nitrogen isotope variations in santa barbara basin sediments: implications for denitrification in...

PALEOCEANOGRAPHY, VOL. 15, NO. 4, PAGES 377-387, AUGUST 2000

Nitrogen isotope variations in Santa Barbara Basin sediments: Implications for denitrification in the eastern tropical North Pacific during the last 50,000 years

Edwin Emmer and Robert C. Thunell

Department of Geological Sciences, University of South Carolina, Columbia

Abstract. Nitrogen isotope variations preserved in Santa Barbara Basin sediments are used to evaluate changes in denitrification in the eastern tropical North Pacific (ETNP) during the last 50,000 years. A significant component of the subsurface waters (-•100-400 m) that presently fill the Santa Barbara Basin is derived from the low-oxygen,j denitrifying zone in the ETNP, and the nitrate in these waters has a •SN value of 8-9%0. During the last glacial, the •SN values of Santa Barbara Basin sediments were typically 6-7%0, indicating decreased denitrification in the ETNP and a better oxygenated intermediate water mass in the Santa Barbara Basin at this time. This reduced denitrification during the last glacial would have increased the pool of fixed nitrogen and may have contributed to the higher productivity previously reported for various regions of the global ocean during this period. At the onset of deglaciation, sediment •SN values increase by more than 2%0, indicating increased denitrification in the ETNP. During Younger Dryas time, •SN values decreased by 3%0 and record a brief return to better ventilated conditions in the subsurface waters of the ETNP. This is followed by an increase in •SN to over 99/oo at -•10,000 years ago, indicating intense denitrification at the beginning of the Holocene.

1. Introduction

Recent studies suggest that ventilation of the ocean's subsurface waters and, consequently, the degree of marine denitrification have varied significantly on glacial-interglacial timescales [Altabet et al., 1995; Ganeshram et al., 1995; Pride et al., 1999]. The fi•SN record preserved in marine sediments can be used to document such changes since the nitrogen isotopic composition of nitrate provides a useful tracer of water derived from zones of denitrification [Brandes et al., 1998]. In the Arabian Sea the fi•SN record of organic matter is characterized by lower values during glacial intervals than during interglacials [Altabet et al., 1995], suggesting that the source of oxygen-poor water flowing into the Arabian Sea oxygen-minimum zone from the Persian Gulf and the Gulf of Aden may have diminished or been completely cut off during glacial periods. Without this source of oxygen-depleted water, less denitrification occurred in the northern Indian Ocean during the last glacial period [Altabet et al., 1995]. Similar decreases in sediment fi•N values during glacial intervals have been observed at several locations in the eastern tropical North Pacific (ETNP) [Ganeshram et al., 1995; Pride et al., 1999]. An extensive oxygen minimum zone presently exists in this region between -•200 and 600 m (Figure 1) [Cline and Richards, 1972; Brandes et al., 1998], and the observed changes during the last glacial period have been attributed to a decrease in denitrification within this zone. Changes in the intensity of denitrification can have a significant impact on the global nitrogen cycle by changing the rate at which fixed nitrogen is removed from the marine system and released to the

Copyright 2000 by the American Geophysical Union.

Paper number 1999PA000417. 0883-8305/00/1999PA000417512.00

atmosphere. During glacial intervals, when global denitrification is thought to be reduced [Altabet et al., 1995; Ganeshrarn et al., 1995], a larger supply of nitrate would have been available for utilization. This could contribute to the general increase in marine productivity reported for the last glacial period [Sarn- thein et al., 1988; Pedersen et al., 1991; Falkowski, 1997].

In addition to possible changes in the O2-minimum region in the ETNP, several lines of evidence suggest that the degree of ventilation of North Pacific Intermediate Water (NPIW) (-•500- 1000 m) has changed through time in response to global changes in ocean circulation. For example, in both Santa Barbara Basin and Guaymas Basin (Gulf of California), oxygen-deficient NPIW presently inhibits bioturbation, allowing laminated sediments to accumulate. However, during the last glacial period and the ensuing Younger Dryas time interval,' sediments in both basins were bioturbated at water depths that presently are anoxic, suggesting that oxygen levels within NPIW were higher than at present [Keigwin and Jones, 1990; Behl and Kennett, 1996; Pride et al., 1999]. Changes in foraminiferal Cd/Ca ratios from the northeastern Pacific also indicate a change in NPIW ventilation during the Younger Dryas period [van Geen et al., 19961.

