Chorng-Shern Horng

Detection of noninteracting single domain particles using first‐order reversal curve diagrams

We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the Hc axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

Magnetic properties of sedimentary greigite (Fe3S4): An update

Greigite (Fe3S4) is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction. It can also grow at any time when dissolved iron and sulfide are available during diagenesis. Greigite is important in paleomagnetic, environmental, biological, biogeochemical, tectonic, and industrial processes. Much recent progress has been made in understanding its magnetic properties. Greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites. The crystallographic c axis is the easy axis of magnetization, with magnetic properties dominated by magnetocrystalline anisotropy. Robust empirical estimates of the saturation magnetization, anisotropy constant, and exchange constant for greigite have been obtained recently for the first time, and the first robust estimate of the low-field magnetic susceptibility is reported here. The Curie temperature of greigite remains unknown but must exceed 350°C. Greigite lacks a low-temperature magnetic transition. On the basis of preliminary micromagnetic modeling, the size range for stable single domain behavior is 17–200 nm for cubic crystals and 17–500 nm for octahedral crystals. Gradual variation in magnetic properties is observed through the pseudo-single-domain size range. We systematically document the known magnetic properties of greigite (at high, ambient, and low temperatures and with alternating and direct fields) and illustrate how grain size variations affect magnetic properties. Recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite, which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records, including widespread remagnetizations.

What do the HIRM and S-ratio really measure in environmental magnetism?

The “hard” isothermal remanent magnetization (HIRM) and the S-ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations, respectively, of antiferromagnetic minerals (hematite and goethite) in mineral mixtures. We demonstrate that synthetic Al-substituted hematite and goethite exhibit a wide range of coercivities, which significantly influences the HIRM and S-ratio. These parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite/goethite. To circumvent this problem, we propose a new parameter (the L-ratio), which is the ratio of two remanences after alternating field (AF) demagnetization of an IRM imparted in a 1 T field with a peak AF of 100 mT and 300 mT: IRM AF@300mT /IRM AF@100mT . These parameters are easily measured using modern vibrating sample or alternating gradient magnetometers. Changes in HIRM only reflect changes in the absolute concentration of hematite and/or goethite if the L-ratio is relatively constant. Conversely, L-ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite/goethite. Corresponding HIRM and S-ratio variations should be interpreted with caution in such cases. The L-ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols. For example, the HIRM is equivalent to IRM AF@300mT . Likewise, 0.5*(SIRM + IRM -100mT ), where IRM -100mT represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back-field of -100 mT, is equivalent to IRM AF@100mT . The HIRM/[0.5*(SIRM + IRM -100mT )] ratio is therefore a suitable substitute for the L-ratio when measurements are made with a long-core magnetometer. The newly proposed L-ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism, the HIRM and S-ratio.

Characterization of hematite (α‐Fe2O3), goethite (α‐FeOOH), greigite (Fe3S4), and pyrrhotite (Fe7S8) using first‐order reversal curve diagrams

[1] First-order reversal curve (FORC) diagrams have become a standard tool in rock magnetism, yet magnetite is the only magnetic mineral that is well characterized using FORC diagrams. We present FORC diagrams for predominantly single-domain (SD) synthetic aluminous hematite (a-Fe2-xAlxO3) and goethite (a-(FeAl)OOH) and natural greigite (Fe3S4) and pyrrhotite (Fe7S8) to constrain interpretation of FORC diagrams from natural samples. Hematite and goethite have low spontaneous magnetizations and negligible magnetic interactions, while greigite and pyrrhotite have higher spontaneous magnetizations and can have strong magnetic interactions. The coercivity of hematite systematically increases with Al content only for samples produced using the same synthesis method, but it is variable for samples produced with different methods even for similar Al content. This precludes use of magnetic coercivity alone to quantify the Al content of natural hematites. Goethite has much higher coercivity than hematite for all measured samples. SD and superparamagnetic (SP) behavior is common in natural greigite samples, with peak coercivities ranging from

Astronomically calibrated ages for geomagnetic reversals within the Matuyama chron

We present a magnetostratigraphic record from the western Philippine Sea that is tied to a marine δ18O record for the past 2.14 million years. The ages of geomagnetic reversals were astronomically calibrated by tuning the oxygen isotopic stratigraphy, yielding a chronology for the following subchrons: Matuyama/Brunhes boundary, 781 ± 3 ka (slightly above δ18O Stage 19.3); top of the Santa Rosa polarity interval, 920 ± 2 ka (Stage 23/24); base of the Santa Rosa polarity interval, 925 ± 1 ka (Stage 24); top of the Jaramillo subchron, 988 ± 3 ka (Stage 27); base of the Jaramillo subchron, 1072 ± 2 ka (Stage 31); top of the Cobb Mountain subchron, 1173 ± 4 ka (Stage 35/36); base of the Cobb Mountain subchron, 1185 ± 5 ka (Stage 36); top of the Olduvai subchron, 1778 ± 3 ka (Stage 63/64); base of the Olduvai subchron, 1945 ± 4 ka (Stage 71/72); top of the Réunion II subchron, 2118 ± 3 ka (Stage 80/81); and base of the Réunion II subchron, 2133 ± 5 ka (Stage 81). This astronomically calibrated chronology independently confirms the ages of major reversals in recently published astronomically calibrated polarity timescales for the Matuyama chron. It also provides the first astronomically calibrated dates for the lower and upper reversals associated with the Cobb Mountain and Santa Rosa polarity intervals, respectively.

Contradictory magnetic polarities in sediments and variable timing of neoformation of authigenic greigite

Abstract In several recent published studies, paleomagnetic results from greigite-bearing sediments reveal characteristic remanences that are anti-parallel to those carried by coexisting detrital magnetic minerals and polarities that are opposite to those expected for the age of the rock unit. These observations have important implications for the reliability of paleomagnetic data from greigite-bearing sediments. We have investigated the origin of such contradictory magnetic polarities by studying the formation mechanisms of greigite in mudstones from the Lower Gutingkeng Formation, southwestern Taiwan. Scanning electron microscope observations indicate that the Gutingkeng greigite has three modes of occurrence, including nodular, framboidal and matrix greigite. Microtextural observations, including transection of bedding by iron-sulfide nodules with no deviation of sediment textures, the presence of partially dissolved edges around detrital and early diagenetic phases, and neoformation of greigite and Fe-rich clays around detrital phyllosilicates, indicate that all three types of greigite have a diagenetic origin that post-dates early diagenetic pyrite. In addition, paleomagnetic data yield contradictory polarities even for greigite-bearing sister samples from the same stratigraphic horizon. The data are collectively interpreted to indicate that neoformation of the Gutingkeng greigite occurred after partial dissolution of syngenetic or early diagenetic pyrite. The timing of greigite formation can apparently vary enough to give contradictory polarities for different greigite components even within a single stratigraphic horizon. Direct petrographic observation of authigenic magnetic iron-sulfide phases, as carried out in this study, can provide important constraints on formation mechanisms and timing of remanence acquisition for these minerals and suggests that care should be taken when interpreting magnetostratigraphic data from greigite-bearing sediments.

Magnetic discrimination of pyrrhotite‐ and greigite‐bearing sediment samples

By using bulk samples, rock magnetic measurements were performed to discriminate between pyrrhotite- and greigite-bearing shallow marine sediments that are now uplifted above sea level in southwestern Taiwan. Thermal demagnetization of a composite isothermal remanent magnetization (IRM) was found to be effective in differentiating between the two types of sediments. To check the thermal instability and estimate the true unblocking temperature (TB) spectra of sediments containing these minerals, saturation IRMs (SIRMs) were imparted at each temperature step during demagnetization. While pyrrhotite-bearing samples showed unambiguous TB temperature spectra, greigite-bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization. Such thermal instability of greigite is a practical and important clue for its identification. Zero-field warming of IRM from 5 to 300 K sensitively indicates the presence of pyrrhotite and trace magnetite in bulk samples without any magnetic separation.

Inconsistent magnetic polarities between greigite- and pyrrhotite/magnetite-bearing marine sediments from the Tsailiao-chi section, southwestern Taiwan

Abstract To establish a magnetobiostratigraphy for a 620-m-thick middle to late Pleistocene mudstone sequence in the Lower Gutingkeng Formation of the Tsailiao-chi (TLC) section in southwestern Taiwan, we conducted paleomagnetic and mineral magnetic measurements, together with sediment granulometry and calcareous nannofossil identification. Paleomagnetic samples from 65 sites revealed two types of thermal demagnetization (25–400°C) behavior: (1) single-component stable characteristic remanence in magnetite- and pyrrhotite-dominated samples (Type S), and (2) abrupt changes in polarity when samples with significant greigite concentrations were heated above 320–340°C (Type C). The characteristic polarities derived from Type S samples and from magnetite-dominated Type C samples (obtained above 340°C) are consistent with those determined from nannofossil biostratigraphy. This implies that the NRM carried by magnetite and pyrrhotite is reliable. The essentially antiparallel remanence components in Type C samples below 340°C are attributed to greigite. The almost antiparallel direction could have resulted from delayed formation of greigite, but in this case, the different direction of this component must have resulted from variable remanence lock-in times. Alternatively, the opposite polarities may result from self-reversal, which warrants further investigation. Pyrrhotite and greigite may have both formed authigenically, but there is no clear explanation for the observed differences in direction.

Magnetic fabric analysis of the Plio-Pleistocene sedimentary formations of the Coastal Range of Taiwan

Abstract Over 2100 samples were collected from 130 sedimentary sites of early Pliocene to Pleistocene age in the Coastal Range of Eastern Taiwan. Their lithology is mainly unmetamorphosed marls, siltstones and sandstones and the formations are folded and faulted at different scales. Different shapes of the susceptibility ellipsoid were observed from an oblate form, with K min closely perpendicular to the bedding plane, to a “pencil” structure, with K min a K int forming a girdle around K max , reflecting an increasing tectonic contribution to the magnetic fabric. At sites where fault tectonic analysis is available, the magnetic lineation is perpendicular to the main compression. These results have a twofold significance: first, confirming previous preliminary results, they show that in such visually undeformed sediments the magnetic fabric may be, at least in part, of tectonic origin. Second, when considered together with recent paleomagnetic and tectonic data, they suggest that in the Plio-Pleistocene sediments of the Coastal Range of Taiwan the magnetic lineation arises from the compression due to the collision of the Philippine Sea and the Eurasian plates. In our interpretation, the stress pattern has rotated clockwise along with the structures during this collision.

Stratigraphic architecture, magnetostratigraphy, and incised-valley systems of the Pliocene-Pleistocene collisional marine foreland basin of Taiwan

Lithofacies analysis, magnetostratigraphy, and seismic profiles of Pliocene-Pleistocene foreland basin deposits of Taiwan provide a framework to evaluate the stratigraphic development of a collisional marine foreland basin. We have recognized several scales of stratigraphic packages and unconformities in deposits of the Taiwan foreland basin. Small-scale (20 to 150 m thick) stratigraphic sequences contain upward-shallowing, marine lithofacies successions that are bracketed by thin coquina sandstones. We interpret the small-scale stratigraphic packages as “parasequences” in the traditional sequence stratigraphy model, the thin coquina sandstones representing marine-flooding intervals. The average duration of individual small-scale packages was in the range of 37.5 k.y., on the basis of our magnetostratigraphy. These sequences are interpreted as the product of eustatic sea- level change possibly related to the orbital time series of obliquity. Intermediate-scale stratigraphic sequences are 150 to 1000 m thick and are bounded by unconformities that are well exposed in outcrop and can be clearly identified in seismic sections. The unconformity surfaces have several hundred meters of relief and represent periods of major fluvial valley incision in the foreland basin. One of the unconformities is locally an angular one that we interpret as representing a growth structure that formed during structural uplift of the proximal margin of the foreland basin at ca. 1.25 Ma. Across this angular unconformity, there were marked increases in rates of sediment accumulation and tectonic subsidence in the foreland basin. Other major unconformities that bound intermediate-scale stratigraphic sequences are high-relief disconformities. These unconformities may be the product of eustatic changes, because there has been little change in rates of sediment accumulation and tectonic subsidence across these unconformities. The duration of individual, intermediate-scale packages ranges from ∼100 000 to 700 000 yr, on the basis of magnetostratigraphy and biostratigraphy. We interpret the intermediate-scale sequences as “sequences” in the traditional sequence stratigraphy model. Our analysis of the Pliocene-Pleistocene deposits of the Taiwan foreland basin has several implications for understanding the stratigraphic evolution of this collisional marine foreland basin. (1) Deposition in the Taiwan foreland basin appears to have been punctuated by at least five episodes of erosion and major fluvial valley incision. Large volumes of sediment were eroded from the proximal margin of the foreland basin and transported to more distal parts of the foreland basin or to depocenters outside the foreland basin system during all stages of basin development. (2) The presence of high-relief unconformities and growth structures in the Pliocene-Pleistocene foreland basin deposits suggests a well-developed wedge-top depozone in the foreland basin system. (3) The Pliocene- Pleistocene strata of the foreland basin of Taiwan record ∼2.3 m.y. of deposition, on the basis of our magnetostratigraphy. Sediment accumulation rate was on the order of ∼950 m/m.y. during the earlier stages of basin development. During the later stages of basin development, sediment accumulation rate increased to ∼1900 m/m.y. Sediment accumulation rates in the collisional marine foreland basin of Taiwan are much higher than previously published rates from more extensively studied retroarc foreland basins and collisional nonmarine foreland basins.