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Mineral magnetic record of paleoclimate variation in loess and paleosol from the Buenos Aires formation (Buenos Aires, Argentina)
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Journal of South American Earth Sciences, Vol. 11, No. 6, pp. 561±570, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0895-9811/98/$ - see front matter S0895-9811(98)00038-8
Mineral magnetic record of paleoclimate variation in loess and paleosol from the Buenos Aires formation (Buenos Aires, Argentina) M.J. ORGEIRA*, 1,5A.M. WALTHER, 1,2,5C.A. VAÂSQUEZ, 5I. DI TOMMASO, 1,4 S. ALONSO, 3G. SHERWOOD, 3HU YUGUAN and 1,2,5J.F.A. VILAS
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Consejo Nacional de Investigaciones Cienti®cas y TeÂcnicas de Argentina, Buenos Aires, Argentina Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina 3 Liverpool John Moores University, Liverpool, UK 4 Centro de Investigaciones en Recursos GeoloÂgicos, Buenos Aires , Argentina 5 Laboratory of Paleomagnetism ``Daniel Valencio'', FCEN, UBA, Ciudad Universitaria Pab II, 1428 Buenos Aires, Argentina 2
(Received December 1997; accepted September 1998) Abstract Ð Environmental magnetic techniques were applied to a loess±paleosol sequence of the Chacopampean plain (Buenos Aires, Argentina). Mineral magnetic carriers and their grain size were identi®ed in order to detect magnetic mineral ¯uctuations associated with climatic changes. Multidomain magnetite of detrital origin dominates the record. In paleosols, a high coercivity fraction was identi®ed. Horizons with no visual evidence of pedogenesis, but showing magnetic behavior analogous to that of paleosols were observed and are thought to represent environmental conditions similar to those prevailing during paleosol formation. The results suggest that the magnetic signal yielded by paleosols in these South American loess deposits is dierent from that in the Chinese loess. This may be due to dierences in parent materials, diagenetic processes and/or dierences in paleoclimatic conditions in both regions. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
intersection with Godoy Cruz street, in Buenos Aires city (34830'S, 58820'W; Fig. 1). A building excavation at this site disclosed a loess section assigned to the Buenos Aires Formation; no anthropogenic modi®cations were found within the sedimentary sequence. The Buenos Aires Formation was originally assigned by Frenguelli (1957) to the ``Bonaerense''. He described the ``Bonaerense'' as a thick homogeneous unstrati®ed loess unit, very ®ne grained, light reddish brown, with homogeneous distribution of calcareous nodules. Loess and paleosols are interbedded showing the alternating cold-arid, and warm-humidic periods.
Environmental magnetic studies have demonstrated that close connection exists between climatic changes and the ferrimagnetic pedogenic mineralogical fraction in loess/paleosol sequences (e.g., Banerjee et al., 1993; Maher and Thompson, 1991; Hunt et al., 1995; Verosub and Roberts, 1995). Environmental magnetic techniques are useful when magnetic minerals are present in low concentration which cannot be detected with common sedimentological techniques. The oxidation state of Fe is a good indicator of soil microclimates (Duchaufour, 1975); therefore, the characterization of the pedogenic magnetic fraction and the ¯uctuations in the detrital ferrimagnetic fractions throughout a sedimentary succession can provide excellent tools to understanding paleoenvironmental changes.
The studied sediments have been traditionally known as ``Bonaerense'', after Ameghino (1889) who used it as a synonym for the upper section of Pampean Formation. Parodi and Parodi (1952), formally named this unit the Buenos Aires Formation. Frenguelli (1957), presented a full description of this unit. He characterized it as a 6±7 m thick layer of aeolian origin, ®ne grained and homogeneous, light brown with reddish tints, extending all over the Pampean loess region.
The Chacopampean plain is one of the largest loess regions in the world. The origin of the pampean loess lies in a continuous dust input carried by west winds from the Andes Cordillera. However, there are no thick natural outcrops of loess: most useful sequences are exposed in excavations at building sites in Buenos Aires Province. The present contribution oers information from a subsoil pro®le of the Buenos Aires Formation, located on the CervinÄo street, near the
A ``Lujanense land mammal age'', tentatively assigned to upper Pleistocene, was proposed by Pascual et al. (1965) on the basis of fossil mammalians found within the unit. Fidalgo et al. (1975), reviewing previous works, found that most of the authors who had studied the
* Corresponding author. 561
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M. J. ORGEIRA et al. Glacial deposits (source of loess material) corresponding to those Oxygen stages have been found in several sites of the Andean range (Clapperton, 1993). However, lack of radiometric data severely hinders a precise correlation. The glacial chronology proposed for the Southern Andes and Southern Patagonia (Rabassa and Clapperton, 1990; Meglioli, 1994) identi®ed deposits which they related to 18O stages 2, 6, 10 and 12. Therefore, no precise correlation between the Buenos Aires Formation and 18O stages can be established upon the basis of the available information.
Fig. 1. Location map.
