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Pedology – Erosion & Weathering during the PETM

by on July 9, 2011

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Pedology is the study of soils in their natural environment. Soils lie at the interface of Earth’s atmosphere, biosphere, hydrosphere and lithosphere. Therefore, a thorough understanding of soils requires some knowledge of meteorology, climatology, ecology, biology, hydrology, geomorphology, geology and many other earth sciences and natural sciences.

Atmospheric carbon dioxide leads to Solution Weathering.

Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO2, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it falls.

One of the most well-known solution weathering processes is Carbonation the process of dissolving carbon dioxide in water. Carbonation can also describe a chemical reaction, one example of which is a key step in photosynthesis.

Carbonation occurs on rocks which contain calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate (the limestone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature, not because low temperatures generally drive reactions faster, but because colder water holds more dissolved carbon dioxide gas. Carbonation is therefore a large feature of glacial weathering. Buildings made of any stone, brick or concrete are susceptible to the same weathering agents as any exposed rock surface.

Comparison of unweathered (left) and weathered (right) limestone.

Thermal stress weathering (sometimes called insolation weathering)results from expansion or contraction of rock, caused by temperature changes. Thermal stress weathering comprises two main types, thermal shock and thermal fatigue. Thermal stress weathering is an important mechanism in deserts, where there is a large diurnal temperature range, hot in the day and cold at night. The repeated heating and cooling exerts stress on the outer layers of rocks, which can cause their outer layers to peel off in thin sheets. Forest fires and range fires are also known to cause significant weathering of rocks and boulders exposed along the ground surface. Intense, localized heat can rapidly expand a boulder. Although temperature changes are the principal driver, moisture can enhance thermal expansion in rock too.

Source Weathering

Orogenic cycle

Although orogeny involves plate tectonics, the tectonic forces result in a variety of associated phenomena, including magmatization, metamorphism, crustal melting, and crustal thickening. Just what happens in a specific orogen depends upon the strength and rheology of the continental lithosphere, and how these properties change during orogenesis.

In addition to orogeny, the orogen once formed is subject to other processes, such as sedimentation and erosion. The sequence of repeated cycles of sedimentation, deposition and erosion, followed by burial and metamorphism, and then by formation of granitic batholiths and tectonic uplift to form mountain chains, is called the orogenic cycle. For example, the Caledonian Orogeny refers to the Silurian and Devonian events that resulted from the collision of Laurentia with Eastern Avalonia and other former fragments of Gondwana. The Caledonian Orogeny resulted from these events and various others that are part of its peculiar orogenic cycle.

Erosion is the process of weathering and transport of solids (sediment, soil, rock and other particles) in the natural environment or their source and deposits them elsewhere. It usually occurs due to transport by wind, water, or ice; by down-slope creep of soil and other material under the force of gravity. In summary, an orogeny is a long-lived deformational episode in which many geological phenomena play a role.

Environmental change during the PETM drives formation of gigantic biogenic magnetite

The discovery of these exceptionally large biogenic magnetite crystals that possibly represent the remains of new micro-organisms that appeared and disappeared with the PETM sheds some light upon the ecological response to biogeochemical changes. Magnetotactic bacteria usually live in the oxic-anoxic transition zone of fresh, brackish, and marine environments including the suboxic zone of sediments. The occurrence of these new forms together with conventional magnetofossils suggests that they shared a similar ecological niche.

Proposal - Biogenic magnetite produced through the reduction of ferric iron within cells.

Magnetotactic bacteria (or MTB) are a polyphyletic group of bacteria that orient along the magnetic field lines of Earth’s magnetic field. To perform this task, these bacteria have organelles called magnetosomes that contain magnetic crystals. The biological phenomenon of microorganisms tending to move in response to the environment’s magnetic characteristics is known as magnetotaxis (although this term is misleading in that every other application of the term taxis involves a stimulus-response mechanism). In contrast to the magnetoception of animals, the bacteria contain fixed magnets that force the bacteria into alignment—even dead cells align, just like a compass needle. The alignment is believed to aid these organisms in reaching regions of optimal oxygen concentration.

Transmission electron micrographs of magnetite and/or maghemite particles extracted from a human hippocampus (well defined crystal faces can seen in some particles). Second image: particles showing some dissolution at the edges. Credit: Dobson and St. Pierre.

It has been suggested MTB evolved in the early Proterozoic Era, as the increase in atmospheric oxygen reduced the quantity of dissolved iron in the oceans. Organisms began to store iron in some form, and this intracellular iron was later adapted to form magnetosomes for magnetotaxis. These early MTB may have participated in the formation of the first eukaryotic cells. Biogenic magnetite not too different from that found in magnetotactic bacteria has been also found in higher organisms, from Euglenoid algae, to salmon, pigeons, and humans.

