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Autonomous platforms yield new insights on North Atlantic bloom phenology

Posted by mmaheigan

· Wednesday, April 22nd, 2020

Phytoplankton produces organic carbon, which serves as a major energy source in marine food webs and plays an important role in the global carbon cycle. Studies of phytoplankton seasonal timing (phenology) have been a major focus in oceanography, especially in the subpolar North Atlantic region, where massive increases in phytoplankton biomass (blooms) occur during the winter-spring transition.

Figure 1. Panel a: Each line represents the trajectory of a profiling Argo float deployed during the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) expeditions (12 total); the initial float deployment location is denoted by a filled circle. The bar chart (inset right bottom) indicates float deployment durations. Panel b: Seasonal climatologies of C_phyto (green), µ (blue), l (red), and r (grey) from Argo floats for all 4 regions (D1-D4 as indicated on map in Panel a).

Many hypotheses based on data from shipboard discrete sampling or satellite remote sensing have been proposed to explain drivers of phytoplankton bloom formation and dynamics. However, discrete shipboard sampling limits both spatial and temporal coverage, and satellite approaches cannot provide direct information at depth. To address this gap in spatiotemporal coverage, a recent study in Frontiers in Marine Science, applied bio-optical measurements from 12 Argo profiling floats to study the year-round phytoplankton phenology in a north-south section of the western North Atlantic Ocean (40° N to 60° N). The authors calculated phytoplankton division rate (µ), loss rate (l), and carbon accumulation rate (r) using the Argo-based Chlorophyll-a (Chl) and phytoplankton carbon (C_phyto) estimates. Latitudinally varying phytoplankton dynamics were observed, with a higher (and later) C_phyto peak in the north, and stronger μ–r decoupling and increased proportion of winter to total annual production in the south (Figure 1). Seasonal phenology patterns arise from interactions between “bottom-up” (e.g., resources for growth) and “top-down” (e.g., grazing, mortality) factors that involve both biological and physical drivers. The Argo float data are consistent with the disturbance recovery hypothesis (DRH) over a full annual cycle. Float-based mixed layer phytoplankton phenology observations were comparable to satellite remote sensing observations. In a data-model comparison, outputs from an eddy-resolving ocean simulation only reproduced some of the observed phytoplankton phenology, indicating possible biases in the simulated physical forcing, turbulent dynamics, and biophysical interactions.

In addition to seasonal patterns in the mixed layer, float-based measurements provide information on the vertical distribution of physical and biogeochemical quantities and therefore are complementary to the satellite measurements. This powerful combination of observing assets enhances spatiotemporal coverage, thus enabling us to better observe, compare, model, and predict seasonal phytoplankton dynamics in the subpolar North Atlantic.

Authors:
Bo Yang (University of Virginia)
Emmanuel S. Boss (University of Maine)
Nils Haëntjens (University of Maine)
Matthew C. Long (National Center for Atmospheric Research)
Michael J. Behrenfeld (Oregon State University)
Rachel Eveleth (Oberlin College)
Scott C. Doney (University of Virginia)

Estimating the large-scale biological pump: Do eddies matter?

Posted by mmaheigan

· Wednesday, December 4th, 2019

One factor that limits our capacity to quantify the ocean biological carbon pump is uncertainty associated with the physical injection of particulate (POC) and dissolved (DOC) organic carbon to the ocean interior. It is challenging to integrate the effects of these pumps, which operate at small spatial (<100 km) and temporal (<1 month) scales. Previous observational and fine-scale modeling studies have thus far been unable to quantify these small-scale effects. In a recent study published in Global Biogeochemical Cycles, authors explored the influence of these physical carbon pumps relative to sinking (gravity-driven) particles on annual and regional scales using a high-resolution (2 km) biophysical model of the North Atlantic that simulates intense eddy-driven subduction hotspots that are consistent with observations.

Figure 1: North Atlantic idealized double gyre ocean biophysical model. Top: Sea surface temperature, surface chlorophyll and mixed-layer depth during the spring bloom (March 21). Bottom: total export of organic carbon (POC+DOC) at 100 and individual contributions from the gravitational (particle sinking) and subduction (mixing, eddy advection and Ekman pumping) pumps for one day during the spring bloom (March 21) and averaged annually. Physical subduction hotspots visible on the daily export contribute little to the annual export due to strong compensation of upward and downward motions.