``Bonaerense'' or equivalent sedimentary units had coincided in assuming an Upper Pleistocene age for them. Tonni and Fidalgo (1978) proposed cold and dry weather conditions and a progressive aridization process that concluded in a pleniglacial stage of the late glaciation (LGM). Clapperton (1993), summarized most of the existing information on the Pampean loess formation (formally named by Teruggi, 1957), within which the ``Bonaerense'' would be the upper section. According to this author, the stratigraphy of the Pampean loess would suggest environmental conditions alternating between dry-windy intervals of loess deposition separated by more humid periods with dominant pedogenesis. Consequently, the episodes of loess have been correlated with Quaternary glaciation, and the humid intervals were assumed to correspond to interglacial or interstadial climatic conditions. On the other hand, paleomagnetic studies performed on the Buenos Aires Formation and other mammalianequivalent age-units (Orgeira, 1990) assigned a Brunhes magnetic age (<0.78 Ma) to the Lujanense land mammal age. The information available at present only allows us to ensure that the Buenos Aires formation was deposited during a middle to late Pleistocene glacial period younger than 0.78 Ma. On the other hand, studies by Cione and Tonni (1995) and Bonadonna et al. (1995), permit us to assume that the Lujan Formation (which overlays the Buenos Aires Formation on several sites throughout the Buenos Aires province), is related to the last glacial maximum and the last interglacial stage. As a consequence, the Buenos Aires Formation should be older than that glaciation, and younger than 0.78 Ma. Upon the basis of information obtained from ice core records and deep sea cores (Imbrie et al., 1992; Jozuel et al., 1993; Tzedakis, 1993; Waelbroeck et al., 1995; and others), cold periods during which the deposition of the Buenos Aires Formation could have taken place, would be equivalent to 18O stages 4, 6, 8, 10 or 12.
The study section of this contribution is 7.6 m thick, and has a Brunhes magnetic age, according to the paleomagnetic study carried out during the present research on samples of the same section. The paleomagnetic study included a traditional analysis of magnetic behaviour with stepwise thermal demagnetization. During the ®eld work, two paleosol levels were visually identi®ed based on geological criteria set by Andreis (1981). Brie¯y, the most important characteristics observed in the ®eld are the following: remarkable color change, prismatic soil structure, clayey texture and small holes left by roots (pedogenic tubes). Levels 13 and 14 are related to the lower paleosol and levels 50±54 to the upper paleosol (Fig. 2). The aim of this paper is to study the magnetic carriers of the Chacopampean loess in order to evaluate the action of climate proxies and the diagenetic processes that occur during pedogenic phases of pampean paleosol formation. In other words, to determine the presence of a magnetic signal associated with climatic changes in this type of loess.
SAMPLING TECHNIQUES Fifty-four consecutive levels at approximately 15 cm intervals were sampled. Bag samples (approximately 150 g) were collected, after cleaning the outcrop face. Subsamples from the bags were air-dried at room temperature at the laboratory and then weighed and packed into plastic boxes (2.5 cm of height and diameter) for magnetic analysis. The remainders were used for non-magnetic treatments. Measurements and analysis Magnetic susceptibility. Magnetic susceptibility at two frequencies (470 and 4700 Hz) was measured using a Bartington MS 2 susceptibilitimeter. A decrease in bulk magnetic susceptibility (Fig. 2a) takes place at both the lower (13 and 14) and upper (50±54) paleosol levels. Likewise, in other levels with no macroscopic pedogenic evidence, particularly at levels 1, 2 and 26, 27, analogous decreases are
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Fig. 2. Magnetic susceptibility at two frequencies (470 and 4700 Hz) (A) and percentage of paramagnetic contribution to bulk susceptibility (B) in a loess±paleosols section of Buenos Aires Formation, Argentina. L: loess levels. S: paleosol levels.
observed. No signi®cant dierences between measurements in frequencies were found (Fig. 2a). The F Factor is lower than 5% in all levels; consequently, no superparamagnetic fraction would be expected (Heller et al., 1991). However, this evidence would not be signi®cant to identify SP (Banerjee et al., 1993). On the other hand, measurements of paramagnetic susceptibility, coercivity and saturation magnetization, using a VSM (vibrating sample magnetometer) were performed at all levels, except odd numbered levels between 40 and 54. Figure 2b shows the variation in percentages of paramagnetic fraction ([paramagnetic susceptibility/total susceptibility] 100). It shows an increase of paramagnetic fraction in paleosol levels. Hysteresis loops and associated parameters. The hysteresis loops reveal the ¯uctuations in the amount of paramagnetic minerals throughout the pro®le, which has already been shown in the paramagnetic susceptibility pro®le. Figure 3 shows the hysteresis loops of samples from the loess level C30 (Fig. 3a) and paleosol level C51 (Fig. 3b). Figure 4a shows the saturation magnetization (Ms) values. This ®gure shows again a decrease of Ms in the paleosol levels, as well as in other levels in which decreases in the susceptibility were recorded. In a broad sense, these drops can be interpreted as a
decrease in the ferromagnetic (sensu lato) fraction in paleosol levels as well as in those with similar magnetic features (levels 1; 26 and 27; 40±45). The coercivity (Hc) shows a rise in the paleosol levels (Fig. 4b; levels 13 and 14, and 54) as well as in those levels with up to now similar magnetic behavior (2; 26 and 27; 42 and 43). At these levels, Hc reaches higher values than those expected for a ferrimagnetic fraction represented by MD magnetite (Roberts et al., 1995). This could mean that in paleosol levels, a mixture of low and high coercivity minerals is present. Isothermal remanent magnetization (IRM) and associated parameters. Samples from all levels were submitted to stepwise acquisition of IRM (from 5 mT to 2.3 T). Figure 5 displays the IRM acquisition curves of a group of representative samples from the sequence. The shape of the curves (i.e., saturation at ca 300 mT) indicates the dominance of the ferrimagnetic fraction (Dankers, 1978). IRM saturation values at 2.3 T (SIRM), displayed in Fig. 6a, show remarkable decreases in the paleosol levels as well as in levels 1 and 2, 26 and 27, 42 and 45. These drops can be related to a decrease in content of ferrimagnetic minerals. Back ®eld experiments were carried out on samples from ten levels, including paleosol levels 14 and 52 and others from levels with
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Fig. 3. Hysteresis loop of a loess sample (A) and of a paleosol sample (B) from a loess±paleosol section of the Buenos Aires Formation, Argentina.
