Introduction

Addressing the significance of recent global climatic variations requires a long-term global-scale frame of reference against which present and future changes may be assessed. Fortunately, proxy records for paleoclimatic reconstruction are available from select high-elevation tropical and subtropical ice caps. These records are of particular interest as they are located where climate has a significant impact on human activity. In 1983, two cores were drilled to bedrock on the Quelccaya ice cap (13°56′ S, 70°50′ W, 5670 m a.s.l.), located on the easternmost rise of the Andes in southern Peru (Fig. 1). These cores provided much valuable information about climatic variations over the last 1500 years in this region (Reference Thompson, Mosley-Thompson, Bolzan and Koci.Thompson and others, 1985, Reference Thompson, Mosley-Thompson, Dansgaard and Grootes.1986); however, they represent the conditions at a single location. Thus, to distinguish large-scale regional or global phenomena from local events it is necessary to retrieve paleoclimatic histories from other tropical locations. In 1993 two cores were recovered to bedrock from the col of Huascarán (9°06′ S, 77°36′ W, 6047 m a.s.l.) in north-central Peru. When complete, the analyses of these cores are expected to provide a climatic history exceeding that from Quelccaya. This paper presents the microparticle (MPC) and nitrate (NO₃ ⁻) concentrations and oxygen-isotopic ratios (δ ¹⁸Ο) for the most recent part of the record (1950–93), extracted from the upper 57 m of the Huascarán core.

Fig. 1. Map of Peru showing the locations of ice caps in the Cordillera Blanca, the Quelccaya ice cap to the south and the meteorological stations at Recuay and Chiquian.

Field and Laboratory Methods

The drilling was accomplished in July–August 1993 using a portable, lightweight, solar-powered system assembled at the Byrd Polar Research Center. It is designed to be broken into components which can be backpacked into remote sites. The first core, 160.3 m long, was cut into samples which were melted and bottled in the field. Core 2, 166.1 m long, was returned frozen to The Ohio Suite University, The upper 33.5 m of core 2, which consisted primarily of firn, was cut from the center of the core into separate samples of equal size for each of the analyses. To handle the fini during inning, several sets of latex gloves were used which had been previously scrubbed in Milli-Q ultra-pure water. Below the firn/ice transition, samples were also cut from the center of the core, but those for MPC and NO₃ ⁻ analyses were transported to a class-100 clean room and washed with Milli-Q water prior to melting.

Concentrations of microparticles with diameters between 2.0 and 40.3 μm were measured with a Coulter Counter Model TAII equipped with a 100 μm aperture tube. Nitrate concentrations were analyzed using a Dionex Model 2010i ion chromatograph, and δ ¹⁸O was measured using a Finnigan Mat Delta E mass spectrometer.

Results and Discussion

At high elevations in the Peruvian Andes, 80–90% of the annual precipitation occurs as snowfall during the austral summer and fall (November–May). On the col of Huascarán an accumulation/strain network of 15 stakes covering an area of 1.2 km × 2.3 km was established in September 1991. The stake heights were remeasured and extended in October 1992 and again in July 1993. The 2 year average of annual accumulation (A _n) is 3.3 m of snow, or 1.29 m w.e.. This is consistent with the results from a 3 m snow pit excavated in July 1993 adjacent to the drill site which contains 1 year of accumulation. The winter dry season is characterized by higher MPC and NO₃ ⁻ concentrations, and generally less negative δ ¹⁸O (Fig. 2a–c). MPC is also higher during the dry season on the Quelccaya ice cap (Reference Thompson, Hastenrath and Morales Arnao.Thompson and others, 1979). Concentrations of Cl⁻ and SO₄ ²⁻ do not display distinct wet/dry season signals.

Fig. 2. (a) Microparticle concentrations (MPC), (b) nitrate concentrations (NO ₃ ⁻), and (c) oxygen-isotopic ratios (δ ¹⁸ O) for a 3 m pit excavated in July 1993 adjacent to the drilling site on Huascarán. The pit record represents 1 year of accumulation. (d) Monthly averages of air temperatures from the AWS on neighboring Hualcán, November 1991–July 1993. The winter dry season (May–October) is marked by high MPC and NO ₃ ⁻ and enriched δ ¹⁸ O and lower air temperatures.

