ALBERT

Description: The southern section of the Agulhas western boundary current system exhibits unique characteristics as regards ocean/atmosphere heat flux processes. The Agulhas Retroflection region's high heat flux core from 37°S to 41°S, 16°E to 22°E does not demonstrate a distinct annual cycle of turbulent heat fluxes (latent and sensible) as is characteristic of its northern hemisphere counterparts. Rather, a weak semiannual heat flux cycle is found with maximum average losses during winter and summer (200 and 211 W/m2 ) and minimum losses during spring and autumn (185 and 162 W/m2 ). Upstream where the Agulhas Current is closer to land, winter heat losses exceed those of summer, but the differences are small. This behavior contrasts with that encountered at the poleward ends of northern hemisphere western boundary currents where winter heat fluxes are several times those of summer. The main reason for this difference is persistent westerly and southwesterly wind flow over the Agulhas Retroflection region throughout the year which ensures that cold, unsaturated maritime air repeatedly forces loss of heat from the ocean's surface. Spatial heat flux gradients associated with the Agulhas‐Subtropical Convergence surface temperature front are more pronounced in summer than in winter, indicating that cyclogenesis locally may be less seasonally dependent than in the northern hemisphere situation. Average oceanic cooling rates in the core region of the Retroflection, based on net heat flux calculations and a mixed surface layer of 75 m, range from 1.35°C/month during winter to 0.25°C/month during summer. Interannual variability in ocean/atmosphere heat fluxes within the Agulhas Retroflection region often exceeds the variability illustrated by the annual cycle. West of the Agulhas Retroflection core region, interannual sea surface temperature (SST) anomalies are more influential in the generation of heat flux anomalies by virtue of their large temporal variability. This high SST variability is primarily attributed to interannual changes in flux of Agulhas Current water into the southeast Atlantic Ocean. Oceanic heat loss within this warm water zone is an important modifying influence to both ocean and atmosphere, thus meriting further research.

Description: Additional data from sonobuoys and the Deep Sea Drilling Project (DSDP) justify separating sound‐velocity‐depth functions and velocity gradients (in the first layer of soft marine sediments) into some geographic areas and sediment types. Based on sonobuoy and core measurements (where V is sound velocity in km/s, and h is depth in sediments in km), the following data are obtained: continental shelf basins off Sumatra and Java—V=1.484+0.710h−0.085h2; U. S. Atlantic continental rise—V=1.513+0.828h−0.138h2; deep‐sea terrigenous sediments—V=1.519+1.227h−0.473h2; and siliceous sediments of the Bering Sea— V=1.509+0.869h−0.267h2. Selected DSDP data (through leg 74) in similar areas yield: continental terrace silt–clays—V=1.505+0.712h; deep‐sea terrigenous sediments—V=1.510+1.019h; and deep‐sea siliceous sediments—V=1.533+0.761h. Computed velocity gradients from sonobuoy measurements are generally supported by the DSDP gradients. Only DSDP data give the following: hemipelagic sediments—V=1.501+1.151h; deep‐sea calcareous sediments—V=1.541+0.928h; and deep‐sea pelagic clay—V=1.526+1.046h. Where fast sediment accumulation occurs, there has not been enough time to reduce sediment pore spaces under overburden pressure; areas of slow accumulation may have relatively high sediment structural strength. Both cases have lower velocity gradients because higher porosities and consequent lower velocities persist to deeper depths.

Description: Tables for the speed of sound in sea water are presented. These tables have been prepared from an empirical formula which was derived to fit measured sound‐speed data obtained over the temperature range −3°C to 30°C, the pressure range 1.033 kg/cm2 to 1000 kg/cm2, and the salinity range 33‰ to 37‰. The discrepancy of −3.0 m/sec found by Del Grosso at 1 atm., as compared to the tables of Kuwahara, is substantiated. In addition, the pressure coefficient of sound speed observed in the present work differs from that predicted by Kuwahara.

Description: The acoustic backscatter of eight well‐curated ferromanganese nodules has been measured in 1 °C seawater at frequencies from 45 to 167 kHz. The nodules have diameters from 37 to 121 mm and are thought to be representative of the Cu–Ni–Co‐rich nodules from the area around 14° 40’ N, 125° 25’ W (DOMES site C). They had been collected in box cores on the Echo 1 expedition and were kept refrigerated and water soaked in air‐tight plastic bags. Acoustic backscatter variations of over 10 dB were observed while the nodule was rotated 10° to 30° about one of its principal axes. The complicated fine structure, as well as the target strength, makes it clear that nodules cannot be modeled as simple spheres.

Description: We examine the closure of the current plate motion circuit between the African, North American, and Eurasian plates to test whether these plates are rigid and whether the Gloria fault is an active transform fault. We also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries, and compare the predicted direction of motion along the African‐Eurasian plate boundary in the Mediterranean with the direction of slip observed in earthquakes. From marine geophysical data we obtain 13 transform fault azimuths and 40 3‐m.y.‐average spreading rates, 34 of which are determined from comparison of synthetic magnetic anomaly profiles to ∼140 observed profiles. Slip vectors from 32 earthquake focal mechanisms further describe plate motion. Detailed magnetic surveys north of Iceland provide 11 rates in a region where prior plate motion models had few data. Magnetic profiles north of the Azores triple junction record a rate of 24 mm/yr, 4 mm/yr slower than used by prior models. Gloria and Sea Beam surveys accurately measure the azimuths of seven transform faults; our plate motion model fits six of the seven within 2°. Two transform faults surveyed by Gloria side scan sonar lie near FAMOUS area transform faults A and B and give azimuths 13° clockwise of them. Because recent studies show that short‐offset transforms, such as transforms A and B, are in many places oblique to the direction of plate motion, we exclude azimuths from transforms with less than 35‐km offset. The best fitting and closure‐enforced vectors fit the data well, except for a small systematic misfit to the slip vectors: On right‐lateral slipping transforms, slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths, whereas on left‐lateral slipping transforms, slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths. We search the long Eurasia‐North America boundary for evidence of an additional plate, but find no systematic misfits to the data. In particular, if a Spitsbergen plate exists and moves relative to Eurasia, its motion is less than 3 mm/yr. An Africa‐Eurasia Euler vector determined by adding the Eurasia‐North America and Africa‐North America Euler vectors is consistent with the Gloria fault trend and with slip vectors from eastern Azores‐Gibraltar Ridge focal mechanisms. A small circle, centered at the Africa‐Eurasia closure‐enforced pole, fits the trace of the Gloria fault. The model in which closure was enforced predicts ∼4 mm/yr slip across the Azores‐Gibraltar Ridge, and west‐northwest convergence near Gibraltar, ∼45° more oblique than suggested by a recent model based on compressive axes of focal mechanisms. Moreover, our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between Gibraltar and Sicily. Because closure‐enforced vectors fit the data nearly as well as the best fitting vectors, we conclude that the data are consistent with a rigid plate model and with the Gloria fault being a transform fault.