Evidence for a volcanic origin of the Younger Dryas cooling event.

The Younger Dryas event occurred from 12.9 to 11.7 thousand years in the Northern Hemisphere with abrupt cooling over a time interval of decades with temperatures possibly reaching 15°C colder than present. This cooling part of succession of climate variability in the Late Pleistocene resulted in progressive megafauna extinction. There are currently four hypotheses for the origins of the Younger Dryas event. The prevailing hypothesis is that the cooling and stratification of the North Atlantic Ocean were a consequence of massive ice sheet discharge of meltwater and icebergs and resulted in reduction orcessation of the North Atlantic Conveyor. This is thought to be augmented by climate forcing with expanded snow cover in the Northern Hemisphere. Another persistent hypothesis is that global cooling was trigged by a bolide impact or airbursts. Stratigraphic markers supporting the Younger Dryas impact hypothesis include elevated concentrations of carbon spherules, magnetic grains, nanodiamonds, and platinum and iridium abundance anomalies. These markers are found singularly or more at various sites globally and appear to reach peak abundances near or at the Younger Dryas basal boundary layer. A meteorite crater potentially associated with the Younger Dryas was recently found in Greenland, although not well dated. A third hypothesis proposes that a supernova explosion in the Vela constellation could have depleted the ozone layer, resulting in greater ultraviolet exposure and atmospheric and surface changes that led to cooling. Last, a megaeruption of the Laacher See volcano, in the Rhineland-Palatinate, Germany, ejected 6.3 km³ dense rock equivalent of zoned sulphur-rich phonolite magma far into the stratosphere at the time of the onset of the Younger Dryas event. Volcanic aerosols and cryptotephra were dispersed throughout the Northern Hemisphere over a period of 2 months and affected the atmospheric optical density for over 1 year. Laacher See released a minimum of 2 metric megatons of sulphur (possibly anging up to 150 megatons) and is suggested to have triggered the sudden lowering of temperature coincident with Younger Dryas climate change in the Northern Hemisphere.

Each of these four possible triggers for the Younger Dryas event is complex, and there is not a clear consensus as to which mechanism or combination of these events initiated the Younger Dryas cold period. Of these explanations, the impact hypothesis has received the most attention, but problems plague this hypothesis. The fundamental issue is delineating if the markers used to support the hypothesis extracted from the Younger Dryas layers at various sites are really impact markers. The grains interpreted as carbon spherules and 'elongates' and 'glass-like carbon' have been instead identified as Fungal sclerotia common in Northern Hemisphere forest litter and soils. In addition, the micrograins interpreted as hexagonal nanodiamonds from Younger Dryas sites of Murray Springs (Arizona) and Arlington Canyon on Santa Rosa Island (California) are instead assessed as graphene/graphene aggregates. These disagreements are compounded by a lack of valid age control at many of the Younger Dryas boundary layer sites. It is now thought that only 3 of the 29 sites dated to the onset of Younger Dryas event were within the prerequisite time period. Furthermore, there are problems in that the reproducibility of observations at the Younger Dryas level has been questioned for the presence of magnetic grains, spherules, and iridium enrichments. A 2009 study by Todd Surovell, Vance Holliday, Joseph Gingerich, Caroline Ketron, Vance Haynes Jr, Ilene Hilman, Daniel Wagner, Eileen Johnson, and Philippe Claeys failed to duplicate the magnetic grain or microspherule peaks associated with the Younger Dryas basal boundary. Thus, there is a lack of consensus on how to interpret the impact markers.

Highly elevated concentrations of iridium together with enrichments of other highly siderophile elements (osmium, iridium, ruthenium, platinum, palladium, and rhenium) in nearly chondritic ratios are considered indicators of a meteoritic contribution delivered when an extraterrestrial object affects the Earth or airbursts over it. These highly siderophile element enrichments may be from an external source because the Earth’s crust has less than 0.1% of the CI chondritic abundances. In addition, chondrites have osmium¹⁸⁷/osmium¹⁸⁸ ratios of around 0.125, whereas continental crust has osmium¹⁸⁷/osmium¹⁸⁸ ratios of more than 1, such that small amounts of extraterrestrial material added to continental crust will shift the osmium¹⁸⁷/osmium¹⁸⁸ ratios of the hybridised material to lower values.

