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Extreme events are accelerating coastal carbon cycling

Posted by mmaheigan

· Monday, March 1st, 2021

The world is getting stormier, and recent evidence shows significant impacts on coastal carbon cycling. The upticks in extreme weather events such as tropical cyclones have resulted in enhanced delivery of nutrients and organic matter across the land-ocean continuum. Lagoonal estuaries such as the Albemarle-Pamlico Sound (APS) in North Carolina and Galveston Bay in Texas are key coastal environments in which we can observe the long-term carbon cycling consequences of these events. Residence times of these coastal environments are on the order of months to over a year, providing ample opportunity for biogeochemical processing. Emerging from studies of Atlantic and Gulf of Mexico hurricanes in 2016 and 2017 is a clear example of the role of terrestrial dissolved organic carbon (DOC) as a key reactant driving the observed carbon cycling and ecosystem effects ( Figure 1).

Figure. 1. The impact of hurricanes on CO₂ fluxes (top) and terrestrial DOC decay constants (bottom) demonstrate the sustained effect on the coastal carbon cycle caused by extreme weather events. Top panel shows results from Hurricane Matthew in 2016, where date is month and day and Km downstream represents observations taken along the main axis of the Neuse River Estuary and lower Pamlico Sound, eastern North Carolina. F_CO2 is the daily sea-to-air flux of CO₂ estimated from measurements of temperature, salinity, dissolved inorganic carbon, and wind speed. The results indicate the Sound existed as a weak yet sustained CO2 source to the atmosphere well after the storm. Outgassing of CO₂ is driven by the rapid mineralization of terrestrial DOC. Bottom panel shows the high bioreactivity of flood-derived terrestrial DOC indicated by elevated microbial decay constants for Galveston Bay and the coastal Gulf of Mexico in 2017 as compared to high and low latitude coastal environments.

In coastal North Carolina, 36 tropical cyclones (TCs), including three floods of historical significance in the past two decades, have occurred in the past 20 years. The lingering effects of these storms include extensive periods of carbon dioxide (CO₂) supersaturation. For example, Hurricane Matthew in 2016 caused the lower Pamlico Sound to emit CO₂ for months after the passage of the storm. With similar results documented for the Pamlico Sound for storms in 2011 and 2012, there is solid evidence that shifts in the ecosystem state of this mesotrophic estuary from net autotrophic to net heterotrophic are a major effect of this process.

Reactive DOC from the landscape appears to be driving the shift in ecosystem state. Large plumes of brown-colored DOC are observable from space in numerous satellite images of the Atlantic and Gulf coasts following these storms. The color is part of a phenomenon known as “coastal darkening"—spectroscopic, stable isotopic, and biomarker evidence show this darkening is related to the flushing of wetlands in the flood-plain adjacent to the rivers draining into these estuaries.

Along the Texas coast, Hurricane Harvey produced the largest rainfall event recorded in US history and caused extensive flooding in 2017. Similar to results from coastal North Carolina, flood-derived terrestrial DOC in Galveston Bay exhibited high bioreactivity, with decay constants exceeding those observed for terrestrial DOC across coastal environments from high and low latitudes by almost three-fold. The rapid processing of terrestrial DOC was linked to an active microbial community capable of decomposing aromatic compounds that are abundant in colored DOC as indicated by genomic analyses. These recent studies clearly demonstrate the impacts of large storm events on coastal carbon cycling via the transport of reactive terrestrial DOC into coastal waters. Climate-driven increases in the frequency and intensity of such storm events warrant more sustained capacity to monitor episodic deliveries of carbon and nutrients and their impacts on coastal marine ecosystems.

Authors:
Chris Osburn (North Carolina State University) @closburn
Hans Paerl (University of North Carolina, Institute of Marine Sciences)
Ge Yan (Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences)
Karl Kaiser (Texas A&M University, Galveston Campus)

Citations:

Yan, G., Labonté, J. M., Quigg, A., & Kaiser, K. (2020). Hurricanes accelerate dissolved organic carbon cycling in coastal ecosystems. Frontiers in Marine Science, 7, 248.

