Over the past two centuries, atmospheric carbon dioxide (CO₂) has increased from ~ 280 ppm to over 400 ppm due to global industrialization, with current concentrations significantly higher than those measured over the past 800,000 years. Furthermore, the rate of increase is an order of magnitude larger than that which has occurred on Earth over the past 100 million years. The magnitude of carbon released into the atmosphere fundamentally impacts the biogeochemistry of marine systems. Not all of the carbon produced from fossil fuels and cement burning ends up in the atmosphere.  Nearly 30% of anthropogenically produced CO₂ is stored in the global ocean. When CO₂ is added to seawater, it is hydrolyzed, thereby releasing hydrogen ions, and decreasing seawater pH, a process known as ocean acidification. Surface ocean pH is estimated to have declined by ~ 0.1 pH units since the preindustrial times, equivalent to a 30% increase in hydrogen ion concentration. As oceans acidify, they reduce the ability of marine organisms, such as coral reefs and planktonic foraminifera, to use calcium carbonate minerals in their skeleton and shells, thus fundamentally impacting marine biodiversity.

The primary goal of this project was to quantify the effect of ocean acidification on calcification rates in planktonic foraminifera since the onset of the Industrial Revolution in the Santa Barbara Basin region, located in the California Current System. Coastal ecosystems like the highly productive waters off of coastal California are particularly vulnerable to ocean acidification due to the combination of atmospheric uptake of CO₂ and seasonal induced upwelling of cold and lower pH deep waters.  We focused our attention on foraminifera that sink to the seafloor, collected using a moored sediment trap in the center of the Basin that has continuously collected samples over the past two decades. Planktonic foraminifera are ubiquitous zooplankton that are responsible for up to 60% of the total calcium carbonate produced in the oceans. Their calcite shells are often well-preserved in marine sediments. As such, they provide an ideal system investigating the influence of global change on marine zooplankton and their skeletons may be used to examine past climate and ecological change. Results were combined with foraminifera shells preserved in age dated Basin sediments to generate a nearly annually resolved record of changes in foraminiferal calcification over the last 300 years.  In addition, our work provided supporting samples and data for a number of related investigations centralized around understanding how environmental signatures are recorded in foraminiferal shells and the source and composition of material buried in Santa Barbara sediments and other regions over long timescales. 

Our research occurred in three phases. First, water column chemistry data and foraminifera collected concurrently in sediment traps were used to calibrate and constrain relationships between foraminiferal shell morphology, shell geochemistry, and water column pH and carbonate chemistry. Second, we compared results over the entire 20 year sediment trap time series with the same time period recorded in the sediment record in order to determine how well foraminiferal skeleton signals are preserved in the sediment record. We then used our results to produce two independent records of changing seawater pH and carbonate chemistry over the last three centuries using the age-dated sediment record.

Our results indicate that there has been a 20% reduction in calcification over the last century, which is equivalent to a 0.22 unit decline in pH (8.22-8.00). This exceeds the estimated global average decline of 0.1 by more than a factor of two. In addition, we observed considerable decadal variability in pH that is significantly correlated with regional scale climate modes, namely the Pacific Decadal Oscillation (PDO). This relationship, until now, has been obscured by the relatively short observational record of most measurements. The PDO modulation of the marine pH/carbonate system suggests that climatic variations will play an important role in amplifying or lessening the anthropogenic signal and progression of OA of ocean acidification in this region.

This funding has supported the research of one female postdoctoral fellow, the Ph.D. and M.S. theses of three female graduate students, and the Honor’s Thesis of one female undergraduate.  Eleven publications have resulted from our efforts as well as numerous conference presentations and seminars. Multiple undergraduates were trained in geochemical analyses and they, along with high school teachers and their students, have had the opportunity to participate in our research cruises off the California Coast. 

In summary, this research has provided fundamental information on the impact of future increases in ocean acidification, flood events on carbon storage, the expansion of oxygen minimum zones, and the life cycle of globally important plankton species. This new knowledge is critical for predicting future changes in climate on our ocean and vice versa.

Last Modified: 11/02/2020
Modified by: Claudia R Benitez-Nelson