The cause of the elevated highly siderophile element concentrations and the osmoium isotopic ratios in Younger Dryas layer sediments remains equivocal and has been used to both support and negate the Younger Dryas impact hypothesis. For example, a 2013 study by Michail Petaev, Shichun Huang, Stein Jacobsen, and Alan Zindler found a platinum enrichment accompanied with an extremely high platinum/iridium but aluminium-poor signature in the Greenland Ice Sheet Project 2 ice core at the Bølling-Allerød/Younger Dryas transition period, which they interpreted to be consistent with an extraterrestrial impactor. Also, the elevated platinum abundance anomalies of 100 to 65,600 parts per trillion at the onset of the Younger Dryas in sites from North America is purportedly consistent with the Greenland ice core platinum data. A 2019 study led by Christopher Moore found platinum and palladium/platinum anomalies in the Younger Dryas basal layer in South Carolina. These data are used to support a model of wide-ranged atmospheric input of platinum-rich dust during the Younger Dryas, potentially related to a bolide impact or airburst. In contrast, the osmium¹⁸⁷/osmium¹⁸⁸ ratios obtained on Younger Dryas basal boundary layers from widely dispersed locales in North America and Europe have largely been similar to those for continental crust or seawater with no evidence of unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios from extraterrestrial or mantle sources, both having osmium¹⁸⁷/osmium¹⁸⁸ ratios of 0.11 to 0.13. This is exceptional because less than 1% of extraterrestrial material from an impactor mixed into continental crust would shift the resultant hybridised material away from terrestrial crustal osmium¹⁸⁷/osmium¹⁸⁸ values toward the less radiogenic values of osmium¹⁸⁷/osmium¹⁸⁸ chondrites. Only one site has been identified with an unradiogenic osmium signature, with an osmium¹⁸⁷/osmium¹⁸⁸ ratio of 0.4 for the Younger Dryas basal boundary layer at Melrose, Pennsylvania. This signature is attributed to surface films on glass spherules with highly elevated osmium concentrations and unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios of 0.113 to 0.121 that may have been caused by mobilization of osmium within a bolide fireball and possibly terrestrial in origin and ejected as molten material following impact. An important question remains: Why are low osmium¹⁸⁷/osmium¹⁸⁸ ratios found only at one site and not more widely dispersed if it is derived from impact or air burst of a bolide?

These studies show that there is no clear consensus on the interpretation of highly siderophile element concentrations and osmium¹⁸⁷/osmium¹⁸⁸ compositions of Younger Dryas basal boundary sediments. A better understanding of their systematics is crucial for determining the role, if any, of a bolide event for the Younger Dryas cooling and to refine conclusive evidence in the rock record for bolide impacts.

In a paper published in the journal Science Advances on 31 July 2020, Nan Sun and Alan Brandon of the Department of Earth and Atmospheric Sciences at the University of Houston, Steven Forman of the Department of Geosciences at Baylor University, Michael Waters of the Center for the Study of the First Americans at Texas A&M University, and Kenny Befus, also of  the Department of Geosciences at Baylor University, present the results of a study which aimed to further examine this issue, by measuring highly siderophile element abundances and osmium¹⁸⁷/osmium¹⁸⁸ isotope ratios in samples from Hall’s Cave, Texas, including those from the Younger Dryas basal boundary layer. Hall’s Cave formed in the Segovia Formation of the lower Cretaceous Edwards Group and contains sediments dating from 20 000 years before the present to the present.

 

Workers excavating Hall's Cave in Central Texas. Mike Waters/Texas A&M University.

The cave has a consistent depositional environment with minimal reworking or disturbance over this time period. The stratigraphy is well dated based on 162 accelerator mass spectrometry carbon¹⁴ dates from Vertebrate fossils, Snails, charcoal, and sediment chemical fractions. The Younger Dryas basal boundary layer at Hall’s Cave also contains purported extraterrestrial proxies including nanodiamonds, aciniform soot, and magnetic spherules. Sun et al. we present osmium isotopes and highly siderophile element abundances from the Younger Dryas basal boundary strata in addition to layers above and below that horizon. Sun et al.'s measurements span about 4000 years of sediment deposition at Hall’s Cave. The highly siderophile element chondrite-normalised patterns combined with osmium¹⁸⁷/osmium¹⁸⁸ at different levels within this section at Hall’s Cave including the Younger Dryas basal boundary layer show a repeating record of osmium concentration enrichment. Multiple occurrences above and below the anticipated Younger Dryas basal boundary layer bring into question the single impact theory for the Younger Dryas climate event. Instead, Sun et al. propose that the five layers containing  highly siderophile element enrichments and osmium isotopic signatures represent volcanic aerosols and cryptotephra contributed from distant volcanic eruptions over the roughly 4000 years.

