Ocean Sciences [OS]

 CC:715A  Wednesday  1400h

AGU Fellows Ocean Sciences Presentations

Presiding:  P Schlosser, Columbia University


Biogeochemical Impacts of a Western Iron Source in the Pacific Equatorial Undercurrent

* Murray, J W (jmurray@u.washington.edu), University of Washington, School of Oceanography Box 355351, Seattle, WA 98195-5351, United States
Slemons, L O (ossianla@u.washington.edu), University of Washington, School of Oceanography Box 355351, Seattle, WA 98195-5351, United States
Gorgues, T (tomgorg@u.washington.edu), University of Washington, School of Oceanography Box 355351, Seattle, WA 98195-5351, United States

Profiles of total acid soluble iron in the western equatorial Pacific (∼155° E) have a maximum associated with the equatorial undercurrent (EUC). This maximum is mostly in the form of particulate iron that appears to originate from rivers and sediments along the northeastern continental margin of Papua New Guinea. There does not appear to be a corresponding maximum for filterable or "dissolved" Fe. We conducted a model simulation (OPA/PISCES) in which this western iron source imposed in the EUC was transported to the east and we evaluated its impact on biogeochemical distributions. We treated all of the total acid soluble iron as if it was 100% bioavailable. A control simulation without the enhanced iron source was run for reference. In the source runs the concentrations of iron decrease from west to east, primarily due to scavenging. The western iron source can explain the maxima in total iron (dissolved plus particulate) previously observed at 140° W. But the control runs did a better job of reproducing the climatological fields of NO3 and chlorophyll. With the source runs NO3 was much lower and chlorophyll is much higher than expected. Diatom production was also excessively enhanced. There were a few examples where the source runs reproduced the data better such as zonal gradients of surface nitrate along the equator and the meridional gradients of primary productivity and carbon export production. Overall, the implications are that most of the total acid soluble iron in the EUC is not bioavailable to phytoplankton in the eastern equatorial Pacific. Even though there is a maximum in acid soluble iron associated with the EUC not all of this iron is available for biological uptake.


Shedding Light on the Ocean's Biological Pump and Twilight Zone Processes

* Buesseler, K O (kbuesseler@whoi.edu), Woods Hole Oceanographic Institution, Mail Stop #25, Clark-447, Woods Hole, MA 02543, United States

Pelagic food webs drive a flux of >10 Gt C yr-1 that exits surface waters, mostly via sinking particles through the ocean's biological pump. Much of this particle flux is remineralized in the poorly studied waters of the twilight zone, i.e. the layer underlying the euphotic zone and extending to 1000 m. Fluxes through this layer are important, as they help set surface to deep ocean gradients in dissolved inorganic carbon, and hence influence the partitioning of carbon between the ocean and atmosphere, and thus the net uptake of CO2 by the sea. These fluxes also provide the organic C supply required to support mid-water heterotrophs, who's current C demand cannot be balanced by most estimates. Also, these sinking materials provide a rapid transport mechanism for all particle reactive elements and bioactive materials to the deep sea, thus setting geochemical constraints on ocean processes. In this talk, a brief history of marine studies of the biological pump will be presented followed by a reanalysis of selected twilight zone data. This reanalysis leads to a new conceptual framework and ecosystem model with which to compare the strength and efficiency of the biological pump between different settings and during seasonal variations in the biological pump (Buesseler and Boyd, Limnology and Oceanography, 2009, in review). We need to be more careful in our consideration of variability in euphotic zone depths, as we compare the biological pump between different regions and seasons. The ratio of POC flux at the base of the euphotic zone (Ez) to net primary production is called the Ez-ratio, to distinguish it from depth normalized flux export ratios (e-ratios). Conventional curve fitting of particle flux data can skew our interpretation of twilight zone processes, and thus attenuation below Ez is parameterized by the ratio of POC flux 100 m below Ez to the flux at Ez. This transfer efficiency, T100, varies from <20% to 100%. These new metrics are used to classify the ocean into regions and times of high and low surface export and subsurface attenuation. Future twilight zone research will benefit from combined studies of geochemical properties and biological processes that consider variability in these new metrics within a unified sampling strategy and model framework.



