Unlike the pockmarked Moon, whose surface has been shaped by impacts large and small for more than 4 billion years, planet Earth has retained a few relics of that cosmic bombardment. Tectonic activity that recycles crust along active plate margins, erosion, and the burial of impact craters underneath layers of sediment and lava have either removed or concealed the majority of the Earth’s cosmic scars. Only 199 impact structures (counting fields of small impact craters produced during the same event as one) and 40 individual horizons of proximal and distal impact ejecta (again, counting layers with the same age at different localities as one) have thus far been recognized on our planet (Fig. 1). Those impact structures and deposits span a time from more than ~3.4 billion years (Ga) before present (Archean impact spherule layers in South Africa and Western Australia) to roughly 6 years ago (the Chelyabinsk airburst in Russia on February 15, 2013, which shattered windows and whose main stony meteorite mass produced a 8-meter-wide circular hole in the frozen Lake Chebarkul; see Issue 133). Although impact rates have dramatically decreased since the early portion of solar system history, we see that meteorite impacts are still an ongoing geologic process and remain a constant threat (Fig. 2).

Despite the limited terrestrial record, the study of impact structures and deposits through geologic field work and drilling is important for the understanding of the collisional evolution of the inner solar system, and particularly the Earth-Moon system. Unlike sample return missions to the Moon and other planetary bodies, collecting impact crater materials on Earth as planetary analogs is convenient and economic. Large amounts and representative collections of sample material can be recovered from the field, processed, and analyzed in the lab. Six fundamental questions are then:

1. How did the structure and/or deposit form and is there compelling evidence for impact?
2. Can we better understand the dynamic processes that create impact craters of different size and in different geologic settings and their associated ejecta deposits?
3. What was the type of impactor that produced the crater?
4. When exactly did the impact occur?
5. How are impact events related to one another and to the impact rate in the Earth-Moon system?
6. How are large impacts related to crises, mass extinctions, and diversification events in the biosphere, and can cooling impact craters serve as a habitat for microbial life?

Questions (1) and (2) require detailed petrographic analysis of rock, including the study of impact-diagnostic shock metamorphic features such as shatter cones, shocked quartz or zircon grains, and the presence of high-pressure polymorphs. Question (3) can be answered through the geochemical analysis of impact melt rocks and breccias that formed under extreme temperature and pressure conditions during the impact event. Trace-element analysis has, in many cases, revealed the nature of the impacting body even though only traces of the projectile were admixed to the mainly crust-derived melt. Likewise, geochemical work is essential when assessing the economic potential of an impact structure, such as the ~1.85-billion-year-old (Ga) and ~200- to 250-kilometer-diameter Sudbury impact basin in Ontario, Canada, which is one of the oldest and largest impact structures on Earth and a world-class reservoir for nickel, copper, and platinum group elements.

Let us now focus on impact geochronology, which has over the past six decades grown into its own field of research and which offers answers to questions (4), (5), and (6) — perhaps those with the most significant and far-reaching global (and extraterrestrial) implications.

Impact Geochronology: Different Methods, One Goal

While the scientific approach for the verification of an impact origin (i.e., the proof of evidence of shock metamorphism in rocks and minerals) remains the same for all candidate impact structures, the timing of impact can be determined using one or more independent methods. The determination of stratigraphic ages, by superposition, can be applied to all impact structures on Earth and elsewhere, where relative host rock ages are to some degree constrained. Every impact structure has a target rock that the impacting body penetrated, and through simple geologic cross-cutting relationships the youngest rock units affected by the impact provide a maximum (oldest possible) age for the impact. In turn, the oldest undisturbed rocks that fill the crater after its formation constrain the minimum (youngest possible) impact age. Sometimes the stratigraphic age for an impact can only be bracketed within several hundred million years, as in the case of the 12-kilometer-diameter Wells Creek impact in Tennessee: The crater must be younger than Mississippian (~323 Ma) and older than Late Cretaceous (~100 Ma), leaving us with a best-estimate age of ~211 ± 111 Ma and a relative error of more than 100%. However, other stratigraphically constrained impact ages are remarkably precise, such as that of the ~14-kilometer-wide marine Lockne crater in Ordovician rocks of Central Sweden. There, the impact age is precisely constrained to be 455 Ma plus and minus a few hundred thousand years, because both the youngest pre-impact and oldest post-impact sequence lie in the late Sandbian L. dalbyensis chitinozoan microfossil zone studied in great detail. (We shall come back to other impact craters produced during the Ordovician later in this article.) We also note that both the Wells Creek and Lockne impact craters have little or no recognized impact melt that could potentially be used as material for radioisotopic analysis.

