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Summer Research Fellowship Programme of India's Science Academies

Environmental Rock Magnetic properties of Sediment Cores from Western Continental Margins of India

Jyotishma Borah

Gauhati University

Guided by:

Dr. Pratima M. Kessarkar

CSIR-National Institute of Oceanography

Abstract

Environmental magnetic measurements are a powerful tool in the assessment of the compositional properties of rocks, sediments and soils. Environmental magnetism is a recent technique which is non-destructive and also cost-effective, used for understanding the environmental conditions during sediment transportation, deposition and transformation of magnetic grains. Among the different enviromagnetic parameters concentration-dependent parameters include SIRM, susceptibility and S-ratio are the most important as far as composition is concerned.

 Keywords: Electromagnetic force, magnetic susceptibility, magnetic field, s-ratio, magnetics, SIRM, lorentz force.

Basic principles of magnetics

Magnetism and magnet

Combined electromagnetic forces are responsible for magnetism. It is a physical phenomenon that arises from the force effected by a magnet. Simply a magnet is a substance which causes repulsion or attraction to other objects due to the magnetic field it produces. A magnetic field exerts a force on particles in the field due to Lorentz force. Every magnet has two poles –one South pole and other North pole. South Pole of one bar magnet is attracted by North Pole of other bar magnet and vice versa. This is because there exists a field called magnetic field around every magnet. This magnetic field can also be called area of influence for a magnet. Magnetic field can be visualized using magnetic field lines (Fig.1) emanating from North Pole of the magnet and entering in the South Pole.

IMG_20180826_111508_1.png
    Magnetic field lines

    Origin of magnetism

    All substances are made up of atoms which are again consist of three elements- electrons, neutron and proton. Electrons which are negatively charges revolve around the positively charged protons and neutrons. If a bar magnet is cut into smaller and smaller pieces indefinitely, still even the smallest piece would show the properties of a magnet. So, magnetism is a property of a substance of atomic level. The negatively charged electrons spin in orbits around the nucleus, much like the Earth revolves around the Sun. The electrons spin as they orbit the nucleus much like the earth spins as it orbits the sun (Fig. 2). As electrons spin and orbit the nucleus they produce a magnetic field. A. M. Ampere first suggested the theory that magnetic fields were due to electric currents continually circulating within.

    IMG_20180826_110925.png
      Origin of Magnetism

      Magnetic field

      Magnetic field is the portion of space near a magnetic body or a current-carrying body, where there is a change of energy. The spinning and orbiting of the nucleus of an atom produces a magnetic field as does electrical current passing through a wire. The direction of the spin and orbit determine the direction of the magnetic field and the magnetic moment is the strength of that magnetic field.

      Types of magnetic materials

      Depending upon the atoms or ions in a substance it shows different magnetic behavior. The net magnetic moments that an atom or ion shows depends upon the strength of the magnetic field or magnetic moment. The net magnetic moment arises from incompletely filled shells that contain unpaired electrons. The atoms or ions in a solid or substance occupy fixed positions in their regular lattice positions in the crystalline structure. It reflects the symmetry of the substance and also controls the interaction between the atoms or ions. The direction of spin and orbit of the electron determines the direction of the magnetic field. The brief description of different types of magnetic materials are given below:

      1. Diamagnetic: Substances that have all paired electrons in the atoms and therefore have zero net magnetic moment is called diamagnetic materials; yet, there are some exceptions. When a diamagnetic material is placed in the magnetic field of a magnet, it will produce a slight magnetic field that opposes the main magnetic field. Both ends of a bar magnet will repel a diamagnetic material. If a diamagnetic material is placed in a strong external magnetic field, the magnetic field strength inside the material will be less than the magnetic field strength in the air surrounding the material. The slight decrease in the field strength is the result of realignment in the orbit motion of the electrons. Diamagnetic materials include zinc, gold, mercury, and bismuth. Another key concept in magnetism is that diamagnetic materials will oppose an applied magnetic field. Both ends of a magnet will repel diamagnetic materials. Diamagnetic substances show weak and negative magnetic susceptibility.

