PhD Theses

SUPERVISOR: Prof. Barbara CAVALAZZI, Univ. Bologna; CO-SUPERVISOR: Dr. Keyron Hickman-Lewis, Imperial College London, London, UK.

ABSTRACT: Stromatolites are ideal ecosystems for how microbe-sediment synergy occurs and, therefore, represent the main resource for the recognition of biosignatures and investigation of astrobiological targets regarding the environment and life interactions. Even though they are one of the oldest records of life on Earth, at present, the modern occurrences of stromatolites are restricted to extreme environments, such as hydrothermal and hypersaline lakes. This PhD research project aims to study three different sites characterized as modern extreme environments due to their high salinity and alkalinity and dominance by microbes, in which occur the development of stromatolites. The selected areas were Lake Ashenge, Ethiopia and Lake Abbe, Djibouti, both localized in the Eastern African Rift System; and Carri Laufquen Lakes System, in the Patagonia Region, Argentina. We seek to understand the morphogenetic and preservational processes of those biostructures in different extreme contexts on Earth, in order to provide new perspectives on the application of Mars-analogue lacustrine environments to the origins and preservation of potential microbial biosignatures in similar environments. In addition, to provide competent material for comparison with samples from the Mars Sample Return mission.

Completion date: March 2025

RESUME: Ce travail regroupe des données recueillies au terme d’une étude stratigraphique, sédimentologique et géochimique des monticules carbonatés sinémuriens dans la portion orientale du Haut Atlas central marocain. Plusieurs objectifs ont orienté ce travail :

1- La définition du cadre lithostratigraphique et biostratigraphique des monticules carbonatés étudiés. Ces derniers affleurent, à Nzala et à Foum Tillicht, dans le tiers supérieur du membre supérieur de la formation d’Idikel. Ils apparaissent localement au sein de la formation d’Aberdouz et peuvent atteindre la base de la formation sus-jacente d’Ouchbis. Les datations fournies par les ammonites permettent de les situer dans l’intervalle allant du passage Sinémurien inférieur - Sinémurien supérieur jusqu’à la base du Carixien inférieur.

2- La reconstitution de l’environnement propice à l’implantation et à la croissance des monticules carbonatés est réalisée par une analyse biosédimentologique à différentes échelles. Ce milieu correspond à une plateforme carbonatée infratidale à circatidale, restreinte en oxygène (OMZ : Zone à oxygène minimum), dys-photique épisodiquement soumis à l’action des vagues de tempêtes et où la turbidité est modérée et quasi-permanente.

3- La confrontation des données géochimiques et diagénétiques et des analyses quantitatives de leurs principaux composants pour comprendre le rôle des différents organismes qui participent directement ou indirectement à l’édification des masses construites, l’origine et le rôle des différentes micrites.

4- La recherche de la terminologie la mieux appropriée pour définir les bioconstructions étudiées. L’étude des processus sédimentaires et diagenétiques a permis d’interpréter ces monticules comme étant des récifs à éponges siliceuses (Hexactinellides et Lithistides), à microbialites (Thrombolites et Leiolites) et à encroûtants (bryozoaires et polychètes). Tous ensembles, ces organismes forment une structure caverneuse dont les cavités labyrinthiques sont remplies par au moins deux types de micrite (polymud): une allomicrite gris clair provenant du piégeage passif des sédiments fins et une automicrite gris foncé induite par l’organominéralisation. Bien que l’importance des trois processus d’accrétion varie verticalement d’un intervalle bioconstruit à l’autre, elle est grandement dominée par la biominéralisation (squelettogénèse et encroûtements biogéniques). L’organominéralisation, rattachée au substrat organique qu'offre le tissu mou non vivant des éponges siliceuses, est relativement moins importante. La cimentation est très faible puisque peu de ciment calcaire comble la porosité restante.

5- La détermination de l’origine de l’automicrite par observations en MEB. Son examen, à cette échelle, indique la présence de substance polymérique extracellulaire (EPS) et des « honeycomb-like structure » qui confirment l’origine microbienne de cette micrite. L’analyse géochimique de l’automicrite par Spectrométrie par Rayon-X (XRF) révèle par ailleurs que celle-ci est dénuée d’éléments authigènes, contrairement à la micrite allochtone.

6- Les constructions récifales étudiées caractérisent une période de bouleversement géodynamique et paléogéographique. Leur apparition et leur grand développement au Sinémurien semblent associés à un changement de la morphologie du fond marin induit par la phase majeure de dislocation de la plate-forme liasique au passage Sinémurien inférieur-Sinémurien supérieur. La disparition définitive de ces constructions récifales, au passage Sinémurien-Carixien, est due à un envasement important par des sédiments terrigènes hémipélagiques du sillon, consécutif à un approfondissement du milieu. Celle-ci coïncide aussi avec la crise lotharingienne de la Téthys.

