Robert F. Stallard

Terrestrial sedimentation and the carbon cycle: Coupling weathering and erosion to carbon burial

This paper examines the linkages between the carbon cycle and sedimentary processes on land. Available data suggest that sedimentation on land can bury vast quantities of organic carbon, roughly 1015 g C yr−1. To evaluate the relative roles of various classes of processes in the burial of carbon on land, terrestrial sedimentation was modeled as a series of 864 scenarios. Each scenario represents a unique choice of intensities for seven classes of processes and two different global wetland distributions. Comparison was made with presumed preagricultural conditions. The classes of processes were divided into two major component parts: clastic sedimentation of soil-derived carbon and organic sedimentation of autochthonous carbon. For clastic sedimentation, masses of sediment were considered for burial as reservoir sediment, lake sediment, and combined colluvium, alluvium, and aeolian deposits. When the ensemble of models is examined, the human-induced burial of 0.6-1.5·1015 g yr−1 of carbon on land is entirely plausible. This sink reaches its maximum strength between 30° and 50° N. Paddy lands stand out as a type of land use that warrants future study, but the many faces of rice agriculture limit generalization. In an extreme scenario, paddy lands alone could be made to bury about 1·1015 g C yr−1. Arguing that terrestrial sedimentation processes could be much of the sink for the so called “missing carbon” is reasonable. Such a hypothesis, however, requires major redesign of how the carbon cycle is modeled. Unlike ecosystem processes that are amenable to satellite monitoring and parallel modeling, many aspects of terrestrial sedimentation are hidden from space.

Soil nutrients influence spatial distributions of tropical tree species

The importance of niche vs. neutral assembly mechanisms in structuring tropical tree communities remains an important unsettled question in community ecology [Bell G (2005) Ecology 86:1757–1770]. There is ample evidence that species distributions are determined by soils and habitat factors at landscape (<104 km2) and regional scales. At local scales (<1 km2), however, habitat factors and species distributions show comparable spatial aggregation, making it difficult to disentangle the importance of niche and dispersal processes. In this article, we test soil resource-based niche assembly at a local scale, using species and soil nutrient distributions obtained at high spatial resolution in three diverse neotropical forest plots in Colombia (La Planada), Ecuador (Yasuni), and Panama (Barro Colorado Island). Using spatial distribution maps of >0.5 million individual trees of 1,400 species and 10 essential plant nutrients, we used Monte Carlo simulations of species distributions to test plant–soil associations against null expectations based on dispersal assembly. We found that the spatial distributions of 36–51% of tree species at these sites show strong associations to soil nutrient distributions. Neutral dispersal assembly cannot account for these plant–soil associations or the observed niche breadths of these species. These results indicate that belowground resource availability plays an important role in the assembly of tropical tree communities at local scales and provide the basis for future investigations on the mechanisms of resource competition among tropical tree species.

Denudation rates determined from the accumulation of in situ-produced 10Be in the luquillo experimental forest, Puerto Rico

We present a simple method for estimation of long-term mean denudation rates using in situ-produced cosmogenic 10Be in fluvial sediments. Procedures are discussed to account for the effects of soil bioturbation, mass wasting and attenuation of cosmic rays by biomass and by local topography. Our analyses of 10Be in quartz from bedrock outcrops, soils, mass-wasting sites and riverine sediment from the Icacos River basin in the Luquillo Experimental Forest, Puerto Rico, are used to characterize denudation for major landform elements in that basin. The 10Be concentration of a discharge-weighted average of size classes of river sediment corresponds to a long-term average denudation of ≈ 43 m Ma−1, consistent with mass balance results.

On the chemical mass-balance in estuaries

Abstract A general model is presented for mixing processes between river and ocean water in which are established criteria for the identification of any non-conservative behavior of the dissolved constituents involved. A review of previous data shows that in no case has removal of silica been demonstrated unambiguously in estuarine regimes. New data for iron which show highly non-conservative behavior are used in an example of the application of the model.

THE FLUVIAL GEOCHEMISTRY AND DENUDATION RATE OF THE GUAYANA SHIELD IN VENEZUELA, COLOMBIA, AND BRAZIL

Abstract The Guayana Shield is composed of Early to Mid-Precambrian igneous and metamorphic basement rocks with a quartzitic platform cover. The complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment. Total erosion rates are extremely slow, ∼10 m/m.y., with equal contributions from the dissolved and suspended loads. However, the former is largely silica with ratios of Si to total cation equivalents [Si:TZ+] ranging to in excess of three. Weathering is “complete” to kaolinite and gibbsite, i.e., the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate. In basins of relatively homogeneous lithology, Rb/Sr isochrons constructed from the river data agree with the whole-rock ages from the drainages; thus, all the common, refractory, Rb-containing minerals (K-feldspar, mica) are completely dissolved. The thick, lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other Southern Hemisphere Shields. In the absence of active tectonics or greatly accelerated mechanical erosion, the weathering rates of these basement rocks must be quite insensitive to environmental change.

