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Magnetism of a speleothem from Midwest Brazil and paleoclimatic implications

Jaqueto, P. F. Magnetism of a speleothem from Midwest Brazil and paleoclimatic implications. 2016. 84 pp. Thesis (Master’s degree) – Departamento de Geofísica, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, 2016. This thesis provides a detailed study of environmental magnetism in a speleothem. It focuses on a stalagmite from Pau D’Alho cave (15°12’20”S, 56°48’41”W) located in Rosário d’Oeste, Mato Grosso State, Brazil. This speleothem grew during the past 1355 years, with average growth rate of ~168 mm/ka and encompasses two key events in the South American Summer Monsoon (SASM), the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA), major dry and wet, respectively. The rock magnetic experiments conducted, include isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM) acquisition curves, thermal demagnetization of three-axis IRM acquisition, hysteresis loops, first order reversal curves (FORC) and low-temperature SQUID magnetometry experiments. The main magnetic remanence carriers in the stalagmite are magnetite and goethite, with a nearly constant relative proportion. Magnetite has remanent coercivities between 14-17 mT, and its magnetic properties are similar to those produced by pedogenic processes. Magnetic remanence is broadly correlated with carbon and oxygen isotope data throughout most of the speleothem, suggesting that precipitation and soil dynamics above the cave exert a strong control on the input of magnetic minerals into the Pau d’Alho cave system. Dry periods like the MCA are associated with less stable soils that result in higher mineral fluxes carried into karst systems via groundwater, while conversely, colder and wetter periods like the LIA are associated with soils topped by denser vegetation that are more capable of retaining micrometer-scale pedogenic minerals, and thus reduce mineral fluxes into karst environments.


Chapter 1: Introduction
The word 'speleothem' has its origin from the Greek spelaion (cave) and thema (deposit).
These geological materials are secondary deposits formed by chemical precipitation in karstic environments (Gornitz, 2009), being usually composed of calcite and/or aragonite (CaCO 3 ). Essentially, calcium carbonate is dissolved in the soil and in the upper epikarst, and is precipitated in the underlying vadose region. As the water descends through the karst, it ultimately encounters a gas phase with a lower pCO 2 . This causes degassing of CO 2 from the solution and precipitation of CaCO 3 forming structures that serve as continuous archives for paleoclimatic studies. Over the last decades, speleothems have become an important record for studies of climate change, due to their long and continuous nature and their potential to preserve orbital, millennial, decadal, annual, or even sub-annual processes (Wong & Breecker, 2015). In recent years, the rapid advance in analytical techniques improved the sensibility and accuracy of equipments that enable us to make multiple measurements in one single record, allowing the study of diverse environmental indicators in a single speleothem (Fairchild & Baker, 2012).
The magnetic study of speleothems started with Latham (1979). The focus of the first studies was to obtain the directional data of the Earth's magnetic field. The challenges at the time involved two main factors: (1) obtain a good dating model, using techniques such as 14 C and 230 Th (Alfred G Latham, Schwarcz, Ford, & Pearce, 1979;Martin, 1990)that at the time presented large uncertainties, (2) detection of the magnetic signal due to the low concentration of magnetic minerals in these geological samples (Lascu & Feinberg, 2011).
The size of samples for 230 Th dating changed from few tens of grams in the 1980s to milligrams in recent years, achieving good analytical precision to as low as 1% (Fairchild & Baker, 2012) providing nowadays a good age control for the continuous age models necessary for paleoclimatic studies (Cheng et al., 2013). At the same time, the evolution of Superconducting Rock Magnetometers (SRM) such as the 2G enterprises system allowed sensitivity in magnetic moment detection in the order of magnitude of 10 -12 Am². The new generation of u-channel magnetometers allows the measurement of continuous sections and specimens with spatial resolution of 5 mm (Jackson, Bowles, Lascu, & Solheid, 2010).
Combining the major advances in 230 Th dating, magnetometer technology and the environmental magnetism research, speleothem magnetism has experienced a revival in the past years (e.g., Lascu & Feinberg, 2011;Font et al., 2014;Bourne et al., 2015). Recent studies in the field are not limited to the study of the Earth's magnetic field, but use the magnetic speleothem record as a tool for high resolution reconstruction of regional climate, erosion or pedogenic histories.
In this present dissertation, the magnetic study of a stalagmite from the cave Pau D'Alho (15°12'20"S, 56°48'41"W) located in Rosário d'Oeste, Mato Grosso State, Brazil, is presented. The main focus is the origin of the magnetic minerals and their relation with climate and vegetation in a karst system. The stalagmite was previously studied by Novello et al. (2016), who presented an age model based on 230 Th dating and a detailed record of δ 18 O and δ 13 C. This stalagmite covers two distinct climate periods in Midwest Brazil, a drier period (Medieval Climatic Anomaly) that occurred between ~900 years CE 1 to ~1100 years CE and a wet period (Little Ice Age) that occurred between ~1600 years CE to ~1820 years CE (Novello et al., 2016). The objective of the magnetic study is to understand the magnetic mineral signal present in the stalagmite, and the meaning of this signal. Magnetic experiments used a sensitive rock magnetometer (Superconducting Rock Magnetometer), thermomagnetic curves, FORC diagrams, and scanning electron microscope images.
The dissertation is structured in six chapters including this Introduction (Chapter 1).
Chapter 2 discusses the processes involved in stalagmite formation and the meaning of its δ 18 O and δ 13 C isotopic composition, in particular the influence of climate, soil and vegetation composition on the isotope signal. Speleothem magnetism is then reviewed in order to establish the major advances in the field since the first studies. Environmental magnetism with a particular focus on soil magnetism is also discussed in this chapter.
Chapter 3 concerns the methods and presents details of sample preparation and magnetic techniques used in the present work. Chapter 4 presents the results obtained and correlations between them. Chapter 5 summarizes key results and presents a discussion on the origin of magnetic minerals found in the Pau d'Alho stalagmite, variations in their 1 Common Era is abbreviated as C.E, and it refers as the same as Anno Domini (AD).
concentration throughout the speleothem and their interpretation in face of the oxygen and carbon isotopic records. In Chapter 6 the conclusions are presented and a framework for future studies to assess the magnetic minerals of stalagmites is presented.
Chapter 2: Isotope records and magnetism of speleothems and soils 2.1 The karst system Klimchouk & Ford, (2000) offer the following definition for the karst system, "It is an integrated mass-transfer system in soluble rocks with permeability structure dominated by conduits dissolved from the rock and organized to facilitate the circulation of fluid". The dynamics of the karst involves surface environments such as climate, soil and vegetation.
So, speleothems are part of a long history of cave space building (Fairchild & Baker, 2012).
In carbonate rocks, the rainfall is acidified with carbon dioxide (CO 2 ) present in the atmosphere and undergoes enrichment in carbonic acid when passes through the soil, as a result of plant respiration and organic matter dissolution along the upper epikarst (high fissured) zone. The underlying vadose region (water table) is the location where the precipitation of CaCO 3 occurs (Figure 2.1). The precipitation region corresponds to the carbonate supersaturation zone. Speleothem precipitation is induced by degassing of CO 2 from descending waters into cave's fissure porosity. The climatic component determines the input of water in the karst system. Climate also influences strongly vegetation (chemistry of the carbonate system) and soil properties (facilitating speleogenesis) . A stalagmite (CaCO 3 ) precipitates from water moving gravitationally and it originated from drip water; this record through time is an excellent archive for the changes that occurred in the hydrological dynamics above and inside the cave. Historically, much of the research on speleothems dealt with the disequilibrium in uranium series isotopes to determine the age of the material, using the age models to constrain the oxygen isotope time series is an attempt to elucidate paleotemperature variations (Fairchild & Baker, 2012). The use of oxygen isotopes as a paleoclimate proxy has begun in the 1960's. It represents the main continental record, together with lake data, that complements the record obtained in marine sediment cores.
Cave environments are stable and their isotopic record reflects the regional mean temperature (Lachniet, 2009). Rapid advances in analytical techniques in the last decade opened the possibility that a single speleothem can provide a combined dataset with time series of δ 18 O, δ 13 C, trace elements, speleothem growth-rates and organic matter fluorescence properties .  (Ruddiman, 2001).