SUPERVISOR: Prof. Barbara CAVALAZZI, Univ. Bologna; CO-SUPERVISOR: Prof. Driss CHAFIKI, Université Cadi Ayyad, Morocco, and Adriano GUIDO, Univ. della Calabria, Italy

Completion date: 29 July 2020

ABSTRACT: Evaluating the nature of the earliest, often controversial, traces of life in the geological record (dating to the Palaeoarchaean, up to ~3.5 billion years before the present) is of fundamental relevance for placing constraints on the potential that life emerged on Mars at approximately the same time (the Noachian period). In their earliest histories, the two planets shared many palaeoenvironmental similarities, before the surface of Mars rapidly became inhospitable to life as we know it. Multi-scalar, multi-modal analyses of fossiliferous rocks from the Barberton greenstone belt of South Africa and the East Pilbara terrane of Western Australia are a window onto primitive prokaryotic ecoystems. Complementary petrographic, morphological, (bio)geochemical and nanostructural analyses of chert horizons and the carbonaceous material within using a wide range of techniques – including optical microscopy, SEM-EDS, Raman spectroscopy, PIXE, µCT, laser ablation ICP-MS, high-resolution TEM-based analytical techniques and secondary ion mass spectrometry – can characterise, at scales from macroscopic to nanoscopic, the fossilised biomes of the earliest Earth. These approaches enable the definition of the palaeoenvironments, and potentially metabolic networks, preserved in ancient rocks. Modifying these protocols is necessary for Martian exploration using rovers, since the range and power of space instrumentation is significantly reduced relative to terrestrial laboratories. Understanding the crucial observations possible using highly complementary rover-based payloads is therefore critical in scientific protocols aiming to detect traces of life on Mars.

SUPERVISOR: Prof. Barbara CAVALAZZI; CO-SUPERVISOR: Dr. Frances WESTALL, CBM-CNRS, France Completion date: October 2019

ABSTRACT: Palaeoarchaean (3.6-3.2 Ga) cherts are commonly associated with hydrothermal activity and retain evidence for an early biosphere. They are rich in disordered carbonaceous matter (CM) whom origin cannot be readily associated to biological activity. This project investigate CM-rich material from the BARB3 core obtained from the c. 3.4 Ga old Buck Reef Chert (BRC) of South Africa, one of the best preserved volcano-sedimentary succession on Earth. CM from the core is characterized through a multiple analytical approach based on in situ complementary techniques. The aim was to reconstruct the CM origin and evaluate its biogenicity. Samples from the BARB3 shallow-platformal lithofacies, are rich in CM occurring within: (i) crinkly laminated chert; (ii) massive black chert; (iii) laminated black chert; and (iii) granular carbonaceous chert. These four facies bear a specific CM microtextures related to their depositional history. The crinkly laminated chert has a planar stromatolitic-like fabric and include CM mat-like laminae and grains. The massive black chert is structureless and include “cloudy” diffuse CM, CM grains and bitumen-interstitial CM originated as a fluid phase. The laminated black chert include CM grains and the granular carbonaceous chert is a mixture of carbonaceous detritus generated by the hydrothermal brecciation of soft crinkly laminated sediments and it is associated to botryoidal-quartz stratiform veins. The structural characterization of such CM microtextures by means of Raman spectroscopy and HRTEM analyses have confirmed their consistency with the BGB regional metamorphic imprint. However a between-facies and, more importantly, a between-microtexture structural heterogeneity occur guiding the reconstruction of the CM and of the depositional facies history. The result is a revisited picture of the BRC as a shallow-water hydrothermal field characterized by complex microbial communities and carbon remobilization now expressed by multiple CM generations. DOI 10.6092/unibo/amsdottorato/8555

SUPERVISOR: Prof. Barbara CAVALAZZI, Univ. Bologna; CO-SUPERVISOR: Prof. Axel HOFMANN, Univ. of Johannesburg, South Africa

Completion date: 8 May 2018

ABSTRACT: Longstanding debates concerning the origin of the Kess Kess Emsian carbonate mounds exposed at Hamar Laghdad Ridge in the eastern Anti-Atlas of Morocco centre around the processes that induced precipitation of carbonate mud and the preservation of the steep morphologies. Although in the last years an origin related to hydrothermalism seemed to be more likely, to date the Kess Kess are still considered unusual or controversial vent deposits, and their origin remains relevant to geoscientists.

