Mineral magnetic characteristics of the late Quaternary coastal red sands of Bheemuni, East Coast (India)

Journal of Applied Geophysics 134 (2016) 77–88

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Mineral magnetic characteristics of the late Quaternary coastal red sands of Bheemuni, East Coast (India) Priyeshu Srivastava a,1, S.J. Sangode a,⁎, Nikita Parmar b, D.C. Meshram a, Priyanka Jadhav a, A.K. Singhvi b a b

Department of Geology, Savitribai Phule Pune University, Pune 411 007, India Physical Research Laboratory, EPSD, OSL Lab, Ahmadabad 380 009, India

a r t i c l e

i n f o

Article history: Received 22 September 2015 Received in revised form 30 July 2016 Accepted 23 August 2016 Available online 26 August 2016 Keywords: Coastal red sands Mineral magnetism Hematite OSL dating Quaternary Bheemuni

a b s t r a c t The voluminous red sand deposits of Bheemuni in the east coast of India provide record of coastal land-sea interaction during the late Quaternary climatic and eustatic oscillations. Limited information on the origin and depositional environments of these red sands and their chronology is available. We studied two inland to coast cross profiles from Bheemuni red sand deposits using mineral magnetism, color characteristics and Citratebicarbonate-dithionite (CBD) extractable pedogenic iron oxides over 23 horizons along with optically stimulated luminescence (OSL) chronology at 6 horizons. The oldest exposed bed had an optical age of ~48.9 ± 1.7 ka. Differential ages between the two parallel sections (SOS = ~48.9 ± 1.7 to 12.1 ± 0.3 ka and IMD = ~29.3 ± 3.5 ka) suggest laterally shifting fluvial sedimentation. Both the profiles show significant amount of antiferromagnetic oxide (hematite) along with ferrimagnetic (magnetite/maghemite) mineral composition. The granulometric (/domain-) sensitive parameters (χFD, χARM, SIRM/χLF and χARM/χLF) indicate variable concentration of superparamagnetic (SP) and single domain (SD) particles between the two profiles. The higher frequency dependent and pedogenic magnetic susceptibilities (χFD and χpedo) in the younger (IMD) profile suggest enhanced pedogenesis under a warm-wet climate post ~ 29.3 ka and also during Holocene. A combination of hard isothermal remanent magnetization (HIRM) and redness rating (RR) index indicates distinct but variable concentration of a) crystalline and b) poorly crystalline (pigmentary) hematites in both the profiles. We consider that the former (#a) is derived from hinterland red soils and possibly due to post-depositional diagenesis, and the latter (#b) precipitated from the dissolved iron under fluvial regime imparting the unique red coloration to Bheemuni sands. Partial to complete alteration of ferromagnesian minerals due to pedogenesis in hinterlands under warm-wet climate was therefore the principal source of reddening of the Bheemuni sands. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The conspicuous red color of red beds, sand dunes, coastal sands and soils/paleosols has been studied for their genesis, rate of reddening, mineralogy, post-depositional diagenetic changes, climate and pedogenesis/ latosolization (Walker, 1967, 1974, 1979; Folk, 1976; Pye, 1981, 1983; Gardner, 1981, 1983; Gardner and Pye, 1981; Schwertmann et al., 1982; Wasson, 1983; Torrent and Schwertmann, 1987; Roskin et al., 2012; Hu et al., 2014). The red color is generally linked to the ferric iron oxidehematite (Walker, 1974; Folk, 1976; Pye, 1983; Gardner, 1983; Gardner and Pye, 1981; Schwertmann et al., 1982; Torrent and Schwertmann, 1987). The hematite coating over quartz grains as rubification process is observed widely (Ben-Dor et al., 2006) and the source of hematite varies ⁎ Corresponding author. E-mail address: [email protected] (S.J. Sangode). 1 Presently: School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067, India.

http://dx.doi.org/10.1016/j.jappgeo.2016.08.005 0926-9851/© 2016 Elsevier B.V. All rights reserved.

from weathering of iron bearing minerals from parent rocks to aeolian dust (Folk, 1976; Walker, 1979; Gardner and Pye, 1981; Anton and Ince, 1986). The iron oxide coating involves release and deposition of free iron under favourable temperature and precipitation conditions (Roskin et al., 2012) and the depositional environments like coastal sands of Quaternary period provide such conditions. The coastal red sands occur in the south-eastern regions of the Indian coast (Gardner, 1981, 1986; Joseph et al., 1999; Nageswara Rao et al., 2006; Thrivikramaji et al., 2008; Jayangondaperumal et al., 2012; Alappat et al., 2013). The detailed study including color characteristics, mineralogy and chronology is available only for the “Teri sands” of Tamil Nadu coast (Gardner, 1981, 1983; Jayangondaperumal et al., 2012; Alappat et al., 2013). The thick and extensive red sand deposits of Bheemuni (locally known as “Erra Matti Dibbalu”) are exposed between Visakhapatnam and Bheemunipatnam, north of Teri sands and are still under debate over its origin and red coloration (e.g. Mahadevan and Sathapathi, 1949; Vishnuvardhana Rao and Durgaprasada Rao, 1968; Durgaprasada Rao and Srihari, 1980; Srihari, 1980; Madabhushi, 1995;