      Example: Quartz (SiO2), Calcite (CaCO3), Water (H2O) etc.

      2. Paramagnetic: In case of paramagnetic materials, the magnetic moment of unpaired electron orbital within individual atoms are uncoupled so that each atom behaves independently. When an external magnetic field is applied the resultant magnetic moment of the electrons aligns in the same direction. The paramagnetic minerals show weak and small positive magnetic susceptibility. Example: Clay minerals, mica minerals.

      IMG_20180826_111351.png
        Representation of magnetic moments in a Paramagnetic material

        3. Ferromagnetic: Ferromagnetism is generally associated with elemental iron, nickel and cobalt but it is also seen in many natural substances such as certain very important iron oxides. The atoms occupy close lattice positions so that the electrons can be exchanged easily. This exchange interaction results in a very strong magnetic field which aligns the magnetic moments of the electrons exactly parallel to the external magnetic field. Example: Fe, Ni, Co and many of their alloys.

        IMG_20180826_112929.png
          Representation of magnetic moments in a Ferromagnetic material

          4. Ferrimagnetic: There are certain materials that behave like ferromagnetic materials in absence of external field but their neighboring pairs of electrons point in opposite direction. These materials are called ferromagnetic materials. In these type of substances there is a generation of antiparallel and unequal magnetizations of the sub-lattice resulting in a net spontaneous magnetization. Example-Magnetite(Fe3+[Fe2+Fe3+]O4, Maghemite etc.  

          IMG_20180826_111241_1.png
            Representation of magnetic moments in a Ferromagnetic materials

            5. Antiferromagnetic:- In these type of materials neighboring valence electrons point in opposite direction of each other therefore produces zero magnetic moment .The magnetic moments produced in these materials is equal and antiparallel to the external magnetic field. The susceptibility of such material is weak and positive. Example- Ilmenite (FeTiO3), Troilite (FeS).

            IMG_20180826_111313.png
              Representation of magnetic moments in a Antiferrimagnetic material

              6. Canted antiferromagnetic:-Canted ferromagnetism, is a result of impurities or lattice defects and also by spin canting. The electron spin shows not completely antiparallel direction with an inclination at small angle. These materials show moderate positive susceptibility. Example:-hematite, goethite.

              IMG_20180826_111108.png
                Magnetic moments of a Canted Antiferromagnetic material

                Magnetic domain

                Magnetic domain is a region of uniform magnetization in a ferromagnetic and ferrimagnetic mineral. These domains are separated by domain walls with a finite thickness. The magnetic properties like frequency dependent susceptibility, coercivity, and remanence controls the size and concentration of magnetic domains.

                A. Single domain (SD): - In small grains with a size of ≤0.2µm, the restricted volume allows only one magnetic domain to form, and so these are termed as single domain grains (SD).

                B. Pseudo-single domain(PSD):- Grains which have a size range of 0.2-110 µm and are large enough to favor more domain although they behave like a SD grain are called as pseudo-single domain or PSD grain.

                C. Multidomain (MD):-Above ~110µm grains are termed as multidomain grains or MD grains.

                D. Super paramagnetic (SP):-The ultrafine particles (≤0.03µm) are referred as SP particles. This behavior strongly depends upon temperature. At room temperature these grains show paramagnetic behavior.

                Magnetic Hysteresis

                After removal of the applied external field, for the diamagnetic and paramagnetic materials the magnetization disappears. On the other hand, for some minerals like ferrimagnetic materials there remains a remnant magnetization after removal of the external magnetic field. So, these samples acquire saturation remnant magnetization (Ms) and the remnant magnetization is called as the saturation remanence (Mrs). Application of magnetic field in opposite direction brings the magnetization to zero. But the magnetization does not fall at the origin, from where it started. This behavior is called as ‘Magnetic Hysteresis’ and ‘hysteresis loop’ is the plot of variation of magnetization (M) with magnetic field (H). The required to decrease the magnetization to zero is called as the coercive force (Hc). When sufficient reverse field is applied in opposite direction, the sample can be saturated. All of these parameters are dependent upon the grain size of the sediment sample, concentration of the magnetic minerals and composition of the sediments.

