Increase in anthropogenic CO2 in the Atlantic Ocean in the last two decades

Data from the first systematic survey of inorganic carbon parameters on a global scale, the GEOSECS program, are compared with those collected during WOCE/JGOFS to study the changes in carbon and other geochemical properties, and anthropogenic CO₂ increase in the Atlantic Ocean from the 1970s to the early 1990s. This first data-based estimate of CO₂ increase over this period was accomplished by adjusting the GEOSECS data set to be consistent with recent high-quality carbon data. Multiple Linear Regression (MLR) and extended Multiple Linear Regression (eMLR) analyses to these carbon data are applied by regressing DIC with potential temperature, salinity, AOU, silica, and PO₄ in three latitudinal regions for the western and eastern basins in the Atlantic Ocean. The results from MLR (and eMLR provided in parentheses) indicate that the mean anthropogenic CO₂ uptake rate in the western basin is 0.70 (0.53) mol m^?2 yr^?1 for the region north of 15°N; 0.53 (0.36) mol m^?2 yr^?1 for the equatorial region between 15°N and 15°S; and 0.83 (0.35) mol m^?2 yr^?1 in the South Atlantic south of 15°S. For the eastern basin an estimate of 0.57 (0.45) mol m^?2 yr^?1 is obtained for the equatorial region, and 0.28 (0.34) mol m^?2 yr^?1 for the South Atlantic south of 15°S. The results of using eMLR are systematically lower than those from MLR method in the western basin. The anthropogenic CO₂ increase is also estimated in the upper thermocline from salinity normalized DIC after correction for AOU along the isopycnal surfaces. For these depths the results are consistent with the CO₂ uptake rates derived from both MLR and eMLR methods.     

Intra-annual variability of extremely rapid sedimentation onto Gardar Drift in the northern North Atlantic

North Atlantic sediment drifts are valuable archives for paleoceanographic reconstructions spanning various timescales. However, the short-term dynamics of such systems are poorly known, and this impinges on our ability to quantitatively reconstruct past change. Here we describe a high-resolution 319-day time-series of hydrodynamics and near-bottom (4 m) particulate matter flux variability at a 2600 m deep site with an extremely high sediment accumulation rate on the southern Gardar Drift in the North Atlantic. We compare our findings with the actual deposits at the site. The total annual particle flux amounted to ～360 g m^?2 yr^?1, varied from ～0.15 to >5.0 g m^?2 day^?1 and displayed strong seasonal compositional changes, with the highest proportion of fresh biogenic matter arriving after the spring bloom in June and July. Flux variability also depended on the changing input of lithogenic matter that had been (re)suspended for a longer time (decades). Active focussing of material from both sources is required to account for the composition and the magnitude of the total flux, which exceed observations elsewhere by an order of magnitude. The enhanced focussing or increased delivery appeared to be positively related to current velocity. The intercepted annual particle flux accounted for only 60% of the sediment accumulation rate of 600±20 g m^?2 yr^?1 (0.20±0.07 cm yr^?1), indicating higher intra- and inter-annual variability of both the biogenic and lithogenic fluxes and/or advection of additional sediment closer to the seafloor (i.e. <4 m). This temporal variability in the composition and amount of material deposited highlights intra-annual changes in the flux of lithogenic material, but also underscores the importance of (reworked) sediment focussing and seasonality of the biogenic flux. All should be taken into account in the interpretation of the paleorecord from such depositional settings.     

Scavenging,cycling and removal fluxes of 210Po and 210Pb at the Bermuda time-series study site

Quantifying relative affinities of Po and Pb in different populations of marine particulate matter is of great importance in utilizing ²¹⁰Po as a tracer for carbon cycling. We collected and analyzed water samples for the concentrations of dissolved and total ²¹⁰Po and ²¹⁰Pb from the upper 600 m of the water column at Bermuda Time-series Study site (September 1999–September 2000) to investigate their seasonality of concentrations and their activity ratio (²¹⁰Po/²¹⁰Pb activity ratio, AR). Sinking particles collected in sediment traps at depths of 500 m, 1500 m, and 3200 m from the Oceanic Flux Program (OFP) time-series sediment traps were analyzed over a period of 12 months (May 1999–May 2000). The objective was to compare the deficiencies of ²¹⁰Po with respect to ²¹⁰Pb in the water column to that measured in the sediment traps and to assess the relative affinities of Po and Pb with different particle pools.Inventories of ²¹⁰Po in the upper 500 m water column varied by a factor of 2, indicating seasonal variations of particulate flux dominated the removal of ²¹⁰Po. The ²¹⁰Po/²¹⁰Pb ARs in the dissolved phase were generally less than the secular equilibrium value (1.0) in the upper 600 m, while were generally greater than 1.0 in the particulate phase, indicating higher removal rates of ²¹⁰Po relative to ²¹⁰Pb by particulate matter. The measured fluxes of ²¹⁰Po and ²¹⁰Pb in the 500 m, 1500 m, and 3200 m traps increased with depth, while the ²¹⁰Po/²¹⁰Pb ARs decreased with depth except from May–August 1999. From the measured fluxes of ²¹⁰Po and ²¹⁰Pb at these three traps and the concentrations of ²¹⁰Po and ²¹⁰Pb in the water column, this region appears to be a sink for ²¹⁰Pb which is likely brought-in by lateral advection.     

