The Oxidants for Organic Matter Degradation: O2, N, and S

There is another group of bioactive elements, the oxidants required for organic matter oxidation (O2, SO43-, and NO3-; see Table 10.1), which are removed directly from seawater by early diagenetic reactions. The net rate of loss of SO43- from seawater due to sulfate reduction during organic matter diagenesis is about 0.5 x 1012 moles/yr (Berner, 1982), a rate which is comparable to, but smaller than, the input rate of SO43- to the oceans by rivers and its removal by hydrothermal activity (Edmond et al., 1979). The residence time of S in the oceans with respect to these processes is long, on the order of 107 years. Oceanic dissolved O2 and fixed nitrogen levels may be affected on shorter time scales. Jahnke and Jackson (1987), for instance, used a compilation of benthic flux measurements to conclude that most of the oxygen consumption occurring below 3 km in the oceans is driven by sedimentary organic carbon degradation; and Emerson et al. (1987) inferred from determinations of the amount and lifetime of reactive organic carbon in pelagic sediments that increases in the sedimentary organic carbon degradation rate during the last glacial (due to increases in the sedimentary organic carbon concentration) could have had a significant influence on deep ocean apparent oxygen utilization rates. Sedimentary organic carbon degradation may also have important effects on the oceanic nitrogen cycle. In continental margin sediments, oxygen is often depleted during early diagenesis, and denitrification begins (Table 10.1). NO3- consumed during denitrification most likely is reduced to N2 (Bender et al., 1977; Froelich et al., 1979; Goloway and Bender, 1982; Jahnke et al., 1982a), and the N released from organic matter during denitrification and sulfate reduction also may ultimately be oxidized from NH3 to N2. Thus, organic matter degradation is a fixed nitrogen sink; in fact, some recent estimates of denitrification on continental shelves exceed estimates of inputs of fixed nitrogen to the ocean plus oceanic nitrogen fixation (Codispoti and Christensen, 1985; Christensen et al., 1987).

In this section, we have illustrated the roles of early diagenetic reactions in modern oceanic geochemical cycles. In the case of the elements affected by aluminosilicate mineral alteration reactions, early diagenesis is a primary source of the element to seawater (Na) or sink from seawater (K, Mg). For the bioactive elements, C, Ca, and Si, early diagenesis is an active player in the cycling of the elements within the ocean and an important determinant of loss rates from the ocean. For the major ions of seawater, the residence times of the elements with respect to input or loss due to early diagenesis are on the order of millions of years. The bioactive elements, C and Si, appear to have shorter residence times with respect to early diagenesis, about 104 years. The oxidants used during organic matter degradation are consumed during early diagenesis: for O2 and fixed nitrogen, the consumption rates may be rapid enough to affect oceanic distributions on thousand to ten thousand year time scales.

EARLY DIAGENETIC FLUXES AND THE SEDIMENTARY RECORD

Introduction

In the preceding section we showed that, as a general rule, diagenetic fluxes are small in the sense that changes in them affect oceanic geochemical cycles on time scales of tens of thousands of years. The significance of early diagenesis to the study of shorter-term processes (i.e., processes with time scales of hundreds to a few thousand years) lies in the fact that the history of past climatic change is an important source of information about the causes and effects of future change. Temporal variability in the concentrations and accumulation rates of the biogenic components of marine sediments are a key source of information on changes in oceanic and atmospheric conditions on the thousand to ten thousand year time scale of the events governing glacial-to-interglacial transitions. Because early diagenetic reactions at the surface of sediments may affect the relationship of observed concentrations and accumulation rates to oceanic and atmospheric conditions, correct interpretation of the sedimentary record requires an understanding of the relationships between rain rates to the sea floor, conditions at the surface of the sediments, and the ultimate concentrations and accumulation rates of sediment components.

The role of early diagenesis in fixing sediment accumulation rates can be viewed in terms of a simple mass balance model. In steady state, the rain rate of a sediment component to the sea floor (R) must equal the sum of its degradation/dissolution rate (F) and its burial rate (B). Then, defining the burial efficiency (E) as the fraction of the rain to the sea floor that is preserved,

If E is constant, or if burial rates are much larger than reaction rates, then early diagenetic reactions have little effect on burial rates, and the latter are easily translated into rain rates to the sea floor. We have shown already that this is not true for organic carbon, CaCO3, and SiO2 (Table 10.6): early diagenetic reaction rates are much greater than burial rates for organic carbon, and significantly larger for SiO2 and CaCO3. Burial rates are slow relative to reaction rates for these biogenic materials, and the relationship between



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