Research

While the sedimentological study of skeletal carbonate factories has experienced an unparalleled progress in the last decades, the process-product understanding of non-skeletal factories (either carbonates, clays and silica) is still in its infancy. A much wider pluridisciplinar approach is required including the use of geomicrobiological, hydrogeochemical, and crystal engineering techniques to better constrain the ultimate mechanisms involved in the chemical sedimentation of non-skeletal carbonates, clays and silica, together with the factors governing the growth of these minerals and their fabrics.

My current research topics include:

NON-SKELETAL CARBONATE FACTORIES: TOWARDS A PROCESS-PRODUCT UNDERSTANDING OF FACIES SUCCESSIONS IN ALKALINE LAKES

Alkaline saline lakes record a wide array of chemical precipitates that form under specific physico-chemical conditions and hydrological (climatic) regimes. Such environments are commonly colonised by prolific microbial communities, despite being unhospitable for metazoans and other organisms. In order to understand the potential mineral paragenesis precipitated in such environments new hydrochemical lacustrine models are required. Such models should explore the impact that water recharge (with spring, hydrothermal and marine fluids), evaporation and leakage to underlying aquifers have in the basin-scale precipitation of evaporite minerals (carbonate-clay-silica-sulphates).

This project analyses the range of depositional ‘chemical’ facies precipitated in alkaline lacustrine environments using a combination of petrographical information, sedimentological datasets, and reactive transport models, and aims at producing state-of-the-art facies models for alkaline lake systems dominated by non-skeletal factories. By so doing, we will understand whether depth and stratification control carbonate-clay-silica production in alkaline lakes, and how sedimentological models would look like under different scenarios.

Alkaline Lake Presalt Carbonate Silica Clay Stevensite Lacustrine Facies Spherulite — Left: East Kirkton calcite spherules are made up of spherulitic calcite formed by dense spheroidal to botryoidal coatings constituted by elongated, rectilinear fibro-radial calcite polycrystals. Silicifications are abundant in laminite facies having either primary (precipitated as benthic gels) or secondary origins (diagenetic fluids dissolving previous carbonate constituents). Right: Chemical models for alkaline saline lacustrine basins require to account for the common processes in such systems: leakage to underlying aquifers, evaporation and mixing of waters. Our reactive transport models explain Presalt mineral paragenesis (calcite, Mg-clay and silica) and their cyclothems as a combination of such processes. All the images have a CCBY attribution.

DISCRIMINATING BETWEEN ABIOTIC AND BIOTIC SIGNATURES IN CONTINENTAL CARBONATES

Ancient and recent terrestrial carbonate-precipitating systems are characterised by a heterogeneous diversity of deposits volumetrically dominated by calcite. In these environments calcite precipitates display an extraordinary morphological variety, from single crystal rhombohedral prisms, to blocky crystalline encrustations, or spherulitic to dendritic aggregates. Despite previous efforts, relating calcite micro-morphology with sedimentary hydrogeochemical conditions remains a challenge. This is because calcite morphogenesis results from the complex interaction between different physico-chemical parameters (abiotic) and microbial-derived substances (biotic).

In order to experimentally address the sedimentological causes of calcite morphogenesis, this project is building a new approach to show the range of microcrystal carbonate products likely nucleated in alkaline, saline lake solutions under specific and experimentally-controlled biogeochemical conditions. This novel methodology will help to clarify the impact that biotic and abiotic mechanisms have in the crystal morphology of continental carbonates (spring, lake, speleothems) or extraterrestrial deposits enabling a refined way to address biogenicity. As some of the most prolific hydrocarbon reservoirs are made up of non-skeletal carbonates, we aim to understand the intimate biotic-abiotic relationships likely attached to the formation of growth-framework to intercrystalline porosity associated with such carbonates. Accordingly, the biochemical water conditions related to allochem nucleation, dissolution and reprecipitation in the Presalt alkaline lakes (e.g., spherulites and shrubs) can be predicted in a basin-scale context.

Alkaline Presalt Carbonate Spherulite Abiotic Biotic Signature Crystal Continental — Our novel experimental programme is aiming at understand the relation between carbonate growth morphologies and the environmental conditions given in alkaline saline waters. We obtain a diversity of microsized calcite including: single-crystal, prismatic and well-defined grains (up-left); spheroidal dumbbell-shaped crystals (up-right); spheroidal and fibro-radial, polycrystalline objects (down-left). We take SEM observations from the millimetre-sized slides used in experiments (down-right). All the images have a CCBY attribution.

UNCOVERING THE DIAGENETIC PROCESSES INVOLVED IN CLAY DISSOLUTION AND CARBONATE PRECIPITATION IN THE PRESALT ALKALINE LAKES (BRAZIL)

The Cretaceous ‘Presalt’ lakes of the South Atlantic margins (Brazil-Angola) display an unusual co-occurrence of chemical precipitates typically accumulated in extensive freshwater and hyper-alkaline volcanic environments (spherulitic calcite, dolomite, Mg-rich smectites and silica). The origin and distribution of these mineral assemblages is still hotly debated and have huge implications for hydrocarbon exploration and production. Indeed, a process-product understanding of the early and late diagenetic transformations affecting carbonate-clay-silica facies is essential to predict the distribution and quality of hydrocarbon reservoirs formed in such rocks.

This project aims at shedding light on the chemical mechanisms involved in Mg-clay dissolution and subsequent carbonate precipitation, and also to recognise the geochemical conditions originating Mg-clay, and the early and late formation of the associated carbonates (calcite, dolomite, aragonite). By performing leading-edge petrographical/ geochemical analyses and tightly constrained reactive transport models (PHREEQC) we aim to understand the specific environmental conditions facilitating such mineral paragenesis. In addition we plan to spatially constrain the clay dissolution-carbonate precipitation process at borehole scale to predict the vertical and lateral distribution of flow-units in a basin-scale context.

Diagenesis Dolomite Microbial Stevensite Dissolution Presalt Reservoir Continental — Left: Calcite spherules floating in a framework of dolomite rhombs filling matrix dissolution porosity (blue epoxy). Photo courtesy Rolando Herlinger. Centre: Back-scatter image showing a meshwork of microbial filaments coated in a stevensite matrix that is being replaced by aragonite crystals (Lake Clifton, Australia). Right: Evolution of mineral paragenesis volumetrically precipitated across the different evaporative cycles in alkaline saline lakes. All the images except the picture on the left have a CCBY attribution.

STRATIGRAPHICAL AND BIOGEOCHEMICAL SIGNATURES OF ENVIRONMENTAL CHANGE RECORDED IN CARBONATE PLATFORMS

The nature, skeletal composition and facies architecture of carbonate platforms is intimately related to the way tectonic, climatic and evolutionary trends have operated during the geological past. These interactions leave a unique combination of sedimentological and geochemical signatures in the carbonate rock record. While identifying environmental perturbations in carbonate platforms dominated by skeletal producers has concentrated much research, the recognition of environmental fluctuations in systems dominated by non-skeletal factories is challenging.

This project aims at explore the stratigraphic and biogeochemical expressions following major environmental perturbations (e.g., massive volcanic eruptions, deep sea anoxia or major sea-level changes) in basins characterised by the production of unusual carbonates (microbial carbonates, ooids, muddy sediments). By integrating stratigraphical, paleontological and geochemical data we aim at reconstruct the environmental and physiographical conditions encouraging the dominance of non-skeletal carbonate factories taking as a case study the Middle Triassic carbonate succession of the Catalan Basin (Spain).