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[BCG06-24] Paleoenvironmental reconstruction using chalcophile element compositions in the Cretaceous–Paleogene boundary clays
Keywords:meteorite impact, chalcophile elements, LA–ICP–MS
The mass extinction at the Cretaceous–Paleogene (K–Pg) boundary was caused by environmental changes associated with a meteorite impact. The following environmental perturbations have been proposed [1]: (1) interception of sunlight by dust particles, followed by a temperature decrease and suppression of photosynthesis; (2) interception of sunlight and acid rain induced by sulfate aerosols originating from SO2/SO3 emitted from the impact site; (3) temperature increase due to aerodynamic heating induced by falling the meteorite and impact debris, followed by global wildfires and the interception of sunlight by the consequent soot emissions; and (4) global warming caused by the CO2 emissions resulting from the aforementioned global wildfires and/or the impact heating of carbonate rocks. However, owing to the lack of materials that record the environment just after the impact event, it has not yet been determined which of these phenomena occurred and at what scales they occurred.
The K–Pg boundary clays were enriched in both siderophile and chalcophile elements. The enrichment of siderophile elements in these clays was due to the incorporation of meteorite condensates, judging from elemental ratios similar to the chondritic values. However, the concentrations of chalcophile elements in the boundary clays were a few orders of magnitude higher than those expected from the chondritic condensates. Therefore, the boundary clays included chalcophile elements unrelated to meteorite condensates. Nevertheless, the coexistence of siderophile and chalcophile elements in the K–Pg boundary clays implies that the processes inducing chalcophile enrichment occurred immediately after the K–Pg meteorite impact. Therefore, the environmental conditions immediately after the K–Pg meteorite impact may be reconstructed using the chemical compositions of chalcophile elements in the K–Pg boundary clays. Because these clays comprise chalcophile-enriched grains of various origins [2], it is important to determine the microscale distribution of the chalcophile elements. Therefore, we applied the laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) method to analyze the chalcophile elements in the K–Pg boundary clays from Stevns Klint, Denmark.
The ion intensity images obtained using LA–ICP–MS indicated that the chalcophile elements were distributed in three phases: pyrite with trace amounts of chalcophile elements and discrete Cu- and Ag-enriched grains, as reported in a previous study [2]. Although correlations between the ion intensities of chalcophile elements (such as Ni, Cu, Zn, As, Ag, Cd, and Pb) and Fe in pyrite grains were observed, the slopes for higher Fe intensities were different from those for lower intensities. This implies that the concentrations of chalcophile elements in pyrites with a higher Fe intensity, that is, larger grains, were different from those in pyrites with a lower Fe intensity, that is, smaller grains. This indicates that the larger pyrite grains were produced in environments different from those in which the smaller pyrite grains were produced.
References [1] Maruoka, T., 2019, Mass Extinction at the Cretaceous–Paleogene (K–Pg) Boundary, in Yamagishi et al. (eds.), Astrobiology: From the Origins of Life to the Search for Extraterrestrial Intelligence, 303-320. [2] Maruoka et al., 2020, GSA Bulletin 132, 2055-2066.
The K–Pg boundary clays were enriched in both siderophile and chalcophile elements. The enrichment of siderophile elements in these clays was due to the incorporation of meteorite condensates, judging from elemental ratios similar to the chondritic values. However, the concentrations of chalcophile elements in the boundary clays were a few orders of magnitude higher than those expected from the chondritic condensates. Therefore, the boundary clays included chalcophile elements unrelated to meteorite condensates. Nevertheless, the coexistence of siderophile and chalcophile elements in the K–Pg boundary clays implies that the processes inducing chalcophile enrichment occurred immediately after the K–Pg meteorite impact. Therefore, the environmental conditions immediately after the K–Pg meteorite impact may be reconstructed using the chemical compositions of chalcophile elements in the K–Pg boundary clays. Because these clays comprise chalcophile-enriched grains of various origins [2], it is important to determine the microscale distribution of the chalcophile elements. Therefore, we applied the laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) method to analyze the chalcophile elements in the K–Pg boundary clays from Stevns Klint, Denmark.
The ion intensity images obtained using LA–ICP–MS indicated that the chalcophile elements were distributed in three phases: pyrite with trace amounts of chalcophile elements and discrete Cu- and Ag-enriched grains, as reported in a previous study [2]. Although correlations between the ion intensities of chalcophile elements (such as Ni, Cu, Zn, As, Ag, Cd, and Pb) and Fe in pyrite grains were observed, the slopes for higher Fe intensities were different from those for lower intensities. This implies that the concentrations of chalcophile elements in pyrites with a higher Fe intensity, that is, larger grains, were different from those in pyrites with a lower Fe intensity, that is, smaller grains. This indicates that the larger pyrite grains were produced in environments different from those in which the smaller pyrite grains were produced.
References [1] Maruoka, T., 2019, Mass Extinction at the Cretaceous–Paleogene (K–Pg) Boundary, in Yamagishi et al. (eds.), Astrobiology: From the Origins of Life to the Search for Extraterrestrial Intelligence, 303-320. [2] Maruoka et al., 2020, GSA Bulletin 132, 2055-2066.