Magnetite rods in plagioclase as the primary carrier of stable NRM in ocean floor gabbros

The ubiquitous occurrence of magnetite rods in plagioclase may account for the stable, intense magnetization of ocean floor gabbros. These single-domain grains are formed by the subsolidus exsolution of Fe²⁺ and Fe³⁺ incorporated into the plagioclase at higher temperatures. We expect that this process will be particularly important in the upper portions of the oceanic gabbro unit because of the relative paucity of femic minerals at these levels. The geometry of accretion prohibits a constructive contribution to marine magnetic anomalies by the lower olivine-rich gabbros. As a result of the presence of these rods, ocean floor gabbros may constitute an important source for marine magnetic anomalies.     

The effects of a photosynthesis inhibitor atrazine,on salt marsh edaphic algae,in culture,microecosystems, and in the field

A 10^?5 M (2.2 ppm) concentration of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) reduced the rate of photosynthesis, chlorophyll content, and cell numbers in unialgal cultures of Nitzschia sigma Grun. and Thalassiosira fluviatilis Hustedt isolated from a salt marsh habitat. Results with lower atrazine concentrations indicated an ability to maintain chlorophyll production and cell division with reduced photosynthesis. The effects of a 10^?5 M concentration of atrazine in unialgal cultures were also evident in microecosystems and in the field at the same concentration, although atrazine effects were less severe in the field than in microecosystems or cultures. Cell number and productivity of the diatoms from microecosystems not treated with atrazine agreed well with field data and previously published data. Diatom species diversity was not affected by 10^?5 M atrazine in microecosystems or in the field but the number of Cymatosira belgica Grun. was increased. Diatom assemblages in atrazine-treated vs non-treated microecosystems were very similar (SIMI>0.838). Results were less conclusive in the field but the trend was toward a lower level of similarity. Based on the least effect level of atrazine to diatoms, the maximum safe level for atrazine in the salt marsh is 10 ppb.     

Deep-sea spherules from Pacific clay: mass distribution and influx rate

From 411 kg of Pacific clay, 22 mg of stony spherules and 50 mg of iron spherules larger than 150 μm were concentrated. The extraterrestrial origin of these particles was evaluated with the aid of both optical and electron microscopy as well as atomic absorption elemental analysis. The integral number (N) of stony particles from this sediment in the mass (M) range 20–300 μg is given by N( >M(g)) = 5.13 × 10^?6 × M^?1.65. The world-wide influx rate of stony particles in the mass range which survive atmospheric heating and ocean sediment storage is calculated to be 90 tons/yr. The relative contributions of ablation debris vs fused interplanetary dust to the influx of stony spherules is discussed, but as yet the question remains unanswered.     

Effects of shock pressure on 40Ar -39Ar radiometric age determinations

The relation of shock to the drop in the ⁴⁰¹Ar/³⁹¹Ar ratio seen at high release temperatures in some neutron-irradiated lunar samples is investigated through measurements of the ⁴⁰¹Ar/³⁹¹Ar ratio in gas samples released by stepwise heating of rock samples previously subjected to shock, either in the laboratory or in nature.Explosives were used to shock solid pieces and powder of a basalt from a diabase dike in Liberia to calculated pressures of 65, 150 and 270 kbar. These, an unshocked sample of the powder, two naturally shocked samples from the Brent impact crater in Canada, one unshocked sample from near the crater, and appropriate monitors were irradiated. Ar from stepwise heating was analyzed.The unshocked basalt shows a good ⁴⁰¹Ar/³⁹¹Ar plateau at age 198 ± 9 m.y. in agreement with a previous result of 186 ± 2 m.y. The shocked samples contain varying amounts of implanted atmospheric Ar, the isotopes of which have experienced mass fractionation. This effect is small enough in four samples so that the linearity of their graphs of ³⁹¹Ar/⁴⁰Ar vs ³⁶Ar/⁴⁰Ar is evidence of a plateau. The ages of these samples are then 201 ± 10, 205 ± 11, 205 ± 12 and 201 ± 9 m.y. It appears that the shock has had little effect on the ⁴⁰Ar-³⁹Ar age spectrum, although the release patterns of the ³⁹¹Ar are shifted downward by the order of 200°C. Shock implantation of Ar was at lower shock pressure, in the presence of less Ar, and into a less porous material than previously demonstrated.The Brent Crater samples do not all show good plateaus, but do indicate an age of ~420 m.y. for the crater event and 795 ± 24 m.y. for the rock formation, in agreement with previous results.None of the ⁴⁰¹Ar/³⁹¹Ar profiles shows a drop at high temperature, but a possible role of shock implantation of Ar is indicated in the production of this effect. Further experiments are suggested.     

A paleomagnetic record from Pyramid Lake,Nevada, and its implications for proposed geomagnetic excursions

A well-dated, 3.3-m section of deep-lake clays from pluvial Lake Lahontan, now exposed along the shore of Pyramid Lake, Nevada, has provided a paleomagnetic record of secular variation during the time interval 25,000–36,000 years B.P. The measured ranges of inclination and declination are 40° and 75°, respectively, and represent values which are comparable to observed secular variation at northern mid-latitudes. These results and those from a previously reported study from Clear Lake, California, together provide strong evidence for the conclusion that northern California and western Nevada were not affected by any geomagnetic excursion during the time interval 21,000–36,000 years B.P. Such a conclusion puts severe constraints on the nature and timing of the proposed Mono Lake, Laschamp and Lake Mungo geomagnetic excursions.