Environmental impact in sandy beaches of copper mine tailings at Chañaral, Chile

Since 1938, untreated copper mine tailings of Potrerillos and El Salvador have been disposed into the sea at Chañaral, Chile (26° 21′ Lat. S., 70° 42′ Long. W.). Over 220×10⁶ t of sediments have been dumped. This pollution has caused drastic geomorphological changes in the c. 16 km contaminated area. The occurrence of new artificial beaches and modification of coastal contours are reported. Sandy beach macrofauna monitoring (1975–1982) demonstrated a progressive lowering of density and biomass in those communities affected by copper mine tailings.     

Cadmium content, accumulation and toxicity in dog whelks collected around the Welsh coastline

The concentration of cadmium in sexually mature Nucella lapillus was investigated around the coast of Wales. Higher concentrations of cadmium were found in whelks located in the Bristol Channel compared with Cardigan Bay Nucella. Accumulation and toxicity of cadmium were investigated at 50–600 μg Cd l⁻¹ over a period of 21 days. At the lower concentrations of 50–100 μg Cd l⁻¹ all Nucella survived the 21 days of experimentation. The rate of cadmium uptake after exposure to cadmium was inconsistent and not related to the degree of site contamination. At all sites the concentration of cadmium in Nucella, after treatment with 50–600 μg Cd l⁻¹ for 21 days, was significantly different from controls, but after 7 days it varied with the site and cadmium concentration. Only samples collected at Fishguard, Dale, and Swansea survived 21 days at 600 μg Cd l⁻¹. Tissue cadmium concentrations did not significantly effect the whole body concentration of zinc.     

Sulfate reduction using methane in sediments beneath a bathyal “cold seep” giant clam community off Hatsushima Island, Sagami Bay, Japan

Two sandy sediment cores (Cores D227-120 and D380) were collected from inside a deep-sea giant clam (Calyptogena soyoae) community off Hatsushima Island, western Sagami Bay, central Japan (35°59.9′N, 139°13.6′E; 1160 m deep) and a muddy sediment core (Core D227-202) was obtained from outside the community by the submersibleShinkai 2000. The chloride concentration of the pore waters is constant vertically and sulfate reduction using sedimentary organic matter occurs in Core D227-202 (21 cm long). The chloride concentrations are lower by 7% at the 7.5–9 cm depth in Core D227-120 (9 cm long) and by 3% at the 11–12 cm depth in Core D380 (16 cm long) than those of the overlying bottom waters in the cores from inside of the community. Sulfate concentration decreases remarkably and dissolved inorganic carbon, alkalinity, ammonium-N, and hydrogen sulfide concentrations increase significantly with increasing depth in Core D380.δ³⁴S values of sulfate ions increase from +20.5 to +35.3‰ andδ¹³C values of dissolved inorganic carbon decrease drastically from −7.0 to −45‰ with increasing depth from the top to the bottom of the core, although theδ¹³C values of the organic carbon of the sediments are−23.7 ± 0.9‰ in Core D380. These results indicate that sulfate reduction using methane is active within the sediments just beneath the living clams and that the hydrogen sulfide produced can be used by endosymbiotic sulfur oxidizing bacteria living in the gills ofC. soyoae in the community.     

Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis

Benthic foraminiferal magnesium/calcium ratios were determined on one hundred and forty core-top samples from the Atlantic Ocean, the Norwegian Sea, the Indian Ocean, the Arabian Sea and the Pacific Ocean, mostly at sites with bottom water temperatures below 5 °C. Mg/Ca ratios are consistently lower, by  0.2 mmol/mol, in samples cleaned using oxidative and reductive steps than using oxidative cleaning. Differences between Cibicidoides species have been identified: Mg/Ca of Cibicidoides robertsonianus > Cibicidoides kullenbergi > Cibicidoides wuellerstorfi. Comparison with bottom water temperatures support observations of lowered Mg/Ca of C. wuellerstorfi at temperature below  3 °C compared with values predicted by published calibrations and from other Cibicidoides species. Hydrographic data shows that carbonate ion saturation (Δ[CO₃²⁻]) decreases rapidly below this temperature. An empirical sensitivity of Δ[CO₃²⁻] on Mg/Ca has been established for C. wuellerstorfi of 0.0086 ± 0.0006 mmol/mol/μmol/kg. A novel application using modern temperatures and Last Glacial Maximum temperatures derived via pore fluid modelling supports a carbonate ion saturation state effect on Mg incorporation. This may significantly affect calculated δ¹⁸O_seawater obtained from foraminiferal δ¹⁸O and Mg/Ca temperature.     