The authors showed that eddy dynamics can transport carbon below the mixed-layer (500-1000 m depth), but this mechanism contributes little (<5%) to annual export at the basin scale due to strong compensation between upward and downward fluxes (Figure 1). Additionally, the authors evidenced that small-scale mixing events intermittently export large amounts of suspended DOC and POC.

These results underscore the need to expand the traditional view of the mixed-layer carbon pump (wintertime export of DOC) to include downward mixing of POC associated with short-lived springtime mixing events, as well as eddy-driven subduction, which can contribute to longer-term ocean carbon storage. High-resolution measurements are needed to validate these model results and constrain the magnitude of the compensation between upward and downward carbon transport by small-scale physical processes.

Authors:
Laure Resplandy (Princeton University)
Marina Lévy (Sorbonne Université)
Dennis J. McGillicuddy Jr. (WHOI)

Industrial era climate forcing drives multi-century decline in North Atlantic productivity

Posted by mmaheigan

· Wednesday, October 2nd, 2019

Phytoplankton respond directly to climate forcing, and due to their central role in global oxygen production and atmospheric carbon sequestration, they are critical components of the Earth’s climate system. There are however few observations detailing past variability in marine primary productivity, particularly over multi-decadal to centennial timescales. This limits our understanding of the long-term impact of climatic forcing on both past and future marine productivity.

Multi-century decline of subarctic Atlantic productivity. From top: standardized (z-score units relative to ad 1958-2016) indices of Continuous Plankton Recorder (CPR)-based diatom, dinoflagellate and coccolithophore relative-abundances; North Atlantic [chlorophyll-α] reconstruction from Boyce et al. (2010, Nature); ice core-based [MSA] PC1 productivity index. The “Industrial Onset” range shows the estimated initiation of declining subarctic Atlantic productivity; reconstructed (Rahmstorf et al., 2015, Nat. Clim. Change) and observed sea-surface temperature-based Atlantic Meridional Overturning Circulation (i.e., AMOC) index, alongside 5-year averaged subarctic Atlantic freshwater storage anomalies (relative to A.D. 1955) from Curry and Mauritzen (2005; Science).

Authors of a new study published in Nature used a high-resolution signal of marine biogenic aerosol emissions (methanesulfonic acid, or “MSA”) preserved within twelve Greenland ice cores to reconstruct a ~250-year record of marine productivity variations across the subarctic Atlantic basin, one of the most biologically productive and climatically sensitive regions on Earth. These results provide the most continuous proxy-based reconstruction of basin-scale productivity to date in this region, illuminating the following major findings: (1) subarctic Atlantic marine productivity has declined over the industrial era by as much as 10 ± 7%; (2) the early 19th century onset of declining productivity coincides with the regional onset of industrial-era surface warming, and also strongly covaries with regional sea surface temperatures and basin-scale gyre circulation strength; (3) there is strong decadal- to centennial-scale coherence between northern Atlantic productivity variability and declining Atlantic Meridional Overturning Circulation (AMOC) strength, as predicted by prior model-based studies.

Future atmospheric warming is predicted to contribute to accelerating Greenland Ice Sheet runoff, ocean-surface freshening, and AMOC slowdown, suggesting the potential for continued declines in productivity across this dynamic and climatically important region. Such declines will, in turn, have important implications for future maritime economies, global food security, and drawdown of atmospheric carbon dioxide.

Authors:
Matthew Osman (Massachusetts Institute of Technology)
Sarah Das (Woods Hole Oceanographic Institution)
Luke Trusel (Rowan University)
Matthew Evans (Wheaton College)
Hubertus Fischer (University of Bern)
Mackenzie Griemann (University of California, Irvine)
Sepp Kipfstuhl (Alfred-Wegener-Institute)
Joseph McConnell (Desert Research Institute)
Eric Saltzman (University of California, Irvine)

Figure references:
Boyce, D. G., Lewis, M. R. & Worm, B. (2010) Global phytoplankton decline over the past century. Nature 466, 591–596.

Curry, R. & Mauritzen, C. (2005) Dilution of the northern North Atlantic Ocean in recent decades. Science 308, 1772–1774.

Rahmstorf, S. et al. (2015) Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nat. Clim. Change 5, 475–480.

Forecasting air-sea CO2 flux variations several years in advance

Posted by mmaheigan

· Tuesday, July 9th, 2019

Year-to-year changes in the flux of CO₂ between the atmosphere and the ocean impact the global carbon cycle and climate system, and challenge our ability to verify fossil fuel CO₂ emissions. A new study published in Earth System Dynamics suggests that these air-sea CO₂ flux variations are predictable several years in advance.

