Abraham Lerman

Global biogeochemical cycles in the geologic past and present; geochemical and transport processes in the surficial and underground environment; natural and anthropogenic controls of geochemical systems.

The concept of the geochemical cycles of the elements emerged in the middle to late 1800s, based on the studies of the water cycle from the 17th century, later understanding of the chemical composition of waters, sediments, and crystalline rocks, and the studies of plant photosynthesis since the late 18th century. The modern concepts of geochemical or biogeochemical cycles date from the 1920s. We are studying the interactions between the global biogeochemical cycles of the life-important elements carbon, phosphorus, and nitrogen by means of conceptual and mathematical models of these cycles that are compatible with the available data for the global C-N-P-O system in the remote and younger geologic past. The industrial and agricultural byproducts of human activity compete in their magnitudes with the sedimentary and geochemical processes on environmentally large scales. Human-made perturbations of the geochemical cycles and their long-term consequences are being studied at a global scale as well as within such specific environments as the land and the oceanic coastal zone. The broader goals of this research, shown in somewhat more detail below, are to understand the natural and human-induced global change in the past, present, and future.

Carbonic and sulfuric acids are the main inorganic acids that react with minerals in the Earth's weathering crust. CO₂ is produced by the oxidation of organic matter in the pore space of soils and H₂SO₄ is the result of oxidation of mineral pyrite (FeS₂). The consumption of CO₂ and production of HCO₃‾ in weathering reactions depends on the carbonic/sulfuric acid ratio in solution and the relative proportions of carbonate and silicate minerals, expressed in the weathering potential, ψ, that usually varies from 0.5 for pure carbonates to 1.0 for pure silicates. This acid consumption model makes it possible to calculate the chemical composition (metal cations and anions) of an average river water, in very good agreement with other estimates.

Since the Last Glacial Maximum, about 18,000 years ago, mean temperature of the Earth's surface has increased by about 4°C and the dry land surface area increased about 10%. In our estimation, these two factors were responsible for an increased flux of CO₂ to mineral weathering, to its pre-industrial value of 22×10¹² mol C/yr.

Interactions of the global reservoirs of the atmosphere, land, coastal ocean, and open ocean under the effects of the major human perturbations have resulted in an increased transport of carbon, nitrogen, and phosphorus to the coastal ocean, in particular since the mid-1900s. The analysis of the global C-N-P cycles, using the models TOTEM (publications 1999-2005) and SOCM (publications 2004-2006) indicates increasing input and subsequent remineralization of organic carbon in the coastal ocean, counteracting to some extent the transport of CO₂ from the atmosphere to coastal ocean waters. Among the results, our estimates of the denitrification rates on land, in the coastal ocean, and open ocean indicate a significant increase in industrial time. Because of the increasing erosion and decay of soil organic matter, the release of N to the atmosphere by denitrification from land became a smaller fraction of total N transport from land to the atmosphere and coastal ocean.

In potassium-bearing clay minerals, the ⁴⁰Ar/⁴⁰K ratio decreases with decreasing particle size, as has been documented by a number of investigators. Smaller values of the ⁴⁰Ar/⁴⁰K ratio correspond to younger K/Ar ages or apparent ages. We quantified the relationship between the apparent age (or the ratio R = ⁴⁰Ar/⁴⁰K) and the particle size in sediments ranging in age from the Cambrian to the Neogene. A relationship of a type log R = constant + b log r makes it possible to determine the fraction of ⁴⁰Ar escaped from the smaller particles relative to the larger sizes. For the sedimentary lithosphere in the Phanerozoic, we estimate the ⁴⁰Ar flux to the atmosphere at 6×10⁶ to 17×10⁶ mol ⁴⁰Ar/yr, approximately 20 to 50% of the flux from the crystalline continental crust.

There are numerous occurrences of clays that show K/Ar ages considerably older than the stratigraphic age of their formations. This suggests that in many sedimentary environments, clays have a long pre-depositional history and the decrease in the apparent age and ⁴⁰Ar/⁴⁰K ratio in the fine-size clay particles are due to the release of ⁴⁰Ar. However, a theoretical analysis shows that a decrease in the apparent age may be due to addition of K to clays in the course of diagenesis, a phenomenon that is difficult to detect. The fact that nearly the same decrease in the apparent age may be achieved by addition of K, without loss of ⁴⁰Ar, as in a case of ⁴⁰Ar production and simultaneous diffusion out of particles, suggests that different diagenetic mechanisms might have been active in different sedimentary sequences.

Consumption of carbonic acid and sulfuric acid, forming from pyrite, in the weathering of carbonates and silicates	A. World-average rivers (different sources), average sediment, and sediment plus continental crust. B. Calculated ion concentrations from CO₂ and H₂SO₄ reactions with sediments and crust.