Field accumulative behavior of organochlorine pesticides. The role of crabs and sediment characteristics in coastal environments

The influence of intertidal crab beds on the concentrations of organochlorine (OC) pesticides in sediment was studied in two different coastal environments in Argentina. Samples of male burrowing crabs (Chasmagnathus granulatus) were collected for this study. Our field data showed lower bioaccumulation of OC pesticides in crabs from sediments with a higher total organic carbon (TOC) and higher clay content. Thus, concentrations in crabs depend on the physico-chemical characteristics of the sediment where they live more than on the OC pesticide concentrations in the environment. The distribution patterns in sediment from inside and outside crab burrows were similar for both coastal areas being HCHs  γ-chlordane > p,p′-DDE for San Antonio Bay (SAO), and HCHs > p,p′-DDE  γ-chlordane for Mar Chiquita (MCh) coastal lagoon. OC pesticide concentrations in sediment were significantly lower inside than outside crab burrows, irrespective of the sediment physico-chemical characteristics due to the bioturbation activity of C. granulatus.     

Seasonal changes in vertical distribution of zooplankton in an oligotrophic, subarctic lake (Lake Takvatn, Norway)

Zooplankton vertical distribution was studied in Lake Takvatn (69°07′ N) by discrete sampling of the water column during the open water season. Attention was focused on predation risk and competition to understand when, where and to what degree a given species should aggregate along the water column. Temperature profiles and phytoplankton abundance and composition were recorded to assess degree of heterogeneity and food availability in the pelagic zone. Vertical segregation was evident during thermal stratification. Rotifers partitioned the water column, with species less susceptible to predation (e.g. Conochilus unicornis) in the epilimnion, where they overlapped with the predators Asplanchna priodonta and Polyphemus pediculus, and species more vulnerable to these predators (e.g. Keratella cochlearis) in the hypolimnion. Cladocerans remained in the epilimnion, in a year when predation by fish was limited. The copepodites of Eudiaptomus graciloides and Cyclops scutifer maintained different depths staying respectively near surface and in the meta-hypolimnion. A broader distribution with increasing density was evident among rotifers and C. scutifer nauplii. Density-dependent habitat selection behaviour is considered as a mechanism explaining the observed distribution of predators and competing prey and promoting species coexistence.     

Effect of hydrostatic pressure on the lattice parameters of Fe2SiO4 olivine up to 70 kbar

The pressure dependence of the three lattice parameters and unit cell volume of fayalite (Fe₂SiO₄ olivine) was determined by X-ray diffraction under hydrostatic pressures up to 70 kbar. In order to eliminate stress inhomogeneity within a composite material consisting of a specimen mixed with an internal-pressure standard, a liquid (1 : 1 mixture of ethanol and methanol) was used as a pressure-transmitting medium. The isothermal bulk modulus calculated on the basis of the second-order Birch-Murnaghan equation of state gives the values K₀ = 1.19 ± 0.10 Mbar and K₀′ = 7 ± 4, and if we assume K₀′ = 5: K₀ = 1.24 ± 0.02 Mbar. Three axes of fayalite were found to be compressible in the following order, b >c >a. Comparisons with the results obtained under non-hydrostatic compression are made.     

