Ch. Hemleben

Sea-level fluctuations during the last glacial cycle

The last glacial cycle was characterized by substantial millennial-scale climate fluctuations, but the extent of any associated changes in global sea level (or, equivalently, ice volume) remains elusive. Highstands of sea level can be reconstructed from dated fossil coral reef terraces, and these data are complemented by a compilation of global sea-level estimates based on deep-sea oxygen isotope ratios at millennial-scale resolution or higher. Records based on oxygen isotopes, however, contain uncertainties in the range of ±30 m, or ±1 °C in deep sea temperature. Here we analyse oxygen isotope records from Red Sea sediment cores to reconstruct the history of water residence times in the Red Sea. We then use a hydraulic model of the water exchange between the Red Sea and the world ocean to derive the sill depth—and hence global sea level—over the past 470,000 years (470 kyr). Our reconstruction is accurate to within ±12 m, and gives a centennial-scale resolution from 70 to 25 kyr before present. We find that sea-level changes of up to 35 m, at rates of up to 2 cm yr-1, occurred, coincident with abrupt changes in climate.

Black Sea impact on the formation of eastern Mediterranean sapropel S1? Evidence from the Marmara Sea

Abstract Water exchange between the Black Sea and the Mediterranean Sea has been a major focus of the paleohydrography of the eastern Mediterranean. Glacial melt water released from the Black Sea is a potential factor in the formation of sapropel S1, an organic-rich sediment layer that accumulated during the Early Holocene. A high-resolution study done on sediments from the Marmara Sea, the gateway between the Mediterranean and the Black Sea, sheds light on the Holocene exchange processes. Past sea surface temperature and sea surface salinity (SSS) were derived from stable oxygen isotope ratios (δ18O) of foraminiferal calcite and alkenone unsaturation ratios (Uk′37). Heavy δ18O values and high SSS in the Marmara Sea suggest absence of low salinity water from the Black Sea during S1. The comparison with data from the Levantine Basin and southern Aegean Sea outlines gradients of freshening in the eastern Mediterranean Sea, whereby the major sources of freshwater were closer to the Levantine Basin. It is thus concluded that the Black Sea was not a major freshwater source contributing to formation of S1. Given the absence of a low salinity layer, the deposition of organic-rich sediments corresponding to S1 in the Marmara Sea is likely the result of the global transgression and the concomitant re-organization of biogeochemical cycles, leading to enhanced productivity as shown by Globigerina bulloides.

Aplanktonic zones in the Red Sea

We use micropalaeontological and stable isotope results for a series of cores from north to south through the Red Sea, to assess temporal and spatial patterns of change in planktonic foraminiferal faunas leading up to the remarkable full-glacial Red Sea aplanktonic zones. Aplanktonic zones reflect salinities in the Red Sea in excess of the lethal 49 p.s.u. limit, caused by reduced exchange transport through the Strait of Bab el Mandab due to glacial sea-level lowering. Concerning the last glacial cycle, aplanktonic conditions began at 39 ka BP in the north, where salinities eventually reached 55‐57 p.s.u. Paradoxically, planktonic faunas are reported to have survived the last glacial maximum in the northern Gulf of Aqaba, suggesting a poorly understood freshwater dilution at that time. Aplanktonic conditionsOS

Time-response of cultured deep-sea benthic foraminifera to different algal diets

49 p:s:u:U reached the south-central Red Sea by 22 ka BP, while planktonic foraminiferal faunas continued, albeit in very low abundances and limited diversityOS # 45 p:s:u:U; in the southernmost Red Sea. During marine isotope stage (MIS) 6, the 49 p.s.u. isohaline appears to have resided in the central Red Sea, between cores KL11 and MD921017. We observe a systematic sequence of species disappearances before all glacial maxima of the last 500 kyr. Absence of a logical relationship with sea levels suggests that the disappearance sequences are not related to a general salinity increase. Instead, we argue that the sequences were driven by complex reorganisations in hydrography (stratification), productivity (food availability) and subsurface oxygenation (reproduction-habitats). The onset is marked by dramatic basin-wide expansion of conditions that today are restricted to only the southern Red Sea, suggesting an expansion of the dominance of NE monsoonal circulation over the entire Red Sea. This expansion occurred 15 or more kyr before the aplanktonic zones of MIS-12, 6 and 2, and also before MIS-10 and 8, which never reach the aplanktonic stage. Regarding the last glacial cycle, this event occurred as early as 75 ka BP (MIS-4/5 boundary). After this major climatic reorganisation, we reconstruct progressive intensification of the new conditions, especially marked by northward expansion and intensification of the subsurface oxygen minimum zone (OMZ). During the last glacial cycle, a shallow and very distinct OMZ affected the central Red Sea as early as 68 ka BP, and the north as late as 55 ka BP. The OMZ expansion/intensification appears to have been interrupted by episodes of increased ventilation. q 2000 Elsevier Science B.V. All rights reserved.

Seasonal variation in the flux of euthecosomatous pteropods collected in a deep sediment trap in the Sargasso Sea

The vertical distribution of benthic foraminifera in the surface sediment is influenced by environmental factors, mainly by food and oxygen supply. An experiment of three different time series was performed to investigate the response of deep-sea benthic foraminifera to simulated phytodetritus pulses under stable oxygen concentrations. Each series was fed constantly with one distinct algal species in equivalent amounts. The temporal reactions of the benthic foraminifera with regard to the vertical distribution in the sediment, the total number, and the species composition were observed and compared within the three series. Additionally, oxygen contents and bacterial cell numbers were measured to ensure that these factors were invariable and did not influence foraminiferal communities. The addition of algae leads to higher population densities 21 days after food was added. Higher numbers of individuals were probably caused by higher organic levels, which in turn induced reproduction. A stronger response is found after feeding with Amphiprora sp. and Pyramimonas sp., compared to Dunaliella tertiolecta. At a constant high oxygen supply, no migration to upper layers was observed after food addition, and more individuals were found in deeper layers. The laboratory results thus agree with the predictions of the TROX-model. An epifaunal microhabitat preference was shown for Adercotryma glomerata. Hippocrepina sp. was spread over the entire sediment depth with a shallow infaunal maximum. Melonis barleeanum preferred a deeper infaunal habitat. Bacterial cell concentrations were stable during the laboratory experiments and showed no significant response to higher organic fluxes.

High rates of sea-level rise during the last interglacial period

A 4-year series of sediment trap samples from a depth of 3.2 km in the Sargasso Sea (32°05′N, 64°15′W) has revealed seasonal variations in the flux of euthecosomatous pteropods. Total pteropod flux is related to seasonal variations of the total particulate and organic carbon flux with a lag of 1–1.5 months. High flux of pteropods (>200 specimens m−2 day−1) occurs in late winter to mid-summer. Shells of individual pteropod species arrive in deep water in a seasonal succession similar to that in the living assemblage. Peak fluxes of Styliola subula, Clio pyramidata and Limacina bulimoides were recorded from February to May. Limacina inflata, Limacina lesueuri and Cuvierina columnella entered the trap in maximum numbers from April to mid-August. Creseis virgula conica and C. acicula were most abundant from June to late August. The latter two are non-migrating, epipelagic pteropods and comprise <10% of the assemblage. Diel migrators dominate the pteropod assemblage (92%). During the summer months they appear to migrate at greater depth, without reaching the surface water. Although many young are produced, only a small fraction, about 4% in the case of L. inflata and L. bulimoides, survives and reaches maturity. Adult shell size of L. inflata and L. bulimoides varies seasonally, reaching maximum size during spring, probably in response to increasing food availability.