Sedimentation of planktonic foraminifera

Abstract

Fossil assemblages of planktonic Foraminifera contain many valuable clues to paleoclimate and paleo-oceanography. Unfortunately, our understanding of production, dissolution, redeposition, and other processes of foraminiferal sedimentation is but rudimentary. Lacking direct observations, information largely rests on comparisons between abundance and composition patterns of life-, death-, and sediment-assemblages. Standing stock and production of life assemblages vary greatly, depending on the fertility of the water. Fertility is greatest near continents and along the equator. The shell supply in these fertile regions is several times greater than in the sterile subtropical areas. The distribution of living and empty shells in the water column suggests that virtually all production takes place within the mixed layer, possibly near its base. Size distributions suggest that most formainiferal production becomes food for predators and is not available for sedimentation. Morphology patterns of living and empty shells confirm this suggestion. Thus, very rapid turnover of populations is required to produce the sedimentation rates observed. A residence time of about one week for living Foraminifera larger than 150 μ is proposed for fertile regions, implying life spans of no more than about 2 weeks. Most of the foraminiferal shells reaching the ocean floor are redissolved. Dissolution is complete below the calcite compensation depth (CCD), but much solution takes place above this level also. This fact has long escaped notice, because percentages of calcite are poor indicators of solution patterns. Any reasonable solution profile, even a linear one, would produce a “sudden” decrease of calcite content at the level where the amount of calcite becomes comparable to the amount of non-calcareous dilutants. This is evident from discussion of the formula: L=100(1− Ro R ) where L is the loss of sediment necessary to increase the insoluble fraction R 0 to R percent. (For example, set R 0 = 5%; R = 10%, yielding L = 50%; i.e., 50% of the sediment, or more than half of the carbonate, was lost while the carbonate content went from 95 to 90%!). The same formula yields minimum dissolution in formainiferal assemblages, if resistant planktonic species (or alternatively, benthonic forms) are assumed insoluble. Resulting solution estimates improve understanding of the sedimentary lysocline, a depth level that separates well-preserved from poorly preserved assemblages. Calcite saturation levels inferred from thermodynamic calculations are conceptually and actually different from both lysocline and CCD. While the depth (i.e., pressure) control of solution patterns is obvious, circulation and fertility patterns also influence preservation of foraminiferal sediments. In general, fertile areas tend to have poorly preserved calcareous assemblages, even at shallow depths, because much CO 2 is developed in the sediments. The associated CCD may be quite deep, however, because of high sedimentation rates of calcium carbonate. The ultimate reason for dissolution of shells is that organisms supply more CaCO 3 to the ocean floor than is being introduced from external sources into the oceanic system. This excess supply leads to undersaturation. Excess supply and dissolution rates must balance for geochemical steady state to prevail. Undersaturation, therefore, proceeds to a value where dissolution rates are just right to provide this balance.