Archive for changing ocean chemistry – Page 4

Sensitivity of future ocean acidification to carbon-climate feedbacks

Posted by mmaheigan 
· Thursday, May 10th, 2018 

There are vast unknowns about the future oceans, from what species or habitats may be most under threat to the continuity of earth system processes that maintain global climate. Modeling can be used to predict future states and explore the impacts of climate change, but several key uncertainties such as carbon-climate feedbacks hamper our predictive power.

Authors of a recent study in Biogeosciences (Matear and Lenton 2018) used a global earth system model to explore the effects of carbon-climate feedbacks on future ocean acidification. Ocean acidification can have wide-ranging impacts on keystone species from reef-building corals to pteropods, a major food web species in the Southern Ocean. The study included four representative scenarios (from IPCC) comparing concentration pathway simulations to emission pathway simulations (RCP2.6, RCP 4.5, RCP6, RCP8.5) to determine carbon-climate feedbacks. The high emission scenarios (RCP8.5 and RCP6) showed surface water undersaturation a decade or more earlier than expected. Surprisingly, the medium (RCP4.5) scenario carbon-climate feedbacks showed the greatest acidification response, doubling the extent of undersaturation and subsequently halving the area that could sustain coral reefs by 2100. The low emissions scenario also showed significant declines in saturation state.

Surface ocean aragonite saturation state for the 2090s for RCP2.6 and RCP 8.5 concentration and emission pathways. The contour line delineates a saturation state of 3 (coral reef threshold), the white line a saturation state of 1, when aragonite becomes unstable and corals dissolve.

The extra atmospheric CO2 from the carbon-climate feedback resulted in accelerated ocean acidification in all emission scenarios. These feedbacks may also affect global warming and deoxygenation. This is particularly important, given that many policymakers are aiming for low emission commitments, but may still be severely underestimating the extent and timing of ocean acidification. There is a great need to improve our ability to predict carbon-climate feedbacks so we do not underestimate projected ocean acidification and its impacts on both sensitive ecosystems and the human communities that rely on them for food, coastal protection and other ecosystem services.

Authors:
Richard Matear (CSIRO Oceans and Atmosphere, Australia)
Andrew Lenton (Antarctic Climate and Ecosystems CRC, Australia)

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)

 

Increased temperatures suggest reduced capacity for carbon

Posted by mmaheigan 
· Thursday, January 18th, 2018 

The ocean’s biological pump works to draw down atmospheric carbon dioxide (CO2) by exporting carbon from the surface ocean. This process is less efficient at higher temperatures, implying a possible climate feedback. Recent work by Cael et al. provides an explanation of why this feedback occurs and an estimate of its severity.

In a highly simplified view, carbon export depends on the balance between two temperature-dependent processes: 1) The autotrophic production and 2) the heterotrophic respiration of organic carbon. Cael and Follows (Geophysical Research Letters 2016) recently developed a mechanistic model based on established temperature dependencies for photosynthesis and respiration to explore feedbacks between export efficiency and climate. Heterotrophic growth rates increase more so than phototrophic rates with increasing temperature, which suggests that at higher temperatures, community respiration will increase relative to production, thereby decreasing export efficiency. Although simplistic, the model captures the temperature dependence of export efficiency observations.

Figure: Schematic of the mechanism on which the Cael and Follows (2016) model is based. (a) Photosynthesis (dark grey) and respiration (light grey) respond to temperature differently, yielding (b) a decline in export efficiency at higher temperatures.

More recently, Cael, Bisson, and Follows (Limnology and Oceanography 2017) applied this model to sea surface temperature records and estimated a ~1.5% decline in globally-averaged export efficiency over the past three decades of increasing ocean temperatures as a result of this metabolic mechanism. This ~1.5% decline is equivalent to a reduced ocean sequestration of approximately 100 million fewer tons of carbon annually, comparable to the annual carbon emissions of the United Kingdom. The model provides a framework in which to consider the relationship between climate and ocean carbon export that might also elucidate large-scale (e.g., glacial-interglacial) atmospheric CO2 changes of the past.

Authors:
B. B. Cael (MIT/WHOI)
Kelsey Bisson (UCSB)
Mick Follows (MIT)

Zooplankton play a key and diverse role in the ocean carbon cycle

Posted by mmaheigan 
· Thursday, December 7th, 2017 

How does the enormous diversity of zooplankton species, life cycles, size, feeding ecology, and physiology affect their role in ocean food webs and cycling of carbon?

