ment representing a given geologic interval divided by the length of the interval, expressed as a length (thickness) per unit time. The most commonly used standard is centimeters per 1000 yr (cm/kyr), but meters per million years (m/m.y.) is also often used. Sedimentation rate does not take the porosity of the sediment into account, and for young sediments, which can be up to 70 percent or more water, it can be very deceptive.
A more objective measure that can be used to compare compacted and uncompacted sediments is the accumulation rate, expressed in term of a mass per unit area per unit time (commonly g/cm2/kyr). Calculation of the accumulation rate requires that the sediment thickness be multiplied by the solidity of the sediment and by the grain density. The solidity of the sediment is one minus the porosity. The accumulation rate may be an accurate expression for the process of accumulation of modern sediments, but a problem arises in using the term in connection with older sediments. An ancient sediment accumulation rarely contains all the sediment that accumulated at a given site during the time represented by the sediment; some of the material that accumulated has been subsequently removed by erosion, dissolution, metamorphism, or subduction. The bulk of the sediment mass is simply eroded and redeposited, or recycled. Gregor (1970) suggested the term ''survival rate" for the measure of mass per unit area per unit time for an ancient deposit. The term survival rate is somewhat deceptive because it is not an expression of the rate of survival of the sediment, which would more properly be a measure of the amount of sediment remaining relative to the amount originally deposited divided by the time elapsed since deposition.
Hay (1985) coined the term "apparent accumulation rate" as an alternative objective expression for the existing mass of sediment per unit area divided by the length of time it represents (= survival rate of Gregor, 1970). Rates calculated from present-day masses are always apparent accumulation rates, that is they recognize that part of the original mass of sediment deposited has been destroyed by erosion, dissolution, subduction, or metamorphism.
The term accumulation rate is often interpreted as a direct reflection of a flux rate, for example, the rain rate of pelagic sediment. Interpretation of an ancient flux rate from an apparent accumulation rate is obviously troublesome, because correction for subsequent loss from erosion and other cause must be made. Analysis of the global mass/age distribution of Phanerozoic sediment indicates that it can be generally described by a decay curve of the form
M1 = Aebt (1.1)
where Mt is the mass in existence today representing sediment of age t, A is the zero-age intercept, b is the decay constant. If t is expressed in million years, then the decay constant for all Phanerozoic sediment is about -0.002. This means, for example, that only 82 percent of the sedimentary material that accumulated 100 m.y. ago is still in existence; the half-life of Phanerozoic sediment is about 350 m.y. The sediment flux at times in the past can be estimated by multiplying the zero-age intercept by the proportional difference between the observed mass of ancient sediment and the value of the decay curve for that time (Wold and Hay, 1988, 1990; Hay and Wold, 1990). Although conversion to flux rates requires caution, it is useful to express accumulation rates in terms of flux rates so that they can be compared with modern process rates. For the Quaternary, with a mean age of 0.8 Ma, the long-term (Phanerozoic) decay constant indicates that the amount of sediment remaining should be about 99.8 percent of the amount deposited, so no correction for long-term sediment cycling is made in the figures given below.
It is likely that there is more rapid recycling on a shorter time scale that may be reflected in estimates of flux rates. Hence, in the discussion below, I distinguish three kinds of flux rates: (1) instantaneous flux rates, (2) short-term flux rates, and (3) long-term flux rates. In geology, instantaneous flux rates are measured over geologically insignificant lengths of time. Short-term flux rates are a measure over a geologically longer but internally homogeneous period of time, such as a 1000-yr episode during the deglaciation. Long-term flux rates are integrated fluxes over a longer, inhomogeneous period of geologic time, such as the Holocene, the Wisconsinan, or the Pleistocene.
Most of the information on apparent accumulation rates is from the marine realm, and most of it is for long-term rates. In a few places estimates of local short-term rates, such as those during deglaciation and during parts of the Holocene have become available.
The deep sea contains more than half of the total Quaternary sediment mass. The flux rates of sediment to the deep sea, based largely on the study of DSDP cores by Sloan (1985), are summarized in Table 1.2; the apparent accumulation rate in the ocean averages about 5 g/cm2/kyr.
The area of continental rise deposits is that given by Sloan (1985) and includes the major deep sea fans. The area of high productivity is after Berner (1982) and includes 23.5 ∞ 106 km2 of the Pacific, 19 ∞ 106 km2 of the Southern Ocean, and 1.5 ∞ 106 km2 of margin high productivity