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Archive for changing ocean chemistry – Page 3

The causes of the 90-ppm glacial atmospheric CO2 drawdown still strongly debated

Posted by mmaheigan

· Tuesday, July 9th, 2019

Joint feature with GEOTRACES

Figure: Illustration of the two main mechanisms identified by this study to explain lower atmospheric CO₂ during glacial periods. Left: present-day conditions; right: conditions around 19,000 years ago during the Last Glacial Maximum. The obvious explanation for lower CO₂ during glacial periods – cooler ocean temperatures (darker blue shade) making CO₂ more soluble, much as a glass of sparkling wine will remain fizzier for longer when it is colder – has long been dismissed as not being a significant factor. However, previous calculations assumed that the ocean cooled uniformly and was saturated in dissolved CO₂. The model, consistent with reconstructions of sea surface temperature, predicts more cooling at mid latitudes compared with polar regions and also accounts for undersaturation. This nearly doubles the effect of temperature change and accounts for almost half the 90 ppm glacial-interglacial atmospheric CO₂ difference. Another quarter is explained in this model by increased growth of marine algae (green blobs and inset) in the waters off Antarctica. Algae absorb CO₂ from the atmosphere during photosynthesis and “pump” it into the deep ocean when they die and sink. But their growth in the present-day ocean, especially the waters off Antarctica, is limited by the availability of iron, an essential micronutrient primarily supplied by wind-borne dust. In our model an increased supply of iron to the Southern Ocean, likely originating from Patagonia, Australia and New Zealand, enhances their growth and sucks CO₂ out of the atmosphere. This “fertilization” effect was greatly underestimated by previous studies. The study also finds that, contrary to the current consensus, a large expansion of sea ice off Antarctica and reconfiguration of ocean circulation may have played only a minor role in glacial-interglacial CO₂ changes. Credit: Illustration by Andrew Orkney, University of Oxford.

Using an observationally constrained earth system model, S. Khatiwala and co-workers compare different processes that could lead to the 90-ppm glacial atmospheric CO₂drawdown, with an important improvement on the deep carbon storage quantification (i.e. Biological Carbon Pump efficiency). They demonstrate that circulation and sea ice changes had only a modest net effect on glacial ocean carbon storage and atmospheric CO₂, whereas temperature and iron input effects were more important than previously thought due to their effects on disequilibrium carbon storage.

Authors:
Samar Khatiwala (University of Oxford, UK)
Andreas Schmittner and Juan Muglia (Oregon State University)

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

Suddenly shallow: A new aragonite saturation horizon will soon emerge in the Southern Ocean

Posted by mmaheigan

· Monday, May 27th, 2019

Earth System Models (ESMs) project that by the end of this century, the aragonite saturation horizon (the boundary between shallower, saturated waters and deeper, undersaturated waters that are corrosive to aragonitic shells) will shoal all the way to the surface in the Southern Ocean, yet the temporal evolution of the horizon has not been studied in much detail. Rather than a gradual shoaling, a new shallow aragonite saturation horizon emerges suddenly across many locations in the Southern Ocean between now and the end of the century (Figure 1, left), as detailed in a new study published in Nature Climate Change.

Figure 1: Maximum step-change in the depth of the aragonite saturation horizon (left), timing of the step-change (center), and cause of the change (right). Xs on the time axis (center) indicate when the shallow horizon emerges in each ensemble member. (click image to enlarge)

The emergence of the shallow aragonite saturation horizon is apparent in each member of an ensemble of climate projections from an ESM, but the step change occurs during different years (Figure 1, center). The shoaling is driven by the gradual accumulation of anthropogenic CO₂ in the Southern Ocean thermocline, where the carbonate ion concentration exhibits a local minimum and approaches undersaturation (Figure 1, right).

The abrupt shoaling of the Southern Ocean aragonite saturation horizon occurs under both business-as-usual and emission-stabilizing scenarios, indicating an inevitable and sudden decrease in the volume of suitable habitat for aragonitic organisms such as shelled pteropods, foraminifers, cold-water corals, sea urchins, molluscs, and coralline algae. Widespread reductions in these habitats may have far-reaching consequences for fisheries, economies, and livelihoods.

