Martin Grosjean

Paleohydrology of the Laguna Lejía (north Chilean Altiplano) and climatic implications for late-glacial times

Paleoenvironmental and sedimentological data from Laguna Lejia (23°30′S, 67°42′W, 4325 m) in the high Altiplano of the Chilean Atacama desert indicate that climatic conditions during late-glacial times were significantly wetter than today. A water and energy budget model was used to simulate climatic scenarios that would have resulted in the observed changes in the water levels, water surfaces, water volumes and salinity during the time interval. Different climatic scenarios for the Laguna Lejia catchment include precipitation increases of over 100% up to 400−>500 mm/yr (today 200 mm/yr) to account for the late-glacial lake levels with shorelines 15–25 m higher than at present, and a lake surface of 9–11 km2 compared to the present 2 km2. During the lake maximum about 13,500–11,300 yr B.P. finely laminated sediments consisting of Mg-calcite, diatoms and sometimes gypsum were deposited. The paleoenvironmental evidence indicates that (north)easterly wind direction prevailed, and the summer rainfall resulted from a seasonal poleward shift of the tropical circulation. The lake history of Laguna Lejia is representative for the Altiplano area between 21° and 24°S.

A 22,000 14C year BP sediment and pollen record of climate change from Laguna Miscanti (23°S), northern Chile

Lake sediments and pollen, spores and algae from the high-elevation endorheic Laguna Miscanti (22°45′S, 67°45′W, 4140 m a.s.l., 13.5 km2 water surface, 10 m deep) in the Atacama Desert of northern Chile provide information about abrupt and high amplitude changes in effective moisture. Although the lack of terrestrial organic macrofossils and the presence of a significant 14C reservoir effect make radiocarbon dating of lake sediments very difficult, we propose the following palaeoenvironmental history. An initial shallow freshwater lake (ca. 22,000 14C years BP) disappeared during the extremely dry conditions of the Last Glacial Maximum (LGM; 18,000 14C years BP). That section is devoid of pollen. The late-glacial lake transgression started around 12,000 14C years BP, peaked in two phases between ca. 11,000 and <9000 14C years BP, and terminated around 8000 14C years BP. Effective moisture increased more than three times compared to modern conditions (∼200 mm precipitation), and a relatively dense terrestrial vegetation was established. Very shallow hypersaline lacustrine conditions prevailed during the mid-Holocene until ca. 3600 14C years BP. However, numerous drying and wetting cycles suggest frequent changes in moisture, maybe even individual storms during the mid-Holocene. After several humid spells, modern conditions were reached at ca. 3000 14C years BP. Comparison between limnogeological data and pollen of terrestrial plants suggest century-scale response lags. Relatively constant concentrations of long-distance transported pollen from lowlands east of the Andes suggest similar atmospheric circulation patterns (mainly tropical summer rainfall) throughout the entire period of time. These findings compare favorably with other regional paleoenvironmental data.

From proxy data to paleoclimate interpretation: the mid-Holocene paradox of the Atacama Desert, northern Chile

Abstract The question whether the mid-Holocene climate (between ca. 9 and 4 cal kyr B.P.) in the Atacama Desert and the Central Andes in general was humid or dry has wide implications with regard to the understanding of long-term climate variability in South America. Paleosols, regional groundwater tables, abiotic proxy data and pollen of aquatic plants in lake sediments show a marked and rapid shift from very humid late-glacial/early Holocene climatic conditions (between ca. 14 and 9.5 kyr B.P.) to extremely dry mid-Holocene conditions (more arid than today between ca. 9 and 4 kyr B.P.). An exception during this hyperarid period is a century-scale more humid interval around ca. 5.5–6 kyr B.P. that appears systematically in lake sediment archives. In contrast, pollen for most terrestrial plants preserved in lake sediments do not show major changes during the Holocene, whereas more humid mid-Holocene conditions (compared with late Holocene conditions) were inferred from plant macrofossils in rodent middens. Is the reason for this disagreement to be attributed to misinterpretation of the paleoenvironments or of the proxy records themselves, or to incomplete paleoclimatic interpretation of the paleoenvironments? We argue that these different paleoclimate archives record different aspects and facets of ‘climate’. While paleosols and groundwater in the Atacama Desert record low-frequency climate variability at century to millennium scales, lake sediments on the Altiplano record decade- to century-scale variability. Terrestrial vegetation responds to shorter high-frequency climate variability at seasonal to inter-annual scales and preferably to humid years. Vegetation remains in ‘hibernation’ or does not germinate during arid years. Thus information from these three types of archives is not a priori comparable and requires careful site-specific, archive-specific and time-scale-specific evaluation. What is natural in modern climatology is also true for paleoclimatology: a comprehensive assessment must account for the complex daily and seasonal cycles, for the range of climate variability and trends at different scales in space and time, for impacts of short-term extreme events, and for specific, often non-linear responses of individual bio-geo-physical archives to any of the numerous aspects of ‘climate’.