A relatively large number of terrestrial impact structures have preserved impact melt-bearing rocks, such as the thick, differentiated crystalline melt sheet at Sudbury (the Sudbury Igneous Complex), the melt sheet at the 100-kilometer-diameter Manicouagan impact structure in Québec, Canada (Fig. 3a), and the glass-rich suevite of the 25-kilometer-diameter Ries crater in Germany (Fig. 3b). The Ries impact also produced green glassy tektites (moldavites; Fig. 3c), distal melt ejecta found ~200–500 kilometers northeast of the crater. Such impact melt lithologies are suitable for geochronologic analysis using a variety of radioisotopic techiques. One method used to determine impact ages is the uranium-lead (U-Pb) and coupled lead-lead (Pb-Pb) geochronometer pioneered by Alfred Nier in the late 1930s–1950s, soon thereafter applied by George Wetherill, Gerald Wasserburg, Fouad Tera, and others, and now being used with several different technical setups. These include, for example, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS) and sensitive high resolution ion microprobe (SHRIMP) analysis, and thermal ionization mass spectrometry after chemically abrading the mineral sample for better results (CA-TIMS).

Each of these techniques has its advantage and disadvantage. While LA-ICP-MS and SIMS/SHRIMP are routinely and rapidly applied to thin-section or grain-mount samples that can preserve the textural context of the sample, producing moderately precise U-Pb and Pb-Pb ages, CA-TIMS completely dissolves the mineral sample but produces much more precise ages with errors commonly in the range of a few thousand to tens of thousands of years. Uranium-lead results are typically visualized in a concordia diagram (Wetherill or Tera-Wasserburg plot) in which minerals from the unmelted target rock tend to yield an older age on or near concordia (the curve along which U-Pb ages from different U-decay series are equal). In contrast, shock-recrystallized and chronometrically reset mineral grains produce either a younger concordia age (Fig. 3d) or, if disturbed by the loss of Pb, a discordant array of dates that then defines a lower intercept with concordia, interpreted as the age of the impact. The U-bearing minerals most commonly used for U-Pb geochronology are either intensely shocked (recrystallized) or melt-grown zircon crystals (Fig. 3e), baddeleyite, monazite, and to a lesser degree titanite and apatite, although recent results for terrestrial impact craters suggest the latter may be a promising target mineral for future studies.

Shocked zircon crystals in melt rock from the ~250- to 300-kilometer-diameter Vredefort impact structure, the largest one on Earth, yielded a U-Pb age of 2023 ± 4 Ma. Zircon grains crystallized from Sudbury’s impact melt sheet produced a U-Pb age of 1850 ± 1 Ma. In a recent study, intensely shock-metamorphosed zircon grains recrystallized into microgranular aggregates yielded a precise age of 77.85 ± 0.78 Ma for the 23-kilometer Lappajärvi impact crater in Finland. This result for Lappajärvi has, as we will see later, implications for the role of impact craters in the origin and evolution of life on the early Earth.

