Which fraction of the organic matter produced in the surface layer reaches the seafloor in the open ocean?
1%
Name 3 biogenic minerals that are produced in the ocean. Also provide one major producer for each mineral.
Calcite: Coccolithophores, Foraminifera
Aragonite: Corals, Pteropods
Opal: Diatoms
Give 3 reasons why the seawater changes from supersaturation for calcite in the surface ocean to undersaturation in the deep ocean. Explain the processes involved.
Calcite concentration decreases with depth because:
pressure effects the CO2 system dissociation reactions -> higher solubility with higher pressure (large effect)
CO2 addition due to respiration of OM - CO2- concentrations: CO32- + H2O + CO2 → 2 HCO3– (Bicarbonate Buffer System)
lower temperature increases solubility (small effect)
Where do we find more calcerous sediments: in the deep North Atlantic or the deep North Pacific
Deep North Atlantic → thermohaline circulation: waters have lower CO2 concentrations (new water, less organic matter decay processes). CO2 is the main driver for CCD. If CO2 levels are low, CCD is deeper.
Why does the Ca2+ concentration have rather little effect on the saturation level of seawater
(Ω) for calcite and aragonite? Explain the relevant processes.
The concentration of CO32- is much more variable (esp. with depth) and in general lower, than the Ca2+ concentration. It is “limiting” and determines the saturation level of calcite and aragonite. Further only a very small fraction of Ca2+ does not behave conservatively (small conc. changes over depth)
general: saturation level: Ω = ([Ca2+] * [CO32-])/Ksp* → Ksp*: apparent, stochiometric solubility product
if Ω > 1→supersaturation, precipitation is favored
if Ω < 1→undersaturation, dissolution is favored
Explain why is there such a clear relationship between the particulate organic carbon flux attenuation and the median temperature in the top 500 m?
The remineralization of organic material is largely temperature dependent (remember rate function: Q10 -> if temperature increases by 10°C -> 2-3x increase of “reaction”-rate (microbial processes)). Therefore, remineralization-rates and thus attenuation of particulate organic material in the upper 500m surface layer is largely temperature dependent. The warmer the surface ocean is, the less POC will be exported to deeper waters, due to enhanced microbial remineralization. However, once POC has been exported from surface layers of warmer oceans, the subsequent down transportation seems to be very effective, as the remaining POC might be more refractory and stable against remineralization.
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