Osburn, C. L., Rudolph, J. C., Paerl, H. W., Hounshell, A. G., & Van Dam, B. R. (2019). Lingering carbon cycle effects of Hurricane Matthew in North Carolina's coastal waters. Geophysical Research Letters, 46(5), 2654-2661.

Does ocean acidification make marine fish grow differently? What about sex-specific effects?

Posted by mmaheigan

· Monday, February 8th, 2021

The question of whether ocean acidification (OA) will impact the growth of marine fish remains surprisingly uncertain. The bulk of available evidence in the form of laboratory experiments suggests that most fish are not impacted by OA-relevant CO₂ levels, but many studies suffer from the inherent methodical constraints of rearing marine fish in captivity. For example, most experiments cover a small fraction of a species’ lifespan and do not employ restricted feeding regimes which may enable fish to increase feeding to offset metabolic deficits associated with high-CO₂ acclimation.

To address these methodological shortcomings, authors of a recent publication in PLOS One synthesized three years of multiple long-term, food-controlled experiments that reared large populations of the model forage fish Menidia menidia (Atlantic silverside) from fertilization to about a third of their lifespan. Results showed modest but consistent negative, temperature-dependent growth effects, in which silversides from high-CO₂ treatments were shorter (-3% to -9%) and weighed less (-6% to -18%) than ambient-CO₂ conspecifics. However, sometimes it takes more than just looking at means and standard deviations to elucidate these effects. Hence, the authors employed powerful shift functions to analyze how the size distributions of experimental populations shifted to smaller quantiles under future CO₂ conditions.

Figure caption: The length of juvenile Atlantic silversides reared from fertilization under control (blue dots) and high-CO₂ treatments (red dots). Exposure to OA conditions imposed a universal shift to a smaller body size across the size frequency distribution. Black vertical bars overlaying each distribution indicate the .1, .25, .5, .75, and .9 quantiles and quantile shifts are indicated by connecting lines.

It took over 100 days of continuous high-CO₂ exposure until size differences were detectable. This means that long-term CO₂effects could exist in other tested species but are missed in relatively short experiments. Furthermore, the authors sexed several thousand fish to enable a rare sex-specific analysis of CO₂ effects. Both sexes were similarly affected by high CO₂, and the hormonal pathways that mediate environmental sex determination in this species are not impacted by CO₂ level. Our results confirm that Atlantic silversides are relatively tolerant of future OA conditions. But even in this robust estuarine species, high CO₂ can reduce growth. This could have cascading effects on population dynamics by impacting size-dependent traits like reproductive success and over-wintering survival of this widespread and ecologically important prey species.

Authors
Christopher S. Murray (University of Washington)
Hannes Baumann (University of Connecticut)

How environmental drivers regulated the long-term evolution of the biological pump

Posted by mmaheigan

· Friday, January 22nd, 2021

The marine biological pump (BP) plays a crucial role in regulating earth’s atmospheric oxygen and carbon dioxide levels by transferring carbon fixed by primary producers into the ocean interior and marine sediments, thereby controlling the habitability of our planet. The rise of multicellular life and eukaryotic algae in the ocean about 700 million years ago would likely have influenced the physical characteristics of oceanic aggregates (e.g., sinking rate), yet the magnitude of the impact this biological innovation had on the efficiency of BP is unknown.

Figure. 1. The impact of biological innovations (left) and environmental factors (atmospheric oxygen level and seawater temperature; right) on the efficiency of marine biological pump (BP). Temperatures are ocean surface temperatures (SST), and atmospheric pO2 is shown relative to the present atmospheric level (PAL). The BP efficiency is calculated as the fraction of carbon exported from the surface ocean that is delivered to the sediment-water interface. The results indicate that evolution of larger sized algae and zooplanktons has little influence on the long-term evolution of biological pump (left panel). The change in the atmospheric oxygen level and seawater surface temperature as environmental factors, on the other hand, have a stronger leverage on the efficiency of biological pump (right panel).

