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Archive for New OCB Research – Page 17

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.

Where the primary production goes determines whether you catch tuna or cod

Posted by mmaheigan

· Friday, September 6th, 2019

Fishes are incredibly diverse, fill various roles in their ecosystems, and are an important resource—economically, socially, and nutritionally. The relationship between primary productivity and fish catches is not straightforward; fisheries oceanographers and managers have long struggled to predict abundances and fully understand the controls of cross-ecosystem differences in fish abundances and assemblages. A recent study in Progress in Oceanography modeled the relationships between fish abundances and assemblages and ecosystem factors such as physical properties and plankton productivity.

The mechanistic model simulated feeding, growth, reproduction, and mortality of small pelagic forage fish, large pelagic fish, and demersal (bottom-dwelling) fish in the global ocean using plankton food web estimates and ocean conditions from a high-resolution earth system model of the 1990s. Modeled fish assemblages were more related to the separation of secondary production into pelagic zooplankton or benthic fauna secondary production than to primary productivity. Specifically, the ratio of pelagic to benthic production drove spatial differences in dominance by large pelagic fish or by demersal fish. Similarly, demersal fish abundance was highly sensitive to the efficiency of energy transfer from exported surface production to benthic fauna.

The model results offer a systematic understanding of how marine fish communities are structured by spatially varying environmental conditions. With global climate change, the expected decrease in exported primary production would lead to fewer demersal fish around the world. This model provides a framework for testing the effect of changing conditions on fish communities at a global scale, which can also help inform managers of potential impacts on economic, social, and nutritional resources worldwide.

Figure 1: (A) Sample food web with three fish types, two habitats, two prey categories, and feeding interactions (arrows). Dashed arrow denotes feeding only occurs in shelf regions with depth <200 m. (B) Fraction of large pelagic vs. demersal fishes (LP/(LP+D)) as a function of the ratio of zooplankton production lost to higher predation (Zoop) to detritus flux to the seafloor (Bent) averaged over large marine ecosystems. Solid line: predicted linear model response, dashed lines: standard error. (Lower panels) Circles=mean biomasses (g m^-2) and lines=fluxes of biomass (g m^-2 d^-1) through the pelagic (top 100m) and benthic components of the food webs at two test locations, (C) Peruvian Upwelling (PUP) ecosystem and (D) Eastern Bering Sea (EBS) shelf ecosystem. Circles and lines scale with the modeled biomasses and fluxes. Circle color key: Gray=net primary productivity (NPP); yellow=medium and large zooplankton; red=forage fish; blue=large pelagic fish; brown=benthos; green=demersal fish.

Authors:
Colleen M. Petrik (Princeton University, Texas A&M University)
Charles A. Stock (NOAA Geophysical Fluid Dynamics Laboratory)
Ken H. Andersen (Technical University of Denmark)
P. Daniël van Denderen (Technical University of Denmark)
James R. Watson (Oregon State University)

Predicting marine ecosystem futures

Posted by mmaheigan

· Wednesday, September 4th, 2019

Earth System Models (ESMs) are powerful and effective tools for exploring and predicting marine ecosystem response to environmental change, including biogeochemical processes that underlie threats to ocean health such as ocean acidification, deoxygenation, and changes in productivity. Seasonal to interannual marine biogeochemical predictions with ESMs hold great promise for exploring links between climate and marine resources such as fisheries but have thus far been challenged by limitations associated with observational initialization, model structure, and computational availability. In a recent study published in Science, authors integrated the Geophysical Fluid Dynamics Laboratory’s (GFDL) COBALT (Carbon, Ocean Biogeochemistry and Lower Trophics) marine biogeochemical model with seasonal to multi-annual climate predictions from GFDL’s CM2.1 climate model to examine marine ecosystem futures on these shorter time scales. The global biogeochemical forecasts were initialized on the first of each month between 1991 and 2017 with 12 ensemble members in each prediction, creating a database of nearly 4000 forecasts and 8000 simulation years. The model skillfully predicted seasonal to multi-annual chlorophyll fluctuations in many ocean regions (Figure 1).

Figure 1: Prediction skill in reproducing observed variations of monthly chlorophyll anomaly. (Top) Correlation coefficient between predicted and observed chlorophyll at 1-3 month lead time during the period 1997-2017. Stippled areas indicate that the correlation is significantly greater than 0 with 95% confidence. Areas with less than 80% satellite chlorophyll coverage are masked in grey. (Lower panels) Correlation coefficient between predicted and observed chlorophyll as a function of forecast initialization month (x-axis) and lead time (y-axis) in tropical Pacific, Indian, North Atlantic, North Pacific, and South Pacific oceans. In all panels, the darker the red, the higher the correlation up to a perfect correlation of 1.0. White indicates no correlation, while blue indicates negative correlation.

