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Archive for ocean carbon uptake and storage

Unexpected global diatom decline in response to ocean acidification

Posted by mmaheigan

· Tuesday, December 13th, 2022

Biological impacts of ocean acidification have been the subject of intense research for more than a decade. While it is known that more acidic seawater will create difficulties for calcifying organisms (e.g. corals or coccolithophores), diatoms have so far been considered to be resilient against, or even benefit from, ocean acidification. But an overlooked biogeochemical feedback mechanism has revealed that diatoms are also under threat from ocean acidification.

Figure 1: Slower solubility of diatom shells in acidified oceans leads to global diatom decline. Diatoms build silica shells and produce organic carbon at the ocean surface. Today, much of the silica dissolves relatively quickly as the particles consisting of dead diatoms sink (e.g. after blooms). The resulting dissolved silicon is returned to the surface by upwelling waters, where it supports the growth of more diatoms. Under ocean acidification, the silica in sinking particles will dissolve slower, thereby reducing the return flux of dissolved silicon to the ocean surface as much of the marine silicon budget will become trapped in deep water. The result is a substantial global decrease in diatom biomass. (Figure source: Nature, Vol. 605, No. 7911, 26 May 2022, DOI: 10.1038/d41586-022-01365-z and 10.1038/s41586-022-04687-0)

Diatoms are the most important primary producers in the ocean and play an important role in transferring carbon dioxide (CO₂) from the atmosphere into the deep ocean. Their most conspicuous feature is a silica shell formed around their cells. A comprehensive study published in Nature dove deep into the impacts of ocean acidification on diatoms and biogeochemical cycling. Their analyses of data from experiments, field observations, and model simulations suggest that ocean acidification could drastically reduce diatom populations. As a result of lower seawater pH, the silica shells of diatoms dissolve more slowly. However, this is not an advantage—it causes diatom shells to sink into deeper water layers before chemically dissolving and being converted back into the inorganic nutrient silicic acid. This means this nutrient is more efficiently exported to the deep ocean and so becomes scarcer in the light-flooded surface layer where diatoms require it to form new shells. Ultimately, this loss of silica from the surface ocean causes a global decline in diatoms, reaching -10% by the year 2100 and -26% by 2200. Since diatoms are one of the most important plankton groups in the ocean, their decline could lead to a significant shift in the marine food web or even a change in the ocean carbon sink.

This finding is in sharp contrast to the previous consensus of ocean research, which sees calcifying organisms as losers, but diatoms being little affected, or even a winner of ocean acidification. This also highlights the uncertainties in predicting ecological impacts of climate change and how small-scale effects can lead to ocean-wide changes with unforeseen and far-reaching consequences for marine ecosystems and matter cycles.

Authors:
Jan Taucher (GEOMAR, Kiel, Germany)
Lennart T. Bach (University of Tasmania, Hobart, Australia)
Friederike Prowe (GEOMAR, Kiel, Germany)
Tim Boxhammer (GEOMAR, Kiel, Germany)
Karin Kvale (GNS Science, Lower Hutt, New Zealand)
Ulf Riebesell (GEOMAR, Kiel, Germany)

Linking the calcium carbonate and alkalinity cycles in the North Pacific ocean

Posted by mmaheigan

· Tuesday, December 13th, 2022

The marine carbon and alkalinity cycles are tightly coupled. Seawater stores so much carbon because of its high alkalinity, or buffering capacity, and the main driver of alkalinity cycling is the formation and dissolution of biologically produced calcium carbonate (CaCO3). In a recent publication in GBC, the authors conducted novel carbon-13 tracer experiments to measure the dissolution rates of biologically produced CaCO3 along a transect in the North Pacific Ocean. They combined these experiment data with shipboard analyses of the dissolved carbonate system, the 13C-content of dissolved inorganic carbon, and CaCO3 fluxes, to constrain the alkalinity cycle in the upper 1000 meters of the water column. Dissolution rates were too slow to explain alkalinity production or CaCO3 loss from the particulate phase. However, driving dissolution with the metabolic consumption of oxygen brings alkalinity production and CaCO3 loss estimates into quantitative agreement (Figure). The authors argue that a majority of CaCO3 production is likely dissolved through metabolic processes in the upper ocean, including zooplankton grazing, digestion, and egestion, and microbial degradation of marine particle aggregates that contain both organic carbon and CaCO3. This hypothesis stems from the basic fact that almost all marine CaCO3 is biologically produced, placing CaCO3 at the source of the acidifying process (metabolic consumption of organic matter). This process is important because it puts an emphasis on biological processing for the cycling of not only carbon, but also alkalinity, the main buffering component in seawater. These results should help both scientists and stakeholders to understand the fundamental controls on calcium carbonate cycling in the ocean, and therefore the processes that distribute alkalinity throughout the world’s oceans.