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Nageswara Rao et al., 2006). Clay mineralogy of these red sands has been reported by Durgaprasada Rao and Srihari (1980). The detailed studies on their color characteristics, magnetic properties and chronology have not been attempted. In this context we present here an integrated study based on mineral magnetism, redness rating (Munsell) and CBD extractable fine-grained pedogenic iron oxides (magnetite/maghemite) along with representative OSL ages from two ~3 km long red sand profiles (~15 m thick) from Bheemuni. Mineral magnetism is established as an efficient tool to characterize magnetic mineralogy (i.e. iron oxides), their concentration and grain size (/domain size) that can be investigated for its sensitivity to the environmental processes governed by a combination of factors including formation, transportation, deposition and post-depositional alterations (Liu et al., 2012). These attributes make mineral magnetism suitable for the characterizations of coastal red sands (Walden and White, 1997; Walden et al., 2000; Gehring et al., 2014). The color characteristics of soil and sands are used to infer the relative ages of sediments, their

provenance and depositional environment (Torrent et al., 1980; Gardner and Pye, 1981; Schwertmann et al., 1982; Lancaster, 1989; Yang et al., 2001; Roskin et al., 2012 and references therein). OSL chronology provides the timing and time scales of these coastal red sands and enable an understanding of their relationship with contemporary climate changes. Luminescence dating has been successfully used to date Teri sands (e.g., Jayangondaperumal et al., 2012). 2. Study area The Bheemuni red sand deposits cover an area of ~ 10 km2 (up to ~ 2 to 5 km inland from shoreline) and are located ~ 20 km North of Visakhapatnam and are exposed from Ramakrishna beach to Bheemunipatnam (Fig. 1). These deposits are bounded by Narasimha hill range in the North, Pedda-Gedda River in the South, Chitti-Gedda River (a tributary of Pedda-Gedda) in the West and Bay of Bengal in the East (Nageswara Rao et al., 2006). The deposits are typically

Fig. 1. The location map of study area with some geomorphological features. The rectangle shows the area studied in detail. (Adopted from Nageswara Rao et al., 2006.)

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N15 m in height from mean sea level (m.s.l). The host rocks in the region are khondalites, leptinites (garnet-biotite gneisses) and charnokites. Although considered to be the product of several geomorphic processes involving multiple cycles of deposition, the more recent deposition is reported as dominated by dune sand (Nageswara Rao et al., 2006). 2.1. Climate The average maximum and minimum temperature of the region are ~32 °C and 23 °C, respectively. The average annual rainfall is ~964 mm with average maximum rainfall of ~238 mm occurring in October. The average relative humidity is ~ 72% categorizing the study area under high humid regions. 2.2. Sampling The basin is exposed by several gullies and four major channels (of different lengths) with their heads in the central part of the exposed upland basin and these debouch into the sea. We selected the two longest profiles from these channels with their highest and youngest

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member being inland and the oldest part exposed near the coast due to deeper incision. The sampling was conducted from this top part and continued up to the mouth of the river that exposed the older lithounits. Thus the lowermost outcrop is also the distal part while the upper most is the proximal part of this small basin. It was not possible to trench the section in the central part due to restrictions from forest departments. Fresh excavations were carried out from two profiles namely SOS and IMD and samples were collected by digging in to the verticalfaces by up to ~20 cm (Fig. 2). For OSL chronology, samples were collected by hammering GI pipes into the sampling horizon with due care for light exposures during sampling. For magnetic analysis, samples were also collected from representative horizons by fresh digs. The aggregated samples were mechanically disintegrated by wooden hammer and dried at room temperature to get ~10 g of specimen by coning and quartering for the magnetic analysis. 3. Methodology The profiles SOS and IMD provide sections from inland towards coast younger to older (see Fig. 2d and e).

Fig. 2. (A) The Google earth image of the study area depicting the distribution of the part of red sand deposits. (B) The badland topography of the red sand deposition here incised by the tributary of Pedda-Gedda River cutting the profile. (C) The red sand exposure showing the rills eroding the profile. (D) The diagram showing the distribution of sampling from inland to coast (vertical scale exaggerated). (E) The SOS and IMD profiles with respective chronology.

