Component composition and benthic foraminifera in sediments of ODP Leg 112

Indicators of surface-water productivity and bottom-water oxygenation have been studied for the age interval from the latest Pleistocene to the Holocene at three holes (679D, 680B, and 68IB) located in the center and at the edges of an upwelling cell at approximately 11°S on the Peruvian continental margin. Upwelling activity was maximal at this latitude during d18O Stages 1 (lower part), 3, the upper part of 5, the lower part of 6, and 7, as documented by high diatom abundance. During these time intervals, the bottom water was poorly oxygenated, as documented by low diversity benthic foraminiferal assemblages that are dominated by B. seminuda s.l. Both surface- and bottom-water-circulation patterns appear to have changed rapidly over short time intervals. Due to changes in surface circulation, the intensity of upwelling decreased, thereby decreasing the concentration of nutrients, and reducing the supply of organic matter to the bottom. Radiolarians became more abundant in the surface waters, and the bottom-water environment was less depleted in oxygen, allowing for the establishment of more diverse benthic foraminiferal assemblages. Surface-water productivity was probably minimal during the early part of d18O Stages 5 and 9, as indicated by the increased abundance of planktonic foraminifers and pteropods and their subsequent preservation.

Distribution of sponge spicule types, Goniothecium odontella and Diploneis cf. bomboides in selected spicule clusters from ODP Leg 112 holes (Table 1)

Clusters of sponge spicules found in Quaternary deep-water sediments at Sites 685 and 688 off Peru represent single individuals of small sponges or fragments of larger sponges. The spicule assemblages constituting these clusters probably represent a few demosponge species of the subclass Tetractinomorpha and order Astrophorida, because triaenes and microscleric euasters, as well as abundant monaxons, are present. As proved by incorporated Neogene diatoms, these spicule clusters are allochthonous. The sponge individuals probably inhabited deeper neritic environments during late Neogene time.

Age models for sediment cores in the eastern tropical Pacific

We present new high-resolution N isotope records from the Gulf of Tehuantepec and the Nicaragua Basin spanning the last 50-70 ka. The Tehuantepec site is situated within the core of the north subtropical denitrification zone while the Nicaragua site is at the southern boundary. The d15N record from Nicaragua shows an 'Antarctic' timing similar to denitrification changes observed off Peru-Chile but is radically different from the northern records. We attribute this to the leakage of isotopically heavy nitrate from the South Pacific oxygen minimum zone (OMZ) into the Nicaragua Basin. The Nicaragua record leads the other eastern tropical North Pacific (ETNP) records by about 1000 years because denitrification peaks in the eastern tropical South Pacific (ETSP) before denitrification starts to increase in the Northern Hemisphere OMZ, i.e., during warming episodes in Antarctica. We find that the influence of the heavy nitrate leakage from the ETSP is still noticeable, although attenuated, in the Gulf of Tehuantepec record, particularly at the end of the Heinrich events, and tends to alter the recording of millennial timescale denitrification changes in the ETNP. This implies (1) that sedimentary d15N records from the southern parts of the ETNP cannot be used straightforwardly as a proxy for local denitrification and (2) that denitrification history in the ETNP, like in the Arabian Sea, is synchronous with Greenland temperature changes. These observations reinforce the conclusion that on millennial timescales during the last ice age, denitrification in the ETNP is strongly influenced by climatic variations that originated in the high-latitude North Atlantic region, while commensurate changes in Southern Ocean hydrography more directly, and slightly earlier, affected oxygen concentrations in the ETSP. Furthermore, the d15N records imply ongoing physical communication across the equator in the shallow subsurface continuously over the last 50-70 ka.

Sulfur in oceans and seas

The book summarizes results of long-term studies of sulfur geochemistry in bottom sediments of seas and oceans. Processes of hydrogen sulfide formation in bacterial reduction of sulfates, its transformation into transient and stable compounds of reduced sulfur in liquid and solid phases of sediments are under consideration. Regularities of distribution of sulfate and reduced sulfur in ocean sediments are shown. Problems of sulfur budget in the modern oceans are discussed.

Stable isotope ratios in shells of marine organisms

Oxygen and carbon isotope analyses have been carried out on calcareous skeletons of important recent groups of organisms. Annual temperature ranges and distinct developmental stages can be reconstructed from single shells with the aid of the micro-sampling technique made possible by modern mass-spectrometers. This is in contrast to the results of earlier studies which used bulk sampIes. The skeletons analysed are from Bermuda, the Philippines, the Persian Gulf and the continental margin off Peru. In these environments, seasonal salinity ranges and thus annual variations in the isotopic composition of the water are small. In addition, environmental parameters are weIl documented in these areas. The recognition of seasonal isotopic variations is dependant on the type of calcification. Shells built up by carbonate deposition at the margin, such as molluscs, are suitable for isotopic studies. Analysis is more difficult where chambers are added at the margin of the shell but where older chambers are simultaneously covered by a thin veneer of carbonate e. g. in rotaliid foraminifera. Organisms such as calcareous algae or echinoderms that thicken existing calcareous parts as weIl as growing in length and breadth are the most difficult to analyse. All organisms analysed show temperature related oxygen-isotope fractionation. The most recent groups fractionate oxygen isotopes in accordance with established d18O temperature relationships (Tab. 18, Fig. 42). These groups are deep-sea foraminifera, planktonic foraminifera, serpulids, brachiopods, bryozoa, almost all molluscs, sea urchins, and fish (otoliths). A second group of organisms including the calcareous algae Padina, Acetabularia, and Penicillus, as weIl as barnacles, cause enrichment of the heavy isotope 18O. Finally, the calcareous algae Amphiroa, Cymopolia and Halimeda, the larger foraminifera, corals, starfish, and holothurians cause enrichment of the lighter isotope 16O. Organisms causing non-equilibrium fractionation also record seasonal temperature variations within their skeletons which are reflected in stable-oxygen-isotope patterns. With the exception of the green algae Halimeda and Penicillus, all organisms analysed show lower d13C values than calculated equilibrium values (Tab. 18, Fig. 42). Especially enriched with the lighter isotope 12C are animals such as hermatypic corals and larger foraminifera which exist in symbiosis with other organisms, but also ahermatypic corals, starfish, and holothurians. With increasing age of the organisms, seven different d13C trends were observed within the skeletons. 1) No d13C variations are observed in deep-sea foraminifera presumably due to relatively stable environmental conditions. 2) Lower d13C values occur in miliolid larger foraminifera and are possibly related to increased growth with increasing age of the foraminifera. 3) Higher values are found in planktonic foraminifera and rotaliid larger foraminifera and can be explained by a slowing down of growth with increasing age. 4) A sudden change to lower d13C values at a distinct shell size occurs in molluscs and is possibly caused by the first reproductive event. 5) A low-high-Iow cycle in calcareous algae is possibly caused by variations in the stage of calcification or growth. 6) A positive correlation between d18O and d13C values is found in some hermatypic corals, all ahermatypic corals, in the septa of Nautilus and in the otoliths of fish. In hermatypic corals from tropical areas, this correlation is the result of the inverse relationship between temperature and light caused by summer cloud cover; in other groups it is inferred to be due to metabolic processes. 7) A negative correlation between d18O and d13C values found in hermatypic corals from the subtropics is explained by the sympathetic relationship between temperature and light in these latitudes. These trends show that the carbon isotope fractionation is controlled by the biology of the respective carbonate producing organisms. Thus, the carbon isotope distribution can provide information on the symbiont-host relationship, on metabolic processes and calcification and growth stages during ontogenesis of calcareous marine organisms.