Weathering of rock is a critical first stage of soil development and the erosional development of a landscape. For soils, rock weathering produces the parent material on which the soil forms and gains its inorganic component; and soil chemistry and fertility is largely determined by what can be weathered from the rock. At the same time, these processes break down rock sufficiently that it can erode to form landform features such as valleys and hillslopes. The type and relative magnitude of weathering processes sets the stage for what follows.
The study I am proposing looks at chemical and weathering processes that are influenced by distance to the ocean -- the chemical effect of marine aerosols and the effect on precipitation due to upwelling -- and orographic effects. With increasing precipitation away from the stable coastal zones comes an increase in precipitation leading to forest and deeper soils increasing the likelihood for high CO2 levels, carbonation and resulting weathering of subsoil rocks. Salt weathering involves chemical breakdown through enhancement of oxidation and mechanical breakdown through the growth of salt crystals in voids (fractures and pores). In geographical extent, carbonation occurs where (and when) soils contain CO2 while salt weathering is limited to sources of salt such as coastal regions and interior desert basins. Another process significant to the coastal zone is the influence of marine aerosols on fog chemistry, and the likely development of sulfurous and sulfuric acid, which may be significant for rock weathering close to the sources in the marine layer.
Coastal sources of weathering effects from salt and possibly sulfuric acid may be the cause of the anomalously weathered granitic rocks observed on Montara Mountain, rock that is the same age as the less weathered granitic rocks of the Sierra Nevada; in fact they are from the same batholith, displaced by the San Andreas Fault. Montara Mountain, a mass of granitic rock rising from the coast at Devils Slide to an elevation of 580 m (1900 feet) a few kilometers inland provides an excellent study area for investigating both processes. CO2 is known in other climates to vary seasonally with temperature and moisture cycles, and also to reflect vegetation controls. A 7-km transect (see map) from the coast inland changes from 500 mm (20 in.) of annual precipitation (with added fog drip) and coastal scrub vegetation, through a band of chaparral generally above the fog, to a Doug Fir-Redwood forest on the lee side of Montara that gets 1150 mm (45 in.) of precipitation -- more than any other place on the San Francisco Peninsula. My hypothesis is that soil CO2 levels will be highest in late spring, and will increase with increasing precipitation inland. In contrast, salt aerosols and thus moisture salinity should decrease with distance from the coast (though surprisingly there's little research on this); furthermore the salinity of derived soil water is expected to be negatively correlated with rainfall intensity and reach a maximum with fog drip. Watershed managers in San Francisco Water Department's Crystal Springs watershed have noted difficulties growing trees in some fog-prone sites due to an assumed effect of salt burn (they are quite interested in the results of this study).
The research plan for this project includes a setup of 2 to 3 days followed by biweekly sampling over a period of one year or more. The setup operation will consist of establishing sampling devices at six sites systematically arranged at significant fog passes and critical sites at varying distances from the coast: (1) Devils Slide (0.2 km inland, 150 m elevation) at Highway 1; (2) Saddle Pass (2 km, 275 m); (3) North Peak Montara Mountain (4 km, 580 m); (4) San Vicente Creek pass (5 km, 470 m); (5) Denniston Creek pass (5 km; 550 m); and (6) Pilarcitos Reservoir (7 km, 210 m), at the maximum-precipitation (1150 mm) site.Sampling equipment will consist of fog drip collectors, precipitation collectors, and soil gas wells. Fog drip collectors will be built based upon a design developed at the University of Hawaii, Hilo, for water resource enhancement on small Pacific islands. Precipitation collectors will be constructed from funnels, tubing, and wide-mouth bottles. Both will be mounted on permanent structures through arrangements with Caltrans, McNee Ranch State Park, and the San Francisco Water Department. Soil gas wells, based upon a design by Kiefer (1990), will be constructed of 3-mm-ID stainless steel tubing, capped to prevent surface air entry, and implaced at 30-50 cm depth, four wells per site. Biweekly measurements will consist of pH and electrical conductivity (EC) of fog drip or precipitation, soil moisture, soil temperature, and air temperature. CO2 samples will be drawn from the soil gas wells using a syringe and vacutainer, and analyzed using a CO2 gas analyzer by a collaborator on the project, Rudi Kiefer of University of North Carolina, Wilmington. EC measurements will be used as a well-established surrogate for salinity. During the course of the study, additional observations will be made of features in the landscape relevant to the study, especially evidence of advanced granitic weathering. Discussions with watershed managers in the San Francisco Water Department's Crystal Springs Watershed initiated this past year will aid in identifying anomalies in sediment yield patterns. For example, patterns of granitic weathering anomalies in stream sediments have been noted, and may be related to salt weathering. As yet unstudied is the significance of carbonate rocks (which are highly susceptible to carbonic acid degradation) found in the watershed.
Some of the equipment required for this research exists in the Department of Geography & Environment: pH and EC meters, a thermocouple probe for soil temperature, and simple survey equipment for locating soil sample locations; GIS software and hardware will be used for creating maps and processing data for reports. New equipment required includes an accurate soil moisture meter and materials for building sample collection devices described above. (Most of this equipment is now in storage, ready for installation.)
Weathering is very slow; but it is critical to soil formation, chemistry, fertility, and erosion processes. The weathering environment is highly influenced by the local climate and biological factors, but in turn influences the plant communities that grow on the soil. I feel it is central to the study of physical geography, and I emphasize it in the courses I teach. Yet our understanding of these vital processes -- especially the effects of seasonality and topography -- is incomplete; process studies like this are badly needed.In terms of professional achievement, the results of this study would be clearly attractive to two journals, Catena and Earth Surface Processes and Landforms. Depending upon the nature of the results, one or two research articles should follow: perhaps one on the CO2 results, the other on the salinity results; or if the combination is more interesting, a single work on the unique combination of processes in influencing the overall weathering environment along the transect.There are many secondary benefits from this study. I hope to do what I have done in my Marble Mountains research: attract graduate students to this and similar studies for thesis topics. This study conforms well to the ideal in that it involves a good demonstration of the scientific method -- there is a clear set of hypotheses to test, for example. If nothing else, it will serve as an excellent example in methods courses. In addition, some of the equipment fills gaps in the Department's field equipment, especially the soil moisture meter, which students and faculty can apply to many other environmental studies.