The mass balance of soil evolution on late Quaternary marine terraces, northern California DOROTHY J. MERRITTS Geosciences Department, Franklin and Marshall College, Lancaster, Pennsylvania 17604-3003 OLIVER A. CHADWICK Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109 DAVID M. HENDRICKS Department of Soil and Water Science, University of Arizona, Tucson, Arizona 85721 CHRISTOPHER^ J^LEWK I department of Geology and Geophysics, University of California, Berkeley, California 94720 ABSTRACT Mass-balance interpretation of a soil chronosequence provides a means of quanti- fying elemental addition, removal, and trans- formation that occur in soils from a flight of marine terraces in northern California. Six soil profiles that range in age from several to 240,000 yr are developed in unconsolidated, sandy-marine, and eolian parent material de- posited on bedrock marine platforms. Soil evolution is dominated by (1) open-system depletion of Si, Ca, Mg, K, and Na; (2) open- system enrichment of P in surface soil hori- zons; (3) relative immobility of Fe and Al; and (4) transformation of Fe, Si, and Al in the parent material to secondary clay minerals and sesquioxides. Net mass losses of bases and Si are generally uniform with depth and substantial—in some cases approaching 100%; however, the rate of loss of each ele- ment differs markedly, causing the ranking of each by relative abundance to shift with time. Loss of Si from the sand fraction by dissolu- tion and particle-size diminution, from -100% to <35% over 240 ky, mirrors a similar gain in the silt (from -2% to 30%-50%) and clay (from -0% to -20%) size fractions. The Fe originally present in the sand fraction de- creases from >80% to <10%, whereas the amount of Fe present in the clay and crystal- line oxyhydroxide fractions increases to 25% and 70%, respectively. Aluminum originally present in the sand fraction decreases from >90% to <10% concurrently with an increase of Al in the organic sesquioxide and clay phases, to 10% and 50%, respectively, while only minor increases occur in the nonorganic sesquioxide phases. INTRODUCTION Processes such as hydrolytic weathering, leaching of soluble elements, accumulation of insoluble residues, and microbial mineralization of organic compounds continually modify the soil mantle at the Earth's surface. At the same time, soil acts to modify mass transfers among the atmosphere, hydrosphere, biosphere, and lith- osphère. Soil is an open system with constant interplay between its surroundings and the evo- lution of soil properties. Many studies define the nature of soil evolution in different environ- ments (compare with Harden and others, 1991), but few quantify mass transfer into and out of the soil environment during soil evolution. The net-mass-change relationships that can be de- rived from a detailed mass-balance accounting of soil evolution would serve to enhance com- munication among earth, water, atmosphere, and biological scientists. The purpose of this paper is to quantify the nature of mass transfers during humid, temper- ate soil evolution in units that can be utilized by geologists and hydrologists to enable greater un- derstanding of weathering and leaching at the Earth's surface. Application of mass-balance analysis to a soil chronosequence (essentially a time series of soil profile development) enables us to determine long-term chemical denudation rates that will complement modern rates derived from stream-water analyses, which are areally extensive. Mass-balance analysis of soils requires comparison of chemical composition, bulk den- sity, porosity, and volume properties between the soil and its parent material. We present a mass-balance analysis of a chronosequence developed on emergent marine terraces in northern California. Specifically, we focus on the net gains (enrichment) and losses (depletion) of rock-forming elements (Fe, Al, Si, Ca, Na, Mg, K, and P) that accompany weathering of littoral marine sand during approximately 240,000 yr of soil evolution. A mass-balance approach enables us to identify shifts in elemen- tal distributions along the time series of profiles. Such shifts can then be compared to (1) minera- logical analyses of the soils and their parent material (X-ray diffraction peak intensities) and (2) the occurrence of elements in different soil fractions (sand and silt, clay, and oxides and oxyhydroxides), in order to determine the fate of each element and the secondary mineral forms in which each might occur. SITE DESCRIPTIONS AND METHODS Site Descriptions and Parent Material Six pedons (1, 2, 3, 4a, 4b, and 5) were sampled on five gently sloping (l°-4°) marine terrace treads between 7 m and 300 m above sea level near the mouth of the Mattole River at the northern end of the King Range, California (Fig. 1; Table 1). Two pedons, 4a and 4b, are 1 km apart on the same terrace tread and allow us to assess variations in pedogenic properties. All sites were at places along the terrace tread where evidence of surface erosion or deposition (for example, mobile dune sands) is minimal or non- existent. Five of the sites were hand-dug rectan- gular pits located on the midsections of terrace treads (sites 2 through 5). Site 1 was excavated from the face of a low sea cliff at Singley Flat. Sedimentary deposits overlying bedrock ma- rine platforms comprise the parent material for soil formation. These deposits are derived from Additional material for this article (three tables) is available free of charge by requesting Supplementary Data 9230 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 104, p. 1456-1470, 10 figs., 2 tables, November 1992. 1456