We report the discovery in Lake Cuitzeo in central Mexico of a black, carbon-rich, lacustrine layer, containing nanodiamonds, microspherules, and other unusual materials that date to the early Younger Dryas and are interpreted to result from an extraterrestrial impact. These proxies were found in a 27-m-long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial. Our attention focused early on an anomalous, 10-cm-thick, carbon-rich layer at a depth of 2.8 m that dates to 12.9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the late Quaternary record. This layer contains a diverse, abundant assemblage of impact-related markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures, all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core. Analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite), in order of estimated relative abundance. This nanodiamond-rich layer is consistent with the Younger Dryas boundary layer found at numerous sites across North America, Greenland, and Western Europe. We have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism. It is, however, consistent with the Younger Dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst(s) and and/or ground impact(s) at 12.9 ka.

We present data from Lake Cuitzeo in central Mexico (19.94 °N, 101.14 °W) in support of evidence for the Younger Dryas (YD) impact hypothesis, as first presented at the 2007 Meeting of the American Geophysical Union in Acapulco, Mexico. There, a consortium of scientists reported geochemical and mineralogical evidence from multiple terrestrial sites ascribed to extraterrestrial (ET) impacts and/or airbursts (1). Their first evidence was the discovery at well-dated Clovis-era archeological sites in North America of abundant magnetic spherules (MSp) and carbon spherules (CSp) in a thin layer (0.5 to 5 cm) called the Younger Dryas boundary layer (YDB), dating to 12.9 ± 0.1 ka BP (calibrated, or calendar years) or 10.9 14C ka BP (radiocarbon years)† (1⇓–3). The YDB is commonly located directly beneath or at the base of an organic-rich layer, or “black mat,” broadly distributed across North America (1). Later, abundant nanodiamonds (NDs) were discovered by Kennett et al. (2, 3) in the YDB layer at numerous locations. NDs also were detected at the margin of the Greenland Ice Sheet in a layer that dates to the approximate YD onset (4). These discoveries led to the hypothesis that one or more fragments of a comet or asteroid impacted the Laurentide Ice Sheet and/or created atmospheric airbursts (1) that initiated the abrupt YD cooling at 12.9 ka, caused widespread biomass burning, and contributed to the extinction of Late Pleistocene megafauna and to major declines in human populations (5).

Some independent workers have been unable to reproduce earlier YDB results for MSp, CSp, and NDs (6⇓–8), as summarized in a “News Focus” piece in Science (9), which claims that the YDB evidence is “not reproducible” by independent researchers. Refuting this view, multiple groups have confirmed the presence of abundant YDB markers, although sometimes proposing alternate hypotheses for their origin. For example, Mahaney et al. (10⇓–12) independently identified glassy spherules, CSps, high-temperature melt-rocks, shocked quartz, and a YDB black mat analogue in the Venezuelan Andes. Those authors conclude the cause was “either an asteroid or comet event that reached far into South America” at 12.9 ka. At Murray Springs, Arizona, Haynes et al. (13) observed highly elevated concentrations of YDB MSp and iridium. Abundances of MSp were 340 × higher than reported by Firestone et al. (1) and iridium was 34 × higher, an extraordinary enrichment of 3,000 × crustal abundance. Those authors stated that their findings are “consistent with their (Firestone et al.’s) data.” In YDB sediments from North America and Europe, Andronikov et al. (2011) reported anomalous enrichments in rare earth elements (REE) and “overall higher concentrations of both Os and Ir [osmium and iridium]” that could “support the hypothesis that an impact occurred shortly before the beginning of the YD cooling 12.9 ka.”‡. Tian et al. (14) observed abundant cubic NDs at Lommel, Belgium, and concluded that “our findings confirm … the existence of diamond nanoparticles also in this European YDB layer.” The NDs occur within the same layer in which Firestone et al. (1) found impact-related materials. Similarly, at a YDB site in the Netherlands, Van Hoesel et al.§observed “carbon aggregates [consistent with] nanodiamond.” Recently, Higgins et al.¶ independently announced a 4- to 4.5-km-wide YDB candidate crater named Corossol in the Gulf of St. Lawrence, containing basal sedimentary fill dating to 12.9 ka. If confirmed, it will be the largest known crater in North and South America within the last 35 million years

Because of the controversial nature of the YD impact debate, we have examined a diverse assemblage of YDB markers at Lake Cuitzeo using a more comprehensive array of analytical techniques than in previous investigations. In addition, different researchers at multiple institutions confirmed the key results.