Another technique prominently used in impact geochronology is the ⁴⁰Ar-³⁹Ar method, an improved variation of the classical K-Ar technique. The Ar-Ar method was pioneered by Heinrich Wänke and Hans König in the late 1950s and Craig Merrihue and Grenville Turner in the 1960s and can today be applied using the total fusion of a sample with a laser or, alternatively, the stepwise heating of a sample using a furnace or laser. The potassium (K)-bearing mineral or rock sample, together with standard minerals, is first irradiated by fast neutrons to produce ³⁹Ar from ³⁹Kr as a proxy for potassium in the sample; the Ar isotope ratios are then measured in a mass spectrometer and ages calculated. Generally, the step-heating method produces a more comprehensive set of data than the total-fusion method and allows for a more robust statistical assessment of resulting ages. Argon-argon results can be disturbed by the effects of sample alteration or inclusions of older material. Statistically robust Ar-Ar results ideally form a “plateau” in the age spectrum (Fig. 3f), a sequence of individual degassing steps with increasing temperature that all overlap within a narrow error limit and include most of the ³⁹Ar extracted from the sample. Precise Ar-Ar ages have been obtained for a number of impacts on Earth, such as 66.051 ± 0.031 Ma for glassy microtektites from the 180-kilometer-diameter Chicxulub crater linked to the end-Cretaceous mass extinction. Ries tektites (Fig. 3c) yielded a precise Ar-Ar age of 14.808 ± 0.038 Ma. An increasingly robust intercalibration between the U-Pb and Ar-Ar geochronometers provides confidence that ages obtained using both techniques are not only precise (with a small error) but also accurate (close to the “true” age) and can be directly compared and correlated.

Additional methods exist for the determination of impact crater ages, such as the rubidium-strontium (Rb-Sr) method (for minerals from impact melt rock), the low-temperature uranium-thorium-helium (U-Th)/He geo-/thermochronometer (mainly using zircon and apatite for cooling studies), the carbon-14 (¹⁴C) method (for charcoal inside a young impact crater), cosmogenic nuclides and exposure ages, luminescence, and fission track analysis (zircon, apatite, or glass).

Double and Multiple Impacts on Earth?

With an increasing number of precise and accurate ages for terrestrial impacts, we can take a closer look at the potential temporal connection between impact events themselves. Classic examples of two closely spaced impact structures are the ~25-kilometer Nördlinger Ries and ~3.8-kilometer Steinheim Basin crater pair in Germany and the two Clearwater Lakes in Québec, Canada (Fig. 4). While the age of the Nördlinger Ries is precisely known, the age of the Steinheim Basin is still somewhat enigmatic. However, the two impact craters are thought to be genetically linked because of the similar age of their (oldest) crater lake sediments and their geometric alignment with the Central European tektite strewn field to the northeast. In Canada, the larger, ~36-kilometer-diameter West Clearwater Lake impact structure has a ring of islands where impact-melt bearing rocks occur. East Clearwater Lake, 26 kilometers in diameter, has a more subtle appearance and melt-bearing rocks are only known from drillings. For almost 50 years, these two impact structures had been considered as a textbook example of an impact crater doublet created simultaneously by the impact of a binary asteroid in the early Permian some 290 million years ago. However, things later turned out to be more complicated. Repeated Ar-Ar analysis, alongside other lines of geologic evidence, eventually made a convincing case against the double impact scenario. While the larger western crater was indeed produced in the Permian at 286.2 ± 2.6 Ma, the eastern crater is almost 180 million years older and, with an age around 465 Ma, dates back to the Ordovician time period (485–443 Ma).