Osmium concentrations and osmium¹⁸⁷/osmium¹⁸⁸ ratios were measured on five bulk samples from the Younger Dryas basal boundary dark layer, and 32 samples were measured from horizons above and below it in Hall’s Cave. In total, samples were collected at high spatial and temporal resolution across depositional ages ranging from 9600 to 13 500 years before present. Hall’s Cave bulk sediments display large variations in osmium abundance from 22.6 to 4478 parts per trillion and osmium¹⁸⁷/osmium¹⁸⁸ ratios from 0.12 to 2.35. The samples are divided into two groups based on their osmium abundances and osmium¹⁸⁷/osmium¹⁸⁸ ratios. In the first group (30 samples, refered to as the 'radiogenic' samples), the samples have osmium¹⁸⁷/osmium¹⁸⁸ ratios from 1.11 to 2.35 and osmium abundances from 22.6 to 56.9 parts per trillion. This combination of high osmium¹⁸⁷/osmium¹⁸⁸ ratios and low 10’s of parts per trillion osmium abundances is typical for continental crust sediments. The second group (7 samples, refered to as the 'unradiogenic' samples) has osmium¹⁸⁷/osmium¹⁸⁸ ratios from 0.12 to 0.42 and osmium abundances from 105 to 4478 parts per trillion. These values are not typical of continental crust and reflect an input from an extraterrestrial or a mantle source. These samples come from five different horizons located at, above, and below the Younger Dryas basal boundary layer. Of the five Younger Dryas basal boundary samples, four are radiogenic with osmium¹⁸⁷/osmium¹⁸⁸ values of 1.49 to 2.22, whereas only HC17_44 has a low, noncontinental crust-like osmium¹⁸⁷/osmium¹⁸⁸ value of 0.41 and  osmium abundance of 105 parts per trillion.

 

Depth below datum (BDт) profiles against total osmium abundances (parts per trillion) and osmium¹⁸⁷/osmium¹⁸⁸ ratios of Hall’s Cave sample section. (A) Depth versus total osmium abundances (parts per trillion). (B) Depth versus osmium¹⁸⁷/osmium¹⁸⁸ ratios. UR, unradiogen. UR1 to UR5 represent five unradiogenic osmium peaks. Depth values are the basal depth of the 1-cm-thick excavation interval relative to the datum. Six ages were calibrated using direct accelerator mass spectrometry carbon¹⁴ measurements with 95.4% confidence intervals and then used to calculate the rest of the dates with linear interpolation between the dated levels. Sun et al. (2020).

Samples of Hall’s Cave sediments with osmium¹⁸⁷/osmium¹⁸⁸ ratios of over 1.11 (i.e. belonging to the radiogenic group) have CI chondrite–normalised highly siderophile element patterns that are indistinguishable from upper continental crust. The unradiogenic samples, with osmium¹⁸⁷/osmium¹⁸⁸ ratios of less than or equal to 0.42, including the one sample in this group at the Younger Dryas basal boundary layer, have low iridium, ruthenium, platinum, palladium, and rhenium abundances that are also similar to upper continental crust. Hence, there is no enrichment in these elements spanning across the Younger Dryas section in Hall’s Cave. This result is consistent with data from eight other Younger Dryas locales. However, the CI chondrite–normalised highly siderophile element patterns for the unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples display a distinct enrichment for osmium concentrations relative to upper continental crust and the radiogenic samples.

 

CI chondrite–normalised incremental mixing model of CI chondrite material into an upper continental crust–like target. From 0.5 to 5.0% mixing lines use 0.5% increment, beneath are the 0.05 and 0.25% lines. highly siderophile element patterns of Hall’s Cave unradiogenic osmium sediments (blue dotted pattern) and radiogenic osmium sediments (pink shadow) compared with average upper continental crust (UCC), Clearwater East Impact melt rock, Canada, and Petriccio Cretaceous/Palaeogene boundary, Italy. Sun et al. (2020).

The osmium isotope systematics and highly siderophile element abundance patterns indicate that exotic materials were contributed during multiple time intervals to the continuous sedimentary record in Hall’s Cave. It is unlikely that multiple impacts/airbursts at these distinct time intervals over about 4000 years had occurred. Furthermore, a bolide compositional signature is unsupported with mass balance calculations for the osmium¹⁸⁷/osmium¹⁸⁸ ratios and highly siderophile element concentrations with end members of CI chondrite and upper continental crust. For osmium, assuming CI chondrite values of osmium¹⁸⁷/osmium¹⁸⁸ equal to 0.127 and osmium of 486 000 parts per trillion and upper continental crust values of osmium¹⁸⁷/osmium¹⁸⁸ equal to 1.3 and osmium of 30 parts per trillion, the amount of meteorite material mixed into upper continental crust to explain the osmium¹⁸⁷/osmium¹⁸⁸ ratios of the Hall’s Cave unradiogenic samples is 0.02 to 0.79%. Similar models using the other highly siderophile elements indicate 0.05 to 5.00% contribution from a CI chondrite impactor. The addition of only 0.05% CI chondrite to the upper continental crust results in gentle positive slopes from osmium to rhenium for CI chondrite–normalizsed patterns. Increasing the amount of CI chondrite to 0.5 to 1.0%, to match the highest amounts of  osmium found in the unradiogenic samples, flattens the slopes of the mixtures. These models do not match the relative distribution or abundances observed in the unradiogenic samples with the osmium-enriched concentrations. The measured abundances are not matched by models using any other chondrite material, including enstatite, ordinary, or other carbonaceous chondrites. Impact melts with purported admixed chondritic material show an even distribution of  highly siderophile element. No iron meteorite groups display the observed distinct enrichments in osmium relative to iridium or the increasing abundances from ruthenium, to platinum, to palladium, and to rhenium relative to CI chondrites. Thus, mixing iron meteorites with upper continental crust will not result in the highly siderophile element patterns exhibited by the unradiogenic samples.