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3.1. OSL chronology Luminescence methods provide the age of deposition of sediments. The method works on the basis that mineral grains constituting the sediment lose their geological luminescence (acquired by them during their geological antiquity) due to daylight exposure in the course of their weathering and transport. On deposition as sediment, burial of grains from overlying strata further sunlight exposure stops and a reinduction of luminescence are initiated through irradiation by natural radiation field arising from the decay of ambient natural radioactivity comprising 238U, 232Th and 40K along with cosmic rays. The samples were extracted using a sequential chemical treatments with 1 N HCl (to remove carbonates), and H2O2 (to remove organic material) followed by sieving to get 90–150 μm grain size fraction. This fraction was passed through Frantz Isodynamic separator to isolate quartz and feldspars from other magnetic minerals. The fraction was etched by 40% HF for 60 min followed by HCl for 30 min to remove the alpha skin and dissolve residual feldspars. The purity of quartz extract was checked via its IRSL stimulation. The pure quartz was spread as monolayer of 3 mm diameter prepared using silicospray and measured on a RISO TL-DA 20 system. Estimation of paleo-dose was carried out using a 5 point SAR protocol (Murray and Wintle, 2000). The radioactivity of samples was measured using thick source alpha counting and NaI (Tl) scintillation spectrometry. 3.2. Redness rating All the three dimensions of color i.e. hue, value and chroma for both dry and moist samples under day light were noted using Munsell color notations (Munsell Color and U.S. Department of Agriculture National Resource Conservation Service, 1998). To express the color coded values quantitatively for comparison, redness rating (RR) was calculated using the equation RR = H × C/V where C and V are numericals of chroma and value of Munsell notation (Torrent et al., 1980). The numerical values for H were adopted after Torrent et al. (1980): 12.5 for Munsell hue of 7.5R; 10 for 10R; 7.5 for 2.5YR; 5 for 5YR; 2.5 for 7.5YR and 0 for 10YR. 3.3. Mineral magnetic analysis The magnetic susceptibility (χ) is analysed to assess the bulk concentration of magnetic minerals (Thompson and Oldfield, 1986; Dearing, 1999). The low and high frequency magnetic susceptibilities (i.e. χLF and χHF) were measured at 0.46 and 4.6 kHz frequencies, respectively, using standard MS2B laboratory sensor of Bartington. The frequency dependent susceptibility (χFD = χLF − χHF) and its percentage (χFD% = [(χLF − χHF / χLF) × 100]) were calculated to infer the qualitative and quantitative estimates of fine grain ferrimagnets present at superparamagnetic (SP) and single domain (SD) boundary (Dearing et al., 1996). Anhysteretic Remanence Magnetization (ARM) was grown using a Magnon AFD-300 instrument with peak alternating field of 100 mT in the presence of a DC bias field of 0.1 mT. ARM was normalized by the applied DC bias field and expressed as ARM susceptibility (χARM, Verosub and Roberts, 1995). The χARM is sensitive to SD ferrimagnetic mineral concentration (Banerjee et al., 1981; King et al., 1982) and the ratio of χARM/χLF indicates relative variation in concentration of fine SP and coarse SD magnetic particles in bulk samples (Maher, 1988). Isothermal Remanence Magnetization (IRM) was grown in different forward fields up to 2200 mT (2.2 T) and in back fields of − 300 mT using an ASC Impulse Magnetizer and remanences were measured using a Molspin Spinner Magnetometer. The IRM analysis was carried out to estimate the concentration and granulometry of ferri- and antiferromagnetic minerals based on various parameters and ratios. The saturation isothermal remanent magnetization (SIRM) was measured at 1000 mT (or 1 T), a value field adopted in most of the

published work. The hard isothermal remanent magnetization [HIRM = 0.5 × (SIRM + IRM− 300 mT)] parameter is commonly used to estimate the concentration of antiferromagnetic minerals with its sensitivity to hematite (Thompson and Oldfield, 1986; King and Channell, 1991; Sangode and Bloemendal, 2004; Nie et al., 2010). The soft isothermal remanent magnetization parameter [Soft-IRM = 0.5 × (SIRM − IRM−20 mT)] is calculated as a proxy for concentration of ferrimagnetic minerals (Thompson and Oldfield, 1986; Lyons et al., 2010). The ratio SIRM/χLF is studied for relative concentration variations of very fine SP and coarse SD-MD magnetic particles (Maher, 1988). The demagnetization parameter S-ratio (IRM−300 mT/SIRM) was calculated to quantify the relative abundance of ferrimagnetic and antiferromagnetic minerals in the bulk sample (Thompson and Oldfield, 1986; Liu et al., 2007). Liu et al. (2007) proposed a magnetic parameter L-ratio for characterization of coercivity variations within antiferromagnetic minerals (i.e. hematite and goethite) to overcome ambiguities in interpretation of HIRM and S-ratio. The L-ratio is defined as the ratio of two remanences after AF field demagnetization of an IRM imparted in a 1 T field with a peak AF of 100 and 300 mT (IRMAF@300 mT/IRMAF@100 mT) (Liu et al., 2007). A modified L-ratio is proposed for a simpler analysis i.e. L-ratio = Hard300 mT/Hard100 mT, where Hard100 mT and Hard300 mT are remaining remanences of SIRM at 1 T after demagnetizing by backward field of − 100 mT and − 300 mT, respectively (Hao et al., 2008, 2009) and was adopted for present study. The relative percentage of magnetization (RPM) acquisition in each forward magnetic fields were plotted to find out the relative concentrations of ferrimagnetic and antiferromagnetic minerals based on their different saturation remanence levels (Srivastava et al., 2015). The calculations were made as Rpm = [(IRM2F − IRM1F/IRM2F) × 100] where IRM2F is the succeeding step of first forward field acquired magnetization (IRM1F).

3.4. Thermo-magnetic analysis Temperature dependent magnetic susceptibility (k-T) was analysed for selective samples in stepwise thermal treatment up to 700 °C using Bartington MS2WFP susceptibility meter. The k-T is commonly analysed for identification of various magnetic minerals in samples based on their Curie temperature (TC).

