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: 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.

Description: One of the most striking features of the upper North Atlantic Ocean is an extensive layer of water with temperature close to 18°C and salinity close to 36.5‰, (ref. 1). This 18°C water is formed by winter convection in the Sargasso sea2,3, but aspects of the annual rate of 18°C water formation remain obscure4. We have simulated this water mass formation by integrating a one-dimensional model along a 4-yr trajectory of a water column circulating around the Sargasso Sea. Winter convection is deep (≥200 m) in regions where the ocean suffers a net annual heat loss to the atmosphere, and shallow (≤lOOm) where the ocean gains heat each year. The origin of the thermostad (nearly isothermal layer) is a thick layer of nearly homogeneous water subducted beneath the seasonal boundary layer in the year that the water column passes through the line dividing annual cooling from annual heating. We estimate the annual production of 18°C water to be 446,000 km3 yr−1. Downstream, more stratified central water is formed each year at a rate that depends more on Ekman pumping (wind-forced convergence) than on the decreasing depth of winter convection

Description: RECENT advances in 40Ar/39Ar dating1,2 have made it possible to date individual K-feldspar grains from Pleistocene tephra, a capability that greatly improves the reliability and temporal resolving power of the method. Here we apply these new techniques to the dating of a phonolite tephra from the East Eifel volcanic field in West Germany, which is sandwiched between loess and palaeosol (alfisol) deposits, and which was therefore erupted during the transition from a glacial to an interglacial period. Our age estimate for this transition is 215±4 kyr (1 σ), which has important implications for the marine δ18O timescale and for models of global climate change during the Pleistocene. The results show that single-grain dating can detect and compensate for the large quantities of xenocrystic contaminants which are found in many tephra deposits. This technique could be used to date the tephra layers found in marine sediment cores and the results could greatly enhance the reliability of the marine δ18O timescale for more rigorous Fourier analysis testing of the Milankovitch hypothesis.

Description: Hurricanes and other strong storms can cause important decreases in sea surface temperature by means of vertical mixing within the upper ocean, and by air–sea heat exchange. Here we use satellite-derived infrared images of the western North Atlantic to study sea surface cooling caused by hurricane Gloria (1985). Significant regional variations in sea surface cooling are well correlated with hydrographic conditions. The greatest cooling (up to 5°C) occurred in slope waters north of the Gulf Stream where the seasonal thermocline is shallowest and most compressed; moderate cooling (up to 3 °C) occurred in the open Sargasso Sea where the thermocline is deeper and more diffused; little or no cooling occurred in shallow coastal waters (bottom depth less than 20 m) which were isothermal before the passage of hurricane Gloria. There is a pronounced right-side asymmetry of sea surface cooling with stronger (by a factor of 4) and more extensive (by a factor of 3) cooling found on the right side of the hurricane track. These qualitative results are consistent with the notion that vertical mixing within the upper ocean is the dominant sea surface cooling mechanism of hurricanes.

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: Perhaps the most significant event in the Cretaceous record of the carbon isotope composition of carbonate1,2, other than the 1–2.5 ‰ negative shift in the carbon isotope composition of calcareous plankton at the Cretaceous/Tertiary boundary3, is the rapid global positive excursion of ~2 ‰ (13C enrichment) which took place between ~91.5 Myr and 90.3 Myr (late Cenomanian to earliest Turonian (C/T boundary event))1,4,5. This excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon (TDC) reservoir resulting from an increase in rate of burial of 13C-depleted organic carbon, which coincided with a major global rise in sea level5 during the so-called C/T oceanic anoxic event (OAE)6. Here we present new data, from nine localities, which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the C/T boundary7,8 is nearly synchronous with that for carbonate and is widespread throughout the Tethys and Atlantic basins (Fig. 1), as well as in more high-latitude epicontinental seas. The postulated increase in the rate of burial of organic carbon may have had a significant effect on CO2 and O2 concentrations in the oceans and atmosphere, and consequent effects on global climate and sedimentary facies.

Description: ROOTH proposed that the Younger Dryas cold episode, which chilled the North Atlantic region from 11,000 to 10,000 yr BP, was initiated by a diversion of meltwater from the Mississippi drainage to the St Lawrence drainage system. The link between these events is postulated to be a turnoff, during the Younger Dryas cold episode, of the North Atlantic's conveyor-belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the North Atlantic region2. This turnoff is attributed to a reduction in surface-water salinity, and hence also in density, of the waters in the region where North Atlantic Deep Water (NADW) now forms. Here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from Orca Basin core EN32-PC4 which reveal a significant reduction in meltwater flow through the Mississippi River to the Gulf of Mexico from about 11,200 to 10,000 radiocarbon years ago. This finding is consistent with the record for Lake Agassiz which indicates that the meltwater from the southwestern margin of the Laurentide Ice Sheet was diverted to the northern Atlantic Ocean through the St Lawrence valley during the interval from ~11,000 to 10,000 years before present (yr BP).

Description: Abrupt changes in climatic conditions have been seen at high latitudes in the North Atlantic1 and the Antarctic2,3 at 13 kyr BP. It is important to determine whether this abrupt change was confined to high-latitude regions or whether it was global. Here we present results demonstrating an abrupt change in the rate and character of sedimentation in the South China Sea at the close of the last glacial period. Radiocarbon dating and its position in the oxygen isotope shift suggest that this change may be coincident with the changes found at high latitudes.