Using magnetic susceptibility to study Permian-Triassic boundar

MAGNETIC SUSCEPTIBILITY STUDY ON THE PERMIAN/TRIASSIC BOUNDARY IN LŨNG CẨM LIMESTONE AT HÀ GIANG PROVINCE, VIỆT NAM

¹LƯU THỊ PHƯƠNG LAN, ²BROOKS B. ELLWOOD,
³JONATHAN H. TOMKIN, ⁴ĐOÀN NHẬT TRƯỞNG

¹Institute of Geophysics, VAST, A8, 18 Hoàng Quốc Việt Str., Cầu Giấy, Hà Nội;
²Dept of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA;
³School of Earth, Society and Environment, University of Illinois, 428 Natural History Building, 1301 W. Green
Street, Urbana, IL 61801, USA; ⁴Institute of Geosciences and Mineral Resources, Thanh Xuân, Hà Nội.

Abstract: Our group has been studying the Permo/Triassic (P/T) boundary interval using the magnetic susceptibility (MS) method to correlate geological sequences in Việt Nam and elsewhere. In this article, we report the results from a 5 m sequence, 101 samples, collected at 5 cm intervals across the P/T boundary in the Lũng Cẩm section exposed at Hà Giang Province, Việt Nam. The MS zonation in Lũng Cẩm is graphically compared with the MS data of Hansen et al. [13] from the Global Boundary Stratotype Section and Point (GSSP) established at Meishan, China. MS zonation for the Lũng Cẩm section is compared closely with the MS zonation at the Meishan GSSP. Results of our graphic comparison identified three Line of Correlation (LOC) segments that are compared well between sequences: LOC 1 from the base of the Lũng Cẩm section to just above the 1^st P/T extinction level observed within the GSSP sequence; LOC 2 from that point to P/T boundary point; and LOC 3 from P/T boundary to the top of the section. There is a distinct change in sediment accumulation rate that is identified in LOC 2 between the Lũng Cẩm and Meishan sequences, where accumulation rate is significantly reduced in the Lũng Cẩm LOC 2 interval, relative to the Meishan section. Graphic comparison of these two sequences, Lũng Cẩm with 5 m of section, and Meishan with 1 m of section, shows that overall the Meishan GSSP is a significantly condensed section relative to Lũng Cẩm. Therefore, our results indicate that the P/T boundary interval (LOC 2) represents a relative decrease in sediment accumulation rate at Lũng Cẩm through the boundary interval. Fourier analysis of the Lũng Cẩm MS data indicates that Milankovitch ~400 kyr and ~100 kyr eccentricity cycles are preserved in both of these P/T boundary sections and provide a means to evaluate absolute sediment accumulation rates for the P/T boundary interval. This rate is 0.3 cm/kyr at the Lũng Cẩm section and 0.28 cm/kyr at the Meishan GSSP, essentially equivalent values.

I. INTRODUCTION

In recent years, our group has been using magnetic susceptibility (MS) measurements of Phanerozoic marine sedimentary rocks, in conjunction with biostratigraphic control, for high-resolution chronocorrelation and named as the method MagnetoSusceptibility Event and Cyclostratigraphy (MSEC). MSEC is a composite of the MS record of marine strata and the coeval biostratigraphic record, and MSEC chronozones have isochronous boundaries. Several Global boundary Stratotype Sections and Points (GSSPs) and co-eval sections have been defined by using MS data, including the Emsian/Eifelian [9], Frasnian/Famennian [3], Silurian/Devonian [2], Middle Carboniferous [10], Permian/Triassic [13, 20], Cretaceous/Tertiary [7], and Paleocene/Eocene [11, 12] boundary GSSPs.

The purpose of this paper is to present the magnetostratigraphic susceptibility record for P/T boundary recorded in the Lũng Cẩm sequence (Hà Giang Province, Việt Nam) and the corresponding new paleontological studies identifying the P/T boundary interval within the section. This work is then correlated to MS data for the P/T GSSP now located within the Meishan quarry system, SE China [13, 15].

II. METHODS

1. Magnetic susceptibility (MS) in marine sequences

All mineral grains are "susceptible" to become magnetized in the presence of a magnetic field, and MS is an indicator of the strength of this transient magnetism within a material sample. MS is very different from remanent magnetism (RM), the intrinsic magnetization that accounts for the magnetic polarity of materials. MS in sediments is generally considered to be an indicator of iron, ferromagnesian or clay mineral concentration.