Authors:
Gabriela Negrete-García (Scripps Institution of Oceanography)
Nicole Lovenduski (University of Colorado Boulder)

See also OCB2019 plenary session: Carbon cycle feedbacks from the seafloor (Wednesday, June 26, 2019)

Pteropod populations stable or increasing according to long-term study along the Western Antarctic Peninsula

Posted by mmaheigan

· Thursday, March 21st, 2019

Shelled pteropods (pelagic snails) are abundant planktonic predators and prey, linking grazers and higher trophic levels and contributing to the carbon cycle via consumption and excretion. Pteropods have been heralded as bioindicators of ocean acidification, given their aragonitic shell’s susceptibility to dissolution, which could ultimately lead to declining abundance. However, pteropod population dynamics are understudied, particularly in the Southern Ocean, a region predicted to be highly impacted by both warming and ocean acidification. In a recent publication in Limnology and Oceanography, long-term data sets from the Western Antarctic Peninsula show that while there is considerable interannual variability in pteropod abundance, populations have remained stable over the past 25 years, with some pteropod species (gymnosomes (non-shelled pteropod) overall, L. antarctica and C. pyramidata (shelled pteropods) regionally) even increasing during this period (Figure 1).

Figure 1. Annual pteropod abundance anomalies for the entire Palmer Antarctica Long-Term Ecological Research (LTER) study region along the Western Antarctic Peninsula. (a) Limacina helicina antarctica (shelled pteropod), (b) Gymnosomes – nonshelled pteropods that prey on shelled pteropods (p = 0.007, r² = 0.27), and (c) Clio pyramidata (shelled pteropod). Effect of environment on pteropod abundance. (d) SST vs. L. antarctica abundance, e) Sea ice advance vs. L. antarctica and Gymnosome abundance, (f) Sea ice retreat vs. C. pyramidata abundance. Data plotted are annual anomalies for each year of the time series (1993–2017). Sea ice advance is lagged 2-yr behind pteropod abundance (e.g., 2017 pteropod annual anomaly is plotted against 2015 sea ice advance annual anomaly) SST are lagged 1-yr behind L. antarctica abundance (e.g., 2017 L. antarctica annual anomaly is plotted against 2016 SST). Regression lines for significant linear relationships are shown, regression statistics are as follows: (d) SST vs. L. antarctica (circles): n = 25, p = 0.006, r² = 0.25 (e) sea ice advance vs. L. antarctica (filled-circles) and Gymnosomes (empty-circles): n = 25, p = 0.003, r² = 0.30 (dashed line); (f) sea ice retreat vs. C. pyramidata (squares): n = 14, p = 0.0003, r² = 0.64.

There was no significant influence of carbonate chemistry parameters (e.g., aragonite saturation state) on pteropod abundance, since the Western Antarctic Peninsula has yet to experience prolonged conditions characteristic of ocean acidification. However, other environmental factors such as warming and associated sea ice retreat were more influential. For example, warmer, ice-free waters in one year typically led to higher pteropod abundances the following year, suggesting that pteropods may be better adapted than expected to warming conditions due to climate change. The authors propose that earlier sea ice retreat promotes recruitment and subsequent expansion of pteropods further South, which could explain their increased abundance in this subregion. These results increase our understanding of pteropod responses to environmental variability, which is important for predicting future effects of climate change on regional carbon cycling and plankton trophic interactions in the Southern Ocean.

Authors:
Patricia S. Thibodeau (VIMS)
Deborah K. Steinberg (VIMS)
Sharon E. Stammerjohn (University of Colorado at Boulder)
Claudine Hauri (University of Alaska Fairbanks)

You better repeat it: Serial ocean acidification experiments on fish early life stages

Posted by mmaheigan

· Tuesday, March 5th, 2019

To detect potential effects of acidification on marine organisms, experimenters most commonly use within-experiment replication, but repeating the experiments themselves is rarely done. While the first approach suffices to detect major CO₂ effects, other potentially important responses may get detected and robustly quantified only via serial experimentation. A study by Baumann et al. in Biology Letters comprises a meta-analysis of 20 standard CO₂ exposure experiments conducted over six years on Atlantic silverside (Menidia menidia) offspring.

Figure 1: Robust estimate of silverside CO₂ sensitivity based on serial experimentation. (A, B) Mean CO₂ effect size calculated as the log-transformed response ratio of six early life history traits measured at 20 standard experiments between 2012-2017 (Error: bootstrapped 95% confidence intervals). (C) Seasonal change in CO₂ sensitivity in silverside early life stages. Each symbol represents an individual experiment, using offspring obtained by fertilizing wild spawners throughout their spring/summer spawning season.

