Paleoenvironmental Reconstruction Final Exam
What are the principal controls on whether a fluvial system undergoes aggradation or degradation? Which ones cause aggradation and which ones cause degradation?
- Base level: drop causes degradation, rise causes aggradation - Tectonics (may cause pulse of incision/degradation) - More discharge (degradation); can be from more storminess or more MAP - Greater sediment load (aggradation) - Steeper stream slope (degradation) Note: flooding events and fires can cause massive aggradation, but does not occur for fires until precipitation/storm events move sediments onto main floodplain terraces.
What changes in soil associated with diagenesis interfere with the ability to extract climate information from paleosols? How/why do these changes matter?
- Changes in organic content with depth may not be preserved in fossil soils. - Cementation by calcite, gypsum, hematite, or silica is also common in diagenesis and post-depositional alteration; this makes it difficult to tell whether a paleosol is an aridosol or has been altered (although secondary alteration often has a coarser crystal size than primary deposition) - Also, compaction (can look at laminae thickness in resistant nodule and ellipticity of buried tree branches to get an idea of effect) - Burial illitization
What are the five factors that must be taken into account when interpreting fluvial records as paleoclimatic indicators?
1. Location 2. Context 3. Convergence 4. Divergence 5. Sensitivity
What are some of the overarching issues underlying all paleoenvironmental proxies?
1. Make sure you understand the spatial and temporal scales your indicator represents. 2. Understanding the processes involved in the deposition of your indicator and what may thus cause changes in it. 3. Understanding processes which might alter your indicator post-deposition. 4. The more proxies, lines of evidence, and samples used in your reconstructions, the better. 5. Adequately constraining and understanding your modern analog. 6. Not attributing correlation to causation or assuming relationships will hold in the past if you don't understand the mechanism causing it. 7. Making sure you're not extending your analogy too far to conditions under which relationship may be fundamentally different. 8. Not being afraid of multi-causal explanations (the climate system is complex!) 9. Thinking in terms of constraints rather than absolutes when necessary 10. Unexpected is sometimes better. 11. Not always basing your interpretation on the match to the big picture/global record.
What are the five influences on soil development?
1. Parent material 2. Climate 3. Topography (relief) 4. Organisms 5. Time These were outlined by Hans Jenny in 1941 and may be remembered by cl-o-r-p-t
Describe the progression of soil orders (i.e., one soil turning into another over time).
1. Parent material generally develops histosol, vertisol, or entisol. 2. If entisol soil, may subsequently develop into inceptisol soil with time. 3. Depending on vegetation and climate, inceptisol may develop into aridosol (dry), mollisol (grassland), spodosol (conifer forest), or alfisol (mixed forest). 4. With time, mollisols, spodosols, and alfisols, all develop into ultisols. 5. With even more time and lots of weathering, ultisols develop into oxisols.
What are the distinguishing characteristics of alfisols?
Alfisols (suffix: -alf) are soils associated with mixed forests (both conifers and deciduous trees) or slightly more acidic bedrock. They have a B horizon that is either clay rich or sodium rich, and they are characterized by a base saturation greater than 35% and less than 50%.
What does amictic mean?
Amicitic is a term referring to lakes that never mix at all. These lakes may be ice covered or may be uniformly warm.
What is an erg?
An erg is a sand sea (word is derived from Arabic term for dune field). Dune fields must cover at least 125 square kilometers to be classified as ergs, but most ergs are greater than 32,000 square kilometers in area. A large supply of sand, winds of at least 4 m/s, and time are needed to form ergs.
What are the distinguishing characteristics of andisols?
Andisols (suffix: -and) are formed from volcanic parent material in places like the Cascades and formerly volcanic terrains. They are variable otherwise, but are very fertile: ash and volcanic glass is so chemically heterogeneous and weathers so quickly that tons of basic ions like Ca are available, forming nutrient-rich soils. Andisols are relatively uncommon.
What are the distinguishing characteristics of aridosols?
Aridosols (suffix: -id) are soils formed in arid climates. They have B horizons that usually contain evaporite salts like gypsum, silica, and carbonates. These evaporites form as water percolates through the soil and subsequently evaporates.
What is base level?
Base level is the lowest elevation to which a land surface can be eroded by a stream. A drop in base level pushes a river system toward degradation, while a rise in base level pushes the system toward aggradation.
What does benthic mean?
Benthic is a term used to describe microorganisms that dwell in deep ocean waters.
What is burial illitization?
Burial illitization is the conversion of many clays to illite through burial alteration and diagenesis. Lots of paleosols are dominated by illite, indicating that this may be a common problem. It may also alter soil chemistry.
What is CBT?
CBT refers to the cyclization of branched tetraethers. It is an index that is based on the relative abundance of cyclopentyl moieties, which are parts of a molecule that may include either whole functional groups or parts of functional groups as substructures (all functional groups are moieties, but not all moieties are functional groups). Functional groups are specific groups of atoms and/or bonds within molecules responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of. The cyclization of branched tetraethers is dominantly a function of pH, with greater amounts of cyclization occurring at lower (more acidic) pHs. Adding or removing cyclopentane rings from membranes changes the packing of membrane lipids, enabling more water molecules to get trapped and consequently increasing the membrane proton permeability (reason for cyclization correlation).
