elements (e.g. barium, radium, and strontium) is carried along with the 

 particulate silicates, carbonates, phosphates, nitrogen compounds and 

 organic materials. Re-solution of sinking carbonates may affect the 

 alkalinity, carbon and oxygen isotope ratios, and trace-element patterns. 

 The kinetics of the dissolution and the true oceanic sinking rates need 

 more study. Likewise, silicate tests are subject to dissolution as they 

 sink, but there is little agreement on the balance between in-transit 

 solution and bottom solution followed by upward mixing. The nature of 

 this balance will affect the associated trace elements as well as the silicate 

 distribution. 



Casts of marine organisms discarded in the surface water during moult- 

 ing may be rich in nutrients or trace metals. The rapid descent of such 

 materials may constitute a significant transport mechanisms, but it also 

 lacks quantitative evaluation. 



Yet another problem is the fluxes of dissolved substances across 

 the sediment-water interface. What are the magnitudes of these fluxes 

 relative to other input or removal processes? Predictive models are 

 needed to estimate the sink and source potentials of different sediment 

 types for a given solute. It is also necessary to understand when a mate- 

 rial that has settled to the seafloor is no longer chemically in contact with 

 the ocean. 



Models for solute flux across the sediment-seawater interface that 

 balance supply and removal by dissolution/precipitation/relax reactions 

 have been proposed (Crank, 1956; Shishkina, 1964; Berner, 1964,1971; 

 Duursma, 1966; Anikouchine, 1967; Lerman, 1971; Tzur, 1971; 

 and Hurd, 1972a,b). The apparent diffusion coefficient of a solute is a 

 function of temperature, tortuosity, and any interaction the solute may 

 have with the surface of the solid phase. These solid-solute interactions 

 are being investigated (Duursma, 1970; Manheim, 1970; and Lerman, 

 1971), but the results have not been rigorously applied to real sediments. 

 Present results, however, suggest that these corrections may reduce the 

 value of the apparent diffusion coefficient of ions by as much as 3 to 

 5 orders of magnitude. If solids are dissolving or precipitating within 

 the sediments, determinations of the heterogeneous solution rate con- 

 stants must be made to compare laboratory data with models based 

 on suspended matter and sediment. In the case of biogenic opal (Hurd, 

 1972a,b), dissolution rate constants of acid-cleaned radiolarians in 

 seawater differed by 3 to 6 orders of magnitude from rate constants 

 for sediments estimated by the models mentioned above. Other biogenic 

 minerals such as calcium carbonate and apatite need to be similarly 

 modeled because of the more rapid turnover through biogenic mineral 

 formation than through inorganic precipitation of the same elements 

 (Ca, C, O, Si, and P). 



38 



