Radiocarbon Dating Of Organic Matter Research Articles

Январская палеотемпература воздуха в период МИС-3-2 в северо-восточной Якутии, реконструированная по высокоразрешающей записи изотопного состава сингенетических повторно-жильных льдов Батагайской едомы

The work is aimed at determining the age of ice wedges, as well as at characterizing winter temperature fluctuations during the formation of the Yedoma based on the isotopic composition of ice wedges in the unique outcrop of the Batagay mega-slump. The researchers carried out a detailed sampling from the Upper Ice Wedge Complex of the Batagay Yedoma, determined the isotopic composition in three ice wedges at different altitudes. The results of radiocarbon dating of organic matter from ice wedge confirm the time of syngenetic formation of the upper-tier ice wedges in the range from 27.2 to 30.8 thousand cal. years. Ice wedges of the lower tier, according to the results of radiocarbon dating of ice accumulated 44.7—47.9 thousand years ago simultaneously at different levels: 274—268 m and 224—227 respectively. The study of yedoma deposits and ice-wedge ice made it possible to obtain data on climatic fluctuations in a region with the most severe continental climate. Direct radiocarbon dating of syngenetic ice wedges provided dating of ice growth in the Batagay yedoma, data on the content of stable isotopes in the ice demonstrated stable and very harsh conditions of the winter season. The air paleotemperature in Batagay in January 47—42 and 30—25 thousand cal. years ago was consistently low about –51°C.

Sartanian (MIS 2) ice wedge pseudomorphs with hydromorphic pedosediments in the north of West Siberia as an indicator for paleoenvironmental reconstruction and stratigraphic correlation

Within the glacier-free scenario of the MIS 2 (Sartanian cryochron) environment in the north of West Siberia, it is important to search for the new regional paleoecological records and stratigraphic markers for this period. We studied large ice-wedge pseudomorphs filled with gleyic pedosediments developed on the high river terraces in the Nadym and Taz rivers basins as a tracer of the surface cryogenic and pedogenic processes during MIS 2. Field morphological observations and measurements, palynological analysis and radiocarbon dating of organic matter from soils and pedosediments were performed. Two sets of paleoecological indicators were discriminated: pseudomorph geometry evidence of the processes and conditions of ice wedge formation during the Last Glacial Maximum whereas the paleopedological and paleobotanic results from the fills recorded the environments of ice wedge melting at the end of MIS 2. Paleotemperatures 7–10 °C lower than at present are inferred during the major part of Sartanian, even at its end continuous permafrost persisted providing development of swampy tundra vegetation and cryohydromorphic palaeosols. We propose to define Taz-Nadym cryopedogenic horizon as a representative regional unit for MIS 2 and to correlate it with synchronous cryogenic and palaeosol units in Central and Eastern Europe.

Soil carbon and microbes in the warming tropics

Abstract Climate warming could destabilise the Earth's largest terrestrial store of reactive carbon (C), by accelerating the decomposition of soil organic matter. A third of that C store resides in the tropics. The potential for tropical soils to sequester C, or to act as an additional source of CO2, will depend on the balance of C inputs and outputs, mediated by the response of soil microbial communities and their activity to perturbation. We review the impact of warming on microbial communities and C storage in humid tropical forest soils over multiple time‐scales. Recent in situ experiments indicate high sensitivity of tropical forest soil C mineralisation to warming in the short term. However, whether this will translate into long‐term soil C decline remains unclear. At decadal time‐scales, high sensitivity of soil C mineralisation to warming is consistent with the correlation between the inter‐annual variation in the tropical land surface temperature and atmospheric CO2 growth rate, and with simulations using the Carnegie‐Ames‐Stanford Approach biosphere model. This observed sensitivity may further contribute to climatic change over millennial time‐scales, suggested by radiocarbon dating of organic matter in river basins showing a twofold acceleration in tropical soil C release during the late‐glacial warming period. However, counter to this evidence, long‐term stability of tropical soil C is suggested by observed steady‐state soil C turnover across temperature gradients with elevation, and by the presence of C in tropical soils that pre‐dates the Holocene Thermal Maximum and late‐glacial warming periods. To help reconcile these recent experimental findings and long‐term observations, we propose mechanisms to explain tropical soil C and microbial responses to warming across multiple time‐scales. Combined in situ experimental and monitoring approaches—large‐scale and cross‐site—are urgently needed to resolve the interplay of these mechanisms across spatial and temporal scales, to shape a better understanding of the relationship between soil microbes and C storage in tropical soils. Read the free Plain Language Summary for this article on the Journal blog.

