Particle fluxes in a coastal upwelling zone: sediment trap results from Santa Barbara Basin, California

Deep-Sea Research II 45 (1998) 1863—1884

Particle fluxes in a coastal upwelling zone: sediment trap results from Santa Barbara Basin, California Robert C. Thunell* Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA Received 7 May 1997; received in revised form 1 December 1997; accepted 27 February 1998

Abstract Particle fluxes were measured in the center of Santa Barbara Basin on a bi-weekly basis for a three-year period beginning in August 1993. Lithogenic material dominates the total flux throughout the entire year, although it is delivered to the basin primarily during the winter rainy period. It appears that both biological and physical processes control the eventual transport of detrital material to the deep part of the basin. Biogenic sedimentation in Santa Barbara Basin is dominated by opaline silica, with highest fluxes during the spring—summer upwelling period. Export ratios (ratios of organic carbon flux to primary production) are inversely related to primary production. This relationship may be due to increased advective transport and/or enhanced regeneration of organic carbon during the highly productive upwelling period. During the first half of the study period, Santa Barbara Basin was influenced by El Nin o conditions and our data suggest that productivity in this region is reduced during such periods. Seasonal changes in the relative contributions of biogenic and lithogenic material to the total particle flux, combined with the lack of bioturbation on the sea floor, results in the accumulation of varved sediments in Santa Barbara Basin.  1998 Elsevier Science Ltd. All rights reserved.

1. Introduction and background The coastal ocean plays an important role in global biogeochemical cycles and processes occurring on the margins are thought to have a significant impact on the interior of the ocean (Walsh et al., 1985; Rowe et al., 1986; Falkowski et al., 1988; Jahnke et al., 1990; Reimers et al., 1992). According to Walsh (1991), continental

*Corresponding author. Fax: 001 803 777 6610; e-mail: [email protected]. 0967-0645/98/$ — see front matter  1998 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 8 ) 0 0 0 5 5 - 1

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margins may be responsible for cycling as much carbon and nitrogen as the open ocean. While sediment trapping has proven to be a useful tool for monitoring seasonal to interannual variability in the flux of both biogenic and lithogenic material in various open ocean settings (Deuser et al., 1981, 1983, 1995; Honjo, 1984; Honjo et al., 1995; Dymond and Collier, 1988; Nair et al., 1989; Fischer et al., 1996; Karl et al., 1996), relatively few studies have attempted to document temporal changes in particle fluxes on continental margins (Biscaye et al., 1988, 1994; Thunell et al., 1994; Pilskaln et. al., 1996). In the open ocean, the delivery of particles (both biogenic and lithogenic) to the seafloor is controlled primarily by vertical transport processes linked to the annual cycle of primary production (Deuser et al., 1981, 1983). The situation is more complicated along continental margins. First, advective processes can result in significant lateral transport of material across the margin (Biscaye et al., 1988; Pilskaln et al., 1996). Second, in addition to seasonal variability in the production and flux of biogenic material, continental margins are strongly impacted by seasonal changes in winds and precipitation-run off which control the input of detrital material to the system (Soutar and Crill, 1977). In this paper we report on a three-year time series (August 1993—September 1996) of particle flux measurements from Santa Barbara Basin, CA. There have been several previous sediment trap studies in Santa Barbara Basin, but all have been based on single collection periods and did not evaluate temporal variability in sediment fluxes (Soutar et al., 1977; Dunbar and Berger, 1981; Dymond et al., 1981). This region was selected for study for several reasons. First, it is an area of upwelling, and several studies (Jahnke et al., 1990; Walsh, 1991) have suggested that a significant fraction of the global carbon flux occurs within eastern boundary upwelling systems. Second, the study area is marked by strong seasonal and interannual changes in climatic and hydrographic conditions, and this allows us to evaluate the effect of physical forcing on particle fluxes. Finally, it has long been recognized that the varved sediments accumulating in Santa Barbara Basin serve as an excellent repository for high resolution records of past climate change (Emery, 1960; Hulsemann and Emery, 1961). The recent recovery of a long sediment record extending back through the penultimate glaciation (&160,000 yr) has generated a great deal of interest in the climate record preserved in Santa Barbara Basin (Kennett et al., 1995). However, to fully exploit the seasonal resolution of these laminated sediments it is necessary to know how and during what season the individual layers are deposited.

