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Archive for ocean carbon uptake and storage

Sea ice loss and the changing Arctic carbon cycle

Posted by mmaheigan

· Friday, September 18th, 2020

Loss of Arctic Ocean ice cover is altering the carbon cycle in ways that are not well understood. Effectively “popping the top off” the Arctic Ocean, ice loss exposes the sea surface to warming and exchange of CO₂ with the atmosphere. These processes are expected to increase CO₂ levels in the Arctic Ocean, changing its contribution to the global carbon cycle, but limited data collection in the region has thus far precluded the establishment of a clear relationship between CO₂ and ice cover. In a recent study published in Geophysical Research Letters, authors report on observed partial pressure of CO₂ (pCO₂) trends from several years of data collection in the surface waters of the Canada Basin of the Arctic Ocean. These data show that the pCO₂ is higher during years when ice cover is low. Uptake of atmospheric CO₂ and heating are the primary sources of the CO₂ increase, with only a small counteracting offset from biological production. These processes vary significantly from year to year, masking the likely increase in pCO₂ over time. Based on these results, we can expect that, while the Arctic Ocean has thus far been a significant sink for atmospheric CO₂, if ice loss continues the uptake of CO₂ will diminish in coming years.

Figure caption: Sea surface pCO₂ increases with decreasing ice concentration (left), determined using the mean of spatially gridded data. The sea surface pCO₂ data were collected on five research cruises on the Canadian icebreaker, CCGS Louis S. St-Laurent, from 2012 to 2017 (shown at right for 2017). The pCO₂ levels are indicated by the color along the ship cruise track (right color bar). The dark shading (left color bar) represents sea ice concentration averaged from the daily satellite data collected during the cruise.

Authors:
Michael DeGrandpre (University of Montana-Missoula)
Wiley Evans (Hakai Institute)
Mary-Louise Timmermans (Yale University)
Richard Krishfield (Woods Hole Oceanographic Institution)
Bill Williams (Institute of Ocean Sciences)
Michael Steele (University of Washington)

Modern OMZ copepod dynamics provide analog for future oceans

Posted by mmaheigan

· Thursday, July 23rd, 2020

Global warming increases ocean deoxygenation and expands the oxygen minimum zone (OMZ), which has implications for major zooplankton groups like copepods. Reduced oxygen levels may impact individual copepod species abundance, vertical distribution, and life history strategy, which is likely to perturb intricate oceanic food webs and export processes. In a study recently published in Biogeosciences, authors conducted vertically-stratified day and night MOCNESS tows (0-1000 m) during four cruises (2007-2017) in the Eastern Tropical North Pacific, sampling hydrography and copepod distributions in four locations with different water column oxygen profiles and OMZ intensity (i.e. lowest oxygen concentration and its vertical extent in a profile). Each copepod species exhibited a different vertical distribution strategy and physiology associated with oxygen profile variability. The study identified sets of species that (1) changed their vertical distributions and maximum abundance depth associated with the depth and intensity of the OMZ and its oxycline inflection points, (2) shifted their diapause depth, (3) adjusted their diel vertical migration, especially the nighttime upper depth, or (4) expanded or contracted their depth range within the mixed layer and upper part of the thermocline in association with the thickness of the aerobic epipelagic zone (habitat compression concept) (Figure 1). Distribution depths for some species shifted by 10’s to 100’s of meters in different situations, which also had metabolic (and carbon flow) implications because temperature decreased with depth. This observed present-day variability may provide an important window into how future marine ecosystems will respond to deoxygenation.

Figure caption: Schematic diagram showing how future OMZ expansion may affect zooplankton distributions, based on present-day responses to OMZ variability. The dashed line indicates diel vertical migration (DVM) and highlights the shoaling of the nighttime depth as the aerobic habitat is compressed. The lower oxycline community and the diapause layer for some species, associated with a specific oxygen concentration, may deepen as the OMZ expands.