CI chondrite-normalised incremental mixing models of various input materials. (a) Enstatite, (b) Ordinary, or (c) Karoonda carbonaceous chondrites and (d) IIAB - Coahuila, (e) IVB - Hoba and (f) IIAB - Filomena iron meteorites into an upper continental crust target, with Hall’s Cave sediment sub-groups in the background as in blue dotted pattern and pink shadow. Confirmed impact melt rocks, Clearwater East Impact melt rock, Canada, Petriccio Cretaceous/Palaeogene boundary, Italy, were plotted for reference. Sun et al. (2020).

This modeling indicates that the combined highly siderophile element abundances and osmium¹⁸⁷/osmium¹⁸⁸ ratios are inconsistent with contribution from an extraterrestrial impactor or bolide airburst in agreement with previous results from other Younger Dryas locales. Failing a meteorite source, the source of the enrichments and isotopic signature remains unresolved. A better explanation must include a terrestrial source of material able to be supplied frequently and episodically across short time intervals (years to decades).

The Hall’s Cave samples are characterised by a two end member mixing model based on osmium¹⁸⁷/osmium¹⁸⁸ ratios and 1/osmium abundances. Data from other Younger Dryas locales with osmium¹⁸⁷/osmium¹⁸⁸ ratios scatter around the Hall’s Cave data, consistent with distinct natural variations in the continental crust Sediment cores from the Gulf of California and Cariaco Basin are not shown because it has been conclusively argued that the osmium¹⁸⁷/osmium¹⁸⁸ ratios here reflect rehomogenisation with seawater and, hence, lack evidence of an unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ component added to the sediments. The bulk sediment, bulk spherules, residues, and leachates from the Melrose, Pennsylvania, Younger Dryas locale cluster around the unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples from Hall’s Cave. The leachates, which are surface films on the bulk spherules, plot to less radiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios and higher  osmium concentrations defined by the Hall’s Cave samples. These relationships are consistent with mixing between Hall’s Cave crust and components broadly similar to the unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ spherule surface films. This hypothesised mixing can yield the variation in osmium isotope systematics for Hall’s Cave unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples. This mixing from two highly siderophile element sources provides a possible causal link between these North American locales for the unradiogenic  osmium¹⁸⁷/osmium¹⁸⁸ ratios, particularly for the Hall’s Cave Younger Dryas layer. The spherules deposited at the Melrose, Pennsylvania, Younger Dryas layer were interpreted to be airborne material The unradiogenic surface films on the spherules are hypothesised to originate from a bolide as it ablated during heating in the Earth’s atmosphere or as terrestrial material that was ejected back into the atmosphere and redeposited following impact. However, these scenarios are problematic. The osmium¹⁸⁷/osmium¹⁸⁸ ratios from 0.112 to 0.120 in the spherule surface films are unlike most meteorites that have osmium¹⁸⁷/osmium¹⁸⁸ ratios of at least 0.124, with a few from 0.120 to 0.124 and two at 0.117. These low values in the spherule surface films are consistent with being ancient subcontinental mantle of at least 2.4 billion years, but no known impact site of a Younger Dryas age is present in areas where this material may be accessible. Thus, if the osmium from these surface films are similar to the material within the Younger Dryas layer in Hall’s Cave, and potentially the other layers with unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios, then it is unlikely that this material came from a bolide, a series of bolides at different times, or from multiple impact sites. This conclusion is also in agreement with the highly siderophile element patterns of the Hall’s Cave sediments that are inconsistent with being derived from a meteoritic source.

 

Osmium isotope results and mixing models between Hall’s Cave crusts and mantle-like osmium end members. (A) Osmium¹⁸⁷/osmium¹⁸⁸ versus 1/osmium concentration (parts per trillion) for Hall’s Cave sediments with four mixing models for this study. (B) Mixing models close-up view on the lower left end of (A). Upper continental crust data for bulk sediments, bulk spherules and spherule surface films, and residues were for the Melrose  Pennsylvania, Younger Dryas site. Sun et al. (2020).