A novel set of initialized forecasts of past air-sea CO₂ flux from an Earth system model (Figure 1a) confidently predicts year-to-year variations in the globally-integrated flux up to two years in advance. At regional scales, the forecast lead time increases. The predictability of CO₂ flux from the initialized forecast system exceeds that obtained solely from foreknowledge of variations in external forcing (e.g., volcanic eruptions) or a simple persistence forecast (e.g., CO₂ flux this year will be the same as next year). The longest-lasting forecast enhancements are in the subantarctic Southern Ocean and the northern North Atlantic (Figure 1b).

Figure 1: (a) Forecasts of the past evolution of air-sea CO2 flux in the South Pacific using an Earth System model indicate the potential to predict the future evolution of this quantity. (b) In each biome, the maximum forecast lead time in which the initialized forecast of air-sea CO2 flux beats out other forecast methods.

These results are particularly meaningful for those forecasting year-to-year changes in the global carbon budget, especially as these forecasting efforts are blind to the externally-forced variability in advance (i.e., the external forcing of the future is unknown). In this way, forecasts of air-sea CO₂ flux variations can help to inform future predictions of land-air CO₂ flux and atmospheric CO₂ concentration.

Authors:
Nicole Lovenduski (University of Colorado Boulder)
Stephen G. Yeager (National Center for Atmospheric Research)
Keith Lindsay (National Center for Atmospheric Research)
Matthew C. Long (National Center for Atmospheric Research)

See also the OCB Ocean-Atmosphere Interactions: Scoping directions for U.S. research Workshop to be held in October 1-3, 2019

Volcanic carbon dioxide drove ancient global warming event

Posted by mmaheigan

· Thursday, March 29th, 2018

A study recently published in Nature suggests that an extreme global warming event 56 million years ago known as the Palaeocene-Eocene Thermal Maximum (PETM) was driven by massive CO₂ emissions from volcanoes during the formation of the North Atlantic Ocean. Using a combination of new geochemical measurements and novel global climate modelling, the study revealed that atmospheric CO₂ more than doubled in less than 25,000 years during the PETM.

The PETM lasted ~150,000 years and is the most rapid and extreme natural global warming event of the last 66 million years. During the PETM, global temperatures increased by at least 5^°C, comparable to temperatures projected in the next century and beyond. While it has long been suggested that the PETM event was caused by the injection of carbon into the ocean and atmosphere, the source and total amount of carbon, as well as the underlying mechanism have thus far remained elusive. The PETM roughly coincided with the formation of massive flood basalts resulting from of a series of eruptions that occurred as Greenland and North America started separating from Europe, thereby creating the North Atlantic Ocean. What was missing is evidence linking the volcanic activity to the carbon release and warming that marks the PETM.

To identify the source of carbon, the authors measured changes in the balance of isotopes of the element boron in ancient sediment-bound marine fossils called foraminifera to generate a new record of ocean pH throughout the PETM. Ocean pH tells us about the amount of carbon absorbed by ancient seawater, but we can get even more information by also considering changes in the isotopes of carbon, which provide information about the carbon source. When forced with these ocean pH and carbon isotope data, a numerical global climate model implicates large-scale volcanism associated with the opening of the North Atlantic as the primary driver of the PETM.

North Atlantic microfossil-derived isotope records from extinct planktonic foraminiferal species M. subbotinae relative to the onset of the PETM carbon isotope excursion (CIE). The negative trend in carbon isotope composition (A) during the carbon emission phase is accompanied by decreasing pH (decreasing δ¹¹B, panel B) and increasing temperature (decreasing δ¹⁸O, panel C). Panels D and E zoom in on the PETM CIE, showing microfossil δ¹³C (D) and δ¹¹B-based pH (E) reconstructions. Also included in E are data from Penman et al. (2014) on their original age model, with recalculated (lab-based) pH values.

These new results suggest that the PETM was associated with a total input of >12,000 petagrams of carbon from a predominantly volcanic source. This is a vast amount of carbon—30 times larger than all of the fossil fuels burned to date and equivalent to all current conventional and unconventional fossil fuel reserves. In the following Earth System Model simulations, it resulted in the concentration of atmospheric CO₂ increasing from ~850 parts per million to >2000 ppm. The Earth’s mantle contains more than enough carbon to explain this dramatic rise, and it would have been released as magma poured from volcanic rifts at the Earth’s surface.

