Geochemical variability

If Vtii/Ptii = 1 then the accumulation of °Thxs in marine sediments would provide an assessment of their sedimentation rate. For instance, if Pm is N dpm m yr , and N dpm are found in the upper 1 cm of 1 m of seafloor, then the sedimentation rate must be lcmyr Sedimentation rate is an important variable in paleoceanographic reconstruction as it provides the timescale for the continuous record of environmental change recorded in marine sediments. Sedimentation rate is also a key geochemical variable as sediments are the major sink for most chemical species in the ocean. A tool allowing assessment of past sedimentation rates is therefore an appealing prospect. 

PCs with high eigenvalues are assumed to best represent the geochemical variability and characteristics of the data. Components 4 and higher returned Eigenvalues below 1.0 and were excluded from further interpretation, as discussed by Mandal et al. (2008). 

Given a geochemical variable y, m measurements at times tu t2,..., tm produce the unevenly spaced time series yl5 y2,..., ym, which we lump together as the vector y. In order to find out eventual periodicities, Lomb (1976) suggests fitting the data by a sine wave using a least-square criterion. For any arbitrary frequency /, the fitting function is written... 

Let us first introduce some important definitions with the help of some simple mathematical concepts. Critical aspects of the evolution of a geological system, e.g., the mantle, the ocean, the Phanerozoic clastic sediments,..., can often be adequately described with a limited set of geochemical variables. These variables, which are typically concentrations, concentration ratios and isotope compositions, evolve in response to change in some parameters, such as the volume of continental crust or the release of carbon dioxide in the atmosphere. We assume that one such variable, which we label/ is a function of time and other geochemical parameters. The rate of change in / per unit time can be written... 

When the geochemical variable is not uniquely determined but is a random variable, we would like to be able to assess how the parameters of the population change... 

We can now write explicitly (t) in terms of the geochemical variable yc(t, t) through equation (7.2.38). Again, the condition that there is no primordial crust at t=0 requires that yc(t, t) and ym(t) are equal and therefore the constant is zero. Rearranging, we get... 

Cronan, D.S., 1997. Some controls on the geochemical variability of manganese nodules widi particular reference to the tropical South Pacific. In Nicholson, K., Hein, J.R., Buhn, B. and Dasgupta, S. (eds). Manganese mineralization Geochemistry and mineralogy of terrestrial and marine deposits. Geological Society Special Publication, 119 139-151. 

Computational requirements limit direct incorporation of geochemical sorption models into current PA codes, and it is likely that the use of PDFs for K s will continue in the next generation of PA codes. One approach to incorporating detailed model results in PA is to use them off-line to provide constraints on parameter uncertainty and account for site-specific geochemical variability. Two approaches arc... 

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