Owen P. Bricker

Geochemical Balance of a Small Watershed and Its Geomorphic Implications

A detailed input-output study of a small forested watershed draining the Wissahickon Formation in the Piedmont of Maryland revealed that chemical solution is five times as effective in removing material as is mechanical erosion. Solution weathering removes 16.9 tons/sq mi/yr of material compared with 3.2 tons/sq mi/yr by mechanical erosion. Plant activity during the growing season increased the concentration of silica, bicarbonate, calcium, and potassium, thus increasing total dissolved solids by one-third. Autumn leaf fall also caused a short-term increase of these ions. Rainfall does not simply dilute floodwaters as the concentration of sulfate, potassium, and calcium increases whereas silica and bicarbonate decrease in concentration during a flood cycle. Our data suggest that during the first half of a flood cycle, both the flood water and the dissolved solids in it come from an area in and immediately adjacent to the flood plain. The weathering model derived from our study suggests that on a long-term basis approximately one-half of the erosion of the Pond Branch watershed is caused by chemical solution of the silicate minerals kaolinite, vermiculite, biotite, and oligoclase. This contrasts to short-term ratio of solutional to mechanical weathering of five to one.

Phosphate in Interstitial Waters of Anoxic Sediments: Oxidation Effects during Sampling Procedure.

Oxidation during sampling procedures significantly decreases the inorganic phosphate concentrations of interstitial water rich in iron (II). All sampling and analytical procedures must be carried out in an inert atmosphere. Orthophosphate in the interstitial water of Cheaspeake Bay sediments, in equilibrium with vivianite, is a potential nutrient source for the overlying water.

Silica in Sea Water: Control by Silica Minerals

Silicate minerals typical of those carried in the suspended load of streams release silica to silica-deficient sea water and abstract silica from silicaenriched sea water. Experimental rates of release and uptake permit the conclusion that the suspended solids carried into the oceans by streams are a major control of the concentration of silica in the ocean.

Hydrogeochemistry of Bermuda: A case history of ground-water diagenesis of biocalcarenites

Bermuda is composed of relatively young skeletal limestones currently undergoing diagenesis by the ground water passing through them. The saturated zone consists of separate fresh-water bodies laterally surrounded and underlain by extensive brackish aureoles, in which the meteoric water is mixed with sea water. The meteoric water enters the aquifer after passing through the soil or through marshes (outcrops of the ground-water bodies), in each case causing an influx of CO 2 to the saturated zone. Examination of the ground-water chemistry enables mapping of (a) the extent of mixing of meteoric ground water and sea water; (b) P CO 2 ; (c) the extent of saturation with calcite and aragonite; (d) concentration of Sr; and (e) the amount of calcium and magnesium derived from the limestones. It is concluded that three processes control the chemistry of Bermudian ground water: (a) generation of elevated CO 2 partial pressures in soils and marshes; (b) dissolution of metastable carbonate minerals (principally aragonite); and (c) mixing with sea water. Bermuda ground water apparently approaches a steady state of aragonite dissolution (at slight subsaturation) and concurrent precipitation of calcite cement. Large Sr/Ca ratios in the ground water indicate that the dissolution of aragonite is incongruent. Dissolution is most pronounced near the marshes where CO 2 content is highest. Mixing with sea water is not significant in controlling calcite saturation. Only small amounts of magnesium enter the ground water by incongruent dissolution of magnesium calcite, an apparently slow process on the time scale of passage of the ground water through the saturated zone. All of the waters are well undersaturated with respect to dolomite. It is estimated that the present rate of recrystallization of aragonite to calcite is about 0.32 cm 3 of aragonite to calcite per m 3 of the saturated zone per year. At the present rate of chemical weathering, 360 m 3 of the saturated zone is lost each year through solution and transported to the sea by ground water.

Chemical Weathering of Serpentinite in the Eastern Piedmont of Maryland

Weathering processes in a small watershed (Soldiers Delight) underlain by Serpentinite in the Piedmont of Maryland were studied by means of a mass balance technique and were compared with the processes operative in a watershed uncertain by schist. The two terranes are downwasting at a rate of 2.4 m per m.y., but chemical weathering much more strongly affects the Serpentinite (2.2 m per m.y.) than the schist (1.2 m per m.y.). The serpentinite lacks a saprolite cover because resistate minerals are absent and alumina in the bedrock is scarce. In contrast, the schist contains both quartz and a source of alumina in the alumino-silicate minerals and, as a result, has a thick saprolite mantle. Relatively small amounts of secondary quartz, chalcedony, and 14A clay minerals are synthesized in the serpentinite watershed, but relatively large amounts of gibbsite and clay minerals (kaolinite and vermiculite) are formed during the weathering of the schist. The hydrologic consequences in the serpentinite terrane compared with the schist watershed are increased flood-flow discharge, greater fluctuation in seasonal, instantaneous base-flow discharge, and pronounced seasonal fluctuations in total discharge. The serpentinite stream water averaged 205 ppm of total dissolved solids in the base flow compared to 25 ppm in the schist. Stream water from the serpentinite is of the magnesium bicarbonate type; that from the schist is sodium–calcium bicarbonate type. On the serpentinite, substantial land-surface reduction (denudation) is effected by chemical weathering; mechanical weathering is secondary. On the schist terrane, mechanical weathering is the primary agent that lowers the land surface, even though chemical weathering has reduced the rock mass ay almost one-half.

Rates of dissolution of aluminosilicates in seawater

Abstract Dissolution of eight clay minerals, four zeolites, and quartz in seawater has been monitored for 81/2 years. For most of the minerals, dissolution can be described as a first-order reaction in which dissolved silica approaches from undersaturation steady concentration values with time. Characteristic reaction rate constants ( k 1 ) are of the order of 10 −7 sec −1 . One of the zeolites, clinoptilolite, shows a different dissolution behavior: SiO 2 concentration in solution reaches a high value within one year, followed by a decline to a lower value, suggestive of precipitation of another silicate phase (possibly sepiolite). A mathematical solution is given for a kinetic equation combining the parabolic-rate and first-order rate processes. It is shown that in a wide range of silicate dissolution reactions taking place over long periods of time, the presence of the parabolic-rate dissolution processes cannot be detected, thereby making its inclusion in the kinetic equations unnecessary. The experimental rates of dissolution are comparable to the SiO 2 − dissolution rates in oceanic sediments near the sediment/water interface. But deeper in the sediment, the calculated dissolution rates are significantly lower than the near-interface and experimental values.

Temporal variation of alkaline earth element/chlorinity ratios in the sargasso sea

Abstract An open ocean hydrographic station located 14 miles SE of the Bermuda Islands was sampled at two week intervals through a vertical profile of 2600 meters and over the period June 1966 to March 1967. 428 samples were analyzed for Ca, Mg, Sr and chlorinity. Large temporal variations in element/chlorinity ratios were observed throughout the water column.