no ®eld evidence of pedogenesis but with similar magnetic behavior (levels 1, 26 and 44), and loess levels 12, 24, 32, 36 and 42. The coercivity of remanence (Hcr, Fig. 6b) displays a remarkable rise in the paleosols (14, 52) and in levels with similar magnetic behavior (26, 42, 44, and to a lower extent in level 1). Hcr values at these levels are, in general, between 90 and 100 mT. They are higher than those expected for most ferrimagnetic minerals, such as magnetite or maghemite (Dankers, 1978; Roberts et al., 1995). These Hcr values are compatible with a mixture of a low coercivity fraction (magnetite) and a high coercivity fraction (hematite and/or goethite). In contrast, Hcr values from the loess levels, ranging between 40 and 50 mT, are compatible with magnetite (Dankers, 1978; Roberts et al., 1995). The S-ratio from the whole pro®le (IRM-300/ SIRM) is shown in Fig. 7a. The ratio in the lower and upper paleosol levels is less than 1. This observation can also be extended to levels 3, 26 and 27. This decrease is associated with a high coercivity fraction, which is present in paleosols and other levels with similar magnetic behavior (King and Channell, 1991; Verosub and Roberts, 1995).
The median acquisition ®eld of the SIRM (H'cr) is shown in Fig. 7b. the H'cr rises in the paleosol and falls in the loess levels. The paleosol H'cr value is higher than that expected for magnetite (Dankers, 1978). This is, again, probably due to the presence of a high coercivity mineral fraction. Finally, the median destructive ®eld (H1/2I) for the paleosol levels ranges between 40 and 50 mT; the H1/ 2I in loess layers is of about 15±25 mT. The value of this parameter in the paleosol levels also indicates the contribution of a high coercivity fraction (Dankers, 1978). Anhysteretic remanent magnetization. All samples were subjected to a d.c. bias ®eld of 0.1 mT in the presence of a decreasing alternating ®eld (peak AF of 100 mT). The anhysteretic remanent magnetization (ARM) is particularly sensitive to ferrimagnetic (low coercivity) single domain or pseudo-single domain grains (Banerjee et al., 1981; Verosub and Roberts, 1995). Decrease in Xarm (Fig. 8c) is associated with decrease in the amount of low coercivity material, which con®rms that both the paleosols and the levels with similar magnetic behavior contain lower concentrations of low coercivity minerals.
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Fig. 4. Saturation magnetization (Ms, A) and coercivity (Hc, B) in a loess±paleosol section of Buenos Aires Formation, Argentina. L: loess levels. S: paleosol levels.
Low temperature experiments. Samples were magnetized in a 1 T ®eld in liquid nitrogen (77 K), and the intensity of remanent magnetism was measured continuously up to room temperature (Fig. 8). In loess samples, the Verwey transition (Verwey and Haayman, 1941; Halgedahl and Jarrard, 1995) was observed.
However, it is not observed in the lower paleosol (level 13) and in a level with similar magnetic behavior (44). The Verwey transition (118 K) indicates a change in the crystal structure of magnetite from orthorhombic to monoclinic (Banerjee, 1991). As a consequence, the magnetization of MD magnetite is destroyed. These
Fig. 5. Isothermal Remanent Magnetization acquisition curves for some representative samples of loess and paleosols levels in a loess±paleosol section of the Buenos Aires Formation, Argentina.
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Fig. 6. Saturation of isothermal remanent magnetization (SIRM, A) and coercivity of remanence (Hcr, B) in a loess±paleosol section of the Buenos Aires Formation, Argentina. L: loess levels, S: paleosol levels.
Fig. 7. S ratio (IRM-300/SIRM, A), medium acquisition ®eld of SIRM (H'cr, B) and anhysteretic susceptibility (XARM, C) in a loess±paleosol section of Buenos Aires Formation, Argentina. L: loess levels. S: paleosol levels.
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Fig. 8. Low temperature demagnetization curves of IRM (1 T, 77 K) (see text for explanation).
results indicate that the ferrimagnetic fraction in the loess layers is dominated by MD magnetite (Banerjee, 1991) which is not detected in the studied paleosol levels.
