Paleosols in Devonian red-beds from northwest China and their paleoclimatic characteristics

Accepted Manuscript Paleosols in Devonian red-beds from Northwest China and their paleoclimatic characteristics

Xuelian Guo, Gregory J. Retallack, Bin Lü, Lusheng He, Ronghua Wang, Hong Song PII: DOI: Reference:

S0037-0738(18)30246-X https://doi.org/10.1016/j.sedgeo.2018.11.001 SEDGEO 5407

To appear in:

Sedimentary Geology

Received date: Revised date: Accepted date:

18 August 2018 2 November 2018 3 November 2018

Please cite this article as: Xuelian Guo, Gregory J. Retallack, Bin Lü, Lusheng He, Ronghua Wang, Hong Song , Paleosols in Devonian red-beds from Northwest China and their paleoclimatic characteristics. Sedgeo (2018), https://doi.org/10.1016/ j.sedgeo.2018.11.001

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1

Paleosols in Devonian red-beds from northwest Chinaand their paleoclimatic characteristics Xuelian Guoa,⁎, Gregory J. Retallack b,⁎, Bin Lüc, Lusheng He a, Ronghua Wanga, Hong Songa

PT

Key Laboratory of Western China’s Mineral Resources of Gansu Province, School of Earth

a

c

Department of Earth Sciences, University of Oregon, Eugene, Oregon 97403, USA

SC

b

RI

Sciences, Lanzhou University, Lanzhou, 730000, China

Research Centre of Global Change, School of Geographical Science, Fujian Normal

MA

ABSTRACT

NU

University, Fuzhou 350007, China

Paleosols have been discovered in the late Devonian (Famennian) Shaliushui Formation near

PT E

D

Pingchuan City, Gansu province, China, and are recognized by evidence of root traces, soil horizons and soil structures. Root traces are remnants of substantial woody plants, reaching

CE

deeply within profiles as clayey infills and as drab-haloed root traces. Soil horizons include thick layers of large calcareous nodules (Bk horizon), and subsurface accumulations of clay

AC

(Bt horizon), and slickensided claystone (vertic Bw horizon). Soil structures include blocky peds and calcareous nodules. The nodules and drab-haloed root traces formed syndepostionally during the late Devonian, because they were also observed in clasts of paleosol and nodules in fluvial conglomerates interbedded with the paleosols. Analyses for Rb/Sr and Ba/Sr ratios and magnetic susceptibility measurements confirm that the more



* Corresponding author. E-mail address: [email protected] (X.L. Guo). E-mail address: [email protected] (G. J. Retallack).

ACCEPTED MANUSCRIPT 2

strongly developed paleosols with larger calcareous nodules and higher clay content are also more chemically differentiated and have higher magnetic susceptibility under semi-arid climates. This suite of paleosols is evidence of semi-arid to sub-humid, highly seasonal climate under dry woodlands and shrublands. Paleosols of the Shaliushui Formation are a

PT

potentially valuable archive of late Devonian palaeoclimates of northwest China.

RI

Key words: Famennian; aridisol; geochemistry; magnetic susceptibility; paleoclimate

SC

1. Introduction

NU

Devonian paleosols have long been recognized from around the world, using the familiar

MA

criteria of root traces, soil horizons and soil structure (Allen, 1973, 1974; Boucot et al., 1982; Retallack, 2009; Narkiewicz and Retallack, 2014; Alekseeva et al., 2016; Genise et al., 2016;

D

Xue et al., 2016). Late Devonian evolution of trees and tetrapods was a major transformation

PT E

of ecosystems, landscapes and climates, including dramatic greenhouse spikes coincident with Frasnian-Famennian (Retallack, 1997; Retallack and Huang, 2011). A remarkably

CE

complete record of global change has been reconstructed from Devonian paleosols in the Old

AC

Red Continent of Euramerica (Retallack, 2011), but Devonian paleosol sequences from China remain little known (Boucot et al., 1982; Guo et al., 2016; Xue et al., 2016). This study introduces a newly discovered record of late Devonian paleosols from the Shaliushui Formation of Gansu (Fig. 1) and their paleoclimatic characteristics. Preservation of Devonian paleosols was compromised by extensive greenschist facies metamorphism, as well as burial decomposition of organic matter, burial gleization and iron oxyhydrate dehydration (Retallack, 2011; Retallack and Huang, 2011). These alterations to

ACCEPTED MANUSCRIPT 3

their appearance make them a gaudy green and red caricature of the original subtle color of the profiles (Retallack, 1991). This paper details field characteristics of the paleosols and employs geochemical and environmental magnetic data to assess whether they are paleosols or not. One reason for failure to recognize paleosols until recently in many red bed sequences

PT

may be lack of appreciation of the nature of soil development. Paleosols vary in predictable

RI

ways from thin and little altered with root traces (very weakly developed) to thick, nodular or

SC

massive clayey (moderate development) (Retallack, 2001). Our geochemical and susceptibility data are used here to examine the degree to which such field categories of soil

NU

development correspond to environmental magnetic and geochemical differentiation during

MA

late Devonian soil formation.