In the 2017 issue of Annual Review of Marine Science, Steinberg and Landry review the fundamental and multifaceted roles that zooplankton play in the cycling and export of carbon in the ocean. Carbon flows through marine pelagic ecosystems are complex due to the diversity of zooplankton consumers and the many trophic levels they occupy in the food web–from single-celled herbivores to large carnivorous jellyfish. Zooplankton also contribute to carbon export processes through a variety of mechanisms (mucous feeding webs, fecal pellets, molts, carcasses, and vertical migrations).


Figure 1.  Pathways of cycling and export of carbon by zooplankton in the ocean.

Climate change and other stressors are already affecting zooplankton abundance, distribution, and life cycles, and are predicted to result in widespread changes in zooplankton carbon cycling in the future. These changes will affect both the larger marine food web that depends upon zooplankton for food (fish) or recycled products for growth (primary producers) and the amount of carbon exported into the deep sea–where far from contact with the atmosphere it no longer contributes to global warming.

 

Authors:

Deborah K. Steinberg, Virginia Institute of Marine Science, The College of William and Mary
Michael R. Landry, Scripps Institution of Oceanography

WBC Series: Decadal variability of the Kuroshio Extension system and its impact on subtropical mode water formation 

Posted by mmaheigan 
· Friday, November 10th, 2017 

Bo Qiu1, Eitarou Oka2, Stuart P. Bishop3, Shuiming Chen1, Andrea J. Fassbender4

1. University of Hawaii at Manoa
2. The University of Tokyo
3. North Carolina State University
4. Monterey Bay Aquarium Research Institute

 

After separating from the Japanese coast at 36°N, 141°E, the Kuroshio enters the open basin of the North Pacific, where it is renamed the Kuroshio Extension (KE). Free from the constraint of coastal boundaries, the KE has been observed to be an eastward-flowing inertial jet accompanied by large-amplitude meanders and energetic pinched-off eddies (see Qiu 2002 and Kelly et al. 2010 for comprehensive reviews). Compared to its upstream counterpart south of Japan, the Kuroshio, the KE is accompanied by a stronger southern recirculation gyre that increases the KE’s eastward volume transport to more than twice the maximum Sverdrup transport (~ 60Sv) in the subtropical North Pacific Ocean (Wijffels et al. 1998). This has two important consequences. Dynamically, the increased transport enhances the nonlinearity of the KE jet, rendering the region surrounding the KE jet to have the highest mesoscale activity level in the Pacific basin. Thermodynamically, the enhanced KE jet brings a significant amount of tropical-origin warm water to the mid-latitude ocean to be in direct contact with cold, dry air blowing off the Eurasian continent. This results in significant wintertime heat loss from the ocean to atmosphere surrounding the Kuroshio/KE paths, contributing to the formation of North Pacific subtropical mode water (STMW; see Hanawa and Talley (2001) and Oka and Qiu (2012) for comprehensive reviews).

Figure 1. Yearly paths of the Kuroshio and KE plotted every 14 days using satellite SSH data (updated based on Qiu and Chen 2005). KE was in stable state in 1993–94, 2002–05, and 2010–15, and unstable state in 1995-2001, 2006–09, and 2016, respectively.

 

Although the ocean is known to be a turbulent medium, variations in both the level of mesoscale eddy activity and the formation rate of STMW in the KE region are by no means random on interannual and longer timescales. One important feature emerging from recent satellite altimeter measurements and eddy-resolving ocean model simulations is that the KE system exhibits clearly defined decadal modulations between a stable and an unstable dynamical state (e.g., Qiu & Chen 2005, 2010; Taguchi et al. 2007; Qiu et al. 2007; Cebollas et al. 2009; Sugimoto and Hanawa 2009; Sasaki et al. 2013; Pierini 2014; Bishop et al. 2015). As shown in Figure 1, the KE paths were relatively stable in 1993–95, 2002–05, and 2010–15. In contrast, spatially convoluted paths prevailed during 1996–2001 and 2006–09. When the KE jet is in a stable dynamical state, satellite altimeter data further reveal that its eastward transport and latitudinal position tend to increase and migrate northward, its southern recirculation gyre tends to strengthen, and the regional eddy kinetic energy level tends to decrease. The reverse is true when the KE jet switches to an unstable dynamical state. In fact, the time-varying dynamical state of the KE system can be well represented by the KE index, defined by the average of the variance-normalized time series of the southern recirculation gyre intensity, the KE jet intensity, its latitudinal position, and the negative of its path length (Qiu et al. 2014). Figure 2a shows the KE index time series in the satellite altimetry period of 1993–present; here, a positive KE index indicates a stable dynamical state and a negative KE index, an unstable dynamical state. From Figure 2a, it is easy to discern the dominance of the decadal oscillations between the two dynamical states of the KE system.

