2 National Deep Sea Center, Ministry of Natural Resources(MNR), Qingdao 266237, China;
3 Dept. of Marine Science & Technology, Federal University of Technology, Akure 340252, Nigeria;
4 University of Chinese Academy of Science, Beijing 100049, China
Environmental magnetism has been widely used to investigate the formation, transportation, deposition, and post-depositional alterations of magnetic minerals under the influences of a wide range of environmental processes (Liu et al., 2003). Environmental magnetic susceptibility method is used because it is simple, fast, efficient, repeatable, and non-destructive to samples (Evans and Heler, 2003; Kars et al., 2017) whether in loess (e.g. Ghafarpour et al., 2016; Guan et al., 2016), lake sediments (Oldfield, 1994; Roza et al., 2016; Kirscher et al., 2018), deep sea sediments (Dong et al., 2016; Lund et al., 2017), or core (Dura et al, 2015; Yang et al., 2016).
Little is known about the subduction zone, especially the abyss that exceeds 6 000 m water depths until human exploration of the abyss began with the invention of echo-sounding technology for submarine detection in the late 19th and 20th centuries (Pautot et al., 1987). In the last 50 years, a series of geological surveys (Kitahashi et al., 2014) in the Mariana Trench and the Puerto Rico Trench have revealed the importance of the Earth system processes such as deep ocean circulation and deep cyclone circulation (Jamieson et al., 2009). They have not only revealed large number of material inputs, rich biodiversity at this great depth but also shown a unique sedimentary environment characterized by high pressure, low temperature, and frequent seismic activity (Gallo et al., 2015).
At present, there are few studies on the use of magnetic susceptibility to investigate the material source and sedimentary environment of the trench. Kawamura et al. (2008) analyzed seven columnar samples from the Ryukyu Trench and found that the magnetic carriers of the sediments were mainly magnetite and maghemite, containing a small amount of hematite. The magnetic mineral content at the top of the sediment was terrigenous debris. Both results of the magnetic and geochemical analyses proved that fine-grained magnetic minerals dissolved with increasing burial depth in anoxic environment. In addition, in the core from the Japanese Trench, magnetic susceptibility was used to correlate the different sliding surfaces of the earthquake, in order to reveal the cumulative effects after several earthquakes (Yang et al., 2016).
Located in the southeastern part of the Philippine Sea, the Yap Trench forms the southeastern boundary of the Philippine Plate with the Izu Ogasawara Trench, the Mariana Trench, and the Palau Trench (Fig. 1). The Yap Trench is 700 km long and has a curved shape that protrudes southeastward. The water depth of the Yap Trench is between 6 000 and 9 000 m and the maximum water depth is 8 946 m (10°29.957′N, 138°40.987′E) (Fujiwara et al., 2000). The magnetic characteristics of the sediments in the Yap Trench are still unknown. In this paper, five magnetic core samples obtained, by the Jiaolong Manned Submersible, from the Yap Trench were used to carry out environmental magnetism and sedimentological studies to reveal the magnetic properties of the Yap Trench. In addition, the mineral species and distribution characteristics, the role of environmental magnetic properties of the source of sediments in the Yap Trench were explored.2 MATERIAL AND METHOD
During the 38th Ocean's Voyage of the Chinese Research Vessel, Xiangyanghong 09, from June 4–13, 2017, five deep push core samples (Core D148, D149, D150, D151, and D152) of sediments were collected by using manned submersible Jiaolong in the southern Yap (Fig. 2, Table 1).
As the Fig. 2 and video records show, the substrate of site D148, D149, and D150 mainly are deep sea clay and a large number of gravel, while that of site D151 and D152 are largely compried of deep sea clay. The same situation was found in our cores (Fig. 3), there are many debris in the middle of D149, D150, and the bottom of D152. The dopsit thickness of D148, D149, and D150 are also thinner than that in site D151 and D152.
We analyzed 126 samples by segmenting each core at 1-cm interval. The samples were dried at 50℃, and then ground into a powder with an agate mortar. About 3 g of each sample was crushed and wrapped in a polyethylene cling film and placed in a 4.74-cm3 plastic cube, crushed and prepared for analysis. The magnetic measurements were carried out at the Environmental Magnetics and Paleomagnetic Laboratory of the Third Institute of Oceanography, Ministry of Natural Resources, China.
All samples expressed on a mass-specific basis, with low (976 Hz) and high (15 616 Hz) frequency magnetic susceptibilities (χlf and χhf, respectively), were measured using an MFK1-FA Multi-Function Kappabridge susceptometer (AGICO, Brno, Czech Republic) with a detection limit of 2×10-8 SI and a measurement accuracy of 0.1 percent, at a field intensity of 200 A/m (peak-to-peak). χlf was taken as the mass-specific low-field magnetic susceptibility (χ).
The isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM) were selected by JR-6A rotating magnetic force, pulse magnetizer and DTECH2000 alternating demagnetizer. The procedure for the remanent magnetization measurements are is as follows: first, the samples were placed at the peak of alternating magnetic field of 100 mT and a DC magnetic field of 0.002 5 mT to obtain a non-hysteresis remanent magnetic ARM. Then isothermal remanent IRM40 mT, IRM100 mT and IRM300 mT, respectively, were obtained for samples with 40 mT, 100 mT, 300 mT after demagnetization; finally, the saturated isothermal remanent magnetization (SIRM) was obtained for the 2T magnetic field.
Magnetic hysteresis loops and back-field isothermal remanent magnetization curves were measured to determine the hysteresis parameters, coercive force (Hc), remanence coercive force (Hcr), saturation remanence (Mrs) and saturation magnetization (Ms) using a MicroMag Model 3900 vibrating sample magnetometer (VSM, Princeton Measurements Corp.). The maximum applied field was 2.0 T.
Frequency dependence of the magnetic susceptibility (χfd%), SOFT, SIRM/χ and S ratio (S300), were calculated as follow:
The k-T measurement of 6 representative samples was completed with a MFK1-FA Kappabridge susceptometer operating at a field of 200 A/m and a frequency of 976 Hz with a CS-4 high-temperature furnace attached to it. Samples were heated to 700℃ in argon of standard laboratory quality (the flow rate was about 100 mL/min) with a heating rate of approximately 6℃/min, and subsequently cooled at room temperature.
The Yap sediment particle size analysis was performed using the Malvern 2000 particle size analyzer at the Geology Laboratory of the First Institute of Oceanography, Ministry of Natural Resources, China. Firstly screening the gravel and very coarse particles (> 2 000 μm) in the samples, dilute hydrochloric acid was used to break down calcium carbonate and then to be washed out, 2 g of each sample was analyzed to obtain the particle size data. The average particle size (Md) was calculated based on the analysis results.3 RESULT 3.1 Particle size analysis
The average particle size (Md) of Yap Trench samples range from 5.59 Φ to 7.92 Φ, the mean value is 7.01 Φ, relatively fine but generally large variation. The Md of Cores D148–D152 is 6.73 Φ, 6.89 Φ, 7.15 Φ, 7.32 Φ, 6.68 Φ, respectively. Among these, the particle size of Core D149 has the largest variation with changing depth, nearly 2 Φ. The particle sizes of Cores D148 and D149 are coarsen along down-core while that of Cores D148 and D149 are fine along down-core. There are no significant correlations between particle size and magnetic parameters (Fig. 4).3.2 Remanent magnetization and hysteresis parameters
The results of each magnetic parameter analysis and its range of variation are shown in Table 2. The distributions of χ, SIRM, and SOFT are more dispersed, and their σ are larger (> 23×10-8 m3/kg, > 850×10-6 Am2/kg, > 334×10-6 Am2/kg, respectively). The maximum values of χ, SIRM, and SOFT appear at the bottom of the Core D150 (Fig. 4). The χ, SIRM, and SOFT exhibit high values in the Core D150, D151, and D152, from the abyss deeper than 6 000 m, and the variation is not large (relative small σ). The remanent magnetic parameters of the Core D148 and D149 display lower values. The values in the Core D149 varied significantly, showing distinct features that display low values at the bottom (Fig. 4). The overall trends of χ, SIRM, and SOFT in the Yap sediments are very consistent in the downcore and between cores. The correlations between them have always been used to indicate types of magnetic mineral in lots of research.
The mean χfd% of the Yap Trench is 3.21%, the χfd% of Core D148 and D149 is relatively lower (mean value are 1.25% and 0.74%). The variance of χfd% in Core D149 is the largest (σ=2.26%), while the χfd% of Core D150 and D151 are relatively stable and above 2%. The mean χfd% of Core D152 is the highest and its χfd% has a relatively distinct variation at the bottom.
As shown in Fig. 4 and Table 2, there are resemblance in the χfd%, χARM, χARM/χ, χARM/SIRM of the Yap Trench. Each magnetic parameter of Core D148 is relatively small. The magnetic parameters of Core D149 is the largest in Yap Trench sediments, while those of Core D149 and D150 are actually smaller. In general, the variance of χfd%, χARM, χARM/χ, and χARM/SIRM are relatively consistent and presented an increasing trend towards the deep trench. It is worth noticing that there are no significant correlations between particle size and magnetic parameters, indicating that study area probably happened sedimentary disturbance, and mixing in large-scale.