Oxygen isotopes in speleothems
The study of oxygen isotopes (δ 18 O) time series in Brazil started in the southern region, which is a well-situated location for investigating the changes in tropical and subtropical atmospheric circulation (Cruz et al., 2005). Stalagmite isotope record in combination with 230 Th age model of the Botuverá cave provided a record of the intensity variation of the South America Summer Monsoon and enabled its correlation with the insolation controlled by orbital forcing (Figure 2.

Carbon isotopes in speleothems
Two nonradioactive isotopes of carbon occur in nature, the 12 C that accounts for more than 98.89%, and the 13 C that comprises the remaining 1.11%. Individual measurements of the 13 C/ 12 C ratio in natural materials are reported as departures in parts per mil (‰) from the standard: where samples with positive δ 13 C values are said to be enriched in 13 C (compared to 12 C) and samples with negative δ 13 C values are said to be 13 C depleted. Carbon isotope ratios can be measured in a wide range of materials such as speleothems, bones, leaves, shells, carbonates, soils, sediments, peats and wood, the isotopic variation being linked to climate effects on the global carbon cycle and photosynthetic pathways of plants ( Figure 2.3) (Gornitz, 2009). All trees as well as most shrubs use a type of photosynthesis called the C 3 pathway, which produces organic tissues with more negative δ 13 C values when compared to some shrubs and most grasses that use a different kind of photosynthesis called C 4 pathway. The carbon isotope signal recorded by a speleothem is controlled by processes occurring mainly in the soil/epikarst and inside the cave (Fairchild & Baker, 2012;Wong and Breecker, 2015), but the interpretation of δ 13 C signal is variable and often region/case specific, including: the δ 13 C value of atmospheric carbon dioxide, the proportions of C 3 /C 4 vegetation, the soil/vegetation productivity controlled by precipitation and temperature, carbon isotope fractionation during decomposition of soil organic matter, the proportion of host rock carbon, and in-cave precipitation (e.g. ventilation, prior calcite precipitation) (Boch, Spötl, & Frisia, 2011;Breitenbach et al., 2015;Meyer, Feng, Breecker, Banner, & Guilfoyle, 2014;Spötl, Fairchild, & Tooth, 2005). In soils, water availability can also control the magnitude of photosynthetic discrimination, where drier and/or hotter climate (higher evaporation and lower soil water content) results in less negative organic carbon values (Wong & Breecker, 2015). The control of rapid biomass shifts in the soil over the cave on the carbon isotope signal of stalagmites can be exemplified by the case study conducted by Baldini et al. 2005). These authors reported an increase in calcite deposition rates in the karst system coeval with a decrease in δ 13 C and δ 18 O in a welldocumented re-vegetation area of an abandoned mine. According to those authors, this signal reveals that the δ 13 C data is able to record vegetation shifts, and suggests that the δ 13 C variation may partly result from changes in the amount of biomass as well as to the shifts in the distribution of C 3 /C 4 plant species.
In Brazil, Cruz et al. (2005) presented a 116 ka carbon stable isotope record from the Botuverá stalagmite, with the aim of investigating the effect of changes in large-scale atmospheric circulation on the temperature-driven biological activity and soil CO 2 productivity during the last glaciation. It was found that the most important determinant of carbon isotope variation is the change in the amount of CO 2 input to the soil waters. The main controls on this parameter are the processes that occur above the cave, such as the rate of organic matter decomposition and the type of vegetation cover. Greater soil CO 2 production results in a greater fraction of the isotopically-depleted soil source being incorporated into the stalagmite. Conversely, when soil CO 2 production is smaller the relatively enriched limestone source of the epikarst is privileged. The fact that more negative δ 13 C intervals in the Botuverá stalagmite were also those with higher growth rates  Changes in  13 C in soils of tropical Brazil are usually driven by changes in the relative contributions of C 3 and C 4 plant communities to the carbon isotope composition of soil organic matter (SOM) (Pessenda, Gouveia, Ribeiro, De Oliveira, & Aravena, 2010). During wetter intervals, as denser forests (C 3 plants) replace savanna grasslands (C 4 plants), soils display a decrease in the  13 C of their SOM. The increase in soil biomass during forest expansion is also likely to generate a higher concentration of isotopically lighter biogenic carbon, regardless of the relative ratio of C 3 and C 4 plants (Baldini et al., 2005).

