Strontium isotope composition of marine carbonates of Middle Triassic to Early Jurassic age, Lombardic Alps, Italy*

Abstract

The s7Sr/R6Sr ratios and strontium concentrations for thirty-three samples of marine carbonate rocks of Middle Triassic to Early Jurassic age have been determined. The samples were collected from four measured sections in the areas of Val Camonica in northern Italy. The strontium concentrations vary from 40 to 7000 ppm. Most of the samples are calcitic limestones containing less than 10% of non-carbonate residues. Dolomitic samples and those containing appreciable non-carbonate residues have significantly diminished strontium concentrations.The n7Sr/R6Sr ratios of the carbonate phases of these rocks appear to be unaffected by dolomitization and by the presence of non-carbonate minerals. The average a'Sr/a6Sr ratios of the formations vary systematically in a stratigraphic sense. The ratio increased from Early Anisian to Early-Middle Ladinian, declined during Late Ladinian and Carnian, rose again during the Norian and then declined throughout the Late Norian (Rhaetian), Hettangian, Sinemurian and Pliensbachian ages. The average B'Sr/s6Sr ratios, relative to 0.7080 for the Eimer and Amendstandard,are: Anisian:0.70805f 0.00019;Early Ladinian:0,7085+0.00038;Late Ladinian: 0.70791 +0.00013; Carnian: 0~70776+0~00015; Norian and Rhaetian: 0,70791 .+ 040014; Hettangian: 0~70762+0~00021 : Sinemurian: 0.7070&0-00038; Pliensbachian: 0.7070+ 0.00015. These variations reflect changes in the isotopic composition of Sr entering the oceans in early Mesozoic time due to varying rates of weathering and erosion of young volcanic rocks (low 87Sr/e6Sr) and old granitic rocks (high s7Sr/asSr). The data presented in this report contribute to a growing body of information regarding the changes that have occurred in the g7Sr/68Sr ratio of the oceans in Phanerozoic time.

Carbon cycle perturbation and stabilization in the wake of the Triassic‐Jurassic boundary mass‐extinction event

“…This formation is overlain by the “Ammonitico Rosso” of Sinemurian age, which consists of a finely bedded, gray to reddish, pelagic limestone with marl alternations. The section of Portirone (Lago d'Iseo, Lombardian Alps, Italy) consists of thick bedded, siliceous limestone of upper Hettangian age (Sedrina Formation), which are followed by lower Sinemurian thinly to thickly bedded, pelagic siliceous limestone [ Gaetani , 1970; Faure et al , 1978]. The section of Pozzo Glaciale (Lago d'Iseo, Lombardian Alps, Italy), which is situated 5 km to the south of Portirone, shows thickly bedded, oolitic and oncolithic carbonates of late Hettangian age at its measured base, which are topped by thinly bedded, marly and partly siliceous limestones of early Sinemurian age [ Gaetani , 1970; Faure et al , 1978].…”

Strontium isotope profile of Carboniferous-Permian Akiyoshi Limestone in southwest Japan.

“…Two possible sources are as follows: large continent with old sialic crust, and young volcanic rocks (mostly basalts) which erupted in the oceanic regions and in the con tinental margin areas (Spooner, 1976). The rocks of the former sources have an average 87Sr/86Sr value of 0.720, and those of the latter show generally low values (0.703-0.704) (Faure et al, 1978). One more important supply into marine Sr is the influx of Sr from carbonate sediments which precipitated in the ocean and those expos ed in the continent interior (Elderfield and Gieskes, 1982;Elderfield, 1986;Hess et al, 1986).…”

“…The Sr isotope com positions (87Sr / 86Sr ratios) of calcareous sediments have been recognized particularly to be a powerful tool in estimating the secular changes in paleoocean chemistry (Peterman et al, 1970;Veizer and Compston, 1974;Faure et al, 1978;Burke et al, 1982 and others). Recent detailed determinations of 87Sr/86Sr ratios on Cenozoic calcareous samples from DSDP drill cores enabled construction of exact 87Sr / 86Sr ratio of sea water-age curve, for the purpose of establishing Sr isotope stratigraphy (e.g., Elderfield and Gieskes, 1982;Koepnik et al, 1985;Palmer and Elderfield, 1985;DePaolo and Ingram, 1985;Depaolo, 1986;Hess et al, 1986;Hodell et al, 1989 and The Akiyoshi Limestone is a thick limestone unit included in the Akiyoshi terrane, an accre tionary complex in southwest Japan.…”

87Sr/86Sr Ratios of the Barremian and Early Aptian Seas

“…A previous study by Peterman et al (1970) has shown that the 87 Sr/ 86 Sr ratio of marine carbonates, which incorporate Sr during their deposition, varied systematically but also irregularly during Phanerozoic time, probably as a result of changes in the balance between continental and submarine weathering. Further studies of the Phanerozoic (Dasch and Biscaye, 1971;Boger et al, 1973;Brass 1973;Veizer and Compston, 1974;Faure et al, 1978) confirmed these conclusions.…”