Silica burial rate

A box model fiar the marine silica cycle is presented in Figure 6.11 with respect to the processes that control DSi and BSi. An oceanic budget is provided in Table 16.3 in which site-specific contributions to oceanic outputs are given. This table illustrates that considerable uncertainty still exists in estimating the burial rate of BSi. Regardless, burial of BSi is responsible for most of the removal of the oceanic inputs of DSi, with the latter being predominantly delivered via river runoff. This demonstrates the importance of the biological silica pump in the crustal-ocean-atmosphere factory. 

Conversely, perturbations in the burial rate of BSi have the potential to alter the marine silica cycle. For example, changes in sea level affect the expanse of continental shelf Since BSi burial is more efficient in shelf sediments (because of better preservation), a topographic change that alters the spatial extent of this depositional environment has the potential to alter the size of the DSi reservoir. 

The offshore advective flux for Si shown in Fig. 17.3 (30 X 10 mol d l) was calculated by difference, based on the total flux of dissolved Si supplied to the shelf system (32 X 10 mol Si d-1), the estimated deltaic burial rate (1-3 x 10 mol Si d ), and the nearshore particulate flux (0.1-0.7 x 10 mol Si d ). This advective flux is in good agreement with the results of Daley (1997), who estimated that 30 x 10 mol d of Si leave the shelf, based on seasonal field data and a multibox model for the shelf. Most of the silicate (94%) supplied to the shelf by external sources appears to be transported to the open ocean in either dissolved or particulate form. Approximately 36% of the Si leaving the outer shelf is in particulate form according to these calculations. Biogenic silica export may have contributed to the lack of closure in the Edmond et al. (1981) silicate budget for the shelf, although deltaic burial also remains as a potentially important sink. 

The rate of the smectite - illite reaction is thus directly proportional to K"" and H+, but is retarded by and by dissolved silica and Na". In deepening sedimentary basins, the extent of the reaction at any depth also depends on the local thermal gradient (temperature) and the sediment burial rate (reaction time). Because smectites of small particle size are the least stable, they alter to illite at lower temperatures than do coarser-grained smectites (Fig. 9.5). 

The geographic distribution of opal in the surfece sediments is controlled by (1) the local rain rate of biogenic silica, (2) the degree of its preservation in the sediments, and (3) the relative rate of accumulation of other types of particles. Preservation is promoted by rapid burial as this isolates BSi from seawater. But if the BSi is buried by other particle types, the relative contribution of BSi to the sediment is diluted. This dilution effect causes the BSi content of most continental margin sediments to be low despite high rain rates. Preservation efficiency is also dependent on (1) the intensity of bioturbation and suspension feeding and (2) the various factors that control... 

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