Geoffrey O. Seltzer

Variability of El Niño/Southern Oscillation activity at millennial timescales during the Holocene epoch

The variability of El Niño/Southern Oscillation (ENSO) during the Holocene epoch, in particular on millennial timescales, is poorly understood. Palaeoclimate studies have documented ENSO variability for selected intervals in the Holocene, but most records are either too short or insufficiently resolved to investigate variability on millennial scales. Here we present a record of sedimentation in Laguna Pallcacocha, southern Ecuador, which is strongly influenced by ENSO variability, and covers the past 12,000 years continuously. We find that changes on a timescale of 2–8 years, which we attribute to warm ENSO events, become more frequent over the Holocene until about 1,200 years ago, and then decline towards the present. Periods of relatively high and low ENSO activity, alternating at a timescale of about 2,000 years, are superimposed on this long-term trend. We attribute the long-term trend to orbitally induced changes in insolation, and suggest internal ENSO dynamics as a possible cause of the millennial variability. However, the millennial oscillation will need to be confirmed in other ENSO proxy records.

Tropical Climate Changes at Millennial and Orbital Timescales on the Bolivian Altiplano

Tropical South America is one of the three main centres of the global, zonal overturning circulation of the equatorial atmosphere (generally termed the ‘Walker’ circulation). Although this area plays a key role in global climate cycles, little is known about South American climate history. Here we describe sediment cores and down-hole logging results of deep drilling in the Salar de Uyuni, on the Bolivian Altiplano, located in the tropical Andes. We demonstrate that during the past 50,000 years the Altiplano underwent important changes in effective moisture at both orbital (20,000-year) and millennial timescales. Long-duration wet periods, such as the Last Glacial Maximum—marked in the drill core by continuous deposition of lacustrine sediments—appear to have occurred in phase with summer insolation maxima produced by the Earths precessional cycle. Short-duration, millennial events correlate well with North Atlantic cold events, including Heinrich events 1 and 2, as well as the Younger Dryas episode. At both millennial and orbital timescales, cold sea surface temperatures in the high-latitude North Atlantic were coeval with wet conditions in tropical South America, suggesting a common forcing.

Holocene paleohydrology and glacial history of the central Andes using multiproxy lake sediment studies

Here we document at century to millennial scale the regional changes of precipitation^evaporation from the late Pleistocene to present with multiproxy methods on a north^south transect of lake sites across the eastern cordillera of the central Andes. The transect of study sites covers the area from V14‡S to 20‡S and includes core studies from seven lakes and modern calibration water samples from twenty-three watersheds analyzed to constrain the down-core interpretations of stable isotopes and diatoms. We selected lakes in different hydrologic settings spanning a range of sensitivity to changes in the moisture balance. These include: (1) lakes directly receiving glacial meltwater, (2) overflowing lakes in glaciated watersheds, (3) overflowing lakes in watersheds without active glaciers, and (4) lakes that become closed basins during the dry season. The results of our current work on multiple lakes in the Bolivian Andes show that while the overall pattern of Holocene environmental change is consistent within the region, conditions were not always stable over centennial to over millennial timescales and considerable decadal- to centuryscale climate variability is evident [Abbott et al., Quat. Res. 47 (1997) 70^80, Quat. Res. 47 (1997) 169^180, Quat. Sci. Rev. 19 (2000) 1801^1820; Polissar, Master’s thesis, University of Massachusetts (1999)]. Comparison of the paleoclimate record from one well-studied site, Lago Taypi Chaka Kkota (LTCK), with others within the region illustrates a consistent overall pattern of aridity from the late glacial through the middle Holocene. Previous work noted a difference between the timing of water-level rise in Lake Titicaca V5.0^3.5 ka B.P. [Abbott et al., Quat. Res. 47 (1997) 169^180; Cross et al., Holocene 10 (2000) 21^32; Rowe et al., Clim. Change 52 (2002) 175^199] and the onset of wetter conditions at 2.3 ka B.P. in LTCK, a lake that drains into the southern end of Lake Titicaca [Abbott et al., Quat. Res. 47 (1997) 70^80]. Sedimentary and oxygen isotope evidence from Paco Cocha (13‡54PS) located in the northern reaches of the expansive 57 000 km 2 Titicaca watershed, which spans V14‡S to 17‡S, indicates that

