ALBERT

Description: An estimate of average river particulate matter (RPM) composition was bàsed on analyses of more than 40 elements in the Amazon, Congo, Ganges, Magdalena, Mekong, Parana and Orinoco rivers, to which were added literature data for 13 other major world rivers, covering the whole spectrum of morphoclimatic features. Geographic variations of major elements in RPM are mostly linked to weathering types and to the balance between weathering rate and river transport. As a result of chemical erosion, Al, Fe and Ti are enriched in RPM with respect to the average parent rock, while Na, Ca, Mg and Sr are strongly depleted. These figures are directly related to the relative importance of dissolved and particulate transport in rivers; this has been computed for each of 40 elements. In order to study weathering on a global scale, the total observed elemental fluxes (dissolved + particulate) have been computed and compared to theoretical ones. The latter were derived from the elemental content in the average parent rock and the total quantity of weathered material, computed from the Al ratio in RPM and in parent rock. Observed and theoretical fluxes are balanced for the less mobilized elements (rare earths, Co, Cr, Cs, Fe, Mn, Rb, Si, Th, Ti, U and V) for which no enrichment relative to Al is noted in RPM, and for B, Ba, Ca, K, Mg, Na, Sr which are relatively depleted in RPM due to their high dissolved transport. Additional fluxes have been found for Br, Sb, Pb, Cu, Mo, Zn and are possible also for Ni and P. This is reflected by marked enrichments in RPM relative to Al for the poorly or moderately dissolved transports (Pb, Cu, Zn). Several hypotheses involving either the natural origin (volcanic dust, marine aerosols, geochemical fractionation) or the artificial origin (worldwide pollution) are discussed to explain these discrepancies, assuming river transport and weathering either to be in a steady state on a global scale or not. However, none of them can fully account for these additional fluxes. It is most likely that these excesses have multiple origins, anthropogenic or natural or both. The comparison between RPM and deep-sea clay compositions emphasizes the prime influence of river input on oceanic sedimentation of Si, Al, Fe, Ti, lanthanides, Sc, Rb, V, etc. A few elements such as Zn, Sb, occur in excess in RPM as compared to deep-sea clays; in order to balance this excess, a remobilization of these elements out of the sediment can be considered. Finally, the enrichment of Co, Cu, Mn and Ni in deep-sea clays compared to RPM is discussed and attributed to several sources and processes.

Description: Several types of abyssal bedforms have been discovered during surveys with a deeply towed instrument package at water depths of 1.5–6 km in the Pacific and Atlantic Oceans. Cores and current-meter records obtained at the same sites provide data for interpreting their dynamics. Wave and current ripples are best portrayed in bottom photographs, but medium-scale bedforms, including sand waves, mud waves and erosional furrows, are described by interpreting high-resolution side-looking sonar records. The largest examples affect surface-ship echograms, though their shape and structure can seldom be resolved without near-bottom observations. Wave ripples are common on the slopes of seamounts and ridges, while current ripples and sand waves occur beneath some fast thermohaline currents whose beds are shallower than the foraminiferal compensation depth. Depositional and erosional bedforms in cohesive sediment have been found beneath the deepest thermohaline currents; they may be restricted to areas where the flow is unusually steady in direction.

