Archive for floats

Partitioning carbon export into particulate and dissolved pools from biogeochemical profiling float observations

Posted by mmaheigan 
· Thursday, December 17th, 2020 

Carbon export from the surface into the deep ocean via the biological pump is a significant sink for atmospheric carbon dioxide. The relative contributions of sinking particles—particulate organic carbon (POC) and dissolved organic carbon (DOC)—to the total export affect the efficiency of carbon export.

In a recent study published in Global Biogeochemical Cycles, the authors used measurements from biogeochemical profiling floats in the Northeast Pacific from 2009 to 2017 to estimate net community production (NCP), an analog for carbon export. In order to close three tracer budgets (nitrate, dissolved inorganic carbon, and total alkalinity), the authors combined these float measurements with data from the Ocean Station Papa mooring and recently developed algorithms for carbonate system parameters. By constraining end-member nutrient ratios of the POC and DOC produced, this multi-tracer approach was used to estimate regional NCP across multiple depth horizons throughout the annual cycle, partition NCP into the POC and DOC contributions, and calculate particulate inorganic carbon (PIC) production, a known ballast material for sinking particles (Figure 1). The authors also estimated POC attenuation with depth, POC export across deeper horizons, and in situ export efficiency via a particle backscatter-based approach.

With the advent of “fully-loaded” biogeochemical profiling floats equipped with nitrate, oxygen, pH and bio-optical sensors, this approach may be used to assess the magnitude and efficiency of carbon export in other ocean regions from a single platform, which will greatly reduce the risks and costs associated with traditional ship-based measurements, while broadening the spatiotemporal scales of observation.

Figure caption: Climatological mean NCP (blue line) over the entire study period (2009-2017); the POC portion of NCP (filled blue area), the DOC portion (white space) and PIC production rate (red line), in the mixed layer (left), and the euphotic zone (right). The numbers in parentheses are the integrated annual NCP rates for each curve and uncertainty reported was determined using a Monte Carlo approach.

 

Authors:
William Haskell (MBARI, now Mote Marine Laboratory)
Andrea Fassbender (MBARI, now PMEL)
Jacki Long (MBARI)
Joshua Plant (MBARI)

How zooplankton control carbon export in the Southern Ocean

Posted by mmaheigan 
· Thursday, December 3rd, 2020 

The Southern Ocean exhibits an inverse relationship between surface primary production and export flux out of the euphotic zone. The causes of this production-export decoupling are still under debate. A recently published mini review in Frontiers in Marine Science focused on zooplankton, an important component of Southern Ocean food webs and the biological pump. The authors compared carbon export regimes from the naturally iron-fertilised Kerguelen Plateau (high surface production, but generally low export) with the iron-limited and less productive high nutrient, low chlorophyll (HNLC) waters south of Australia, where carbon export is relatively high.

Figure 1: The role of zooplankton in establishing the characteristic export regimes at two sites in the Southern Ocean, (a) the highly productive northern Kerguelen Plateau, which exhibits low export, and (b) the iron-limited waters south of Australia with low production, but relatively high carbon export.

Size structure and zooplankton grazing pressure are found to shape carbon export at both sites. On the Kerguelen Plateau, a large size spectrum of zooplankton acts as “gate-keeper” to the mesopelagic by significantly reducing the sinking flux of phytoaggregates, which establishes the characteristic low export regime. In the HNLC waters, however, the zooplankton community is low in biomass and grazes predominantly on smaller particles, which leaves the larger particles for export and leads to relatively high export flux.

Gaps in knowledge related to insufficient seasonal data coverage, understudied carbon flux pathways, and associated mesopelagic processes limit our current understanding of carbon transfer through the water column and export. More integrated data collection efforts, including the use of autonomous profiling floats (e.g., BGC-Argo), stationary moorings, etc., will improve seasonal carbon flux data coverage, thus enabling more reliable estimation of carbon export and storage in the Southern Ocean and improved projection of future changes in carbon uptake and atmospheric carbon dioxide levels.

 

Authors:
Svenja Halfter (University of Tasmania)
Emma Cavan (Imperial College London)
Ruth Eriksen (CSIRO)
Kerrie Swadling (University of Tasmania)
Philip Boyd (University of Tasmania)

Timing matters: Correcting float-based measurements of diurnal oxygen variability

Posted by mmaheigan 
· Friday, November 6th, 2020 

Despite its fundamental importance to the global carbon cycle, climate, and marine ecosystems, oceanic primary production is grossly under-sampled. Autonomous platforms represent an important frontier for expanding measurements of marine primary productivity in time and space, but this requires the establishment of robust, standardized methods to obtain reliable data from these platforms. Using data from profiling floats deployed in the northern Gulf of Mexico, authors of a recent study published in Biogeosciences demonstrated, for the first time, that daily cycles of dissolved oxygen can be observed with Argo-type profiling floats. The floats were instructed to profile continuously, resulting in about one profile every three hours. The floats recorded data both on the ascent (upcast) and the descent (downcast). Adjacent casts showed hysteresis in gradient areas, i.e. a lag in the concentration measurement, due to the slow response time of oxygen sensors.