On Huascarán, as on Quelccaya, the increased MPC and NO₃ ⁻ in the dry season result from high radiation receipt coupled with little snow accumulation (Reference Thompson, Mosley-Thompson, Grootes, Pourchet and Hastenrath.Thompson and others, 1984). Multiple thin radiation crusts are generally associated with the dust peaks. MPC also reflects the nature of and distance from source areas and wind speed. The satellite-linked automatic weather station (AWS) situated on the nearby Hualcán ice cap (9°15′ S, 77°30′ W, 5273 m a.s.l.) has so far provided temperature and wind data from November 1991 to September 1993. Winds vary from strong northeasterlies during the winter (up to 6 m_s ⁻¹ in July) to weaker southeasterlies in summer (2.5–4 m s⁻¹). Throughout the year, the dominant wind direction is from the Amazon basin, a possible source area for much of the dust. This local wind regime is different from that on Quelccaya to the south, where westerlies and northwesterlies dominate in the dry season (Reference Mosley-ThompsonMosley-Thompson, 1982). The observed wind directions at both Huascarán and Quelccaya agree with the large-scale 500 mbar flow pattern described by Reference Chu and Hastenrath.Chu and Hastenrath (1982).

The ice-core time-scale is based on the seasonal fluctuations in MPC, NO₃ ⁻ and δ ¹⁸O and is shown (Fig. 3) for three sections in the upper 57 m of core 2. The chinning of the annual layers with depth is obvious. The annual cycles are depicted as thermal years (i.e. from dry season to dry season) which are easily identified, particularly in NO₃ ⁻ concentrations. This is expected to provide the template for dating the deep core to the depth at which annual resolution is lost due to layer thinning.

Fig. 3. Three intervals from the Huascarán ice core (1950/51 to 1955/56; 1969/70 to 1973/74; 1987/88 to 1989/90). Time lines (dashed) are drawn to depict thermal years (dry season to dry season). The distinct seasonality in MPC and NO₃ ⁻ is illustrated, as is the thinning of the annual layers with depth.

Most of the snow on Huascarán which falls in the Southern Hemispheric summer is characterized by ¹⁸O depletion (Fig. 2c), which is opposite to the temperature– δ ¹⁸O relationship observed in the polar regions (Reference DansgaardDansgaard, 1964). AWS temperature data from Hualcán indicate that the warmest temperatures occur in the summer (Fig. 2d), and the lowest from August to October, at which time snowfall is enriched in ¹⁸O. The transfer functions essential for interpreting δ ¹⁸O in tropical ice caps are not fully understood. Reference Groontes, Stuiver, Thompson and Mosley-ThompsonGrootes and others (1989) analyzed the δ ¹⁸O record from Quelccaya and theorized that on a seasonal basis δ ¹⁸O does not strongly collect temperature as in high latitudes, hut results from a combination of factors including air-mass stability and air circulation. Seasonal differences in the isotopic composition of the moisture from the Amazon basin may also be responsible for the isotopic variations in the ice. A more detailed discussion is provided by Reference Molión.Molión (1975), Reference Lettau, Lettau and Molión.Lettau and others (1979) and Reference Salati, Dall’Olio, Matsui and Gat.Salati and others (1979).

Several studies have noted a seasonal and annual “amount effect”, which is a relationship between high stable-isotopic ratios and Low amounts of precipitation, especially in tropical regions (Reference DansgaardDansgaard, 1964; Reference Yurtserver and Gat.Yurtserver and Gat, 1981; Reference Jouzel, Russell, Suozzo, Koster, White and Broecker.Jouzel and others, 1987). Although on Huascarán and Quelccaya the wet-season snow is isotopically depleted, annually averaged δ ¹⁸O (δ ¹⁸O_an) and A _n (reconstructed from layer thicknesses) are poorly correlated; the coefficient of determination (R ² ) is less than 0.1. For a longer time perspective, Reference Thompson, Mosley-Thompson, Dansgaard and Grootes.Thompson and others (1986) noted that the Little Ice Age on Quelccaya was characterized by depletion in δ ¹⁸O from AD 1500 to 1900, but that accumulation was above average from 1500 to 1700 and below average from 1700 to 1900. The “amount effect” in the tropics was documented using data mainly from warm (> 15°C), low-elevation sites. It is possible that this relationship does not apply to colder, higher elevations, although more investigation is needed to verify this.

It is of interest to note that while the seasonal temperature range determined from an AWS on Quelccaya is about 2°C (Reference Mosley-ThompsonMosley-Thompson, 1982), the AWS data from Hualcán show a 9°C difference between winter and summer temperatures (Fig. 2d). The 1993 Huascarán core and the combined records from the two Quelccaya cores show well-preserved seasonably in δ ¹⁸O, with a seasonal variation of ≈8‰ at the surface. However, the seasonal range in Quelccaya decreases to approximately 4‰ at 40 m (Reference Thompson, Bradley and JonesThompson, 1992), which is dated 1950, while that ou Huascarán does not decrease with depth in the upper 57 m.