Climatic and tectonic significance of Taboche Lake, Khumbu Region, Nepal

SummaryThe Everest region is characterized by its alpine glacial environment. In an effort to understand environmental change and tectonic activity, our team cored Taboche Lake, situated at 4,712 m along the western margin of the Ngozumpa Glacier. This research catalogs past earthquakes using geological records of the lake core that are important for the assessment of future earthquake hazards in the region and provides information for tectonic risk of glacial lake floods. Core grain size characteristics and internal sedimentary structures from computed tomographic scan were coupled with radiocarbon dating of organic matter preserved in the core to reconstruct the environmental history of the area. The 58-cm-long core consists of laminated silty sands and sandy silts with particle diameters <2 mm. The core records a syn-sedimentary deformational structure, folded sediments, rhythmically alternating dark- and light-colored laminations, and turbidites, which indicate coeval climatic and tectonic variations over the past ∼1,600 years.

Climatic and Tectonic Significance of Taboche Lake, Khumbu Region, Nepal

The Mount Everest region is characterized by its alpine glacial environment. In an effort to understand environmental change in the area, our team cored Taboche Lake, situated at 4,712 m along the western margin of the lateral moraine of the Ngozumpa Glacier. This research catalogues past earthquakes using geological records of the lake core that are important for the assessment of future earthquake hazards in the region. To achieve the aims of the study, core grain size characteristics and internal sedimentary structures from CT scan tomography, were coupled with radiocarbon dating of organic matter preserved in the core to reconstruct the environmental history of Loboche Lake. The 58 cm-long core consists of laminated silty sands and sandy silts with particle diameters less than 2 mm, covering ∼ 2,000 years of sedimentation history. Syn-sedimentary deformational structure, folding of sediments, rhythmically alternating dark- and light-coloured laminations, and turbidites are discussed in terms of their origins as past climatic and tectonic indicators.

Debris Flow Morpholithogenesis in the Central Part of the Sayan-Baikal Stanovoi Highlands

Presented are the results from studying transported and suspended load formed during the debris flows and floods within the basins of small rivers in the central part of the Sayan-Baikal Stanovoi Highlands. In studying deposits of suspended and transported load exposed by profiles on alluvial fans, the age of the debris flow event was determined by radiocarbon dating of organic matter of buried soils. The turbidity of the flow, its speed, water discharge, pH, total mineralization of water and mudflow suspensions, and particle size distribution of suspended material were estimated for suspended load as part of the research. With a normal and reduced runoff, the water in the rivers of the central part of the Sayan-Baikal Stanovoi Highlands is transparent and clean. Visible turbidity of water appears with the formation of a high water resulting from atmospheric precipitation and snowmelt. In the rivers under investigation, the turbidity at the peak of the flood or debris flow was about 7 thou g/m3, with the maximum turbidity not exceeding 25 thou g/m3 (the Slyudyanka river, 1960). The particle size of suspended material varied from silt sand to heavy sandy loam and light loam. The research has shown that suspended load, along with tractional load, plays a significant role in the formation of accumulative landforms. This is especially evident in the lower parts of the cones, where they are able to be deposited as layers reaching several meters.

Petrology, geochemistry, and correlation of tephra deposits from a large early-Holocene eruption of Mentolat volcano, southern Chile

