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 d18O 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 D14C 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
Click on the video image to view the video from Inagaki et al., 2006.
The liquid CO2 and CO2-rich fluids are produced at subducting plate boundaries where carbonate carried down the subducting limb is de-carbonated. The CO2 is released at depth and then migrates upwards through the structural conduits of the volcanic center 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). In the modern ocean CO2 released from these hydrothermal systems forms a solid (hydrate) at the sediment/water interface where ocean temperatures are below 9oC and depths are below 600 meters. The hydrate forms a cap that restricts the flux of CO2 into the overlying sea water.
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.
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.
To read about this hypothesis and get a glimpse into our on-going research follow this link: pdf
Broecker, W., and Barker, S., 2007, A 190o/oo drop in atmosphere's D14C 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.