Grain size of magnetic carriers. Figure 9 shows the SIRm/Ms vs Hcr/Hc ratios for a representative group of samples from paleosol and loess. Most of the samples display a behavior that is consistent with MD magnetite (Day et al., 1977). Observed Hcr/Hc values (between 5 and 15) could indicate a bimodal grain size distribution of magnetite; in this case, the coarser particles would constitute an overwhelmingly large fraction of the total volume of magnetite in the samples (Parry, 1982). Finally, SIRM/susceptibility ratios (Fig. 10) vary between a range that is compatible with samples in which the main component is MD magnetite (Thompson and Old®eld, 1986). All of the above magnetic determinations indicate the presence of multidomain magnetite. Multidomain behavior is attributed to grain sizes larger than 8 mm (Hunt et al., 1995), equivalent to ®ne silt-sized particles. Consequently, the magnetite detected along the pro®le is most likely of detrital origin.
DISCUSSION Fig. 9. SIRM/Ms vs Hcr/Hc of some representative samples of loess and paleosol levels.
As stated above, the paleosols (levels 13, 14 and 50±54) display a conspicuous magnetic behavior.
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M. J. ORGEIRA et al. altered loess levels (with decrease of detrital magnetite and an associated high coercivity fraction), climatic changes similar to those that produce the paleosols were recorded. Probably these processes were not long enough to be conspicuously recorded as paleosols. Agreement among magnetic parameters in the upper levels (42, 43, 44 and 45) is not as good as it is in the middle and lower levels. However, the Hc and Hcr from levels 42 and 44 reach values higher than in paleosols, which suggests the de®nite presence of a high coercivity fraction. The IRM curves from the loess levels, as well as Hc, Hcr, H'cr and the S-ratio, indicate the dominance of a low coercivity fraction represented by magnetite. These data and ratios (low temperature, Day diagram and SIRM/susceptibility) indicate that magnetite is multidomain, with a grain size not smaller than ®ne silt size (i.e., 8 mm), which denotes a detrital origin. Brie¯y, in the studied section two conspicuous magnetic signals were identi®ed. A pedogenic signal represented by a high coercivity fraction (goethite? and/ or hematite), and a detrital one, carried by MD magnetite.
Fig. 10. SIRM/Susceptibility ratio of the whole loess±paleosol section of the Buenos Aires Formation, Argentina.
Decreases in susceptibility, Ms, ARM and SIRM are associated with rises in Hcr, H'cr and Hc. In addition, S-ratios lower than 1, the lack of a Verwey transition, IRM acquisition curves and the mean values of Hcr, Hc, H1/2I and H'cr indicate that: Two distinctive magnetic fractions have been detected, a low coercivity fraction which is represented by magnetite (and/or titanomagnetite) and another high coercivity fraction. The decrease of magnetic parameters, such as susceptibility and intensity of SIRM, suggests a decrease in detrital magnetite content in the paleosol. Associated with this decrease, a high coercivity fraction is present. This association could indicate that under warm and humid climatic conditions, acidi®cation (by humic acids) and oxygenation of the environment can produce partial oxidation of the detrital magnetite and its replacement by a high coercivity fraction (pedogenic goethite? and/or hematite). Three layers with similar magnetic behavior to these paleosols were observed: a lower (1, 2 and 3), middle (26 and 27) and upper (42, 43, 44 and 45) level. In the lower and middle levels, the magnetic data are consistent with the presence of pedogenic processes. It is possible to suggest that in these less
Finally, these results suggest that the magnetic signal of pedogenic events in the South American loess is dierent from that in the northern Chinese loess (e.g. Banerjee et al., 1993). The cause of this dierent magnetic behavior could be the dierence in the parent materials in both loess (Frenguelli, 1957). Dierences in past climatic conditions could also produce the same dierences in magnetic signatures, as already recorded in the magnetic results from the northern and southern China loess (Han et al., 1996). Furthermore, the type of diagenesis, conversion of non-magnetic clays and weakly magnetic hematite to more magnetic magnetite, suered in China may not have occurred in Argentina loess. Only diagenesis of more magnetic magnetite to less magnetic hematite and goethite may have happened here.
CONCLUSIONS ÐThe presence of MD magnetite of assignable detrital origin was determined in the whole loess section (grain size not smaller than ®ne silt size, i.e., 8 mm). ÐIn paleosols a high coercivity fraction (probably goethite) of pedogenic origin was determined. ÐIn some levels with no visual evidence of the presence of soils, magnetic behavior analogous to the paleosols was observed. ÐThese mineralogical changes were interpreted to have been produced by more temperate climatic conditions (warm temperature and higher humidity) that modi®ed the physicochemical environment and led to the generation of a high coercivity magnetic fraction.
Mineral magnetic record of paleoclimate variations Acknowledgements Ð The authors thank the Consejo Nacional de Investigaciones Cienti®cas y Tecnologicas (CONICET), the Universidad de Buenos Aires (UBA), Argentina, and the Liverpool John Moores University, United Kingdom; and Dr Banerjee, Dr Verosub, Dr Rapalini and Dr Vizan for helpful reviews of the manuscript.
