Santa Barbara Sediment Trap Time-Series Program
- Claudia Benitez-Nelson
University of South Carolina
- Robert Thunell
University of South Carolina
Since August 1993, a moored sediment trap has been located near the center of the Santa Barbara Basin (SBB) (34˚14’ N, 120˚02’W). Over the course of the time series, the deep trap was deployed between 500 m and 540 m in a total water depth of approximately 590 m. A second shallow trap was added in 2009 and is located at ~ 250 m depth. Sinking particles have been continuously collected by an automated Mark VI sediment trap (0.5 m2 trap opening) equipped with 13 sampling cups poisoned with sodium azide on a rotating carousel. Each trap sample represents approximately two-weeks of collection time. Occasional disruptions in the time series data set are typically due to trap clogging associated with periods of high mass flux or due to loss of the sediment trap.
Trap samples have been used for a number of investigations ranging from foraminifera proxy development for paleo-reconstructions of ocean acidity to understanding the composition, magnitude and efficiency of downward particle flux. Archived sediment trap samples (both wet and dry) are available by request.
Summary to DateSince 1993, the SBB has been the site of a deep-moored sediment trap (500 m) that has continuously collected the flux of sinking particles. In September of 2009, a second trap was added at a shallower depth of 150 - 300 m (recently deepened to reduce clogging). On a seasonal basis, increases in diatom, and to a lesser extent, haptophyte abundance in surface waters during upwelling (Anderson et al., 2008) lead to increases in the export of opal, carbonate, particulate organic carbon (POC), organic phosphorus (POP) and nitrogen (PN) to depth (Thunell et al., 2007; Sekula-Wood et al., 2012). During the nonupwelling period, terrigenous fluxes dominate, mainly as a result of increased continental runoff from higher rates of precipitation (Thunell et al., 1995, 1998). Closer inspection of the relationships between POC, PN, and POP, with opal and carbonate suggest that C and nutrient fluxes to depth are controlled by both mineral phases that act as ballast, with carbonate having a carrying capacity almost twice that of opal (Thunell et al., 2007; Sekula-Wood et al. 2012).
Over longer timescales, previous work has linked variations in surface water properties and underlying sediments to large scale climate modes such as the El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), the Northern Oscillation (NO) and the North Pacific Gyre Oscillation (NPGO). Such trends are difficult to observe given the myriad processes that influence the physics and biogeochemistry within SBB. For example, direct long-term correlations (with and without lags) between water column upwelling and stratification with ENSO and PDO are poor, even though the NO and PDO correlate with upwelling-favorable winds in the region (Garcia-Reyes and Largier, 2010). As a result of these combined physical processes, direct relationships between bulk particle fluxes and climate indices are poor, despite some limited studies that have directly linked assemblages of both diatoms and planktonic foraminifera to specific El Niño events (Black et al, 2001; De Bernardi et al., 2005; Vennrick et al. 2008).
Nonetheless, there is strong general evidence that elemental fluxes and their relative composition have changed in the SBB over the past two decades (Bograd et al. 2014). While POC and carbonate fluxes have remained relatively constant, opal fluxes have increased significantly over the past 10 years (P < 0.01 for the linear trend) in concert with increasing molar Si/N ratios (P < 0.01), and as mentioned previously, with increasing Pseudo-nitzschia cell abundance and DA fluxes. These results, combined with an increase in the molar C/N ratio and δ 15N of sinking particles (P < 0.005), suggest that observations of changing plankton speciation and increasing nutrient utilization and diatom fluxes in overlying waters, are reflected in the composition of sinking organic matter. In other words, increasing nutrients in surface waters have resulted in larger scale diatom blooms that are subsequently captured within the traps. At the same time, this change in plankton assemblage has resulted in what we hypothesize as more efficient nutrient utilization, which has increased the δ15N of particles while also allowing the proliferation of species, such as Pseudo-nitzschia, that thrive in low Si/N environments. These results also suggest, however, that increasing opal fluxes do not result in a concomitant increase in sinking particulate organic carbon. Thus, over longer timescales, even though primary production and nutrient concentrations in surface waters have increased in the SBB, the relative efficiency of organic matter export has actually decreased, supporting the importance of carbonate as a ballast material for the transport of POC to depth (Thunell et al., 2007). Although still speculative, these results also influence our understanding of how increasing biological production in surface waters feeds into deep water oxygen concentrations.