Instead, the unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios from 0.12 to 0.42 in seven of the Hall’s Cave samples indicate that some type of mantle-derived material was likely incorporated into some sediment layers at different time intervals. The elevated osmium concentrations in these samples of 105 to 4478 parts per trillion relative to continental crust with 30 parts per trillion osmium, and the radiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples with 22 to 55 parts per trillion osmium, are consistent with mantle-derived osmium material being added to these layers. If it is not impact ejecta from ancient subcontinental mantle, as discounted in a 2013 study by Yingzhe Wu, Mukul Sharma, Malcolm LeCompte, Mark Demitroff, and Joshua Landis, then one possibility is that exposed ancient mantle peridotite or komatiite (1000s of parts per trillion osmium) with low osmium¹⁸⁷/osmium¹⁸⁸ and high osmium abundance was eroded and episodically provided a supply of sediment at these time intervals where the unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios are found in the layers. However, there are no known exposures of peridotite or komatiite within 100s km of Hall’s Cave rendering this possibility unlikely. Another possibility is local mafic volcanic material with mantle-derived osmium¹⁸⁷/osmium¹⁸⁸ ratios that is subsequently eroded and deposited in Hall’s Cave. Mafic volcanic material is also problematic from the perspective of not having a source of this material nearby to erode. 

One final scenario is that the source of the unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ may be airborne material from concurrent volcanic eruptions, which would alleviate the need for nearby older volcanic sources to erode and deposit material in Hall’s Cave. If the unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios in the Younger Dryas layer and, potentially, the other layers in Hall’s Cave are of volcanic origin, then the very high abundances of osmium in the Melrose surface films of about 19 000 to 43 000 parts per trillion could reflect aerosols enriched in highly siderophile element associated with airborne volcanic tephra from distant eruptions, which are enriched in osmium with mantle-like osmium¹⁸⁷/osmium¹⁸⁸ ratios.

To test this hypothesis, Sun et al. developed mixing models between Hall’s Cave crust compositions and hypothetical mantle-like osmium end members. These models were parameterised to examine the range of potential variation in these components when mixed together, simulating the unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ and high osmium concentrations in the Hall’s Cave samples. In mix 1 the average of the leachates for objects 2, 4, 5, 11, and 13 from the Melrose locale was used as the unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ end member. When this is mixed with Hall’s Cave crust with 21 parts per trillion osmiumand osmium¹⁸⁷/osmium¹⁸⁸ of 2.1, the mixing curve passes through the unradiogenic osmium sample from the Younger Dryas layer in Hall’s Cave and three of the samples from the other layers in Hall’s Cave. Three unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples from other layers in Hall’s Cave plot to higher values of osmium¹⁸⁷/osmium¹⁸⁸ and not along this mixing curve.  These mixing models are all hyperbolic, and all seven unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples from Hall’s Cave fall within the band of possible values generated by mixing this range of end-member compositions. This modeling indicates that the observed compositions of the Hall’s Cave unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples reflect a range in osmium concentrations and osmium¹⁸⁷/osmium¹⁸⁸ values for unradiogenic osmium end members when mixed with Hall’s Cave crustal component. Thus, the different layers in Hall’s Cave with unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios are unrelated to a single source of osmium, as expected given the different depositional times but instead likely related to a process of high osmium concentration material deposited having a range of osmium¹⁸⁷/osmium¹⁸⁸ values from around 0.115 to 0.20.

 

This range of osmium¹⁸⁷/osmium¹⁸⁸ values needed for the mantle end member are within the range present in volcanic lava samples worldwide, for example, the Cascade arc (0.129 to 0.253), Ocean Island lavas (0.110 to 0.176), Ethiopian lavas from East African Rift (0.124 to 0.427), and numerous other volcanic rock locales. Aerosols measured at Mauna Loa in Hawaii have osmium¹⁸⁷/osmium¹⁸⁸ ratios of 0.136 to 0.140, consistent with the compositions of lavas from this volcano. Condensates from aerosols from Piton de la Fournaise fumarole on Reunion Island show that different sources can be tapped and that decoupling in the osmium¹⁸⁷/osmium¹⁸⁸ ratio can occur between lavas and aerosols. The samples from the lowest temperature condensates (less than 350°C) have osmium¹⁸⁷/osmium¹⁸⁸ ratios of 0.130 to 0.135 and overlapping those of the associated lavas. The highest temperature condensates (384 to 400°C) have osmium¹⁸⁷/osmium¹⁸⁸ ratios of 0.123 to 0.129, indicating derivation from older mantle sulphides and show that osmium can be derived from different sources than those for the lavas resulting in distinct osmium¹⁸⁷/osmium¹⁸⁸ values. Hence, the lavas are not always a direct indication of the osmium¹⁸⁷/osmium¹⁸⁸ ratios for those of the associated aerosols, and these sources for osmium can change during evolution of the volcano magmatic systems. What these relationships indicate is that if it is not unexpected, then that airborne cryptotephra with high osmium concentration surface films derived from volcanic aerosols would have osmium¹⁸⁷/osmium¹⁸⁸ ratios consistent with the range of those needed to explain the range of unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ sample values from Hall’s Cave.

