Physical and chemical oceanography in the Arctic Ocean

Data from sections across the Eurasian Basin of the Arctic Ocean occupied by the German Research Vessel Polarstern in 1987 and by the Swedish icebreaker Oden in 1991 are used to derive information on the freshwater balance of the Arctic Ocean halocline and on the sources of the deep waters of the Nansen, Amundsen and Makarov basins. Salinity, d18O and mass balances allow separation of the river-runoff and the sea-ice meltwater fractions contained in the Arctic halocline. This provides the basis for tracking the river-runoff signal from the shelf seas across the central Arctic Ocean to Fram Strait. The halocline has to be divided into at least three lateral regimes: the southern Nansen Basin with net sea-ice melting, the northern Nansen Basin and Amundsen Basin with net sea-ice formation and increasing river-runoff fractions, and the Canadian Basin with minimum sea-ice meltwater and maximum river-runoff fractions and water of Pacific origin. In the Canadian Basin, silicate is used as a tracer to identify Pacific water entering through Bering Strait and an attempt is made to quantify its influence on the halocline waters of the Canadian Basin. For this purpose literature data from the CESAR and LOREX ice camps are used. Based on mass balances and depending on the value of precipitation over the area of the Arctic Ocean the average mean residence time of the river-runoff fraction contained in the Arctic Ocean halocline is determined to be about 14 or 11 years. Water column inventories of river-runoff and sea-ice meltwater are calculated for a section just north of Fram Strait and implications for the ice export rate through Fram Strait are discussed. Salinity, tritium, 3He and the d18O ratio of halocline waters sampled during the 1987 Polarstern cruise to the Nansen Basin are used to estimate the mean residence time of the river-runoff component in the halocline and on the shelves of the Arctic Ocean. These estimates are done by comparing ages of the halocline waters based on a combination of tracers yielding different time information: the tritium 'vintage' age which records the time that has passed since the river-runoff entered the shelf and the tritium/3He age which reflects the time since the shelf waters left the shelf. The difference between the ages determined by these two methods is about 3 to 6 years. Correction for the initial tritium/3He age of the shelf waters (about 0.5 to 1.5 years) yields a mean residence time of the river-runoff on the shelves of about 3.5 ± 2 years. Comparison of the 18O/16O ratios of shelf water, Atlantic water and the deep waters of the Arctic Ocean indicate that the sources of the deep and bottom waters of the Eurasian Basin are located in the Barents and Kara seas.

Pelagic phosphorus accumulation in the Pacific Ocean

As a limiting nutrient to marine life, phosphorus (P) is an effective tracer of today's marine productivity. The distribution of P in marine sediments likewise tracks the history of marine productivity because of its relative insolubility in seawater. CaCO3, biogenic opal, terrigenous sediment, and total P have been measured in cores from nine Pacific sites (Deep Sea Drilling Project (DSDP) 65, 66, 310, 77, 62, 572, 463, 586, and GPC-3) and one subantarctic (DSDP 266) site. These sites were specifically chosen to provide information on biota burial flux changes with time for sedimentary sinks that represent key oceanographic variables, i.e., rate of upwelling, water depth, and carbonate dissolution gradient. The accumulation rates of these components for the last 10 Ma were then calculated from determined core age versus depth plots, core bulk density, and porosity data. The accumulation of P weakly correlates with that of CaCO3, moderately with that of total sediment, and very strongly with carbonate-free accumulation. Two prominent peaks for all components occur at 2-3 Ma and 5-6 Ma, and record the chemical loading of dissolved CaCO3, SiO2, and P from glacially emergent continental shelves. These results indicate that continental shelf phosphorites form during interglacially high sea levels and correspond to low deep-sea P accumulation rates, whereas glacially lowered sea levels allow for shelf bypassing and greater deep-sea P accumulation rates.

Barium in seawater and sediments from the Artic Ocean

As part of ongoing circulation studies in the Arctic, seawater samples for dissolved Ba concentrations were obtained during Sep.-Oct., 1992 at several locations in the Bering Strait, Eastern Chukchi and Southern Beaufort Seas. The results reveal a dynamic rang (10 to 150 nmol/kg) for this element in the Arctic equal to or greater than that in combined Atlantic, Indian and Pacific oceans. Lowest levels are observed in surface waters, with values tending to decrease northwards in the direction of currents generally flowing frorn the Bering Strait along the Alaskan coast. Low surfacc concentrations tend to be accompanied by relatively enriched near bottom levels. On the basis of these spatial distributions, hydrographic observations and a knowledge of its behavior in other marine settings, it appears that Ba can be significantly depleted from surface waters as a result of the highly seasonal biological aclivities over Arctic marginal shelves. Removal at the surface is counteracted to some extent by regeneration at depth or in the sediments and by riverine inputs. The biologically related drawdown is likely to enhance the contrast between 'background' surface Ba levels in the Arctic and waters imprinted by regeneration and/or rivers, These preliminary findings suggest that Ba holds particular promise for tracing river waters and the ventilation of halodine waters hy laterally sinking brines produced during ice formation over the shelves.

