ALBERT

Description: Marine organic carbon is heavier isotopically (13C enriched) than most land-plant or terrestrial organic C1. Accordingly, δ 13C values of organic C in modern marine sediments are routinely interpreted in terms of the relative proportions of marine and terrestrial sources of the preserved organic matter2,3. When independent geochemical techniques are used to evaluate the source of organic matter in Cretaceous or older rocks, those rocks containing mostly marine organic C are found typically to have lighter (more-negative) δ 13C values than rocks containing mostly terrestrial organic C. Here we conclude that marine photosynthesis in mid-Cretaceous and earlier oceans generally resulted in a greater fractionation of C isotopes and produced organic C having lighter δ 13C values. Modern marine photosynthesis may be occurring under unusual geological conditions (higher oceanic primary production rates, lower P CO2) that limit dissolved CO2 availability and minimize carbon isotope fractionation4.

Description: The climate record obtained from two long Greenland ice cores reveals several brief climate oscillations during glacial time. The most recent of these oscillations, also found in continental pollen records, has greatest impact in the area under the meteorological influence of the northern Atlantic, but none in the United States. This suggests that these oscillations are caused by fluctuations in the formation rate of deep water in the northern Atlantic. As the present production of deep water in this area is driven by an excess of evaporation over precipitation and continental runoff, atmospheric water transport may be an important element in climate change. Changes in the production rate of deep water in this sector of the ocean may push the climate system from one quasi-stable mode of operation to another.

Description: Cephalopods may be divided into five types according to their buoyancy. Members of several families such as the Octopodidae, Loliginidae and Ommastrephidae are negatively buoyant and must swim to stay in midwater and are therefore highly muscular animals. Others have mechanisms to make them neutrally buoyant so they can remain suspended in midwater without effort. Nautilus, Spirula and cuttlefishes have low pressure gas-filled chambers and their flesh is muscular and non-buoyant (Denton & Gilpin-Brown, 1973). Squids of one family, the Gonatidae, have a low density oil in their livers to give buoyancy but most of their body is muscular. Some oceanic octopods have very watery tissues in which lighter chloride ions replace sulphate ions (Denton & Shaw, 1961). In 12 of the 26 teuthoid families the buoyancy is provided by low-density ammonia-rich solution in their body and head tissues or in an expanded coelomic cavity (Clarke, Denton & Gilpin-Brown, 1979). These ammoniacal squids are extremely abundant in the oceans of the world and form a large part of the diet of birds, cetaceans, seals and fish (Clarke, 1977). When their biomass is estimated from their utilization by predators it is important to know their properties as food and, in particular, their calorific values. As pointed out by Croxall & Prince in a review of the calorific values of cephalopods (1982), all the known values are of muscular, negatively buoyant species because they are of value as food for humans but no measurements have been made on the ammoniacal or oily species which are probably as important, or even more important, in the economy of the ocean (Clarke, 1983).

Description: The dissolution rates of spheres of two magnesian olivines, two plagioclases, and quartz in tholeiitic basalt have been determined at three super-liquidus temperatures and one-atmosphere pressure. There are considerable differences in the rates among the minerals, e.g. at 1210°, 12° above the liquidus temperature of the basalt, labradorite dissolves at 86 µm/h. and the magnesian olivines at 9 and 14 µm/h. The rates are not time dependent and this, coupled with the existence of concentration gradients in the composition of quenched melt adjacent to partially dissolved crystals, indicates that the dissolution rates are dictated by a combination of diffusion and convection of components to and from the crystal-liquid interface. Values for the activation enthalpy of dissolution are small for quartz and plagioclase (40–50 kcal mol−1) but large for olivine 73–118 kcal mol−1). Dissolution of plagioclase in rock melts seems to be a much more rapid process than crystal growth, whereas olivines apparently dissolve and grow at similar rates. Crystal dissolution is sufficiently slow that ascending, crystal-bearing magma may become superheated and yet fail to dissolve the crystal fraction before quenching; this may be the reason that olivine phenocrysts are often rounded.