2. Study site 2.1. Depositional setting Santa Barbara Basin is the northernmost basin of the Southern California Borderlands (Fig. 1). It is bounded to the north by the California coastline and to the south by the Channel Islands. To the east, the Anacapa Sill (&200 m deep) separates Santa Barbara from Santa Monica Basin. A 475 m deep sill on the western edge of the basin separates it from the open ocean. These sills restrict the flow of subsurface waters into

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Fig. 1. Bathymetric map (in meters) of the Santa Barbara Basin and the location of the sediment trap mooring (triangle).

the basin and control its ventilation. In particular, dysaerobic to anoxic conditions exist within the basin at depths below the deepest sill. It has been reported that flushing of the deep basin (maximum depth(600 m) occurs periodically (Sholkovitz and Gieskes, 1971; Reimers et al., 1990), but high oxygen demand associated with organic matter regeneration quickly depletes the bottom waters of oxygen. As a result of these low oxygen conditions, the benthic fauna is scarce and there is little bioturbation. Under these depositional conditions, seasonally varying inputs of sediment are preserved as varves (Hulsemann and Emery, 1961; Thunell et al., 1995). The mineralogy and provenance of recent sediments accumulating in the basin have been reported on by Fleischer (1972). 2.2. Climatic and hydrographic setting Oceanographic conditions within Santa Barbara Basin are strongly influenced by the southward flowing, highly productive California Current. Seasonal changes in the positions of the North Pacific High and the adjacent continental thermal low result in changes in wind speed and direction, which in turn control the strength of the California Current (Huyer, 1983). The winds are strongest and from the north during spring and early summer (Fig. 2), causing offshore Ekman transport and upwelling during this period. This upwelling is clearly seen in Santa Barbara Basin as a shoaling of isotherms during April—June in each of our three study years (Fig. 3). The expected increases in primary productivity associated with the upwelling periods are reflected

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Fig. 2. Upper panel: daily average wind speed and direction measured at the Santa Barbara Airport for the period January 1993 to June 1996. Data provided by the National Climate Data Center. Lower panel: quarterly surface water chlorophyll concentration measurements made at CalCOFI Station 82.47 in Santa Barbara Basin. Data provided by Scripps Institution of Oceanography.

Fig. 3. Temperature time series of the upper 100 m for July 1993 through December 1996 based on bi-weekly CTD casts at the sediment trap mooring site. Isotherms are in degrees centigrade.

in elevated surface chlorophyll concentrations during the spring at CalCOFI Station 82.47 in Santa Barbara Basin (Fig. 2). During the fall and winter, the northerly component of the winds weakens (Fig. 2), upwelling is diminished and surface productivity decreases. These large scale circulation features vary on an interannual basis in response to El Nin o-Southern Oscillation (ENSO). It has been reported that the flow of the California Current weakens and productivity decreases during El Nin o

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Fig. 4. Upper panel: daily precipitation at the Santa Barbara Airport for the period July 1993 through December 1996. Data provided by the National Climate Data Center. Lower Panel: River discharge for the Ventura River for the period July 1993 through December 1996. Data provided by the United States Geological Survey.

events (Bernal, 1981; McGowan, 1983; Chavez, 1996). This is important since strong El Nin o conditions existed in the eastern Pacific during the first half of our study period. The major features of the surface circulation in Santa Barbara Basin have recently been reported by Hendershott and Winant (1996). Flow within the basin is highly complex and surface waters are derived from several sources. Warm, saline surface waters enter the basin from the south, while colder, less saline waters that upwell off Point Conception enter the basin from the north. The relative importance of water from each of these sources varies seasonally, as does the general direction of transport through the basin. During periods of upwelling, a counterclockwise surface eddy exists in the basin, with a net flow of surface waters into the basin from the west and out of the basin to the east. The climate of the adjacent continental drainage basin is arid and the source rocks are primarily clastics (Fleischer, 1972). A number of rivers empty into the basin, with the largest being the Santa Clara and Ventura Rivers. These two rivers deliver over 90% of the terrigenous sediment being deposited in the basin (Thornton, 1984), with most of it being fine-grained gray lutite (Fleisher, 1972). Precipitation and river discharge into Santa Barbara Basin are typically greatest during the winter (Fig. 4; Soutar and Crill, 1977). For our three-year study period, 1995 seems to be anomalous in that the period of high rainfall and runoff extended well into the spring (Fig. 4).

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Fig. 5. Relationship between river discharge and suspended sediment concentration for the Ventura River. Measurements were made between 1968 and 1985 by the United States Geological Survey.

