Fossil forams provide surprising insight into ice age oceans

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.

Fossil foraminifera, a popular organism for radiometric dating studies to reconstruct past climates

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.

The rise and fall of human society with climate change

Scientists are becoming increasingly skilled at reconstructing past climates through use of a variety of “climate proxies”, including ice cores, lake sediments, fossils and tree pollen assemblages. Dendochronology, the analysis of tree rings, is now providing powerful evidence for the connection between human welfare and climate. Climate variations have influenced agricultural productivity, warfare and health of preindustrial peoples. A recent study in Science reports a high-resolution reconstruction of Central European summer precipitation and temperature for the past 2500 years, providing direct evidence that periods of social stability correspond with climactic stability, while periods of social upheaval, famine and even plague correspond with dramatic climate shifts and increased climactic variation.

To reconstruct a long-term climate record, researchers examined nearly 9000 pieces of wood from living and dead trees. Over 7000 were oak samples from France and Germany. Many of these were collected from historic buildings, or rivers and bogs that preserve ancient wood. To obtain the earliest dates possible, samples were obtained from archaeological sites. Researchers separately collected 1500 stone pine and larch wood samples from high altitudes in Austria.

To make a continuous record from the present to the past, dendochronologists first examine tree rings from live wood samples to provide a baseline for dating. From there they work back to older and older samples. The width of spacing between consecutive rings corresponds to the amount of growth experienced that year. In particularly bad years, a ring may be broken, fuzzy, or barely present. The isotopic composition of wood samples can also be taken and several important climate metrics can be extracted from it. O18, for example, is a heavy isotope of oxygen. In rainy years, paleoclimatologists expect a sample to be relatively enriched in O18. Taken together, these two sources of data provide strong evidence for both temperature and moisture conditions in a particular year.

To calibrate such a climate record, human records are extremely valuable. Weather records providing temperature and moisture data over the past 200 years were collected for Central Europe. These records allow scientists to precisely determine how weather affects ring growth in that year.

The result?  A continuous, 2500-year climate record that, when compared with archaeological and historical data, showed a stark pattern. Times of social stability and prosperity corresponded with warm, wet summers that led to high agricultural yields. Warm, stable climates coincide with the rise of the Roman Empire and peak years of medieval Europe. The opposite was also true. For example, a dramatic cold snap around ~1300 A.D. occurred directly prior to the famines and plague that spread across Europe half a century later. The decline of the Holy Roman Empire from AD ~250-600 coincides with a marked increase in climactic variability.

Though ancient peoples were clearly more susceptible to the affects of a bad harvest year than modern societies are, the powerful and fundamental connection between human welfare and climate still manifests itself today. Many scientists believe that the Holocene, the geological era of relatively mild, stable climates that led to the global dominance of the human species, is now ending. The Anthropocene, an era in which human actions are the primary climate driver, has just begun. We may do well to consider the effect of climate on human society throughout a climactically stable era when making decisions that will affect our climate in the future.

 

Buntgen et al. 2011. 2500 Years of European Climate Variability and Human Susceptibility. Science. In press.