Fish farming enhances biomass and nutrient loss in Posidonia oceanica (L.) Delile

Fish farming impact on the seasonal biomass, carbon and nutrient (nitrogen and phosphorus) balance of the endemic Mediterranean seagrass Posidonia oceanica was assessed in the Aegean Sea (Greece) in order to detect changes in magnitude and fate of seagrass production and nutrient incorporation with organic loading of the meadows. Phosphorus concentration in the leaves, rhizomes and roots was enhanced under the cages throughout the study. Standing biomass was diminished by 64% and carbon, nitrogen and phosphorus standing stock by 64%, 61% and 48%, respectively, under the cages in relation to those at the control. Seagrass production decreased by 68% and element (C, N, P) incorporation by 67%, 58% and 58%, respectively, under the cages. Leaf shedding was reduced by 81% and loss of elements (C, N, and P) through shedding by 82%, 74% and 72%, respectively, under the cages. Leaf and element (C, N, P) residual loss rate, accounting for grazing and mechanical breakage of leaves, was decreased by 79%, 85%, 100% and 96%, respectively, at the control station. At the control station, 13.98 g C m^?2 yr^?1, 1.91 g N m^?2 yr^?1 and 0.05 g P m^?2 yr^?1 were produced in excess of export and loss. In contrast, under the cages 12.69 g C m^?2 yr^?1, 0.31 g N m^?2 yr^?1 and 0.04 g P m^?2 yr^?1 were released from the meadow. Organic loading due to fish farm discharges transformed the seagrass meadow under the cages from a typical sink to a source of organic carbon and nutrients.     

A carbon budget for the Barents Sea

The first carbon budget constructed for the Barents Sea to study the fluxes of carbon into, out of, and within the region is presented. The budget is based on modelled volume flows, measured dissolved inorganic carbon (DIC) concentration, and literature values for dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations. The results of the budget show that ～5600±660×10⁶ t C yr^?1 is exchanged through the boundaries of the Barents Sea. If a 40% uncertainty in the volume flows is included in the error calculation it resulted in a total uncertainty of ±1600×10⁶ t C yr^?1. The largest part of the total budget flux consists of DIC advection (～95% of the inflow and ～97% of the outflow). The other sources and sinks are, in order of importance, advection of organic carbon (DOC+POC; ～3% of both in- and outflow), total uptake of atmospheric CO₂ (～1% of the inflow), river and land sources (～0.2% of the inflow), and burial of organic carbon in the sediments (～0.2% of the outflow). The Barents Sea is a net exporter of carbon to the Arctic Ocean; the net DIC export is ～2500±660×10⁶ t C yr^?1 of which ～1700±650×10⁶ t C yr^?1 (～70%) is in subsurface water masses and thus sequestered from the atmosphere. The net total organic carbon export to the Arctic Ocean is ～80±20×10⁶ t C yr^?1. Shelf pumping in the Barents Sea results in an uptake of ～22±11×10⁶ t C yr^?1 from the atmosphere which is exported out of the area in the dense modified Atlantic Waters. The main part of this carbon was channelled through export production (～16±10×10⁶ t C yr^?1).     

Population dynamics and secondary production of the cockle Cerastoderma edule: A comparison between Merja Zerga (Moroccan Atlantic Coast) and Arcachon Bay (French Atlantic Coast)

Cockle (Cerastoderma edule) population dynamics were studied at the southern limit of the distribution of this marine bivalve in Merja Zerga, Morocco. Parameters such as growth, mortality, and production were compared with those of a population at Arcachon Bay (France) a site in the center of the cockle's range. At each sampling period between two and three cohorts were simultaneously observed at each site and the average total abundance was usually higher at Merja Zerga. Recruitment occurred at both sites in spring when temperature rose above 19 °C, independently of the month. In Merja Zerga, winter recruitment was also observed at one occasion, following high sediment disturbance. The first year (2005–06) at Merja Zerga, the mortality rate was close to nil for juveniles and was Z = 1.5 yr^? 1 for adults, providing a high production (64 g dry weight m^? 2 yr^? 1). At Arcachon during the same period, the juvenile mortality rate was Z = 10.9 yr^? 1, the adult mortality rate was 3.4 yr^? 1 and production was 26 gDW m^?2 yr^? 1. The second year (2006–07), mortality after recruitment was much higher (Z = 8.6 yr^? 1, for juveniles) and similar to what was observed at Arcachon (Z = 8.4 yr^? 1). Mortality rate of adults was higher at Merja Zerga (Z = 3.0 yr^? 1) than at Arcachon (Z = 1.5 yr^? 1). Production was lower at Arcachon than at Merja Zerga although growth performances were higher at Arcachon. The higher growth performance at Arcachon (Φ′ = 3.3) was mainly due to high asymptotic length (L_∞ = 38 mm) and was related to low intraspecific competition compared to Merja Zerga where cockle abundance was higher (Φ′ = 3.1, L_∞ = 31 mm). P/B was low in both sites and slightly higher at Arcachon (1.1–1.5 against 1.0–1.1 yr^? 1). At Arcachon, recruitment was correlated with temperature, a peak occurring when temperature rose above 19 °C (June–July). At Merja Zerga, recruitment was already 2–3 months earlier but was not significantly correlated to temperature.This study showed that population dynamics of cockles at the southern limit of this distribution fell in the range of what was observed elsewhere in the North-Eastern Atlantic coast. Most factors that were involved in population regulation (intraspecific competition, predation and sediment dynamics) were not strictly dependent on latitude. The direct role of temperature (latitude dependent factor) was not obvious. Variation in temperature could explain the recruitment delay between Arcachon and Merja Zerga and the low maximum shell length at Merja Zerga.     