Dynamics of soil carbon during deglaciation of the Laurentide ice sheet

Deglaciation of the Laurentide Ice Sheet in North America was accompanied by sequestration of organic carbon in newly exposed soils. The greatest rate of land exposure occurred around 12,000 to 8,000 years ago, and the greatest increase in the rate of carbon sequestration by soils occurred from 8,000 to 4,000 years ago. Sequestration of carbon in deglaciated peat lands continues today, and a steady state has not been reached. The natural rate of carbon sequestration in soils, however, is small relative to the rate of anthropogenic carbon dioxide production.

Dissolution at dislocation etch pits in quartz

Abstract Several samples of quartz were etched hydrothermally at 300°C in etchams of controlled dissolved silica concentration in order to measure the critical concentration, Ccrit, above which dislocation etch pits would not nucleate on the quartz surface. Ccrit for 300°C was theoretically predicted to be 0.6C0 and the measured Ccrit, was 0.75 ± 0.15C0 (C0 is the equilibrium concentration). Above this value, some dislocation etch pits formed, but the rate of formation significantly decreased. These results are the first experimental validation of etch pit formation theory under hydrothermal conditions. Dune sands showed a generally angular and pitted surface when etched in dilute solutions, while sands etched at C ~ Ccrit showed less angular pitting. Analysis of a soil profile developed in situ on the Parguaza granite, Venezuela, revealed a gradual change from angular, pitted grain surfaces at the top of the profile to rounded surfaces on grains sampled just above bedrock. Since quartz dissolution without surface pitting continues deep in the profile, the Si concentration must exceed Ccrit, at depth. These results indicate that for C >Ccrit, dissolution occurs at edges and kinks on the surface of quartz and very few pits form; in contrast, at C ⪡ Ccrit, dislocation etch pits grow rapidly, contributing to the overall dissolution rate.

First-Cycle Quartz Arenites in the Orinoco River Basin, Venezuela and Colombia

Modern first-cycle quartz arenites are forming in the Orinoco drainage basin by at least two distinct mechanisms. Common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur. In the Llanos (Andean foreland basin), extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains. In lowland portions of the Guayana Shield, long soil mineral residence times are produced by very low erosion and transport rates, resulting from low relief and tectonic quiescence. Both weathering intensity and the duration of weathering are important in the modification of sand composition. First-cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences. Furthermore, although tectonic setting is of great importance in determining sand composition, modifications during transport and deposition can overprint the effects of the tectonic regime. In extreme cases, such as first-cycle quartz arenites, the tectonic signal may be virtually obliterated. Many criteria used in the past to discriminate between first-and multi-cycle quartz arenites are invalid when applied to the quartz arenites of the Orinoco drainage basin. At present, the only unambiguous criterion is that the presence of sedimentary lithic fragments and syntaxial quartz overgrowths is clearly indicative of a multi-cycle origin for at least one component of a sand.

The chemical mass balance in the Amazon plume I: The nutrients

In May and June 1976, the Amazon plume was dominated by a diatom blood centered on the 10 x 10−3 isohaline. The bloom was apparently initiated by the increased transparency produced by the rapid settling of the fluvial detrial load. The bloom removes the nitrate and phosphate from the surficial layer. The underlying salt wedge was enriched by remineralisation of planktonic debris. A balance between the dissolved and particulate material shows that the regeneration was essentially complete for carbon and phosphorus; but about 50% of the nitrogen was unaccounted for, having been solubilised to species other than nitrate and nitrite. Only 20% of the silica removed as diatoms (∼25% of the river dissoled flux) could be accounted for as salt wedge enrichment. The rest must have been incorporated in the sediment. The composition of the river waters was strongly affected, before entry to the mixing zone, by remineralisation of a large fraction of the fluvial particulate organic material in the broad area of multi-channel flow above the mouth.

Methane emission by bubbling from Gatun Lake, Panama

We studied methane emission by bubbling from Gatun Lake, Panama, at water depths of less than 1 m to about 10 m. Gas bubbles were collected in floating traps deployed during 12- to 60-hour observation periods. Comparison of floating traps and floating chambers showed that about 98% of methane emission occurred by bubbling and only 2% occurred by diffusion. Average methane concentration of bubbles at our sites varied from 67% to 77%. Methane emission by bubbling occurred episodically, with greatest rates primarily between the hours of 0800 and 1400 LT. Events appear to be triggered by wind. The flux of methane associated with bubbling was strongly anticorrelated with water depth. Seasonal changes in water depth caused seasonal variation of methane emission. Bubble methane fluxes through the lake surface into the atmosphere measured during 24-hour intervals were least (10-200 mg m{sup -2} d{sup -1}) at deeper sites (>7 m) and greatest (300-2000 mg m{sup -2} d{sup -1}) at shallow sites (<2 m). 37 refs., 11 figs., 5 tabs.