Speleothem magnetism
The first studies of speleothem magnetism focused on the paleosecular variation (PSV) of the Earth's Magnetic Field (EMF) (Latham et al. 1979;Latham 1981;Latham et al. 1989;Morinaga et al. 1986;Morinaga 1989;Morinaga et al. 1992;Martin 1990;Openshaw et al., 1997), because these geological objects provide continuous and instantaneous records of the EMF direction, being thus an alternative to other magnetic archives such as lake sediments and volcanic rocks.
The research that first demonstrated speleothem magnetism as a potential recorder of the EMF was made by Latham et al. (1979). The authors used stalagmites and flowstones from   Morinaga et al. (1986Morinaga et al. ( , 1989Morinaga et al. ( , and 1992) studied stalagmites from western Japan. The magnetic carrier determined was again magnetite, through studies of Isothermal Remanent Magnetization (IRM) curves and thermomagnetic curves. Although Morinaga and his coauthors did not have access to a dating method (Morinaga et al., 1986(Morinaga et al., , 1989, it was possible to compare the speleothem data with available archaeomagnetic curves for the last 2000 years in Southwest Japan. This is the first study to relate the dominant magnetic mineral found in the stalagmites to that observed on the overlying soils and sediments, in their case the soils and sediments exposed in the Akioshi Plateau (Morinaga et al., 1986).
The authors suggested that the magnetic mineral (magnetite) has been deposited after being carried either as a detrital grain or in solution within the dripping water into the cave, the magnetization being acquired at the time of crystallization of the stalagmite.
Finally, in the 1990's, some studies of speleothem magnetism were consecrated to the origin of speleothem remanence (e.g. Perkins & Maher, 1993;Perkins, 1996;Openshaw et al., 1997). The potential of speleothems as recorders of PSV has been successfully demonstrated before, but the origins for the speleothem magnetism were not well documented. Perkins & Maher (1993) listed the advantages of using speleothems as a recorder of the EMF : (1) the remanence acquisition timethe remanence of the top of active speleothems were close to the EMF at the cave location indicating that acquisition time-lag is negligible; (2) sedimentary problemsspeleothems are not subject to compaction or slumping, and dissolution or gaps in the record can be easily identified in the laminae; (3) the low intensity of their natural remanent magnetization is a problem, but a careful choice of samples can solve it; (4) diagenetic problemsthe only diagenetic problem is the occurrence of recrystallization, that is easily detectable in thin section by crystallographic changes; (5) biological problemsbiogenically derived magnetite could overprint a sedimentary remanence, but no organisms are known to burrow into subaerial speleothems; (6) sampling problemsno disturbances occur with collecting speleothems because they are solid rocks, and the sample can be oriented in-situ; (7) dating problemsfluid movements caused by sedimentary processes and bioturbation in sediments can lead to incorrect radiocarbon dates, whereas in speleothems these processes do not occur, their dating being made by 230 Th techniques (eventual contamination with detrital thorium can be corrected). This work also demonstrates that two stalagmites from the same cave reproduce the magnetic record of contemporaneous deposits of lake sediments (Figure 2.6). Extraction of magnetic minerals was carried by Perkins (1996) for electron microscopy and energy dispersive X-ray analyses. Three major groups were identified: (1) detrital grains composed of magnetite, hematite and titanomagnetite; (2) unabraded grains composed of magnetite, with two possible origins (bacterial magnetite from the overlying soil or inorganic authigenic magnetite precipitated in the cave); (3) needle-like grains, probably composed of goethite. The last research that appeared in 1990's was made by Openshaw et al. (1997), where five stalagmites from China were used to investigate the paleosecular variation. The comparison between the speleothem record and the modern field suggested a westward drift of non-dipole sources during the last 2.8 kyrs. This result confirms that speleothem magnetism is a valuable alternative tool for the study of paleosecular variation.  The origin of magnetic remanence and its relation to hydrologic and climatic variation is the topic of the majority of studies after 2010 (Bourne et al., 2015;Font et al., 2014;Strauss et al., 2013;Strehlau et al., 2014;Zhu et al., 2012). Determination of grain size, shapes and elemental chemistry of magnetic carriers in stalagmites were reported by Strauss et al. (2013). In all samples, magnetite and goethite were found in varying abundances and morphologies. Magnetite grains displayed features characteristic of transport (evidence for detrital grains), including titanomagnetite grains and exsolved iron-titanium oxides identified through microscope analysis. Goethite grains were found in three different morphologiesneedle aggregates, solitary needles and polycrystalline aggregates. The main conclusion was that only magnetite and titanomagnetite can hold depositional remanence, and that goethite may be precipitated in situ on the stalagmite's drip surface.
More recently, Bourne et al. (2015) identified a connection between large-scale climatic changes (precipitation) and the more localized processes of magnetic mineral cycling in soils overlying the cave systems. These authors found that changes in magnetite concentration follow changes in stable isotopes and other global climate proxies. The magnetic concentration time series was interpreted as a variation in local pedogenic processes, controlled by regional precipitation. The main magnetic assemblage in the studied caves is magnetite, produced by pedogenic processes, then transferred via dripwaters into the underlying cave-systems and incorporated into stalagmite. A similar conclusion was achieved by Zhu et al. (2012) using the remanence record of a Chinese stalagmite.

