Abstract

Discerning the timing and pattern of late Quaternary glacier variability in the tropical Andes is important for our understanding of global climate change. Terrestrial cosmogenic nuclide (TCN) ages (48) on moraines and radiocarbon-dated clastic sediment records from a moraine-dammed lake at Nevado Huaguruncho, Peru, document the waxing and waning of alpine glaciers in the Eastern Cordillera during the past ∼15 k.y. The integrated moraine and lake records indicate that ice advanced at 14.1 ± 0.4 ka, during the first half of the Antarctic Cold Reversal, and began retreating by 13.7 ± 0.4 ka. Ice retreated and paraglacial sedimentation declined until ca. 12 ka, when proxy indicators of glacigenic sediment increased sharply, heralding an ice advance that culminated in multiple moraine positions from 11.6 ± 0.2 ka to 10.3 ± 0.2 ka. Proxy indicators of glacigenic sediment input suggest oscillating ice extents from ca. 10 to 4 ka, and somewhat more extensive ice cover from 4 to 2 ka, followed by ice retreat. The lack of TCN ages from these intervals suggests that glaciers were less extensive than during the late Holocene. A final Holocene advance occurred during the Little Ice Age (LIA, ca. 0.4 to 0.2 ka) under colder and wetter conditions as documented in regional proxy archives. The pattern of glacier variability at Huaguruncho during the Late Glacial and Holocene is similar to the pattern of tropical Atlantic sea-surface temperatures, and provides evidence that prior to the LIA, ice extent in the eastern tropical Andes was decoupled from temperatures in the high-latitude North Atlantic.

INTRODUCTION

The highlands of Peru are at a location where atmospheric changes may be affected by climatic processes that originate in both the tropical Pacific and Atlantic Oceans. The Eastern Cordillera is adjacent to the Amazon Basin, and glaciers there should respond primarily to temperature and precipitation changes that are driven by conditions in the Atlantic Ocean rather than by those in the Pacific Ocean. However, our understanding of how climatic variability in the Eastern Cordillera responded to shifting tropical and high-latitude ocean-atmospheric conditions during the Late Glacial and Holocene has not been clear because records of past glacial changes from this region are scarce.

Here we present a combined data set of proglacial lake sediments and terrestrial cosmogenic nuclide (TCN) ages from Nevado Huaguruncho in the central Peruvian Andes (Fig. 1) to develop a continuous and robustly dated record of glacial activity in a previously unstudied region. Lake sediment records provide continuous archives of upvalley glacial erosion but do not precisely delimit changes in glacier length, whereas moraine ages correspond to discrete ice margin positions. Combining these independent proxies thus provides a comprehensive perspective of glacial variability.

SITE DESCRIPTION

Modern interannual rainfall patterns at Huaguruncho (Fig. 1) are driven by tropical, subtropical, and extratropical features that vary in importance depending on time scales (e.g., Garreaud et al., 2009). The El Niño Southern Oscillation in the Pacific Ocean leads to wetter and colder conditions during the La Niña phase, and to warmer and drier conditions during the El Niño phase (Garreaud et al., 2009), and also affects the position of the Intertropical Convergence Zone (ITCZ). In the paleorecord, however, decadal- to centennial-scale precipitation variability in the Amazon Basin and adjacent regions is affected more by Northern Hemisphere temperature changes, resulting in a displacement of the Atlantic ITCZ and shifts in the strength of the South American Summer Monsoon (Vuille et al., 2012). Glacier mass-balance changes in the Andes are driven by shifts in both temperature and precipitation. However, glaciers on the eastern side of the tropical Andes receive high amounts of precipitation, and are therefore especially sensitive to tropical Atlantic temperature variability (e.g., Sagredo and Lowell, 2012).

