Discuss the evidence for environmental change since the beginning of Marine Isotope stage 5 in the Tropical and Subtropical Andes.


The Andes mountain range, located on the west coast of South America, is the longest continental mountain range in the world, extending for 7000Km from ~10° North to ~53° South and with peaks in the tropical and subtropical sections of the mountain range having altitudes in excess of 4000m a.s.l. (Garreaud, 2009). The physical configuration of the mountain range means that almost all atmospheric currents are truncated by the mountain range and diverted North or South. This physical configuration has been recognisable since the Miocene (28 to 6Myr), and thus the environmental impact of the Andes mountain range on the area was long established by the Pleistocene (Anderson et al, 2013).  Redirection of prevailing winds, and the subsequent alteration to precipitation patterns has enabled the Atacama Desert, which lies to the West of the Andes and subsequently leeward side, to be deemed the driest desert (none polar) on the planet with modern recorded precipitation rates averaging <20mm per year (Rech et al, 2006).

Modern day variance in seasonal conditions, and the subsequent use of this as an analogue for past climatic conditions, is heavily influenced by migration seasonally of the Inter Tropical Convergence Zone (ITCZ), and the impact this has on precipitation levels through the shifting of prevailing winds (Figure 1) (Ortiz-Royero et al, 2013). Also, climate modelling and continued high resolution recording of sea surface temperatures (SST’s) has seen the increased importance of the El Nino Southern Oscillation (ENSO) to world climatic variance be demonstrated, but the regional impact of these changes to the area under examination cannot be understated (Eichler and Londoño, 2013), and thus will be a continuing influence within this discussion.


Figure 1. Geographical shift of the Inter Tropical Convergence Zone (ITCZ) seasonally. Source: Modified from Ortiz-Royero et al, 2013

To reconstruct, examine and discuss the environmental changes occurring in this region over the last ~130,000 years, studies in to palaeolakes, marine cores, fossil dunes, aeolian sediments and other secondary sources will be considered. From these disparate and sometimes contradicting sources a history of environmental change since MIS 5 (Marine Isotope Stage) within the tropical and subtropical Andes will be produced.


Palaeo-climatic evidence derived from lacustrine sources often presents a well stratified record of climatic conditions of a region, but interpretation of the deposits must be considered carefully as many factors can conspire to produce a particular stratigraphic representation (Fritz, 2008). Historic lake heights may be a result of increased precipitation, catchment area modification, or reduced evaporation and the results of changes rather than the changes themselves are what is witnessed.

Lacustrine evidence from the Andes region is extensive and due to the large latitudinal range of the area examined within the studies detailed below, they may produce sometimes contradictory evidence (Figure 2).


Figure 2. Location of studies using lacustrine deposits (circles), and marine cores (triangles). Lake sediment studies: 1. Fritz et al, 2004; 2. Bush et al, 2004; 3. Hillyer et al, 2009; 4. Baker et al, 2001; Bird et al, 2011; Moy et al, 2002; 7. Mayle et al, 2000; 8. Mayle et al, 2004; 9. Bao et al, 2012. Marine cores: 10. Pena et al, 2008; 11. Rein et al, 2005.

Modified from Metcalfe and Nash, 2012.


Deep cores taken from Salar de Uyuni (20°14.97”S, 67°30.03”W, Figure 2: site 1), produced 220.6m of almost continuous deposition (Fritz et al, 2004), and using radiocarbon dating for the uppermost (recent) deposits and the U-series disequilibrium method for deeper (older) deposits allows for dating up to ~300,000 yrs. B.P. (Ku, 2000). Results from the dating process show 100m depth of the core to be ~170,000 yrs. old, with marked stages of increased desiccation shown by the development of more saline lake conditions which at times developed in to salt pan conditions (Figure 3). Borehole Gamma analysis of the core was carried out to assess the mineralogical composition of the deposits as the drilling process had disrupted the retrieved cores, and to produce a more accurate result gamma analysis was conducted within the borehole post core retrieval.


Figure 3. Mineralogical analysis of the borehole Gamma results produces demarked strata of lake deposits that then change to shallow saline lake and/or salt pan, describing periods of climatic desiccation. Source: Frits et al, 2004.


