Klimat i oceanografia przełomu jury i kredy zachodniej Tetydy
Abstract
Climate and oceanography of the Jurassic/Cretaceous transition in the Western Tethys.
A b s t r a c t. The key aspect for evaluation of potential effects of ongoing environmental changes is identification of their controls on one hand, and understanding of their mutual relations on other. In this context, the best source of information about medium and long term consequences of various environmental processes is the geologic record. Numerous different-scale palaeoenvironmental events took place during the Jurassic/Cretaceous transition; amongst them, the best documented so far are: long term marine regression during the Tithonian–early Berriasian, climate aridization during the late Tithonian–early Berriasian, and tectonic activity in western parts of the NeoTethys Ocean during the late Berriasian–Valanginian. This study, which is based on the PhD dissertation of Damian Gerard Lodowski, attempts to reconstruct the latest Jurassic–earliest Cretaceous paleoenvironment and its evolution in the area of the Western Tethys, with special attention paid to cause-and-effect relationships between climate changes, tectonic activity and oceanographic conditions (perturbations in marine circulation and bioproductivity). Here are presented the basic results of high resolution geochemical investigations performed in the Transdanubian Range (Hárskút and Lókút, Hungary), High-Tatric (Giewont, Poland) and Lower Sub-Tatric (Pośrednie III, Poland) series, Pieniny Klippen Belt (Brodno and Snenica, Slovakia; Velykyi Kamianets, Ukraine) and Western Balkan (Barlya, Bulgaria) sections. The sections were correlated and compared in terms of paleoredox conditions
(authigenic U), accumulation of micronutrient-type element (Zn) and climate changes (chemical index of alteration, CIA), providing a consistent scenario of the Tithonian–Berriasian palaeoenvironment evolution in various western Tethyan basins. Amongst the first-order trends and events, characteristic of studied sections are the two intervals recording an oxygen deficient at the seafloor: 1) the upper Tithonian–lowermost Berriasian (OD I); and 2) at the lower/upper Berriasian transition (OD II). Noteworthy, this phenomena cooccurred with elevated accumulations of nutrient-type elements (i.e. enrichment factor of Zn). Besides, collected data document the late Tithonian–early Berriasian trend of climate aridization, as well as the late Berriasian humidification. Such record is explained by a model, in which decreasing intensity of atmospheric circulation during the late Tithonian–early Berriasian was directly connected with climate cooling and aridization. This process resulted in lesser efficiency of up- and/or downwelling currents, which induced seawater stratification, seafloor hypoxia and perturbations in the nutrient-shuttle process during the OD I. On the other hand, the OD II interval may correspond to tectonic reactivation in the NeoTethyan Collision Belt. This process might have led to physical cutoff of Alpine Tethys basins from the NeoTethyan circulation (both atmospheric and oceanic), driving the limited stratification in the former, and limiting the effect of gradual humidification of global climate (i.e. due to increasing strength of monsoons and monsoonal upwellings).
A b s t r a c t. The key aspect for evaluation of potential effects of ongoing environmental changes is identification of their controls on one hand, and understanding of their mutual relations on other. In this context, the best source of information about medium and long term consequences of various environmental processes is the geologic record. Numerous different-scale palaeoenvironmental events took place during the Jurassic/Cretaceous transition; amongst them, the best documented so far are: long term marine regression during the Tithonian–early Berriasian, climate aridization during the late Tithonian–early Berriasian, and tectonic activity in western parts of the NeoTethys Ocean during the late Berriasian–Valanginian. This study, which is based on the PhD dissertation of Damian Gerard Lodowski, attempts to reconstruct the latest Jurassic–earliest Cretaceous paleoenvironment and its evolution in the area of the Western Tethys, with special attention paid to cause-and-effect relationships between climate changes, tectonic activity and oceanographic conditions (perturbations in marine circulation and bioproductivity). Here are presented the basic results of high resolution geochemical investigations performed in the Transdanubian Range (Hárskút and Lókút, Hungary), High-Tatric (Giewont, Poland) and Lower Sub-Tatric (Pośrednie III, Poland) series, Pieniny Klippen Belt (Brodno and Snenica, Slovakia; Velykyi Kamianets, Ukraine) and Western Balkan (Barlya, Bulgaria) sections. The sections were correlated and compared in terms of paleoredox conditions
(authigenic U), accumulation of micronutrient-type element (Zn) and climate changes (chemical index of alteration, CIA), providing a consistent scenario of the Tithonian–Berriasian palaeoenvironment evolution in various western Tethyan basins. Amongst the first-order trends and events, characteristic of studied sections are the two intervals recording an oxygen deficient at the seafloor: 1) the upper Tithonian–lowermost Berriasian (OD I); and 2) at the lower/upper Berriasian transition (OD II). Noteworthy, this phenomena cooccurred with elevated accumulations of nutrient-type elements (i.e. enrichment factor of Zn). Besides, collected data document the late Tithonian–early Berriasian trend of climate aridization, as well as the late Berriasian humidification. Such record is explained by a model, in which decreasing intensity of atmospheric circulation during the late Tithonian–early Berriasian was directly connected with climate cooling and aridization. This process resulted in lesser efficiency of up- and/or downwelling currents, which induced seawater stratification, seafloor hypoxia and perturbations in the nutrient-shuttle process during the OD I. On the other hand, the OD II interval may correspond to tectonic reactivation in the NeoTethyan Collision Belt. This process might have led to physical cutoff of Alpine Tethys basins from the NeoTethyan circulation (both atmospheric and oceanic), driving the limited stratification in the former, and limiting the effect of gradual humidification of global climate (i.e. due to increasing strength of monsoons and monsoonal upwellings).