Coral bleaching on high-latitude marginal reefs at Sodwana Bay,South Africa

Coral bleaching, involving the expulsion of symbiotic zooxanthellae from the host cells, poses a major threat to coral reefs throughout their distributional range. The role of temperature in coral bleaching has been extensively investigated and is widely accepted. A bleaching event was observed on the marginal high-latitude reefs of South Africa located at Sodwana Bay during the summer months of 2000. This was associated with increased sea temperatures with high seasonal peaks in summer and increased radiation in exceptionally clear water. The bleaching was limited to Two-mile Reef and Nine-mile Reef at Sodwana Bay and affected <12% of the total living cover on Two-mile Reef. Montipora spp., Alveopora spongiosa and Acropora spp. were bleached, as well as some Alcyoniidae (Sinularia dura, Lobophytum depressum, L. patulum). A cyclical increase in sea temperature (with a period of 5–6 years) was recorded during 1998–2000 in addition to the regional temperature increase caused by the El Niño Southern Oscillation phenomenon. The mean sea temperature increased at a rate of 0.27 °C year⁻¹ from May 1994 to April 2000. High maximum temperatures were measured (>29 °C). The lowest mean monthly and the mean maximum monthly temperatures at which coral bleaching occurred were 27.5 and 28.8 °C, respectively, while the duration for which high temperatures occurred in 2000 was 67 days at 27.5 °C (4 days at 28.8 °C). Increased water clarity and radiation appeared to be a synergistic cause in the coral bleaching encountered at Sodwana Bay.     

Geochemical morphology of the North Mid-Atlantic Ridge, 10°–24°N: Trace element-isotope complementarity

The new data presented here from a 10–24°N segment of the North Mid-Atlantic Ridge show that this segment is the most depleted of the 10–70°N ridge section. They also show the existence of: (1) a geochemical gradient from the 14°N anomaly to 17°10′N; (2) a very depleted mantle source (the lowest Sr isotopic ratios found so far in the North Atlantic); and (3) a geochemical limit located at about 17°10′N without any obvious relation with any structural feature. The 15°20′N fracture zone does not show any relationship with respect to this gradient. The basalts located north of 17°10′N have very homogeneous features, which allow their characteristics to be averaged (i.e., ⁸⁷Sr/⁸⁶Sr= 0.70238 ± 0.00004, (Nb/Zr)_N = 0.28 ± 0.1) and they are defined as normal mid-ocean ridge basalts. The basaltic glasses located south of 17°10′N present a wide spectrum of isotopic compositions and extended rare earth element patterns (from depleted to enriched). Despite this, they have a constant K/Nb of 233 ± 9 (1s_M, n = 18) whereas this ratio is 344 ± 29 north of 17°10′N. These observations illustrate the strong coherence of behaviour between K and Nb (Ta) during the petrogenic processes involved in the generation of these mid-ocean ridge basalts and also their fractionation during previous mantle processes. Possible interpretations of mixing processes are discussed and sources at the ridge segment scale are favoured. However, when looking in detail, local heterogeneities are still common and can even be traced back off-axis to 115 my.

Placed in the context of the North Atlantic Ridge from 10° to 70°N, the Sr isotopic ratios reveal the Azores superstructure (23–50°N), whereas the trace element ratios (La/Sm-Nb/Zr) trace the second-order structures (33–40°N, 42–48°N) superimposed on the superstructure. This study illustrates the complementarity of information given by certain well chosen trace element ratios on the one hand and by isotopic ratios on the other. Since there is evidence of decoupling between isotopic ratios and/or trace element ratios, it introduces the notion of complementary “chemical memory” as recorded by a given type of trace element ratio or a given type of isotopic ratio     

Calcium isotope fractionation in coccoliths of cultured Calcidiscus leptoporus, Helicosphaera carteri, Syracosphaera pulchra and Umbilicosphaera foliosa

Four species of marine calcifying algae, the coccolithophores Calcidiscus leptoporus, Helicosphaera carteri, Syracosphaera pulchra and Umbilicosphaera foliosa were grown in laboratory cultures under temperatures varying between 14 and 23 °C, and one species, C. leptoporus, under varying [CO₃²⁻], ranging from 105 to 219 μmol/kg. Calcium isotope compositions of the coccoliths resemble in both absolute fractionation and temperature sensitivity previous calibrations of marine calcifying species e.g. Emiliania huxleyi (coccolithophores) and Orbulina universa (planktonic foraminifera) as well as inorganically precipitated CaCO₃, but also reveal small species specific differences. In contrast to inorganically precipitated calcite, but similar to E. huxleyi and O. universa, the carbonate ion concentration of the medium has no statistically significant influence on the Ca isotope fractionation of C. leptoporus coccoliths; however, combined data of E. huxleyi and C. leptoporus indicate that the observed trends might be related to changes of the calcite saturation state of the medium. Since coccoliths constitute a significant portion of the global oceanic CaCO₃ export production, the Ca isotope fractionation in these biogenic structures is important for defining the isotopic composition of the Ca sink of the ocean, one of the key parameters for modelling changes to the marine Ca budget over time. For the present ocean our results are in general agreement with the previously postulated and applied mean value of the oceanic Ca sink (Δ_sed) of about − 1.3‰, but the observed inter- and intra-species differences point to possible changes in Δ_sed through earth history, due to changing physico-chemical conditions of the ocean and shifts in floral and faunal assemblages.     

