Steven J. Manganini

Sedimentation of biogenic matter in the deep ocean

The major constituents of biogenic particles that settle through the water column of the ocean are carbonate tests, opaline shells, and particulate organic matter (cellular and amorphous). This paper describes the quality and quantity of such fluxes to the deep ocean and compares them with biogenic remains in the bottom sediment. Studies of samples collected during PARFLUX sediment trap experiments in the Atlantic and Pacific have shown that 60 to 90% of the total particulate flux is of biogenic origin; the contribution of biogenic materials decreases with increasing depth. Carbonate hard parts accounted for 30 to 60% and were the largest constituents in pelagic sediments at temperate and tropical areas. Combustible components ranged from 10 to 30% with zooplankton remains and fecal pellets accounting for the bulk of the organic flux. Amorphous fine particles were also significant in the organic flux. The atomic ratios of organic constituents in settling particles deviated systematically from the ‘Redfield ratio’ of 106:16:1 (C:N:P) for marine plankton. In the mesopelagic depths the ratio was 200:21:1, and in the bathypelagic depths the ratio was 300:33:1, with a wide range of variation in phosphorus. The residence time of biogenic particles in the deep-ocean water columns is relatively short and the particles can be expected to arrive at the abyssal floor without major dissolution and remineralization (excepting small opal particles). Fine organic particles such as cell remains and pigmented granules are a major source of organic carbon and nitrogen at deep traps. Microscopic study suggests that the fine organic particles were once included in larger but loosely-formed particles that settled rapidly. Such particles dispersed from settling fecal pellets or similar particles are reintroduced into the water column resulting in ‘secondary’ suspended particles. Such particles may eventually be remineralized while in suspension. The decomposition rate of organic carbon in the deep water was estimated to be about 2.2 mg C m−2 day−1, a rate consistent with rates of oxygen consumption estimated by other investigators. The bottom sediment is enriched in refractory lithogenic particles because of the remineralization of biogenic matter. A hypothetical benthic transition layer serves as a reservoir of benthic activity and can exist under certain conditions at the abyssal floor.

Export production of particles to the interior of the equatorial Pacific Ocean during the 1992 EqPac experiment

Abstract Twenty-four time-series, moored sediment traps were deployed between 2/2/92 and 1/27/93 along 140°W at 9°N, 5°N, 2°N, 0°, 2°S, 5°S and 12°S at water depths of approximately 1200 m and 2200 m, and 700 m above the bottom. The opening/closing of the traps was synchronized at 17-day periods, for 21 events, covering a total of 357 days. The average annual particle flux in the oceans interior (2.2 to 4.4 km deep) from 5°N to 5°S was 28.5 g m−2 year−1, with 34.8 g−2 year−1 the maximum annual flux at the equator. Sixty-six per cent of settling particles were carbonate; 24% biogenic SiO2 and 5% organic carbon. The onset of tropical instability waves, marking the years El Nino/post-El Nino boundary, was associated with a succession of intervals with greater organic carbon and opal at 5°N, 2°S and 5°S that occurred synchronously with a meridional oscillation of instability waves, while net carbon flux during El Nino and post-El Nino periods did not change. Although organic carbon flux increased at 5°N, 2°S and 5°S during the post-El Nino period, it was counterbalanced by decreases at the upwelling stations (2°N and the equator), resulting in no net carbon flux increase across the 5°N to 5°S region. In February/March 1992, only 0.34% of the organic carbon fixed by primary production over the 5°N to 5°S zone arrived in the oceans interior. In August/September that year, zonal average of organic carbon flux increased slightly to 0.5% of primary production. Very little carbon reached the interior depths of the upwelling stations; however, the fraction of export was higher at the 5°N, 2°S and 5°S stations. The pattern of variability of particle flux at the shallow depths was observed also in deeper traps, without temporal offsets, suggesting a settling particle residence time shorter than the 17-day timeseries resolution during most of this experiment.

Annual biogenic particle fluxes to the interior of the North Atlantic Ocean; studied at 34°N 21°W and 48°N 21°W

