Sea ice concentration impacts dissolved organic gases in the Canadian Arctic

The marginal sea ice zone has been identified as a source of different climate active gases to the atmosphere due to its unique biogeochemistry. However, it remains highly undersampled and the impact of changes in sea ice concentration on the distributions of these gases is poorly understood. To address this, we present measurements of dissolved methanol, acetone, acetaldehyde, dimethyl sulfide and isoprene in the sea ice zone of the Canadian Arctic from the surface down to 60 m. The measurements were made using a Segmented Flow Coil Equilibrator coupled to a Proton Transfer Reaction Mass Spectrometer. These gases varied in concentrations with depth, with the highest concentrations generally observed near the surface. Underway (3–4 m) measurements showed broadly higher concentrations in partial sea ice cover compared to ice-free waters. The large number of depth profiles at different sea ice coverages enables proposition of the likely dominant production processes of these compounds in this area. Methanol concentrations appear to be controlled by specific biological consumption processes. Acetone and acetaldehyde concentrations are influenced by the penetration depth of light and the mixed layer depth, implying dominant photochemical sources in this area. Dimethyl sulfide and isoprene both display higher surface concentrations in partial sea ice coverage compared to ice-free waters due to ice edge blooms. Dimethyl sulfide concentrations sometimes display a subsurface maximum in ice -free conditions, while isoprene displays more reliably a subsurface maximum. Surface gas concentrations were used to estimate their air – sea fluxes. Despite obvious in situ production, we estimate that the sea ice zone is absorbing methanol and acetone from the atmosphere. In contrast, DMS and isoprene are consistently emitted from the ocean, with marked episodes of high emissions during ice-free conditions, suggesting that these gases are produced in ice-covered areas and emitted once the ice has melted. Our measurements show that the seawater concentrations and air-sea fluxes of these gases are clearly impacted by sea ice concentration. These novel measurements and insights will allow us to better constrain the cycling of these gases in the polar regions and their effect on the oxidative capacity and aerosol budget in the Arctic atmosphere.

A model of mercury cycling and isotopic fractionation in the ocean

Mercury speciation and isotopic fractionation processes have been incorporated into the HAMOCC offline ocean tracer advection code. The model is fast enough to allow a wide exploration of the sensitivity of the Hg cycle in the oceans, and of human exposure to Hg via monomethyl-Hg incorporation into fish. Vertical particle transport of Hg appears to play a discernable role in setting present-day Hg distributions, which we surmise by the fact that in simulations without particle transport, the high present-day Hg deposition rate leads to an Hg maximum at the sea surface, rather than a subsurface maximum as observed. Hg particle transport has only a relatively small impact on anthropogenic Hg uptake, but it sequesters Hg deeper in the water column, so that excess Hg is retained in the model ocean for longer after anthropogenic Hg deposition is stopped. The concentration of monomethyl Hg is sensitive to its production rate, with model experiments suggesting that human impacts on ocean oxygen concentrations could have as significant an impact on oceanic MMHg concentration as the anthropogenic Hg emission itself. Eight different isotopic fractionation mechanisms are simulated, independently and combined together, to predict their expression in the spatial distributions of isotopic signatures of Hg species in the ocean.

An audio-magnetotelluric investigation of the Otjiwarongo and Katima Mulilo regions, Namibia

As an additional opportunistic component to the southern African magnetotelluric experiment, natural-source audio-magnetotelluric (AMT) data were acquired during phase IV to investigate the local-scale electric conductivity subsurface structure in the Otjiwarongo and Katima Mulilo regions (Namibia) as an aid in locating the installation points for high-voltage direct current earth electrodes. The study showed that the shallow subsurface of areas containing one measurement site in the Otjiwarongo region and three sites in the Katima Mulilo region have appropriate high conductivities for the optimal placement of the earth electrodes. Both of the AMT surveys are situated close to the edge of the orogenic Neo-Proterozoic Damara mobile belt (DMB). Previous studies all suggest the existence of a highly conductive midcrustal zone, which correlates well with the spatial location of the DMB. Two-dimensional inverse modeling of the Otjiwarongo AMT data confirms the existence of the high-conductive zone at midcrustal depths (10–15 km). The high conductivity of the DMB is explained by the presence of interconnected graphite in the marble units present. The Katima Mulilo inversion results are characterized by a conductive upper crustal layer that does not form part of the DMB conductive belt. It was deduced that at the uppermost subsurface (maximum [Formula: see text]), Kalahari sediments are responsible for the high conductivity observed, whereas at greater depth (up to 6 km), its cause remains enigmatic, albeit the hypothesis of ironstone or graphite being present and causing the observed conductive upper crust.