What kinds of paleoclimatic information can be extracted from paleosols using soil carbonates? What are some challenges in using this and how can it be used responsibly?
Carbonates can generally be used to reconstruct precipitation. Pedogenic carbonate is generally associated with drier climates, as in super humid climates, all of the carbonates are leached out of the soil. The depth from the surface of a soil to the top of the carbonate horizon in a soil is generally a function of rainfall, with greater amounts of rainfall corresponding to deeper carbonate horizons. Challenges with this include the compaction of soils (alters depth reconstructions), the possibility of truncation through erosion of top of soil, the partial pressure of CO2 in the atmosphere affecting the solubility of carbonate and the rainfall vs. depth relationship, and the possible precipitation of carbonate from carbonate-rich groundwater through capillary action. It is generally best to use this on moderately-developed soils, soils with unconsolidated parent material that does not affect water movement, and soils in seasonally warm climates (assuming the depth measured is to where abundant nodules occur, not solitary nodules and within the low points of gilgai microrelief). It does not work for "petrocalcic horizons," calcretized bedrock, dolocretes, carbonate collars, or erosion-susceptible hillslope soils. - carbonate C isotopic comp. can also be used to reconstruct soil CO2
What is cation exchange capacity (CEC)?
Cation exchange capacity (CEC) refers to the ability of a soil to retain cations in a form that is available to plants. A soil's CEC is tied to organic materials and clays. Clays are sheet silicates within which there is usually some charge left over between the sheets. Many clays are willing to exchange adsorbed cations between their sheets with cations in the soil solution, and plant-needed nutrients can often be stored in clays. Many clays would happily give up Na or Ca ions needed by plants for H ions given up by the plant. In a way, CEC is thus almost like the ability of a soil to act as a refrigerator and store nutrients. CEC both influences and is influenced by soil pH. CEC is high for smectite (TOT), low for mica (TOT) and kaolinite (TO), and moderate to low for (TOT)O clays like chlorite.
What kinds of paleoclimatic information can be extracted from paleosols using clay mineralogy? What are some challenges in using this and how can it be used responsibly?
Clay mineralogy is in some cases a function of temperature and humidity (stable weathering products) and may be used to reconstruct climate in that manner. Also, clays have hydroxides as part of their mineral formula that are formed primarily in the soil, and the oxygen and hydrogen isotopes from these hydroxides can be used to indicate temperature or precipitation amount by reconstructing water isotopic composition from temperature-dependent water-clay fractionation (depending on latitude). However, evaporation and hydrothermal alteration also may influence these isotopes and modify values, and the clays must be authigenic (formed in soil). Also, the wetter a climate is, the more clay and red, less weatherable stuff its soils tend to have. With weathering extent, the sequence of clays tends to be as follows: muscovite/illite/chlorite -> smectites -> kaolinite -> oxides. We cannot use clays as indicators if the parent material was sedimentary with clay content, and we must be careful with seasonality of rainfall versus climate change affecting clay mineralogy. - Certain clays like palygorskite and palygorskite are indicative of very dry climates, while others like kaolinite (clay), boehmite, and gibbsite (aluminum oxides) are indicative of very humid climates.
Name some ways in which the influences on soil development are interrelated.
Climate & TIme -> weathering Parent material & climate -> acidity Climate and Organisms -> acidity
How does climate affect soil development?
Climate affects soil development in that as precipitation increases, more leaching occurs and more water moves through the soil profile. Easily-removed base cations tend to get leached out in this process while acidic ions stay, making the soil more acidic through time. Greater precipitation may also make the soil more favorable for organisms and thus cause a greater organic component to develop in the soil (acidifying even more). Climate may be partially inferred from paleosols through clay mineralogy, as clay mineralogy (which weathering products are stable) is tied in part to temperature and humidity.
What is El Nino?
El Niño is a warm surface current that usually appears in the Pacific Ocean off Ecuador and Peru around Christmas, and lasts about three months. Every three to seven years it remains for as long as a year-and-a-half as part of a southern oscillation. In North America, this contributes to warmer temperatures along the Pacific coast and weaker hurricanes on the Atlantic. It is a system of coupled ocean-atmosphere dynamics; a pressure gradient in the atmosphere causes stronger easterly winds across the Pacific and allows a warm body of water in the western Pacific to move toward South America. This shuts off the upwelling of cool waters off the Peruvian coast, leading to higher sea surface temperatures (SST) in that are and impeding biological productivity.
What are the distinguishing characteristics of entisols?
Entisols (suffix: -ent) are very young and recent soils with no distinctive horizons. They are poorly developed and do not contain a B horizon. They may exist due to very little time passing in the formation of the soil, a highly resistant parent material, or a high degree of slope that results in too much erosion for the soil to develop fully.
What conditions favor the deposition of evaporites? How can we use evaporites to constrain climate conditions?