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Journal of South American Earth Sciences, Vol. 11, No. 6, pp. 561±570, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0895-9811/98/$ - see front matter S0895-9811(98)00038-8
Mineral magnetic record of paleoclimate variation in loess and paleosol from the Buenos Aires formation (Buenos Aires, Argentina) M.J. ORGEIRA*, 1,5A.M. WALTHER, 1,2,5C.A. VAÂSQUEZ, 5I. DI TOMMASO, 1,4 S. ALONSO, 3G. SHERWOOD, 3HU YUGUAN and 1,2,5J.F.A. VILAS
1,5
1
Consejo Nacional de Investigaciones Cienti®cas y TeÂcnicas de Argentina, Buenos Aires, Argentina Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina 3 Liverpool John Moores University, Liverpool, UK 4 Centro de Investigaciones en Recursos GeoloÂgicos, Buenos Aires , Argentina 5 Laboratory of Paleomagnetism ``Daniel Valencio'', FCEN, UBA, Ciudad Universitaria Pab II, 1428 Buenos Aires, Argentina 2
(Received December 1997; accepted September 1998) Abstract Ð Environmental magnetic techniques were applied to a loess±paleosol sequence of the Chacopampean plain (Buenos Aires, Argentina). Mineral magnetic carriers and their grain size were identi®ed in order to detect magnetic mineral ¯uctuations associated with climatic changes. Multidomain magnetite of detrital origin dominates the record. In paleosols, a high coercivity fraction was identi®ed. Horizons with no visual evidence of pedogenesis, but showing magnetic behavior analogous to that of paleosols were observed and are thought to represent environmental conditions similar to those prevailing during paleosol formation. The results suggest that the magnetic signal yielded by paleosols in these South American loess deposits is dierent from that in the Chinese loess. This may be due to dierences in parent materials, diagenetic processes and/or dierences in paleoclimatic conditions in both regions. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
intersection with Godoy Cruz street, in Buenos Aires city (34830'S, 58820'W; Fig. 1). A building excavation at this site disclosed a loess section assigned to the Buenos Aires Formation; no anthropogenic modi®cations were found within the sedimentary sequence. The Buenos Aires Formation was originally assigned by Frenguelli (1957) to the ``Bonaerense''. He described the ``Bonaerense'' as a thick homogeneous unstrati®ed loess unit, very ®ne grained, light reddish brown, with homogeneous distribution of calcareous nodules. Loess and paleosols are interbedded showing the alternating cold-arid, and warm-humidic periods.
Environmental magnetic studies have demonstrated that close connection exists between climatic changes and the ferrimagnetic pedogenic mineralogical fraction in loess/paleosol sequences (e.g., Banerjee et al., 1993; Maher and Thompson, 1991; Hunt et al., 1995; Verosub and Roberts, 1995). Environmental magnetic techniques are useful when magnetic minerals are present in low concentration which cannot be detected with common sedimentological techniques. The oxidation state of Fe is a good indicator of soil microclimates (Duchaufour, 1975); therefore, the characterization of the pedogenic magnetic fraction and the ¯uctuations in the detrital ferrimagnetic fractions throughout a sedimentary succession can provide excellent tools to understanding paleoenvironmental changes.
The studied sediments have been traditionally known as ``Bonaerense'', after Ameghino (1889) who used it as a synonym for the upper section of Pampean Formation. Parodi and Parodi (1952), formally named this unit the Buenos Aires Formation. Frenguelli (1957), presented a full description of this unit. He characterized it as a 6±7 m thick layer of aeolian origin, ®ne grained and homogeneous, light brown with reddish tints, extending all over the Pampean loess region.
The Chacopampean plain is one of the largest loess regions in the world. The origin of the pampean loess lies in a continuous dust input carried by west winds from the Andes Cordillera. However, there are no thick natural outcrops of loess: most useful sequences are exposed in excavations at building sites in Buenos Aires Province. The present contribution oers information from a subsoil pro®le of the Buenos Aires Formation, located on the CervinÄo street, near the
A ``Lujanense land mammal age'', tentatively assigned to upper Pleistocene, was proposed by Pascual et al. (1965) on the basis of fossil mammalians found within the unit. Fidalgo et al. (1975), reviewing previous works, found that most of the authors who had studied the
* Corresponding author. 561
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M. J. ORGEIRA et al. Glacial deposits (source of loess material) corresponding to those Oxygen stages have been found in several sites of the Andean range (Clapperton, 1993). However, lack of radiometric data severely hinders a precise correlation. The glacial chronology proposed for the Southern Andes and Southern Patagonia (Rabassa and Clapperton, 1990; Meglioli, 1994) identi®ed deposits which they related to 18O stages 2, 6, 10 and 12. Therefore, no precise correlation between the Buenos Aires Formation and 18O stages can be established upon the basis of the available information.
Fig. 1. Location map.
