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Moraines to reconstruct paleoclimate and limitations
Moraines indicate glacier advances and involve a glacier period in equilibrium with climate (i. e. when mass balance is equal 0) and then, a period with positives mass balances. Benn and Evans, (2010); Lukas et al, (2012) proposed that the moraine size can be related to: a) glacier equilibrium duration with climate, b) glacier front speed (which controls the deposition rate) and c) capacity of the glacier to erode rock material (glacier size vs lithology).
Paleoclimate reconstructions are commonly based on former glaciers using lateral and frontal moraines analyses. This approach relies on the assumption that the distribution of moraines in the modern landscape is an accurate reflection of former ice margin positions during climatically controlled periods of glacier margin stability (Barr and Lovell, 2014). However, the validity of this assumption is debated because a number of additional, no climatic factors are known to influence the moraine distribution. For example topography, could be an important factor (Barr and Lovell, 2014).
Topographic control on the moraines distribution in the landscape
Topography controls the location and timing of moraines formation, glacier extension and dynamics, as well as the margin stability (Barr and Lovell, 2014). Glaciers are developed in the topography above the regional climatic-ELA (i.e., in the accumulation zone upon which snow and ice can accumulate and persist interannually) (Kessler et al., 2006; Kaplan et al., 2009). Mountains topography can vary significantly, from flat plateaus to high relief peaks which directly regulate the size and shape of glaciers (Manley, 1955; Ives et al., 1975; Sugden and John, 1976; Golledge, 2007). Mountains topography thereby determines morphology and the moraines distribution. For example, plateaus located above the ELA provide large areas for snow and ice accumulation, whereas the steepest slopes have little capacity to produce snow accumulation. Thus, flat topography allows more extensive glaciers and moraines development, whereas non plateau topography will restrict glaciation to smaller and not abundant ice masses and moraines (Sugden and John, 1976) (Figure II-3). Despite same climatic conditions prevail in both cases.
Moreover steep slopes in the accumulation areas will influence the debris provision for moraine formation. Steep slope walls are more instable and debris collapses are higher, so available material is higher for the moraine formation (Figure II-3) (Kessler et al., 2006; Kaplan et al., 2009).
Geochronological methods used to study Quaternary glaciations
Quaternary glaciations studies are based on geomorphological glacial analysis and dating. Also are based on pro-glacial sediments sequence (lacustrine or continental) descriptions and timing. Glacial landforms commonly dated are moraines, polished surfaces and roches moutonnées (erosional glacier landforms).
Fig. II-3. Illustration of how, under uniform climatic conditions (reflected by a uniform ELA), (A) mountain height and (B) plateau and nonplateau topography can lead to variations in glacier dimensions and thereby control moraine location. Blue zones represent glacier accumulation areas (Ac) (From Barr and Lovell, 2014).
Various dating techniques are commonly used (lichenometry, dendrochronology, radiocarbon dating and Terrestrial Cosmogenic Nuclide (TCN) dating) (Figure II-4). The selection of geochronological techniques depends of the available material to date and the time window of interest.
Lichenometry is a surface-exposure dating method that uses lichen-growth rates to infer the age of recent glacial landforms (Briner, 2011). The method is particularly useful in regions above and beyond the tree-line and especially in Arctic-Alpine environments because erosion of glacial landforms is low (Armstrong, 2004). However, in high elevation areas of the tropical Andes, this method has been successfully used (e.g. Rabatel et al., 2005; Jomelli et al., 2009). This is because the external conditions are equivalent to those of artic areas (low erosion). This method is restricted to date only Neoglacial deposits (within the last 500 years) and require knowing the lichen ecology (i.e. thallus growth rate) in the studied area (Armstrong, 2004). Dendrochronology technique dates current or sub fossils logs. Moraines can be dated until 11 ka (Briner, 2011). Dendrochronology was
mainly used in boreal and temperate regions because the annual growth of the trees rings is completely understood. However, during the last decade this technique has been also used in the tropical regions (e.g. Wils, et al., 2010). Lichenometry and dendrochronology are dating techniques suitable for Holocene glacier dynamics.
Figure II-4. Common methods used to date glacial landforms. Targets for radiocarbon and dendrochronology dating are labeled: LT living tree, L log, SS sheared stump, ROM reworked organic material, D/O OM deformed/overridden organic material. Targets for cosmogenic exposure dating (boulders preferably located on the moraine crest) (Modified from Briner, 2011).
Radiocarbon dating involves 14C determination in dead organisms. Assuming that the organism death and landform deposition was contemporaneous, age of the deposit could be estimated (Libby, 1955; Taylor and Lloyd, 1992). Deposit ages can be estimated for the last 45.00 kyr (Siame et al., 2000). Studies have applied radiocarbon dating to sediments below, within, and above moraines to provide maximum (below and within) and minimum (above) age constraints (Brinier, 2011). The radiocarbon dating provides only bracketing ages (i.e. an age interval) of the glaciers advances (Balco, 2011). In high elevation area, organic remains are scarce leading to low preservation of available materials to be dated (Balco, 2011).