Soil magnetism
Natural rocks and soils comprise a complex assemblage of magnetic minerals with different compositions and grain sizes that result from iron oxide formation, dissolution or transformation. Almost all rocks contain some iron. During weathering, iron is released from rock-forming minerals and "secondary" pedogenic iron minerals are formed (Cornell & Schwertmann, 2003). The assemblages of magnetic minerals formed in well-drained soils are iron oxides, most frequently magnetite, maghemite, hematite, and goethite (Maxbauer, Feinberg, & Fox, 2016). The pedosphere is an environment of active mineral formation and transformation and exhibits a large variation in formation parameters in space and time over a range of scales. It openly and permanently communicates with neighboring compartments of the ecosystem (atmosphere, biosphere, hydrosphere, and lithosphere) so that equilibrium is usually not reached.
In oxic soils with pH > 3 the dissolution of ferrous iron-bearing primary minerals releases Fe 2+ ions into the soil, and rapid oxidation occurs with the poorly soluble Fe 3+ ions rapidly undergoing hydrolysis to precipitate ferrihydrite (Maxbauer, Feinberg, & Fox, 2016). When soil's pH is lower than 3, the formation of Fe 3+ minerals must be mediated by microorganisms (Cornell & Schwertmann, 2003). Ferrihydrite is unstable with respect to the more crystalline ferromagnetic and antiferromagnetic minerals and, with time, will progressively be converted into these more stable phases (Maxbauer, Feinberg, & Fox, 2016).
In the study of soils through rock magnetic techniques it is common to observe a magnetic enhancement between the upper A and B soil horizons. This arises primarily from the presence of superparamagnetic (SP) and stable single-domain (SSD) grains (Maher 2007;Liu et al. 2012, and references therein). There are two mechanisms to explain the 'magnetic enhancement' in well-drained soils: (1) Redox oscillations and the fermentation mechanism: the fermentation (anoxic condition) is the most significant pathway, where a redox oscillation during wet/dry cycles by reducing bacteria produces ultrafine magnetite, prone to oxidation towards maghemite (Maher, 1997). Under anoxic conditions, the oxidation of organic matter is often coupled to the microbial reduction of Fe 3+ -oxides in a process referred as Dissimilatory Iron Reduction (DIR). The DIR bacterium Geobacter metallireducinis (GS-15) was the first organism discovered that coupled degradation of organic matter (acetate) to the reduction of ferric iron leading to the extracellular precipitation of SP/SSD magnetite (Maxbauer, Feinberg, & Fox, 2016).
(2) Aging pathways of ferrihydrite to hematite: the slow transformation of ferrihydrite into hematite, during which an intermediate ferromagnetic phase is produced is an alternative pathway that can lead to magnetic enhancement. The timescale for a full transformation of ferrihydrite into hematite is in the order of 10 5 -10 6 years. Anoxic conditions are not necessary for this aging mechanism, therefore during a prolonged dry season the magnetic mineral production is primarily associated with ferrihydrite aging while in the rainy season the fermentation process is favored (Maxbauer, Feinberg, & Fox, 2016).
Transformation of ferrihydrite directly into goethite and hematite is common in soils and is dependent on the soil physicochemical conditions. Goethite is favored in cool, moist soils that only rarely experience prolonged aridity. By contrast, hematite is more abundant in subtropical, tropical soils with frequent episodes of prolonged dryness. Soils with near neutral pH and low organic content tend to favor hematite over goethite, and vice-versa (Maxbauer, Feinberg, & Fox, 2016). The presence of goethite and hematite in most of soils suggests that climate play an important role in the iron mineralogy of soils.
The major iron oxides in soils are magnetite, maghemite, hematite and goethite. Below there is a brief description of the magnetic properties of these minerals.  (Dunlop & Özdemir, 1997).
The magnetic susceptibility (χ; defined in Chapter 3) is usually 56500x10 -8 m³/kg; two important temperature transitions characterize pure magnetite grains: the Curie point at 580°C, where the thermal energy overcomes the exchange coupling and the ferrimagnetism is lost, and the Verwey transition at -150°C, a change in the crystallographic distribution of the iron ions, the previous cubic structure being slightly distorted into a monoclinic symmetry (Evans & Heller, 2003). Saturation magnetization (Ms; defined in Chapter 3) is ~92 Am²/kg; coercivity (Hc) is low (tens of mT) and coercivity of remanence (Hcr) is usually 300mT (Maxbauer, Feinberg, & Fox, 2016 (Dunlop & Ozdemir, 1997).
Goethite (α-FeOOH): it is orthorhombic and possesses a weak ferromagnetism originated from defects and substitutions within the crystal structure. Due to its 22 defect moment and the extremely weak saturation magnetization (~0.05-0.30 Am²/kg), extreme high fields must be applied to deflect its moments. Susceptibility is usually ~70 x10 -8 m³/kg; the Néel temperature (which coincides with the Curie point) is about 120-150°C; it has a high coercivity (Hc) beyond 100 mT (Dunlop and Özden Özdemir., 1997;Maxbauer et al., 2016).

Rock magnetic measurements and environmental magnetic significance
Magnetic susceptibility is one of the most common properties used for rock magnetism and environmental magnetism studies. It relates the applied field (H) with the resulting induced magnetization (M). The volume susceptibility (κ) is expressed as the magnetization acquired per unit field: where κ is dimensionless in the Système International (SI) and M and H are expressed in A/m. Due to porosity variations that are commonly observed in speleothems (e.g. Strauss et al., 2013) and its irregular shape, mass normalized susceptibility (χ) is usually adopted. It is defined as the volume susceptibility divided by the density (ρ) of the material: where χ is expressed in m³/kg.  with peculiar shapes, such as "wasp-wasted" and pot-bellied" (Tauxe, Mullender, & Pick, 1996), but unmixing the different magnetic contributions in hysteresis loops is always a challenging task (e.g. Dunlop, 2002;Jackson, Worm, & Banerjee, 1990). A summary of the critical sizes of the main magnetic grains can be found in Table 1. First-order reversal curves (FORC) were first proposed in an attempt to remove the ambiguity in hysteresis measurements (Pike, Roberts, & Verosub, 1999; a. P. Roberts, Pike, & Verosub, 2000). This diagram is based on partial hysteresis cycles and represents the most efficient method for mapping distributions of coercivities and interaction fields that exist at the nanometer scale (Harrison & Feinberg, 2008 where the expression is divided by two because the magnetization switch from +Ms to -Ms has a magnitude of 2Ms. In Figure 3.2, there is an example of a FORC diagram with each derivative. A smoothing procedure is required in this process because the second derivative tends to amplify the noise.  Anhysteretic Remanence Magnetization (ARM) is an artificial remanence commonly used in environmental magnetism. It is obtained by subjecting the assembly of magnetic particles of a sample to an alternating and declining magnetic field (AF) superimposed to a small and constant direct field (DC). The sample will acquire an anhysteretic remanence magnetization (Collinson, 1983). ARM is a useful property for studies of SD particles containing ferrimagnetic materials because they acquire more remanence than the same volume of particles having domain walls because of their lower magnetostatic energy configuration. Based on this fact, (Cisowski, 1981) proposed a method to investigate the by comparing the ARM (or IRM) acquisition and ARM (or IRM) demagnetization curves.
Non-interacting particles show symmetrical curves of magnetization acquired/ demagnetized against the applied field. The crossover between both acquisition and demagnetization curves lies close to the remanent coercivity field in the magnetic field axis and also provides the ratio R along the normalized magnetization axis. R-values less than 0.5 indicate interactions between magnetic grains. Another parameter obtained from demagnetization/acquisition curves is the median destructive field (MDF) which corresponds to the field necessary to demagnetize half of the total magnetization.
Chemical and physical processes which affect the magnetic properties of a mixture of magnetic minerals in a sediment can often be associated to the climatic conditions during sedimentation (Egli, 2004). Magnetic unmixing techniques aim to identify these magnetic mineral components on the basis of their rock magnetic properties. Robertson and France (1994) proposed that acquisition/demagnetization curves of remanence magnetization with appropriate model functions could be used to separate the different magnetic components.
According to these authors at first order each magnetic contribution in a given sample can be modeled as an independent cumulative log-Gaussian function (CLG). The parameters of the CLG (saturation remanence, remanent coercivity and deviation, DP) define the shape of each function. Therefore, a linear combination of model functions is expected to reproduce the acquisition/demagnetization curves. The individual cumulative log-Gaussian functions adjusted (Figure 3.4) provide information about coercivity distributions and total ARM (or IRM) of the different magnetic mineral components, and can be used to infer mineral type, grain size distribution and concentration of mixture components (Heslop, 2015).
Isothermal Remanence Magnetization (IRM) is acquired by exposing a sample to a strong steady field (pulsed field or DC field) at ambient temperature. By subjecting the sample to increasing applied fields the magnetization achieves its saturation, or the saturation remanent magnetization (SIRM). Larger fields (>1T) will tend to saturate antiferromagnetic materials (hematite, goethite), whereas smaller fields (≤0.3T) will saturate ferrimagnetic minerals (magnetite, titanomagnetite). Magnetic mineral unmixing can be performed in an IRM acquisition curve using an approach similar to that described above for ARM curves (Figure 3.4). Because the SIRM is independent of magnetic grain size, this technique provides a good parameter that reflects bulk magnetic mineral concentration (Liu et al., 2012). To separate the signal of the "hard" coercivity minerals (e.g., hematite, goethite) from the "soft" coercivity minerals (e.g., magnetite, maghemite), we can use IRM acquired with contrasting inducing fields. This separation makes possible to obtain two parameters, the HIRM (Hard-IRM) and the soft-IRM, expressed as: where HIRM is divided by two due to the contribution of the hard coercivity minerals in both SIRM and IRM @300mT . Both parameters can be divided by the mass to reflect concentration of these magnetic minerals phases.

