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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 first year were fourfold:
(1) she sampled an existing varved sediment core, a process that involved extracting and counting the benthic foraminifera at intervals that represent roughly two years of time. The counts currently span the interval AD1635-present.
(2) she prepared and analyzed approximately 60 samples for radiocarbon. The radiocarbon analyses were performed 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. Though there are a few gaps owing to the availability of sufficient sample mass, the time series of benthic foraminifera now extends from the present through the early 19th century.
(3) she measured the stable isotopic composition of benthic foraminiferal samples, including taxa that dwell at the sediment-water interface as well as within the sediment pore water zone. As with the radiocarbon, this time series now extends through the early 19th century.
(4) she collected water samples from the deep Santa Barbara Basin for radiocarbon analysis, by "piggybacking" on an unrelated cruise to the region.
The results thus far show two main features of the record. First, there is a large trend towards increasing radiocarbon over the course of the 20th century, beginning in the early 1950's. Though a part of this increase may very well be the result of bomb radiocarbon infiltration (most likely delivered to the sea floor by raining organic particles), this bomb radiocarbon clearly cannot explain entirely the timing and magnitude of the benthic foraminiferal increase. Thus, the radiocarbon record joins other tracers in suggesting a secular change in conditions of the basin over the 20th century. Second, there are significant decadal-scale fluctuations in the radiocarbon content of the benthic foraminifera, even in the pre-bomb era. The correlation between these radiocarbon fluctuations and other indices of the large scale climate, such as the Pacific Decadal Oscillation, is striking. It suggests that the ventilation of the Santa Barbara Basin is connected to the water mass conversion processes of the North Pacific as a whole.
Work in the next year will include extension of radiocarbon time series (though the radiocarbon fluctuations captured in the sediment decay through time, the apparent decadal signal of more than 20 per mil is still strong enough to be analytically significant into the early portion of the last millennium), quantitative analysis of the results, and writing of at least two manuscripts for publication.