Radiocarbon dating is used to date sedimentary material as lake sediments or peatbog samples, directly related to glacial landforms (i.e. proglacial lake sediments developed upstream moraines ridges which provide minimum-limiting ages for down valley moraines) (Robdell et al., 2009). Furthermore, organic matter in outwash material (sand and gravel) down valley from moraines allow the calculation of maximum-limiting ages (e.g., Gonzalez et al., 1965; Mercer and Palacios, 1977; Helmens, 1988). This requires the assumption of evident relationship between dated outwash materials and the nearby moraine ridge. When this assumption is not clear, radiocarbon ages from organic matter in these stratigraphic settings are ambiguous (Robdell et al., 2009).
Terrestrial Cosmogenic Nuclide (TCN) dating (the methodological aspect will be discuss in detail in Chapter IV- Methods and Materials) is based on the quantification of isotopes produced by the interaction of cosmic rays with Earth chemical targets (Gosse and Phillips, 2001; Dunai, 2010). Geological surfaces exposure time to the cosmic rays are determined using the TCN dating. Glacial landforms exposure times can be interpreted as a chronological markers for the former glacier activity reconstruction. It is easily related to the deglaciation age of a glacial landform and overcomes the absence of organic material trapped necessary for radiocarbon dating.
This thesis is based on TCN dating because this method coupled to geomorphological investigations provides more accurate ages for glaciations reconstructions. Specifically 10Be in-situ nuclide dating was used because of the regional lithology (granites and gneisses). These rocks have high quartz content; 10Be is the most suitable nuclide (see more in Chapter IV- Methods and Materials). Uncertainties in the 10Be in-situ nuclide dating are lower than 15% (Dunai, 2010). Contrary to radiocarbon dating, the TCN dating allows dating exposure ages of glacial landforms from the Late, Middle and Early Pleistocene (e.g. Smith et al., 2005b; Heyman, 2014).
Table of contents :
PART I. SECTION I. GLACIATIONS, TROPIC CLIMATE, TROPICAL GLACIERS AND ANDEAN GLACIATIONS
I-1.0 QUATERNARY GLACIATIONS BACKGROUND
I-1.1 GENERALITIES AND CAUSES
I-1.2 GLACIALS AND INTERGLACIALS CLASSIFICATIONS AND LIMITATIONS
I-1.3 LAST GLACIATION AND CLIMATE EVENTS INVOLVED
I-1.3.1 Last Glaciation
I-1.3.2 Dansgaard-Oeschger (D-O) and Heinrich events
I-1.3.3 Last Glacial Maximum
I-1.3.4 Late Glacial (LG)
I-2.0 TROPICAL ANDES, CLIMATE AND GLACIERS BEHAVIORS
I-2.1 CLIMATE OF THE TROPICAL ANDES, OVERVIEW
I-2.2 TROPICAL ANDEAN GLACIERS
I-2.2.1 Mass balance
I-2.2.2 Equilibrium Line Altitudes (ELA)
I-3.0 TROPICAL ANDEAN GLACIATIONS
I-3.1 PREVIOUS MIS 5 AND MIS 5 GLACIERS ADVANCES (SMITH ET AL., 2008)
I-3.2 MIS 3, MIS 2 AND MIS 1 GLACIERS ADVANCES (CORONATO AND RABASSA, 2007; SMITH ET AL., 2008)
SECTION II. A BACKGROUND GLACIATIONS RECONSTRUCTION METHODS
II-1.0 METHODS FOR GLACIATIONS RECONSTRUCTION STUDIES
II-1.1 PALEOGLACIOLOGY
II-1.1.1 Moraines, an important glacial feature in paleoglaciology studies
II-1.1.1.1 Moraines to reconstruct paleoclimate and limitations
II.1.1.1.2 Topographic control on the moraines distribution in the landscape