Two correlated tephra deposits, each 13 cm thick in the Aisén region of southern Chile, one in a lacustrine sediment core from the Mallín el Toqui (MET) peat bog and another from a subaerial soil exposure ∼10 km to the west in the Río Maniguales (RM) valley, preserve evidence for a large explosive eruption of Mentolat volcano, one of the five stratovolcanoes of the southernmost portion of the Andean Southern Volcanic Zone (SVZ). This eruption is constrained in age to ≥11,728 cal years BP by radiocarbon dating of organic matter from the MET sediment core and is termed the ∼11.7 ka MEN event. The two tephra deposits are identical and based on their petrology, bulk tephra, glass, and amphibole geochemical characteristics, are attributed to an eruption of Mentolat volcano. Both contain pumice lapilli with glass compositions that ranges from 59 to 76 wt. % SiO2, with medium to low-K2O calc-alkaline composition and trace element abundances similar to both lavas and other tephras derived from Mentolat. They have abundant amphibole, plagioclase, orthopyroxene, and clinopyroxene phenocrysts, with a smaller proportion of olivine and Fe-Ti oxides, and a minor amount of distinctive crustal xenoliths with both unfoliated and foliated textures. Amphiboles have low K2O (0.20–0.37 wt. %) and TiO2 (1.5–3.8 wt. %) and are similar geochemically to amphiboles from other Mentolat-derived tephra (K2O = 0.14–0.43 wt. % and TiO2 = 1.9–2.4 wt. %), but distinct from amphiboles in lavas and tephra derived from other volcanoes in the southernmost SSVZ including Cay (K2O = 0.47–0.55 wt. % and TiO2 = 2.4–3.1 wt. %) and Melimoyu (K2O = 0.39–0.52 wt. % and TiO2 = 2.8–4.5 wt. %). Amphiboles from the ∼11.7 ka MEN tephra formed over a broad range of pressures (154–406 MPa), temperatures (834-969 °C), and magma water contents (4.9–7.0 wt. %), which overlap with the physical-chemical conditions for the formation of amphiboles from other Mentolat-derived tephra. The two correlated tephra deposits, which are located ∼95 km southeast of Mentolat, are correlative with tephra of similar age identified in 12 other lacustrine sediment cores from the region, for which, based on their petrology and the geochemistry of their tephra glass and amphiboles, Mentolat is also the likely source volcano. This eruption produced approximately 1.8 km3 of bulk material with an estimated magnitude of 5.2. Mentolat has produced numerous (>18) explosive eruptions since glacial retreat from the region and future explosive eruptions from this center could potentially impact local population centers and the agricultural industry in southern Chile and Argentina.

Exploring the dating of “dirty” speleothems and cave sinters using radiocarbon dating of preserved organic matter

Abstract Speleothems and other carbonate deposits such as tufa containing high proportions of detrital material can be difficult to chemically date due to detrital thorium levels causing a high level of error in conventional U-Th disequilibrium dating. Here we investigate the use of an alternative technique centring on radiocarbon dating of organic matter preserved within the detrital fraction. Non-acid soluble humic, particulate and detritally absorbed organic matter was recovered from eight samples from a flowstone sinter formed within a roman aqueduct at Trento in Italy with a maximum age of 100 CE (1850 cal yr BP), and two repeat samples from a dripstone formed within the 20th Century on a wire fence at Lilly-Pilly Cave, Buchan Caves Reserve in Victoria, Australia. In the aqueduct samples the median calibrated 14 C ages ranged from 2232 to 2889 cal yr BP, with 95.4% probability age range in the youngest and oldest samples of 2153–2337 and 2342–3449 cal yr BP respectively. The median age of the more modern dripstone was 336 cal yr BP, with a 95.4% probability age range of 148–486 cal yr BP. These results provide very approximate ball-park estimates of the age of the sample, but are consistently too old when compared to the known maximum ages of formation. It is hypothesised that this offset is due to a combination of the nature of the organic carbon transported from the source organic matter pools, and reworking of both modern and old organic carbon by in situ microbial communities.

A multi-proxy reconstruction from Lutomiersk–Koziówki, Central Poland, in the context of early modern hemp and flax processing

During an archaeological investigation at Lutomiersk–Koziówki in central Poland, deposits indicative of an old rettery from the 16th–17th century AD were discovered. The artifacts found in the lacustrine deposits, together with historical sources and radiocarbon dates of organic matter, show that the pond at a local mill was present from ca. AD 1525 to at least AD 1620. The high content of Cannabis and Linum subfossil macro- and micro-remains in the sediment indicate that the pond was most probably used as a rettery for hemp and flax fibre production. Pollen analysis revealed strong deforestation of the local landscape at the beginning of the pond history. Despite high pollution caused by plant retting, species-rich chironomid, cladoceran and diatom communities occupied the pond. Our investigations reveal that the rettery was situated on the artificial channel of a local stream. High abundance of yellow flatsedge (Cyperus flavescens) fruit remains and coprophile beetle subfossils indicate that pond was also used as a watering place for cattle. Decline in the concentration of aquatic invertebrate subfossils, diatoms, aquatic and cultivated plant macrofossils, reveals rapid abandonment of the rettery in the mid-17th century AD. For some time after the basin was a telmatic ecosystem overgrown by sedges and bulrush. The basin was finally filled by a high-energy overbank deposition not later than in the beginning of 19th century AD.

Isotopes tell the story of lead in ancient Rome

The toxic element leached out of pipes into the water and left its record in harbor sediments.

