The Gulf of St. Lawrence is a large, partially enclosed sea that connects the St. Lawrence Estuary to the North Atlantic Ocean and is home to a significant portion of Canada’s fisheries. Less dense, warmer waters sit near the surface, with denser, cooler bottom-waters intruding from the North Atlantic, leading to a permanently stratified water column. This stratification allows for metabolic carbon dioxide (a by-product of the microbial degradation of organic matter) to accumulate in these bottom-waters, causing them to become progressively more acidic. Consequently, the bottom-waters in the Gulf of St. Lawrence have reached pH levels expected for the global surface ocean by the end of the century, providing a natural analogue for studying future effects of ocean acidification. Understanding how ocean and coastal acidification impacts the preservation of carbonate minerals found below these bottom waters provides important clues that not only indicate how organisms with calcium carbonate skeletons may be impacted, but also how the buffering capacity provided these sediments will continue to change over time.
William Nesbitt, Scientific Coordinator for the TReX Deep Experiment (CERC.OCEAN Lab at Dalhousie University), and lead-author of a new study examining potential dissolution of carbonate sediments in the Gulf of St. Lawrence, was finishing his undergraduate degree in Geological Sciences at Queen’s University when he had the opportunity to attend an oceanographic field course in Bermuda. It was while snorkeling amongst the unique cup reefs and underwater cities of corals that he became inspired to pursue research in carbonate chemistry and oceanography. William completed an M.Sc. in oceanography at McGill where he examined how oceanographic conditions and acidification in the Gulf of St. Lawrence have influenced carbonate minerals in the underlying sediments. William gives us some perspectives on the results of his research, published in the Canadian Journal of Earth Sciences in January.
“I am an aqueous geochemist and oceanographer with research interests in carbon cycling and the carbonate-carbonic acid system. I conducted my M.Sc. research under the supervision of Dr. Alfonso Mucci at McGill University where I investigated the effects of bottom water acidification in the Gulf of St. Lawrence on the preservation of detrital carbonate minerals in the seafloor sediments,” Mr. Nesbitt says.
“The coastal ocean is the most productive marine environment on the planet, making it at particular risk of eutrophication from the increased flux of land-derived nutrients and the resulting increase of organic matter generation.”
Eutrophication presents numerous consequences to marine ecosystems, such as the depletion of dissolved oxygen (and the creation of oxygen minimum zones) and acidification driven by the accumulation of metabolic carbon dioxide, which can exacerbate ocean acidification in the coastal ocean.
“When [eutrophication] occurs in a stratified water column, such is the case in the Gulf of St. Lawrence, the increased load of organic matter can lead to the depletion of dissolved oxygen and the accumulation of metabolic carbon dioxide. The generation of the latter makes coastal environments particularly vulnerable to acidification, as the isolated bottom waters are pre-acidified from when they were previously in contact with the atmosphere. Therefore, adding metabolic carbon dioxide to these bottom waters drives the effects of ocean acidification beyond that of the anthropogenic atmospheric contribution. The preservation of carbonate minerals is greatly impacted by the acidification of seawater as it drives their dissolution.”
Mr. Nesbitt says that his research highlights the connections between biological and chemical processes when it comes to ocean acidification and the ocean’s ability to neutralize absorbed anthropogenic carbon dioxide. It is well established that shelled organisms that build their skeletons from calcium carbonate are vulnerable to ocean acidification, but what happens if these materials dissolve more quickly?
“Beyond the ecological importance of carbonate minerals as the material many marine invertebrates (bivalves, corals etc.) precipitate to construct their exoskeleton, their dissolution provides a critical short-term buffering method and sink of anthropogenic carbon dioxide in the ocean.”
“Beyond the ecological importance of carbonate minerals as the material many marine invertebrates (bivalves, corals etc.) precipitate to construct their exoskeleton, their dissolution provides a critical short-term buffering method and sink of anthropogenic carbon dioxide in the ocean.”
In other words, as acidification continues over time, carbonates will experience more dissolution, meaning that there will be less material left to buffer against carbon dioxide driven changes in pH.
“The combination of carbonate minerals being biologically important as well as acting as a sort of anti-acid tablet for the ocean is what got me interested in studying the carbonate system in an acidifying ocean,” Mr. Nesbitt says.
But, there have been few studies which have documented acidification-driven carbonate dissolution in natural settings, or over longer modern time-series.
“Several previous studies had explored carbonate precipitation/dissolution and calculated rates in controlled, lab-based environments but very few had quantified them in a natural setting,” Mr. Nesbitt explains. “The pH of the bottom waters of the Gulf of St. Lawrence have decreased by 0.3-0.4 units, which is proportionate to the variation expected for the global surface ocean by the end of the century. This provided us a perfect natural laboratory to investigate the effects of these conditions on the preservation of carbonate minerals.”