Covering 300–400 km 2 , Cuitzeo is the second largest lake in Mexico, located at high elevation of 1,820 m in the northern part of the state of Michoacán within the tectonically active Trans Mexico volcanic arc ( 15 ) ( SI Appendix, Fig. 1 ). Situated in the tropics, this lake currently experiences a semiarid climate with annual temperatures ranging from 10 to 28 °C (avg 19 °C) and annual rainfall ranging from 60 to 100 cm. This shallow lake currently varies in depth from 0.8 to 2.2 m (avg 1.9 m).

Organic matter in the anomalous interval is enigmatic and not the normal plant-derived kerogenous organic matter, dominating the rest of the 27-m core over the last 100 kyr; instead, it appears to be very old and radiocarbon-dead. Pyrolysis analysis (Rock/Eval) was conducted on samples from the carbon-rich layer between 2.90 and 2.55 m and then compared with a carbon-rich sample from the YD-aged black mat at Murray Springs, Arizona ( SI Appendix, Table 3 ), as previously analyzed by Bischoff in Haynes ( 25 ). This comparison suggests that much of the TOC is unreactive carbon, whereas, according to GC-MS analysis, the remaining extractable carbon fraction is typical of immature plant-derived compounds, mostly n-alkanes. Contamination of carbon from petroleum seeps, such as those in Lake Chapala 300 km to the west, was explored as a possible source of the anomalous carbon ( 26 ). However, pyrolysis analysis showed no detectable petroleum hydrocarbons in the sequence analyzed. This carbon does not appear to be derived from typical immature plant compounds, and its origin is unknown.

Changes in carbon for the upper 6 m of the Lake Cuitzeo sequence. There YD onset peaks in TOC wt%, C/P, and δ 13 C. The dark gray band denotes the YD interval, and the light gray band is the interval between 4.0 and 2.0 m.

Our attention was first drawn to the anomalous interval of unusually high values of TOC (5–16%) occurring between 4.0 and 2.6 m, particularly a TOC of 15.8% in the thin, 1-cm-thick layer at 2.75 m, close to the YD onset ( Fig. 3 ). This value is the highest TOC in the entire 27-m core, which has a background average of only 1.2%. We performed δ 13 C analyses, and below 2.75 m, there are minor fluctuations near an average of −2‰, which is typical for algal matter. Above 2.75 m, δ 13 C values increase 10 × to −19‰ at 2.7 m in the dark layer, followed by heavier values above 2.0 m (after approximately 10 ka) in the Holocene. The 2.75-m layer is depleted in phosphorus, producing a distinct carbon/phosphorus ratio (C/P) peak that is the highest in core. This material may be analogous to the YD carbon-rich black mat observed at many North American sites ( 25 ). When viewed with the scanning electron microscope (SEM), the 2.75-m layer contains thin millimeter-sized interbeds of black organic carbon that appear without form or structure. Analysis by energy-dispersive X-ray spectroscopy (EDS) indicates these bands are almost pure elemental carbon.

In summary, from widely separated lakes in the highlands of Costa Rica to the lowlands of Guatemala and Panama, there is only one stratigraphic interval that displays extraordinary environmental and biotic changes, and in each case, this interval occurs at or near the YD onset. For Lake Cuitzeo, the age-depth model indicates the YD onset occurs between 1.95 and 3.35 m, representing a 9-kyr span. Within this span, only one level displays extraordinary environmental and biotic changes, as in other regional lakes, and that level is at 2.8 m. Therefore, we conclude that the Cuitzeo age-depth model is robust and that the YD onset is correctly identified at 2.8 m.

In Fig. 2 , we compare the pollen records to a temperature proxy (δ 18 O) from a Greenland ice core, GISP2 ( 22 ). We have also correlated paleoceanographic records from the Cariaco Basin in the Caribbean ( Fig. 2 ), in which titanium represents terrigenous input due to continental runoff ( 23 ) and molybdenum varies in response to deglacial climate change ( 24 ). The YD onset is identified in all three records and corresponds well with the pollen records from Cuitzeo and other lakes.