Figure Caption: Sinking-dissolution model results compared with tracer-based alkalinity regeneration rates (TA*-CFC, Feely et al., 2002). We also plot alkalinity regeneration rates using updated time transit distribution ages (TA*- and Alk*-TTD). The modeled alkalinity regeneration rate uses our measured dissolution rates for biologically produced calcite and aragonite, and is driven by a combination of background saturation state and metabolic oxygen consumption. The dissolution rate is split up into a calcite component (produced mainly by coccolithophores) and an aragonite component (produced mainly by pteropods). Aragonite does not contribute significantly to the overall dissolution rate. Driving dissolution by metabolic oxygen consumption produces alkalinity regeneration rates that are in quantitative agreement with tracer-based estimates.

Authors:
Adam Subhas (Woods Hole Oceanographic Institution) et al.

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How does the competition between phytoplankton and bacteria for iron alter ocean biogeochemical cycles?

Posted by mmaheigan

· Friday, August 26th, 2022

Free-living bacteria play a key role in cycling essential biogeochemical resources in the ocean, including iron, via their uptake, transformation, and release of organic matter throughout the water column. Bacteria process half of the ocean’s primary production, remineralize dissolved organic matter, and re-direct otherwise lost organic matter to higher trophic levels. For these reasons, it is crucial to understand what factors limit the growth of bacteria and how bacteria activities impact global ocean biogeochemical cycles.

In a recent study, Pham and colleagues used a global ocean ecosystem model to dive into how iron limits the growth of free-living marine bacteria, how bacteria modulate ocean iron cycling, and the consequences to marine ecosystems of the competition between bacteria and phytoplankton for iron.

Figure 1: (a) Iron limitation status of bacteria in December, January, and February (DJF) in the surface ocean. Low values (in blue color = close to zero) mean that iron is the limiting factor for the growth of bacteria; (b) Bacterial iron consumption in the upper 120m of the ocean and (c) Changes (anomalies) in export carbon production when bacteria have a high requirement for iron.

Through a series of computer simulations performed in the global ocean ecosystem model, the authors found that iron is a limiting factor for bacterial growth in iron-limited regions in the Southern Ocean, the tropical, and the subarctic Pacific due to the high iron requirement and iron uptake capability of bacteria. Bacteria act as an iron sink in the upper ocean due to their significant iron consumption, a rate comparable to phytoplankton. The competition between bacteria and phytoplankton for iron alters phytoplankton bloom dynamics, ocean carbon export, and the availability of dissolved organic carbon needed for bacterial growth. These results suggest that earth system models that omit bacteria ignore an important organism modulating biogeochemical responses of the ocean to future changes.

Authors:
Anh Le-Duy Pham (Laboratoire d’Océanographie et de Climatologie: Expérimentation et Approches Numériques (LOCEAN), IPSL, CNRS/UPMC/IRD/MNHN, Paris, France)
Olivier Aumont (Laboratoire d’Océanographie et de Climatologie: Expérimentation et Approches Numériques (LOCEAN), IPSL, CNRS/UPMC/IRD/MNHN, Paris, France)
Lavenia Ratnarajah (University of Liverpool, United Kingdom)
Alessandro Tagliabue (University of Liverpool, United Kingdom)

Carbon fluxes in the coastal ocean: Synthesis, boundary processes and future trends

Posted by mmaheigan

· Friday, August 26th, 2022

A vital part of mitigating climate change is the coastal and open ocean carbon sink, without this, it is not possible to meet the target set by the Paris Agreement. More research is needed to better understand the ocean carbon cycle and its future role in the uptake of anthropogenic carbon. A review provides an analysis of the current qualitative and quantitative understanding of the coastal ocean carbon cycle at regional to global scales, with a focus on the air-sea CO₂ exchange. It includes novel findings obtained using the full breadth of methodological approaches, from observation-based studies and advanced statistical methods to conceptual and theoretical frameworks, and numerical modeling.

