Archive for alkalinity

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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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)

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)

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