3.5. Citrate-bicarbonate-dithionite (CBD) extraction The Citrate-bicarbonate-dithionite (CBD) method of iron oxide extraction was adopted from van Oorschot and Dekkers (1999) and Mehra and Jackson (1960). The technique is based on reductive iron dissolution with sodium dithionite as reducing agent and sodium citrate as chelating agent to bind the dissolved iron. Sodium bicarbonate was used to buffer the H+ loss during the reaction. Approximately 4 g of ovendried (at ~ 35 °C) samples were placed in a 100 ml plastic centrifuge tubes and solutions of 40 ml 0.3 M sodium citrate (Na3C6H5O7.2H2O) and 5 ml 1 M sodium bicarbonate (NaHCO3) were added. The tubes were placed in a water bath at 80 °C temperature. Further, 1 g sodium dithionite (Na2S2O4) was added and stirred thoroughly. The samples were kept at 80 °C temperature for 15 min and stirred at every 5 min. After heating for 15 min, the samples were centrifuged for 10 min at 3500 g (4417 rpm) to separate the liquid from sample. The samples were then rinsed with de-mineralized water (~50 ml) and again centrifuged at the same frequency to separate the solids from rinsing liquid. The liquid was decanted and samples were placed in an oven to dry at ~40 °C for overnight. The extraction was repeated again. The magnetic susceptibility (χ at 0.46 kHz) of dry samples was measured before and after the CDB extraction to distinguish the contribution by pedogenic fine-grained magnetite/maghemite. The pedogenic magnetic susceptibility (χpedo) was calculated as =Pre CBD (χ) − Post CBD (χ).

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Table 1 The summary of OSL chronology of both IMD and SOS profiles of Bheemuni red sands, India. Sample name

U (ppm)

Th (ppm)

K (%)

CR (μGy/a)

ED (Gy) weighted mean

Dose rate (Gy/ka)

Age (ka)

IMD/OSL/1 IMD/OSL/2 IMD/OSL/3 SOS/OSL/1 SOS/OSL/2 SOS/OSL/3

8.8 ± 0.9 4.3 ± 1.29 5.5 ± 0.5 8.6 ± 0.8 4 ± 0.4 7.6 ± 0.7

75.1 ± 12 41.3 ± 5.5 124 ± 12.3 183.8 ± 18.3 68.3 ± 6.8 178.5 ± 17.8

0.33 ± 0.1 0.24 ± 0.1 0.4 ± 0.1 0.78 ± 0.12 0.8 ± 0.12 0.79 ± 0.12

191.5 ± 38.3 48.3 ± 9.6 34.6 ± 6.9 165.3 ± 33.1 45.6 ± 9.1 29.9 ± 5.9

6.4 ± 0.04 118. ± 1.87 21.7 ± 0.18 184.6 ± 3.4 196 ± 6.8 29.6 ± 0.24

7.557 ± 0.842 4.037 ± 0.478 10.095 ± 0.068 15.353 ± 0.108 4.010 ± 0.025 14.644 ± 0.099

0.85 ± 0.1 29.3 ± 3.5 2.2 ± 0.1 12.1 ± 0.3 48.9 ± 1.7 2.02 ± 0.02

4. Results

4.2. Mineral magnetic variations in the profiles

4.1. Chronology

4.2.1. SOS profile The χLF for SOS profile samples varies from ~ 2 to 11 (×10−8m3 kg−1) with a mean value of ~4.56 depicting variable concentration of magnetic minerals in different horizons. The χFD% ranges from ~ 3 to 8% broadly suggesting SD-SP mixed particle size of magnetic minerals (Dearing, 1999). The χFD values range from ~ 0.06 to 0.72 × 10−8m3 kg−1 also indicate poor concentration of SP particles. The decreasing trend in χLF from inland towards coast suggests a progradation from proximal to distal distribution of heavy minerals. This is considering that χLF is controlled by detrital modes (Fig. 3a). The SIRM/χLF and χARM/χLF however do not show any change from inland to coast suggesting no relative variation in concentration of fine SP and coarse SD magnetic particles. The S-ratio showing nearly constant values further depict negligible variation in the abundance of ferri- and antiferromagnetic minerals. The other magnetic concentration parameters SIRM, HIRM, Soft-IRM and χARM co-vary with magnetic susceptibility (χLF). However, an anomalously high χLF is noted for SOS11 sample (near the coast) suggesting high magnetic mineral concentration (Fig. 3a). This high concentration of magnetic minerals can be due to the sediment mixing from gullying activity and/or from the sea wave intrusions. Heavy mineral rich sand layers are present in the coastal profiles of Bheemuni (Laxmi et al., 2011) which may be the source for high χLF in this sample. As alluded to, OSL chronology also yielded the anomalous age (~2.02 ± 0.02 ka) for this sample which further reflects the influence of modern sands. The L-ratio shows minor variations (~0.37 to 0.44) indicating lower effect of coercivity variation

The experimental data and ages of samples from both the profiles are summarized in Table 1 and are plotted along with the stratigraphic sequence in Fig. 2. A maximum age of ~48.9 ± 1.7 ka was obtained in the SOS profile suggesting that deposition of these red sands occurred during marine isotope stage (MIS 3). The youngest age for this profile shows ~12.1 ± 0.3 ka indicating that the sedimentation continued up till Holocene. The IMD profile shows ~ 29.3 ± 3.5 ka as the oldest age and ~0.82 ± 0.1 ka for the top. Whereas the younger age at the top of the profile may be contaminated and influenced by the modern pedogenic activity and mixing; the older age of ~29.3 ± 3.5 ka depict deposition during MIS 3. The two anomalous ages ~ 2.02 ± 0.02 ka and ~ 2.2 ± 0.1 ka were recorded in the lower part of the sections in both SOS and IMD profiles. The lowermost i.e., the oldest part of the sections are exposed near the beach where anthropogenic activity is common. This part is also influenced by the coastal/beach processes, and there can be significant mixing of the Bheemuni sands with the recycled beach sands. These effects are not perceptible due to red coloration of both the sands and diffused bedding contacts. This inference is buttressed by anomalously enhanced magnetic concentration in the sample from this part which suggests its mixing with the heavy mineral rich beach sands resulting into the erroneous ages besides enhanced magnetic mineral concentration. Therefore we consider these anomalous ages from the lower part as being contaminated and were neglected.

