The Mid-atlantic Ridge (31°S–34°30′S): Temporal and spatial variations of accretionary processes

The ridge located between 31° S and 34°30′S is spreading at a rate of 35 mm yr⁻¹, a transitional velocity between the very slow (≤20 mm yr⁻¹) opening rates of the North Atlantic and Southwest Indian Oceans, and the intermediate rates (60 mm yr⁻¹) of the northern limb of the East Pacific Rise, and the Galapagos and Juan de Fuca Ridges. A synthesis of multi-narrow beam, magnetics and gravity data document that in this area the ridge represents a dynamically evolving system. Here the ridge is partitioned into an ensemble of six distinct segments of variable lengths (12 to 100 km) by two transform faults (first-order discontinuities) and three small offset (< 30 km) discontinuities (second-order discontinuities) that behave non-rigidly creating complex and heterogeneous morphotectonic patterns that are not parallel to flow lines. The offset magnitudes of both the first and second-order discontinuities change in response to differential asymmetric spreading. In addition, along the fossil trace of second-order discontinuities, the lengths of abyssal hills located to either side of a discordant zone are observed to lengthen and shorten creating a saw-toothed pattern. Although the spreading rate remains the same along the length of the ridge studied, the morphology of the spreading segments varies from a deep median valley with characteristics analogous to the rift segments of the North Atlantic to a gently rifted axial bulge that is indistinguishable from the shape and relief of the intermediate rate spreading centers of the East Pacific Rise (i.e., 21°N). Like other carefully surveyed ridge segments at slow and fast rates of accretion, the along-axis profiles of each ridge segment are distinctly convex upwards, and exhibit along-strike changes in relief of 500m to 1500 between the shallowest portion of the segment (approximate center) and the segment ends. Such spatial variations create marked along-axis changes in the morphology and relief of each segment. A relatively low mantle Bouguer anomaly is known to be associated with the ridge segment characterized by a gently rifted axial bulge and is interpreted to indicate the presence of focused mantle upwelling (Kuo and Forsyth, 1988). Moreover, the terrain at the ends of each segment are known to be highly magnetized compared to the centers of each segment (Carbotte et al, 1990). Taken together, these data clearly establish that these profound spatial variations in ridge segment properties between adjoining segments, and along and across each segment, indicate that the upper mantle processes responsible for the formation of this contrasting architecture are not solely related to passive upwelling of the asthenosphere beneath the ridge axis. Rather, there must be differences in the thermal and mechanical structure of the crust and upper mantle between and along the ridge segments to explain these spatial variations in axial topography, crustal structure and magnetization. These results are consistent with the results of investigations from other parts of the ridge and suggest that the emplacement of magma is highly focused along segments and positioned beneath the depth minimum of a given segment. The profound differences between segments indicate that the processes governing the behavior of upwelling mantle are decoupled and the variations in the patterns of axis flanking morphology and rate of accretion indicate that processes controlling upwelling and melt production vary markedly in time as well. At this spreading rate and in this area, the accretionary processes are clearly three-dimensional. In addition, the morphology of a ridge segment is not governed so much by opening rate as by the thermal structure of the mantle which underlies the segment.     

黄海西部底栖海藻记录的评论

黄海西部是指北起鸭绿江口,南至长江口(包括渤海)的中国沿岸。这一带的海藻研究,迄今将近百年,目前已掌握的海藻分类研究文献共有45篇。我们在研究和分析这些文献后,把这些海藻记录归的为3类。(1)可以接受的记录：根据我们十多年来在黄海沿岸的调查采集经验证实所记录的产生于黄海西部的海藻,或虽尚未采到,但就目前的资料分析,估计该项记录可能是正确的。(2)待证实的记录：有些海藻记录无法肯定其鉴定是否正确,需待重新检查其原标本后始能确定的。(3)不能接受的记录：这类记录中,有些是已被藻类学家所否定,有些经检查原定名标本后证实是错误的,有些则根据地理分布观点,被认为是不可能生长在这个海区的种类。 根据黄海西部的海藻研究的发展情况,我们把过去的工作划分为两个历史阶段。     