Figure 2. (a) Time series of the KE index from 1993‑present; available at http://www.soest.hawaii.edu/oceanography/bo/KE_index.asc. (b) Year-mean SSH maps when the KE is in stable (2004 and 2011) versus unstable (1997 and 2008) states. (c) SSH anomalies along the zonal band of 32°-34°N from satellite altimetry measurements. (d) Time series of the PDO index from 1989-present; available at http://jisao.washington.edu/pdo/PDO.latest.

 

Transitions between the KE’s two dynamical states are caused by the basin-scale wind stress curl forcing in the eastern North Pacific related to the Pacific Decadal Oscillation (PDO). Specifically, when the central North Pacific wind stress curl anomalies are positive during the positive PDO phase (see Figure 2d), enhanced Ekman flux divergence generates negative local sea surface height (SSH) anomalies in 170°–150°W along the southern recirculation gyre latitude of 32°–34°N. As these wind-induced negative SSH anomalies propagate westward as baroclinic Rossby waves into the KE region after a delay of 3–4 years (Figure 2c), they weaken the zonal KE jet, leading to an unstable (i.e., negative index) state of the KE system with a reduced recirculation gyre and an active eddy kinetic energy field (Figure 2b). Negative anomalous wind stress curl forcing during the negative PDO phase, on the other hand, generates positive SSH anomalies through the Ekman flux convergence in the eastern North Pacific. After propagating into the KE region in the west, these anomalies stabilize the KE system by increasing the KE transport and by shifting its position northward, leading to a positive index state.

The dynamical state of the KE system exerts a tremendous influence upon the STMW that forms largely along the paths of the Kuroshio/KE jet and inside of its southern recirculation gyre (e.g., Suga et al. 2004; Qiu et al. 2006; Oka 2009). Figure 3a shows the monthly time series of temperature profile, constructed by averaging available Argo and XBT/CTD/XCTD data inside the KE southern recirculation gyre (see Qiu and Chen 2006 for details on the constructing method). The black line in the plot denotes the base of the mixed layer, defined as where the water temperature drops by 0.5°C from the sea surface temperature. Based on the temperature profiles, Figure 3b shows the monthly time series of potential vorticity. STMW in Figure 3b is characterized by water columns with potential vorticity of less than 2.0 x 10-10 m-1s-1 beneath the mixed layer. From Figure 3, it is clear that both the late winter mixed layer depth and the low-potential vorticity STMW layer underwent significant decadal changes over the past 25 years. Specifically, deep mixed layer and pronounced low-potential vorticity STMW were detected in 1993–95, 2001–05, and 2010–15, and these years corresponded roughly to the periods when the KE index was in the positive phase (cf. Figure 2a).

 

Figure 3. Monthly time series of (a) temperature (°C) and (b) potential vorticity (10-10 m-1 s-1) averaged in the KE’s southern recirculation gyre. The thick black and white lines in (a) and (b) denote the base of the mixed layer, defined as where the temperature drops by 0.5°C from the surface value. Red pluses (at the top of each panel) indicate the individual temperature profiles used in constructing the monthly T(z, t) profiles. The potential vorticity, Q(z,t) = fα∂T(z,t)/∂z, where f is the Coriolis parameter and α the thermal expansion coefficient.

