Archive for DIC

What drives decadal changes in the Chesapeake Bay carbonate system?

Posted by mmaheigan 
· Tuesday, May 3rd, 2022 

Understanding decadal changes in the coastal carbonate system (CO2-system) is essential for predicting how the health of these waters is affected by anthropogenic drivers, such as changing atmospheric conditions and terrestrial inputs. However, studies that quantify the relative impacts of these drivers are lacking.

A recent study in Journal of Geophysical Research: Oceans identified the primary drivers of acidification in the Chesapeake Bay over the past three decades. The authors used a three-dimensional hydrodynamic-biogeochemistry model to quantify the relative impacts on the Bay CO2-system from increases in atmospheric CO2, temperature, oceanic dissolved inorganic carbon (DIC) concentrations, terrestrial loadings of total alkalinity (TA) and DIC, as well as decreases in terrestrial nutrient inputs. Decadal changes in the surface CO2-system in the Chesapeake Bay exhibit large spatial and seasonal variability due to the combination of influences from the land, ocean and atmosphere. In the upper Bay, increased riverine TA and DIC from the Susquehanna River have increased surface pH, with other drivers only contributing to decadal changes that are one to two orders of magnitude smaller. In the mid- and lower Bay, higher atmospheric CO2 concentrations and reduced nutrient loading are the two most critical drivers and have nearly equally reduced surface pH in the summer. These decadal changes in surface pH show significant seasonal variability with the greatest magnitude generally aligning with the spring and summer shellfish production season (Figure 1).

Figure 1: Overall changes in modeled surface pH (ΔpHall) due to all global and terrestrial drivers combined over the past 30 years (i.e., 2015–2019 relative to 1985–1989). ΔpHall includes changes in surface pH due to increased atmospheric CO2, increased atmospheric thermal forcing, increased oceanic dissolved inorganic carbon concentrations, decreased riverine nitrate concentrations, decreased riverine organic nitrogen concentrations, and increased riverine total alkalinity and dissolved inorganic carbon concentrations.

 

These results indicate that a number of global and terrestrial drivers play crucial roles in coastal acidification. The combined effects of the examined drivers suggest that calcifying organisms in coastal surface waters are likely facing faster decreasing rates of pH than those in open ocean ecosystems. Decreases in surface pH associated with nutrient reductions highlight that the Chesapeake Bay ecosystem is returning to a more natural condition, e.g., a condition when anthropogenic nutrient input from the watershed was lower. However, increased atmospheric CO2 is simultaneously accelerating the rate of change in pH, exerting increased stress on estuarine calcifying organisms. For ecosystems such as the Chesapeake Bay where nutrient loading is already being managed, controlling the emissions of anthropogenic CO2 globally becomes increasingly important to decelerate the rate of acidification and to relieve the stress on estuarine calcifying organisms. Future observational and modeling studies are needed to further investigate how the decadal trends in the Chesapeake Bay CO2-system may vary with depth. These efforts will improve our current understanding of long-term change in coastal carbonate systems and their impacts on the shellfish industry.

 

Authors:
Fei Da (Virginia Institute of Marine Science, William & Mary, USA)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science, William & Mary, USA)
Pierre St-Laurent (Virginia Institute of Marine Science, William & Mary, USA)
Elizabeth H. Shadwick (CSIRO Oceans and Atmosphere, Australia)
Raymond G. Najjar (The Pennsylvania State University, USA)
Kyle E. Hinson (Virginia Institute of Marine Science, William & Mary, USA)

Multiyear predictions of ocean acidification in the California Current System

Posted by mmaheigan 
· Thursday, August 20th, 2020 

The California Current System is a highly productive coastal upwelling region that supports commercial fisheries valued at $6 billion/year. These fisheries are supported by upwelled waters, which are rich in nutrients and serve as a natural fertilizer for phytoplankton. Due to remineralization of organic matter at depth, these upwelled waters also contain large amounts of dissolved inorganic carbon, causing local conditions to be more acidic than the open ocean. This natural acidity, compounded by the dissolution of anthropogenic CO2 into coastal waters, creates corrosive conditions for shell-forming organisms, including commercial fishery species.

A recent study in Nature Communications showcases the potential for climate models to skillfully predict variations in surface pH—thus ocean acidification—in the California Current System. The authors evaluate retrospective predictions of ocean acidity made by a global Earth System Model set up similarly to a weather forecasting system. The forecasting system can already predict variations in observed surface pH fourteen months in advance, but has the potential to predict surface pH up to five years in advance with better initializations of dissolved inorganic carbon (Figure 1). Skillful predictions are mostly driven by the model’s initialization and subsequent transport of dissolved inorganic carbon throughout the North Pacific basin.

Figure 1. Forecast of annual surface pH anomalies in the California Current Large Marine Ecosystem for 2020. Red colors denote anomalously basic conditions for the given location and blue colors indicate anomalously acidic conditions.

These results demonstrate, for the first time, the feasibility of using climate models to make multiyear predictions of surface pH in the California Current. Output from this global prediction system could serve as boundary conditions for high-resolution models of the California Current to improve prediction time scale and ultimately help inform management decisions for vulnerable and valuable shellfisheries.