D

2. Geological setting

PT E

Our three study sections are 14 km northeast and 24 km northwest of Pingchuan City, in Baiyin County, Gansu province, China (Fig. 1). Devonian rocks of the Northern Qilian

CE

Orogenic Belt include two formations: the early-middle Devonian Laojunshan Formation and

AC

the late Devonian Shaliushui Formation (Zhai, 1981). The early-middle Devonian Laojunshan Formation formed during the rapid uplift of the Northern Qilian Orogenic Belt. The orogeny of the Northern Qilian Orogenic Belt was diachronous in the trending direction, the eastern sector had stronger tectonic intensity compared to the western sector, due to the oblique collision between the middle Qilian block and the Alxa block, the Laojunshan Formation was not present in our measured sections (Xu et al., 2010). The thickness of the Shaliushui Formation is ~250 m. The Shaliushui Formation in our sections overlies an angular

ACCEPTED MANUSCRIPT 4

unconformity with grey Silurian schists of the Hanxia Formation (Zhai, 1981). The uppermost Shaliushui Formation is conformably overlain by the Carboniferous Qianheishan Formation of gray-white gravelly sandstone and purple siltstone. In contrast, the Shaliushui Formation is composed of clastic rocks with red and green mottled claystones, with interbedded sandstone

PT

and conglomerate (Fig. 1B). According to the tempestite in sandstone in Shaliushui Formation,

RI

Du et al (2001) suggested that the study region was located in a low latitude (5º~20º) zone,

SC

and Shaliushui Formation was mainly composed of fluvial and lacustrine deposits. A late Devonian (Famennian) age of the Shaliushui Formation is evident from lycopsid

NU

stems of Leptophloeum rhombicum (Zhai, 1981; Xu et al., 2011). An earliest Carboniferous

MA

(earliest Tournaisian) ostracod Chamishaella lysei in the conformably overlying Qianheishan Formation confirms that this sequence includes the Devonian-Carboniferous boundary (Yao et

PT E

D

al., 2016).

3. Materials and methods

CE

Three paleosol sections from different sites including Ciyao section (CY), Dahongmen

AC

section 2 (DHM 2) and Dahongmen section 4 (DHM 4) were measured (Fig. 1B). CY section (N36.79085o E104.94379o) is 4.3 m thick, while DHM 2 (N36.90899o E104.67810o) and DHM 4 (N36.90774o E104.68212o) are 1 m and 4 m thick respectively. Powder samples were collected from DHM 2 and DHM 4 sections with 10 cm interval and from CY section with 50 cm interval. Totally 35 samples were obtained from three sections. The samples were air-dried, then weighed and packed in a nonmagnetic plastic box (2×2×2 cm). Magnetic susceptibility (χ, mass-specific) was measured using Bartington MS-2

ACCEPTED MANUSCRIPT 5

meter (low frequency χ with 470 Hz). Hysteresis loop and thermomagnetic curves were measured using a Magnetic Measurements Variable Field Transition Balance (MMVFTB). The hysteresis parameters, including saturation magnetization (Ms), remanence saturation magnetization (Mrs), coercivity (Bc) and remanent coercivity (Bcr) of paleosols were

PT

determined on hysteresis loops.

RI

Major and trace element concentrations were measured using XRF spectroscopy. For

SC

each XRF measurement, a whole-rock sample was powdered by an agate mortar, sieved to a size-fraction of <200 µm mesh, afterwards were weighed 4 g, and then compacted into a round

NU

disk and measured with a VP-320 XRF spectrometer. The relative standard deviation is about

MA

1%, and the relative errors estimated from the measured values and the commended values for the standards are less than 2%.

D

Six block samples were obtained from red beds. A thin sliver of soil was cut from the

PT E

sample with a diamond saw and ground optically flat. It was mounted on a glass slide with epoxy resin, and ground smooth using progressively finer abrasive grit until the sample was

CE

only 30 μm thick, then analyzed and described in thin sections with a stereomicroscope.

AC

4. Results and analyses

The stratigraphy of sections in this study belong to the late Devonian Shaliushui Formation (Zhai, 1981), which is composed of red and green mottled claystones, with interbedded sandstone and conglomerate. The conglomerate is thin bedded (0.1-0.4 m thick), while the grey green sandstone is a continuous, thinly laminated (0.1-0.5 m thick) layer with planar bedding, trough cross bedding and occasional ripple marks. The red claystones is about

ACCEPTED MANUSCRIPT 6

1-4 m thick dominated by paleosols at various stages of maturity. The most notable paleosol features in this study are roots trace, pedogenic carbonate nodules, vertebrate burrows, and degree of development of the profile. Paleosols were described using the Retallack (1988) nomenclature and interpreted with the modern soil classification system of the Soil Survey

PT

Staff (2014).

SC

RI

4.1. Paleosol field observations

4.1.1. Root traces

NU

Root traces are the most diagnostic evidence of paleosols, and are common in the

MA

Shaliushui Formation. They have rootlets branching from a central tap root penetrating deeply into paleosol, with irregular striated surface like woody roots of progymnosperms such as

D

Callixylon-Archaeopteris (Retallack and Huang, 2011). Some large cylindrical root traces (5

PT E

cm diameter, 60 cm deep, n = 3) show a central dark cylindrical woody streak after the root protostele and a carbonaceous epidermis separated by a zone filled with mud and calcite (Fig.

CE

2A). In other many tubular root traces (1-3 cm diameter, 5-30 cm deep), the tubular cavity from the rotted stele is filled with sparry calcite.