The Fritzs et al (2004) study found that salt structures older than ~90,000 yrs. demonstrated significantly more arid conditions, and this was also supported in the diatoms present and their concentrations, with abundance of planktonic diatoms increasing between 60,000 and 20,000 yrs. B.P. Overflow and run-off from Lake Titicaca has been seen as the historical source of much of the water for the Salar de Uyuni, and therefore lake depths for the core area could be strongly linked to water depths and subsequent discharge volumes from Lake Titicaca (Abbott et al, 1997).

Cores examined from Lake Titicaca (within the Altiplano region), being the deepest lake within South America has provided maximum ages of 25,000 yrs. B.P., and through the analysis of halophilic, planktonic and benthic diatoms and their variance in relative abundancy, lake levels have been reconstructed and show a net reduction in lake level from ~21,000 yrs. B.P. with a minimum lake level ~15,000 yrs. B.P. when the lakes overflow ceased (Baker et al, 2001). These findings are supported from studies at Lake Pacucha, Peru, where diatom, and lake level evidence suggested abrupt warming ~19,500 yr.s B.P. with conditions suggesting a wetter period around the LGM (Hillyer et al, 2009). Further studies within the Altiplano also show climatic change synchronous with the last glacial maximum (LGM) through analysis of pollen deposits showing steady increases in temperature through the migration of species intolerant of lower temperatures in to higher altitudes (Bush et al, 2004). Migration of species could also be affected by the influence of clouds on the ecosystem, and comparison of arboreal and aquatic species tolerance to desiccation were identified within the study which may demonstrate the influence of cloud cover to species survival rather than purely changes in precipitation or temperature (León‐Vargas et al, 2006).

Lake Pacucha sediments point to a continued drying of the region around 10,000 yrs B.P., with the lake shallow and saline with high CaCO3 deposition (Figure 4). Reduced sedimentation rates suggest a reduced input of river discharge, as river discharge volumes could be considered a primary controller of suspended sediment volumes being carried to the lake and subsequently deposited (Milliman and Syvitski, 1992). Abrupt changes to the strata within the cores examined, it has been suggested, may have been caused by El Nino Southern Oscillation (ENSO) weakening and its interaction with the tropical Atlantic dipole causing wetter conditions in a typically more arid climate (Hillyer et al, 2009).


Figure 4. Lake Pacucha core showing increased organic matter and CaCO3 content at the same time as a marked reduction in deposition ~13,000 to 4,500 yrs. B.P., suggesting marked reduction in precipitation. Source: Hillyer et al, 2009.


ENSO influence on the climate of South America through the Holocene may be a major factor in determining the causes of climatic oscillations in cores throughout the region. Cores taken from Lake Laguna Pallcacocha in Ecuador may be able to determine the influence ENSO has had upon the region (Moy et al, 2002) as modern light-coloured sediment strata within the lake have found to be correlated to documented ENSO events. Analysis of a 9m deep core which has been dated to 12,000 yrs. B.P. at its deepest has thus been able to correlate frequency of ENSO events over time up to a maximum of 12,000 years B.P. (Figure 5).


Figure 5. Frequency of ENSO events as recorded in sedimentation beds in Lake Laguna Pallcacocha, Ecuador. Number of ENSO events per 100 years listed over the last 12,000 years. Source: Moy et al, 2002.


Data suggests that ENSO events have increased dramatically over the last 5,000 years in comparison to before this boundary and would support evidence from Lake Titicaca that suggested a warmer/wetter climate since 5,500 yrs. B.P. (Baker et al, 2001). The method of data analysis should be considered however, as ENSO events are recorded through sedimentation changes, and other factors could have altered the sediment record within the lake before 5,000 yrs. B.P. resulting in the paucity of events in the older section of the core.

Further high definition studies of lacustrine deposits at Lago Chungara, South of Lake Titicaca using radiocarbon dated diatom assemblages has demonstrated periods of increased humidity (12,400 to 10,000 yrs. B.P.) and periods of increased aridity (10,000 to 9,600 yrs. B.P. and 7,400 to 3550 yrs. B.P.) with the most recent period of aridity being termed the Andean Mid-Holocene Aridity Period (Bao et al, 2015). The comparison of these results to those at Lake Laguna Pallcacocha (Moy et al, 2002) may demonstrate the importance of the ITCZ location and how general climatic conditions, and specifically precipitation levels, can vary within the tropical/subtropical zone.

