Oceanic redox and nutrient cycling in the mid-Silurian

This thesis investigates the complex interplay of redox dynamics, nutrient
cycling, and biotic responses during the mid-Silurian, focusing on the Ireviken
Biogeochemical Event (IBE) within the Welsh Basin and Lakesman Basin.
Chapter 1 provides an introduction to the PhD project and a brief overview of
previous progress in the relevant field. Chapter 2 describes in detail the
methodologies applied to address the project objectives. Chapter 3 explores
early Silurian deep-water sedimentary systems, revealing how substrate
consistency and oxygenation influenced bioturbation. Macrofaunal activity in
the Aberystwyth Grits Group (AGG) was restricted to firm substrates exposed
by sediment gravity flows, while meiofaunal burrows dominated the soupy
substrates of the Borth Mudstone Formation (BMF). These findings challenge
traditional interpretations of laminated sediments as dysoxic indicators,
emphasising substrate properties over oxygen availability.
Chapters 4 and 5 highlight the progression of redox instability from oxic-
ferruginous to euxinic conditions, documenting the impact of marine redox
evolution on marine biodiversity and primary productivity during the IBE. The
Banwy River section demonstrates how dynamic redox oscillations and
localized euxinia triggered biotic stress, particularly in benthic fauna and
graptolites. A mass balance model shows that cooling-induced intensification
of ocean circulation drove nutrient recycling, organic carbon burial, and
prolonged deoxygenation. This kind of marine deoxygenation during the mid-
Paleozoic contrasts with the rapid hyperthermal-driven Oceanic Anoxic Events
(OAEs) of the Mesozoic and Cenozoic, offering a unique perspective on mid-
Paleozoic anoxia.
Chapter 6 examines phosphorus and cadmium cycling as proxies for
nutrient dynamics and productivity. Redox-controlled phosphorus release
during euxinic conditions sustained high productivity, reflected in elevated
Corg/P ratios and cadmium isotope trends. The results underscore the role of
nutrient feedbacks in amplifying marine productivity and deoxygenation during
the IBE. Additionally, climatic cooling enhanced terrigenous phosphorus
delivery, linking terrestrial weathering processes to marine redox evolution.
Collectively, this research demonstrates the interconnectedness of redox
shifts, nutrient cycling, and climatic perturbations in shaping mid-Silurian ocean
chemistry and ecosystems. It provides new insight into the drivers of Paleozoic
anoxia and highlights the significance of transient cooling events in the long-
term oxygenation of Earth's oceans.