Figure 3.4: Cumulative log-Gauss (CLG) function proposed by Robertson and France (1994), the individual component is adjusted with median acquisition/demagnetization field (B 1/2 ); dispersion parameter (DP); saturation remanence magnetization (M). The gradient acquisition plot (GAP) obtained through the derivative of CLG. Standardised acquisition plot (SAP), ARM (or IRM) acquisition curve on a probability scale (modified from Heslop, 2015)
Another parameter that is widely used in environmental magnetic study is the Sratio. This parameter reflects the change in the type of magnetic mineral. The S-ratio is given by: The S-ratio is usually presented as percentage (%), representing the percentage contribution of the soft component.
The magnetic response of different magnetite and maghemite samples using the median destructive field (MDF) and the dispersion parameter (DP) after CLG function adjustment was obtained by (Egli, 2004). This study comprises samples from wind-blown dust, soils, loess, detrital grains, extracellular magnetite, aeolian dust, pedogenic magnetite and others.
The MDF vs. DP diagram shows significant difference between the studied populations of grains and provide thus a "template" for the interpretation the origin of fine magnetite and maghemite particles.
The identification of magnetic populations with different coercivities can also be done using the approach proposed by Lowrie (1990), which consists in the three-axial IRM acquisition followed by stepwise thermal demagnetization of the sample. High (>1.2 T), medium (0.3 T) and low (0.1 T) magnetic fields are applied successively to the z, y and x axes of a sample and separate the contributions of hard (z-axis), medium (y-axis) and soft (x-axis) populations. Then, the unblocking temperatures revealed by the thermal demagnetization provide an independent parameter to identify the magnetic carriers.

Sample preparation
The Alho 6 stalagmite sample is 240 mm long (z-axis) and 60 to 80 mm in diameter (top to bottom) (Figure 4.1). For magnetic measurements, the stalagmite was sliced with a diamond wire-saw into two halves. One half was subsampled for arbitrarily oriented specimens with 7 mm height (Z-axis, corresponding to the growth axis of the stalagmite), 20 mm length (X-axis) and 15 mm width (Y-axis), resulting in 34 discrete specimens  In one sample (2 cm height from 9-11cm from the top), the magnetic extraction protocol of Strehlau et al. (2014) was applied. The protocol consists of a neodymium magnet coupled to an orbital shaker. Acetate buffer solution was used as a 4:1 ratio of 2 M CH 3 COOH and 1 M NaCH 3 COO for dissolution. A flask extraction is made with the neodymium magnet resulting in two subsamples: the extract, that contains strongly magnetic minerals such as magnetite and maghemite, and the remainder, that tends to provide the weakly magnetic minerals (e.g., hematite and goethite). Both subsamples were studied with low-temperature magnetic techniques (see below).
In addition to the samples collected from the stalagmite, sediments inside and outside the cave were sampled to better constrain the origin of magnetic minerals in the stalagmite.
These samples were only analyzed for ARM acquisition curves (see below).

Remanence measurements
Remanence measurements were made in a shielded room with ambient field <500 nT at the Am². The Superconducting QUantum Interference Device (SQUID) monitors the differential change in the electric current flowing through a pair of Helmholtz-geometry superconductor pickup rings that is proportional to the magnetic moment of a sample in the center of the sense region (Kirschvink et al., 2015). The shape of the sample is not important, as long as the material being measured is contained within the uniform sense region (Kirschvink, 1992). In older magnetometers (e.g. astatic, induction-spinning and fluxgate-ring sensors) the shape of the sample is important.
Stalagmites have low concentrations of magnetic minerals. In order to obtain the best quality data, a study of the different configurations of the 2G TM SRM was carried. In this equipment, the user is able to select over how many counts the magnetic moment will be averaged between 1 and 90. For counts equal to 1 the measurement takes ~58 seconds to be completed, whereas for counts equal to 90 the measurement is completed in ~308 seconds.
Using a standard sample provided by 2G TM , we performed tests varying progressively the number of counts (MM in Figure 4.2a). We observed a clear gain of sensitivity until count number of 6. After count number higher than 10 there is no clear increase in sensitivity ( Figure 4.2b). We therefore set our counting number at 6, which was the most timeeffective configuration. Hysteresis loops and direct current demagnetization curves (DCD) were obtained in an