II-1.2 GEOCHRONOLOGICAL METHODS USED TO STUDY QUATERNARY GLACIATIONS
SECTION III. GENERAL GEOLOGICAL, CLIMATE AND PALEOCLIMATE
III-1.0 GENERAL GEOLOGICAL SETTINGS
III-2.0 GLACIATIONS RECONSTRUCTIONS IN THE CENTRAL MÉRIDA ANDES
III-3.0 PALEOCLIMATE SETTINGS
III-4.0 PRESENT-DAY CLIMATE IN THE VENEZUELAN ANDES
SECTION IV. METHODOLOGY
IV-1.0 GEOCHRONOLOGY METHOD BASED ON TERRESTRIAL COSMOGENICS NUCLIDES (TCN)-10BE NUCLIDE DATING
IV-1.1 INTERACTION BETWEEN COSMIC RAYS AND MATTER
IV-1.2 TERRESTRIAL COSMOGENICS NUCLIDES (TCN)-10BE NUCLIDE DATING
IV-1.3 SLHL TCN PRODUCTION RATE SCALING (LOCAL PARAMETERS CONTROLLING THE
10BE PRODUCTION RATE)
IV-1.3.1 Magnetic field and latitudinal dependence
IV-1.3.2 Altitudinal dependence
IV-1.3.3 Topographic dependence
IV-1.3.4 Depth production
IV-1.3.5 Temporal evolution of TCN concentration
IV-2.0 STUDY AREA LOCATION AND SAMPLES COLLECTION
IV-3.0 SAMPLES PREPARATION
IV-3.1 QUARTZ SEPARATION FROM BULK MATERIAL
IV-3.2 CHEMICAL 10BE EXTRACTION IN THE BEO FORM
IV-4.0 10BE CONCENTRATION DETERMINATION USING ACCELERATOR MASS SPECTROMETRY (AMS)
IV-5.0 10BE PRODUCTION RATES, EROSION VALUE, SCALING SCHEME AND EXPOSURE
AGES CALCULATION
IV-6.0 GEOMORPHOLOGICAL ANALYSIS AND FORMER GLACIERS RECONSTRUCTIONS
IV-7.0 PALEO ELA RECONSTRUCTION
IV-7.1 ACCUMULATION-AREA RATIO (AAR)
IV-7.2 AREA–ALTITUDE BALANCE RATIOS (AABR)
IV-7.3 PALEO ELA CORRECTIONS
PART II. SECTION V. RESULTS
V-1.0 GENERALITIES ABOUT STUDIED GLACIAL LANDFORMS, 10BE CONCENTRATIONS,
10BE PRODUCTION RATES AND EXPOSURE AGES
V-1.1 WHY GLACIAL LANDFORMS STUDIED?
V-1.2 10BE CONCENTRATIONS
V-1.3 10BE PRODUCTION RATES USED AND INFLUENCES IN THE EXPOSURE AGES 109
V-2.0 DETAILED GLACIAL GEOMORPHOLOGICAL FEATURES AND DEGLACIATION CHRONOLOGIES
V-2.1 SIERRA NEVADA
V-2.1.1 Mucubají and Los Zerpa
V-2.1.2 Gavidia valley
V-2.1.3 Mucuchache valley, El Caballo and Las Tapias moraines
V-2.1.3.1 Geomorphological descriptions and previous studies
V-2.1.3.2 Exposure ages and outliers
V-2.2 SIERRA DEL NORTE:
V-2.2.1 La Culata moraines/Mucujún valley
V-2.2.1.1 Geomorphological descriptions and previous studies
V-2.2.1.2 Exposure ages and outliers
V-2.2.2 Mifafí valley and El Desecho moraine
V-2.2.2.1 Geomorphological descriptions
V-2.2.2.2 Exposure ages and outliers
V-2.3 CORDILLERA DE TRUJILLO (PUEBLO LLANO VALLEY/LA CANOA)
V-2.3.1 Previous studies
V-2.3.2 Glacial geomorphological features
V-2.3.2 Exposure ages and outliers
V.3.0 PALEO ELA VALUES
V- 3.1 PREVIOUS STUDIES
V- 3.2 RESULTS
PART III. SECTION VI. DISCUSSIONS
VI-1 DEGLACIATION HISTORIES IN THE CENTRAL MÉRIDA ANDES AND PRINCIPAL GEOMORPHIC PARAMETERS DRIVING THE FORMER GLACIERS DYNAMICS
VI-1.1 DEGLACIATION HISTORIES IN THE CENTRAL MÉRIDA ANDES
VI-1.1.1 Mucubají valley (Sierra Nevada)
VI-1.1.2 Mucuchahe valley (Sierra Nevada)
VI-1.1.3 Gavidia valley (Sierra Nevada)
VI-1.1.4 Mifafí valley (Sierra del Norte)
VI-1.2 NON CLIMATIC PARAMETERS DRIVING DIFFERENT DYNAMICS OF THE FORMER
GLACIERS
VI-2 DEGLACIATION CHRONOLOGIES AND GLACIATION RECONSTRUCTIONS
IMPLICATIONS IN THE CENTRAL MA
VI-3.0 PALEOGLACIOLOGY CONTRIBUTIONS TO THE PALEOCLIMATE RECORD IN THE
CENTRAL MA
VI-3.1 REGIONAL AND GLOBAL CLIMATE FORCING DRIVING CENTRAL MÉRIDA ANDES
GLACIER VARIABILITIES