Authors:
Karen F. Wishner (University of Rhode Island)
Brad Seibel (University of South Florida)
Dawn Outram (University of Rhode Island)

Blue hole in the South China Sea reveals ancient carbon

Posted by mmaheigan

· Wednesday, July 8th, 2020

Blue holes are unique depositional environments that are formed within carbonate platforms. Due to an enclosed geomorphology that restricts water exchange, blue hole ecosystems are typically characterized by steep biogeochemical gradients and distinctive microbial communities. For the past three decades, studies have described vertical gradients in physical, chemical, and biological parameters that typify blue hole water columns, but their elemental cycles, particularly carbon, remain poorly understood.

Figure 1. Aerial photo of the Yongle Blue Hole in the South China Sea (Credit: P. Yao et al./JGR Biogeosciences)

In July 2016, the Yongle Blue Hole (YBH) was discovered to be the deepest known blue hole on Earth (~300 m). YBH is located in the Xisha Islands of the South China Sea. The unique features and ease of accessibility make YBH an ideal natural laboratory for studying carbon cycling in marine anoxic systems. In a recent study published in JGR Biogeosciences, the authors reported extremely low concentrations of dissolved organic carbon (DOC) (e.g., 22 µM) and very high concentrations of dissolved inorganic carbon (DIC) (e.g., 3,090 µM) in YBH deep waters. Radiocarbon dating revealed that the YBH DOC and DIC were unusually old, yielding ages (6,810 and 8270 years BP, respectively) that are much more typical of open ocean deep water. Based on H₂S and microbial community composition profiles, the authors concluded that sharp redox gradients and a high abundance of sulfur cycling bacteria were likely responsible for much of the DOC consumption in YBH. The unusually low concentrations and old DOC ages in the relatively shallow YBH suggest short-term cycling of recalcitrant DOC in oceanic waters, which has been recognized as a long-term microbial carbon sink in the global ocean. The stoichiometry of DIC and total alkalinity changes suggested that the accumulation of DIC in the deep layer of the YBH was largely derived from both the dissolution of carbonate and OC decomposition through sulfate reduction. However, the role of carbonate dissolution from the walls of the blue hole in affecting the old ages of carbon in this system remain uncertain, yet there appears to no evidence of subterranean freshwater into the bottom waters of the blue hole. In the face of expanding oxygen minimum zones and anthropogenically-induced coastal hypoxia, blue holes such as YBH can provide an accessible natural laboratory in which to study the microbial and biogeochemical features that typify these low-oxygen systems.

Authors:
Peng Yao (Ocean University of China)
Thomas S. Bianchi (University of Florida)
Xuchen Wang (Ocean University of China)
Zuosheng Yang (Ocean University of China)
Zhigang Yu (Ocean University of China)

A Methane-Charged Carbon Pump in Shallow Marine Sediments

Posted by mmaheigan

· Wednesday, June 3rd, 2020

Ocean margins are often characterized by the transport of methane, a potent greenhouse gas, entering from the subsurface and moving towards the seafloor. However, a significant portion of subsurface methane is consumed within shallow sediments via microbial driven anaerobic oxidation of methane (AOM). AOM converts the methane carbon to dissolved inorganic carbon (DIC) and reduces the amount of sulfate that diffuses down from the seafloor towards a sediment interval known as the sulfate-methane transition zone (SMTZ). The SMTZ is where the upward flux of methane encounters the downward diffusive sulfate flux (Figure 1). While the mechanisms of methane production and consumption have been extensively studied, the fate of the DIC that is produced in methane-charged sediments is not well constrained.