Description: Deep‐sea drilling in the Antarctic region (Deep‐Sea Drilling Project legs 28, 29, 35, and 36) has provided many new data about the development of circum‐Antarctic circulation and the closely related glacial evolution of Antarctica. The Antarctic continent has been in a high‐latitude position since the middle to late Mesozoic. Glaciation commenced much later, in the middle Tertiary, demonstrating that near‐polar position is not sufficient for glacial development. Instead, continental glaciation developed as the present‐day Southern Ocean circulation system became established when obstructing land masses moved aside. During the Paleocene (t = ∼65 to 55 m.y. ago), Australia and Antarctica were joined. In the early Eocene (t = ∼55 m.y. ago), Australia began to drift northward from Antarctica, forming an ocean, although circum‐Antarctic flow was blocked by the continental South Tasman Rise and Tasmania. During the Eocene (t = 55 to 38 m.y. ago) the Southern Ocean was relatively warm and the continent largely nonglaciated. Cool temperate vegetation existed in some regions. By the late Eocene (t = ∼39 m.y. ago) a shallow water connection had developed between the southern Indian and Pacific oceans over the South Tasman Rise. The first major climatic‐glacial threshold was crossed 38 m.y. ago near the Eocene‐Oligocene boundary, when substantial Antarctic sea ice began to form. This resulted in a rapid temperature drop in bottom waters of about 5°C and a major crisis in deep‐sea faunas. Thermohaline oceanic circulation was initiated at this time much like that of the present day. The resulting change in climatic regime increased bottom water activity over wide areas of the deep ocean basins, creating much sediment erosion, especially in western parts of oceans. A major (∼2000 m) and apparently rapid deepening also occurred in the calcium carbonate compensation depth (CCD). This climatic threshold was crossed as a result of the gradual isolation of Antarctica from Australia and perhaps the opening of the Drake Passage. During the Oligocene (t = 38 to 22 m.y. ago), widespread glaciation probably occurred throughout Antarctica, although no ice cap existed. By the middle to late Oligocene (t = ∼30 to 25 m.y. ago), deep‐seated circum‐Antarctic flow had developed south of the South Tasman Rise, as this had separated sufficiently from Victoria Land, Antarctica. Major reorganization resulted in southern hemisphere deep‐sea sediment distribution patterns. The next principal climatic threshold was crossed during the middle Miocene (t = 14 to 11 m.y. ago) when the Antarctic ice cap formed. This occurred at about the time of closure of the Australian‐Indonesian deep‐sea passage. During the early Miocene, calcareous biogenic sediments began to be displaced northward by siliceous biogenic sediments with higher rates of sedimentation reflecting the beginning of circulation related to the development of the Antarctic Convergence. Since the middle Miocene the East Antarctic ice cap has remained a semipermanent feature exhibiting some changes in volume. The most important of these occurred during the latest Miocene (t = ∼5 m.y. ago) when ice volumes increased beyond those of the present day. This event was related to global climatic cooling, a rapid northward movement of about 300 km of the Antarctic Convergence, and a eustatic sea level drop that may have been partly responsible for the isolation of the Mediterranean basin. Northern hemisphere ice sheet development began about 2.5–3 m.y. ago, representing the next major global climatic threshold, and was followed by the well‐known major oscillations in northern ice sheets. In the Southern Ocean the Quaternary marks a peak in activity of oceanic circulation as reflected by widespread deep‐sea erosion, very high biogenic productivity at the Antarctic Convergence and resulting high rates of biogenic sedimentation, and maximum northward distribution of ice‐rafted debris.

Description: A complete set of linearly independent relationships among the different cross spectral components obtained from pairs of moored instruments is derived which can be utilized to test whether or not the observed fluctuations within the internal wave frequency band represent a field of propagating internal waves. A further complete set of relationships is derived which enables to test whether or not the internal wave field is horizontally isotropic and (or) vertically symmetric. These relations are compared with corresponding relations for alternative models (standing internal wave modes, three-dimensional isotropic turbulence) and their capability to discriminate between the various models is investigated. The tests are applied to a set of data for which it is found that the observed fluctuations are consistent with both propagating and standing internal waves whereas isotropic turbulence must be rejected for the most part of the internal wave frequency band.

Description: This chapter discusses the carbon turnover, calcification, and growth in coral reefs. Carbon turnover within a total reef community is a function of two distinct, biochemically interacting cycles. The first is the metabolic cycle consisting of the photosynthetic fixation of CO2 and the release of CO2 by respiration and decomposition processes. Superimposed on this are the direct incorporation of organic compounds (dissolved or particulate; living or non-living) originating outside the reef systems (in the adjacent ocean waters), and the loss of organic compounds from the reef system into the out-flowing water. The second is the inorganic carbonate cycle involving the biological and non-biological precipitation and dissolution of carbonates. Superimposed on this is the loss of particulate carbonates in suspension in the out-flowing water. The main chemical component of a coral-reef system is calcium carbonate, which occurs either as high-Mg calcite, aragonite, or low-Mg calcite. The mean calcification values in various environments at One Tree Reef are presented in the chapter. These data may be converted to an implied vertical growth rate potential assuming that accrual is dominantly aragonite (density = 2.89 g cm–3) and that there is 50% porosity after normal compaction.