Three-year assessment of particulate organic carbon fluxes in Amundsen Gulf (Beaufort Sea): Satellite observations and sediment trap measurements

Surface concentrations and vertical fluxes of particulate organic carbon (POC) were assessed in the Amundsen Gulf (southeastern Beaufort Sea, Arctic Ocean) over the years 2004 to 2006 by using ocean color remote-sensing imagery and sequential sediment traps moored over the ca. 400 m isobath. Environmental conditions (sea ice, wind) and oceanographic variables (temperature, salinity, fluorescence and currents) were investigated to explain the variability of POC data. Annual downward POC fluxes in 2004, 2005 and 2006 cumulated, respectively, to 3.3, 4.2 and 6.0 g C m^?2 yr^?1 at ～100 m depth, and to 1.3, 2.2 and 3.3 g C m^?2 yr^?1 at ～210 m depth. The fraction of settling POC attributable to autochthonous processes occurring at or next to ice break-up was estimated to be 75–84% of the 100 m annual fluxes and to be 61–75% of the 210 m fluxes. Over the three ice-reduced seasons, distinct scenarios between ice conditions, surface POC pools and vertical POC export at 100 m were identified: (1) in 2004, despite a normal ice break-up, a weak primary production was measured and low vertical fluxes were collected as old ice moved across the region; (2) in 2005, a lengthened ice-free period allowed an extended season of surface POC production near-shore, while an intermediate increase of vertical fluxes was recorded offshore; and (3) in 2006, a late ice melt gave rise to a pulsed ice edge bloom and to large vertical fluxes also associated with extra ice-flushed material. Linear regressions of vertical POC fluxes against satellite-derived surface POC concentrations suggested that the pelagic POC retention in the upper 100 m of the Amundsen Gulf ranged from ca. 70% to 90% depending on the timing of ice cover melt. Regardless of the inter-annual variability, the estimated fraction of the surface POC reservoir reaching the 210 m water depth was reduced to ～5%. Therefore, as the Arctic Ocean warms up, our results support the expectation that the increasing extent of the seasonal ice zone will promote the POC pathways that benefit pelagic webs rather than benthic communities.     

Natural and anthropogenic sources of Hg,Cd, Pb,Cu and Zn in seawater and sediment of Mljet National Park,Croatia

Distributions of Hg, Cd, Pb, Cu and Zn in seawater and sediment from Mljet National Park, Adriatic Sea are presented for the first time. Natural and anthropogenic factors play an important role in determining resultant trace metals' concentrations in the region. We place particular emphasis on the saline “lakes” of Malo Jezero and Veliko Jezero, which have restricted flows of seawater. In Malo Jezero lake, fresh karstic spring water generated by flooding, and weathering of dolomites are the main sources of naturally elevated Cd, Pb and Zn concentrations (20.7 ± 1.6, 289 ± 19, 1260 ± 0.08 ng L^?1, respectively); anthropogenic input is negligible. In Veliko Jezero lake enhanced Cu and Zn contents originate from anthropogenic input (tourism and agriculture). Distributions of the Pb and Zn in the water columns of both lakes are influenced by natural aragonite precipitation and sedimentation. Exceptionally high total Hg concentrations of 24.2 and 33.7 ng L^?1 in the water column of Malo Jezero, sampled during periods of high rainfall associated with strong eastern winds, suggest an airborne input. Total Hg concentrations in waters of both lakes are elevated because of inefficient mixing. Two different metal distribution patterns exist in the sediment columns. First, Hg, Pb, Cu and Zn show elevated concentrations in recent sediments due to anthropogenic input. Second, Cd content increases with depth due to reprecipitation via a downward redox boundary shift.Described natural processes, as well as anthropogenic influence, enhance levels of trace metals. Careful study followed by suitable interpretation based on geochemical data were necessary to distinguish natural from anthropogenic sources.     