Because climate change also affects diatom populations, we compared the diatom record at Cuitzeo with other lakes, and as with pollen, the diatom record reveals a period of extraordinary change at the YD onset. Stephanodiscus niagarae and Aulacoseira spp. display major YD abundance peaks that are among the largest in the last 100 kyr ( SI Appendix, Fig. 3 and Biostratigraphy). For Cuitzeo diatom assemblages, we also plotted the change in diversity (“δ diversity”), defined as the value derived from subtracting the total number of diatom species in one sample from the adjacent sample above it. This demonstrates that the greatest change in diversity within > 50 kyr occurred at the YD onset at 2.8 m in Lake Cuitzeo (δ diversity in SI Appendix, Fig. 3 ), and this correlates well with the Lake La Yeguada record where the greatest turnover in diatom species occurred at approximately 12.8 ka (10.8 14 C ka) ( 21 ). In addition, for Lakes Cuitzeo and La Yeguada, especially large YD peaks are evident in other aquatic taxa, including cattails (Typha) and the algal forms Botryococcus and Coelastrum ( SI Appendix, Fig. 4 ). High abundances for these taxa are consistent with major ecological change at the YD onset.

Graphs for pollen, Cariaco Basin proxies, and GISP2 temperatures. (A–C) compare Lake Cuitzeo pollen abundances to two regional lakes. Warmer Bølling–Allerød (BA) in light gray, and YD in dark gray. D and E show that Lake Cuitzeo Quercus abundances are similar to those of Lake Petén Itzá. All lake plots correspond well to graph F from a Cariaco Basin core displaying ppm abundances of titanium (orange; smoothed 30×) and molybdenum (red; smoothed 3×) ( 23 , 24 ). Graph G is a GISP2 temperature proxy plot (‰ δ 18 O; smoothed 10 × ) ( 22 ). Black diamonds are depth of 14 C dates. All graphs are similar, demonstrating that the YD onset is consistent at all sites.

In these regional lakes, the pollen records typically form a “peak-trough-peak” pollen pattern with the trough representing very low pollen levels during the YD ( Fig. 2 ; SI Appendix, Table 2 ). At Cuitzeo, total pollen and Quercus reveal a similar pattern in which high pollen abundances below 2.9 m correspond to the BA when warmer temperatures supported abundant biota around the lake. Low pollen abundances in the trough correspond to the YD interval. For all lakes, the unique, distinctive BA pollen peaks are among the largest in the last 40 to 100 kyr and are followed during the YD by some of the lowest total pollen values, consistent with a cooler and/or drier climate. After the YD, a rebound of varying magnitude occurred at all lakes after the Holocene began at 11.5 ka.

For Lake La Yeguada, although the YD episode was not expressly identified, workers there recognized a major, abrupt environmental and ecological change (a “time of crisis”) close to the onset of the YD at approximately 12.8 ka (10.8 14 C ka) ( 20 ). This is reflected in dramatic changes in the pollen and diatom records, biotic turnover, clay mineralogy, sedimentary geochemistry, and particulate carbon flux. At the same time, Quercus and Myrtaceae (myrtle) were replaced by Poaceae. These changes represent the most distinctive layer in this record ( 21 ).

Pollen changes in Lake Petén Itzá at the YD onset display a decrease in Quercus (oak), a persistence of Pinus (pine), a dominance of Poaceae (grasses), and low diversity and productivity for plants in general ( 19 ). At or near the YD onset, the record at Petén Itzá exhibits exceedingly large, abrupt, and unprecedented changes (both in magnitude and rate of change) for temperature, rainfall, and biotic turnover. These changes produced the most distinctive layer in the Late Quaternary record ( 18 , 19 ).

Although the stratigraphic position of the YD onset has been reasonably extrapolated using numerous 14 C dates, as is standard practice, we have examined other stratigraphic data to assist with this placement of the YD onset. This is achieved using biostratigraphic correlation of pollen sequences from Lake Cuitzeo with those from YD-aged regional lakes that have been independently dated. Islebe and Hooghiemstra ( 17 ) reported that evidence for YD climate change is either present or likely in some, but not all, lakes in Mexico, Guatemala, Costa Rica, Colombia, Ecuador, and Peru. Of those mentioned, we have examined pollen records from Lake Petén Itzá in Guatemala ( 18 , 19 ), La Chonta Bog in Costa Rica ( 20 ), and Lake La Yeguada in Panama ( 21 ) ( SI Appendix, Fig. 2 ). The pollen sequences from these lakes reflect climate changes, including the Late Glacial, Bølling–Allerød (BA), (14.5 to 12.9 ka), YD stadial (12.9 to 11.5 ka), and the Holocene (11.5 ka to present), comprising a distinctive cold-warm-cooler-warmer climatic sequence. The YD interval was specifically identified by previous investigators at Lakes La Chonta and Petén Itzá.

