Event- and biostratigraphic evidence for two independent Ries and Steinheim asteroid impacts in the Middle Miocene.

For decades, the Nördlinger Ries and Steinheim Basin in southern Germany have been regarded as a textbook example of a terrestrial impact crater doublet, although the oldest crater lake deposits in both craters suggest a biostratigraphic age difference of ~ 0.5 to 1 Myr. We previously presented stratigraphic arguments that challenged the double impact scenario and favoured a model of two temporally independent impact events in the Mid-Miocene. We here present, for the first time, four localities within a distance of ~ 50-100 km from the Ries and ~ 50-70 km from the Steinheim crater that expose two independent seismite horizons, together unique within the Upper Freshwater Molasse of the North Alpine Foreland Basin, each one featuring impressive water escape structures. The seismite horizons are separated by ~ 10 to 15 m of undisturbed Molasse deposits and, biostratigraphically, by an entire European Land Mammal Zone, thus providing evidence for two independent major seismic events within a time span of ~ 0.5-1 Myr. Both the lower and the upper seismite horizons can be correlated litho- and biostratigraphically with the basal crater lake sediments at the Ries and Steinheim craters, respectively, deposited immediately after the impacts. From a biostratigraphic point of view, the impact event that formed the Steinheim Basin probably occured around 14 Ma, some 0.8 Myr after the ~ 14.81 Ma Ries impact event.

Sand spikes pinpoint powerful palaeoseismicity.

Sand spikes, pin-shaped, carbonate-cemented sandstone bodies of variable size widely interpreted as sedimentary concretions, have been enigmatic for nearly two centuries. We here present a high-energy mechanism for their formation. Two classic sand spike occurrences are found in the North Alpine Foreland Basin of Central Europe and at Mount Signal in southern California, USA. A distinct seismite horizon in Mid-Miocene Molasse sediments of southern Germany, genetically linked with the Ries impact event, exhibits dewatering structures and contains numerous sand spikes with tails systematically orientated away from the Ries crater. Sand spikes at Mount Signal, strikingly similar in shape to those found in Germany, have tails that point away from the nearby San Andreas Fault. Based on their structural and stratigraphic context, we interpret sand spikes as a new type of seismite and a promising tool to identify strong impact-induced or tectonic palaeo-earthquakes and their source regions in the geologic record.

Microbial Sulfur Isotope Fractionation in the Chicxulub Hydrothermal System.

Target lithologies and post-impact hydrothermal mineral assemblages in a new 1.3 km deep core from the peak ring of the Chicxulub impact crater indicate sulfate reduction was a potential energy source for a microbial ecosystem (Kring et al., 2020). That sulfate was metabolized is confirmed here by microscopic pyrite framboids with δ34S values of -5 to -35 ‰ and ΔSsulfate-sulfide values between pyrite and source sulfate of 25 to 54 ‰, which are indicative of biologic fractionation rather than inorganic fractionation processes. These data indicate the Chicxulub impact crater and its hydrothermal system hosted a subsurface microbial community in porous permeable niches within the crater's peak ring.

Globally distributed iridium layer preserved within the Chicxulub impact structure.

The Cretaceous-Paleogene (K-Pg) mass extinction is marked globally by elevated concentrations of iridium, emplaced by a hypervelocity impact event 66 million years ago. Here, we report new data from four independent laboratories that reveal a positive iridium anomaly within the peak-ring sequence of the Chicxulub impact structure, in drill core recovered by IODP-ICDP Expedition 364. The highest concentration of ultrafine meteoritic matter occurs in the post-impact sediments that cover the crater peak ring, just below the lowermost Danian pelagic limestone. Within years to decades after the impact event, this part of the Chicxulub impact basin returned to a relatively low-energy depositional environment, recording in unprecedented detail the recovery of life during the succeeding millennia. The iridium layer provides a key temporal horizon precisely linking Chicxulub to K-Pg boundary sections worldwide.

Earth's Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits.

This article presents a current (as of September 2019) list of recommended ages for proven terrestrial impact structures (n = 200) and deposits (n = 46) sourced from the primary literature. High-precision impact ages can be used to (1) reconstruct and quantify the impact flux in the inner Solar System and, in particular, the Earth-Moon system, thereby placing constraints on the delivery of extraterrestrial mass accreted on Earth through geologic time; (2) utilize impact ejecta as event markers in the stratigraphic record and to refine bio- and magneto-stratigraphy; (3) test models and hypotheses of synchronous double or multiple impact events in the terrestrial record; (4) assess the potential link between large impacts, mass extinctions, and diversification events in the biosphere; and (5) constrain the duration of melt sheet crystallization in large impact basins and the lifetime of hydrothermal systems in cooling impact craters, which may have served as habitats for microbial life on the early Earth and, possibly, Mars.

New discovery of two seismite horizons challenges the Ries-Steinheim double-impact theory.

The Nördlinger Ries and the Steinheim Basin are widely perceived as a Middle Miocene impact crater doublet. We discovered two independent earthquake-produced seismite horizons in North Alpine Foreland Basin deposits potentially related to both impacts. The older seismite horizon, demonstrated to be associated with the Ries impact, is overlain by distal impact ejecta in situ, forming a unique continental seismite-ejecta couplet within a distance of up to 180 km from the crater. The younger seismite unit, also produced by a major palaeo-earthquake, comprises clastic dikes that cut through the Ries seismite-ejecta couplet. The clastic dikes may have formed in response to the Steinheim impact, some kyr after the Ries impact, in line with paleontologic results that indicate a time gap of about 0.5 Myr between the Ries and Steinheim events. This interpretation suggests the Ries and Steinheim impacts represent two temporally separate events in Southern Germany that, thus, witnessed a double disaster in the Middle Miocene. The magnitude-distance relationship of seismite formation during large earthquakes suggests the seismic and destructive potential of impact-induced earthquakes may be underestimated.

Probing the hydrothermal system of the Chicxulub impact crater.

The ~180-km-diameter Chicxulub peak-ring crater and ~240-km multiring basin, produced by the impact that terminated the Cretaceous, is the largest remaining intact impact basin on Earth. International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drilled to a depth of 1335 m below the sea floor into the peak ring, providing a unique opportunity to study the thermal and chemical modification of Earth's crust caused by the impact. The recovered core shows the crater hosted a spatially extensive hydrothermal system that chemically and mineralogically modified ~1.4 × 105 km3 of Earth's crust, a volume more than nine times that of the Yellowstone Caldera system. Initially, high temperatures of 300° to 400°C and an independent geomagnetic polarity clock indicate the hydrothermal system was long lived, in excess of 106 years.