Isotopic evidence for late Quaternary climatic change in tropical South America

The tropical hydrologic cycle affects atmospheric trace gases and global climate change, and thus records of hydrologic change encompassing a variety of time scales from the low latitudes are important in paleoclimatology. Isotopic analysis of calcite from Lake Junin, Peru, provides a record of hydrologic variability that spans the last glacial-interglacial transition in the southern tropics. The record reveals a 6‰ enrichment in δ 18 O calcite during the late glacial followed by a gradual depletion during the Holocene, which can be interpreted as a decrease followed by a long-term increase in effective moisture. Close agreement between δ 18 O calcite and rainy season insolation indicates that long-term changes in tropical hydrology were linked to orbital variations. Furthermore, hydrologic change was out of phase in the northern and southern tropics over this time period.

A new estimate of the Holocene lowstand level of Lake Titicaca, central Andes, and implications for tropical palaeohydrology

New evidence from piston cores and high-resolution seismic reflection data shows that water levels in Lake Titicaca were as much as 100 m below the present level during the early to mid-Holocene (between .6 and 3.8 14C kyr BP). Climatological and modelling studies indicate that Lake Titicaca rainfall depends on convective activity in upwind Amazonia; the lake-level data therefore suggest a drier Amazon Basin during this time. This view is bolstered by an excellent match between the Titicaca lake-level curve and decreased methane concentrations in Greenland ice, previously ascribed to drying of low-latitude wetlands (Blunier et al., 1995). The postglacial history of Lake Titicaca fits a global pattern of lake-level change in the tropics, characterized by opposite phasing between the Southern and Northern Hemispheres. This pattern is most likely the result of orbital controls over the intensity of summer insolation.

A Vegetation and Fire History of Lake Titicaca since the Last Glacial Maximum

Fine-resolution fossil pollen and charcoal analyses reconstruct a vegetation and fire history in the area surrounding Lake Titicaca (3810 m, Peru/Bolivia) since ca. 27 500 cal yr BP (hereafter BP). Time control was based on 26 accelerator mass spectrometer (AMS) radiocarbon dates. Seventeen AMS dates and 155 pollen and charcoal samples between ca. 17 500 BP and ca. 3100 BP allow a centennial-scale reconstruction of deglacial and early- to mid-Holocene events. Local and regional fire signals were based on the separation of two charcoal size fractions, ≥180 μm and 179–65 μm. Charcoal abundance correlated closely with the proportion of woody taxa present in the pollen spectra. Little or no pollen was detected in the sedimentary record prior to ca. 21 000 BP. Very cold climatic conditions prevailed, with temperatures suggested to be at least 5–8°C cooler than present. Increases in pollen concentration suggest initial warming at ca. 21 000 BP with a more significant transition toward deglaciation ca. 17 700 BP. Between 17 700 BP and 13 700 BP, puna brava is progressively replaced by puna and sub-puna elements. The most significant changes between the Pleistocene and the Holocene floras were largely complete by 13 700 BP, providing an effective onset of near-modern conditions markedly earlier than in other Andean records. Fire first occurs in the catchment at ca. 17 700 BP and becomes progressively more important as fuel loads increase. No evidence is found of a rapid cooling and warming coincident with the Younger Dryas chron. A dry event between ca. 9000 BP and 3100 BP, with a peak between 6000 and 4000 BP, is inferred from changes in the composition of aquatics, and the marsh community as pollen of Cyperaceae is replaced by Poaceae, Apiaceae, Plantago and the shrub Polylepis. Human disturbance of the landscape is evident in the pollen spectra after ca. 3100 BP with the appearance of weed species.

