Eighth Year Report 2009
Carbon Science: Paleoclimate
The Sarmiento and Sigman groups continue to develop new models and tools for studying ancient changes in carbon fluxes and climate.
Long-Term Variations in Oceanic and Atmospheric Radiocarbon
It has become increasingly clear over the last decade that the Southern Ocean plays a critical role in the global carbon cycle, and that it may be expected to play a key role in determining carbonclimate feedbacks under climate change. Recent modeling has suggested a direct mechanistic link between changes in the strength of the winds over the Southern Ocean and perturbations to the exchange of CO2 between the oceanic and atmospheric reservoirs. Given this potentially strong link, and the lack of knowledge about how Southern Ocean winds have changed in the past, the Sarmiento Group has been motivated to test the idea that past changes in atmospheric Δ14C (inferred from paleo-proxies) can reveal past changes in Southern Ocean winds.
Paleo-proxy records reveal large Δ14C variations for both the atmosphere and the ocean on centennial to millennial timescales. One of the most pronounced examples is the onset phase of the Younger Dryas, when atmospheric Δ14C rose by 70 per mil in only 200 years. This change coincided with a large and rapid decrease in the Δ14C of intermediate waters of the North Pacific. Another example is the most recent deglaciation transition between full glacial and interglacial states of the climate system. During this time atmospheric Δ14C decreased by more than 200 per mil over 3000 years (this interval has been referred to by Broecker and others as the "Mystery Interval", with the mystery being a specific reference to radiocarbon). For both atmospheric and marine reservoirs of CO2, paleo-proxies indicate that many of the significant variations and transients in atmospheric CO2 are mirrored in atmospheric Δ14C on centennial to millennial timescales over the last 50,000 years.
These past changes are important as they correspond to changes in the state of the climate system during times when mechanistic understanding of climate change is at best incomplete. In order to address this problem, Rodgers, Mikaloff-Fletcher, and Slater used an ocean model to test the idea that variations in the strength of the Southern Ocean winds provides a means to explain both the changes in Δ14C of the atmosphere and the ocean on centennial to millennial timescales. Previous modeling efforts have sought to explain such changes through perturbations of the meridional overturning circulation (MOC) of the North Atlantic, but in models MOC perturbations have thus far proven unable to account for the amplitude of the changes in Δ14C revealed in the paleo-proxy records.
The model experiments conducted reveal that only relatively modest (20%) changes in the Southern Ocean winds are able to produce changes in both atmospheric Δ14C and ocean interior Δ14C that are consistent with the amplitude of the changes found in the paleo-proxy records. The results obtained thus far provide a "test-of-concept" for a Southern Ocean mechanism that serves to motivate further work with more sophisticated models. Future work will also focus on relating these changes to changes in CO2.
Studying Glacial-Interglacial Changes and the Southern Ocean
Daniel Sigman and his collaborators continue to pursue evidence for reduced vertical exchange (i.e. "stratification") in the halocline-bearing polar ocean regions under colder climates of the past 3 million years. A major motivation for this focus is that the reconstructed polar ocean changes have the capacity to lower atmospheric CO2 during ice ages by reducing the natural leak of biologically sequestered CO2 out of the ocean and into the atmosphere.
The work also bears on the expected response of the different polar ocean regions to anthropogenic climate change, with implications for regional weather, fisheries, and the oceanic sink of anthropogenic carbon. Global models of anthropogenic warming have traditionally indicated that polar oceans would become more stratified with warming, whereas the changes reconstructed by Sigman and his colleagues from paleoclimate data show increased stratification in cooler climates. However, this picture of disagreement is starting to change because of recent realizations regarding the response of the Southern Ocean to 20th century wind shifts, which appear to agree with the sensitivities inferred from the paleoclimate data.
In 2008, former Princeton postdoc Rebecca Robinson published the largest set to date of diatom microfossil nitrogen (N) isotope records from the Southern Ocean over the last ice age/interglacial transition, which are intended to reconstruct surface nutrient conditions in the past. These records suggest a remarkable scenario for ice age conditions in the Antarctic Zone of the Southern Ocean, the domain closest to the Antarctic continent. While the entire modern Antarctic surface ocean layer is nutrient-rich, there is a weak decrease in nutrient concentrations toward Antarctica. The downcore N isotope data suggest that this southward decrease became much stronger during ice ages, leading to nutrient-deplete conditions close to the continent (Figure 20). The decrease supports the existence of stronger stratification in the South that limited upward transport of nutrients from deep water, which would also prevent CO2-rich deepwater from contact with the atmosphere. A reasonable modern analogue is the Arctic Ocean, although the cause for this nutrient-deplete, highly stratified condition in the Arctic is different.
If this hypothesis bears up under testing with further modern and ice age data for the Antarctic, it will do much to explain the decline in atmospheric carbon dioxide during ice ages.