The East Clearwater Lake impact in the Ordovician is, however, not a unique structure of that time. In fact, as more impact structures are discovered and their ages refined, the list of Ordovician impacts steadily grows. Twenty-two of the currently known 199 impact structures on Earth (i.e., more than 10%) have proven or very likely Ordovician ages, creating a distinct spike in the terrestrial impact cratering record (Fig. 2). Recent additions to the list of Ordovician impacts, based on new U-Pb and Ar-Ar geochronologic results, include, for example, the 54-kilometer-diameter Charlevoix impact structure, the 50-kilometer-diameter Carswell impact structure, and the 8-kilometer-diameter La Moinerie impact structure, all located in Canada. Those impact structures — six in the United States, nine in Canada, five in Sweden, and one in Estonia, Ukraine, and Australia, respectively — were produced over a period of several million years. Among the Swedish impact structures, the 14-kilometer-diameter Lockne and 0.7-kilometer-diameter Målingen impact craters may represent a true crater doublet within the framework of multiple impacts. In addition, a number of fossil meteorites found in Ordovician limestone in Sweden and the impact-produced Osmussaar Breccia in Estonia testify to a period of enhanced bombardment of Earth by asteroids at that time. Analysis of the fossil meteorites and impact breccias suggests that most of the Ordovician impacts are linked to the collisional breakup of the L-chondrite parent asteroid in space some 470 million years ago, which then sent large masses of shock-melted stony meteorites into Earth-crossing orbit. However, compared to the largest terrestrial impacts, such as Vredefort, Sudbury, and Chicxulub, the asteroids that created the Ordovician impact structures were rather small.

While the Ordovician can be regarded as a time of intense impact cratering, there is currently no evidence for true multiple impact events resulting in the formation of larger-scale impact crater chains on Earth. Although such a scenario had been proposed for at least five impact structures with overlapping ages (Manicouagan and Lake Saint Martin in Canada, Red Wing Creek in the United States, Rochechouart in France, and Obolon in Ukraine) in the Late Triassic some 214 million years ago, more recent Ar-Ar age determinations on the Lake Saint Martin (227.8 ± 0.9 Ma) and Rochechouart (206.92 ± 0.32 Ma) impacts and refined stratigraphic age constraints for Obolon (<185 Ma) demonstrated that all those craters have very different ages and are therefore unrelated. We conclude that the Late Triassic Earth did not see a multiple impact event similar to the impact of the tidally disrupted Comet Shoemaker-Levy 9 on Jupiter as observed by the Hubble Space Telescope in July 1994 (see Issue 152). While there are true impact crater chains on the Moon and other planetary bodies, no such chain is known to exist on Earth.

The Impact-Biosphere Connection Through High-Resolution Geochronology

With the advent of the “New Catastrophism” in the wake of Luis and Walter Alvarez’ impact-mass extinction hypothesis (1980), according to which the Earth’s Mesozoic life — most prominently the dinosaurs — were wiped out due to the impact of a large asteroid that was also the source of a global iridium anomaly, larger meteorite impacts have been discussed as potential triggers for most, if not all, of the “big five” extinction events in the geologic past. While the end-Ordovician extinction (~443 Ma) was most likely related to climatic effects, some researchers argue that frequent impacts in the Mid-Ordovician (~470–458 Ma) may have, in fact, boosted biodiversification. The Late Devonian Frasnian/Famennian transition, associated with an extinction event, has an age (~372 Ma) that is similar to the age of the ≥52-kilometer-diameter Siljan impact structure in Sweden, Europe’s largest impact structure. However, current Ar-Ar results suggest the Siljan impact occurred at either ~400 Ma or ~380 Ma, and therefore a causal link with the Frasnian/Famennian boundary event seems implausible. Likewise, there is currently no convincing evidence of global-scale impacts at the end-Permian (~252 Ma), the biggest of all life crises on Earth during which more than 95% of marine species and 70% of terrestrial vertebrates went extinct, and the end-Triassic (~201 Ma). Although the ~40-kilometer-diameter Rochechouart impact structure in France previously had an age that overlapped with the Triassic/Jurassic boundary, new Ar-Ar results suggest the impact occurred some ~5 million years before the transition. Instead, the end-Permian and end-Triassic extinction events may have been caused by volcanic activity in large igneous provinces, such as the Emeishan and Siberian Traps in the final stages of the Permian, and the Central Atlantic Magmatic Province at the end of the Triassic, and potentially other compounding environmental factors. Thus far, the only convincing case for impact as the trigger of a mass extinction remains the giant Chicxulub impact on the Yucatán Peninsula in Mexico (see Issue 144), which has been stratigraphically, micropaleontologically, geochemically, and in terms of precise U-Pb and Ar-Ar ages linked with the Cretaceous/Paleogene boundary at ~66.05 Ma. At the time of impact, the contemporaneous Deccan trap volcanism in India had already been active.

