Carbon Export Flux Research Articles

Seasonal export fluxes of particulate organic carbon from 234Th/238U disequilibrium measurements in the Ulleung Basin1 (Tsushima Basin) of the East Sea1 (Sea of Japan)

Export fluxes of particulate organic carbon (POC) were estimated from the 234Th/238U disequilibrium in the Ulleung Basin1 (UB) of the East/Japan Sea1 (EJS) over four seasons. The fluxes were calculated by multiplying the average POC/234Th ratio of sinking particles larger than 0.7 μm at 100- and 200-m water depths to 234Th fluxes by the integrated 234Th/238U disequilibrium from the surface to 100-m water depth. In spring, the 234Th profiles changed dramatically with sampling time, and hence a non-steady-state 234Th model was used to estimate the 234Th fluxes. The 234Th flux estimated from the non-steady-state model was an order of magnitude higher than that estimated from the steady-state model. The 234Th fluxes estimated using the steady-state model showed distinct seasonal variation, with high values in summer and winter and low values in autumn. In spring, the phytoplankton biomass had the highest value, and primary production was higher than in summer and autumn, but the 234Th fluxes were moderate. However, these values might have been significantly underestimated, as the 234Th fluxes were estimated using the steady-state model. The POC export fluxes estimated in autumn were about four times lower than those in other seasons when they were rather similar. The annually averaged POC flux was estimated to be 161 ± 76 mgC m−2 day−1, which was somewhat lower than that in highly productive coastal areas, and higher than that in oligotrophic regions. The export/primary production (ThE) ratios ranged from 7.0 to 56.1%, with higher values in spring and summer and lower values in autumn and winter. In summer, a high ThE ratio of 48.4 ± 7.0% was measured. This may be attributed to the mass diatom sinking event following nitrate depletion. In the UB1, the annually averaged ThE ratio was estimated to be 34.4 ± 12.9%, much higher than that in oligotrophic oceans. The high ThE ratio may have contributed to the high organic carbon accumulation in the UB1.

Carbon export production in the subantarctic zone and polar front zone south of Tasmania

We studied the water column distribution of total 234Th in subantarctic (SAZ) and polar front (PFZ) zone systems south of Tasmania during mid-austral summer 2007. The objective was to assess whether the observed zonal differences in biomass in this sector translated into variability of the carbon export and sequestration potential, and to identify possible causes inducing this variability. This study is part of a broader investigation focusing on macro- and micronutrient availability controlling ecosystem functioning in this area. Surface deficits in 234Th activities were observed at every station. 234Th export fluxes calculated from the 234Th activity deficits assuming steady state conditions showed higher 234Th export fluxes in the western than eastern SAZ, both of which were higher than those in the PFZ. 234Th fluxes sampled by free-drifting IRS and PPS-3 sediment traps at 150 and 170 m during short-term deployments (∼ 6 days) at the three process stations were significantly lower than those obtained by the 234Th-deficit method. Possible reasons for this discrepancy are discussed. Carbon export fluxes were calculated based on the total 234Th fluxes ( 234Th deficit method) and the C:Th ratio for the >54 μm particle size fraction from the appropriate export depth. The >54 μm C:Th ratio was significantly lower in the eastern SAZ than the western SAZ or PFZ, resulting in carbon export fluxes that were lowest in the eastern SAZ. Overall, export fluxes range from 3.6±1.5 to 13.2±3.1 mmol C m −2 d −1. Carbon export fluxes are compared with gross primary production, new production and mesopelagic remineralization fluxes obtained by others during the same cruise. Contrary to expectations, we found higher export production in the PFZ and the western SAZ where biomass and dissolved Fe were lower than in the eastern SAZ. Significant differences in community structure of both primary producers and consumers likely contributed to this difference between the three regions. This pattern of higher shallow export continued into the mesopelagic, with lower remineralization efficiency in PFZ and SAZ-West compared to SAZ-East. Within the SAZ, east and west sites thus differed in their efficiency of carbon sequestration into the deep (>600 m) water column, with SAZ-West exceeding the sequestration capacity of SAZ-East. If the SAZ-East region can be used as an analog for a future climate warming scenario, our results suggest that export production at high latitudes could decrease, despite increased primary production.