The k-T cycles heated from room temperature to 700℃ of representative samples are shown in Fig. 5. On the heating branches, k gradually increases up to approximately 300℃, then is stable from 300℃ to 400℃. This might have resulted from the transformation of some magnetic minerals into weak magnetic hematite, as it decreased dramatically to nearly zero at about 700℃. The cooling k-T cycles have two patterns: one pattern as D148-2, D149-12, D150-1, D151-2. Here, k increase sharply as temperature decreased from 700℃ to 300℃, and at the same time higher than the same temperature during heating. As temperature decreased from 300℃ to the room temperature, the values of k decrease slowly and become steady. The other pattern as D149-3 and D152-3. Here, the values of k increase distinctly as the temperature decreased from 700℃ to 350℃ and at the same time even lower than the same temperature during heating.
The Mrs/Ms and Hcr/Hc ratios (Day plot) of the Yap Trench sediments are mainly located in the PSD (pseudo-single-domain) region, between two SD (single-domain)+MD (multi-domain) mixing curves of Dunlop and Özdemir (2001).4 DISCUSSION 4.1 Inference from Environmental magnetism
In the present study, χ and SIRM have been used to reflect the content of magnetic minerals. The distributions of χ, SIRM and SOFT are shown in Fig. 4. As can be seen, magnetic minerals of Yap Trench sediments are mainly concentrated in the Cores D150, D151 and D152 and the middle of D149. Among these, the magnetic minerals content of Core D150 is the highest. As shown in Fig. 6a, the SIRM of Yap Trench sediments correlates well with χ (R2=0.97). On the one hand, sediments have magnetic uniformity, and on the other hand, χ can be used as an indicator for representing magnetic minerals. The magnetic strength of magnetite is several orders of magnitude higher than that of hematite. Therefore, in this study, χ is used as an indicator of the level of magnetite in the sample. SIRM and SOFT differ in mineral directivity: SIRM is mainly affected by ferrimagnetic minerals and incomplete antiferromagnetic minerals, but not by paramagnetic minerals and diamagnetic minerals, while SOFT is mainly affected by ferrimagnetic mineral (Liu et al., 2010). The correlation between SIRM and SOFT is R2=0.99, which indicates that the magnetic properties of the Yap Trench sediments are dominated by ferrimagnetic minerals, and the contribution of incomplete antiferromagnetic minerals is limited. As shown in Fig. 6a, the incomplete diamagnetic minerals in the Core D148 and D149 samples are relatively higher than others, while the D150, D151, D152 ferrimagnetic minerals are relatively higher. The SIRM value is the largest among the SD minerals and zero in the SP minerals. The SIRM distribution of the Yap Trench indicates that the magnetic domains of the three sites in the ultra-abyss are finer, and the number of SD magnetic particles is more than that of D148 and D149. Unlike other studies that particle size has significant correlation with magnetic parameters (Liu et al., 2007; Prajith A et al., 2015), the particle size of sediments in Yap Trench has different trend from magnetic parameters, showing that there probably happened sedimentary disturbance and mixing in large-scale.
The k-T cycles can be used to effectively determine the type of magnetic mineral (Dunlop et al., 1997; Geissman, 2004). For instance, maghemite is usually transformed into weakly magnetic hematite at the temperature that ranges from 300℃ to 400℃. Temperature between 400℃ and 590℃ is the unblocking temperature when maghemite is heated to Curie temperature and k is distinctively decreased. At 700℃ is the Curie temperature of maghemite when k is closed to 0 (Thompson and Oldfield, 1995). As shown in Fig. 5, all these characteristics prove that the maghemite is the main mineral in Yap Trench sediments. During the cooling process, the k value increases as the temperature decreased from 590℃ to 430℃, but this temperature range is larger for the k value during heating, indicating that magnetite is formed during the heating process. We observed that the phase characteristics of the k-T cycle are consistent with the maghemite in the deep-sea basalt of the drilling report of Riisager et al. (2002). In summary, the mineral composition of the Yap Trench sediments is dominated by ferrimagnetic maghemite, containing a small amount of antiferromagnetic hematite and ferromagnetic magnetite.4.2 Magnetic domain characteristics of sediments in Yap Trench
Generally, the magnetic domain structure of magnetic minerals can be subdivided into SP, SD, PSD and MD grains from fine to coarse (Liu, 2010).
