Air–sea exchanges of CO2 in the world's coastal seas

[摘要] The air–sea exchanges of CO₂ in the world's 165 estuaries and 87continental shelves are evaluated. Generally and in all seasons, upperestuaries with salinities of less than two are strong sources of CO₂(39 ± 56 mol C m⁻² yr⁻¹, positive flux indicates that thewater is losing CO₂ to the atmosphere); mid-estuaries with salinities ofbetween 2 and 25 are moderate sources(17.5 ± 34 mol C m⁻² yr⁻¹) and lower estuaries withsalinities of more than 25 are weak sources(8.4 ± 14 mol C m⁻² yr⁻¹). With respect to latitude,estuaries between 23.5 and 50° N have the largest flux per unit area(63 ± 101 mmol C m⁻² d⁻¹); these are followed bylower-latitude estuaries (23.5–0° S:44 ± 29 mmol C m⁻² d⁻¹; 0–23.5° N:39 ± 55 mmol C m⁻² d⁻¹), and then regions north of50° N (36 ± 91 mmol C m⁻² d⁻¹). Estuaries southof 50° S have the smallest flux per unit area(9.5 ± 12 mmol C m⁻² d⁻¹). Mixing with low-pCO₂shelf waters, water temperature, residence time and the complexity of thebiogeochemistry are major factors that govern the pCO₂ in estuaries,but wind speed, seldom discussed, is critical to controlling the air–waterexchanges of CO₂. The total annual release of CO₂ from the world'sestuaries is now estimated to be 0.10 Pg C yr⁻¹, which is much lowerthan published values mainly because of the contribution of a considerableamount of heretofore unpublished or new data from Asia and the Arctic. TheAsian data, although indicating high pCO₂, are low in sea-to-airfluxes because of low wind speeds. Previously determined flux values relyheavily on data from Europe and North America, where pCO₂ is lower butwind speeds are much higher, such that the CO₂ fluxes are higher than inAsia. Newly emerged CO₂ flux data in the Arctic reveal that estuariesthere mostly absorb rather than release CO₂.

Most continental shelves, and especially those at high latitude, areundersaturated in terms of CO₂ and absorb CO₂ from the atmospherein all seasons. Shelves between 0 and 23.5° S are on average a weaksource and have a small flux per unit area of CO₂ to the atmosphere.Water temperature, the spreading of river plumes, upwelling, and biologicalproduction seem to be the main factors in determining pCO₂ in theshelves. Wind speed, again, is critical because at high latitudes, the windstend to be strong. Since the surface water pCO₂ values are low, theair-to-sea fluxes are high in regions above 50° N and below50° S. At low latitudes, the winds tend to be weak, so thesea-to-air CO₂ flux is small. Overall, the world's continental shelvesabsorb 0.4 Pg C yr⁻¹ from the atmosphere.

[学科分类] 地球化学与岩石