Archive for rivers

The curious role of organic alkalinity in seawater carbonate chemistry

Posted by mmaheigan 
· Wednesday, August 5th, 2020 

The marine chemistry community has measured organic alkalinity in coastal and estuarine waters for over two decades. While the common perception is that any unaccounted alkalinity should enhance seawater buffer capacity, the effects of organic alkalinity on this buffering capacity, and hence the potential CO2 uptake by coastal and estuarine systems are still not well quantified.

In a thought experiment recently published in Aquatic Geochemistry, the author added organic alkalinity to model seawater (salinity=35, temperature=15˚C, pCO2=400 µatm) in the form of 1) organic acid (HOA) and 2) its conjugate base (OA–). Results suggest that the weaker organic acid/conjugate base pair (pKa ~8.2-8.3) yields the greatest buffering capacity under the simulation conditions. However, the HOA addition first displaces dissolved inorganic carbon (DIC) and causes CO2 degassing; the resultant seawater buffer capacity can be greater or less than the original seawater, depending on the pKa. In comparison, OA– addition leads to CO2 uptake and elevated seawater buffer capacity. As the organic anions are remineralized via biogeochemical processes, a “charge transfer” results in quantitative conversion to carbonate alkalinity (CA), which is overpowered by the concomitant CO2 production (∆DIC>∆CA). Overall, the complete process (organic alkalinity addition and remineralization) results in a net CO2 release from seawater, regardless of whether it is added in the form of HOA or OA–.

Figure caption: A schematic illustration of the role of organic alkalinity on seawater carbonate chemistry in an open system (constant CO2 partial pressure). Organic acid (HOA) addition leads to CO2 degassing and varying seawater buffer (greater or lower than the original seawater) as a function of Ka. Organic base (OA–) addition causes initial CO2 uptake and overall elevated seawater buffer. Regardless, upon complete remineralization, more CO2 is produced than the amount of net gain in carbonate alkalinity (OA– addition only). Therefore, the complete process (organic acid/base addition and its ultimate remineralization) should result in net CO2 degassing.

While the presence of organic alkalinity may increase seawater buffer capacity to some extent (depending on the pKa values of the organic acid), CO2 degassing from the seawater, because of both the initial organic acid addition and eventual remineralization of organic molecules, should be the net result. However, modern alkalinity analysis precludes the bases of stronger organic acids (pKa < 4.5). This fraction of “potential” alkalinity, especially from river waters, remains a relevant topic for future alkalinity cycle studies. The potential alkalinity can be converted to bicarbonate through biogeochemical reactions (or charge transfer at face value), although it is unclear how significant this potential alkalinity is in rivers that flow into the ocean.

 

A backstory
The author used an example of vinegar and limewater (calcium hydroxide solution), which is employed by many aquarists to dose alkalinity and calcium in hard coral saltwater tanks, to demonstrate the conversion of organic base (acetate ion) to bicarbonate and CO2 via complete remineralization. It is also known the added vinegar helps microbes to remove excess nitrate. This procedure had been in the author’s memory for the past nine years, ever since his previous research life when he participated in a study at a coral farm in a suburb of Columbus, Ohio. A strong vinegar odor would arise every now and then at the facility. However, a recent communication with the facility owner suggests that this memory was totally false and the owner simply used vinegar to get rid of lime (CaCO3) buildup in the water pumps. Nonetheless, the chemistry in this paper should still hold, with that false memory serving as the inspiration.

 

Author:
Xinping Hu (Texas A&M University-Corpus Christi)