Bacterial abundance and production in epipelagic and mesopelagic waters in the Subantarctic and Polar Front zones south of Tasmania

Heterotrophic bacteria influence the carbon export and consequently the efficiency of the biological carbon pump through the remineralization of organic matter. Bacterial remineralization was investigated during the SAZ-Sense cruise (January–February 2007) in the Subantarctic (SAZ) and Polar Front Zones (PFZ) of the Southern Ocean south of Tasmania, by combining bacterial biomass (BB) and bacterial production (BP) measurements in the epipelagic (0–100 m) and mesopelagic (100–700 m) water column. Bacterial carbon demand (BCD) was assessed using different conversion factors and growth efficiencies and was confronted to primary production and carbon export flux estimates. Surface layer bacterial biomass and production were higher in SAZ waters east of Tasmania (SAZ-East) compared to SAZ waters west of Tasmania (SAZ-West), while values at the PF were similar to those for the SAZ-West. At the PF, subsurface maximum values of bacterial production were observed. Bacterial parameters followed chl a and dissolved organic carbon distributions. Bacterial abundance, biomass and production drastically decreased below 100–200 m. However, depth-integrated biomass and activity rates revealed that the mesopelagic zone contributed significantly to the upper 700 m water column stocks (41–68% for BB) and rates (10–74% for BP). Highest and lowest contributions of mesopelagic BP to epi-plus mesopelagic water column BP were observed at the PF and in the SAZ-East, respectively. Results show that the SAZ-East region had a poor carbon sequestration efficiency compared to the SAZ-West and the PFZ. Despite some uncertainties in carbon flux estimations and discrepancies between methods the present study highlights the importance of studying bacterial dynamics in the twilight zone because of their significant role in shaping the carbon fluxes through the water column.

234Th in surface waters: Distribution of particle export flux across the Antarctic Circumpolar Current and in the Weddell Sea during the GEOTRACES expedition ZERO and DRAKE

As part of the GEOTRACES Polarstern expedition ANTXXIV/3 (ZERO and DRAKE) we have measured the vertical distribution of 234Th on sections through the Antarctic Circumpolar Current along the zero meridian and in Drake Passage and on an EW section through the Weddell Sea. Steady state export fluxes of 234Th from the upper 100 m, derived from the depletion of 234Th with respect to its parent 238U, ranged from 621±105 to 1773±90 dpm m −2 d −1. This 234Th flux was converted into an export flux of organic carbon ranging from 3.1 to 13.2 mmol C m −2 d −1 (2.1–9.0 mmol C m −2 d −1) using POC/ 234Th ratio of bulk (respectively >50 μm) suspended particles at the export depth (100 m). Non-steady state fluxes assuming zero flux under ice cover were up to 23% higher. In addition, particulate and dissolved 234Th were measured underway in high resolution in the surface water with a semi-automated procedure. Particulate 234Th in surface waters is inversely correlated with light transmission and pCO 2 and positively with fluorescence and optical backscatter and is interpreted as a proxy for algal biomass. High resolution underway mapping of particulate and dissolved 234Th in surface water shows clearly where trace elements are absorbed by plankton and where they are exported to depth. Quantitative determination of the export flux requires the full 234Th profile since surface depletion and export flux become decoupled through changes in wind mixed layer depth and in contribution to export from subsurface layers. In a zone of very low algal abundance (54–58°S at the zero meridian), confirmed by satellite Chl-a data, the lowest carbon export of the ACC was observed, allowing Fe and Mn to maintain their highest surface concentrations. An ice-edge bloom that had developed in December/January in the zone 60–65°S as studied during the previous leg had caused a high export flux at 64.5°S when we visited the area 2 months later (February/March). The ice-edge bloom had then shifted south to 65–69°S evident from uptake of CO 2 and dissolved Fe, Mn and 234Th, without causing export yet. In this way, the parallel analysis of 234Th can help to explain the scavenging behavior of other trace elements.