The Huaguruncho massif (10.531°S; 75.931°W; 5723 m above sea level, asl) is a glacial horn surrounded radially by five major valleys. The Jaico valley extends south-southeastward from the main summit and contains at least three distinct groups of moraines inside a prominent moraine that likely formed as part of the last local glacial maximum. Laguna Yanacocha (10.560°S, 75.930°W, 4360 m asl) is located in a southeast-facing cirque (Fig. 2) that drains into Laguna Jaico. The small glaciers that feed Yanacocha have headwalls of ∼5000 m asl. A moraine was identified outboard of Yanacocha, and a small moraine ridge extends into the lake from the south (Fig. 2). The lake has a maximum water depth of ∼21.5 m, and the well-protected cirque basin keeps it isolated from wind mixing. Low oxygen conditions of bottom waters limit bioturbation and allow for preservation of laminated sediments.

METHODS

The timing of glacial variability at Huaguruncho is based on a combination of lake sediment records and TCN ages (see the GSA Data Repository1 for details). A continuous percussion sediment core was collected from the depocenter of Yanacocha. The age-depth model is based on 210Pb dating and 13 radiocarbon ages that were calibrated to calendar year ages. Depth was converted to age based on both point-to-point and polynomial interpolations between modeled median ages (Fig. 3) that were generated using Bayesian statistical methods (Bronk Ramsey, 2008). Cores were also collected in the distal shallow basin of Yanacocha to acquire a basal age of the sediments. Samples for 10Be exposure dating were collected from quartz monzonite erratics following previously described field protocols (e.g., Licciardi et al., 2009), and returned to the lab for physical and chemical processing.

RESULTS

The Yanacocha sediments mostly consist of laminated angular to subrounded silt to fine sand alternating between light to dark gray (Gley 5/1) in sections with highest clastic sediment content, and less clastic-rich layers with higher organic matter that are grayish-brown (2.5Y 3/1). The elements that covary with the first principal component (PC1) of the Yanacocha sediment geochemical data are consistent with minerogenic material derived from quartz monzonite bedrock (see the Data Repository).

The mean TCN ages of moraines in the Jaico valley comprise three distinct groupings centered on ca. 14.1 ka, 11.6–10.3 ka, and 0.4–0.3 ka (Fig. 2; Table DR2; see the Data Repository). Boulders on the moraine enclosing Laguna Yanacocha yield a mean age of 14.1 ± 0.4 ka. A moraine trending south from Yanacocha and inboard of the enclosing moraine dates to 11.0 ± 0.4 ka. The distal, intermediate, and proximal zones of the multiridged moraine belt west of Laguna Jaico yield mean ages of 11.6 ± 0.2 ka, 11.5 ± 0.3 ka, and 10.6 ± 0.3 ka, respectively. Other moraines dating from this interval are located downvalley of Jaico (11.0 ± 0.3 ka), near the eastern shore of Jaico (10.5 ± 0.4 ka), and north of Jaico (10.3 ± 0.2 ka). Three moraine ridges north of Jaico and inside the early Holocene positions yield mean ages of 0.4, 0.3, and 0.3 ka.

MORAINE AND SEDIMENT RECORDS OF GLACIAL VARIABILITY

The Yanacocha sediment core provides a clastic sediment record that spans the end of the Late Glacial through the Holocene, and periods of elevated values of clastic sediment proxies are in close agreement with the distribution of TCN moraine ages (Figs. 2 and 4). Fine-grained clastic sediment yield is generally greater during periods of expanded ice coverage for regions like Huaguruncho with warm-based glaciers and high amounts of precipitation (e.g., Rodbell et al., 2008). Moreover, relatively small glaciers in regions of high precipitation have mass balances that respond quickly (within decades) to shifting atmospheric conditions, and the associated clastic sedimentary response in the lake basin should correspond closely to the timing of changes in upvalley ice extent, although this may be complicated by paraglacial and nonglacial sediment sources (Church and Ryder, 1972; Stansell et al., 2013).

