Foraminifera dissolution

Dating of speleothems from western Malaysian cave were recently reported55 in addition to those of Brazilian,56 Taiwan,43 Indian shells57 and Chinese calcitic shells with Mn2+ impurities.58 In the last one, a new intensity standard sample of CuS04. 5H20 was developed as will be described later. Past carbonate dissolution in deep sea sediments has been studied through ESR spectroscopy of microfossil foraminifera.59... 

Figure 2.14. The log of dissolution rate in percent per day versus the log of (1-fi). A = whole Indian Ocean sediment dissolved in deep-sea sediment pore water B = whole Pacific Ocean sediment dissolved in Atlantic Ocean deep seawater C = whole Atlantic Ocean sediment dissolved in Long Island Sound seawater (Morse and Berner, 1972) D = > 62 pm size fraction of the Indian Ocean sediment dissolved in Atlantic Ocean deep seawater, E = the 125 to 500 pm size fraction of Pacific Ocean sediment dissolved in Atlantic Ocean deep seawater F = 150 to 500 pm Foraminifera dissolved in the Pacific Ocean water column. (After Morse, 1978.)...

Betzer et al. (1984, 1986) studied the sedimentation of pteropods and foraminifera in the North Pacific. Their sediment trap results confirmed that considerable dissolution of pteropods was taking place in the water column. They calculated that approximately 90% of the aragonite flux was remineralized in the upper 2.2 km of the water column. Dissolution was estimated to be almost enough to balance the alkalinity budget for the intermediate water maximum of the Pacific Ocean. It should be noted that the depth for total dissolution in the water column is considerably deeper than the aragonite compensation depth. This is probably due to the short residence time of pteropods in the water column because of their rapid rates of sinking. 

The foraminiferal lysocline (FL) was defined by Berger (1968) as the depth where the predominant type of foraminifera shifts in surface sediments, because of dissolution, from "soluble" to "resistant" species ( 50% change in ratio). To determine the depth of the foraminiferal lysocline, three conditions must be met (Parker and Berger, 1971) ... 

Adelseck C.G., Jr. and Berger W.H. (1975) On the dissolution of planktonic foraminifera and associated microfossils during settling and on the sea floor. In Dissolution of Deep-Sea Carbonates (eds. A. Be and W. Berger), pp. 70-81. Cushman Foundation for Foraminiferal Research, Special Publication 13, W. Sliter. 

Berger W.H. and Piper D.J.W. (1972) Planktonic foraminifera Differential settling, dissolution, and redeposition. Limnol. Oceanogr. 17, 275-287. 

Keir R.S. and Hurd D.C. (1983) The effect of encapsulated fine grained sediment and test morphology on the resistance of planktonic foraminifera to dissolution. Mar. Micropaleontol. 8, 183-214. 

Berger (12) has identified another sediment marker level in the sediments. This is the level at v hich the proportion of "resistant" planktonic foraminifera show the first significant increase. Berger s estimate of minimum dissolution loss at the Rq level is 10 percent. Adelseck s (11) experimentally determined amount of dissolution necessary to produce the Rq level is approximately 50 percent dissolution. 

Figure 17. Log of the dissolution rate vs. total carbonate ion concentration for synthetic aragonite, pteropods, calcitic Pacific Ocean sediment, and foraminifera in the 125-500 iim size fraction. (A) indicates ihe aragonite equilibrium total carbonate ion concentration at 25°C, 1 atm (26). (C) indicates the calcite equilibrium total carbonate ion concentration at 25°C, 1 atm (25).

Planktonic shell thickness. A third indicator of CaC03 dissolution within the water column comes from a more subtle examination of the shells of foraminifera, the thickness of their walls in fact. This technique was pioneered by Lohmann (1995) and further pursued by Broecker and Clark (2001). What they find is that the shell thickness of forams increases throughout the first kilometer of the water column, and decreases below 3 km, representative of dissolution within the water column. 

Broecker W. S. and Clark E. (2001) An evaluation of Lohmann s foraminifera weight dissolution index. Paleo-ceanography 16, 531-534. 

Lxthmann G. P. (1995) A model for variation in the chemistry of planktonic foraminifera due to secondary calcification and selective dissolution. Paleoceanography... 

Brown S. and Elderfield H. (1996) Variations in Mg/Ca and Sr/ Ca ratios of planktonic foraminifera caused by postdeposi-tional dissolution—evidence of shallow Mg-dependent dissolution. Paleoceanography 11(5), 543-551. 

Caron D. A., Anderson O. R., Lindsey J. L., Faber J. W. W., and Lin Lim E. (1990) Effects of gametogenesis on test structure and dissolution of some spinose planktonic foraminifera and implications for test preservation. Mar. Micropaleontol. 16, 93-116. 

Wu G. and Berger W. H. (1989) Planktonic foraminifera differential dissolution and the Quaternary stable isotope record in the west equatorial Pacific. Paleoceanography 4(2), 181-198. 

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... 

Figure 5 Results of in situ dissolution experiments. Peterson (1966) re-weighed polished calcite spheres after a 250 d deployment on a mooring in the North Pacific. Honjo and Erez (1978) observed the weight loss for calcitic samples (coccoliths, foraminifera and reagent calcite) and an aragonitic sample (pteropods) held at depth for a period of 79 d. While Peterson hung his spheres directly in seawater, the Honjo-Erez samples were held in containers through which water was pumped. The results suggest that the calcite saturation horizon lies at 4,800 200 m in the North Atlantic and at about 3,800 200 m in the North Pacific. For aragonite, which is 1.4 times more soluble than calcite, the saturation horizon in the North Atlantic is estimated to be in the range 3,400 200 m.

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