 

The close connection between the dynamical state of the KE system and the STMW formation has been detected by many recent studies based on different observational data sources and analysis approaches (Qiu and Chen 2006; Sugimoto and Hanawa 2010; Rainville et al. 2014; Bishop and Watts 2014; Oka et al. 2012; 2015; Cerovecki and Giglio 2016). Physically, this connection can be understood as follows. When the KE is in an unstable state (or a negative KE index phase), high-regional eddy variability infuses high-potential vorticity KE and subarctic-gyre water into the southern recirculation gyre, increasing the upper-ocean stratification and hindering the development of deep winter mixed layer and formation of STMW. A stable KE path with suppressed eddy variability (in the positive KE index phase), on the other hand, favors the maintenance of a weak stratification in the recirculation gyre, leading to the formation of a deep winter mixed layer and thick STMW.

Since the STMW is renewed each winter, due to combined net surface heat flux and wind stress forcing that modulate on interannual timescales, a question arising naturally is the timescale on which the dynamical state change of the KE system is able to alter the upper ocean stratification and potential vorticity inside the recirculation gyre. If the influence of the KE dynamical state acts on interannual timescales, one may expect a stronger control on the STMW variability by the wintertime atmospheric condition (e.g., Suga and Hanawa 1995; Davis et al. 2011). Intensive observations from the Kuroshio Extension System Study (KESS) program, spanning the period from April 2004 to July 2006, captured the 2004–05 transition of the KE system from a stable to an unstable state. The combined measurements by profiling Argo floats, moored current meter, current and pressure inverted echo sounder (CPIES), and the Kuroshio Extension Observatory (KEO) surface mooring revealed that the KE dynamical state change was able to change the STMW properties both significantly in amplitude and effectively in time (Qiu et al. 2007; Bishop 2013; Cronin et al. 2013; Bishop and Watts 2014). Relative to 2004, the low-potential vorticity signal in the core of STMW was diminished by one-half in 2005, and this weakening of STMW’s intensity occurred within a period of less than seven months. These significant and rapid responses of STMW to the KE dynamical state change suggests that the variability in STMW formation is more sensitive to the dynamical state of the KE than to interannual variations in overlying atmospheric conditions over the past 25 years.

The decadal variability of STMW in the KE’s southern recirculation gyre is able to affect the water property distributions in the entire western part of the North Pacific subtropical gyre (Oka et al. 2015). Measurements by Argo profiling floats during 2005–14 revealed that the volume and spatial extent of STMW decreased (increased) in 2006–09 (after 2010) during the unstable (stable) KE period in its formation region north of ~28°N, as well as in the southern, downstream regions with a time lag of 1-2 years. Such decadal subduction variability affects not only physical but also biogeochemical structures in the downstream, interior subtropical gyre. Shipboard observations at 25°N and along the 137°E repeat hydrographic section of the Japan Meteorological Agency exhibited that, after 2010, enhanced subduction of STMW consistently increased dissolved oxygen, pH, and aragonite saturation state and decreased potential vorticity, apparent oxygen utilization, nitrate, and dissolved inorganic carbon. Changes in dissolved inorganic carbon, pH, and aragonite saturation state were opposite their long-term trends.

KE State and the Ocean Carbon Cycle

Western boundary current (WBC) regions display the largest magnitude air-to-sea carbon dioxide (CO2) fluxes of anywhere in the global ocean. STMW formation processes are thought to account for a majority of the anthropogenic CO2 sequestration that occurs outside of the polar, deep water formation regions (Sabine et al. 2004; Khatiwala et al. 2009). Once subducted and advected away from the formation region, mode waters often remain out of contact with the atmosphere on timescales of decades to hundreds of years, making them short-term carbon silos relative to the abyssal carbon storage reservoirs. One of the physical impacts on carbon uptake via air-sea CO2 flux is due to the temperature dependence of the solubility of pCO2 in the surface waters. Cooler surface waters during the wintertime months reduce the oceanic pCO2 and subsequently enhance the CO2 flux into the ocean. This carbon uptake corresponds with the timing of peak STMW formation.

