John R. Southam

Estimates of Cenozoic Oceanic Sedimentation Rates

Estimation of average Cenozoic sedimentation rates for the Atlantic, Indian, and Pacific oceans indicates global synchronous fluctuations. Paleocene-early Eocene and late Eocene-early Miocene rates are only a fraction of middle Eocene and middle Miocene-Recent rates. These changes must reflect significantly different modes of continental weathering, which may be due to alternate states of atmospheric circulation marked by reduction of global precipitation.

CARBPLAT - A computer model to simulate the development of carbonate platforms

This paper presents a computer model with two new main components that simulates the development of carbonate platforms. First, it uses a function for the decrease of carbonate production with depth that combines the photosynthetic growth of carbonate-producing organisms and the extinction of light with depth. Platform growth is simulated with a differential equation incorporating sea level. These processes therefore act continuously rather than in steps. Second, it attempts a more realistic approach to predicting carbonate-platform slopes by incorporating variation of slope angle with textural composition of the sediment. In this model, the angle at which slope sedimentation begins depends on the radio between sediment produced on the platform interior (mud) and that produced on the platform margin (sand). The shape of the slope is represented by an exponential function that can be changed to accommodate the total amount of slope sediment. In the model, stratigraphic unconformities due to changing textural composition are produced on the fore-reef slopes.

Correlation of stratigraphic sections by continuous variables

Abstract A technique is presented to obtain greater stratigraphic resolution than methods based on unique events. The information content of the stratigraphic record between unique events in any two sections is used to calculate a cross-correlation coefficient. Stratigraphic correlation is obtained by varying the stretching or shrinking of the records to maximize the cross-correlation coefficient. This procedure yields the relative sedimentation rates of the two sections being compared.

Dynamical formulation of Broecker's model for marine cycles of biologically incorporated elements

Broecker presented a steady-state, two-box model which emphasizes the role of kinetic factors in determining the chemical composition of sea water. Unlike thermodynamic models, Broeckers model suggests that sizable temporal variation in the composition of sea water and rates of sedimentation may have taken place in the past 100 million years. To describe the evolution of the oceans chemical system and interpret variations observed in the sedimentary record, we have formulated a dynamical model. Mass-balance consideration leads to a set of coupled nonlinear differential equations. The equations are linearized and solved for step function changes in the rate of river input and in the rate of vertical mixing. This simple model of the oceans chemical system is shown to be stable against oscillations. Using data for the modern ocean, the response times for P, C, Si, Ca, and Ba are calculated to be of the order of 104 to 106 yr for changes in river and other inputs and of the order 101 to 102 yr for a change in the rate of vertical mixing. Analog-simulation techniques, discussed for the situation of the two-box model, provide a powerful tool for treating nonlinearities and systems with more than two components. In the final section, consequences of the dynamical formulation are compared with parameters appearing in Broeckers steady-state formulation.