This study combines in updated research review information from previous work and from results from detailed field observations and new analytical results to define a consistent framework and some new insight of current knowledge about the origin of the Emsian mounds. We obtain a complete minero-petrographic and palaeobiological data set and a detailed geochemical characterization of the different lithologies and facies of the Hamar Laghdad stratigraphic succession, including Kess Kess mounds, and we compared the results with the data from Maïder

Basin mounds (Anti-Atlas, Morocco). Our data point out and support the hydrothermal model proposed for the genesis and development of the Kess Kess mounds. The mechanisms linked to the mounds formation and growth are discussed in the light of the new finding of fluid-sediment interaction within a scenario driven by late magmatic fluids circulation. Conical mounds and other fluids related morphologies were also reported from Crommelin crater area (Arabia Terra, Mars). These mounds consist in meter-sized conical buildups hosted in the Equatorial Layered Deposits (ELDs) deposed during a regional groundwater fluid upwelling. The geometries and the geological conditions that might have controlled the development of such morphologies were discussed. According to our new data the morphological and stratigraphical characteristics of the mounds in the area of the Crommelin crater are most consistent with a formation by fluids advection. Then we compare terrestrial and Martian data and examine the geological settings of hydrothermal mound occurrences on Earth in order to describe potential target areas for hydrothermal structures on Mars.

SUPERVISOR: Prof. Roberto Barbieri, Univ. of Bologna; CO-SUPERVISOR: Dr.ssa Barbara Cavalazzi, Univ. of Johannesburg, South Africa.

Completion date: May 2014

ABSTRACT: When large whales die and sink to the sea floor provide a huge and concentrated food source to the marine ecosystem. Whale falls are mainly known from the deep sea, both in the modern and in the fossil record, where they host a rich and specialized community similar to those living at other deep sea reducing habitats, like hydrothermal vents and hydrocarbon seeps. On the contrary little is known on what happens in shallow waters. This study contributes novel data to our knowledge of shallow water whale fall communities (WFC). Fossil whale falls from the Neogene of Italy were studied in detail, together with a modern analogue ecosystem from the North Sea. A multidisciplinary approach was applied to the study of fossil shelfal WFC, based on a variety of tools, ranging from taphonomy to petrography and isotope analysis, to quantitative benthic paleoecology. Results concern different taxonomic groups that took advantage of the large quantity of energy stored in whale tissues, from the microbial consortium at the base of the food web, to benthic macro-invertebrates, to larger pelagic scavengers.

The field excavation of a 10 m long baleen-whale from the Pliocene of Tuscany is at the base of the study. Taphonomy and the position of macrofaunal whale associates with respect to the still articulated bones allowed to reconstruct the main tracts of the ecological succession at the whale fall. Sediment bulk samples collected next to whale bones and from the surrounding sediments were quantitatively analyzed for their mollusc content. Quantitative species-level data on bivalves, gastropods and scaphopods allowed to compare WFC and background communities. A similar approach was applied to the study of the mollusc community associated with a 5 m long minke whale sunk at 125 m depth in the North Sea. To further widen the field of enquiry other twenty-four, more or less complete fossil whales present in Italian museum collections were studied, guided by the experience previously gained during our own field work. The data include taphonomy of fossil bones and qualitative and partial information on the associated fauna. Petrographic microfacies techniques were applied to the study of whale bones. Taphonomy at the microscopic scale was approached through optical and scanning microscopy, Raman spectroscopy and stable isotope geochemistry in order to analyze the signature of microorganisms participating to whale bone degradation, mainly bacteria and fungi, and understand microenvironmental conditions within and around larger bones. Some of the outcrops from which museum specimens had been extracted were studied and sampled to reconstruct local environmental conditions. Bulk samples were analyzed to gather abundance data that were then compared within a larger data set of intertidal to bathyal samples from the literature. As expected, the main factor conditioning the distribution of molluscs around WFC and in other normal settings is water-depth. Absolute depth estimates and considerations based on lithology and paleoecology showed that at least seven whale falls out of twenty-four were located in open shelf settings and possibly associated with high-nutrient conditions. The general results are consistent with the hypothesis that shallow water whale falls are different from their deep counterparts. On the shelf obligate taxa of families typical of deep sea reducing environments are small-sized and rare, possibly occurring only in offshore settings. The organic input concentrated in a large whale sunken to the bottom becomes food for generalist taxa commonly living on the shelf.

SUPERVISOR: Prof. Simonetta Monechi, Univ. Firenze; CO-SUPERVISORS: Dr. Stefano Dominici, Univ. Firenze, AND Dr. Barbara Cavalazzi, CBM-CNRS-Olreans, France

Complection date: 31 December 2010