Atlantic Ocean carbonate saturation

More recent calculations such as those in this book indicate substantially lower saturation depths. Those calculated here are plotted in Figure 4.21. The SD is generally about 1 km deeper than that presented by Berger (1977). Clearly the new SD is much deeper than the R0 and appears only loosely related to the FL. Indeed, in the equatorial eastern Atlantic Ocean, the FL is about 600 m shallower than the SD. If these new calculations are even close to correct, the long cherished idea of a "tight" relation between seawater chemistry and carbonate depositional facies must be reconsidered. However, the major control of calcium carbonate accumulation in deep sea sediments, with the exceptions of high latitude and continental slope sediments, generally remains the chemistry of the water. This fact is clearly shown by the differences between the accumulation of calcium carbonate in Atlantic and Pacific ocean sediments, and the major differences in the saturation states of their deep waters. 

Figure 4.21. Latitudinal variation of saturation depths (SD) and carbonate sediment facies in the eastern and western Atlantic Ocean basins. (Modified after Berger, 1977.)...

The Geochemical Ocean Section Program (GEOSECS) has produced data from which it is possible to profile the saturation state of seawater with respect to calcite and aragonite in the Atlantic and Pacific oceans. Representative north-south calcite saturation profiles for the Western Atlantic and Central Pacific oceans are presented in Figures 5 and 6 (based on 39). It was observed that the saturation state of seawater with respect to calcite at the CCD was close to constant ( 2 = 0.70 I" 0,05) except in the southern extremes (39). Broecker and Takahashi (31) have recently found that the carbonate ion concentration is close to constant at the FL, when appropriate corrections are made for pressure. The saturation state of seawater at the FL, calculated by the method presented in this paper, is 0.80 0.05. Berger (40) has presented profiles for Rq, FL, CCD and CSL (calcite saturation level) in the eastern and western Atlantic ocean (see... 

Figure 7. The depth distribution of the Ro and calcite saturation levels, the foraminiferal lysocline and the calcium carbonate compensation depth in the Western and Eastern Atlantic Ocean (after Ref. 40)...

Mackensen, A. Licari, L. 2004. Carbon isotopes of live benthic foraminifera from the eastern South Atlantic Ocean sensitivity to bottom water carbonate saturation state and orgnaic matter rain rates. In Wefer, G., Mulitza, S. Ratmeyer, V. (eds) The South Atlantic in the Late Quaternary - Reconstruction of Material Budget and Current Systems. Springer-Verlag, Berlin, Heidelberg, 623 -644. 

Figure 3 Bathymetric profiles of calcium carbonate (calcite) saturation for hydrographic stations in the Atlantic and Pacific Oceans (data from Takahashi etai 1980). Carbonate saturation here is expressed as ACOa ", defined as the difference between the in situ carbonate ion concentration and the saturation carbonate ion concentration at each depth ACOa " = [C03 ]seawater - [COa Jsaturation)-The saturation horizon corresponds to the transition from waters oversaturated to waters undersaturated with respect to calcite (A 003 = 0). This level is deeper in the Atlantic than in the Pacific because Pacific waters are COa-enriched and [C03 ]-depleted as a result of thermohaline circulation patterns and their longer isolation from the surface. The Atlantic data are from GEOSECS Station 59 (30°12 S, 39°18 W) Pacific data come from GEOSECS Station 235 (16°45 N,161°23 W).

North Atlantic to 500 m in the North Pacific. This reflects an increasing addition of CO2 to deep waters as meridional overturning circulation moves them from the Atlantic to the Indian and then to the Pacific Ocean. Thus, as a water mass ages, it becomes more corrosive to calcium carbonate. Since aragonite is more soluble than calcite, its saturation horizon lies at shallower depths, rising from 3000 m in the North Atlantic to 200 m in the North Pacific. 

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