Rates and processes of soil evolution on uplifted marine terraces, northern California

Abstract

Time-dependent changes in soil development have been evaluated for two flights of uplifted marine terraces with similar site characteristics and a range in terrace ages (∼ 3.9 ka to 330 ka) in northern California. Six pedogenic properties of soils developed in unconsolidated deposits overlying bedrock marine platforms have systematic, time-dependent trends, but each property differs in rate of change and in the time period over which its changes are useful as relative age dating or correlation tools. Changes in accumulated mass of organic carbon and pH are useful properties only for the youngest soils, and probably for soils less than 10–20 ka in this climatic region. Soils on terraces older than 29 ka have nearly similar amounts of mass of organic carbon, with values of about 3–6% by weight for the upper meter of the soil profile. Soil reaction (pH) in the top 20 cm of the soil profile decreases at a rapidly declining rate, reaching and maintaining a value of ∼ 5 by 39–40 ka. Total mass of accumulated clay is an excellent indicator of relative soil age during all stages of soil development. The rate of accumulation of clay is about 0.4 g/cm 2 soil column per thousand years, so that pedons on terraces with inferred ages of about 124 ka have accumulated at least 50 g/cm 2 soil column of clay. Systematic and linear increases in accumulated mass of several forms of pedogenic iron make them very useful for chronological purposed for at least 240 ky of soil development. Crystalline forms of iron enable greater distinction among young soils than amorphous forms, which have negligible accumulations until at least 40 ka. The rate of accumulation of crystalline forms of iron (Fe d−Fe o) is about 0.02 g/cm 2 soil column per thousand years, so that pedons on terraces with inferred ages of about 124 ka have accumulated about 3 g/cm 2 soil column of crystalline forms of pedogenic iron. Changes with time in accumulated mass of amorphous forms of pedogenic aluminum, Al o, are negligible within the first 29 ka, then reflect a rapid increase in rate of accumulation until at least 124 ka. Two pedons from the same terrace (inferred age 124 ka) contain strikingly different amounts of pedogenic Al o, however, and suggest that Al o may be an unreliable indicator of the degree of soil development. In addition to mass values of different soil substances, maximum percent values were calculated. Soils developed on marine terraces older than 29 ka have at least 10% maximum clay, older than 100 ka at least 30% maximum clay, and older than 240 ka at least 40–50% maximum clay. Soils on terraces younger than 29 ka have less than 1% maximum Fe d, and soils older than 240 ka at have at least 4% maximum Fe d. These easily obtained values provide textural and chemical boundaries for separating marine terraces younger or older than about 29, 100, and 240 ka in this region, but are useful only as very general tools for correlation. Maximum percent clay is a more reliable indicator of soil age than maximum percent Fe d, as the latter is greatly influenced by local groundwater flux, and percent values of Fe d have a high degree of scatter.