Research Highlights

Ocean Acidification - A Pteropod's Story  

Since the industrial revolution, burning of fossil fuels has caused atmospheric CO2 concentrations to steadily increase. Two CO2 induced stressors currently challenge the Ocean's Surface Waters: Ocean Warming and Ocean Acidification. Both of these have especially strong effects in high-latitude surface waters, such as the Arctic Ocean, where the rate of warming is nearly double the global average. Furthermore, high CO2 solubility due to cold surface waters leads to an increased uptake of CO2, changing the carbonate chemistry of the ocean. The associated decrease in the oceans pH is is referred to as "Ocean Acidification". This can have severe consequences for marine organisms, especially for calcifiers, who produce chalky structures (e.g. corals in reefs). Pteropods are snails living in the open ocean and are also called "sea butterflies", since they have wings enabling them to "fly" through the oceans. They produce their shell out of a special form of

chalk, which is extremely susceptible to Ocean Acidification. As they form a major dietary component for zooplankton and higher predators (e.g. whales), they are key components of high-latitude ecosystems. The calcium carbonate ("chalk") produced by marine calcifiers is a source of information for paleoceanographical studies. The environmental conditions (temperature, salinity, ocean carbonate chemistry, etc.) under which the calcium carbonate has been formed are recorded in its geochemical composition (e.g. trace metal concentrations). This is especially interesting to researchers, since fossilized shells buried in the oceans sediments can then be used to extract information about past climates.

In this study Dr. Keul analyzes the geochemical signal of Ocean Acidification and Warming in the shells of Arctic pteropods during the last decade, a time period which has seen the most rapid and drastic change in Ocean Chemistry and Temperature in the Arctic (temperature has increased 0.8 °C). Pteropods were obtained from a 13-year sediment trap series located in an unique location in the Arctic, were both warmer, North-Atlantic waters as well as colder, Arctic waters are brought into. Two different species of pteropods, Limacina helicina (Arctic) and Limacina retroversa (North Atlantic) are present in these sediment traps. Dr. Keul is using a special setup, where a laser is used to drill small holes into the shells. The material extracted is then analysed for its geochemical composition, which will then tell us something about Ocean Acidification and Ocean Warming from a pteropod perspective.

SEM (scanning electron microscopy) images of two pteropod species from the sediment traps.

Mg/Ca SST Proxy Insights

(A, B) Mean annual SST (solid black line, 0 m) and monthly           maximum and minimum ranges (solid grey lines) for coretop           locations in the Atlantic transect. (A) Isotopic calcification           temperatures are shown as grey symbols and closely           correspond to mean annual SST. (B) Mg/Ca derived           temperatures are shown as grey symbols and overestimate           the SST in the subtropics. (C) “Excess Mg/Ca” (measured           Mg/Ca – expected Mg/Ca) is plotted as grey symbols, mean           annual salinity is shown as a solid black line. Excess Mg/Ca           and salinity are well correlated.

Recent DEES Ph.D. graduate Jenny Arbuszewski based her thesis work on a collection of sediment core-top samples from the Lamont-Doherty Core Repository. Using decades-old cores from a meridional transect in the tropical to subtropical Atlantic Ocean, Jenny and colleagues analyzed the Mg/Ca ratios and δ¹⁸O composition of the shells of the surface- dwelling planktonic foraminifer, Globigerinoides ruber (white). The Mg/Ca ratios of this species’ shells are often used as a proxy for sea surface temperatures. They discovered that the Mg/Ca ratios for these samples were significantly elevated above expected values in the saltiest regions of the study area. In a 2010 Earth and Planetary Science Letters paper, found here, Jenny and coauthors explain how this implies a significant, and previously unrecognized, influence of salinity on shell Mg/Ca content and provide a new calibration that takes into account this effect. Understanding this effect is important because it influences the accuracy of past temperature estimates.

 Jenny is now a NOAA Climate and Global Change Postdoctoral Fellow at the Woods Hole Oceanographic Institute and plans to continue to look into the influences of salinity on shell Mg/Ca as a part of her post-doctoral work, in addition to several other projects. You can read more about Jenny's work here or contact her directly at jarbuszewski@whoi.edu.