Around 19,000 years ago, oceanic conditions underwent dramatic changes that coincided with a shift in global climate, marking the onset of the Holocene warming. In the North Atlantic, major changes in the Meridional Overturning Circulation (MOC), which carries warm and highly saline surface water north to cooler regions, played a substantial role in regulating climate and levels of atmospheric carbon dioxide. Scientists are now convinced that the ocean absorbed, stored, and released vast quantities of carbon in the past, playing a major role in the end of the last Pleistocene Ice Age glacial period. Understanding the ocean's role in the past is important to understanding how it may influence climate in the future. A new report in Science shows that the MOC experienced a series of abrupt changes that lasted from decades to centuries, and may have stored and released more CO₂ than previously thought.

It is beginning to look like the Atlantic ocean's overturning current flow has had a very active and varied history during the last deglaciation, 18,000-10,000 years ago. Two articles in the January 14, 2011, issue of the journal Science explain the latest discoveries about the part ocean circulation in the Atlantic has played in climate change. Reporting in a perspective article, “Northern Meltwater Pulses, CO₂, and Changes in Atlantic Convection,” Professor Michael Sarnthein, a researcher in Paleoclimatology and Paleoceanography at the University of Kiel, reviews “impressive and detailed evidence of how the North Atlantic MOC behaved after the Last Glacial Maximum (LGM).” According to Sarnthein, some old oceanographic dogma needs to be changed:

Today, intensive convection in the North Atlantic quickly transfers surface waters to depth, and newly formed deep water typically has a ventilation age of ∼500 years. In contrast, Thornalley et al. report that during the Heinrich stadial 1 (HS1), a cold interval that occurred between about 17,500 and 14,700 years ago, a mass of extremely old water, up to 5200 years old, reached the Atlantic Ocean's northern “dead end.” They suggest this was intermediate water that originated in the Southern Ocean near Antarctica, and its age would once have been considered unreasonable. In addition, they found that the ventilation ages of past surface waters could vary by as much as 2100 years, which agrees with results of other methods and challenges a common, but little-substantiated dogma: that past surface waters had ventilation ages of ∼400 years, similar to a current age average.

As regular readers of this blog know, this is but the latest chapter in science's changing view of the MOC (see “Conveyor Belt Model Broken” and “Ocean Conveyor Belt Dismissed”). According to Sarnthein, such old ages and long cycling times are important because they suggest the ocean absorbed, stored, and released vast quantities of carbon in the past. Furthermore, researchers can use this new information to estimate MOC and CO2 storage patterns during the LGM and HS1 (see the figure below). This new work suggest that deep waters at those times were, on average, ∼1000 to ∼2000 years older than they are today, suggesting that the ocean absorbed and stored a massive amount of atmospheric CO₂ during the LGM and early HS-1. During those periods, deepwater CO₂ concentrations in about half of the ocean's volume were markedly higher than today's levels.

Average apparent ¹⁴C ventilation ages and possible short-term circulation changes of ocean deep and intermediate waters during the Bølling interstadial, Heinrich stadial 1 (HS1), and the Last Glacial Maximum (LGM). Ages in blue are as low as or lower than current ages, indicating good ocean ventilation. Ages in orange are higher than current ages, and represent CO₂-enriched waters. Numbers in green are ¹⁴C ages of topmost intermediate waters outside the Nordic Sea, and numbers in black are water depths (rounded). Source M. Sarnthein/Science.

The basis for these conclusions is a research paper by David J. R. Thornalley, Stephen Barker, Wallace S. Broecker, Henry Elderfield and I. Nick McCave entitled “The Deglacial Evolution of North Atlantic Deep Convection.” In it, Thoranalley et al. describe using water-column radiocarbon reconstructions to examine changes in northeast Atlantic convection since the Last Glacial Maximum. The authors explain the importance of their work:

Open-ocean deep convection in the North Atlantic occurs in the Labrador and Greenland Seas, transforming well-ventilated, nutrient-poor surface waters into North Atlantic Deep Water (NADW), which spreads southward to occupy much of the deep Atlantic. Paleoceanographic studies suggest that deep convection in the North Atlantic was altered during the last ice age as compared with today. Glacial convection was shallower, forming Glacial North Atlantic Intermediate Water (GNAIW), and possibly weaker, leading to poorer ventilation of the deep Atlantic. Rapid fluctuations between weak and strong modes of deep convection could also be linked with abrupt climate changes across the North Atlantic region because of the associated changes in the poleward flux of warm surface waters. Furthermore, mode switches in deep convection might be triggered by changes in the input of fresh water, with the Younger Dryas (YD) cold reversal being the archetypal example. Yet there is growing evidence that the YD may be part of an intrinsic oscillation associated with deglaciation, rather than peculiar to the last termination. To test these hypotheses, we require precise constraints on the timing (and rate) of deep convection changes relative to abrupt climate events and freshwater perturbations.

The main tool used for this study was analysis of ¹⁴C/ ¹²C ratios found in various proxy sources—planktonic and benthic foraminifera. Cosmogenic radiocarbon (¹⁴C) is produced in the upper atmosphere when cosmic rays interact with nitrogen. The ¹⁴C is taken up by the ocean through air-sea gas exchange with the levels seeking equilibrium. Over time the radioactive decay of ¹⁴C into ¹²C alters the ratio between the isotopes and allows determination of the apparent time elapsed since a water mass was last in equilibrium with the atmosphere. This is called the ¹⁴C ventilation age.

