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Arabian Journal of Geosciences ISSN 1866-7511 Arab J GeosciDOI 10.1007/s12517-014-1328-8

Early Eocene paleosol developed frombasalt in southeastern Australia:implications for paleoclimate

Yu Zhou, Gregory John Retallack &Chengmin Huang

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ORIGINAL PAPER

Early Eocene paleosol developed from basalt in southeasternAustralia: implications for paleoclimate

Yu Zhou & Gregory John Retallack & Chengmin Huang

Received: 23 September 2013 /Accepted: 7 February 2014# Saudi Society for Geosciences 2014

Abstract The Early Eocene Climate Optimum (EECO, 51–53 Ma) is one of several short-term greenhouse spikes punc-tuating long-term Cenozoic paleoclimatic cooling. This studycharacterized major element geochemical and mineral com-position and micromorphological textures of the Ngaiur claypaleosol at Bridle Creek (Australia) formed in the early Eo-cene (∼52 Ma). The Ngaiur paleosol has unusually thick andstrongly developed soil horizons. Gibbsite and kaolinite arethe dominant minerals in the paleosol. The A horizon of thepaleosol has a pectic microtexture dominated by sesquioxidicspherical micropeds. The content of iron oxides is the highestand skeleton grains of primary minerals are least abundant inthe B horizon. A variety of weathering indices (includingchemical index of alteration without potash, weathering index,and silica/sesquioxide ratio) are evidence of intenseweathering. Alkalis and alkaline earths (Ca, Mg, K, and Na)were exhausted by weathering. Significant ferrallitization wasobserved with desilication of 85 % (Ti as immobile element).The paleosol is classified as an Oxisol, and is considered tohave formed under a hot and humid tropical climate. Such awarm, wet paleoclimate at a paleolatitude of 57°S in the earlyEocene (∼52 Ma) may be due to a greenhouse paleoclimaticanomaly at the time of the EECO oxygen isotope excursion.

Keywords Basalt . Oxisol . Paleoclimate . Early EoceneClimate Optimum (EECO) . Australia

Introduction

Paleosols buried within thick sequences of sediments and vol-canics can provide detailed archives of paleoclimatic conditions(Retallack 1992, 2008; Mack and James 1994; Retallack et al.2002; Sheldon 2003). Much is known about paleoclimaticcontrols on the morphology, chemical and mineral composi-tion, geochemical weathering trends, and micromorphology ofsoils (Jenny 1941; Birkeland 1999; Bockheim and Gennadiyeu2000; Rasmussen et al. 2010), and this information can be usedto interpret paleoclimate from comparable observations ofpaleosols (Retallack 2001; Sheldon and Tabor 2009).

Because basalts cover a terrestrial area of over 6.8×106 km2, or approximately 5 % of global terrestrial area(Dessert et al. 2003), much has been written on the geochem-istry, clay mineralogy, and carbon cycle implications of basaltweathering and soil development (Chesworth et al. 1981;Eggleton et al. 1987; Nesbitt andWilson 1992; Nieuwenhuyseand van Breemen 1997; Pillans 1997; Stefánsson andGíslason 2001; Dessert et al. 2003; Huang et al. 2004; Zhanget al. 2007; Rasmussen et al. 2010; Marin-Spiotta et al. 2011).Thick sequences of basaltic rocks commonly includepaleosols developed between flows (Singer and Ben-Dor1987; Rye and Holland 2000; Sheldon 2003; Retallack2008; Mitchell and Sheldon 2009; Jutras et al. 2012;Srivastava et al. 2012). Unlike paleosols in sedimentary se-quences, which are developed on sediment already weatheredto an indeterminate degree, the unweathered parent materialsof interbasaltic paleosols can be unambiguously identified inthin section, which is a great advantage in applying geochem-ical indices of weathering to paleoclimate reconstruction (Hillet al. 2000a; Sheldon et al. 2002; Sheldon 2003; Tabor et al.2004; Ghosh et al. 2006; Retallack 2008; Driese et al. 2011).