Silicon flux and distribution of biogenic silica in deep-sea sediments in the western North Pacific Ocean

We investigated biogenic silica, several biological components, and silicate in pore-water in the abyssal sediment to determine silicon flux of western North Pacific during several cruises. The surficial sediment biogenic silica content was high at high latitudes with the boundary running along the Kuroshio Extension, and maximum values (exceeding 20%) were found in the Oyashio region. In the subtropical region to the south, most stations showed less than 5% biogenic silica content. This distribution pattern reflected primary production and ocean currents in the surface layer very well. Pore-water samples were collected from 4 stations along the east coast of Japan. The highest asymptotic silicic acid concentration (670 μmol L^?1) in pore-water was observed at the junction of Kuroshio and Oyashio, followed by samples from the Oyashio region. It is at the southern station that the lowest value (450 μmol L^?1) was observed, and the primary production is low under the influence of Kuroshio there. The diffusive flux followed the same geographic trend as the asymptotic silicic acid concentrations did, ranging 77–389 mmol m^?2 yr ^?1. Multiple sampling of pore-water was conducted throughout the year at one station at high latitude. The average annual biogenic silica rain flux observed using sediment traps was 373 mmol m^?2 yr^?1; the diffusive flux and burial flux at the sediment–water interface were 305 and 9 mmol m^?2 yr^?1, respectively. We concluded that most of the settling silica particles dissolved and diffused at the sediment–water interface and approximately 3% only were preserved in this area. In addition, the obvious time lag observed between the peak rain flux and the maximum diffusive flux suggested that primary production in the surface layer has a great influence on the sedimentation environment of abyssal western North Pacific. These transitions of Si flux at the sediment–water interface were considerably greater in northwestern North Pacific than in southwestern North Pacific. In addition, a station in the Philippine Sea indicated high biogenic silica content because of Ethmodiscus ooze, which are scattered randomly on the sea floor in the subtropical region.     

Impact of submarine hydrothermal vents on the metal composition of krill and its excretion products

We have measured the metal composition and estimated the excretion rate of trace elements (Ag, Cd, Co, Cu, Ni, Pb, V and Zn) by Antarctic krill (Euphausia superba) in three locations (two located within a submarine hydrothermal vent field and one away from it) along the Antarctic Peninsula region of the Southern Ocean. Results indicated that krill excreted large amounts of Ag, Cu, Pb and Zn, (range: 1.9–41.2 pmol Ag g DW^? 1 h^? 1, 15.3–26.8 nmol Cu g DW^? 1 h^? 1, 308.7–1118.3 pmol Pb g DW^? 1 h^? 1 and 24.4–76.5 nmol Zn g DW^? 1 h^? 1), compared with the non-significant or undetectable release rates of Cd, Co, Ni and V. The metal composition of the excreted material from krill collected in the area of hydrothermal activity was similar to the metal composition reported for suspended particles emitted from those vents. Our results suggest that krill recycling of Ag, Cu, Pb and Zn could potentially influence trace metal concentrations in the water column of the Bransfield region of the Southern Ocean, and that the original source of metals to these waters may be hydrothermal vents.     

Isotopic constraints on the Si-biogeochemical cycle of the Antarctic Zone in the Kerguelen area (KEOPS)

Estimation of the silicon (Si) mass balance in the ocean from direct measurements (Si uptake-dissolution rates …) is plagued by the strong temporal and spatial variability of the surface ocean as well as methodological artifacts. Tracers with different sensitivities toward physical and biological processes would be of great complementary use. Silicon isotopic composition is a promising proxy to improve constraints on the Si-biogeochemical cycle, since it integrates over longer timescales in comparison with direct measurements and since the isotopic balance allows to resolve the processes involved, i.e. uptake, dissolution, mixing. Si-isotopic signatures of seawater Si(OH)₄ and biogenic silica (bSiO₂) were investigated in late summer 2005 during the KEOPS experiment, focusing on two contrasting biogeochemical areas in the Antarctic Zone: a natural iron-fertilized area above the Kerguelen Plateau (< 500 m water depth) and the High Nutrient Low Chlorophyll area (HNLC) east of the plateau (> 1000 m water depth). For the HNLC area the Si-isotopic constraint identified Upper Circumpolar Deep Water as being the ultimate Si-source. The latter supplies summer mixed layer with 4.0 ± 0.7 mol Si m^? 2 yr^? 1. This supply must be equivalent to the net annual bSiO₂ production and exceeds the seasonal depletion as estimated from a simple mixed layer mass balance (2.5 ± 0.2 mol Si m^? 2 yr^? 1). This discrepancy reveals that some 1.5 ± 0.7 mol Si m^? 2 yr^? 1 must be supplied to the mixed layer during the stratification period. For the fertilized plateau bloom area, a low apparent mixed layer isotopic fractionation value (?³⁰Si) probably reflects (1) a significant impact of bSiO₂ dissolution, enriching the bSiO₂ pool in heavy isotope; and/or (2) a high Si uptake over supply ratio in mixed layer at the beginning of the bloom, following an initial closed system operating mode, which, however, becomes supplied toward the end of the bloom (low Si uptake over supply ratio) with isotopically light Si(OH)₄ from below when the surface Si(OH)₄ pool is significantly depleted. We estimated a net integrated bSiO₂ production of 10.5 ± 1.4 mol Si m^? 2 yr^? 1 in the AASW above the plateau, which includes a significant contribution of bSiO₂ production below the euphotic layer. However, advection which could be significant for this area has not been taken into account in the latter estimation based on a 1D approach of the plateau system. Finally, combining the KEOPS Si-isotopic data with those from previous studies, we refined the average Si-isotopic fractionation factor to ? 1.2 ± 0.2‰ for the Antarctic Circumpolar Current.     