For this scenario, powerful volcanic eruptions provide the most likely source for the observed high osmium abundances and low osmium¹⁸⁷/osmium¹⁸⁸ signatures at Hall’s Cave and potentially explain the surface films on the Melrose spherules in the Younger Dryas layer. The record of punctuated volcanism would be preserved as cryptotephra horizons with unique chemical fingerprints within the stratigraphy. Mafic melts contain the highest concentrations of highly siderophile element with the concentration of at 0.1 to 1000 parts per trillion. Fractional crystallisation produces more silicic, evolved melts with lower highly siderophile element concentrations (e.g. osmium of 10 to 34 parts per trillion). These values do not completely overlap with the osmium concentrations preserved in the seven possible cryptotephra samples at Hall’s Cave, which range from 105 to 4478 parts per trillion. Volcanic degassing may provide the required mechanism that enriches low magmatic highly siderophile element concentrations to much higher values. During ascent and depressurisation, volatiles exsolve from magma. Common volatiles include carboon dioxide, water, sulphur dioxide, hydrogen sulphide, hydrochloric acid, and hydrofluoric acid. Highly siderophile element partition strongly to an exsolved vapor phase, with estimated melt-vapour distribution coefficients as high as 105 to 106. This process can be accentuated by both the increasing solubility of sulphur and increasing oxidation state of the magma during decompression. In oxidised magmas with lower amounts of hydrogen sulphide, Highly siderophile elements concentrate in the melt where they are available to degas. This is because the sulphide minerals in which highly siderophile elements would partition are unstable. Sulphur becomes increasingly soluble with decreasing pressure, which causes the melt to become sulphur undersaturated and forces highly siderophile element-bearing sulphides to dissolve back into the melt with subsequent gas release.

The highly siderophile element during an eruption is lost from the melt, added to the eruption column, exists in sulphur and other compounds, and adheres to ash particles (here, referred to as volcanic aerosols). The magnitude of a volcanic eruption controls how high the plume penetrates the atmosphere, which subsequently controls the geographic distribution of the tephra. Large eruptions may thus entrain highly siderophile element far into the atmosphere where they can be transported long distances before deposition. Eruptions with magnitudes of over 5 on the  volcanic explosivity index reach the stratosphere and can be globally distributed as volcanic aerosols. Volcanic aerosols have a significant impact on climate interactions, ozone chemistry, and vapor transport, and therefore, these particles have been the focus of observation and modeling. Particulate entrainment within the planetary wave train results in global distribution of volcanic aerosols from high-latitude volcanoes, with spread of the cryptotephra across the hemisphere, whereas equatorial eruptions may distribute volcanic aerosols in both hemispheres.

Sun et al. contend from the analysis of Hall’s Cave sediments that horizons with unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios are cryptotephra with surface films consisting of volcanic aerosols. Thus, the highly siderophile element concentrations should correspond to observed values in volcanic aerosols from eruptions in modern environments. The highly siderophile element values in Hall’s Cave follow a trend showing a progressive increase in concentrations from ruthenium to iridium to osmium and from ruthenium to platinum and palladium that closely matches the patterns of the Kudryavy volcano gas condensates, Kurile Islands. In particular, iridium and platinum have been used to support the bolide hypothesis in other Younger Dryas locales.

Using pluatinum, iridium, osmium, and palladium, concentrations and ratios, the Hall’s Cave unradiogenic osmium samples instead closely match those for the range and variation of gas condensates from three different volcanoes. The range in highly siderophile element concentrations and ratios for all Hall’s Cave samples plot as well-defined trends or clusters in these variation diagrams. These variations are consistent with the mixing scenario between volcanic aerosols with a range of highly siderophile element systematics related to specific supplies of material from different volcanoes, with upper continental crust, within any given unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ layer, observed for osmium¹⁸⁷/osmium¹⁸⁸ versus 1/osmium and that for osmium¹⁸⁷/osmium¹⁸⁸ versus osmium/iridium.

