As the concentration of carbon dioxide (CO2) in the atmosphere continues to climb, a group of researchers at Lamont-Doherty Earth Observatory are investigating how this change will impact the Earth system, with a focus on oceans and marine life.
Atmospheric CO2 exchanges with carbon dissolved in the surface ocean, depending on the relative concentrations of each. This gas exchange is similar to the process that allows ‘bubblers’ in fish tanks exchange gases for fish to thrive. As CO2 enters the surface ocean, the acidity of surface seawater generally increases, a process commonly referred to as ocean acidification.
With support from World Surf League PURE, Lamont paleoceanographer Bärbel Hönisch and postdoctoral researcher Kelsey Dyez are studying the respective effects of ocean warming and acidification on plankton collected from a global suite of sediment cores. The time period of this study covers the last ice age (characterized by colder and more alkaline seawater) and into the current warm period (warmer and more acidic seawater).
Influence on Ocean Life
Scientists are interested in what influence warming temperatures and ocean acidification may have on marine life. To test these effects on marine communities, researchers have mapped the changing composition of species across the globe and in all types of ecosystems. A group of calcifying marine plankton called foraminifera has proven particularly useful for tracing ocean changes: The group as a whole is ubiquitous in the ocean, but individual species are sensitive to specific environmental conditions. The foraminifera community consists of up to 40 different species, and past changes have been catalogued from every ocean basin and for past time periods.
The foraminifer species on the left are typical of those found in cooler waters and include Globigerina bulloides, Neogloboquadrina pachyderma and Globorotalia inflata. The foraminifer species on the right are typical of warm waters and include Globigerinoides ruber and Trilobatus sacculifer. The foraminifera are positioned on the date of a dime for scale. Photo: Kelsey Dyez
An analogy for the terrestrial environment is as follows: If you found a cliff made out of old soil and a certain layer had bones from wolves, arctic hares, and reindeer, you would infer that the climate at that time was fairly cold, given their modern preferences. On the other hand, if you found the fossil remains of alligators, parrots, and tree frogs, you would infer a warmer climate.
To date, the primary interpretations of foraminifera assemblages have focused on their sensitivity to ocean temperature, which is closely related to past climate changes. From such data, scientists have learned a great deal about how and where in the ocean the impacts of past ice ages and interglacial warm periods were felt most keenly.
However, because surface ocean acidification and warming went hand in hand at the end of ice ages, part of the observed changes in foraminifera assemblage may have been caused by ocean acidification. To decipher whether and which of these two parameters affects foraminifera growth more than the other, Hönisch and Dyez aim to quantify the respective assemblage response to ocean warming and acidification. To do so, they leverage the vast database of foraminifera assemblages with geochemical reconstructions of past ocean temperature and seawater acidity from a wide range of locations in the ocean.
Since the start of the project, Dyez (with the help of Columbia University undergraduate student Hailey Riechelson) has selected, obtained, and washed samples from twelve sediment cores from tropical to polar latitudes, which contain material from the last glacial maximum (the most extreme portion of the last ice age) and from the more recent warm interval.
The samples are currently being analyzed for their boron isotopic composition (a seawater acidity indicator), their Mg/Ca ratio (a temperature indicator) and their radiocarbon and oxygen isotopic composition (as time markers). As the climate warmed from the last ice age to the present, the plankton’s geochemical composition also changed in response to environmental changes in the seawater.
Data collected to date indicate that while some of the core sites reflect both warming and acidification into our current warm period, some core sites located in upwelling areas — places where cold, nutrient-rich waters from the deep ocean rise to the surface — show little variation in acidity but large changes in temperature and foraminiferal species composition. If this trend is corroborated in further analyses, it would suggest that warming is more important for ecosystem changes than acidification, at least over the slow rate of acidification and warming from the last ice age to the preindustrial.
Carbon Pathways Through the Ocean
Ocean acidity can also be a signal of where carbon is entering and exiting the surface ocean. By estimating surface ocean acidity of past time periods, these geochemical data will also help the scientists map carbon dioxide as it enters and exits the ocean surface.
Today, the ocean is a net sink of carbon dioxide from the atmosphere; nearly one quarter of anthropogenic carbon emitted into the atmosphere has already been dissolved in the surface ocean. As surface ocean acidity is being mapped for the last ice age with geochemical data, scientists may discover more precisely how and where carbon was released from the ocean to the atmosphere at the end of the most recent ice age.
Kelsey Dyez, Columbia University News, 28 March 2017. Article.
