Physical and biogeochemical drivers of Modern carbonate diagenesis in intertidal sediments of Abu Dhabi

Abstract

Authigenic carbonates are common early-diagenetic features of sedimentary rocks and have been linked directly to the global carbon, sulphur, and iron cycles. However, physical, chemical, and biological drivers of authigenic carbonate formation remain contentious. The sea floor of the modern Arabian Gulf is characterised by extensive areas of recently-lithified carbonate sediments, with firmgrounds and hardgrounds cemented by acicular aragonite and high Mg-calcite cements. This study presents the first comprehensive analysis of biologically-influenced aragonite mineralisation resulting in subsurface firmground formation within the sediment column in two contrasting shallow coastal lagoons in Abu Dhabi with varying degrees of restriction and hydrodynamic complexity. Physical and chemical characteristics of shallow sediment cores and firmground samples, corresponding porewater chemistry, and microbial community composition (via 16S SSU rRNA gene sequencing) were analysed to achieve a mechanistic understanding of the drivers of firmground formation.
Evidence suggests firmground formation is linked to the redox boundary and driven by increased porewater pH generated as a by-product of microbial respiratory processes. Archaeal Marine Benthic Group D (Thermoprofundales) in association with known sulphate-reducing bacteria and rare archaeal taxa are likely involved in sulphate reduction which, in conjunction with pyrite formation, could drive aragonite supersaturation. Additionally, Fe/Mn-oxide reduction may drive increased pH and precipitation of carbonate cements to form the firmground. Preliminary modelling results suggest that firmground formation may also be favoured in warmer summer months over winter months, particularly in the upper 20 cm of sediment in the lower-to-middle intertidal zone, as temperature is a key thermodynamic control on carbonate precipitation. These findings have implications for interpreting the role of biological processes in hardground formation in the geologic record. Additionally, these results help unravel the complex role microbes play in biogeochemical cycles within the coastal marine subsurface environment and provide constraints for reactive transport models of early diagenesis.

Date of Award	28 Sep 2021
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Fiona F Whitaker (Supervisor), Dan Lunt (Supervisor) & Sarah E Greene (Supervisor)