χfd% can reflect the contribution of fine-viscous SP particles near the SP and SD boundaries to the magnetic susceptibility. When χfd% < 2%, the sample is basically free of SP particles. But when χfd% > 5%, it indicates there are more SP particles in the sample (Maher and Thompson, 1999). The overall average χfd% is 3.21%, further indicating that the SP particles in these sediments are very low. The Cores D148 and D149 sediments are basically below 2% (Mean 1.25% and 0.74%) indicating that there are few SP particles at the top of Core D148, which are basically free of SP particles (Fig. 7).
The χARM has high sensitivity to SD crystals. The high value of χARM in the Yap trench sediments appears in the super abyss, and at the higher-position sites are relatively low, indicating that the sediments in the deep trenches have high SD particles. χARM/χ is commonly used to indicate the grain size of ferrimagnetic minerals. Low values reflect more MD and SP grains, while high values indicate more single domain particles. The χARM/SIRM is extremely limited by SP, and the low value reflects the coarser MD grain content (Kim et al., 2013). As illustrated in Fig. 4, the χARM/χ and χARM/SIRM of the Yap Trench are small, with average values of 8.57×10-2 and 2.43×10-5 m/A, respectively (Table 2), less than the standard χARM/χ > 10, χARM /SIRM > 60×10-5 m/A, indicating that the ferrimagnetic mineral grains in the Yap Trench sediment are mainly coarser PSD.
The Day plot can visually reflect the magnetic domain information of ferrimagnetic minerals (Day et al., 1977; Dunlop and Özdemir, 2001). We calculated the Mrs/Ms and Hrc/Hc based on the hysteresis loop measurements using the MicroMag 3900 (Fig. 7). As shown in Fig. 7, the sediment samples of the Yap Trench basically fall in the PSD region, and based on the magnetic domain boundary curve of Dunlop and Özdemir (2001), the hysteresis loop of the Yap sediment mainly falls on the SD+MD mixing curves within the range of 70%–90%. This is basically consistent with the previous χARM and χfd% indicator analysis results. In addition, Figure 7 shows the distribution of Mrs/Ms and Hrc/Hc in other surface columnar sediments of the Philippine Seas. It is found that the core magnetic domain characteristics of the West Philippine Sea and the Shikoku Basin are similar to those of the Yap Trench, mainly, falling within the range of 75%–90% of SP (7 nm)+SD mixing curves.4.3 Source of sediments from their magnetic characteristics
The factors affecting the source of sediment are mainly debris forces, biological forces, dust sedimentation and early diagenesis, etc. The use of magnetic properties to analyze the source of sediment must clarify the genesis of minerals in sediments because only the magnetic minerals of debris has information about the source. The formation of biological forces in early diagenesis may not represent the original magnetic characteristics (Canfield et al., 2006; Sim et al., 2011; Li et al., 2016). Biological forces are usually accompanied by early diagenesis. Under the Sulfate-reducing bacteria, pyrite is formed, of which melnikovite and pyrrhotite are intermediate products. So, in the hadal sediments that under reduction environment, pyrite can be regarded as an indication of biological forces (Novosel et al., 2006; Borowski et al., 2013). The SIRM/χ of the melnikovitebearing samples is often greater than 80 kA/m (Dewangan et al., 2013), while the SIRM/χ values of the Yap Trench sediments are between 1.48 and 4.58 kA/m, and the parameters such as the k-T cycle do not reflect the inclusion. Information on melnikovite and pyrrhotite can be considered to be unaffected by biological forces. Early diagenesis will dissolve low coercivity minerals (ferromagnetic minerals), resulting in a rapid decline in S300. Low coercivity minerals and high coercivity mineral ratios decrease (Liu et al., 2007) with large SOFT values. The magnitude is decreasing. It can be seen from Figs. 6 & 8 that this phenomenon does not exist in Yap Trench sediments, which proves that the Yap Trench sediments is largely unaffected by the early diagenesis, and the magnetic mineral preserves the information of the original sediment. The SIRM/χ, k-T cycle, and SOFT indicate that the sediment is mainly low-coercivity magnetite, while the generally higher S300 value and low HIRM indicate that there are few hard magnetic minerals, such as hematite and goethite, in the sediment. The main source of hard magnet minerals in marine sediments is dust, so there are few wind and dust deposits in the Yap Trench. Combined with the previous analysis, it can prove that the Yap Trench sediments are mainly debris sources. Not only that, by comparing SIRM/χ and S300 of Yap Trench with these of other places, we can further determine the resource of sediment in the Yap Trench.