Pb and Cs in sediments from Sagami Bay, Japan: sedimentation rates and inventories

Profiles of the radioisotopes ²¹⁰Pb and ¹³⁷Cs were determined in 15 sediment cores collected from Sagami Bay, Japan. The activities of ²¹⁰Pb_ex (unsupported) in core top sediments increased with water depth from 25 dpm g⁻¹ on the upper continental slope off the mouth of Tokyo Bay to an average of 283 dpm g⁻¹ at the deep-sea station SB. The high ²¹⁰Pb trapping efficiency of settling particles expected from the results of the sediment trap experiment near the SB site suggests that effective ²¹⁰Pb enrichment in surface sediments may occur during resuspension and lateral transportation of particles via the benthic nepheloid layer on the continental slope. In several cores, ¹³⁷Cs profiles showed an increase, a distinct peak, and then a decrease to an undetectable level downcore. These profiles can be compared with the temporal change of bomb-produced ¹³⁷Cs fallout.The mean sedimentation rates estimated by the ²¹⁰Pb_ex inventory method, rather than using ²¹⁰Pb_ex profiles, ranged from 0.06 g cm⁻² y⁻¹ to 0.14 g cm⁻² y⁻¹. The average value of the rates in SB cores was calculated to be 0.11 g cm⁻² y⁻¹, which was similar to that calculated under the assumption that the age of the ¹³⁷Cs peak corresponds to its maximum fallout year in 1963.Although ¹³⁷Cs inventories represented one tenth of the anthropogenic fallout of ¹³⁷Cs until 1997, they correlated with the increase in ²¹⁰Pb_ex inventory. This suggests that the scavenging of refractory ¹³⁷Cs as well as ²¹⁰Pb by settling particles in the water column can lead to the formation of a time marker layer even in deep-sea sediment core, such as at the SB site.     

Contrasting life-histories,secondary production,and trophic structure of Peracarid assemblages of the bathyal suprabenthos from the Bay of Biscay (NE Atlantic) and the Catalan Sea (NW Mediterranean)

The life-histories and the secondary production of four dominant peracarid crustaceans (the mysids Boreomysis arctica and Parapseudomma calloplura, the amphipod Rhachotropis caeca, and the isopod Ilyarachna longicornis) in bathyal depths of the Bay of Biscay (NE Atlantic; between 383 and 420 m) and the Catalan Sea (Northwestern Mediterranean; between 389 and 1355 m) were established. Both the Atlantic and the Mediterranean populations of the major part of the target-species had two generations/year with mean cohort-production intervals (CPI) ranging from 5.5 mo for Ilyarachna longicornis to 6.3 mo for Parapseudomma calloplura. The Hynes method showed secondary production to vary in the Bay of Biscay between 0.113 mg DW m⁻² yr⁻¹ for I. longirostris and 3.069 mg DW m⁻² yr⁻¹ for P. calloplura, with P/B ratios between 4.57 (I. longirostris) and 7.93 (Boreomysis arctica). In the Catalan Sea, production varied between 0.286 mg DW m⁻² yr⁻¹ for I. longirostris and 1.096 mg DW m⁻² yr⁻¹ for P. calloplura, with P/B between 5.72 (I. longirostris) and 6.66 (P. calloplura). Application of two different empiric models to the whole peracarid assemblage gave similar levels of secondary production in both study areas (between 29.26 and 32.14 mgDWm⁻² yr⁻¹ in the Bay of Biscay; between 26.23 and 26.54 mg DW m⁻² yr⁻¹ in the Catalan Sea). From the analysis of gut contents of 22 species the dominant species in each study area were assigned to two basic trophic levels, detritus feeders and predators. Also, cumulative curves of dominance showed high diversity (low dominance) for peracarid assemblages distributed at mid-bathyal depths (524–693 m) both in the Bay of Biscay off Arcachon and in the Catalan Sea off Barcelona. We also discuss and compare, both within and between areas, how environmental features may explain the observed diversity patterns, the trophic structure, and the production results obtained for the suprabenthos assemblages.     