How the ancient Earth system responded to this carbon injection at the PETM can tell us a great deal about how it might respond in the future to man-made climate change. Earth’s warming at the PETM was about what we would expect given the CO₂ emitted and what we know about the sensitivity of the climate system based on Intergovernmental Panel on Climate Change (IPCC) reports. However, the rate of carbon addition during the PETM was about twenty times slower than today’s human-made carbon emissions.

In the model outputs, carbon cycle feedbacks such as methane release from gas hydrates—once the favoured explanation of the PETM—did not play a major role in driving the event. Additionally, one unexpected result was that enhanced organic matter burial was important in ultimately drawing down the released carbon out of the atmosphere and ocean and thereby accelerating the recovery of the Earth system.

Authors:
Marcus Gutjahr (National Oceanography Centre Southamption, GEOMAR)
Andy Ridgwell (Bristol University, University of California Riverside)
Philip F. Sexton (The Open University, UK)
Eleni Anagnostou (National Oceanography Centre Southamption)
Paul N. Pearson (Cardiff University)
Heiko Pälike (University of Bremen)
Richard D. Norris (Scripps Institution of Oceanography)
Ellen Thomas (Yale University, Wesleyan University)
Gavin L. Foster (National Oceanography Centre Southamption)

Role for iron in controlling microbial phosphorus acquisition in the ocean

Posted by mmaheigan

· Thursday, October 12th, 2017

In the subtropical North Atlantic, dissolved inorganic phosphorus (DIP) concentrations are depleted and might co-limit N₂ fixation and microbial productivity. There are relatively large pools of dissolved organic phosphorus (DOP), but microbes need an enzyme to access this P source. One such alkaline phosphatase (APase) enzyme requires zinc (Zn) as its activating cofactor. This has been known for almost 30 years. However, recent crystallography studies revealed that two other widespread APase enzymes contain Fe. Via this requirement, Fe availability could regulate microbial access to the DOP pool.

As detailed in a recent publication in Nature Communications (Browning et al. 2017), this hypothesis was tested on a cruise across the tropical North Atlantic by adding Fe and Zn to incubated seawater and monitoring changes in bulk APase using a simple fluorescence assay. Adding Fe significantly increased APase activity in seawater samples collected in areas that were far-removed from coastal and aerosol Fe sources. Despite seawater Zn concentrations being much lower than Fe, it appeared not to be limiting.

Iron (Fe) and zinc (Zn) enrichment experiments conducted in the DIP-depleted tropical North Atlantic suggested that Fe, not Zn, could limit alkaline phosphatase activity (APA). DIP*=DIP–DIN/16, and represents excess DIP availability assuming a 16-fold higher microbial N requirement. Results in the bar chart represent a subset of treatments from one experiment (out of eight conducted).

DIP is depleted in surface waters of the tropical North Atlantic because inputs of North African aerosol Fe stimulates N₂ fixation and leads to microbial drawdown of DIP. If the modern ocean is a good analog for the past, the lack of APase stimulation following experimental Zn addition could reflect limited evolutionary selection for Zn-containing APase. In general, DIP is only substantially depleted where there is enhanced Fe input fueling N₂ fixation; it therefore follows that any significant requirement for APases might be restricted to these relatively high-Fe, low-Zn waters.

On a shorter timescale, growing anthropogenic nitrogen input to the ocean relative to phosphorus could result in more prevalent oceanic phosphorus deficiency. Corresponding iron inputs might then serve as an important control on phosphorus availability for microbes in these regions.

Authors:

Tom Browning (GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany)
Eric Achterberg (GEOMAR)
Jaw Chuen Yong (GEOMAR)
Insa Rapp (GEOMAR)
Caroline Utermann (GEOMAR)
Anja Engel (GEOMAR)
Mark Moore (Ocean and Earth Science, University of Southampton, Southampton, UK)

Tiny marine animals strongly influence the carbon cycle

Posted by mmaheigan

· Thursday, August 31st, 2017

What controls the amount of organic carbon entering the deep ocean? In the sunlit layer of the ocean, phytoplankton transform inorganic carbon to organic carbon via a process called photosynthesis. As these particulate forms of organic carbon stick together, they become dense enough to sink out of the sunlit layer, transferring large quantities of organic carbon to the deep ocean and out of contact with the atmosphere.

