Magnetic Mineral Dissolution in Heqing Core Lacustrine Sediments and Its Paleoenvironment Significance

Abstract

The magnetic parameters within lacustrine sediments serve as invaluable proxies for deciphering the paleoenvironmental and paleoclimatic conditions. However, the dissolution of magnetic minerals can significantly alter detrital magnetic mineral assemblages, thereby complicating their interpretation in paleoenvironmental reconstructions. In an effort to clarify the impact of this dissolution on the grain size of magnetic minerals in lacustrine sediments, we undertook a thorough analysis of the rock magnetic properties on samples from the interval characterized by low ARM (anhysteretic remanent magnetization)/SIRM (saturation isothermal remanent magnetization) values between 140 and 320 ka in the Heqing (HQ) lacustrine drill core, located in Southwest China. Temperature-dependent magnetic susceptibility and FORC diagrams revealed a predominance of single-vortex and pseudo-single domain (PSD) magnetite and maghemite within the sample. When compared to samples from both the glacial and interglacial periods, the high SIRM, elevated magnetic susceptibility, and low ARM/SIRM ratio intervals from 140 to 320 ka suggested a high concentration of magnetic minerals coupled with a relatively low concentration of fine-grained particles in the sediments. The reductive dissolution of the fine-grained magnetic oxides is responsible for the reduction in the fine-grained magnetic particles in this interval. Our findings indicate that pedogenic fine-grained magnetite and maghemite are the first to dissolve, followed by the dissolution of coarser-grained iron oxides into finer particles. This process underscores the complex interplay between magnetic mineral dissolution and grain size distribution in lacustrine sediments, with significant implications for the reliability of paleoenvironmental interpretations derived from magnetic parameters.

1. Introduction

The Indian monsoon is a pivotal component of the tropical/subtropical climate system [1,2,3,4,5]. A thorough understanding of its dynamics and evolutionary history is essential for accurate future climate predictions [6,7,8,9,10]. Southwest (SW) China acts as the primary conduit for heat and moisture transport from the tropical/subtropical Indian Ocean to continental China [6,11]. Previous studies have demonstrated that the lacustrine sedimentary sequences in this region provide exceptional records of the Indian summer monsoon (ISM) evolution [2,11,12,13].

Magnetic parameters are acknowledged as significant indicators of paleoenvironmental and paleoclimatic conditions within lacustrine sediments [14,15,16,17]. However, these sedimentary magnetic properties are susceptible to influences such as erosion, sediment transport [18,19,20,21], and depositional and post-depositional diagenesis [14,15,22,23,24,25]. This can introduce ambiguities in interpreting magnetic parameters [26]. The extensive occurrence of depositional and post-depositional diagenesis can lead to the dissolution of magnetic iron oxides, thereby compromising the integrity of sedimentary magnetic assemblages [14,15,16,27] and raising concerns about the reliability of magnetic parameters derived from sediments [14,15,25,28].

Previous studies have shown that the magnetic parameters from the HQ drill core are excellent indicators of ISM activity variations [23,27]. Our analysis of the HQ drill core’s magnetic parameters reveals an abrupt interval characterized by extremely low ARM values between 140 and 320 ka. We evaluate the potential causes of this interval and explore the impact of reductive dissolution on magnetic mineral grain size. This investigation is essential for understanding the implications of magnetic mineral dissolution on the paleoenvironmental records derived from the lacustrine sediments in the study area.

2. Materials and Methods

The Heqing Basin is located at the southeastern margin of the Tibetan Plateau in Southwest China (Figure 1). The local climate is dominated by the Indian Monsoon, which is characterized by warm and wet summers with cool and dry winters. The HQ drill core primarily consists of laminated grayish-green calcareous clay and silty clay with thin-bedded silt and fine sand layers [2].

U-channels are typically 1.5 m in length with a 2 × 2 cm² cross-section and an arrow on one of the sides. Remanence measurements were conducted on U-channel samples using a 2G Enterprises Model 755R cryogenic magnetometer (Beijing Hotan Star Technology Development, Beijing, China) in a magnetically shielded space (<150 nT). Anhysteretic remanent magnetization (ARM) was induced using an alternating 80 mT peak field superimposed with a 0.05 mT DC field using 2G-755R ARM system. Isothermal remanent magnetization (IRM) was imparted with a pulse magnetizer (2G Model 660), with the IRM imparted at 1 T regarded as the SIRM. A SIRM demagnetized with a 300 mT backfield (IRM_−300mT) was used to calculate the S-ratio (=−IRM_−300mT/SIRM). All measurements were conducted at the Environmental Magnetism Laboratory at the Institute of Earth Environment, Chinese Academy of Sciences (Xi’an, China). Temperature-dependent magnetic susceptibility (χ-T) was measured using an MFK1-FA Kappabridge (AGICO, Brno, Czech Republic) system with a temperature range of 40–700 °C. To minimize the oxidation of magnetic minerals during heating and cooling, all the measurements were carried out in an argon environment. Hysteresis parameters and first-order reversal curve (FORC) diagrams were measured using a Model 3900 vibrating sample magnetometer (VSM) (Princeton Measurement, Irvine, NJ, USA) with a maximum applied field of 1 T.

