ALBERT

Description: The well-known caldera of Thira (Santorini), Greece, was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a NE trending line of vents. Before the Minoan eruption of 1400 B.C., Thira consisted of three Java shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half. Andesitic lavas formed the overlapping shields of the north and were contemporaneous with and, in many places, interbedded with the southern tuff deposits. Although there appears to be little difference between the composition of magmas erupted, differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial, producing lava flows, whereas in the south, eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs. Many of these tuffs are plastered onto the walls of what appears to have been an older caldera, most probably associated with an eruption of rhyodacitic tephra 100,000 years ago. The Minoan eruption of about 1400 B.C. had four distinct phases, each reflecting a different vent geometry and eruption mechanism. The Minoan activity was preceded by minor eruptions of fine ash. (1) The eruption began with a Plinian phase, from subaerial vent(s) located on the easternmost of the lava shields. (2) Vent(s) grew toward the SW into the flooded depression. Subsequent activity deposited large-scale base surge deposits during vent widening by phreatomagmatic activity. (3) The third eruptive phase was also phreatomagmatic and produced 60% of the volume of the Minoan Tuff. This activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding. This deposit contains 5–40% lithic fragments that are typical of the westernmost lava shield and appears to have been erupted when caldera collapse began. (4) The last phase consisted of eruption of ignimbrites from vent(s) on the eastern shield, not yet involved in collapse. Collapse continued after eruption of the ignimbrites with foundering of the eastern half of the caldera. Total volume of the collapse was about 19 km3, overlapping the older caldera to form the caldera complex visible today. Intracaldera eruptions have formed the Kameni Islands along linear vents concomitant with vents that may have been sources for the Minoan Tuff.

Description: A population dynamics analysis for planktonic diatoms is presented that allows estimates of the net rate of increase (k), the death rate (δ), the sedimentation rate (σ) and, in absence of grazing, the growth rate (μ). It requires counts of live and dead cells suspended in the euphotic part of the water column and accumulated in sedimentation traps. The application of the model is demonstrated for the three dominant summe diatom species in Lake Constance. Asterionella formosa Hass, Fragilaria crotonensis Kitton and Stephanodiscus binderanus Krieger. Only during the first two weeks of the summer bloom of diatoms the loss rates were unimportant in comparison to the growth rates. Thereafter diatom population dynamics was strongly influenced by sedimentation and mortality, which sometimes led to a decrease in population density even when cell division continued at high rates. There were two periods of extraordinarily high death rates, which were associated in the case of A. formosa with silicon depletion and in the case of F crotensis with fungal parasitism.

Description: Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of Regional Seismic Test Network station RSCP are inverted for vertical velocity structure beneath the Cumberland Plateau, Tennessee. The detailed broadband receiver functions are obtained by stacking source‐equalized horizontal components of teleseismic P waveforms. The resulting receiver functions are most sensitive to the shear velocity structure near the station. A time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin, flat‐lying, homogeneous layers. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. The results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the East Continent Gravity High located northeast of RSCP. The results from events arriving from the northeast show a high‐velocity midcrustal layer not present in results from the southeast azimuth. This velocity structure can be shown to support the idea that this feature is part of a Keweenawan rift system. Another interesting feature of the derived velocity models is the indication that the crust‐mantle boundary beneath the Cumberland Plateau is a thick, probably laminated transition zone between the depths of 40 and 55 km, a result consistent with interpretations of early refraction work in the area.

Description: From 27 Janua1y to 23 June 1979 R. V. "Meteor" surveyed the central equatorial Atlantic on a section along 22° W from 3° N to 2° S. During the observation period, a hydrographic section down to 600 m was repeated ten times with a continuous "Howaldt-Bathysonde" CTD and a rosette sampler. The station distance was 10 to 15 nm. The water samples were used to recalibrate salinity and to determine oxygen, nutrients and chlorophyll a. An undulating CTD system ("Delphin") was towed on 11 sections. A profiling distance of one to two nautical miles and a profile depth of 90 m was obtained. Five current meter arrays were moored along 22° W between 3° N and 1° S from January to March 1979. In May and June two moorings were installed at 2° N and at the equator. On the buoys measurements of wind speed and direction were obtained. At 43 stations a wire-guided Aanderaa profiling current meter was successfully lowered. Drifting buoy experiments were repeated three times with clusters of 5 to 10 buoys. A fourth experiment took place in 1978 in the Gulf of Guinea. On the way from and to port XBT sections were carried out. The data sets obtained by these instruments are presented in this data report.

Description: Along the Sierra Leone continental shelf edge, bedded rocks, presumably mostly calcareous sandstone, calcarenites and siltstones form exposures up to 10 km long in water depths ranging from 80 to 110 m. These outcrops are found in a 6 km broad zone along the shelf edge; landward they get covered by recent shelf sands. Their apparent strike direction parallds the shelf edge indicating nearly horizontal bedding which locally is interrupted by faulting. On the middle shelf, a terrace is well developed at a water depth of 56 m. Ripples, megaripples, pockmarks 1-5 m in diameter, and canyon-heads form other significant features of the shelf.

