H.C. de Stigter

The depth dependency of planktonic/benthic foraminiferal ratios: Constraints and applications

Abstract Study of the ratio between planktonic and benthic foraminifera in a great number of areas shows that variation of this ratio with depth can be described using organic matter flux equations. Essentially, the share of benthic foraminifera in the total association is inversely proportional to depth, as is the amount of organic matter resulting from primary productivity reaching the sea bottom. Variation in the P B ratio from area to area appears to be strongly dependent on the amount of inbenthic living foraminifera. If the ratios are corrected for this, the regressions between the proportion of planktonic foraminifera (%P) and depth appear to be near-identical in the Gulf of Mexico, the Gulf of California, the west coast of the USA and the Adriatic Sea. This regression can be described by: Depth = e (3.58718+(0.03534%P)) A study of the behaviour of this function in nine cores from the Adriatic Upper Quaternary reveals that the palaeodepth reconstructions are not influenced by fluctuations in productivity and temperature but that they are significantly influenced by redeposition of fine-grained deposits.

200 Year interruption of Holocene sapropel formation in the Adriatic Sea

An interruption of Holocene sapropel S1 is found in cores from various subbasins of the eastern Mediterranean. In core IN68-9 from the Adriatic Sea, sapropel S1 is dated between 8300 and 6340 BP, interrupted between 7100 and 6900 BP (14C years uncorrected for reservoir age). Lithology and variations in the foraminiferal faunas suggest that the interruption is genuine, and not the result of resedimentation. The results indicate that S1 was deposited within a period of enhanced levels of productivity (resulting from increased seasonal contrasts) which started around 9300 BP and ended around 5200 BP. The onset, interruption, and final ending of S1 deposition in the Adriatic Sea, however, appear to have been triggered by changes in ventilation of the basin related to changes in sea surface temperature (SST). Although the rough estimates of SST change are relatively small (< 2°C), they still are significant when compared with the relative SST changes considered necessary to upset convection in the Adriatic. Moreover, recent studies show that the influence of the inferred temperature changes should be viewed in combination with that of reduced salinities due to (1) the deglaciation, and (2) increased humidity in the eastern Mediterranean area during the deposition of S1. The lithological and benthic foraminiferal evidence that sapropel formation in the Adriatic Sea ended around 6340 BP contrasts with the conclusion from a recent geochemical study that sapropel formation in the open eastern Mediterranean would have ended as late as 5000 BP. More significantly, the results of the present study combined with other reports on sapropel interruptions suggest that the process of sapropel formation is not a very stable mode in the basin, but that it may be relatively easily interrupted in response to subtle rearrangements in the balance between productivity and, especially, deep water ventilation.