Here, a single strongly developed interbasaltic paleosol fromthe early Eocene Monaro Volcanics in southeastern Australiawas selected for detailed study (Fig. 1). The Monaro Volcanics

Y. Zhou : C. Huang (*)Department of Environmental Science and Engineering, SichuanUniversity, Chengdu, Sichuan 610065, Chinae-mail: huangcm@scu.edu.cn

C. Huange-mail: cmhuangscu@gmail.com

G. J. RetallackDepartment of Geological Sciences, University of Oregon, Eugene,OR 97403, USA

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aremainly basalts, with a total volume of∼630 km3, covering anarea of∼4,200 km2, to a depth of over 100m, and ranging in agefrom latest Paleocene to early Oligocene (Taylor et al. 1990;Roach 2004; Sharp 2004; Retallack 2008). The particularpaleosol studied in outcrop is correlative with the Ngaiurpedotype profile dated at 52 Ma (early Eocene) by interpolationof an age model of Monaro Volcanics based on radiometric (K-Ar) ages of basalt flows at different stratigraphic level within thenearby Bega 7 core (Retallack 2008). Paleolatitude of theMonaro Volcanics in the early Eocene was 57°±4°S (Idnurm1985). This research aims (1) to carry out a comprehensivepedogenetical study on elemental geochemisty and micromor-phology aswell asmineral composition and (2) to reconstruct theregional paleoclimate at the formation of this paleosol profile.

Materials and methods

Sample collection

A paleosol in the early Eocene Monaro Volcanics was sampledfrom 200m east of Bridle Creek (36°13′ S, 148°58′E, elevationof 955 m) along the road between Cooma and Adaminaby,13 km northwest of Cooma (Fig. 1). This is an unusually thick(7 m) bauxite-bearing laterite and was sampled at 13 levels,

with morphological description of the horizons made in thefield (Fig. 2). The boundaries between the different paleosolhorizons are delineated based on the variation of the color (hue,value, and chroma), texture (clay, silt, and sand), abundance ofbasalt fragments, plasticity between the fingers and traces ofroots and burrows along this paleosol profile.

Analytical methods

The bulk density of the samples was determined using the clodmethod with paraffin (Retallack 1997). The Na2CO3 fusionmethod was used to determine the loss on ignition and bulkcomposition of SiO2, Al2O3, Fe2O3, CaO, Na2O, MgO, K2O,and TiO2 in the samples (Rettig et al. 1983), and the analysiswas executed at Institute of Mountain Hazards and Environ-ment, Chinese Academy of Sciences. The content of traceelements was measured on ICP-MS (Canadian Perkin ElmerCompany ELAN DRC-e type) at Institute of Geochemistry,Chinese Academy of Sciences. The mineral composition ofpaleosols and basalts was identified using X-ray diffractometer(XRD) model X-Pert of the Philips Company under the voltageof 50 kV and current of 35 mA at Sichuan University, China.After the production of petrographic thin sections, a NikonE600POL polarizing microscope was used to observe andphotograph micromorphological characteristics at the State

NorthernTerritory

Western Australia

SouthAustralia

New SouthWales

Queensland

Victoria

Adaminaby

Cooma km 200 10

Location of theBridle Creek

Monaro basalts

N

Bridle Creek

Bega No.7 core

36°00 S

149°00 E

36°30 S

149°30 E

Fig. 1 Sampling location and geological map (modified after Taylor et al. (1992) and Retallack (2008))

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Key Laboratory of Oil and Gas Reservoir Geology and Exploi-tation, Chengdu University of Technology, China.

Proxies for assessing the degree of basalt weathering and soildevelopment

Reddening rate (R)

Reddening rate values can be calculated using the followingequation (Torrent et al. 1980, 1983):

R ¼ 12:5−Hð Þ � C=V ð1Þ

whereH,C, and V represent soil hue, chroma, and value in theMunsell color system, respectively. Here, hues 2.5Y, 10YR,7.5YR, 5YR, 2.5YR, and 10R were set as 12.5, 10, 7.5, 5, 2.5,and 0, respectively. The larger the reddening rate value, thehigher the hematite content in the soil (Torrent et al. 1983).