The authors of a recent paper in Nature Geoscience constructed a particle-based stochastic model to explore the change in the efficiency of the BP in response to biological and physical changes in the ocean over geologic time. The model calculates the age of organic particles in each aggregate based on their sinking rates, and considers the impact of primary producer cell size, aggregation, temperature, dust flux, biomineralization, ballasting by mineral phases, oxygen, and the fractal geometry (porosity) of aggregates. The model results demonstrate that while the rise of larger-sized eukaryotes led to an increase in the average sinking rate of oceanic aggregates, its impact on BP efficiency was minor. The evolution of zooplankton (with daily vertical migration in the water column) had a larger impact on the carbon transfer into the ocean interior. But results suggest that environmental factors most strongly affected the marine carbon pump efficiency. Specifically, increased ocean temperatures and greater atmospheric oxygen abundance led to a significant decrease in the efficiency of the BP. Cumulatively, these results suggest that while major biological innovations influenced the efficiency of BP, the long-term evolution of the marine carbon pump was primarily controlled by environmental drivers such as climate cooling and warming. By enhancing the rate of heterotrophic microbial degradation, our results suggest that the anthropogenically-driven global warming can result in a less efficient BP with reduced power of marine ecosystem in sequestering carbon from the atmosphere.

Authors:
Mojtaba Fakhraee (Yale University, Georgia Tech, and NASA Astrobiology Institute)
Noah J. Planavsky (Yale University, and NASA Astrobiology Institute)
Christopher T. Reinhard (Georgia Tech, and NASA Astrobiology Institute)

Sea ice loss amplifies CO2 increase in the Arctic

Posted by mmaheigan

· Thursday, January 7th, 2021

Warming and sea ice loss over the past few decades have caused major changes in sea surface partial pressure of CO₂ (pCO₂) of the western Arctic Ocean, but detailed temporal variations and trends during this period of rapid climate-driven changes are not well known.

Based on an analysis of an international Arctic pCO₂ synthesis data set collected between 1994-2017, the authors of a recent paper published in Nature Climate Change observed that summer sea surface pCO₂ in the Canada Basin is increasing at twice the rate of atmospheric CO₂ rise. Warming, ice loss and subsequent CO₂ uptake in the Basin are amplifying seasonal pCO₂ changes, resulting in a rapid long-term increase. Consequently, the summer air-sea CO₂ gradient has decreased sharply and may approach zero by the 2030s, which is reducing the basin’s capacity to remove CO₂from the atmosphere. In stark contrast, sea surface pCO₂ on the Chukchi Shelf remains low and relatively constant during this time frame, which the authors attribute to increasingly strong biological production in response to higher intrusion of nutrient-rich Pacific Ocean water onto the shelf as a result of increased Bering Strait throughflow. These trends suggest that, unlike the Canada Basin, the Chukchi Shelf will become a larger carbon sink in the future, with implications for the deep ocean carbon cycle and ecosystem.

As Arctic sea ice melting accelerates, more fresh, low-buffer capacity, high-CO₂ water will enter the upper layer of the Canada Basin, which may rapidly acidify the surface water, endanger marine calcifying organisms, and disrupt ecosystem function.

Figure. 1: TOP) Sea surface pCO₂ trend in the Canada Basin and Chukchi Shelf. The grey dots represent the raw observations of pCO₂, black dots are the monthly mean of pCO₂ at in situ SST, and red dots are the monthly means of pCO₂ normalized to the long-term means of SST. The arrows indicate the statistically significant change in ∆pCO₂. BOTTOM) Sea ice-loss amplifying surface water pCO₂ in the Canada Basin. Black dots represent the initial condition for pCO₂ and DIC at -1.6 ℃. The arrows indicate the processes of warming (red), CO₂ uptake from the atmosphere (green), dilution by ice meltwater (blue). The yellow shaded areas indicate the possible seasonal variations of pCO₂, which are amplified by the synergistic effect of ice melt, warming and CO₂ uptake.

