OS11B-0501
Estimates of Export production in Sargasso Sea During Winter Storms Prior to Seasonal Stratification
Direct estimates of primary production in the subtropical North Atlantic Ocean appear to be too low when compared to geochemical based estimates of carbon export. It has been suggested that difficult to measure episodic inputs of new nutrients to surface waters by eddies or storms may solve this discrepancy. In this study, we investigated carbon export using 234Th:238U disequilibria and free-floating sediment traps during and immediately following two weather systems encountered in February and March 2004. These storms resulted in a 2-4 fold increase in mixed layer NO3 inventories and a subsequent increase in autotrophic biomass. The average of 234Th based and trap based particulate organic carbon (POC) and biogenic silica (bSiO2) fluxes at 200 m during this study period were 3.39 ± 1.39 mmol C m-2 d-1 and 0.36 ± 0.15 mmol Si m-2 d-1, respectively. While POC export was elevated by a factor of two relative to average winter estimates in this region, it still remains well within the range of non-storm winter fluxes observed in the region. The C export efficiency (export/primary production) was ~ 7%, similar to the ~ 4% observed under ambient conditions. Thus, while the passage of weather systems perturb the system by leading to increased productivity, autotrophic biomass (particularly diatoms), and export, they do not appear to increase the efficiency at which carbon is removed from the system by particle sinking. In contrast, the three fold increase in bSiO2 fluxes observed during storms is significantly higher (p < 0.007) than that typically observed. This suggests that, given the vastness of the North Atlantic mid-ocean gyre, winter storms may significantly impact silica cycling in this region.
OS11B-0502
Organic nutrient enrichment in the oligotrophic ocean: Impacts on remineralization, carbon sequestration, and community structure
In oligotrophic seas where inorganic nitrogen (N) and phosphorus (P) are below the limits of detection, organic forms of these nutrients may constitute greater than 90% of the total N and P in the euphotic zone. The combined enzymatic activity of phytoplankton and heterotrophic bacteria determines the rate of nutrient remineralization, thereby influencing phytoplankton growth rates and carbon sequestration in these regions. In this study we investigated the effects of fertilization with ammonium (NH4), nitrate (NO3), nitrite (NO2), and phosphate (PO4) as well as various forms of organic N (urea, glycine) and P (deoxyribonucleic acid, 2- aminoethyl phosphonic acid, phytic acid) on the growth and taxonomic composition of the phytoplankton community in the Gulf of Aqaba, Red Sea. The impacts of these changes on nutrient cycling and biological assimilation were also assessed. Organic N additions led to phytoplankton growth when given together with PO4, yielding 2-3 fold increases in chlorophyll a (Chl a) and cell density relative to initial levels. Moreover, our results show that addition of NH4 or NO3 led to accumulation of extra-cellular NO2, suggesting that incomplete assimilatory reduction of NO3 by phytoplankton as well as chemoautotrophic oxidation of NH4 by ammonium oxidizing microbes contributed to NO2 formation. These findings conflict with earlier studies in the Gulf that attributed NO2 formation solely to the phytoplankton community. Organic P additions also led to 2-3 fold increases in Chl a and cell density relative to initial levels when given together with NH4 and NO3. Compared to other P additions, DNA led to the rapid accumulation of extra-cellular PO4, indicating substantial nucleotidase activity in excess of the amount needed to meet phytoplankton growth requirements. These results show the importance and interconnectivity of phytoplankton and heterotrophic bacteria communities in contributing to nutrient cycling and carbon sequestration in oligotrophic marine regions.