The δ ¹⁸O_an values from the Huascarán core are similar to those found in the Quelccaya ice cap (Reference Thompson and Mosley-ThompsonThompson and Mosley-Thompson, 1992) for the period 1950–83 (Fig. 4). Not only the trends but also the ranges of the annual averages over the last four decades are comparable at both sites, indicating that they may lie affected by the same processes, for example, they share a common wet/dry season moisture source, the Amazon basin and ultimately the Atlantic Ocean (Reference TaljaardTaljaard, 1972). Similarly, both sites are affected by the same annual cycle of large-scale circulation (Reference Chu and Hastenrath.Chu and Hastenrath, 1982: Reference Thompson, Mosley-Thompson, Grootes, Pourchet and Hastenrath.Thompson and others, 1984). Quelccaya at 5670 m a.s.l. has an average δ ¹⁸O of −17.6‰ from 1950 to 1983, which is very close to that from Huascarán at 6050 m a.s.l. (−17.3‰) from 1950 to 1993. Dansgaard (1964), Reference Ambach, Darogaard, Eisner and Mailer.Ambach and others (1968) and Reference Burk and Stuiver.Burk and Stuiver (1981) suggest that altitude effects are important at lower latitudes, and they indicate an average ¹⁸O depletion of 0.2‰ per 100 m of elevation increase. However, Quelccaya is located 4.5° south of Huascarán, and generally δ ¹⁸Ο becomes more negative with increasing distance from the equator (Reference Thompson, Mosley-Thompson, Grootes, Pourchet and Hastenrath.Thompson and others, 1984).

Fig. 4. Annually averaged δ¹⁸ O (1950–92) from the Huascarán we cure, compared with annually averaged δ ¹⁸ O from the Quelccaya ice cap (1950–83).

How δ ¹⁸O in tropical ice caps is affected by meteorological processes is a relatively new area of investigation. A close relationship between long-term changes in stable-isotopic ratios and surface air temperatures has been established for mid- and high-latitude regions (Reference Rozanski, Araguás-Araguás and Gonfiantini.Rozanski and others, 1991). The establishment of the AWS on Hualcán and the ongoing collection of pit and surface snow samples will help to establish transfer functions for this tropical location when enough data are collected. Air-mass circulation and the nature and source of precipitation undoubtedly alfcct δ ¹⁸O in the ice fields in the Andes; however, the evidence discussed below establishes a linkage with temperature.

Recent warming

Figure 5 compares the δ ¹⁸O_an from Huascarán with annually averaged temperatures from two meteorological stations in the region: Recuay (09°43′ S, 77°27′ W, 3394 m a.s.l.) and Chiquian (10°09′ S, 77°09′ W, 3350 m a.s.l.). Although the stations are located approximately 2500 m below the col drill site, all three records contain similar-general trends since 1965, the first complete year in which temperature was recorded. Regression analyses yield the following R ² values: Recuay and Chiquian, 0.70; Chiquian and Huascarán, 0.40; Recuay and Huascarán. 0.04. The very low R ² between Recuay and Huascarán is surprising, given that their profiles appear qualitatively very similar. This may be due in part to the large ranges in seasonal isotopic values and seasonal temperatures on Huascarán (at least 9°C), which are much greater than the ranges of seasonal temperatures from Recuay and Chiquian (2–3°C). These large intra-annual differences may contribute to an exaggerated range of annual averages of isotopic values seen in Figure 5, compared with the smaller range in temperatures at the meteorological stations. As with the seasonal variations in δ ¹⁸O and temperature, the δ ¹⁸O_an–lemperature relationship can be more firmly established when sufficient data have been collected from the Hualcán AWS which, unlike the meteorological stations, is at an elevation comparable to Huascarán.

Fig. 5. Annual averages of δ ¹⁸ O form Haascarán, compared with annually averaged temperatures from two high-altitude meteorological stations in the region. All the records display increasing trends since 1975.

