Metamorphosed palaeosols associated with Cretaceous fossil forests, Alexander Island, Antarctica

Fossil soils (palaeosols) are excellent indicators of ancient terrestrial environments. They not only provide evidence for ancient vegetation and ecosystems, but also contain signals of palaeoclimates that are independent of evidence from other associated sediments and fossils (Retallack 2001). Palaeosols are often difficult to identify because the soil features can be subtly defined, and obvious features such as in situ plants and diagnostic soil structures such as peds are often not well preserved. In addition, later diagenetic and metamorphic processes can destroy or modify original soil fabrics.

Fossil soils of mid-Cretaceous age on Alexander Island, Antarctica, are exceptionally well preserved. They contain large upright fossil trees in their original growth positions with their roots within the fossil soils. Distinctive soil structures such as peds and soil horizons are apparent, and even though the palaeosols have suffered some burial metamorphism, the original soil structures have not been destroyed. Despite this good preservation, the palaeosols have not been well described nor their important palaeoenvironmental record properly deciphered, even though the fossil plants and associated sediments have received attention (e.g. Jefferson 1982a; Moncrieff & Kelly 1993; Falcon-Lang et al. 2001; Nichols & Cantrill 2002).

This paper presents detailed descriptions of the palaeosols and their environmental significance, plus deductions about regional burial regimes from the metamorphic overprint.

Geological setting

Alexander Island is situated off the west coast of the Antarctic Peninsula at c. 70°S, 70°W (Fig. 1a). The island contains a suite of rocks deposited in a forearc setting to the west of a magmatic arc, formed as the Pacific plate was subducted beneath Antarctica (Barker et al. 1991). The sequence of Mesozoic rocks on Alexander Island includes the Upper Jurassic-Lower Cretaceous Fossil Bluff Group, a series of marine, fluviatile and deltaic sediments that formed as the infill of the subsiding forearc basin (Butterworth et al. 1988). They are dominated by volcaniclastic components, derived from the eroding volcanic arc.

The fossil forests and their palaeosols are preserved within the 800-1000m thick Triton Point Formation of the Fossil Bluff Group in the SE of Alexander Island (Fig. 1b). These sediments represent braided river systems (Citadel Bastion Member) and meandering river systems (Coal Nunatak Member) with broad river belts and extensive floodplains (Cantrill & Nichols 1996; Nichols & Cantrill 2002; Fig. 2). Both the floodplain and midchannel bars of the wide river channels supported diverse vegetation, the remains of which are now preserved as in situ fossil tree stumps and rooted plants.

In southeastern Alexander Island the Citadel Bastion Member crops out on Citadel Bastion, Titan Nunataks and the basal 60 m of Coal Nunatak (Fig. Ic). The Coal Nunatak Member crops out on the upper sections of Coal Nunatak. The strata dip c. 40 -10° to the SE.

Both the Citadel Bastion and Coal Nunatak members are dominated by sequences of channel and over-bank sandstones (Fig. 3). The over-bank sandstones are up to 10m thick, are typically featureless and have sharp, non-erosive bases. These are interpreted as sheet flood and crevasse splay deposits and indicate that the river systems were prone to flooding (Nichols & Cantrill 2002). These sandstones encase the trunks of upright fossil trees (Fig. 4a), their roots preserved in palaeosols below. Individual channel sandstones range from 1 to 4 m thick and form units up to 15m thick. They characteristically have scoured erosional bases with rip-up clasts. In the Citadel Bastion Member the channel sandstones form ribbon deposits characteristic of braided river systems but in the overlying Coal Nunatak Member they have lenticular morphologies typical of meandering river systems (Reading 1986; Cantrill & Nichols 1996). No in situ fossilized trees were found preserved within channel sandstone bodies.

Between the sandstone units are carbonaceous-rich finegrained sandstones and siltstones (Fig. 3) that represent deposition from sediment-laden floodwaters on the floodplains. The 0.2-0.75 m thick sandstones are finely laminated and contain abundant plant debris and some fossil tree trunks. The siltstones are also finely laminated, the laminations often draped over fossil plant material that is preserved in situ in palaeosols below. Palaeosol horizons are interbedded with these units, representing the pedogenic alteration of exposed floodplain sediments.

