College of Oceanic and Atmospheric Sciences, Oregon State University Corvallis, OR 97331
The map in Figure 1 identifies four sites in the Arabian Sea where sediment trap moorings (see X symbols) were deployed as part of the JGOFS program. The following poster presents results from analysis of long-chain alkenones in a six month time series of sediment trap materials collected 2229 m below the ocean surface at the most offshore mooring site, AST-4. AST-4 is located 560 km off the coast of Oman (15°59'N 61°30'E). Total water depth at this open ocean site is 3974 m.
Long chain alkenones are biosynthesized by only a few species within the algal Class Prymnesiophyceae. The most notable representative of this biomarker in the ocean is the cosmopolitan coccolithorphid Emiliania huxleyi. Since their biological discovery, alkenones in marine particulate samples have been analyzed for absolute concentration, relative composition and individual d13C composition in order to gain insight into questions about primary productivity, surface water temperature and pCO2 in the atmosphere, respectively. In this poster, discussion of alkenones focuses on the primary productivity application and plays upon an early finding that these compounds comprise a large and relatively constant percentage of total cellular organic carbon in E. huxleyi (i.e., 5-10%: Prahl et al., 1988).
Alkenone flux at AST-4 was measured with ~8.5 day resolution over the 170 day time interval from November 11, 1994 to April 30, 1995. A time series plot of results from these twenty-one biomarker analyses displays two conspicuous features: a minor flux maximum in late December and a major flux maximum in March (Figure 2). Water temperatures measured continuously at five depths (1.5 m, 20 m, 40 m, 90 m, 125 m) in the upper ocean at a nearby physical oceanographic mooring (see * in Figure 1) are also plotted on Figure 2. Comparison of the biomarker record with these water temperature records shows the minor and major peaks in alkenone flux correspond in time, respectively, with the period of rapid breakdown and buildup of thermal stratification in surface waters of this open ocean region of the Arabian Sea.
The length of time between the two alkenone flux events corresponds to the period of the NE monsoon (Figure 2). During this meteorological event, the mixing of surface water was very deep (³90 m) and coldest water temperatures were evident at the sea-surface (~25°C). The apparent temporal correspondence of the alkenone flux events with the beginning and end of the NE monsoon suggests a causal linkage. This feature of primary productivity is not unique to the Arabian Sea, however. It was also apparent in our earlier study of alkenones in sediment trap times series for the NE Pacific (Prahl et al., 1993). Throughout much of the world ocean, enhanced alkenone flux and, therefore, enhanced production of prymnesiophytes such as E. huxleyi may typify the pattern for primary productivity during periods of seasonal transition.
The map in Figure 3 identifies three sites (Nearshore, Midway, Gyre) in the NE Pacific where samples from a one-year time series of sediment traps deployed at 1000 m below the ocean surface were analyzed for alkenones. Nearshore, Midway and Gyre are located 120, 270 and 620 km off the coast of northern California, respectively. The map also identifies the location of a NOAA weather buoy (#46002) where a continuous record for sea-surface temperature was obtained concurrently with the sediment trap time series.
Alkenone flux displayed a distinct maximum in late spring at all three sites in the NE Pacific (Figure 4). Comparison with the time series record for sea-surface temperature in this region shows the alkenone flux event occurred during the spring transition when surface waters start to thermally stratify after a long period of deep winter mixing. The magnitude of the alkenone flux event increased 2 to 3-fold with distance offshore and curiously, at the most oceanic site in the NE Pacific study area, Gyre, attained a value (~0.7 mg/cm2-yr) nearly the same as that observed at AST-4 in the Arabian Sea during an analogous period of thermal stratification (0.9 mg/cm2-yr, see Figure 2) .
Alkenone flux was measurable throughout the AST-4 time series. Although alkenone flux was reasonably well-correlated with TOC flux (r2 = 0.89, n = 21), alkenone concentrations (mg/gOC) enriched up to 6-fold during the periods of peak flux (Figure 5). A similar feature was previously documented in the NE Pacific (Prahl et al., 1993). Alkenone enrichment relative to TOC indicates prymnesiophyte productivity is preferentially stimulated by the physical processes acting to breakdown and, in particular, buildup stratification in these ocean settings. Given alkenones comprise 5-10% of cellular organic carbon in E. huxleyi (Prahl et al., 1988) and several additional simplifying assumptions, it can be shown that prymnesiophyte production accounts for a small but nonetheless, significant percentage of the TOC flux (i.e. £3%) during these peak events.
In contrast, alkenone flux correlated poorly with CaCO3 flux (Figure 6) regardless whether the two highest biomarker data points were included (r2 = 0.20, n = 21) or excluded (r2 = 0.26, n = 19) from consideration. Unless this poor correlation results from preferential degradation / dissolution during the sedimentation process, this observation has two possible biological implications:
1) the export of CaCO3 from surface waters at this site in the Arabian Sea is either not primarily derived from primary producers, or
2) alkenone-biosynthesizing prymnesiophytes like E. huxleyi are not the dominant coccolithophorids in this open ocean environment.
Research Plans for the Immediate Future
- Extend alkenone times series at AST-4 for one complete year to determine how the perceived prymnesiophyte productivity pattern relates to climatic forcing of the Arabian Sea by both the NE and SW monsoons.
- Analyze other biomarkers in the same AST-4 sample set and compare the perceived prymnesiophyte productivity pattern with that for other algal components of the phytoplankton community.
- Examine the same biomarker information in the sediment trap time series for the other three Arabian Sea sites (AST-1, -2, -3) to determine the extent of spatial variability in the biomarker-perceived algal productivity patterns and the relationship of such variability to monsoonal forcing conditions in this region.
References
Prahl F.G., Muehlhausen L.A. and Zahnle D.L. 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochim. Cosmochim. Acta 52, 2303-2310.
Prahl F.G., Collier R.W., Dymond J., Lyle M. and Sparrow M.A. 1993. A biomarker perspective on prymnesiophyte productivity in the northeast Pacific Ocean. Deep-Sea Res. Part I 40, 2061-2076.
Acknowledgements. We thank Patricia Collier for initial processing of sediment trap samples, Roberta Conard for all bulk chemical analyses, Margaret Sparrow for alkenone analyses and Dr. Robert Weller (WHOI) for access to water temperature data from the Arabian Sea JGOFS mooring.