Intra-regional comparison of productivity, carbon flux and ecosystem composition within the northern Barents Sea

We assess inter-annual and spatial variability and pathways of carbon flow within lower trophic levels and between plankton and benthos. Simulations were performed of gross primary production over a 13-yr period (1995–2007) using a physical–biological coupled model (SINMOD). Model results are compared with interdisciplinary field research in the Northern Barents Sea, spanning a 3 yr period (2003–2005). Modelled productivity, presented for seven sub-regions around Spitsbergen, reveals that Atlantic influenced regions are highly productive and less influenced by inter-annual variability in comparison to seasonally ice-covered regions. In Atlantic Water influenced regions, the range of annual gross primary production is 106–134 g C m −2 yr −1, while seasonally ice-covered regions exhibit higher inter-annual variability but lower productivity (54–67 g C m −2 yr −1). We evaluate the pathways of primary production through the planktonic food web to the benthic compartment based on six stations from the field investigation representing different sub-regions and ecological scenarios. Comparing different ecological groups in terms of biomass and carbon turnover, we find that smaller planktonic organisms are major players in terms of carbon turnover (stations range 67–91%, average 77%), despite that larger groups often dominated in terms of biomass (stations range 22–72%, average 50%). Carbon budgets illustrated the importance of grazers for vertical flux regulation, as highest downward export coincided with lowest heterotroph consumption relative to autotroph biomass. Sediment carbon burial rates reflect longer time-scale (years), and these rates are relatively constant among the sampled field stations. Thus simulated patterns in annual productivity do not correspond with an increase in carbon burial in high-productivity regions. However, benthic biomass was higher in Atlantic Water influenced stations in good agreement with higher annual primary production and carbon export flux. While simulated primary production patterns in the sub-regions point to important physical drivers for productivity, field investigations provide important insights into the pathways of carbon flow and the relationships among lower trophic level groups. When combined, these approaches provide information on key ecological links to aid in evaluating the sensitivity of the Arctic Barents Sea ecosystem to longer term climatic variability.

Evidence for aggregation and export of cyanobacteria and nano-eukaryotes from the Sargasso Sea euphotic zone

Abstract. Pico-plankton and nano-plankton are generally thought to represent a negligible fraction of the total particulate organic carbon (POC) export flux in oligotrophic gyres due to their small size, slow individual sinking rates, and tight grazer control that leads to high rates of recycling in the euphotic zone. Based upon recent inverse modeling and network analysis however, it has been hypothesized that pico-plankton, including the cyanobacteria Synechococcus and Prochlorococcus, and nano-plankton contribute significantly to POC export, via formation and gravitational settling of aggregates and/or consumption of those aggregates by mesozooplankton, in proportion to their contribution to net primary production. This study presents total suspended particulate (>0.7 μm) and particle size-fractionated (10–20 μm, 20–53 μm, >53 μm) pigment concentrations from within and below the euphotic zone in the oligotrophic subtropical North Atlantic, collected using Niskin bottles and large volume in-situ pumps, respectively. Results show the indicator pigments for Synechococcus, Prochlorococcus and nano-eukaryotes are; (1) found at depths down to 500 m, and; (2) essentially constant, relative to the sum of all indicator pigments, across particle size fractions ranging from 10 μm to >53 μm. Based upon the presence of chlorophyll precursor and degradation pigments, and that in situ pumps do not effectively sample fecal pellets, it is concluded that these pigments were redistributed to deeper waters on larger, more rapidly sinking aggregates likely by gravitational settling and/or convective mixing. Using available pigment and ancillary data from these cruises, these Synechococcus, Prochlorococcus and nano-plankton derived aggregates are estimated to contribute 2–13% (5 ± 4%), 1–20% (5 ± 7%), and 6–43% (23 ± 14%) of the total sediment trap POC flux measured on the same cruises, respectively. Furthermore, nano-eukaryotes contribute equally to POC export and autotrophic biomass, while cyanobacteria contributions to POC export are one-tenth of their contribution to autotrophic biomass. These field observations provide direct evidence that pico- and nano-plankton represent a significant contribution to the total POC export via formation of aggregates in this oligotrophic ocean gyre. We suggest that aggregate formation and fate should be included in ecosystem models, particularly as oligotrophic regions are hypothesized to expand in areal extent with warming and increased stratification in the future.

