Platinum

Petaev et al.5 report a large Pt anomaly at the YD onset in the Greenland Ice Sheet Project (GISP2) ice core across an ice interval beginning precisely at the YD onset. They considered multiple potential sources for the Pt anomaly and conclude that the most likely explanation is multiple atmospheric injections of platinum-rich dust by an extraterrestrial impact, followed by fallout of Pt-rich dust during the next 21 years. Geochemical data for meteorites (n = 167), including chondrites, achondrites, irons, and urelites, show Pt abundances ranging from 39,300 to 0.2 ppb (avg: 16,077 to 1198 ppb)4. This indicates that all classes of meteorites are possible sources of YDB Pt enrichment. However, Bunch et al.8 and Moore et al.33,34 conclude that the evidence is inconsistent with the normal influx of meteoritic dust and consistent only with a rare impact by an asteroid or comet.

A later study by Moore et al.4 found the same Pt anomaly at the YD onset in 11 widely-spaced sedimentary sequences across North America. A few displayed smaller secondary peaks interpreted as a result of redeposition or depositional variability. Moore et al. also show that volcanism is an unlikely contributor to the Pt anomaly given the lack of evidence for continental-scale volcanism at the YD onset, and geochemical studies demonstrating a lack of tephra or sulfur anomalies associated with Pt in the YDB layer (see discussion of Potential Sources of YDB Platinum in SI from Moore et al.4). At White Pond, analyses of cryptotephra and Hg confirm this and provide no evidence in support of a volcanic source.

As found at many terrestrial archaeological sites by Moore et al.4, only the YDB layer at White Pond contains both a large Pt anomaly and a coeval Pt/Pd anomaly. The Pt anomaly is ~5.5x higher than the natural Pt background for muddy stratigraphic units (Units I and II) containing the YDB and earlier muddy Pleistocene sediments, and the anomaly is more than 3x background for the entire core sequence tested for Pt, including Holocene peaty sediments that have a slightly higher Pt background level. Pt also is elevated throughout Unit III, likely due to preferential uptake and redeposition of Pt upward in the solum by aquatic plants rooted and intrusive into Unit II, as observed in previous studies35. The Pt and Pt/Pd anomalies also display no relationship with fine grain sizes, loss-on-ignition (LOI), total organic carbon (TOC), and mercury (Hg) contents from correlative intervals in core WP-16-3 (Supplementary Table 12). While trace amounts of both Pt and Pd are present at very low levels throughout, the presence of an anomalous ratio of platinum to palladium only in the YDB indicates an influx of Pt-enriched material from a exogenic source at the YD onset4.

The combination of Pt, other PGEs, microspherules with scanning-electron microscope (SEM) confirmed melt textures, soot, and/or nanodiamonds, among other impact proxies, found at White Pond and at YDB-dated terrestrial sites on 4 continents suggests an extraterrestrial source via impact and/or atmospheric airbursts3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18. The usefulness of the Pt anomaly as a precise chronostratigraphic datum is demonstrated here, and its presence is best explained as the result of an extraterrestrial event.

Coprophilous fungi

Coprophilous spore frequencies are widely used as indicators of megaherbivore population sizes. For example, declines in Sporormiella observed in sediments younger than ~14,800 cal yr BP in Ohio and northern Indiana27,36 and New York37,38 were interpreted as demonstrating evidence of a sharp decline in megafaunal abundance. Alternatively, it is possible that declining numbers of coprophilous fungal spores may signify microenvironmental fluctuations or a decline in small herbivores. At White Pond, we used multiple taxa of strongly and semi-coprophilous fungal spores to minimize these possibilities39,40. All fungi types in this study generally followed the same temporal pattern as Sporormiella (see Supplementary Figs 1–4).