AC

Especially common and conspicuous are drab-haloed root traces, with a central clayey thread surrounded by a diffuse halo of green-grey claystone (Fig. 2B-C). These are most common in surface horizons where they are truncated above by overlying sediments, and they reach deeply into the subsurface. They are not burrows filled with grey sand or silt from above, because they have a similar clayey texture as their matrix. Features similar to drab-haloed root traces are formed in modern soils by surface water

ACCEPTED MANUSCRIPT 7

gleying above impermeable soil layers (Lindbo et al., 2010), but this is an unlikely explanation for drab-haloed root traces in the Shaliushui Formation for several reasons. First there are no clearly impermeable layers in the paleosols that would have deflected root penetration and ponded water (Fig. 3). Nor are there rims of iron-manganese or pyrite as

PT

evidence of gleization during soil formation (Retallack, 1983; Retallack et al., 2000). Deeply

RI

penetrating root traces and abundant carbonate nodules are evidence instead that these

SC

paleosols were originally well drained and porous (Retallack, 2001). Drab mottles can form by burial gleization–the chemical reduction of iron hydroxides and oxides by anaerobic

NU

bacteria consuming organic matter buried with the soil below or near the water table

MA

(Retallack, 1991). Reduction haloes around buried organic matter can form in only a few thousand years (Allen, 1986). The drab-haloed root traces are formed from chemical

D

reduction of matrix by microbes decomposing organic matter of roots buried in the paleosol

PT E

as it subsided below water table, the drab-haloed root traces are thus evidence of burial gleization (Retallack, 1991; Retallack et al., 2000). Such early diagenetic drab-haloed root

CE

traces are widely known from Devonian paleosols (Retallack and Huang, 2011; Xue et al.,

AC

2016). Furthermore, there are also non-drab-haloed root traces in the same paleosols. The non-drab-haloed root traces are evidence of roots that had rotted in the soil prior to burial and filled with oxidized soil, like other Devonian root traces (Hillier et al., 2008).

4.1.2. Vertebrate burrows

Some paleosols and paleochannels have large (10-13 cm diameter and 25-30 cm deep) subvertical to oblique burrows filled with calcareous silt and sand (Fig. 2D-E). Indeterminate bone fragments were found within one of these.

ACCEPTED MANUSCRIPT 8

Similar burrows are made by small vertebrates such as amphibians (Hembree et al., 2005), reptiles (Miller et al., 2001) and mammals (Hembree and Hasiotis, 2008), but no late Devonian quadrupedal fossorial vertebrates are known (Retallack, 2007). Burrows of the Shaliushui Formation are most like aestivation burrows of lungfish (Berman, 1976; Dubiel et

PT

al., 1987; Graham, 1997), including a lariat-like shape in some cases. In Africa today, lungfish

RI

burrow into soft soils and lake bottoms before the dry season, and pass the summer in torpor

SC

until rains return again (Janssens, 1964). All burrows of vertebrates require well drained,

NU

oxygenated soil, and lungfish burrows are indicators of a long hot dry season.

4.1.3. Soil horizons

MA

Paleosols have recognizable soil horizons in the field, truncated sharply by overlying sandstone or siltstone horizon (Fig. 3C, D). Below sandstone or siltstone horizon, boundaries

PT E

D

of different soil horizons and underlying parent material have diffuse contacts (Retallack, 1991). Normal and reverse graded bedding can also show similar gradational contacts, but

CE

lacks the slickensided clay skins (Fig. 2H) in both normal-below-reverse grading of an argillic horizon. Three kinds of soil horizon were recognized in paleosols of the Shaliushui Formation:

AC

horizon of calcareous nodules (calcic Bk) (Fig. 3D), horizons of subsurface clay enrichment (argillic Bt) and horizons of deformed and slickensided claystone (vertic Bw) (Fig. 3E). Paleosols with B horizons qualifying as calcic and argillic (Soil Survey Staff, 2014) are very weakly to moderately developed (Retallack, 1988). Especially obvious are calcareous nodules of unusually thick calcic (Bk) horizons (Fig. 3D), like those found in Devonian red beds of Britain (Allen, 1973, 1974) and elsewhere in China (Boucot et al., 1982). Nodules in paleosols of the Shaliushui Formation are 1-5 cm in

ACCEPTED MANUSCRIPT 9

diameter, and form horizons 11-110 cm thick at depths to carbonate nodular horizon in soils of 0-72 cm. Some of these calcic horizons are truncated by sandstone (Fig. 3D), and many of the conglomerates contain calcareous nodules as clasts and proof of erosion of nodules that came from paleosols below the conglomerates (Fig. 3A). Calcic horizons of this thickness and

PT

depth are indications of an arid and highly seasonal palaeoclimate (Retallack, 2005).

RI

The argillic and vertic horizons are dark red and slickensided claystone (Fig. 3E),

SC

unusual in this sequence of siltstone, sandstone and conglomerate (Fig. 2H). Large blocks of red claystone containing drab-haloed root traces are also found in paleochannel deposits (Fig.

NU

3A) as evidence that these horizons formed during deposition and not as late diagenetic

MA

structures. The argillic and vertic horizons are weakly to non-calcareous above weakly calcareous parent material, and so are evidence of subhumid to humid palaeoclimates

D

(Retallack, 2005). The argillic and vertic paleosols may reflect short episodes of global

CE

4.1.4. Soil structures

PT E

greenhouse palaeoclimate noted in other Devonian sequences of paleosols (Retallack, 1997).

Soil structures gradually form and replace the structures of parent materials such as

AC

bedding, crystal structure, and schistosity of parent materials, by the activity of plants and animals, wetting and drying and other soil-forming processes. Mud cracks from desiccation are an obvious structure of very weakly developed soils. Mud cracks in the Shaliushui Formation are atop thin paleosols with drab surface and red subsurface (Fig. 3B). The original desiccation cracks are filled with sand from above, and show strong taper characteristic of desiccation, as opposed to synaeresis cracks (Plummer and Gostin, 1981; Weinberger, 2001). Clayey (argillic and vertic) horizons of the Shaliushui Formation have blocky subangular

ACCEPTED MANUSCRIPT 10

peds and blocky angular peds (soil clods) defined by slickensided clay skins (Fig. 2D, E). These clayey cutans (argillans) are evidence of illuviation of clay within a profile, which is possible in soils but not saturated sediments (Buurman et al., 1998). Comparable blocky peds and argillans are widely known in paleosols (Retallack, 1988; Caudill et al., 1996).