Fig. 3. Various mineral magnetic parameters plotted against OSL chronology for the two parallel coastal sand profiles (SOS and IMD, for details see text). The units of χLF and χFD = 10−8 m3 kg−1, χARM = 10−5 m3 kg−1, for SIRM, Soft-IRM and HIRM = 10−5 Am2 kg−1, χARM/χLF = k and for SIRM/χLF = kA/m.

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amongst antiferromagnetic minerals suggesting suitability of HIRM and S-ratio parameters for estimation of antiferromagnetic minerals concentration.

sands and suggests a negligible variation in coercivity of antiferromagnetic minerals (Liu et al., 2007). 4.3. IRM acquisition

4.2.2. IMD profile The χLF of IMD profile samples vary between ~ 3.5 to 11.8 (×10−8m3 kg−1) with a mean value of ~7.64 indicating higher concentration of magnetic minerals compared to the adjoining SOS profile. The χFD% varies between ~7 to 12.7% and suggests significant presence of SP magnetic particles. The χFD values between ~0.25 to 1.2 × 10−8m3 kg−1 indicate poor but comparatively higher concentration of SP magnetic particles than in the SOS profile. The χLF shows a decreasing trend from inland towards coast (IMD-1 to IMD-8) and co-vary with SIRM, Soft-IRM and HIRM concentration parameters suggesting bulk reduction in magnetic minerals concentration (Fig. 3b). However, χFD and χARM for IMD-1 to IMD-3 shows minor increasing trend suggesting enhanced SP-SD magnetic mineral concentrations. The SIRM/χLF further shows minor decrease in values for the samples from IMD-1 to -3, suggesting higher SP content along with higher χARM/χLF to depict increased SD particles. The high χLF and Soft-IRM for IMD-9 sample depict increased ferrimagnetic concentration. The S-ratio (−0.90) further supports relative abundance of ferrimagnetic concentration for IMD-9 and 10 samples. The increased χFD, χARM and lower SIRM/χLF suggest higher SP and SD ferrimagnetic concentration for IMD-9 and 10, and decreased χLF, χFD, χARM and Soft-IRM suggest reduced ferrimagnetic concentration for IMD-11(Fig. 3b). However slightly increased HIRM and SIRM along with higher S-ratio suggest increased antiferromagnetic concentration for IMD-11. The L-ratio shows no change from inland to coastal red

The IRM spectra show that both SOS and IMD profile samples do not acquire saturation up to the maximum applied field of 2.2 T (Fig. 4a and c). This suggests that antiferromagnetic minerals (e.g., hematite and goethite) are significant contributors in the magnetic remanence of studied samples considering that saturation of these minerals occur above 1 T while most of ferrimagnetic minerals (magnetite and maghemite) are saturated up to 300 mT (Thompson and Oldfield, 1986). The Rpm for the SOS samples shows higher acquisition between 0.1 and 0.7 T (Fig. 4b). Rpm for IMD profile samples show lower intensity of acquisition after 0.5 T and continue to acquire the remanence up to 2.2 T depicting lower concentration of antiferromagnetic minerals (Fig. 4d). 4.4. Correlations of mineral magnetic parameters The χLF and χFD show strong linear correlation (R2 N 0.80) for both SOS and IMD profiles (Fig. 5a). The magnetic parameters χARM vs. χLF and SIRM vs. χLF for the SOS profile show strong linear correlations (R2 = 0.94 and R2 = 0.96, respectively) and weaker correlations (R2 = 0.64 and R2 = 0.66, respectively) for the IMD profile (Fig. 5b and c). This indicates that SD particles are important susceptibility and remanence carrier in the SOS profile whereas grain size distribution in the IMD profile is not constant and show relative variations in concentration of SD to SP grains. The SIRM and χARM bivariate plot show strong

Fig. 4. The IRM acquisition curves showing under-saturation up till 2.2 T for the bulk samples from IMD and SOS profiles. The RPM shows high intensity of saturation remanence acquisition between 0.1 and 0.7 T for the SOS profile whereas relatively lower intensity can be observed of remanence acquisition after 0.5 T in IMD profile.

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Fig. 5. The bivariate plots of magnetic parameters used to derive the inferences on granulometry and concentration (for details see text).

correlation (R2 = 0.84) for the SOS profile further affirming that remanence magnetization is dominantly carried by SD particles. On the other hand a weak correlation (R2 = 0.10) for the IMD profile suggests mixed mineral concentrations of SD and MD particles (Fig. 5d). Overall, mineral magnetic results indicate that both the profiles contains significant amount of antiferromagnetic minerals (hematite/goethite) along with the ferrimagnetic minerals (magnetite/maghemite). The magnetic granulometric/domain parameters (χFD, χARM, SIRM/χLF and χARM/χLF) indicate variable concentration of SP and SD magnetic minerals in both SOS and IMD profiles. 4.5. Redness rating (RR) The details of Munsell color notations and redness rating is summarized in Table 2. The RR index for the SOS profile ranges from ~7.75 to 15 whereas that for the IMD profile varies between ~13.5 to 17.5 suggesting distinct redness for both the profiles. The increasing RR has been directly related to increasing concentration of hematite (Torrent et al., 1980, 1983). Therefore, higher average RR for the IMD profile suggests high hematite concentration compared to the SOS profile. The RR plotted against litho-section and chronology of SOS profile show decreasing trend from inland to coast suggesting decreasing concentration of hematite whereas for the IMD profile it does not show any significant proximal to distal variability (Fig. 6a and b). 4.6. Temperature dependent magnetic susceptibility (k-T) The magnetic susceptibility was analysed at constant incremental and reversible temperature intervals for the identification of magnetic minerals. Magnetic ordering of minerals above specific temperatures