To compensate for these anomalously old radiocarbon dates, we excluded the six dates that form the reversal between 3.10 and 2.05 m and utilized the remaining 16 dates to generate an age-depth curve with a fifth-order polynomial regression ** . The resulting curve ( Fig. 1 ), predicts that the 12.9-ka YD onset is at a depth of approximately 2.9 to 2.7 m, consistent with the earlier identification by Israde et al. ( 15 ).

The samples at 3.35 and 1.95 m have calibrated ages of 18.8 and 9.9 ka, respectively, consistent with the linear extrapolation of the rest of the core. However, six samples between these two levels provided radiocarbon ages older than the interpolation predicts. They represent a major radiocarbon reversal of thousands of years, with older sediment overlying younger, a situation that can result from reworking of older organic material. The reversal begins with a date of 18.8 ka, shifts anomalously older by approximately 20 kyr above, and then normalizes to 9.9 ka higher in the section. At 2.75 m within this interval, total organic carbon (TOC) is 15.8 wt%, the highest percentage in approximately 100 kyr ( 15 ). This sample yields a date of approximately 32 ka, but linear interpolation indicates it should date to approximately 13 ka, a difference of approximately 20 kyr. Accounting for this shift requires major contamination of the TOC by radiocarbon-dead or very old carbon (92 wt%). Currently, the source of this old carbon remains unclear.

Previously, Israde et al. ( 15 ) published an age-depth model for the uppermost 9 m at Lake Cuitzeo comprised of 16 accelerator mass spectroscopy (AMS) 14 C dates on bulk sediment and used in a linear interpolation with the YD onset identified at approximately 2.8 m. To test and refine that model, we acquired six more AMS 14 C dates on bulk sediment, for a total of 22 dates, and calibrated them using the IntCal04 calibration curves in CalPal07 ∥ ( Fig. 1 ; SI Appendix, Table 1 ). A 20-cm-thick tephra layer at 4.7 to 4.5 m has been identified as the Cieneguillas rhyolitic tephra, originating from nearby Las Azufres volcano and 14 C dated by others at three locations to approximately 31 ka (26.8 ± 0.9 14 C ka) ( 16 ). The age of this tephra serves as an anchor for the chronology. The dates from 9 to 3.35 m and from 2 to 0 m show a relatively consistent linear increase with depth, ranging in age from approximately 46 to 0 kyr.

(Left). Lake Cuitzeo lithostratigraphy from 4.0 to 2.0 m. Red brackets indicate the carbon-rich layer corresponding to the YD. Blue tick marks at left indicate sample depths. (Right) Graph of calibrated 14 C dates. A regression polynomial (black line) of accepted dates (red circles) and tephra date (black dot); blue circles are excluded dates. Error bars are less than circle widths. Dark gray band denotes YD interval; lighter gray band corresponds to interval between 4.0 and 2.0 m. Cal ka BP, calibrated kiloannum before present; char, charcoal.

A 27-m-long, 10-cm-diameter core was extracted in 1997 from thick deposits in Lake Cuitzeo as part of an interdisciplinary, multiproxy effort to acquire a detailed paleoclimate record extending back to the last interglacial [130,000 (130 kyr)] ( 15 ). The core consists of interbedded sands, silts, clays, and epiclastites, along with 6-kyr-old, 20-cm-thick tephra between 1.7 and 1.4 m and 31.5-kyr-old, 20-cm-thick tephra between 4.7 and 4.5 m, with several more volcanic deposits below 10 m. A conspicuous, dark, carbon-rich layer, dominantly comprised of clay and silt, occurs between 2.82 and 2.50 m ( Fig. 1 ) and is the focus of this study because of its similarity to the black mat at YDB sites across North America. Sediment samples of approximately 1 cm thickness were taken every 5 cm across the critical section between 2.80 and 2.65 m and at 10 cm intervals above and below this section. These samples were quantitatively analyzed for diatoms and pollen assemblages, carbonate (%TIC), organic carbon (%TOC), bulk major-element composition, stable carbon isotopes (both organic and inorganic), organic nitrogen, MSp, NDs, CSp, charcoal, and aciniform soot.