Biogenic Magnetite and Biomagnetism. A large spherical cluster of spearheads was dubbed the "Magnetic Death Star" by researchers. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)


From a thermodynamic point of view, in the presence of a neutral pH and a low redox potential, the inorganic synthesis of magnetite is favoured when compared to those of other iron oxides. It would thus appear microaerophilic or anaerobic conditions create a suitable potential for the formation of BMPs. Moreover, all iron absorbed by the bacteria is rapidly converted into magnetite, indicating the formation of crystals is not preceded by the accumulation of intermediate iron compounds; this also suggests the structures and the enzymes necessary for biomineralisation are already present within the bacteria. These conclusions are also supported by the fact that MTB cultured in aerobic conditions (and thus nonmagnetic) contain amounts of iron comparable to any other species of bacteria.

Microorganisms belonging to the genus Thioploca, for example, use nitrate, which is stored intracellularly, to oxidize sulfide, and have developed vertical sheaths in which bundles of motile filaments are located. It is assumed that Thioploca uses these sheaths to efficiently move in a vertical direction in the sediment, thereby accumulating sulfide in deeper layers and nitrate in upper layers.For some MTB, it might also be necessary to perform excursions to anoxic zones of their habitat to accumulate reduced sulfur compounds. 

Physically, the development of a magnetic crystal is governed by two factors: one is moving to align the magnetic force of the molecules in conjunction with the developing crystal, while the other reduces the magnetic force of the crystal, allowing an attachment of the molecule while experiencing an opposite magnetic force. In nature, this causes the existence of a magnetic domain, surrounding the perimeter of the domain, with a thickness of approximately 150 nm of magnetite, within which the molecules gradually change orientation. For this reason, macroscopically, the iron is not magnetic in the absence of an applied field. Similarly, extremely small magnetic particles do not exhibit signs of magnetisation at room temperature; their magnetic force is continually altered by the thermal motions inherent in their composition. Instead, individual magnetite crystals in MTB are of a size between 35 and 120 nm, that is, large enough to have a magnetic field and at the same time small enough to remain a single magnetic domain. Aerotaxis is the response by which bacteria migrate to an optimal oxygen concentration in an oxygen gradient. It has been shown that, in water droplets, one-way swimming magnetotactic bacteria can reverse their swimming direction and swim backwards under reducing conditions (less than optimal oxygen concentration), as opposed to oxic conditions (greater than optimal oxygen concentration). The behaviour that has been observed in these bacterial strains has been referred to as magneto-aerotaxis.

Source Magnetotactic bacteria

As melted material moves about in the Earth’s core, a magnetic field is created around the globe.  The ability to detect, or otherwise be effected by the natural magnetic field is called biomagnetism. Intensive research has been conducted on the correlation between biogenic magnetite bearing creatures and their ability to navigate seemingly without any other cue than biomagnetism. Floating crystals of magnetite in receptor cells, normally found in the front/snout portion of organisms, will react to magnetic fields. The torque placed upon the magnetite causes the depolarization of nearby nerve cells. The action is analogous. The Earth’s magnetic field is weaker near the equator (around 27,000 nT) and strengthens nearer to the poles (roughly 80,000 nT). An animal would be able to sense its latitude along the varying field.

The magnetization of the ocean floor, primarily in banded iron formations or BIFs, and other places could be due to the magneto-fossils of Precambrian magnetotactic bacteria. Formation of Magnetic Single-Domain Magnetite in Ocean Ridge Basalts with Implications for Sea-Floor Magnetism  Source Skywalker

The development of a thick suboxic zone with high iron bioavailability – a product of dramatic changes in weathering and sedimentation patterns driven by severe global warming – may have resulted in diversification of magnetite-forming organisms, likely including eukaryotes.

Nitrogen Cycle

Denitrification is a microbially facilitated process of nitrate reduction that may ultimately produce molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products.

This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favorable include nitrate (NO3), nitrite (NO2), nitric oxide (NO), and nitrous oxide (N2O). In terms of the general nitrogen cycle, denitrification completes the cycle by returning N2 to the atmosphere.

The process is performed primarily by several species of bacteria, involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway have been identified in the reduction process.

Potential for feedbacks between climate and tectonics mediated by geomorphic processes

An example of thin-skinned deformation (thrust faulting) of the Sevier Orogeny in Montana. Note the white Madison Limestone repeated, with one example in the foreground (that pinches out with distance) and another to the upper right corner and top of the picture.

Modern geomorphology can be thought of as the study of the divergence of fluxes of material on a planetary surface, and as such is closely allied with sedimentology, which can equally be seen as the convergence of that flux.

Geomorphic processes are influenced by tectonics, climate, ecology, and human activity, and equally, many of these drivers can be affected by the ongoing evolution of the Earth’s surface, for example, via isostasy or orographic precipitation. Many geomorphologists are particularly interested in the potential for feedbacks between climate and tectonics mediated by geomorphic processes.

Orography has a major impact on global climate, for instance the orography of East Africa substantially determines the strength of the Indian monsoon.[3] In geoscientific models, such as general circulation models, orography defines the lower boundary of the model over land.