River discharge and suspended sediment concentration are highly correlated (Fig. 5), and thus most of the terrigenous input to the basin also occurs in winter. 3. Materials and methods 3.1. Sample and data collection A time series sediment trapping program was started in the center of Santa Barbara Basin (34°14N, 102°02W; Fig. 1) in August 1993. Two-week long samples have been collected continuously at this site using an automated, conical sediment trap positioned at 540 m water depth, approximately 50 m off the bottom. The sediment trap was recovered and redeployed every six months. The data presented in this paper are for the period August 1993 to September 1996. Two three-month gaps exist in our sample collection (November 1994—January 1995 and June—August 1995) and are attributed to clogging of the trap during two different deployment periods. Biweekly conductivity-temperature-density (CTD) casts were taken of the upper 100 m of the water column at the mooring location These data are used to evaluate changes in upper ocean thermal structure during the study period. Chlorophyll-a concentrations have been measured quarterly at CalCOFI Station 82.47 (Scripps

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Institution of Oceanography) which is in close proximity to the sediment trap mooring site. Wind and precipitation data measured daily at the Santa Barbara Airport were provided by the National Climate Data Center. River discharge and suspended load concentration data for Santa Clara and Ventura Rivers are from the United States Geological Survey Water Resources Division. 3.2. Sample processing and analyses A buffered sodium azide solution was placed in each sample cup prior to trap deployment and acted as a poison. Upon recovery, samples were stored in sealed containers and refrigerated. Whole samples were split using a precision rotary splitter and a quarter split was used for bulk geochemical analyses. These subsamples were studied under a dissection microscope and all obvious swimming organisms were removed before analysis. Organic carbon, nitrogen, carbonate, opaline silica, and lithogenic contents were determined for each sample and converted to fluxes (g m\ d\). A Perkin Elmer 2400 Elemental Analyzer was used to measure organic carbon and nitrogen contents following the procedures described in Froelich (1980). Carbonate content was measured using an automated system in which samples are reacted under vacuum in 100% phosphoric acid and the CO generated is measured  by a pressure transducer (Osterman et al., 1990). Opaline silica contents were determined using the wet chemical leaching techniques described by Mortloch and Froelich (1989) and measured spectrophotometrically. Lithogenic content was determined indirectly by subtracting the total biogenic component (carbonate, opal, and organic matter) from the total sample weight. Organic matter was estimated to be 2.5 times the organic carbon content. The carbon isotopic composition of organic matter (dC ) was determined on decalcified samples. The samples were analyzed using  a VG Optima stable isotope ratio mass spectrometer interfaced with a Carlo Erba CN Analyzer.

4. Results Over the three-year study period 62 samples were collected and analyzed. The calculated flux data for these samples are presented in Table 1. A significant amount of temporal variability exists in both total mass flux and the fluxes of the individual particulate components (Fig. 6). However, an examination of the various flux records suggests that a common seasonal pattern exists for all of the flux constituents; fluxes tend to be high in spring and summer and low in winter. Indeed, linear regression analyses indicate that significant (p(0.05) correlations exist between all of the flux components (Table 2). Total mass flux varied by an order of magnitude (0.47—4.39 g m\ d\) between August 1993 and September 1996 (Table 1, Fig. 6). Although the total mass flux time series is characterized by high fluxes in spring—summer, additional brief periods of high total mass fluxes also occur during the winter. In fact, the highest total mass flux measured during the entire three year study occurred in December 1995.

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Table 1 Santa Barbara Basin sediment flux data Sample C

Duration (d)

Begin date

Total mass flux (g m\ d\)

Org. carbon Carbonate flux flux (g m\ d\) (g m\ d\)

Opal flux (g m\ d\)

Lithogenic flux (g m\ d\)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 13 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14 14

08/12/93 08/26/93 09/09/93 09/23/93 10/07/93 10/21/93 11/04/93 11/18/93 12/02/93 12/16/93 12/30/93 01/13/94 01/27/94 02/11/94 02/25/94 03/11/94 03/25/94 04/08/94 04/22/94 05/06/94 05/20/94 06/03/94 06/17/94 07/01/94 07/15/94 07/29/94 08/23/94 09/06/94 09/20/94 10/04/94 10/18/94 02/21/95 03/07/95 03/21/95 04/04/95 04/18/95 05/02/95 08/26/95 09/09/95 09/23/95 10/07/95 10/21/95 11/04/95 11/18/95 12/02/95 12/16/95 12/30/95

2.74 2.16 1.89 1.65 1.48 1.27 1.82 1.32 2.44 1.92 1.28 1.53 1.37 1.47 0.81 0.84 0.98 1.44 1.55 2.26 2.16 2.18 3.55 2.45 1.65 2.31 1.95 2.07 1.52 1.70 1.99 2.29 1.52 2.50 3.35 2.38 2.09 2.21 2.59 2.94 1.86 1.39 1.75 0.71 1.45 4.39 1.43