 

 

CI chondrite–normalised highly siderophile element patterns of all gas condensates and the average of the Kudryavy volcano gas condensates, Kurile Islands. The highly siderophile element patterns for the osmium isotope radiogenic and unradiogenic groups from Hall’s Cave are in pink shadow and blue dotted pattern, respectively. All Kudryavy volcano gas condensates (abbreviated to vlc. gas cond.) are plotted as yellow shadow whose average is presented as the yellow line. Sun et al. (2020).

 

The one highly siderophile element that is not showing a similar enrichment in the Hall’s Cave unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples to that expected from volcanic aerosol addition is rhenium. Rhenium, like osmium, shows up to a two order of magnitude enrichment in volcanic aerosols relative to upper continental crust, but this is not observed in the Hall’s Cave samples. If the volcanic aerosol hypothesis is viable, then the question arises as to why there is no rhenium enrichment. There are two possible scenarios. In the first scenario, rhenium was redistributed in the sediments thereby erasing the aerosol signature. Rhenium can be highly mobile in the surface environment in oxidising conditions where it can form perrhenate. However, this is discounted because the platinum/osmium and rhenium/osmium ratios strongly correlate with osmium¹⁸⁷/osmium¹⁸⁸ in the Hall’s Cave samples, indicating little to no highly siderophile element mobility.

 

Binary plots of highly siderophile elements and ratios of Hall’s Cave samples. (A) Iridium versus iridium/platinum, (B) platinum/osmium versus osmium, (C) platinum versus palladium. compared to upper continental crust, Clearwater East Impact melt rock, Canada; Petriccio Cretaceous/Palaeogene boundary, Italy; Kudryavy volcano gas condensates, Kurile Islands; Tolbachik volcano gas condensate, Kamchatka; Erta Ale volcano gas (abbreviated to vlc. gas), Ethiopia and confirmed impact craters, Popigai, Russia; Morokweng, South Africa; Serenitatis basin, Moon; MORB, mid-ocean ridge basalts; OIB, ocean island basalts; Volcanic arcs; and komatiites. Sun et al. (2020).

In the second scenario, rhenium from volcanic aerosols is not condensing on cryptotephra surfaces that are transported long distances before deposition in locations such as Hall’s Cave. The limited studies on volcanic systems to date indicate that the highly siderophile element can strongly fractionate from each other during volatile phase transport and condensation from the aerosols. The highly siderophile element fractionations and minerals condensed, such as rhenite, potasium-rhenium-perrhenate, and those for platinum-palladium and osmium alloys, are dependent on the temperature of condensation and volatile compounds that are present in the aerosols. These can vary greatly from volcano to volcano as do the variations for highly siderophile element relative to each other in the aerosols. Further work will be required to better constrain the highly siderophile element relationships during these gaseous volcanic processes and as they relate to the observed aerosol fingerprints observed in the Hall’s Cave unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples and elsewhere.

 

CI chondrite-normalised incremental mixing models of various input materials. (a) Enstatite, (b) Ordinary, or (c) Karoonda carbonaceous chondrites and (d) IIAB - Coahuila, (e) IVB - Hoba and (f) IIAB - Filomena iron meteorites into an upper continental crust target, with Hall’s Cave sediment sub-groups in the background as in blue dotted pattern and pink shadow. Confirmed impact melt rocks, Clearwater East Impact melt rock, Canada, Petriccio Ceretaceous/Palaeogene boundary, Italy were plotted for reference. Sun et al. (2020).

Notably, the highly siderophile element systematics show only minor overlap with those for mafic and ultramafic lavas from different tectonic settings, further supporting a scenario where the highly siderophile element are fingerprinting the aerosols from volcanic systems and unlikely sourced from respective silicate magmas. In addition, impact-related materials have clearly different highly siderophile element systematics, consistent with a non-extraterrestrial source with low osmium¹⁸⁷/osmium¹⁸⁸ ratios for the unradiogenic samples. The highly siderophile element systematics in the Hall’s Cave unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples are thus best explained by input of cryptotephra with surfaces laden with highly siderophile element-enriched aerosols from distal, large-volume Plinian eruptions occurring at different times and added to the continental crust-derived sediments.

Distant volcanic eruptions may provide both the compositional control and physical mechanism to produce the highly siderophile element-enriched cryptotephra horizons in Hall’s Cave. The multidecadal to century scale time resolution for sedimentation in Hall’s Cave obviate correlation with specific volcanic eruptions. However, this section documents significant eruptions within the time frame of the cryptotephra horizons fingerprinted by osmium¹⁸⁷/osmium¹⁸⁸ ratios and highly siderophile element systematics. These times indicate active volcanism in regions in the Northern Hemisphere that could have contributed cryptotephra to Hall’s Cave sediments. The cryptotephra horizon at 151 cm at the Younger Dryas basal boundary layer at 13.11 to 12.90 thousand years ago was likely sourced from the approximately 13.10 thousandf year ago eruption of Laacher See. The Laacher See eruption ejected 6.3 km³ dense rock equivalent of sulphur-rich magma far into the stratosphere and likely dispersed volcanic aerosols throughout the Northern Hemisphere. Laacher See released from 2 to 150 megatons of sulphur. Although under debate, this may have triggered the temperature decline associated with Younger Dryas climate change in the Northern Hemisphere.