Recent sediment transport and accumulation on the NW Iberian margin

Abstract Five transects across the NW Iberian margin were studied in the framework of the EU-funded Ocean Margin EXchange II (OMEX II) project, to determine and establish recent sediment and organic carbon transport and accumulation processes and fluxes. On the Galician shelf and shelf edge, resuspension of sediments resulting in well-developed bottom nepheloid layers was observed at all stations, but transport of suspended sediment appears largely confined to the shelf. On the continental slope, only very dilute bottom nepheloid layers were present, and intermediate nepheloid layers were only occasionally seen. This suggests that cross-slope transfer of particles is limited by the prevailing northerly directed shelf and slope currents. Optical backscatter and ADCP current measurements by the BOBO lander, deployed at 2152 m depth on the Galician slope, indicated that particles in the bottom boundary layer were kept in suspension by tidal currents with highest speeds between 15–25 cm s−1. Net currents during the recording period August 6th–September 10th 1998, were initially directed along-slope toward the NNW, but later turned off-slope toward the SW. The separation of the water masses on the slope from the sediment-laden shelf water by the along-slope current regime is reflected in the recent sedimentary deposits of the Galician shelf and slope. Apart from compositional differences, shelf deposits differ from those on the slope by their higher flux of excess 210Pb (0.57–5.37 dpm cm−2y−1 versus 0.11–3.00 dpm cm−2y−1), a much higher sediment accumulation rate (315.6–2295.9 g m−2y−1 versus 10.9–124.7 g m−2y−1) and organic carbon burial rate (1.01–34.30 g m−2y−1 versus 0.01–0.69 g m−2y−1). In contrast to the observations on the Galician margin, pronounced nepheloid layers occurred in the Nazare Canyon, which extended to considerably greater water depths. This indicates that significantly greater transport of fine-grained particles in both the INL and the BNL was occurring within the canyon, as reflected in the exceptionally high 210Pb excess flux (up to 34.09 dpm cm−2y−1), mass accumulation rates (maximum 9623.1 g m−2y−1) and carbon burial fluxes (up to 180.91 g m−2y−1) in the sediment. However, radioisotope fluxes in the lower canyon were only slightly higher than at comparable depths on the Galician margin. This suggests that transport and rapid accumulation is focused on the upper and middle part of the canyon, from where it is episodically released to the deep sea. Compared to the Galician margin, the Nazare Canyon may be considered as an important organic carbon depocenter on short time-scales, and a major conduit for particulate matter transport to the deep sea on >100 y time-scales.

Structure and development of giant carbonate mounds at the SW and SE Rockall Trough margins, NE Atlantic Ocean

Abstract High-resolution seismic reflection profiling carried out in 1997–1999 showed that giant carbonate mounds occur between 500 and 1200 m water depth along both the SE and SW margins of Rockall Trough. The mounds rise 5–300 m above the surrounding seafloor and have diameters at their bases of up to 5 km. Buried mounds, at relatively shallow depths below the seafloor, are also found. Both individual and complex clusters of mounds can be recognized. Smaller and individual, sometimes buried mounds are found at the upper slope. On the SW Rockall Trough margin, higher, steeper and individual mounds are found deeper downslope (900–1100 m). At the middle slope the mounds merge into a complex structure and form complex clusters with a very irregular upper surface and an apparent lack of internal reflectors (600–1000 m depth). Initial results of high-resolution 3D-seismic profiling at 30 m line spacing in August 2000 in two boxes of 750 m×14.5 km covering a small, central part of the SW Rockall Trough mound area indicate two stages of mound development here, the latest probably of Pliocene–Holocene age. This age, based on the presence of a regionally recognizable unconformity below the mounds, is also assumed for the mounds of the SE Rockall Trough (Porcupine Bank) margin.

The role of the benthic biota in sedimentary metabolism and sediment-water exchange processes in the Goban Spur area (NE Atlantic)

We provide an overview of the role of biological processes in the Benthic boundary layer (BBL) and in sediments on the cycling of particulate organic material in the Goban Spur area (Northeast Atlantic). The benthic fauna, sediment and BBL characteristics were studied along a transect ranging from 208 to 4460 m water depth in different seasons over 3 years. Near-bottom flow velocities are high at the upper part of the slope (1000–1500 m), and high numbers of filter-feeding taxa are found there such that organic carbon normally passing this area during high flow conditions is probably trapped, accumulated, and/or remineralised by the fauna. Overall metabolism in shelf and upper slope sediments is dominated by the macrofauna. More than half of the organic matter flux is respired by macrofauna, with a lower contribution of metazoan meiofauna (4%) and anoxic and suboxic bacterial mineralisation (21%); the remainder (23%) being channelled through nanobiota and oxic bacteria. By its feeding activity and movement, the macrofauna intensely reworks the sediments on the shelf and upper slope. Mixing intensity of bulk sediment and of organic matter are of comparable magnitude. The benthos of the lower slope and abyssal depth is dominated by the microbiota, both in terms of total biomass (>90%) and carbon respiration (about 80%). The macrofauna (16%), meiofauna (4%) and megafauna (0.5%) only marginally contribute to total carbon respiration at depths below 1400 m. Because large animals have a lower share in total metabolism, mixing of organic matter within the sediments is reduced by a factor of 5, whereas mixing of bulk sediment is one to two orders of magnitude lower than on the shelf. The food quality of organic matter in the sediments in the shallowest part of the Goban Spur transect is significantly higher than in sediments in the deeper parts. The residence time of mineralisable carbon is about 120 d on the shelf and compares well with the residence time of the biota. In the deepest station, the mean residence time of mineralisable carbon is more than 3000 d, an order of magnitude higher than that of biotic biomass.

