ALBERT

Description: Benthic foraminifera from surface Sediments of the Ross Sea were studied to determine modern distributions of important assemblages. Factor analysis of the raw data distinguished nine significant factor assemblages which account for 86% of the raw data. These factor assemblages provide a means of understanding modern oceanographic and ecologic conditions because they show the response of this faunal group to different environments. Environmental conditions are in turn controlled by the modern climatic regime of the region. Four benthic assemblages from the relatively shallow (500 to 700 m) eastern Ross Sea Continental shelf are predominantly arenaceous. This may be because the relatively late seasonal breakup of pack ice inhibits productivity in the surface waters and permits a buildup of CO, thus causing the CCD to occur at shallow depths. On the Western part of the Continental shelf, three assemblages are composed primarily of calcareous species even though water depths are often greater there than they are in the east. One of these calcareous assemblages occurs in samples from water depths as great as 755 m in the southwestern part of the region, below the CCD as defined for the Ross Sea by previous workers. We relate this depressed CCD to early seasonal breakup of pack ice in the Western Ross Sea. Within the eastern arenaceous and Western calcareous regions on the Continental shelf, distributions of benthic assemblages are probably related to water depth and other ecologic variables. Two benthic assemblages from the Continental slope north of the Ross Sea also are calcareous. We attribute their distributions to high rates of productivity in the overlying surface waters, where the interface between eastward-flowing Circumpolar Deep Water and Ross Sea water masses is marked by a narrow zone of intense upwelling.

Description: Helium isotope measurements in six major basins in the Gulf of California show that the deep Guaymas Basin has 3He/4He 65–70% higher than atmospheric helium, clear evidence of mantle helium injection. Smaller 3He excesses observed in the Carmen and Farallon basins may be derived from this Guaymas Basin anomaly. The 3He concentrations in the Mazatlan Basin in the mouth of the Gulf of California are similar to average eastern Pacific values, indicating that the Gulf does not provide a significant flux of 3He into the general Pacific circulation. On the basis of temperature and salinity measurements an upper limit of 0.28°C can be placed on the amount of geothermal heating observed in any of the basins. The isotopic ratio of the injected Guaymas Basin helium is found to be 3He/4He = (1.10±0.06) × 10−5, almost identical to the helium signature observed at the Galapagos Rift but somewhat lower than the average ratio in oceanic basalt glasses.

Description: Processes from above and below influence the sea bottom and its sedimentary cover. The evolution of the lithosphere is being as passionately discussed today as was the evolution of the biosphere a century ago. The physical, chemical and biological interactions at the sea bottom, of "benthic boundary", call for interdisciplinary research.

Description: Fifty-four new heat flow measurements in the central troughs of the Guaymas basin support the hypothesis that they are sites of active intrusion. In the northern trough a distinct pattern of hydrothermal cooling is revealed, with venting along the western boundary fault of the trough. In the southern trough an analogous pattern is apparently superimposed upon a conductive cooling anomaly associated with a recent central intrusion. The discharge of thermal waters occurs along the boundary faults and through other faults associated with a possible horst block located in the north central floor of the southern trough. The heat flow patterns suggest that the intrusions are episodic and do not occur simultaneously along the length (15–40 km) of a spreading segment. A review of all available heat flow measurements for the Guaymas basin suggests that most of the recharge for a pervasive regional hydrothermal system is limited to the central depressions, with perhaps some contribution from pore water. The discharge of thermal waters occurs predominantly in the central depressions and possibly along the boundary transform faults and fracture zones. The regions of the basin more than a few kilometers in distance from the spreading axis, although presumably underlain by a hydrothermal system, are probably not the location of numerous vents or recharge zones.