Silversides are an abundant and ecologically important forage fish in the North Atlantic. The study revealed that during early life stages, Atlantic silversides tolerate pCO₂levels up to ~2,000 µatm, with seasonal shifts in sensitivity. However, this early exposure to high pCO₂ levels reduces embryo survival by 9% and overall survival by 13% (Figure 1). Future ocean acidification could cause reduced survival of these and other forage fish, and thus impact their diverse marine predators, including seabirds and commercially important fish species. This sustained experimental work resulted in the most robustly constrained estimates of average CO₂ effect sizes for a marine organism to date, demonstrating the utility of serial experimentation as a powerful tool for assessing organism responses to changing CO₂.

Authors:
Hannes Baumann
Emma L. Cross
Chris S. Murray
(all University of Connecticut)

Rapid warming and salinity changes mask acidification in Gulf of Maine waters

Posted by mmaheigan

· Wednesday, February 20th, 2019

Why don’t we see ocean acidification in over a decade of high-frequency observations in the Gulf of Maine? The answer lies in a recent decade of changes that raised sea surface temperature and salinity, and in turn dampened the expected acidification signal and caused the saturation states of calcite minerals to increase. From 2004 to 2014, sea surface temperatures in the Gulf of Maine were higher than any observations recorded in the region over the past 150 years. This greatly impacted both CO₂ solubility and the sea surface carbonate system, as detailed in a recent paper in Biogeochemistry.

Over the 34 years of the time-series, the recent event is extreme, but interannual and decadal salinity and temperature variability also influenced carbonate system parameters, which makes it difficult to isolate and quantify an anthropogenic ocean acidification signal, especially if relying on shorter-term observations (Figure 1).

Figure 1: Modeled Ω_Aragonite (top panel) and pH (bottom panel) anomalies relative to monthly 2004 data. The red lines show trends prior to and after 2004, after which warming accelerated.

For those with a stake in profiting from or managing extractive resources that are susceptible to ocean acidification such as commercially important lobster and bivalves, understanding how ecosystems will be affected is critical. These analyses clearly demonstrate how physical processes can either accelerate or mitigate ocean carbonate system changes, thus confounding the detection of ocean acidification that is expected from increasing atmospheric carbon dioxide. To assess whether an ecosystem or species is at risk or aided by such processes, it is important to observe, understand, and be able to model all sources of carbonate system variability.

Authors:
Joe Salisbury and Bror Jönsson (Both at Ocean Processes Analysis Laboratory, University of New Hampshire)

Biological and physical controls on estuarine nitrous oxide emissions

Posted by mmaheigan

· Tuesday, February 5th, 2019

Nitrous oxide (N₂O) is a potent greenhouse gas with rising atmospheric concentrations. Atmospheric emissions of N₂O are predicted to increase with continued anthropogenic perturbation of the nitrogen cycle, yet the magnitude of these emissions is uncertain, particularly in coastal systems where N₂O fluxes are poorly constrained. How do N₂O emissions from a eutrophic estuary vary in space and time?

Figure 1: Depth profiles of nitrous oxide (N₂O) (circles), salinity (dashed line), and dissolved oxygen (solid line) in the Chesapeake Bay at three stations. Solid circles indicate oversaturation of N₂O with respect to equilibrium with the atmosphere, and open circles indicate undersaturation.

In a recent publication in Estuaries and Coasts, Laperriere et al. (2018) examined how physical and biological processes influence the distribution of N₂O in Chesapeake Bay using dissolved gas measurements (N₂O and N₂/Ar) and stable isotope tracer incubations. During stratified summer conditions, the mesohaline region of the Chesapeake Bay was always a source of N₂O to the atmosphere. The highest N₂O concentrations occurred in the pycnocline at the interface between reducing bottom waters and oxygenated surface waters (Figure 1). Vertical mixing of surface waters across the pycnocline caused elevated rates of ammonia oxidation, a biological source of N₂O, and resulted in the accumulation of nitrite (NO₂^–) below the pycnocline. During periods of weak mixing, ammonia oxidation rates and N₂O concentrations were lower, and low dissolved oxygen concentrations below the pycnocline set the stage for N₂O consumption via denitrification (Figure 1). The interplay between biological and physical processes controlling fluctuations in N₂O concentration was examined using a mass balance approach. Mass balance estimates indicated that both biological processes and physical transport contribute to local changes in N₂O concentration. The authors suggest that the fate of N₂O during stratified summer conditions is governed by vertical mixing across the pycnocline, controlling whether N₂O is released to the atmosphere or consumed at depth.