                IMG_20180826_111038.png
                  Magnetic hysteresis loop and initial magnetization curve showing saturation magnetization, saturation remnence, coercivity, remnence coercivity

                  Description of the area

                  Goa, a coastal state of India, lies within the geographical coordinates located between 14.90-15.800N and 73.68-74.200E with an area of about 3701 km2 and within the geographical coordinates 8.40-12.300N and 75.00-77.000E another south-western coastal state Kerala is situated. Geologically, Goa is a part of the Indian shield mostly covered by metamorphic rocks belonging to the Dharwar Craton of Archean-Proterozoic age. Along the NE border of Goa Late Cretaceous to Eocene Deccan trap rocks are exposed which are further covered by thick layers of aluminous- laterite and laterite of Pliocene to Pleistocene time (Gokul et al., 1985). There are three main geological formations in Kerala namely, the Archeans (oldest rocks), the Warkalli beds of Tertiary Age (Upper Miocene to Pliocene) and the recent deposits (Quaternary) with North-South alignment. The state of Goa (7 rivers) and Kerala (18 Rivers) debouch sediments into the Arabian Sea. The two states have similar weather and are surface exposed rocks/sediments mostly lateritic. The continental margin of India also receive sediments from the other marginal landmass of the Arabian Sea that may be transported via currents, biogenic material from water column. For the present study I have taken two sediment cores off Goa and one core off Kerala from continental margin with following objective

                  Objective

                  To study rock magnetic properties of sediments off Goa and Kerala.

                  IMG_20180826_111015.png
                    Map showing the study area and location of the sample.

                    Methodology

                    Methods of sample collection

                    Sample coring technique is used in underground or undersea exploration. Prospecting coring is employed to recover unconsolidated samples of the several layers with the sequence preserved as it is in the depositional sequence. There are different sample coring devices based on which they are named. The off Goa sediment samples were collected by box corer and off Kerala samples were collected by gravity corer.

                    Box corer:- The box corer is a simple and reliable geological sampling tool for unconsolidated or soft sediments in the lakes and oceans. It is designed in such a way that the sediment surface is not disturbed by the wave action. A box corer is deployed from a research vessel and with a wire and is also suitable for any water depth.

                    IMG-20180825-WA0012.jpg
                      Box corer

                      Gravity corer:-It is a geological tool to collect sediments cores from deepwater sites and coastal areas. The corer consists of steel core barrel, which uses the pull of gravity to collect sample from seabed of up to six meters in length.

                      IMG-20180825-WA0009.jpg
                        Gravity corer

                        Sample preparation

                        The required amount of oven-dried samples were taken, crushed and wrapped by thin polythene films followed by packing in the cylindrical non-magnetic pre-weighted sample holders. The sample weight was taken by the digital laboratory balance (Fig12a)

                        Instruments used

                        a. Magnetic susceptibility system: For the measurement of magnetic susceptibility Bartington MS2B magnetic Susceptibility meter is used (Fig12; sensitivity 2×10-6SI). The magnetic susceptibility system comprises of one dual frequency sensor and a magnetic susceptibility meter. The dual frequency sensor can operate between two frequencies-low frequency (0.465 KHz) and high frequency (4.65 KHz), which allows frequency dependence of the samples. Low frequency alternating magnetic field is applied to the samples, a change in frequency occurs. This is then converted to magnetic susceptibility measurement and will be displayed in CGS or SI unit, as selected. The internal batteries of the instrument can be recharged from mains and vehicle dashboard with indicators for charging and battery status. For zeroing or taking measurements push button or a toggle switch is used. A manually operating platen is there which allows the sample to be inserted inside and positioned centrally within this central cavity. By using the calibration sample and adjuster tool dual frequency cross calibration can be accomplished. The sensor that was used accepts 10ml cylindrical bottles.  

