Continental shelf pump

In oceanic biogeochemistry, the continental shelf pump is proposed to operate in the shallow waters of the continental shelves, acting as a mechanism to transport carbon (as either dissolved or particulate material) from surface waters to the interior of the adjacent deep ocean.

Overview

Originally formulated by Tsunogai et al. (1999),

• the dense, carbon-rich shelf waters sink to the shelf floor and enter the sub-surface layer of the open ocean via isopycnal mixing

Modern Continental Shelf Pump Theory

Continental shelves make up approximately 7% of the oceans area yet have significant roles in oceanic biogeochemical processes. Continental shelves have a large input of terrestrial nutrients and shallow waters that provide productive conditions for biological organisms, and they can be variable due to inputs of dissolved inorganic carbon (DIC) from estuaries, which can influence both the salinity and alkalinity. The coastal processes were largely thought to have an insignificant impact on the ocean's carbon cycling processes compared to the vast open ocean. Combining an Atlantic Margin Model simulation and Proudman Oceanographic Laboratory Coastal-Ocean Modeling System allowed them to reproduce conditions from 1960-2004, with the main focus on hydrodynamics and calculating the correlating biogeochemical effects. They found that 40% of carbon sequestered was heterogeneously removed in a single growing season, with variable removal in some areas, and that only 52% of this carbon was redirected to the deep ocean. In this case, shelf and deep sea circulation must be coupled.

Significance

Based on their measurements of the CO₂ flux over the East China Sea (35 g C m⁻² y⁻¹), Tsunogai et al. (1999) and modelling of anthropogenic emissions of CO₂ estimates suggest that the ocean is currently responsible for the uptake of approximately 2 Gt C y⁻¹, and that these estimates are poor for the shelf regions, the continental shelf pump may play an important role in the ocean's carbon cycle.

One caveat to this calculation is that the original work was concerned with the hydrography of the East China Sea, where cooling plays the dominant role in the formation of dense shelf water, and that this mechanism may not apply in other regions. However, it has been suggested that other processes may drive the pump under different climatic conditions. For instance, in polar regions, the formation of sea-ice results in the extrusion of salt that may increase seawater density. Similarly, in tropical regions, evaporation may increase local salinity and seawater density.

The strong sink of CO₂ at temperate latitudes reported by Tsunogai et al. (1999) the Middle Atlantic Bight and the North Sea. On the other hand, in the sub-tropical South Atlantic Bight reported a source of CO₂ to the atmosphere.

Recently, work has compiled and scaled available data on CO₂ fluxes in coastal environments, and shown that globally marginal seas act as a significant CO₂ sink (-1.6 mol C m⁻² y⁻¹; -0.45 Gt C y⁻¹) in agreement with previous estimates. However, the global sink of CO₂ in marginal seas could be almost fully compensated by the emission of CO₂ (+11.1 mol C m⁻² y⁻¹; +0.40 Gt C y⁻¹) from the ensemble of near-shore coastal ecosystems, mostly related to the emission of CO₂ from estuaries (0.34 Gt C y⁻¹).

An interesting application of this work has been examining the impact of sea level rise over the last de-glacial transition on the global carbon cycle. During the last glacial maximum sea level was some lower than today. As sea level rose the surface area of the shelf seas grew and in consequence the strength of the shelf sea pump should increase.

The effect of warming is of particular concern around the Antarctic ice shelves, as the ice sheets are the largest of the Earth’s ice reservoirs and changes in their mass has the greatest potential to have a significant impact on rising sea levels. An eddying global climate model revealed that the shelf is governed by different mechanisms: the Circumpolar Deep Water (CDW) initiates with deep shelf warming with vertical mixing and the Antarctic Slope Front (ASF) utilizes a lateral density gradient near the shelf break. The disconnect between the CDW and ASF can complicate heat transfer across the ASF and prevent heat from escaping deeper waters. But in areas where this transport is less inhibited, heat is able to move to shore and disperse. Gaining a more rounded understanding of this shelf pump could help researchers to better anticipate the effect of warming on ice sheets.

References

See also

• Biological pump
• Ocean acidification
• Solubility pump