3. Results

Saturation isothermal remanent magnetization (SIRM) is dependent on the concentration of magnetic minerals [26,29], while ARM is sensitive to stable single-domain (SD) ferromagnetic particles [29]. The ARM/SIRM ratio serves as a reliable estimate for the relative concentration of fine-grained particles within lacustrine sediments [16,19]. As depicted in Figure 2, an interval characterized by exceptionally low ARM values between 140 and 320 ka suggests a marked decrease in the fine-grained particles, predominantly SD, to about 10% of their concentrations during glacial intervals, as illustrated by the blue dashed line in Figure 2c. When compared to the average values from both the glacial and interglacial periods, the sediments from this interval show the lowest concentrations of fine-grained particles, as indicated by both ARM and ARM/SIRM, along with moderate concentrations of low-coercivity magnetic minerals, as reflected by the S-ratio, SIRM, and χ.

In order to delve into the magnetic mineralogy of this particular interval, a selection of representative samples were gathered for assessment through temperature-dependent magnetic susceptibility and FORC diagnostics. The heating curves of these samples indicated a significant drop in χ at approximately 585 °C, aligning with the Curie temperature of magnetite. Furthermore, the decrease noted between 320 °C and 350 °C is indicative of the thermal transformation of maghemite into hematite [19,27,30]. From these observations, we deduce that magnetite, and possibly maghemite, are the predominant magnetic carriers. However, those samples with weaker magnetic signals exclusively contained magnetite, with no significant maghemite signatures detectable (as shown in Figure 3). The closed contours aligned along the horizontal axis suggest primary SD particle distribution [29], whereas the vertical contours diverging beyond 30 mT thresholds are likely indicative of either a single-vortex magnetic domain state [31] or PSD characteristics [16,29]. The FORC diagrams revealed that those samples with a strong magnetic signature were dominated by single-vortex magnetization, accompanied by a weaker SD contribution. In contrast, the samples with weaker magnetization exhibited minimal SD magnetic mineral concentrations.

The magnetic parameters have revealed that, during the glacial periods, the lacustrine sediments in the HQ drill core were marked by a higher concentration of magnetic minerals and a reduced proportion of fine-grained particles. Conversely, in the interglacial periods, there were decreases in the magnetic mineral concentration alongside increases in the proportion of fine-grained particles. According to the rock magnetic results, we deduce that the magnetic carriers in the sediments from the interval of 140 to 320 ka in the HQ drill core were primarily in a single-vortex state, consisting of magnetite and maghemite, along with a signal from the SD particles. Compared to the samples from both the glacial and interglacial periods, the sediments from this particular interval showed moderate concentrations of low-coercivity magnetite and maghemite. Nevertheless, the lower χ_fd and ARM/SIRM values suggest a significantly diminished concentration of finer-grained particles, encompassing both the SP and SD fractions.

4. Discussion

4.1. The Availability of the Low ARM in the HQ Drill Core

ARM serves as a pivotal parameter for estimating the concentration of SD ferromagnetic particles [29]. However, the strikingly low ARM values between 140 and 320 ka cast doubt on their reliability within this interval. Possible contributing factors could involve instrument calibration issues or an extraordinarily low concentration of SD ferromagnetic particles at that time. Throughout the ARM testing, the drift measurements on all the axes were negligible, not exceeding 10⁻⁷ SI, while the recorded ARM values were around 10⁻⁴ SI. This leads us to conclude that there were no instrumental malfunctions during the ARM tests. The FORC diagrams revealed that the samples from this interval were mainly characterized by single-vortex and/or PSD particles, with no significant signals suggesting superparamagnetism. Moreover, the parameter χ_fd, often used to indicate the relative concentration of SP particles, also showed reduced values (as indicated in Figure 2d). Therefore, the observed decrease in the ARM values for this interval can be attributed to a reduction in the concentration of fine-grained magnetic particles, including both the SD and SP types.

Figure 4, marked with dotted lines, shows that lower ARM values are associated with reduced χlevels and increased ARM/SIRM ratios during the intervals of a relatively strong Indian summer monsoon (ISM), as indicated by the higher ISM indices. Even when the ARM values are at their lowest, these correlations hold true. Hence, we confirm that the low ARM values for the 140–320 ka interval in the HQ drill core are valid indicators. The reduction in the fine-grained magnetic particles is most likely due to reductive dissolution processes that impact magnetic minerals [14,15,27].