                        IMG_20180826_110904.png
                          Magnetic susceptibility meter and the sensor

                          b. Molspin alternating field demagnetizer:-

                          IMG_20180826_121037.png
                            Alternating field demagnetizer

                            Anhysteretic remnant magnetization is imparted on the sediment samples by superposing a constant biasing field of 0.05mT intensity on a smooth decreasing AC field from peak of 100 mT in Alternating field magnetizer. All particles with remanent coercivity equal to or less than the maximum will become magnetized along the constant biasing DC field. Through various Geological processes over a period of time rocks acquire their Natural Remanent Magnetism (NRM). NRM is vector sum of various components. So it becomes very difficult to identify its various components. Using Molspin alternating field demagnetizer (Fig.14) allows us to cancel

                            Components one at a time and thus we can isolate them.

                            c. Pulse magnetizer:-

                            Using this Pulse magnetizer (Fig.15) instrument we can apply various magnetic field to the

                            sediment samples at constant room temperature. At first the samples are subjected to a set of magnetic field in forward direction from 20mT to 1000mT. After that the direction of the samples are reversed to get backward magnetic fields from -20mT to -1000mT. After applying magnetic fields IRM is measured using spinner magnetizer.  

                            IMG_20180802_121135.jpg
                              Pulse magnetizer

                              d. Spinner magnetometer:-

                              A spinner magnetometer (Fig 16) rotates the sample around a fixed axis inside an annular-shaped magnetometer. Variations of the sample's magnetic field are picked up by the magnetometer and converted into voltage variations which provide output. Since the formation of the old sediments and rocks, they have been subjected to different types of environments which results in different levels and orientation of magnetization of the samples. As they have different levels and orientations of magnetization they act as vectors. A spinner magnetometer is able to measure the total natural remanent magnetism, which is the sum of all the vectors.

                              IMG_20180816_112156.jpg
                                Spinner magnetometer

                                Results and Discussions

                                 Total of 219 samples from three cores (two off Goa and one off Kerala) were analyzed for the different rock magnetic properties and calculations were carried out using standard formulas given in Thompson and Oldfield (1986). The data are presented in the table 1 and Figures 17-18.

                                Range and mean values of magnetic parameters in sediment cores from Goa and Kerala.

                                 

                                 

                                Goa core 1(GS-3)

                                Goa core 2(GS-9)

                                M3 core samples(Kerala)

                                Length of the recovered sediments in cores

                                 

                                27-28cm

                                27-28cm

                                57-58cm

                                Χlf(10-8m3kg-1)

                                Min

                                12.02

                                24.16

                                4.94

                                 

                                Max

                                30.39

                                33.62

                                14.56

                                 

                                Mean

                                20.52

                                28.12

                                10.08

                                χARM(10-5m3kg-1)

                                Min

                                1005.17

                                1227.89

                                9.02

                                 

                                Max

                                2222.56

                                1828.35

                                449.80

                                 

                                Mean

                                1631.11

                                1561.74

                                146.58

                                SIRM(10-3Am2kg-1)

                                Min

                                697.87

                                795.17

                                36.26

                                 

                                Max

                                1445.40

                                1168.56

                                172.25

                                 

                                Mean

                                1093.03

                                981.66

                                90.26

                                 

                                 

                                 

                                 

                                 

                                 

                                 

                                 

                                 

                                 

                                 

                                 

                                 

                                 

                                 

                                χARMlf

                                Min

                                47.43

                                43.05

                                0.88

                                 

                                Max

                                107.67

                                64.51

                                46.53

                                 

                                Mean

                                80.02

                                55.88

                                13.54

                                SIRM/χlf

                                Min

                                36.65

                                26.82

                                3.48

                                 

                                Max

                                66.21

                                42.85

                                19.57

                                 

                                Mean

                                53.65

                                35.13

                                8.85

                                χARM/SIRM(10-5mA-1)

                                Min

                                1.25

                                1.36

                                0.18

                                 

                                Max

                                1.63

                                1.71

                                2.89

                                 

                                Mean

                                1.48

                                1.60

                                1.15

                                S-ratio

                                Min

                                96.26

                                95.76

                                24.63

                                 