In general, marine sediments are composed of terrigenous, detrital and eolian constituents, and a biogenic component of the carbonate and/or siliceous tests of marine organisms. The MS signature observed in these sediments mainly represents contributions from the terrigenous and biogenic components. Because the biogenic component is usually weakly diamagnetic, in general, the terrigenous component dominates the MS. Therefore, those processes controlling the influx into the marine environment of terrigenous components will usually account for the MS variations observed. Thus, climate cycles, that cause erosion and transport of terrigenous materials, will result in a cyclic signal in the MS recorded in the deposited sediments. There are many factors that can modify this signature, but while the expected variations may be reduced in magnitude, usually the character of the cycles will remain and can be extracted. Superimposed on these cyclic variations are contributions from unique events, such as volcanic eruptions or meteorite impacts [7], which may be of local, regional, or global significance [1, 2].

2. MSEC method

a. Fundaments of the method: MSEC applies a type of magnetostratigraphy that is dependent on the induced magnetization of rock instead of the often altered and less reliable remanent magnetization used in polarity studies. MSEC data are particularly useful in the identification of events for event-stratigraphy and orbital forcing cycles for cyclostratigraphy [26, 10]. Interregional and other types of long-distance chronocorrelation between MSEC data sets are possible when the lithological variations producing the MS variations are the result of "random" events or regional and global (orbital forced) climate changes [1, 5, 6]. In the case of cyclostratigraphy, the MSEC signature of a marine sedimentary sequence records the complexity of sedimentary processes, often driven by climatic/orbital forcing cycles. These cycles exhibit a rhythmicity in the MSEC data that exhibits an apparent lack of uniqueness. For this reason the utility of the MSEC method is dependent on biostratigraphy for temporal indexing within sequences. With high-density sampling, MSEC data can provide event-stratigraphic chronocorrelations at resolutions exceeding those of the best biostratigraphy.

Event stratigraphy develops from unusual events, such as meteorite impacts [7] or during time of sea-level regression/transgression when the base level changes and erosion rates, bringing detrital components into the marine system, primarily from rivers, varies [11, 12]. This material is then dispersed by bottom currents throughout ocean basins [9], with the result that MS magnitudes fluctuate. Locally, sampled sections that were deposited near the mouths of ancient river systems will have MS values elevated relative to sections far removed from rivers, but while the magnitude may be different, the variations (trends) resulting from erosional events will be identifiable in all sections within basins. If the event is global, then MS effects will be global. In the case of transgression, MS magnitudes are usually reduced, because detrital sediments are trapped in nearshore environments, while during regression, MS values rise. High-density data sets provide a large number of samples where overall MSEC trends can be identified. Such data sets allow long-term trends or events to dominate over single, essentially instantaneous sediment influxes, like those associated with storms. MSEC trends must be characterized by multiple data points to define MS data that result from long-term geological processes.

In this paper, we interpret MSEC events in the following ways. Increases in MS magnitude correspond to relative or net drops in sea level. Highest magnitudes represent low stands in sea level. Decreasing magnitudes correspond to relative or net rises in sea level. Lowest magnitudes represent high stands in sea level. Trends that show increasing magnitudes correspond to relative or net falls in sea level. Commonly a number of consecutive MS lows or highs will coalesce into broad episodes indicative of sea level stasis. The character of the increase or decrease in MS magnitudes within a signature represents the rate of the process or processes responsible for the change.

b. Advantages of the MSEC Method: Using the MSEC method it is possible to quickly and easily measure small unoriented samples, so that samples are relatively easy to collect. In the very low inducing magnetic fields that are generally applied, MS is largely a function of the concentration and composition (mineralogy and grain morphology) of the magnetizable material in a sample. The method works even when the sampled rocks are somewhat weathered and/or have been heated to moderate temperatures (in some cases up to >350^o C). This method can be measured for a wide range of sediment and sample types, including cuttings and friable outcrop materials. In conjunction with biostratigraphy, MS can provide high-resolution relative dates [8].

III. MS DATA FOR THE P/T BOUNDARY GSSP AT MEISHAN, SOUTH CHINA

The P/T boundary interval represents a time of major extinction, sometimes informally called the Great Dying, that occurred ~251 M.a. ago. It was the Earth's most severe extinction event, with up to 96 % of all marine species and 70 % of terrestrial vertebrate species becoming extinct. Because fewer than 10 % of living species survived the event, the recovery of life on earth took significantly longer than did recovery after other extinction events. This event has been described as the "mother of all mass extinctions" and therefore is the focus of many studies.