MODERN AND LAST LOCAL GLACIAL MAXIMUM SNOWLINES IN THE CENTRAL ANDES OF PERU, BOLIVIA, AND NORTHERN CHILE

Late Pleistocene snowlines in the central Andes were 500–1200 m lower than at present. Radiocarbon dates imply that the late-Pleistocene glacial maximum in the region occurred prior to 20 14C ka, but lack of maximum limiting ages adds considerable uncertainty to the exact timing. Snowline modeling demonstrates that snowlines in the eastern and western cordilleras of the central Andes respond differently to temperature and precipitation changes. In the eastern cordillera, the snowline is near the level of the annual 0°C isotherm and melting occurs throughout the year. Here snowlines are sensitive to temperature changes, but relatively insensitive to accumulation changes. In the western cordillera, the snowline rises 1000 higher owing to increasing aridity, and the snowline exhibits a much stronger sensitivity to accumulation changes. The consistent 1200 m snowline depression along the eastern cordilleras of the central Andes can be modeled by a mean annual cooling of 5–9°C. This is inconsistent with the <2°C cooling in tropical sea-surface temperatures suggested by CLIMAP reconstructions. The 800–1000 m snowline depression in the western cordillera cannot be accounted for solely by cooling, but also requires an increase in precipitation.

A Late Quaternary diatom record of tropical climatic history from Lake Titicaca (Peru and Bolivia)

Abstract A composite high-resolution diatom stratigraphy from three piston cores and one box-core in the deep sub-basin of Lake Titicaca reveals large moisture variations during the past 30 kyr in the Altiplano region. Diatom sequences indicate orbital and millennial-scale variability in water level and salinity. The pelagic freshwater diatom species Cyclotella andina and Cyclotella stelligera dominate Glacial-age sediments, suggesting that the lake was above its present outlet. Generally, wet conditions continued until 11 000 cal yr BP, as indicated by high percentages of freshwater planktonic diatoms. Large pulses of benthic diatom species between about 11 000 and 10 000 cal yr BP suggest brief intervals of large-amplitude declines in lake level. During the early Holocene (10 000–8500 cal yr BP), a freshwater diatom assemblage suggests overflowing conditions. Pelagic freshwater diatoms are replaced ca. 8500 cal yr BP by the salinity-indifferent species Cyclotella meneghiniana and by benthic taxa, indicating the beginning of lake regression. During the mid-Holocene (6000–3500 cal yr BP), the abundance of the saline taxon Chaetoceros muelleri , coupled with high abundances of epiphytic and epipelic diatoms, indicates maximum salinity and lowest lake levels in the entire 30 000 year record. Lake transgression began ca. 4000 cal yr BP, and the lake achieved modern levels by about 1500 cal yr BP. These water-level changes imply changes in effective moisture, most likely resulting from large precipitation changes. Precipitation was high throughout the Last Glacial Maximum (21 000–18 000 cal yr BP), likely due to an enhanced South American Summer Monsoon during peak summer insolation in the Southern Hemisphere. In contrast, the mid-Holocene transition was dryer than today in association with an austral summer insolation minimum and the subsequent weakening of the summer monsoon.

Tropical glacier meltwater contribution to stream discharge: a case study in the Cordillera Blanca, Peru

Discharge measurements, climate observations and hydrochemical samples gathered monthly (1998/99) in the Yanamarey and Uruashraju glacier-fed catchments of the Cordillera Blanca, Peru, permit an analysis of the glacier meltwater contribution to stream-flow. These glacier catchments feed the Rio Santa, which discharges into the Pacific Ocean. Based on a water-balance computation, glacier melt contributes an estimated 35% of the average discharge from the catchments. For comparison, a volumetric end-member mixing model of oxygen isotopes shows glacier melt contributes 30-45% to the total annual discharge. Based on stream geochemistry, discharge from the Yanamarey glacier catchment provides 30% of the annual volume discharged from the Querococha watershed, which is < 10% glacierized. By analogy, the larger Rio Santa watershed, also < 10% glacierized, receives at least 12% of its annual discharge from melting glacier ice. Tributary watersheds to the Rio Santa with larger fractions of glacier cover have less variable runoff and enhanced discharge, demonstrating that the glaciers effectively buffer stream discharge seasonally. With continued glacier melting, stream-flow will likely become more variable, and there will be less dry-season runoff.

High-resolution seismic reflection profiles from Lake Titicaca, Peru-Bolivia: Evidence for Holocene aridity in the tropical Andes

High-resolution seismic reflection profiles of the sediments of Lake Titicaca, Peru-Bolivia, suggest that lake levels in the recent past were considerably lower than today. Incised channels on the major deltas extend to depths of 85 m below modern lake level. Erosional truncation of onlapping seismic reflectors is found at similar depths. This interpretation of the seismic data is supported by analyses of sediment cores from the lake, which indicate that there was a significant drop in lake level during the early to mid-Holocene.