Distributions of hydrothermal activity along the Mid-Atlantic Ridge: interplay of magmatic and tectonic controls

Hydrothermal activity has been investigated along three different sections of the slow-spreading Mid-Atlantic Ridge (MAR): 11°20′–30°N, 36–38°N and the Reykjanes Ridge, 57°45′–63°06′N. When considered in total, the incidence of venting along these three sections of the MAR compares well with the predictions of a model in which frequency of venting is linearly related to ridge-crest spreading-rate. At the scale of individual study areas, however, departure from the model is observed by up to an order of magnitude. Venting is anomalously rare along the Reykjanes Ridge but anomalously abundant along the MAR 36–38°N. Whilst such variability may be within the error of the linear spreading-rate model, we note that the interplay between magmatic and tectonic processes also differs between the three study areas. In the case of the Reykjanes Ridge we propose that the low incidence of venting reported may reflect a limitation of the sampling/investigative strategy because the style of venting which predominates may not give rise to conventional black-smoker hydrothermal plumes. Along the oblique and broadly segmented MAR 36–38°N, we propose that vigorous hydrothermal venting in broad segment-end non-transform discontinuities may be focussed along deeply penetrating active faults with the requisite heat supply being supported through some combination of along-axis magmatic intrusions and thermal release associated with the serpentinisation of crustal peridotites.     

U,U andTh in seawater

We have developed techniques to determine²³⁸U,²³⁴U and²³²Th concentrations in seawater by isotope dilution mass spectrometry. U measurements are made using a²³³U²³⁶U double spike to correct for instrumental fractionation. Measurements on uranium standards demonstrate that²³⁴U/²³⁸U ratios can be measured accurately and reproducibly.²³⁴U/²³⁸U can be measured routinely to ± 5‰ (2σ) for a sample of 5 × 10⁹ atoms of²³⁴U (3 × 10⁻⁸ g of total U, 10 ml of seawater). Data acquisition time is 1 hour. The small sample size, high precision and short data acquisition time are superior to-counting techniques.²³⁸U is measured to ± 2‰ (2σ) for a sample of 8 × 10¹² atoms of²³⁸U ( 3 × 10⁻⁹ g of U, 1 ml of seawater).²³²Th is measured to ± 20‰ with 3 × 10¹¹²³²Th atoms (10⁻¹⁰ g²³²Th, 1 1 of seawater). This small sample size will greatly facilitate investigation of the²³²Th concentration in the oceans. Using these techniques, we have measured²³⁸U,²³⁴U and²³²Th in vertical profiles of unfiltered, acidified seawater from the Atlantic and²³⁸U and²³⁴U in vertical profiles from the Pacific. Determinations of²³⁴U/²³⁸U at depths ranging from 0 to 4900 m in the Atlantic (7°44′N, 40°43′W) and the Pacific (14°41′N, 160°01′W) Oceans are the same within experimental error (± 5‰,2σ). The average of these²³⁴U/²³⁸U measurements is 144 ± 2‰ (2σ) higher than the equilibrium ratio of 5.472 × 10⁻⁵. U concentrations, normalized to 35‰ salinity, range from 3.162 to 3.281 ng/g, a range of 3.8%. The average concentration of the Pacific samples (31°4′N, 159°1′W) is 1% higher than that of the Atlantic (7°44′N, 40°43′W and 31°49′N, 64°6′W).²³²Th concentrations from an Atlantic profile range from 0.092 to 0.145 pg/g. The observed constancy of the²³⁴U/²³⁸U ratio is consistent with the predicted range of²³⁴U/²³⁸U using a simple two-☐ model and the residence time of deep water in the ocean determined from¹⁴C. The variation in salinity-normalized U concentrations suggests that U may be much more reactive in the marine environment than previously thought.     

