45571-AC2
Ventilation of the Santa Barbara Basin and The North Pacific Thermocline Over the Past Two Millennia
The main goal of this project is to generate a detailed and continuous record of the radiocarbon content of benthic foraminifera over the last few hundred years in the varved sediments of the Santa Barbara Basin. The rationale is that radiocarbon represents a highly discriminatory tracer of changes in water mass characteristics, specifically the ventilation of this silled basin. The radiocarbon content of the bottom water can be reconstructed faithfully through decades to millennia, because benthic foraminifera effectively lock in the C-14 content of ambient bottom water and because the annual laminations, from which the foraminifera are derived, provide an independent clock. Once completed, this time series of deep radiocarbon can be compared to other evidence for change in the basin, such as the benthic assemblage variability or nutrient flushing. Because radiocarbon is primarily a reflection of physical processes, this comparison will provide important constraints on the driving forces for oceanographic change in the region.
This project has supported entirely the work of graduate student Lydia Roach. Her activities during this second year were:
(1) She organized a cruise to the Santa Barbara Basin (funding for ship time provided by University of California ship funds). On this cruise she collected a new suite of varved sediment cores and water samples from in and around the basin.
(2) She processed the new sediment cores and sampled the upper 20 years, isolating carbonate material (including benthic foraminifera) for further analysis.
(3) She prepared and analyzed seven benthic foraminiferal samples for radiocarbon measurement. Radiocarbon analysis was preformed at accelerator facility of Lawrence Livermore National Laboratory. Tom Guilderson, director of the Center for Accelerator Mass Spectrometry, served as her mentor for this work, and he guided her through all phases of the measurement process. These measurements extended the existing radiocarbon time series through 2007.
(4) She prepared and analyzed five water samples for radiocarbon measurement. Carbon dioxide extraction was carried out in the lab of Ralph Keeling under the supervision of lab personal, and radiocarbon analysis was preformed as described above
(5) She investigated the potential physical driving mechanisms of the observed benthic radiocarbon variability in the Santa Barbara Basin through simple models and comparisons with instrumental and reconstructed climate records.
Radiocarbon measurements from the new sediment cores effectively extend the existing record to the modern day, thus increasing the overlap with the instrumental climate database. Overlapping portions of the new samples with existing measurements match up satisfactorily, although a slight consistent enrichment of the new samples relative to those previously measured may reflect a minor source of modern carbon contamination. Nevertheless, the value of bottom water C-14 is, within analytical error, identical to that of the uppermost benthic foraminifera, indicating that these organisms do faithfully capture bottom water radiocarbon. Water sample measurements have also allowed for greater constraint in determining influence of oxidation of photosynthate organic material. Using water sample and benthic foraminiferal radiocarbon as input values, simple modeling of sedimentary diagenetic processes indicate that this oxidation has a negligible effect on the radiocarbon record prior to the presence of “bomb carbon” in the environment, and cannot entirely explain variability observed following the incursion of the bomb transient to the surface ocean. In addition to the correlations observed between the radiocarbon record and a reconstruction of the Pacific Decadal Oscillation, further correlations have been observed with large scale sea surface temperature and sea level pressure variability captured by instrumental measurements. These striking relationships suggest ventilation of the Santa Barbara Basin is connected to the water mass conversion processes of the North Pacific as a whole.
For the remainder of this project (extended at no-cost), student Roach and P.I. Charles will concentrate on finalizing manuscripts for publication.
