Minimum Off Research Articles

The marine record of deglaciation from the continental margin off Nova Scotia

Continental margin sediments off Nova Scotia accumulate at high rates (up to 360 cm kyr−1) and contain a history of millennial‐scale environmental changes which are dominated by the proximity of the Laurentide ice sheet during the latest Quaternary. Using stable isotope ratios of oxygen, accelerator mass spectrometer radiocarbon dating, micropaleontology, and sedimentology, we document these changes in six piston cores ranging in water depth from ∼450 to ∼4300 m. We find that maximum δ18O in N. pachyderma occurred about 15 ka and preceded the maximum abundance of this species in these cores by ∼1000 years. Between 13 and 14 ka we find a second peak in abundance of N. pachyderma, minimum δ18O, and two pulses of ice rafting. The sediment lithology supports terrestrial studies which indicate that there was a general withdrawal of ice beyond the upper Paleozoic and Mesozoic red beds by 14 ka in southeastern Canada, so the ice rafting events between 13 and 14 ka probably reflect ice stream activity in the St. Lawrence valley. The Younger Dry as event is recognized as a peak in abundance of N. pachyderma and ice rafting (dated as ∼11.3 ka), but meltwater discharge to the Gulf of St. Lawrence was either too small or occurred over too long a time to leave a distinct δ18O minimum off Nova Scotia. At 7.1 ka, in the middle of Holocene warming, we find a third peak in abundance of N. pachyderma and another δ18O minimum but no ice rafting. We interpret these data as evidence of a late‐occurring meltwater event which, if correct, could have originated in the Great Lakes, in the Labrador‐Ungava region, or in both. The final millennial‐scale phenomenon off Nova Scotia is the onset of “Neoglaciation,” marked by increased ice rafting and increased % N. pachyderma beginning about 5 kyr ago.

A chlorofluorocarbon section in the eastern North Atlantic

We present the distributions of two chlorofluorocarbons (CFCs) CFC-11 and CFC-12, measured as part of a hydrographic section between Iceland and the equator during July and August 1988. CFC-tagged water has filled the entire subpolar water column and subtropical thermocline in the eastern North Atlantic. Measurable CFC concentrations are observed at the ocean bottom as far south as 35°N, and the CFC penetration depth shoals to ≈750 m in the tropics. Specific features in the CFC distributions include a clear signal of Labrador Sea mid-depth ventilation, a CFC-enriched overflow water boundary current along the Iceland slope, and a mid-depth, equatorial plume of upper North Atlantic Water. The CFC data are used, in conjuction with the hydrographic data from the cruise, to illustrate the ventilation time-scales and pathways for the water masses in the eastern basin. A subsurface CFC maximum at about 100–200 m depth in the subtropics is shown to be a by-product of the heating and degassing of the seasonal thermocline and of the temperature sensitivity of CFC solubility. The CFC concentrations in the subpolar mode water are undersaturated by 15–18% relative to the atmosphere, reflecting the age of the mode waters and the very deep winter mixed layers in the eastern subpolar gyre. The CFC concentrations in the oxygen minimum off tropical Africa are much lower than the concentrations in the subtropical gyre, supporting previous work that suggests that isolation and enhanced productivity both contribute to the formation of the tropical oxygen minimum. In addition, the CFC inventories at the tropical stations have increased between 1982 and 1983 (TTO/TAS) and the summer of 1988 at a slower rate relative to the subtropical inventories over the same period. Thermocline oxygen utilization rates calculated from the CFC concentration data range from 5 to 10 μmol kg −1y −1 and are in line with previous estimates for the eastern subtropical thermocline. The low CFC concentrations in Mediterranean Water, about one-quarter those in the Labrador Sea Water, are shown to result from entrainment near the Straits of Gibraltar of a large component of low CFC, lower Atlantic thermocline water. Based on the CFC and other transient tracer distributions, the deep eastern basin can be divided into two regions: the Iceland Basin and surrounding area influenced by Iceland-Scotland Overflow Water that is ventilated on a decadal time-scale, and the area south of ≈50°N that has little or no CFC and is ventilated from a southern source on a much longer time-scale. A northward flowing boundary current of low CFC, modified Eastern Basin Bottom Water is also found along the Rockall Plateau and in the Iceland Basin.