Chemical index of alteration minus potassium (C)

Chemical index of alteration minus potassium (C) is an indexof chemical weathering devised by Maynard (1992) forpaleosols in which potash may be enriched during burialdiagenesis. It is based on the usual trajectory of hydrolyticweathering of feldspars, which enriches alumina in clays, andmobilizes alkalis and alkaline earths as soluble cations. Thisindex is calculated from molar proportions of major elementoxides according to the following formula:

C ¼ Al2O3= Al2O3 þ CaOþ Na2Oð Þ½ � � 100 ð2Þ

Weathering index (W)

Weathering index is another index of hydrolytic alteration offeldspars, calculated by the following expressions (Price et al.1991), accounting for losses of cations and gain of alumina ina sample compared with its parent material.

W ¼ Asample=Aparent material ð3Þ

A ¼ Al2O3= CaOþ Na2Oð Þ ð4Þ

Silica/sesquioxide ratio (S)

Silica/sesquioxide ratio is the molar ratio of silica (SiO2) tosesquioxides (Fe2O3, Al2O3) in soils or rocks, useful as aguide to the relative importance of weathering residues suchas clay and hematite on the one hand (enriched by non-acidicor weakly acidic intense weathering processes such asferrallization) and quartz on the other hand (enriched byhighly acidic weathering processes such as podzolization)(Jenny 1941; Retallack 2001).

S ¼ SiO2= Fe2O3 þ Al2O3ð Þ ð5Þ

Element migration

Losses and gains of particular elements during weathering canbe calculated by comparison with an element assumed to havebeen immobile (Brimhall et al. 1991; Merrits et al. 1992;Langley-Turbaugh and Bockheim 1998; Bern et al. 2011;Ma et al. 2012). Here, Ti was selected as the immobile elementbecause it is mainly in weather-resistant minerals such as

Fig. 2 Bulk density, R and clayminerals within the paleosolprofile. K, G, Gt, Hm, Py, Pl, andBi, respectively representkaolinite, gibbsite, goethite,hematite, pyroxene, plagioclase,and biotite. R representsreddening rate

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ilmenite (Hill et al. 2000b; Huang et al. 2004). The enrichmentor loss as a percentage of any element (ΔC) in a soil horizonrelative to its concentration in the parent rock can be given by:

ΔC %ð Þ ¼ Cs=I sð Þ= Cr=I rð Þ−1½ � � 100 ð6Þ

In Eq. 6, Cs and Cr are the concentrations of a chosenelement in soil and parent rock, respectively, and Is and Irare the concentrations of Ti in soil and rock. Cases of positiveΔC indicate accumulation of the element relative to Ti duringthe soil-forming process, and negative values indicate lossesof the element.

Pedogenic features of paleosol

Morphology

The hue of the A and B horizons in paleosol profile ranges5YR-10YR and 2.5YR-5YR, respectively, and the reddeningrate value is between 8 and 12 except for two subhorizons(BC2 and BC3). Within the C horizon, the soil hue is 10YRwhich decreased reddening rate to 2.5, and less in areas of lessweathered basaltic corestones (BC10, BC11 subhorizons).The hue of rock fragments is mainly 5BG, and the infillingin the pores between the fragments is principally brown andthe RR value is between 0 and 2 (Fig. 2).

