We are conducting research in collaboration with colleagues at the University of Hawaii, University of Colorado and the University of California, Santa Cruz, which will investigate the predictability of hydroclimate variability, particularly drought on decadal time scales.
The southwestern US experienced several decadal-length droughts during the 20th century. The recurrence of drought constitutes a major challenge to resource managers who must plan for adequate water resources for a growing population. But predicting when drought will occur and how long it will persist is not yet possible. Our goal is to develop methods that will lead to a better understanding of the causes of drought and extreme precipitation variability.
We use the stable isotope composition of precipitation (d18O an dD) that falls over the southwest to fingerprint sources of atmospheric moisture. The precipitation from storms that originate in the North Pacific has a d<sup>18</sup>O compoistion that is 4 to 8O/oo lower than the precipitation from subtropical storms. We assess how these sources of moisture change in response to varying atmospheric behavior and how contributions of moisture from these sources changes during periods of drought. This information is being used to evaluate climate model experiments that are designed to evaluate what factors may cause drought, including changing sea surface temperature changes that may accompany a warming climate.
Climate models used to assess how the climate system will respond to rising greenhouse gas concentrations simulate northward dispacement of winter season storm tracks. Such a shift is expected to reduce the number of North Pacific storms (with lower isotopic values) reaching California. Hence, we expect a warming climate to increase the average isotope ratio of precipitation over California because the proportion of tropical moisture will increase. We are establishing a transect of precipitation monitoring stations along the west coast of North America that will collect precipitation samples for isotope measurement. These samples will be used to test climate model simulations we use to investigate storm track behavior. Ultimately, we hope to improve the ability of models to simulate the climated behavior over western North America and in doing so, aid efforts to plan appropriately for the region's water needs.
During the last glacial termination the rise in atmospheric pCO2 was accompanied by a precipitous drop in surface ocean ∆14C that cannot be explained by changes in 14C production and therefore appears to require a flux of 14C-depleted carbon into the surface ocean. The magnitude of this Δ14C excursion is hard to reconcile with an ocean-only mechanism of CO2 regulation.
Figure above is taken from Broecker and Barker (2007) showing the large -190o/oo drop in Δ14C during the last deglaciation.
Recent discoveries have revealed large accumulations of liquid CO2 in the sediments that cover the margins of active submarine volcanic centers in the Pacific Ocean. One spectacular example of these accumulations was caught on film by a crew investigating an active site near the Okinawa Trough. Note the bubbles streaming from the seafloor. These are bubbles of liquid CO2
The liquid CO2 and CO2-rich fluids are produced at hydrothermal centers where CO2 released at depth migrates upwards through the structural conduits where it reacts with water. The figure above shows how liquid CO2-rich fluids are thought to be accumulating in the sediments that blanket the vents (Lupton et al, 2008)
Click on the video image from Inagaki et al., 2006 to see a how a reservoir of liquid CO2 is trapped within sediments beneath a cap of hydrate-CO2 located in the Back Arc Basin of the Okinawa Trough. When the hydrate cap is punctured the bouyant liquid CO2 is released to the overlying water column.
We are exploring the possibility that hydrothermal sources of CO2 contributed to glacial/interglacial CO2 variability and to the Δ14C variations during the last deglaciation.
The Stott and Timmermann hypothesis was motivated by the discovery of liquid CO2 reservoirs at intermediate water depth in the oceans and by the magnitude of the deglacial Δ14C anomalies that have been observed at intermediate water depth core sites in the Pacific
Figure above show the deglacial Δ14C anomalies at intermediate water depth sites in the eastern Pacific. Note that there appears to be late glacial excursion that preceded the deglacial excursions. The timing of the late glacial excursion corresponds closely with the timing of Heinrich Event 2 in the Atlantic. Similarly, the deglacial excursions coincided in time with Heinrich Event 1 and the Younger Dryas. The correspondence between the timing of Δ14C excursions with the Heinrich Events could imply that the trigger for the release of carbon was the temperature increase that accompanied reduced North Atlantic Deep Water (NADW) formation during Heinrich Events. The reduced NADW shifts heat to the southern hemisphere where it is entrained in intermediate waters that flow towards the equator in the Pacific.
We hypothesize that as the ocean cooled during glaciations CO2-hydrate stability expanded upward to shallower depths and over a broader region of the sea floor, reducing the flux of 14C-depleted CO2 into the ocean from sediment reservoirs that blanket active vents throughout the ocean. Conversely, as the oceans warmed during deglaciation, the CO2-hydrate stability horizon deepened and caused a transient release of stored 14C-depleted CO2 from the sediment reservoirs.
Using a transient glacial-interglacial simulation conducted with the earth system model of intermediate complexity LOVECLIM we estimate ~3oC temperature increase at intermediate water depths in the Pacific during the last deglaciation would have been large enough to lower the hydrate stability horizon by several hundred meters and significantly reduce the areal extent of sea floor where hydrate was stable. This hypothesis, while provocative, would explain why 14C ages of abyssal water masses were not anomalously old during the last glacial and why there was a large radiocarbon activity (Δ14C) anomaly during the last deglaciation at intermediate (500-700m) water depths. If correct, this hypothesis implies a total carbon flux from hydrothermal systems significantly larger than is currently estimated.
Figure above shows the phase stability for CO2 in the modern ocean. Today hydrate CO2 is stable at water depths below about ~700m and at temperatures below 9oC. During the last glacial maximum the stabiliy of hydrate would have been as much as 300m shallower and thereby greatly expanded throughout the shallow, intermediate depth ocean. The increased area of the ocean seafloor over which hydrate was stable would have provided a resevoir for the accumulation and storage of carbon. Upon deglacial warming, the hydrate stability horizon would have deepened and thereby released the more bouyant liquid CO2 from sediments and to the overlying water column.
Broecker, W., and Barker, S., 2007, A 190o/oo drop in atmosphere's Δ14C during the "Mystery Interval" (17.5 to 14.5 kyr): Earth and Planetary Science Letters, v. 256, no. 1-2, p. 90-99.
Inagaki, F., Kuypers, M. M. M., Tsunogai, U., Ishibashi, J.-i., Nakamura, K.-i., Treude, T., Ohkubo, S., Nakaseama, M., Gena, K., Chiba, H., Hirayama, H., Nunoura, T., Takai, K., JÃ¸rgensen, B. B., Horikoshi, K., and Boetius, A., 2006, Microbial community in a sediment-hosted CO2 lake of the southern Okinawa Trough hydrothermal system: Proceedings of the National Academy of Sciences, v. 103, no. 38, p. 14164-14169.
Lupton, J., Butterfield, D., Lilley, M., Evans, L., Nakamura, K.-i., Chadwick, W., Jr., Resing, J., Embley, R., Olson, E., Proskurowski, G., Baker, E., de Ronde, C., Roe, K., Greene, R., Lebon, G., and Young, C., 2006, Submarine venting of liquid carbon dioxide on a Mariana Arc volcano: Geochem. Geophys. Geosyst., v. 7.
Stott, L., and Timmermann, A. Hypothesized Link Between Glacial/Interglacial Atmospheric CO2 Cycles and Storage/Release of CO2-Rich Fluids From Deep-Sea Sediments. In: Abrupt Climate Change: Mechanisms, Patterns, and Impacts Geophysical Monograph Series 193
Copyright 2011 by the American Geophysical Union. 10.1029/2010GM001052