Analysis of historical shorelines for lakes within the Andes also allows for palaeo-climatic reconstruction, and assessment of Lake Titicaca using shoreline evidence, organic matter and clay deposits within cores shows extensive lake depth fluctuations through the late Holocene (Abbott et al, 1997) possibly linked to solar radiation variance (Clement et al, 2000). Extrapolation of causes from observed data must always be treated with caution however as multiple factors may have influenced lake levels (Anderson et al, 2013).

Marine Cores

Sampling taken from cores within the Pacific can provide a record of marine conditions and through comparison to modern analogues allow us to reconstruct palaeo-climatic conditions. Through the analysis of oxygen isotopic fractionation within the tests of pelagic and benthic species it has been possible to reconstruct SST’s over a considerably longer timescale than a typical terrestrial record (Shackelton and Opdyke, 1973). Site 1240 of the Ocean Drilling Program, a marine core drilled in the Panama Basin, produced a marine record extending back 275,000 yrs. B.P.. Preferential fractionation during evaporation of seawater means that lighter δ16O is removed from ocean water in greater volumes than δ18O, and during glacial conditions this lighter isotope of oxygen is trapped within ice sheets. The reduction in δ16O means that changes within oxygen isotope ratios allows us to estimate global temperature changes and Ice sheet expansion/retreat. Through analysis of δ18O within the tests of species within the marine core recovered, as well as the type of species deposited (and their tolerance to differing water temperatures) it is demonstrated that rapid and extensive changes to SST’s, and by extrapolation climatic conditions, are observed (Figure 6) with particularly dramatic temperature changes seen during MIS 5 and MIS 2 (Pena González et al, 2008).


Figure 6. Marine sediment core from ODP site 1240, Panama Basin, showing oxygen isotope in G. ruber (Purple) demonstrating variance over the last 275 Kyr, with estimated sea surface temperatures (SST)(Orange), and Vostok ice core recordings for comparison (Black). MIS stages listed across the top. Source: Pena González et al, 2008.


Modern ocean currents within the region (ocean currents which have been in place since before MIS 5, McCave et al, 2008)) have a strong impact on the climatic conditions of the Andes region (Bates et al, 2008), and subsequently identifying changes to this coupled ocean-atmospheric relationship is key to understanding past climatic changes, and in particular the dramatic effect ENSO events can produce (Hebbeln et al, 2002). The nutrient rich waters of the Western coast of tropical and sub-tropical South America are fed by cold nutrient rich upwelling currents (Bakun and Weeks, 2008), and changes to this upwelling through ENSO events produces marked changes in marine productivity that can be identified within marine sediment cores. Marine core 106KL, taken 80km off the Peru/Lima coastline provides dating evidence going back to ~20,000 yrs. B.P., and has attempted to reconstruct SST’s and bio productivity in an attempt to demonstrate the strength and frequency of ENSO events and their impact on regional and global climate (Rein et al, 2005), demonstrating an increase in frequency and strength of ENSO events since the LGM.

Fossil Dunes & Aeolian Sediments

Aeolian processes and the subsequent formation of dune systems are heavily dependent on sources of fine-grained sand, regional precipitation (both average and seasonal), and prevailing wind direction and strength (Anderson et al, 2013). With the extended aridity during glacial periods contributing to the formation of extensive dune fields (Clapperton, 1993). Examination of aeolianites and associated dune systems in the Atacama Desert, Chile, have produced records of periodic dune formation dating back 130,000 yrs. B.P. (Nash et al, 2018). Using optically stimulated luminescence dating (OSL), the historical record of dune formation has been dated with periods of growth (aridity) identified in four main periods: 130, 111 to 98, 77 to 69 and 41 to 28 Ka (Figure 7). Further analysis using oxygen and carbon isotope ratios has been able to identify the source of dune growth to be predominantly marine in origin (especially in the three oldest phases identified), and orientation analysis shows the prevailing wind direction during formation to have been from directions very different to modern prevailing winds, thus helping to reconstruct palae-climatic conditions during these periods.

