CO2 leaking into the shallow sediments and overlying seawater is partitioned in different forms, each
migrating at its own rate and having potentially different impacts. To begin with the CO2 gas will
migrate through the shallow subsurface either alone as a free gas or together with associated deep
fluids (e.g. brines), with the free-phase CO2 equilibrating with the surrounding pore waters/associated
brines. Migrating upward these fluids will enter the base of the water column, with the release of gas
bubbles (and possibly associated waters) from the sediments into the overlying seawater. The bubbles
will rise in the water column creating what is known as a bubble “flare” with the CO2 in the bubbles
dissolving in the surrounding surface water as they rise. Depending on the depth and the
chemical/physical characteristics of the water column, these bubbles may or may not reach the water
surface. Any co-migrating water/brine will also be released into the water column, creating a plume
having a chemical composition that is distinct from the surrounding seawater, consisting of dissolved
gases (mainly CO2), elements in the original brine, and elements liberated via CO2-induced water-rock
interaction. The height that this dissolved plume will reach in the water column will depend on the
original flow rate across the sediment-water interface and the density contrast between the plume
and surrounding seawater. Both the gas-induced and water plumes will then migrate laterally and
vertically as a result of the local currents, water column stratification, and density effects, meaning
that there is the potential for impact both in the near and far field for pelagic organisms, both in terms
of a lower pH and the possibility of elevated concentrations of toxic elements. This study was carried
out in the framework of two EC funded projects, RISCS and ECO2 related to research on sub-seabed
CO2 storage as climate change mitigation strategy, and potential impact on marine ecosystems. Here,
we investigated how CO2-leakage, a risk associated with subseafloor CO2-storage, can affect physical
and chemical characteristics of the surrounding ecosystem. We studied the Panarea natural
laboratory site (Aeolian Islands), where natural CO2 is leaking from the seafloor into the overlying
water column, as an analogue for a leakage scenario