Table 2 The Munsell color notations and respective redness rating (RR) for the coastal red sand samples of Bheemuni. The wet color notation represents the Munsell color of the residue water after rinsing the coastal red sands. Sample

Munsell color (dry)

Munsell color (wet)

RR (Dry)

RR (Wet)

RR (Avg.)

SOS profile SOS-1 SOS-2 SOS-3 SOS-4 SOS-5 SOS-6 SOS-7 SOS-8 SOS-9 SOS-10 SOS-11 SOS-12

2.5YR, 2.5/4 2.5YR, 3/8 2.5YR, 4/8 2.5YR, 5/8 2.5YR, 4/6 5YR, 4/6 2.5YR, 4/8 2.5YR, 4/6 2.5YR, 5/8 5YR, 5/8 2.5YR, 4/8 5YR, 5/8

2.5YR, 3/6 5YR, 5/8 5YR, 5/8 5YR, 5/8 2.5YR, 5/6 5YR, 5/8 5YR, 5/8 5YR, 5/8 5YR, 5/8 5YR, 5/8 2.5YR, 4/8 5YR, 5/8

12 20 15 12 11.25 7.5 15 11.25 12 8 15 8

15 8 8 8 9 8 8 8 8 8 15 8

13.5 14 11.5 10 10.13 7.75 11.5 9.63 10 8 15 8

IMD profile IMD-1 IMD-2 IMD-3 IMD-4 IMD-5 IMD-6 IMD-7 IMD-8 IMD-9 IMD-10 IMD-11

10R, 4/8 10R, 4/6 10R, 3/6 10R, 3/6 10R, 4/6 10R, 4/8 10R, 4/8 10R, 4/8 10R, 4/6 10R, 3/6 10R, 3/6

2.5YR, 3/6 2.5YR, 3/6 2.5YR, 3/6 2.5YR, 3/6 2.5YR, 5/8 2.5YR, 3/6 2.5YR, 3/6 2.5YR, 3/6 2.5YR, 3/6 2.5YR, 4/8 2.5YR, 4/8

20 15 20 20 15 20 20 20 15 20 20

15 15 15 15 12 15 15 15 15 15 15

17.5 15 17.5 17.5 13.5 17.5 17.5 17.5 15 17.5 17.5

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Fig. 6. The redness rating (RR) is plotted against chronology of inland to coast sections of the SOS and IMD red sand profiles.

is lost and the temperature is known as Curie Temperature (TC) [analogues Néel temperature (TN) for antiferromagnetic minerals] (Dunlop and Özdemir, 1997). Due to weak susceptibilities majority of the k-T results have shown noisy curves in the Bartington sensor. Therefore only representative samples are produced here for the interpretations. The k-T characteristics of sample SOS-1 show paramagnetic behavior after ~640 °C indicating presence of maghemite (Fig. 7a). The low magnetic susceptibility and noisy curve for sample SOS-3 do not permit identification of the magnetic minerals. The SOS-11 sample shows TC at ~600 °C indicating presence of magnetite (Fig. 7a). All the IMD samples represent similar thermal characteristics showing TC between ~640 and 650 °C indicating presence of maghemite (Fig. 7b).

(Fine et al., 1993, 1995; Verosub et al., 1993; Hunt et al., 1995). The CBD extraction of iron is used to estimate the pedogenic iron oxides (i.e. fine-grained magnetite/maghemite) which lead to the decrease in magnetic susceptibility. The reduced magnetic susceptibility (χpedo) is therefore directly proportional to the concentration of pedogenic magnetic minerals. The χpedo result shows an average 40% loss for the SOS and ~73% loss for the IMD profile suggesting relatively higher concentration of pedogenic magnetic minerals in the IMD profile (Fig. 8a and b). 5. Discussion 5.1. Ages of Bheemuni red sands

4.7. Pedogenic magnetic susceptibility (χpedo) The combined CBD and magnetic susceptibility (χ) analysis is used to distinguish the pedogenic and lithogenic magnetic components

There are few attempts to constrain the ages for the Bheemuni sands. Carbonate concretions from the upper horizons of the Bheemuni produced the age of ~ 5840 years BP (Srihari, 1980; Durgaprasada Rao

Fig. 7. The temperature-dependent magnetic susceptibility (k-T) of samples from SOS and IMD profiles.