Periods of extended aridity in the region (Atacama) and changes to prevailing wind direction, was further examined in a study of Chilean fossil dunes from ~30°S to ~35°S (Veit et al, 2014) in which aridity and dune formation was hypothesised to be controlled and dominated by solar radiation, predominantly obliquity (Milankovitch, 1998). Alternative studies highlight the importance of hydrology preceding dune formation, as the main supplier of sediment subsequently used in dune formation, and also the affect tectonic uplift and/or eustatic sea level changes can have on sediment supply, with the increased availability of suitably sized sand particles being essential to extended development of dune fields (Londoño et al, 2012). Thus, it is important to consider the interpretation of aeolian evidence in light of the factors required for dune formation, which extend beyond just average precipitation levels.

As discussed with lacustrine evidence, the regional importance of the migration of the ITCZ, and the impact this has on aridity levels has also been suggested as a major factor affecting dune development, particularly in the late Pleistocene and early Holocene. With the use of OSL dating of dunes in Venezuelan Llanos, climatic changes possibly brought on by a Heinrich event in the North Atlantic could have shifted the ITCZ and caused arid conditions to facilitate dune expansion from fluvial sediment sources (Carr et al, 2016).


Figure 7. Eustatic sea level and MIS numbers with orange banding overlaid showing periods of dune building, and blue banding showing periods of aeolianite cementation. Black points with error bars show OSL ages, and brown horizontal bars at the top show other dune field development in South America. Source: Nash et al, 2018.


Significantly lower eustatic sea level through the last glacial period means that aeolian sediments can and are found within marine cores, with the subsequent submersion enabling preservation of strata. Stuut and Lamy (2004) examined marine cores from off the coast of the Atacama Desert (GeoB 3375-1) and using grain-size distribution of terrigenous sediment developed a palaeo-history of climatic change over the last 120 Ka (Figure 8), which the authors suggested followed processional solar radiance maxima, but also highlighted the influence ENSO events may have played on their results.


Figure 8. Intensity of aridity within the Atacama region of Chili/Peru over the last 120 Ka yrs. derived from grain size analysis within marine cores derived from the South American continental shelf. Source: Stuut and Lamy, 2004.


Rodent middens

Arid and hyper-arid conditions within a region produce a unique set of issues for palaeo-climatic investigation as the majority of evidence available for examination is usually related to fluvial transportation or sedimentation, a process devoid in an arid environment. The dry conditions however do enable the retention and protection of evidence which is available, such as rodent middens (Holmgren et al, 2001; Betancourt and Saavedra, 2002). Middens provide evidence of pollen, macrofossils, cuticles, and other faunal remains that the rodents have collected or eaten from the surrounding area, with preservation being enhanced by the arid conditions (Figure 9). The organic nature of the material contained within the middens enables radiocarbon dating to produce quantifiable timescales for the finds, and comparison of species to modern analogues allows for climatic conditions to be inferred (Diaz et al, 2012).


Figure 9. Midden finds demonstrating the variety and quality of preservation from a) 500 yrs. and b) 17,500 yrs. which enables easier classification of finds to enable climatic reconstruction for the periods the midden was in use. Source: Diaz et al, 2012.


Increasing numbers of studies in the last twenty years on middens within the Atacama have started to produce evidence of climatic change over the late Pleistocene and Holocene, with Maldonado et al (2005) extending midden evidence back to 52,000 yr. B.P.. Evidence for climatic variations in summer, winter, and all year-round moisture levels were inferred through the analysis of pollen identified and dated using radiocarbon techniques. Midden deposits showed episodic periods of higher moisture availability at 40 to 33 Kyr. B.P. and 24 to 17 Kyr. B.P. from winter rainfall, 17 to 14 Kyr. B.P. from all year-round rainfall, and 14 to 11 Kyr. B.P. from increased summer rainfall (Figure 10). Slightly contradictory data from a study by Diaz et al (2012) also within the same approximate geographical area (24-26°S), found a more fragmented diversity of evidence around the LGM, with low taxonomic richness dated to 19.3 Kyr. B.P. suggested a more arid climate at this time (Figure 10).