Composition of hydrothermal manifestations in the World Ocean

The monograph summarizes results of studies of hydrothermal fields on the ocean floor, hydrothermal plumes and metalliferous sediments. Hydrothermal ore manifestations formed in different geodynamic settings, with different character of volcanism in different facial conditions of deposition are described. Causes of non-uniformity of hydrothermal system functioning in different parts of the ocean and therefore variability of hydrothermal deposits are under consideration. On the base of found relationships of these irregularities with geodynamics, volcanism and sedimentation a new classification of hydrothermal processes and genetic models of hydrothermal ore formation in the ocean have been created. Regularities of hydrothermal sedimentary material dispersion in bottom waters are discussed.

Radionuclides in sediment cores of the Atlantic sector of the Southern Ocean

We present time series of export productivity proxy data including 230Thex-normalized deposition rates (rain rates) of 10Be, dissolution-corrected biogenic Ba, and biogenic opal as well as authigenic U concentrations which are complemented by rain rates of total (detrital) Fe and sea ice indicating diatom abundances from five sediment cores across the Atlantic sector of the Southern Ocean covering the past 150,000 years. The results suggest that 10Be rain rates and authigenic U concentration cannot serve as quantitative paleoproductivity proxies because they have also been influenced by detrital particle fluxes in the case of 10Be and bulk sedimentation rates (sediment focussing) and deep water oxygenation in the case of U. The combined results of the remaining productivity proxies of this study (rain rates of biogenic opal and biogenic Ba in those sections without authigenic U) and other previously published proxy data from the Southern Ocean (231Pa/230Th and nitrogen isotopes) suggest that a combination of sea ice cover, shallow remineralization depth, and stratification of the glacial water column south of the present position of the Antarctic Polar Front and possibly Fe fertilization north of it have been the main controlling factors of export paleoproductivity in the Southern Ocean over the last 150,000 years. An overall glacial increase of export paleoproductivity is not supported by the data, implying that bioproductivity variations in the Southern Ocean are unlikely to have contributed to the major glacial atmospheric CO2 drawdown observed in ice cores.

Sedimentation at Manop Site H in the eastern equatorial Pacific

Gravity cores recovered from Manganese Nodule Project site H (6°33'N, 92°49'W) show marked downcore variations in the abundance of calcium carbonate, organic carbon, opal, manganese, and other components deposited over the past 400,000 years. Variations in the downcore abundance of organic carbon, which ranges from 0.2 to 1.0%, can be used to hindcast redox conditions in the surface sediments over this time. The results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles. Downcore variations in solid phase Mn, Ni, and Cu can be produced by such changes in redox conditions. A model which predicts that solid phase Mn can be trapped and buried when the Mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and Mn records and supports the reconstructed redox variations. The history of redox variations at site H can be explained by changes with time in surface water productivity. Major productivity variations at the site occur over 100-kyr cycles, with relatively higher productivity occurring during glacial stages. Thus Quaternary climate changes influence surface water productivity, redox conditions in sediments, and the cycling of transition metals.

Biogenic silica in the coastal plume of the Mississippi River

The surface distributions of dissolved silicic acid, chlorophyll and diatom abundance were measured in the plume of the Mississippi River and adjacent waters during spring (late April and early May 1993) and summer (July 1992). In spring, the time of maximum river flow, there was an intense diatom bloom with a mean diatom abundance of 1.5 x 10**7 cells/l, more than an order of magnitude higher than in summer. Mixing curves of silicic acid concentration ([Si(OH)4]) versus salinity indicate that biological uptake within the river plume removed >99% of the Si(OH)4 supplied by the river in spring and 80 to 95% in summer. In spring [Si(OH)4] was occasionally depleted to <0.2 µM-among the lowest values ever reported from the ocean-with extensive depletion to >=0.5 µM over the shelf. In summer [Si(OH)4] was less severely depleted; the lowest measured was 0.93 µM and all others were >=2.4 µM. 30Si kinetic experiments were performed during both spring and summer to measure the degree to which the rate of Si uptake by the natural diatom assemblages was limited in situ by substrate availability. In spring the dependence of the specific uptake rate (V) on extracellular [Si(OH)4] conformed much more closely to the Michaelis-Menten saturation function than has been observed in past studies. Strong dependence of V on [Si(OH)4] was observed throughout the most Si(OH)4-depleted (<0.5 µM) region, where V was limited to 12 to 45% of the diatom assemblages' maximum uptake rate (Vmax). Half-saturation concentrations for Si uptake (Ks) averaged 0.85 uM (range = 0.48 to 1.71; n = 7) in spring, with the lowest values equal to the lowest previously reported for natural diatom assemblages. There was only 1 station in summer where V was limited by [Si(OH)4], and at that station Ks was 5.3 µM-quite high in comparison with previous studies. At stations where V was limited by [Si(OH)4], in both spring and summer, Chaetoceros spp. were numerically dominant; where there was no Si limitation other diatoms, usually Skeletonema costatum, dominated. The data thus indicate strong Si limitation in spring, with diatom assemblages well adapted to low [Si(OH)4], but little or no Si limitation in summer. Historical data suggest that coastal Si(OH)4 depletion and Si limitation may be recent phenomena in the northern Gulf of Mexico, resulting from increasing [NO3-] and decreasing [Si(OH)4] in the Mississippi River during the past 30 to 50 yr.