Authors:
Zhangxian Ouyang (University of Delaware, USA),
Di Qi (Third Institute of Oceanography, China),
Liqi Chen (Third Institute of Oceanography, China),
Taro Takahashi† (Columbia University, USA),
Wenli Zhong (Ocean University of China, China),
Michael D. DeGrandpre (University of Montana, USA),
Baoshan Chen (University of Delaware, USA),
Zhongyong Gao (Third Institute of Oceanography, China),
Shigeto Nishino (Japan Agency for Marine-Earth Science and Technology, Japan),
Akihiko Murata (Japan Agency for Marine-Earth Science and Technology, Japan),
Heng Sun (Third Institute of Oceanography, China),
Lisa L. Robbins (University of South Florida, USA),
Meibing Jin (International Arctic Research Center, USA),
Wei-Jun Cai* (University of Delaware, USA)

Partitioning carbon export into particulate and dissolved pools from biogeochemical profiling float observations

Posted by mmaheigan

· Thursday, December 17th, 2020

Carbon export from the surface into the deep ocean via the biological pump is a significant sink for atmospheric carbon dioxide. The relative contributions of sinking particles—particulate organic carbon (POC) and dissolved organic carbon (DOC)—to the total export affect the efficiency of carbon export.

In a recent study published in Global Biogeochemical Cycles, the authors used measurements from biogeochemical profiling floats in the Northeast Pacific from 2009 to 2017 to estimate net community production (NCP), an analog for carbon export. In order to close three tracer budgets (nitrate, dissolved inorganic carbon, and total alkalinity), the authors combined these float measurements with data from the Ocean Station Papa mooring and recently developed algorithms for carbonate system parameters. By constraining end-member nutrient ratios of the POC and DOC produced, this multi-tracer approach was used to estimate regional NCP across multiple depth horizons throughout the annual cycle, partition NCP into the POC and DOC contributions, and calculate particulate inorganic carbon (PIC) production, a known ballast material for sinking particles (Figure 1). The authors also estimated POC attenuation with depth, POC export across deeper horizons, and in situ export efficiency via a particle backscatter-based approach.

With the advent of “fully-loaded” biogeochemical profiling floats equipped with nitrate, oxygen, pH and bio-optical sensors, this approach may be used to assess the magnitude and efficiency of carbon export in other ocean regions from a single platform, which will greatly reduce the risks and costs associated with traditional ship-based measurements, while broadening the spatiotemporal scales of observation.

Figure caption: Climatological mean NCP (blue line) over the entire study period (2009-2017); the POC portion of NCP (filled blue area), the DOC portion (white space) and PIC production rate (red line), in the mixed layer (left), and the euphotic zone (right). The numbers in parentheses are the integrated annual NCP rates for each curve and uncertainty reported was determined using a Monte Carlo approach.

Authors:
William Haskell (MBARI, now Mote Marine Laboratory)
Andrea Fassbender (MBARI, now PMEL)
Jacki Long (MBARI)
Joshua Plant (MBARI)

Chesapeake Bay acidification partially offset by submerged aquatic vegetation

Posted by mmaheigan

· Wednesday, September 30th, 2020

Ocean acidification is often enhanced by eutrophication and subsequent hypoxia and anoxia in coastal waters, which collectively threaten marine organisms and ecosystems. Acidification is particularly of concern for organisms that form shells and skeletons from calcium carbonate (CaCO₃) such as commercially important shellfish species. Given that CaCO₃ mineral dissolution can increase the total alkalinity of water and neutralize anthropogenic and metabolic CO₂, it is important to include CaCO₃ cycle in the coastal water acidification study. However, very few studies have linked CaCO₃ dissolution to the timing and location of its formation in coastal waters. A recent study from the Chesapeake Bay published in Nature Geoscience reveals the occurrence of a bay-wide pH-buffering mechanism resulting from spatially decoupled CaCO₃ mineral cycling (Figure 1). Photosynthesis by submerged aquatic vegetation at the head of the Bay and in other shallow, nearshore waters can remove nutrient pollution from the Bay, generate very high pH, and elevate the carbonate mineral saturation state (Figure 1). This facilitates the formation of CaCO₃ minerals, which are then transported downstream along with other biologically produced carbonate shells into acidic subsurface waters, where they dissolve. This dissolution of carbonate minerals helps “buffer” the water against pH decreases and even drive pH increases. This finding has great ecological and natural resource management significance, in that coastal nutrient management and reduction via the resurgence of submerged aquatic vegetation can help mitigate low oxygen and acidification stress for these environments and organisms.