Mr. Nesbitt says that the main questions they wanted to address were:
Can we detect evidence of diagenetic dissolution of carbonate minerals in the Gulf of St. Lawrence sediments?
If so, can we quantify the magnitude of dissolution and calculate dissolution rates?
Have carbonate dissolution rates changed temporally?
To answer these questions, Mr. Nesbitt and his co-author, Dr. Alfonso Mucci, explored samples dating back several years.
“We derived evidence of diagenetic dissolution from the analyses of pore water chemistry and solid sediments sampled from three box cores recovered over a 13-year time frame (2003, 2013, 2016) at a specific station (Station 18) to the south of Anticosti Island in the Gulf of St. Lawrence. Pore-water pH and alkalinity measurements (along with phosphate and silica) were used to calculate saturation states with respect to calcite (one of the main polymorphs of carbonate). Pore-water calcium concentrations were measured to compliment saturation state calculations,” Mr. Nesbitt writes.
“This showed that when undersaturation occurred within the pore-waters, a significant increase in calcium was observed, suggesting that active dissolution is occurring.”
Other measurements, including manganese and iron concentrations of the pore-waters, also provided information on the depth of oxygen in the column, or box cores, indicating the extent of vertical dissolution. Inorganic carbon content was also measured in each sediment core to quantify, through an integration method, the amount of carbonate that had already dissolved and to calculate dissolution rates.
There were three key outcomes of this research:
Carbonates within the sediments at the bottom of the Gulf of St. Lawrence are actively dissolving. “Pore-water chemistry provides strong evidence of active calcium carbonate dissolution in the sediments of the Gulf of St. Lawrence,” Mr. Nesbitt explains. Just below the sediment water interface (the top of the seafloor), they observed an increase in calcium concentration in conjunction with decreasing pH and carbonate saturation state, suggesting active dissolution is occurring.
The carbonate deficit in solid sediment content within the first few centimeters of the core relative to constant concentrations at depth compellingly supports the pore-water data. Furthermore, the calculated magnitude of carbonate deficit can be paired with sedimentation rates to quantify the rate of dissolution.
The rate at which carbonates are dissolving in the sediments of the Gulf of St. Lawrence is increasing over time. “Calculated dissolution rates in each core shows that dissolution has significantly accelerated in the sediments of the Gulf of St. Lawrence between 2003 and 2016,” Mr. Nesbitt indicates.
The results of this study are critical to understanding how such an economically and ecologically important Canadian water body, the Gulf of St. Lawrence, will be impacted as ocean acidification progresses.
“Often when we hear about the impacts of ocean acidification in the media it is focused on tropical reef environments. Our study provides an example of a Canadian environment that has become progressively more acidic on a decade time scale and quantifies the impacts of this change on a crucial mineral to the marine system.”
Mr. Nesbitt says that one of the biggest things he learned from conducting this research was the interconnectedness biological, physical, and chemical properties of the ocean. Changes to one part of the system can have huge impacts elsewhere in the system.
“We see the ocean as a vastly large system, but it’s very sensitive,” he points out.
He says that his work has made him realise the significant impact humans have had on our oceans, and that our connections and reliance on the ocean means that all of us will be affected by the consequences of these anthropogenic changes. The timeline of the study (13 years) demonstrates the drastic changes he observed have happened in his lifetime. He suggests that this rapid rate of change means that mitigation and adaptation will be important priorities moving forward.
Mr. Nesbitt is motivated to continue research in oceanography. He says that the project not only allowed him to pursue his interests in chemical oceanography, but also allowed him to develop a new skillset and geochemical tools that can be applied to other projects.
“Following my MSc, I worked as a research assistant in the Department of Biology at McGill on the IDRC-NutriFish project, exploring micronutrient utilization in East African fisheries. Now I am the scientific coordinator for MEOPAR’s Tracer Release Deep Experiment (TReX) where I oversee the coordination of logistics and planning of the deep release experiment.”
Mr. Nesbitt plans to start a Ph.D. program next year, where he hopes to continue to conduct research on the oceanic carbon cycle and forecasting the impacts of ocean acidification.
Read Nesbitt and Mucci, 2021 here (journal access required):
Or
Request a full text on Research Gate here.
Citation: Nesbitt, W. A. and A. Mucci. 2021. Direct evidence of sediment carbonate dissolution in response to bottom-water acidification in the Gulf of St. Lawrence, Canada. Canadian Journal of Earth Sciences 58:84–92.
Acknowledgments:
Thanks to William Nesbitt for his virtual “in-person” interview and written responses.
To learn more about William and his research, please visit his Research Gate profile.