There are noteworthy similarities and differences between the TCN ages and sediment core data from Huaguruncho (Fig. 4). Clastic sediment concentrations (PC1) and flux were generally elevated during the Late Glacial and Holocene whenever glaciers were in expanded positions and moraines were constructed. Conversely, sharp decreases in clastic sediment flux and concentration correspond to intervals that lack TCN ages. This general pattern prevails throughout the Holocene record, but there are intervals during which PC1 values are high and correspond to clusters of moraine ages, while clastic sediment flux is low. This inconsistency between clastic concentration and clastic flux may be attributed to uncertainties in the applied age model, as sediment flux is strongly influenced by small differences in bulk sedimentation rate. There are also glacial episodes that are apparent in both clastic sediment flux and concentration, but not in the TCN ages of moraines, and this may reflect the obliteration of some moraines by younger, more extensive glacial advances.

LATE GLACIAL AND EARLY HOLOCENE (15–10 ka)

The TCN ages from multiple valleys indicate that glaciers in Huaguruncho were in an advanced position ca. 14.1 ± 0.4 ka, at the start of the Antarctic Cold Reversal. Clastic sediment flux is represented differently in the polynomial and point-to-point age models, but a basal radiocarbon age of 13.7 ± 0.4 ka (Table DR2) was obtained from the shallow basin in Yanacocha where a clear a transition occurs from high concentrations of sand and silt to elevated values of organic matter. This provides an independent minimum limiting age for the timing of local ice retreat. The period from ca. 13.7 to 12 ka is marked by an absence of moraines and declining clastic sediment concentration and flux, probably indicating an interval of retreating ice margins and waning paraglacial sedimentation. Beginning ca. 12 ka, clastic sediment concentration and flux increased abruptly; this heralds an expansion of ice in the Jaico cirque that culminated in multiple moraines constructed from 11.6 ± 0.2 ka to 10.3 ± 0.2 ka at progressively diminished ice extents. These glacial chronologies align well with a recent compilation of 10Be and 3He moraine ages across the tropical Andes (Jomelli et al., 2014).

MIDDLE-LATE HOLOCENE (10 ka TO PRESENT)

Proxy indicators of glacigenic sediment input suggest oscillating ice extent from ca. 10 to 4 ka and somewhat more extensive ice cover from 4 to 2 ka, followed by a substantial ice retreat. The lack of TCN ages from this interval suggests that glaciers were less extensive than during the subsequent Little Ice Age (LIA, ca. 0.4–0.2 ka). A final Holocene advance occurred during the LIA, during which clastic sediment indicators increased abruptly. The LIA culminated in the deposition of three moraine belts within 1 km of the north end of Laguna Jaico (Fig. 2); these moraines mark the most extensive ice cover in the Jaico cirque since the early Holocene.

GLACIER VARIABILITY AND CLIMATE CHANGE IN THE EASTERN CORDILLERA

The mountain glacier records from the Eastern Cordillera provide a unique perspective on climatic shifts when compared to regional proxy data (Fig. 4). The Late Glacial and early Holocene were intervals of relatively dry conditions in the Peruvian Andes (Seltzer et al., 2000), and yet glaciers at Huaguruncho advanced or stabilized multiple times. Oxygen isotope records from nearby Laguna Pumacocha (Bird et al., 2011) and Nevado Huascarán (Thompson et al., 1995) likewise indicate a trend toward drier and likely warmer conditions after ca. 13 ka and that a pronounced shift to even drier conditions occurred after ca. 10.7 ka. Therefore, the Huaguruncho records indicate that there must have been discrete and perhaps short-lived intervals of lower temperatures in order for glaciers to advance during the early Holocene. For comparison, tropical Atlantic sea-surface temperature (SST) values (Rühlemann et al., 1999) were detrended and plotted as residuals to highlight deviations that overprint the influence of orbital forcing. Periods of higher clastic sediment values and TCN moraine ages from Huaguruncho during the Late Glacial and early Holocene generally correspond to the timing of colder conditions in the tropical Atlantic Ocean (Fig. 4).

