Early deglaciation in the tropical Andes.

Tropical Andes The record of ancient glaciers is the principal source of information on high-altitude paleoclimate in the tropics, but the age of late Pleistocene glaciation throughout the tropics is poorly constrained, which complicates estimating past climate from glacial records. Seltzer et al. (1) established that maximum late Pleistocene glaciation in the tropical Andes was broadly synchronous with the global Last Glacial Maximum (LGM) 21,000 calendar years before present (cal yr B.P.), which supports inferences of LGM cooling of 4 to 6C at high altitudes in the tropics (2). Comparing their records to a record of glaciation in the Sierra Nevada and to Greenland and Antarctic ice core records, Seltzer et al. also concluded that deglaciation in the tropical Andes preceded substantial post-LGM warming at high northern latitudes by several thousand years, which in turn led them to speculate that earlier warming in the tropics triggered the last deglaciation. Here, I show that the Sierra Nevada record of glaciation is inappropriate for comparison with the Andean records used by Seltzer et al. and that a number of other well-dated records of Northern Hemisphere glaciation have an early-deglaciation phase at the same time as that of the tropical Andes. I also show that different published chronologies of Greenland and Antarctic ice-core records lead to different conclusions on the relationship between high-latitude climate change and that of the tropical Andes. Seltzer et al. dated tropical deglaciation based on proxies in Lake Junin, Peru, and Lake Titicaca, Peru/Bolivia. Nearby LGM glaciers supplied the lakes with sediment, but retreat of the glaciers resulted in the formation of lakes behind the LGM moraines that trapped glaciogenic sediments. This sediment trapping not only amplified the deglacial signal in lakes Junin and Titicaca, but also attenuated any subsequent evidence of continued glaciation in the watersheds. Their lake records thus cannot reveal the magnitude of glacier retreat. Indeed, other work on the glacial history of this region suggests that late Pleistocene glaciers were near their LGM limit at 17,550 300 cal yr B.P. and at 50% of LGM size 16,650 cal yr B.P. (3), but in general the details of deglaciation remain poorly understood. Seltzer et al. compared their records with a record of Sierra Nevada glaciation from Owens Lake, California (4), to argue that tropical deglaciation occurred several thousand years before Northern Hemisphere deglaciation. In contrast to those of Junin and Titicaca, however, retreat of LGM Sierran glaciers did not result in similar sediment trapping in moraine-dammed lakes; thus, Owens Lake continued to receive glaciogenic sediment as long as the watershed remained glaciated. Consequently, the South American lake records should not be directly compared with those from Owens Lake to contrast the timing of deglaciation. More appropriate comparisons to Northern Hemisphere deglaciation should be based on records that directly date the timing of ice-margin fluctuations. Several such records from western North America reveal that an early deglacial phase occurred at the same time as that of the tropical Andes (Fig. 1A). Similar ages of deglaciation are also suggested for sectors of the Laurentide (5), British (6), and Scandinavian (7) ice sheets. Early deglaciation following the LGM thus was not unique to the tropical Andes. By comparing the timing of tropical Andean deglaciation to methane-synchronized ice-core records of temperature from Greenland (GRIP) and Antarctica (Byrd Station) placed on the GRIP time scale (8), Seltzer et al. concluded that “the onset of deglaciation in the tropical Andes follows the record of Antarctic warming.” However, the GRIP time scale used by Seltzer et al. is younger than the GISP2 time scale (9) for ages 15,000 yr B.P. Recent highresolution dating of a Chinese speleothem supports the GISP2 time scale (10), and Johnsen et al. (11) revised the GRIP time scale so that it now agrees with GISP2 back to 40,000 yr B.P. The methane-synchronized ice-core records on the GISP2 timescale (12, 13) show that initial warming at Greenland began 24,000 yr B.P., 2000 to 3000 years before Antarctic and tropical warming, and that nearly one-third of deglacial warming had occurred by 19,000 yr B.P., well before the onset of the Bølling-Allerød warming (14) (Fig. 1). In contrast, the ice-core record shows no change in atmospheric methane concentration during this time (Fig. 1A), which suggests that the climate change responsible for early tropical deglaciation was insufficient to affect low-latitude methane source regions. In summary, the onset of tropical Andean deglaciation from LGM positions occurred at the same time as deglaciation widely recorded at higher northern latitudes, in contrast to the conclusion by Seltzer et al. that tropical warming led and thus caused Northern Hemisphere deglaciation. The timing of this early phase of deglaciation was coincident with high-latitude warming and increased summer insolation in the