Holocene deposits in the Mangyshlak Peninsula, North Caspian Sea region

Comprehensive analysis of the data of high-precision seismoacoustic profiling, drilling and sampling of deposits using seabed corers, biostratigraphic studies, and radiocarbon age data was performed for the first time for Mangyshlak sediments in several bottom sites of the North Caspian. It was found that the Mangyshlak sediments comprise numerous linearly stretched depressions of 5–10 m in depth (morphologically similar to modern substeppe ilmen areas in the Volga River delta), which are covered by the Novocaspian sedimentary cover, and river incisions (among them the largest Volga River valley). In addition, the Mangyshlak sediments comprise the deltaic alluvial fans of different sizes along the shelf zone of the North Caspian. Analysis of mollusks and biogenic remains indicates that accumulation of the Mangyshlak sediments occurred in freshwater and slightly salty water environments under various hydrodynamic and hydrochemical conditions. According to radiocarbon dating of organic matter, the Mangyshlak sediments formed during sea regression in the range of 10-8 ka (isotopic age) or 11.5-8.5 ka (calendar age). Several types of sediments are distinguished: clayey-carbonate sediments, enriched with organic matter up to the formation of sapropel and peat, accumulated at the lowest sea level; weakly calcareous silty-clayey silts, formed during the subsequent intense filling of paleodepressions with terrigenous material. The features of the mineral composition of sediments are as follows: polymineral composition of clayey material with a high proportion of hydromica and disordered mixed-layered formations, a high content of minerals of the epidote group, amphiboles, and other accessory minerals. All of this indicates a genetic relationship between the Mangyshlak sediments and the Volga terrigenous material.

Holocene reactivations of catastrophic complex flow-like landslides in the Flysch Carpathians (Czech Republic/Slovakia)

Complex flow-like landslides (CFLLs) are important geomorphic agents of Late Quaternary mountain evolution in the Flysch Belt of the Outer Western Carpathians. The CFLLs are characterised by the upper section of deep-seated, retrogressive landslide of structurally unfavourably oriented rocks and lower sections composed of earthflows originated due to liquefaction of material accumulated from the upper slopes. Radiocarbon dating of organic matter incorporated into landslide debris or related deposits suggests that most of the CFLLs collapsed repeatedly throughout the Holocene with typical recurrence intervals of approximately 1–2ka. Catastrophic landslides that occurred during extreme hydrometeorological events in recent decades displayed evidence of Holocene activity. Most of the CFLLs dammed and steepened adjacent valleys. Our chronological dataset is biased by erosion of older landforms, but most of the dated reactivations correlate to regional increases in humidity identified by previous paleoenvironmetal studies.

Ice genesis and its long-term mass balance and dynamics in Scărişoara Ice Cave, Romania

Abstract. The paleoclimatic significance of the perennial ice deposit in Scărişoara Ice Cave has been remarked on since the early 20th century, but a lack of understanding of the processes involved in the genesis, age and long-term dynamics and volume fluctuations of ice hampered all attempts to extract valuable data on past climate and vegetation changes. In this paper, we present a model of ice genesis and dynamics, based on stable isotopes, ice level monitoring (modern and archived) and radiocarbon dating of organic matter found in the ice. Ice in this cave mostly consists of layers of lake ice, produced as liquid water freezes from top to bottom in mid-autumn, and floor ice, produced as inflow water in winter freezes on top of the lake ice. This mechanism was also acting in the past, during the Medieval Warm Period and the Little Ice Age. The ice block is not stable in shape and volume, being continuously modified by ablation on top and sides, basal melting and lateral flow. Radiocarbon dating shows that the ice block is older than 1000 years, but ice flow and differential basal melting suggesting that the ice could be much older.

Investigations on buried soils and colluvial layers around Bronze Age burial mounds at Bornhöved (northern Germany): an approach to test the hypothesis of `landscape openness' by the incidence of colluviation

The idea of open landscapes around prehistoric burial mounds is founded on their topographical position and findings of plaggen clots within the hills at some sites. We have investigated the surroundings of four Bronze Age burial mounds at Bornhöved (northern Germany) to test whether this assumed landscape openness enabled soil erosion and colluviation or not. The soils and colluvia within a watershed below the burial mounds were investigated in six large exposures and additional auger cores. The chronology is based on 21 AMS-radiocarbon dates, complemented by charcoal analysis and the content of selected heavy metals in the sediments. Colluvia were deposited in the Late Neolithic (~2500—2200 cal. BC), Roman Emperor Times (~ 250—400 cal. AD), Mediaeval Times (~ AD 600—1400) and Modern Times (~ AD 1800—2000). Our findings indicate that the soil surfaces were protected against soil erosion during the phase of Bronze Age funeral use (~ cal. 1800—600 BC). Either the prehistoric gravediggers practised a well suited form of pasturing to keep the scenery open or the surroundings of the burial mounds were forested at the time of funeral use. The results of charcoal analysis reflect the known succession of woody taxa in the region and underline the great potential of anthracology on colluvial layers for palaeoenvironmental reconstructions apart from traditional archives such as lakes or mires. Bulk radiocarbon dating of organic matter overestimated the time of burial of a buried soil as well as the time of deposition of colluvial layers considerably.