Carbon export associated with free-drifting icebergs in the Southern Ocean

Enrichment of the pelagic ecosystem associated with the proliferation of free-drifting icebergs prompts questions about increased productivity and the export flux of organic carbon to the deep ocean with continued climate warming. Lagrangian Sediment Traps (LST) were deployed autonomously beneath a large tabular, free-drifting iceberg (C-18a) in the NW Weddell Sea during March and April 2009 to collect sinking particles at a depth of 600 m. Three LST deployments associated with C-18a, within a 30-km radius, collected sinking diatom frustules, dominated by Corethron pennatum and Fragilariopsis nana, euphausiid fragments, crustacean and fish fecal material, detrital aggregates and mineral grains. One LST deployment at a “control” site 74 km away in open water devoid of icebergs collected diatom frustules, euphausiid molts, crustacean fecal material and detrital aggregates. Phytoplankton abundance, microbial abundance and biomass were significantly higher in the LST samples than in open-water collections at 500 m depth. The mean mass flux and organic carbon flux associated with iceberg C-18a were twice as high, 124 mg m −2 d −1 and 5.6 mg C org m −2 d −1, respectively, than at the control site. A similar trend was observed in C org/ 234Th activity, being highest near C-18a and lowest at the control site. Extrapolation of the area of enrichment to 30 km radius around C-18a, 2826 km 2, produces an estimated mass flux of 350 tons d −1 and carbon flux of 15.8 tons C org d −1. Five similar sized icebergs to C-18a were identified in satellite images in a surrounding 47,636 km 2 area at the same time of sampling. Assuming a 30-km radius as the area of influence around each of these five icebergs, 46% of the total area would be enhanced with an export flux at 600 m of 122.4 tons C org d −1. The large numbers of smaller icebergs identified visually in this area would only increase this area of influence. Icebergs serve as areas of local enrichment and with increased proliferation, must be considered in the cycling of carbon in the Southern Ocean.

Sources and export fluxes of inorganic and organic carbon and nutrient species from the seasonally ice-covered Yukon River

National Science Foundation [0554781, 0436179, 0850957]; Frontier Research System for Global Change/Japan through the International Arctic Research Center at the University of Alaska Fairbanks; Northern Gulf Institute; University of Southern Mississippi

234Th-derived particulate organic carbon export flux in the western Arctic Ocean

To evaluate the particle dynamics and estimate the POC (particulate organic carbon) export flux from the euphotic zone in the western Arctic Ocean, 234Th-238U disequilibrium was applied during the second Chinese National Arctic Research Expedition (July 15–September 26, 2003). The POC export fluxes are estimated from the measured profiles of the 234Th/238U activity ratios and the POC/PTh ratios. The average residence times of the particulate and dissolved 234Th in the euphotic zone are 33 d and 121 d, and their average export fluxes are 480 dpm/m2d and 760 dpm/m2d, respectively. The scavenging and removal processes of particle reactive elements are active in the upper layer of the Chukchi Sea. The average residence time of 234Th increases from shelf to basin, while the export fluxes of 234Th decrease. The estimated POC export fluxes from the euphotic zone vary from 2.1 to 20.3 mmol/m2d, indicating that the western Arctic Ocean is an important carbon sink in summer due to efficient biological pump.

Low export flux of particulate organic carbon in the central Arctic Ocean as revealed by 234Th:238U disequilibrium

The loss of Arctic sea ice has accelerated in recent years. With the decline in sea ice cover, the Arctic Ocean biogeochemistry is undergoing unprecedented change. A key question about the changing Arctic Ocean biogeochemistry is concerning the impact of the shrinking sea ice cover on the particulate organic carbon (POC) export from the upper Arctic Ocean. Thus far, there are still very few direct measurements of POC export in the permanently ice‐covered central Arctic Ocean. A further issue is that the magnitude of the POC export so far documented in this region remains controversial. During the ARK‐XXII/2 expedition to the Arctic Ocean from 28 July to 7 October in 2007, we conducted a high‐resolution study of POC export using 234Th/238U disequilibrium. Depth profiles of total 234Th in the upper 200 m were collected at 36 stations in the central Arctic Ocean and its adjacent seas, i.e., the Barents Sea, the Kara Sea and the Laptev Sea. Samples were processed using a small‐volume MnO2 coprecipitation method with addition of a yield tracer, which resulted in one of the most precise 234Th data sets ever collected. Thorium‐234 deficit with respect to 238U was found to be evident throughout the upper 100 m over the Arctic shelves. In comparison, 234Th deficit was confined to the upper 25 m in the central Arctic Ocean. Below 25 m, secular equilibrium was approached between 234Th and 238U. The observed 234Th deficit was generally associated with enhanced total chlorophyll concentrations, indicating that in situ production and export of biogenic particles are the main mechanism for 234Th removal in the Arctic Ocean. Thorium‐234‐derived POC fluxes were determined with a steady state model and pump‐normalized POC/234Th ratios on total suspended particles collected at 100 m. Results showed enhanced POC export over the Arctic shelves. On average, POC export fluxes over the various Arctic shelves were 2.7 ± 1.7 mmol m−2 d−1 (the Barents Sea), 0.5 ± 0.8 mmol m−2 d−1 (the Kara Sea), and 2.9 ± 1.8 mmol m−2 d−1 (the Laptev Sea) respectively. In comparison, the central Arctic Ocean was characterized by the lowest POC export flux ever reported, 0.2 ± 1.0 mmol m−2 d−1 (1 standard deviation, n = 26). This value is very low compared to prior estimates and is also much lower than the POC export fluxes reported in other oligotrophic oceans. A ThE ratio (234Th‐derived POC export/primary production) of <6% in the central Arctic Ocean was estimated using the historical measurements of primary production. The low ThE ratio indicates that like other oligotrophic regimes, the central Arctic Ocean is characterized by low POC export relative to primary production, i.e., a tightly coupled food web. Our study strongly suggests that the current role of the central Arctic Ocean in C sequestration is still very limited. Meanwhile, this role might be altered because of global warming and future decline in sea ice cover.