The Impact of the Herbicide Diuron on Photosynthesis in Three Species of Tropical Seagrass

The impact and recovery from exposure to the herbicide diuron [DCMU; 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea] was assessed for three tropical seagrasses, maintained in outdoor aquaria over a 10-day period. Photosynthetic stress was detected using chlorophyll a fluorescence, measured with a Diving-PAM (pulse amplitude modulated fluorometer). Exposure to 10 and 100 μg l⁻¹ diuron resulted in a decline in effective quantum yield (ΔF/F_m^′) within 2 h of herbicide exposure in Cymodocea serrulata, Halophila ovalis and Zostera capricorni. Effective quantum yield also declined over the first 24 h of exposure in H. ovalis at even lower diuron concentrations (0.1 and 1.0 μg l⁻¹). Effective quantum yield in H. ovalis and Z. capricorni was significantly depressed at all diuron concentrations (0.1–100 μg l⁻¹) after 5 days exposure, whereas effective quantum yield in C. serrulata was only significantly lower in plants exposed to highest diuron concentrations (10 and 100 μg l⁻¹). Effective quantum yield depression was present 5 days after plants exposed to 10 and 100 μg l⁻¹ diuron were returned to fresh seawater. These results indicate that exposure to herbicide concentrations present in nearshore Queensland sediments present a potential risk to seagrass functioning.     

The effects of a spillage of diesel fuel on a rocky shore in the sub-Antarctic region (Macquarie Island)

On 3 December 1987, the supply ship Nella Dan ran aground at Macquarie Island (54°29′S, 158°58′E) releasing about 270 000 I of oil, mostly light marine diesel, into the sea. At the time of the incident, many marine invertebrates were washed up dead along 2 km of shoreline. Twelve months later, the shore community was investigated using 1. algal and invertebrate populations of the littoral and sublittoral rocky shore, and 2. the invertebrate communities living in the holdfasts of the giant kelp Durvillaea antarctica, which were collected for later examination. Investigations were undertaken at both affected and control locations. Analyses of differences in community structure involved nested ANOVA and multidimensional scaling techniques. On the rocky substrate, the effect of the spill was restricted to some biota of the lower littoral and sublittoral zones—particularly echinoderms and the patellid limpet Nacella macquariensis. There were differences in cover for some algal species between locations. Within the kelp holdfasts, communities were dominated by peracarid crustaceans at control locations and by polychaetes (particularly the opportunistic groups—capitellids, cirratulids and spionids) at oil-affected locations.

The communities have recently been re-surveyed (in the summer of 1994–1995) to assist in the interpretation of the results and to gauge the extent of recovery of the affected biota.     

Imposex in Morula Granulata as Bioindicator of Tributyltin (TBT) Contamination in the Dampier Archipelago, Western Australia

This study was undertaken to examine the use of imposex in the tropical neogastropod whelk Morula granulata as bioindicator of tributyltin (TBT) contamination. Samples were collected from sites throughout the Dampier Archipelago; both impact sites close to intensive shipping activity and remote control sites. TBT in this area originates mainly from commercial vessels using the Port of Dampier that are >25 m in length and exempt from legislation controlling the use of TBT in antifouling paints. A field survey was undertaken in July 1997; 100 M. granulata were taken at each of 18 sites with varying vessel activity, and examined for imposex. The level of TBT contamination in the Archipelago was low; ranging from <0.3 to 25 ng Sn l⁻¹ in water,<0.3 to 80 ng Sn g−¹ in oyster tissue (Saccostrea cuccullata – prey of M. granulata), and <0.3 to 33 ng Sn g⁻¹ in whelks. Percentage of imposex in M. granulata ranged from 0% to 57%, and was correlated with distance to vessel activity (r=0.646, p<0.05). The only significant relationship of imposex with butyltin concentrations was with TBT in oysters (r=0.826, p<0.05), and dibutyltin (DBT) in whelks (r=0.783, p<0.05). It is postulated that this is because of interspecies variation in ability to metabolize TBT to DBT and MBT (monobutyltin), as well as induction of imposex in earlier life stages, so that imposex in adults could be a reflection of prior contamination. M. granulata is a suitable bioindicator of TBT contamination in the Dampier Archipelago, which could be used throughout the Indo-Pacific given its abundance in the region.     

