Carbonate accumulation in deep sea sediments

The Oceanic Carbonate System and Calcium Carbonate Accumulation in Deep Sea Sediments... 

One of the most controversial areas of carbonate geochemistry has been the relation between calcium carbonate accumulation in deep sea sediments and the saturation state of the overlying water. The CCD, FL, R0, and ACD have been carefully mapped in many areas. However, with the exception of complete dissolution at the CCD and ACD, the extent of dissolution that has occurred in most sediments is difficult to determine. Consequently, it is generally not possible to make reasonably precise plots of percent dissolution versus depth. In addition, the analytical chemistry of the carbonate system (e.g., GEOSECS data) and constants used to calculate the saturation states of seawater have been a source of almost constant contention (see earlier discussions). Even our own calculations have resulted in differences for the saturation depth in the Atlantic of close to 1 km (e.g., Morse and Berner, 1979 this book). 

More recent calculations such as those in this book indicate substantially lower saturation depths. Those calculated here are plotted in Figure 4.21. The SD is generally about 1 km deeper than that presented by Berger (1977). Clearly the new SD is much deeper than the R0 and appears only loosely related to the FL. Indeed, in the equatorial eastern Atlantic Ocean, the FL is about 600 m shallower than the SD. If these new calculations are even close to correct, the long cherished idea of a "tight" relation between seawater chemistry and carbonate depositional facies must be reconsidered. However, the major control of calcium carbonate accumulation in deep sea sediments, with the exceptions of high latitude and continental slope sediments, generally remains the chemistry of the water. This fact is clearly shown by the differences between the accumulation of calcium carbonate in Atlantic and Pacific ocean sediments, and the major differences in the saturation states of their deep waters. 

Two major types of variability in the relationship between overlying water chemistry and carbonate accumulation in deep sea sediments occur. The first is the previously discussed relation of the saturation state of the water to the R0, FL and CCD. The second is the relative separation of these different sedimentary features. In some areas of the ocean these relations can be influenced by transitions in water masses having different chemical and hydrographic characteristics (e.g., Thunell, 1982), but in many areas of the ocean the only major variable influencing the saturation state over wide areas is pressure, which leads to a nearly uniform gradient in saturation state with respect to depth. 

A major benthic process, that had only casually been considered for its potential influence on carbonate accumulation in deep sea sediments, is the oxidation of organic matter. The general reaction for this process involving marine organic matter is (Emerson and Bender, 1981) ... 

Work needs to be continued on sediment-water interfacial processes that control carbonate accumulation in deep-sea sediments. 

The results of these studies have shown a surprising degree of variability and have further demonstrated the complexity of calcium carbonate accumulation in deep-sea sediments. Several studies by different groups of investigators appeared at about the same time (Emerson et al, 1980 Murray et al, 1980 Sayles, 1980). The results of Emerson etal (1980) and Sayles (1980)... 

It is convenient to divide the deposition of carbonates in marine sediments into those being deposited in shallow (shoal) water (water depths of a few hundred meters or less) and those being deposited in deep sea sediments, where the water depth is on the order of kilometers. The primary reasons for this division are the differing sources, dominant mineralogies, and accumulation processes operative in these environments. Shoal water carbonates are the topic of Chapter 5. Naturally, there are "grey" areas of intermediate characteristics between these two extremes, such as continental slopes and the flanks of carbonate banks and atolls. 

Before proceeding with an examination of the oceanic carbonate system and the accumulation of calcium carbonate in deep sea sediments, it is useful to consider briefly the general relationships among the different components and their most basic characteristics. The various components of first order importance to the system are shown in Figure 4.1. These can be divided into external components,... 

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

As previously mentioned, the primary processes responsible for variations in the deep sea C02-carbonic acid system are oxidative degradation of organic matter, dissolution of calcium carbonate, the chemistry of source waters and oceanic circulation patterns. Temperature and salinity variations in deep seawaters are small and of secondary importance compared to the major variations in pressure with depth. Our primary interest is in how these processes influence the saturation state of seawater and, consequently, the accumulation of CaC03 in deep sea sediments. Variations of alkalinity in deep sea waters are relatively small and contribute little to differences in the saturation state of deep seawater. 

Factors controlling the accumulation of calcium carbonate in deep sea sediments... 

FACTORS CONTROLLING THE ACCUMULATION OF CALCIUM CARBONATE IN DEEP SEA SEDIMENTS... 

Figure 5 Abundance of radiocarbon age of black carbon in slowly accumulating ( 2.5 cm kyr ) deep-sea sediments from the Southern Ocean (54 °S 176° 40 W) (a) a plot of the ratio of black carbon to total organic carbon (BC/OC) with sediment depth and (b) A C (per mil) and C age (kyr BP) of BC (solid symbols) and non-BC sedimentary OC (open symbols) as a function of depth (after Masiello and Druffel, 1998).

One of the most important applications of uranium-series methods of age determination has been the dating of fossil corals and other carbonate materials. In contrast to deep-sea sediments, which accumulate excess h and Pa that decay over time, carbonates accumulate uranium by co-precipitation from seawater that is essentially free of °Th and Pa. The radioactive ingrowth of °Th and Pa over time toward secular equilibrium with and is the basis of the two methods. 

Fig. 3.25 Inverse relationship between carbon content of sediments (normalized to grain surface area) and exposure time to oxygen (after Gelinas et al. 2001). Oxygen exposure time (OET) = (depth of 02 penetration in sediment pore waters)/(sediment accumulation rate). Ctotaloiganic (mgm 2) = —0.161ogeOET + 1.28 and Cnoll protein (nignE2) = -0.0431ogeOET + 0.32 (r2 = 0.96 for both OET measured in years). Low OETs are typical of coastal sediments and high OETs of deep-sea sediments.

This section primarily focuses on the description of the deposition and accumulation of carbonates in shallow waters and in the deep ocean. The main depocenters for calcium carbonates are the continental shelf areas, as well as island arcs or atolls, which are the typical shallow water environments for massive carbonate formation, and the pelagic deep-sea sediments above the calcite compensation depth catching the rain of small calcareous tests formed by marine plankton in the surface waters. 

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