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(Singhvi et al., 1986). Jayangondaperumal et al. (2012) produced various younger ages for different depositional environments of the Teri sands from N25 to 5.6 ka. Nageswara Rao et al. (2006) proposed that the wave cut terrace in the Bheemuni area lying at + 12 m asl could be product of higher sea levels during the last major interglacial and can be considered as old as 125 ka. The oldest ages produced for the SOS (~48.9 ± 1.7 ka) and for IMD profile (~29.3 ± 3.5 ka) in the present study suggests that deposition of these red sands was initiated at least in the period corresponding to the marine isotope stage 3 (MIS 3). The present results are therefore in accord with the sedimentologic, geomorphic and sea level related attributes (Nageswara Rao et al., 2006; Durgaprasada Rao et al., 1982; Gardner, 1995). 5.2. Magnetic granulometry A summary of mineral magnetic variations of SOS and IMD profiles is produced in Table 3. The OSL chronology depicting younger age (~ 29.3 ± 3.5 ka) of deposition for the IMD profile relative to the SOS profile (~ 48.9 ± 1.7 ka) suggests laterally shifting deposition from SOS to IMD. The magnetic properties distinctly suggest presence of SP and SD particles in variable amount amongst the two profiles. The source of SP and SD particles in both the profiles appears to be the red soils in the hinterlands. The low concentrations of SP and SD particles in the SOS profile (low χFD and χARM) indicate lower pedogenic content relative to IMD profile. The comparatively higher amount of pedogenic magnetic minerals in the younger IMD profile (i.e., ~29.3 ka) suggests prevalence of warm-wet conditions which became more pronounced during Holocene as enhanced pedogenic fraction is discovered in the younger, near inland red sand (Fig. 3). The present study area is in the tropical warm and wet climate that facilitates intense weathering of parent silicate rocks. The ferrihydrite formation occurs under wet climate and its transformation to hematite during warm-dry climate which is anticipated as the source for red coloration. The results therefore suggest that pedogenic hematite is from the hinterland red soils (developed under warm-wet climate) in both the profiles imparting the red colorations. Fig. 8. The magnetic susceptibility of pre- and post-CBD treated samples of the SOS and IMD profiles. The relatively higher loss in magnetic susceptibility (χpedo) of IMD profile suggests higher concentration of pedogenic magnetic concentration.

et al., 1982) and the depositional ages of host red sands was considered to be older (late Pleistocene). Rath (1996) indicated middle Paleolithic age (~ 20–30 ka) for the artefacts collected in red sands of the east coast. The red sand coastal dunes at Patirajavela in northern Sri Lanka area indicated two possible episode of deposition as 25 ka and 70 ka

5.3. Color and magnetic characteristics of the coastal red sand deposits Soil and sediment color is commonly attributed to mineral composition, climates and depositional environment (Torrent et al., 1980, 1983; Schwertmann et al., 1982; Singh and Gilkes, 1992; Yang et al., 2001; Bullard and White, 2002; Hu et al., 2014). The colorimetric quantification by RR index in the present study suggested a relatively higher concentration of hematite in IMD. This contradicts with the magnetic

Table 3 The summary of results of mineral magnetic parameters used in the present study (see text and references). SOS profile

IMD profile

Inferences

Low χLF and SIRM = Low magnetic concentration in the profile Low χFD = lower concentration of pedogenic SP ferrimagnetic particles Low χpedo = low concentration of pedogenic magnetic minerals Strong χARM and SIRM correlation = dominant SD particles as magnetic remanence carrier

High χLF and SIRM = High magnetic concentration in the profile High χFD = higher concentration of pedogenic SP ferrimagnetic particles High χpedo = high concentration of pedogenic magnetic minerals Poor χARM vs. SIRM correlation = SD-MD mixed granulometry

Higher pedogenic magnetic concentration in younger IMD profile compared to the older SOS profile.

Lower RR = low hematite concentration Higher HIRM = high concentration of crystalline hematite Strong HIRM and χFD correlation = paragenesis of SP ferrimagnetic particle and hematite

Higher RR = high hematite concentration Lower HIRM = low concentration of crystalline hematite Weak HIRM and χFD correlation = No significant paragenesis of SP ferrimagnetic particle and hematite Poor correlation of HIRM and RR = Presence of poorly crystallized hematite OSL Chronology: Sand depositions after ~29 ka.

Strong correlation of HIRM and RR = Significant contribution of crystalline hematite in reddening OSL Chronology: Sand depositions from ~50 ka to 12 ka

SD ferrimagnetic particles are important remanence carrier in the SOS profile whereas mixed SD-MD particles in the IMD profile. The contrary behavior of the RR and HIRM for hematite concentration in both profiles indicates presence of crystalline and poorly crystalline hematite in both profiles. The crystallized hematite sourced from hinterland red soils and post-depositional diagenetic changes. The poorly crystalline hematite sourced from precipitation of dissolved iron in the fluvial regime. Significant differences in the chronology suggest lateral shifting phases of the sand depositions.

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Fig. 9. The HIRM and RR plot shows significant linear correlation for the SOS profiles whereas weak correlation was observed for the IMD section. Note here that we have removed possibly contaminated sample (SOS-11) with highest HIRM (an outlier) in the correlation to avoid any over/under estimation of correlation coefficients.