Sediments of the Triton Point Formation contain volcaniclastic components with palaeocurrent structures indicative of palaeoflow towards the SW, suggesting sediment supply from the magmatic arc. During the mid-Cretaceous a phase of plutonism in Palmer Land caused uplift and erosion that supplied sediment from the arc. Coeval volcanism supplied air-fall ashes that now occur as beds of tuff within the sediments (Vaughan 1995).

The Fossil Bluff Group is considered late Albian in age, being under- and overlain by marine sequences containing faunas that indicate a late Albian age (Kelly & Moncrieff 1992). At this time Alexander Island was situated at about 70°S palaeolatitude (Lawvere et al. 1985).

The palaeosols

Field and petrographie descriptions

Over 90 palaeosol horizons were identified throughout the strata on Citadel Bastion, Titan Nunataks and Coal Nunatak on Alexander Island. Palaeosol horizons were identified using the following criteria: (1) the presence of in situ tree trunks and other plants with rootlets in the palaeosols; (2) recognizable palaeosol features such as ped structures; (3) distinctive organicrich layers containing plant material. The palaeosols have a generally consistent structure throughout all exposures, although some profiles appear less mature than others. Where well developed, the palaeosols consist of two distinct horizons, an upper and a lower horizon.

Upper horizon. The upper horizon is black or dark brown in colour, and varies in thickness from 0.05 to 0.6 m with lateral undulations (Fig. 4b). These horizons are mainly massive in structure but with a crumbly texture. Occasional fine laminations of aligned organic material are present but in general all sedimentary structures have been lost. The upper horizon is frequently mottled (mottled areas average 3-4 mm long and 26 mm wide) by structures that could be burrows or rootlets, filled with white, fine-grained sandstone or brown clay.

The matrix is dominantly organic- or clay-rich, with either an agglomeroplasmic fabric (local matrix areas surrounding skeletal grains; terminology of Brewer 1976) or porphyroskelic fabric (larger grains set within a fine matrix; Brewer 1976). In places the matrix contains the clay mineral smectite, which replaces some mineral grains. Detrital grains of quartz, altered feldspars, laumontite, chlorite, volcanic glass shards and rock fragments make up 5-45% of this horizon. The chlorite crystals are iron-rich and are often split and contorted. In addition, they are also sometimes aligned (skelsepic fabric), the result of realignment caused by percolating water.

Organic matter (5-40%) occurs either as brown amorphous material or as identifiable plant structures, such as branched rootlets (Fig. 5a). In most of the upper horizon large pieces of plant debris are coated by laumontite. Laumontite also occurs as small particles within the matrix.

Lower horizon. The lower horizon typically consists of lightcoloured, fine- to coarse-grained sandstone. This horizon is massive, up to 2 m thick and occasionally banded (20 mm wide bands) as a result of variations in grain size. These sandstones are similar to the fluvial sandstones common throughout the sequence. The upper parts of the lower horizon are commonly darker than the sediment below and are cracked and jointed, forming well-defined ped structures (Fig. 6a). The joints around the peds extend vertically for 18-30 mm.

Within the palaeosol horizons on Alexander Island two types of ped structures were seen: (1) prismatic peds, taller than they are wide, with flat tops and of medium size with diameters of 20-50 mm (Fig. 6a); (2) angular blocky peds that are more irregular in shape but have interlocking faces, occurring as both fine in size with a diameter of 5-10 mm and very coarse with a diameter >50 mm (using the classification scheme of Retallack 2001). Blocky peds probably resulted from both cracks that formed around rootlets and the presence of swelling clays.

Mottling (irregular patterns of two or more colours with sharp boundaries) is also common in the lower horizon, occurring as grey or white patches and elongate interconnected areas. Using the classification of the Soil Survey Staff (1975), the mottles can be described as prominent (outstanding feature of the horizon), many (occupying more than 20% of the exposed surface) and medium (5-15 mm) to coarse (15-20 mm) in size.