In a recent study published in Frontiers in Marine Science, authors used existing reports of methane and sulfate flux values to the SMTZ and synthesized a carbon flow model to quantify the DIC cycling in diffusive methane flux sites globally. They report an annual average of 8.7 Tmol (1 Tmol = 10¹² moles) of DIC entering the diffusive methane-charged shallow marine sediments due to sulfate reduction coupled with AOM and organic matter degradation, as well as DIC input from depth (Figure 1). Approximately 75% (average of 6.5 Tmol year^–1) of this DIC pool flows upward toward the water column, making it a potential contributor to oceanic CO₂and ocean acidification. Further, an average of 1.7 Tmol year^–1DIC precipitates as methane-derived authigenic carbonates. This synthesis emphasizes the importance of the SMTZ, not only as a methane sink but also an important biogeochemical front for global DIC cycling.

Figure 1: A simplified representation of DIC cycling at diffusive methane charged settings.

The study highlights that regions characterized by diffusive methane fluxes can contribute significantly to the oceanic inorganic carbon pool and sedimentary carbonate accumulation. DIC outflux from the methane-charged sediments is comparable to ~20% global riverine DIC flux to oceans. Methane-derived authigenic carbonate precipitation is comparable to ~15% of carbonate accumulation on continental shelves and in pelagic sediments, respectively. These pathways must be included in coastal and geologic carbon models.

Authors:
Sajjad Akam (Texas A&M University-Corpus Christi)
Richard Coffin (Texas A&M University-Corpus Christi)
Hussain Abdulla (Texas A&M University-Corpus Christi)
Timothy Lyons (University of California, Riverside)

Light matters for biological pump assessments

Posted by mmaheigan

· Thursday, May 7th, 2020

Organic carbon produced during photosynthesis in the sunlit euphotic zone is transported to the deep ocean via the ocean’s biological carbon pump (BCP). Even small changes in the BCP efficiency changes the carbon dioxide gradient across the ocean‐atmosphere interface, thus influencing global climate. A recent study in PNAS demonstrate that prior studies that estimate BCP efficiencies at a fixed depth fail because they do not consider the varying depth of light penetration, which ultimately controls production of sinking organic carbon and varies by location and season. Subsequently, the fixed depth approach introduces regional biases and underestimates global estimates of BCP efficiency by two-fold (Figure 1). These new findings make the case for using euphotic zone‐based metrics rather than applying a fixed depth to compare BCP efficiencies between sites. Improved estimates of BCP efficiency will lead to a better understanding of the mechanisms that control ocean carbon fluxes and its feedbacks on climate.

Figure 1: Carbon loss from the surface ocean shows more variability and is twice as high if measured at the depth where sunlight penetrates (left) vs. 150 meters (about 500 feet; right) where it is commonly measured. One Pg is 10¹⁵ grams with close to 6 Pg of carbon being transported to depth per year in left panel. In comparison, about 10 Pg C/yr is released to the atmosphere as a result of human activity.

Authors:
Ken Buesseler (WHOI)
Philip Boyd (IMAS Univ. Tasmania)
Erin Black (Dalhousie University)
David Siegel (University of California, Santa Barbara)

Also see: Tiny plankton drive processes in the ocean that capture twice as much carbon as scientists thought on The Conversation.

Featured on the cover of the PNAS May 5, 2020 issue:

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)

Little big exporters

Posted by mmaheigan

· Wednesday, April 8th, 2020

In the Southern Ocean, coccolithophores are thought to account for a major fraction of marine carbonate production and export to the deep sea. Despite their importance in the ocean carbon cycle, we lack fundamental information about Southern Ocean coccolithophore abundance, species composition, and contribution to carbonate export.

Figure caption: Heliscosphaera carteri (left), Coccolithus pelagicus (right) and Emiliania huxleyi (bottom right, partially behind C. pelagicus) coccospheres retrieved from the subantarctic waters south of Tasmania. Image Ruth Eriksen, courtesy AAD EMU.