``Bonaerense'' or equivalent sedimentary units had coincided in assuming an Upper Pleistocene age for them. Tonni and Fidalgo (1978) proposed cold and dry weather conditions and a progressive aridization process that concluded in a pleniglacial stage of the late glaciation (LGM). Clapperton (1993), summarized most of the existing information on the Pampean loess formation (formally named by Teruggi, 1957), within which the ``Bonaerense'' would be the upper section. According to this author, the stratigraphy of the Pampean loess would suggest environmental conditions alternating between dry-windy intervals of loess deposition separated by more humid periods with dominant pedogenesis. Consequently, the episodes of loess have been correlated with Quaternary glaciation, and the humid intervals were assumed to correspond to interglacial or interstadial climatic conditions. On the other hand, paleomagnetic studies performed on the Buenos Aires Formation and other mammalianequivalent age-units (Orgeira, 1990) assigned a Brunhes magnetic age (<0.78 Ma) to the Lujanense land mammal age. The information available at present only allows us to ensure that the Buenos Aires formation was deposited during a middle to late Pleistocene glacial period younger than 0.78 Ma. On the other hand, studies by Cione and Tonni (1995) and Bonadonna et al. (1995), permit us to assume that the Lujan Formation (which overlays the Buenos Aires Formation on several sites throughout the Buenos Aires province), is related to the last glacial maximum and the last interglacial stage. As a consequence, the Buenos Aires Formation should be older than that glaciation, and younger than 0.78 Ma. Upon the basis of information obtained from ice core records and deep sea cores (Imbrie et al., 1992; Jozuel et al., 1993; Tzedakis, 1993; Waelbroeck et al., 1995; and others), cold periods during which the deposition of the Buenos Aires Formation could have taken place, would be equivalent to 18O stages 4, 6, 8, 10 or 12.
The study section of this contribution is 7.6 m thick, and has a Brunhes magnetic age, according to the paleomagnetic study carried out during the present research on samples of the same section. The paleomagnetic study included a traditional analysis of magnetic behaviour with stepwise thermal demagnetization. During the ®eld work, two paleosol levels were visually identi®ed based on geological criteria set by Andreis (1981). Brie¯y, the most important characteristics observed in the ®eld are the following: remarkable color change, prismatic soil structure, clayey texture and small holes left by roots (pedogenic tubes). Levels 13 and 14 are related to the lower paleosol and levels 50±54 to the upper paleosol (Fig. 2). The aim of this paper is to study the magnetic carriers of the Chacopampean loess in order to evaluate the action of climate proxies and the diagenetic processes that occur during pedogenic phases of pampean paleosol formation. In other words, to determine the presence of a magnetic signal associated with climatic changes in this type of loess.
SAMPLING TECHNIQUES Fifty-four consecutive levels at approximately 15 cm intervals were sampled. Bag samples (approximately 150 g) were collected, after cleaning the outcrop face. Subsamples from the bags were air-dried at room temperature at the laboratory and then weighed and packed into plastic boxes (2.5 cm of height and diameter) for magnetic analysis. The remainders were used for non-magnetic treatments. Measurements and analysis Magnetic susceptibility. Magnetic susceptibility at two frequencies (470 and 4700 Hz) was measured using a Bartington MS 2 susceptibilitimeter. A decrease in bulk magnetic susceptibility (Fig. 2a) takes place at both the lower (13 and 14) and upper (50±54) paleosol levels. Likewise, in other levels with no macroscopic pedogenic evidence, particularly at levels 1, 2 and 26, 27, analogous decreases are
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Fig. 2. Magnetic susceptibility at two frequencies (470 and 4700 Hz) (A) and percentage of paramagnetic contribution to bulk susceptibility (B) in a loess±paleosols section of Buenos Aires Formation, Argentina. L: loess levels. S: paleosol levels.
observed. No signi®cant dierences between measurements in frequencies were found (Fig. 2a). The F Factor is lower than 5% in all levels; consequently, no superparamagnetic fraction would be expected (Heller et al., 1991). However, this evidence would not be signi®cant to identify SP (Banerjee et al., 1993). On the other hand, measurements of paramagnetic susceptibility, coercivity and saturation magnetization, using a VSM (vibrating sample magnetometer) were performed at all levels, except odd numbered levels between 40 and 54. Figure 2b shows the variation in percentages of paramagnetic fraction ([paramagnetic susceptibility/total susceptibility] 100). It shows an increase of paramagnetic fraction in paleosol levels. Hysteresis loops and associated parameters. The hysteresis loops reveal the ¯uctuations in the amount of paramagnetic minerals throughout the pro®le, which has already been shown in the paramagnetic susceptibility pro®le. Figure 3 shows the hysteresis loops of samples from the loess level C30 (Fig. 3a) and paleosol level C51 (Fig. 3b). Figure 4a shows the saturation magnetization (Ms) values. This ®gure shows again a decrease of Ms in the paleosol levels, as well as in other levels in which decreases in the susceptibility were recorded. In a broad sense, these drops can be interpreted as a
decrease in the ferromagnetic (sensu lato) fraction in paleosol levels as well as in those with similar magnetic features (levels 1; 26 and 27; 40±45). The coercivity (Hc) shows a rise in the paleosol levels (Fig. 4b; levels 13 and 14, and 54) as well as in those levels with up to now similar magnetic behavior (2; 26 and 27; 42 and 43). At these levels, Hc reaches higher values than those expected for a ferrimagnetic fraction represented by MD magnetite (Roberts et al., 1995). This could mean that in paleosol levels, a mixture of low and high coercivity minerals is present. Isothermal remanent magnetization (IRM) and associated parameters. Samples from all levels were submitted to stepwise acquisition of IRM (from 5 mT to 2.3 T). Figure 5 displays the IRM acquisition curves of a group of representative samples from the sequence. The shape of the curves (i.e., saturation at ca 300 mT) indicates the dominance of the ferrimagnetic fraction (Dankers, 1978). IRM saturation values at 2.3 T (SIRM), displayed in Fig. 6a, show remarkable decreases in the paleosol levels as well as in levels 1 and 2, 26 and 27, 42 and 45. These drops can be related to a decrease in content of ferrimagnetic minerals. Back ®eld experiments were carried out on samples from ten levels, including paleosol levels 14 and 52 and others from levels with
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Fig. 3. Hysteresis loop of a loess sample (A) and of a paleosol sample (B) from a loess±paleosol section of the Buenos Aires Formation, Argentina.