parameter HIRM which estimates a higher concentration of antiferromagnetic mineral (hematite) in the SOS profile (Fig. 3). Further, higher concentration of pedogenic magnetic mineral in the IMD profile compared to SOS suggests high production of pedogenic hematite in the hinterlands during the IMD sedimentation. This postulation is further attested by significant positive correlation between HIRM and RR index (R2 = 0.73) for SOS profile as compared to that of notably weak correlation (R2 = 0.06) for IMD profile (Fig. 9). Furthermore, HIRM and χFD also showed strong correlation (R2 = 0.84) for the SOS profile which suggest paragenesis or coeval formation of fine grained SP ferrimagnets and hematite (Fig. 10) (e.g., Torrent et al., 2010). A weak correlation of HIRM with χFD (R2 = 0.02; Fig. 10) is observed for the IMD profile. This behavior therefore can be explained here by the relative abundance of crystalline and poorly crystalline hematites in both the profiles leading to distinct reddening. The crystalline pedogenic hematite is suggested to have been formed in the hinterland red soils whereas poorly crystalline hematite formed by precipitation of dissolved iron (available from weathering of iron bearing minerals in hinterland) under fluvial regime. The poorly crystalline hematite acquires negligible remanence magnetization and will have no significant contribution to the bulk magnetic remanence of the samples, limiting the HIRM. Therefore poorly crystalline hematite precipitation may enhance the color of sand/sediments (which is well explained by the higher reddening recorded by RR in the IMD profile) but could not be quantified using routine magnetic parameters. This behavior justifies weak correlation between HIRM and RR for the IMD profile. The present assumption

therefore can be applicable to other studies reporting weak correlations between color characteristics and magnetic properties of hematite coated sediments. Further, the above discussed processes can only explain higher RR index but cannot explain lower content of crystalline hematite in the IMD profile which is expected to be higher than SOS profile due to increased pedogenic production of hematite in the hinterland during sedimentation. This behavior shown by both profile is possibly due to the post-depositional diagenetic changes. The older SOS profile would have undergone more intense diagenesis producing higher hematite leading to higher HIRM. It is also necessary to mention here that visual errors are limiting factor in determining the Munsell color although such results are found to be consistent with the spectroscopic methods (Torrent et al., 1983; Bullard and White, 2002). Thus a better quantification is warranted using detailed spectroscopic approaches like diffuse reflectance spectroscopy for precise quantification of the color and type of iron oxides. Detailed analysis like Ammonium oxalate iron extraction (e.g., Jackson et al., 1986) is further required to distinguish the poorly crystalline hematites. 5.4. Origin of reddening in the Quaternary coastal sands of Bheemuni Durgaprasada Rao and Srihari (1980) based on similar clay mineral assemblages in the Bheemuni red sands and the upland soils suggested detrital sedimentation with their transportation from upland soils. The present study based on mineral magnetism endorses this observation and further suggests three phases of hematite occurrence and reddening in the coastal sands of Bheemuni as below; (1) The hematite production by partial to complete oxidation of ferromagnesian and ferrimagnetic minerals in the hinterland due to weathering (pedogenesis) under tropical warm and wet climate. (2) Subsequent fluvial transportation and deposition of this hematite bearing source (pedogenic crystalline hematite and the dissolved iron) into the basin. The dissolved iron precipitated as poorly crystalline hematites. (3) The in situ production of hematite in the basin by oxidation of detrital iron silicates during diagenesis.

We therefore infer multiple stages of hematite formation and sources of reddening of the Bheemuni sands. The partial to complete alteration of ferromagnesian minerals during pedogenesis in the hinterlands (during late Quaternary) and the post-depositional diagenetic changes in the basin therefore appears to be prime factors of reddening of the Bheemuni sands. The study is based on the exposed sections while a large thickness of Bheemuni sands remains unexposed to the surface and detailed work is warranted by obtaining sediment cores from this basin. 6. Conclusions

Fig. 10. The bivariate plot of HIRM and χFD shows strong linear correlations for SOS and comparatively weak correlation for IMD profile samples. The strong correlation suggests paragenesis of fine-grained SP ferrimagnetic (magnetite/maghemite) and antiferromagnetic (hematite) minerals. The contaminated sample (SOS-11) with highest HIRM (an outlier) in the correlation is removed to avoid any over/under estimation of correlation coefficients.

The OSL ages indicate that the deposition of Bheemuni red sands in the east coast of India mainly occurred during MIS 3 of late Quaternary. The laterally shifting nature of sedimentation depicted by the variability amongst mineral magnetic parameters between the two adjacent sections and endorsed by the OSL ages depicts dominantly fluviatile depositional environments. The Bheemuni sands contain significant amount of antiferromagnetic oxide (hematite) along with variable amounts of SP and SD ferrimagnets (magnetite/maghemite) in both the studied sections. The higher frequency dependent magnetic susceptibility (χFD) and pedogenic magnetic susceptibility (χpedo) in the younger section indicate enhanced pedogenesis under warm-wet climate post ~ 29.3 ka and also during Holocene. Hematite, causing reddening of these sands shows a polymodal origin as a result of pedogenesis in the hinterlands producing a) pigmentary and b) crystalline varieties with

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a possible post-depositional/diagenetic occurrence within the basin. Since a larger part of the sedimentary record is subsurface, a detailed analysis on sediment cores from Bheemuni is warranted to understand the climatic-eustatic significance of the Bheemuni red sands. Acknowledgements The authors thank the Head, Department of Geology, Savitribai Phule Pune University (SPPU), Pune. Priyeshu Srivastava acknowledges the Council of Scientific and Industrial Research (CSIR), Government of India for the support of Senior Research Fellowship (09/263/1031/ 2014-EMR-1). AKS thanks the Indian Department of Science and Technology, for a JC Bose National fellowship and the Department of Atomic Energy for a Raja Ramanna Fellowship. NP thanks PRL for facilities to carry out her work. We acknowledge Department of Science and Technology, New Delhi for support to Rock Magnetic Lab, Department of Geology, SPPU, under DST-FIST program (grant SR/FST/ESII-101/2010). 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