A recent study in Biogeosciences has generated annual observations of coccolithophore species composition and contribution to calcium carbonate fluxes at two sites that are representative of a large portion of the Subantarctic zone. Coccolithophores account for roughly half of the annual calcium carbonate exported to the deep sea. Notably, it is not the most abundant species (Emiliania huxleyi), but rather the less abundant and larger species (e.g. Calcidiscus leptoporus, Helicosphaera carteri and Coccolithus pelagicus) that make the greatest contribution to carbonate export to the deep sea. Since these larger species exhibit substantially different ecological traits from the opportunistic E. huxleyi, predictions of future response of Southern Ocean coccolithophore communities should not be based on the physiological results from experiments with E. huxleyi. Rather, new physiological response experiments of those less abundant, larger coccolithophore species are urgently needed to constrain responses of these important carbonate exporters to environmental change in the Southern Ocean. This study underscores the importance of phytoplankton ecological traits on the regulation of the marine carbon cycle and emphasizes the need for more species-specific studies to improve predictions of marine ecosystem response to ongoing climate change.

Authors
Andrés S. Rigual Hernández (Universidad de Salamanca)
Thomas W. Trull (CSIRO and ACE CRC)
Scott D. Nodder (NIWA)
José A. Flores (Universidad de Salamanca)
Helen Bostock (University of Queensland,)
Fátima Abrantes (Portuguese Institute for Sea and Atmosphere and CCMAR)
Ruth S. Eriksen (CSIRO and IMAS)
Francisco J. Sierro (Universidad de Salamanca)
Diana M. Davies (CSIRO and ACE CRC)
Anne-Marie Ballegeer (Universidad de Salamanca)
Miguel A. Fuertes (Universidad de Salamanca)
Lisa C. Northcote (NIWA)

Tiny, but effective: Gelatinous zooplankton and the ocean biological carbon pump

Posted by mmaheigan

· Wednesday, March 25th, 2020

Barely visible to the naked eye, gelatinous zooplankton play important roles in marine food webs. Cnidaria, Ctenophora, and Urochordata are omnipresent and provide important food sources for many more highly developed marine organisms. These small, nearly transparent organisms also transport large quantities of “jelly-carbon” from the upper ocean to depth. A recent study in Global Biogeochemical Cycles focused on quantifying the gelatinous zooplankton contribution to the ocean carbon cycle.

Figure 1. Processes and pathways or gelatinous carbon transfer to the deep ocean.

Using >90,000 data points (1934 to 2011) from the Jellyfish Database Initiative (JeDI), the authors compiled global estimates of jellyfish biomass, production, vertical migration, and jelly carbon transfer efficiency. Despite their small biomass relative to the total mass of organisms living in the upper ocean, their rapid, highly efficient sinking makes them a globally significant source of organic carbon for deep-ocean ecosystems, with 43-48% of their upper ocean production reaching 2000 m, which translates into 0.016 Pg C yr^-1.

Figure 2. Mass deposition event of jellyfish at 3500 m in the Arabian Sea (Billett et al. 2006).

Sediment trap data have suggested that carbon transport associated with large, episodic gelatinous blooms in localized open ocean and continental shelf regions could often exceed phytodetrital sources, in particular instances. These mass deposition events and their contributions to deep carbon export must be taken into account in models to better characterize marine ecosystems and reduce uncertainties in our understanding of the ocean’s role in the global carbon cycle.

Links:

Jellyfish Database Initiative http://jedi.nceas.ucsb.edu, http://jedi.nceas.ucsb.edu-dmo.org/dataset/526852 )

Authors:
Mario Lebrato (Christian‐Albrechts‐University Kiel and Bazaruto Center for Scientific Studies, Mozambique)
Markus Pahlow (GEOMAR)
Jessica R. Frost (South Florida Water Management District)
Marie Küter (Christian‐Albrechts‐University Kiel)
Pedro de Jesus Mendes (Marine and Environmental Scientific and Technological Solutions, Germany)
Juan‐Carlos Molinero (GEOMAR)
Andreas Oschlies (GEOMAR)

Can phytoplankton help us determine ocean iron bioavailability?