The middle Holocene pattern of glacial variability at Huaguruncho likewise suggests that tropical Atlantic Ocean conditions influenced local ice margin fluctuations (Fig. 4). Summer insolation values increased progressively during the Holocene, leading to a generally stronger South American Summer Monsoon than existed during the early Holocene (e.g., Bird et al., 2011), but clastic sediment values are only modestly high during the middle Holocene and there are no TCN ages within this interval. Notably, reconstructed temperatures in the tropical Atlantic fluctuated during the middle Holocene, and were higher during intervals of reduced ice cover, as recorded by decreased clastic sediment input to Laguna Yanacocha.

There is a close correspondence in late Holocene records of glacial variability in the Eastern Cordillera and proxy records of regional temperature and precipitation (Fig. 4). Glaciers advanced during the late Holocene ca. 4–2 ka, and retreated after ca. 2 ka. Glaciers then advanced to their greatest extent of the late Holocene during the LIA (Fig. 2), and clastic sediment indicators were high during the last ∼600 yr. Stable isotope records from the region indicate that the LIA was relatively wet (Vuille et al., 2012), and ice advances at Huaguruncho provide further supporting evidence that the LIA was also the coldest time since the early Holocene (e.g., Thompson et al., 1995).

The tropical Pacific and North Atlantic Oceans both influence interannual climate and circulation variability in tropical South America, although the teleconnections between these systems over longer time scales are not well understood. The combined moraine and lake records presented here reflect oscillating and reduced glacier extents in the Eastern Cordillera during the middle Holocene, at a time when independent proxy records indicate a general pattern of fewer El Niño events, whereas glaciers at Huagurucho were advancing in the late Holocene when El Niño events were more frequent (Moy et al., 2002). This pattern of glaciation therefore does not appear to be associated with the El Niño Southern Oscillation because El Niño events generally lead to warmer and drier conditions in the Eastern Cordillera, and the opposite during La Niña (Garreaud et al., 2009). Regarding potential Atlantic teleconnections, the Late Glacial to Holocene record of glaciation documented here appears to align more closely with tropical Atlantic SSTs than with temperature trends in the high-latitude North Atlantic, except during the LIA, when glaciation coincided with cold temperatures in both tropical and northern sectors of the Atlantic basin. This suggests that changes in tropical Atlantic SSTs exerted a strong influence on glacial variability at Huaguruncho through their role in modulating atmospheric temperature and circulation.

CONCLUSIONS

Lake sediments and TCN exposure ages provide a detailed integrated record of the timing of Late Glacial and Holocene glacial variability in the Eastern Cordillera of the tropical Peruvian Andes. Glaciers expanded ca. 14.1 ± 0.4 ka, during the first half of the Antarctic Cold Reversal. This was followed by an interval of ice retreat from 13.7 to 12 ka, an interval that spans most of the Younger Dryas. Glacigenic clastic sediment proxies indicate an abrupt glacial expansion starting ca. 12 ka that culminated in moraines constructed from 11.6 ± 0.2 ka to 10.3 ± 0.2 ka. Ice advanced or stabilized under colder and drier atmospheric conditions during the early Holocene. During the middle Holocene, the combined clastic sediment and moraine records indicate that glaciers were limited in extent, but minor advances may have occurred under colder conditions when stable isotope records from the region indicate that conditions were wetter than during the early Holocene. Glaciers advanced during the late Holocene ca. 4–2 ka, followed by retreat. A final late Holocene period of glacier advance occurred during colder LIA conditions. While precipitation changes linked to high-latitude North Atlantic temperatures were important drivers of climate change in tropical South America, the observed centennial- to millennial-scale glacial variability in the Eastern Cordillera of Peru is best explained by temperature changes in the tropical Atlantic Ocean.

We thank D. Bain, J. Dalakos, and D. Pompeani for laboratory assistance and J. Schaefer for providing the low-level 9Be carrier. Funding was provided by the National Science Foundation (grants EAR-1003780 and EAR-1003711), and the Keck Geology Consortium. This is Lawrence Livermore National Laboratory contribution LLNL-JRNL-667538.

1GSA Data Repository item 2015255, methods, radiocarbon ages and terrestrial cosmogenic nuclide data, is available online at www.geosociety.org/pubs/ft2015.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.