Deep-Ocean Carbon Sequestration Through Enhanced Mineral Weathering in Abyssal Plains

Assessing the Viability of Accelerating Silicate Mineral Reactions in Ocean Trenches for Gigaton-Scale CO₂ Removal

Geochemical Foundations of Oceanic Mineral Carbonation

The fundamental chemical reactions underlying mineral weathering for carbon sequestration have been understood since the work of Urey (1952) and subsequent researchers. The general weathering reaction for silicate minerals can be represented as:

In marine environments, these reactions are influenced by several critical factors:

• Pressure effects: Increased hydrostatic pressure at depth affects reaction kinetics and mineral stability fields
• Temperature gradients: Cold deep waters (2-4°C) versus warmer surface waters create complex reaction dynamics
• Mineral surface area: Particle size distribution and reactive surface area dramatically impact reaction rates
• Biological factors: Microbial communities may catalyze or inhibit certain reaction pathways

The Abyssal Plain Environment

The abyssal plains, covering approximately 70% of the ocean floor at depths between 3,000-6,000 meters, present unique characteristics for enhanced weathering:

• Consistent low temperatures (2-4°C)
• High pressures (300-600 atmospheres)
• Relatively stable sedimentation rates (2-10 cm/kyr)
• Low organic carbon content in sediments
• Near-saturation conditions for carbonate minerals

Potential Mineral Candidates for Deep Ocean Weathering

Olivine Group Minerals

The magnesium-rich endmember of the olivine series (forsterite, Mg₂SiO₄) has been extensively studied due to its high reactivity and abundance. The carbonation reaction proceeds as:

Experimental studies under simulated deep-ocean conditions (Hangx & Spiers, 2009) suggest reaction rates may be enhanced by 1-2 orders of magnitude compared to surface conditions due to pressure effects on water structure and mineral surface chemistry.

Pyroxenes and Amphiboles

These chain silicate minerals offer alternative reaction pathways with different kinetic profiles:

• Enstatite (MgSiO₃): Slower reaction kinetics but greater thermodynamic stability at depth
• Wollastonite (CaSiO₃): Faster initial reaction rates but potential passivation issues
• Tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂): Complex dissolution behavior with possible catalytic effects from hydroxyl groups

Engineering Approaches for Large-Scale Implementation

Mineral Delivery Systems

The logistics of introducing gigaton-scale mineral quantities to abyssal plains presents substantial engineering challenges:

Reaction Enhancement Techniques

Several methods have been proposed to accelerate the weathering process in deep-sea environments:

• Electrochemical stimulation: Applying small voltage potentials to drive ion migration through sediments (estimated 3-5x rate enhancement in lab studies)
• Microbial augmentation: Introducing specific bacterial strains known to catalyze silicate dissolution (e.g., Bacillus species)
• Particle engineering: Creating optimized grain size distributions and surface defects to maximize reactive surface area
• Hydrothermal cycling: Leveraging natural thermal gradients near spreading centers to boost reaction kinetics

Theoretical Carbon Sequestration Potential

A first-order estimate of global sequestration capacity can be derived from:

Where:

• A: Available abyssal plain area (~3.5×10⁸ km²)
• ρ: Average sediment density (~1.5 g/cm³)
• f: Mineral fraction in sediment (optimistically ~10%)
• k: Weathering rate constant (10^-12-10^-10/s under deep-sea conditions)
• t: Time scale of operation (years)

Theoretical calculations suggest that with optimized mineral distribution and reaction enhancement, the abyssal plains could potentially sequester 1-10 Gt CO₂/yr, though practical implementation would likely achieve only a fraction of this theoretical maximum.

Environmental Monitoring and Risk Assessment

Potential Ecosystem Impacts

The introduction of large quantities of reactive minerals to deep-sea environments could affect benthic ecosystems through several mechanisms:

• pH changes: Localized increases in alkalinity may benefit some organisms while harming others
• Turbidity effects: Suspended particles could interfere with filter-feeding organisms
• Trace metal release: Nickel and chromium content in some ultramafic minerals may exceed toxicity thresholds
• Settlement alteration: Changes in sediment composition could disrupt benthic community structures

Monitoring Strategies for Large-Scale Deployment

A comprehensive monitoring program would require:

• AUV-based sensor networks: Deployable arrays measuring pH, dissolved inorganic carbon, and redox potential at multiple depths in the water column and sediment pore waters.
• Tracer studies: Using isotopically labeled minerals (e.g., ²⁶Mg-enriched olivine) to track reaction progress and byproduct migration.
• benthic observatories: Long-term stations equipped with cameras and environmental DNA samplers to monitor ecological changes.
• Synthetic aperture radar: Satellite-based monitoring of surface expression of deep-sea processes (e.g., possible upwelling signals).

The Carbon Accounting Challenge

The verification of actual carbon removal presents several unique challenges in deep-sea environments:

• Temporal decoupling: The time between mineral deployment and complete carbonation may span decades to centuries.
• Spatial variability: Heterogeneous reaction rates across the seafloor complicate mass balance calculations.
• Coupled biogeochemical cycles: Changes in alkalinity may affect other elemental cycles (e.g., nitrogen, phosphorus).
• Cascade effects: Secondary precipitation of carbonates could temporarily sequester additional carbon before eventual dissolution.

A proposed verification framework includes:

• Tier 1: Direct measurement of mineral dissolution rates in controlled mesocosm experiments.
• Tier 2: Water column alkalinity budgets using tracer-release techniques.
• Tier 3: Long-term sediment core analysis for carbonate accumulation.
• Tier 4: Full ecosystem modeling incorporating all major biogeochemical fluxes.

The Path Forward: Research Priorities and Scaling Challenges

The development of deep-ocean enhanced weathering as a viable carbon dioxide removal approach requires progress in several key areas:

Scientific Understanding Gaps

• Coupled reaction-transport modeling:
High-resolution models incorporating turbulent boundary layer dynamics, sediment porewater flow, and mineral surface chemistry.
• The role of extreme pressures: