Revisiting the Lowrie–Fuller test: alternating field demagnetization characteristics of single-domain through multidomain glass–ceramic magnetite

This paper reports the alternating field demagnetization characteristics of glass–ceramic magnetite assemblages carrying weak-field thermoremanent magnetization (TRM), weak-field anhysteretic remanent magnetization (ARM), and saturation remanence (J_rs). Average grain sizes vary from less than 0.1 μm to approximately 100 μm, and hysteresis parameters indicate that these assemblages encompass single-domain (SD) through truly multidomain (MD) behavior. In all assemblages, weak-field TRM and weak-field ARM are more stable to alternating field demagnetization than is (J_rs). This response is especially remarkable in the 100 μm assemblage, which otherwise displays truly MD behavior. Although the SD samples pass the Lowrie–Fuller test for SD behavior, calculations presented here show that populations of noninteracting, uniaxial SD grains should behave in just the opposite sense to that reported originally by Lowrie and Fuller. This discrepancy could indicate that SD, glass–ceramic magnetite populations are more affected by magnetic interactions than would be expected for magnetite crystals that nucleated individually from a silicate matrix. This interpretation is supported by the SD assemblages failing the ‘Cisowski' test: that is, the curves for acquisition and AF demagnetization of (J_rs) intersect well below the 50% mark. However, a second and intriguing explanation of the SD-like results obtained from all samples is that alternating field demagnetization characteristics reflect a strong dependence of local energy minimum domain state, and its associated stability, on the state of magnetization.     

Characteristic magnetic properties of titanomagnetites in continental igneous rocks

Curie temperatures, hysteresis, alternating field properties and anhysteretic and ordinary susceptibilities have been used to characterize the titanomagnetites in a large collection of continental granites, diorites, syenites, anorthosites, gabbros, diabases and basalts. Low-Curie-point titanomagnetites or titanomaghemites were found only in basalts. In all shallow and deep-seated intrusive rocks, the predominant magnetic phase was nearly-titanium-free titanomagnetite with a Curie point of 520–580°C. Most felsic plutonic rocks owed their magnetic properties to coarse, discrete titanomagnetites with truly multidomain properties. Many mafic plutonic rocks (anorthosites, gabbros, norites) displayed bimodal magnetic properties, strong-field properties being due to the discrete titanomagnetites and weak-field properties being due to fine magnetite inclusions in deuterically altered silicates. The Lowrie-Fuller test and the anhysteretic induction curve were the most diagnostic tests of this bimodal behaviour. Grain-size variation within a single diabase dike or sill had a strong expression in all magnetic properties, except H_R/H_c and the Lowrie-Fuller test. On the other hand, the Lowrie-Fuller test was a sensitive indicator of changes in “effective” grain size in basalts due to the subdivision of grains by ilmenite lamellae.     

Magnetic domain observations of nucleation processes in natural pyrrhotite and titanomagnetite

Magnetic domain patterns have been observed on particles of natural pyrrhotite and titanomagnetite undergoing hysteresis. These observations indicate that hysteresis properties are governed by two distinct mechanisms: (1) wall-pinning and (2) nucleation of reverse domains. Particles which are dominated by wall-pinning spontaneously nucleate reverse domains in saturation remanence (J_rs). The coercivity of such grains is determined by the presence of potential wells encountered by the wall in its traverse across the grain. However, many pseudosingle-domain particles (PSD) between 5 and 30 μm in diameter do not nucleate reverse domains in J_rs, but remain as saturated single-domains. These particles require a reverse field H_n to nucleate domain walls. When H_n is sufficiently large, the nucleating field controls magnetization reversal by driving the wall across the particle in a single Barkhausen jump, and the muscopic coercivity is nucleation-dominated.

The proportion P (w=0) of particles of a given size d which fail to nucleate walls in J_rs is found to be given by A exp(−Bd^1/2), where A and B are experimentally determined constants. The nucleation field H_n in pyrrhotite is observed to increase with decreasing grain size, exceeding 500 Oe in 5 μm particles.

The difficulty with which reverse domains are nucleated subsequent to saturation may thus provide a mechanism for achieving the high values of J_rs/J_s and coercive force observed in fine, pseudosingle-domain particles.     

Some magnetic properties of synthetic and natural haematite of different grain size

The behaviour of some magnetic properties of natural and synthetic haematite of different grain size is examined. The natural haematite was obtained from the hydrothermal deposit Kada (Czech and Slovak Federal Rep.). Six grain-size fractions ranging from 120 to 40 μm were prepared by means of sieving and two further fractions down to 5 μm by wet ultrasonic sieving. Since the behaviour of the fractions is similar, that of only four representative samples is reported. In addition, the behaviour of one submicron synthetic haematite fraction (0.5 μm) prepared by oxidation of ferrous sulphate (uniform in size and shape) was investigated.

The initial remanence value (J_r) seems to increase with decreasing grain size. During alternating field (AF) demagnetization, all fractions behaved similarly, except for the submicron fraction which is considerably softer than the others. Normalized (isothermal remanent magnetization) IRM acquisition curves were similar for all fractions.

Parameters of the anisotropy of magnetic susceptibility (AMS) display significant changes, mainly during IRM acquisition. During AF demagnetization, the anisotropy degree P exhibits a slight increase (some %), while the behaviour of the shape factor T is complicated. The anisotropy ellipsoid exhibits a tendency to rotate. Significant changes in the AMS parameters occur during IRM acquisition. Curves of P and T vs. IRM acquisition field, for various grain-size fractions, show no coherent pattern. For all the samples studied, the T vs. H curve exhibits a threshold value at which change in the type of arrangement of easy axis of magnetization occurs. For the IRM acquisition fields higher than some 320 kA m⁻¹, the minimum susceptibility axis parallels the direction of the IRM acquisition field.

Hysteresis curves of the fractions are similar to each other. The Preisach distribution function was determined and it indicates that the reversible part of the magnetization process plays an important role comparatively. Based on the coercivity data presented no unambiguous conclusion could be drawn from the single-domain (SD)-multidomain (MD) transition, associated with a coercivity maximum.     

Determination of domain structure in igneous rocks by alternating field and other methods

AF (alternating field) demagnetization, ARM (anhysteretic remanent magnetization) and strong-field hysteresis properties of a large collection of mostly continental igneous rocks are reported here. The collection included rocks whose magnetic carriers were believed from previous work to be of one of three types: MD (multidomain); SD/PSD (single-domain/pseudo-single-domain); or a bimodal mixture of MD grains (e.g., discrete opaques) and SD/PSD material (e.g., silicate inclusions). Two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour. SD/PSD rocks have relatively hard inflected AF decay curves (decay rate initially increasing, then decreasing), MD rocks have soft, exponential-like decay curves, and bimodal rocks have a combination of these characteristics. Relative hardnesses of normalized decay curves of remanences acquired in weak, intermediate and strong fields (the Lowrie-Fuller test) are also distinctively different for the three classes, and the results support the theory developed in an accompanying paper [1] that Lowrie-Fuller characteristics are an expression of the shapes of decay curves. The Lowrie-Fuller test, although its result can be expressed as a numerical parameter, is not capable of fine-scale classification of domain structure or grain size. The shape of the ARM induction curve does have a quasi-continuous variation with grain size, however. The parameter χ_ar/J_rs (initial anhysteretic susceptibility normalized to saturation remanence), which is easily measured with standard paleomagnetic instrumentation, is potentially useful for magnetic granulometry, although χ_ar itself was not diagnostic of grain size.     

Southern limit of mantle-derived geothermal helium emissions in Tibet: implications for lithospheric structure

The isotopic composition of helium emitted from geothermal springs in the southern Tibetan plateau, reported as R_c/R_A (R_c=air corrected sample ³He/⁴He, R_A=air ³He/⁴He), ranges from 0.013 to 0.38, and defines two principal domains. In southernmost central Tibet, helium isotope ratios are typical of radiogenic helium production in the crust (R_c/R_A<0.05, crustal helium domain). Further north, there is a resolvable ³He anomaly consistent with a mantle contribution (R/R_A>0.1, mantle helium domain). The highest values of 0.27–0.38 R_A occur at the southern end of the Karakoram fault. The boundary between the two domains lies 50–100 km north of the Indus-Zangpo suture zone. There seems to be no association between the ³He anomaly and zones of active normal faulting and litho-tectonic crustal units, such as the ultramafic rocks of the Indus-Zangpo suture zone and the Gangdese intrusive belt. Although scavenging of mantle-derived helium, stored in large ultrabasic and basic intrusions in the crust, cannot be ruled out entirely, we argue that the ³He anomaly most plausibly reflects degassing of volatiles from young (Quaternary) mantle-derived melts intruded into the crust. As such, it defines the southern limit of recent mantle melting and mantle melt extraction beneath the Tibetan plateau. The southern limit of the ³He anomaly coincides with the junction between the Indian and Asian plates, in the region where the Indian lithospheric slab steepens and is subducted beneath Tibet as suggested by seismic studies. Recent mantle melting and melt extraction is confined to the Asian mantle, but the southern limit of the melt zone may have migrated northwards during the last 10 Ma as the Indian lithosphere has progressively underthrust the Himalayas and Tibet.     

Mathematical modeling of PCB bioaccumulation in Perna viridis

In the present work, we built a mathematical model of polychlorinated biphenyl (PCB) bioaccumulation in Perna viridis, namely, a one-compartment model with a time dependent incorporation rate R (μg g⁻¹ lipid per ppb water per day), with positive substrate cooperativity as the underlying physical mechanism. The temporal change of the PCB concentration Q (μg g⁻¹ lipid) in the soft tissues of the mussel depends on the competition of the input rate RW and the output rate kQ, where W is the concentration of PCB in water (ppb water) and k is the elimination rate (per day). From our experimental data, k=0.181±0.017 d⁻¹. The critical concentration in water W_c for positive substrate cooperativity was found to be 2.4 ppb. Below W_c, R is a constant. For a water concentration of 0.5 ppb Aroclor 1254, R=24.0±2.4 μg g⁻¹ lipid ppb⁻¹ d⁻¹. Above W_c, positive substrate cooperativity comes into effect and R becomes a function of time and dependent on the concentration Q in a form R=γQ/(Q+δ). This is the case for a water concentration of 5 ppb Aroclor 1254, where γ=15.1 μg g⁻¹ lipid ppb⁻¹ d¹ and δ≈200 μg g⁻¹ lipid. From this model, the uptake is exponentially increasing when the PCB concentration in the mussel is small compared to 200 μg g⁻¹ lipid, and hyperbolically increasing when the concentration is large compared to 200 μg g⁻¹ lipid, which are consistent with the experimental data. The model is useful for understanding the true processes taking place during the bioaccumulation and for risk assessment with higher confidence. Future experimental data which challenge the present model are anticipated and in fact desirable for improvement and perfection of the model.     

Size effects of suspended particles on gill damage in green-lipped mussel Perna viridis

The green-lipped mussels Perna viridis were exposed to <500 μm suspended solids (SS) with concentrations of 0 (control), 250, 500, 750 and 1000 mg/L for 14 days, followed by transferring to clean, filtered seawater for 28 days. Results of scanning microscopy showed significantly higher damages to the cilia on the frontal surface of the gill filaments than that on the abfrontal surface in both demibranchs. Percent ciliary depletion varied with SS concentrations and time. No sign of recovery of the gill filaments was observed after the mussels were transferred to clean seawater. In a second experiment, mussels were exposed to SS with size range from <63, >125–<250 and >250–<500 μm at 600 mg/L, together with a control (0 mg/L) for 14 days, followed by transferring to clean, filtered seawater for 28 days. Results of scanning microscopy showed significant ciliary damages in both the ascending and descending lamellae under the three particle size groups as compared with the control. Percent depletion of frontal cilia was most serious for the >250–<500 μm size group and least for the <63 μm size group. However, percent depletion of abfrontal cilia was most serious for the >125–<250 μm size group and least for the <63 μm size group. No recovery of ciliary damages was observed. The effects of particle size of suspended sediments on the morphological damages of gill filaments in the green-lipped mussels were discussed.     

The variation of the blocking temperature in models of thermoremanence (TRM)

The simplest non-interactive forms of Néel's domain-reversal and wall-motion models are examined and the variations of their respective blocking temperatures as functions of applied field and coercivity are compared. Both models show remarkably similar blocking temperature behavior, but the remanence acquisition mechanisms are so different as to produce diametrically opposed results when the Lowrie-Fuller stability criterion is considered. In particular, in low fields the wall-motion model selectively activates high-coercivity fractions while the domain-reversal model favors the low coercivities. This results in the paradox that the wall-motion model predicts behavior experimentally associated with the single-domain and pseudo-single-domain size range while the domain-reversal model predicts behavior usually associated with the large multidomain range. The paradox can be partly resolved within the multidomain model alone, but this would not apply for the smallest grain sizes. Several alternative approaches to the problems of this size range are suggested.     

宁夏地区地震目录最小完整性震级分析

收集宁夏测震台网地震目录资料,采用MBS、EMR、震级-序号、MAXC、GFT等方法,对宁夏境内1970-2017年最小完整性震级M_c时序变化进行综合分析,确定采用EMR方法测定的M_c较为理想,并进一步分析该区M_c时空分布特征。结果显示,宁夏测震台网测震模拟观测、数字化改造及"十五"数字化运行阶段,M_c分别为M_L 2.2±0.1、M_L 1.7±0.1、M_L 1.6±0.1;"十五"数字化运行阶段以来,M_c空间分布不均匀,整体维持在M_L 1.3左右。     

Anhysteretic remanent magnetization (ARM) in magnetite grains

Summary ARM has been measured in a range of inducing, steady fields up to 50 oersteds and for 6 sizes of magnetite grains with average diameters 5 m to 174 m. For all sizes a slight non-linearity of ARM with inducing field was found, apparently comprising a non-linear contribution independent of grain size plus a linear contribution which increased with decreasing grain size. In the largest grains induced ARM agreed well with multidomain grain theory. Relative enhancement of ARM in smaller grains is comparable to the enhancement of thermoremanence and therefore appears to indicate a pseudosingle domain contribution to ARM in small grains. However the observations allow an alternative explanation in terms of more extreme dimension ratios in the smaller grains. Presentation of the equations for multidomain ARM and TRM using observed instead of intrinsic susceptibilities makes it appear that the inadequacy of multidomain theory (and consequent necessity for pseudo-single domain theory) are less serious than has been supposed.     

多方法联合评估河套地震带的台网监测能力

使用删除余震后的地震目录, 综合运用多种方法进行M_c值的定量计算和对比分析, 对河套地震带的台网监测能力的时空变化进行分析判定。 结果表明, 1970年以来, 河套地震带的台网监测能力整体呈现逐步增强趋势。 在Bootstrap次数为200的条件下, MAXC方法确定的研究区整体M_c值为M_L2.7±0.07, δM_c的分布基本控制在M_L0.1～0.4之间, EMR方法给出的M_c时序变化范围是M_L1.6～2.8; 多方法联合测定的M_c值变化范围为M_L1.8～3.3, R/S检验给出的结果是M_L3.4。 对比发现, 余震删除对最小完整性震级的计算有一定影响, R/S检验求取的最小完整性震级在余震删除前后相差M_L0.9; 整体来看, M_c值随时间演化逐渐减小, 较小时空范围内的M_c值仍有显著的非均匀性变化现象, 不同时段内台网布局的空间调整以及地震事件的时空非均匀性活动可能是主要原因。     

Physical basis of earthquake magnitudes: an extreme value of seismic amplitudes from incoherent fracture of random fault patches

A relation between surface-wave magnitude M_s and fault area S for great earthquakes has been proposed

log S 2M_{s − 11.5 (}M_{s > 7.5)}

A similar formula has been also derived for body-wave magnitude m_b^* redetermined from maximum amplitudes of short-period P-wavetrains

log S ∝ 1.7 m_b^*

These are quite different from a theoretical relation expected on the basis of long-wave approximation. Because wavelengths of seismic waves used for the above magnitudes are very short compared to the size of earthquake sources, these relations represent the short-period nature of the earthquake process. The statistical theory of extreme values has been applied to understand the relations considering that the component waves which constitute the wavetrains for m_b^* and M_s determinations originate from the random fracture of fault heterogeneities.     

Spatial variation of effective porosity and its implications for discharge in an upland headwater catchment in Scotland

Spatial and temporal measurements of shallow sub-surface soil physical properties were made within a 1 km² upland catchment. The surface soil layer of the catchment was organic rich (>70% organic matter) with a corresponding total porosity of 81%. Monthly point observations of volumetric water content (θ) were combined with point estimates of total porosity () and the porosity <50 μm (_residual), to define the ratio of water filled pore volume:pore volume in pores <50 μm (=θ/_residual). Values of θ/_residual were compared with discharge to test whether mass flow occurred when θ/_residual>1. A correlation between water content and discharge was found, with discharge increasing rapidly when θ/_residual approached unity. Similar relationships between water content and catchment discharge were identified for soil units adjacent to the stream when θ/_residual approached unity. These data suggest that soil pores >50 μm are of crucial importance in determining catchment discharge. Spatial and temporal variations in soil properties related to moisture content of the soil were also observed. Under dry conditions, a clear division based on aspect was noted, the west-facing side of the catchment being wettest. In wetter months, total porosity and soil water content were significantly affected by soil type and the spatial pattern of soil water content was more variable than in the dryer months. The physical quantification of soil properties in the shallow sub-surface layer proved important in explaining different initial changes in discharge from the catchment in response to a rainfall event.     

The crust and upper mantle discontinuity structure beneath Alaska inferred from receiver functions

In this study, three receiver function stacking methods are used to study the detailed crust and upper mantle structure beneath south-central Alaska. We used teleseismic waveform data recorded by 36 stations in the Broadband Experiment Across the Alaska Range (BEAAR) and 4 permanent stations in Alaska. H − κ stacking method using P-to-S converted wave and its multiply reflected waves between the Earth's surface and the Moho discontinuity is adopted to estimate the crustal thickness (H) and average crustal V_P/V_S ratio (κ) in this region. The receiver function results for 24 stations show that the crustal thickness under Alaska ranges from 26.0 to 42.6 km with an average value of 33.8 km, and the V_P/V_S ratio varies from 1.66 to 1.94 with an average value of 1.81 which corresponds to an average Poisson's ratio of 0.277 with a range from 0.216 to 0.320. High Poisson's ratios under some stations are possibly caused by partial melting in the crust and the uppermost mantle. Common converted point (CCP) stacking results of receiver functions along three lines show clear Moho and slab images under this subduction zone. The depths of the slab from our CCP stacking images are consistent with those estimated from the Wadati–Benioff Zone (WBZ). In the area between two stations DH2 (147.8°W, 63.3°N) and DH3 (147.1°W, 63.0°N), a Moho depth offset of about 10 km is found by both the H − κ and CCP stacking techniques. Common depth point (CDP) stacking of receiver functions shows not only the 410-, 520- and 660-km discontinuities, but also significant variations (−30 to 15 km) in the transition zone thickness under the southwest and southeast parts of the study region. The transition zone becomes thinner by 20–30 km, indicating that the temperature there is 150–200 K higher than that of the normal mantle.     

Sensitivity of bacterioplankton nitrogen metabolism to eutrophication in sub-tropical coastal waters of Key West, Florida

Expression of intracellular ammonium assimilation enzymes were used to assess the response of nitrogen (N) metabolism in bacterioplankton to N-loading of sub-tropical coastal waters of Key West, Florida. Specific activities of glutamine synthetase (GS) and total glutamate dehydrogenase (GDH_T) were measured on the bacterial size fraction (<0.8 μm) to assess N-deplete versus N-replete metabolic states, respectively. Enzyme results were compared to concentrations of dissolved organic matter and nutrients and to the biomass and production of phytoplankton and bacteria. Concentrations of dissolved inorganic N (DIN), dissolved organic N (DON), and dissolved organic carbon (DOC) positively correlated with specific activities of GDH_T and negatively correlated with that of GS. Total dissolved N (TDN) concentration explained 81% of variance in bacterioplankton GDH_T:GS activity ratio. The GDH_T:GS ratio, TDN, DOC, and bacterial parameters decreased in magnitude along a tidally dynamic trophic gradient from north of Key West to south at the reef tract, which is consistent with the combined effects of localized coastal eutrophication and tidal exchange of seawater from the Southwest Florida Shelf and Florida Strait. The N-replete bacterioplankton north of Key West can regenerate ammonium which sustains primary production transported south to the reef. The range in GDH_T:GS ratios was 5–30 times greater than that for commonly used indicators of planktonic eutrophication, which emphasizes the sensitivity of bacterioplankton N-metabolism to changes in N-bioavailability caused by nutrient pollution in sub-tropical coastal waters and utility of GDH_T:GS ratio as an bioindicator of N-replete conditions.     

SiO2 precipitation in olivine: ATEM investigation of two dunites annealed at 300 MPa in hydrous conditions

Two natural dunites were annealed at pressure P=300 MPa, temperature T=1373, 1473 and 1573 K, and fO₂ within the stability field of olivine. The starting materials contained small amounts of hydroxyls in the form of minor phases of hydrated minerals, which released an aqueous phase during the experiments. A detailed analytical transmission electron microscopy (ATEM) investigation of these materials revealed that small quantities of two types of silica-rich glass formed during heat treatment. The first type of glass, found at triple junctions as rare partially crystallized glass pockets, results from melting dehydration reactions involving the hydrous phases. The second type of glass is found as pure silica precipitates (0.1–0.5 μm in size, for a total of a few 0.1 vol%) within the olivine grains of specimens heated to ≥1473 K. From considerations of the kinetics of the precipitation at 1473 K, we interpret this silica precipitation as resulting from the condensation of olivine metallic vacancies promoted by increasing fluid fugacities during the runs. Our observations, thus, demonstrate that metastable silica can precipitate in olivine from dunites experiencing rapid changes in their thermodynamical environment.     

