Karina L. Lecomte

Weathering and the Riverine Denudation of Continents

Denudation, volcanism, and tectonics are intertwined Earth system processes that constitute the main driving forces intervening in shaping the Earth’s landscape. Clearly, the wearing away of the Earth’s surface cannot occur unless a series of synergistic processes, collectively known as ‘‘weathering,’’ are initiated. This term, in use for a long time, promotes the idea that climate (weather) always plays a major role in rock breakdown; since this is not the case in every instance the change for ‘‘rock decay’’ has been proposed recently. At any rate, the linkage between weathering and denudation is not straightforward because the latter may be limited by the former (‘‘weathering-limited denudation’’) or, in contrast, it may be restricted by the hindered transport of the weathering-produced debris (‘‘transport-limited denudation’’). In addition to these possible scenarios, two new approaches have been gaining growth in the recent past: one is the study of the ‘‘regolith’’ as a convenient research framework, and the other is the notion of ‘‘the critical zone,’’ where the dynamic interaction with the atmosphere and vegetation is emphasized and added to the materials collectively defined as regolith.

Magnetic, chemical and radionuclide studies of river sediments and their variation with different physiographic regions of Bharathapuzha River, southwestern India

Bharathapuzha River is the second longest river in southwest India, where three physiographic regions show a distinctive spatial variation and their bed sediments can be considered environmental hosts for end-products generated by human activities and natural radionuclide components. Thus, the study of this river sediments in SW India is important not only because they are recorders of adverse human impacts (e.g., intense agricultural activities and urban pollution), but also because of their potential health hazards due to their common use as construction materials. Magnetic (e.g., magnetic susceptibility, anhysteretic remanent magnetisation and isothermal remanent magnetisation), radionuclide (226Ra, 232Th and 40K) and chemical (trace and major elements) measurements were carried out in bed sediment samples along 33 sites from the uppermost catchment downstream. Magnetic measurements show the dominance of ferrimagnetic minerals; their concentration ranges widely along the river and between regions, showing up to 7-fold higher values for concentration-dependent magnetic parameters, e.g., mean values of saturation of isothermal remanent magnetisation acquisition are 67.9 and 9.4 × 10-3 Am2 kg-1 for highland and lowland regions, respectively. Multivariate statistical analyses show the existence of relationships between magnetic, radioactivity and chemical variables. In particular, magnetic concentrationdependent parameters are significantly correlated with radioactivity variables 40K and 226Ra (with concentrations about 20% higher than the worldwide mean values), as well as with some elements: Fe, Ca and P. Such analyses also show differences between physiographic regions where samples from the highland (and lowland) region are well grouped showing higher (lower) magnetic concentrations and lower (higher) coercivity minerals. The spatial variation of magnetic parameters along the river can be related to the influence of both natural sources and human activities, i.e. urbanisation and intense agricultural activities. In this sense, environmental magnetism data provide very useful tools to investigate adverse human activities occurring in the riverine environment.

Sedimentary analysis and magnetic properties of Lake Anónima, Vega Island

Abstract During the summer Lake Anónima experiences important changes in salinity and lake level fluctuations. Physicochemical data and field observations indicate that evaporative processes are dominant and that the water inflow is mainly provided by snow meltwater and streams. A multiproxy analysis of data from lake bottom sediments suggests that the main surface stream located south-west of the lake controls the clastic input and the spatial variation of sediment composition. Through an integrated analysis (magnetic, X-ray diffraction and Fourier transform infrared spectroscopy studies) magnetite and greigite minerals were identified in these lake sediments. Such ferrimagnetic minerals have ultra-fine grain sizes (<0.1 μm). Magnetic parameters and non-magnetic variables analysed by multivariate statistics reveal significant differences between silt facies (e.g. mass-specific susceptibility χ=109.6×10-8 m3 kg-1, remanent coercivity Hcr=49.2 mT and total organic carbon (TOC)=1.11%) and sand facies (e.g. χ=82.1×10-8 m3 kg-1, Hcr=44.7 mT and TOC=0.70%), and four recent depositional sub-environments were identified and characterized in Lake Anónima. This multiparameter analysis contributes to the understanding of present-day lacustrine dynamic and sedimentary processes. Lake Anónima may provide a useful analogue for the interpretation of other lacustrine basins in the Antarctic region.

