Daniele Masetti

Rare earth elements in limestone/marlstone couplets from the Albian-Cenomanian Cismon section (Venetian region, northern Italy) : assessing REE sensitivity to environmental changes

Abstract We have studied the REE distribution through the Albian-Cenomanian interval of a stratigraphic succession (Cismon section, Scaglia Variegata Formation, Venetian Prealps) consisting of rhythmically alternating carbonate-rich and carbonate-poor strata which are the sedimentary expression of systematic variations of surface fertility and bottom-water redox conditions in the depositional basin. Limestone semicouplets show seawater-like shale-normalized patterns (HREE-enriched, with negative Ce anomalies and high Er/Nd ratios) indicating a clear dependence on the REE chemistry of surface seawaters. Marlstones and shales exhibit flatter patterns, more moderate Ce anomalies and low Er/Nd. Good correlations between Er/Nd, Ce/Ce∗ and Mn∗ suggest that, superimposed on REE signatures imparted by detrital and biogenic phases, the REE composition shows evidence of their sensitivity to palaeoredox conditions resulting in REE fractionation and Ce cycling. REE data are consistent with results from previous stratigraphic and geochemical (major and trace elements) investigations in reflecting palaeoenvironmental conditions rhythmically varying from efficiently mixed seawater during the limestone deposition to oxygen-depleted bottom waters at the time of marlstone deposition.

Orbitally induced limestone/marlstone rhythms in the Albian—Cenomanian Cismon section (Venetian region, northern Italy): Sedimentology, calcareous and siliceous plankton distribution, elemental and isotope geochemistry

Abstract A multidisciplinary study of the upper Albian—Cenomanian portion of the Cismon section (Venetian region, northern Italy) was undertaken in order to characterize the cyclic alternations of carbonate-rich and carbonate-poor layers and to investigate their possible origin and cyclic patterns. Limestone semicouplets are characterized by abundant radiolarians and micarbs (micron-sized calcitic fragments), common planktonic foraminifera, strong bioturbation, good oxygenation as expressed by the Mn* and V/(V+Ni) parameters, high Si/Al ratio, low K/Al, in the absence of pyrite and organic matter. The marlstone semicouplets are, on the contrary, frequently laminated, rich in pyrite and organic matter with common fish remains in the absence of radiolarians. The Mn* and V/(V+Ni) parameters indicate that the marlstone deposition occurred in dysoxic to anoxic conditions, which allowed the preservation of organic matter. The low Si/Al ratio and high K/Al ratio, the latter related to abundant feldspars, indicate that marlstones received a substantial contribution from a terrigenous source. The abundance of biogenic silica suggests that during limestone deposition surface waters were rather fertile due to an efficient recycling of nutrients from deeper waters. The enhanced fertility was coupled with an efficient current system at the welloxygenated sea floor which prevented the organic matter from accumulating. During marlstone deposition, surface waters were less fertile than in the limestone semicouplets, but still within the mesotrophic spectrum as indicated by the presence of fertility index species among calcareous nannofossils, whereas the bottom waters were oxygen-depleted. To accommodate fertility at the surface with stagnation at the bottom the scenario implies an enhanced stratification caused by an increase in freshwater runoff monitored by 18O-depleted δ values. In the meantime, the water discharge supplied the nutrients to the surface waters necessary for the calcareous nannofossils to proliferate. The cyclic organization of limestone/marlstone couplets as revealed by power spectral analysis seems to represent climatic cycles. The alternating conditions are (1) efficient mixing, highly fertile surface and near-surface waters, and a well-oxygenated seafloor to (2) enhanced water runoff or decreased evaporation, or both, and oxygen-depleted bottom waters. We suggest that this deterministic sedimentary cyclicity could be explained by periodic orbital-climatic cycles, in tune with the Milankovitch cycles. Within this scenario, the latest Cenomanian Bonarelli Level is an exceptional event marked by a large positive δ13C spike. It is characterized by high to very high fertility conditions in surface waters caused by increased upwelling that engendered high nutrient levels in near-surface waters. The enhanced production of organic matter was in excess for the oxygen available at the bottom floor, thus allowing the organic matter to accumulate in the sediments.