Figure 1: Example of raw oxygen measurements from a downcast (dark purple line) and an upcast (dark green line) and corrected profiles (lighter purple and green lines) in (a) density and (b) pressure coordinates. (c) Upcasts and downcasts (top 150 m) plotted against each other with raw data (purple) and data corrected according to the new method (red). (d) The root-mean-square difference (RMSD) between the upcast and downcast after correcting casts for a range of time constants (τ), showing an optimal τ value in this case of 76 s (red dot).

To correct for these measurement errors, the authors developed a method to determine sensor response time in situ, using an established process for correcting sensor response time errors. This method requires a timestamp associated with each observation. The response time parameter (τ) was determined by correcting consecutive profiles taken in opposite directions using a range of possible values and finding the minimum root-mean-square-difference between them (Figure 1). In light of these findings, future oxygen measurements from Argo floats should be transmitted with time stamps for a calibration period during which up- and downcasts are recorded to facilitate response time correction. The method developed here will contribute to more accurate measurement of dissolved oxygen, thus improving the quality of derived quantities such as primary productivity.

 

Authors
Christopher Gordon (Dalhousie University)
Katja Fennel (Dalhousie University)
Clark Richards (Fisheries and Oceans Canada)
Nick Shay (University of Miami)
Jodi Brewster (University of Miami)

Unexpected patterns of carbon export in the Southern Ocean

Posted by mmaheigan 
· Tuesday, July 7th, 2020 

The Southern Ocean is a major player in driving global distributions of heat, carbon dioxide, and nutrients, making it key to ocean chemistry and the earth’s climate system. In the ocean, biological production and export of organic carbon are commonly linked to places with high nutrient availability. A recent paper, published in Global Biogeochemical Cycles, highlighting new observations from robotic profiling floats demonstrates that areas of high carbon export in the Southern Ocean are actually associated with very low concentrations of iron, an important micronutrient for supporting phytoplankton growth. This suggests a decoupling between the production and export of organic carbon in this region.

Figure caption: (A) Meridional pattern of Annual Net Community Production (ANCP) (equivalent to carbon export) (± standard deviation) in the Southern Ocean (blue line with circles and shaded area), carbon export estimates from previous satellite-based analyses (blue dashed line), and silicate to nitrate (Si:NO3) ratio of the surface water (black continuous line). Grey dotted line shows a Si:NO3 = 1 mol mol−1, characteristic of nutrient-replete diatoms. (B) Meridional pattern of Southern Ocean nutrient concentrations, including dissolved iron (Fe) concentration (black line), nitrate (red line), and silicate (blue line). (C) Mean 2014–2015 annual zonally averaged air-sea flux of CO2 computed using neural network interpolation method. STF = Subtropical Front, PF = Antarctic Polar Front, SIF = Seasonal Ice Front.

Using observations of nutrient and oxygen drawdown from a regional network of profiling Biogeochemical-Argo floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM), the authors calculated estimates of Southern Ocean carbon export. A meridional pattern in biological carbon export emerged, showing peak export near the Antarctic Polar Front (PF) associated with minima in surface iron concentrations and dissolved silicate to nitrate ratios. Previous studies have shown that under iron-limiting conditions, diatoms increase their uptake ratio of silicate with respect to other nutrients (e.g., nitrogen), resulting in silicification. Here, the authors hypothesize that iron limitation promotes silicification in Southern Ocean diatoms, as evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic Polar Front. High diatom silicification increases ballasting of particulate organic carbon and hence overall carbon export in this region. The resulting meridional pattern of organic carbon export is similar to that of the air-sea flux of carbon dioxide in the Southern Ocean, underscoring the importance of the biological carbon pump in controlling the spatial pattern of oceanic carbon uptake in this region.

Authors:
Lionel A. Arteaga (Princeton University)
Markus Pahlow (Helmholtz Centre for Ocean Research Kiel, GEOMAR)
Seth M. Bushinsky (University of Hawaii)
Jorge L. Sarmiento (Princeton University)

 

Building ocean biogeochemistry observing capacity, one float at a time: An update on the Biogeochemical-Argo Program

Posted by mmaheigan 
· Thursday, July 5th, 2018 

By Ken Johnson (MBARI)

The Biogeochemical-Argo (BGC-Argo) Program is an international effort to develop a global network of biogeochemical sensors on Argo profiling floats that has emerged from over a decade of community discussion and planning. While there is no formal funding for this global program, it is being implemented via a series of international research projects that harness the unique capabilities provided by BGC profiling floats. The U.S. Ocean Carbon and Biogeochemistry (OCB) Program maintains and supports a U.S. BGC-Argo subcommittee as a focal point for U.S. community input on the implementation of the global biogeochemical float array and associated science program development.

