New OCB Research :: Ocean Carbon & Biogeochemistry

Studying marine ecosystems and biogeochemical cycles in the face of environmental change

Archive for New OCB Research – Page 6

Adaptive emission pathways to stabilize global temperatures

Posted by mmaheigan

· Thursday, May 11th, 2023

Around the world, countries have agreed in the Paris Agreement to limit global warming well below 2°C and to pursue efforts to reduce global warming to 1.5°C. However, large uncertainties remain about which emission pathways will allow us to reach this goal. A recent paper presents a new adaptive approach to create emission pathways and estimate the necessary emission reductions every five years, following the stocktake process of the Paris Agreement. This Adaptive Emissions Reduction Approach (AERA) is solely based on past warming rates, and emissions of CO₂ and non-CO₂ radiative agents, and explicitly does not rely on projections by Earth System Models. Updating the emission pathways every five years, circumvents uncertainties in the climate system and its transient response to cumulative emissions (TCRE). Testing with the Bern3D-LPX Earth System Model of Intermediate Complexity shows that the approach works robustly across a wide range of TCREs, avoids large overshoots, and only small changes to the emission pathways are necessary every five years. This approach will allow policymakers to estimate emission pathways and create a base for international negotiations. Furthermore, it allows simulations with Earth System Models that all converge to the same temperature target to compare the climate at stabilized warming levels.

Figure caption: The three steps of the Adaptive Emission Reduction Approach: 1) Estimating the past anthropogenic warming, 2) estimating the remaining emission budget, and 3) redistributing it over the future years.

Authors
Jens Terhaar (University of Bern, now Woods Hole Oceanographic Institution)
Thomas L Frölicher (University of Bern)
Mathias T Aschwanden (University of Bern)
Pierre Friedlingstein (University of Exeter, Ecole Normale Superieure)
Fortunat Joos (University of Bern)

Twitter @JensTerhaar @froeltho @PFriedling @unibern @snsf_ch @4C_H2020 @ExeterUniMaths @Geosciences_ENS @IPSL_outreach

Hydrostatic pressure substantially reduces deep-sea microbial activity

Posted by mmaheigan

· Thursday, May 11th, 2023

Deep sea microbial communities are experiencing increasing hydrostatic pressure with depth. It is known that some deep sea microbes require high hydrostatic pressure for growth, but most measurements of deep-sea microbial activity have been performed under atmospheric pressure conditions.

In a recent paper published in Nature Geoscience, the authors used a new device coined ‘In Situ Microbial Incubator’ (ISMI) to determine prokaryotic heterotrophic activity under in situ conditions. They compared microbial activity in situ with activity under atmospheric pressure at 27 stations from 175 to 4000 m depths in the Atlantic, Pacific, and the Southern Ocean. The bulk of heterotrophic activity under in situ pressure is always lower than under atmospheric pressure conditions and is increasingly inhibited with increasing hydrostatic pressure. Single-cell analysis revealed that deep sea prokaryotic communities consist of a small fraction of pressure-loving (piezophilic) microbes while the vast majority is pressure-insensitive (piezotolerant). Surprisingly, the piezosensitive fraction (~10% of the total community) responds with a more than 100-fold increase of activity upon depressurization. In the microbe proteomes, the authors uncovered taxonomically characteristic survival strategies in meso- and bathypelagic waters. These findings indicate that the overall heterotrophic microbial activity in the deep sea is substantially lower than previously assumed, which implies major impacts on the carbon budget of the ocean’s interior.

Figure caption: Deep sea microbial activity under varying pressure. (a) In situ bulk leucine incorporation rates normalized to rates obtained at atmospheric pressure conditions. (b) A microscopic view of a 2000 m sample collected in the Atlantic and incubated under atmospheric pressure conditions. The black halos around the cells are silver grains corresponding to their activities. The highly active cells (indicated by arrows) were rarely found in in situ pressure incubations. (c) Depth-related changes in the metaproteome of three abundant deep sea bacterial taxa (Alteromonas, Bacteroidetes, and SAR202). The number indicates shared and unique up- and down-regulated proteins in different depth zones.

Authors
Chie Amano (University of Vienna, Austria)
Zihao Zhao (University of Vienna, Austria)
Eva Sintes (University of Vienna, IEO-CSIC, Spain)
Thomas Reinthaler (University of Vienna, Austria)
Julia Stefanschitz (University of Vienna, Austria)
Murat Kisadur (University of Vienna, Austria)
Motoo Utsumi (University of Tsukuba, Japan)
Gerhard J. Herndl (University of Vienna, Netherlands Institute for Sea Research)

Twitter @microbialoceanW

Does dark carbon fixation supply labile DOC to the deep ocean?