Planktonic foraminifera collected as a part of the trapping program have also been used in various paleo proxy development projects. δ18O and Mg/Ca ratios, which can be used to reconstruct past temperature, have been measured in trap samples to develop species-specific calibration relationships between temperature (CTD casts PnB) and foram geochemistry (i.e., Bemis et al., 2002; Pak et al., 2004). In response to recent studies have indicated that trace amounts of boron are incorporated in to foraminiferal shells as a function of pH, the most recent SBB recoveries have been used to evaluate the relationship between B/Ca ratios and seawater chemistry. Flux studies have also been done using SBB trap material to identify the species flux of the dominant foraminifera and other plankton to identify changes in the seasonal assemblage in response to upwelling (Kincaid et al., 2000).
DiscussionWe are in the initial stages of formulating a cooperative research program with Dr. Debora Iglesias-Rodriguez (UCSB) and Dr. Nina Bednarsek (NOAA PMEL) to study the impact of ocean acidification on plankton calcification in Santa Barbara Basin during the last 100 years. This study will utilize the 20 year time series of sediment trap samples from the basin and will examine all of the major carbonate producing groups of plankton: coccolithophores (Iglesias-Rodriguez), pteropods (Bednarsek) and foraminifera (Osborne and Thunell). Specifically, this study aims to evaluate the influence of declining seawater pH as a result of anthropogenic carbon emissions on the calcification efficiency of these three plankton groups. The shell wall thickness of planktonic foraminifera has previously been linked to the ambient carbonate ion concentration of seawater, which is directly linked to seawater pH. By evaluating the change in shell thickness of planktonic foraminifera over the >20 year time-series we hope to develop a proxy to estimate carbonate ion concentration over the last several centuries to constrain the human influence on the carbonate chemistry of seawater.
Selected Publications resulting from this work: Bemis, B., Spero, H. and Thunell, R., 2002. Using species-specific paleotemperature equations with foraminifer: a case study in the Southern California Bight. Marine Micropaleontology 46: 405-430. Kincaid, E., et al., 2000. Planktonic foraminifera fluxes in the Santa Barbara Basin: response to seasonal and interannual hydrographic changes. Deep-Sea Research II, 47: 1157-1176. Pak, D., et al., 2004. Seasonal and interannual variation in Santa Barbara Basin water temperatures observed in sediment trap foraminiferal Mg/Ca. Geochemistry, Geophysics, Geosystems 5. Sekula-Wood, E. and others 2009. Rapid downward transport of the neurtoxin domoic acid in coastal waters. Nature Geoscience 2(4): 342-350. Sekula-Wood, E. Benitez-Nelson, C., Morton, S., Anderson, C.R., Burrell, C., Thunell, R.C., 2011. Pseudo-nitzschia and domoic acid fluxes in Santa Barbara Basin (CA) from 1993 to 2008. Harmful Algae, 10, 567-575. Sekula-Wood, E. et al., 2012. Magnitude and composition of sinking particulate phosphorus fluxes in Santa Barbara Basin, California. Global Biogeoch. Cycles, 26, GB2023,doi:10.1029/2011GB004180 Thunell, R., Tappa, E. and Anderson, D., 1995. Sediment fluxes and varve formation in Santa Barbara Basin, offshore California, Geology 23, 1083-1086. Thunell, R., 1998. Particle fluxes in a coastal upwelling zone: sediment trap results from Santa Barbara Basin, California, Deep-Sea Research 45, 1863-1884.
- Other nutrients
Study MethodsAll methods are available by request. For routine measurements, see:
Thunell, R., 1998. Particle fluxes in a coastal upwelling zone: sediment trap results from Santa Barbara Basin, California, Deep-Sea Research 45, 1863-1884.
Immediately after collection, sediment trap samples are sent back to the laboratory at the University of South Carolina, split with a rotary splitter and a fraction stored in a refrigerated container. Samples are routinely analyzed for particulate organic carbon, nitrogen, total and inorganic phosphorus, carbonate, opal, and lithogenic material. Additional samples have been analyzed for a variety of other components, including domoic acid concentrations and Pseudo-nitzschia abundance and speciation, and paleoceanographic proxies including stable isotopes, Mg/Ca and B/Ca ratios, etc.