Figure 1: Updated sea-air CO₂ flux density (mol C m⁻² year⁻¹) in the global coastal oceans that reveals that the global coastal ocean is an integrated CO₂ sink with the strongest CO₂ uptake at high latitudes. The challenges associated with identifying current and projected responses of the coastal ocean and it source/sink role in the global carbon budget require observational networks that are coordinated and integrated with modeling programs; development of this capability is a priority for the ocean carbon research and management communities.

Based on a new quantitative synthesis of air-sea CO₂ exchange, this study yields an estimate for the globally integrated coastal ocean CO₂ flux of −0.25 ± 0.05 Pg C year⁻¹, with polar and subpolar regions accounting for most of the CO₂ removal (>90%). A framework that classifies river-dominated ocean margin (RiOMar) and ocean-dominated margin (OceMar) systems is used in to conceptualize coastal carbon cycle processes. Ocean carbon models are reviewed in terms of the ability to simulate key processes and project future changes in different continental shelf regions. Concurrent trends and changes in the land-ocean-atmosphere coupled system introduce large uncertainties into projections of ocean carbon fluxes, in particular into defining the role of the coastal carbon sink and its evolution, both of which are of fundamental importance to climate science and climate policies developed before and after achievement of net-zero CO₂ emissions. The major gaps and challenges identified for current coastal ocean carbon research have important implications for climate and sustainability policies. This study is a contribution to the Regional Carbon Cycle Assessment and Processes Phase 2 supported by the Global Carbon Project.

Authors:
M. H. Dai, J. Z. Su, Y. Y. Z., E. E. Hofmann, Z. M. Cao, W.-J. Cai, J. P. Gan, F. Lacroix, G. G. Laruelle, F. F. Meng, J. D. Müller, P. A.G. Regnier, G. Z. Wang, and Z. X. Wang

The most important 234Th disequilibrium compilation you ever saw

Posted by mmaheigan

· Thursday, August 25th, 2022

Thorium-234 (²³⁴Th), a naturally radioactive element present in nature, is one of the most actively used tracers in oceanography. ²³⁴Th is widely used to study the removal rate of material on sinking particles from the upper ocean, known as “scavenging,” and for determining the downward flux of carbon. Starting in 1969, ocean measurements of the ²³⁴Th temporal distribution in the hydrologic cycle comprise an indispensable component of oceanographic expeditions. However, even after five decades and extensive use of ²³⁴Th to understand natural aquatic processes, there are major gaps in this tool, no unified compilation of ²³⁴Th measurements and no centralized source for ²³⁴Th data.

A new study aims to fill these gaps with a comprehensive global oceanic compilation of ²³⁴Th measurements in a single open-access, long-term, and dynamic repository. They collated over 50 years of results from researchers and laboratories, 379 oceanographic expeditions, and more than 56 600 ²³⁴Th data points from over 5000 locations spanning every ocean. These data are archived on PANGAEA® (Ceballos-Romero et al., 2021, see references below).

This paper introduces the dataset in context via informative and descriptive graphics and a broad overview of the data sets, with potential uses for future studies. A historical review of 50 years of the ²³⁴Th technique is included also, covering four well-distinguished eras that are marked by four seminal publications that changed the course of the ²³⁴Th technique and impact on oceanography.

Map showing the distribution of sampling stations cataloged as i) unpublished (yellow diamonds), ii) published exclusively in repositories (blue square), and iii) published in referred journals (magenta circles).