Bayesian analysis of 22 AMS dates from cores 2016-1, 2016-2, and 2016-3 reveals a significant hiatus or interval of very slow sedimentation (~2,400 years) immediately above the stratigraphic unit containing the large soot anomaly, Pt and Pt/Pd anomaly, and pre-Holocene Sporormiella and strongly coprophilous spore minimum (Fig. 2 and Supplementary Fig. 3). This hiatus occurs at the top of Unit IIB and base of Unit IIIA, with a 1-cm section of core representing a modeled age range of ca. 12,733 to 10,383 cal yr BP (see Fig. 2 and Supplementary Table 2). A sharp decline in Sporormiella occurs in Units IIA and IIB; however, given the lengthy hiatus separating Units II and III, we may or may not be seeing the final pre-Holocene Sporormiella decline at White Pond that has been attributed elsewhere to megaherbivore extinction (Supplementary Figs 2, 3). On the other hand, at the Page-Ladson site in Florida, the major decline indicating megaherbivore extinction occurs ca. 12,700 cal yr BP41. This timeframe is consistent with the observed pre-Holocene minimum in Sporormiella and strongly coprophilous spores at White Pond with a Bayesian modeled age of 12,763 to 12,745 cal yr BP (Supplementary Figs 1–3). The Bayesian age/depth statistical model places the upper part of Unit II (containing the soot peak, Pt anomaly, and pre-Holocene spore minimum) as all being deposited during the YD onset—between ca. 12,835 to 12,735 cal BP (Fig. 2 and Supplementary Fig. 3). Based on a recent study from the Page-Ladson site, it appears that the final extinction event did not occur until sometime after the YD onset during the early YD—a time for which we apparently lack a sedimentological record in the White Pond core. Based on evidence from White Pond, Page-Ladson, and other sites3,41, we speculate that the proposed YD impact was just one of several coeval factors, along with overhunting and climate change, that contributed to the megafaunal declines at 12.8 ka, followed by a long, multi-century slide into full extinction41.

An examination of spore influx (spores/annum) based on the Bayesian age/depth model, shows relatively uniform spore input through most of the late Pleistocene record, with a large spike and then rapid decline to near zero during the YD onset (Supplementary Fig. 3d). One possible interpretation is that there was a period during the YD onset with a sudden increase in megaherbivore abundance at White Pond, followed by a rapid decline represented by the lowest spore abundance in the entire pre-Holocene record. However, there are considerable difficulties with any interpretive model for multiple reasons, including the presence of the hiatus in deposition, uncertainties in the age-depth model, rapid changes in sedimentation, variability in spore preservation, potential sampling and processing errors, and high variability in the lake level. In particular, an episode of drought is suggested by the presence of oxidized sediments in Unit IIb, possibly indicative of subaerial exposure from low water levels and drought across the YD interval. If drought is confirmed, this could explain the hiatus in sedimentation immediately above the YDB layer. All of these issues potentially affect the spore record at White Pond during this time and therefore, spores may not reflect actual megaherbivore abundance. Additional research is necessary to resolve this issue.

Organic and pyrogenic carbon

In general, the possible routes of entry for soot into a watershed tend to be from fluvial input, local shoreline erosion, and eolian deposition. In the White Pond system, however, the punctuated interval of elevated soot in the absence of a similarly elevated influx of OC, suggests an eolian source. Thus, the presence of a soot anomaly of this magnitude in the White Pond core is consistent with large-scale regional fires coincident with the YD onset. This finding is also consistent with the results reported by Wolbach et al.18, who presented multiple lines of evidence for large-scale biomass burning on a widespread but discontinuous continental scale at the YD onset.

The largest soot anomaly slightly precedes/predates the Pt and Pt/Pd anomaly in Unit IIA based on nearly identical core lithologies between duplicate cores analyzed for soot and Pt, respectively (Fig. 2 and Supplementary Fig. 11). Under the YDIH scenario, we speculate that this is the order of events at the YD onset: multiple impacts and airbursts occurred over a brief period of time3,17,18,19,20, along with subsequent climate changes, regional wildfires, an impact winter, and soot deposition immediately after the impact17,18. This was followed by deposition of atmospheric Pt over several decades, as indicated by GISP2 ice core data reported by Petaev et al.5. Furthermore, the chronological sequence in the cores from the pyrogenic carbon peak, Pt anomaly, and extended Sporormiella decline is consistent with expected environmental and ecological changes resulting from multiple impacts and airbursts of a fragmented comet or asteroid3,17,18,19,20. Secondary soot peaks occur in Unit II, including several that overlap stratigraphically with the Pt and Pt/Pd anomalies. Exact correlation by depth for soot peaks and the Pt and Pt/Pd anomalies are impossible due to use of duplicate cores; however, the Bayesian age/depth model for White Pond is consistent with a brief interval of rapid sedimentation in Unit II—implying all proxies could reasonably have been deposited in the core within a ~100-year window (ca. 12,835 to 12,735 cal yr BP).

