Archive for AMOC

The ocean’s smallest phytoplankton may be bigger than we thought

Posted by mmaheigan 
· Tuesday, December 17th, 2019 

Flow cytometry can sort hundreds of thousands of phytoplankton cells in minutes, a tool that has been exploited for over thirty years in marine science. However, skilled analysts are still needed for manual interpretation of these cells into different types and then further into size distributions and optical properties.

In a recent study published in Applied Optics, the authors developed and implemented an automated scheme on the large Atlantic Meridional Transect flow cytometric database, which contains around 104 samples and 109 cells (the entire AMT flowcytometric dataset which spans a decade of transects (AMT18 – AMT27). This unique, well-calibrated dataset spans 100° of latitude between the UK and the Falklands, with multiple samples between 0-200m. The results clearly show that Prochlorococcus, very small marine cyanobacteria, are consistently larger than previously thought (>0.65 µm), and their size distribution reveals a distinctive double peak (0.75 µm and 1.75 µm) that varies strongly with depth. This is coupled with changes in Prochlorococcus optical properties, a term we have coined “opto-types.”  By contrast, Synechococcus are typically 1.5 µm in diameter and more homogeneously dispersed.

Figure 1: North to South transect (bottom left) of the Atlantic Ocean showing the variability in the abundance (top left), size (top right) and refractive index (bottom right) of Prochlorococcus

This work has uncovered new information regarding the size distribution of the ocean’s smallest phytoplankton, which has implications for how energy is transferred between different biological organisms.

 

Authors:
Tim Smyth (Plymouth Marine Laboratory)
Glen Tarran (Plymouth Marine Laboratory)
Shubha Sathyendranath (Plymouth Marine Laboratory)

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.

Constraints on glacial overturning circulation and export production lead to answers about the carbon cycle

Posted by mmaheigan 
· Friday, January 4th, 2019 

One of the biggest unsolved mysteries in climate science concerns the dynamics and feedbacks of the ice age carbon dioxide (CO2) cycle.

At the height of the Pleistocene ice ages, the atmospheric CO2 concentration was about 1/3 lower than during the warm interglacial periods. Most scientists think that the CO2 that was missing from the atmosphere was in the deep ocean, but how and why remains unclear. In a study published in Earth and Planetary Science Letters, we compared different computer simulations of the ice age ocean with δ13C, radiocarbon (14C), and δ15N data from sea floor sediments.

We find that a weak and shallow Atlantic Meridional Overturning Circulation (6-9 Sv, or approximately half of today’s overturning rate) best reproduces the glacial sediment isotope data. Increasing the atmospheric soluble iron flux in the model’s Southern Ocean intensifies export production, carbon storage, and further improves agreement with glacial δ13C and δ15N reconstructions.

Figure Caption: Depth profiles of global mean δ13C, calculated using only grid boxes for which there exists Last Glacial Maximum data. Blue: Weak Atlantic circulation; Red: Strong Atlantic circulation; Green: Collapsed Atlantic circulation; Dashed: Extra iron in the Southern Ocean; Orange: Last Glacial Maximum Data.

Our best-fitting simulation (blue, dashed line in the figure) is a significant improvement over previous studies and suggests that both circulation and export production changes were necessary to maximize carbon storage in the glacial ocean. These findings provide an equilibrium glacial state, consistent with a combination of proxies, that can be used as a basis for simulations covering the last deglaciation time period. Understanding the different states that the global climate system can transit, and the characteristics of the transitions, is crucial to project possible outcomes of current climate change processes.

 

Authors:
Juan Muglia (Oregon State University)
Luke C. Skinner (Godwin Laboratory for Palaeoclimate Research, University of Cambridge)
Andreas Schmittner (Oregon State University)

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