Zonal and meridional patterns of phytoplankton biomass and carbon fixation in the Equatorial Pacific Ocean, between 110°W and 140°W

Primary production (P prim) and calcification (C calc) were measured in the eastern and central Equatorial Pacific during December 2004 and September 2005, between 110°W and 140°W. The design of the field sampling allowed partitioning of P prim and total chlorophyll a (B) between large (>3 μm) and small (0.45-3 μm) phytoplankton cells. The station locations allowed discrimination of meridional and zonal patterns. The cruises coincided with a warm El Niño Southern Oscillation (ENSO) phase and ENSO-neutral phase, respectively, which proved to be the major factors relating to the patterns of productivity. Production and biomass of large phytoplankton generally covaried with that of small cells; large cells typically accounted for 20-30% of B and 20% of P prim. Elevated biomass and primary production of all size fractions were highest along the equator as well as at the convergence zone between the North Equatorial Counter Current and the South Equatorial Current. C calc by >0.4 μm cells was 2-3% of P prim by the same size fraction, for both cruises. Biomass-normalized P prim values were, on average, slightly higher during the warm-phase ENSO period, inconsistent with a “bottom-up” control mechanism (such as nutrient supply). Another source of variability along the equator was Tropical Instability Waves (TIWs). Zonal variance in integrated phytoplankton biomass (along the equator, between 110° and 140°) was almost the same as the meridional variance across it (between 4° N and 4° S). However, the zonal variance in integrated P prim was half the variance observed meridionally. The variance in integrated C calc along the equator was half that seen meridionally during the warm ENSO phase cruise whereas during the ENSO-neutral period, it was identical. No relation could be observed between the patterns of integrated carbon fixation (P prim or C calc) and integrated nutrients (nitrate, ammonium, silicate or dissolved iron). This suggests that the factors controlling integrated P prim or C calc are more complex than a simple bottom-up supply model and likely also will involve a top-down grazer-control component, as well. The carbon fixation within the Equatorial Pacific is well balanced with diatom and coccolithophore production contributing a relatively steady proportion of the total primary production. This maintains a steady balance between organic and inorganic production, relevant to the ballasting of organic matter and the export flux of carbon from this important upwelling region.

Bathymetric zonation of deep-sea macrofauna in relation to export of surface phytoplankton production