4.2. Reductive Dissolution and Its Influence on the Magnetic Mineral Grain Size

The reductive dissolution of magnetic iron oxides is a common phenomenon in lacustrine sediments [14,15,27], which significantly modifies the detrital magnetic mineral assemblages and complicates their paleoenvironmental interpretations [15,32]. In the primary study, the rock magnetic analyses indicated a low concentration of magnetic minerals, with maghemite notably absent in the weakly magnetic sections of the HQ drill core [19,20,21,22,23,24,25,26,27]. The dissolution of these minerals profoundly affected the magnetic properties of the sediment [23,27], aligning with observations from both natural sedimentary environments [14,22,27,31] and laboratory studies [33,34]. It is suggested that fine-grained iron oxides, particularly fine-grained maghemite, are more prone to dissolution compared to their coarser counterparts [14,15,33,34].

The ARM/SIRM ratio serves as a preliminary indicator of the relative abundance of fine-grained particles, especially those in the SD state [14,16,19,25]. As illustrated in Figure 2, interglacial periods have a higher prevalence of fine-grained particles compared to glacial periods, implying that the dissolution of magnetic minerals contributes to a reduction in their grain size [14,15,32]. Moderate SIRM and χ values, when correlated with ISM indices, indicate that an average level of magnetic mineral dissolution occurred between approximately 140 and 320 ka. However, the lowest ARM/SIRM ratio points to a significantly reduced relative concentration of fine-grained particles. The impact of magnetic mineral dissolution on particle grain size appears to be nonlinear and complex. In this context, we have categorized glacial periods, the interval from 140 to 320 ka, and interglacial periods as having weak, moderate, and strong magnetic mineral dissolutions, respectively, to formulate a corresponding model. Figure 5 demonstrates that, during glacial conditions, there was a high concentration of magnetic minerals, while the relative abundance of the fine-grained particles was moderate. The elevated χ_fd observed in the modern surface soils of the Heqing Basin [19] suggests that primary terrigenous magnetic minerals contain a larger proportion of fine-grained particles, consistent with the findings from the pedogenic loess-paleosols in the Chinese Loess Plateau [35]. Consequently, only a portion of the pedogenic fine-grained particles underwent reductive dissolution when the dissolution of the magnetic minerals was minimal [27]. As the dissolution of the magnetic minerals increased, nearly all the pedogenic fine-grained particles were dissolved, as evidenced by the low ARM/SIRM ratios and χ_fd values during the 140 to 320 ka interval in the HQ drill core. Subsequently, under conditions of intense magnetic mineral dissolution, the coarse-grained iron oxides were reduced to finer grains [14,27] and the fine-grained particles generated by this process were also dissolved [14]. Thus, during interglacial periods, intense magnetic mineral dissolution led to lower concentrations of magnetic minerals and relatively higher concentrations of fine-grained particles.

Magnetic mineral dissolution is influenced by the supply of reactive organic matter and iron bearing minerals, and by the chemical composition of the water column [14,15,23]. These factors are significantly correlated with variations in local temperature and precipitation, which are primarily driven by the ISM [19,27,36]. Consequently, the extent of magnetic dissolution serves as a mirror of ISM fluctuations [27]. According to the global marine oxygen isotope records, the period between 140 and 320 ka encompassed both glacial and interglacial climate stages. However, the moderate level of magnetic mineral dissolution suggests a moderate to weak ISM during this interval. Previous studies of the pollen assemblages [36], environmental magnetic indices [19,27], and geochemical indices [19,37] in the HQ basin have indicated that the local climate was likely arid [27,36], with a reduced lake level [37] since approximately 320 ka. This suggests that regional climates can exhibit distinct responses to global climatic changes.

5. Conclusions

In the interval of 140 to 320 ka in the HQ drill core, characterized by low ARM/SIRM values, rock magnetic analyses suggest that single-vortex and pseudo-single domain (PSD) magnetite and maghemite are the predominant magnetic minerals. The significantly reduced concentration of fine-grained magnetic particles is attributed to the reductive dissolution of these fine-grained magnetic oxides. The dissolution process initially targets pedogenic fine-grained magnetite and maghemite. Subsequently, coarse-grained iron oxides are broken down into finer grains, with the dissolution of these newly formed fine-grained particles occurring concurrently. The magnetic grain size directly responds to the intensity of the magnetic mineral dissolution. Consequently, the ARM/SIRM ratio is intimately linked to changes in the sedimentary environment and serves as a valuable indicator for paleoenvironmental reconstruction in lacustrine sediments.