Part of the global DOC versus AOU (dissolved organic carbon/apparent oxygen utilization) data compilation, Azores-I

Temperate, transitional and subtropical waters of the remote Azores Front region east of Azores (24-40°N, 22-32°W) were sampled during three cruises conducted under increasing stratification conditions (April 1999, May 1997 and August 1998). Despite the temporal increase of surface temperature (by 5 °C) and stratification (by 2.1 1/min**2), as well as the thermocline shoaling (by ~15 m), dissolved organic carbon (DOC) and nitrogen (DON) in the surface layer were not significantly different for the early spring, late spring and summer periods, with average concentrations of 69±2 µM-C and 5.2±0.4 µM-N, respectively. The surface excess of semi-labile DOC, compared with the baseline DOC concentration in the deep ocean (47±2 µM-C), represents 33% of the bulk DOC concentration and as much as 85% of the TOC (=POC+DOC) excess. When compared with the winter baseline (56±2 µM-C), the seasonal surface DOC excess is 20% of the bulk DOC concentration and 87% of the seasonal TOC excess. These results confirm the major role played by DOC in the carbon cycle of surface waters of the Azores Front region. The total amount of bioreactive DOC transported from the temperate to the subtropical North Atlantic by the Ekman flux between March and December represents only ~15% of the average annual primary production, and ~15% and ~30% of the measured sinking POC flux+vertical DOC eddy diffusion during early spring and summer, respectively. Vertical eddy diffusion is 35% and 2% of the spring and summer sinking POC flux, respectively. On the other hand, DOC only contributes 13% to the local oxidation of organic matter in subsurface waters (between the pycnocline and 500 m) of the study region.

Grain size investigations of sediments off the coast of Istria

Composition, grain-size distribution, and areal extent of Recent sediments from the Northern Adriatic Sea along the Istrian coast have been studied. Thirty one stations in four sections vertical to the coast were investigated; for comparison 58 samples from five small bays were also analyzed. Biogenic carbonate sediments are deposited on the shallow North Adriatic shelf off the Istrian coast. Only at a greater distance from the coast are these carbonate sediments being mixed with siliceous material brought in by the Alpine rivers Po, Adige, and Brenta. Graphical analysis of grain-size distribution curves shows a sediment composition of normally three, and only in the most seaward area, of four major constituents. Constituent 1 represents the washed-in terrestrial material of clay size (Terra Rossa) from the Istrian coastal area. Constituent 2 consists of fine to medium sand. Constituent 3 contains the heterogeneous biogenic material. Crushing by organisms and by sediment eaters reduces the coarse biogenic material into small pieces generating constituent 2. Between these two constituents there is a dynamic equilibrium. Depending upon where the equilibrium is, between the extremes of production and crushing, the resulting constituent 2 is finer or coarser. Constituent 4 is composed of the fine sandy material from the Alpine rivers. In the most seaward area constituents 2 and 4 are mixed. The total carbonate content of the samples depends on the distance from the coast. In the near coastal area in high energy environments, the carbonate content is about 80 %. At a distance of 2 to 3 km from the coast there is a carbonate minimum because of the higher rate of sedimentation of clay-sized terrestrial, noncarbonate material at extremely low energy environments. In an area between 5 and 20 km off the coast, the carbonate content is about 75 %. More than 20 km from the shore, the carbonate content diminishes rapidly to values of about 30 % through mixing with siliceous material from the Alpine rivers. The carbonate content of the individual fractions increases with increasing grain-size to a maximum of about 90 % within the coarse sand fractions. Beyond 20 km from the coast the samples show a carbonate minimum of about 13 % within the sand-size classes from 1.5 to 0.7 zeta¬? through mixing with siliceous material from the alpine rivers. By means of grain-size distribution and carbonate content, four sediment zones parallel to the coast were separated. Genetically they are closely connected with the zonation of the benthic fauna. Two cores show a characteristic vertical distribution of the sediment. The surface zone is inversely graded, that means the coarse fractions are at the top and the fine fractions are at the bottom. This is the effect of crushing of the biogenic material produced at the surface by predatory organisms and by sediment eaters. lt is proposed that at a depth of about 30 cm a chemical solution process begins which leads to diminution of the original sediment from a fine to medium sand to a silt. The carbonate content decreases from about 75 % at the surface to 65 % at a depth of 100 cm. The increase of the noncarbonate components by 10 % corresponds to a decrease in the initial amount of sediment (CaC03=75 %) by roughly 30 % through solution. With increasing depth the carbonate content of the individual fractions becomes more and more uniform. At the surface the variation is from 30 % to 90 %, at the bottom it varies only between 50 % and 75 %. Comparable investigations of small-bay sediments showed a c1ear dependence of sediment/faunal zonation from the energy of the environment. The investigations show that the composition and three-dimensional distribution of the Istrian coastal sediments can not be predicted only from one or a few measurable factors. Sedimentation and syngenetic changes must be considered as a complex interaction between external factors and the actions of producing and destroying organisms that are in dynamic equilibrium. The results obtained from investigations of these recent sediments may be of value for interpreting fossil sediments only with strong limitations.