C and N isotopes and magnetic susceptibility

At White Pond, sudden increases in carbon and nitrogen content are contemporaneous with decreases in δ15N and δ13C values at the mud-to-peat transition (Unit IIB to Unit III), indicating a general increase in nutrient availability and primary productivity within and around White Pond (see Supplementary Fig. 13). The concomitant rise in C:N ratios indicates a general increase in terrestrial organic matter (litter) contributions to the SOM pool for White Pond that may have also increased overall N availability. Similar patterns were reported by Spencer et al.42 and Lane et al.43 in southeastern North Carolina at roughly this time and are interpreted as a significant increase in terrestrial biomass during the latest Pleistocene and early Holocene as a result of increased moisture delivery to the region. Close correspondence between δ15N and magnetic susceptibility during the Pleistocene-Holocene transition supports this interpretation; increased terrestrial biomass likely stabilized soils in the White Pond watershed, thus leading to decreases in allochthonous mineral influx to the core site.

Of relevance to this study, the observed shift in δ15N that occurs in the middle of Unit II is interpreted as signaling an overall change in watershed biogeochemical cycles indicative of wholesale ecological transitions concurrent with the Pt and soot anomalies (Supplementary Fig. 13). While the δ15N values are not directly indicative of the cause of the YD or subsequent changes, they are direct evidence of dramatic changes in nitrogen cycling in the watershed concurrent with the Pt anomaly.

Environmental disruption and extinction

A “one-two punch” of human overhunting and rapid climate change has been proposed as a cause of the extinction of Pleistocene megafauna44,45,46. A related possibility is that megafaunal overhunting was directly triggered by the widespread environmental disruption of traditional human food sources through geographically heterogeneous, impact-related biomass burning17,18 and by abrupt, impact-related YD climate change. Presumably, some areas less severely affected than others became refugia for remnant herds of megafauna. If so, stressed animal populations in these refugia were more easily targeted by equally stressed human populations, all struggling to survive in the aftermath of an environmental calamity. Afterward, intensive overhunting continued for decades to few centuries as Paleoindians adapted to the sudden loss of vegetative biomass for subsistence. All of these interrelated influences contributed to the final extinction of most of the remaining Pleistocene megafauna across North America before the end of the Younger Dryas (~11,700 cal BP). Small remaining populations of megafauna persisted into the Holocene in isolated refugia away from human predation, before declining into full extinction.

Hiawatha crater, Patagonia, and YD environmental change

The recent discovery of a massive (31 km) impact crater under the Hiawatha Glacier in northwestern Greenland by Kjaer et al.19 has garnered great interest because of the presumed recent age. While its exact age is yet to be determined, the crater represents the largest known impact event in the last 5 million years (after Kara-Kul) and the second largest in the last 36 million years (after Chesapeake Bay). If a YD age is eventually confirmed, the Hiawatha impact was energetic enough to have triggered a brief impact winter, abrupt YD climate and oceanographic change, widespread biomass burning, and deposition of multiple impact proxies, as found at more than 50 sites across large portions the globe. In addition, data from Pilauco in Patagonia20 add to a growing body of evidence supporting multiple airburst/impacts at the YD onset with synchronous deposition of YDB impact proxies and Sporormiella decline. When viewed in the context of rapid environmental change, these new data, along with all previous paleo-environmental reconstructions of the YD, suggest that the trigger was geologically instantaneous. An impact event in the high-latitude Northern Hemisphere (i.e., Greenland) may have been the mechanism to generate the 1st- and 2nd-order environmental feedbacks within the Earth system that have been identified over the past two decades, suggesting that Earth’s global environment may be sensitive to abrupt (<101 yr) stimuli with lasting effects on the order of at least 103 yrs. As suggested by Moore et al.4, regardless of whether the YD triggered the observed widespread environmental change, the Pt anomaly at White Pond, as later supported by Bayesian-modeled radiocarbon ages, represents a precise chronostratigraphic datum for the YD onset. Multiple lines of synchronous evidence indicate that a cosmic impact event was a causative mechanism of catastrophic environmental change at the YD onset.