Evaporites generally form under conditions of: - Greater evaporation than input through precipitation, runoff, and groundwater over the course of a year; this is often helped by high temperature and wind, which brings in less humid air mass (see esp. rain shadow desert and basinal relief). - Warm, seasonal climates (if you are super dry, you don't have any water coming in to evaporate). They may also form near glaciers in cold, dry climates, but I believe this is relatively rare. Besides telling you that the area once (or currently does) experienced a seasonally dry climate with greater evaporation than input, specific evaporite minerals may indicate specific conditions. For example, "chickenwire nodular anhydrite" requires temperatures of at least 35 degrees C for nucleation and at least 20 degrees C to be maintatined, while glauberite and mirabilite are also have specific humidity and/or temperature requirements. Evaporites may also inform us of past changes in atmospheric circulation and ocean currents.
What kinds of paleoclimatic information can be extracted from paleosols using Fe/Mn nodules in vertisols? What are some challenges in using this and how can it be used responsibly?
Fe/Mn nodules in vertisols can be used to estimate mean annual precipitation (MAP). The total Fe content of Fe/Mn nodules in vertisols increases with increasing precipitation, so if one can demonstrate that a paleosol is a vertisol through the presence of smectite clays, MAP can be estimated from these nodules. This is a better method than using carbonates because it is not dependent on soil depth. However, smectite clays can be secondarily altered, and the mechanism behind this correlation is not very well understood.
What is field capacity?
Field capacity is the amount of soil moisture or water content held in soil after excess water has drained away and the rate of downward movement has materially decreased, which usually takes place within 2-3 days after a rain or irrigation in pervious soils of uniform structure and texture.he water is drawn away by gravity. (i.e., maximum amount of water soil can hold via surface tension and max available to plants)
What do foram oxygen isotopes tell us about climate? How do we know what controls isotopic variation in forams?
Foram oxygen isotopes serve as a proxy for ocean water composition. In turn, ocean water composition tracks ice volume (oceans get heavier with more ice), temperature change with time (temperature dependence of calcite-water fractionation), and changes in the temperature profile of the oceans (using combination of planktonic and benthic forams). During warm times in the past in which no continental ice existed, isotope changes can largely be interpreted as temperature changes, while icehouse conditions make ice volume drive composition changes. We can get an idea of what controls isotopic variation in forams (temp. vs. ice) by keeping in mind the fact that the heaviest benthic 18O values possible in an ice free world are 1.8 per mil at 0 degrees C. ...? Forams are generally good for helping to identify abrupt events (where ice volume or temperature change may be easier to distinguish).
What is a foram?
Forams (or foraminifera) are single-celled microorganisms that live in the oceans and produce tests made of calcite. Foram tests can be used in a variety of paleoclimate studies, including carbon and oxygen isotopes and strontium, barium, and magnesium substitution into the calcite structure (there is a temperature dependence for the Mg substitution coefficient for calcite). Foram species differ in the thicknesses of their tests, a considerable preservation issue (some species may be preserved while others aren't, possibly introducing bias and inaccuracies). Biogeography, tolerances, and morphological changes (?) are all considered in foram studies. Looking at their distributions with knowledge of their tolerances may be especially helpful. Also, cadmium in foram tests is thought to track d13C and thus productivity and upwelling (amount, proximity; it is related to P taken up into soft tissue).
What is GDGT?
GDGT refers to branched glycerol dialkyl glycerol tetraethers. GDGT are "components with 4-6 methyl groups attached to the n-alykl chains and 0 to 2 cyclopentyl moieties in the alkyl chain." Branched glycerol diakyl glycerol tetraethers are soil derived, and the relative abundances of different types of GDGTs tell us something about soil ph and temperature. GDGTs are produced globally by anaerobic bacteria, but we do not know the exact bacteria that produce them. However, we do know that the organisms that produce branched glycerol diakly glycerol tetraethers prefer acidic conditions to neutral conditions.
What are the distinguishing characteristics of gelisols?
Gelisols (suffix: -el), also known as cryosols, are organic rich soils that are formed in very cold climates with permafrost. There is not much horizon formation in these soils, as the soil is moved via cryoturbation (freezing and thawing). These soils are restricted to Alaska in the U.S..
What are the distinguishing characteristics of histosols?
Histosols (suffix: -ist) are highly organic-rich soils associated with wetlands (swampy soils). These soils are waterlogged, providing anoxic conditions that allow for the preservation of organic materials.
What does holomictic mean?
Holomictic is a term referring to lakes that fully mix at least once over the course of a year.
What is humus?
Humus is decomposed organic material (i.e., plant and animal matter) that is often present in the O horizon of soils. It adds nutrients to the soil that are important for plant growth.
How does location affect one's interpretation of fluvial records as paleoclimatic indicators?
If a river (ex: Nile River) crosses multiple climate zones, we cannot use it as a good local climatic indicator unless we are lucky enough to have sediment differences in the different climate zones. Giant river systems don't function all as one, so the location of the response to climate change is not necessarily the location at which the climate change occurred.
What do you know about climate if you identify a particular type of dune?