0.10 0.09 0.08 0.06 0.06 0.05 0.06 0.05 0.08 0.07 0.05 0.06 0.05 0.06 0.04 0.04 0.04 0.06 0.07 0.11 0.09 0.10 0.18 0.11 0.08 0.12 0.09 0.08 0.06 0.07 0.08 0.07 0.06 0.08 0.11 0.09 0.09 0.12 0.17 0.16 0.11 0.08 0.08 0.04 0.07 0.12 0.05

0.42 0.57 0.29 0.26 0.22 0.17 0.17 0.19 0.21 0.18 0.19 0.21 0.20 0.31 0.16 0.16 0.23 0.38 0.49 0.70 0.31 0.50 0.98 0.57 0.41 0.55 0.35 0.31 0.22 0.21 0.20 0.19 0.13 0.26 0.66 0.50 0.74 0.47 0.62 0.44 0.30 0.19 0.19 0.09 0.15 0.25 0.11

1.86 1.18 1.24 1.10 1.00 0.84 1.37 0.87 1.75 1.39 0.80 0.99 0.85 0.85 0.44 0.45 0.52 0.77 0.79 1.15 1.44 1.26 1.91 1.43 0.93 1.34 1.24 1.43 0.96 1.16 1.48 1.77 1.13 1.90 2.25 1.51 0.99 1.26 1.34 1.82 1.13 0.91 1.23 0.46 1.03 3.60 1.11

0.21 0.17 0.16 0.13 0.11 0.13 0.13 0.13 0.27 0.19 0.15 0.18 0.18 0.15 0.12 0.14 0.12 0.13 0.09 0.14 0.19 0.17 0.22 0.18 0.11 0.14 0.14 0.14 0.19 0.16 0.12 0.15 0.10 0.14 0.19 0.13 0.14 0.19 0.20 0.27 0.15 0.11 0.12 0.06 0.10 0.24 0.08

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Table 1. Continued. Sample C

Duration (d)

Begin date

Total mass flux (g m\ d\)

Org. carbon Carbonate flux flux (g m\ d\) (g m\ d\)

Opal flux (g m\ d\)

Lithogenic flux (g m\ d\)

48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

14 14 13 14 14 14 14 14 14 14 14 14 14 14 14

01/13/96 01/27/96 02/10/96 03/26/96 04/09/96 04/23/96 05/07/96 05/21/96 06/04/96 06/18/96 07/02/96 07/16/96 07/30/96 08/13/96 08/27/96

0.47 1.38 2.29 2.00 2.12 2.90 3.57 1.53 3.48 0.94 1.98 1.78 2.81 2.15 1.28

0.02 0.06 0.09 0.08 0.13 0.20 0.16 0.07 0.18 0.05 0.10 0.08 0.13 0.13 0.07

0.05 0.23 0.30 0.38 0.77 0.96 0.65 0.23 0.84 0.15 0.30 0.32 0.45 0.46 0.21

0.33 0.91 1.62 1.26 0.91 1.26 2.28 0.99 1.94 0.59 1.21 1.10 1.83 1.20 0.81

0.03 0.10 0.16 0.16 0.13 0.18 0.24 0.12 0.24 0.08 0.22 0.15 0.21 0.17 0.10

Lithogenic material is the single largest contributor to the total mass flux, typically accounting for 50—80% of the total (Table 1, Fig. 7). As a result, lithogenic flux and total mass flux are strongly correlated (r"0.93). The annual cycle of lithogenic input to Santa Barbara Basin appears to be bimodal, with high lithogenic fluxes recorded during both the late fall—early winter and the spring—early summer. Large changes in lithogenic flux can occur on very short time scales, as evidenced by the fact that the highest (3.60 g m\ d\) and lowest (0.33 g m\ d\) fluxes recorded during the three year study occurred within a six week period from December 1995 to January 1996. It was anticipated that the fluxes of the three biogenic components (organic carbon, carbonate and opaline silica) would reflect the seasonal cycle of primary productivity in the basin. However, since we are dealing with a continental margin, there is the possibility of significant terrestrial carbon input to the basin. Organic matter C : N ratios and the carbon isotopic composition of the organic carbon are used to identify the source(s) of organic carbon collected in the sediment traps. Marine organic matter typically has C : N ratios of 6—7, while organic matter from terrestrial sources usually has a C : N ratio greater than 20 (Hedges et al., 1986). Similarly, there is a distinguishable difference in the typical dC of marine phytoplankton (!18 to !22) and terrestrial C plants (!26 to !28) (Degens, 1969). For the samples on which both  of these measurements were made, the C : N and dC values vary from 7.0 to 8.5  and !20.0 to !22.5, respectively (Fig. 8), indicating that the carbon is derived primarily from a marine source. Both opal and organic carbon fluxes mirror the productivity cycle in that fluxes are high during the spring—summer upwelling period and low during the fall and winter (Fig. 6), with a strong linear correlation existing between the two (r"0.84). Organic carbon fluxes vary by an order of magnitude and range from a low of 0.02 g m\ d\

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Fig. 6. Bi-weekly fluxes of total mass, lithogenic material, organic carbon, carbonate and opaline silica in Santa Barbara Basin for the period August 1993 to September 1996. All fluxes are in g m\ d\.