Mid-latitude Southern Indian Ocean response to Northern Hemisphere Heinrich events

The lack of temporal resolution and accurate chronology of Southern Ocean marine cores has hampered comparison of glacial millennial-scale oscillations between the Southern Ocean, Antarctic ice and other records from both hemispheres. In this study, glacial climate variability is investigated over the last 50 ka using a multi-proxy approach. A precise chrono-stratigraphy was developed on the high-sedimentation rate core MD94-103 (Indian Southern Ocean, 45°35′S 86°31′E, 3560 m water depth) by geomagnetic synchronization between the later core and NAPIS75, and ¹⁴C dates. High-resolution time-series of δ¹⁸O in planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma, and sea surface temperatures (SSTs) estimated from the alkenone U^K′₃₇ index and foraminifera assemblages have been generated. Temporal evolution of the two temperature proxy records is notably different during the last glacial period. While foraminifera data indicate a consistent cooling towards the last glacial maximum, anomalous warm glacial alkenone temperatures suggest a strong advection of warm “detrital” alkenones by surface waters of the Agulhas current. Superimposed to this general trend, during Heinrich events, foraminiferal SSTs point to warmer surface waters, while concurrent alkenone SSTs exhibit apparent coolings probably caused by enhanced local alkenone production. By analogy to modern observations, possible influence of ENSO-like conditions on the subantarctic Southern Ocean SSTs is discussed.     

The distribution characteristics of bacterial beta-glucosidase activity in Taiwan strait

Twenty stations were established in the near-shore regions of South Fujian Shoal (116°10′–119°00^′ E, 21°20′–24°10′ N) on summer and winter cruises during the period from August 1997 and February to March 1998. The distribution pattern of marine bacterial β-glucosidase activity (β-GlcA) has been investigated by using fluorogenic model substrate (FMS) technique in order to have better understanding of the β-GlcA, as well as its relation to marine bacterial biomass, productivity and environmental factors in Taiwan strait. The results showed that: (1) In summer, the average of β-GlcA at the Southern stations of Taiwan strait was 1.94 nmol/1 h. While in winter, the average of β-GlcA at the Northern stations was 0.86 nmol/1 h and the range of variation (0.34–1.89 nmol/1 h) was much more narrow than that in summer (0.31–8.1 nmol/1 h). (2) According to the carbon conversion factor, the β-GlcA was 0.14 and 0.062 ugc/1 h in summer and winter respectively. These β-GlcA values were higher than the bacterial production of the two seasons respectively. (3) The β-GlcA gradually rises from offshore water to near-shore water. (4) The correlation between the β-GlcA and the bacterial secondary production was not so obvious. (5) The correlation between the section distributions, daily varying of the β-GlcA and the bacterial production was not obvious. (6) In the surface water, the distribution character of free-state β-GlcA from bacteria was equal to that of the total β-GlcA in the whole sea area.     

Seasonal studies on cadmium concentration and toxicity, oxygen consumption, digestive gland glycogen, and lipid in dog whelks collected at Aberystwyth, Wales

The concentration of cadmium in Nucella was investigated at different times of the year at one site. There was no significant difference in tissue cadmium at the times of the year investigated but tissue zinc was more variable. Accumulation and toxicity of cadmium were studied using 50–600 μg Cd l⁻¹ for 21 days. There was no significant seasonal difference in the rate of cadmium uptake but mortality appeared to vary with the season. Nucella did not survive 21 days at 600 μg Cd l⁻¹, but at lower concentrations 50–100 μg Cd l⁻¹ all survived.

Although there was significant drop in oxygen consumption between 7 and 14 days it did not appear to be related to the cadmium exposure. Glycogen levels on the other hand showed a decrease after cadmium treatment, while lipid values remained unchanged. Results are discussed in relation to energy source, season, and adaptation to the environment.     