As mentioned above, the formation of STMW is modulated by the dynamic states of the KE, with less STMW forming during unstable states and more during stable states. To complicate matters, more enhanced levels of surface chlorophyll (Chla) have also been observed from satellite ocean color during unstable states (Lin et al. 2014), which points to the potential importance of biophysical interactions on carbon uptake. Elevated levels of Chla can further modify the pCO2 of surface waters and enhance carbon export at depth from sinking of particulate organic matter following an individual bloom. Given that submesoscale processes result from deep wintertime mixed layers and from the presence of the larger mesoscale lateral shear and strain fields (McWilliams 2016), it is expected that submesoscale processes are also important in STMW formation during unstable states of the KE. An open question in the research community is to what extent do elevated levels of mesoscale and submesoscale eddy activity modulate STMW formation and carbon uptake during unstable states of the KE? With large variations in STMW formation occurring in concert with decadal variability in the mesoscale eddy field, it is possible that submesoscale processes may impact STMW formation through restratification of the mixed layer within density classes encompassing STMW and timing of the spring bloom. These mesoscale and submesoscale processes may then also impact the uptake of CO2 in the North Pacific on interannual to decadal timescales.

 

 

References

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Cronin, M. F., N. A. Bond, J. T. Farrar, H. Ichikawa, S. R. Jayne, Y. Kawai, M. Konda, B. Qiu, L. Rainville, and H. Tomita, 2013: Formation and erosion of the seasonal thermocline in the Kuroshio Extension Recirculation Gyre. Deep-Sea Res. II, 85, 62-74, doi:10.1016/j.dsr2.2012.07.018.

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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)

 

Arctic surface waters release methane but also absorb 2,000 times the CO2 for a net cooling effect

Posted by mmaheigan 
· Thursday, September 28th, 2017 

A recent study by Pohlman et al. published in PNAS showed that ocean waters near the surface of the Arctic Ocean absorbed 2,000 times more carbon dioxide (CO2) from the atmosphere than the amount of methane released into the atmosphere from the same waters. The study was conducted near Norway’s Svalbard Islands, which overly numerous seafloor methane seeps.

Methane is a more potent greenhouse gas than CO2, but the removal of CO2 from the atmosphere where the study was conducted more than offset the potential warming effect of the observed methane emissions. During the study, scientists continuously measured the concentrations of methane and CO2 in near-surface waters and in the air just above the ocean surface. The measurements were taken over methane seeps fields at water depths ranging from 260 to 8530 feet (80 to 2600 meters).

Figure 1. Ocean waters overlying shallow-water methane seeps (white dots) offshore from the Svalbard Islands absorb substantially more atmospheric carbon dioxide than the methane that they emit to the atmosphere. Colors indicate the strength of the negative greenhouse warming potential associated with carbon dioxide influx to these surface waters relative to the positive greenhouse warming potential associated with the methane emissions. Gray shiptracks have background values for the relative greenhouse warming potential.

Analysis of the data confirmed that methane was entering the atmosphere above the shallowest (water depth of 260-295 feet or 80-90 meters) Svalbard margin seeps. The data also showed that significant amounts of CO2 were being absorbed by the waters near the ocean surface, and that the cooling effect resulting from CO2 uptake is up to 230 times greater than the warming effect expected from the methane emitted.

Most previous studies have focused only on the sea-air flux of methane overlying seafloor seep sites and have not accounted for the drawdown of CO2 that could offset some of the atmospheric warming potential of the methane. Phytoplankton appeared to be more active in the near-surface waters overlying the seafloor methane seeps, which would explain why so much carbon dioxide was being absorbed. Physical and biogeochemical measurements of near-surface waters overlying the seafloor methane seeps showed strong evidence of upwelling of cold, nutrient-rich waters from depth, stimulating phytoplankton activity and increasing CO2 drawdown. This study was the first to document this CO2 drawdown mechanism in a methane source region.

“If what we observed near Svalbard occurs more broadly at similar locations around the world, it could mean that methane seeps have a net cooling effect on climate, not a warming effect as we previously thought,” said USGS biogeochemist John Pohlman, the paper’s lead author. “We are looking forward to testing the hypothesis that shallow-water methane seeps are net greenhouse gas sinks in other locations.”

 

Authors:
John W. Pohlman (USGS Woods Hole Coastal & Marine Science Center)
Jens Greinert (GEOMAR, Univ. of Tromsø, Royal Netherlands Institute for Sea Research)
Carolyn Ruppel (USGS Woods Hole Coastal & Marine Science Center)
Anna Silyakova (Univ. of Tromsø)
Lisa Vielstädte (GEOMAR)
Michael Casso (USGS Woods Hole Coastal & Marine Science Center)
Jürgen Mienert (Univ. of Tromsø)
Stefan Bünz (Univ. of Tromsø)

Sinking particles as biogeochemical hubs for trace metal cycling and release

Posted by mmaheigan 
· Thursday, September 14th, 2017 

The extent to which the return of major and minor elements to the dissolved phase in the deep ocean (termed remineralization) is decoupled plays a major role in setting patterns of nutrient limitation in the global ocean. It is well established that major elements such as phosphorus, silicon, and carbon are released at different rates from sinking particles, with major implications for nutrient recycling. Is this also the case for trace metals?