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Table 2 Correlation coefficient (r) matrix for the various flux components

Total mass Org. carbon Carbonate Opal Lithogenic

Total mass

Org. carbon

Carbonate

Opal

Lithogenic

1.00 0.83 0.77 0.67 0.95

0.83 1.00 0.67 0.84 0.66

0.77 0.67 1.00 0.45 0.71

0.67 0.84 0.45 1.00 0.41

0.93 0.60 0.69 0.35 1.00

All correlations are significant at p(0.05; N"62.

Fig. 7. Relative proportions of lithogenic material, opaline silica, carbonate and organic carbon in the total particulate fluxes measured in Santa Barbara Basin for the period August 1993 to September 1996.

in January 1996 to a high of 0.20 g m\ d\ in April 1996 (Table 1, Fig. 6), and almost always account for less than 5% of the total flux (Fig. 7). Opaline silica is by far the dominant biogenic sediment produced in Santa Barbara Basin, periodically accounting for up to 35% of the total flux (Fig. 7). The magnitudes of the opal fluxes measured during the upwelling periods (up to 0.98 g m\ d\) are similar to those recently reported for Monterey Bay (Pilskaln et al., 1996), and in general, are comparable to those measured in the highly productive regions around Antarctica (Dunbar and Leventer, 1986; Gersonde and Wefer, 1987; Wefer et al., 1988). In contrast, the carbonate flux record bears more resemblance to the lithogenic and total mass flux records in that all three are characterized by flux peaks in the winter as

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Fig. 8. Relationship between organic matter C : N ratio and the carbon isotopic composition of organic carbon for sediment trap samples collected between August 1993 and August 1994. A hypothetical mixing line between typical marine organic matter and typical terrestrial organic matter is plotted.

Table 3 Annual flux estimates for Santa Barbara Basin (g m\ yr\) Year

Total mass

Org. carbon

Carbonate

Opal

Lithogenic

1994 1995 1996

644 814 732

29 36 36

54 56 54

132 126 146

385 543 441

well as the spring—summer (Fig. 6). While much of the carbonate is derived from calcareous plankton (foraminifera and coccolithophores), the high correlation between carbonate and lithogenic fluxes (r"0.71) suggests that there may be some input of detrital carbonate into Santa Barbara Basin. Carbonate fluxes also vary by an order of magnitude from 0.08 to 0.27 g m\ d\, and typically account for 5—10% of the total flux (Fig. 7). Although data gaps exist in the flux records for each year, we have estimated annual fluxes (g m\ yr\) for each sediment component (Table 3). This was done by calculating average daily fluxes from the available data for each year and then normalizing them to a one year period. Over the three year sampling period the total annual flux varied by approximately 25%, from a low of 644 g m\ yr\ in 1994 to a high of 814 g m\ yr\ in 1995. Of the various biogenic components, the annual carbonate flux was the most constant during this period (54—56 g m\ yr\). In contrast, the annual fluxes of organic carbon and opal varied by 15—20% from year to year.