Authors:
Sarah M. Laperriere (University of California, Santa Barbara)
Nicholas J. Nidzieko (University of California, Santa Barbara)
Rebecca J. Fox (Washington College)
Alexander W. Fisher (University of California, Santa Barbara)
Alyson E. Santoro (University of California, Santa Barbara)

Evidence against an Arctic Ocean methane bomb

Posted by mmaheigan

· Tuesday, February 5th, 2019

Gas hydrates are an ice-like storehouse of the greenhouse gas methane found in continental margins of the world ocean. Warming waters can cause hydrates to decompose and release ancient methane to overlying sediment and waters. The continental shelves of the Arctic Ocean have been thought of as “ground zero” for the potential release of methane from hydrates, since the Arctic is warming rapidly and hydrates are found at relatively shallow water depths there. Another potential ancient methane input to Arctic shelf waters is the methane produced by microorganisms from the gradual thawing of permafrost carbon within seafloor sediment and/or transported to the shelf from terrestrial permafrost via rivers. But, can large stores of ancient-sourced methane reach surface waters and enter the atmosphere, contributing to greenhouse warming?

Figure caption: Map showing the fraction of methane in each surface water sample that was derived from ancient hydrate or permafrost, on a scale from 0 (modern, 0% ancient; indigo) to 1 (100% ancient; yellow). While some of the near-shore surface methane samples have a significant (~50%) ancient component, in waters deeper than 20 m, the surface water methane was mostly (90-95%) derived from modern sources.

To answer this question and understand the role of these ancient sources of methane (hydrates and permafrost), the authors of a 2018 study in Science Advances measured the natural abundance of radiocarbon (¹⁴C) in dissolved methane in the shallow shelf waters of the Alaskan Arctic Ocean (U.S. Beaufort Sea); methane derived from ancient sources has little to no measurable ¹⁴C because of radioactive decay over time. The ¹⁴C-methane results show that ancient sources are contributing methane to the study area’s waters, as the authors predicted. However, ancient methane emitted to seawater can be consumed by microorganisms or transported away by currents before reaching the atmosphere, though these mechanisms have not been known to be effective at removing methane in waters <100 m. This study revealed that these removal processes are surprisingly efficient in shallow shelf waters, especially at the study area’s deepest stations of 30 and 40 m depth, where only ~10% of the methane in surface waters was derived from ancient sources. These results add to a growing body of evidence against the likelihood of a large methane emission to the atmosphere occurring from ancient sources like hydrates, since the authors expect that methane removal processes in the water column are much more efficient in waters 100s of meters deep, where the bulk of the hydrate reservoir resides.

Authors:
K.J. Sparrow (University of Rochester; current address: Florida State University)
J.D. Kessler (University of Rochester)
J.R. Southon (University of California Irvine)
Garcia-Tigreros (University of Rochester)
K.M. Schreiner (University of Minnesota Duluth)
C.D. Ruppel (USGS)
J.B. Miller (University of Colorado Boulder; NOAA)
S.J. Lehman (University of Colorado Boulder)
Xu (University of California Irvine)

The past, present, and future of artificial ocean iron fertilization experiments

Posted by mmaheigan

· Wednesday, January 23rd, 2019

Since the beginning of the industrial revolution, human activities have greatly increased atmospheric CO₂ concentrations, leading to global warming and indicating an urgent need to reduce global greenhouse gas emissions. The Martin (or iron) hypothesis suggests that ocean iron fertilization (OIF) could be a low-cost effective method for reducing atmospheric CO₂ levels by stimulating carbon sequestration via the biological pump in iron-limited, high-nutrient, low-chlorophyll (HNLC) ocean regions. Given increasing global political, social, and economic concerns associated with climate change, it is necessary to examine the validity and usefulness of artificial OIF (aOIF) experimentation as a geoengineering solution.