Europe’s Grand Canyon: Nazaré submarine canyon

The Nazare submarine canyon extends similar to 210 km westward from the coast of Portugal, down to a water depth of > 4300 m. The considerable habitat heterogeneity found throughout the canyon is affected by strong currents and high turbidity, especially in the upper parts of the canyon. The canyon morphology comprises steep slopes, scarps, terraces, and overhangs, and a deeply incised thalweg is found in the lower part of the canyon. The seabed within the canyon is composed of varying proportions of rock and sediments that range from sand to fine mud. This great variation in physical environment is reflected by the varied fauna inhabiting the canyon. Diversity tends to decrease with depth, but there is also continual replacement of species with increasing water depth. Certain groups, such as the gorgonians and sea lilies, tend to be found on rocky surfaces, while large protozoans dominate the sediments at 3400-m depth. In addition to describing the fauna of Nazare Canyon, we discuss experiments undertaken as part of the HERMES project to elucidate the ecosystem function processes operating in the deeper parts of the canyon.

Feeding types of the benthic community and particle transport across the slope of the N.W. European Continental Margin (Goban Spur)

Densities and biomass of feeding guilds of benthic foraminifera, macrofauna and megafauna were estimated at seven stations ranging from 208 m to 4460 m water depth along the OMEX-transect at the continental margin of the Goban Spur N.E. Atlantic. At the same stations flow velocities in the Bottom Boundary Layer (BBL) were measured at 30 cm height above the bottom. Overall densities of all three faunal groups decreased with increasing water depth, but a peak in density and biomass of suspension-feeding taxa was observed in all groups at similar to 1000-1500 m water depth. At these depths the highest how velocities were measured in all seasons of the year. At station II at 1470 m flow velocities of similar to 35 cm s(-1) were measured during autumn/winter, but in spring/summer flow velocities did not exceed 10 cm s(-1), but were still highest at this station. At this station a very high biomass of suspension feeders was found within the megafauna (mainly sponges), high densities of Astrorhizid foraminifera and high densities of hydrozoids, sponges and tunicates within the macrofauna. At all other stations deposit feeders predominate and much lower flow velocities occurred. It was concluded that a high load of (re)suspended material at similar to 1470 m water depth provide good feeding conditions for suspension feeders and hence that flow velocities are important in structuring the benthic community. These high numbers of suspension feeders, on the other hand, actively capture particles that would otherwise have been transported past this highly energetic region and the relative high numbers of surface- and interface-feeding infauna then bury them in the sediment. Feeding and tube structures seen on the sediment surface can locally change the flow velocities and cause resuspension and passive biodeposition of particles. [KEYWORDS: Seabight northeast atlantic; area ne atlantic; deep-sea floor; boundary-layer; vertical-distribution; seasonal deposition; sediment transport; particulate matter; foraminifera; phytodetritus]

Recent sediments, sediment accumulation and carbon burial at Goban Spur, N.W. European Continental Margin (47–50°N)