Figure 20. Modern and ice age modes of ocean circulation in the Southern Ocean
Sigman's group has recently begun to consider physical mechanisms for the apparent climate/polar stratification link described above. Former postdoc Agatha de Boer adapted the current GFDL ocean model (MOM4) for paleoclimate-scale simulations and used this platform to investigate the sensitivities of deep ocean ventilation to fundamental climate parameters, including the mean temperatures of the ocean and atmosphere and the strength of the southern hemisphere westerly winds. In a paper published in 2008, de Boer considered together her model experiments on three important forcings of polar ocean overturning: mean ocean temperature, the atmosphere's hydrological cycle, and the westerly winds. This leads to the proposal of two end-members of ocean overturning behavior, "thermal" and "haline" limits, in which temperature and salinity, respectively, are the dominant controls on the distribution of deep water formation among the major polar ocean regions (Figure 21).
Figure 21. Climate sensitivities of ocean overturning, showing "thermal" and "haline" states.
At the "thermal" limit, convection is shared among the North Atlantic, North Pacific, and Southern Ocean. At the haline limit, convection is restricted to the North Atlantic. The modern ocean appears to lie between these end-members, with similar rates of overturning in the North Atlantic and Southern Ocean, but little formation of interior water in the North Pacific. The effect of a more vigorous hydrological cycle is to produce stronger salinity gradients, favoring the haline state of North Atlantic dominance in deep water formation. In contrast, a higher mean ocean temperature will increase the importance of temperature gradients because the thermal expansion coefficient is higher in a warm ocean, tilting the system toward the thermal limit of globally distributed polar ocean overturning. An increase in westerly winds tends to weaken the salinity gradients more strongly than it does the temperature gradients, also pushing the ocean toward the thermal state of globally distributed overturning. This perspective unifies the behavior from otherwise disconnected model experiments, providing a useful conceptual framework for more specific, hypothesis-driven studies of the climate sensitivities of ocean overturning. From this perspective, the models of global warming predicting increased stratification with anthropogenic warming are likely driven by the hydrological cycle; the paleoclimate data suggest that the models may be overly sensitive to it, relative to the wind and ocean temperature forcings. Alternatively, the models' increased stratification with anthropogenic warming may be a transient phenomenon that is replaced by decreased stratification once the mean ocean temperature comes into equilibrium with the atmosphere.
Stability of the Ocean Nutrient Reservoir
Nitrogen (N) is a limiting algal nutrient in the low latitude ocean, and an increase in the oceanic N inventory has been seen as the leading alternative explanation to polar ocean changes for the lower atmospheric CO2 content observed during ice ages. Sediment records have shown clearly that that rate of N loss form the ocean was lower during ice ages, and it has also been argued that the input of N from N fixation was greater at those times, raising the possibility of a large increase in N inventory and global ocean productivity during ice ages.
Over the last two years, graduate student Haojia Ren in Sigman's group has been developing a new method for reconstructing past nutrient conditions in the low latitude ocean, using the N isotopes of organic matter trapped within calcium carbonate microfossils. Her first application of the method is to the question of N fixation (i.e. N input) changes over glacial cycles [Ren et al., in press]. She has found that N fixation in the Atlantic was sharply reduced during ice ages. The N fixation decrease was most likely a response to the known ice age reduction in ocean N loss, due to a long-posed but weakly substantiated feedback involving the phosphorus demands of N fixing organisms. This feedback would have worked to balance the ocean N budget and to curb ice ageto- interglacial change in the N inventory. While this avenue of research has just opened, the results to date indicate that natural and human pressures to a change in the N inventory will be countered by this negative feedback, which will act to stabilize global ocean fertility.
Mechanisms of Younger Dryas Cooling in Western Europe
Sigman's group is also working to improve understanding of the mechanisms by which past ocean changes have interacted with conditions on land. As part of a long term collaboration with close colleague Gerald Haug, Professor in the Geological Institute at ETH Zurich, sediment from a German crater lake was used in a high resolution study of western European climate at the onset of the Younger Dryas cold period ~12,700 years ago. The Younger Dryas cold period was likely fundamentally driven by a sharp reduction in North Atlantic overturning circulation. However, previous climate model studies have suggested that the overturning circulation alone could not fully explain the degradation in European climate.
Data from the lake indicate an abrupt increase in winter storminess at the onset of the cold period, occurring from one year to the next at 12,679 yr BP, providing one of the best dated records of this climate transition. The storminess change is best explained by an abrupt change in the North Atlantic westerlies towards a stronger and more zonal jet due to a cooling of the subpolar North Atlantic. The reconstructed wind shift may explain the temporal coincidence of the overturning decrease with European cooling, providing an important part of the missing link between ocean and land.