PT

Purple to black Fe-Mn coatings (mangans) are another common cutan in claystones and

RI

around carbonate (Fig. 2D, E). These kinds of coatings are common in slowly permeable parts

SC

of soils, and when thicker and more abundant than in the Shaliushui Formation are called

NU

placic horizons (Retallack et al., 2000; Soil Survey Staff, 2014; Lindbo et al., 2010).

4.2. Petrographic observations

MA

Observations in petrographic thin section confirm several field observations of paleosols in the Shaliushui Formation. The carbonate nodules that appear like pedogenic carbonate in

PT E

D

the field also show both replacive (Fig. 4F) and displacive (Fig. 4A) structures diagnostic of soil carbonate. Soil carbonate is characteristically micrite replacing soil matrix which remains

CE

as isolated quartz grains with marginal texture (Wright, 1986; Searl, 1989). Displacive fabrics on the other hand are where angular pieces of nodule or matrix have fallen into voids later

AC

filled with sparry calcite, so are evidence of dilations formed in low pressure soil rather than deeply buried rocks (Braithwaite, 1989). Sparry calcite in veins, burrows and old root traces (Fig. 4A-F) also form in modern soils, but may also have been late diagenetic cements (Wright, 1986). Clayey matrix to paleosols of the Shaliushui Formation also shows birefringence fabrics such as porphyroskelic insepic (Fig. 4C, D) in the terminology of Brewer (1964). The matrix-supported quartz and feldspar grains (porphyroskelic) were produced by weathering of

ACCEPTED MANUSCRIPT 11

matrix grains to clay, and the high birefringence oriented clay in random arrangement (insepic) produced by fine scale shrink-swell deformation by cracks, roots and burrows. These birefringence fabrics are unique to soils (Retallack, 1988) and can be contrasted with intertextic asepic fabrics (Fig. 2G).

PT

Thin sections also reveal details of clay skins. The laminated interior of some clay skins

RI

mark them as illuviation argillans (Fig. 4C, D). Other cutans coating original voids filled with

SC

sparry calcite are hematite diffusion ferrans (Fig. 4E, F) and Fe-Mn diffusion mangans (Fig. 4F). This form of cutan is produced by diffusion of oxygen from the void into the matrix,

NU

abundant oxygen in the case of ferrans and limited oxygen in the case of mangans (Brewer,

MA

1964). In both cases these are evidence of an oxygenated environment of soil formation which is a kind of early diagenesis, rather than the poorly oxygenated local environments of late

D

diagenesis (Retallack, 1988).

PT E

4.3. Geochemical characteristics of paleosol sections

CE

Our chemical analyses were designed to examine whether the degrees of paleosol development observed in the field (Fig. 3) corresponded with degree of chemical weathering.

AC

Two molar ratios in particular are useful indices of chemical weathering: Ba/Sr and Rb/Sr ratios (Retallack, 1988). Chemical index of alteration (CIA = Al2O3 / (Al2O3+CaO+K2O+Na2O) in which CaO is non-carbonate: Nesbitt and Young 1982) is a popular weathering proxy, but not appropriate for our paleosols because they have so much calcite. As would be expected for paleosols, the profiles showing greatest textural differentiation and homogenization due to soil formation also had the highest Ba/Sr, and Rb/Sr ratios (Fig. 5). None of these paleosols are deeply weathered chemically, as expected

ACCEPTED MANUSCRIPT 12

for their likely arid palaeoclimate and abundant pedogenic carbonate. The Ba/Sr and Rb/Sr ratios are based on relative solubilities of two chemically similar elements, and so are proxies for water/rock ratios of weathering. The least chemically weathering paleosols have high Sr and CaCO3, due to late dry

PT

season accumulation of low-magnesium carbonate in the paleosols (Retallack, 2005). The

RI

parent material reflected in weakly and very weakly developed paleosols also has high

SC

amounts of Sr and CaCO3, derived from calcic soils of an arid upland.

Amounts of Ti and SiO2 are also proxies for parent material and are uniform in clayey

NU

and silty parts of the measured profiles, but very different in sandstone paleochannels (Fig. 5).

MA

This is an indication, that as in many aridlands, the parent material of many of the paleosols

PT E

4.4. Magnetic characteristics

D

was not alluvial sand but calcareous eolian dust (Guo et al., 2013; Retallack, 2013).

A thermomagnetic curve is generally preferred for determining composition of magnetic

CE

minerals in a sample (Evans and Heller, 2003). The thermomagnetic curves of CY-2 and DHM 2-4 samples are shown in Fig. 6A. The magnetizations have steep decrease at about

AC

658 ℃ and gradually reduction from 658 to 700 ℃ on heating curves, suggesting that hematite is the major magnetic minerals of samples. Hysteresis loop measurements also give information about the composition of magnetic mineral and particle size (Evans and Heller, 2003). No closure of loops in Fig. 6B at 300 mT and lack of saturation (which requires near zero slope of the loop at the highest fields) above 300 mT indicate mainly magnetic minerals are hard magnetic minerals such as hematite and/or goethite. Bcr >350 mT and the pot-bellied loops indicate that the high coercivity

ACCEPTED MANUSCRIPT 13

mineral is hematite in both samples (Dunlop and Özdemir, 1997). Taken together, both hysteresis loops and thermomagnetic curves show that the dominant magnetic mineral is hematite. Hematite is an indication of soil maturity and an arid climate, which is consistent with lungfish burrows (Fig. 2D-E).

PT

Magnetic susceptibility (χ) is another indication of degree of soil development, as it

RI

increases with increasing pedogenic degree in aerobic soil with low and moderate rainfall.

