Andrea I. Pasquini

The hydrological signal of the Perito Moreno Glacier damming of Lake Argentino (southern Andean Patagonia): the connection to climate anomalies

Abstract The periodic damming of Lake Argentino by the Perito Moreno Glacier (50°30′S, 73° 00′W) in Argentinas southern Patagonian Andes has been recorded seventeen times since the beginning of this century. Such events are significant factors controlling discharge anomalies (seasonal component removed) of the Santa Cruz River, the sole outlet of Lake Argentino. Power spectrum analysis of the deseasonalized discharge revealed significant period peaks in the 33- to 36-month range and in the 42- to 58-month range. The first frequency is probably determined by the anomalous position of the subtropical anticyclones in the Pacific (with 2–5 years recurrence intervals), whereas the remaining frequencies are coincidental with the multivariate ENSO index (MEI) frequency spectrum. Significant squared coherency (>0.78) between the Santa Cruz River discharge anomalies and the MEI suggests that there is a significant teleconnection between ENSO occurrences in the Pacific and the Perito Moreno Glacier dynamics. El Nino events, for example, appear to have fostered the advancement of the glaciers snout and influenced the recorded damming–rupture sequence.

Material Sources, Chemical Weathering, and Physical Denudation in the Chubut River Basin (Patagonia, Argentina): Implications for Andean Rivers

The Chubut is a medium‐size (42,000 km2) river basin that drains the arid‐to‐semiarid Patagonian seaboard and pours its waters into the southwestern Atlantic Ocean (ca. lat 43°20′S, long 65°04′W). The materials eroded from the continent and deposited in the sea are scarcely affected by chemical weathering (the chemical index of alteration of riverbed sediments is ∼55) and bear a typical chemical and mineralogical signature characteristic of volcanic arcs. Clearly, flowing toward a passive margin, the river carries the mineralogical and chemical signature of an active margin. Physically weathered andesites and basalts occupy only about 25% of the drainage area, and therefore most exported material must be supplied by outcropping sedimentary beds of variable age. The Chubut River headwaters are placed in a tectonically active region, soil formation is incipient (“weathering‐limited regime”), and the rate of denudation (24.6 t km−2 yr−1) is much lower than the rates exhibited by similar rivers in other parts of the world. The depleted dissolved and particulate load is determined by scarce atmospheric precipitations (i.e., the drainage basin is in the Andean rain shadow) and by the protective effect of Cenozoic lava flows that often shield sedimentary formations from denudation. Although the index of chemical variability suggests that materials exported are products of the first denudational cycle, the geological history supports the view that most materials may have passed two or even three times through the exogenous cycle without acquiring a chemical or mineralogical signature indicative of repeated weathering. This is probably also true for other basins in temperate Andean climates.

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.

The Geochemistry of the Paranamp;#x00E1; River: An Overview

River basins are not inert continental features. Moreover, it can be said that rivers have a life, and their evolution is usually predictable. In biological terms, for example, rivers are processors of materials as the biota they contain take up, convert, use, and release resources that come to them. In other words, rivers are active biological systems that metabolize the organic matter they transport. From a geological point of view, rivers transport sediments and solutes whose dynamics is also determined by a set of complex interacting variables, such as lithology, climate, and relief. Consequently, the water that reaches a rivers mouth is far different, both qualitatively and quantitatively, from the water that entered the system as rain or snowfall. Summarizing, the chemical signatures of rivers are reflections of complex natural and interdependent relationships involving the chemistry of precipitation, the weathering of minerals, the cycling of vegetation, and the evolution or history of its water. The recently published geochemical treatise (Drever 2005) is a major step towards the elucidation of such complexities.

Chemical Weathering Processes on the Earth’s Surface

The exhumation of rocks from the Earth’s crust implies that they must adjust thermodynamically to the conditions existing on the surface, which are extremely different from those prevailing during their formation, with higher temperature, pressure, and often exposed to chemically aggressive fluids. The processes involved in chemical weathering, such as dissolution, hydrolysis, etc., are at the core of the adjustment mechanism, transforming solid, and usually refractory rock material, into particles-typically stripped from part of their original components and dissolved phases, both of which are amenable to be transported from the continents to the sea. Although anthropogenic actions have altered natural denudation rates, still a relatively minor portion of the material thus produced stays for a longer period on the continents, temporarily sequestered in depositional systems. The most important participants in the weathering scenario are mineral dissolution, silicate hydrolysis, and redox reactions.

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.