Abstract In order to clarify the annual quality, quantity and export processes of biogenic particles from the euphotic zone to the deep ocean interior, an array of automated time-series sediment traps were deployed for 1 year from 4 April 1989 to 17 April 1990 at 34°N 21°W and 48°N 21°W as part of the Joint Global Ocean Flux Program (JGOFS) North Atlantic Bloom Experiment (NABE). Three sediment traps with 13 time-series sediment collectors were placed at both stations approximately 1 and 2 km below the surface and 0.7 km above the bottom. They collected settling particles during 26 14-day intervals for 376 days with an 20-day hiatus in September-October 1989 for changeover of the trap moorings. The collection periods of the six traps were synchronized, forming a spatio-temporal matrix of 156 samples. The annual mass flux at about 2 km deep during this experiment was 22 and 27 g m−2 y−1 at the 34 and 48°N stations, consisting of biogenic particles with traceable quantities of lithogenic particle flux. The spring particle bloom, characterized by the sedimentation of particles relatively enriched by Norg, began in January at the 34°N station and in March at the 48°N station. The bloom continued for 4.5 and 3 months and provided 62 and 50% of the annual biogenic particle mass flux at 2 km at the 34 and 48°N stations. The surface bloom penetrated to the ocean interior within a few weeks, with apparently accelerated settling speed at deeper layers. The order of susceptibility of biogenic elements to mineralization while settling in the 0.7–1 km a.b. water column was, from least to most resistant: P, Norg, Corg, Si and Ca. The C/N/P ratio at 0.7 km a.b. was 154:18:1 at the 34°N station and 148:18:1 at the 48°N station.

Biogenic barium fluxes to the deep sea: Implications for paleoproductivity reconstruction

Dymond et al. (1992) have recently proposed an algorithm to reconstruct paleoproductivity from biogenic Ba (bio-Ba) accumulation rates in sediments. Their equation is based on sediment trap data which indicate that Corg/bio-Ba ratios in settling particles are higher in the western Atlantic compared to the Pacific. From this observation they have suggested that the flux of bio-Ba to the seafloor may depend on dissolved Ba concentrations in intermediate and deep waters which are significantly higher in the Pacific compared to the Atlantic. Accordingly, they have introduced a factor related to dissolved Ba concentration in their equation as a variable which strongly influences paleoproductivity estimates. In an attempt to confirm the proposed dependency of bio-Ba fluxes to the seafloor on dissolved Ba concentrations in seawater we have compiled additional data on organic carbon and bio-Ba fluxes in the deep sea. These data confirm Dymond et al.s findings that settling particles have significantly higher Corg/bio-Ba in the western Atlantic compared to the Pacific. However, we also found lower ratios in traps deployed in the North Atlantic, similar to those found in the Pacific, while in the Panama Basin we found ratios as high as those in the western Atlantic. From these observations we conclude that dissolved Ba concentration is not an important factor in regulating the flux of bio-Ba to the seafloor. Instead, we propose that high Corg/bio-Ba ratios found in the western Atlantic, the Panama Basin, the Arabian Sea, and some stations in the Nordic Seas result from the addition of refractory organic carbon from nearby continents, shelves, or slopes. If that is confirmed, the algorithm proposed by Dymond et al. (1992) could be simplified and could provide a powerful means to estimate paleoproductivity. In addition, deviations from the Corg/bio-Ba ratios in settling particles could be used to estimate the input of continental or shelf-derived refractory organic matter into the deep sea.

Moored sediment trap measurements of carbon export in the Subantarctic and Polar Frontal Zones of the Southern Ocean, south of Australia

Sediment trap moorings were deployed from September 21, 1997 through February 21, 1998 at three locations south of Australia along 140°E: at ∼47°S in the central Subantarctic Zone (SAZ) with traps at 1060, 2050, and 3850 m depth, at ∼51°S in the Subantarctic Front with one trap at 3080 m, and at ∼54°S in the Polar Frontal Zone (PFZ) with traps at 830 and 1580 m. Particle fluxes were high at all the sites (18–32 g m−2 yr−1 total mass and 0.5–1.4 g organic carbon m−2 yr−1 at −1000 m, assuming minimal flux outside the sampled summer period). These values are similar to other Southern Ocean results and to the median estimated for the global ocean by Lampitt and Antia [1997], and emphasize that the Southern Ocean exports considerable carbon to the deep sea despite its “high-nutrient, low chlorophyll” characteristics. The SAZ site was dominated by carbonate (>50% of total mass) and the PFZ site by biogenic silica (>50% of total mass). Both sites exhibited high export in spring and late summer, with an intervening low flux period in December. For the 153 day collection period, particulate organic carbon export was somewhat higher in all the traps in the SAZ (range 0.57–0.84 gC m−2) than in the PFZ (range 0.31–0.53), with an intermediate value observed at the SAF (0.60). The fraction of surface organic carbon export (estimated from seasonal nutrient depletion, Lourey and Trull [2001]) reaching 1000 m was indistinguishable in the SAZ and PFZ, despite different algal communities.