This compilation is especially relevant to present and future investigations of the biological carbon pump (BP), which transports carbon to the deep ocean and regulates atmospheric CO₂ levels. In the last few decades, scientists have made considerable progress on unraveling the behavior of the BP. However, many questions on how the mechanisms function and shape carbon dynamics and the ocean carbon cycle remain unknown. The authors emphasize that many analyses of BP processes could benefit from utilizing ²³⁴Th data. The authors list a number of applications that could derive from this impressive data set, such as establishing the distribution of the probability of ²³⁴Th reaching equilibrium (or not) with its parent at 100 m. This distribution allows extracting i) the number of data points in the compilation that could be used to evaluate processes in the upper ocean (e.g., export flux and export efficiency) or ii) scavenging rates of trace metals or particle sinking velocities using “deficit” ratios, as well as those that could be used to study processes such as particle remineralizations by using the “excess” ratios. This compilation provides a valuable resource to better understand and quantify how the contemporary oceanic carbon uptake functions and how it may change in the future. This tool can be served as a focal point for the ²³⁴Th community under the principles of openness and reproducibility.

Authors

Elena Ceballos-Romero (University of Sevilla and WHOI)
Ken O. Buesseler (WHOI)
María Villa-Alfageme (University of Sevilla)

References
Ceballos-Romero, E., Buesseler, K. O. and Villa-Alfageme, M. (2022) ‘Revisiting five decades of 234Th data: a comprehensive global oceanic compilation’, Earth System Science Data, 14(6), pp. 2639–2679. doi: 10.5194/essd-14-2639-2022.

Ceballos-Romero, E., Buesseler, K. O., Muñoz-Nevado, C., and Villa-Alfageme, M. (2021) ‘More than 50 years of Th-234 data: a comprehensive global oceanic compilation‘, PANGAEA. doi: 10.1594/PANGAEA.918125.

How do coccolithophores survive the darkness?

Posted by mmaheigan

· Friday, April 1st, 2022

Coccolithophores have survived several major extinction events over geologic time. The most significant was the asteroid impact at the K/T boundary, followed by months of darkness. Additionally, coccolithophores regularly reside in the twilight zone, just beyond the reach of sunlight. A paper recently published in the New Phytologist addresses how these photosynthetic algae can persist and grow, albeit slowly, in darkness using osmotrophy.

The authors discovered that the osmotrophic uptake of certain types of dissolved organic carbon (DOC) can support survival in low light. They completed a 30-day darkness experiment to determine how the concentration of several DOC compounds affects growth. The coccolithophore species Cruciplacolithus neohelis growth rate increased with the increasing concentration of dissolved organic compounds. They also examined the kinetics of short-term uptake of radiolabeled DOC compounds and found that the uptake rate generally showed Michaelis-Menten-like saturation kinetics. All radiolabeled DOC compounds were incorporated into the POC fraction, but surprisingly also into the particulate inorganic carbon (PIC) fraction (i.e., calcite coccoliths).

These results suggest that osmotrophic uptake in coccolithophores may be significant enough to be included in carbon cycle models, especially if they can simultaneously take up a wide range of organic compounds. Surprisingly, we detected ¹⁴C-DOC in the PIC fraction after only 24 hours. This remarkably rapid incorporation is most likely due to the respiration of radiolabeled DOC into dissolved inorganic carbon (DIC), subsequently used by coccolithophores for calcification. These results have implications for the biological carbon pump and alkalinity pump paradigms, as we confirmed that both POC and PIC originate from DOC on short time scales.

Predators Set Range for the Ocean’s Most Abundant Phytoplankton

Posted by mmaheigan

· Friday, April 1st, 2022

Prochlorococcus is the world’s smallest phytoplankton (microscopic plant-like organisms) and the most numerous, with more than ten septillion individuals. This tiny plankton lives ubiquitously in warm, blue, tropical waters but is conspicuously absent in more polar regions. The prevailing theory was the cold: Prochlorococcus doesn’t grow at low temperatures. In a recent paper, the authors argue ecological control, in particular, predation by zooplankton. Cold polar waters are greener because they contain more nutrients, leading to more life and more organic matter production. This production feeds more and larger heterotrophic bacteria, who then feed larger predators—specifically the same zooplankton that consume Prochlorococcus. If the shared zooplankton increases enough, it will consume Prochlorococus faster than it can grow, causing the species to collapse at higher latitudes. These results show that an understanding of both ecology and temperature is required to predict how these ecosystems will shift in a warming ocean.