no ®eld evidence of pedogenesis but with similar magnetic behavior (levels 1, 26 and 44), and loess levels 12, 24, 32, 36 and 42. The coercivity of remanence (Hcr, Fig. 6b) displays a remarkable rise in the paleosols (14, 52) and in levels with similar magnetic behavior (26, 42, 44, and to a lower extent in level 1). Hcr values at these levels are, in general, between 90 and 100 mT. They are higher than those expected for most ferrimagnetic minerals, such as magnetite or maghemite (Dankers, 1978; Roberts et al., 1995). These Hcr values are compatible with a mixture of a low coercivity fraction (magnetite) and a high coercivity fraction (hematite and/or goethite). In contrast, Hcr values from the loess levels, ranging between 40 and 50 mT, are compatible with magnetite (Dankers, 1978; Roberts et al., 1995). The S-ratio from the whole pro®le (IRM-300/ SIRM) is shown in Fig. 7a. The ratio in the lower and upper paleosol levels is less than 1. This observation can also be extended to levels 3, 26 and 27. This decrease is associated with a high coercivity fraction, which is present in paleosols and other levels with similar magnetic behavior (King and Channell, 1991; Verosub and Roberts, 1995).
The median acquisition ®eld of the SIRM (H'cr) is shown in Fig. 7b. the H'cr rises in the paleosol and falls in the loess levels. The paleosol H'cr value is higher than that expected for magnetite (Dankers, 1978). This is, again, probably due to the presence of a high coercivity mineral fraction. Finally, the median destructive ®eld (H1/2I) for the paleosol levels ranges between 40 and 50 mT; the H1/ 2I in loess layers is of about 15±25 mT. The value of this parameter in the paleosol levels also indicates the contribution of a high coercivity fraction (Dankers, 1978). Anhysteretic remanent magnetization. All samples were subjected to a d.c. bias ®eld of 0.1 mT in the presence of a decreasing alternating ®eld (peak AF of 100 mT). The anhysteretic remanent magnetization (ARM) is particularly sensitive to ferrimagnetic (low coercivity) single domain or pseudo-single domain grains (Banerjee et al., 1981; Verosub and Roberts, 1995). Decrease in Xarm (Fig. 8c) is associated with decrease in the amount of low coercivity material, which con®rms that both the paleosols and the levels with similar magnetic behavior contain lower concentrations of low coercivity minerals.
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Fig. 4. Saturation magnetization (Ms, A) and coercivity (Hc, B) in a loess±paleosol section of Buenos Aires Formation, Argentina. L: loess levels. S: paleosol levels.
Low temperature experiments. Samples were magnetized in a 1 T ®eld in liquid nitrogen (77 K), and the intensity of remanent magnetism was measured continuously up to room temperature (Fig. 8). In loess samples, the Verwey transition (Verwey and Haayman, 1941; Halgedahl and Jarrard, 1995) was observed.
However, it is not observed in the lower paleosol (level 13) and in a level with similar magnetic behavior (44). The Verwey transition (118 K) indicates a change in the crystal structure of magnetite from orthorhombic to monoclinic (Banerjee, 1991). As a consequence, the magnetization of MD magnetite is destroyed. These
Fig. 5. Isothermal Remanent Magnetization acquisition curves for some representative samples of loess and paleosols levels in a loess±paleosol section of the Buenos Aires Formation, Argentina.
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Fig. 6. Saturation of isothermal remanent magnetization (SIRM, A) and coercivity of remanence (Hcr, B) in a loess±paleosol section of the Buenos Aires Formation, Argentina. L: loess levels, S: paleosol levels.
Fig. 7. S ratio (IRM-300/SIRM, A), medium acquisition ®eld of SIRM (H'cr, B) and anhysteretic susceptibility (XARM, C) in a loess±paleosol section of Buenos Aires Formation, Argentina. L: loess levels. S: paleosol levels.
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Fig. 8. Low temperature demagnetization curves of IRM (1 T, 77 K) (see text for explanation).
results indicate that the ferrimagnetic fraction in the loess layers is dominated by MD magnetite (Banerjee, 1991) which is not detected in the studied paleosol levels.