Figure 1. (a) Global annual distribution of surface chlorophyll concentrations (mg m^-3) with locations of 13 aOIF experiments. Maximum and initial values in (b) maximum quantum yield of photosynthesis (Fv/Fm ratios) and (c) chlorophyll-a concentrations (mg m^-3) during aOIF experiments. (d) Changes in primary productivity (ΔPP = [PP]_{post-fertilization (postf)} ‒ [PP]_{pre-fertilization (pref)}; mg C m^-2 d^-1). (e) Distributions of chlorophyll-a concentrations (mg m^-3) on day 24 after iron addition in the Southern Ocean iron experiment-north (SOFeX-N) from MODIS Terra Level-2 daily image and on day 20 in the SOFeX-south (SOFeX-S) from SeaWiFS Level-2 daily image (white dotted box indicates phytoplankton bloom during aOIF experiments). (f) Changes in nitrate concentrations (ΔNO₃^– = [NO₃^–]_postf ‒ [NO₃^–]_pref; μM). (g) Changes in partial pressure of CO₂ (ΔpCO₂ = [pCO₂]_postf ‒ [pCO₂]_pref; μatm). The color bar indicates changes in dissolved inorganic carbon (ΔDIC = [DIC]_postf ‒ [DIC]_pref; μM). The numbers on the X axis indicate the order of aOIF experiments as given in Figure 1a and are grouped according to ocean basins; Equatorial Pacific (EP) (yellow bar), Southern Ocean (SO) (blue bar), subarctic North Pacific (NP) (red bar), and subtropical North Atlantic (NA) (green bar).

A review paper published in Biogeosciences on aOIF experiments provides a thorough overview of 13 scientific artificial OIF experiments conducted in HNLC regions over the last 25 years. These aOIF experiments have demonstrated that iron addition stimulates substantial increases in phytoplankton biomass and primary production, resulting in drawdown of macro-nutrients and dissolved inorganic carbon (Figure 1). Many of the aOIF experiments have also precipitated community shifts from smaller (pico- and nano-) to larger (micro) phytoplankton. However, the impact on the net transfer of CO₂ from the atmosphere to below the winter mixed layer via the biological pump is not yet fully understood or quantified and appears to vary with environmental conditions, export flux measurement techniques, and other unknown factors. These results, including possible side effects, have been debated among those who support and oppose aOIF experimentation, and many questions remain about the effectiveness of scientific aOIF, possible side effects, and international aOIF law frameworks. Therefore, it is important to continue undertaking small-scale, scientifically controlled studies to better understand natural processes in the HNLC regions, assess the associated risks, and lay the groundwork for evaluating the potential effectiveness and impacts of large-scale aOIF as a geoengineering solution to anthropogenic climate change. Additionally, this paper suggests considerations for the design of future aOIF experiments to maximize the effectiveness of the technique and begin to answer open questions under international aOIF regulations.

Authors:
Joo-Eun Yoon (Incheon National University)
Il-Nam Kim (Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)

Unexpected acidification of deep waters in the Sea of Japan due to global warming

Posted by mmaheigan

· Tuesday, May 22nd, 2018

Oceans worldwide are warming up, and thermohaline circulation is expected to slow down. At the same time, ocean acidity is increasing due to the influx of anthropogenic carbon dioxide (CO₂) from the atmosphere, a phenomenon called ocean acidification that has primarily been documented in shallow waters. In general, deeper waters contain less anthropogenic CO₂, but predicted reductions in ventilation of deep waters may impact deep ocean chemistry, as described in a recent study in Nature Climate Change.

Figure caption: Secular trend of total scale pH at in-situ temperature and pressure at various depths between 1965 and 2015 in the Sea of Japan.

The Sea of Japan is a marginal sea with its own deep- and bottom-water formation that maintains relatively elevated oxygen levels. However, time-series data from 1965-2015 (the longest time-series available) reveal that oxygen concentrations in these deep waters are declining, indicating a reduction in ventilation that increases their residence time. As organic matter decomposition in these waters continues to accumulate more CO₂, the pH decreases. As a result, the acidification rate near the bottom of the Sea of Japan is 27% higher than at the surface. As a miniature ocean with its own deep- and bottom-water formation, the Sea of Japan provides insight into how future warming might alter deep-ocean ventilation and chemistry.

Authors:
Chen-Tung Arthur Chen (National SunYat-sen University, Taiwan and Second Institute of Oceanography, China)
Hon-Kit Lui (National SunYat-sen University and Taiwan Research Institute)
Chia-Han Hsieh (National SunYat-sen University, Taiwan)
Tetsuo Yanagi (International Environmental Management of Enclosed Coastal Seas Center, Japan)
Naohiro Kosugi (Japan Meterological Agency)
Masao Ishii (Japan Meterological Agency)
Gwo-Ching Gong (National Taiwan Ocean University)

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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