The Sweet Aftermath: Environmental Changes and Biotic Restoration Following the Marine Mjølnir Impact (Volgian-Ryazanian Boundary, Barents Shelf)

Abstract

During the Late Jurassic and earliest Cretaceous the Barents Shelf. was dominated by fine-grained clay sedimentation, with mostly anoxic to hypoxic depositional conditions. The stratified water-masses contained typically relatively rich, but low diversity, nectonic faunas and marine microfloras above the pycnocline. In contrast the benthic faunas contained only a few bivalve species and low diversity communities of foraminifera. At the time of the Volgian-Ryazanian boundary (142.2 +/- 2.6 Ma) a 1.5-2 km-diameter bolide hit the paleo-Barents Sea and created the 40 km-diameter Mjolnir Crater. The central peak of the crater formed an island, and the high standing crater rims and annular ridges further led to significant changes in the sea-bed topography. The impact and crater fori-nation led to significant disturbance and environmental changes, both at the crater site and over large distances of the paleo-Barents Shelf. Tsunamis were formed and travelled back and forth across the seas for a day or two after the impact. Continuing collapse of unstable, unconsolidated highs and rims formed avalanches, slumps and slides that developed into gravity flows in the crater surroundings. Computer simulations of ejecta formation and distribution indicate that major ejecta transportation occurred along the trajectory of the incoming bolide, i.e., toward the northeast. No evidence exists of any major biotic extinction or changes in diversity related to the impact event, but the overall compositions of the microfossil assemblages show a significant change within the impact-influenced strata. In the lowermost post-impact deposits in the Mjolnir Crater, and in association with the ejecta-bearing strata on the adjacent shelf, a conspicuous acme of the marine prasinophyte Leiosphaeridia combined with an influx of abundant juvenile freshwater algae of the genus Botryococcus occur. The prolific blooms of Leiosphaeridia suggest that these algae had a behavioral pattern typical for so-called disaster species. The recovery of the algal bloom in deposits off Troms, 500 kin to the south of the Mjolnir Crater, and on Svalbard, 450 km to the north, suggest that a regional eutrophication event was induced in the impact-ocean. The duration of the environmental change and the biotic turnover is currently difficult to estimate, but was most likely relatively short. Depositional conditions comparable to those found on the shelf prior to the impact (i.e., stratified water-masses, with anoxic hypoxic bottom conditions and low diversity marine benthic faunas) were restored during the earliest Ryazanian (i.e., prior to the time corresponding to the Heteroceras kochi ammonite zone).