Interhemispheric linkage of paleoclimate during the last glaciation

Abstract

Combined glacial geologic and palynologic data from the southern Lake District, Seno Reloncaví, and Isla Grande de Chiloé in middle latitudes (40°35’–42°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 14C yr BP in late Llanquihue time, (2) that within this late Llanquihue interval mean summer temperature was depressed 6°–8°C compared to modern values during major glacier advances into the outer moraine belt at 29,400, 26,760, 22,295–22,570, and 14,550–14,805 14C yr BP, (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 Chiloé that represent cooling at 44,520–47,110 14C yr BP (T‐11), 32,105–35,764 14C yr BP (T‐9), 24,895–26,019 14C yr BP (T‐7), 21,430–22,774 14C yr BP (T‐5), and 13,040–15,200 14C yr BP (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 14C yr BP and another at 12,700–13,000 14 C yr BP, and (7) that a late‐glacial climate reversal of ≥2–3° C set in close to 12,200 14C yr BP, 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 BP, followed by a reversal of trend that culminated in Younger‐Dryas‐age glacier readvance at 11,400–10,200 14C 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 14,700 14C 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 (∼14,600 14C yr BP) and near the Oldest Dryas/Bölling transition (∼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 modern mode, with strong formation of Lower North Atlantic Deep Water in the Nordic Seas. In both regions, the maximum Bölling‐age warmth was achieved at 12,200–12,500 14 C yr BP, 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/Bölling transition then couldhave triggered another increase in tropical water‐vapor production to near‐interglacial values.