Description: The sediments of a 10.67 m long piston core recovered from the Sierra Leone Rise ("Meteor" Core 13519) have been analysed for their paleomagnetic properties. Using detailed demagnetization techniques, the geomagnetic Brunhes/Matuyama boundary could be identified between 9.81 and 9.89 m sub-bottom. This is in excellent agreement with the oxygen isotope stratigraphy by SARNTHEIN et al., this volume. Down to this level, therefore, the average apparent accumulation rate amounts to 13.5 m/m.y. assuming an age of 0.73 m.y. for the reversal boundary (BERGGREN et al. 1983). A second transition in magnetization polarity was recognized between 10.51 and 10.54 m sub-bottom. According to its magnetic signature it was tentatively correlated to the beginning of the Jaramillo event (0.98 m.y.). This would imply that in the lowermost part of the core the apparent accumulation rate decreases to less than 20% of that found in the upper sequences. However, as obviously only a small portion of the Jaramillo event is recorded in the sediment column, a more plausible solution is to keep the overall sedimentation rate about constant by introducing a hiatus of some 2·105 years shortly after the lower Jaramillo/Matuyama reversal boundary. Alternative interpretations are also discussed.

Description: From a 10.7 m long gravity core from the Sierra Leone Rise (5° 39,5' N, 19° 51'W) a detailed oxygen and carbon isotope record of both planktonic and benthonic foraminifera species was obtained extending from the Recent to the Jaramillo event. The analysis yielded six major results. 1. Benthos oxygen isotopes varied by 1.8-2.2‰ from interglacial to glacial times and may indicate a synglacial cooling of North Atlantic Deep Water at 2800 m depth by 1-3° C. 2. Variable anomalies between the benthos and plankton δ18 O record indicate a cooling of sea-surface temperatures (SST) by up to 6° C during some glacial stages. 3. Southerly trade winds and equatorial upwelling may exert the primary control of SST variations, in particular of extreme values of cold and warm stages and of the abrupt character of climate transitions and their leads and lags, and finally, of variable sedimentation rates. 4. The benthos δ13C record correlates well with the flux and preservation of organic matter. 5. A new time scale, CARPOR, was established from the assumption that terrigenous sediment supply was ± constant but CaC03 varied considerably. When applied to the δ18O record, three major and numerous short-term variations of sedimentation rates (0.8 to 4.0 cm/kyr) can be distinguished. 6. The climatic record was modified by bioturbation much more strongly during cold than during warm stages.

Description: Planktonic foraminiferal tests of the spinose species Orbulina universa, of the non-spinose Globorotalia tumida-menardii complex, and of a mixed species assemblage (grain size fraction 200-400 μm) were isolated from Sierra Leone Rise core 13 519-2 and analyzed for free, total, and bound (by difference) amino acids to study the isoleucine epimerization mechanism in fossil foraminiferal tests and to define empirical calibration curves for dating deep-sea sediments over the past 900,000 years. Total isoleucine epimerization curves typically separate into three "linear" segments of decreasing apparent rates with increasing time and exhibit a pronounced "species effect". The degree of epimerization attained at time is considerably lower in 0. universa than in G. tumida-menardii while the mixed species results scatter between the limits delineated by the two monospecific curves. Total allo/iso ratios are closely related to the proportion of free to total isoleucine accumulating in the tests indicating that the rate of hydrolysis of matrix proteins and peptides controls the overall epimerization reaction. The results are consistent with experimental evidence (KRIAUSAKUL & M1TTERER 1978, 1980 a, b) whereupon isoleucine epimerizes at a rapid rate in terminal positions but at slow rates in interior positions as well as in the free state. Notwithstanding free isoleucine exhibits the highest degree of epimerization due to preferential hydrolysis of extensively epimerized terminal isoleucine. Species-specific hydrolysis and epimerization rates are maintained until about 50% of bound isoleucine have been hydrolyzed to the free state corresponding to a total allo/iso ratio of about 0.5. Remaining peptide units appear to be more resistent against hydrolysis and separate species then show the same apparent epimerization rate dominantly controlled by the slow conversion rate in the free state until equilibrium is achieved in Miocene samples under deep-ocean temperature conditions (KING & HARE 1972 a). The degree of epimerization attained at comparable time in separate species will, however, remain different due to different initial rates of hydrolysis. Selective leaching of free isoleucine from the tests results in a lowering of total allo/iso ratios and apparent rate constants when significant amounts have been generated by diagenetic hydrolysis. Comparison with results based on Pacific core V 28-238 (KING & NEVILLE 1977) indicates an inverse relationship between the intensity of surficial carbonate dissolution and post-depositional leaching. Tests deposited under less corrosive conditions (Atlantic core 13519-2) will more significantly deviate from a closed system during diagenesis than those deposited in more corrosive bottom waters (Pacific core V 28-238). Carbonate dissolution removes proteinaceous lamellae from more external shell structures, which otherwise - if preserved during deposition - will preferentially be subject to leaching of free amino acids during diagenesis. This effect may qualify the applicability of empirical epimerization calibration curves to different depositional environments although the general agreement of the epimerization curves obtained for both sites is promising for future research. Identical apparent epimerization rates in G. tumidamenardii tests younger than 120,000 years (when leaching is stiH insignificant) at both Atlantic and Pacific sites are not consistent with the presently different bottom water temperatures in these regions. It is concluded that glacial North Atlantic Deep Water was cooler than at the present at the Sierra Leone Rise.