Of particular interest is the increasing temperature trend beginning in the early 1970s which is contemporaneous with ¹⁸O enrichment in the ice core, ibis recent warming has adversely affected other ice caps in the Cordillera Blanca. The location of an appropriate deep-drilling site was the goal of the 1990 and 1991 field seasons, when reconnaissance expeditions were conducted to several sites: Huascarán, Hualcán, Copap, Pucahirca and Caullaraju (Fig. 1). Shallow cores were collected and analyzed for MPC, NO₃ ⁻ and δ ¹⁸O. δ ¹⁸O was most valuable for evaluating the potential quality of the proxy climate record at each site (Fig. 6). Only the δ ¹⁸O profile from the Huascarán shallow core drilled in 1991 contains distinct seasonably which does not smooth with depth. δ ¹⁸O profiles from the lower-elevation sites display both diminished seasonal ranges and loss of seasonality below a few meters. This isotopic alteration has occurred recently and over a short period of time. For example, the δ ¹⁸O profile from Pucahirca displays large isotopic enrichment and signal smoothing between 1984, when an 8 m core was retrieved, and 1990 (Fig. 7). The enrichment which occurred during these 6 years was about 3.5‰. and the seasonal range decreased from 18‰ to 4.5‰. Also, during shallow drilling at these lower-elevation sites, layers of percolated meltwater were observed below the surface. This did not occur on Huascarán, where surface melting and meltwater percolation through the firn were never observed. In fact, these observations were critical for the selection of Huascarán, the highest of these mountains, as the best site for obtaining a preserved proxy climatic record.

Fig. 6. δ¹⁸ O profiles from lower-elevation sites in the Cordillera Blanca show the smoothing and enrichment of the seasonal signal with depth, and are compared with the well presented record in Huascarán.

Fig. 7. δ¹⁸O records from shallow cures recovered from the Pucahirca ice cap in 1984 and 1990 demonstrate the significant isotopic enrichment and smoothing of the seasonal signal over the intervening 6 year interval.

This recent warming is occurring not only in northern regions of the Andes but in the southern Andes as well. During a return trip to the Quelccaya ice cap in 1991, the margins, including the Qori Kalis outlet glacier, were found to have drastically receded (Reference Brecher and Thompson.Brecher and Thompson, 1993). The δ ¹⁸O profile from a shallow core drilled at the summit of Quelccaya in 1991 was noticeably smoothed in comparison with the profile retrieved from the summit in 1976 (Reference ThompsonThompson and others, 1993). These data indicate that the elevation of the zone of melting and subsequent water percolation have risen in this area. Recent warming, marked by increases in annual temperatures measured at Recuay and Chiquian, the contemporaneous oxygen-isotopic enrichment in Huascarán and the disappearance of the climate records in the lower-elevation ice caps in the Cordillera Blanca, is also reflected in the Quelccaya ice cap to the south.

Conclusions

The new cores from Huascarán contain the best-preserved seasonably in MPC, NO₃ and δ ¹⁸O yet seen from the Peruvian Andes. Temperature, wind speed and wind direction recorded by the satellite-linked AWS established in 1991 on neighboring Hualcán also vary seasonally. High MPC and NO₃ ⁻ occur in the winter when strong northeasterlies prevail at the 500 mbar level. Seasonal variations in these two parameters are instrumental for dating the core.

Oxygen-isotopic ratios vary in a similar manner to those on the Quelccaya ice cap, However, consistent with other Andean ice cores, and opposite to polar cores. ¹⁸O is enriched during the winter. This reflects additional factors such as precipitation source and transport and atmospheric-circulation effects. Annually averaged temperatures from two meteorological stations in the region show an increasing trend over the previous two decades contemporaneous with increasing δ ¹⁸O_an values from Huascarán. Although the processes affecting δ ¹⁸O in tropical snowfields are urn fully understood, a positive correlation exists on an inter-annual basis between increasing temperature and less negative oxygen-isotopic ratios.

Annually averaged δ ¹⁸O from the Huascarán ice core is very negative, while that from Copap, Pucahirca, Caullaraju and Hualcán, retrieved in 1990 and 1991, is more enriched (less negative). These differences primarily reflect elevation effects. Many of these ice caps exist at or near the thermal limit, and small increases in atmospheric temperatures are expected to elevate the zones of melting and percolation. This is observed on lower-elevation ice caps in the Cordillera Blanca where seasonal δ ¹⁸O signals are smoothed just a few meters below the surface due to percolation of meltwater. Fortunately, the paleo-climate record on Huascarán is intact. It is the highest tropical mountain in the world, and the drill site is located well above the present thermal limit. The paleoclimatic records from ice caps in this region of the world aie disappearing rapidly in response to the recent warming. If this warming trend continues, Huascarán may also cease to preserve a climate record.