Abstract

Combined glacial geologic and palynologic data from the southern Lake District, Seno Reloncavi, and Isla Grande de Chiloe in middle latitudes (40 degrees 35'-42 degrees 25'S) of the Southern Hemisphere Andes suggest (1) that full-glacial or near-full-glacial climate conditions persisted from about 29,400 to 14,550 (14)C yr BP in late LIanquihue time,(2) that within this late Llanquihue interval mean summer temperature was depressed 6 degrees-8 degrees C compared to modem values during major glacier advances into the outer moraine belt at 29,400,26,760, 22,295-22,570 and 14,550-14,805 (14)C yr sp, (3) that summer temperature depression was as great during early Llanquihue as during late Llanquihue time,(4) that climate deteriorated from warmer conditions during the early part to colder conditions during the later part of middle Llanquihue time,(5) that superimposed on long-term climate deterioration are Gramineae peaks on Isla Grande de Chiloe that represent cooling at 44,520-47,110 (14)C yr sp (T-11), 32,105-35,764 (14)C yr BP(T-9),24,895-26,019 (14)C yr BP (T-7), 21,430-22,774 (14)C yr BP (T-5), and 13,040-15,200 (14)C yr sp (T-3),(6) that the initial phase of the glacial/interglacial transition of the last termination involved at least two major steps, one beginning at 14,600 (14)C yr sp and another at 12,700-13,000 (14)C yr sp, and (7) that a late-glacial climate reversal of less than or equal to 2-3 degrees C set in close to 12,200 (14)C yr sp, after an interval of near-interglacial warmth, and continued into Younger Dryas time. The late-glacial climate signal from the southern Chilean Lake District ties into that from proglacial Lago Mascardi in the nearby Argentine Andes, which shows rapid ice recession peaking at 12,400 14C yr sp, followed by a reversal of trend that culminated in Younger-Dryas-age glacier readvance at 11,400-10200 (14)C yr BP. Many full- and late-glacial climate shifts in the southern Lake District match those from New Zealand at nearly the same Southern Hemisphere middle latitudes. At the last glacial maximum (LGM), snowline lowering relative to present-day values was nearly the same in the Southern Alps (875 m) and the Chilean Andes (1000 m). Particularly noteworthy are the new Younger-Dryas-age exposure dates of the Lake Misery moraines in Arthur's Pass in the Southern Alps. Moreover, pollen records from the Waikato lowlands on North Island show that a major vegetation shift at close to 14;100 (14)C yr BP marked the beginning of the Last glacial/interglacial transition (Newnham et al. 1989). The synchronous and nearly uniform lowering of snowlines in Southern Hemisphere middle-latitude mountains compared with Northern Hemisphere values suggests global cooling of about the same magnitude in both hemispheres at the LGM. When compared with paleoclimate records from the North Atlantic region, the middle-latitude Southern Hemisphere terrestrial data imply interhemispheric symmetry of the structure and timing of the last glacial/interglacial transition. In both regions atmospheric warming pulses are implicated near the beginning of Oldest Dryas time (similar to 14,600 (14)C yr BP) and near the Oldest Dryas/Bolling transition (similar to 12,700-13,000 (14)C yr BP). The second of these warming pulses was coincident with resumption of North Atlantic thermohaline circulation similar to that of the modem mode, with strong formation of Lower North Atlantic Deep Water in the Nordic Seas. In both regions, the maximum Bolling-age warmth was achieved at 12,200-12,500 (14)C yr sp, and was followed by a reversal in climate trend. In the North Atlantic region, and possibly in middle latitudes of the Southern Hemisphere,this reversal culminated in a Younger-Dryas-age cold pulse. Although changes in ocean circulation can redistribute heat between the hemispheres, they cannot alone account either for the synchronous planetary cooling of the LGM or for the synchronous interhemispheric warming steps of the abrupt glacial-to-interglacial transition. Instead, the dominant interhemispheric climate linkage must feature a global atmospheric signal. The most likely source of this signal is a change in the greenhouse content of the atmosphere. We speculate that the Oldest Dryas warming pulse originated from an increase in atmospheric water-vapor production by half-precession forcing in the tropics. The major thermohaline switch near the Oldest Dryas/Bolling transition then could have triggered another increase in tropical water-vapor production to near-interglacial values.