Bulk density increases with the increasing depth of the soilprofile, and the values within the A and B horizons are lowerthan those in the C and CR horizons and the fresh basalts. Thebulk density for the unweathered basalts (BC12, BC13, andBC14) is ∼2.80 g/cm3, and that in the C horizon is 2.0 g/cm3,but for the saprolites (CR horizon, BC10, BC11) it is between2.40 and 2.60 g/cm3 (Fig. 2). Although the value in the A andB horizons is the lowest in the profile (1.65–2.0 g/cm3), it isstill higher than that in modern soils of the same type, becausecompaction and diagenesis after the burial of paleosol as voidspaces, organisms, and water within the soil are crushed by theweight of overlying strata (Birkeland 1999; Retallack 1991a,2001; Sheldon and Retallack 2001).

Mineralogy

Kaolinite is the predominant mineral in the A horizon of thispaleosol, followed by gibbsite. Despite the high reddening ratevalue, hematite was undetected by XRD in the A horizon, due tothe low degree of iron oxide crystallization. Easily weatheringminerals such as plagioclase and pyroxene were not detected inthe A horizon (Fig. 2). In the B horizon, the major mineralsinclude gibbsite, kaolinite, hematite, and goethite. Gibbsite isdominant, and kaolinite is less common than in the A horizon.Moreover, crystallized goethite and hematite were identifiedusing XRD, and goethite content is higher than hematite (Fig. 2).

Plagioclase, pyroxene, and biotite are major minerals in theC horizon, where pedogenic kaolinite and gibbsite were notdetected, but hematite was found. In saprolites (BC10 andBC11) and fresh basalts (BC12, BC13, and BC14), mainlyprimary basaltic minerals, such as plagioclase, biotite, andpyroxene, were detected (Fig. 2).

Micromorphology

The paleosol A horizon has a highly pectic micromorphology(Cao 1986), characterized by microaggregates consistingmainly of sand and silt size nodules of clay and of iron oxides(Fig. 3a–b), known as spherical micropeds (Eswaran 1972;Retallack 1991b). The B horizon has abundant blockypeds of brown-red iron and clay and gibbsite resultedfrom highly oxidization in matrix, mostly with a lengthor width over 200 μm (Fig. 3c–f). Soil matrix of the Bhorizon is dark red, due to strong rubification, andreflects the presence of large amounts of hematite(Cao 1986; Dick and Schwertmann 1996). Weatheringin the C horizon is apparent as dark red and rusty ironoxide alteration rinds (diffusion sesquans) on the surfaceand edge of the mineral grains (Fig. 3g). Unweatheredplagioclase, pyroxene, and other primary minerals wereobserved in the fresh basalt (Fig. 3h).

Elemental geochemistry

Element content

Silica content and silica/sesquioxide ratios are high and variedslightly in the C horizon, saprolite, and fresh bedrock (Fig. 4).Silica and silica/sesquioxide ratios are much lower in the

�Fig. 3 Micromorphological features in various horizons within thepaleosol profile. a Highly pectic matrix with ferric concretions (Fe),spherical grains (fecal pellets) (Sg) and numerous voids. Microcrystallinequartz grains (Q) were also observed (BC1, 0–20 cm in depth, plane-polarized light). b Highly pectic matrix with numerous voids and moreand larger ferric concretions (Fe) than those in Sample BC1, and thespherical fecal pellets (Sg) are distinctively observed (BC2, 20–45 cm indepth, plane-polarized light). c Nodules and concretions of iron oxides(Fe) affected by aging gelatination due to the contraction and coagulationunder the alternation of wetting and drying, and no weatherable mineralsare observed (BC3, 45–119 cm in depth, plane-polarized light). d Blockyand circular nodules and concretions of iron oxides (Fe), and noweatherable minerals are observed (BC4, 119–178 cm in depth, plane-polarized light). e Agglutinic iron oxides predominant in matrix with fewweatherable minerals and numerous and large voids (BC5, 178–305 cmin depth, plane-polarized light). f Aging coagulation of iron oxides withfew weatherable minerals (BC7, 305–420 cm in depth, plane-polarizedlight). gWeathering alteration occurred on edge of primary minerals likeplagioclase, and iron oxides observed on mineral surface and edge (BC9,530–700 cm in depth, cross-polarized light). h Plagioclase, pyroxene, andother mineral are in well-crystallized form in fresh basalt, and noweathering evidence in primary minerals is observed (BC13, 700–900 cm in depth, cross-polarized light)

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paleosol than the parent material, and the lowest values are inthe B horizon. Higher silica and silica/sesquioxide ratios in theA than B horizon may be due to eolian and other sedimentaryadditions during soil formation (Fig. 3a) (Merrits et al. 1992;Porder et al. 2007; Yoo et al. 2007), because sedimentaryrocks associated with the Monaro Volcanics are quite quartzrich (Retallack 2008).