The lower horizon is generally dominated by mineral grains and has a granular fabric (skeletal grains touching with little matrix; Brewer 1976) to intertextic fabric (skeletal grains dominant with intergranular braces of matrix; Brewer 1976). Quartz is the dominant mineral, commonly forming 40-50% of the unit. The feldspars (30-40%) are often weakly to moderately altered to smectite with the loss of the original crystal shape. Other minor components of the lower horizon with variable abundances include organic matter, volcanic rock fragments, chlorite, a laumontite or calcite matrix, and the metamorphic minerals laumontite and prehnite.

The organic matter is often scattered throughout the rock, forming fine (<1 mm) discontinuous laminae and occasionally forming concentrations. Fossil plant material and rootlets are also scattered throughout the finer-grained matrix, with cell anatomy visible in some plant fragments. Rootlets commonly occur as black carbonaceous material extending for <10 mm, either down from the upper surface or running parallel to bedding.

Volcanic rock fragments are sub-angular to sub-rounded in shape. The chlorite crystals are iron-rich and occur in a hydrated state, contorted and exfoliated and regularly split, with laumontite growing between the split sections (Fig. 5b). In places they occur parallel to bedding, giving the rock a laminated appearance, or concentrated around rootlet traces and voids. The laumontite or calcite matrix is a minor component of the lower horizon and fills gaps between grains or occurs as plugs and coatings.

Within the matrix, clay minerals (smectite, illite and chlorite) occur with a skelsepic microfabric (highly birefringent clay crystals growing parallel to mineral grains; Brewer 1976), occasionally forming embedded grain or ped cutans (concentration of a soil constituent or in situ modification forming a 'skin' around a grain or ped surface; Brewer 1976).

Significant environmental features

The palaeosols described above contain important features comparable with those of modern soils, such as rootlet horizons, ped structures and mottling. These features provide important evidence for environmental conditions and their significance is discussed in more detail below.

Rootlet horizons. Nearly all palaeosols examined on Alexander Island are associated with large in situ tree stumps, finer rootlets or rootlet traces. The tree roots branch and extend down into the palaeosols to a depth of =1 m (commonly 0.2-0.3 m) (Fig. 6b), and radiate from single tree trunks for up to 2 m, frequently on a horizontal plane. Finer rootlets, up to 10mm in diameter, were found concentrated in rootlet masses (150mm thick) directly beneath the upper horizon, commonly visible as dark, organicrich material within the lighter-coloured lower horizon.

The presence of such a diverse range of roots confirms that these palaeosols were fertile and supported substantial vegetation. Plant roots need oxygen for the uptake of water and nutrients and so do not penetrate below the water table (apart from those such as mangrove types with special adaptations, not seen here). Fossil roots penetrating to a depth of 1 m suggest that the Cretaceous water table was at least 1 m below the surface. Waterlogging as a result of flooding clearly did not last for such significant periods of time that plant growth was inhibited.

Ped structures. Peds are blocky structures formed in soils as a result of wetting and drying processes under seasonal climate regimes (Retallack 2001). The peds have clay cutans (clay coatings), which were formed either as material washed down into cracks or from alteration of the ped surface, the former process being most likely. clay cutans are characteristic of soils formed above the water table and are common in well-drained soils (Brewer 1976), indicating that these palaeosols were not waterlogged but received water input mostly as precipitation. The presence of ped structures indicates that the Cretaceous climate in which these soils developed consisted of periods of relatively wet conditions during which the rainwater infiltrated the soil and percolated downwards, carrying material into cracks. The wet season was followed by prolonged dryness, similar to mid-latitude temperate climates today.

Mottling. Mottling is present within the upper and lower horizons of the palaeosols. The mottling does not have the classic red and grey colours of gley soils, typically formed in waterlogged soils (Retallack 2001). These drab-haloed root traces (Retallack 2001) present here are areas of reduction owing to anaerobic bacterial decay of organic matter in the rootlets. The extensive occurrence of these mottles, found up to Im beneath the upper horizon, indicates that there was a significant amount of vegetation and biological activity deep within the soil profile, indicating that the soil was rich and fertile.

