Senescence- and death-related alteration of chlorophylls and carotenoids in marine phytoplankton

This report extends previous work ([Louda et al., 1998a] and [Louda et al., 1998b]. Chlorophyll degradation during senescence and death. Organic Geochemistry 29, 1233–1251.) in which we detailed type-I (alteration) and -II (destruction) degradation of chlorophyll with representative fresh water phytoplankton. The present study covers similar experiments with marine phytoplankton, namely, a cyanobacterium (“ANA” Anacystis sp), a coccolithophore (“COC” Coccolithophora sp.), a dinoflagellate (“GYM” Gymnodinium sp.) and two diatoms (“CYC” Cyclotella meneghiniana and “THAL” Thalassiosira sp.). Mg loss (‘pheophytinization') was rapid and continuous in all species under room-oxic conditions and slow or sporadic under anoxic conditions. The proportion of dephytylated pigments (pheophorbides-a, chlorophyllides-a), relative to the phytylated forms (chlorophyll-a, pheophytins-a), increased over the first year under room-oxic conditions and in room-anoxic conditions only in “CYC”. Pheophorbide-a was converted to pyropheophorbide-a within 15 months only in “THAL” and “ANA”, and slightly in “COC”. After 9–15 months of oxic incubation, “COC” was found to contain traces of purpurin-18 phytyl ester. Consideration of carotenoid pigments is also included herein. All fucoxanthin containing species, except “THAL”, exhibited conversion of fucoxanthin to fucoxanthinol in room-oxic conditions. Diadinoxanthin was rapidly de-epoxidized to give diatoxanthin within the first 2–4 weeks. Diatoxanthin then disappeared from all species by 15 months with a concurrent increase in a pigment which we tentatively identify as a cis-zeaxanthin. Incubations of pure cultures are found to be an effective way by which to model the early type-I reactions for both chlorophylls and carotenoids. The influence of oxygen during senescence-death and the onset of early diagenesis is of paramount importance. The absence of oxygen and, by inference, aerobic microbiota, retards the breakdown of these pigments dramatically.     

Sea-level pressure variability around Antarctica since A.D. 1750 inferred from subantarctic tree-ring records

 A tree-ring chronology network recently developed from the subantarctic forests provides an opportunity to study long-term climatic variability at higher latitudes in the Southern Hemisphere. Fifty long (1911–1985), homogeneous records of monthly mean sea-level pressure (MSLP) from the southern latitudes (15–65^°S) were intercorrelated on a seasonal basis to establish the most consistent, long-term Trans-Polar teleconnections during this century. Variations in summer MSLP between the South America-Antarctic Peninsula and the New Zealand sectors of the Southern Ocean are significantly correlated in a negative sense (r=−0.53, P<0.001). Climatically sensitive chronologies from Tierra del Fuego (54–55^°) and New Zealand (39–47^°) were used to develop verifiable reconstructions of summer (November to February) MSLP for both sectors of the Southern Ocean. These reconstructions, which explain between 37 and 43% of the instrumentally recorded pressure variance, indicate that inverse trends in MSLP from diametrically opposite sides of Antarctica have prevailed during the past two centuries. However, the strength of this relationship varies over time. Differences in normalized MSLP between the New Zealand and the South America-Antarctic Peninsula sectors were used to develop a Summer Trans-Polar Index (STPI), which represents an index of sea-level pressure wavenumber one in the Southern Hemisphere higher latitudes. Tree-ring based reconstructions of STPI show significant differences in large-scale atmospheric circulation between the nineteenth and the twentieth centuries. Predominantly-negative STPI values during the nineteenth century are consistent with more cyclonic activity and lower summer temperatures in the New Zealand sector during the 1800s. In contrast, cyclonic activity appears to have been stronger in the mid-twentieth than previously for the South American sector of the Southern Ocean. Recent variations in MSLP in both regions are seen as part of the long-term dynamics of the atmosphere connecting opposite sides of Antarctica. A detailed analysis of the MSLP and STPI reconstructions in the time and frequency domains indicates that much of the interannual variability is principally confined to frequency bands with a period around 3.3–3.6 y. Cross spectral analysis between the STPI reconstruction and the Southern Oscillation Index suggests that teleconnections between the tropical ocean and extra-tropical MSLP variations may be influencing climate fluctuations at southern latitudes. Received: 18 December 1996/Accepted: 10 January 1997     

Interference effect of two closely-spaced shallow strip foundations on geogrid-reinforced sand

Summary Geotextiles and geogrids are now being used extensively in many civil engineering construction works. This study presents some laboratory model test results for the ultimate bearing capacity of an isolated, and two closely-spaced, strip foundations resting on unreinforced sand, and sand reinforced with layers of geogrid. Based on the model test results, the variation of the group efficiency with the centre-to-centre spacing of the foundation has been determined.     

The application of electromagnetic techniques to groundwater recharge investigations

The use of electrical methods for estimating spatial patterns of groundwater recharge was evaluated at a field site in southeastern Australia. Here, recharge increased from less than 0.2 mm year⁻¹ under native Eucalyptus vegetation, to between 1 and 14 mm year⁻¹ under dryland agriculture. This increase in recharge results in progressive leaching of salts in the soil profile. Differences in recharge can be estimated from differences in depth of leaching. The estimated recharge rates are correlated with soil texture, with higher recharge rates generally occurring through sandier soils. The relationships of recharge to salt content and soil texture both contribute to lower apparent electrical conductivities for higher recharge rates.