Arctic rivers as carbon highways

Posted by mmaheigan 
· Tuesday, June 16th, 2020 

Rapid environmental changes in the Arctic will potentially alter the atmospheric emissions of heat-trapping greenhouse gases such as methane (CH4) and carbon dioxide (CO2). A recent study on the Canadian Arctic published in Geophysical Research Letters reveals that spring meltwater delivery drives episodic outgassing events along the lake-river-bay continuum. This spring runoff period is not well-represented in prior studies, which, due to ease of sampling access, have focused more on summertime low-ice conditions. Study authors established a community-based monitoring program in Cambridge Bay and an adjacent inflowing river system in Nunavut, Canada from 2017-2018. These time-series data revealed that at the onset of the melt season river water contains methane concentrations up to 2000 times higher than observed in the bay from late summer through early spring (Figure 1 panel a). In addition, the authors deployed a novel robotic chemical sensing kayak (the ChemYak) in the Bay for five days in 2018 to densely sample water CH4 and CO2 levels in space and time during the spring thaw (Figure 1 panel b). The ChemYak observations revealed that river water containing elevated levels of both of these greenhouse gases flowed into the bay and outgassed to the atmosphere over a period of 5 days! The authors estimate that river inflow during the short melt season drives >95% of all annual methane emissions from the bay. These results demonstrate the need for seasonally-resolved sampling to accurately quantify greenhouse gas emissions from polar systems.

Figure 1: Panel a) Measurements of methane concentration in Cambridge Bay and an adjacent river showed strong seasonality; elevated concentrations were associated with river inflow at the start of the freshet. Panel b) Observations with the ChemYak robotic surface vehicle in Cambridge Bay revealed that excess methane was rapidly ventilated to the atmosphere following ice melt in the bay.

 

Authors
Cara Manning (University of British Columbia)
Victoria Preston (Woods Hole Oceanographic Institution and Massachusetts Institute of Technology)
Samantha Jones (University of Calgary)
Anna Michel (Woods Hole Oceanographic Institution)
David Nicholson (Woods Hole Oceanographic Institution)
Patrick Duke (University of Calgary and University of Victoria)
Mohamed Ahmed (University of Calgary)
Kevin Manganini (Woods Hole Oceanographic Institution)
Brent Else (University of Calgary)
Philippe Tortell (University of British Columbia)

Impacts of atmospheric nitrogen deposition and coastal nitrogen fluxes on oxygen concentrations in Chesapeake Bay

Posted by mmaheigan 
· Tuesday, April 30th, 2019 

How do atmospheric and oceanic nutrients impact oxygen concentrations in the Chesapeake Bay? Generally, researchers focus on how terrestrial nutrients impact hypoxia. The relative importance of river, atmosphere, and ocean inputs have not been quantified, largely because estimates of nitrogen fluxes from the atmosphere and ocean are limited.

A recent study in Journal of Geophysical Research: Oceans quantified the relative impacts of atmospheric and oceanic nitrogen inputs on dissolved oxygen (DO) in the Chesapeake Bay. The authors combined a 3-D biogeochemical model and estimates of atmospheric deposition from the Community Multiscale Air Quality model and interpolations of nitrogen concentrations along the continental shelf from the Ocean Acidification Data Stewardship Project. Atmospheric nitrogen deposition and coastal nitrogen fluxes most impact Chesapeake Bay DO concentrations during the summer when surface waters are depleted in nitrogen. Overall, atmospheric nitrogen deposition has about the same gram-for-gram impact on Chesapeake Bay DO as riverine loading. Although all three nutrient sources vary spatially and temporally, in the central bay, where summer hypoxia is most prevalent, coastal nitrogen fluxes and atmospheric nitrogen fluxes have roughly the same impact on bottom oxygen as a ~10% change in riverine nitrogen loading (Figure 1).

Figure caption: (Left) Four-year (2002–2005) average increase in DO in the summer by removing the atmospheric nitrogen deposition (AtmN), reducing the riverine loading (ΔRiverN) by ~10% (roughly equivalent to turning off the atmospheric deposition), and removing the nitrogen fluxes from the continental shelf (CoastalN). (Right) Relative impacts of the three nitrogen modification scenarios on summertime bottom DO.

These results indicate that two often-neglected sources of nitrogen—direct atmospheric deposition and fluxes of nitrogen from the continental shelf—substantially impact Chesapeake Bay DO, especially in the summer. Future study is needed to investigate the long-term trend of these relative impacts by continued coordination between modeling and observational work, such as applying higher-resolution atmospheric deposition products and integrating more in situ data along the model ocean boundary when they are available. These efforts will improve our understanding of the impacts of different nutrient sources on biogeochemical cycles in coastal water bodies.