These results suggest that annual fish catches in selected large marine ecosystems can be predicted from chlorophyll and sea surface temperature anomalies up to 2-3 years in advance. Given that fisheries predictions sometimes failed to the point of commercial stock collapse in the past, this prediction capacity could be vital for fisheries managers. Biogeochemical prediction systems can extend beyond sea surface temperature and chlorophyll to include other potential drivers (e.g., oxygen, acidity, net primary production, zooplankton, etc.) as highly valuable tools for marine resource managers of dynamic and changing ecosystems.

Authors:
Jong-Yeon Park (Princeton Univ, NOAA GDFL, Chonbuk National Univ., Korea)
Charles A. Stock, John P. Dunne, Xiaosong Yang, and Anthony Rosati (NOAA GFDL)

Upwelling and solubility drive global surface dissolved inorganic carbon (DIC) distribution

Posted by mmaheigan

· Tuesday, August 20th, 2019

What drives the latitudinal gradient in open-ocean surface DIC concentration? Understanding the processes that drive the distribution of carbon in the surface ocean is essential to the study of the ocean carbon cycle and future predictions of ocean acidification and the ocean carbon sink.

Authors of a recent study in Biogeosciences investigated causes of the observed latitudinal trend in DIC and salinity-normalized DIC (nDIC) (Figure 1). The latitudinal trend in nDIC is not driven solely by the latitudinal gradient in temperature (through its effects on solubility), as is commonly assumed. Careful analysis using the Global Ocean Data Analysis Project version 2 (GLODAPv2) database revealed that physical supply from below (upwelling, entrainment in winter) at high latitudes is another major driver of the latitudinal pattern. The contribution of physical exchange explains an otherwise puzzling observation: Surface waters are lower in nDIC in the high-latitude North Atlantic than in other basins. This cannot be accounted for by temperature difference but rather is explained by a difference in the carbon content of deeper waters (lower in the subarctic North Atlantic than in the subarctic North Pacific or Southern Ocean) that are mixed up into the surface during winter months.

Figure caption: (Top) spatial distributions of surface ocean DIC and (bottom) salinity-normalised (nDIC). Both, most notably nDIC, increase towards the poles. Values are normalised to year 2005 to remove bias from changing levels of atmospheric CO2 in some observations before and after 2005. Data are from GLODAPv2.

These results also suggest that the upwelling/entrainment of water that is high in alkalinity generates a large and long-lasting effect on DIC, one that persists beyond the timescale of CO₂ gas exchange equilibration with the . That is to say, the impact of changes in upwelling on the ocean’s carbon source-sink strength depends not only on the DIC content of the upwelled water but also on its TA content.

Authors:
Yingxu Wu (University of Southampton)
Mathis Hain (University of California, Santa Cruz)
Matthew Humphreys (University of East Anglia and University of Southampton)
Sue Hartman (National Oceanography Centre, Southampton)
Toby Tyrrell (University of Southampton)

Air-sea gas exchange estimates biased by multi-day surface trapping

Posted by mmaheigan

· Tuesday, August 20th, 2019

Routine measurements of air-sea gas exchange assume a homogeneous gas concentration across the upper few meters of the ocean. But is this assumption valid? A recent study in Biogeosciences revealed substantial systematic gradients of nitrous oxide (N₂O) in the top few meters of the Peruvian upwelling regime. These gradients lead to a 30% overestimate of integrated N₂O emissions across the entire region, with local emissions overestimated by as much as 800%.

Figure caption: Air-sea gas exchange estimates can be biased by gas concentration gradients within the upper few meters of the ocean; in particular, surface trapping over several days’ duration can generate substantial gradients.

The N₂O gradients off Peru form during multi-day events of surface trapping, in which near-surface stratification dampens turbulent mixing. Until now, surface trapping was assumed to be a diurnal (driven by solar warming) process without memory, whereby only weak gradients would form during the hours of trapping and then dissipate. It is likely that multi-day surface trapping occurs in other ocean regions as well. The total impact on emission estimates of different greenhouse gases is yet to be quantified, but given the findings in the Peruvian upwelling system, could be significant globally.

Authors:
Tim Fischer, Annette Kock, Damian L. Arévalo-Martínez, Marcus Dengler, Peter Brandt, Hermann W. Bange (GEOMAR)

Regional circulation changes and a growing atmospheric CO2 concentration drive accelerated anthropogenic carbon uptake in the South Pacific

Posted by mmaheigan

· Tuesday, August 6th, 2019

About one tenth of human CO₂ emissions are currently being taken up by the Pacific Ocean, which makes the seawater more corrosive to the calcium carbonate shells and skeletons of the plants and animals that live there. Now, thanks to hard work by international teams of scientists from the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP), there are decades of data, enough to test how much this anthropogenic CO₂ accumulation varies throughout the Pacific Ocean and regionally on the timescales of decades.