In this study, we evaluate changes in the nature of Santa Bar- bara Basin subsurface waters during the past 50,000 years using the nitrogen isotope composition of organic matter preserved in the sediments from Ocean Drilling Program (ODP) hole 893-A and consider the implications of these changes for large-scale changes in denitrification in the eastern Pacific. In addition, we compare the sediment fi•5N record with the bioturbation index for this Santa Barbara Basin ODP hole [Behl and Kennett, 1996]. This latter record reflects changes in the ventilation of NPIW and thus allows us to determine whether ventilation

changes in the ETNP oxygen-minimum zone and NPIW were coupled over the last 50,000 years.

377

378 EMMER AND THUNELL: EASTERN PACIFIC DENITRIFICATION

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Figure 1. Map of the eastern North Pacific showing the present-day geographic distribution of the subsurface oxygen- minimum zone (shaded region) where intense alenitrification occurs [Deuser, 1975]. The insert is a bathymetric map of the Santa Barbara Basin showing the locations of Ocean Drilling Program (ODP) hole 893-A (square) and the sediment trap mooring (cone). Also shown are the locations of cores NH22P [Ganeshram et al., 1995] and AII125-8-56 [Pride et al., 1999].

2. Oceanographic and Depositional Setting of Santa Barbara Basin

Santa Barbara Basin is one of several semi-enclosed basins

within the Southern California Borderland Province (Figure 1). The northwestern entrance to the basin has a sill depth of---475 m [Marsaglia et al., 1995] and is---4 km wide [Gardner and Dartnell, 1995]. The southeastern part of the basin has a sill depth of---250 m and is ---3 km wide. These sills inhibit ventilation of the basin, and deep waters presently are dysaerobic (<0.1 mL/L oxygen) below-•475 m.

Two currents dominate the near-surface circulation (upper 100 m) in the Santa Barbara Basin: the southward flowing Cali- fornia Current and related northward flowing countercurrents [Hickey, 1992; Henderschott and Winant, 1996]. The interac- tion between these two systems creates a semipermanent cy- clonic gyre, which incorporates cold California Current water from the northwest and warmer Davidson Current water from

the southeast. Seasonal variations in the strength of the two surface currents affect the degree of upwelling and biological productivity within the basin. During the late spring and summer (April to August), the southward flowing California Current dominates as a result of strong northerly winds. These winds increase Ekman transport of surface waters and result in greater upwelling and productivity in the spring-summer. This upwelled water comes from depths as great as 300 m [Etnery and H•lse-

mann, 1962]. The opposite holds from October to February, when the northerly winds weaken and the Davidson Current increases in strength, reducing upwelling [Lynn and Simpson, 1987]. An in-depth discussion of seasonal changes in Santa Bar- bara Basin surface circulation is provided by Hendershott and Winant [ 1996].

Waters entering the basin between ---100 and 400 m depth are derived primarily from the south and carried northward by the California Undercurrent. This current originates off Baja, Cali- fornia, and contains a significant component of subtropical subsurface water from the oxygen-deficient, denitrifying zone in the ETNP [Wyrtki, 1967; Wooster and Jones, 1970]. The strong 02- minimum zone in the ETNP is maintained by high surface productivity and high rates of oxygen consumption in the subsurface waters. In contrast, Santa Barbara Basin bottom waters

(>475 m) are derived from oxygen-deficient NPIW which periodically flows over the deepest sill [Sholkovitz and Gieskes, 1971; Reimers et al., 1990] and originates in the high northern latitudes of the Pacific [ Warsh and Warsh, 1973].

Water column values of •5•5N of nitrate from the Santa Bar- bara Basin reflect the input and sources locations of these different water masses (Figure 2). Surface water •lSN values vary seasonally and may be as low as 5-696o [œiu, 1979]. These values are typical of water from similar depths in the open North Pa- cific [œiu and Kaplan, 1989]. Subsurface waters down to the sill depth (-475 m) are characterized by •515N values of 8-996o.

