ALBERT

Description: The ventilatory system of cephalopods has evolved so that the animals minimise the energetic cost of either oxygen extraction or jet propulsion. Optimal design for jet propulsion requires a large stroke volume moving water through the system with minimal acceleration, so the oxygen-carrying capacity of the ventilatory stream is always greatly in excess of requirements. Oxygen uptake in a jet-propelled animal must be independent of the volume of the jet to avoid locking locomotion to oxygen uptake. Any such link is incompatible with the repayment of an oxygen debt after exercise, with added oxygen demand during digestion, and with regulation of uptake under hypoxic conditions. Cephalopods evolved for efficient jet propulsion must be able to alter the rate of oxygen extraction from the ventilatory stream. Squids and Nautilus do this; oxygen extraction is normally low, in the range 5–10% during jet-propelled cruising at subcritical speeds, but can rise to as much as 25% in acute hypoxia or at rest after exhausting exercise. Other cephalopods, such as Octopus, minimise the cost of oxygen extraction by propelling a minimal volume of water through the gills, extracting 35–50% (exceptionally 75%) of the available oxygen. This leaves little scope for a further increase in extraction, and any added demand is met by increasing the ventilation stroke volume. A consequence is that jet propulsion becomes extravagant; octopuses show much greater jet pressures than squid when they choose to swim. The two alternative specialisations are linked to life-style and cut across systematic boundaries.

Description: Cylindrical samples of sand consolidated with tetrahydrofuran hydrate were tested for their compressive strength and creep behavior under uniaxial compression. The samples were 15 cm in length and 7.5 cm in diameter and were tested at −10 °C. The results, when combined with our previous measurements on similar samples at −6 °C, show that the material becomes stronger by about 10% with decrease in temperature; otherwise, the slopes of the peak stress – strain rate curves are the same. These results are similar to those of sand consolidated with ice, except that in the latter case the increase in strength over the same temperature range is about 30%. Furthermore, the slope of the peak stress – strain rate curve for the hydrate-consolidated sand is almost zero, whereas for the ice-consolidated sand it is quite steep. Consequently, at strain rates below 10−5 s−1 the hydrate-consolidated sand is stronger, whereas at strain rates above 10−5 s−1 the ice-consolidated sand is the stronger material. Noticeable differences were also observed in the creep behavior of the hydrate- and ice-consolidated sands. At −10 °C, ice-consolidated sand failed in about 15 h under a stress of about 7 MPa, whereas hydrate-consolidated sand failed after 52.3 h under a stress of 12.2 MPa and some samples did not fail even after 540 h when subjected to a stress of 9.3 MPa.

Description: The distribution of gold has been examined in sub-seafloor stockworks from two sections of hydrothermally altered oceanic crust beneath the axial zones of fossil spreading centers. Deep Sea Drilling Project hole 504B (Leg 83) contains a narrow zone of stockwork-like sulfides in 6 Ma old basalt from the transition zone between sheeted dikes and overlying pillow lavas (910-928 m). Mineralization occurred as a result of local mixing between ascending hydrothermal fluids (350-degrees-C) and seawater that penetrated the top of the dike section. Previous studies indicate that a significant amount of gold was leached from the sheeted dikes during high-temperature greenschist-facies alteration, but mineralized wall rock in the transition zone is not substantially enriched in gold. Sulfide concentrates from the narrow stockwork average 26 ppb Au, and one sample of As-rich pyrite from a quartz-epidote breccia contains 100 ppb Au. Cyprus Custal Study Project hole CY-2a contains an equivalent section of altered Cretaceous pillow lavas from the Troodos Ophiolite but includes a near-surface stockwork from the ore zone of the Agrokipia B deposit. The combined effects of hydrothermal metasomatism and regional metamorphism are represented by zeolite facies mineralogy above the ore zone (0-154 m), intense silicification and argillic alteration within the stockwork (154-300 m), and propylitic alteration at depth (300-400 m). The sheeted dikes below 400 m are altered uniformly to greenschist-facies mineralogy. Extensive sulfide mineralization in the pillow lavas occurred within a few hundred metres of the seafloor in response to a steep thermal gradient caused by mixing of high-temperature fluids with cold seawater. Pyrite from the stockwork ore contains up to 480 ppb Au and averages 160 ppb Au. A narrow zone of quartz-sulfide veinlets also occurs at the pillow-dike transition. Two samples of As-rich pyrite (up to 0.75 wt.% As) from this zone contain 980 ppb Au, but gold contents in fracture-filling and disseminated pyrite throughout most of the transition zone and sheeted dikes are 〈 20 ppb Au. Despite local enrichment within specific sulfide phases, deep sub-seafloor mineralization does not appear to have been an important sink for gold in either CY-2a or 504B. At higher levels in the crust, as in Agrokipia B, the locus and extent of mixing may be important controls on the intensity of mineralization and the deposition of gold within near-surface stockworks. Without an effective means of interrupting the flow of high-temperature fluids to vents on the seafloor, gold may be carried through the upflow zone at the time when high-temperature stockworks are forming.