SC

However, when the moisture is high, the susceptibility decreases (Maher and Thompson, 1994; Balsam et al., 2011). The susceptibility of DHM 2 and CY sections are higher and increase

NU

with decreasing depth (Fig. 5A, B), indicating that the pedogenic degree enhance. However,

MA

magnetic susceptibility of DHM 4 section is low with almost uniform value from bottom to top (Fig. 5C), which may be due to magnetic mineral transformation under high soil moisture

D

content (Liu et al., 2012; Wang et al., 2013; Guo et al., 2018). Grimley and Vepraskas (2000)

PT E

take 20 ×10-8m-3kg-1 as an upper threshold for soils, when fine-grained strongly magnetic minerals (mainly magnetite and/or maghemite) were converted into weakly magnetic

CE

minerals (mainly hematite) by pedogenesis, which resulted in a decline in magnetism and

AC

decreasing susceptibility (Guo et al., 2018). The low susceptibility may be due to burial gleization, as documented for magnetic susceptibility of other paleosols (Retallack et al., 2003).

5. Discussion

Studies show that the Shaliushui Formation was composed of clastic rocks with red and green mottled claystones, with interbedded sandstone and conglomerate (Zhai, 1981; Du et al.,

ACCEPTED MANUSCRIPT 14

2001; Xu et al., 2010). But we discover that the red and green mottled claystones show paleosol characteristics, so we assess whether or not they are paleosols by evidence of root traces, soil horizons and soil structures. Formerly woody root traces are ubiquitous in the red beds (Fig. 2A-C). Many of these show drab haloes due to burial gleization, but the

PT

non-drab-haloed root traces are evidence of roots that had rotted in the soil prior to burial and

RI

filling with oxidized soil, like other Devonian root traces (Hillier et al., 2008). Many red beds

SC

display diffuse changes down from a truncated top, similar to soil horizons of calcareous nodules (calcic Bk) or of illuviated clay (argillic Bt) or deformed clay (vertic Bw) (Fig. 3).

NU

The calcareous nodules have both replacive and displacive textures of pedogenic carbonate

MA

(Fig. 3). Soil structures such as blocky peds are outlined by clay skins, and thin section study reveals these to be illuviation argillans (Fig. 4). Other planar features in the soil are diffusion

D

ferrans and diffusion mangans created during soil formation and exposure to air rather than

PT E

deep burial diagenesis. A varieties of field observations and thin sections analyses confirm that the Shaliushui Formation has a long sequence of clayey and calcareous red paleosols.

CE

Paleosols of the Shaliushui Formation are a potentially valuable archive of late Devonian

AC

palaeoclimates, life and palaeogeography of northwest China. The dominant hard magnetic hematite in red beds (Fig. 6) and abundant carbonate nodules at shallow depth in the profiles (Fig. 3D) are evidence of arid climates, but the spread of nodules within the profiles are indications of strongly seasonal, monsoonal climates (Retallack, 2005). Devonian tropical climate potentially exhibited an even more intense monsoonal circulation (Streel et al., 2000), characterized by seasonally wet-and-dry climates (Cecil, 1990). The Ba/Sr values show that paleosol weathering intensity of the DHM 4 section was

ACCEPTED MANUSCRIPT 15

stronger than DHM 2 and CY sections (Fig. 5). However, the magnetic susceptibility as an indication of degree of soil development is lower within the DHM 4 profile than DHM 2 and CY profiles (Fig. 5), and hematite is the dominant magnetic mineral in red beds. The argillic (Bt) and vertic (Bw) paleosols reflect short episodes of global greenhouse palaeoclimate (Fig.

PT

3E). Orgeira et al. (2011) proposed that soil water balance is the most important factor that

RI

controls magnetic enhancement because it dominates the annual soil ‘wetting’ and ‘drying’

SC

cycle. When the moisture is high and over an upper threshold (Grimley and Vepraskas, 2000), the parts of the fine-grained strongly magnetic minerals (mainly magnetite and/or maghemite)

NU

were converted into weakly magnetic hematite by pedogenesis, which resulted in a decline in

MA

magnetism and a significant decreasing in susceptibility of DHM 4 section (Liu et al., 2012; Wang et al., 2013; Guo et al., 2018). Scattered through the sequence are argillic and vertic

D

paleosols with little carbonate (Fig. 3E) also reflect the brief episodes of humid climates

PT E

attributed to global greenhouse spikes in other Devonian paleosol sequences (Retallack, 1997, 2011). Overall, the soil horizons and magnetic susceptibility reflect semi-arid to sub-humid,

CE

highly seasonal palaeoclimates in the late Devonian in northwest China. This is compatible

AC

with the study of Zhao et al. (1986), who speculated that the climate of the late Devonian Zhongning Formation in the southern Ningxia Autonomous Region, northwest China, might be a warmer and humid interval after a long dry period, according to the lithological variations and fossil flora. The dramatic greenhouse spikes commonly occur in other late Devonian strata (Vleeschouwer et al., 2013). These vertic and argillic paleosols have deeply reaching stout root traces of woodlands (Fig. 2A-C), but the root traces of calcic paleosols also are deeply reaching and with nodules

ACCEPTED MANUSCRIPT 16

so shallow (Fig. 3D) that their vegetation would have been an arid shrubland, by comparison with other Devonian paleosols (Retallack and Huang, 2011). Such highly seasonal monsoonal palaeoclimates also imply a large North Qilian Mountain range near at hand (Retallack, 2005), as a source for the course conglomeratic paleochannel sandstones.

PT

6. Conclusions

RI

A variety of field observations and laboratory analyses confirm that the red beds of

SC

Shaliushui Formation has a long sequence of clayey and calcareous red paleosols. Weathering ratios and magnetic susceptibility confirm that the more strongly developed

NU

paleosols with larger calcareous nodules, more clay and higher susceptibility under low and

MA

moderate soil moisture. Some large woody root traces and abundant carbonate nodules at shallow depth in the profiles indicate dry woodlands and shrublands in a coastal plain to a

D

range of high mountains.