OS11B-0503
Of The Effect Of Mesoscale Structures On The Deep Chlorophyll And Particle Maxima At The Entrance To The Gulf Of California
The entrance to the Gulf of California is characterized by the convergence of three surface water masses resulting in a complex mesoscale field. The effects of such structures on the deep chlorophyll maximum, an ubiquitous feature of many oceanic regions, is not well known. An interdisciplinary oceanographic survey was carried out in February-March 2005. The main goal was to study the effect of mesoscale eddies on the biology of an oligotrophic ocean. Several cyclonic and anticyclonic structures were found as well as a narrow and intense current jet off the coast of Baja California. Inside cyclonic/anticyclonic eddies a relative increase/decrease of both chlorophyll and dissolved oxygen concentration is clearly observed. The deep chlorophyll maximum is found positioned above the thermocline throughout the survey. Higher concentrations are found inside the deep chlorophyll maximum inside the cyclonic eddy and in the deep chlorophyll maximum of the coastal jet. The latter case is consistent with the enhancement of chlorophyll concentrations by advective processes. This confirms that advection plays an important role in the enhancement observed inside such mesoscale structures. These eddies are advecting cold, upwelling-enriched California Current waters. Anticyclonic eddies show the opposite behavior because they contain warm tropical waters, typically poor in nutrients and low in chlorophyll concentrations. A one-dimensional model is used to determine the effect of dynamical variations generated by mesoscale structures, when growth balances the sinking of phytoplankton in a mixed vertical structure within the surface mixed layer, model results suggest that vertical diffusion is an important mechanism capable of diluting phytoplankton concentrations.
OS11B-0504
Diel Cycle of Photosynthetic Electron Transport and Fluorescence Characteristics in Natural Phytoplankton Communities.
Phytoplankton photosynthetic performance is strongly controlled by the daily irradiance cycle. The most pronounced effects are the photoinhibition of photosynthetic activity in the morning and noon hours, and the development of non-photochemical quenching throughout the day. These two effects are extensively investigated as they significantly diminish the daily production rates. Less obvious, but equally important are the daily changes in the kinetics of rate-limiting electron transport within Photosystem II and Photosystem I of the photosynthetic apparatus. Using a fast repetition rate (FRR) fluorometer operating in the continuous flow-through mode in the Eastern Tropical Pacific, we observed theses rates to decelerate by a factor of five during the night, but recovering to a full speed just before the dawn. We characterized the effects of these changes on the photosynthetic performance of phytoplankton by continuously recording the fast light curves (variable fluorescence versus irradiance relationship). Besides controlling photochemistry, these changes strongly affect the chlorophyll fluorescence yield, especially when measured with a multiple turnover excitation. We will discuss how the knowledge of these rate-limiting steps may improve the fluorescence-based estimates of photosynthesis and chlorophyll biomass.
OS11B-0505
Living coccolithophores from the Bay of Bengal (NE Indian Ocean)
Sediment traps collected from three mooring sites located longitudinally at northern (NBBT), central (CBBT) and southern (SBBT) Bay of Bengal were analyzed for coccolithophores. It yielded twenty five species of coccolithophores represented both by coccospheres and disintegrated individual coccoliths. Coccolithophore fluxes showed that the highest fluxes occurred at CBBT with an average flux of 114.68 x105 m-2 d-1 followed by SBBT and NBBT with the average values of 109.35 x105 m-2 d-1 and 41.55 x105 m-2 d-1 respectively in the shallow traps. Similarly, except for a significant increase in F.profunda at SBBT and CBBT and its decrease at NBBT, the remaining species continued to display similar trend in the deep traps (2146-3011m depths) also. It was also observed that Gephyrocapsa oceanica, was the most abundant species followed by Umbilicosphaera sibogae, Umbellosphaera irregularis, Florisphaera profunda, Emiliania huxleyi, Calcidiscus leptopora and Olithotus antillarum in the shallow traps (1156 to 2558m depths). Overall fluxes of coccolithophores observed in the Bay of Bengal were comparatively lower than those reported from the Arabian Sea. Besides, temporal distribution clearly displays distinct seasonality. Total fluxes increased with depth from shallow to deep traps due to lateral advection, which was very conspicuous at SBBT. In general species such as G. oceanica, U. sibogae¬ and E. huxleyi were dominant displaying a distinct seasonality with peak fluxes associated with SW and NE monsoons, reflecting their affinity to nutrient rich waters accomplished by river plumes, upwelling and advection at NBBT, CBBT and SBBT respectively. While U. irregularis, an indicator of oligotrophic conditions recorded peak fluxes only during spring intermonsoon period at all the three trap locations demonstrated its preference for nutrient depleted waters caused by increased sea surface temperatures, stratification and lack of supply of nutrients. F. profunda a deep dwelling species recorded its peak fluxes during fall intermonsoon (Oct/Nov) in the northern and the southern traps and during spring intermonsoon (Apr/May) in the central and the southern trap indicating the presence of a deep nutricline. Interestingly, coccoliths affected by very conspicuous dissolution and mechanical break down were encountered at all the three moorings most part of the year. This may by attributed to intense biological activity.
