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[BCG07-07] Paleoenvironmental reconstruction using microscale distribution of chalcophile elements in the Cretaceous–Paleogene boundary clays
Keywords:meteorite impact, acid rain, chalcophile element
Cretaceous–Paleogene (K–Pg) boundary clays are enriched in both siderophile [1] and chalcophile elements[2]. The enrichment of siderophile elements in these clays is owing to the incorporation of meteorite condensates, as the elemental ratios of siderophile elements are similar to the chondritic values [1]. However, the ratios of chalcophile and siderophile elements (such as Zn/Ir, As/Ir, and Sb/Ir) are one to two orders of magnitude higher than those of chondrites [2], indicating that these clays include chalcophile elements that are unrelated to meteorite condensates. In contrast, 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 by using the chemical compositions of chalcophile elements in the K–Pg boundary clays, as the dissolution and precipitation of chalcophile elements is controlled by environmental factors, such as pH and oxidation–reduction potential. As these clays comprise chalcophile-enriched grains of various origins [3], it is important to obtain the microscale distribution of chalcophile elements. Therefore, we applied the laser-ablation inductively coupled plasma mass-spectrometry (LA–ICP–MS) method to analyze chalcophile elements in the Cretaceous-Paleogene (K–Pg) boundary clays from Stevns Klint, Denmark.
The ion intensity images obtained using LA–ICP–MS indicated that 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 [3]. Although both Cu- and Ag-enriched grains existed as discrete grains, there was a positive correlation between the concentrations of Cu and Ag in bulk samples from the K–Pg boundary, suggesting that a common process was involved in supplying these elements to sediments. As Cu and Ag generally exist in acid-soluble sulfides on Earth surface, they might have been supplied to seawater by intense acid rain immediately after the K–Pg impact.
The strong correlations between the ion intensities of chalcophile elements (such as Ni, Cu, Zn, As, Ag, Cd, and Pb) and Fe for pyrite grains imply a constant concentration of chalcophile elements in pyrite grains in the K–Pg boundary clays. The concentrations of chalcophile elements in bulk K–Pg boundary samples were higher than those in samples below and above the K–Pg boundary, whereas the concentrations of chalcophile elements in pyrite grains in K–Pg boundary samples were lower than those in samples below and above the K–Pg boundary. This indicates that these pyrite grains might have been produced in environments with higher Fe2+ concentrations than under normal conditions, i.e., before and after the K–Pg impact event. Such Fe might be supplied to oceans as iron oxide condensates after the K–Pg meteorite impact [4].
References [1] Kyte et al., 1980, Nature 288, 651-656; Schmitz, 1985, Geochim. Cosmochim. Acta, 49, 2361-2370; Schmitz, 1988, Geology, 16, 1068-1072; Schmitz, 1992, Geochim. Cosmochim. Acta 56, 1695-1703. [2] Gilmour and Anders, 1989, Geochim. Cosmochim. Acta, 53, 503-511. [3] Maruoka et al., 2020, GSA Bulletin 132, 2055–-2066. [4] Wdowiak et al., 2001, Meteorit. Planet. Sci., 36, 123-133; Ferrow et al., 2011, Geochim. Cosmochim. Acta, 75, 657-672.
The ion intensity images obtained using LA–ICP–MS indicated that 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 [3]. Although both Cu- and Ag-enriched grains existed as discrete grains, there was a positive correlation between the concentrations of Cu and Ag in bulk samples from the K–Pg boundary, suggesting that a common process was involved in supplying these elements to sediments. As Cu and Ag generally exist in acid-soluble sulfides on Earth surface, they might have been supplied to seawater by intense acid rain immediately after the K–Pg impact.
The strong correlations between the ion intensities of chalcophile elements (such as Ni, Cu, Zn, As, Ag, Cd, and Pb) and Fe for pyrite grains imply a constant concentration of chalcophile elements in pyrite grains in the K–Pg boundary clays. The concentrations of chalcophile elements in bulk K–Pg boundary samples were higher than those in samples below and above the K–Pg boundary, whereas the concentrations of chalcophile elements in pyrite grains in K–Pg boundary samples were lower than those in samples below and above the K–Pg boundary. This indicates that these pyrite grains might have been produced in environments with higher Fe2+ concentrations than under normal conditions, i.e., before and after the K–Pg impact event. Such Fe might be supplied to oceans as iron oxide condensates after the K–Pg meteorite impact [4].
References [1] Kyte et al., 1980, Nature 288, 651-656; Schmitz, 1985, Geochim. Cosmochim. Acta, 49, 2361-2370; Schmitz, 1988, Geology, 16, 1068-1072; Schmitz, 1992, Geochim. Cosmochim. Acta 56, 1695-1703. [2] Gilmour and Anders, 1989, Geochim. Cosmochim. Acta, 53, 503-511. [3] Maruoka et al., 2020, GSA Bulletin 132, 2055–-2066. [4] Wdowiak et al., 2001, Meteorit. Planet. Sci., 36, 123-133; Ferrow et al., 2011, Geochim. Cosmochim. Acta, 75, 657-672.