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 399:1-14 (2010) - DOI: https://doi.org/10.3354/meps08388 FEATURE ARTICLE Bathymetric zonation of deep-sea macrofauna in relation to export of surface phytoplankton production Chih-Lin Wei1,*, Gilbert T. Rowe2, G. Fain Hubbard2,†, Amélie H. Scheltema3, George D. F. Wilson4, Iorgu Petrescu5, John M. Foster6, Mary K. Wicksten7, Min Chen8, Roe Davenport7,†, Yousria Soliman2, Yuning Wang9 1Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA 2Department of Marine Biology, Texas A&M University at Galveston, Galveston, Texas 77551, USA 3Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 4Australian Museum, Sydney, New South Wales 2010, Australia 5National Museum of Natural History ‘Grigore Antipa’, Bucharest 011341, Romania 6Department of Coastal Sciences, University of Southern Mississippi, Ocean Springs, Mississippi 39564, USA 7Department of Biology, Texas A&M University, College Station, Texas 77843, USA 8ExxonMobil Biomedical Sciences, Annandale, New Jersey 08801, USA 9Oceanside Biology Lab, San Francisco Public Utilities Commission, San Francisco, California 94132, USA *Email: weic@tamug.edu†Deceased ABSTRACT: Macrobenthos of the deep, northern Gulf of Mexico (GoM) was sampled with box cores (0.2 m2) along multiple cross-depth transects extending from depths of 200 m to the maximum depth of the basin at 3700 m. Bathymetric (depth) zonation of the macrofaunal community was documented for 6 major taxa (a total of 957 species) on the basis of shared species among geographic locations; 4 major depth zones were identified, with the 2 intermediate-depth zones being divided into east and west subzones. Change of faunal composition with depth reflects an underlying continuum of species replacements without distinct boundaries. The zonal patterns correlated with depth and detrital particulate organic carbon (POC) export flux estimated from remotely-sensed phytoplankton pigment concentrations in the surface water. The Mississippi River and its associated mesoscale eddies, submarine canyon, and deep sediment fan appear to influence the horizontal zonation pattern through export of organic carbon from the ocean surface and the adjacent continental margin. On the local scale, near-bottom currents may shape the zonation pattern by altering sediment grain size, food availability, and larval dispersal. This study suggests a macroecological relationship between depth, export POC flux, and zonation; parsimonious zonal thresholds need to be tested independently for other continental margin ecosystems. KEY WORDS: Northern Gulf of Mexico · Deep sea · Macrofauna · Zonation · Biogeography · Community structure · POC export flux · Macroecology Full text in pdf format Information about this Feature Article Supplementary material NextCite this article as: Wei CL, Rowe GT, Hubbard GF, Scheltema AH and others (2010) Bathymetric zonation of deep-sea macrofauna in relation to export of surface phytoplankton production. Mar Ecol Prog Ser 399:1-14. https://doi.org/10.3354/meps08388Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 399. Online publication date: January 28, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Climate, carbon cycling, and deep-ocean ecosystems

Climate variation affects surface ocean processes and the production of organic carbon, which ultimately comprises the primary food supply to the deep-sea ecosystems that occupy approximately 60% of the Earth's surface. Warming trends in atmospheric and upper ocean temperatures, attributed to anthropogenic influence, have occurred over the past four decades. Changes in upper ocean temperature influence stratification and can affect the availability of nutrients for phytoplankton production. Global warming has been predicted to intensify stratification and reduce vertical mixing. Research also suggests that such reduced mixing will enhance variability in primary production and carbon export flux to the deep sea. The dependence of deep-sea communities on surface water production has raised important questions about how climate change will affect carbon cycling and deep-ocean ecosystem function. Recently, unprecedented time-series studies conducted over the past two decades in the North Pacific and the North Atlantic at >4,000-m depth have revealed unexpectedly large changes in deep-ocean ecosystems significantly correlated to climate-driven changes in the surface ocean that can impact the global carbon cycle. Climate-driven variation affects oceanic communities from surface waters to the much-overlooked deep sea and will have impacts on the global carbon cycle. Data from these two widely separated areas of the deep ocean provide compelling evidence that changes in climate can readily influence deep-sea processes. However, the limited geographic coverage of these existing time-series studies stresses the importance of developing a more global effort to monitor deep-sea ecosystems under modern conditions of rapidly changing climate.

Enhancement of particulate organic carbon export flux induced by atmospheric forcing in the subtropical oligotrophic northwest Pacific Ocean

The effects of extreme atmospheric forcing on the export flux of particulate organic carbon (POC) in the warm oligotrophic nitrogen-limited northwest Pacific Ocean were examined in 2007 during the spring Asian dust storm period. Several strong northeast monsoon events (maximum sustained wind speeds approaching 16.7 m s − 1 , and gusts up to 19.0 m s − 1 ) accompanied by dust storms occurred during a 1-month period. The cold stormy events decreased surface water temperature and induced strong wind-driven vertical mixing of the water column, resulting in nutrient entrainment into the mixed layer from subsurface waters. As a result, the export flux of POC ranged from 49 to 98 (average value = 71 ± 16) mg m − 2 day − 1 , approximately 2–3 times greater than average values in other seasons. As dry and wet deposition of nitrogen attributable to Asian dust storm events does not account for the associated increases in POC stocks in this N-limited oligotrophic oceanic region, the enhancement of POC flux must have been caused by nutrient entrainment from subsurface waters because of the high winds accompanying the dust storm events.

