Global Ballistic Sedimentology (W. Alvarez, 1996)

Global Ballistic Sedimentology (GBS) is a phrase coined by renown geochemist and biostratigrapher Walter Alvarez in the 1996 paper "Trajectories of ballistic ejecta from the Chicxulub Crater". The phrase describes the study of impact ejecta alteration, suborbital ballistic transport and fall patterns resulting from large planetary impact events on Earth. Fall pattern convolution over the rotating Earth, along with how the ejecta is altered and where it is found in the column relative to other co-located impact-related material are each important factors affecting the observable imprint.

Individual scientific camps or disciplines often develop their own language and sub-cultures within the larger scientific community. These factors can lead to 'fence building' or barriers between camps, impeding or preventing healthy collaboration and idea sharing. In his 1990 paper regarding the importance of interdisciplinary research for planetary impact problems like crater hunting and identification, Dr. Alvarez explains the need for collaboration between different scientific camps, in order to leverage the strength of all camps in a structure larger than each single discipline. Even as a geochemist, Dr. Alvarez knew he needed dynamically correct suborbital transport assessment to treat the problem in references and of the mid 1990s, referring back to a 1981 planetary science paper by Dr. Anthony Dobrovolskis, "Ejecta patterns diagnostic of planetary rotations". Reference is the original public sector treatment of inverse square gravity with planetary rotation, presented cleanly in non-dimensional format for application to various bodies in the solar system to estimate planetary rotation rate history and evolution.

Dr. Alvarez realized the suborbital ejecta transport problem is a critically important part of the geologic imprint, just like the location of the ejecta, the stratigraphic description of ejecta in the geologic column, and the imprinted petrographic content of impact and transport alteration. While working on the 66 million year old K-Pg stratigraphic boundary and related extinction of the dinosaurs, Walter Alvarez realized a single study needed to include suborbital transport of shocked and heated geo-materials over global distances, as well as the forensic details of how they became emplaced upon falling back to Earth, and their state of alteration or petrographic history. This was the only way to unlock unknown source locations of globally significant impact ejecta layers buried within Earth's geologic history, with such energetic governing processes often responsible for extinctions and global climatological upheaval at the larger scales being considered.

The dinosaur extinction of the K-Pg layer is just one relevant example, with the location of that impact eventually located at Chicxulub Mexico thanks to tireless work of Walter Alvarez and his father Luis Walter Alvarez. A much more recent unresolved giant terrestrial impact event corresponds to the Australasian tektite distal impact ejecta melt of only 789 thousand years ago, with some presentations in impact surge deposits of shocked, unmelted impact clastics covering large areas across northeast Thailand per Tada et al.. The Australasian tektite event produced tens of billion of tons of vacuum processed silicate melt from near-surface sediments, ejected at high-energy early in the excavation, outside the regime of proximal blanket deposits ejected later in cratering excavation process. A majority of Australasian tektite melt was accelerated to 10 km/s or more, suggesting vastly distal fall locations across a convoluted global pattern from the source per reference , which is sometimes referred to as 'the sword of Dobrovolskis' for its hard truths of suborbital physical science. Applications of proximal ejecta blanket models to the central area of the tektite strewnfield have proved fruitless in the source structure search, most likely due to application of a proximal ejecta coverage model (ejecta blanket) to tektites which are distal ejecta. The Australasian tektite event is one of the few large impact events on Earth lacking dynamically correct suborbital modeling of ejecta transport - apparently there are no peer-reviewed papers using the accepted reference for that task of modeling that strewnfield based on NASA's body of 1960s tektite ablation research described in .

In the 1996 paper , Dr. Alvarez explained the unique combination of inverse-square gravity and Earth's rotation relative to the plane of a suborbital trajectory in inertially fixed space. The 1996 work and companion paper Alvarez et al. dated 1995 both demonstrate 'fold-back' of the ejecta fall pattern originating from K-Pg-contemporary Chicxulub location initiating in the fall pattern by 9 km/s based on the dynamically correct simplified two-body model, per Figure 8C of reference , and Figure 1 of reference . This is problematic when assessing ejecta fall sites containing both distal ejecta melt (vacuum processed) and unmelted target fragments. Due to fold-back convolution also identified in reference [4], area concentrations and ejecta alteration may vary widely across Earth's surface, including zero-coverage areas and areas of increasing concentration with increasing radial distance from the ejecta source or impact site (the opposite of proximal ejecta blanket trends). In the case of "reentry traffic pile-up", the sequence of geologic strata must be considered to assess relative arrival time of various ejecta, and therefore relative loft duration or Time of Flight.

