Planktonic foraminiferal abundance and flux in the Porcupine Seabight, Northeast Atlantic

Modern planktonic foraminifera collected with a sediment trap and subfossil assemblages from surface sediments from Galway Mound in the Porcupine Seabight off southwestern Ireland, northeastern Atlantic, were studied to show recent assemblage variations. The sediment trap operated from April to August 2004 and covers the spring bloom and early summer conditions with sampling intervals of 8 days. Eleven different species were recorded. Glorotalia hirsuta, Turborotalita quinqueloba and Globigerinita glutinata appeared predominately in spring. Neogloboquadrina incompta, Globigerina bulloides and Globorotalia inflata were abundant in spring and summer. The highest foraminiferal tests flux occured in June. The faunal composition was similar to subfossil assemblages from surface sediments, but the species proportions were different. This was mainly affected by the subtropical G. hirsuta, which was frequent in 2004 and rare in surface sediment samples and in earlier plankton collections from the southern Porcupine Seabight that were performed during the 1990s. The weight of deposited foraminifera is mainly influenced by spring bloom as indicated by sea-surface chlorophyll-a data. The top three-ranked species, G. hirsuta, N. incompta and G. bulloides contributed 87 % to the foraminiferal carbonate flux at Galway Mound. Foraminiferal carbonate and shell flux as well as the shell size revealed variations, which are related to lunar periodicity. The data infer a lunar pacing of reproduction for the main species as well as for G. glutinata and G. inflata, which was not recorded before.

Methane, helium isotope and hydrogen data from the Nibelungen hydrothermal area, Mid Atlantic Ridge

Hydrothermal emission of mantle helium appears to be directly related to magma production rate, but other processes can generate methane and hydrogen on mid-ocean ridges. In an on-going effort to characterize these processes in the South Atlantic, the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8°17.9' S, 13°30.4' W. The vent lies on the shoulder of an oblique offset in the Mid-Atlantic Ridge and discharges high concentrations of methane and hydrogen. Measurements during expeditions in 2004 and 2006 show that the ratio of CH4 to 3He in the neutrally buoyant plume is quite high, 4 x 10**8. The CTD stations were accompanied by velocity measurements with lowered acoustic Doppler current profilers (LADCP), and from these data we estimate the methane transport to have been 0.5 mol/sec in a WSW-trending plume that seems to develop during the ebb tidal phase. This transport is an order of magnitude greater than the source of CH4 calculated from its concentration in the vent fluid and the rise height of the plume. From this range of methane fluxes, the source of 3He is estimated to be between 0.14 and 1.2 nmol/sec. In either case, the 3He source is significantly lower than expected from the spreading rate of the Mid-Atlantic Ridge. From the inventory of methane in the rift valley adjacent to the vent, it appears that the average specific rate of oxidation is 2.6 to 23/yr, corresponding to a turnover time between 140 and 16 days. Vertical profiles of methane in the surrounding region often exhibited Gaussian-like distributions, and the variances appear to increase with distance from the vent. Using a Gaussian plume model, we obtained a range of vertical eddy diffusivities between 0.009 and 0.08 m2m2/sec. These high values may be due to tidally driven internal waves across the promontory on which the vent is located.

Planktic foraminiferal populations and test flux in the eastern North Atlantic Ocean

Planktic foraminiferal assemblages vary in response to seasonal fluctuations of hydrographic properties, between water masses, and after periodical changes and episodic events (e.g. reproduction, storms). Distinct annual variability of the planktic foraminiferal flux is also known from sediment trap data. In this paper we discuss the short-term impacts on interannual flux rates based on data from opening-closing net hauls obtained between the ocean surface and 500 m water depth. Data were recorded during April, May, June, and August at around 47°N, 20°W (BIOTRANS) in 1988, 1989, 1990, 1992, 1993, and during May 1989 and 1992 at 57°N, 20-22°W. Species assemblages closely resemble each other when comparing the mixed layer fauna with the fauna of the upper 100 m and the upper 500 m of the water column. In addition, species assemblages >100 µm are almost indistinguishable from assemblages that are >125 µm in test size. The standing stock of planktic foraminifers at BIOTRANS can vary by more than one order of magnitude over different years; however, species assemblages may be similar when comparing corresponding seasons. Early summer assemblages (June) are distinctly different from late summer assemblages (August). Significant variations in the species composition during spring (April/May) are independent of the mixed layer depth. Spring assemblages are characterized by high numbers of Globigerinita glutinata. In particular, day-to-day variations of the number of specimens and in species composition may have the same order of magnitude as interannual variations. This appears to be independent of the reproduction cycle. Species assemblages at 47°N and 57°N are similar during spring, although surface water temperatures and salinities differ by up to 10°C and 0.7 (PSU). We suggest that the main factors controlling the planktic foraminiferal fauna are the trophic properties in the upper ocean productive layer. Planktic foraminiferal carbonate flux as calculated from assemblages reveals large seasonal variations, a quasi-annual periodicity in flux levels, and substantial differences in timing and magnitude of peak fluxes. At the BIOTRANS station, the average annual planktic foraminiferal CaCO3 fluxes at 100 and 500 m depth are estimated to be 22.4 and 10.0 g/m**2/yr, respectively.

Organic carbon flux through the benthic community in the temperate abyssal Northeast Atlantic

In order to assess the carbon flux through the deep-sea benthic boundary layer, sediment community oxygen consumption (SCOC) was measured in different months and years at the BIOTRANS area in the abyssal northeastern Atlantic. SCOC varied seasonally with a maximum in July/August. Evidence is given for a direct coupling between a substantial sedimentation of phytodetritus and the seasonal increase in SCOC. Rapid colonization, growth and decomposition rates indicate that the deep-sea benthic microbial and protozoan biota can react quickly to substantial falls of particulate organic matter. They seem to be the most important groups to generate seasonal changes in deep-sea benthic carbon flux rates.