Estimates of sulphate reduction rates in Lake Vanda, Antarctica support the proposed recent history of the lake

Lake Vanda, a perennially ice-covered Antarctic lake has a highly stratified structure with a pronounced density gradient from 45 m and is anoxic from 68 m down. In order to gain a greater understanding of the carbon cycling in the lake we attempted to measure sulphate reduction in the summer of 1994, but rates were below detectable limits of 1.2 × 106 mol sulphate yr−1. Therefore sulphate reduction was estimated by calculating the rate of sulphide diffusion from the anoxic zone. Sulphate reduction rates were estimated to be 17.7 × 103 mol sulphate yr−1, accounting for 30% of planktonic primary production over the whole lake, but over 100% of primary productivity in the deep chlorophyll maximum. Radiocarbon dating of organic matter in the sediment (565 ± 50 yr bp), suggests that little new organic carbon reaches the sediment from the upper water column. Oxygen diffusion into the anoxic zone could account for only 44% of sulphide reoxidation; therefore, alternative oxidizing potential presumably accounted for the remainder. Sulphate concentration may be controlled by the solubility limit for a mineral phase, perhaps gypsum, as it shows a positive correlation with conductivity. The sulphur species in the anoxic zone are highly enriched in 34S (sulphate δ34S = + 42 to + 46%0, sulphide δ34S = + 13.9%0) which may be explained by either selective removal of sulphide at the oxic/anoxic interface by precipitation of metal sulphides or previous loss ofH2S to the atmosphere. While sulphate reduction is an important part of the carbon cycle within the lake, the oligotrophic nature of Lake Vanda means that all microbiological process rates are very low. The data presented suggest a dynamic and complex history for Lake Vanda and are entirely consistent with present theories of the lake's history.

The role of 14C dating in ironpan formation

The results of novel experiments undertaken to attempt to date ironpans using AMS radiocarbon dating are reported. In the absence of clear stratigraphic control, for example where buried soils are present, it is difficult to put into sequence the events which have led to the formation of present-day soil profiles. Micromorphology can give some indications of sequence, particularly when evidence of one process has been superimposed onto another. However, this still does not give absolute dates that can be tied in accurately with archaeological and other environmental (particularly palynological) information. Problems exist in the interpretation of radiocarbon dates of organic matter from mineral soils. However, some of the organic C fraction of a soil can become protected, particularly by clay minerals and pedological features such as ironpans,and dating of this should, in theory, provide a date for the formation of that feature. This technique was applied to ironpans of the ironpan stagnopodzols from the Hiraethog Moors in North Wales. This is a novel use of 14C dating which has begun the process of clarifying the techniques which need to be employed in the preparation of such samples and in the interpretation of the results.

Radiocarbon Age Constraints on Rates of Advance and Retreat of the Puget Lobe of the Cordilleran Ice Sheet during the Last Glaciation

Calibrated radiocarbon dates of organic matter below and above till of the last (Fraser) glaciation provide limiting ages that constrain the chronology and duration of the last advance–retreat cycle of the Puget Lobe in the central and southeastern Puget Lowland. Seven dates for wood near the top of a thick proglacial delta have a weighted mean age of 17,420 ± 90 cal yr B.P., which is the closest limiting age for arrival of the glacier near the latitude of Seattle. A time–distance curve constructed along a flowline extending south from southwestern British Columbia to the central Puget Lowland implies an average glacier advance rate of ca. 135 m/yr. The glacier terminus reached its southernmost limit ca. 16,950 yr ago and likely remained there for ca. 100 yr. In the vicinity of Seattle, where the glacier reached a maximum thickness of 1000 m, ice covered the landscape for ca. 1020 yr. Postglacial dates constraining the timing of ice retreat in the central lowland are as old as 16,420 cal yr B.P. and show that the terminus had retreated to the northern limit of the lowland within three to four centuries after the glacial maximum. The average rate of retreat was about twice the rate of advance and was enhanced by rapid calving recession along flowline sectors where the glacier front crossed deep proglacial lakes.