If you identify a particular type of dune, you can often infer something about predominant wind directions(s) and sand supply. Dune form is largely tied to sand supply and wind direction variability. These in turn may be influenced by vegetation, grain size, and topography. The foreset dip, or angle of cross-bedding, within the dune is a reasonably reliable indicator of wind direction.
What do you know about climate if you identify an erg?
If you identify an erg, you know that the climate in the region was very dry for an extended period of time (time is needed for the sand to accumulate) and had moderate wind speeds (at least 4 m/s winds are needed to reliably move sand. Also, the drier it is, the more area you have covered by dunes (and the less area attributed to interdunal space).
What do you know about climate if you identify loess?
If you identify loess, you know that the regional climate was quite windy (a lot of wind is needed to pick up and eventually deposit loess) and that either a desert or glaciers were probably located nearby (you at least know that an unvegetated area with silty sediments was nearby).
What are the distinguishing characteristics of inceptisols?
Inceptisols (suffix: -ept) are soils that are still quite young, but have a bit more going on than entisols do. They exhibit some distinctive horizons and may have some vegetation growing in them, but not much. No clay is present in the B horizons of inceptisols (clay takes time to develop, and inceptisols are young).
How may lakes be classified by nutrient status? What are the categories, and where do lakes of each type tend to occur?
Lakes may be classified by nutrient status into the following categories: - eutrophic (nutrient rich) - mesotrohpic (nutrient intermediate) - oligotrophic (nutrient poor) While no general trends in location exist, oligotrophic lakes are more common in watersheds that are composed of felsic bedrock that contributes to higher degrees of acidity (and lesser general productivity).
How may lakes be classified by mixing status? What are the categories, and where do lakes of each type tend to occur?
Lakes may classified by mixing status into the following categories: - Amictic: never mix - Holomictic: mix fully at least once during the year - Meromictic: only ever partially mix Amictic lakes are generally found in very cold climates (constantly ice-covered), but they may also be uniformly warm. Meromictic lakes tend to occur in saline lakes or lakes that are very deep (?). Holomictic lakes can further be divided into the following subcategories based on frequency of mixing and where they tend to occur: - Oligomictic lakes are warm tropical lakes with brief/sporadic mixing that occurs based on occasional cooling - Polymictic lakes are shallow, warm or high altitude lakes that stratify/destratify multiple times throughout the year due to strong diurnal (daytime) temperature changes - Dimictic lakes are average temperate zone lakes that tend to follow similar criteria as above - Monomictic lakes are lakes that mix only once during the year
Why do lakes mix?
Lakes mix due to changes in density over the course of a year. Wind may aid mixing by churning things up. Near-homogenous density is best for conditions of thorough mixing .(?)
What is leaching?
Leaching is dissolution and removal of material from soils by organic acids. Over time, the process may form an E horizon in a soil.
What is loam?
Loam is soil composed of sand, silt, and clay in relatively even concentration (about 40-40-20% concentration respectively). Loam soils generally contain more nutrients and humus than sandy soils, have better infiltration and drainage than silty soils, and are easier to till than clay soils.
What is loess?
Loess is the finer-grained fraction of eolian sediment (between 20 and 60 micrometer grains) that often is deposited in large accumulations at the margins of deserts (it is too windy within the deserts to settle out). Loess deposits may also be associated with glaciers, where rock flour at the terminal end of glaciers is picked up and later deposited by the wind. We know of loess deposits mostly from the Quaternary, as the glacial conditions experienced during the Quaternary are particularly conducive to loess formation and as loess deposits may easily be confused with near-shore marine deposits following diagenesis. Vegetation often serves as traps for loess (i.e., causes it to settle out).
What changes in soil associated with low grade metamorphism interfere with the ability to extract climate information from paleosols? How/why do these changes matter?
Low grade metamorphism may contribute to the process of burial illitization by converting some other types of clays to illite. One can usually distinguish metamorphic illite (which may have previously been smectities) from natural illite in that natural illites and clays tend to be more finely crystalline. For any analyses with clays, one should ideally use soils developed on igneous or metamorphic bedrock.
What is MBT?
MBT refers to the methylation of branched tetraethers. It is an index that considers the relative abundance of methyl branches. The abundance of methyl branches is tied to both pH and temperature, with greater abundance occurring at higher temperatures and lower pHs (membrane lipid adaptation to temperature change is a well-known feature).
What does meromictic mean?
Meromictic is a term that refers to lakes that only ever partially mix; lakes that have a monimonlimnion layer are meromictic.
How do we use minor element geochemistry in corals to reconstruct climate? What are the alternative factors you have to consider for interpretation?
Minor element geochemistry in corals is primarily used in paleothermometry applications. Mg/Ca and Sr/Ca ratios in corals are both used to directly reconstruct SST, as there is a temperature-dependent substitution coefficient of Mg into coral aragonite, with more Mg being incorporated at higher temepratures; similarly, there is an inverse relationship between SST and coral Sr, with lesser amounts of Sr substituting for Ca at higher temperatures. Cd/Ca ratios may also be used to reconstruct nutrient availability and biological productivity (and upwelling) in the regions near corals.