Northward transport of pacific summer water along the Northwind Ridge in the Western Arctic Ocean

The recent sea-ice reduction in the Arctic Ocean is not spatially uniform, but is disproportionally large around the Northwind Ridge and Chukchi Plateau compared to elsewhere in the Canada Basin. In the Northwind Ridge region, Pacific Summer Water (PSW) delivered from the Bering Sea occupies the subsurface layer. The spatial distribution of warm PSW shows a quite similar pattern to the recent ice retreat, suggesting the influence of PSW on the sea-ice reduction. To understand the regionality of the recent ice retreat, we examine the dynamics and timing of the delivery of the PSW into this region. Here, we adopt a two-layer linearized potential vorticity equation to investigate the behavior of Rossby waves in the presence of a topographic discontinuity in the high latitude ocean. The analytical results show a quite different structure from those of mid-latitude basins due to the small value of β. Incident barotropic waves excited by the sea-ice motion with large annual variation can be scattered into both barotropic and baroclinic modes at the discontinuity. Since the scattered baroclinic Rossby wave with annual frequency cannot propagate freely, a strong baroclinic current near the topographic discontinuity is established. The seasonal variation of current near the topographic discontinuity would cause a kind of selective switching system for shelf water transport into the basin. In our simple analytical model, the enhanced northward transport of summer water and reduced northward transport of winter water are well demonstrated. The present study indicates that these basic dynamics imply that a strengthening of the surface forcing during winter in the Canada Basin could cause sea-ice reduction in the Western Arctic through the changes of underlying Pacific Summer Water.     

Paleomonsoon precipitation deduced from a sediment core from the equatorial Indian Ocean

Rapid shifts in past climate recorded in polar ice sheets have elicited various explanations relating to either thermohaline circulation changes by ice-rafting or natural greenhouse gas concentrations modulated by climatic conditions in the tropics. To compare the tropical paleoclimate record with the polar record, one must choose sediment cores from highly productive ocean regions. Necessarily, such regions reflect the wind records in the tropics, because high productivity is associated with upwelling driven by winds. Comparing tropical precipitation records with high-latitude records is, however, a more difficult task because sediments recording paleoprecipitation usually have low sedimentation rates, and offer coarser resolution relative to polar ice cores. Here, we present δ ¹⁸O data of three planktonic species of Foraminifera (a proxy for precipitation) from such a sediment core, spanning the past 35 ka for the equatorial Indian Ocean, which falls under the southwest monsoon (SWM) realm. Results show that minimum SWM precipitation occurred at the Last Glacial Maximum, with a subsequent increase at Termination IA. During the Holocene, SWM precipitation intensified uniformly up to the core top (∼2.2 ka b.p.), as revealed by generally decreasing δ ¹⁸O values. Variations in precipitation are consistent with climate changes recorded in polar ice sheets. Although the different resolutions of the two records preclude a rigorous comparison, abrupt cooling/warming events appear to be accompanied by sudden reduction/enhancement in (SWM) rainfall. Thus, mechanisms with time scales much shorter than a millennium, such as natural greenhouse warming (e.g., CH₄ concentration), controlled by emissions from the tropics, could have played a major role in high-latitude climate change.     

The Potential for Environmentally Sound Development of Marine Deposits of Potassic and Phosphatic Minerals Offshore, Southern Africa