To identify possible volcanic sources for the other horizons in Hall’s Cave that have osmium¹⁸⁷/osmium¹⁸⁸ ratios and highly siderophile element signatures consistent with cryptotephra, the carbon¹⁴ ages of large-magnitude volcanic eruptions during the late Pleistocene to Holocene are compared to the carbon¹⁴ ages of the five highly siderophile element-enriched unradiogenic horizons (UR1 to UR5) in Hall’s Cave. The five interpreted cryptotephra horizons can be grouped into three volcanic mixing events that correlate well with known eruptions. The couplet of UR1 and UR2 horizons at 176 and 171 cm depth below datum, with a depositional age of 13.33 ± 0.19 thousand years, is similar to the Glacier Peak volcano in Washington, USA erupted at 13.71 to 13.41 thousand years ago and/or the J Swift tephra from Mount Saint Helens erupted at 13.75 to 13.45 thousand years ago. These eruptions demonstrate that the Cascade volcanic arc was highly active at these times and would likely have dispersed volcanic aerosols and cryptotephra widely across the Northern Hemisphere.

There are possibly two eruptive candidates for Horizon UR5 at 140 cm depth below datum, which dates to 10.98 thousand years ago. Both the Fisher Tuff eruption from the Aleutian Arc and the Lvinaya Past eruption from the Kuril Arc occurred during the appropriate time interval. Each of these large-volume arc volcano eruptions produced a Plinian eruption column that reached the stratosphere and distributed volcanic aerosols across North America.

The mass balance models using a range of chondrite and iron meteorite highly siderophile element abundances indicate that it is highly improbable that the addition of meteoritic impactor components into continental crust could reproduce the observed chondrite-normalised highly siderophile element patterns for the Hall’s Cave unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ samples. A two end member mixing relationship between Melrose (Pennsylvania) spherule surface films and Hall’s Cave bulk sediments demonstrate a distinctly different osmium isotope systematics for the Younger Dryas layer. The four other layers above and below the Younger Dryas layer with unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratios are consistent with mixing between Hall’s Cave sediment and mantle-derived end members with a range of osmium¹⁸⁷/osmium¹⁸⁸ ratios from 0.115 to 0.2. This range of osmium¹⁸⁷/osmium¹⁸⁸ ratios overlaps those for lavas from different tectonic settings worldwide and those measured on volcanic aerosols. The highly siderophile element ratios and chondrite-normalized patterns for these samples from Hall’s Cave display close similarity with volcanic aerosols. The five unradiogenic osmium peaks, including the Younger Dryas layer, fall within a roughly 4000-year time interval. The unradiogenic osmium¹⁸⁷/osmium¹⁸⁸ ratio and highly siderophile element abundance data from Hall’s Cave sediments are inconsistent with the Younger Dryas impact hypothesis. Alternatively, these levels contain cryptotephra and associated aerosols derived from large Plinian volcanic eruptions. The Younger Dryas horizon correlates in time with the Laacher See eruption with a volcanic explosivity index of 6 and a 6.3-km³ eruptive volume that was dispersed throughout the Northern Hemisphere. Previously found Younger Dryas markers, such as nanodiamonds and other wildfire products, are not necessarily solely impact-induced. Instead, these could originate from high-temperature, large-scale volcanic eruptions whose explosive conditions are capable of producing molten silica and carbon spherules and possibly nanodiamonds (Lonsdaleite).

These observations from the Hall’s Cave section also explain the lack of an osmium isotope extraterrestrial signature, or for the interpretation of a cryptotephra signature, at many Younger Dryas locales across the Northern Hemisphere. Deposits of cryptotephra at any given time would be dependent on dispersal of the eruptive products from the volcanoes emitting them, which could be localized or found in narrow bands depending on the dispersal patterns. This issue requires further investigation using highly siderophile element and osmium isotopes to track dispersal dynamics.

The results here have implications for not only the Younger Dryas event but also other platinum and iridium enrichment events in Earth history and where other supposed bolide markers have been used to support impacts at these times. This analysis indicates that coupled osmium isotope and highly siderophile element concentration data are needed at close stratigraphic intervals above and below suspected bolide events to evaluate an alternative volcanic origin.

 

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