Finally, it is worth noting that large impacts, capable of causing widespread destruction and mass extinctions, are not only detrimental to the biosphere. As LPI’s scientist David Kring formulated in his “Impact Origin of Life Hypothesis,” cooling impact craters that hosted hydrothermal systems are thought to have served as a habitat for thermophilic and hyperthermophilic microbial life on the early Earth (and possibly Mars). Although large impacts were much more abundant during the Hadean and Archean before ca. 3.7 Ga, impact craters and their hydrothermally altered rocks and minerals accessible today are valuable analog sites for this type of habitat. Two critical factors in hot-fluid systems as biologic habitats are their temperature and lifetime. Numerical modeling suggests that the largest terrestrial impact craters, such as Sudbury and Chicxulub, may have sustained initially hot hydrothermal activity for more than 2 million years, whereas medium-sized impact craters around 20–30 kilometers in diameter were generally thought to cool down more rapidly, perhaps over a few tens of thousands of years. Recent high-precision U-Pb and Ar-Ar results for the 23-kilometer Lappajärvi impact crater in Finland, mentioned earlier, suggest those initial estimates may have been too conservative. An older zircon U-Pb age, recording lead diffusion at ~900°C, in combination with significantly younger Ar-Ar results for K-feldspar that record Ar diffusion over several hundreds of thousands of years, indicate that even the comparatively small Lappajärvi crater cooled to ~200°C over a period of at least 1.3 million years. This is substantially longer than estimated previously and makes Lappajärvi-sized impact craters, which are much more common over geologic time than Sudbury- or Chicxulub-sized craters, an important type of habitat for thermophilic and hyperthermophilic microbes.

In summary, high-precision geochronology has refined the timeline for a number of impact events on Earth whose ages can be correlated with other impacts and geologic events in Earth history. Based on the latest geochronologic results, synchronous double impacts on Earth seem to be rare and evidence for a large-scale multiple impact event on our planet is currently missing. However, the Ordovician marks a time period of intense bombardment over several millions of years, supported by a growing number of Ordovician U-Pb, Ar-Ar, and stratigraphic impact ages. Only the Chicxulub impact has been firmly linked to a mass extinction event, in part based on high-precision U-Pb and Ar-Ar results. The latter can also be used to determine the lifetime of hydrothermal systems in cooling impact craters, as recently done for the slowly cooled Lappajärvi impact crater in Finland as an analog for impact-crater-hosted habitats for microbial life on the early Earth.

 

 

 

 

 

 

 

About the Author:  Dr. Martin Schmieder is a Postdoctoral Fellow/Visiting Scientist at the Lunar and Planetary Institute in Houston, Texas. He has, over the past decade, worked extensively on the petrology, shock metamorphism, and geochronology of impact crater materials using both the Ar-Ar and U-Pb methods, and has led and co-authored more than 40 peer-reviewed papers in this field of research. He has studied 19 terrestrial impact structures on six continents in the field and is, alongside LPI’s Dr. David Kring, actively involved in the analysis of IODP-ICDP Expedition 364 drill core samples from the large, end-Cretaceous Chicxulub impact crater. While geochronologic results discussed in this article are in part sourced from the literature, several of the impact crater ages presented herein are the product of Schmieder’s geochronologic work. His mission is to constantly improve our knowledge of the terrestrial impact cratering record, to better understand the timing of large impacts through geologic time, their fingerprint in different isotopic systems, and their implications for early life.