PRESERVATION OF FLORAL AND FAUNAL REMAINS IN IMPACT MELTS

The Argentine Pampas contains a record of melt breccias formed by at least 8 impacts over the last 10 Myr. We have confirmed the impact origin of these melts through detailed petrographic/geochemical studies and have begun to identify the craters likely associated with them. We also have documented the well-preserved remains of late Miocene-Pleistocene vegetation in glasses from 6 deposits. One mid-Pleistocene glass even contains rare examples of cartilaginous material. These unique remains are not impressions or casts. They largely are intact inorganic skeletons, which commonly display fine, internal, 3D anatomical details. We are investigating whether they may sequester a signature of organics and volatiles.

We refer to these glasses as “impact amber” because, like resinous amber, they entomb organisms in a viscous liquid. Evidence supports the conclusion that our examples captured samples of extant, or very recently deceased, biota rather than entraining pre-existing fossils. First, there is a paucity of preserved grasses in target strata. Second, many of the remains retain structural details that do not survive long exposure and reworking in most sedimentary environments. Finally, most exhibit evidence that volatiles were released in the preservation process.

Dramatically demonstrated by the experiments discussed below, these fossils exist due to three key factors: 1) rapid, ultra-high heating; 2) high heat capacity of original volatiles; and 3) rapid quenching.

We conducted high T experiments up to 1600ºC in order to investigate the response of grass and cartilage to: 1) flash heating; 2) melting of encasing sediments; and 3) contact with a melt surface (simulating ejecta landing on an organism). The experiments showed not only that the inorganic parts of plants and animals can withstand extreme temperatures, but their delicate structures are more likely to survive at ultra-high temperatures (>1500ºC) than at lower temperatures (<1200ºC) where slow combustion rips them apart. Melting under reducing conditions may even preserve some organics.

The realization that impact melts can trap and preserve biologic remains suggests an obvious strategy for astrobiology. We propose that impact melts on other planetary surfaces, particularly Mars, should be primary targets to search for evidence of past life.