Why are mixing status and nutrient status important in understanding the paleoclimatic records that come from lakes?
Mixing status and nutrient status are important in understanding the paleoclimate records that come from lakes because they help us to interpret the environment in which the lake was found. The presence of preserved organic matter, for example, generally indicates anoxic conditions in a monimolimnion layer, and this type of layer generally occurs in saline lakes (or deep lakes?).
What are the distinguishing characteristics of mollisols?
Mollisols (suffix: -oll) are soils associated with grasslands and prairies. They have organic-rich A horizons and high base saturation (greater than 50%). Mollisols are either young or are formed from an easily soluble parent material that provides a constant supply of base ions (i.e., carbonate bedrock).
How do we interpret organic rich lake sediments? How about laminated lake sediments?
Organic rich lake sediments can generally be interpreted as being deposited in a saline lake or in otherwise anoxic conditions (merimictic lakes; good assumption for large lakes). It also tells you that the lake was biologically productive enough for those organics to be preserves. Laminated lake sediments generally indicate seasonal variations within the lake, whether is be in the form of the mineralogy of evaporites or organic-rich versus organic poor laminations (which generally indicate closed basin with higher salinity and better preservation in dry season).
How do organisms affect soil development?
Organisms tend to make soils more acidic, as the breakdown of organic material forms acids. Thus, greater productivity and more organisms tends to leads to more acidic soils. Organisms also affect soil thickness; deeper rooting structures and greater amounts of bioturbation tend to lead to thicker soils.
What are the distinguishing characteristics of oxisols?
Oxisols (suffix: -ox) are old soils that have experienced so much weathering that nothing really remains in the soil except oxides (Bo horizon). They have thin A horizons and commonly develop in tropical, humid climates like those of Hawaii and Puerto Rico, although they may be present in places that are drier today (torrox).
What is an oxygen isotope stage?
Oxygen isotope stages, more recently known as marine isotope stages (MIS), are alternating warm and cool periods in earth's paleoclimate corresponding to global ice volume changes from glaciation and deglaciation. They are derived from oxygen isotope data from benthic forams in deep sea marine sediment cores and are used mainly as a reference for time.
How do we use oxygen isotopes in corals to reconstruct climate? What are the alternative factors you have to consider for interpretation?
Oxygen isotopes in corals are used to reconstruct both water temperature (from the temperature dependent fractionation between water and calcium carbonate) and the oxygen isotopic composition of marine water. Marine d18O generally changes over shorter periods of time with evaporation, rainfall, and runoff. A priori, we usually don't know whether temperature or runoff will be the dominant control on coral 18O unless we are out in the middle of the ocean, in which case 18O is probably related to temperature; this necessitates modern calibrations on active corals rather than just assuming 18O variability is due to either one factor or the other and also makes multi-proxy studies valuable.
How does parent material affect soil development?
Parent material is the main determinant of soil pH, perhaps the most important soil variable. Parent material that is rich in Ca, Mg, Na, and K ions (ex: limestone and mafic rocks) will tend to form basic soils upon weathering, while parent material that is rich in Al, Fe, or exclusively Si ions (ex: felsic rocks containing more quartz) will tend to form acidic soils. The influence of parent material on soil development diminishes over time with greater amounts of weathering.
What are peds?
Peds are prismatic, blocky, granular soil aggregates (clumps). They are formed under conditions of translocation (the movement of dissolved material within a plant) and a water gradient and are indicative of the water table being just below.
What does planktonic mean?
Planktonic is a term used to describe microorganisms that dwell in shallow water. Planktonic organisms can be classified as living in either the photic (light-penetrating) or aphotic zone.
What is plant available water?
Plant available water refers to the amount of water in a soil that is available for plants to intake. It spans the continuum between field capacity and the wilting point. Pore spaces in the soil are partially drained after it stops raining, but a certain amount of water adheres to the surfaces of objects in the soil due to surface tension (and maybe capillary surface). Plants growing in the soil essentially set up a negative pressure gradient that sucks up water into the plants, but there is a point as the soil dries out that the surface tension of the water remaining on the soil objects (film water) becomes so great that the plants cannot suck up the water (and the plant wilts).
What is regolith?
Regolith is the unconsolidated mantle of weathered rock and soil material on the earth's surface. It includes soil, which is layers of mineral and/or organic constituents that are different from the parent material on which they rest and are formed through the chemical and physical alteration of parent material. They layers are formed and altered in situ. No organic rock (like coal) is present in regolith.
How does sensitivity affect one's interpretation of fluvial records as paleoclimatic indicators?
Sensitivity affects one's interpretation of fluvial records of paleclimatic indicators in that fluvial systems are non-linear; a small change in the system can result in a large change in the river (thresholds), and it is tough to distinguish whether a big climate change occurred or whether the system was near a threshold to begin with.
What is soil texture?
Soil texture is described by the composition of the soil in terms of sand, silt, and clay percentages. It determines the porosity and permeability of the soil, which are collectively integral to the rate of the development of the soil and of the vegetation community.