Hydrochemical Heterogeneity in Karst Tributaries on Siliciclastic Boundaries: Corrente River, Bahia—Brazil

The karst aquifers in the San Franciscan depression belong to the sandstone aquifer and subjected to intense fresh water pumping. The Corrente River provides nearly 30 % of the Sao Francisco River’s water flow. The main objective of the present paper was to define sources and distribution of dissolved elements and to describe the geochemical processes that govern their mobility within the Urucuia Aquifer. Water samples are classified into three groups, depending on the dominant weathering process. When carbonate dissolution dominates waters are of bicarbonate–calcium-type, whereas, when the atmospheric precipitation signal is present, the samples in siliciclastic terrain are mostly of Cl−–Na+ type. Ground waters reflect bicarbonate—mixed-type, with the highest dissolved concentrations. In contrast to major elements, trace elements, including rare earth elements (REEs), show seasonal behavior: their concentrations increase in the beginning of the wet season, due to re-mobilization and release into the solution of adsorbed elements from the system and the atmospheric dust. The total dissolved REE concentration (800–7500 ng L−1) is one order of magnitude more concentrated in karst than in siliciclastic rocks.

Weathering: Intensity and Rate

The evaluation of the intensity of weathering is usually achieved by means of numerous procedures, which may be absolute or relative; may be tackled by examining the solid residue left by weathering or by establishing the nature of the dissolved fraction, whose largest proportion is exported from the continents via streams, rivers, and ground waters. Absolute methods are feasible when weathering profiles are complete and clearly exposed whereas relative methodologies are useful when the weathered product is not necessarily near its source. The use of multivariate methodologies and modeling appears as promising techniques to assess weathering intensity. Laboratory experimentation, on the other hand, has supplied useful information on weathering rates of minerals. The field-supported mass balance approach, however, has furnished the most reliable information on weathering rates.

The Wearing Away of Continents

Denudation is a substantial part of a cycle in which flowing water associates the terrestrial sector of the global hydrological cycle with continental wearing down, of which it is a chief agent. Tectonic uplift stimulates fluvial erosion, on contact with the water cycled by solar energy and clustered in surface channels by catchment processes. Progressive sediment transfers occur between upper and lower catchments, and subsequently between lower catchments and the marine environment. Big river flood plains store sediment in larger systems for longer periods, where reworking continues mechanical and chemical sorting before reaching the onward transfer of mature sediments to the coastal zone. This is a continuous process where coarse, raw fluvial sediments are eventually swept as molasse into trenches and back-arc basins, close to orogens. About 19.1 Gt of sediment and 3.8 Gt of dissolved phases are annually transferred to coastal oceans by river discharge. This huge amount of material is, directly or indirectly, the result of the action of weathering, which is a significant link in the cycling of carbon and, hence, a participant in controlling the Earth’s climate. A significant portion of this material is, however, an indirect product of weathering because it is rock debris that has been recycled, having passed two or more times through the Earth’s exogenous cycle. Also, anthropogenic activities are responsible for opposing actions, which increase the denudation rate through soil erosion, on one hand, and sequester sediments in human-made reservoirs, on the other.

The Biological Path to Rock Breakdown

Biological weathering is exerted through both, biophysical and biochemical corridors. Considering the recent scientific advances in applied microbiology, it is now possible to attain a more accurate view on the role of biology in the breakdown of minerals and rocks in the Earth’s material cycle. Roots, lichens, mosses, algae, and bacteria are significant agents in mineral and rock breakdown, even exerting in some cases a comminuting action that promotes further the ensuing bio- or geochemical effect. Microorganisms tackle such action by producing aggressive substances (e.g., organic acids) that dissolve minerals and produce secondary solid and soluble phases which participate, through their riverine exportation to world oceans, in the process of continental denudation. The role of bacteria in dissolving metal sulfides, as in tailing impoundments resulting from mining operations, is particularly important to contribute to the understanding of the interaction of biota with the inorganic realm.