Figure 1: Conceptual model of the self-regulated pH-buffering mechanism in the Chesapeake Bay. Calcium carbonate is formed within the high-pH submerged aquatic vegetation beds in shallow waters (top left and upper part of diagram, all Shoals with SAV), where it could be subsequently transported longitudinally and/or laterally into the deep main channel of the mid and lower bay (center) and upon dissolution, increase pH buffering capacity and alleviate coastal acidification (lower section).

Authors:
Jianzhong Su (University of Delaware, Xiamen University)
Wei-Jun Cai (University of Delaware)
Jean Brodeur (University of Delaware)
Baoshan Chen (University of Delaware)
Najid Hussain (University of Delaware)
Yichen Yao (University of Delaware)
Chaoying Ni (University of Delaware)
Jeremy Testa (University of Maryland Center for Environmental Science)
Ming Li (University of Maryland Center for Environmental Science)
Xiaohui Xie (University of Maryland Center for Environmental Science, Second Institute of Oceanography)
Wenfei Ni (University of Maryland Center for Environmental Science)
K. Michael Scaboo (University of Delaware)
Yuanyuan Xu (University of Delaware)
Jeffrey Cornwell (University of Maryland Center for Environmental Science)
Cassie Gurbisz (St. Mary’s College of Maryland)
Michael S. Owens (University of Maryland Center for Environmental Science)
George G. Waldbusser (Oregon State University)
Minhan Dai (Xiamen University)
W. Michael Kemp (University of Maryland Center for Environmental Science)

Sea ice loss and the changing Arctic carbon cycle

Posted by mmaheigan

· Friday, September 18th, 2020

Loss of Arctic Ocean ice cover is altering the carbon cycle in ways that are not well understood. Effectively “popping the top off” the Arctic Ocean, ice loss exposes the sea surface to warming and exchange of CO₂ with the atmosphere. These processes are expected to increase CO₂ levels in the Arctic Ocean, changing its contribution to the global carbon cycle, but limited data collection in the region has thus far precluded the establishment of a clear relationship between CO₂ and ice cover. In a recent study published in Geophysical Research Letters, authors report on observed partial pressure of CO₂ (pCO₂) trends from several years of data collection in the surface waters of the Canada Basin of the Arctic Ocean. These data show that the pCO₂ is higher during years when ice cover is low. Uptake of atmospheric CO₂ and heating are the primary sources of the CO₂ increase, with only a small counteracting offset from biological production. These processes vary significantly from year to year, masking the likely increase in pCO₂ over time. Based on these results, we can expect that, while the Arctic Ocean has thus far been a significant sink for atmospheric CO₂, if ice loss continues the uptake of CO₂ will diminish in coming years.

Figure caption: Sea surface pCO₂ increases with decreasing ice concentration (left), determined using the mean of spatially gridded data. The sea surface pCO₂ data were collected on five research cruises on the Canadian icebreaker, CCGS Louis S. St-Laurent, from 2012 to 2017 (shown at right for 2017). The pCO₂ levels are indicated by the color along the ship cruise track (right color bar). The dark shading (left color bar) represents sea ice concentration averaged from the daily satellite data collected during the cruise.

