BIOMINERALIZATION AND GLOBAL CHANGE: BIOTIC RESPONSE TO THE GONDWANAN GLACIATION AND SUBSEQUENT CLIMATE WARMING

The late Palaeozoic, a time-interval of roughly 100 million years comprising the Carboniferous and the Permian, has been the theatre of dramatic climate changes. Starting from greenhouse climatic conditions, this time-interval witnessed the transition to icehouse conditions in the late Visean, with the onset of the Late Palaeozoic Ice Age, one of the major glacial events occurred in the Phanerozoic. By the Sakmarian (Early Permian) this long and dynamic glacial phase came to an end, and Earth’s climate returned to greenhouse conditions, punctuated by sporadic cool phases, before the occurrence of the final hothouse in the end-Permian, which lead to the most severe mass extinction of the Phan-erozoic. Given this wide array of climatic conditions that have been recorded by stratigraphic successions all over the world, the late Palaeozoic represents one of the best laboratories to analyse past climate changes in order to contribute to the understanding of present climate change challenges. Brachio-pods are considered one of the best archives for palaeoclimatic proxies, and their shell microstruc-ture, biomineralization processes, and geochemistry are intensively studied to refine our ability to in-fer past climatic and environmental changes from their shell, in order to understand and manage cur-rent climatic changes. In this perspective, seasonality represents one of the important climatic pa-rameters to investigate, as it influences species evolution and distribution and it is deeply intertwined with the climate and its changes. However, studies of palaeoseasonality in deep time are very scarce. For this reason, this thesis deals with the analysis of the record of seasonality in brachiopod shells from the late Palaeozoic, in order to assess their accuracy in recording climatic changes at the high-est resolution possible. Chapter 1 of the thesis presents an overview of the current knowledge on brachiopod shell micro-structure and biomineralization, and the state of the art of brachiopod shell geochemistry-based pal-aeoenvironmental and palaeoclimatic reconstructions. In this chapter a description of the late Palaeo-zoic climate, and the relationship between seasonality and climate changes are also presented. Chapter 2 describes the work phases in which the research was divided, with detailed description of the methods used for microstructural analysis, diagenetic screening and geochemical analyses. The full list of the material studied in this thesis and the geological setting of each locality from which the brachiopod specimens were collected is also provided in this chapter. Chapter 3 includes the results of the microstructural analyses of the 88 brachiopod shells analysed in this thesis, as well as the results of the first step of the diagenetic screening, the cathodolumines-cence analysis. The resulting data have been used to select the best-preserved brachiopod shells for further diagenetic screening analyses. The microstructural analyses of these brachiopods showed that, as expected, the primary layer is very rarely or never preserved in the fossil material. The sec-ondary layer preservation is not correlated to the type of fabric, but the presence of tertiary layer was observed more frequently in species with a fibrous secondary layer (Rhynchonellata) than in those with a laminar fabric (Strophomenata). Intercalations of secondary and tertiary layer were also more frequently observed in the species with a fibrous fabric. Silicification is highly locality-dependant, and, among other, the brachiopods from the upper Tournaisian Mobarak Fm. (Bahrammanesh et al., 2011) and the upper Kungurian-lower Roadian Qarari Unit (Viaretti et al., 2022) are silicified. Silici-fication is however almost always incomplete, and in these cases the microstructure unaffected ap-pears to be well-preserved. Chapter 4 presents a pioneering geochemical study of the partially silicified shells from the Mobarak Fm. and the Qarari Unit. Silicification is yet to be fully understood, but it is more frequently ob-served in the Palaeozoic. Limited research was conducted in this field, and it was mainly focused on the crystallographic and mineralogical features of the silica replacements or the stratigraphic pat-terns of silicification occurrence. Partially silicified brachiopod shells have been rarely reported (Holdaway & Clayton, 1982) and the geochemistry of the calcite shell remains has never been ana-lysed. In order to understand whether these well-preserved microstructure relics preserve a pristine geochemical signal, their microstructure was accurately analysed with SEM. Also, a new technique, based on δ11B analyses of these brachiopod shells with laser ablation, has been tested on the calcite shell remains. Boron is a highly mobile element that can be used as a tracer of imprint of diagenetic alteration on fossil shells (Joachismki et al., 2005). The results of these analyses show that silicifica-tion can affect in different ways the brachiopod shells, and three silicification patterns have been de-scribed: amorphous film-like, fully crystalline plague-like and isolated megaquartz. The