Both chemical weathering indices are the lowest in thesaprolites and fresh bedrocks (C<6 and W<1). These ratiosare much higher within the A and B horizons, where theyexceed 7 and 15, respectively (Fig. 4).

Particular ratios of trace elements, such as Sr/Be (Huanget al. 2004) and Ba/Nb (Price et al. 1991; Huang et al. 2004)also can be used to assess weathering intensity of soils devel-oped from basalt. During weathering, Sr and Ba are dissolved,Ba less readily than Sr, while Be and Nb are stable. Asexpected, Sr/Be and Ba/Nb were lower in the paleosol thanin the bedrock. In the C horizon, saprolite and fresh bedrock ofthe Bridle Creek paleosol profile, Sr/Be value is 350, whilethat in the A and B horizons is ∼10 (Fig. 4). During the initial

phase of basalt weathering, Ba is enriched relative to Nb (Priceet al. 1991), so that Ba/Nb ratios in the C horizon and sapro-lites are higher than those of both fresh basalts and in the Aand B horizons (Fig. 4). Barium leaching in soils may beresponsible for lower values in the paleosol than basalt(Huang et al. 2004).

Major element migration and redistribution

Intense chemical weathering of the Bridle Creek paleosol isindicated by profound mobilization and enrichment of majorelements relative to Ti. Depletion of (ΔC) of alkali and alkali-earth metals is the largest. Depletion of Ca and Mg exceeds94 %. With the exception of K in the A horizon, the mobilityrate of K andNa is 81 to 97%. Themobility of P andMn is alsoremarkable: ΔC=31–82 and 50–89 %, respectively (Table 1).Desilication was also profound, ranging from 61 to 85 %, andwas also supported by petrographic evidence that the ratio ofsilicates to the minerals dramatically decreased in the A and Bhorizons relative to the core basalt (Fig. 2). Al and Fe are often

Fig. 4 Variation in SiO2 content,chemical weathering indices,Sr/Be and Ba/Nb within thevarious horizons. S, C, Wrepresent silica/sesquioxide ratio,chemical index of alterationminus potassium, and weatheringindex, respectively

Table 1 Variation rate of majorelements within the paleosolprofile

The mean value in basalt (BC12and BC13) is selected as theelement content in parent rock tocalculate the ΔC value

Sample no. Soilhorizon

ΔC (%)

Si Al Fe K Na Ca Mg Mn P

BC1 A −63 −1 −11 −67 −92 −97 −95 −76 −82BC2 −63 7 −14 −67 −96 −98 −95 −86 −81BC3 B −85 25 28 −95 −87 −98 −94 −50 −31BC4 −78 52 −19 −81 −86 −97 −97 −86 −63BC5 −81 2 1 −97 −82 −98 −97 −89 −71BC6 −61 3 40 −96 −97 −96 −94 −81 −45BC7 −71 −20 −8 −96 −96 −97 −95 −67 −55BC9 C −25 0 −39 −11 −92 −57 −70 −39 1

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conservative elements, but may accumulate in the solum (Aand B horizons) during chemical weathering of fresh basalt(Bain et al. 1980; Chesworth et al. 1981; Nesbitt and Wilson1992). Presumably ΔC value of Al and Fe should be positive.However,ΔC varied within negative and positive values in thedifferent samples of the paleosol, and the largest enrichment ofFe and Al in the B horizons of the soil profile reached 40 and52 %, respectively (Table 1). Sesquioxides (Al and Fe) arecommonly enriched and depleted in different horizons of thesoils derived from basalt (Sanematsu et al. 2011), syenite(Braun et al. 1993), or sediments (Merrits et al. 1992). Titaniacan also be mobilized and fractionated in different parts of soilprofiles during the intensive weathering (Braun et al. 1993; Cheet al. 2012).

