Deviations of δ13C:PO4 and Cd:PO4 ratios from the global trend are observed within the density range 26.8>σΘ>27.7 close to where the isopycnal surfaces outcrop at the sea surface. In these areas tracer concentrations seem to be decoupled from nutrient concentrations presumably because of the effects of gas exchange and varying recycling depths for tracers and nutrients. Temperature exerts strong control on the surface water δ13C anomalies in high latitudes but results from an ocean-atmosphere δ13C -simulating box model imply that the anomalies still exist if air-sea δ13C -fractionation is held constant for the warm and cold surface ocean. The warm and northern Atlantic surface waters acquire a lower δ13C than expected from biological fractionation alone, whereas the Antarctic surface δ13C is somewhat more positive. A dynamic balance is achieved where CO2 evading from Antarctic waters to the atmosphere has a lower 13C/12C ratio than that invading, while the isotope ratio evading from the warm ocean is greater than that of the CO2 return flux.Box model simulations of Cd and PO4 distribution suggest that differential uptake of Cd in Antarctic waters affects the ocean Cd distribution globally because these waters are entrained into intermediate waters which spread about the global ocean. Stripping Cd from Antarctic surface waters produces the greatest intermediate water depletion in the Atlantic, deeper recycling produces the greatest intermediate water Cd depletion in the North Pacific with noticeable lower Cd:PO4 ratios than those of the deep waters in that ocean. Because of the polar mediterranean nature of the circulation in the northern Atlantic differential uptake of Cd in boreal Atlantic surface waters would only be detected locally. In the rest of the ocean that is capped by other surface waters, a linear correlation of Cd and PO4 would be observed throughout the water column. Changing parameters related to biological cycling in Antarctic surface waters exerts by far the largest control on the distribution of Cd and PO4 in the world ocean.Because δ13C and Cd are not uniquely tied to phosphorus a reliable interpretation of foraminiferal δ13C and Cd/Ca records is difficult, especially if they come from shallow-ocean core sites at high latitudes or other areas of oceanic upwelling. When estimating nutrient concentrations from benthic δ13C and/or Cd/Ca ratios we need to know if we are on the low-nutrient or high-nutrient side of the break in the global regression lines. It may be possible to obtain this information by measuring a number of different proxies with different boundary conditions. Subtle differences between independent proxy records may tell us more about past ocean chemical cycling than coherent paleo-proxy records.
Weitere Kapitel dieses Buchs durch Wischen aufrufen
- Tracer-Nutrient Correlations in the Upper Ocean: Observational and Box Model Constraints on the Use of Benthic Foraminiferal δ13C And Cd/Ca As Paleo-Proxies for the Intermediate-Depth Ocean
- Springer Berlin Heidelberg