However, all is not still in the dark ocean. Microbial organisms such as bacteria, and zooplankton consume the sinking, carbon-rich particles and convert the organic carbon back to its original inorganic form. Depending on how deep this occurs, the carbon can be physically mixed back up into the surface layers for exchange with the atmosphere or repeat consumption by phytoplankton. In a recent study published in Biogeosciences, researchers used field data and an ecosystem model in three very different oceanic regions to show that zooplankton are extremely important in determining how much carbon reaches the deep ocean.

Figure 1. Particle export and transfer efficiency to the deep ocean in the Southern Ocean (SO, blue circles), North Atlantic Porcupine Abyssal Plain site (PAP, red squares) and the Equatorial Tropical North Pacific (ETNP, orange triangles) oxygen minimum zone. a) particle export efficiency of fast sinking particles (Fast PEeff) against primary production on a Log10 scale. b) transfer efficiency of particles to the deep ocean expressed as Martin’s b (high b = low efficiency). Error bars in b) are standard error of the mean for observed particles, error too small in model to be seen on this plot.

In the Southern Ocean (SO), zooplankton graze on phytoplankton and produce rapidly sinking fecal pellets, resulting in an inverse relationship between particle export and primary production (Fig. 1a). In the North Atlantic (NA), the efficiency with which particles are transferred to the deep ocean is comparable to that of the Southern Ocean, suggesting similar processes apply; but in both regions, there is a large discrepancy between the field data and the ecosystem model (Fig. 1b), which poorly represents particle processing by zooplankton. Conversely, much better data-model matches are observed in the equatorial Pacific, where lower oxygen concentrations mean fewer zooplankton; this reduces the potential for zooplankton-particle interactions that reduce particle size and density, resulting in a lower transfer efficiency.

This result suggests that mismatches between the data and model in the SO and NA may be due to the lack of zooplankton-particle parameterizations in the model, highlighting the potential importance of zooplankton in regulating carbon export and storage in the deep ocean. Zooplankton parameterizations in ecosystem models must be enhanced by including zooplankton fragmentation of particles as well as consumption. Large field programs such as EXPORTS could help constrain these parameterisation by collecting data on zooplankton-particle interaction rates. This will improve our model estimates of carbon export and our ability to predict future changes in the biological carbon pump. This is especially important in the face of climate-driven changes in zooplankton populations (e.g. oxygen minimum zone (OMZ) expansion) and associated implications for ocean carbon storage and atmospheric carbon dioxide levels.

Authors:
Emma L. Cavan (University of Tasmania)
Stephanie A. Henson (National Oceanography Centre, Southampton)
Anna Belcher (University of Southampton)
Richard Sanders (National Oceanography Centre, Southampton)

The changing ocean carbon cycle

Posted by mmaheigan

· Thursday, July 6th, 2017

Since preindustrial times, the ocean has removed from the atmosphere 41% of the carbon emitted by human industrial activities (Figure 1). The globally integrated rate of ocean carbon uptake is increasing in response to rising atmospheric CO₂ levels and is expected to continue this trend for the foreseeable future. However, the inherent uncertainties in ocean surface and interior data associated with ocean carbon uptake processes make it difficult to predict future changes in the ocean carbon sink. In a recent paper, McKinley et al. (2017), review the mechanisms of ocean carbon uptake and its spatiotemporal variability in recent decades. Looking forward, the potential for direct detection of change in the ocean carbon sink, as distinct from interannual variability, is assessed using a climate model large ensemble, a novel approach to studying climate processes with an earth systems model, the “large ensemble.” In a large ensemble, many runs of the same model are done so as to directly distinguish natural variability from long-term trends.

This analysis illustrates that variability in CO₂ flux is large enough to prevent detection of anthropogenic trends in ocean carbon uptake on at least decadal to multi-decadal timescales, depending on location. Earliest detection of trends is most attainable in regions where trends are expected to be largest, such as the Southern Ocean and parts of the North Atlantic and North Pacific. Detection will require sustained observations over many decades, underscoring the importance of traditional ship-based approaches and integration of new autonomous observing platforms as part of a global ocean carbon observing system.

Please see a relevant OCB outreach tool on ocean carbon uptake developed by McKinley and colleagues:
OCB teaching/outreach slide deck Temporal and Spatial Perspectives on the Fate of Anthropogenic Carbon: A Carbon Cycle Slide Deck for Broad Audiences – also download explanatory notes

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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