Clay minerals. The clay minerals illite, chlorite and montmorillonite were identified within the palaeosols by X-ray diffraction. Montmorillonite is a swelling clay formed from the degradation of silicates such as feldspars, volcanic minerals, chlorite and illite, and is commonly found in alkaline soils today (Retallack 2001). It occurs in well-drained soils where leaching is not a dominant process, suggesting that there was no significant transport of soluble minerals in these palaeosols. Smectite is also a major component of modern soils that receive <500 mm mean annual rainfall (Retallack 2001). Illite and chlorite are more stable clay minerals, formed mainly during diagenesis from the dehydration of smectite minerals (Dunoyer de Segonzac 1970).

Hydrated chlorite. Iron-rich chlorite was identified within the palaeosols but its optical properties make it difficult to distinguish from weathered biotite, and it is likely that both are present. The chlorite frequently shows signs of alteration, such as exfoliation (caused by expansion as a result of hydration) and contortion, reflecting the in situ weathering processes that had occurred within the palaeosol profile. In these palaeosols laumontite intergrowths are commonly present within split chlorites (Fig. 5b). Chlorite is found as aligned grains, a result of the realignment of clay particles during wet phases; this skelsepic fabric is seen in modern temperate soils that are subject to expansion and contraction during seasonal wet and dry periods (Kemp 1985).

Classification of the palaeosols

The Alexander Island palaeosols have several features that are characteristic of certain types of modern soils, although because of the metamorphic overprint and burial effects a definite comparison with a modern soil type cannot be made. In the most well-developed palaeosols the upper horizon (A horizon) resembles a mollic epipedon, a dark brown or black organic-rich (but not peaty) layer >25 cm in thickness. There is a range from this to less well-developed palaeosols that have an upper layer with a high detrital (volcanic) mineral component, classified as a melanic epipedon. The lower horizon (E/B horizon) is probably a cambic horizon, with relic bedding and features of the underlying bedrock but more weathered, and with peds and mottling in the upper part of this horizon.

There is little evidence in the Alexander Island palaeosols for significant movement of clays through the soils. No argillic or bleached horizons are present below the epipedon, and there is no significant alignment or draping features of clays within the matrix that would suggest extensive leaching of clays from layers above. The occurrence of hydrated chlorite indicates chemical weathering by hydrolysis and the clay fabric within the matrix is evidence of in situ weathering of primary minerals. The lack of evidence for significant leaching suggests that these Cretaceous soils were not saturated with water for long periods of time, also supported by the lack of red-grey mottling within the cambic horizon, which would indicate a waterlogged soil if present.

According to the US soil taxonomy scheme (Soil Survey Staff 1975), the palaeosols studied here are similar to modern mollisols, which, by definition, have a dark, humus-rich upper horizon (mollic epipedon), formed by the underground decomposition of organic residues of roots and plant material, and a lower horizon (cambic horizon), usually of fine-grained sand that has undergone alteration and weathering. Mollisols are formed today under climates that have moderate to strong seasonality in rainfall. Today they develop in sub-humid to semi-arid, predominantly mid-latitude areas, such as the plains of North America, Europe, Asia and South America, and occur on a range of landscapes from flat alluvial plains to mountain slopes (Wilding et al. 1983). Most develop today under grassland vegetation but some mollisols occur under well-drained forest vegetation (formerly classified as Brown Forest soils; Brady 1999), such as beneath mixed forests of the Southern Hemisphere characterized by trees such as Araucaria, Podocarpus and Agathis (Pritchett & Fisher 1987).

Some of the less well-developed palaeosols may be compared with modern Inceptisols. These are relatively young soils and include soils that have a high content of volcaniclastic material (sub-order Andepts).

The Alexander palaeosols are not comparable with the modern soil orders Entisols (which lack distinctive horizons), Vertisols (clay-rich, mixed, with slickensides and gilgai), Oxisols (very weathered minerals), Histosols (peat-like), Alfisols (clay layers), Aridsols (much carbonate), Ultisols (clay layers) or Spodosols (usually iron- and aluminium-rich).