 

Authors:
Fei Da (VIMS, College of William & Mary)
Marjorie A. M. Friedrichs (VIMS, College of William & Mary)
Pierre St-Laurent (VIMS, College of William & Mary)

Ocean’s heat cycle shows that atmospheric carbon may be headed elsewhere

Posted by hbenway 
· Thursday, August 16th, 2018 

Studies over the past 25 years have supported the existence of a large net land biosphere CO2 sink of 0.5–2 PgC yr-1. Significant uncertainties remain, however, regarding the long-term partitioning between northern, tropical, and southern land sinks, in part connected to the uncertain ocean carbon sink. These uncertainties limit our capacity to predict earth system response to anthropogenic changes and design effective mitigation strategies.

Land sinks from atmospheric inversion (1990-2010 average) with two different ocean/river fluxes: (top) previous ocean inversion-based carbon fluxes; and (bottom) updated pCO2-based air-sea flux with a scaled-up river flux of 0.78 PgC /yr.

In a recent study published in Nature Geoscience, Resplandy et al. (2018) used models and field observations to demonstrate that the world’s oceans transport heat between the northern and southern hemispheres in the same way that carbon is transported. The transport of heat, however, is easier to observe. By tracking this heat, they showed that the Southern Ocean — while still a substantial carbon sink —may not take up as much carbon as previously thought, and that ocean currents might transport 20 to 100% more carbon from the northern to the southern hemisphere. To maintain this additional transport of carbon, they showed that the amount of carbon entering the ocean from rivers may be as much as 70% higher than estimated in previous global carbon budget studies. These changes in the ocean and river carbon transport imply that up to 40% of the world’s atmospheric carbon absorbed by land ecosystems needs to be reallocated from existing estimates.

Authors
L. Resplandy, Princeton University
Ralph Keeling, Scripps Institution of Oceanography/ UCSD
Christian Rödenbeck, Max Planck Institute
Briton Stephens, NCAR
Matthew Long, NCAR
Samar Khatiwala, University of Oxford
Keith Rodgers, Princeton University
Laurent Bopp, ENS Paris
Pieter Tans, NOAA’s ESRL

Do rivers supply nutrients to the open ocean?

Posted by mmaheigan 
· Wednesday, May 24th, 2017 

Rivers carry large amounts of nutrients (e.g., nitrogen and phosphorus) to the sea, but we do not know how much of that riverine nutrient supply escapes biological and chemical processing in shallow coastal waters to reach the open ocean. Most global ocean biogeochemical models, which are typically unable to resolve coastal processes, assume that either all or none of the riverine nutrients entering coastal waters actually contribute to open ocean processes.

While we know a good deal about the dynamics of individual rivers entering the coastal ocean, studies to date have been limited to a few major river systems, mainly in in developed countries. Globally, there are over 6,000 rivers entering the coastal ocean. In a recent study, Sharples et al (2017) devised a simple approach to obtain a global-scale estimate of riverine nutrient inputs based on the knowledge that low-salinity waters entering the coastal ocean tend to form buoyant plumes that turn under the influence of Earth’s daily rotation to flow along the coastline. Using published data on such flows and incorporating the effect of Earth’s rotation, they obtained estimates of typical cross-shore plume width and compared them to the local width of the continental shelf. This was used to calculate the residence time of riverine nutrients on the shelf, which is the key to estimating how much of a given nutrient is consumed in shelf waters vs. how much is exported to the open ocean.

Global distribution of the amount of riverine dissolved inorganic nitrogen that escapes the continental shelf to reach the open ocean.

The results indicate that, on a global scale, 75% (80%) of the nitrogen (phosphorus) supplied by rivers reaches the open ocean, whereas 25% (20%) of the nitrogen (phosphorus) is consumed on the shelf (e.g., fueling coastal productivity). Limited knowledge of nutrient cycling and consumption in shelf waters represents the primary source of uncertainty in this study. However, well-defined global patterns related to human land use (e.g., agricultural fertilizer use in developed nations) emerged from this analysis, underscoring the need to understand how land-use changes and other human activities will alter nutrient delivery to the coastal ocean in the future.

 

Authors:
Jonathan Sharples (School of Environmental Sciences, University of Liverpool, UK)
Jack Middelburg (Department of Earth Sciences, Utrecht University, Netherlands)
Katja Fennel (Department of Oceanography, Dalhousie University, Canada)
Tim Jickells (School of Environmental Sciences, University of East Anglia, UK)

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