Foraminifera shells

There has been a long discussion in the literature as to the extent to which foraminifera shells are in oxygen-isotopic equilibrium. The precise state of equilibrium cannot be defined with sufficient precision from theory, so the general practice is to compare observed foraminiferal calibrations to inorganic experiments... 

Foraminifera shells differ from the thermodynamic prediction in two fundamental ways. 

Lea and Boyle (1989) showed that the barium content in foraminifera shells is, like cadmium, controlled by the barium content in bottom waters. Barium, in a broad sense, also cycles like a nutrient (depleted in surface waters, and higher in deep waters), but its regeneration occurs deeper than the organic matter. This results in a close correlation between barium and alkalinity in today s ocean (Lea, 1993). Like Cd/Ca, Ba/Ca in foraminifera has also been used, as a paleo-tracer of water masses (e.g.. Lea and Boyle (1990), but suffers the same carbonate-saturation-state-linked effect as Cd/Ca (McCorkle et al., 1995). 

Along with the silicate debris carried to the sea by rivers and wind, the calcitic hard parts manufactured by marine organisms constimte the most prominent constituent of deep-sea sediments. On high-standing open-ocean ridges and plateaus, these calcitic remains dominate. Only in the deepest portions of the ocean floor, where dissolution takes its toll, are sediments calcite-free. The foraminifera shells preserved in marine sediments are the primary carriers of paleoceano-graphic information. Mg/Ca ratios in these shells record past surface water temperatures temperature corrected 0/ 0 ratios record the volume of continental ice ratios yield information... 

Three possible dissolution processes come to mind. The first of these is termed water column dissolution. As foraminifera shells fall quite rapidly and as they encounter calcite undersaturated water only at great depth, it might be concluded that dissolution during fall is unimportant. But it has been suggested that... 

This Waterloo was unfortunate for the author considers the boron method to be basically sound and potentially extremely powerful. The answer to the benthic enigma may lie in species-to-species differences in the boron isotope vital effect for benthic foraminifera. The measurement method use by Sanyal et al. (1996) required a large number of benthic shells in order to get enough boron to analyze. This created a problem because, as benthics are rare among foraminifera shells, mixed benthics rather than a single species were analyzed. If the boron isotope pH proxy is to... 

The other bottom-water CO ion concentration proxy is based on the Zn/Cd ratio in benthic foraminifera shells. As shown by Marchitto et al. (2000), the distribution coefficient of zinc between shell and seawater depends on COl ion... 

HONISCH, B. Hemming, N. G. 2004. Ground-tmthing the boron isotope paleo-/)H proxy in planktonic foraminifera shells partial dissolution and shell size effects. Paleoceanography, 19 PA4010, doi 10.1029/2004PA001026. 

Honisch, B., Bijma, j., Russell, A. D., Spero, H. J., Palmer, M. R., Zeebe, R. E. Eisenhauer, A. 2003. The influence of symbiont photosynthesis on the boron isotopic composition of foraminifera shells. Marine Micropaleontology, 49, 87-96. 

Microscopic examination of seafloor sediments (if shallow enough that the CaCOs does not dissolve) and of material caught in sediment traps has revealed that much of the calcium carbonate in the samples consists of coccoliths. The flux of coccoliths probably accounts for c. 50% of the total vertical CaCOs flux in open ocean waters (in other words, about 50% of the inorganic carbon pump), with foraminifera shells responsible for most of the rest. It is usually not the most numerous species (E. huxleyi) but rather larger species (e.g., Calci-discus quadriperforatus and Coccolithus pelagicus) that make the greatest contributions to the total coccolith flux. 

Figure 1.11 Foraminifera shells from Great Barrier Reef, Australia (top figure) and an enlarged surface micrograph (bottom) showing unique interconnected porosity structure.

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