Figure caption: Map of the concentration of human-emitted CO₂ along the sections where data were available from more than one decade, estimated for the year 2015.

Using a new take on an old technique, along with a wide variety of repeat biogeochemical measurements, a study in Biogeochemical Cycles revealed that Pacific anthropogenic CO₂ accumulation increased from the 1995-2005 decade to the 2005-2015 decade. While the magnitude of the decadal increase was consistent with increases in human CO₂ emissions over this period for most of the Pacific, the rate of change was greater than expected in the South Pacific subtropical gyre. The authors suggest that recent increases in circulation in the gyre region could have delivered an unexpectedly large amount of anthropogenic CO₂-laden seawater from the surface to the ocean interior. Programs like GO-SHIP will continue to be critical for tracking the fate of human CO₂emissions and associated feedbacks on climate and marine ecosystems.

Authors:
B. R. Carter (Univ. Washington and PMEL)
R. A. Feely, G. C. Johnson, J. L. Bullister (PMEL)
R. Wanninkhof (NOAA AOML)
S. Kouketsu, A. Murata (JAMSTEC
R. E. Sonnerup, S. Mecking (Univ. Washington)
P. C. Pardo (Univ. Tasmania)
C. L. Sabine (Univ. Hawai‘i, Mānoa)
B. M. Sloyan, B. Tilbrook (CSIRO, Australia)
K. Speer (Florida State University
L. D. Talley (Scripps Institution of Oceanography)
F. J. Millero (Univ. Miami)
S. E. Wijffels (CSIRO and WHOI)
A. M. Macdonald (WHOI)
N. Gruber (ETH Zurich)

A new era of observing the ocean carbonate system

Posted by mmaheigan

· Tuesday, August 6th, 2019

Amidst a backdrop of natural variability, the ocean carbonate system is undergoing a massive anthropogenic change. To capture this anthropogenic signal and differentiate it from natural variability, carbonate observations are needed across a range of spatial and temporal scales (Figure 1), many of which are not captured by traditional oceanographic platforms. A new review of autonomous carbonate observations published in Current Climate Change Reports highlights the development and deployment of pH sensors capable of in situ measurements on autonomous platforms, which represents a major step forward in observing the ocean carbonate system. These sensors have been rigorously field-tested via large-scale deployments on profiling floats in the Southern Ocean (Southern Ocean Carbon and Climate Observations and Modeling, SOCCOM), providing an unprecedented wealth of year-round data that have demonstrated the importance of wintertime outgassing of carbon dioxide in the Southern Ocean.

Figure 1: Observational capabilities and carbonate system processes as a function of time and space. Ocean processes that affect the carbonate system (solid color shapes—labeled in the legend) are depicted as a function of the temporal and spatial scales over which they must be observed to capture important variability and/or long-term change.

Most current autonomous platforms routinely measure only a single carbonate parameter, which then requires an algorithm to estimate a second parameter so that the rest of the carbonate system can be calculated. However, the ongoing development of sensors and systems to measure, rather than estimate, other carbonate parameters may greatly reduce uncertainty in constraining the full carbonate system. It is critical that the community continue to develop and adhere to best practices for calibration and data handling as existing sensors are deployed in increasing numbers and new sensors become available. Expanding autonomous carbonate measurements will increase our understanding of how anthropogenic change impacts natural variability and will provide a means to monitor carbon uptake by the ocean in real-time at high spatial and temporal resolution. This will not only help to understand the mechanisms driving changes in the ocean carbonate system, but will allow new insights in the role of mesoscale processes in regional and global biogeochemical cycles.

Authors:
Seth M. Bushinsky (Princeton University/University of Hawai’i Mānoa)
Yuichiro Takeshita (Monterey Bay Aquarium Research Institute)
Nancy L. Williams (Pacific Marine Environmental Laboratory – NOAA / University of South Florida)

Can microzooplankton shape the depth distribution of phytoplankton?

Posted by mmaheigan

· Tuesday, July 23rd, 2019

Photosynthetic, single-celled phytoplankton form the base of many marine and lacustrine (lake) food webs. These microscopic algae typically occur in the sunlit surface layer, but in many ecosystems, there are also sub-surface peaks in phytoplankton and chlorophyll-a, their key photosynthetic pigment. Historically, scientists have explained deep chlorophyll maximum (DCM) formation by invoking “bottom-up” processes such as nutrient and light co-limitation, while less attention has been paid to “top-down” controls such as predation.