A recent publication by Boyd et al. in Nature Geoscience provides new insights into the biotic and abiotic processes that drive remineralization of metals in the ocean.  Particle composition changes rapidly with depth with both physical (disaggregation) and biogeochemical (grazing; desorption) processes leading to a marked decrease in the total surface area of the particle population. The proportion of lithogenic metals in sinking particles also appears to increase with depth, as the biogenic metals may be more labile and hence more readily removed.

Findings from GEOTRACES process studies revealed that release rates for trace elements such as iron, nickel, and zinc vary from each other. Microbes play a key role in determining the turnover rates for nutrients and trace elements. Decoupling of trace metal recycling in the surface ocean and below may result from their preferential removal by microbes to satisfy their nutritional requirements. In addition, the chemistry operating on particle surfaces plays a pivotal role in determining the specific fates of each trace metal. Teasing apart these factors will take time, as there is a complex interplay between chemical and biological processes. Improving our understanding is crucial, as these processes are not currently well represented by state-of-the-art ocean biogeochemical models.

Figure caption: Rapid changes in the characteristics of sinking particles over the upper 200 m as evidenced by: a) differential release of trace metals from sinking diatoms; b) changes in proportion of lithogenic versus biogenic materials; and c) ten-fold decrease in total particle surface area.

 

Authors:
Philip Boyd (IMAS, Australia)
Michael Ellwood (ANU, Australia)
Alessandro Tagliabue (Liverpool, UK)
Ben Twining (Bigelow, USA)

 

Relevant links:
GEOTRACES Digest: Iron Superstar

Joint workshop with GEOTRACES in August 2016: Biogeochemical Cycling of Trace Elements within the Ocean

An autonomous approach to monitoring coral reef health

Posted by mmaheigan 
· Thursday, July 20th, 2017 

Coral reefs are diverse, productive ecosystems that are highly vulnerable to changing ocean conditions such as acidification and warming. Coral reef metabolism—in particular the fundamental ecosystem properties of net community production (NCP; the balance of photosynthesis and respiration) and net community calcification (NCC; the balance of calcification and dissolution)—has been proposed as a proxy for reef health. NCC is of particular interest, since ocean acidification is expected to have detrimental effects on reef calcification.

Traditionally, these metabolic rates are quantified through laborious methods that involve discrete sampling, which, due to a limited number of observations, often fails to characterize natural variability on time scales of minutes to days. In a recent paper in JGR, Takeshita et al. (2016) presented the Benthic Ecosystem and Acidification Measurement System (BEAMS), a fully autonomous system that simultaneously measures NCP and NCC at 15-minute intervals over a period of weeks. BEAMS utilizes the gradient flux method to quantify benthic metabolic rates by measuring chemical (pH and O2) and velocity gradients in the turbulent benthic boundary layer.

Two BEAMS were simultaneously deployed on Palmyra Atoll located approximately one km apart over vastly different benthic communities. One site was a healthy reef with approximately 70% coral cover, and the other was a degraded reef site with only 5% coral cover that was dominated by a non-calcifying invasive corallimorph Rhodactis howesii. Over the course of two weeks, BEAMS collected over 1,000 measurements of NCP and NCC from each site, yielding significantly different ratios of NCP to NCC between the two sites. These initial results suggest that BEAMS is capable of detecting different metabolic states, as well as patterns consistent with degrading reef health.

BEAMS is an exciting new autonomous tool to monitor reef health and study drivers of reef metabolism on timescales ranging from minutes to months (and potentially years). Additionally, autonomous measurement tools increase the potential for widespread and comparable observations across reefs and reef systems. Such knowledge will greatly improve our ability to predict the fate of coral reefs in a changing ocean.

 

Authors: 
Yui Takeshita (Monterey Bay Aquarium Research Institute)

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

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