Early Eocene paleoclimate at high paleolatitudein the southern hemisphere

Gibbsite and kaolinite dominate with common or few goethiteand hematite in the B horizon of Bridle Creek paleosol (Fig. 2)and are the typical clay minerals of Oxisols (ferrallitic soils) intropical regions (Eswaran et al. 1977; Eggleton et al. 1987;Schwartmann and Taylor 1989; van Wambeke 1992; Goulartet al. 1998; Birkeland 1999; Bockheim and Gennadiyeu 2000;Huang et al. 2002; Retallack 2010). Solum (A plus B horizon)thickness of the Bridle Creek paleosol is also exceptional (∼3 m)compared with other paleosols (Alfisol, Inceptisol, Entisol, Ulti-sol in USDA Soil Taxonomy ) with a thickness of no more than∼1.5 m in the Monaro Volcanics, and supports identification ofthis Ngaiur pedotype paleosol as an Oxisol (Retallack 2008).Micromorphologically, the presence of highly pectic matrix insoils is the specific feature in ferrallitic soils developed frommetamorphosed ultrabasic rocks such as serpentinite and basicrocks such as basalt (Eswaran 1972; Cao 1986). Sphericalmicropeds like those observed in the paleosol are commonlyfecal and oral pellets of ground-dwelling termites, which arerestricted in distribution to the tropical climates which favor theformation of Oxisols (Retallack 1991b).

Intense weathering of a warm wet paleoclimate is alsosupported by strong leaching of alkali and alkaline earthelements, as well as an array of weathering indices (chemicalindex of alteration and weathering index) and selected ele-mental ratios (Sr/Be and Ba/Nb). Silica/sesquioxide ratios areevidence of warm and wet soil-forming processes offerrallitization at moderate pH, rather than the cold and wethighly acidic podzolization (Retallack 2001). The highestmobility rate for Si in the paleosol reaches 85 % with anaverage value of ∼80 %. This is comparable with 80 % (Tias the immobile element) in Oxisols on basalt some 1 millionyears old in northern Hainan Island under mean annual tem-perature of 23–24 °C and mean annual precipitation of 1,400–1,800 mm (Huang et al. 2004). In the perhumid tropics of

Costa Rica with mean annual temperature of 25 to 26 °C andmean annual precipitation of 3,500 to 5,500 mm, the maximalmobility rate of Si also is just ∼85% in the soil, with an age of450 ka, formed from basaltic andesite (Nieuwenhuyse andVan Breemen 1997).

Thus, the Bridle Creek paleosol is like other Ngaiurpaleosols of the Monaro Volcanics formed under a hot andhumid climate. The paleosol is currently at a latitude of 36°S,but formed much further south before the northward conti-nental drift of Australia away fromAntarctica. A paleolatitudeof 57°S during the early Eocene has been calculated from thepaleomagnetic inclination for the Monaro Volcanics (Idnurm1985). The presence of such an intensely weathered ferralliticpaleosol at high paleolatitude is anomalous because intenselyweathered ferrallitic soils are not found at such high latitudes.Although it has been proposed the highly weathered paleosols(laterites and bauxites) in the Monaro Volcanics were formedunder cool climate over long periods of time (Taylor et al.1992), this view is falsified by the known eruption frequencyof the volcanic sequence, which includes many less weatheredtemperate paleosols and only a few anomalous tropicalpaleosols (Retallack 2008). Modern Oxisols are developedon ice-free land, ∼98 % of which occur in the tropical zone,and are not found under cold and wet conditions (Birkeland1999; Soil Survey Staff 1999; Ugolini and Dahlgren 2003).Oxisols developed from basalt have not yet been reported inthe humid subtropical climate of China, where gibbsite is rarein Ultisols (Wu and Lu 1989; Zhang et al. 1998). Under aclimate with plentiful precipitation and low temperature,gibbsite may be produced by means of hydrolysis and aggre-gation of free Al3+ because Si(OH)4 from parent materials isquickly separated from Al3+ resulting from intensive eluvia-tion due to abundant rainfall (Huang and Gong 2000). How-ever, losses of Ca2+, Mg2+, Si4+and other ions may be minorunder this acidic cool environment (Ezzaïm et al. 1999; Huangand Gong 2000; Rasmussen et al. 2010), unlike rapid andsignificant loss of these cations under a climate with abundantprecipitation but hot temperature (e.g. Brimhall et al. 1991;Huang et al. 2004; Che et al. 2012).

