Patricio I. Moreno

Interhemispheric correlation of Late Pleistocene glacial events.

A radiocarbon chronology shows that piedmont glacier lobes in the Chilean Andes achieved maxima during the last glaciation at 13,900 to 14,890, 21,000, 23,060, 26,940, 29,600, and ≥33,500 carbon-14 years before present (14C yr B.P.) in a cold and wet Subantarctic Parkland environment. The last glaciation ended with massive collapse of ice lobes close to 14,00014C yr B.P., accompanied by an influx of North Patagonian Rain Forest species. In the Southern Alps of New Zealand, additional glacial maxima are registered at 17,72014C yr B.P., and at the beginning of the Younger Dryas at 11,050 14C yr B. P. These glacial maxima in mid-latitude mountains rimming the South Pacific were coeval with ice-rafting pulses in the North Atlantic Ocean. Furthermore, the last termination began suddenly and simultaneously in both polar hemispheres before the resumption of the modern mode of deep-water production in the Nordic Seas. Such interhemispheric coupling implies a global atmospheric signal rather than regional climatic changes caused by North Atlantic thermohaline switches or Laurentide ice surges.

Interhemispheric Linkage of Paleoclimate During the Last Glaciation

Combined glacial geologic and palynologic data from the southern Lake District, Seno Reloncavi, and Isla Grande de Chiloe 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 Chiloe 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 Arthurs 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/Bolling 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 Bolling-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/Bolling transition then couldhave triggered another increase in tropical water-vapor production to near-interglacial values.

Global climate evolution during the last deglaciation

Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth’s climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO2 and CH4 to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the Atlantic meridional overturning circulation.

Geomorphology, Stratigraphy, and Radiocarbon Chronology of LlanquihueDrift in the Area of the Southern Lake District, Seno Reloncaví, and Isla Grande de Chiloé, Chile

Glacial geomorphologic features composed of (or cut into) Llanquihue drift delineate former Andean piedmont glaciers in in the region of the southern Chilean Lake District,Seno Reloncavi, Golfo de Ancud, and northern Golfo Corcovado during the last glaciation. These landforms include extensive moraine belts, main and subsidiary outwash plains, kame terraces, and meltwater spillways. Nt Numerous radiocarbon dates document Andean ice advances into the moraine belts during the last glacial maximum (LGM) at 29,363-29,385 14 C yr BP, 26,797 14 C yr BP, 22,295-22,570 14 C yr BP, and 14,805-14,869 14 C yr BP.Advances may also have culminated at close to 21,000 14 C yr BP, shortly before 17,800 14 C yr BP, and shortly before 15,730 14 C yr BP. The maximum at 22,295-22,567 14 C yr BP was probably the most extensive of the LGM in the northern part of the field area, whereas that at 14,805-14,869 14 C yr BP was the most extensive in the southern part. Snowline depression during these maxima was about 1000 m. Andean piedmont glaciers did not advance into the outer Llanquihue moraine belts during the portion of middle Llanquihue time between 29,385 14 C yr BP and more than 39,660 14 C yr BP. In the southern part of the field area, the Golfo de Ancud lobe, as well the Golfo Corcovado lobe, achieved a maximum at the outermost Llanquihue moraine prior to 49,892 14 C yr BP. Pollen analysis of the Taiquemo mire,which is located on this moraine, suggests that the old Llanquihue advance probably corresponds to the time of marine isotope stage 4. The implication is that Andean snowline was then depressed as much as during the LGM. A Llanquihue-age glacier expansion into the outer moraine belts also occurred more than about 40,000 14 C yr BP for the Lago Llanquihue piedmont glacier.

Precise radiocarbon dating of Late-Glacial cooling in mid-latitude South America

Abstract Variability of atmospheric 14C content often complicates radiocarbon-based chronologies; however, specific features such as periods of constant 14C age or steep changes in radiocarbon ages can be useful stratigraphic markers. The Younger Dryas event in the Northern Hemisphere is one of those periods, showing conspicuous 14C wiggles. Although the origin of those variations is not fully understood, we can make practical use of them and determine: (i) whether the Younger Dryas was global in extent; if so, (ii) were the initial cooling and the final warming synchronous worldwide; and (iii) what are the implications of these similarities/differences? Here we report high-resolution AMS 14C chronologies from the mid-latitudes of South America that pinpoint a cool episode between 11,400 and 10,20014C yr B.P. The onset of the final cool episode of the Late Glacial in the southern mid-latitudes, i.e., the Huelmo/Mascardi Cold Reversal, preceded the onset of the Younger Dryas cold event by ∼550 calendar years. Both events ended during a radiocarbon-age plateau at ∼10,20014C yr B.P. Thus, the Huelmo/Mascardi Cold Reversal encompasses the Younger Dryas, as well as a couple of short-term cool/warm oscillations that immediately preceded its onset in the North Atlantic region.