OS11B-0506
Flow-Through Leaching of Marine Barite: New Insights on its Composition and Diagenesis
The distribution of stable mineral barite (BaSO4) in marine sediments has long been studied as a proxy for paleoproductivity. It is important to investigate the variation in Sr/Ba ratios of crystal barite, as it has a great influence on barite solubility and its early diagenetic processes. In addition, the role of alternative barium carriers to the sediments (e.g. aluminum silicates and oxyhydroxides) and their contributions to overall barium budget and burial efficiency need to be resolved. The techniques currently used to describe and quantify barium phases are all based on batch leaching techniques that define barium phases operationally, not chemically. Because during batch analyses each phase is characterized by a single-point measurement, variations due to phase heterogeneities cannot be resolved; nor can the results of these experiments be related in any systematic way to what happens in nature. To overcome this problem, we are developing a flow-through method that makes use of automated chromatographic techniques, which allows complete monitoring of the dissolution of barite samples with time-resolved analysis (TRA) as each phase is sequentially leached using different reagents. We have analyzed a barite sample recovered from seeps along the San Clemente escarpment, and show that we can attain complete dissolution of the sample (>85%) in 2 hours, using DTPA at 80°C. Approximately 100 μg of barite are first leached with distilled water (pH 5) for 30 minutes. During this step ~2% of the barite is removed. This highly soluble phase has Sr/Ba ratios that range from 30 to 120 mmol/mol. Acid leaching of the samples with 10 mM HNO3 removes an additional 4~8% of the barite, and this phase has Sr/Ba ratios ranging from 13 to 35 mmol/mol. Higher acid concentration (100 mM HNO3) dissolves up to 40% of the barite. These results are consistent with electron microprobe data that show clear oscillatory zoning of the (Ba,Sr)SO4. Unlike the barite sample, sediment samples collected at the base of the escarpment did not show a Ba release in the water leach. We might speculate that the highly susceptible Sr-rich barium phase present in the barite sample, dissolved during transport from a seep site leaving a barite with a lower Sr/Ba ratio, as found in the sediment samples. Our analytical approach has the potential to further address a variety of outstanding questions on the complex geochemical cycle of barium and its applications to climate change, upper ocean fertility and ocean circulation through time.
OS11B-0507
Determination of Trace Aldehydes and Ketones in Seawater Using a Derivatization-Solid- Phase Microextraction (SPME) Method
Volatile aldehydes and ketones are thought to originate from the photochemistry of dissolved organic matter (DOM) in surface ocean waters. They are of interest both because they represent a sink of a portion of marine DOM and because, after transfer across the sea-air interface, they may affect oxidative processes in the marine troposphere, including the tropospheric HOx budget. Therefore, facile, low-cost methods to determine their typical concentrations in seawater are needed. We here present a method based on derivatization of these carbonyl compounds to their pentafluorobenzyl oximes followed by solid phase microextraction (SPME) method. The method allows the facile, low-cost, portable, direct characterization of C1 - C9 carbonyl compounds in seawater with sub-nanomolar detection limits, and its optimization and application to surface seawaters from selected locations is presented.