Falcon-Lang et al. (2001) briefly described palaeosol horizons from the Triton Point location on Alexander Island. 'Fully developed' (p. 714) palaeosols were described as having 'an upper leaf litter layer (O-horizon), a middle, medium brown carbonaceous mudstone layer exhibiting closely spaced vertical fractures (c. 8 cm thick; A-horizon) and a lower bleached sandstone or siltstone layer (<60 cm thick; E/C-horizon)'. They did not record the presence of peds, cutans, or other significant features seen in the palaeosols described here, although their observation of vertical fractures may in fact be the fractures associated with peds. They did not provide sufficient detail to confirm this, nor did they provide details of the 'bleached' layer to establish its origin. However, Falcon-Lang et al. (2001) concluded these palaeosols closely resemble leached podzolic soils, similar to those supporting conifer forests in volcanic terrains in New Zealand (although, according to Retallack (2001), podzols tend to have a white bleached zone overlying dark, subsurface horizons). This wetter environment implied by these podzols contrasts with the much more seasonally drier climate described in this paper, and may represent a more northerly part of the delta that was more susceptible to waterlogging and poor drainage.

Palaeoenvironmental significance of the palaeosols

Features of the palaeosols described above are indicative of seasonally dry climates. However, the sedimentary succession of the Triton Point Formation is dominated by fluvial sandstones and floodplain sediments, leading to interpretation of this highlatitude environment as one of unstable conditions with frequent flooding under a rather wet climate regime (Jefferson 1982a; Moncrieff 1989; Cantrill & Nichols 1996; Falcon-Lang et al. 2001; Nichols & Cantrill 2002). These flood sediments are likely to have been deposited rapidly in short periods of time; Moncrieff (1989) suggested a matter of days or weeks whereas Jefferson (1981) suggested up to several months.

However, the palaeosols represent much longer periods of time, possibly from hundreds to thousands of years for soil development and forest growth, if not longer. Growth rings in the tree stumps indicate that the trees lived for more than 100200 years (Falcon-Lang et al. 2001), providing a minimum age for many of the palaeosols. The presence of unaltered feldspars and volcanic glass suggests that weathering had not occurred over more than perhaps 5000 years (Fitzpatrick 1980).

The palaeosols thus provide a more realistic picture of the general prevailing climate at that time. Palaeosol structures such as the development of peds and clay cutans, plus the lack of bleached horizons, imply little water flow through the profile; the penetrating plant roots suggest low water tables; and the absence of gley features excludes waterlogging. In summary, the palaeosols indicate that drainage was good and the climate seasonally dry, and that any flood events were intermittent.

The floral composition and productivity of the broad-leaved evergreen araucarian-podocarp forests that grew in the soils support a warm temperate climate (Falcon-Lang et al. 2001). This region also falls into the area classified as the Southern High-latitude Temperate Humid belt (Chumakov et al. 1995), based on the presence of a range of geological climate indicators. Climate models for the mid-Cretaceous predicted warm humid climates for this region. Valdes et al. (1996) predicted high summer temperatures of 20-24 °C and low winter temperatures just above freezing, whereas Barren et al. (1994) predicted somewhat lower temperatures of about 0-10°C for the summer and near-freezing temperatures in the winter. There is no evidence of freezing in the palaeosols. However, it is likely that the more significant climate parameter that influenced the soils was rainfall and soil moisture. The models of Valdes et al. (1996) of seasonal mean surface soil moisture distribution (the balance between precipitation and evaporation) predict a seasonal moisture regime with dry conditions in summer but wet in winter, supporting the seasonal signature seen in the palaeosols.

Metamorphic overprint

Zeolite minerals. The palaeosols on Alexander Island contain the zeolite-facies mineral laumontite, plus prehnite and pumpellyite. The laumontite occurs as very fine-grained crystals in the matrix, as round crystals (<200 µm) and as laumontite cutans (mineral coatings) a few millimetres thick around plant remains (Fig. 7a). Prehnite occurs as small (<50 µm) globular masses overgrowing laumontite cutans and feldspar crystals, or as tabular groups of crystals forming rosette shapes (1500 µm) overgrowing a laumontite matrix, resulting in a spotted appearance (Fig. 7b). Pumpellyite is a rarer mineral within the palaeosols, found between fragments of organic matter within laumontite cutans or as small tabular crystals within albite crystals.

Zeolitic minerals and prehnite have previously been described in sandstones of the Fossil Bluff Group (Home 1968; Jefferson 1982e; Browne 1996), but pumpellyite has not been described from the Triton Point Formation. Jefferson (19826) also described laumontite growths around fossil leaves. Zeolite minerals are diagenetic products formed from the breakdown of volcanic glass and minerals by fluids circulating through the rocks. It is interesting to note that the zeolite metamorphism has not destroyed many of the original soil structures, but simply added a metamorphic overprint.

