THE ROLE OF BATHYMETRY IN UPWELLING INDUCED PRODUCTIVITY IN ICEHOUSE AND GREENHOUSE WORLDS

We use a Global General Circulation Model (GCM) to show that variations in seabed rugosity brought about by eustatic changes in sea-level have a profound impact on the localised efficiency of the oceanic carbon pump.

Vertical mixing in the oceans promotes nutrient uplift (e.g. phosphorus etc.) from the deep to the surface and increases biodiversity and productivity. This mixing is driven by climate, tides and the local interaction of currents with seabed bathymetry. Regions of elevated mixing experience enhanced carbon export to the deep ocean and sediment archive, anoxia and the deposition of silica and phosphate.

In epicontinental seas vertical mixing at the small and medium scale is driven by the interaction of currents with bathymetry. Coastal geomorphology changes dramatically during a sea-level cycle. The formation of incised river valleys during low-stands creates roughness whereas sediment blanketing during the transgression and highstand forms smoother bathymetry.

The Massachusetts Institute of Technology general circulation model (MITgcm) is used here to model the impact of turbulence during the key stages of an idealised transgression in an icehouse and greenhouse world.

Results indicate that the upwelling produced by flow over rough bathymetry during the early part of an ice-house transgression is likely to be 20x higher than from flow over a smoother seabed. To put this in perspective, during an early transgression a stretch of coastline just 20 km long by 5 km wide could result in an annual upward phosphorus flux of 10⁵ tonnes, equivalent to the phosphorus input by the modern Mississippi.

Importantly mixing associated with seabed rugosity is highly localised and likely to create a coastal ocean with a spatially variable level of productivity. If this was matched with habitat heterogeneity then we might expect this to impact upon biodiversity.

Seabed rugosity is likely to change during a transgression due to sediment blanketing, and also vary with the frequency and magnitude of eustatic change and this will affect the biological carbon pump. If the reduced levels of eustatic variation in a greenhouse world are accompanied by lower levels of coastline rugosity (ignoring the effects of local tectonics) then coastal vertical mixing will be lower in a greenhouse world than it is in an icehouse world.