We investigated sediment cores from Salada Chiprana, a saline lake in the central Ebro basin in Spain, using pollen, charcoal, sedimentological, geochemical and radiometric dating techniques. The sequence indicates a rapid evolution from an ephemeral playa lake during the Late Holocene to a permanent saline lake a few centuries ago. The limnological evolution correlates with changes in agricultural practices and provides evidence of the strong impact of irrigation on the lakes hydrological balance from the XVth century. The work demonstrates that the Salada Chiprana, the only permanent, relatively deep, hypersaline lake in Spain has been created by a long history of human interaction with the landscape.

Patterns of regional hydrological variability in central-southern Altiplano (18°–26°S) lakes during the last 500 years

Abstract Paleohydrological reconstructions based on sedimentological, geochemical, and isotopic records from a lake transect in the central-southern Altiplano (18°–26°S) indicate abrupt moisture fluctuations during the last 500 years. A change to modern conditions occurred in the late 19th century in all the records, from northern Chile (Lago Chungara, 18°15′S) and the Atacama (Laguna Miscanti, 23°45′S) to the southern tip of the Altiplano (Laguna El Peinado, NW Argentina, 26°30′S). A previous drier period shows different patterns of timing, duration, and intensity. In Chungara, the arid period was shorter and occurred during the late 19th and early 20th centuries, while in Miscanti, it occurred earlier and ended at the beginning of the 20th century. In El Peinado, conditions were wetter during the 17–19th centuries and the arid period occurred prior to the 17th century. Other records from the region show abrupt paleohydrological and paleoclimatic changes synchronous with the termination of the Little Ice Age. Despite local differences and dating uncertainties, the Little Ice Age stands out as a significant though complex climatic event in the Andean Altiplano. The discrepancies between the northern and southern Altiplano records during the last few centuries may reflect contrasting responses to external forcing in two areas with different climatic regimes.

Glacial and Fluvial Deposits in the Aragón Valley, Central-Western Pyrenees: Chronology of the Pyrenean Late Pleistocene Glaciers

Abstract The ragón alley glacier (entral estern yrenees) has been studied since the late nineteenth century and has become one of the best areas in the yrenees to study the occurrence of leistocene glaciations and the relationships between moraines and fluvial terraces. New morphological studies and absolute ages for moraines and fluvial terraces in the ragón alley allow a correlation with other yrenean glaciers and provide solid chronologies about the asynchroneity between global last glacial maximum and the maximum ice extent. Six frontal arcs and three lateral morainic ridges were identified in the illanúa basin terminal glacial complex. The main moraines (1 and 2) correspond to two glacial stages (oxygen isotopic stages 6 and 4), dated at 171 ± 22-ka and 68 ± 7-ka, respectively. From a topographical point of view, moraine M1 appears to be linked to the 60 m fluvioglacial terrace, dated in a tributary of the ragón iver at 263 ± 21-ka. The difference in age between 1 moraine and the 60 m fluvioglacial terrace suggests that the latter belongs to an earlier glacial stage ( 8). Moraine 2 was clearly linked to the fluvioglacial 20 m terrace. Other minor internal moraines were related to the 7–8 m terrace. The dates obtained for the last glacial cycle (20–18-ka) are similar to other chronologies for Mediterranean mountains, and confirm the occurrence of an early in the entral yrenees that does not coincide with the global .