 

Authors:
Riley X. Brady (University of Colorado Boulder)
Nicole S. Lovenduski (University of Colorado Boulder)
Stephen G. Yeager (National Center for Atmospheric Research)
Matthew C. Long (National Center for Atmospheric Research)
Keith Lindsay (National Center for Atmospheric Research)

A Methane-Charged Carbon Pump in Shallow Marine Sediments

Posted by mmaheigan 
· Wednesday, June 3rd, 2020 

Ocean margins are often characterized by the transport of methane, a potent greenhouse gas, entering from the subsurface and moving towards the seafloor. However, a significant portion of subsurface methane is consumed within shallow sediments via microbial driven anaerobic oxidation of methane (AOM). AOM converts the methane carbon to dissolved inorganic carbon (DIC) and reduces the amount of sulfate that diffuses down from the seafloor towards a sediment interval known as the sulfate-methane transition zone (SMTZ). The SMTZ is where the upward flux of methane encounters the downward diffusive sulfate flux (Figure 1). While the mechanisms of methane production and consumption have been extensively studied, the fate of the DIC that is produced in methane-charged sediments is not well constrained.

In a recent study published in Frontiers in Marine Science, authors used existing reports of methane and sulfate flux values to the SMTZ and synthesized a carbon flow model to quantify the DIC cycling in diffusive methane flux sites globally. They report an annual average of 8.7 Tmol (1 Tmol = 1012 moles) of DIC entering the diffusive methane-charged shallow marine sediments due to sulfate reduction coupled with AOM and organic matter degradation, as well as DIC input from depth (Figure 1). Approximately 75% (average of 6.5 Tmol year–1) of this DIC pool flows upward toward the water column, making it a potential contributor to oceanic CO2 and ocean acidification. Further, an average of 1.7 Tmol year–1 DIC precipitates as methane-derived authigenic carbonates. This synthesis emphasizes the importance of the SMTZ, not only as a methane sink but also an important biogeochemical front for global DIC cycling.

Figure 1: A simplified representation of DIC cycling at diffusive methane charged settings.

The study highlights that regions characterized by diffusive methane fluxes can contribute significantly to the oceanic inorganic carbon pool and sedimentary carbonate accumulation. DIC outflux from the methane-charged sediments is comparable to ~20% global riverine DIC flux to oceans. Methane-derived authigenic carbonate precipitation is comparable to ~15% of carbonate accumulation on continental shelves and in pelagic sediments, respectively. These  pathways must be included in coastal and geologic carbon models.

Authors:
Sajjad Akam (Texas A&M University-Corpus Christi)
Richard Coffin (Texas A&M University-Corpus Christi)
Hussain Abdulla (Texas A&M University-Corpus Christi)
Timothy Lyons (University of California, Riverside)

Upwelling and solubility drive global surface dissolved inorganic carbon (DIC) distribution

Posted by mmaheigan 
· Tuesday, August 20th, 2019 

What drives the latitudinal gradient in open-ocean surface DIC concentration? Understanding the processes that drive the distribution of carbon in the surface ocean is essential to the study of the ocean carbon cycle and future predictions of ocean acidification and the ocean carbon sink.

Authors of a recent study in Biogeosciences investigated causes of the observed latitudinal trend in DIC and salinity-normalized DIC (nDIC) (Figure 1). The latitudinal trend in nDIC is not driven solely by the latitudinal gradient in temperature (through its effects on solubility), as is commonly assumed. Careful analysis using the Global Ocean Data Analysis Project version 2 (GLODAPv2) database revealed that physical supply from below (upwelling, entrainment in winter) at high latitudes is another major driver of the latitudinal pattern. The contribution of physical exchange explains an otherwise puzzling observation: Surface waters are lower in nDIC in the high-latitude North Atlantic than in other basins. This cannot be accounted for by temperature difference but rather is explained by a difference in the carbon content of deeper waters (lower in the subarctic North Atlantic than in the subarctic North Pacific or Southern Ocean) that are mixed up into the surface during winter months.

Figure caption: (Top) spatial distributions of surface ocean DIC and (bottom) salinity-normalised (nDIC). Both, most notably nDIC, increase towards the poles. Values are normalised to year 2005 to remove bias from changing levels of atmospheric CO2 in some observations before and after 2005. Data are from GLODAPv2.

These results also suggest that the upwelling/entrainment of water that is high in alkalinity generates a large and long-lasting effect on DIC, one that persists beyond the timescale of CO2 gas exchange equilibration with the . That is to say, the impact of changes in upwelling on the ocean’s carbon source-sink strength depends not only on the DIC content of the upwelled water but also on its TA content.

Authors:
Yingxu Wu (University of Southampton)
Mathis Hain (University of California, Santa Cruz)
Matthew Humphreys (University of East Anglia and University of Southampton)
Sue Hartman (National Oceanography Centre, Southampton)
Toby Tyrrell (University of Southampton)

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