Salinity variability in shelf waters near Otago Peninsula, New Zealand, on a time scale of hours

Abstract A transect crossing the continental shelf adjacent to Otago Peninsula was occupied on six occasions each for c. 8 h. Substantial variability in both high‐salinity and low‐salinity waters was found. On five occasions, a clearly identifiable low‐salinity pulse, which took less than a tidal cycle to pass the transect, was observed. No correlation with any part of the tidal cycle was identified. These low‐salinity events are probably generated by ebb tide release of water from the Clutha River. The position and form of the surface salinity minimum across the continental shelf was highly variable on occasion. Movement up to 3 km cross‐shelf, splitting into two minima, and the combining of two minima were events observed within the timescale of the observations. The results allow prediction of salinity minimum off Otago Peninsula over a Clutha River flow range of 500–1000 m3 s−1 : S = ‐0.00201F + 35.53, where S = salinity minimum (psu) and F = mean daily Clutha River flow 4–12 days earlier (m3 s−1 ).

Molecular recognition of amiloride analogs: a molecular electrostatic potential analysis. 1. Pyrazine ring modifications.

Ab initio molecular electrostatic potential (MEP) patterns are used to determine the electrostatic requirements for the formation of a stable blocking complex between amiloride analogs and the epithelial sodium channel of Rana ridibunda. MEP maps calculated in the 3-21G(*) and STO-3G basis sets for amiloride and analogs with pyrazine ring modifications are used to interpret differences in the microscopic rate constants for analog-channel binding determined by Li et al. MEP maps of the protonated analogs are correlated to differences in the value of kon, the microscopic association constant. Those analogs with kon values similar to amiloride are found to have a MEP maximum that is localized over the side chain, as well as strong, distinguishing minima in the MEP pattern off the carbonyl oxygen and positions 3, 4, and 5 of the pyrazine ring. MEP maps of a model-encounter complex (protonated analog and formic acid anion) are correlated to differences in koff, the microscopic dissociation constant. The major conclusions of this work are that (1) a stable blocking complex is formed with analogs which have a deep, localized minimum off the 6 position of the pyrazine ring, (2) the stability of the blocking complex is directly related to the depth of that minimum, (3) substitution at position 5 affects not only the depth but also the location and size of the minimum off position 6, and (4) steric factors may influence the optimal binding of the 6-position ligand to the ion channel. The MEP analysis also suggests that the distance between the proton donors of the chelating guanidinium moiety and the deep, localized minimum off position 6 of the pyrazine ring may define an important spatial requirement for all those analogs which form a stable blocking complex with the channel.

Time-domain asymptotics and the method of stationary phase

A technique is described for obtaining the asymptotic behaviour at largeλof integrals having the formI(λ) = ∫g(x) p{λf(x)} dx, wherepis an arbitrary periodic function with mean zero. It is based on the fact that the method of stationary phase may be applied directly top{λf(x)}, without decomposition ofpinto Fourier components: thus a simple minimum offatx = x0gives a term containing a fractional integral of order one-half, proportional top̂+(y) = {∞y{p(x)/ (x — y)1/2} dxevaluated aty=λf(x0); a simple maximum gives a similar term. In many physical problem,fdepends linearly on a parameter, sayt, in such a way thatIis periodic intand the quantity of interest isq(t)= dl/dt. The theory of how the shape ofqis determined by that ofpwhenλis large but fixed is here called waveform asymptotics and its main features investigated using ‘barber’s pole’ integrals. For example, the singularity inqproduced by a discontinuity inpis found explicitly as an inverse square-root multiplied by a coefficient, so need not be inferred from the tail of a Fourier series. More generally, the effect onqof any rapid change inpmay be obtained by the present method of stationary phase in the time domain, without resolution into components; since Gibbs’ phenomenon is thereby avoided the method is suited to highly non-sinusoidal wave problems. An asymptotic representation ofqby zeta functions is possible. Four extensions of the basic theory are analysed in detail: coalescence of a maximum and minimum off; contributions from the end-points of the range of integration; collision of a maximum or minimum with an end-point ; and the behaviour of integrals with no stationary points or end-points. The first and third of these lead to time-domain Airy functions and Fresnel integrals, respectively, with singularity structures dual to the smooth patterns found in diffraction catastrophes; the second recovers the original waveform; and the fourth gives exponential asymptotics. The theory is illustrated throughout by analysis and computation for functionspdescribing a square wave and an intermittentN-wave, and by diagrams of the resulting waveforms.