Fate of biogenic carbon in the upper 200 m of the central Greenland Sea

Sedimentation of particulate carbon from the upper 200–300 m in the central Greenland Sea from August 1993 to June 1995 was less than 2 g C m⁻² yr⁻¹. Daily rates of sedimentation of particulate organic carbon reached highest values of about 18 mg m⁻² d⁻¹ in fall 1994. For total particulate material, maximum rates of sedimentation of about 250 mg m⁻² d⁻¹ were recorded in spring and fall 1994. For chlorophyll equivalent, highest rates of sedimentation of about 140 μg m⁻² d⁻¹ were recorded in spring 1994. As reported in related investigations, the transient accumulation of DOC in surface waters during summer, as well as respiration and mortality of deep overwintering zooplankton stocks, appeared to dominate the fate of photosynthetically fixed organic carbon. The above processes may account for roughly 43 g C m⁻² in the upper 200 m of the central Greenland Sea. For comparison, the seasonal deficit in dissolved inorganic carbon was reported to be about 23 g C m⁻² in the upper 20 m of surface water, and estimates for new annual production were reported to be about 57 g C m⁻². In our investigation, the biological carbon pump was not unusually effective in transporting carbon out of the productive surface layer.     

Biogenic fluxes in the Cariaco Basin: a combined study of sinking particulates and underlying sediments

The fluxes of total mass, organic carbon (OC), biogenic opal, calcite (CaCO₃) and long-chain C₃₇ alkenones (ΣAlk₃₇) were measured at three water depths (275, 455 and 930 m) in the Cariaco Basin (Venezuela) over three separate annual upwelling cycles (1996–1999) as part of the CARIACO sediment trap time-series. The strength and timing of both the primary and secondary upwelling events in the Cariaco Basin varied significantly during the study period, directly affecting the rates of primary productivity (PP) and the vertical transport of biogenic materials. OC fluxes showed a weak positive correlation (r²=0.3) with PP rates throughout the 3 years of the study. The fluxes of opal, CaCO₃ and ΣAlk₃₇ were strongly correlated (0.6<r²<0.8) with those of OC. The major exception was the lower than expected ΣAlk₃₇ fluxes measured during periods of strong upwelling. All sediment trap fluxes were significantly attenuated with depth, consistent with marked losses during vertical transport. Annually, strong upwelling conditions, such as those observed during 1996–1997, led to elevated opal fluxes (e.g., 35 g m⁻² yr⁻¹ at 275 m) and diminished ΣAlk₃₇ fluxes (e.g., 5 mg m⁻² yr⁻¹ at 275 m). The opposite trends were evident during the year of weakest upwelling (1998–1999), indicating that diatom and haptophyte productivity in the Cariaco Basin are inversely correlated depending on upwelling conditions.The analyses of the Cariaco Basin sediments collected via a gravity core showed that the rates of OC and opal burial (10–12 g m⁻² yr⁻¹) over the past 5500 years were generally similar to the average annual water column fluxes measured in the deeper traps (10–14 g m⁻² yr⁻¹) over the 1996–1999 study period. CaCO₃ burial fluxes (30–40 g m⁻² yr⁻¹), on the other hand, were considerably higher than the fluxes measured in the deep traps (∼10 g m⁻² yr⁻¹) but comparable to those obtained from the shallowest trap (i.e. 38 g m⁻² yr⁻¹ at 275 m). In contrast, the burial rates of ΣAlk₃₇ (0.4–1 mg m⁻² yr⁻¹) in Cariaco sediments were significantly lower than the water column fluxes measured at all depths (4–6 mg m⁻² yr⁻¹), indicating the large attenuation in the flux of these compounds at the sediment–water interface. The major trend throughout the core was the general decrease in all biogenic fluxes with depth, most likely due to post-depositional in situ degradation. The major exception was the relatively low opal fluxes (∼5 g m⁻² yr⁻¹) and elevated ΣAlk₃₇ fluxes (∼2 mg m⁻² yr⁻¹) measured in the sedimentary interval corresponding to 1600–2000 yr BP. Such compositions are consistent with a period of low diatom and high haptophyte productivity, which based on the trends observed from the sediment traps, is indicative of low upwelling conditions relative to the modern day.     

Factors controlling silicon and nitrogen biogeochemical cycles in high nutrient,low chlorophyll systems (the Southern Ocean and the North Pacific): Comparison with a mesotrophic system (the North Atlantic)