OS11B-0508
Vertical Distributions of Iron Complexing Ligands in the Southern Ocean
In order to better understand the origins of iron-(III) complexing ligands in the open ocean it is essential to first investigate their vertical distributions over the water column. This paper presents profiles of iron-(III) complexing ligands through the upper 1000 m of a high nitrate low chlorophyll (HNLC)region of the Southern Ocean, south of Australia in austral summer (January – February, 2007). Ligand concentrations were high (0.86 nM) in the upper 25 m, but decreased with depth towards the deep chlorophyll maximum (DCM), which was centered at 40 m. Ligand concentrations again peaked immediately below the DCM but remained relatively constant at about 0.6 nM below 200 m. The high ligand concentrations in the upper 25 m surface mixed layer is indicative that ligands are produced by the biota in iron-depleted surface waters. Additional high ligand concentrations measured just below the DCM may suggest that the remineralization of phytoplankton and detritus pool also contribute to the total ligand pool.
OS11B-0509
Volcanic ash as an iron-fertilizer in ocean surface water
Surface ocean fertilisation with iron may affect the marine primary productivity, C-cycles and eventually climate development. Volcanic ash has the potential to release iron on contact with seawater and to stimulate phytoplankton growth (1,2) but the relative importance of volcanism at destructive plate margins (subduction zones, SZ) and intraplate volcanic settings (ocean islands at hot spots) remains unknown. Here we present new results from geochemical experiments with natural seawater and numerous volcanic ash samples from SZ volcanoes in the Pacific Ring of Fire (Alaska, Japan, Kamchatka, Northern and Central America and Papua New Guinea) and hot spot volcanoes (on Iceland and Hawaii). The release of iron as a function of time was determined in situ in seawater by means of Cathodic Stripping Voltammetry. Our experiments show that: A) volcanic ash from both SZ and hot spot volcanic areas mobilise significant amounts of iron, B) with the highest mobilisation rates within the first 10-20 minutes and C) indicate that volcanic ash from hot spot volcanoes mobilise less iron than volcanic ash from SZ. We propose that the higher iron-mobilisation potential of SZ volcanic ash results from higher HCl/HF ratios in SZ volcanic gases that seem to be involved in the formation of Fe-bearing soluble salt coatings (condensed gases and adsorbed aerosols) on ash particles (1,2,3). Higher HCl/HF ratios in SZ volcanic gases thus appear to be linked to the recycling of seawater through subduction of oceanic lithosphere at destructive plate margins. Together, taking into account differences in ash-fluxes from SZ and hot spot volcanoes into the oceans, our study suggests that SZ volcanic ash plays a more important role for the global surface ocean iron budget than ash from volcanoes in hot spot areas. 1 Frogner, Gislason, Oskarsson (2001). Geology, 29, 487-490. 2 Duggen, Croot, Schacht, Hofmann (2007) Geoph. Res. Letters 34, 5. 3 Oskarsson (1980), J. Volc. and Geoth. Res. 8, 251-266.
OS11B-0510
Lead and lead isotopes in the North Pacific: mid-depth maxima and deep water anthropogenic source
Lead (Pb) concentrations and isotopic ratio in seawater samples that were collected in a section from 7 „aS to 30 „aN along 158 „aW in the central Pacific ocean were determined using Mg(OH)2 coprecipitation ICPMS. Surface water Pb concentrations decrease by 30-50% since 1980's, probably due to the phasing-out of leaded gasoline in Japan. There is pronounced mid-depth maxima for Pb at ~ 600 m in subtropical oceans which coincides with salinity minima and the maxima of CFC, tritium Pu, Cs, Co, Bi and Fe, indicating the influence by the advection of the North Pacific intermediate water. 206Pb/207Pb ratios at 30¢XN are relatively constant in the upper 500 m, increase with increasing depth below 500 m and are lower than those for the Pleistocene sediments and Mn nodule, suggesting that anthropogenic Pb may have contaminated some of oldest waters of the world ocean, possibly via Pb regeneration during microbial degradation of sinking organic debris.