                                Max

                                99.94

                                98.43

                                97.70

                                 

                                Mean

                                98.16

                                97.18

                                92.11

                                IMG_20180826_122116.png
                                  Down core variation of different magnetic parameters (concentration-Xlf. XARM, SIRM grain size- XARM /Xlf. XARM/SIRM, SIRM/Xlf and mineralogy-S-ratio of the magnetic minerals) for core Sample ID-SSK-046-GS-3, Goa.
                                  IMG_20180826_122141.png
                                    Down core variation of different magnetic parameters (of concentration-Xlf. XARM, SIRM, grain size- XARM /Xlf. XARM/SIRM, SIRM/Xlf and mineralogy-S-ratio magnetic minerals) for core Sample ID-SSK-046-GS-9, Goa.
                                    IMG_20180826_123632.png
                                      Down core variation of different magnetic parameters (of concentration -Xlf. XARM, SIRM, grain size- XARM /Xlf. XARM/SIRM, SIRM/Xlf and mineralogy-Sratio of the magnetic minerals) for M3 core Sample

                                      Variations in magnetic properties in Core sample (SSK-046-GS-3), Goa

                                      Concentration parameters

                                      The down core variations in profile of the parameters like χlf, χARM SIRM reveals values gradually decreasing with depth and showing abrupt decrease within depth range of 3-4cm. The average values of χlf,’ χARM, SIRM are 20.52 ×10-8m3kg-1, 1631.11×10-5m3kg and 1093.03×10-5Am2kg-1respectively. Higher concentration of the XARM and SIRM indicate high concentration of the fine grained minerals.

                                      Grain size parameters

                                      High value of SIRM and ARM values indicate the presence of grains with single-domain (Thompson 1986). In this core the values of SIRM is decreasing gradually with depth with an abrupt decrease within 3-4 cm depth. The higher values of χARMlf, , χARM/SIRM are indicating fine grain size. Whereas higher values of SIRM/χlf ratio indicate coarse grain sized sediment (Oldfield, 1991;Thompson and Oldfield, 1986). These ratios have average values of 80.02, 1.48×10-3mA-1.

                                      Mineralogy of the magnetic grains

                                      IRM is a direct measure of magnetic mineralogy and grain size (Oldfield et al., 1999). S ratio has mean of 98.16 indicating that magnetite to me the dominant mineral.

                                      Variation of magnetic properties in Core sample (ID-SSK-046-GS-9), Goa;

                                      Concentration parameters

                                      The χlf value of the sediments varies distinctly from 33.615114 ×10-8m3kg-1 to 24.15663 ×10-8m3kg-1.The average value of χlf is 28.11612×10-8m3kg-1.For the middle depth range the value of χlf is almost constant. On the surface this value is little less, increase with depth and again shows decreasing value of concentration of magnetic minerals. At around 24-25cm depth the maximum concentration is achieved for this core. SIRM values ranges from 1168.56×10-5Am2kg-1 to 795.17×10-5Am2kg-1, which also indicate direct concentration of fine grained magnetic minerals. ΧARM is showing values varying from 1227.89×10-8m3kg-1 to 1828.35×10-8m3kg-1. Only near the surface and about 26-28 cm depth range these values are in decreasing trend with relatively same values in middle depth. High Xlf values average 28.12 x10-8m3kg-1 indicate higher concentration of the magnetic grains as compared to the core GS-03. The low average values of XARM and SIRM indicate that in GS-09 core concentration of the fine grain minerals are lower as compared to the core GS-03.

                                      Grain size parameters

                                      The average χARM/SIRM, χARMlf and SIRM/χlf are 1.598633×10-5mA-1, 55.88 and 35.13.

                                      The lower value of χARM/SIRM near the surface indicates the presence of coarser magnetic grains in the sample. With increasing depth from the surface, relatively higher values infer the presence of finer magnetic mineral grains.

                                      Mineralogy

                                      The S-ratio, which is a mineralogy indicator, varies from 95.76 to 98.43 with an average value of about 97.18 .The high S-ratio indicate the of magnetite to be the dominant mineral.