A 1-D coupled physical-biogeochemical model is used to study the seasonal cycles of silicon and nitrogen in two High Nutrient Low Chlorophyll (HNLC) systems, the Antarctic Circumpolar Current (ACC) and the North Pacific Ocean, and a mesotrophic system, the North Atlantic Ocean. The biological model consists of nine compartments (diatoms, nano-flagellates, microzooplankton, mesozooplankton, two types of detritus, nitrate, ammonium and silicic acid) forced by irradiance, temperature, mixing and deep nitrate and silicic acid concentrations. At all sites, nanophytoplankton standing crop variations are low, in spite of variations in primary production, because of a “top–down” control by microzooplankton. Although nanophytoplankton sustain more than 60% of the annual primary production in these areas, their contribution to the export production does not exceed 1% of the total. The differences in the seasonal plankton cycle among these regions come mainly from differences in the dynamics of large phytoplankton (here diatoms). In the ACC, the chlorophyll maximum remains <1.5 mg m⁻³, as an unfavourable light/mixing regime and a likely trace-metal limitation keep diatoms from blooming. In the northeast Pacific, trace-metal limitation seems to keep diatoms from blooming throughout the year. In both these systems, light or iron limitations induce high Si/N uptake ratios. Incidentally these high Si/N uptake ratios lead to a net excess of silicic acid utilization over nitrate, and to a subsequent silicic acid limitation during the summertime. In the North Atlantic, under favourable light/mixing regime and nutrient-replete conditions at the onset of the growing period, diatoms outburst and sustain a bloom >3.5 mg Chl-a m⁻³. Thereafter, mesozooplankton grazing pressure and silicic acid limitation induce the collapse of the chlorophyll maximum and the persistence of lower chlorophyll concentrations in summer. Although the ACC and the North Pacific show HNLC features, they support a high biogenic silica production (1.9 and 1.07 mol Si m⁻² yr⁻¹) and export flux (0.79 and 0.61 mol Si m⁻² yr⁻¹), compared to the North Atlantic (production: 0.23 mol Si m⁻² yr⁻¹, export: 0.12 mol Si m⁻² yr⁻¹). The differences in Si production and export between the HNLC systems and the mesotrophic North Atlantic come from both higher Si concentrations and Si/N uptake ratios in the HNLC areas compared to the North Atlantic. Also, the low dissolution rate of biogenic silica compared to nitrogen degradation rate, and the inhibition of nitrate uptake by ammonium, reinforce the net excess of silicic acid utilization over nitrate. As a result, the model also illustrates the efficiency of the silica pump for the three sites: about 50% of the biogenic silica synthesized in the euphotic layer is exported out of the first 100 m, while only 4–11% of the particulate organic nitrogen escapes recycling in the surface layer.     

Late Quaternary siliciclastic/carbonate sedimentation model for the Capricorn Channel,southern Great Barrier Reef province,Australia

A model is presented for hemipelagic siliciclastic and carbonate sedimentation during the last glacial–interglacial cycle in the Capricorn Channel, southern Great Barrier Reef (GBR). Stable isotope ratios, grainsize, carbonate content and mineralogy were analysed for seven cores in a depth transect from 166 to 2892 m below sea level (mbsl). Results show variations in the flux of terrigenous, neritic and pelagic sediments to the continental slope over the last sea level cycle.During the glacial lowstand terrigenous sediment influenced all the cores down to 2000 mbsl. The percentages of quartz and feldspar in the cores decreased with water depth, while the percentage of clay increased. X-ray diffraction analysis of the glacial lowstand clay mineralogy suggests that the siliciclastic sediment was primarily sourced from the Fitzroy River, which debouched directly into the northwest sector of the Capricorn Channel at this time. The cores also show a decrease in pelagic calcite and an increase in aragonite and high magnesium calcite (HMC) during the glacial. The influx of HMC and aragonite is most likely from reworking of coral reefs exposed on the continental shelf during the glacial, and also from HMC ooids precipitated at the head of the Capricorn Channel at this time. Mass accumulation rates (MARs) are high (13.5 g cm^? 3 kyr^? 1) during the glacial and peak at ~ 20 g cm^? 3 kyr^? 1 in the early transgression (16–14 ka BP). MARs then decline with further sea level rise as the Fitzroy River mouth retreats from the edge of the continental shelf after 13.5 ka BP. MARs remain low (4 cm^? 3 kyr^? 1) throughout the Holocene highstand.Data for the Holocene highstand indicate there is a reduction in siliciclastic influx to the Capricorn Channel with little quartz and feldspar below 350 mbsl. However, fine-grained fluvial sediments, presumably from the Fitzroy River, were still accumulating on the mid slope down to 2000 mbsl. The proportion of pelagic calcite in the core tops increases with water depth, while HMC decreases, and is present only in trace amounts in cores below 1500 mbsl. The difference in the percentage of HMC in the deeper cores between the glacial and Holocene may reflect differences in supply or deepening of the HMC lysocline during the glacial.Sediment accumulation rates also vary between cores in the Capricorn Channel and do not show the expected exponential decrease with depth. This may be due to intermediate or deep water currents reworking the sediments. It is also possible that present bathymetry data are too sparse to detect the potential role that submarine channels may play in the distribution and accumulation of sediments.Comparison of the Capricorn Channel MARs with those for other mixed carbonate/siliciclastic provinces from the northeast margin of Australia indicates that peak MARs in the early transgression in the Capricorn Channel precede those from the central GBR and south of Fraser Island. The difference in the timing of the carbonate and siliciclastic MAR peaks along the northeast margin is primarily related to differences in the physiography and climate of the provinces. The only common trend in the MARs from the northeast margin of Australia is the near synchronicity of the carbonate and siliciclastic MAR peaks in individual sediment cores, which supports a coeval sedimentation model.     