Latest Pleistocene lake shorelines and glacial chronology in the Western Basin and Range Province, U.S.A.: insights from AMS radiocarbon dating of rock varnish and paleoclimatic implications

Accelerator mass spectrometry radiocarbon dating of organic matter in the basal layer of rock varnish provides a minimum surface exposure age for the underlying landform. Varnishes 14C ages indicate that the latest Pleistocene highstand of Lake Lahontan was abandoned before 12.7 ka, Searles Lake before 13.6 ka, Lake Manly before 13.0 ka, and Lake Mojave before 11.6 ka. The maxima of the Tioga glaciation at Pine Creek in the eastern Sierra Nevada, California, and the Middle Creek glaciation in the adjacent White Mountains of California-Nevada was just before 18–19 ka; major recessional moraines were deposited in these ranges before 12–13 ka. Penecontemporaneous glacial retreat and high levels in closed-basin lakes cannot be explained by glacial melt water, but probably reflects higher snowlines and more precipitation. A relatively minor glacial advance, perhaps related to the “Younger Dryas” in Europe, may be penecontemporaneous with a slight transgression in lake levels about 10–11 ka in the western Basin and Range, suggesting a temporary return to cooler or wetter conditions, or both at the termination of the Pleistocene.

Late Holocene debris cone evolution in Glen Feshie, western Cairngorm Mountains, Scotland

ABSTRACTRecent river erosion of three coalescing debris cones in Glen Feshie has exposed a complex sequence of debris flow units. Radiocarbon dating of organic matter from interbedded buried soils reveals that the soil at the base of the sequence was buried atc.2000 yr BP, but that the bulk of the cones accumulated since the fifteenth century AD. The episodic nature of cone development is attributable to lateral migration of the River Feshie, with periods of cone accumulation when the river occupied the far side of its floodplain alternating with periods of erosion when the river impinged on the cones. There is no evidence to suggest that recent cone accumulation is related to anthropogenic vegetation disturbance, but phases of cone accumulation show a broad temporal correspondence with periods of Late Holocene climatic deterioration. The cones are essentially paraglacial in that their continuing accumulation depends on a supply of sediment derived from glacial and periglacial deposits upslope. The form of debris-flow units indicates that flows at this site were less viscous than most ‘hillslope’ flows, and cone volumes indicate an average annual accumulation ofc.50–60 m3of sediment over the pastc.300years.

Pleistocene Glaciation of Volcano Ajusco, Central Mexico, and Comparison with the Standard Mexican Glacial Sequence

Three Pleistocene glaciations and two Holocene Neoglacial advances occurred on volcano Ajusco in central Mexico. Lateral moraines of the oldest glaciation, the Marqués, above 3250 m are made of light-gray indurated till and are extensively modified by erosion. Below 3200 m the till is dark red, decomposed, and buried beneath volcanic colluvium and tephra. Very strongly to strongly developed soil profiles (Inceptisols) have formed in the Marqués till and in overlying colluvia and tephra. Large sharp-crested moraines of the second glaciation, the Santo Tomás, above 3300 m are composed of pale-brown firm till and are somewhat eroded by gullies. Below 3250 m the till is light reddish brown, cemented, and weathered. Less-strongly developed soil profiles (Inceptisols) have formed in the Santo Tomás till and in overlying colluvia and tephra. Narrow-crested moraines of yellowish-brown loose till of the third glaciation, the Albergue, are uneroded. Weakly developed soil profiles (Inceptisols) in the Albergue till have black ash in the upper horizon. Two small Neoglacial moraines of yellowish-brown bouldery till on the cirque floor of the largest valley support weakly developed soil profiles with only A and Cox horizons and no ash in the upper soil horizons. Radiocarbon dating of organic matter of the B horizons developed in tills, volcanic ash, and colluvial volcanic sand includes ages for both the soil-organic residue and the humic-acid fraction, with differences from 140 to 660 yr. The dating provides minimum ages of about 27,000 yr for the Marqués glaciation and about 25,000 yr for the Santo Tomás glaciation. Dates for the overlying tephra indicate a complex volcanic history for at least another 15,000 yr. Comparison of the Ajusco glacial sequence with that on Iztaccíhuatl to the east suggests that the Marqués and Santo Tomás glaciations may be equivalent to the Diamantes glaciation First and Second advances, the Albergue to the Alcalican glaciations, and the Neoglacial to the Ayolotepito advances.