Predictability of biomass burning in response to climate changes

Climate is an important control on biomass burning, but the sensitivity of fire to changes in temperature and moisture balance has not been quantified. We analyze sedimentary charcoal records to show that the changes in fire regime over the past 21,000 yrs are predictable from changes in regional climates. Analyses of paleo- fire data show that fire increases monotonically with changes in temperature and peaks at intermediate moisture levels, and that temperature is quantitatively the most important driver of changes in biomass burning over the past 21,000 yrs. Given that a similar relationship between climate drivers and fire emerges from analyses of the interannual variability in biomass burning shown by remote-sensing observations of month-by-month burnt area between 1996 and 2008, our results signal a serious cause for concern in the face of continuing global warming.

Covariability of the Southern Westerlies and atmospheric CO2 during the Holocene

A suite of mechanisms has been proposed to account for natural variations in atmospheric CO 2 during the Holocene; all of which have achieved limited success in reproducing the timing, direction, and magnitude of change. Recent modeling studies propose that changes in the latitudinal position and strength of the Southern Hemisphere Westerly Winds (SWW) can greatly influence large-scale ocean circulation and degassing of the deep ocean via changes in wind-driven upwelling in the Southern Ocean. The extent to which the hypothesized SWW–Southern Ocean coupled system could account for changes in atmospheric CO 2 is uncertain, because of a lack of observations on the behavior of the SWW in the past, the paucity of appropriate records of productivity changes in the Southern Ocean, and our limited understanding of the sensitivity of the Southern Ocean biological and/or physical system to SWW forcing. Here we report a reconstruction of the behavior of the SWW during the past 14 k.y. based on terrestrial ecosystem proxies from western Patagonia, South America. The reconstructed variations in the intensity of zonal flow correspond to the timing and structure of atmospheric CO 2 changes, and are consistent with the modeled magnitude of CO 2 changes induced by varying strengths of the SWW. The close match between data and models supports the view that the SWW–Southern Ocean coupled system underpins multimillennial CO 2 variations during the current interglacial and, possibly, during glacial cycles over the past 800 k.y.

Renewed glacial activity during the Antarctic cold reversal and persistence of cold conditions until 11.5 ka in southwestern Patagonia

Resolving the timing, direction, and magnitude of paleoclimate changes in the southern midlatitudes is a prerequisite for determining the mechanisms underlying abrupt and widespread climate changes between the hemispheres during the Last Glacial-Interglacial transition (LGIT). This issue is still debated, with previous studies producing apparently discordant fi ndings. Here we show evidence for a glacial readvance and a cold episode between ca. 14.8 and 12.6 ka in southwestern Patagonia (50°S), contemporaneous with the Antarctic cold reversal. This was followed by ice recession under cold but relatively milder conditions until ca. 11.5 ka, when paleovegetation records indicate the onset of warm interglacial conditions. These fi ndings differ from those reported in northern Patagonia (~40°S), where deteriorating conditions before 13.5 ka were followed by the coldest part of the LGIT that lasted until ca. 11.5 ka. We interpret the apparent blend of Greenlandic and Antarctic cold phases as evidence for their co-occurrence in the southern middle latitudes in Patagonia, and hypothesize that the position of the Antarctic Polar Front modulated the strength of these cold events in regions to the north or south of it.

Vegetation and climate near Lago Llanquihue in the Chilean Lake District between 20200 and 9500 14C yr BP

Radiocarbon-dated pollen records of two adjacent sediment cores from Canal de la Puntilla (40°579090S, 72°549180W) in the Chilean Lake District reveal that a sparsely vegetated landscape prevailed during the portion of the Last Glacial Maximum between 20 200 and about 14 800 14 C yr BP. Dominating the vegetation was Nothofagus, Gramineae and Compositae, along with taxa commonly found today above the Andean treeline (Perezia-type, Valeriana) and in Magellanic Moorlands (Donatia, Astelia). Nothofagus expanded between 20 200 and 15 800 14 C yr BP, interrupted by a reversal at 19 200 14 C yr BP and followed by a prominent increase in Gramineae pollen between 15 800 and about 14 800 14 C yr BP. A major increase in Nothofagus started at about 14 800 14 C yr BP, followed by an abrupt expansion of thermophilous Valdivian/North Patagonian Rain Forest taxa (Myrtaceae, Lomatia/Gevuina, Hydrangea, etc.) at about 14 000 14 C yr BP. An opening of the rain forest and an expansion of Podocarpus nubigena, Misodendrum, and Maytenus disticha-type subsequently occurred between 11 000 and 10 000 14 C yr BP. These results suggest that mean annual temperature was 6-7°C colder than at present, with twice the modern annual precipitation between 20 200 and 14 000 14 C yr BP, implying a northward shift and intensification of the westerlies storm-tracks. Slight climate warming occurred between 20 200 and 15 800 14 C yr BP, featuring cooling reversals at 19 200 14 C yr BP, and later at 15 800 14 C yr BP. The warming of the last termination started at about 14 800 14 Cy r BP, and reached a total temperature rise of

Climatic control of the biomass-burning decline in the Americas after ad 1500:

5°C by 12 400 14 C yr BP, followed by cooling between 11 000 and 10 000 14 C yr BP.