Figure 1: Surface populations of Prochlorococcus collapse (dashed lines) moving northward from Hawaii as seen in transects (transect line shown in red on map, lower left) from cruises in April 2016 (black dots) and September 2017 (green triangles). This collapse of the Prochlorococcus emerges in dynamical computer models (lower right, color indicates Prochlorococcus biomass in mgC/m3) when heterotrophic bacteria and Prochlorococcus share a grazer (top schematic). Increased organic production heading poleward first increases the heterotrophic bacterial population, increasing the shared zooplankton population which eventually consumes Prochlorococcus faster than it can grow (dashed contour).

Authors
Christopher L. Follett (MIT)
Stephanie Dutkiewicz (MIT)
François Ribalet (UW)
Emily Zakem (USC)
David Caron (USC)
E. Virginia Armbrust (UW)
Michael J. Follows (MIT)

New Data Standard for Oceanographic Research

Posted by mmaheigan

· Friday, February 18th, 2022

Effective data management is paramount in oceanographic research. The ocean is a global system, and research to understand regional and global oceanographic processes often involves compiling cruise-based data from different laboratories and expeditions.

The new international data standard covers column header abbreviations, quality control flags, missing value indicators, and standardized calculation of numerous parameters. Released alongside this paper are newly developed tools to calculate some oceanographic properties, and recommendations for dissociation constants of the seawater carbon system calculations. In addition, the use of “content” instead of “concentration” is recommended for mass-based properties.

The column header abbreviation standards presented here are based on the 30-year-old Exchange format of the World Ocean Circulation Experiment (WOCE) Hydrographic Program (Joyce and Corry, 1994; Swift and Diggs, 2008) with updates and refinements by the Climate and Ocean-Variability, Predictability, and Change (CLIVAR) and the Carbon Hydrographic Data Office (CCHDO) of the Scripps Institution of Oceanography. This format has been used as a data file standard for discrete chemical oceanographic observations for several decades.

The new international data standards will facilitate data sharing, quality control, and synthesis efforts to promote climate change and ocean acidification research at regional to global scales. This product is a significant step forward in terms of (a) creating common data standards for the international oceanographic research community to streamline data management, quality control, and data product developments; and (b) bringing the subject matter expertise from the research community to the data management world.

Authors (partial, see full list on publication)
Li-Qing Jiang (Univ Maryland, NOAA/NCEI)
Denis Pierrot (NOAA/AOML)
Rik Wanninkhof (NOAA/AOML)
Richard A. Feely (NOAA/PMEL)
Bronte Tilbrook (CSIRO Oceans and Atmosphere and Australian Antarctic Program Partnership)
Simone Alin (NOAA/AOML)
Leticia Barbero (Univ Miami; NOAA/AOML),
Robert H. Byrne (Univ South Florida),
Brendan R. Carter (Univ Washington, NOAA/PMEL)
Andrew G. Dickson (Scripps Institution of Oceanography)
Jean-Pierre Gattuso (CNRS, Laboratoire d’Océanographie de Villefranche, Sorbonne Univ; Institute for Sustainable Development and International Relations, Sciences Po, France)
Dana Greeley (NOAA/PMEL)
Mario Hoppema (Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Sciences Po,)
Matthew P. Humphreys (NIOZ Royal Netherlands Institute for Sea Research, Netherlands)
Johannes Karstensen (GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany)
et al.

Ocean Acidification drives shifts in global stoichiometry and carbon export efficiency

Posted by mmaheigan

· Friday, November 19th, 2021

Marine food webs and biogeochemical cycles react sensitively to increases in carbon dioxide (CO₂) and associated ocean acidification, but the effects are far more complex than previously thought. A comprehensive study published in Nature Climate Change by a team of researchers from GEOMAR dove deep into the impacts of ocean acidification on marine biota and biogeochemical cycling. The authors combined data from five large-scale field experiments with natural plankton communities to investigate how carbon cycling and export respond to ocean acidification.