Distributions and stoichiometry of dissolved nitrogen and phosphorus in the iron fertilized region near Kerguelen (Southern Ocean)

During KEOPS2 (Kerguelen Ocean and Plateau Compared Study 2), we determined dissolved inorganic and organic nitrogen and phosphorus species in the naturally fertilized region of Kerguelen Island (Southern Ocean). Above 150 m, stations were clearly separated by the Polar Front (PF), with concentrations of NO3–, NO2– and PO43– overall lower north than south of the PF. Though less pronounced, a similar trend was detectable for dissolved organic nitrogen (DON) and phosphorus (DOP). At all stations offshore and above the plateau, a subsurface maximum of NH4+ was observed between 50 and 150 m. We examined nutrient stoichiometry by calculating the linear combination N* = [NO3–] − 16[PO43– ]. The majority of stations and depths revealed N* close to −3 μM, however, for surface waters north of the PF N* increased up to 6 μM. This suggests a preferential uptake of PO43– vs. NO3– by fast growing diatoms. Using the tracer TNxs = [TDN] − 16[TDP] revealed that the dissolved organic fraction significantly contributed to changes in TNxs. TNxs were negative for most stations and depths, and relatively constant in the layer 0–500 m. As for N*, the stations north of the PF had higher TNxs in the layer 0–100 m. We discuss this stoichiometric anomaly with respect to possible external sources and sinks of N and P. Additional data collected in February 2013 at two sites revealed the occurrence of a subsurface of N* located just below the pycnocline that denotes a~layer where preferential remineralization of P vs. N persists throughout the season.

The Kuroshio Nutrient Transport during Winter of 2009

To confirm Spatial variations of Kuroshio nutrient transport from the East China Sea to South of Japan, we apply an inverse method to hydrographic data from sections across the Kuroshio path from the East China Sea (Sections PN and TK) to south of Japan (Sections ASUKA and 137E) to get absolute geostrophic velocity, then the nutrient flux (velocity times concentration) and nutrient transport (integration of flux over a section) were calculated. In addition, Section OK east of the Ryukyu Islands was also examined. The nitrate flux during winter of 2009 shows a subsurface maximum core with a value of 11, 15, 7, 19,and 10 mol m-2s-1at Sections PN, TK, OK, ASUKA and 137E, respectively. The depth of subsurface maximum core is about 280, 470, 800, 200, and 470 m at Sections PN, TK, OK, ASUKA and 137E, respectively. The eastward nitrate transport is 248.6,213.3,97.7,804.3,879.0 k mol s-1at Sections PN, TK, OK, ASUKA and 137E, respectively. Comparisons between nitrate transport through Section ASUKA and the sum of transports through Sections TK and OK and nitrate transport of Section 137E, suggest that the Kuroshio recirculation south of Shikoku can significantly intensify the eastward nitrate transport by the Kuroshio and therefore plays an important role in the nitrate transport in the Kuroshio region.

Spatial variations in the Kuroshio nutrient transport from the East China Sea to south of Japan

Based on absolute geostrophic velocity calculated from repeated hydrographic data of 39 cruises from 2000 to 2009 and nitrate concentrations measured at the same sections from 1964 to 2011, we obtained temporally averaged nitrate flux (the product of velocity and nitrate concentration) and nitrate transport (integration of flux over a section) through 4 sections along the Kuroshio path from the East China Sea (sections PN and TK) to south of Japan (sections ASUKA and 137E). In addition, we examined section OK east of the Ryukyu Islands in order to understand the contribution of Ryukyu Current to the Kuroshio nutrient transport south of Japan. The mean nitrate flux shows a subsurface maximum core with a value of 10, 10, 11, 11, and 6 mol m–2 s–1 at sections PN, TK, ASUKA, 137E, and OK, respectively. The depth of subsurface maximum core changes among five sections and is approximately 400, 500, 500, 400, and 800 m at sections PN, TK, ASUKA, 137E, and OK respectively. The mean downstream nitrate transport is 199.3, 176.3, 909.2, 1385.5, and 341.2 kmol m–1 at sections PN, TK, ASUKA, 137E, and OK respectively. The nutrient transports at these sections suggest the presence of Kuroshio nutrient stream from its upstream region to downstream. The deep current structure of Ryukyu Current (section OK) makes it contribute more nitrate transport than the Kuroshio in the East China Sea (section TK) to the Kuroshio south of Japan. In addition, the positive difference between the downstream nitrate transport through section ASUKA and the sum of nitrate transports through sections TK and OK, as well as the positive difference of downstream nitrate transport between sections 137E and ASUKA, suggest that the Kuroshio recirculation significantly intensifies the downstream (eastward) nitrate transport by the Kuroshio.