The P/T boundary is formally defined in China at the Meishan GSSP locality [15] and is based on the first appearance of the conodont H. parvus found in the upper part of Bed 27 (Bed 27c), in an MS zone that we have labelled In1 (Fig. 1), a time representing low MS values. There are two extinction events immediately preceding the P/T boundary level and these are identified within Bed 24 (1^st extinction level in Fig. 1) associated with our MS zone ChY, and within Bed 26 (2^nd extinction level in Fig. 1) associated with our MS zone ChZ, respectively.

1. Paleontological study of the Lũng Cẩm section

The lithostratigraphy and biostratigraphy of the Lũng Cẩm section is given below. During our preliminary studies of the section we did not find conodonts, thus the P/T boundary was tentatively determined based on a comparison between changes in foraminiferal assemblages within the Lũng Cẩm section with those found in the Meishan GSSP (China). The lower part of the Lũng Cẩm section (from Bed 1 to Bed 12) consists of bioclastic limestone containing Permian foraminifers (including fusulinids). In Bed 13, just above Bed 12, fusulinids are absent. Therefore we place the first extinction level at the top of the bed 12, where we see the last occurrence of fusulinids. However, beds 13 to 15 contain a relict Permian fauna including Nodosaria, Geinitzina, Rectostipulina, Globivalvulina and are therefore considered as transitional beds. Above these beds this relict of Permian fauna disappears, indicating that the second extinction level lies somewhere within the upper part of Bed 15 and the P/T boundary is tentatively placed between beds 15 and 16.

The Lũng Cẩm section (23⁰14'11'' N; 105⁰13'02'' E; Fig. 1) is exposed near Road 4C, ~ 4 km southeast of the Phó Bảng tower (Fig. 2). Beds 1 to bed 13 in the section belong to the Đồng Đăng Formation and bed 14 to bed 30 to the overlying Hồng Ngài Formation. In detail, the section is seen as follows (in ascending order):

- Beds 1 (29 cm), 2 (18 cm): Dark-grey organic limestone;

- Bed 3 (41 cm): Dark-grey organic limestone, yielding the foraminifers Nankinella spp., Frondina sp., Climacammina sp.;

- Bed 4 (14 cm): Dark-grey organic limestone and clayish limestone, yielding the foraminifers: Nankinella spp.;

- Beds 5 (9 cm), 6 (11 cm): Dark-grey cherty limestone and clay;

- Bed 7 (18 cm): Dark-grey organic limestone yielding the foraminifers: Nankinella spp., Dagmarita spp.;

- Bed 8 (17 cm): Dark-grey organic limestone yielding gastropods;

- Bed 9 (8 cm): Dark-grey organic limestone and cherty limestone;

- Bed 10 (27 cm): Dark-grey organic limestone;

- Bed 11 (29 cm): Grey organic limestone yielding the foraminifers: Dagmarita spp., Globivalvulina spp., Paraglobivalvulina spp.;

- Bed 12 (9 cm): Dark-grey cherty limestone;

1^st extinction event level

- Bed 13 (11 cm): Clay shale, yielding the foraminifers: Nodosaria spp., Geinitzina spp., Rectostipulina quadrata;

- Bed 14 (5 cm): Dark-grey clayish limestone, yielding the foraminifers: Nodosaria spp., Globivalvulina spp.;

- Bed 15 (15 cm): Grey, poorly dolomitic limestone yielding the foraminifers: Globivalvulina spp., Ammodiscus sp.;

2^nd extinction event level within bed 15

P/T boundary at the top of bed 15?

- Bed 16 (34 cm): Beginning of Triassic? Grey limestone containing organic residues and the foraminifers Ammodiscus sp.;

- Bed 17 (25 cm): Grey limestones;

- Bed 18 (20 cm): Grey limestone yielding the foraminifers: Ammodiscus sp.;

- Beds 19 (7 cm), 20 (9 cm), 21(24 cm), 22 (14 cm), 23 (17 cm), 24 (26 cm), 25 (17 cm), 26 (30 cm), (5 cm), 28 (2 cm), 29 (5 cm): Grey limestones;

- Bed 30 (5 cm): Grey limestone yielding gastropods.