Alternating Gradient Magnetometer (AGM) by Princeton Measurements Corporation
Micromag TM 2900 also housed at the Laboratório de Paleomagnetismo of Universidade de São Paulo (USPmag). We carried the experiment in 23 samples (0.5x0.5 cm) varying the field from +1 to -1 T in 0.01 T steps. The FORC distribution included in this study was obtained after stacking the result of five independent measurements in order to improve the signal-to-noise ratio in these extremely weak magnetic rocks (Heslop & Roberts, 2012).
The optimal choice of FORC parameters were the same as described in Egli et al., (2010) for smoothing a factor of five. FORC distributions were computed with FORCinel package (Harrison and Feinberg, 2008).
For the anhysteretic remanence magnetization (ARM) a direct bias field of 0.05 mT in Zaxis was adopted, with the alternating field varying from 1.5 to 200 mT in 32 steps. Using this procedure an acquisition curve of ARM and demagnetization curve (AF) were generated enabling the Cisowski test. This procedure was performed with the u-channel SRM, where the user is able to write a script and run eight samples per time.
The isothermal remanence magnetization (IRM) acquisition curves were obtained at room temperature using the same u-channel SRM and the pulse magnetizer (MMPM10). The 34 specimens were subjected to pulsed fields along the Z-axis varying from 0.01 to 2.8 T in 37 steps, then three pulsed fields were imparted in the opposite direction (-Z-axis) at 0.01, 0.1 and 0.3 T to obtain the back-field values.
In addition to the Alho 6 samples, we also obtained ARM acquisition curves for sediments collected inside and outside de Pau d'Alho cave to allow a comparison between the magnetic mineralogy of the speleothem and that of eventual sources of magnetic carriers.
The Lowrie (1990) test was performed by thermally demagnetizing a three-axis IRM in 8 representative samples from the first quarter of the stalagmite (~15 mm height). Fields of 2.8 T, 0.4 T, and 0.12 T were successively induced along three perpendicular directions of the specimens using the MMPM10. Eighteen steps of thermal demagnetization were conducted up to 580 ºC in a dual-chamber paleomagnetic oven manufactured by ASC Scientific. Magnetization measurements were performed using the u-channel SRM.

Low temperature experiments on bulk rock and magnetic extracts
Low Room-temperature saturation isothermal remanent magnetization (RTSIRM) curves were obtained in a field of 2.5 T at room-temperature, with the samples continuously measured in zero-field when cooled down to 10 K and warmed up to 300 K. For the FC-ZFC protocol, Field-Cooling (FC) curves were performed by applying a constant field of 2.5 T down to 10 K, whereas Zero-Field Cooling (ZFC) curves were performed in zero-field down to 10 K. In both cases, stepwise measurements were obtained at 5 K intervals.

Age of the Alho 6 stalagmite
The dating method and results used for the construction of the age-model of the Alho 6 stalagmite is described in Novello et al. (2016). These results were integrated in the present study since it corresponds to the same stalagmite studied here. In figure 5.01, we show the correspondence between samples, their age and the average time that each sample represents. The equation used is the one provided by Novello et al. (2016Novello et al. ( ): = 1917 where y is the age in years CE, and x is the distance from the top in mm.

Oxygen and Carbon Isotope records
The oxygen and carbon isotope record has 1174 points. The oxygen isotope ratios have an average value of -6.17‰, varying from -4.59 to -7.89 ‰, and the carbon isotope ratios have an average value of -6.58 ‰, varying from -3.84 to -8.72 ‰.

Hysteresis and FORC diagrams
The 23    FORC data were treated using the FORCInel software (Harrison & Feinberg, 2008). The smoothing factor selected was five, and the diagram was obtained after a stack of five independent measurements. Vertical and horizontal profiles were made to provide a better visualization of the diagrams (Figure 5.05). The FORC diagram and the profiles indicate a narrow central ridge, implying a low-coercivity mineral with single domain behavior.

ARM acquisition curves and Cisowski test
ARM acquisition curves for the 34 samples show the same behavior, indicating that the main magnetic carrier does not change throughout the stalagmite. In figure 5.06, four representative samples are shown and the results indicate a consistency with hysteresis and FORC analysis, where the low-coercivity fraction dominates de magnetic signal.

IRM acquisition curves and Lowrie test
The isothermal remanence magnetization (IRM) curves also show a similar behavior throughout the stalagmite. Four representative samples are plotted in Figure 5.09. The lowcoercivity magnetic phase (<300mT) contributes with more than 90% of the magnetization and a high-coercivity magnetic phase (>300mT) is clearly present because the magnetization does not reach the saturation with the highest field. Thermal demagnetization of 3-axis IRMs typically show the dominance of a soft magnetic component, which demagnetizes at ~560°C. The medium coercivity phase is almost negligible, but when present it unblocks at roughly the same temperature ( Figure   5.10). The hard component is also small to negligible in most samples. In some samples there is a slight increase in magnetization at ~120°C that may be due to the presence of goethite (see low-temperature MPMS results below). It is worth noting the absence of remanent carriers with unblocking temperatures higher than 600°C, suggesting that hematite is not a significant remanence carrier in these samples. Four representative samples are shown in Figure 5.10 with their respective applied fields.

Unmixing methods and remanence ratios
The analysis of ARM curves for all 34 specimens shows a low-coercivity fraction with B 1/2 of 15.1-17.0 mT (average of 16 mT) and dispersion parameter (DP) between 0.27-0.31 (average of 0.29) (Figure 5.11). A cumulative log-Gaussian function (CLG) was calculated with Kruiver et al. (2001) workbook for all samples. An example of the gradient is plotted in Figure 5.11 with the respective histogram of the adjusted component. Results from the unmixing of IRM acquisition curves are in good agreement with the parameters derived from ARM curves, with the B 1/2 values of 24-28 mT (average of 26.2 mT) and DP between 0.25-0.31 (average of 0.28), and a high-coercivity fraction with B 1/2 of 1260-3980 mT (average of 2557 mT) and DP between 0.8-1.0 (average of 0.9). The high-coercivity fraction was not accurately defined given its low content and the fact that the magnetization did not reach saturation with the maximum applied field of 2800 mT. In Figure 5.12, an example of a gradient of cumulative log-Gaussian function (CLG) used to adjust the IRM is plotted together with histograms of B 1/2 and DP.  1 for a table showing the values of remanence magnetizations). NRM varies from ~1.1x10 -7 to ~1.5 x10 -6 Am 2 /kg, ARM varies from ~4.5x10 -7 to 3.6x10 -6 Am 2 /kg, SIRM varies from ~1.2x10 -5 to ~7.5x10 -5 Am²/kg, HIRM varies from ~6.4x10 -7 to ~3.5x10 -6 and soft-IRM varies from ~1.1x10 -5 to 7x10 -5 Am 2 /kg. All magnetic parameters covary throughout the stalagmite ( Figure 5.14), including those that highlight the low-coercivity fraction (e.g., ARM, soft-IRM). A linear regression coefficient (r²) was calculated for all the parameters that express variation in concentration. All r 2 values are positive and higher than 0.67, indicating that the magnetic minerals expressed by natural and artificial remanences and by the different remanence ratios can be explained by the same origin.
The S-ratio parameter varies from 0.87 to 0.92 (mean of 0.90) (Figure 5.13) and are similar to those found in other speleothems (e.g. Font et al., 2014). This result implies that 90% of the magnetization can be attributed to the low-coercivity fraction.