PT E

Argillic and vertic paleosols with little carbonate reflect the brief episodes of humid climates, and abundant carbonate nodules at shallow depth reflect arid climates, this suite of

CE

paleosols reflect semi-arid to sub-humid, highly seasonal climate in the late Devonian in

AC

northwest China.

Acknowledgments

We thank M.S. Qian Li for help in the field, thank Guoyong Zhao of College of Urban and Environmental Science, Xinyang Normal University for his help in element concentrations. This work was supported by the National Natural Science Foundation of China (Grant no. 41772168, 41877435).

ACCEPTED MANUSCRIPT 17

References

Alekseeva, T., Kabanov, P., Alekseev, A., Kalinin, P., Alekseeva, V., 2016. Characteristics of early earth’s critical zone based on Middle-Late Devonian paleosol properties (Voronezh

PT

High, Russia). Clays and Clay Minerals 64, 677–694. Allen, J.R.L., 1973. Compressional structures (patterned ground) in Devonian pedogenic

RI

limestones. Nature 243, 84.

SC

Allen, J.R.L., 1974. Geomorphology of Siluro-Devonian alluvial plains. Nature 249, 644.

NU

Allen, J.R.L., 1986. Time scales of colour change in late Flandrian intertidal muddy sediments of the Severn Estuary. Proceedings of the Geologists’ Association 97, 23–38.

MA

Balsam, W.L., Ellwood, B.B., Ji, J.F., Williams, E.R., Long, X.Y., Hassani, A.E., 2011. Magnetic susceptibility as a proxy for rainfall: worldwide data from tropical and

PT E

D

temperate climate. Quaternary Science Reviews 30, 2732–2744. Berman, D.S., 1976. Occurrence of Gnathorhiza (Osteichthyes: Dipnoi) in aestivation

CE

burrows in the Lower Permian of New Mexico with description of a new species. Journal of Paleontology 50, 1034–1039.

AC

Boucot, J., Gray, J., Fang, R.S., Yang, X. C., Li Z.P., Zhang, N., 1982. Devonian calcrete from China: its significance as the first Devonian calcrete from Asia. Canadian Journal of Earth Sciences 19, 1532–1534. Braithwaite, C.J.R., 1989. Displacive calcite and grain breakage in sandstones. Journal of Sedimentary Petrology 57, 587–601. Brewer, R., 1964. Fabric and mineral analysis of soils. Soil Science 100, 4–6. Buurman, P., Jongmans, A.G., PiPujol, M.D., 1998. Clay illuviation and mechanical clay

ACCEPTED MANUSCRIPT 18

infiltration: is there a difference? Quaternary International 51/52, 66–69. Caudill, M.R., Driese, S.G., Mora, C.I., 1996. Preservation of paleo-Vertisol and an estimate of Late Mississippian paleoprecipitation. Journal Sedimentary Research 66, 58–70. Cecil, C.B., 1990. Paleoclimate controls on stratigraphic repetition of chemical and

PT

siliciclastic rocks. Geology 18, 533–536. Du, Y.S., Zhou, D.H., Gong, S.Y., Han. X., 2001. Tempestite and its palaeogeographical

SC

Mineral Petrology 21, 69–73. (in Chinese).

RI

significance of Devonian in Jingyuan and Jingtai County, Gansu province. Journal of

NU

Dubiel, R.F., Blodgett, R.H., Bown, T.M., 1987. Lungfish burrows in the Upper Triassic Chinle and Dolores Formations, Colorado Plateau. Journal Sedimentary Research 57,

MA

512–521.

Dunlop, D.J., Ozdemir, O., 1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge

PT E

D

University Press, New York, 573pp.

Evans, M.E., Heller, F., 2003. Environmental Magnetism: Principles and Applications of

CE

Enviromagnetics. Academic Press, New York, 299pp. Genise, J.F., Bedatou, E., Bellosi, E.S., Sarzetti, L.C., Sánchez, M.V., Krause, J.M., 2016. The

AC

Phanerozoic four revolutions and evolution of paleosol ichnofacies. In: Mángano, M.G., Buatois, L.A. (Eds.), The trace-fossil record of major evolutionary events. Dordrecht: Springer-Verlag, pp. 301–370. Graham, J.B., 1997. Air-breathing fishes. Evolution, diversity and adaptation. San Diego, California, Academic Press, 299pp. Grimley, D.A., Vepraskas, M.J., 2000. Magnetic susceptibility for use in delineating hydric soils. Soil Science Society of America Journal 64, 2174–2180.

ACCEPTED MANUSCRIPT 19

Guo, X.L., Banerjee, S.K., Wang, R.H., Liu, X.M., Song, H., Zhao, G.Y. and Lü, B., 2018. Why magnetite is not the only indicator of past rainfall in the Chinese loess plateau? Geophysical Journal International 213, 2128–2137. Guo, X.L., Liu Y.X., Liu X.M., Wang, R.H., Wang, S.H., Lü, B., Zhao, G.Y., 2016. The

PT

identification of late Devonian paleosol and pedogenetic character. Journal of

RI

Subtropical Resources and Environment 11, 1–5. (in Chinese).

SC

Guo, X.L., Liu, X.M., Li, P.Y., Lü, B., Guo, H., Chen, Q., Ma, M.M., 2013. The magnetic

Quaternary International 306, 129–136.

NU

mechanism of paleosol S5 in the Baoji section of the southern Chinese Loess Plateau.