Authors:
Michael DeGrandpre (University of Montana-Missoula)
Wiley Evans (Hakai Institute)
Mary-Louise Timmermans (Yale University)
Richard Krishfield (Woods Hole Oceanographic Institution)
Bill Williams (Institute of Ocean Sciences)
Michael Steele (University of Washington)

An Important Biogeochemical Link between Organic and Inorganic Carbon Cycling: Contributions of Organic Alkalinity

Posted by mmaheigan

· Wednesday, April 8th, 2020

As a part of dissolved organic carbon (DOC), organic acid charge groups can contribute significantly to total alkalinity (TA) in natural waters. Such a contribution is termed as organic alkalinity (OrgAlk). Beyond being part of TA, OrgAlk represents an important biogeochemical linkage between organic and inorganic carbon cycling. In other words, the biogeochemical cycling of organic acid charge groups – i.e. their sources, sinks, and biogeochemical behaviors – directly impacts pH and carbonate speciation, which may ultimately influence air-water CO₂ exchange and inorganic carbon fluxes. However, the effects of OrgAlk is often ignored or treated as a calculation uncertainty in many aquatic CO₂ studies. How we treat and study OrgAlk may need a new paradigm under biogeochemical cycles.

Based on direct titration data of OrgAlk, the authors of a recent study conducted a comprehensive assessment of OrgAlk variability, sources, and characteristics in a sub-estuary of Waquoit Bay (Massachusetts). The sub-estuary is influenced by a salt marsh, groundwater input, and offshore water. Both the salt marsh and groundwater OrgAlk contributed up to 4.3% of the TA across all sampled seasons. Estuarine OrgAlk:DOC ratios varied across space and time, which suggests that their abundances are controlled by different biogeochemical processes. In addition, the study demonstrates the insufficiency of using a fixed proportion of DOC to account for OrgAlk, as well as the challenge of using measured pH, TA, and dissolved inorganic carbon (DIC) to estimate OrgAlk. The effects of OrgAlk in these waters are equivalent to a pH change of ~ 0.03 – 0.26, or a pCO₂ change of ~30–1600 matm. If extrapolating OrgAlk results to other coastal systems ranging from estuaries to continental shelves, OrgAlk would exert a strong control on both carbonate speciation and, ultimately, air-sea CO₂ fluxes. This study provides a new conceptual framework for cycling of OrgAlk species and associated links between DOC and DIC pools in coastal systems (Figure 1).

Figure caption: A conceptual model of organic alkalinity cycling in coastal systems. BioP and ChemP represent in-situ biological production and chemical production of organic acid charge groups, respectively. Alk denotes total alkalinity. Arrows with dashed lines indicate processes that were not studied in the present study. The values in the boxes of pH, pCO₂, and buffer capacity represent the magnitude of OrgAlk effects on pH, pCO₂, and buffer capacity in the range of OrgAlk% in TA observed in this study (0.9 – 4.3%).

Authors
Shuzhen Song (East China Normal University)
Zhaohui Aleck Wang (Woods Hole Oceanographic Institution)
Meagan Eagle Gonneea (U. S. Geological Survey)
Kevin D. Kroeger (U. S. Geological Survey)

The past, present, and future of the ocean carbon cycle: A global data product with regional insights

Posted by mmaheigan

· Tuesday, January 21st, 2020

A new study published in Scientific Reports debuts a global data product of ocean acidification (OA) and buffer capacity from the beginning of the Industrial Revolution to the end of the century (1750-2100 C.E.). To develop this product, the authors linked one of the richest observational carbon dioxide (CO₂) data products (6^th version of the Surface Ocean CO₂ Atlas, 1991-2018, ~23 million observations) with temporal trends modeled at individual locations in the global ocean. By linking the modeled pH trends to the observed modern pH distribution, the climatology benefits from recent improvements in both model design and observational data coverage, and is likely to provide more accurate regional OA trajectories than the model output alone. The authors also show that air-sea CO₂ disequilibrium is the dominant mode of spatial variability for surface pH, and discuss why pH and calcium carbonate mineral saturation states (Omega), two important metrics for OA, show contrasting spatial variability. They discover that sea surface temperature (SST) imposes two large but cancelling effects on surface ocean pH and Omega, i.e., the effects of SST on (a) chemical speciation of the carbonic system; and (b) air-sea exchange of CO₂ and the subsequent DIC/TA ratio of the seawater. These two processes act in concert for Omega but oppose each other for pH. As a result, while Omega is markedly lower in the colder polar regions than in the warmer subtropical and tropical regions, surface ocean pH shows little latitudinal variation.