OS11B-0511
Anthropogenic Osmium in Precipitation
Here we report the Os isotopic composition for precipitation from Hanover (NH), Soda Springs (CA) and the Ross Sea (Antarctica) as determined by negative thermal ionization mass spectrometry. All samples yielded non- radiogenic Os isotopic compositions. Snow and rain samples from Hanover, NH had Os concentrations of 0.8 - 12.2 fg/g (1 fg/g = 1E-15 g/g) and 187Os/188Os from 0.16 - 0.24. Snowpack from the high Sierra Nevada (Central Sierra Snow Laboratory, Soda Springs, CA) yielded Os concentration and isotopic composition of 3.6 fg/g and 0.21, respectively; Antarctic snow deposited above first year pack ice had [Os] = 0.8 fg/g and 187Os/188Os = 0.42. The isotopic ratios indicate that potential natural sources of Os to the atmosphere, such as continental mineral aerosols (187Os/188Os = 1.26) and seawater (187Os/188Os = 1.05) do not contribute bulk of Os to the precipitation. Instead, the isotopic ratios are identical to the platinum ores from the Merensky Reef in the Bushveld Igneous Complex, South Africa and Noril'sk Ni-Cu sulfide deposit associated with the Siberian Flood Basalts, Russia. These two deposits produce greater than 95 percent of the total Pt, Pd and Rh consumed annually primarily by the automotive industry. We infer that anthropogenic Os contribution dominates the isotopic composition of precipitation. The similar and non-radiogenic Os isotopic compositions observed in precipitation from disparate locations suggest that contamination of the troposphere with anthropogenic Os may be global in scale. We think that processing of ore to extract Pt, Pd, and Rh from PGE ores (PGE: group of six closely related elements Os, Ir, Pt, Pd, Rh, and Ru), which involves smelting and converting at high temperature and in the presence of oxygen, releases the volatile, toxic compound OsO4 into the troposphere, where it is mixed and then scavenged by precipitation, thus explaining both the non-radiogenic isotopic composition and the high and variable Os concentrations of Os in the precipitation. The magnitude of this contamination is likely sufficient to alter the global Os cycle and impact the isotopic composition of Os in the surface ocean.
OS11B-0512
137Cs inventory in semi-isolated basins of the western South Pacific
The main introduction routes of 137Cs into the Pacific Ocean are worldwide global fallout from atmospheric nuclear weapons testing and close-in fallout from U. S. tests conducted on the Bikini and Enewetak Atolls. The objectives of this study are to measure the 137Cs activities in water columns of the western South Pacific Ocean and to discuss the processes controlling the 137Cs inventory. The 137Cs activities were determined for seawater samples from the East Caroline, Coral Sea, New Hebrides, South Fiji and Tasman Sea Basins of the western South Pacific Ocean. The 137Cs activities in surface waters ranged from 1.7 Bq m- 3 in the Tasman Sea Basin to 2.3 Bq m-3 in the East Caroline Basin. The latitudinal 137Cs distributions in surface waters showed the opposite trend to the expected deposition density from global fallout. The distribution profiles of 137Cs activity at these six western South Pacific Ocean stations did not differ from each other significantly. The total 137Cs inventories in the western South Pacific Ocean ranged from 850 Bq m-2 in the Coral Sea Basin to 1270 Bq m-2 in the South Fiji Basin. Higher 137Cs inventories were observed at middle latitude stations in the subtropical gyre than at low latitude stations. The 137Cs inventories were 1.9 - 4.5 times higher than that of the expected deposition density of atmospheric global fallout at the same latitude. The possible sources of excess 137Cs inventories in the western South Pacific Ocean might be attributable to both the inter-hemisphere dispersion of the atmospheric nuclear weapons testing 137Cs from the northern stratosphere to the southern one and its subsequent deposition, and water- bearing transport of 137Cs from the North Pacific Ocean to the South Pacific.