MA

Hembree, D.I., Hasiotis, S.T., 2008. Miocene vertebrate and invertebrate burrows defining compound paleosols in the Pawnee Creek Formation, Colorado, USA. Palaeogeography,

D

Palaeoclimatology, Palaeoecology 270, 349–365.

PT E

Hembree, D.I., Hasiotis, S.T., Martin, L.D., 2005. Torridorefugium eskridgensis (new ichnogenus and ichnospecies): amphibian aestivation burrows from the Lower Permian

CE

Speiser Shale of Kansas. Journal of Paleontology 79, 583–593.

AC

Hillier, R.D., Edwards, D., Morrissey, L.B., 2008. Sedimentological evidence for rooting structures in the Early Devonian Anglo-Welsh Basin (UK), with speculation on their producers. Palaeogeography, Palaeoclimatology, Palaeoecology 270, 366–380. Janssens, P.A., 1964. The metabolism of the aestivating African lungfish. Comparative Biochemistry & Physiology 11, 105–117. Lindbo, D.L., Stolt, M.H., Vepraskas, M.J., 2010. Redoximorphic features. In: Stoops, G., Marcelino, V., Mees, F. (Eds.), Interpretation of micromorphological features of soils and

ACCEPTED MANUSCRIPT 20

regoliths. Elsevier, Amsterdam, pp. 129−147. Liu, C.C., Deng, C.L., Liu, Q.S., 2012. Mineral magnetic studies of the vermiculated red soils in

southeast

China

and

their

paleoclimatic

significance.

Palaeogeography,

Palaeoclimatology, Palaeoecology 329–330, 173–183.

PT

Maher, B.A., Thompson, R., 1994. Comments on: pedogenesis and paleoclimateinterpretation

RI

of the magnetic susceptibility record of Chinese loess-paleosol sequences. Geology 23,

SC

857–858.

Miller, M.F., Hasiotis, S.T., Babcock, L.E., Isbell, J.L., Collinson, J.W., 2001. Tetrapod and

NU

large burrows of uncertain origin in Triassic high paleolatitude floodplain deposits,

MA

Antarctica. Palaios 16, 218–232

Narkiewicz, M., Retallack G.J., 2014. Dolomitic paleosols in the lagoonal tetrapod

D

track-bearing succession of the Holy Cross Mountains (Middle Devonian, Poland).

PT E

Sedimentary Geology 299, 74–87.

Nesbitt, H., Young, G.M., 1982. Early Proterozoic climates and plate motions inferred from

CE

major element chemistry of lutites. Nature 299, 715–718.

AC

Orgeira, M.J., Egli, R., Compagnucci, R.H., 2011. A quantitative model of magnetic enhancement in loessic soils. In: Petrovsky, E., Ivers, D., Harinarayana, T., Herrero-Bervera, E. (Eds.), The Earth’s Magnetic Interior. Springer, Netherlands, pp. 361–397. Plummer, P.S., Gostin, V.A., 1981. Shrinkage cracks: desiccation or synaeresis? Journal of Sedimentary Research 51, 1147–1156. Retallack, G.J., 1983. Late Eocene and Oligocene paleosols from Badlands National Park,

ACCEPTED MANUSCRIPT 21

South Dakota. Geological Society of America Special Paper 193, 82pp. Retallack, G.J., 1988. Field recognition of paleosols. In: Reinhardt, J., Sigleo, W.R. (Eds.), Paleosols and weathering through geologic time: principles and applications. Geological Society of America, Special Paper 216, pp. 1–19.

RI

Annual Review of Earth and Planetary Sciences 19, 183–206.

PT

Retallack, G.J., 1991. Untangling the effects of burial alteration and ancient soil formation.

SC

Retallack, G.J., 1997. Early forest soils and their role in Devonian global change. Science 276, 583–585.

NU

Retallack, G.J., 2001. Soils of the Past: An Introduction to Paleopedology. Second Edition,

MA

Blackwell, Oxford, 600pp.

Retallack, G.J., 2005. Pedogenic carbonate proxies for a mount and seasonality of

D

precipitation in paleosols. Geology 33, 333–336.

PT E

Retallack, G.J., 2009. Early Paleozoic pedostratigraphy and global events in Australia. Australian Journal of Earth Sciences 56, 569–584.

CE

Retallack, G.J., 2011. Woodland hypothesis for Devonian evolution of tetrapods. Journal of

AC

Geology 119, 235–258. Retallack, G.J., 2013. Ediacaran life on land. Nature 493, 89–92. Retallack, G.J., Bestland, E.A., Fremd, T.J., 2000. Eocene and Oligocene paleosols and environmental change in central Oregon. Special Paper of the Geological Society of America 344, 192pp. Retallack, G.J., Huang, C.M., 2011. Ecology and evolution of Devonian trees in New York, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 299, 110–128.

ACCEPTED MANUSCRIPT 22

Retallack, G.J., Sheldon, N.D, Cogoini, M., Elmore, R.D., 2003. Magnetic susceptibility of early Paleozoic and Precambrian paleosols. Palaeogeography, Palaeoclimatology, Palaeoecology 198, 373–380. Searl, A., 1989. Podogenic columnar calcite from the Oolite Group (Lower Carboniferous),

PT

South Wales. Sedimentary Geology 62, 47–58.

RI

Soil Survey Staff, 2014. Kellogg Soil Survey Laboratory Methods Manual. Soil Survey

SC

Investigations Report No. 42, Version5.0. U.S. Department of Agriculture, Natural Resources Conservation Service, 372pp.

NU

Streel, M., Caputo, M.V., Loboziak, S., Melo, J.H.G., 2000. Late Frasnian–Famennian climates based on palynomorph analyses and the question of the Late Devonian

MA

glaciations. Earth Science Reviews 52, 121–173.

