Underhill, Vivian RÌý1
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With rising global temperatures, a major concern is that fresh water from melting glaciers and the Greenland ice sheet will slow down global oceanic circulation patterns, in particular the North Atlantic Current (NAC). The NAC, an extension of the Gulf Stream, carries warm, saline Atlantic waters north to mix with cold, fresh Arctic water. It then cools and sinks, forming the North Atlantic Deep Waters and the deep limb of thermohaline circulation (Broecker et al, 1985).
I investigated a marine sediment core collected off the southwest coast of Iceland (see Fig. 1) to reconstruct conditions during the mid-Holocene (4,000-8,000 years ago). This epoch is important because it is the only time when the North Atlantic oceanic circulation was not being influenced by freshwater melting from nearby ice sheets, which inhibits deep water formation (Broecker et al, 1985). This allows me to research the natural variability of NAC water-mass characteristics, such as temperature and salinity, in the absence of addition of external freshwater. Knowing more about North Atlantic circulation without additional fresh water will help predict the effects of increased freshwater forcing.
To do this, I am using the oxygen isotopic composition and magnesium/calcium ratios in the calcium carbonate shells of a benthic foraminiferal species, Cassidulina laevigata, a single-celled organism that lives in the upper few centimeters of ocean sediments. When precipitating their shells, foraminifera preferentially take up the isotope Oxygen-16, but can also use the heavier Oxygen-18. The ratio of Oxygen-16 and Oxygen-18 incorporated into the shells is correlated with both water temperature and salinity (Katz et al, 2010). The Mg/Ca ratio is a function only of temperature, with a higher ratio correlated with higher temperatures. I use this ratio, a relatively new development in the field (Katz et al, 2010), to separate temperature and salinity components of the δ18O signal and obtain far more accurate results than were possible in the past. (See Fig. 2). Preliminary results of these analyses will be ready to be presented by mid-March.
Broecker, W.S., Peteet, D.M., Rind, D, 1985, Does the ocean-atmosphere system have more than one stable mode of operation?: Nature, v. 315, p. 21-26.
Jennings, A.E., Hald, M., Smith, M., & Andrews, J.T, 2006, Freshwater forcing from the Greenland ice sheet during the Younger Dryas: evidence from Southeastern Greenland shelf cores: Quaternary Science Review, v. 25, p. 282-298.
Katz, M.E., Cramer, B.S., Franzese, A., Honisch, B., Miller, K.G., Rosenthal, Y., Wright, J.D, 2010, Traditional and Emerging Geochemical Proxies in Foraminifera: Journal of Foraminiferal Research, v. 40, p. 165-192.