Annual particle flux and a winter outburst of sedimentation in the northern Norwegian sea

Monthly samples were collected by a sediment trap deployed for one year at 473 m above the sea floor in water 2123 m deep at a station located at 75°N, 11°E, southwest of Spitsbergen. This station was positioned at the northernmost extension of the Norwegian Current and was not covered by sea ice throughout the year of the experiment. The annual particle flux was 28.3 g m−2, of which 49% was biogenic and 51% was lithogenic particles. The annual fluxes of organic carbon, calcium carbonate, and biogenic opal were 2.9, 6.6 and 2.0 g m−2, respectively. Biogenic particles settling in the northern Norwegian Sea are dominated by carbonate, not opal. There were three distinct seasonality phases in sedimentation: phase 1, May to July was a period of relatively small flux, reflecting the spring bloom material in the surface layer; phase 2, August to November, showed the largest flux of carbon and other biogenic particles; and phase 3, December to May, was a period of outburst of lithogenic particle sedimentation which peaked during mid-January to mid-February. We assume that this outburst is related to cold deep water generated on the Barents Sea shelf and flowing southwestward through the Storfjord Trough into the Norwegian Sea.

Sedimentation of lithogenic particles in the deep ocean

Abstract Investigation of lithogenic particles collected by sediment traps in open-ocean stations revealed that the sediment flux increased linearly with depth in the water column. This rate of increase decreased with distance of the station from the continent; it was largest at the Panama Basin station and almost negligible at the E. Hawaii Abyssal Plain station. At the Panama Basin station, smectite flux increased with depth. We suggest that smectite resuspended from bottom sediments of the continental slope west of the sediment-trap station is advected by easterly deep currents, and the suspended particles are then possibly entrapped by large settling particles. On the other hand, the flux of hemipelagic clay particles, kaolinite and chlorite, was nearly constant at all depths; this can be explained by incorporation of these particles in fecal pellets which then settle from the surface water. At the Demerara Abyssal Basin Station, flux of illite and chlorite particles increased with depth and the flux of smectite was constant. A sudden increase of the flux of illite and chlorite was observed near the bottom traps at the Sohm Abyssal Plain station. The flux of quartz and feldspar was 10 to 15% of the clay flux.

Lithogenic fluxes to the deep Arabian Sea measured by sediment traps

Abstract Particle fluxes measured continously for one year at three locations in the Arabian Sea using time-series sediment traps show that lithogenic sedimentation processes are strongly coupled to biological processes. The vertical flux of lithogenic matter is controlled by episodic production and fluxes of biogenic matter. Illite and quartz are the dominant clay minerals in the traps at all three locations. Smectites generally range between 2 and 8%, but show higher fluxes up to 25% in the central and eastern Arabian Sea during the southwest monsoon period. Most of the river discharge is retained on the continental shelf, and less than 5% of the annual input of lithogenic material to the Arabian Sea is deposited in the deeper part as hemipelagic sediments.

Amino acid, hexosamine and carbohydrate fluxes to the deep subarctic Pacific (Station P)

Abstract Sediment trap samples covering the period from September 1982 to September 1983 at Station P in the subarctic Pacific were analysed for organic carbon, nitrogen, amino acids, hexosamines and carbohydrates. One trap was deployed at 3800 m water depth for the whole period and a second trap was deployed at 1000 m from March to September 1983. Peaks of particle fluxes were observed in September–November 1982. May–June 1983 and July–September 1983. Organic compounds, used as indicators for organic matter sources and degradation, revealed that organic matter is generally least degraded during periods of maximum particle fluxes. During most of the year organic matter is derived from phytoplankton (mainly diatoms). In July to October concentrations of all measured organic compounds peak simultaneously at both trap depths, and amino acid and hexosamine fluxes are higher in the deeper trap. Spectral distributions of amino acids and hexosamines suggest that their increase is due to the addition of organic matter derived from copepods. In this case about 15% of the organic matter in the shallow trap and about 60% of the organic matter in the deep trap is contributed by copepods. The considerable enrichment of the more resistant hexosamines in the deep trap indicates that the copepods entering the traps are not active swimmers, but their decayed remains or molts.

Evidence of old carbon in the deep water column of the Panama Basin from natural radiocarbon measurements

Radiocarbon (Δ14C) analyses were performed on sinking and suspended particulate matter, dissolved inorganic carbon (DIC), sediment and sediment fluff from a deep-water station in the Panama Basin. Δ14C signatures for sinking particulate inorganic and organic carbon (PIC, FOC) collected at 3354 meters depth were lower than surface DIC Δ14C values, and higher than the inorganic and organic carbon from the sediment (SIC, SOC). We compare these data with those obtained for other Pacific and Atlantic sites and show that the gradient between surface DIC Δ14C and deep suspended POC Δ14C is the same irrespective of distance from the coast. This indicates that: 1) resuspended SOC is an equally important contributor to deep POC at locations close to and far from continental slopes, and/or 2) DOC sorption is a significant contributor for causing low Δ14C values of deep POC.