Seasonal and spatial patterns of sedimentary denitrification rates in the Chukchi sea

Abstract Recent constructions of the global nitrogen budget estimate that at least half of the ocean's fixed nitrogen is lost by sedimentary denitrification, the majority of which occurs in continental shelves. The Arctic contains approximately 20% of the world's continental shelf, suggesting it is a substantial contributor to the global sedimentary denitrification rate. During two cruises in the summer and spring of 2002 and 2004, respectively, denitrification rates were calculated from the downward diffusive flux of nitrate in the shelf and slope sediments of the Chukchi Sea in the western Arctic. Additionally, in the spring of 2004, denitrification rates were determined by whole-core incubations in which the flux of nitrogen gas out of the sediments was measured. Measurements were made along three transects crossing the shelf and slope (50–3000 m), each transect having different overlying water characteristics. Denitrification rates generally decreased with increasing water depth: rates varied from about 1.6 mmol N m−2 d−1 for the shallow-water sediments to undetectable in deep-water sediments. Rates showed little variation between the two seasons. However, rates were found to correspond with differences in annual overlying primary productivities and particulate organic carbon export fluxes. An extrapolation to the whole Arctic yielded an average Arctic sedimentary denitrification rate of 13 Tg N yr−1. Taken in the context of the global nitrogen budget, it is about 4–13% of the total sink of fixed nitrogen in the ocean.

Tracing abyssal food supply back to upper‐ocean processes over a 17‐year time series in the northeast Pacific

Detrital aggregates episodically deposited on the seafloor represent an underestimated food source to deep‐sea communities. A 17‐yr time‐series study was conducted from 1990 to 2006 in the abyssal northeast Pacific (Sta. M, 4100 m in depth) to evaluate the importance of this food source and its temporal relationship to water column and surface ocean processes. Detrital aggregates appeared on the seafloor from June through December, with the highest peaks in 1990, 1994, 2001, and 2002 reaching a maximum density of 23 m−2 in fall 2001. A total of 15,816 aggregates were measured, most less than 20 cm2 in area and with a mode of 9 cm2. Density of detrital aggregates was highly correlated with particulate organic carbon (POC) flux at 600 and 50 m above the bottom (p < 0.001) with no time lag. Export flux of organic carbon from the euphotic zone was significantly correlated with aggregate density, lagged earlier by 1‐4 months (p ≤ 0.001). Zooplankton displacement volume was significantly correlated with POC flux (p = 0.023) and with detrital aggregate density (p = 0.028) on the seafloor when lagged earlier by ≤1 month. The Bakun upwelling index computed for the region around Sta. M was significantly correlated with detrital aggregate density when lagged earlier by 2‐5 months (p < 0.001). A strong correlation exists between surface ocean processes and abyssal food supply, including POC flux and detrital aggregates. This direct coupling through the entire water column must be considered in resolving the marine carbon cycle.

Benthic oxygen fluxes and denitrification rates from high-resolution porewater profiles from the Western Antarctic Peninsula continental shelf

Abstract Benthic fluxes of dissolved oxygen and nitrate were calculated from high-resolution porewater profiles collected on the continental margin of the Western Antarctic Peninsula. Profiles were collected in four seasons between March 2000 and February 2001 as part of the FOODBANCS program. Oxygen consumption rates ranged from 0.92 to 3.11 mmol O2 m−2 d−1 over the course of the year with an average annual oxygen consumption rate of 1.74 mmol O2 m−2 d−1. The oxygen fluxes follow a trend similar to the particulate carbon export flux with smaller fluxes during the winter and larger fluxes during the spring bloom period. However, the range in oxygen fluxes is substantially smaller than the range in the particulate carbon export. Denitrification rates ranged from 0.66 to 1.46 mmol N m−2 d−1, and the average annual denitrification rate was 1.29 mmol N m−2 d−1. The O2 consumption and denitrification rates are of similar magnitude to rates measured on other deep (∼500 m) continental margins. Denitrification rates are strongly coupled to nitrification rates, with coupled nitrification–denitrification accounting for more than 80% of the total denitrification rate in these sediments. The Antarctic continental-margin sediment denitrification rates correspond to ∼3–5 Tg N yr−1, and thus these continental-margin sediments account for roughly 1–2% of the global sediment denitrification signal.

