ALBERT

Description: A model for interstitial silica concentrations is derived, incorporating biological mixing of sediments. This model predicts concentrations and gradients and can account for the observed geographical variations in interstitial silica on the basis of a dynamic balance between solution of silica particles and diffusion from the sediments. The flux of particulate biogenous silica into the sediments is confirmed as an important parameter controlling interstitial silica concentrations. Biological mixing of sea floor sediments also has an important influence on interstitial composition by modifyirig the depth at which dissolving particles react. Faster mixing raises the interstitial concentration. The rate at which siliceous particles dissolve also plays a role; the slower they dissolve, the greater the interstitial silica concentration. Measurements on near‐bottom waters of the Atlantic show no consistent gradients in dissolved silica, but antarctic bottom water seems significantly more variable in the benthic boundary layer than in the water mass above or in the benthic zone of North Atlantic deep water.

Description: Biological mixing in deep‐sea sediments is described in terms of a time‐dependent eddy diffusion model where mixing takes place to a depth L at constant eddy diffusivity D. The differential equation that describes this model has been solved for an impulse source of tracer delivered to the plane surface that forms the top of the mixed layer. The solution then serves as a Green's function, which can be used to determine the distribution of tracer in depth and in time for a surface input of tracer specified as any arbitrary function of time. The characteristic properties of the solution are dependent on the dimensionless parameter D/Lυ, where υ is the sedimentation rate. If D/Lυ is greater than 10, the surface layer becomes homogeneous, and the model is identical to the homogeneous layer model proposed by Berger and Heath (1968). If D/Lυ is less than 0.1, little mixing can take place before the sediments are buried, and so the surface concentration propagates downward into the sediments with little dispersion. For all values of D/Lυ the weighted mean depth of the concentration distribution is the depth at which an impulse source would be found in the sediment if no mixing had taken place. The microtektite data of Glass (1969, 1972) and Glass et al. (1973) indicate that abyssal sediments are mixed from the surface to a maximum mixing depth that ranges between 17 and 40 cm below the surface. Mixing occurs at rates between 1 and 100 cm2 kyr−1. Higher mixing rates may occur nearer the surface, but microtektite distributions cannot be used to estimate these rates in the presence of the deeper, slower mixing. Estimates for D based on dimensional analysis of sediment reworking rates for nearshore organisms (103–106 cm2 kyr−1) are used to predict abyssal mixing rates between 1 and 103 cm2 kyr−1 by invoking the assumption that mixing is proportional to biomass. Plutonium distributions in deep‐sea sediments (Noshkin and Bowen, 1973) indicate abyssal mixing rates ranging from 100 to 400 cm2 kyr−1.

Description: THE class Cephalopoda is represented by over 10,000 fossil species and about 1,000 living species. Almost the entire fossil record so far described consists of external or internal calcareous shells of the Ammonoidea, the Nautiloidea and some members of the Coleoidea. The subclass Coleoidea includes all but three (Nautilus spp.) of the living species of cephalopods, and is represented in the fossil record largely by the order Belemnitida which is important from the early Jurassic to the Eocene. Another coleoid order, the Sepioidea, including the living Sepia and Spirula have internal calcareous shells, and have left traces from the Upper Jurassic (Voltzia) to the present. The three remaining coleoid orders, the Teuthoidea, the Vampyromorpha and the Octopodida include all the remaining living cephalopods, comprising 29 teuthoid or squid families, one vampyromorph family and 12 octopod families.