Figure 1. Steve Riser deploying a SOCCOM float from the R/V Palmer

About BGC-Argo Floats
BGC-Argo floats can carry a suite of chemical and bio-optical sensors (Figure 1 – Float Schematic).  They have enough energy to make about 250 to 300 vertical profiles from 2000 m to the surface.  At a cycle time of 10 days, that corresponds to a lifetime near 7 years.  The long endurance allows the floats to resolve seasonal to interannual variations in carbon and nutrient cycling throughout the water column.  These time scales are difficult to study from ships and ocean interior processes are hard to resolve from satellites.  BGC profiling floats extend the capabilities of these traditional observing systems in significant ways.

Figure 2. Images of Navis and APEX floats used in the SOCCOM program. These floats carry oxygen, nitrate, pH, and bio-optical (chlorophyll fluorescence and backscatter) sensors.

All of the data from profiling floats operating as part of the Argo program must be available in real-time with no restrictions on access.  The Argo Global Data Assembly centers in France and the USA both provide complete listings of all BGC profiles (argo_bio-profile_index.txt) and access to the data.  Extensive documentation of the data processing protocols is available from the Argo Data Management Team.  Individual research programs, such as SOCCOM (see below), may also provide direct data access to the observed data along with value added products such as best estimates of pCO2 derived from pH sensor data.

Regional Deployments
In 2018, it is projected that 127 profiling floats with biogeochemical sensors are will be deployed, including ~40 floats by U.S. projects. Most of the U.S. deployments (30+) will be carried out by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project (Figure 2 – Float Deployment). These floats will carry oxygen, nitrate, pH, chlorophyll fluorescence, and backscatter sensors. As part of the NOAA Tropical Pacific Observing System (TPOS) program, Steve Riser’s group (Univ. Washington) will deploy 3 BGC-Argo floats per year in the equatorial Pacific over the next 4 years. These floats will be equipped with oxygen, pH, bio-optical sensors and Passive Acoustic Listener (PAL) sensors, which provide wind speed estimates at 15-minute intervals while the floats are parked at 1000 m.  Wind speed is derived from the noise spectrum of breaking waves. Steve Emerson (Univ. Washington), with NSF support, is also deploying floats equipped with oxygen, nitrate and pH sensors in the equatorial Pacific. With funding from NSF, Andrea Fassbender (MBARI) will deploy two floats at Ocean Station Papa in the northeast Pacific in collaboration with the EXPORTS program. These floats will also carry oxygen, nitrate, pH, and bio-optical sensors.

Nearly 90 BGC floats will be deployed in 2018 by other nations in multiple ocean basins.  Much of this effort will focus on the North Pacific and North Atlantic.  The sensor load on these floats is somewhat variable. Some will be deployed with only oxygen sensors or bio-optical sensors for chlorophyll fluorescence and particle abundance. Others will carry the full suite of six sensors (oxygen, nitrate, pH, chlorophyll fluorescence, backscatter, and irradiance) that are outlined in the BGC-Argo Implementation Plan (BGC-Argo, 2016). These floats will contribute to the existing array of 305 biogeochemical floats (Figure 3 BGC Argo Map).

Community Activities
In response to the tremendous interest in the scientific community in the capabilities of profiling floats, OCB is sponsoring a Biogeochemical Float Workshop at the University of Washington in Seattle from July 9-13, 2018 to begin the process of transferring this expertise to the broader oceanographic community, bringing together potential users of this technology to discuss biogeochemical profiling float technology, sensors, and data management and begin the process of the intelligent design of future scientific experiments. The workshop will provide participating scientists direct access to the facilities of the Float Laboratory operated by Riser. This workshop builds on a previous OCB workshop Observing Biogeochemical Cycles at Global Scales with Profiling Floats and Gliders (Johnson et al., 2009). BGC-Argo will also have a prominent presence at the 6th Argo Science Workshop (October 22-24, 2018, Tokyo, Japan) and OceanObs19 (September 16-20, 2019, Honolulu, HI).

Figure 3. May 2018 map of the location of BGC-Argo floats that have reported in the previous month and sensor types on these floats. From jcommops.org.

BGC-Argo Publications
Several resources now highlight the capabilities of profiling floats to accomplish scientific observing goals. A web-based bibliography of biogeochemical float papers hosted on the Biogeochemical-Argo website currently includes >100 publications and continues to grow. A special issue of Journal of Geophysical Research: Oceans focused on the SOCCOM program is in progress with 11 papers now available and a dozen more forthcoming. These papers include summaries of the technical capabilities of floats and the biogeochemical sensors, comparisons of float bio-optical data with satellite remote sensing observations, seasonal and interannual assessments of air-sea oxygen flux, under-ice biogeochemistry, carbon export, comparisons of pCO2 estimated from floats with pH vs. time-series data, and net community production. The connection of float observations with numerical models is a special focus of the program and this is highlighted in several papers, including a description of the Biogeochemical Southern Ocean State Estimate (SOSE), which is a data assimilating BGC model. Results from Observing System Simulation Experiments (OSSEs) used to assess the number of floats needed for large-scale observations are also reported. The BGC-Argo steering committee is developing a community white paper for the OceanObs19 conference in September 2019. BGC-Argo also develops and distributes a community newsletter.

For more information, visit the BGC-Argo website or reach out to the U.S. BGC-Argo Subcommittee.

 

 

 

 

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