Adaptations to the Deep-Sea Oxygen Minimum Layer: Oxygen Binding by the Hemocyanin of the Bathypelagic Mysid, Gnathophausia ingens Dohrn.

The bathypelagic mysid, Gnathophausia ingens Dohrn, lives aerobically at oxygen partial pressures as low as 6 torr in the oxygen minimum layer off southern California. This study is concerned with the O2 binding properties of this mysid's hemocyanin and the function of the pigment in O2 uptake at low Po2. The effect of temperature on in vivo hemolymph pH (ΔpH/ΔT = -0.018) was measured from 2.5 to 12.5°C. Hemocyanin concentration was estimated to be 24 mg/1, corresponding to an O2 binding capacity of about 0.3 mmol O2/l. Freezing of hemolymph samples significantly decreased the affinity and cooperativity of HcO2 binding, necessitating the use of fresh hemolymph. The HcO2 affinity was high (P50 of 1.4 torr at 5.5°C, pH 7.87), allowing the loading of O2 even at 6 torr. The cooperativity of HcO2 binding was also high (n50 = 3.5 at 5.5°C, pH 7.87); presumably allowing the pigment to function effectively as an O2 transporter within the small Po2 difference between the environment and the tissues. Temperature differences within the environmental range (2-10°C) had no significant effect on the oxygen affinity (ΔH = -6.7 kJ/mol, pH 7.7) or on the cooperativity of O2 binding. A large Bohr shift (Δ log P50/ΔpH = -0.80 to -0.81) was present at all temperatures. L-lactate produced moderate increases in HcO2 affinity (Δ log P50/Δ log [lactate] = -0.13 at pH 7.9) and in cooperativity. Regional and ontogenetic comparisons suggest that regional and ontogenetic differences in HcO2 affinity occur in this species. This mysid has a hemocyanin of unusually high O2 affinity and cooperativity of O2 binding for a crustacean living at low temperatures, and this appears to be an adaptation for oxygen loading and transport at the cold, low oxygen conditions in deep-sea oxygen minimum layers. The reduced temperature sensitivity of HcO2 affinity may also be an adaptation to low oxygen.

Line search by curve fitting in minimization algorithms

Consider an unconstrained minimization of an uniformly convex functionf(z). Let Ω be an algorithm that, for solving it constructs a sequence {zi} withzi+1 =z i + λ(i)h i ,h i ∈R n , λ(i) ∈R = and −h i T ∇f(z i ) > 0. For any algorithm Ω that converges linearly and that uses parabolic or cubic interpolations for the line search, upper bounds on the number of function evaluations needed to approximate the minimum off(z) within a given accuracy, are calculated. The obtained results allow to compare the line search procedure under investigation.

Internally insulated pipes for district-cooling systems

A method is presented whereby, for any set of prescribed conditions, an optimal thickness of a thermally insulating internal lining may be determined for pipes to be used for the conveyance of chilled water. The optimum investigated in this paper corresponds with the minimum total rate off energy dissipation costs as a result of refrigeration and pumping. The overall optimal design cannot be decided at this juncture without a knowledge of such unknowns as the capital cost of internally lining the pipe.

Vertical distribution of zooplankton in relation to the oxygen minimum off Peru

In April 1977, discrete depth zooplankton samples were collected off central Peru with a high-yield pump at shelfbreak, slope, and offshore stations. Upwelling brought nearly anoxic (⪕0.1 ml O 2 1 −1) water from the eastern Pacific oxygen minimum to within 50 to 65 m of the surface at the slope station and to within 20 to 35 m at the shelfbreak station. Offshore oxygen concentrations exceeded 1.0 ml 1 −1 throughout the upper 85 m. Most zooplankton taxa were limited in vertical distribution by the 0.1-ml O 2 1 −1 isopleth, and onshore shoaling of the oxygen minimum resulted in unusually high near-surface zooplankton concentrations relative to offshore waters. Eucalanus inermis was exceptional in concentrating at times within the oxygen minimum. A temporary deepening of the minimum at shelfbreak was accompanied by a general downward dispersal of zooplankton.