Posted by mmaheigan

· Thursday, March 30th, 2023

Nitrifying microbes are the most abundant chemoautotrophs in the dark ocean. Though better known for their role in the nitrogen cycle, they also fix dissolved inorganic carbon (DIC) into biomass and thus play an important role in the global carbon cycle. The release of organic compounds by these microbes may represent an as-yet unaccounted for source of dissolved organic carbon (DOC) available to heterotrophic marine food webs. Quantifying how much DIC these microbes fix and release again into the ambient seawater is critical to a complete understanding of the carbon cycle in the deep ocean.

To address this knowledge gap, a recent study grew ten diverse nitrifier cultures and measured their cellular carbon (C) content, DIC fixation yields and DOC release rates. The results indicate that nitrifiers release between 5 and 15% of their recently fixed DIC as DOC (Figure 1). This would equate to global ocean fluxes of 0.006–0.02 Pg C yr⁻.

Figure 1. DOC release by ten different chemoautotrophic nitrifying (ammonia- and nitrite-oxidizing) microbes. The diversity of marine nitrifiers used in this study comprises all genera currently available as axenic cultures. Species and strain names are given for completeness.

Our results provide values for biogeochemical models of the global carbon cycle, and help to further constrain the relationship between C and N fluxes in the nitrification process. Elucidating the lability and fate of carbon released by nitrifiers will be the crucial next step to understand its implications for marine food-web functioning and the biological sequestration of carbon in the ocean.

Authors:
Barbara Bayer (University of California, Santa Barbara and University of Vienna)
Kelsey McBeain (University of California, Santa Barbara)
Craig A. Carlson (University of California, Santa Barbara)
Alyson E. Santoro (University of California, Santa Barbara)

Enhanced-warming Kuroshio Current experiences rapid seawater acidification and CO2 increase

Posted by mmaheigan

· Thursday, March 30th, 2023

In order to project the future states of the climate and the marine ecosystem it is vital to understand the long-term changes in ocean carbon chemistry driven by anthropogenic influence. A paucity of data make the rates of seawater acidification and partial pressure of CO₂ (pCO₂) rise on ocean margins highly uncertain.

Figure 1. Graphic summary of 9 years of data from the Kuroshio Current time-series: (a) under the influences of only atmospheric CO2 increase, (b) the combined effect of atmospheric CO2 increase, SST increase, and additional DIC supply, (c) annually averaged air-sea CO2 flux decrease, (d) Projected seawater pCO2 increase under SST rise and sustained DIC increase.

A recent study in Marine Pollution Bulletin documented the rapid increase of seawater pCO₂ (3.70±0.57 matm year^-1) and acidification (pH at -0.0033±0.0009 unit year^-1) along Kuroshio in the East China Sea (Figure 1). These findings were based on nine years of time-series data ( 2010-2018) which are now available on the website of Japan Meteorological Agency (JMA). These trends are significantly greater than the expected rates from CO₂air-sea equilibrium and those reported from other oceanic time-series studies. Interestingly, they showed the contribution of each parameter such as sea surface temperature (SST), sea surface salinity (SSS), and normalized dissolved inorganic carbon (nDIC) and total alkalinity (nTA) to the pCO₂ variability. Seawater warming caused rapid rates of pCO₂increase and acidification under sustained DIC increase. The faster pCO₂growth relative to the atmosphere resulted in the CO₂uptake through the air-sea exchange declining by ~50% (~-0.8 to -0.4 mol C m^-2 y^-1) over the study period.

If this trend continues and the atmospheric CO₂increases at its current rate, the rapid warming Kuroshio regions could change from a sink to a source of CO₂ , and cause a loss of oceanic CO₂ uptake in the near future (ca. 2030-2040). Further, other “warming hotspots” in the global ocean along western boundary currents with a continuous DIC supply may exhibit similarly accelerated acidification and pCO₂ rise. This could lead to a significant reduction in ocean CO₂ uptake.

Authors:
Shou-En Tsao (Institute of Oceanography, National Taiwan University, Taiwan)
Po-Yen Shen (Institute of Oceanography, National Taiwan University, Taiwan)
Chun-Mao Tseng^* (Institute of Oceanography, National Taiwan University, Taiwan)

Severe warming = 15% increase in bacterial respiration: Southern Ocean most impacted

Posted by mmaheigan

· Thursday, March 30th, 2023

The utilization, respiration, and remineralization of organic matter exported from the ocean surface to its depths are key processes in the ocean carbon cycle. Marine heterotrophic Bacteria play a critical role in these activities. However, most three-dimensional (3-D) coupled physical-biogeochemical models do not explicitly include Bacteria as a state variable. Instead, they rely on parameterization to account for the bacteria’s impact on particle flux attenuation.

