Carbonate Deposition and Degassing

Any transfer of carbonate deposition from shallow-water shelves and platforms to the deep sea should have resulted in increased rates of global degassing in the geological past. This is because deep sea carbonates rest on oceanic crust that undergoes subduction and possible degassing at active plate boundaries. In chapter 3 it was shown that the rise and spread of calcareous plankton, chiefly coccoliths and forams, should have caused a shift of carbonate burial from shallow to deep water starting at about 150 Ma in the Jurassic. This shift has been demonstrated quantitatively by Wilkinson and Walker (1989) based on rock abundance data. The effect on CO2 degassing over the past 150 million years depends on how fast the shift has occurred, whether it has been accelerating or decelerating, and whether the pelagic carbonate has been subducted and, if subducted, also decarbonated and degassed. For example, present pelagic CaCO3 accumulation is much higher than the rate of CaCO3 subduction because a large CaCO3 fraction accumulates on the Atlantic sea-floor, which is surrounded by passive continental margins (Schrag, 2002). Berner (1991) originally assumed a decelerating transfer of carbonate to the deep-sea floor based on DSDP (Deep Sea Drilling Project) paleonto-logical data (Hay et al., 1988), but the rock abundance data of Wilkinson and Walker has caused a linerarly increasing effect to be assumed in later work (Berner, 1994; Berner and Kothavala, 2001). In the latter case the effect on degassing of the shift from shallow water to deep water deposition, resulting in increasing carbonate in subducting crust, is represented by the dimensionless parameter fC(t):

For the past 150 million years:

Before 150 Ma:

where fC(t) is the effect of carbonate content of subducting oceanic crust on the rate of CO2 degassing.

The constant value at and before 150 Ma, fC(150), is assumed to represent only nonsubduction zone degassing plus subduction of only that carbonate that infills basaltic crust because of the absence of convincing data for the existence of pelagic deep sea carbonates before this time (see chapters 2 and 3).

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Organic Gardening

Gardening is also a great way to provide healthy food for you and your loved ones. When you buy produce from the store, it just isnt the same as presenting a salad to your family that came exclusively from your garden worked by your own two hands.

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