Alvarez, Claeys, Kieffer propose a multi-modal ejection sequence for the K-Pg blast, modulated by warm/hot volatile expansion behind the ejecta curtain boosting ejecta to 6 & 7 km/s or more, top explain the observed sequences they describe. More recently, DePalma et al. (2019) estimate that the fall rate of solidified Chicxulub impact melt spherules at the Tanis site in North Dakota would have peaked in intensity between 1 and 2 hours after the ~3000 km-distant event. The melt spherules arrived after seismic disruption had displaced fish in an onshore surge deposit and the water had shallowed sufficiently to strand the long narrow fish with their bodies aligned to the runoff flow. The fish drew shallow water with impact melt spherules into their gills during their final living moments, collecting the spherules neatly for geologists to find 66 million years later.

The greater the distance from a giant impact on Earth, the greater the chance that seismic energy will arrive at that location before suborbital ejecta, mainly due to slow speed and associated extended loiter around apogee compared to seismic waves through Earth that start out fast near the surface and have higher wave speed through Earth's interior. These are the types of relationships that help researchers doing Global Ballistic Sedimentology. Because the study involves geologic interpretation down to the microscopic level within the geologic column as well as some fundamentals of astrodynamics, practitioners must 'look both ways, both up and down, to get the big picture'.

Each of the imprinted features of cosmic impact ejecta transport provides requirements and constraints on the process and location of impact event. Kinetic Energy (KE) "partitioning" during impact event takes place where the cosmic projectile reaches Earth, and it's astronomically high KE is divided into various forms of work and heat acting on itself and the portion of Earth it directly collides with. The huge energy release of partitioning causes geo-materials to be broken up, melted or even vaporized, with some ejected into the atmosphere and beyond into space. When more-altered ejecta is dispatched from the impact site with higher speed, this fraction stays aloft the longest, which can also lead to odd distribution across Earth's surface in the governing dynamic of large impacts. This is because suborbital launch at speed of 10 km/s or 80% of Earth's escape KE guarantees loft duration of 3.5 to 12 hours depending on launch angles and location. Reference show a strong dependence of semi-major axis a as a function of launch speed in Figure 3, with loft duration primarily a function of semi-major axis and also eccentricity e. This brings counterintuitive results when ejecta goes up to the east but loiters in the overhead for so long while Earth is rotating beneath that the ejecta ends up landing to the west of the launch point, per Figure 8C of reference , and Figure 1 of reference . It is a unique regime when objects slam into Earth so hard that near-Earth space gets congested with debris launched above Earth's atmosphere by the impact.

References

[1] Walter Alvarez (1996), Trajectories of ballistic ejecta from the Chicxulub Crater, Geological Society of America Special Papers 307, https://doi.org/10.1130/0-8137-2307-8.141

[2] Walter Alvarez (1990), Interdisciplinary aspects of research on impacts and mass extinctions; A personal view, Geological Society of America Special Papers 247, https://doi.org/10.1130/SPE247-p93

[3] Walter Alvarez, Philippe Claeys, Susan W. Kieffer (1995) Emplacement of Cretaceous-Tertiary Boundary shocked Quartz from Chicxulub Crater, Science Vol 269, ppp. 930-935, 18 Aug 1995, DOI: 10.1126/science.269.5226.930

[4] Anthony Dobrovolskis (1981), Ejecta patterns diagnostic of planetary rotations, Icarus vol. 47, issue 2, https://doi.org/10.1016/0019-1035(81)90167-6

[5] Tada et al. (2022), Identification of the ejecta deposit formed by the Australasian Tektite Event at Huai Om, northeastern Thailand, Meteoritics and Planetary Science, vol. 57, issue 10, https://doi.org/10.1111/maps.13908

[6] N. Artemieva, (2012) Numerical Modeling of the Australasian Tektite Strewn Field, 44th Lunar and Planetary Science Conference (2013), https://www.lpi.usra.edu/meetings/lpsc2013/pdf/1410.pdf

[7] D. K. Pal, C. Tuniz, R. K. Monist, T. H. Kruse, G. F. Herzog, (1982) Beryllium-10 in Australasian Tektites: Evidence for a Sedimentary Precursor, Science vol. 218, 19 Nov 1982, https://www.jstor.org/stable/1689753

[8] D. R. Chapman, H. K. Larson (1963), Lunar Origin of Tektites (aerodynamic heating) NASA Technical Note D-1556, https://ntrs.nasa.gov/api/citations/19630003053/downloads/19630003053.pdf

[9] R. A. DePalma, J. Smit, D. A. Burnham, W. Alvarez, (2019) A seismically induced onshore surge deposit at the KPg boundary, North Dakota, PNAS, https://doi.org/10.1073/pnas.1817407116

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