SEM, CL, and EDS analyses revealed that the calcite microstructure is well-preserved in most cases, and that the silica-replaced shell portions are enriched in Sr. The δ11B results from the shell remains showed that these shell portions retain measurable concentrations of [B], and the isotopic signature is in line with the values measured on well-preserved brachiopods from the same time-interval (Jurikova et al., 2020a). These results demonstrate that partial silicification can preserve portions of shell microstruc-ture and shield them from the influence of other diagenetic processes. These results unlock the poten-tial of several renowned brachiopod collections that in the past have always been excluded from pal-aeoclimatic studies. Chapter 5 presents a case study of brachiopod sclerochemistry. Eleven specimens of Araxilevis in-termedius, a large sized and thick shelled brachiopod species, have been selected from correlatable beds of the Wuchiapingian Julfa and Hambast Fms. After the diagenetic screening which involved SEM, petrographic analysis, CL, EBSD and element/Ca analyses the shells were samples on a section along the growth axis to collect powders for δ18O and δ13C analyses. The results of the sclerochemi-cal analyses revealed a decreasing trend in Δδ18O from the base to the top of the analysed interval. The brachiopod-based palaeothermometer of Brand et al. (2019) was applied to the resulting Δδ18O and revealed a high amplitude of temperature seasonality (~6°C) in the lower Wuchiapingian Clarki-na asymmetrica Zone, higher than the seasonal variation currently experienced at similar low palaeo-latitudes and settings. A return to the expected values of the seasonal variation is then recorded in the C. transcaucasica Zone, at the top of the studied interval. The high amplitude of temperature season-ality in the early Wuchiapingian is consistent with the occurrence of a Wuchiapingian cooling phase, resulting from the emplacement of Emeishan LIP and its weathering (e.g., Chen et al., 2011; Wang et al., 2020; Sun et al., 2022). The return to low seasonality indicates the end of the cooling phase, sug-gesting a ~2 My duration for this event. These results agree with the timing proposed for the cooling phase by studies based on different proxies, demonstrating that brachiopods record changes in tem-perature seasonality and can be used to refine at a high-resolution the climatic events occurred in the deep time. Chapter 6 follows the case study of Chapter 5 and illustrates the sclerochemical analyses performed on selected brachiopod shells from different stratigraphic successions from the late Tournaisian to the late Sakmarian, bracketing the Late Palaeozoic Ice Age. The analysis was performed on nine shells belonging to six different species and selected, after a careful microstructural and diagenetic screening, from different stratigraphic successions recording different time-intervals and palaeolati-tudinal and depositional settings: Unispirifer (Unispirifer) striatoconvolutus from the upper Tour-naisian Mobarak Fm. (Iran), Frechella sp. from the lower Visean Mobarak Fm. (Iran), Frechella sp. and Echinoconchus sp. from the Serpukhovian Dozdehband Fm. (Iran), Choristites sp. from the lower Moscovian Qezelqaleh Fm. (Iran), Choristites aff. mosquensis from the middle Moscovian Jandaq Fm. (Iran) and Pachycyrtella omanensis from the middle-upper Sakmarian Saiwan Fm (Oman). The results showed that some of these shells accurately recorded the climatic changes occurred in the time-interval, with the exception of the specimens from the Tournaisian which are probably altered. In particular, brachiopods from the lower Visean to the Moscovian recorded an increasing trend in Δδ18O, indicating an increase in temperature seasonality from ~5°C to ~13°C. Compared with data from modern oceans at similar latitudes and depth, the scherochemical profiles record the transition to icehouse conditions after the early Visean, with glacial advance in the Serpukhovian-early Mos-covian. The specimens from the middle Moscovian, on the other hand, indicate warmer average tem-peratures and lower seasonality, possibly recording a warmer interglacial episode which was fol-lowed by the late Moscovian warming. The seasonality observed in the specimen Pachycyrtella omanensis (OL129-1) from the upper Sakmarian Saiwan Fm. is high and it is also observed in the δ13C profile. The high amplitude oscillations recorded by the δ18O and δ13C profiles are likely result-ing from the interplay of temperature, continental runoff, productivity and possibly glacial meltwater following the intense deglaciation occurring from the early Sakmarian. A sclerochemical δ11B profile was also measured on this specimen, and shows that δ11B also record oscillating values, which trans-late to pH variations. The frequency of these oscillations, however, is double compared to the δ18O and δ13C profiles and might be an indication of vital effects superimposed on the seasonal signal. Finally, in Chapter 7 the main results achieved with this work are synthetised, which prove that bra-chiopod shells, if analysed in detail and properly screened, are good archives of climatic proxies to understand paleoseasonality and climate change.