BIOMINERALIZATION AND GLOBAL CHANGES: BRACHIOPOD SHELLS AS ARCHIVES OF THE END PERMIAN EVENTS

The Permian has been the theatre of major global changes in the Earth’s geodynamics, climate, seawater and atmosphere geochemistry, and thus it represents an interesting case study to understand the response of organisms to environmental changes, a topic which is of increasing interest to the scientific community, who has to face the current global change. In fact, in the Permian the biotic response was dramatic, culminating at the end of the period with the greatest mass extinction of the Phanerozoic. Noteworthy, the end Permian mass extinction coincided with one of the largest known continental eruptions, the Siberian trap basalts, that are considered to have generated more than 100,000 Gt of CO2 as well as CH4, leading to ocean acidification and global warming. Brachiopods, which are low buffered organisms with a heavily calcified shell, can be the perfect candidates to record the trends related to changes in seawater chemistry during this critical interval. The aim of this research is thus to study the biomineralization of brachiopod shells to unravel the patterns of biotic changes caused by the extreme Late Permian events. To reach this goal, I organized my research in three different phases, starting to investigate the main differences in the shell fabric of the brachiopod groups ruling the benthic communities in the Late Permian, that are the classes Rhynchonellata and Strophomenata (phase 1); then comparing the stratigraphic distribution of brachiopod genera during the Late Permian in a paleogeographic perspective (phase 2); finally, analyzing in great details, both qualitatively and quantitatively, the shell fabric of several taxa from Tethyan Permian-Triassic Boundary (PTB) successions, to unravel the biomineralization activity at generic level(phase 3). To develop this research I investigated brachiopods belonging to different paleogeographic localities in the Tethyan realm. The specimens were in part collected by myself during field activity, in part already available from the collections of Dipartimento di Scienze della Terra “A. Desio” and also provided by external partners. The studied brachiopods come from: 1. Nesen Formation, Alborz Mountains, northern Iran; 2. Julfa Formation, Ali Bashi Formation and Boundary Clay, Ali Bashi Mountains, Northwestern Iran; 3. Selong Group, southern Tibet; 4. Gyanyima Formation, southwestern Tibet; 5. Bulla Member, Dolomites, Northern Italy; 6. Gomaniibrik Formation, Hazro, Turkey; 7. Changhsing Limestone and Dalong Formation, South China These data were integrated with the analysis of the available published literature on Upper Permian brachiopods, in particular to develop step 1 and 2. The methods used to develop this research may be grouped in four main categories: 1. Microscopical analysis of the shell structure using SEM (phases 1 and 3) 2. Geochemical analysis of the calcitic shell contents for trace elements (Mg, Sr, Fe, Mn) and stable isotopes (C and O)(phase 1) 3. Image analysis to acquire quantitative parameters of the shell ultrastructure (phase 3) 4. Statistical analysis of the stratigraphic distribution of brachiopod taxa using the logistic regression in order to test association between environmental variable and taxonomic composition (phase 2). Performing phase 1, I discovered important differences in the structural and chemical composition of the shell in the two main Upper Permian brachiopod classes: the Strophomenata and the Rhynchonellata. These taxa bear a different calcitic shell fabric: the former possesses a double or triple layer shell consisting of a primary layer, a secondary layer with cross-bladed laminae and a prismatic tertiary layer; the latter have a shell succession similar to extant ones, which is composed of a primary layer of crystallites, a secondary layer of discrete fibers and, eventually, a tertiary layer of prisms. Their different fabric corresponds to differences in the chemical composition. In particular the Strophomenata, which have a laminar fabric enriched in organic compounds, have higher Sr and Mg contents and a lower δ13C in their shells than co-occurring Rhynchonellata. In phase 2, the logistic regression analysis has shown that important changes in terms of taxonomic composition took place from the Wuchiapingian to the Changhsingian, with the Strophomenata being the dominant group in terms of abundance, but the Rhynchonellata being more prone to high rank diversification. In phase 3, the detailed study of the shell structure at the SEM, has revealed that Upper Permian genera can produce different type of shells, especially regarding the ratio between the organic and inorganic content. In particular, the taxa occurring during the first part of the Late Permian (Wuchiapingian and early Changhsingian) biomineralized thick shells with a relatively high inorganic content. Instead, in the late Changhsingian, brachiopod taxa produced shells with a higher organic content. In addition, the quantitative analysis of the fabric, based on the measured size of its structural units, revealed different trends in the two classes. Rhynchonellata reduced the size of the structural units (fibers) of their shell as approaching the PTB. On the other hand, the Strophomenata show a more complex response, either continuing in their normal biomineralization activity or increasing the size of their structural units (laminae). Through this research, two important conclusions were reached: the first concerns the paleobiological implications of the different biomineralization processes performed by brachiopods, and the second is related to the brachiopod response to the end Permian global environmental changes. It is now clear that the brachiopod classes of Strophomenata and Rhynchonellata have profound differences in terms of the structural and elemental composition of their shell. These differences are likely related to the biomineralization process responsible for the formation of their shell, a collective process where arrays of mantle cells secrete the biocomposite in the Strophomenata, versus a discrete, single cell driven process in the Rhynchonellata . The observed changes in brachiopod shell biomineralization in the latest Permian are compatible with a change in the carbonate saturation state of seawater and thus with ocean acidification, related to Siberian Traps flood basalt volcanism. In fact, a general trend toward production of calcitic shells with higher organic content is recorded up to the PTB in most brachiopod groups. This may have been likely the result of changes in the physical and chemical composition of seawater that produced an increase in the energetic cost for carbonate precipitation in low buffered organisms such as brachiopods.