In our study, the sedimentary magnetic properties of the study area are compared with those in the surface column of the Core F090201 from the East Philippine Sea (Meng et al., 2006), Core GX149 from the West Philippine Sea (Li et al., 2015), and the Core GX168 from the East China Sea Basin (Li et al., 2016); and also with sediment at the top of C0012 core from the Shikoku Basin. As shown in Fig. 8, the D149 lower layer sediment is very close to the magnetic characteristics of the Core F090201 from the East Philippine Sea, while the magnetic characteristics of the Cores D150, D151, and D152 from the Yap Trench sediments are relatively close to the Core GX149 from the West Philippine Sea and the Core GX168 from the East China Sea Basin surface. It is worth noting that the relationship between Mrs/Msand Hcr/Hc (Day plot) of the Core GX168, the Core GX149 and the Yap trench sediment are very close (Fig. 7). With reference to similar magnetic characteristics, the characteristics of χ, SIRM, and SOFT are lower than those of the high coercivity hematite content of the volcanic debris. The volcanic debris are mainly from the Philippine island and caused by currents and the volcanic material debris of the nearby Mariana Trench. The Core GX149 sediments from the Western Philippine Sea are greatly affected by dust deposits, which are inconsistent with the situation in the Yap Trench. The turbidity developed in the Core GX168 from the Huatung Basin are very similar to the Yap Trench sediments in terms of sedimentary magnetic mineral types, k-T cycle, and SIRM/x characteristics and stratification characteristics.
SIRM-χ scatter plot for Cores D148 and D149 from the Yap Trench are significantly different from other sites, indicating that the source composition is different. Dots of Core D149 are disperse, indicating that there are many debris, large particle size variations, and magnetic domain MD crystallites. More important, the abnormally high value of the χ in the middle of the core indicates that the sedimentary environment has changed greatly (Fig. 4). The upper color of Core D149 is lighter than the lower part, the sand is thick, and the silt clay has a small specific gravity, exhibiting homologous turbidity characteristics (Bridge and Demicco, 2011).
Figure 9 illustrates plot the C-M map of the sandstone-like granularity data of the Yap Trench sediments and the linear distribution characteristics of the gravity flow. The slope and position of the sediments are different, and the overall gravity flow characteristics are atypical. Among these, the gravity flow characteristics of the Core D149 are more significant. We infer that it may be that the deep-sea basalt is broken at 4 000 m (D148), and the sedimentation occurred after the turbidity of the coastal gully at 4 500 m (D149), covering the volcanic clastic sediments deposited by current before the sedimentation site. This situation is consistent with that there are no significant correlations between particle size and magnetic parameters.
Turbidity current is very similar to the conditions generated by gravity flow: sufficient water depth, sufficient material, necessary slope, and trigger mechanism. The first two are the source of matter, and the third is the source of turbidity (Li et al., 2016; Jiang, 2010). In the marine environment, the main stream occurs mostly in the deep-sea area such as the seabed valley. The trigger mechanism includes sea level fluctuations, earthquakes, volcanic eruptions, storm surges, etc., causing loose deposits on steep terrain to mix down the slope, thereby forming a turbid stream (Arai et al., 2013). Cores D148 and D149 are located on the steep slope of the west side of the Yap Trench at 4 000 m and 4 500 m depths, respectively. They are close to the magma eruption of the Mariana Trench and close to the volcanic seismic belt, thereby providing a rich source of material and favorable terrain for gravity flow development.
In summary, the main sources of sediments in the Yap Trench are nearly-source clastics and volcanic debris. The Core D150, Core D151, Core D152, and the upper layers of Cores D149 is the deposition of basalt and diabase debris, which is triggered by earthquakes and volcanic eruptions. The sediments are mainly composed of terrestrial material debris such as basalt and pyrozenite. The Core 148 and the bottom of Core D149 are mainly volcanic dust and debris.5 CONCLUSION
The magnetic minerals in the surface columnar sediments of the Yap Trench largely include magnetite, maghemite, and hematite. The maghemite dominates the sediment magnetic properties of the study area, and the sediment is more magnetic.
The magnetic particles of Yap Trench sediment are mostly pseudo single domains (PSD), and the hysteresis loop is mainly located within the range of 70%–90% of the SD+MD hybrid line.
The main sources of sediments in the Yap Trench are volcanic debris and nearly-source debris. The lower layers of Cores D148 and D149 are mainly volcanic debris transported by ocean currents; while the upper layers of Cores D149, D150, D151, and D152 are primarily nearly-source debris. The sediments are dominated by basalt and pyrozenite debris deposition near the trench caused by earthquakes and volcanic eruptions.6 DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available on request from the corresponding author.7 ACKNOWLEDGMENT
The authors would like to thank Professor Wang Weiguo and Dr Liu Jianxing for providing insightful comments on this paper, and the anonymous reviewers for their constructive comments and helpful suggestions, which have improved the manuscript.
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