Low-temperature SQUID magnetometry (FC-ZFC, RTSIRM)
Six bulk samples distributed along the stalagmite produced similar MPMS low-temperature curves, which are typical of a mixture between magnetite and goethite ( Figure 5.15).
RTSIRM curves typically show the Verwey transition ( Figure 5.15) characteristic of magnetite. In addition, these data also indicate the presence of goethite, identified by the separation between FC-ZFC curves between 10 and 300 K (high-coercivity component) (Guyodo et al., 2006;Maxbauer et al., 2016) and is similar to results obtained in other stalagmites (Lascu & Feinberg, 2011;Strauss et al., 2013). This interpretation is reinforced by the decreasing trend on warming observed in RTSIRM data (Strauss et al., 2013). The absence of Morin transitions in low-temperature curves suggests that hematite does not contribute significantly to the high-coercivity signal in Alho 6, although it should be noted that many forms of natural hematite show muted Morin transitions. These results allow us to identify the two phases that dominate the remanence throughout Alho 6 stalagmite as magnetite and goethite, which correspond respectively to the low-and high-coercivity phases observed in other measurements.  Chapter 6: Discussion

The source of magnetic minerals
Magnetic measurements indicate that the main magnetic remanence carriers in Alho 6 stalagmite are magnetite and goethite, occurring in a nearly constant relative proportion.
The same phases have been identified in other stalagmites by Perkins et al. (1996), Lascu & Feinberg (2011), Strauss et al. (2013) and Strehlau et al. (2014). Magnetite grains have remanent coercivities varying between 14-17 mT (from ARM acquisition curves and FORC diagrams). To infer the origin of the low-coercivity phase, our data can be further compared with Egli's (2004) database for detrital, pedogenic and organically formed magnetite in which the median destructive field (MDF) is plotted against the dispersion parameter (DP) of ARM acquisition curves (Figure 6.1). In this diagram, we can separate two major groups: biogenic and detrital (plus pedogenic) magnetite. Biogenic magnetite (soft and hard components) has MDF values higher than 30 mT and DP values below 0.28. Detrital magnetite (plus extracellular and pedogenic magnetite) has MDF values below 32.5 mT and DP values higher than 0.22. Figure 6.1 also includes results obtained from other stalagmite studies (Lascu & Feinberg, 2011;Osete et al., 2012;Font et al., 2014;Bourne et al., 2015) and those from sediments collected inside and outside the Pau d'Alho cave. All stalagmite results plot together with the detrital group, except for a handful of data points reported by Lascu & Feinberg (2011), who used unmixing to demonstrate contributions from both pedogenic and loess sources. The data from Alho 6 form a tight cluster and overlap the field of detrital (plus pedogenic or extracellular) magnetite. Sediments related to the cave also plot together with the stalagmite data. So far, the data available for all stalagmites define distinct clusters in the diagram and suggest that these parameters can be used as fingerprints of specific processes of magnetic mineral assembly formation for each cave system (Figure 6.1). The similarity between magnetic properties of pedogenic and extracellular magnetite suggests a common origin for them, possibly related to bacterial dissimilatory iron reduction (Egli, 2004). The magnetic particles reaching the Alho 6 stalagmite are likely to have been formed during soil development. A similar pedogenic derivation can be tentatively attributed to goethite. This mineral seems to be pervasive in soils and sediments (e.g. Till et al. 2014;Maxbauer et al. 2016) and also in stalagmites (e.g. Perkins, 1996;Lascu & Feinberg, 2011;Strauss et al., 2013). The fact that goethite and magnetite are present always in similar proportion in our samples, in spite of variations in the total concentration of magnetic minerals throughout the stalagmite suggests a common origin for both minerals. However, goethite may also form during speleogenesis, because dissolved iron in groundwater re-equilibrates with the Eh-pH conditions of the cave system at a speleothem drip point (Strauss et al., 2013).