A recent study examined how bacteria respond to climate change by employing a 3-D coupled ocean biogeochemical model that incorporates explicit bacterial dynamics. The model (CMCC-ESM2) is a part of the Coupled Model Intercomparison Project Phase 6. The authors first evaluated the reliability of century-scale forecasts (2015-2099) for bacterial stocks and rates in the upper 100 m layer against the compiled measurements from the contemporary period (1988-2011). Next the authors analyzed the predicted trends in bacterial stocks and rates under diverse climate scenarios and explored their association with regional differences in temperature and organic carbon stocks. Three crucial findings were revealed. There is a global-scale decrease in bacterial biomass of 5-10%, with a 3-5% increase in the Southern Ocean (Figure 1). In the Southern Ocean, the rise in semi-labile dissolved organic carbon (DOC) leads to an increase in DOC uptake rates of free-living bacteria; in the northern high and low latitudes, the increase in temperature drives the increase in their DOC uptake rates. Importantly, extreme warming could result in a global increase (up to 15%) and even higher in the Southern Ocean (21% increase) in bacterial respiration (Figure 1), potentially leading to a decline in the biological carbon pump.

This analysis is an unprecedented and early effort to understand the climate-induced changes in bacterial dynamics on a global scale in a systematic manner. This study takes us one step closer to comprehending how bacteria influence the functioning of the biological carbon pump and the distribution of organic carbon pools between surface and deep layers, especially their response to climate change.

Figure 1. Global projections of bacterial carbon stocks and rates during the baseline period (1990-2013) and their changes as anomalies under the most-severe climate change scenario (i.e., SSP5-8.5) relative to the baseline period (2076-2099). The stocks and rates during the baseline period (a, b, c, g, h, i) and their changes as anomalies under the most-severe climate change scenario (d, e, f, j, k, l). All variables are depth-integrated in the upper 100 m. Solid-line contours as standard deviation from averaging over 1990-2013. Anomalies are 2076-2099 average values relative to 1990-2013 average values. Global bacterial biomass has decreased by 5-10%, with a 3-5% increase in the Southern Ocean. However, extreme warming may increase bacterial respiration worldwide, thereby reducing the efficiency of the biological carbon pump. This study provides an early attempt to understand the response of bacteria to climate change and their impact on the distribution of organic carbon in the ocean.

Author
Heather Kim, Woods Hole Oceanographic Institution

Small particles contribute significantly to the biological carbon pump in the subpolar North Atlantic

Posted by mmaheigan

· Monday, February 13th, 2023

The ocean’s biological carbon pump (BCP) is a collection of processes that transport organic carbon from the surface to the deep ocean where the carbon is sequestered for decades to millennia. Variations in the strength of the BCP can substantially change atmospheric CO₂ levels and affect the global climate. It is important to accurately estimate this carbon flux, but direct measurement is difficult so this remains a challenge.

Figure 1. (a) A schematic illustrating the downward transport of small and large POC into the deep ocean and the subsequent remineralization and fragmentation which breaks large POC into small POC. (b) Trajectories of BGC-Argo float segments. (c) Relative contributions to the annually averaged vertical carbon flux show the dominant role of gravitational sinking flux of large POC as well as the significant contributions from small POC at 100 m due to different mechanisms and at 600 m due to fragmentation.

A recent paper published in Limnology and Oceanography performed a novel mass budget analysis using observations of dissolved oxygen and particulate organic carbon (POC) from BGC-Argo floats in the subpolar North Atlantic. The authors assessed relative importance of different mechanisms contributing to the BCP and related processes, the sinking velocity and remineralization rate of different particle size classes as well as the rate of fragmentation which breaks large particles into smaller ones. Results suggest that on annual timescales, the gravitational settling of large POC is the dominant mechanism. Small POC supplements the vertical carbon flux at 100 m significantly, through various mechanisms, and contributes to carbon sequestration below 600 m due to fragmentation of large POC. In addition, sensitivity experiments highlight the importance of considering remineralization and fragmentation when estimating the vertical carbon flux of small POC.

This novel method provides additional independent constraints on current estimates and improves our mechanistic understanding of the BCP. In addition, it demonstrates the great potential of BGC-Argo float data for studying the biological carbon pump.