Vleeschouwer, D.D., Rakociński, M., Racki, G., Bond, D.P.G., Sobień, K., Claeys, P., 2013.

D

The astronomical rhythm of Late-Devonian climate change (kowala section, holy cross

PT E

mountains, Poland). Earth and Planetary Sciences Letters 365, 25–37. Wang, S.Y., Lin, S., Lu, S.G., 2013. Rock magnetism, iron oxide mineralogy and

CE

geochemistry of Quaternary red earth in central China and their paleopedogenic

AC

implication. Palaeogeography, Palaeoclimatology, Palaeoecology 379–380, 95–103. Weinberger, R., 2001. Evolution of polygonal patterns in stratified mud during desiccation: the role of flaw distribution and layer boundaries. Geological Society of America Bulletin 113, 20–31. Wright, V.P., 1986. The role of fungal biomineralization in the formation of early carbinoferous soil fabrics. Sedimentology 33, 831–838. Xu, Y.J., Du, Y.S., Yang, J.H., Chen, J.C., 2011. Geochemistry and provenance of the Upper

ACCEPTED MANUSCRIPT 23

Devonian

shaliushui

formation

in

Jingyuan

county,

Gansu

province.

Acta

Sedimentologica Sinica 19, 41–54. (in Chinese). Xu, Y.J., Du, Y.S., Yang, J.H., Huang, H., 2010. Sedimentary geochemistry and provenance of the lower and middle Devonian Laojunshan Formation, the North Qilian Orogenic Belt.

PT

Science China Earth Sciences 53, 356–367.

RI

Xue, J.Z., Deng, Z.Z., Huang, P., Huang, K.J., Benton, M.J., Cui, Y., Wang, D.M., Liu, J.B.,

SC

Shen, B., Basinger, J.F., Hao, S.G., 2016. Belowground rhizomes in paleosols: The hidden half of an Early Devonian vascular plant. Proceedings of the National Academy

NU

of Sciences of the United States of America 113, 9451–9456.

MA

Yao, L., Aretz, M., Chen, J.T., Webb, G.E., Wang, X.D., 2016. Global microbial carbonate proliferation after the end-Devonian mass extinction: Mainly controlled by demise of

D

skeletal bioconstructors. Scientific Reports 6, 39694, doi: 10.1038/srep39694.

PT E

Zhai, Y.P., 1981. An outline of the Devonian stratigraphy of Gansu province. Journal of Stratigraphy 5, 148–155. (in Chinese).

CE

Zhao, X.H., Wu, X.Y., Gu, Q.C., 1986. Late Devonian flora from southern Ningxia. Acta

AC

Palaeontologica Sinica 25, 544–559. (in Chinese).

ACCEPTED MANUSCRIPT 24

Fig. 1. Map and measured sections. (A) Map showing the study site in Gansu province, China; (B) Simplified geological map and the location of the measured sections of study area. ① CY section; ② DHM 2 section; ④ DHM 4 section. Fig. 2. Field characteristics of late Devonian: (A) root traces; (B-C) drab-haloed root traces; (D-E) calcareous lungfish burrows; (F) sandy lungfish burrows; (G) slickensided and

PT

patchily reduced clay skins; (H) interbedded red clayey and grey-green sandstone

RI

palaeochannels.

Fig. 3. Field scale of palaeosol profile development in the late Devonian (Famennian)

SC

Shaliushui Formation of northwest China: (A) clast of palaeosol redeposited in conglomerate with redeposited calcareous soil nodules (DHM 4); (B) thin paleosol with

NU

mudcracks (near DHM 4); (C) measured DHM 2 ; (D) calcareous nodular paleosol near CY; (E) measured CY.

MA

Fig. 4. The thin section characters of late Devonian; (A) spar filled root trace in micrite; (B) clay skin around micrite in clayey plasma matrix; (C) spar filled root trace with diffusion

D

ferran in micrite; (D) diffusion ferran around micritic nodule; (E) intertextic silasepic

PT E

parent siltstone; (F) calcite-filled tapering root trace in micrite. Red bars = 200 µm in A, B, D and F, 50 µm in C and E.

CE

Fig. 5. Vertically distributed characteristics of geochemical elements of late Devonian from measured section of paleosols classified in terms of degree of development. χ represent

AC

magnetic susceptibility. A represents horizon of organic mineral mixed with mineral fracion; Bk represents horizon of calcareous nodules; Bt represents horizon of subsurface clay enrichment; Bw represents horizon of deformed and slickensided claystone. Fig. 6. Thermomagnetic curves and hysteresis loops of red-beds samples from the measured sections. (A) the upper panel, thermomagnetic curves of CY-2 and DHM 2-4 samples, red is warming curves, blue is cooling curves; (B) the lower panel, corresponding to hysteresis loops. the values of saturation remanence (Mrs), saturation magnetization (Ms), coercivity (Bc) and coercivity of remanence (Bcr) are listed inside the hysteresis

ACCEPTED MANUSCRIPT 25

AC

CE

PT E

D

MA

NU

SC

RI

PT

loop plots, all values are prior to paramagnetic correction.

ACCEPTED MANUSCRIPT 26

Highlights: 1. Laboratory analysis confirms field recognition of paleosols in the Shaliushui Formation 2. Paleosols with large nodules and more clay have higher Ba/Sr ratios and magnetic susceptibility 3. Woody root traces and carbonate nodules at shallow depth indicate dry woodlands and

PT

shrublands 4. These paleosols are evidence of semi-arid to sub-humid, highly seasonal, Late Devonian

RI

climate

AC

CE

PT E

D

MA

NU

SC

5. Paleosols are a novel archive of Late Devonian paleoclimate in northwest China

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6