Interhemispheric linkage of paleoclimate during the last glaciation

Combined glacial geologic and palynologic data from the southern Lake Distr ict, Seno Reloncavi, and Isla Grande de Chiloe in middle latitudes (40 degr ees 35'-42 degrees 25'S) of the Southern Hemisphere Andes suggest (1) that full-glacial or near-full-glacial climate conditions persisted from about 2 9,400 to 14,550 C-14 yr BP in late LIanquihue time,(2) that within this lat e Llanquihue interval mean summer temperature was depressed 6 degrees-8 deg rees C compared to modem values during major glacier advances into the oute r moraine belt at 29,400,26,760, 22,295-22,570 and 14,550-14,805 C-14 yr sp , (3) that summer temperature depression was as great during early Llanquih ue as during late Llanquihue time,(4) that climate deteriorated from warmer conditions during the early part to colder conditions during the later par t of middle Llanquihue time,(5) that superimposed on long-term climate dete rioration are Gramineae peaks on Isla Grande de Chiloe that represent cooli ng at 44,520-47,110 C-14 yr sp (T-11), 32,105-35,764 C-14 yr BP(T-9),24,895 -26,019 C-14 yr BP (T-7), 21,430-22,774 C-14 yr BP (T-5), and 13,040-15,200 C-14 yr sp (T-3),(6) that the initial phase of the glacial/interglacial tr ansition of the last termination involved at least two major steps, one beg inning at 14,600 C-14 yr sp and another at 12,700-13,000 C-14 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 C-14 yr sp, after an interval of near-interglacia l warmth, and continued into Younger Dryas time. The late-glacial climate s ignal from the southern Chilean Lake District ties into that from proglacia l Lago Mascardi in the nearby Argentine Andes, which shows rapid ice recess ion peaking at 12,400 14C yr sp, followed by a reversal of trend that culmi nated in Younger-Dryas-age glacier readvance at 11,400-10200 C-14 yr BP. Many full- and late-glacial climate shifts in the southern Lake District ma tch those from New Zealand at nearly the same Southern Hemisphere middle la titudes. At the last glacial maximum (LGM), snowline lowering relative to p resent-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-a ge exposure dates of the Lake Misery moraines in Arthur's Pass in the South ern Alps. Moreover, pollen records from the Waikato lowlands on North Islan d show that a major vegetation shift at close to 14;100 C-14 yr BP marked t he beginning of the Last glacial/interglacial transition (Newnham et al. 19 89). The synchronous and nearly uniform lowering of snowlines in Southern Hemisp here middle-latitude mountains compared with Northern Hemisphere values sug gests global cooling of about the same magnitude in both hemispheres at the LGM. When compared with paleoclimate records from the North Atlantic regio n, the middle-latitude Southern Hemisphere terrestrial data imply interhemi spheric symmetry of the structure and timing of the last glacial/interglaci al transition. In both regions atmospheric warming pulses are implicated ne ar the beginning of Oldest Dryas time (similar to 14,600 C-14 yr BP) and ne ar the Oldest Dryas/Bolling transition (similar to 12,700-13,000 C-14 yr BP ). The second of these warming pulses was coincident with resumption of Nor th Atlantic thermohaline circulation similar to that of the modem mode, wit h strong formation of Lower North Atlantic Deep Water in the Nordic Seas. I n both regions, the maximum Bolling-age warmth was achieved at 12,200-12,50 0 C-14 yr sp, and was followed by a reversal in climate trend. In the North Atlantic region, and possibly in middle latitudes of the Southern Hemisphe re,this reversal culminated in a Younger-Dryas-age cold pulse. Although changes in ocean circulation can redistribute heat between the hem ispheres, they cannot alone account either for the synchronous planetary co oling of the LGM or for the synchronous interhemispheric warming steps of t he abrupt glacial-to-interglacial transition. Instead, the dominant interhe mispheric climate linkage must feature a global atmospheric signal. The mos t likely source of this signal is a change in the greenhouse content of the atmosphere. We speculate that the Oldest Dryas warming pulse originated fr om an increase in atmospheric water-vapor production by half-precession for cing in the tropics. The major thermohaline switch near the Oldest Dryas/Bo lling transition then could have triggered another increase in tropical wat er-vapor production to near-interglacial values.