Particle export within cyclonic Hawaiian lee eddies derived from 210Pb–210Po disequilibrium

Abstract Particle export from the upper waters of the oligotrophic ocean may play a crucial role in the global carbon cycle. Mesoscale eddies have been hypothesized to inject new nutrients into oligotrophic surface waters, thereby increasing new production and particle export in otherwise nutrient deficient regimes. The E-Flux Program was a large multidisciplinary project designed to investigate the physical, biological and biogeochemical characteristics of cold-core cyclonic eddies that form in the lee of the Hawaiian Islands. There, we investigated particle dynamics using 210Pb–210Po disequilibrium. Seawater samples for 210Pb and 210Po were collected both within (IN) and outside (OUT) of two cyclones, Noah and Opal, at different stages of their evolution as well as from the eddy generation region. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO2) export fluxes were determined using water-column PC, PN, and bSiO2 inventories and the residence times of 210Po. PC and PN fluxes at 150 m ranged from 1.58±0.10 to 1.71±0.16 mmol C m−2 d−1 and 0.22±0.02 to 0.30±0.02 mmol N m−2 d−1 within Cyclones Opal and Noah. PC and PN fluxes at OUT stations sampled during both cruises were of similar magnitudes, 1.69±0.16 to 1.67±0.16 mmol C m−2 d−1 and 0.30±0.03 to 0.26±0.03 mmol N m−2 d−1. The bSiO2 fluxes within Cyclone Opal were 0.157±0.010 mmol Si m−2 d−1 versus 0.025±0.002 mmol Si m−2 d−1 at OUT stations. These results of minimal PC and PN export, but significant eddy-induced bSiO2 fluxes, agree very well with other studies that used a variety of direct and indirect methods. Thus, our results suggest that using elemental inventories and residence times of 210Po is another independent and robust method for determining particle export and should be investigated more fully.

Interannual hypoxia variability in a coastal upwelling system: Ocean–shelf exchange, climate and ecosystem-state implications

In this study we use multi-year time series to examine the dynamic characteristics of coupled physical–biogeochemical processes that modulate interannual coastal hypoxia in the Benguela upwelling system in the southeast Atlantic. The results confirmed earlier findings on the role of advection to explain much of the seasonal–decadal variability. These results challenge the predominantly biogeochemical basis, namely benthic–pelagic coupling, to understand the variability of hypoxia and its ecosystem implications. Unexpectedly, the results showed that the variability was insensitive to changes in the electron-donating capacity (carbon export fluxes) but strongly dependent on the advected oxygen fluxes. The dynamics of the interaction of equatorial and polar boundary conditions (ocean–shelf exchange) and seasonally phased shelf advection were the key forcing functions that explained hypoxia variability in seasonal–decadal time scales. The vulnerability of the system to climate change lies in the long-term response of the equatorial system that governs seasonal and interannual warming at the Angola–Benguela front as well as wind stress in the Luderitz southern boundary that governs ventilation. The proposed model was able to explain most of the decadal scale variability of two different ecosystem-state indicators. The model predicts a long-term decline of present ecosystem function with climate change.

Seasonal and interannual changes in particulate organic carbon export and deposition in the Chukchi Sea

Particulate organic carbon (POC) export fluxes were estimated in the shelf‐slope region of the Chukchi Sea using measurements of 234Th−238U disequilibria and the POC/234Th ratio in large (>53‐μm) particles. These export fluxes were used in conjunction with rates of primary productivity and benthic carbon respiration to construct a POC budget for this shelf‐slope region. Samples were collected along a series of shelf‐basin transects in the spring (May–June) and summer (July–August) of 2004. These stations were previously occupied during the ice covered (spring) and open water (summer) seasons of 2002, allowing for an interannual comparison of export flux. In contrast to 2002, when open water POC fluxes were significantly higher than in the ice‐covered period, POC export fluxes in 2004 were similar during the spring (average = 19.7 ± 24.8 mmol C m−2 d−1) and summer (average = 20.0 ± 14.5 mmol C m−2 d−1). The high POC fluxes measured during the spring are attributed to a plankton bloom, as evidenced by exceptionally high rates of primary productivity (average = 124.4 ± 88.1 mmol C m−2 d−1). The shelf‐slope budget of particulate organic carbon indicates that 10–20% of primary productivity was exported below 50 m but was not consumed during benthic carbon respiration or burial and oxidation in underlying sediments. Furthermore, a water column−sediment budget of 234Th indicates that particulate material is retained in shelf sediments on a seasonal basis.