Oscillating redox conditions in the Middle–Late Jurassic Alpine Tethys: Insights from selected geochemical indices and 57Fe Mössbauer spectroscopy

Abstract

Abstract: The Middle–Late Jurassic greenhouse conditions included CO2 levels that have been estimated to be four to five times higher than they are presently. By analysing the pattern of selected trace elements as redox proxies (V, U, and Mo) and the variations in the chemical state of iron using 57Fe Mossbauer spectroscopy, we demonstrate the occurrence of major changes in sediment redox conditions from the Late Bajocian through the Late Oxfordian in the pelagic environment of the Alpine Tethys in its Carpathian domain. Our interpretation is based on analysis of siliceous limestones rich in radiolarians intercalated with red nodular limestones of Ammonitico Rosso-type facies from the Križna Nappe in the eastern part of the Tatra Mountains, Poland. The presented redox potential (Eh values) shows its changeable pattern along the section (from −0.74 V to 0.16 V). The low values of Eh (from −0.74 V to −0.4 V) create the long-term (ca. 7 Myr) trend of the medium-scale fluctuations comprising the Upper Bajocian through the Lower Oxfordian interval of predominately grey and green radiolarian siliceous limestones. This interval comprises the three short-term (less than 0.5 Myr) periods of slightly increasing Eh (from −0.35 V to −0.17 V) observed in the uppermost Bajocian, Lower Bathonian, and Lower Callovian, as well as the significant changes in sediment redox conditions from oxic to dysoxic in the Upper Bajocian. The overlying succession corresponding to the Middle–Upper Oxfordian (ca. 4 Myr) is characterized by red radiolarian siliceous limestones with positive Eh values that indicate deposition under more oxic conditions than those during any other period in the study section. The negative values of Eh, attributed to the upwelling periods, coincide with the decreasing sea surface water temperature (SST) and increasing organic matter flux. The largest negative shift is noted in the Upper Callovian, which coincided with the cool climatic phase. The positive Eh values indicate oxic conditions due to the lowering of organic matter input, coinciding with an increased SST caused by surface water stratification. Based on the observed long-term variation in the redox, we postulate thermally driven east–west atmospheric circulation along the equatorial Tethys Ocean, caused by the sharp contrast in sea surface temperature, similar to the Walker Circulation operating across the modern Pacific. This phenomenon would have been the dominant periodical mode over the tropics and subtropics regions of the NW Tethys during the Middle–Late Jurassic. Global warming reduces the long-term strength of the mean tropical atmospheric circulation, leading to sea water stratification near the surface and consequently lowering the organic matter flux. However, additional seaways of heat transport could also exist, as they are associated with the opening/closing of the Hispanic Corridor.