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5. Discussion 5.1. Seasonal variability in particle fluxes The supply of sediments to Santa Barbara Basin is largely a function of two processes: (1) seasonal changes in primary productivity, and (2) local climate changes that control the delivery of allochthonous material to the basin. As already discussed, primary productivity in Santa Barbara Basin is highest in the spring-early summer when upwelling is most intense. Organic carbon and opal are characterized by flux patterns that clearly reflect these seasonal changes in productivity, while that for carbonate is more complicated. The C : N ratios and dC data (Fig. 8) indicate that  the carbon is derived primarily from marine phytoplankton. In addition, the strong positive correlation between organic carbon and opal flux (r"0.84; Table 2) indicates that seasonal blooms of siliceous phytoplankton are the primary mechanism controlling the export of carbon in Santa Barbara Basin. Diatoms are the main contributor to the opal flux, with more minor contributions from radiolaria and silicoflagellates (Lange et al., 1997). Chaetocerus resting spores dominate the diatom assemblage during the productive spring upwelling period. Based on these sediment trap results, it appears that opal accumulation rates in sediments should be a good proxy for past changes in productivity in this region. Using historical records, Soutar and Crill (1977) demonstrated a strong relationship between local rainfall and varve thickness in Santa Barbara Basin. During our three year study period, river discharge into Santa Barbara Basin was consistently high during the winter, in conjunction with periods of high rainfall (Fig. 4). During 1995 this period of peak discharge was prolonged and extended from January through April. A comparison of the river discharge record and the lithogenic flux time series reveals no strong covariance between the two (Fig. 9). Specifically, times of high discharge are not periods of particularly high detrital sediment fluxes in the center of the basin, and vice versa. For example, the high river discharge during January— February in both 1994 and 1996 coincides with times of relatively low lithogenic sediment fluxes. Similarly, the very high terrigenous fluxes measured in December 1995 actually precede the period of high discharge. What is responsible for the apparent decoupling of these two processes? One explanation is that much of the suspended sediment is initially deposited on the shelf and not transported to the more offshore part of the basin. The width of the shelf (distance from shoreline to 100 m isobath) varies considerably within the basin, being 5—10 km wide between Point Conception and Santa Barbara Point and 15—20 km wide down to Point Hueneme (Fig. 1). Both the Ventura and Santa Clara rivers empty into Santa Barbara Basin in this region where the shelf is relatively wide, and much of the suspended-load may be deposited rapidly on this part of the shelf. A recent study on the northern California margin off of the Eel River has demonstrated that fine-grained suspended sediment carried by small river systems is rapidly dispersed over the margin during periods of high river discharge (Wheatcroft et al., 1997). Subsequent resuspension of this finegrained material by storm or wave activity and down slope transport may be an important mechanism for delivering detrital material to the center of the basin. In

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Fig. 9. Upper panel: Santa Clara River discharge record for July 1993 to September 1996 (data provided by the United States Geological Survey). Lower panel: lithogenic flux record for August 1993 to September 1996.

contrast, the coarse sand fraction stays trapped on the shelf and represents a very minor component of the deeper basin sediments (Thornton, 1984). Such a delay in the delivery of fine-grained terrigenous material to the deep part of the basin was observed by Drake and others (1972) following the large floods of 1969. According to these authors, the flood sediment carried by the Santa Clara and Ventura rivers was deposited first on the shelf and subsequently remobilized and transported down slope to the deep basin. A second possible explanation for this lack of correlation between river discharge and lithogenic sediment flux is that the terrigenous material tends to be very fine-grained and remains suspended in the water column until it is scavenged. According to Fleischer (1972), the mean grain size of the gray lutite being deposited in the deep Santa Barbara Basin is 2.2 lm. Such particles have very low sinking velocities and will be removed from the water column when incorporated into some form of aggregate. The packaging of lithogenic material into fecal pellets or the scavenging of this material from the water column by marine snow would provide an important mechanism for delivering terrigenous material to the center of the basin. Indeed, Dunbar and Berger (1982) have estimated that fecal pellets may serve as the transport mechanism for over 60% of the material delivered to the sea floor in the center of Santa Barbara Basin. This would explain why lithogenic fluxes are high during spring—summer when productivity is high but river runoff is low.