The biological pump is a key mechanism in transferring carbon to the deep ocean and contributes significantly to the oceans’ function as a carbon sink. The carbon-to-nitrogen ratio of sinking biogenic particles, here termed (C:N_export), determines the amount of carbon that is transported from the euphotic zone to the ocean interior per unit nutrient, thereby controlling the efficiency of the biological pump. The authors demonstrate for the first time that ocean acidification can change the elemental composition of organic matter export, thereby potentially altering the biological pump and carbon sequestration in a future ocean (Figure 1).

Figure 1: Until now, the common assumption is that changes in C:N (and biogeochemistry, in general) are mainly driven by phytoplankton. In a series of in situ mesocosm experiments in different biomes (left), Taucher et al., (2020) found distinct impacts of ocean acidification on the C:N ratio of sinking organic matter (middle). By linking these observations to analysis of plankton community composition, the authors found a key role of heterotrophic processes in controlling the response of C:N to OA, particularly by altering the quality and carbon content of sinking organic matter within the biological pump (right).

Surprisingly, the observed responses were highly variable: C:N_export increased or decreased significantly with increasing CO₂, depending on the composition of species and the structure of the food web. The authors found that heterotrophic processes driven by bacteria and zooplankton play a key role in controlling the response of C:N_export to ocean acidification. This contradicts the widespread paradigm that primary producers are the principal driver of biogeochemical responses to ocean change.

Considering that such diverse pathways, by which planktonic food webs shape the elemental composition and biogeochemical cycling of organic matter, are not represented in state-of-the-art earth system models, these findings also raise the question: Are currently able to predict the large-scale consequences of ocean acidification with any certainty?

Authors:
Jan Taucher (GEOMAR, Kiel, Germany)
Tim Boxhammer (GEOMAR, Kiel, Germany)
Lennart T. Bach (University of Tasmania, Hobart, Australia)
Allanah J. Paul (GEOMAR, Kiel, Germany)
Markus Schartau (GEOMAR, Kiel, Germany)
Paul Stange (GEOMAR, Kiel, Germany)
Ulf Riebesell (GEOMAR, Kiel, Germany)

The ephemeral and elusive COVID blip in ocean carbon

Posted by mmaheigan

· Monday, September 20th, 2021

The global pandemic of the last nearly two years has affected all of us on a daily and long-term basis. Our planet is not exempt from these impacts. Can we see a signal of COVID-related CO₂ emissions reductions in the ocean? In a recent study, Lovenduski et al. apply detection and attribution analysis to output from an ensemble of COVID-like simulations of an Earth system model to answer this question. While it is nearly impossible to detect a COVID-related change in ocean pH, the model produces a unique fingerprint in air-sea DpCO₂ that is attributable to COVID. Challengingly, the large interannual variability in the climate system makes this fingerprint difficult to detect at open ocean buoy sites.

This study highlights the challenges associated with detecting statistically meaningful changes in ocean carbon and acidity following CO₂ emissions reductions, and reminds the reader that it may be difficult to observe intentional emissions reductions — such as those that we may enact to meet the Paris Climate Agreement – in the ocean carbon system.

Figure caption: The fingerprint (pink line) of COVID-related CO2 emissions reductions in global-mean surface ocean pH and air-sea DpCO2, as estimated by an ensemble of COVID-like simulations in an Earth system model. While the pH fingerprint is not particularly exciting, the air-sea DpCO2 fingerprint displays a temporary weakening of the ocean carbon sink in 2021 due to COVID emissions reductions.

Authors:
Nikki Lovenduski (University of Colorado Boulder)
Neil Swart (Canadian Centre for Climate Modeling and Analysis)
Adrienne Sutton (NOAA Pacific Marine Environmental Laboratory)
John Fyfe (Canadian Centre for Climate Modeling and Analysis)
Galen McKinley (Columbia University and Lamont Doherty Earth Observatory)
Chris Sabine (University of Hawai’i at Manoa)
Nancy Williams (University of South Florida)

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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