Cyclicities in Triassic platform carbonates: synchronizing radio‐isotopic and orbital clocks

Abstract

Sedimentary cycles recorded in young sediments are often attributed to fluctuations of the Earth's climate on a 104−106‐year scale which in turn is governed by periodic variations in solar insolation linked to orbital (Milankovitch) parameters. A spectacular example of cyclic stratal patterns in ancient deposits is the Middle Triassic Latemar carbonate platform (W Dolomites, N Italy). Based on spectral analyses from previous studies, a superimposition of precession (∼20 ka) and eccentricity (∼100 ka) controlled sea‐level fluctuations has been suggested to account for the stacking hierarchy at Latemar, with ∼20 ka being assigned to each highest‐order depositional cycle. Zircon U–Pb isotopic ages from volcanic‐ash layers within the cyclic succession, corroborated by biostratigraphic constraints, suggest that the average time interval for every individual cycle is significantly smaller than the shortest Milankovitch period and therefore challenge previously published interpretations relating distinct spectral peaks to the above mentioned hierarchy. However, our new spectral data indicate that cyclicities resembling Milankovitch characteristics might exist, but on an entirely different scale. Our findings show that frequency spectra should only be interpreted in combination with robust age control. They also encourage the search for complementary mechanisms controlling carbonate deposition.

Bio- and chronostratigraphy of the Middle Triassic Reifling Formation of the westernmost Northern Calcareous Alps

“…Accounting for this complication the latter ages were deemed minimum ages in Mundil et al (1996a). The recent re-analysis of a Latemar sample (Brack et al 2007) shows that similar shifts towards slightly older ages may be expected also for values obtained on older levels in the Buchenstein Formation and at Latemar (Mundil et al 1996a(Mundil et al , 2003. Therefore we are confident that our new radio-isotopic age from Flexenpass is consistent with the bio-and lithostratigraphy-based correlation of the upper Reifling and the Buchenstein Formations.…”

Forward modelling of carbonate platforms: Sedimentological and diagenetic constraints from an application to a flat-topped greenhouse platform (Triassic, Southern Alps, Italy)

“…The number of cycles in the platform interior facies of the Esino Limestone is between 700-800 (all thicker cycles) and c.2500 (all thinner cycles). Considering the thickness of the Esino Limestone and the number of cycles (even if considering no missed beats), the average duration of the cycles results sub-Milankovian, in agreement with values reported for other Triassic platforms in the Tethyan realm (Schwarzacher & Haas, 1986, Reijmer et al, 1991Mundil et al, 2003;). To simulate the observed stacking pattern of the platform interior facies we have added a high-frequency asymmetric sea-level curve (frequency: 5 ka).…”

The origin and early radiation of dinosaurs

“…The base of the Late Triassic is poorly dated: there are no precise radioisotopic ages from near the Ladinian-Carnian boundary and there is no published magnetostratigraphic record that crosses the boundary. An approximate age of 235 Ma for the Ladinian-Carnian boundary has been interpolated using records from earlier in the Ladinian (e.g., Mundil et al, 1996;Muttoni et al, 1997;Mundil et al, 2003;Brack et al, 2005). The Carnian-Norian boundary is constrained by a new U-Pb single crystal zircon age of 230.91 ±0.33 Ma from the Upper Carnian marine section at Pignola, Italy (Furin et al, 2006).…”