Late-glacial and early Holocene lake sediments, ground- water formation and climate in the Atacama Altiplano 22-24° S *

Precipitation rates in the Atacama Altiplano 22–24°S were 400–500 mm yr−1 during late glacial and early Holocene times as opposed to 200 mm yr−1 today. This humid phase (Tauca phase) was likely due to strengthened tropical (monsoonal) circulation, which brought continental moisture to the Atacama Altiplano. The lake level of Laguna Lejía (23°30′S, 4350 m) at that time was up to 25 m higher than it is today. Mg/Ca and Sr/Ca data from lake sediments show that, what today is a highly saline lake was a freshwater lake at that time. Seasonally-laminated calcareous sediments were deposited between 13 500 and <10 400 yr B.P. indicating the maximum of the humid phase. Climatic changes in the past are important for current groundwater resources.14C and3H data from lake-, ground- and well water suggest that modern groundwater formation (i.e. water <40 years) in the Altiplano is very limited under current arid conditions. We conclude that significant amounts of the water resources in this area originated during the time of the late-glacial and early Holocene humid climate. Tritium data from snow samples show that the moisture in the Altiplano at 22–24°S is mainly of continental origin, whereas precipitation from the westerlies hardly contributes to the water supply in this area. This precipitation pattern matches the paleodata, and we suggest that current precipitation formation may provide an analogue framework for late-glacial circulation in this area.

Limnogeology of Laguna Miscanti: evidence for mid to late Holocene moisture changes in the Atacama Altiplano (Northern Chile)

Sedimentological, mineralogical and geochemical analyses of sediment cores from 9 m-deep, saline Laguna Miscanti, Chile (23 ° 44′S, 67 °46′W, 4140 m a.s.l.) together with high-resolution seismic profiles provide a mid to late Holocene time series of regional environmental change in the Atacama Altiplano constrained by 210Pb and conventional 14C dating. The mid Holocene was the most arid interval since the last glacial maximum, as documented by subaerial exposure and formation of hardgrounds on a playa surface. Extremely low lake levels during the mid Holocene appear consistent with lower effective moisture recorded at other sites along the Altiplano and in the Amazon Basin. Termination of this arid period represented a major shift in the regional environmental dynamics and inaugurated modern atmospheric conditions. The cores show a progressive upward increase in effective moisture interrupted by numerous century-scale drier periods of various intensities and durations that characterize a fluctuating late Holocene climate. In spite of chronological uncertainties, the major environmental changes seem to correlate with the available paleorecords from the region providing a coherent account of effective moisture variability in the tropical highlands of South America.

Late Pleistocene climate conditions in the north Chilean Andes drawn from a climate-glacier model

A climate-glacier model was used to reconstruct Late-glacial climate conditions from two case-study glaciers at 18° and 22° S in the arid (sub) tropical western Andes of northern Chile. The model uses (i) the geometry of the Late-glacial maximum glaciation, (ii) modern diurnal and annual cycles, amplitudes and lapse rates of the climate, (iii) empirical-statistical sublimation, melt and accumulation models developed for this area, and (iv) dynamic ice flow through two known cross-sections for steady-state conditions. The model is validated with modern conditions and compares favorably with the glaciological features of today. The mass-balance model calculates the modern equilibrium-line altitude at 18° S as high as 5850 m (field data 5800 m), whereas no glaciers exist in the fully arid southern area at 22° S despite altitudes above 6000 m and continuous permafrost. For Late-glacial times, the model results suggest a substantial increase in tropical summer precipitation (AP = +840 (-50/+ 10) mm a -1 for the northern test area; +1000 (- 10/+ 30) mm a -1 for the southern test area) and a moderate temperature depression (ΔT = -4.4 (-0.1/+0.2) °C at 18° S; -3.2 (±0.1) °C at 22° S). Extratropical frontal winter precipitation (June-September) was <15% of the total annual precipitation. A scenario with higher winter precipitation from the westerlies circulation belt does not yield a numerical solution which matches the observed geometry of the glaciers. Therefore, we conclude that an equatorward displacement of the westerlies must be discarded as a possible explanation for the late Pleistocene glaciation in the Andes of northern Chile.