Chlorophyll a in Arctic sediments implies long persistence of algal pigments

Sediment cores were collected from the shelf, slope, and basin of the Bering, Chukchi, and Beaufort Seas during May–June (under ice cover) and July–August (largely ice-free) 2004. Measurements of chlorophyll a (chl a), total organic carbon (TOC), and C/N ratios were made in surface and some subsurface core increments. Surface sediment chl a decreased with increasing water depth. Significant positive correlations were found between chl a and TOC and chl a and C/N ratios in the basin (>2000 m), but there were significant negative correlations between chl a and C/N ratios on the shelf (⩽200 m). Chl a values generally declined in down-core profiles, but in some deeper slope and basin cores, measurable inventories of subsurface chl a were present at depth. In some cases, these subsurface chlorophyll inventories coincident with peak activities of the anthropogenic radionuclide ¹³⁷Cs were detected, which had maximal deposition following the atmospheric nuclear weapons testing era in the 1960s. A sedimentation rate independently determined for one of these cores using ²¹⁰Pb was consistent with the depths of subsurface ¹³⁷Cs peaks in slope sediments reflecting steady, relatively undisturbed deposition over a several-decade period. The depth of penetration of ¹³⁷Cs in some continental slope sediments, together with detectable chl a, suggests that chl a can be buried in some of these deeper-water sediments under cold conditions for decadal periods in the absence of deposit feeders. Because organic deposition from the water column is episodic at high latitudes and concentrated following the spring bloom, these buried sources of organic materials, whether on the shelf or in deeper basin sediments, may ultimately be important for benthic invertebrates that could utilize this food source during times of the year when primary production flux from the overlying water column is reduced.     

Cross-slope zonation of erosion and deposition in the Faeroe-Shetland Channel,North Atlantic Ocean

Boundary currents and internal waves determine cross-slope zonation of erosion and deposition in the Faeroe-Shetland Channel. Currents were measured at 8 and 34–50 m above the bottom at three mooring sites (502, 595 and 708 m depth) for 14 days. The structure of the water column was evaluated from CTD sections, and included nepheloid layers and particulate matter concentrations. Indicators for recent deposition in the sediment (organic carbon, phytopigments, ²¹⁰Pb) were measured at eight stations across the slope. Strong near-bottom currents at the upper slope sustain down-slope particle transport in a benthic nepheloid layer, which is eroded under the influence of critically reflecting M₂ internal tidal waves at 350–550 m, where the major pycnocline meets the sloping bottom. Beam attenuation profiles confirmed the presence of intermediate nepheloid layers intruding into the Channel along the major pycnocline, and elevated concentrations of particulate matter and chlorophyll-a were measured at this depth. Near-bottom currents decreased with depth, thus allowing particle deposition down the slope. Inventories of excess ²¹⁰Pb activity in the sediment deeper than 600 m were higher than what was expected on the basis of atmospheric input of ²¹⁰Pb and production in the water column, thus indicating additional lateral inputs. Simple calculations showed that off-slope input of particles from areas shallower than 600 m may be responsible for the enhanced deposition at greater depths.     

Distribution and burial of organic carbon in sediments from the Indian Ocean upwelling region off Java and Sumatra,Indonesia

Sediments were sampled and oxygen profiles of the water column were determined in the Indian Ocean off west and south Indonesia in order to obtain information on the production, transformation, and accumulation of organic matter (OM). The stable carbon isotope composition (δ¹³C_org) in combination with C/N ratios depicts the almost exclusively marine origin of sedimentary organic matter in the entire study area. Maximum concentrations of organic carbon (C_org) and nitrogen (N) of 3.0% and 0.31%, respectively, were observed in the northern Mentawai Basin and in the Savu and Lombok basins. Minimum δ¹⁵N values of 3.7‰ were measured in the northern Mentawai Basin, whereas they varied around 5.4‰ at stations outside this region. Minimum bottom water oxygen concentrations of 1.1 mL L^?1, corresponding to an oxygen saturation of 16.1%, indicate reduced ventilation of bottom water in the northern Mentawai Basin. This low bottom water oxygen reduces organic matter decomposition, which is demonstrated by the almost unaltered isotopic composition of nitrogen during early diagenesis. Maximum C_org accumulation rates (CARs) were measured in the Lombok (10.4 g C m^?2 yr^?1) and northern Mentawai basins (5.2 g C m^?2 yr^?1). Upwelling-induced high productivity is responsible for the high CAR off East Java, Lombok, and Savu Basins, while a better OM preservation caused by reduced ventilation contributes to the high CAR observed in the northern Mentawai Basin. The interplay between primary production, remineralisation, and organic carbon burial determines the regional heterogeneity. CAR in the Indian Ocean upwelling region off Indonesia is lower than in the Peru and Chile upwellings, but in the same order of magnitude as in the Arabian Sea, the Benguela, and Gulf of California upwellings, and corresponds to 0.1–7.1% of the global ocean carbon burial. This demonstrates the relevance of the Indian Ocean margin off Indonesia for the global OM burial.