Modelling the vertical distribution of bromoform in the upper water column of the tropical Atlantic Ocean

The relative importance of potential source and sink terms for bromoform (CHBr3) in the tropical Atlantic Ocean is investigated with a coupled physical-biogeochemical water column model. Bromoform production is either assumed to be linked to primary production or to phytoplankton losses; bromoform decay is treated as light dependent (photolysis), and in addition either vertically uniform, proportional to remineralisation or to nitrification. All experiments lead to the observed subsurface maximum of bromoform, corresponding to the subsurface phytoplankton biomass maximum. In the surface mixed layer, the concentration is set by entrainment from below, photolysis in the upper few meters and the outgassing to the atmosphere. The assumed bromoform production mechanism has only minor effects on the solution, but the various loss terms lead to significantly different bromoform concentrations below 200 m depth. The best agreement with observations is obtained when the bromoform decay is coupled to nitrification (parameterised by an inverse proportionality to the light field). Our model results reveal a pronounced seasonal cycle of bromoform outgassing, with a minimum in summer and a maximum in early winter, when the deepening surface mixed layer reaches down into the bromoform production layer.

ESTIMACIÓN DE LA PRODUCTIVIDAD PRIMARIA EN DOS BAJOS DE LA PARTE SUR DEL GOLFO DE CALIFORNIA, MÉXICO

Primary productivity estimation in two seamounts in the southern Gulf of California, México Vertical profiles of temperature and natural fluorescence from 100 m deep were made during February 2005. Water transparency was measured using Secchi’s disc, as well samples of superficial water and at maximum of fluorescence deep were collected to analyze inorganic nutrients. In “El Bajo Espiritu Santo” temperature (20 °C at surface) diminished gradually with depth, without significant stratification.Primary productivity shows superficial values close to 6 mg C m-3 h-1, recahing undetectable values at 20 m of depth. In “El Bajo Gorda” surface temperature reached 22 °C and the water column shows a thermocline between 35 m and 45 m of depth. The profiles of primary productivity presented a subsurface maximum (approximately 2 mg C m-3 h-1) associated with the thermocline.

Modelling the vertical distribution of bromoform in the upper water column of the tropical Atlantic Ocean

The relative importance of potential source and sink terms for bromoform (CHBr3) in the tropical Atlantic Ocean is investigated with a coupled physical-biogeochemical water column model. Bromoform production is either assumed to be linked to primary production or to phytoplankton losses; bromoform decay is treated as light dependent (photolysis), and in addition either vertically uniform, proportional to remineralisation or to nitrification. All experiments lead to the observed subsurface maximum of bromoform, corresponding to the subsurface phytoplankton biomass maximum. In the surface mixed layer, the concentration is set by entrainment from below, photolysis in the upper few meters and the outgassing to the atmosphere. The assumed bromoform production mechanism has only minor effects on the solution, but the various loss terms lead to significantly different bromoform concentrations below 200 m depth. The best agreement with observations is obtained when the bromoform decay is coupled to nitrification (parameterised by an inverse proportionality to the light field). Our model results reveal a pronounced seasonal cycle of bromoform outgassing, with a minimum in summer and a maximum in early winter, when the deepening surface mixed layer reaches down into the bromoform production layer.

In situ evidence for a nutrient limitation of phytoplankton growth in pelagic Antarctic waters

Studies in a large (30000 km2) sampling grid around Elephant Island, Antarctica, during January–March of four successive years (1990–1993) have shown that one of the water types within the sampling area (Drake Passage water) shows low chlorophyll a in surface waters and a subsurface maximum between 50 and 80 m depth. Ancillary data (beam attenuation, in situ chl a fluorescence) support the view that the extracted chl a values actually do represent increased phytoplankton biomass at depth; other data (oxygen concentrations and upwelling radiance at 683 nm) suggest that the phytoplankton within this subsurface maximum layer are photosynthetically active and do not represent a senescent, sinking population of cells. Such deep chl a maxima were found only in Drake Passage waters; in the other four water types sampled, chl a concentrations were maximal in surface waters and decreased with depth. Phytoplankton biomass and activity in Drake Passage waters is suggestive of a nutrient limitation for phytolankton growth in surface waters. Nutrient concentrations of N, P, and Si were high throughout the euphotic zone at all stations, and hence it is unlikely that any macronutrient would be limiting. The data presented in this paper support the hypothesis of Martin and colleagues that availability of Fe may limit phytoplankton biomass in pelagic Antarctic waters, but not in coastal waters where Fe concentrations are relatively high. All other reports on the effects of Fe on Antarctic phytoplankton have utilized deck incubations from which it is difficult to extrapolate such evidence of nutrient limitation to in situ conditions. Our data represent the first in situ evidence linking Fe limitation to the paradox of high macronutrient concentrations and low phytoplankton biomass in Antarctic pelagic waters.