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Laminated sediments presently accumulate in Santa Barbara Basin, with a sedimentary couplet (one dark layer and one light layer) representing a varve or an annual unit of sedimentation. Previous studies (Hulsemann and Emery, 1961; Lange and Schimmelmann, 1994) have demonstrated that the dark layers are predominantly fine-grained clay particles and that the light layers are enriched in biogenic silica. Lange and others (1995) have reviewed the various models that have been proposed to explain the origin of these varves. Hulsemann and Emery (1961) were the first to suggest that the dark layers are deposited during the winter months when most of the detrital material is brought to the basin and that the light layers are formed during the spring—summer when productivity in Santa Barbara Basin is high. Our data clearly show that strong seasonal differences exist in the relative proportions of lithogenic and biogenic material settling through the water column, and that there is a repeatable seasonal pattern from year to year (Fig. 7); the total flux is dominated by terrigenous material (70—80%) during the fall and winter, while biogenic material accounts for a significantly higher fraction (40—50%) of the total flux during the spring and summer. These seasonally varying inputs of lithogenic and biogenic sediments, together with the lack of bioturbation on the sea floor, result in varve formation in a manner consistent with that originally proposed by Hulsemann and Emery (1961). 5.2. Interannual variability The bi-weekly flux records do not show a significant amount of variability from year to year in either seasonal trends or magnitudes of the fluxes (Fig. 6). The maximum and minimum flux values for the various sedimentary components appear to be quite similar over the three year sampling period. However, when the time series data are used to estimate annual fluxes (Table 3), some interannual variability is apparent. The most striking differences are in annual terrigenous fluxes; during 1995, lithogenic fluxes were nearly 40% higher than during the previous year and 20% higher than in 1996. This interannual variability appears to be directly related to precipitation and runoff. The very high annual terrigenous flux in 1995 is due to significantly higher precitation during winter of that year and an ensuing prolonged period of high river discharge which lasted from January to May (Fig. 4), which undoubtedly delivered large quantities of suspended material to the basin. In contrast, precipitation was very low during 1994, and this manifested itself in a low annual terrginous flux. One consequence of these changes in lithogenic input is that total sediment fluxes for 1995 were approximately 25% and 10% higher than for 1994 and 1996, respectively (Table 3). Within the limits of the data, there is virtually no change in annual carbonate flux over the three-year period, while organic carbon and opal fluxes are marked by moderate changes from year to year (Table 3). The highest annual fluxes of both opal and organic carbon occurred in 1996. Could ENSO be responsible for the interannual variability observed in our particle flux records? El Nin o conditions developed in the eastern Pacific in early 1991 and persisted through 1994, making this one of the longest ENSO events on record. Thus, for the first part of our study period (1993—1994) the Santa Barbara Basin was influenced by El Nin o conditions. Several previous studies have concluded that much

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of the interannual variability in productivity along the California margin is related to changes in the transport of the California Current and that this in turn is coupled to ENSO-driven changes in the eastern equatorial Pacific (Bernal, 1981; Chelton et al., 1982; McGowan, 1983). In particular, it has been suggested that reduced flow of the California Current during El Nin o events results in a decrease in productivity in this region (Chavez, 1996). From the El Nin o conditions of 1994 to the non-El Nin o conditions of 1996, we observe a 10% increase in annual opal flux and a 20% increase in organic carbon flux (Table 3). Furthermore, surface chlorophyll concentrations in Santa Barbara Basin were considerably lower during the spring upwelling period of 1993 and 1994 (El Nin o years) than during the spring of 1995 and 1996 (non-El Nin o years; Fig. 2). Thus, our findings support the concept that productivity in the region is reduced during El Nin o periods. 5.3. Export production Export production is the carbon flux measured at depth in the water column and when expressed as a proportion of the original primary production is referred to as the export ratio (e-ratio; Murray et al., 1989; Baines et al., 1994). E-ratios vary nonlinearly with depth and are a function of primary productivity and carbon regeneration rates in the water column (Suess, 1980; Betzer et al., 1984; Martin et al., 1987; Pace et al., 1987), with the highest rates of carbon regeneration typically occurring immediately below the photic zone in the depth range of 100—500 m. A limited number of primary productivity measurements were made at CalCOFI Station 82.47 during the study period and are combined with our carbon flux measurements at 540 m water depth to estimate e-ratios (Table 4). The primary productivity measurements range from a high of 1.49 gC m\ d\ in July 1995 to a low of 0.31 gC m\ d\ in February 1994. The calculated e-ratios vary from 0.07 to 0.16 and are inversely related to primary productivity; the lowest ratio is associated with the highest measured productivity. These values compare well with previously determined e-ratios for the central California coastal region (Knauer et al., 1979; Knauer and Martin, 1981; Pilskaln et al., 1996). Why does proportionately less carbon make it to depth in Santa Barbara Basin during periods of high productivity? Pilskaln and others (1996) observed the same relationship between productivity and e-ratio in Monterey Bay and attributed it to Table 4 Primary productivity estimates, carbon fluxes and calculated export-ratios Date

Primary productivity (g m\ d\)

Carbon flux (g m\ d\)

Export-ratio

February 1994 August 1994 July 1995

0.31 0.79 1.49

0.05 0.09 0.11

0.16 0.11 0.07

for CalCOFI Station 82.47.