Authors:
Bin Wang (Dalhousie University)
Katja Fennel (Dalhousie University)

An expanding understanding of Southern Ocean productivity and export

Posted by mmaheigan

· Monday, February 13th, 2023

Biology in the Southern Ocean is known to help regulate Earth’s climate by capturing and eventually sequestering carbon from its surface. Unfortunately, accurate estimates of the magnitude of the Southern Ocean (SO) biological carbon sink are limited and subject to ongoing debate. However, a recently published study used the expanding Southern Ocean BGC-Argo fleet to provide new estimates of SO Annual Net Community Production (ANCP) and export production.

Over long enough time and space scales (>1000 km and seasons), ANCP is equal to the amount of carbon fixed during photosynthesis that is not remineralized in the surface layer. What remains is available to be exported to depth. As this organic matter sinks out of the surface ocean, most of it is eventually remineralized by bacteria, leaving behind a signature of depleted oxygen. With enough floats, basin-scale ANCP can be estimated from the seasonal oxygen drawdown measured across their profiles. While similar studies have been carried out on single floats, here, the authors construct a composite of all available profiles and include a greater depth range than previously considered.

Figure 1. All available BGC-ARGO float profiles (25,512) were used to create an A) ensemble seasonal cycle in surface chlorophyll and sub-surface oxygen. B) Annual Net Community Production (ANCP) was then estimated by computing the depth-integrated oxygen depletion during the productive period. C) ANCP was estimated across 12 major regions, separated by the Indian, Pacific and Atlantic basins and Subantarctic (SAZ), Polar (PFZ), Antarctic (AZ), and Southern (S) frontal zones. Each region used 100s-1000s of individual float profiles (color-coded scatter points).

Results from this novel approach estimate SO ANCP (and ~export) at 3.89 GT C year^-1, with basin-scale regional estimates as much as a factor 2.8 larger than previous studies. Moreover, nearly 30% of remineralization was measured at depths not typically considered, with 14% below 500 m and another 15% immediately below the euphotic depth but above 100 m. These values suggest a more critical role for the Southern Ocean in regulating oceanic carbon storage, atmospheric CO₂ exchange, and climate than previously thought.

Authors:
Jiaoyang Su (University of Tasmania, Australia)
Christina Schallenberg (University of Tasmania, and Australian Antarctic Program Partnership)
Tyler Rohr (Australian Antarctic Program Partnership)
Peter G. Strutton (University of Tasmania, Australia)
Helen E. Phillips (University of Tasmania, and Australian Antarctic Program Partnership)

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)

Towards using historical oxygen observations to reconstruct the air-sea flux of biological oxygen

Posted by mmaheigan

· Tuesday, December 13th, 2022

Dissolved oxygen (O₂) is a central observation in oceanography with a long history of relatively high precision measurements and increasing coverage over the 21^st century. O₂ is a powerful tracer of physical, chemical and biological processes (e.g., photosynthesis and respiration, wave-induced bubbles, mixing, and air-sea diffusion). A commonly used approach to partition the processes controlling the O₂ signal relies on concurrent measurements of argon (an inert gas), which has solubility properties similar to O₂. However, only a limited fraction of O₂ measurements have paired argon measurements.

Figure 1. (a) The newly developed empirical model to parameterize the physical oxygen saturation anomaly (ΔO_2[phy]) in order to separate the biological contribution from total oxygen, and (b-c) regional, inter-annual, and decadal variability of air-sea gas flux of biological oxygen (F[O2]bio as) reconstructed from the historical dissolved oxygen record.

A recent study published in the Journal of Global Biogeochemical Cycles presents semi-analytical algorithms to separate the biological and physical O₂ oxygen signals from O₂ observations. Among the approaches, a machine-learning algorithm using ship-based measurements and historical records of physical parameters from reanalysis products as predictors shows encouraging performance. The researchers leveraged this new algorithm to reconstruct regional, inter-annual, and decadal variability of the air-sea flux of biological oxygen (from historical O₂ records.

The long-term objective of this proof-of-concept effort is to estimate from historical oxygen records and a rapidly growing number of O₂ measurements on autonomous platforms. In regions where vertical and horizontal mixing is weak, the projected approximates net community production, providing an independent constraint on the strength of the biological carbon pump.

Authors:
Yibin Huang (Duke University)
Rachel Eveleth (Oberlin College)
David (Roo) Nicholson (Woods Hole Oceanographic Institution)
Nicolas Cassar (Duke University)

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.

Also see Eos highlight here

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.

Archive for New OCB Research – Page 6

Filter by Keyword