Reports: G2
46695-G2 Examining the Biogeochemistry Behind the Organic-rich Sediments in Guaymas Basin, Gulf of California
Synoposis of Activities
The principle goal of the project is to evaluate the potential for using diatom-bound N isotope measurements (as d15N) to record relative variations in rates of N-fixation in the Guaymas Basin over time. There are two avenues to our approach, 1). Examine the seasonality of diatom-bound and bulk d15N in the modern Guaymas Basin using sediment trap materials and water column samples; and 2). Sample downcore materials at extremely high resolution in order to resolve seasonal-scale cycles in the diatom and bulk sedimentary d15N.
Initially, we invested a significant amount of time in adapting existing cleaning methodology to the organic-rich sediments of the Guayamas Basin. This work proved essential. We learned a good deal about the effects of each chemical treatment and we became adept at modifying the procedure to accommodate the various sample types that we measured. The methodology that will be published at a result of this work will be an improvement over the existing protocol.
Previous work suggests that bulk sedimentary d15N largely reflects changes in the isotopic composition of sub-euphotic zone nitrate. Here we found that diatom-bound d15N from a less than 63 µm size fraction represents annual nutrient utilization. Diatom bound d15N from the greater than 63 µm size fraction, provides new evidence for the importance summer diatoms that are exported once stratification breaks down, fall dump species, in nitrogen dynamics.
Six sections of sediment core, dating between ~10 to 8 kya, each block spanning ~30-40 years, were sampled on cm and mm scale resolution. All samples were analyzed for %wt opal, %Corg, bulk sedimentary d15N and diatom bound d15N from two size fractions, greater and less than 63μm. Modern water column diatom bound and particulate matter d15N samples collected within the Gulf of California and in adjacent waters were measured for direct comparison to the sediments.
The two sediment sampling scales showed slight differences, with the mm scale showing more seasonal like variance, while the coarser resolution variance was similar to a longer time scale productivity signal. With proper technique, it would be possible to reconstruct seasonal variations down core. The bulk sedimentary d15N ranged between 10-11.5 and <63µm the diatom bound size fraction ranged between 7-12. In all but one of the sediment sections, the d15N of the > 63µm size fraction was significantly lower than the other two fractions, ranging between 1-7. Negative correlations between %Opal and %Corg and <63μm diatom bound d15N showed low frequency (decadal) variations in productivity and related changes in nitrate utilization. Water column d15N data and sedimentary diatom bound d15N from the large size fraction, which contained diatoms associated with the fall dump, suggest that their low d15N values are contributed by the fall dump species. This is tentatively hypothesized to reflect an ecological preference for living at depth, in a low light and more nitrate-replete horizon of the water column. In addition, the larger size fraction appears to contribute significantly to the bulk sedimentary d15N, suggesting that nutrient sources other than seasonal upwelling may be contributing to the high rates of export production in the Gulf of California.
This work is the basis for Ms. Julie Fliegler's Masters of Science thesis, which was completed in September of 2009. At present, we are finalizing our interpretations and preparing a manuscript for publication. Any additional measurements will be made as needed. In addition, the results will be presented at the American Geophysical Unions Fall meeting in December of 2009.