Deep-water asymmetric cycles and progradation of carbonate platforms governed by high-frequency eustatic oscillations (Triassic of the Dolomites, Italy)

The basinal San Cassiano Formation (Triassic, Dolomites, Italy) is interfingered with clinostratified megabreccia slope deposits of coeval carbonate platforms, and to a large extent is composed of metre-scale thickening, coarsening-upward cycles. These asymmetrical cycles, often representing bundles of five coarsening-upward sequences, are interpreted as platformbasin interactions governed by fourth- and fifth-order eustatic oscillations. According to this model, progradation of Triassic platforms of the Dolomites occurred mainly during fourthorder sea-level lowstands.

THE TRIASSIC/JURASSIC BOUNDARY IN A PERITIDAL CARBONATE PLATFORM OF THE PELAGONIAN DOMAIN: THE MOUNT MESSAPION SECTION (CHALKIDA, GREECE)

In the Mount Messapion area (Chalkida, Greece) a continuous and expanded section of Triassic/Jurassic (T/J) limestone is exposed. This section consists of a 710 m thick pile of shallowing-upward peritidal cycles; the persistence of the facies and the lack of paleoenvironmental changes across the T/J boundary allowed studying the distribution of shallow water microfossils. The T/J boundary is placed in the upper part of the section defined by correspondence with (i) the last occurrence of Triasina hantkeni , (ii) the abrupt disappearance of megalodontid faunas and (iii) the presence of Early Jurassic microfossil assemblages. This paleontological reorganization happens suddenly, it is not controlled by any facies change, and surprisingly, it seems to produce no evident modification in the vertical stacking pattern of the cycles. A detailed facies analysis, performed along a 290 m thick stratigraphic interval (60 m above the T/J boundary and 230 m below), allowed the recognition of peritidal cycles: five different elementary cyclothemes are described and their distribution along the section is given. This integrated stratigraphic study attempts to highlight the relationship between the changes of carbonate producers and sea level fluctuations across the T/J boundary.

Carbon-isotope anomalies and demise of carbonate platforms in the Sinemurian (Early Jurassic) of the Tethyan region: evidence from the Southern Alps (Northern Italy)

Despite its global impact on ecosystems, the Triassic/Jurassic boundary event had only a modest effect on the carbonate depositional systems of the Southern Alps, whereas a fundamental reorganization of the same palaeogeographic area took place during the Sinemurian Age. This paper investigates whether or not the well-documented demise of Sinemurian carbonate platforms in the Tethyan region was a response to a global event by examination of carbon-isotope anomalies in successions of different facies that record this interval of time. A chemostratigraphic transect from Lake Garda up to the eastern Italian border is illustrated by four stratigraphic sections; high-resolution (20 cm over key intervals) chemostratigraphic sampling allowed detection of a major negative δ 13 C anomaly of ~ 1.5‰, preceded by a positive excursion, both in shallow- and deep-water successions, over the stratigraphical range of the ammonite genus Arnioceras . A comparison with sections from the UK suggests that the positive excursion belongs to the turneri Zone and the succeeding negative excursion falls within the obtusum Zone. In the deep-water Belluno Basin, the negative anomaly occurs in a biogenic chert-rich unit recording the onset of mesotrophic conditions in the basin. In the platform-carbonate successions, this major negative carbon-isotope excursion is developed within a calcarenitic unit corresponding to the lowest occurrence of the foraminifer Paleomayncina termieri . This evidence for deepening and transgression across the carbonate platform suggests pre-conditioning for drowning. Hence, rather than tectonic subsidence alone, environmental factors may have aided the demise of Tethyan carbonate platforms during the Early Jurassic Sinemurian Age.