What are the distinguishing characteristics of spodosols?
Spodosols (suffix: -od) are conifer forest soils that are characterized by low base saturation. They usually have an E horizon, as conifer needles and other organic material form enough acids to promote leaching. Sandy parent material and a humid climate also helps develop spodosols.
What changes in soil associated with burial interfere with the ability to extract climate information from paleosols? How/why do these changes matter?
Substantial losses in soil organic material occur over time due to the decay of organic content in paleosols, but deep burial where temperatures are higher quickens this process. Organic carbon generally decreases with soil depth. Compaction is also an issue and varies with the texture of the soil. Because of compaction as well as erosion and deposition, it is difficult to reconstruct how deeply a soil may have been buried at one point. Buried paleosols may also appear red like oxisols due to them being extremely old and are not necessarily indicative of hot and humid climates. Iron oxides like goethite and hematite often formed in soils post-burial. - Also, burial illitization
What are the 12 soil orders?
The 12 soil orders are: 1. Entisols (-ent) 2. Inceptisols (-ept) 3. Histosols (-ist) 4. Andisols (-and) 5. Vertisols (-ert) 6. Aridosols (-id) 7. Gelisols (-el) 8. Mollisols (-oll) 9. Alfisols (-alf) 10. Spodosols (-od) 11. Ultisols (-ult) 12. Oxisols (-ox) Soil order is tied to the basic structure of the soil and the presence of distinctive horizons or a distinctive climate. The suffix of the last word in a soil name indicates the soil order.
What is the A horizon?
The A horizon is the second horizon from the top in soils. It is composed of a mixture of organic material and mineral (inorganic; bedrock constituent) material.
What is the B horizon?
The B horizon is the soil horizon found under the E horizon. It is the horizon in which the material leached from the E horizon is deposited and accumulates. Multiple subhorizons within the B horizon exist depending on the type of material that is accumulating there and its resulting color (ex: Bhs -> organics [lighter brown] and Bs -> iron oxides [orangish]).
What is the C horizon?
The C horizon is the second soil horizon from the bottom. It is composed of weathered and broken down parent material to which nothing has been added. It is located above the R horizon, which is simply unaltered parent material (bedrock).
What is CCD?
The CCD refers to the carbonate compensation depth. The carbonate compensation depth is the depth within the oceans at which the partial pressure of dissolved CO2 in bottom waters becomes high enough (or temperature becomes low enough) that calcite tests begin to dissolve. Calcite tests that deposited on a seafloor above the carbonate compensation depth are usually preserved very well, while those deposited below the CCD may not be preserved at all (they dissolve). The presence/absence of carbonate foram tests in sediment cores can help one reconstruct the CCD through time and calculate the depth or place of the seafloor at a given time.
What is the CCD and why does it vary? Why do we care?
The CCD, or carbonate compensation depth, is the depth at which calcite begins to dissolve due to the higher partial pressures of dissolved CO2 in ocean bottom waters. It varies due to ocean temperature (colder temperatures result in calcium carbonate becoming more soluble), ocean pressure (higher pressures correspond to higher partial pressures of carbon dioxide, which create more acidic conditions under which calcium carbonate is more soluble), and the amount of dissolved carbon dioxide in ocean bottom waters (increases with the age of the water and the amount of decomposition of organic matter). The Pacific CCD is around 3.5km, while the Atlantic CCD is around 5km (Atlantic deeper because deep water is younger and has less dissolved CO2). We care about this because it affects the preservation of foram tests from which we can gain paleoclimatic information and because the presence or absence of preserved forams can help us infer changes in ocean conditions that would cause changes in the CCD.
What is the E horizon?
The E horizon, or "eluvial horizon," is the third horizon from the top in soils. It is a leached horizon from which material has been removed and takes a while to fully develop. The organic material in the O and A horizons above the E horizon generates a lot of organic acids (like humic acid) that solubilize and remove (leach) things from the E horizon as they move down the soil profile.
What is the O horizon?
The O horizon is the uppermost horizon in soils. It is dominated by organic material (leaf litter and humus).
How does context affect one's interpretation of fluvial records as paleoclimatic indicators?
The context of the river (its large scale setting) plays an important part in the interpretation of fluvial records, as disregarding context may result in an incorrect interpretation. For example, glaciation lowers sea level and thus should also lower the base levels of many rivers, predictably leading to degradation. However, we generally see aggradation in the western U.S. going into glaciations due to huge increases in sediment load.
What is the epilimnion?
The epilimnion is the top layer of the lake (lakes can be subdivided based on physical and chemical structure). It consists of generally warm, well-oxygenated water that is warmed by the sun and mixed by the wind. It is fairly constant in temperature, and it may be less oxygenated in the summer due to the fact that warmer water is less capable of holding dissolved oxygen.
What is the hypolimnion?
The hypolimnion is the bottom layer of a lake. It is characterized by cold temperatures and anoxic conditions (respiration by cold-loving organisms and the decomposition of organic matter create these little to no oxygen conditions). The existence of this layer in a lake often determines whether or not the organic material from lakes or bogs is preserved.
What is the metalimnion?