Through painstaking and detailed analysis, combining many data sources, Thornalley al. have built up a new and more complete picture of ocean ventilation during the end of the last glacial period and the onset of the Holocene warming. Deglacial intermediate/deep (I/D) ventilation ages show similar trends throughout the deglaciation, alternating rapidly (within ~100 to 200 years) between close-to-modern values (500 years or less) and extremely old ages (3000 to 5200 years). This implies abrupt changes in deep convection during this period.

“In general, young I/D ventilation ages are seen during warm intervals [e.g., the Bølling-Allerød (B-A) and early Holocene] and reflect the rapid transfer of equilibrated surface waters to depth, similar to today’s ocean,” the authors state. “However, the occurrence of extremely ¹⁴C-depleted waters between 1.2 and 2.3 km during cold intervals [e.g., Heinrich stadial 1 (HS1), the Intra-Allerød Cold Period (IACP), and YD] demands not only a shoaling of convection but also the incursion of a very depleted water mass that must already have been in existence.”

This implies that during cold intervals of the deglacial period, the influence of ¹⁴C-depleted Antarctic Intermediate Water (AAIW), which formed along the northern margin of the Southern Ocean, reached as far north as ~60°N in the North Atlantic, much farther than today. Today, the influence of AAIW reaches 20 to 30°N in the Atlantic Ocean. The complex and previously undocumented interaction of ocean circulation patterns with deep reservoirs of Antarctic and Arctic water hints that the ocean holds other mysteries within its depths. The figure below provides a summary view of the studies ventilation age findings.

Cartoon contour plots for the ventilation age.

One of the things bandied about by global warming alarmists is the possibility of a shutdown of the MOC, bringing a sudden cold spell to the Norther Hemisphere similar to the Younger Dryas. It has been argued by some that the Dryas shutdown was a onetime event triggered by the release of a huge volume of fresh water into the arctic ocean as a result of a glacial dam break. Thornalley et al. suggest that the timing of these events may be a bit different than conventional wisdom proposes:

A long-standing view of Northern Hemisphere deglaciation has been that the strong resumption of deep convection in the North Atlantic during the B-A was interrupted by a freshwater rerouting or flood event, which reduced convection and triggered the YD cold reversal. In contrast to this paradigm, our I/D ocean ventilation results show that after the vigorous convection of the early B-A, there was a shutdown in open-ocean deep convection in the high-latitude NE Atlantic for a sustained interval beginning ~600 years before the onset of the YD, that is, during the IACP. The occurrence of an earlier shutdown in deep convection in the NE Atlantic before the YD suggests that in terms of NE Atlantic convection, the YD was not a unique event during the deglaciation. Instead, the amplified circulation associated with the Bølling warming was probably a transient feature of deglaciation, and the stability of deep convection in the NE Atlantic gradually weakened throughout the B-A, with intervals of vigorous deep convection being punctuated by several freshwater events. This interpretation is consistent with the recent recognition of “YD equivalents” during earlier terminations of the Late Pleistocene.

This conclusion does not dismiss a freshwater release as the proximate cause for the Younger Dryas, but, rather, suggests that such events were frequent during the deglaciation. They served as aberrations causing rapid, dramatic climate changes on top of a general warming trend reflected by stability in the Atlantic circulation. Even though large variations are detected in the present day MOC, it still looks like a sudden, Dryas like cold spell requires a massive infusion of glacial melt water that could not occur today.

The significance of this work should not be underestimated. “Thornalley et al.'s findings represent real progress,” concludes Sarnthein. “For instance, their extremely old ventilation ages for intermediate and deep waters—together with other high ages—may help explain what happened during the "mystery" interval, a period of rapid decrease in atmospheric ¹⁴C that occurred 17,500 to 14,500 years ago.” While the exact mechanisms that caused the last deglaciation remain unknown, this works may help scientists to narrow down the possibilities.

Atmosphere and ocean exchange CO₂ more rapidly than thought.

Additionally, it was found that the ocean holds deep reservoirs of ancient water that, under certain conditions, can shift north out of the Antarctic during cold periods. This echos findings of a Southern Ocean source for the deglacial radiocarbon-depleted CO₂ detected in the eastern North Pacific (see “Southern Ocean source of ¹⁴C-depleted carbon in the North Pacific Ocean during the last deglaciation”).

In the Atlantic, this can impact currents and convection by interacting with the flow of North Atlantic Deep Water (NADW), which spreads southward to occupy much of the deep Atlantic. The resulting rapid fluctuations between weak and strong modes of deep convection causes changes in the poleward flow of warm surface waters, and that can impact sea-ice cover in the Nordic Seas.

“We have suggested that differences in the timing of changes in open-ocean convection and sea-ice coverage between the NE and NW Atlantic may be an important control on atmospheric circulation,” the authors conclude, “therefore, further investigation into the nature of the atmospheric reorganizations associated with this heterogeneity is warranted.”

It seems that when it comes to controlling Earth's climate, the ocean plays a very deep game indeed. Thornalley et al.'s work also shows that the world's oceans can release or absorb atmospheric CO₂ much more rapidly that previously thought. It is the shifting of ocean currents, both on the surface and deep below, that cause Earth's temperature to fluctuate and weather patterns to change. Atmospheric CO₂ doesn't control climate, the ocean controls both climate and the amount of CO₂ in the air.

Be safe, enjoy the interglacial and stay skeptical.