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. We had the opportunity to participate in a cruise to the Guaymas Basin investigating fixation rates in July-August of 2008. During the cruise, filtered particulate samples from either the surface or a subsurface diatom-rich layer were collected for measurement of diatom bound d15N at each of the 6 stations occupied. These samples are associated with in-situ measurements of N-fixation, organic biomarker and phytoplankton assemblage data. Our interpretations will benefit from a direct comparison to estimates of fixation. We will use these data in the broader context of a seasonal cycle. Sediment trap materials from the Guaymas will be used to make this comparison. The sediment work, including measurement of bulk sedimentary nitrogen and organic carbon concentrations, biogenic opal concentrations, bulk d15N, and diatom-bound d15N, is underway at present. Initially, a small subset of samples was used to develop a sampling strategy, to determine ideal sample size, and to adapt the diatom-bound sample preparation procedures in order to apply the established methodology to the organic-rich Guaymas Basin sediments. Adaptation of the existing cleaning methodology to the organic-rich sediments of the Guayamas Basin was a huge effort. Additional cleaning steps are required in order to remove all external organic N from the diatom frustules prior to measurement of the diatom-bound N. Preliminary Findings We have generated multi-proxy data from several of the sediment blocks at centimeter and laminae-scale intervals. Inter-annual variability in organic carbon and biogenic opal contents overlays the seasonal patterns exhibited by the varve pairs. Biogenic opal and organic carbon concentrations tend to be negatively related. Opal-rich layers are more organic matter poor than the more clay-rich sediments. There is no clear relation between the abundance of either biogenic component and bulk sediment d15N at centimeter scale sampling. However, on the laminae-scale, bulk sediment d15N is elevated in the opal-poor layers and relatively low in the diatom-dominated layers. We are just beginning to generate diatom-bound d15N data. Our preliminary data shows that the diatom fraction is enriched in d15N relative to the bulk sediment, as expected from other settings and recent culture work. One key result that is emerging is that a systematic offset occurs between the diatom-bound d15N of the >63µm and the <63µm size fractions. The larger diatoms record a d15N that is on average 4ä lower than their smaller counterparts. This pattern of lower d15N in the larger fractions fits with the generalization that larger diatoms tend to be associated with the "Fall dump". The fall dump is a large depositional event caused by the break down of summer stratification. The larger diatoms may be recording a d15N value associated with nitrogen fixation that occurs during the summer months when vertical nutrient supply is limited. We will be pursuing this by combining the size fractionated isotope measurements with diatom identifications in order to better understand the origins of the low d15N signature. This is the Master of Science research of URI graduate student Julie Fliegler.