A recent study in Nature Communications challenges this conventional wisdom by arguing that microzooplankton (top-down control) can cause the formation of DCMs by preferentially consuming phytoplankton near the surface. This can occur when microzooplankton exhibit light-dependent grazing—a known but not well-understood phenomenon in which prey consumption rates increase with increasing light intensity. By incorporating this phenomenon into mathematical models, the authors showed that this can create a “spatial refuge” for phytoplankton in deeper, darker parts of the water column, where there is enough sunlight to photosynthesize, but too little for efficient microzooplankton predation. Furthermore, when light-dependent grazing is incorporated into a global ocean biogeochemistry model (COBALT: Carbon, Ocean Biogeochemistry and Lower Trophics – planktonic ecosystem model), DCMs that are already present due to bottom-up controls deepen, improving agreement between model predictions, satellite data, and in situ observations.

Figure legend: Global comparison of annual mean deep chlorophyll maxima (DCM) depths (A) predicted by the unmodified COBALT model, (B) predicted by the COBALT model modified to include light-dependent microzooplankton grazing, and (C) estimated based on satellite data. Incorporating light-dependent grazing deepens the DCM, especially in oligotrophic gyres, and improves agreement with observational data.

These findings highlight the importance of higher trophic levels in regulating aquatic primary productivity. The model predictions suggest that not only can microzooplankton suppress primary production near the surface, but by shifting phytoplankton abundances deeper, they may increase carbon export via the biological pump. Future field tests of this hypothesis—i.e. detailed grazing measurements in stratified water columns with DCMs—can elucidate the extent to which light-dependent grazing shapes phytoplankton distribution in real biological systems.

Authors:
Holly Moeller (University of California Santa Barbara)
Charlotte Laufkötter (University of Bern and Princeton University)
Edward Sweeney (Sea Education Association and Santa Barbara Museum of Natural History)
Matthew Johnson (Woods Hole Oceanographic Institution)

Deep ocean carbon reconstruction helps decipher a million-year-old climate mystery

Posted by mmaheigan

· Tuesday, July 23rd, 2019

Approximately one million years ago, Earth’s periodic ice ages increased in strength and duration, shifting from a 41,000-year pacing to a 100,000-year pacing, both linked to Earth’s orbital variations. The causes of this climate shift known as the mid-Pleistocene transition (MPT) have been debated for decades.

A recent study in Nature Geoscience addresses how the ocean carbon cycle contributed to the MPT by quantifying the carbon inventory of the deep Atlantic Ocean during this time. Using trace element and isotope ratios of fossil marine foraminifera, the authors demonstrate that an abrupt weakening of deep ocean overturning circulation between 950,000 and 900,000 years ago occurred alongside a pronounced increase in carbon content of the deep Atlantic Ocean. This study revealed significantly higher carbon concentrations in the deep North and South Atlantic basins during the post-MPT 100,000-year ice ages relative to the 41,000-year ice ages prior to the MPT (Figure 1).

Figure 1 caption: The last two million years of glacial cycles, with present day on left and age increasing from left to right. Orange data are from 41,000-year ice ages; blue data are from,100,000-year ice ages. (A) Glacial-interglacial cycles demonstrated in benthic oxygen isotopes (green), with warmer interglacials up and peak ice ages downward. (B) Atmospheric CO₂ from ice core measurements (gray lines) and reconstructed from boron isotopes (circles) (C), Peak ice age neodymium isotope ratios indicating strength of density-driven deep ocean circulation (squares and triangles indicate two different sediment cores). (D) Peak ice age deep ocean carbon content (squares and diamonds indicate two independent reconstructions from the same South Atlantic sediment core).

These data indicate that since 950,000 years ago, the deep Atlantic Ocean has stored an extra 50 billion tons of carbon during peak ice ages. This study hypothesizes that this extra carbon was sequestered from the atmosphere via a feedback between Antarctic ice sheet extent and the efficiency of air-sea carbon exchange in the Southern Ocean. The authors propose that intensification of ice ages one million years ago was closely linked to enhanced ocean carbon storage and resultant lowering of atmospheric CO₂ levels.

While paleoclimatologists consider the MPT to be the most recent major climate transition, the magnitude of carbon perturbation at the MPT pales in comparison to today’s human emissions. Today, humans produce 50 billion tons of carbon in only five years. Studies of the carbon cycle across past climate transitions like the MPT provide key insights on how future climate may respond to today’s carbon cycle disruption.

Authors
Jesse Farmer (LDEO Columbia University; now at Princeton University and Max Planck Institute for Chemistry)
Bärbel Hönisch, Laura Haynes, Maureen Raymo, Steven Goldstein, Maayan Yehudai, Joohee Kim (LDEO Columbia University)
Heather Ford (Queen Mary University of London)
Dick Kroon, Simon Jung, Dave Bell (University of Edinburgh)
Maria Jaume-Seguí, Leopoldo Pena (University of Barcelona)

See this related popular article and video in the Washington Post.

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)

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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