2. Magnetic susceptibility of the Lũng Cẩm Section

We present the MS data from the Lũng Cẩm section in Fig. 4 (samples were measured at Louisiana State University). Both raw and smoothed data using smoothing splines are given, and the smoothed data (solid line in Fig. 4) have been used to develop a graphic presentation in the form of bar-logs similar to those used in magnetic polarity studies. These bar-logs are accompanied by both the raw and smoothed MS data sets. The following bar-log plotting convention was used; if the MS cyclic trends increase or decrease by a factor of two or more, and if the change is represented by two or more data points, then this change is assumed to be significant and the highs and lows associated with these cycles are differentiated by filled (high MS values) or open (low MS values) bar-logs (shown in figures). The large numbers of closely spaced samples provide highresolution, somewhat redundant data sets, thus helping resolve variations associated with anomalous samples.

Figure 4. The magnetic susceptibility results of the Lũng Cẩm section.

We have identified the proposed P/T boundary level as well as the two extinction event horizons in the Lũng Cẩm section and these are given and labelled in Fig. 4. Using the MSEC method we then graphically compared the Meishan MS zonation with the Lũng Cẩm MS zonation and the result is given in Fig. 5. The MS data show three, fairly well-defined Line of Correlation segments (LOC): LOC 1 from the base of ChW to the base of ChZ; LOC 2 from the base of ChZ to the P/T boundary; and LOC 3 from the P/T boundary to the top of In4. These results indicate that the sedimentation rates were consistent during deposition of each LOC segment, but there is a distinctive and abrupt change between these segments. Both LOC1 and LOC3 exhibit similar sediment accumulation rates (SARs) among the two section reported, but LOC2 is very different. The change seen in LOC2 is toward much reduced sediment accumulation at Lũng Cẩm during this P/T boundary interval segment, indicating the boundary interval at Lũng Cẩm was significantly condensed during the P/T boundary interval. However, immediately following the end of the Permian, rates at Lũng Cẩm in the Triassic recover to pre-boundary rates.

Lũng Cẩm also exhibits an offset from LOC1 and LOC2 of the two pre-Triassic extinction levels identified in both sections. This offset is most pronounced for the 1^st Extinction Level at the upper end of LOC1 and would require significant offset to conform with the Meishan GSSP equivalent extinction level (represented by the arrow at that level in Fig. 5). Five explanations for this offset are possible: this offset is the result of: 1) onset of condensation within the Lũng Cẩm section in the upper portion of LOC1; 2) the fact that the 1^st Extinction Level has been misidentified by us in the Lũng Cẩm section and adequate extinction evidence is missing in the section; 3) the severe condensation in the Lũng Cẩm section that has obscured the MS data for the section leading to missing or unrecognized MS zones; 4) a real difference in the timing of the extinction events between the Lũng Cẩm location in Việt Nam and the Meishan GSSP site; or 5) the MS data from the GSSP are problematic.

2. Climate Standardized Zonation and Sediment Accumulation Rates (SARs)

It is now welknown and has been demonstrated in a number of studies that MS variations are due to detrital fluxes into the marine environment [15], that were caused by climate variability, and therefore MS data sets can be used as climate proxies [12]. This work has demonstrated that MS bar-logs are not uniform because of local SAR variations, and these variations can be identified if a standard climate zonation is established and used for comparison to natural data sets. We have done this in Fig. 6, where we have created an uniform bar-log zonation (climate standardized MS zonation in Fig. 6) by assuming that MS bar-logs reflect relatively constant climate periodicities and therefore represent relatively uniform periods of time. This follows logically from the Fourier Transform work done by a number of investigators, where clear Milankovitch eccentricity, obliquity and precession bands are well-defined in cyclic MS data sets [2, 11, 12, 14, 18, 22, 25, 26]. To test the climate model for the Lũng Cẩm section, we have performed Fourier analysis on the raw MS data from the section and the results are presented in Fig. 7 [12]. There are two statistically significant peaks represented in the data set. In our analysis we have assigned the 2^nd peak, E2, a value of 100 kyr (shaded E2 peak at ~1.2 cy/m in Fig. 7). This assignment is based on the assumption that while the main Milankovitch eccentricity peak (E1; Fig. 7) at ~400 kyr is robust and expected in cyclic geological data sets [23], the Lũng Cẩm record is too short to resolve this peak with high precision. Therefore we used the second peak for age assignment yielding an age range for the Lũng Cẩm section of ~0.6 myr. Because the MS bar-log zonation represents ~4.5 cycles and the collected section is 5 m long, each full MS cycle in Fig. 6 represents deposition during ~100 kyr, and each half cycle represents ~50 kyr. Because there is a reasonable correlation between the Lũng Cẩm and Meishan GSSP sections (Fig. 5), we infer that the MS zonation for the GSSP also reflects Milankovitch E2 eccentricity timing.