The effect of recharge rate on measured apparent electrical conductivities was modelled for various geophysical devices (including frequency-domain (FEM) and time-domain (TEM) electromagnetic instruments and direct current resistivity). The soil-texture effect was shown to have a greater effect than the solute leaching effect in determining the correlation between recharge and apparent electrical conductivity. Analysis of sensitivity to geological noise showed that variations in soil type below 2 m could disguise any correlation.

Correlations between recharge rate, measured at core sites from chloride tracer techniques, and apparent electrical conductivity, measured with FEM electromagnetic devices, supported the conclusions of the model. For DC resistivity and TEM methods, correlations between recharge and apparent electrical conductivity were not significant, although for resistivity this may be due partly to the small number of measurements made. The FEM device most sensitive to variations in recharge had an operating frequency of 9.8 kHz. At lower frequencies the sensitivity is reduced, as the instruments are sensing too deeply. The poor correlations for TEM, as compared with FEM, are due probably to the relatively deeper penetration of the TEM instrument used in the study, rather than any inherent differences between the techniques.

Because the major reason for the correlation between recharge and apparent electrical conductivity is soil texture, in this area the geophysical devices are mostly mapping soil type.     

Diorites and associated rocks in the Anglem Complex at The Neck, northeastern Stewart Island, New Zealand: an example of magma mingling

Exposures of the Anglem Complex (Cretaceous) at The Neck, northeastern Stewart Island consist dominantly of diorite, microdiorite, tonalite and granite. Dyke-rocks include microgranite, microdiorite, andesite and microgranodiorite.

A Diorite Suite composed of hornblende gabbro, diorite and quartz-bearing diorite forms a complex association of basic to intermediate lithologies with highly variable grainsize. The Heterogeneous Suite formed in part by mingling of diorite and tonalite intrudes the Diorite Suite. Enclaves within this suite show highly variable margins, from sharp sometimes crenulated and apparently chilled contacts, through to diffuse contacts. Prior to, and during intrusion of this heterogeneous magma, significant textural modifications of dioritic enclaves occurred, and some important controls on this modification process are discussed. Aspects considered include contrasts between the host melt and enclaves in temperature, viscosity, chemistry, volatile content, deformation and recrystallisation. A liquid immiscibility model is discounted in favour of the magma mingling model. Subsequent intrusion of granite was followed by a porphyritic microgranodiorite. Microdiorite and microgranite dykes intrude all of the above rocks with the exception of the microgranodiorite. Coexisting microdiorite, microgranite, and composite microdiorite-microgranite dykes in the one dyke swarm also provide compelling evidence for magma mingling.     

High temperature telescoped tungsten-antimony mineralization,Guatemala

In the course of a geologic mapping project in northwestern Guatemala an area of unusual stibnite and scheelite mineralization was encountered near Ixtahuacan. The ore is localized along two intersecting faults in the upper part of the Pennsylvanian-Permian Tactic shale and is apparently a telescoped xenothermal deposit. The source of the hydrothermal tungsten-antimony-bearing solutions is uncertain, but field evidence suggests that the fluids could have been associated with a volcanic center less than 4 km to the south. On the basis of the estimated temperature of deposition and experimentally-determined factors controlling scheelite and stibnite solubility, it is suggested that this deposit formed at 400 to 500°C by an extremely rapid reaction between an alkaline hydrothermal solution and Chochal limestone or calcareous layers in the upper Tactic shale. This result contrasts with those for similar deposits which contain minerals thought to form at lower temperatures by a rapid decrease in temperature. Strontium isotopic evidence on Guatemalan volcanic rocks related to those thought to have been the source of the Ixtahuacan metallization suggests that the metals had their source in the upper mantle or basaltic lower crust.

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Zusammenfassung Während einer geologischen Kartierung in Nordwestguatemala wurde ein Gebiet ungewöhnlicher Antimonit- und Scheelitvererzung vorgefunden. Das Erzvorkommen befindet sich an zwei sich kreuzenden Verwerfungen im oberen Teil der permokarbonen Tacticschiefer und stellt wahrscheinlich eine Teleskop-artige xenothermale Ablagerung dar. Der Ursprung der hydrothermalen Wolfram- und Antimon- führenden Lösungen steht nicht fest, aber Beobachtungen im Gelände deuten auf einen Zusammenhang mit einem vulkanischen Zentrum hin, das etwa 4 km südlicher liegt. Temperaturschätzungen der Ablagerung und experimentell festgestellte Faktoren, die die Löslichkeit von Stibnit und Scheelit beherrschen, deuten darauf hin, daß diese Lagerstätte bei 400–500°C durch schnelle chemische Reaktion einer hydrothermalen Alkalilösung mit den Chochalkalken oder kalkigen Schichten in den oberen Tacticschiefern entstand. Diese Resultate stehen im Gegensatz zu denjenigen von Lagerstätten mit Tieftemperaturmineralen, die sich unter schneller Abkühlung bildeten. Strontiumisotopmessungen weisen auf eine Herkunft der Lösungen aus dem oberen Erdmantel oder der unteren Basaltschale hin.