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two mechanisms that may be working in concert. The first mechanism involves seasonal changes in upper water column flow regime. During the spring/summer upwelling period, offshore Ekman transport is greatest and particles are advected out of the coastal region before settling to depth. In contrast, during the fall/winter, offshore transport is reduced and a higher percentage of the export production settles vertically through the water column. The idea that carbon is advectively transported away from coastal regions has been proposed for a number of other areas. Using hydrographic, drifter, and satellite data, Washburn and others (1993) demonstrated a significant offshore transport of particles along northern California in response to wind forcing. Ortiz and Mix (1992) also concluded that there is advective transport of carbon from the coastal region to the offshore region along Oregon. A similar model involving advective transport was proposed by Suess (1980) to explain carbon fluxes associated with the Peru coastal upwelling system. If this mechanism is at work along the margin of western North America and other coastal upwelling regimes, it could result in significant removal of carbon from the coastal ocean to the deep sea. The second mechanism proposed by Pilskaln and others (1996) to explain the e-ratio/productivity relationship is biologically driven and is a function of seasonal changes in plankton dynamics. According to these authors, the relatively low export production values during periods of high primary productivity are due to an increase in the abundance of mid-water zooplankton and associated higher rates of carbon utilization/regeneration. This suggestion is supported by the fact that zooplankton biomass is significantly higher during upwelling periods in the Southern California Boarderland region (Chelton et al., 1982). For Santa Barbara Basin, both mechanisms may be at work simultaneously to produce the inverse relationship between productivity and export production. 5.4. Regional trends The recent study by Pilskaln and others (1996) of sediment fluxes in Monterey Bay (&36°45N) on the California margin provides a data set that can be directly compared to our Santa Barbara Basin results and allows us to evaluate regional trends in these data. Like Santa Barbara Basin, Monterey Bay is influenced by the California Current (Rosenfeld et al., 1994) and is marked by strong upwelling and high productivity in the spring and summer (Chavez, 1996). The magnitudes of the fluxes measured in Santa Barbara Basin and Monterey Bay are quite similar, particularly for biogenic particles (Table 5). The major difference between the two study areas is in terrigenous input. Although lithogenic material represents a quantitatively significant component (i.e., 40—80%) of the total flux for both regions, Santa Barbara Basin has lithogenic fluxes that are twice as high as those measured in Monterey Bay. This is primarily due to the greater total mass fluxes measured in Santa Barbara Basin vs Monterey Bay (Table 5; Fig. 10). The relationships between carbon fluxes and the fluxes of other sedimentary components are also quite similar for Santa Barbara Basin and Monterey Bay (Fig. 9). In particular, the slopes of the best fit lines derived from linear regression analyses of the various flux data sets are virtually identical for the two regions. This implies that

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Table 5 Comparison of minimum—maximum fluxes (g m\ d\) for Santa Barbara Basin and Monterey Bay

Total mass flux Organic carbon flux Carbonate flux Opal flux Lithogenic flux

Santa Barbara Basin

Monterey Bay

0.47—3.57 0.02—0.20 0.03—0.27 0.05—0.96 0.33—3.60

0.18—2.93 0.01—0.18 0.01—0.22 0.01—0.80 0.10—1.61

data from Pilskaln et al. (1996).

Fig. 10. Scatter plots of organic carbon flux vs total mass flux, lithogenic flux, opaline silica flux and carbonate flux for Santa Barbara Basin and Monterey Bay (data from Pilskaln et al., 1996). All fluxes are in g m\ d\.

for a given change in organic carbon flux, there are comparable changes in carbonate, opal and lithogenic flux in both settings. From this it is concluded that the same basic mechanism(s) control particle fluxes in Santa Barabra Basin and Monterey Bay, and that both regions probably have similar plankton dynamics.

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6. Summary This study in Santa Barbara Basin represents one of the first attempts to monitor seasonal to interannual variability in particle fluxes in a coastal upwelling environment. Primary productivity in Santa Barbara Basin is highest during the spring— summer upwelling period and is dominated by silica-secreting phytoplankton. The fluxes of biogenic opal and particulate organic carbon clearly reflect this seasonal surface productivity signal. However, as productivity increases, the associated vertical carbon flux decreases, possibly due to enhanced offshore transport associated with Ekman flow. If this process is typical of coastal upwelling areas, it could result in significant export of carbon from continental margins to the deep sea. Riverine input of terrigenous material primarily occurs during the winter rainy period but the actual delivery of lithogenic material to the deep basin occurs throughout the year. It appears that lithogenic sediment fluxes are controlled by both physical (resuspension and downslope transport) and biological processes (incorporation of particles into aggregates). Annual fluxes of organic carbon and opaline silica increased from 1994 to 1996. This is attributed to more vigorous upwelling and higher productivity associated with the cessation of El Nin o conditions in late 1994. The biogenic particle fluxes measured in Santa Barbara Basin are similar in composition, magnitude and seasonal variability to those reported for Monterey Bay (Pilskaln et al., 1996). This suggests that similar processes control the production and flux of this material over a broad region of the California Current.

Acknowledgements I thank Eric Tappa for coordinating each of the sediment trap recovery/redeployment cruises and for sample analyses. This work was supported by NOAA Grants NA36GP0239 and NA56GP0243.

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