The metalimnion is the middle layer of a lake. It is characterized by a rapid change in density and temperature with depth (thermocline), and as a result of this density/temperature gradient, it inhibits mixing between the epilimnion and the lower hypolimnion and (if present) monimolimnion.
What is the mixolimnion?
The mixolimnion refers to the mixing layers in a lake (i.e., all of the layers except the monimolimnion if it is present). It is partially characterized by the chemocline, the chemical difference as one goes from the mixolimnion to the monimolimnion.
What is the monimolimnion?
The monimolimnion is the bottom anoxic water layer in a lake. The water in this layer is too dense to ever mix with the above layers, and this layer typically occurs in saline lakes (it can grow during a time when road salt is flushed into the lake).
What can isotopic analysis of lacustrine carbonates tell you? What are some of the difficulties in using these parameters to reconstruct paleoclimate?
The oxygen isotopic composition of lacustrine carbonates can allow you to reconstruct the oxygen isotopic composition of the lake water and infer precipitation/evaporation in the lake (also must consider temperature-dependent fractionation factor). The carbon isotopic composition of lacustrine carbonates can be used to reconstruct the dissolved inorganic carbon in the lake water and infer biological productivity (higher degrees of biological productivity lead to heavier inorganic carbon and lacustrine carbonate compositions). Covariance of C and O may also indicate a closed basin lake, as we cannot see O enrichment due to evaporation in open basin lakes because the residence time is too short. Difficulties in using these parameters include the carbon problem: while biological productivity toward the surface of the lake usually makes surface-water carbonate C heavier, the deep-water carbonate C may become lighter with greater biological productivity if the bottom of the lake is dominated by the decomposition of organic matter. Paleoclimatic interpretation thus depends on in what part of the lake the carbonate is believed to have formed (although greater temperatures at the surface of the lake usually makes carbonate precipitation more likely there). Also, changes in the size of the lake are likely to affect productivity, with lakes becoming more productive as they become more shallow. In general w/lakes, distinguishing between local and regional influences might be impossible.
What does eutrophic mean?
The term eutrophic refers to lakes that are nutrient rich. Eutrophic lakes are characterized by high productivity and a high depth of organic sediment, with food for bacteria and an anoxic lower layer.
What does mesotrophic mean?
The term mesotrophic refers to lakes with intermediate levels of nutrients. Production of plankton is intermediate in mesotrophic lakes, with some organic sediment accumulating and some loss of oxygen in the lower waters (decomposers decompose some organic matter when the waters are warm [above?]). Mesotrophic lakes are devoid of oxygen in late summer.
What does oligotrophic mean?
The term oligotrophic refers to lakes that are nutrient poor. They tend to be clear, deep, and free of weeds or large algae blooms. Oligotrophic lakes can have some large game fish, but their populations are small. Little to no organic matter is present at the bottom of oligotrophic lakes, and no food or bacteria is present. There is no consumption of oxygen, and they are often characterized by a sandy, rocky bottom (perhaps due to a rocky watershed?).
What is the wilting point?
The wilting point is the point at which the surface tension of film water in a drying-out soil becomes so great that plants cannot suck up the water and begin to wilt.
How does time affect soil development?
Time affects soil development primarily through weathering. All soils increase in FeO content through time, but the rate at which this increase occurs depends on climate. Carbonates and base ions tend to decrease through time in most soils, while biomass, humus, and clays increase through time until the soil becomes quite old.
How does topography affect soil development?
Topography affects soil development in multiple ways. The steepness of the slope affects whether erosion or deposition occurs and thus is a major factor in soil thickness. Slope position is also a factor; the nearness to the water table affects the degree of development aeration and color. Slope aspect (direction) is crucial, as different amounts of solar insolation may be felt on slopes facing different directions, fueling soil moisture and productivity differences. Side note: variation in a soil across a landscape with everything but topography held constant is called a catena.
What are the distinguishing characteristics of ultisols?
Ultisols (suffix: -ult) are mature, late-stage mixed forest soils that are characterized by low base saturation. They MUST have a clay (Bt) horizon and also often have an E horizon. Ultisols occur in the Southeastern U.S>, where soils are old enough that they have developed.
What is upwelling?
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased nutrient availability in upwelling regions results in high levels of primary productivity and thus fishery production.
What are varves?
Varves are regular, alternating layers of sediments generally formed in glacial lakes. Coarse grained layers in glacial varves are associated with the coarse sediments brought in by meltwater streams in the summer, while the fine grained layers are associated with the settling out of fine grained material in the winter when the lake freezes over. The thickness of the summer layer is proportional to the amount of meltwater (and thus indirectly summer temperature). Although they are commonly associated with glacial lakes, changes in evaporite mineralogy in lakes in dry environments may record changes in brine composition in the lake from the wet season to the dry season. Carbonate evaporites often form in the wet season, while anhydrite evaporites generally form in the dry season. The thickness of each layer in a varve may give an indication of the length of the wet season versus the dry season, and counting and looking at the cyclicity in thickness variations can help one identify possible climate forcers (ex: Milankovic cycles) from cycle ratios even if the age range of the layers is not known.
