title: The role of changing water geochemistry in mineral formation and distribution in estuaries
Creators: name: Byrne, Gemma Mary
Publisher: name: University of Liverpool Repository
Year: 2013

description: In deeply buried petroleum reservoirs, authigenic iron-rich chlorite coats on sand grains act as barriers between silica-rich waters and the host grain and inhibit the generation of authigenic quartz cements leading to anomalously high porosity and permeability in sandstones. The origin and distribution of iron-rich chlorite grain coats in the deep subsurface is poorly constrained. To address this, the formation processes and geographic distribution of potential precursor minerals to authigenic iron-rich chlorite grain coats, such as berthierine, have been investigated in two modern estuarine analogues. Estuarine environments are important locations of soluble iron-loss and mineral flocculation, and links between water geochemistry variation, climate, local geology and clay mineral distributions are here addressed. The two estuaries investigated, in NW Spain and SW Iceland, have contrasting climates and hinterland geology, which are major controls in clay mineral types present in the local soils and within the estuary. The formation and distribution of all minerals present in the estuarine environment have been considered. In NW Spain, kaolinite, illite-muscovite and chlorite are the dominant clay minerals in the fine fraction of both the soils and estuary sediments and are present in the estuary surface sediments in similar bulk ratios to the local soils. In SW Iceland, smectite, vermiculite and chlorite are the dominant clay minerals in the fine fraction of the local soils and estuary sediments. They are also present in similar bulk ratios in both the soils and in-estuary sediments. Precipitation of neoformed carbonate mineral occurs in the lower estuary reaches of the Spanish estuary but is not evident in the Icelandic estuary. Evaporation and the corresponding supersaturation of mixed waters are responsible for carbonate mineral formation, highlighting the significance of climate on mineral type within estuarine systems. Dissolved iron is lost within the estuarine turbidity maximum zone, in the presence of relatively small proportions of seawater in the upper reaches of the two estuaries. In NW Spain, 37% is removed from the water by the upper reaches and 98% is removed by the middle reaches. In SW Iceland, 91% is removed from the water in the upper reaches while all is removed by the middle reaches. The estuarine turbidity maximum zone is responsible for the re-suspension of solid iron phases, resulting in elevated particulate iron in the upper estuary reaches. In NW Spain, particulate iron increases from the riverine input by 54% in the upper reaches while in SW Iceland, particulate iron increases from the river and stream inputs by 242% in the upper reaches of the estuary. Within the surface sediments of both estuaries, there is minor variation in clay mineral distribution of the fine fraction of the surface sediments from the upper reaches to the lower estuary. Provenance is a dominant control on the clay mineral type present in modern environments. Climatic regime and estuarine circulation processes are also factors with regards to clay distribution within the estuarine environment. This study has led to an enhanced understanding of early diagenesis, such as the location of iron loss and deposition in estuaries, the role of provenance, climate and freshwater input in clay mineral type formation and how evaporation may introduce early diagenetic cements into modern estuaries and will lead to future research development and predictive models relative to iron-rich chlorite grain coat distribution.