Temperature and depth of burial. Sediments of the Fossil Bluff Group containing the palaeosols were deposited during the Albian on the eastern parts of a synclinal forearc basin during a period of increased subsidence and basin inversion (Doubleday 1994; Doubleday & Storey 1998). The sediments were then overlain by c. 3.8 km of sediment of unknown character during which time they would have reached maximum burial temperatures (Storey et al. 1996). Previous investigations of the maximum temperatures reached during burial of the Fossil Bluff Group suggested a range of 170-235°C based on vitrinite reflectance measurements (Doubleday 1994), using a burial depth of 3.8-5.2 km and a geothermal gradient of 45 °C km^sup -1^.

The palaeosols themselves contain evidence of burial depths and temperatures. Laumontite tends to occur at depths between 1 and 14 km and temperatures between 150 and 250 °C (Deer et al. 1992). Pumpellyite and prehnite usually overlap the laumontite zone with minimum depths of burial of 3-14 km and temperatures between 150 and 300 °C (Hay 1966). Reflectance analysis of vitrinite (woody tissue) from the palaeosols gave values of Ro 1.6%, indicative of palaeotemperatures of between 150 and 170 °C and a burial depth of c. 3.3-3.7 km (using a geothermal gradient of 45 °C km^sup -1^) (Bostick 1979). In addition, most of the illites and chlorites formed during burial diagenesis at maximum depths of 5-7 km and temperatures of 200 °C. The presence of smectite minerals suggests burial depths closer to 2 km and temperatures of 120 °C, as smectite is unstable in natural systems below this depth and above this temperature (Velde 1985).

Conclusions

Palaeosols are an important component of the mid-Cretaceous (Albian) sedimentary sequence on Alexander Island, Antarctic Peninsula. They can be clearly identified by the presence of in situ tree stumps and other rooted plants. The palaeosols typically have an upper organic-rich horizon with identifiable plant rootlets and other organic debris and contain a variety of mineral grains, including volcanic glass, quartz and chlorite. The lower horizon has a sandstone matrix and an organic-rich upper part, which is mottled and features well-developed prismatic or blocky peds. The palaeosols are comparable with modern soils of the mollisol type, typical today of some forests and grasslands, which generally form today under seasonally dry climates. They also have features characteristic of soils that form on volcanic terrains.

The palaeosols provide important information about environmental conditions during the Albian at high latitudes. Features of the palaeosols, such as the presence of peds, clay cutans around the peds, and lack of well-developed bleached horizons, suggest that the soils formed under seasonal climates with a well-defined dry season in which the percolation of water and the transportation of clay minerals was minimal. There is little evidence for regular flooding or waterlogging.

In contrast, the interbedded fluvial sediments have been interpreted as representing waterlogged environments dominated by flood events. The fluvial sediments, however, represent rapid deposition in relatively short periods (days or months). In contrast, the palaeosols represent much longer periods of time, of the order of hundreds to thousands of years, and so provide a better indication that the prevailing climate was in fact much drier, with well-defined seasons.

The palaeosols have subsequently acquired a metamorphic overprint, obtained during burial diagenesis, although soil characters have not been destroyed. The presence of zeolites such as laumontite, prehnite and pumpellyite indicates diagenetic alteration from original volcanic components of the palaeosols such as volcanic glass. The presence of these zeolites and vitrinite reflectance studies on plant material indicate burial temperatures of c. 150-200 °C at depths of >3 km.

This work was part of an NERC-funded PhD by J. Howe, supervised by J. Francis and D. Cantrill. We thank the British Antarctic Survey for financial and field support for this case studentship. C. Day is thanked for support in the field. We thank C. Booth (University of Sheffield) for her help with vitrinite reflectance, and L. Neve, E. Condliffe and N. Cundall (University of Leeds) for XRD, SEM and thin-section help. We are grateful for useful comments from G. Whiteley, J. A. Crame and B. Yardley. J.H. thanks the Trans-Antarctic Association for financial support.

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