A probabilistic algorithm for global optimization

In this paper the authors validate the Wiener process as a suitable stochastic model of the global behaviour of a functionf (x), x∈XCR 1. An algorithm is given, which exploits the properties of the model, for an effective approximation of the global minimum off (x).

Respiration and the effects of pressure on the mesopelagic vertically migrating squid Histioteuthis heteropsis1

The effects of hydrostatic pressure and dissolved oxygen concentration on oxygen consumption rates were examined in the mesopelagic, vertically migrating squid Histioteuthis heteropsis from southern California offshore waters. Elevated hydrostatic pressures (to 136 atm) had no significant effects on oxygen consumption rates at 5°C. Low oxygen concentrations (<0.5 ml O2 · liter−1) depressed rates of oxygen consumption. These findings suggest that metabolism is pressure insensitive during vertical migrations and that at least partial anaerobiosis may be necessary to supply the total energy needs of this species at daytime depths in the oxygen minimum layer off southern California.

OXYGEN UPTAKE AND RESPIRATORY QUOTIENT OF FIELD SOIL CORES IN RELATION TO THEIR AIR‐FILLED PORE SPACE

SummaryThe oxygen uptake and respiratory quotient (RQ) of field cores from three soils of different texture classes were studied using electrolytic respirometers. Critical values of air‐filled pore space (f), above which RQ was always near unity, that is, corresponding to the transition from anaerobic to aerobic respiration, were defined for each soil. These critical values agreed with minimum values off required for plant growth as reported in the literature, suggesting that RQ can be used as an index of adequate soil aeration.Oxygen uptake by the three soils reached a maximum at f values slightly above the critical values, corresponding to the highest moisture content at which oxygen diffusion was non‐limiting.

The respiratory rates of midwater crustaceans as a function of depth of occurrence and relation to the oxygen minimum layer off Southern California

1. 1. Oxygen consumption rate and critical oxygen partial pressure were measured in twenty-eight species of midwater crustaceans and two species of midwater fishes. 2. 2. The P c's were compared to the environmental oxygens at which these species are found. This comparison indicates that all species, except the two parasites and the one diurnal vertical migrator, are able to regulate their QO 2 to at least the lowest pO 2 they encounter in nature (as low as 4 mm Hg O 2 in some cases). The exceptions have considerable anaerobic abilities. 3. 3. The oxygen consumption rate of these species is shown to decline rapidly with increasing depth of occurrence and the factors involved are discussed.

On a problem of Bonar concerning Fatou points for annular functions

The purpose of this paper is to study the distribution of Fatou points of annular functions introduced by Bagemihl and Erdös [1]. Recall that a functionf(z), regular in the open unit diskD: |z| < 1, is referred to as an annular function if there exists a sequence {Jn} of closed Jordan curves, converging out to the unit circleC: |z| = 1, such that the minimum modulus off(z)onJnincreases to infinity. If theJncan be taken as circles concentric withC, f(z)will be called strongly annular.

Optimal Gate Closure Schedule for Hydroelectric Turbine System

A procedure is presented for the generation of the optimal gate closure schedule for the case of full load rejection on an isolated hydroelectric system. Constraints are placed on turbine overspeed and penstock overpressure. The optimization is carried out on the basis of minimum integral square off speed and implemented digitally by the method of steepest descent. The computation indicates that a substantial performance improvement is achieved with optimal control as opposed to the conventional.

A push-to-talk rating system

This rating system (one minute maximum ON and four minutes minimum OFF) recognizes the trend toward shorter transmission time and higher power without impairing the useful service life of the tube. Continuous operation under these conditions is not to result in catastrophic failure. The mechanisms of failure are discussed. Existing types can be re-rated wlth an approximate increase in power output of 30%; the actual value varies greatly according to the conservatism of the basic design of each tube. New tubes designed especially for Push-to-Talk Service with such features as high thermal inertia anodes have shown that as much as an 80% increase in power output can be obtained with respect to the prototype.

Shorewards Upslope of the Layer of Minimum Oxygen Off Bombay: Its Influence on Marine Biology, Especially Fisheries

Shorewards Upslope of the Layer of Minimum Oxygen Off Bombay: Its Influence on Marine Biology, Especially Fisheries