Figure 10. Dated midden deposits found within the arid Atacama Desert from five separate studies demonstrating the varied results chronologically. Compiled from: De Porras et al, 2017; Diaz et al, 2012; Maldonado et al, 2005; Latorre et al, 2003; Latorre et al, 2002.


Studies from De Porras et al (2017), and Lattore et al (2002; 2003) also produce sometimes contradictory evidence which demonstrates that evidence contained within a rodent midden is considered to have been collected from an area <50m distance from the animals’ burrow (Spaulding et al, 1990), but the transport method (fluvial, aeolian), local environmental conditions and rodent species can all have an impact on the midden contents. When compared to lacustrine and marine evidence previously discussed, the faunal and floral evidence contained within rodent middens may be detailing a seasonal, annual or decadal change in climatic conditions through the ability for species to respond rapidly to change, compared to the century and millennial records often contained within more geomorphological evidence (Grosjean et al, 2003).

Human Occupation

Migration to the Americas by Humans is considered to have occurred via the Bering Sea land bridge at the end of the LGM possibly as late as 16,500 yrs. B.P. (Goebel et al, 2008; Meltzer, 2009), with migration through North America seeing human occupation in the Andes region being dated to ~13,000 Yrs. B.P. (Figure 11) (Jackson et al, 2007; Nunez et al 2002). Human occupation, particularly inland away from marine sources of food (Jackson et al, 2007; Sandweiss et al, 1998), would indicate a less arid environment capable of sustaining populations, and the regionally extensive evidence of human occupation around 13,000 to 9,500 yrs. B.P. suggests a less arid climate (Nunez et al, 2002; Borrero, 2008; Steele and Politis, 2009; Grosjean et al, 2005). Again, the localised climatic conditions affecting occupation, such as local water supply, must be considered as the area examined covers areas of differing altitude and prevailing wind direction, which could produce very localised climatic conditions (Latorre et al, 2013). As seen earlier whilst discussing rodent midden evidence, we must therefore be careful not to extrapolate climatic conditions for extended areas on localised information (Nunez et al, 2002).


Figure 11. Archaeological sites with dated evidence demonstrating the human occupation of the area through the late Pleistocene and early Holocene. Source Lattore et al, 2013.


Environmental change within the tropical and subtropical region of the Andes since MIS 5 (~130,000 yrs.) is a complex and disjointed history in which multiple inputs such as solar radiation, geomorphology, oceanic and atmospheric currents must all be considered. The size of the region under discussion and the changing location and subsequent influence of both the ITCZ and ocean-atmosphere prevailing currents makes reconstruction of the entire area very difficult. Solar insulation variance in the form of Obliquity and Procession have been identified in lacustrine records to collate with aridity for long term trends (Bradley, 1985), with ENSO events influencing short term, higher resolution changes seen in rodent middens and higher resolution lacustrine records.

Paucity of data compared to Northern Hemisphere regions (Tripaldi and Zarate, 2016) means that on-going research in to historical climate change will aid the development of understanding in to the regions history but also the global climatic system and the bi-polar see-saw phenomenon (Broecker, 1998). Fossil dune (Nash et al, 2018), marine core (Pena González et al, 2008), and lacustrine evidence (Frits et al, 2004) points to predominantly arid conditions through MIS 5 & 4 (up to ~50,000 yrs. B.P.) but interspersed with periods of lowered aridity. All records examined suggest a colder but moister environment at the LGM, with conditions rapidly approaching modern day equivalents from ~14 to 12,000 yrs. B.P. (Figure 12). With the transition in to MIS 1 (~10,000 yrs. B.P.) so the influence, strength and frequency of ENSO events seems to have increased (Moy et al, 2002).


Figure 12. Combined records for the last ~120,000 yrs. from: A) fossil dunes (Stuut and Lamy, 2004), B) Lake basin evidence (Groot et al, 2011), C) Marine core (Pena Gonzalez et al, 2008), D) Fossil dune systems (Nash et al, 2018), and E) Lacustrine aridity (Frits et al, 2004). Green bar used to highlight synchronicity of events through ~105,000 yrs. B.P.. Blue box highlighting strong correlation of evidence through the LGM and in to the Younger Dryas. Sourced from papers referenced above.



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