EAGER: The 170-excess of dissolved O2 in the deep sea: A possible tracer of Little Ice Age oceanography

DESCRIPTION: From previous work, it is known that oxygen isotope measurements of dissolved oxygen (O2) in seawater make it possible to distinguish between two types of oxygen, one derived from gas exchange with air O2 and the other produced by photosynthesis in the ocean. The excess of 17O in dissolved oxygen (del 17O) makes it possible to estimate the proportions of each type of oxygen in a parcel of seawater. And importantly, del 17O of dissolved O2 is a unique conservative tracer that is not affected by respiration and thus preserves the signature acquired at the upper water source regions of deep water masses. In general, del 17O is high in upper waters situated in the photic zone but that are not affected by gas exchange with the atmosphere. Such water is found in the seasonal thermocline all over the ocean.

With funding from this Early Concept Grant for Exploratory Research (EAGER), researchers at the Lamont Dougherty Earth Observatory of Columbia University will measure del 17O in seawater of the deep North Pacific and NE Atlantic in order to evaluate the extent of sea-ice cover in the North Atlantic during the Little Ice Age (LIA). They hypothesize that the del 17O of seawater under sea ice will also be high due to light penetration and absence of air-water gas exchange. This hypothesis can explain new observations showing high del 17O in Antarctic Bottom Water (AABW) that originate from regions where sea ice is abundant, but not in North Atlantic Deep Water (NADW) originating from sea-ice free surface water. A surprising observation is that NADW in the South Atlantic is high in del 17O, and it is suggested that the high values in the south are a remnant from the Little Ice Age when the North Atlantic had more extensive sea ice.

OUTCOMES: Two profiles from the North and South Atlantic Ocean have been completed.