(Table 2) Annual mass fluxes, 230Th composition and organic geochemistry of sediment trap samples in the South Altantic

The scavenging of 231Pa and 230Th was investigated in the Atlantic Sector of the Southern Ocean by combining results from sediment trap and in situ filtration studies. We present the first high-resolution profile of dissolved 230Th and 231Pa in surface waters across the ACC, showing a dramatic southward increase of both radionuclides around the southern ACC Front at 533S. High dissolved 231Pa/230Th ratios combined with low 230Th/231Pa fractionation factors (F) in these surface waters result in extremely high 231Pa94/230Th94 ratios of material collected in the shallow traps. Particulate 231Pa94/230Th94 ratios in a shallow trap near Bouvet Island increase continuously during the productive period in austral summer, and drop back in the low flux period. This behavior, following the Rayleigh fractionation principle, is interpreted to be due to an increase in the dissolved 231Pa/230Th ratio in the euphotic zone resulting from preferential scavenging of 230Th relative to 231Pa, even in opal-dominated regions. In the post-bloom stage, the depleted radionuclide concentrations are replenished by upwelling of Circumpolar Deep Water. The high particulate 231Pa94/230Th94 signal is weakened during downward transport of the bloom particles in the water column by incorporation of deep suspended particles, which have a lower 231Pa94/230Th94 ratio. It is shown that under the special hydrographic conditions in the Southern Ocean scavenging from the upper water column significantly influences the budgets of 230Th and 231Pa in the sediment. Nevertheless, the budgets are still made up primarily by scavenging from the large standing stock of deep suspended particles.

Flux data in the Greenland Basin

Pelagic processes and their relation to vertical flux have been studied in the Norwegian and Greenland Seas since 1986. Results of long-term sediment trap deployments and adjoining process studies are presented, and the underlying methodological and conceptional background is discussed. Recent extension of these investigations at the Barents Sea continental slope are also presented. With similar conditions of input irradiation and nutrient conditions, the Norwegian and Greenland Seas exhibit comparable mean annual rates of new and total production. Major differences can be found between these regions, however, in the hydrographic conditions constraining primary production and in the composition and seasonal development of the plankton. This is reflected in differences in the temporal patterns of vertical particle flux in relation to new production in the euphotic zone, the composition of particles exported and in different processes leading to their modification in the mid-water layers. In the Norwegian Sea heavy grazing pressure during early spring retards the accumulation of phytoplankton stocks and thus a mass sedimentation of diatoms that is often associated with spring blooms. This, in conjunction with the further seasonal development of zooplankton populations, serves to delay the annual peak in sedimentation to summer or autumn. Carbonate sedimentation in the Norwegian Sea, however, is significantly higher than in the Greenland Sea, where physical factors exert a greater control on phytoplankton development and the sedimentation of opal is of greater importance. In addition to these comparative long-term studies a case study has been carried out at the continental slope of the Barents Sea, where an emphasis was laid on the influence of resuspension and across-slope lateral transport with an analysis of suspended and sedimented material.

Particle flux determined from traps in the Canary Island region

We present a 3 year record of deep water particle flux at the recently initiated ESTOC (European Station for Time-series in the Ocean, Canary Islands) located in the eastern subtropical North Atlantic gyre. Particle flux was highly seasonal, with flux maxima occurring in late winter-early spring. A comparison with historic CZCS (Coastal Zone Colour Scanner) data shows that these flux maxima occurred about 1 month after maximum chlorophyll was observed in surface waters in a presumed primary source region 100 km * 100 km northeast of the trap location. The main components of the particles collected with the traps were mineral particles and carbonate, both correlating strongly with organic matter sedimentation. Mineral particles in the sinking matter are indicative of the high aeolian input from the African desert regions. Comparing particle fluxes at 1 km and 3 km depth, we find that particle sedimentation increased substantially with depth. Yearly organic carbon sedimentation was 0.6 g m**-2 at 1 km depth compared with 0.8 g m**-2 at 3 km. We hypothesize that higher phytoplankton biomass observed further north could be a source of laterally advecting particles that interact with fast sinking particles originating from the primary source region. This hypothesis is also supported by the differences in size distribution of lithogenic matter found at the two trap depths.