In the North Atlantic, ocean water circulation patterns have far-reaching effects on global climate. Convective mixing is a dominant process due to thermal stratification of the water column. At low latitudes, warm, low-density surface waters float over a mass of much colder, high-density subsurface water. As warm surface water travels north, the temperature difference between surface and subsurface is diminished. Nutrient-depleted surface water cools and sinks, forcing deep water to rise. As deep water rises to the ocean surface, it brings a fresh pulse of nutrients that causes enhanced ocean productivity near the poles.
The formation of North Atlantic deepwater, or NADW, and the continual circulation of warm, subtropical water, play an important role in moderating Arctic climates. In colder intervals of Earth’s history such as the Last Glacial Maximum (LGM) 20,000 years ago, diminished thermal stratification reduces open ocean convection. Less surface water is transported poleward, and the water that is does not have the same warming effect on the local atmosphere and land surfaces.
This much about the interaction between North Atlantic circulation and climate is well understood. However, the timing of changes in NADW circulation and corresponding changes in climate remains something of a mystery. Scientists essentially face a chicken and egg problem- do climate changes shut down this oceanic conveyor belt, or does the shutdown of the conveyor belt occur first, by some other means entirely, but cause subsequent feedbacks on climate?
Currently, the climate change-induced NADW breakdown theory is popular and has been used to explain a number of abrupt climate reversals. The most prominent example is the Younger Dryas (YD), a brief cold-snap that occurred some 12 millions years ago following the end of the LGM and the retreat of continental glaciers. Proponents of this theory argue that glacial melting caused huge pulses of low-density freshwater into the north Atlantic, in precisely the region where vertical stratification is weak today and convective mixing occurs. This influx of low-density water effectively shut down NADW formation, leading to a rapid cold reversal and a brief but dramatic rebound of continental glaciers.
A recent study using carbon isotopes found in fossil foraminifera, or forams, to date ocean water columns suggests otherwise. 14C is a heavy isotope of carbon that is produced in the upper atmosphere due to cosmic ray activity, and enters the surface ocean as a dissolved gas. It is a popular isotope for radiometric dating, as it decays to 12C over a known period of time. The quantity of 14C remaining in a sample can thus be used to determine the sample’s age. A decreased 14C/12Cratio indicates an older sample. Indeed, numerous studies suggest that 14C depleted water is associated with decreases in convective mixing.
But how does one find 10,000 year old water to date and study in the first place? Scientists can’t simply put a bucket into the ocean and pull up 20,000 year old water to- they need a fossil or preserved object from the time period of interest. Some planktonic organisms such as forams leave behind a calcareous exoskeleton when they die. If buried quickly, these can be preserved for thousands or millions of years. While many planktonic organisms preferentially take up 12C over 14C, skewing the natural ratio of the two isotopes in their body tissue, forams do not significantly alter the natural 14C abundance. Examining fossil forams buried in ocean sediments thus provides a window into the past, allowing an accurate date to be ascribed to the ocean that the tiny creature existed in.
What are fossil forams from the North Atlantic telling us about ice age oceans? Proponents of the glacial melt water-induced NADW shutdown theory, and fans of “The Day After Tomorrow”, will no doubt be surprised by the finding that deepwater from the YD era actually dates to 600 years prior to the cold reversal. The shutdown of the oceanic conveyor belt prior to global cooling suggests that an unknown mechanism may in fact be driving ocean circulation, and in doing so exerting a powerful control on global climate.
Thornalley et al. 20110. The Deglacial Evolution of North Atlantic Deep Convection. Science 331: 202-205.