Advances in Chemical Oceanography Made With Microelectrodes

* Reimers, C E (creimers@coas.oregonstate.edu), Oregon State University, College of Oceanic and Atmospheric Sciences Hatfield Marine Science Center, Newport, OR 97365, United States

Many of the remarkable biogeochemical processes that regulate the transfer of mass and energy between the atmosphere, ocean waters, the benthos and the Earth's crust take place at small spatial scales, e.g., within single cells, pores of sediments and rocks, aggregates, microbial mats, or biofilms. Over the past two decades, with advances in electroanalytical chemistry and microelectronics technology, it has become progressively possible to probe marine microenvironments and interfaces, and to discover how marine chemistry and life interact. Some of this exploration has been in the laboratory, but a significant portion has been possible because of in situ techniques designed for extreme or dynamic environments (e.g., euxinic seas, the seafloor, hydrothermal vents). This presentation will review past and present roles of microelectrodes in providing fundamental information about the chemical reactions which structure the marine environment.


A Diagnostic Model of Mixed Layer Depth Variability with Application to the Northeast Pacific

* Thomson, R E (Richard.Thomson@dfo-mpo-gc.ca), Fisheries and Oceans Canada, Institute of Ocean Sciences 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
Fine, I V (Isaac.Fine@dfo-mpo.gc.ca) AB: Direct determination of the surface mixed layer depth requires the measurement of turbulent mechanical mixing or observations from high precision temperature-conductivity profilers. Long-term measurements of these variables are difficult to sustain so that investigators typically seek estimators for mixed layer depth that can be derived from more readily available oceanic and meteorological time series. This study uses a one- dimensional heat balance equation and remotely sensed surface data to formulate a simple diagnostic model for determining the depth of the mixed layer. Daily time series of mixed layer depth from early spring to late fall can be closely approximated using only records of the sea surface temperature and surface heat flux. A test of the diagnostic model based on the 55-year series of oceanographic and meteorological data from Ocean Station "P" (50 N, 145 W) and data from Argo drifters for the northeast Pacific shows that the model provides more accurate estimates of mixed layer depth and is simpler to apply than established models. Application of the model to Station "P" shows that, contrary to what has been reported for late winter, there is no significant trend in the summer mixed layer depth at this mid-ocean location over the observation period 1951 to 2007 despite significant trends in the buoyancy and turbulent energy fluxes used as input to the model. The lack of trend has implications for studies of climate-induced changes in upper ocean productivity.


Characterizing Open Ocean Ecosystems Using Satellite Observations: Beyond the Remote Assessment of Chlorophyll

* Siegel, D A (davey@icess.ucsb.edu), Institute for Computational Earth System Science and Department of Geography, University of California, Santa Barbara, Santa Barbara, CA 93106-3060, United States

Satellite observations have brought a new vantage for characterizing open ocean ecosystems on local to global spatial scales and from intraseasonal to interannual time scales. These satellite observations are most often used to assess the time/space distribution of the chlorophyll concentration, the primary photosynthetic pigment found in all phytoplankton. However, there are additional optically active constituents regulating the color of the open ocean besides chlorophyll. Here, we use novel satellite retrieval algorithms to partition the ocean color spectrum into three primary optically active constituents - the phytoplankton chlorophyll concentration (Chl), the absorption due to chromophoric dissolved organic matter (CDOM) and the particulate backscatter coefficient (BBP). We assess the variability and co-variability of these three optical properties on regional to global scales and evaluate the potential processes controlling these variations. We conclude that the chlorophyll concentration is the worst of the three primary bio-optical properties for characterizing ocean ecosystems. This is because of the extreme plastic nature of cellular chlorophyll concentrations in response to light and nutrient conditions and the difficulty in separating light absorption in phytoplankton Chl concentrations from CDOM. I suggest that future satellite missions consider the primary optically active constituents in the open ocean and provide a path for their robust determination. In this way we can maximize the scientific returns from our satellite infrastructure investments.