We have reasonable confidence that the Lũng Cẩm data set graphically compared with the uniform climate standard, given in Fig. 6, represents a climate comparison model. Therefore, the LOC segments developed can be used to estimate accumulation rates for the section. This comparison identifies four LOC segments that we have labelled LOC1, 2, A and 3 in Fig. 6 and the sediment accumulation rates (SARs) are given in Table 1 for these line segments. SAR extremes include the low values in LOC2, a relatively condensed rate at 0.3 cm/kyr, and the very high values in LOC A (3.7 cm/kyr), just above the P/T, where MS zone In2 is very expanded. This higher SAR may be the result of the destruction of plants during the Permo-Triassic extinctions resulting in high erosion rates during the earliest Triassic. The SARs for LOC1 and LOC3, below and above the P/T boundary interval, respectively, represent a range of more normal rates for marine sediments [19].

Table 1. Sediment Accumulation Rate (SAR) data

a) Lũng Cẩm
LOC Segment	SAR (cm/kyr)
1	1.37
2	0.30
A	3.70
3	0.60
b) Meishan GSSP
LOC Segment	SAR (cm/kyr)
1	0.20
2	0.28
3	0.12

We have also compared the Meishan GSSP MS zonation (Fig. 1) with the standard climate zonation that we have developed from the Lũng Cẩm FT data in Fig. 7. Because the GSSP section, only 1 m in length, is essentially equivalent in time to the Lũng Cẩm section that is 5 m in length. Clearly, overall, the GSSP section is very condensed relative to the Lũng Cẩm data set. However, when contrasted with the standard climate zonation (Fig. 8), the Meishan GSSP section appears to be more uniform, with three LOC segments that are not distinctively different. One of these, LOC1, is very uniform over more than half of the sequence, and LOC2, above, exhibits only a slightly higher SAR (Table 1). In addition, LOC2 is relatively uniform through the P/T boundary interval, indicating that there is no obvious hiatus in sedimentation across the boundary. The SAR for the GSSP through this interval is essentially identical to that in the equivalent line segment, LOC2, in the Lũng Cẩm section (Table 1). LOC3 in the GSSP exhibits the lowest SAR for these sections, indicating that the upper part of the GSSP interval sampled is very condensed (Fig. 8; Table 1).

Because the GSSP LOC data look good, even though highly condensed, we are able to project the two observed extinction levels through LOC1 and into the climate standard zonation in Fig. 8. This result is slightly different from the Lũng Cẩm extinction locations and thus further complicates our understanding of the extinction level locations within the Lũng Cẩm section, where the extinctions appear to be poorly defined, although the boundary location is well defined.

VI. CONCLUSIONS

In this paper, we present new data from the Lũng Cẩm Permo/Triassic (P/T) boundary section, located in North Việt Nam, to the Meishan P/T GSSP in China. Graphic comparison, using magnetic susceptibility (MS) zone data, indicates that the two data sets are highly correlated. A Fourier Transform (FT) of the raw MS data from the Lũng Cẩm section exhibits Milankovitch eccentricity in two significant bands, E1, ~400 kyr, and E2, ~100 kyr climate cycles. Because smoothed MS cycles correspond to the E2 FT data, we have established a climate standardized MS zonation exhibiting uniform length half-cycles representing 50 kyr. This climate standard was then graphically compared to the Lũng Cẩm and Meishan MS zonations to estimate sediment accumulation rates and rate changes for the two sections analyzed. This comparison indicated that even though the Meishan GSSP is highly condensed, with very low sediment accumulation rates, much of the section appears to have been deposited relatively uniformly. In contrast, the Lũng Cẩm section appears more variable, with the P/T boundary representing a zone of very high condensation at the site. This condensation has made identification of extinction levels in the Lũng Cẩm section difficult, and is the focus of ongoing research. We conclude from our work that no unconformity exists within the P/T boundary interval.

Acknowledgements: Funding for this project was provided by the National Fund for Basic Research Program of Việt Nam, N^o 7.120.06. We are grateful to Prof. Nguyễn Thị Kim Thoa for her help to this project.

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