Grain size of magnetic carriers. Figure 9 shows the SIRm/Ms vs Hcr/Hc ratios for a representative group of samples from paleosol and loess. Most of the samples display a behavior that is consistent with MD magnetite (Day et al., 1977). Observed Hcr/Hc values (between 5 and 15) could indicate a bimodal grain size distribution of magnetite; in this case, the coarser particles would constitute an overwhelmingly large fraction of the total volume of magnetite in the samples (Parry, 1982). Finally, SIRM/susceptibility ratios (Fig. 10) vary between a range that is compatible with samples in which the main component is MD magnetite (Thompson and Old®eld, 1986). All of the above magnetic determinations indicate the presence of multidomain magnetite. Multidomain behavior is attributed to grain sizes larger than 8 mm (Hunt et al., 1995), equivalent to ®ne silt-sized particles. Consequently, the magnetite detected along the pro®le is most likely of detrital origin.
DISCUSSION Fig. 9. SIRM/Ms vs Hcr/Hc of some representative samples of loess and paleosol levels.
As stated above, the paleosols (levels 13, 14 and 50±54) display a conspicuous magnetic behavior.
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M. J. ORGEIRA et al. altered loess levels (with decrease of detrital magnetite and an associated high coercivity fraction), climatic changes similar to those that produce the paleosols were recorded. Probably these processes were not long enough to be conspicuously recorded as paleosols. Agreement among magnetic parameters in the upper levels (42, 43, 44 and 45) is not as good as it is in the middle and lower levels. However, the Hc and Hcr from levels 42 and 44 reach values higher than in paleosols, which suggests the de®nite presence of a high coercivity fraction. The IRM curves from the loess levels, as well as Hc, Hcr, H'cr and the S-ratio, indicate the dominance of a low coercivity fraction represented by magnetite. These data and ratios (low temperature, Day diagram and SIRM/susceptibility) indicate that magnetite is multidomain, with a grain size not smaller than ®ne silt size (i.e., 8 mm), which denotes a detrital origin. Brie¯y, in the studied section two conspicuous magnetic signals were identi®ed. A pedogenic signal represented by a high coercivity fraction (goethite? and/ or hematite), and a detrital one, carried by MD magnetite.
Fig. 10. SIRM/Susceptibility ratio of the whole loess±paleosol section of the Buenos Aires Formation, Argentina.
Decreases in susceptibility, Ms, ARM and SIRM are associated with rises in Hcr, H'cr and Hc. In addition, S-ratios lower than 1, the lack of a Verwey transition, IRM acquisition curves and the mean values of Hcr, Hc, H1/2I and H'cr indicate that: Two distinctive magnetic fractions have been detected, a low coercivity fraction which is represented by magnetite (and/or titanomagnetite) and another high coercivity fraction. The decrease of magnetic parameters, such as susceptibility and intensity of SIRM, suggests a decrease in detrital magnetite content in the paleosol. Associated with this decrease, a high coercivity fraction is present. This association could indicate that under warm and humid climatic conditions, acidi®cation (by humic acids) and oxygenation of the environment can produce partial oxidation of the detrital magnetite and its replacement by a high coercivity fraction (pedogenic goethite? and/or hematite). Three layers with similar magnetic behavior to these paleosols were observed: a lower (1, 2 and 3), middle (26 and 27) and upper (42, 43, 44 and 45) level. In the lower and middle levels, the magnetic data are consistent with the presence of pedogenic processes. It is possible to suggest that in these less
Finally, these results suggest that the magnetic signal of pedogenic events in the South American loess is dierent from that in the northern Chinese loess (e.g. Banerjee et al., 1993). The cause of this dierent magnetic behavior could be the dierence in the parent materials in both loess (Frenguelli, 1957). Dierences in past climatic conditions could also produce the same dierences in magnetic signatures, as already recorded in the magnetic results from the northern and southern China loess (Han et al., 1996). Furthermore, the type of diagenesis, conversion of non-magnetic clays and weakly magnetic hematite to more magnetic magnetite, suered in China may not have occurred in Argentina loess. Only diagenesis of more magnetic magnetite to less magnetic hematite and goethite may have happened here.
CONCLUSIONS ÐThe presence of MD magnetite of assignable detrital origin was determined in the whole loess section (grain size not smaller than ®ne silt size, i.e., 8 mm). ÐIn paleosols a high coercivity fraction (probably goethite) of pedogenic origin was determined. ÐIn some levels with no visual evidence of the presence of soils, magnetic behavior analogous to the paleosols was observed. ÐThese mineralogical changes were interpreted to have been produced by more temperate climatic conditions (warm temperature and higher humidity) that modi®ed the physicochemical environment and led to the generation of a high coercivity magnetic fraction.
Mineral magnetic record of paleoclimate variations Acknowledgements Ð The authors thank the Consejo Nacional de Investigaciones Cienti®cas y Tecnologicas (CONICET), the Universidad de Buenos Aires (UBA), Argentina, and the Liverpool John Moores University, United Kingdom; and Dr Banerjee, Dr Verosub, Dr Rapalini and Dr Vizan for helpful reviews of the manuscript.
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