A crystal-chemical basis for Pb retention and fission-track annealing systematics in U-bearing minerals, with implications for geochronology

This study develops an empirical crystal-chemical framework for systematizing the kinetics of Pb loss and fission-track annealing in U-bearing minerals. Ionic porosity, Z (the fraction of a mineral's unit-cell volume not occupied by ions) potentially accounts for kinetic behavior by monitoring mean metal-oxygen bond length/strength. Various tests of a general kinetics-porosity relationship are presented, based upon diverse mineral data including: (1) Pb diffusion parameters; (2) measured closure temperatures (T_C) for fission-track annealing and (3) retentivities of both Pb and fission tracks, from apparent-age data. Every kinetic parameter (including T_C and mineral age for both the U/Pb and fission-track systems) is inversely correlated with Z within the sub-assemblage: zircon (Z ≈ 29%), titanite ( 34%) and apatite ( 38%). Assuming a diffusional closure model, Pb isotopic transport phenomena are described by a T_C-Z scale “calibrated” with field-based T_C data for titanite (≥ 680 ± 20°C) and apatite ( 500°C). Extrapolation of this scale yields T_C estimates for the following minerals: staurolite (T_C ≥ 1060°C, Z ≈ 25%); garnet (≥ 1010°C, 26.5%); zircon (≥900°C); monazite, xenotime, and epidote (≥ 750°C, 32%); and Ca-clinopyroxene (≥ 670 ± 30°C, 34 ± 1%, depending on composition). These empirical results imply that a (U/)Pb/Pb date for staurolite or garnet records the time of mineral growth, not post-growth isotopic closure, as also concluded in recent field studies. Because Z systematizes fission-track annealing, this recrystallization process, like volume-diffusion, must also be rate-limited by the strength of chemical bonds. The extent to which other recrystallization processes are likewise rate-limited is important to U/Pb geochronology because they potentially compete with diffusion as mechanisms for Pb-isotopic resetting in nature.     

The Mid-Atlantic Ridge between 29°N and 31°30′N in the last 10 Ma

The segmentation of the Mid-Atlantic Ridge between 29°N and 31°30′ N during the last 10 Ma was studied. Within our survey area the spreading center is segmented at a scale of 25–100 km by non-transform discontinuities and by the 70 km offset Atlantis Transform. The morphology of the spreading center differs north and south of the Atlantis Transform. The spreading axis between 30°30′N and 31°30′N consists of enéchelon volcanic ridges, located within a rift valley with a regional trend of 040°. South of the transform, the spreading center is associated with a well-defined rift valley trending 015°. Magnetic anomalies and the bathymetric traces left by non-transform discontinuities on the flanks of the Mid-Atlantic Ridge provide a record of the evolution of this slow-spreading center over the last 10 Ma. Migration of non-transform offsets was predominantly to the south, except perhaps in the last 2 Ma. The discontinuity traces and the pattern of crustal thickness variations calculated from gravity data suggest that focused mantle upwelling has been maintained for at least 10 Ma south of 30°30′ N. In contrast, north of 30°30′N, the present segmentation configuration and the mantle upwelling centers inferred from gravity data appear to have been established more recently. The orientation of the bathymetric traces suggests that the migration of non-transform offsets is not controlled by the motion of the ridge axis with respect to the mantle. The evolution of the spreading center and the pattern of segmentation is influenced by relative plate motion changes, and by local processes, perhaps related to the amount of melt delivered to spreading segments. Relative plate motion changes over the last 10 Ma in our survey area have included a decrease in spreading rate from 32 mm a⁻¹ to 24 mm a⁻¹, as well as a clockwise change in spreading direction of 13° between anomalies 5 and 4, followed by a counterclockwise change of 4° between anomaly 4 and the present. Interpretation of magnetic anomalies indicates that there are significant variations in spreading asymmetry and rate within and between segments for a given anomaly time. These differences, as well as variations in crustal thickness inferred from gravity data on the flanks of spreading segments, indicate that magmatic and tectonic activity are, in general, not coordinated between adjacent spreading segments.