Posted by mmaheigan

· Wednesday, March 11th, 2020

Iron (Fe) is a key element to sustaining life, but it is present at extremely low concentrations in seawater. This scarcity limits phytoplankton growth in large swaths of the global ocean, with implications for marine food webs and carbon cycling. The acquisition of Fe by phytoplankton is an important process that mediates the movement of carbon to the deep ocean and across trophic levels. It is a challenge to evaluate the ability of marine phytoplankton to obtain Fe from seawater since it is bound by a variety of poorly defined organic complexes.

Figure 1: Schematic representation of the reactions governing dissolved Fe (dFe) bioavailability to phytoplankton (a) Bioavailability of dFe in seawater collected from various basins and depth and probed with different iron-limited phytoplankton species under dim laboratory light and sunlight (b) (See paper for further details on samples and species)

A recent study in The ISME Journal proposes a new approach for evaluating seawater dissolved Fe (dFe) bioavailability based on its uptake rate constant by Fe-limited cultured phytoplankton. The authors collected samples from distinct regions across the global ocean, measured the properties of organic complexation, loaded these complexes with a radioactive Fe isotope, and then tracked the internalization rates from these forms to a diverse set of Fe-limited phytoplankton species. Regardless of origin, all of the phytoplankton acquired natural organic complexes at similar rates (accounting for cell surface area). This confirms that multiple Fe-limited phytoplankton species can be used to probe dFe bioavailability in seawater. Among water types, dFe bioavailability varied by ~4-fold and did not clearly correlate with Fe concentrations or any of the measured Fe speciation parameters. This new approach provides a novel way to determine Fe bioavailability in samples from across the oceans and enables modeling of in situ Fe uptake rates by phytoplankton based simply on measured Fe concentrations.

Authors:
Yeala Shaked (Hebrew University of Jerusalem)
Kristen N. Buck (University of South Florida)
Travis Mellett (University of South Florida)
Maria. T. Maldonado (University of British Columbia)

Untangling microbial evolution in the oceans: How the interaction of biological and physical timescales determine marine microbial evolutionary strategies

Posted by mmaheigan

· Wednesday, March 11th, 2020

Marine microbes are the engines of global biogeochemical cycling in the oceans. They are responsible for approximately half of all photosynthesis on the planet and drive the ‘biological pump’, which transfers organic carbon from the surface to the deep ocean. As such, it is important to determine how marine microbes will adapt and evolve in response to a changing climate in order to understand and predict how the global carbon cycle may change. However, we still lack a mechanistic understanding of how and how fast microorganisms adapt to stressful and changing environments. This is particularly challenging due to the diversity of organisms that live in the ocean and the dynamic nature of the oceans themselves—microbes are at the whim of ocean currents and so get transported large distances fairly quickly. For the first time, a new study published in PNAS provides a prediction on the controls of microbial evolutionary timescales in the oceans. The authors hypothesize that there is a trade-off for marine microbes between ability to evolve to long-term changes versus respond to shorter term variability. Their results suggest that marine microbes commonly experience conditions that favor a short-term strategy at the cost of long-term adaptation. This trade-off determines evolutionary timescales and provides a foundation for understanding distributions of microbial traits and biogeochemistry.

Illustration of trade-off in evolutionary strategy as a function of environmental variability. Trajectories where individuals perceived high environmental variability (a & b) exhibited low selective pressure for any one environment but allowed for high environmental tracking. Trajectories where individuals perceived a more stable environment (c&d) had high selective pressure for ’new environments’ (high probability of a selective sweep) but these individuals exhibited poor environmental tracking. Panels a and c show trajectories where selective sweeps were highly probable (red), likely (yellow), and had a low probability (grey). Panels b and d show the estimated persistence of non-genetic modifications necessary for environmental tracking, where grey indicates unrealistically long timescales.

Authors:
Nathan G. Walworth (University of Southern California)
Emily J. Zakem (University of Southern California)
John P. Dunne (Geophysical Fluid Dynamics Laboratory, NOAA)
Sinéad Collins (University of Edinburgh)
Naomi M. Levine (University of Southern California)

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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