Figure 1. Spatial distribution of global surface ocean pH_T (total hydrogen scale, annually averaged) in past (1770), present (2000) and future (2100) under the IPCC RCP8.5 scenario.

This data product, which extends from the pre-Industrial era (1750 C.E.) to the end of this century under historical atmospheric CO₂ concentrations (pre-2005) and the Representative Concentrations Pathways (post-2005) of the Intergovernmental Panel on Climate Change (IPCC)’s 5^th Assessment Report, may be helpful to policy-makers and managers who are developing regional adaptation strategies for ocean acidification.

The published paper is available here: https://www.nature.com/articles/s41598-019-55039-4

The data product is available here: https://www.nodc.noaa.gov/oads/data/0206289.xml

Authors:
Li-Qing Jiang (University of Maryland and NOAA NCEI)
Brendan Carter (NOAA PMEL and University of Washington JISAO)
Richard Feely (NOAA PMEL)
Siv Lauvset, Are Olsen (University of Bergen and Bjerknes Centre for Climate Research, Norway)

A role for tropical nitrogen fixers in glacial CO2 drawdown

Posted by mmaheigan

· Wednesday, December 4th, 2019

Iron fertilization of marine phytoplankton by Aeolian dust is a well-established mechanism for atmospheric carbon dioxide (CO₂) drawdown by the ocean. When atmospheric CO₂ decreased by 90-100 ppm during previous ice ages, fertilization of iron-limited phytoplankton in the high latitudes was thought to have contributed up to ¹/₃ (30 ppm) of the total CO₂ drawdown. Unfortunately, recent modeling studies suggest that substantially less CO₂ (only 2-10 ppm) is sequestered by the ocean in response to high latitude fertilization.

The limited capacity for high latitude CO₂sequestration in response to iron enrichment motivated the authors of a new study published in Nature Communications to address how lower latitude phytoplankton could contribute to CO₂ drawdown. The authors used an ocean model to show that in response to Aeolian iron fertilization, dinitrogen (N₂) fixers, specialized phytoplankton that introduce bioavailable nitrogen to tropical surface waters, drive the sequestration of an additional 7-16 ppm of CO₂ by the ocean.

Figure 1: Scenarios of Fe supply to the tropical Pacific. In the low iron scenario, analogous to the modern climate, N₂ fixation (yellow zone and dots) is concentrated in the Northwest and Southwest subtropical Pacific where aeolian dust deposition is greatest. Non-limiting PO₄ concentrations (green zone and dots) exist within the tropics and spread laterally from the area of upwelling near the Americas and at the equator (blue zone). In the high Fe scenario, analogous to the glacial climate, N₂ fixation couples to the upwelling zones in the east Pacific, enabling strong utilisation of PO₄, the vertical expansion of suboxic zones (grey bubbles) and a deeper injection of carbon-enriched organic matter (downward squiggly arrows).

These results provide evidence of a tropical ocean CO₂ sequestration pathway, the mere existence of which is hotly debated. Importantly, the study describes an additional mechanism of CO₂ drawdown that is complementary to the high latitude mechanism. When combined, their contributions elevate iron-driven CO₂ drawdown towards the expected 30 ppm, making iron fertilization a driver of a stronger biological pump on a global scale.

Authors:
Pearse Buchanan (University of Liverpool, University of Tasmania, CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate System Science)
Zanna Chase (University of Tasmania)
Richard Matear (CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate Extremes)
Steven Phipps (University of Tasmania)
Nathaniel Bindoff (University of Tasmania, CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate Extremes, Antarctic Climate and Ecosystems Cooperative Research Centre)

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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