Extensive potassium and phosphorous-rich mineralization occurs on the outer continental shelf of the southern and west coasts of South Africa and Nambia. This article reviews the potential of exploiting these deposits in an environmentally sound manner for the manufacture of fertilizer. At present, reserves of potash and phosphate fertilizer are exploited from terresterial deposits, the majority being surface mined. The use of fertilizer in South Africa has shown no growth in the past 13 years, and, in some years, usage has even declined. On average, over the last decade, South Africa has consumed 2 million Mt of fertilizer (including nitrogen) per annum, the vast majority of phosphate fertilizer being produced by FOSKOR from the Phalaborwa Igneous Complex. Potash fertilizer is imported into South Africa. Although fertilizer consumption is expected to decrease in the short-term, there are good future prospects for the domestic and international fertilizer market. Considerable research into both glauconitic (containing K 2 O) and phosphatic deposits along the southern African continental shelf indicate that these sedimentary deposits have a complex genesis and mineralization. Of the total K 2 O reserves of 1300 million Mt on the southern African margin, 1000 million Mt is located off the southern African west coast, and the remainder situated on the Agulhas Bank. The largest glauconite concentration ( ±300 million Mt K 2 O) off southern Africa lies west of Saldanha Bay, South Africa. The distribution of P 2 O 5 off southern Africa is dominated by the vast deposit between Walvis Bay and Luderitz, Nambia. This reserve is estimated to contain 1000 million Mt of greater than 5% P 2 O 5 in a relatively small area of about 10000 km2. The phosphorite deposit south of Saldanha Bay constitutes a reserve of ±3500 million Mt of apatite and the deposit on the Agulhas Bank comprises 5500 million Mt. The phosphate deposit off Saldanha Bay occurs as an extensive, low to medium grade deposit. Although vast resources of potash and phosphatic minerals occur along the southern African outer continental shelf the expensive nature of marine exploitation may render most of these deposits, especially the phosphates, subeconomic. The low price of fertilizer andextensive natureontheonland deposits, although confined to asmallnumber of countries, mean that it will not be financially viable to extract these deposits. Assuming high grade glauconitic sand with the right composition can be located, the high market price indicates good future prospects for these potash deposits.     

Current and temperature observations in the east Tsushima channel and the Sea of Genkai

Data on the Tsushima Current and its neighboring coastal current are analyzed to examine short-term variability of the currents and storm events due to typhoons. A three current-meter array was deployed in a strong current region of the east Tsushima channel during summer in 1983 and 1984, and other two current-meter arrays in the eastern coastal area of the channel (the Sea of Genkai) in the summer and autumn in 1983. The observations of coastal current show that the kinetic energy of the subtidal current component was larger in summer than in autumn by a factor of about 2. A comparison of the wind stresses and the estimated values of mixed layer depth in the summer and autumn season suggest that this seasonal change is closely associated with that of the mixed layer depth rather than with that of the wind stress. The Tsushima Current was greatly influenced by two storm events: its speed increased by a factor of 2 in one event and decreased to nearly zero in the other. Such a large variation of mean current during the storm was observed only for the Tsushima Current and not for the coastal current, suggesting that the Tsushima Current may temporarily change its regular course as a result of a storm.     

藻类分子生物技术两年评─—与藻类活性物质研究有关的生物技术

根据１９９２年６月－１９９４年６月国内外发表资料，评述海洋藻类活性物质研究的生物技术及最新成果。认为，海洋生物活性物质研究的重心正转向与海洋生物技术有关的研究。运用生物技术筛选、提取并生产藻类活性物质已经成为这一领域的主流。综合文献表明，藻类活性物质的生物技术正走向成熟，大量的活性物质被发现，光生物反应器将成为主要的下游生产技术，通过藻类基因工程生产药物已初现端倪。     

Incorporating Māori values into land management decision tools

ABSTRACT

Environmental sustainability and the long-term wellbeing of Māori (the indigenous people of New Zealand) are interdependent and degradation of landscapes risks the progressive degradation of Māori wellbeing. The present study developed an analysis framework based on Ki Uta Ki Tai (holistic-mountains to the sea- management philosophy advocated by Ngāi Tahu) for exploring relationships between landcover and Māori values to enable predictions of cultural values through space and time. We used this framework to predict how two Māori values (Overall Health and Cultural Land Use) have been altered as a result of landcover change between 2001–2012 in three Canterbury catchments. The area of native vegetation declined while exotic pasture increased between 2001–2012, and there were corresponding declines in both cultural health scores. These results suggest that the change in landcover has reduced the ability of the landscape to support Māori values. This framework for assessing changes in Māori values with respect to changing environmental conditions may identify opportunities for Māori to better engage in land use management decisions.