Stable oxygen isotope record and age determination of sediments from the North Atlantic Ocean

Sea surface temperature and salinity estimates reconstructed using planktonic foraminiferal abundance and delta18O records from core SU90-03 (40°N, 32°W, 2475 m water depth) reveal large climatic fluctuations linked to major instabilities in Northern Hemisphere ice sheets over the last 150 000 years. Episodes of enhanced ice rafted detritus (IRD) input were accompanied by discrete temperature minima, representing coolings of between 4 and 8°C, and reductions in surface salinity of up to 2.5-3.5 per mil. Several additional cooling episodes of a similar magnitude were documented during intervals of low IRD input that appear to be synchronous, within the limits of dating, with ice rafting events spatially confined to higher latitudes. Accelerator mass spectrometer 14C dates for Heinrich events (H1 - 14.2 ka, H2 - 21.4 ka, H3 - 26.7 ka, H4 - 34.8 ka, H5 - 47.2 ka) obtained from core SU90-03 agree well with other published age estimates and suggest a contemporaneous pattern of climate change throughout the North Atlantic during the last glacial period. This interpretation is supported by a comparison of IRD and palaeotemperature records from DSDP site 609 and core SU90-03, which clearly shows that the major climatic fluctuations identified at high latitudes were transmitted toward the subtropics. However, 14C dates suggest that ice rafting episodes may be diachronous to some extent. The northward migration of the polar front after the H1 event at 40°N in the mid-Atlantic occurred at 14 ka, approximately 500 years earlier than along the Portuguese margin, where the southerly advection of polar waters persisted within eastern boundary current system.

High-resolution isotope stratigraphy of ODP Site 184-1144

Narrow-spaced oxygen and carbon stable isotope records of the planktonic foraminifer Globigerinoides ruber (white) were obtained at Ocean Drilling Program Leg 184 Site 1144 to establish a first record of high-resolution Pleistocene monsoon variability on orbital to centennial timescales in the northern South China Sea. The new records extend from the Holocene back to marine isotope Stage (MIS) 34 (1.1 Ma). Sedimentation rates average 0.56 m/k.y. for the upper Matuyama and Brunhes Chrons and increase to 1.8 m/k.y. over the last 100 k.y. Stable isotope records thus reach an average time resolution of 270-500 yr for the last 375 k.y. and 570 yr further back to 700 ka. On the other hand, major stratigraphic gaps were identified for peak warm Stages 5.5, 7.5 (down to 8.4), 11.3, and 15.5. These gaps probably resulted from short-lasting events of contour current erosion induced by short-term enhanced incursions of Upper Pacific Deep Water near the end of glacial terminations. A further major hiatus extends from MIS 34 to MIS 73(?). The long-term variations in monsoon climate were largely dominated by the 100-k.y. eccentricity cycle. Planktonic delta13C values culminated near 30, 480, and 1035 ka and reflect an overlying 450-k.y. eccentricity cycle of minimum nutrient concentrations in the surface ocean. Superimposed on the orbital variations, millennial-scale cycles were prominent throughout the last 700 k.y., mainly controlled by short-term changes in monsoon-driven precipitation and freshwater input from mainland China. During the last 110 k.y. these short-lasting oscillations closely match the record of 1500-yr Dansgaard-Oeschger climate cycles in the Greenland ice core record.