                                      Variation of magnetic properties in M3 Core samples, Kerala

                                      Concentration parameters

                                      The value of χlf varies from 4.939055×10-8m3kg-1 to 14.56031×10-8m3kg-1 and the average value is 10.08543×10-8m3kg-1. The minimum value for SIRM and χARM are 36.26×10-5Am2kg-1 , 9.02×10-8m3kg-1and the maximum values are 172.25×10-5Am2kg-1, 449.80×10-8m3kg-1.These values are direct reflection of magnetic mineral concentration. The mean χ lf values are much lower than the cores off Goa indicate lower magnetic minerals as compared to Goa. The χ ARM and SIRM values shows lower values suggesting higher concentration of coarse grains as compared to the Goa cores.

                                      Grain size parameters

                                      The average value of χARMlf and χARM/SIRM are 13.54 and 1.15×10-5mA-1. As the higher values of χARMlf and χARM/SIRM indicate presence of fine grain sized magnetic minerals and higher value of SIRM/χlf indicates coarse grained mineral grains(Oldfield, 1991;Thompson and Oldfield,1996;Walden,1999). In the upper depth ranges or near the surface the higher values of χARMlf and χARM/SIRM ratios give information about presence of fine grained magnetic minerals in the samples, whereas the lower values of these ratios in the lower depth indicate presence of grains with coarse grain size. In about 25-26 cm depth there is a abrupt increase in these ratios which infers that the grain size is fine.

                                      Mineralogy

                                      The maximum S-ratio value is 92.11 with the range is from 24.63 to 97.70. It is almost constant throughout the whole depth of sediment core indicating same mineralogy (Magnetite) for the whole core sample.

                                      Conclusion

                                      The enviromagnetic properties of the sediment cores off Goa and Kerala, west coast of India, provide clear information about the change in mineralogy of the sediments, grain size and concentration of magnetic minerals.

                                      Magnetite is found to be the dominant mineral in all three cores.

                                      Concentration of magnetic minerals is higher in the cores off Goa as compared to the Kerala.

                                      Finer magnetic grain concentration is higher in the off Goa cores whereas off Kerala cores coarser magnetic grains are dominated.

                                      High magnetic mineral concentration off Goa cores can be related to the iron mining/ iron ore bodies in Goa.  

                                      Bibliography  

                                       1. Kessarkar, P.M., Suja, S., Sudheesh, V., Srivastava, S., Rao, V.P., 2015. Iron ore pollution in Mandovi and Zuari estuarine sediments and its fate after mining ban. Environ. Monit. Assess. 187:572. http://dx.doi.org/10.1007/s10661-015-4784-z.

                                      2. Tyson Sebastian, B.Nagender Nath,, Sangeeta Naik, D.V. Borole , Salou Pierre , Armoury Kazip Yazing 2016.Offshore sediments record the history of onshore iron ore mining in Goa. Marine Pollution Bulletin 114 (2017) 805–815, www.elsevier.com/locate/marpolbul .

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                                      4. J. Walden, F. Oldfield &J. Smith ____ Environmental Magnetism A Practical Guide-pp

                                      5. http://www.bartington.com/spinner-magnetometry

                                      6. 1.http://www.nims.go.jp/mmu/tutorials/magnetism.html

                                      7. http://www.thepinsta.com/electron-spin-theory_9WWTr9V*X4WFnBjKiEUyyok13Abayqe2dEOzeW6FOqE/

                                      8. (https://www.sciencedirect.com/science/article/pii/0025322788900746

                                      9. (http://www.kerenvis.nic.in/Database/Geological_1345.aspx).  

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                                      11. https://en.wikipedia.org/wiki/Box_corer

                                      12.https://upload.wikimedia.org/wikipedia/commons/7/77/Giant-box-corer_hg.jpg

                                      13.http://www.mooringsystems.com/sediment.htm

                                      15.https://gbank.gsj.jp/pb-rock21/index2f_b.html

                                      16. https://www.prokerala.com/kerala/kerala-rivers.htm

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