In summary, gibbsite forms under high precipitation, butthorough loss of alkali and alkaline-earth metals and theintense eluviation of Si in Oxisols requires warm temperatureas well. An Oxisol formation at the high latitude (57°S) in theearly Eocene (∼52 Ma) can be explained by the global peakwarming during The Early Eocene Climate Optimum. Theglobal temperature in the EECOwas highest since 65Ma, andsea surface temperatures in high latitudes were 14–16 °Cgreater than current temperatures whilst the poles were ice-free during this episode (Zachos et al. 2001, 2008; Yapp 2008;Pross et al. 2012). The EECO led to extensive distribution oflaterites at high latitudes, including the Bighorn Basin (Krausand Riggins 2007), the Williston Basin (Clechenko et al.2007) and Sierra Nevada (Yapp 2008) in the USA, Patagonia

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in Argentina (Krause et al. 2010), and Soriano and Durazno inUruguay (Genise et al. 2011).

Conclusions

This study characterized major element geochemical, mineral-ogical, morphological, and micromorphological features of anintrabasaltic lateritic paleosol in outcrop near Bridle Creek,New South Wales Australia (Fig. 1). This thick profile (∼7 m)is correlated with the Ngaiur pedotype of theMonaro Volcanicsin the Bega 7 core nearby (Retallack 2008), where it is dated byinterpolation of radiometric ages as the early Eocene (∼52 Ma).

The difference between pedogenetic horizons is clear withinthe paleosol profile. The bulk density in the A and B horizons isbetween 1.65 and 2.0 g/cm3, much lower than that in the freshbedrock as 2.80 g/cm3. The hue of the A and B horizons rangesbetween 10YR and 2.5YRwhile the reddening rate value is 8 to12, in contrast with values of the fresh bedrock which is only 0to 2. Gibbsite and kaolinite are the dominant minerals inpaleosols. Highly pectic matrix micromorphology (Cao 1986)with spherical micropeds (Retallack 1991b) was observed inthe A horizon, but the B horizon has the highest content of ironoxides (Fig. 2) and no skeleton grains of feldspar or pyroxene(Fig. 3c–f). A variety of weathering indexes reflect intenseweathering and strong ferrallitization in this paleosol. Themaximal migration rate of Si was 85 % (Ti as immobileelement). This paleosol is classified as an Oxisol, and formedin a hot and humid tropical climate, like that of comparablemodern soils in Hainan (Huang et al. 2004) and Costa Rica(Nieuwenhuyse and Van Breemen 1997). This hot and humidclimate for a short time between lava flow eruptions atpaleolatitude (57 ˚S) during the early Eocene (∼52 Ma)(Retallack 2008) is supported by a transient warm- wet climaticspike (EECO) recognized from stable isotopic anomalies indeep sea cores (Zachos et al. 2001, 2008).

Acknowledgements This study was funded partly by the NationalBasic Research Program of China (grant no. 2012CB822003) and theNational Natural Science Foundation of China (grant no. 41371225).

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