Variation in the magnetic signal and paleoclimatic significance
The results provide support for a relationship between isotopic and magnetic signals of the Alho 6 speleothem across decadal to centennial time scales. In general, more negative (positive) carbon and oxygen isotope ratios correspond to lower (higher) values in all magnetic remanence measurements. To simplify the visualization only the parameters ARM and HIRM are plotted with the isotope records and the growth rate in Figure 6 This correlation between drier environmental conditions and elevated concentrations of magnetic minerals runs contrary to conventional thinking associated with the production of magnetite in soils, where a higher concentration of magnetic minerals would be expected in the humid periods not in the drier ones. Earlier studies have proposed a link between the concentration of pedogenic magnetite and local precipitation rates in soils and speleothems (e.g. Bourne et al., 2015;Maher, 1998;Maxbauer et al., 2016). However, while the rate of magnetite production in soils has been shown to vary with precipitation, it is unclear whether elevated concentrations of magnetite in speleothems are a result of increased pedogenic activity (e.g., Bourne et al., 2015), or due to the changes in the rate of downward translocation of micrometer scale minerals during periods of increased precipitation (Maher and Taylor, 1988), or a combination of the two. As in Bourne et al. (2015), we are able to discuss the main environmental factors controlling the incorporation of detrital magnetic minerals into Alho 6 speleothem by comparing the magnetic data with stable carbon and oxygen isotope ratios measured on the same samples. This study is unique in that we examine these relationships on far shorter timescales than previously possible (e.g. Bourne et al., 2015).
The magnetization record in Alho 6 shows an inverse correlation with precipitation rate as determined by  18 O values (Figure 6.2). This suggests that fine scale variations in the concentration of magnetite and goethite are not necessarily driven by increased rainfall in the study area. On the contrary, higher concentrations of magnetic minerals correspond to relative drier periods as inferred from the more positive  18 O values observed in Alho 6.
Thus, a higher flux of groundwater through overlying soil is not the main factor controlling magnetic mineral delivery to the speleothem of Pau d'Alho cave.
Further insights can be provided by examining the relationship between the  13 C record and magnetic remanence in Alho 6. The carbon isotope value recorded by a speleothem is controlled by processes occurring in the soil/epikarst and inside the cave (Fairchild & Baker, 2012;Wong & Breecker, 2015). Changes in  13 C in soils of tropical Brazil are usually driven by changes in the relative contributions of C 3 and C 4 plants to the carbon isotope composition of the soil organic matter (SOM) (e.g. Silva et al.;2008, Pessenda et al., 2010. During wetter intervals, as denser forests (C 3 plants) replace savanna grasslands (C 4 plants), the soils displays a decrease in the  13 C of the SOM. The increase in soil biomass during forest expansion is also likely to generate a higher concentration of isotopically lighter biogenic carbon, regardless of the relative ratio of C 3 and C 4 plants (Baldini et al., 2005). Consequently, it is possible that the carbon isotope composition recorded by the Alho 6 speleothem moves towards more negative values during times of forest expansion, in agreement with the generally more negative values of  18 O that usually covary with decreases in  13 C. On the timescales studied here, we suggest that the increased soil biomass associated with the formation of denser vegetation retains pedogenic magnetic minerals more effectively than grassland soils, leading to a reduced flux of magnetite into the underlying karst system.
While the  13 C values in the Alho 6 record (ranging between -5 to -8 ‰) indicate that carbon from SOM played an important role, other fractionation processes are likely to have contributed to the  13 C record as well; especially those processes that occur inside the cave or in the unsaturated aquifer above the cave system. For example, seasonal variations in cave venting can change the rate of CO 2 degassing that occurs during speleothem formation (Boch et al., 2011;Breitenbach et al., 2015;Meyer et al., 2014;Spötl et al., 2005). This is particularly effective at caves situated in temperate climates, which are characterized by large temperature gradients between the cave's interior and the external environment, as well as by high summer soil CO 2 productivity. Studies of modern speleothems show that when precipitation of CaCO 3 occurs in cave atmospheres depleted in 13 C, speleothems record higher  13 C values. The opposite is true when carbonate precipitation occurs in cave atmospheres enriched in 13 C. Thus, for temperate climates, periods of higher and lower  13 C values are associated with cold winters and warm summers, respectively (Meyer et al., 2014).
Large fluctuations in cave CO 2 are observed at our tropical site, which often experiences higher CO 2 levels in cave air during wet and slightly warmer summer months. One way to test if the climate conditions inside the cave exerted a dominant control on the  13 C of Alho 6 speleothem is by making a comparison with its growth rates (GR), because  13 C and GR are both thought to be affected by seasonal fluctuations in cave pCO 2 (Banner, Guilfoyle, James, Stern, & Musgrove, 2007;Baldini, 2010). In most caves, the soil zone is the main source of CO 2 , which enters directly through fractures or from degassing of cave drip waters. At our study site, periods of elevated CO 2 in the soil and the cave are likely to occur during episodes of increased rainfall and warmer temperatures. Because this process could result in high pCO 2 levels in the cave, it may inhibit the average speleothem growth rate and also laminae thickness (Boch et al., 2011;Banner, Guilfoyle, James, Stern, & Musgrove, 2007;Baldini, 2010). While it is important to consider these mechanisms here, it is important to note the lack of a relationship between  13 C and GR in Alho 6. In fact, this speleothem shows similar values of GR during periods of anomalously high and low  13 C values during MCA and LIA events ( Figure 6.2), suggesting that progressive CO 2 degassing during carbonate precipitation in the cave and possibly in the vadose zone is not likely to exert a major control on the  13 C of Alho 6, at least on multidecadal to centennial time scales. In this light, the close correspondence between  13 C and the concentration of magnetic minerals supports the idea that carbon isotope variability is mainly associated with the production of biogenic carbon in soil, where the decreased flux of magnetic minerals into Alho 6 at periods of higher rainfall (as denoted by low  18 O values) is by explained by the greater retentivity of magnetic minerals by soils with denser vegetative cover. This notion is consistent with the lower  13 C values expected to occur in soils during warmer and wetter periods with high biogenic CO 2 production. Thus on multidecadal to centennial time-scales, while  18 O values track the variability of regional precipitation patterns near Pau d'Alho cave, we suggest that the records of  13 C and mineral magnetism capture short term soil dynamics. A scheme of the soil dynamics in a karst system summarizing the results found here is presented in figure 6.3 (dry periods) and in figure 6.4 (wet periods).

Chapter 7: Conclusion and Perspectives
The magnetic mineral assembly in stalagmite Alho 6 consists of fine-grained magnetite and goethite in a nearly constant proportion along the whole speleothem. The magnetic properties of the soft-coercivity phase show very little variability across the entire stalagmite and are essentially identical to those of pedogenic magnetite. In line with recent work showing that goethite is a pervasive phase in soils and stalagmites, we suggest that goethite in Alho 6 is pedogenic as well, but cannot rule out the possibility of goethite nucleation and growth during speleogenesis. The variations in the concentration of these magnetic minerals can be used as a proxy for the flux of suspended material delivered to the Pau d'Alho cave system via drip waters.
We conclude that the combined variability of magnetic parameters,  13 C and  18 O in the Alho 6 speleothem can provide new insights into the processes controlling the soil production in tropical regions. Our results indicate that a soil's retentiveness of its magnetic minerals is more strongly influenced by variations in vegetation cover than by changes in the rate of precipitation on multidecadal to centennial timescales of the last millennium in Midwest Brazil. The data suggest that the expansion of dense vegetation during wet periods might reduce the downward translocation of micrometer-scale pedogenic minerals from soils, resulting in speleothems characterized by low concentrations of magnetic minerals and low  13 C values.
This study offers a new perspective for speleothem researchers on how environmental magnetic techniques may be combined with stable isotope methods to arrive at a more nuanced paleoclimate reconstruction that accounts for short-term variability in soil dynamics.