burial rates and rain rates is no longer simple. A numerical example illustrates the point. If 95 percent of the rain of, for instance, organic carbon is degraded during early diagenesis (F/R=0.95), then a 5 percent reduction in F due to a change in the sedimentary environment (F/R=0.9) will cause a doubling of the fraction of the rain that is preserved: the accumulation rate will double with no change in the rain rate. Although early diagenetic reactions affect oceanic concentrations of the chemical species of interest here only on long time scales, they cannot be ignored when sedimentary concentrations are considered: an understanding of early diagenetic reactions is essential to the interpretation of the sedimentary record.
The simple mass balance of Eq. (10.1) hides the complexity of the relationships that determine burial efficiency for biogenic materials: relationships between solid phase reactivity, the chemical and biological environment of the sediments, and the transport processes that operate within the sediment column. The relationships can be demonstrated using simple diagenetic models, but application of the models to the problem of predicting spatial and temporal variability in burial efficiency, as well as to the more difficult problem of estimating burial efficiency variations through time from the sedimentary record, requires a wealth of data about early diagenesis. The data base does not yet exist. In the following paragraphs, we will review existing knowledge and present a framework for the examination of relevant sedimentary properties.
Measuring burial efficiencies is a difficult problem. It requires measurement of two of the three variables in Eq. (10.1), coupled with a steady-state assumption. All of the measurement uncertainties we mentioned previously—sampling resolution, stoichiometric uncertainties, possible sampling artifacts—apply to the benthic reaction rate determinations we will use. Additional uncertainties arise from the varying temporal resolution of the different measurements. Sediment trap based rain rate measurements typically average over a one or two year period. Because of the rate at which benthic organic carbon degradation and silica dissolution occur, benthic flux measurements of species involved in these reactions, and also in carbonate dissolution, are averaged over periods which may range from years to hundreds of years. The resolution of burial rate (accumulation rate) measurements is determined by sediment mixing, and can be limited to 103 to 104 years (Dymond and Lyle, 1985). Thus, comparisons of these different measurements must be interpreted with some caution. Where possible, we apply calculations from different pairs of measurements in order to compare results.
The simplest case among the three we examine is CaCO3. Sedimentary CaCO3 dissolves when the pore waters surrounding it are undersaturated with respect to the mineral phase (calcite or aragonite). By most accounts, resaturation is very rapid. Keir (1983), for example, estimated rate constants for dissolution of about 1000 percent/day. Recently, Archer et al. (1989) have estimated that the rate constant is 10 to 100 times slower; this is still rapid on the time scale of early diagenesis. The result of the rapidity of CaCO3 dissolution in undersaturated pore waters is that transport processes within the sediment column are of secondary importance in determining calcite burial efficiency.
Dissolution in sediments occurs for two reasons: because the sediments lie underneath undersaturated bottom water, and in response to acids released by the oxidation of organic matter by O2 (see the stoichiometry in Table 10.1). If only the first process is operating, dissolution will cease within a few millimeters of the sediment/water interface. The effects of the second process are to drive dissolution at water depths shallower than the CaCO3 saturation horizon and beneath the level in the sediments where the pore waters would reach saturation in the absence of organic carbon oxidation. Emerson and Bender (1981) used kinetic models of CaCO3 preservation and dissolution to suggest that it could lead to dissolution as much as 1000 m above the saturation horizon; Sayles (1981) used measurements of Ca2+ fluxes from sediments to demonstrate the occurrence of dissolution as much as 1500 m above saturation horizon in the North Atlantic. We will use calculations of the burial efficiency for CaCO3 to evaluate the importance of dissolution due to the introduction of acids from the oxidation of organic matter in the sediments and to demonstrate the variability in burial efficiency. The data are shown in Table 10.7, which lists burial efficiencies along with the carbonate content of the sediments and the method used for the calculation.
The stations in the Atlantic at which E is estimated all lie above the lysocline. Two stations lie on the continental margin, the rest in the deep Northwest Atlantic. All stations show dissolution of a significant fraction of the CaCO3 rain to the sea floor, with the largest fractional dissolution (50 to 74 percent) occurring in margin sediments. The average fractional dissolution for deep Atlantic stations is lower, 20 percent. However, these values were obtained by in situ sampling techniques with limited resolution near the sediment-water interface; as a result, the benthic Ca2+ flux, and therefore the CaCO3 dissolution rate, may be underestimated by more than a factor of two. Since all of these stations lie above the saturation horizon, all dissolution is due to the effects of organic matter oxidation, emphasizing the importance of this process.