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.
If the hypothesis is correct, then the deep North Pacific would contain dissolved O2 with high values of del 17O. Likewise, old deep water in the east basin of the North Atlantic would also have high del 17O. This project will determine whether or not that is indeed the case.
Broader Impacts: If the hypothesis above is proven, this work will open a way to gaining new insights on climatic history of the recent past. As such, it will benefit the broad oceanographic community by introducing a unique way to trace the history of water masses. Likewise, it will benefit researchers dealing with paleo-climatology by providing a new way to reconstruct conditions of the North Atlantic during the Little Ice Age
The Little Ice Age (LIA) is a period between the 14th and 19th centuries during which Europe and North America were subjected to colder climate than during the 20th and 21st centuries. Among the hardest hit areas during this period is Iceland. Sea ice, which today is far to the north of this island, came down around its coasts and in some years it was difficult to bring a ship ashore. Sea ice reflects sun light and thus prevents warming of seawater below and causes climate in general to cool. Thus, scientists that study LIA climate include sea ice in their calculations. However, other than the reports on sea ice along the coasts of Iceland, there is no other record on the extent of sea ice over broad regions of the North Atlantic. In our project we found evidence for more extensive sea ice in the North Atlantic several hundred years ago. Our evidence comes from studies of isotopes of oxygen gas which is dissolved in deep Atlantic seawater. Deep water in this ocean originates from shallow depth when the ocean surface between Norway and Greenland becomes very cold in winter and seawater sinks to the abyss. Water sinking to the deep sea carries with it dissolved oxygen from the ocean surface. From the ratios among the isotopes of this gas we can tell how much of the dissolved oxygen originates from the atmosphere and how much from photosynthesis by marine organisms. In winter there are high winds in the high latitude North Atlantic causing fast exchange of oxygen between air and sea and there is practically no photosynthesis. This is evident in deep water sinking to the abyss today. Our study in the sea-ice covered Arctic Ocean shows that the situation there is different. In summer, marine life blooms and much oxygen is produced by photosynthesis and stored as dissolved gas beneath the floating ice. Arctic sea-ice prevents air penetration to seawater below and thus surface water that sinks to the deep Arctic Ocean is different in the isotopic composition of its dissolved oxygen from dissolved oxygen in the N. Atlantic. We can, thus, tell apart deep water whose origin is from an ocean covered with sea ice from deep water that sank from an ice free ocean surface. The origin of most deep water in the Atlantic is from the north high latitude region, but not from the Arctic Ocean. From radiocarbon measurements, we know that close to the high latitude source in the north the deep seawater is young, but in the South Atlantic the deep seawater is older and it sank during the Little Ice Age. In our project we found that dissolved oxygen in deep seawater of the S. Atlantic is similar to that in the Arctic Ocean. This discovery leads us to conclude that during the LIA, sea ice in the N. Atlantic was much more extensive than today.