Practical applications of geomorphology include hazard assessment (such as landslide prediction and mitigation), river control and restoration, and coastal protection.

Biology – The study of orogeny (mountain genesis or general Earth’s crust deformation), coupled with biogeography (the study of the distribution and evolution of flora and fauna),geography and mid ocean ridges, contributed greatly to the theory of plate tectonics. Even at a very early stage, life played a significant role in the continued existence of oceans, by affecting the composition of the atmosphere. The existence of oceans is critical to sea-floor spreading and subduction.

Soil erosion and climate change

The warmer atmospheric temperatures observed over the past decades are expected to lead to a more vigorous hydrological cycle, including more extreme rainfall events.In 1998 Karl and Knight reported that from 1910 to 1996 total precipitation over the contiguous U.S. increased, and that 53% of the increase came from the upper 10% of precipitation events (the most intense precipitation). The percent of precipitation coming from days of precipitation in excess of 50 mm has also increased significantly.

Studies on soil erosion suggest that increased rainfall amounts and intensities will lead to greater rates of erosion. Thus, if rainfall amounts and intensities increase in many parts of the world as expected, erosion will also increase, unless amelioration measures are taken.

Soil erosion rates are expected to change in response to changes in climate for a variety of reasons. The most direct is the change in the erosive power of rainfall. Other reasons include: a) changes in plant canopy caused by shifts in plant biomass production associated with moisture regime; b) changes in litter cover on the ground caused by changes in both plant residue decomposition rates driven by temperature and moisture dependent soil microbial activity as well as plant biomass production rates; c) changes in soil moisture due to shifting precipitation regimes and evapo-transpiration rates, which changes infiltration and runoff ratios; d) soil erodibility changes due to decrease in soil organic matter concentrations in soils that lead to a soil structure that is more susceptible to erosion and increased runoff due to increased soil surface sealing and crusting; e) a shift of winter precipitation from non-erosive snow to erosive rainfall due to increasing winter temperatures; f) melting of permafrost, which induces an erodible soil state from a previously non-erodible one; and g) shifts in land use made necessary to accommodate new climatic regimes.

Studies by Pruski and Nearing indicated that, other factors such as land use not considered, we can expect approximately a 1.7% change in soil erosion for each 1% change in total precipitation under climate change. The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the lower crust and mantle. This can cause tectonic or isostatic uplift in the region. Research undertaken since the early 1990s suggests that the spatial distribution of erosion at the surface of an orogen can exert a key influence on its growth and its final internal structure (see erosion and tectonics).

Climate forcing of geological and geomorphological hazards

Evidence for periods of exceptional past climate change eliciting a dynamic response from the Earth’s crust, involving enhanced levels of potentially hazardous geological and geomorphological activity. The response, McGuire notes, is expressed through the triggering, adjustment or modulation of a range of crustal and surface processes, which include gas-hydrate destabilization, submarine and subaerial landslides, debris flows and glacial outburst floods, and volcanic and seismic activity.

Gas hydrates may present a serious threat as the world warms, primarily through the release of large quantities of methane into the atmosphere, thus forcing accelerated warming, but also as a consequence of their possible role in promoting submarine slope failure and consequent tsunami generation.

Source Wikipedia Related Magnetic Death StarMagnetiteOrogenWeatheringErosionDecompositionErosion and tectonicsPETM Paleocene Eocene Thermal MaximumAprubt CC – Runaway CCEpeirogenic movementLithosphereGeomorphologySedimentology –  PedologyStructural geology – GeochemistryHypoxiaOrographySiltation

  1. “Faivre & Schüler (2008) estimated that, in some locations on Earth, up to 10% of the iron contained within sediment may be contributed by magnetotactic bacteria. Recently, Haltia-Hovi and colleagues suggested that fossil magnetosome concentrations could be used as a climate indicator (Haltia-Hovi et al. 2010). Magnetosome production was influenced by changes in the bacteria’s natural habitat (e.g., organic matter), which is directly controlled by climate.

    As we begin to understand Earth’s magnetotactic bacteria and the mechanism by which these organisms synthesize iron oxides and sulfides, we may also be able to answer whether the magnetite observed in the Martian meteorite ALH84001 (or others) may be biotic in origin as well. Such insight might help us understand the origin of life on Earth and elsewhere in the universe.”

  2. Environmental change during the PETM drives formation of gigantic biogenic magnetite

  3. Magnetotactic Bacteria

    Almost all cultured species exhibit nitrogenase activity and thus fix atmospheric nitrogen and many denitrify. Magnetotactic bacteria thus show a great potential for iron, nitrogen, sulfur, and carbon cycling in natural environments.

Trackbacks & Pingbacks

  1. Connecting the Dots: Climate Change drives Earthquake / Seismic activity « The Climax
  2. Soil carbon and climate change: from the Jenkinson effect to the compost-bomb instability « The Climax

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