To quantify recent sediment accumulation, carbon fluxes and cycling, three N.W. European Continental Margin transects on Goban Spur and Meriadzek Terrace were extensively studied by repeated box- and multicore sampling of bottom sediments. The recent sediment distribution and characteristics appear directly related to the near-bed hydrodynamic regime on the margin, which at the upper slope break on the Goban Spur results in along-slope and periodic off-slope directed transport of particles, possibly by entrainment of particles in a detached bottom or intermediate nepheloid layer. From the shelf to the abyssal plain the surface sediments on the Goban Spur change from terrigenous sandy shelf sediments into clayey silts. 210Pb activity decreases exponentially down core, reaching a stable background value at 10 cm (shallower stations) to 5 cm (deeper stations) sediment depth. 210Pb profiles of repeatedly sampled stations indicate negligible annual variability of mixing and flux. The 210Pbxs flux to the sediment shows a decreasing trend with increasing water depth. Below about 2000 m the average 210Pbxs flux is about 0.3 dpm cm−2 y−1, a third of the fluxes measured on the shelf and upper slope stations. Sediment mixing rates (Db) correlate with macro- and meiofaunal density changes and are within the normal oceanic ranges. Lower mixing rates on the lower slope likely reflect lower organic carbon fluxes there. Mass accumulation rates on Meriadzek Terrace are at maximum 80 g m−2 y−1, almost twice as high as at Goban Spur stations of comparable depth. A minimum accumulation rate of 16.6 g m−2 y−1 is found at the Goban Spur upper slope break. Organic carbon burial rates are low compared to other margins and range from a lowest value of 0.05 g m−2 y−1 at the upper slope break to 0.11 g m−2 y−1 downslope. A maximum organic carbon burial rate of 0.41 g m−2 y−1 is found on Meriadzek Terrace. Carbonate burial rates increase along the northern transect from the shelf (13 g m−2 y−1) via a low (9.3 g m−2 y−1) on the upper slope break to the deep sea (30.7 g m−2 y−1). Carbonate burial is highest on Meriadzek Terrace (44.5 g m−2 y−1). The N.W. European Margin at Goban Spur and Meriadzek Terrace cannot be considered a major carbon depocenter.

Differential rates of benthic foraminiferal test production in surface and subsurface sediment habitats in the southern Adriatic Sea

Abstract Live communities and corresponding dead assemblages of benthic foraminifera recovered in box cores from the southern Adriatic Sea differ significantly with respect to the relative abundances of species and higher-order groups of foraminifera. Taphonomic destruction of disintegration-prone arenaceous species is held responsible for the observed strong reduction in relative abundance of arenaceous foraminifera in the dead assemblage. Systematic differences in live and dead relative abundances of calcareous and calcite-cemented arenaceous species may be due to differential rates of test production rather than selective taphonomic destruction. We observed that epifaunal or shallow-infaunal foraminifera are often overrepresented in the dead assemblage, compared to their relative number in the live community, whereas deeper-infaunal species tend to be underrepresented. On this basis we hypothesize that test production rate of benthic foraminiferal species may be related to their living depth in the sediment.

Efficient burial of carbon in a submarine canyon

Burial of organic carbon (OC) in marine sediments moderates atmospheric CO2 levels on geological time scales, but uncertainties remain about how much OC is buried and about the efficiency of OC burial, particularly in heterogeneous seafloor environments such as ocean margins. Here we describe OC burial in Nazare submarine canyon and the adjacent continental slope off Portugal, an area within which sedimentation rates vary by three orders of magnitude. Using a nested series of observations at different scales, ranging from regional bathymetry to sediment cores, we estimate the annual sediment and OC deposited in the canyon at 620,000 t and 12,500 t, respectively. Nazare Canyon is thus a significant sink of both sediment and OC. Canyon sediments typically contain ~2% OC, both in surface sediments and at depth, and there is a limited correlation between sedimentation rate and OC content. The likely explanation is that the OC has already survived a lengthy period of degradation prior to deposition in the canyon, such that additional exposure to oxygenated water has minimal effect. Burial efficiency is difficult to calculate because of extensive resuspension and reworking of OC in the upper canyon, but probably exceeds 30% in areas of high sedimentation. These areas are shown to be 30 times more effective in burying OC than adjacent areas of the continental slope, indicating that Nazare Canyon is a hitherto overlooked sink of OC on a continental margin where OC burial is otherwise low.