Full and Late Glacial Lake Records Along the PEP 1 Transect: Their Role in Developing Interhemispheric Paleoclimate Interactions

Publisher Summary Lacustrine records that encompass the full, and late glacial periods, extends into the early Holocene document climate changes through the effects on limnology. Wind intensity, temperature, and water balance are the principal climate effects recorded by lake sediments. A selection of lake records along the Pole-Equator-Pole: Americas transect from Alaska to southern Patagonia reveals broad zonal regions of climate-controlled paleolimnology. In the Arctic, lakes record climate mostly as temperature signals that change the effective moisture balance, and their productivity. South of the Laurentide ice sheet, lakes also responded to warming temperatures, and reduced anticyclonic wind systems between the full glacial, and the early Holocene. A related effect, the disruption of Intertropical Convergence Zone (ITCZ), and the moisture-bearing trade winds responsible for maintaining lakes in Central America, and northern South America, caused basins in this region to be shallow, saline, or dry during the full glacial period, and to become re-established only by the latest glacial, and early Holocene. The single site in southern South America apparently did not receive moisture in significant amounts until the early Holocene, as westerly precipitation finally became established in the region. This analysis indicates that lakes recorded generally synchronous climate changes by similar but sometimes opposite responses. This synchroneity does not imply the same causality, even though the dynamics of climate change between the full glacial and the Holocene were clearly linked and mutually interdependent.

Modeling Modern and Late Pleistocene Glacio-Climatological Conditions in the North Chilean Andes (29–30 °)

An empirical-statistical climate-glacier model is used to reconstruct Late Pleistocene climate conditions in the south-central Andes of northern Chile (29–30° S). The model was tested using modern climate data and the results compare favorably with key glaciological features presentlyobserved in this area. Using several glaciers at 29° S as casestudies, the results suggest an increase in annual precipitation(Δ P = 580 ± 150 mm, today 400 mm), and a reduction inannual mean temperature (Δ T = −5.7 ± 0.7 ° C).These data suggest full glacial LGM (Last Glacial Maximum) conditionsfor the maximum glacier advances at 29° S, a scenario that is asynchronous with the timing of maximum advances north of the Arid Diagonal (18–24° S) where late-glacial climate was moderately cold but very humid.The reconstructed case study glaciers at 29° S do not allow conclusions to be drawn about the seasonality of precipitation. However, comparison with regional paleodata suggests intensified westerly winter precipitation and a stable position for the northern boundary of the westerlies at ∼ 27° S. However, the meridional precipitation gradients were much steeper than today while the core area of the Arid Diagonal remained fixed between 25–27° S.

A quantitative high-resolution summer temperature reconstruction based on sedimentary pigments from Laguna Aculeo, central Chile, back to AD 850

We present a pigment-based quantitative high-resolution (five years) austral summer DJF (December to February) temperature reconstruction for Central Chile back to AD 850. We used non-destructive in situ multichannel reflection spectrometry data from a short sediment core of Laguna Aculeo (33°50′S/70°54′W, 355 m a.s.l., central Chile). Calibration-in-time (period AD 1901—2000, cross-validated with split periods) revealed robust correlations between local DJF temperatures and total sedimentary chlorin (relative absorption band depth (RABD) centred in 660—670 nm RABD660;670: r=0.79, P<0.01; five-years triangular filtered) and the degree of pigment diagenesis (R 660nm/670 nm: r=0.82, P<0.01; five-years triangular filter). Root Mean Squared Error values are small (between 0.24 and 0.34°C) suggesting that most of the reconstructed decadal-scale climate variability is significant. Our data provide quantitative evidence for the presence of a Medieval Climate Anomaly (in this case, warm summers between AD 1150 and 1350; ΔT = +0.27 to +0.37°C with respect to (wrt) twentieth century) and a very cool period synchronous to the ‘Little Ice Age’ starting with a sharp drop between AD 1350 and AD 1400 (−0.3°C/10 yr, decadal trend) followed by constantly cool (ΔT = −0.70 to −0.90°C wrt twentieth century) summers until AD 1750. The structure of variability is consistent in great detail with annually resolved tree-ring based warm-season temperature and river discharge reconstructions from northern Patagonia (42°S) for the past 400 years, with qualitative climate reconstructions from Andean glacier fluctuations, and with hydrological changes in Patagonian lake sediment records.

Holocene climate variability and change; a data-based review

The Holocene Thermal Maximum with peak temperatures prior to 7 ka BP is mostly accentuated in the Northern Hemisphere, still visible in the Southern Hemisphere and possibly did not exist in the tropics. Between this period and the modern warming a remarkable negative temperature trend occurred in the Northern Hemisphere, which was probably the effect of a decreasing orbital-induced insolation during the boreal summer. On average the Northern Hemisphere humidity–precipitation records do not show any significant trend. A mechanistic explanation for the multi-decadal- to century-scale Holocene cold relapses, which mainly occurred in the Atlantic–European regions, exists for only the early Holocene cooling events, which are probably the result of a collapse of the meridional overturning circulation owing to freshwater outbursts from the Laurentide ice sheet. Possibly, the late Holocene cold events after c. 4 ka BP are influenced by the covarying influence of major tropical volcanic eruptions and Grand Solar Minima.