Impact Proxies

CSp, Charcoal, and Aciniform Soot. Black CSp, 20 to 260 μm, averaging 90 μm diameter, were observed at Cuitzeo, appearing as ovoid-to-round with cracked, roughened surfaces and typically revealing a thin rind, with spongy, vesicular interiors surrounded by a smooth, homogeneous matrix (SI Appendix, Fig. 5). SEM-EDS indicates that CSp are dominantly carbon (> 87%) with minor particulates, such as Si, Al, and Fe, concentrated in the rind. They reach a significant peak of approximately 680/kg at 2.75 m within the YDB layer. Above 2.75 m, the CSp persist at an average of approximately 120/kg. No CSp were detected below 2.8 m. Charcoal microparticles (> 125 μm) were counted between 3.6 and 2.2 m, an interval dating from approximately 21 to 10 ka (Fig. 4), displaying background levels of approximately 5,000 particles/kg. There is a minor charcoal peak at 3.1 m of approximately 29,000 particles/kg, dating to approximately 16 ka, and there is a major rise in charcoal beginning near the YD onset and reaching a maximum peak at 2.65 m of 77,000 particles/kg (15 × background) in a major episode in biomass burning. The main charcoal peak is about 5 cm above the impact proxies discussed below, and the lack of tephra within this interval indicates the biomass burning is unrelated to volcanism. There is also a major peak in particulate carbon (charcoal) in Lake Le Yeguada that dates close to 12.8 ka (21), near the YD onset. Fig. 4. Markers over the interval between 3.6 and 2.2 m. The YD episode (12.9 to 11.5 ka) is represented by dark band. YDB layer is at 2.8 m. NDs and magnetic impact spherules both peak at the YD onset, whereas framboidal spherules, CSps, and charcoal peak higher in the sequence. Magnetic grains peak just prior to the YD onset. NDs are in ppb; Msps, framboidal spherules, CSps, and charcoal are in no./kg; magnetic grains in g/kg.

Twinned NDs. These NDs are made up of two or more crystals that share a common lattice plane (the twin plane) and grow symmetrically away in different orientations (Fig. 11). Twinning is commonly observed in commercial NDs formed by carbon vapor deposition (CVD), during which NDs crystallize from gaseous carbon, typically at high temperatures in an inert atmosphere. Twinned cubic NDs are common in meteorites (38), having formed in space through a process possibly analogous to CVD (39). They also are found in impact craters (39)‡‡, where they formed upon impact from terrestrial carbon. Twinned lonsdaleite has been observed in meteorites and associated with impact craters‡‡. Twins can form in numerous configurations, including “accordion twins,” which exhibit folded, pleat-like lattice planes, and fivefold “star twins” (38), as observed by Tian et al. (14) in the YDB layer from Lommel, Belgium. At Cuitzeo, most NDs were twinned n-diamond and i-carbon and only occasionally were monocrystalline NDs observed. Twinned lonsdaleite with d-spacings of 2.06 Å and 1.93 Å was observed occasionally (Fig. 11B). Fig. 11. HRTEM images of twinned NDs from the 2.8 m layer. Double yellow lines represent lattice planes and the numbers indicate d-spacings in Å. Arrows are parallel to common twinning plane. (A) Star-twin ND with fivefold star-like morphology. (B) Accordion twin lonsdaleite with pleated morphology. (C) Twin with multiple folds. (D) “Scalloped” twin.

Cubic NDs. Cubic NDs were previously identified in the YDB (2⇓–4), and subsequently, Tian et al. (14) confirmed cubic NDs in the YDB. In the Cuitzeo section, however, we could not unequivocally identify the cubic allotrope. This may be due to masking by i-carbon and/or n-diamonds, which share some d-spacings with cubic NDs (SI Appendix, Table 8). Also, cubic NDs possess so-called “forbidden reflections,” such as the 1.78 Å d-spacing, that are typically invisible in cubic SAD patterns but are sometimes apparent in twinned cubic NDs, most likely due to double diffraction (38). Because n-diamonds also display these forbidden reflections, twinned n-diamonds cannot be easily differentiated from twinned cubic NDs. Thus, it is possible that some of the apparent n-diamonds from Cuitzeo are actually twinned cubic NDs.