Effect of exchangeable Mg on saturated hydraulic conductivity, disaggregation and clay dispersion of disturbed soils

Different opinions exist regarding the specific effect of Mg on soil physical and chemical properties. We hypothesized that Mg²⁺, compared with Ca²⁺, reduces saturated hydraulic conductivity (K_s) via promoting clay swelling, disaggregation, and clay dispersion. Two soils (mixed, mesic Typic Hapludalfs) in packed soil columns were leached with either Ca- or Mg-containing solutions at the successive concentrations of 250, 10, 2, 0.5, and 0 mM. Critical flocculation concentration (CFC) in either Ca or Mg systems was determined with flocculation series tests. Aggregate stability and mean weight diameter (MWD) were assessed by wet-sieving. The CFCs were higher in Mg than in Ca for both soils, indicating that Mg is more dispersive than Ca. The MWDs measured using 1–2 mm aggregates of both soils were significantly larger for Ca-soils than for Mg-soils (P=0.05). The K_sr (normalized with initial K_s) started to decline at higher concentrations for Mg than for Ca, and the reduction was much greater in Mg than in Ca above 0.5 mM. The K_sr and percent transmittance (inversely related to turbidity) of leachate at a given eluted pore volume following ‘steady state’ were higher in Ca than in Mg for both soils (P=0.1), indicating lower permeability and more clay dispersion with the Mg treatment. Swelling and disaggregation, which reduced large pores, appeared to be the dominant process causing the rapid initial decline of K_sr. Clay dispersion and subsequent pore plugging became progressively important when electrolyte concentration was reduced to below CFCs.     

Low-temperature exsolution in refractory siderophile element-rich opaque assemblages from the Leoville carbonaceous chondrite

A model for the formation of opaque assemblages in refractory inclusions in C3V chondrites was proposed by Blum et al. [1,2], in which the assemblages formed not by direct condensation but by crystallization from siderophile element-rich droplets within molten silicate inclusions, followed by low-temperature exsolution and oxidation and/or sulfidation. Although the data presented do not address the high-temperature histories of these objects, we describle herein two opaque assemblages in a Leoville refractory inclusion which provide the strongest textural and chemical evidence yet found in natural samples for the relatively low-temperature equilibration called for in this model. The assemblages have fine ( < 1 μm across) lamellae of εRuFe in γNiFe, with sharp contacts between phases. The larger (35 × 35 μm) of the two assemblages, OA1, has sets of regularly spaced, parallel lamellae, demonstrating crystallographic control. In the other, OA2, Ir and Pt partitioning between lamellae and host is consistent with equilibration at 873 K, as are the Fe, Ni and Ru contents of coexisting εRuFe and γNiFe and the narrow widths of the lamellae in both assemblages. The bulk compositions of the two assemblages bracket a previously inferred phase boundary in the FeNiRu system at 873 K. In these assemblages, the observed phases and their compositions are as predicted by the inferred phase relations at this temperature, suggesting that the position of the inferred phase boundary is correct. This is the same equilibration temperature estimated by Blum et al. [2] for another opaque assemblage from Leoville, also on the basis of alloy compositions and inferred phase equilibria in the FeNiRu system. The similarity of results for different Leoville samples is consistent with the suggestion [2] that final equilibration of the assemblages was post-accretionary.     

Clam burrowing behaviour: Inhibition by copper-enriched sediment

Burrowing behaviour is adaptive and allows clams to escape predation; yet the effects of potentially toxic metals on such behaviour have not been adequately investigated. In natural marine sediment contaminated with copper the time for littleneck clams (Protothaca staminea) to achieve complete burial was recorded. Above a threshold of 5.8 μg g⁻¹ Cu added to dry sediment, the time for 50% of the clams to burrow (ET₅₀) increased logarithmically with increasing sediment copper concentration according to:

logET₅₀ = 0.15 (CU) - 1.37 (n = 4, r² = 0.98)

where ET₅₀=time in hours for 50% of clams to burrow and Cu=μg g⁻¹ Cu in dry sediment. Previously exposed clams had both a lower threshold to Cu and a longer reburrowing time (ET₅₀). Clams exposed to sediment mixed with Chelex-100®-sorbed copper showed no significant change in burrowing time. Bioassays based on claim burrowing behaviour can measure both bioeffectiveness of sediment-sorbed metals and a sublethal effect with ecological meaning.