Why do varves form? What is the nature of the environmental record they contain?
Varves form from differences in the characteristics of deposited sediments over a given time cycle. They often record amount of meltwater (and thus summer temperature) for glacial varves or the lengths of the wet and dry seasons (in more arid varves).
What are the distinguishing characteristics of vertisols?
Vertisols (suffix: -ert) are soils that have developed with a high fraction of smectite clays. They have high CECs and are excellent at soaking up water, allowing the soil to expand and contract with water content. Deep cracks develop in vertisols due to these wetting and drying cycles, allowing mixing to occur and keeping distinctive soil horizons from forming. Vertisols commonly form in humid places.
What specific climatological issue(s) are we generally trying to understand using corals? Why/how are corals well suited for each of these things?
We are generally trying to understand SST changes associated with El Nino events (well suited because cadmium incorporation into coral calcium carbonate tracks nutrient availability well, because they often exhibit aragonite bands with annual seasonal resolution, because they are located in tropical [Pacific and Indian] ocean regions dramatically affected by El Nino, and because they preserve very well), esp. through SST. They can also be used to reconstruct the oxygen isotopic composition of the ocean water. In dealing with all things corals, we must make sure that we understand "vital effects," that we understand what d18O is or isn't telling us in a given coral, that we are careful in crafting and applying transfer functions (should we use intraannual variability to calibrate transfer functions we then use to reconstruct decadal/century scale variation), and that we consider diagenesis (usually we can assert a coral as primary if it is still aragonite, as aragonite is not stable and any secondary alteration would change it to calcite).
How do we reconstruct sequences of aggradation and degradation in fluvial systems?
We often reconstruct sequences of aggradation and degradation in fluvial systems from floodplain terraces. Fill terraces are the result of fundamentally strong aggradation in which a valley fills with alluvium over time and are created when aggradtion stops and the stream begins downcutting through the alluvium. Strath terraces result when a stream cuts through bedrock instead of alluvium and are generally characterized by flat terraces with thin layers of alluvium. Most strath terraces are uplift controlled. The sediments themselves on terraces record aggradation, while the terrace faces record degradation.
How do we use the growth rates of corals to reconstruct climate? What are the alternative factors you have to consider for interpretation?
We use the growth rates of corals to reconstruct climate based on whether the conditions at a location were ideal for coral growth during a given time period. We generally consider conditions to have been more ideal during time periods associated with thicker and/or denser aragonite bands.
What are we measuring with the MBT and CBT proxies? What are the issues with the techniques currently under investigation?
With the methylation of branched tetraethers (MBT) and cyclization of branched tetraethers (CBT) proxies, we are measuring past soil temperature and pH conditions. CBT is a function of exclusively soil pH, with greater cyclization abundance occurring at lower pH values; MBT, on the other hand, is a function of both temperature and pH, with greater methylation abundances occurring at higher temperatures and lower pHs. While CBT can be used directly to infer soil pH, using CBT and MBT together allows one to also infer temperature by factoring out pH-related change in MBT using CBT (which depends exclusively on pH). Issues with these techniques include the fact that we don't know the exact organisms or bacteria that are making these branched glycerol dialkyl glycerol tetraethers (GDGT), the modern temperature/pH relationships may not hold through time depending on genetics, and the growth rate of the organisms that produce GDGT may affect the nature and amount of their production over time. We also know little about possible diagenetic effects on GDGT and what the best conditions are for their preservation. Local or regional calibrations are best; we get the right trend using global calibration, but we need local calibrations to be more quantitatively accurate. Also, there has been evidence of in situ GDGT production in lakes with T reconstructions different than soil T; this is BAD!
Why are zooxanthellae important for corals? What can harm zooxanthellae (and consequences thereof)?
Zooxanthellae are a form of algae that often gives living corals a brown color. They are important to corals because zooxanthellae and corals live in a symbiotic relationship: the algae are able to get some of the nutrients digested by corals that are necessary for their growth, while the algae in turn help the corals build their carbonate substructures. A decrease in the density of zooxanthellae symbionts or the concentration of photosynthetic pigments within the zooxanthellae due to stress on the corals or the algae may cause coral bleaching, the loss of these symbionts from the tissues of the coral. Corals can recover from these bleaching events if they are brief, but they may die if unfavorable conditions persist. Zooxanthellae are harmed by high temperature and irradiance stressors, which can disrupt their enzyme systems that protect against oxygen toxicity, impair their photosynthetic pathways, and cause the detachment of cnidarian endodermal cells with their zooxanthellae. Consequences = bleaching. Other things that can harm zooxanthellae and corals include sedimentation (a little adds nutrients, but too much can cut down on sunlight penetration into the photic zone and prevent photosynthesis), cold temperatures, freshwater dilution (esp. due to flood of riverine input), pollution (causing infestation of bacteria), and too warm temperatures (reef building corals tend to live near their upper thermal tolerance limits). However, corals may be recolonized after bleaching with a different algael symbiont that is bettwer at living under the conditions that originally caused the bleaching.