EMMER AND THUNELL: EASTERN PACIFIC DENITRIFICATION 379

6.0

8"n (%) 8.0 10.0

I , I

12.0

100 --

200 --

.c: 300 --

_

400 --

500 --

600 --

Station Location

34 ø16'N, 119 ø 59'W

Sill Depth

I

Figure 2. Water column profile of 8•SN of dissolved nitrate for a station in Santa Barbara Basin [Liu, 1979]. The sill depth is at 475 m.

Similar values are found at these depths offshore southern Cali- fornia [Liu and Kaplan, 1989]. This ]SN-enriched nitrate originates in the ETNP and is carried to the Santa Barbara Basin by the California Undercurrent. During upwelling these waters are brought to the surface and result in an increase in the •515N of surface waters. The oxygen-deficient conditions that exist below the sill depth result in denitrification and further increase the •515N of nitrate in the bottom waters (Figure 2).

Presently, the extremely low oxygen concentrations and the lack of bioturbation below 500 m in the Santa Barbara Basin re-

sult in the preservation of sediment varves [Hiilsemann and Em- ery, 1961]. Each varve is a sedimentary couplet consisting of light and dark laminae produced from seasonally variable input of sediments [Hiilsemann and Emery, 1961; Thunell et al., 1995; Thunell, 1998]. Dark bands form during winter, when increased rains cause an increase in terrigenous sediment input into the basin [Hiilsemann and Emery, 1961; Thunell et al., 1995; Thunell, 1998]. Lighter laminae are deposited during the spring-summer when runoff is reduced and biogenic silica production dominates the sediment flux [Soutar, 1975; Thunell et al., 1995; Thunell, 1998].

3. Factors Controlling the Isotopic Composition of Nitrogen in the Ocean

An understanding of the factors controlling the isotopic fractionation of nitrogen in the ocean is necessary in order to evaluate the use of •515N of marine organic matter as a proxy for the isotopic signal of the nitrate source waters. Isotopic fractiona-

tion of nitrogen occurs when organisms preferentially utilize the isotopically "lighter" InN during nutrient assimilation [Faure, 1986]. In a nutrient-rich setting, organisms are able to preferentially uptake nitrate containing inN, leaving the remaining nitrate enriched in 15N. In regions where nitrate availability is limited, phytoplankton are not able to discriminate between •4NO3- and •NO3-, resulting in the incorporation of isotopically "heavier" dissolved inorganic nitrogen during assimilation. In the Califor- nia Current region, all of the upwelled nitrate is utilized on an annual basis, and as a result, phytoplankton utilization does not leave an isotopic signature on the average annual •515N value of particulate organic nitrogen (PON) [Altabet et al., 1999]. Rather, Altabet et al., [ 1999] concluded that the nitrogen isotope composition of PON produced along the margin of the eastern North Pacific primarily is a function of the 8•N of subsurface nitrate.

The intense denitrification that occurs within the oxygen- deficient regions of the ETNP also causes a fractionation of nitrogen isotopes. Bacteria preferentially remove 14NO3', leaving the remaining nutrient pool enriched in 15NO3'. Particulate organic nitrogen produced from this isotopically enriched nitrate will have a high •515N value. As a result of denitrification, the •515N values for both the source waters and PON in the eastern Pacific are higher than most regions of the open ocean. Sub- euphotic zone nitrogen isotope values average 4.5-596o for most oceanic regions [Liu and Kaplan, 1989; Sigman, 1997]. In contrast, filSNnitrat ½ for eastern tropical North Pacific waters average ~11.796o, with values ranging from 7.5 to 18.59/oo. As a result, it is possible to use the •515N of organic matter in sediments from this region as a proxy for denitrification.

In the open ocean, there is a significant difference in the •515N of sinking particles and surface sediments, with sediments being enriched by 3-496o [Altabet, 1996]. However, studies in coastal regions of the eastern North Pacific [Altabet et al., 1999] have shown that there is little fractionation between sinking particles and surface sediments (<196o) from both oxic and anoxic basins. Thus the degree of bottom water oxygenation is not controlling this lack of diagenesis. Rather, this difference between open ocean and continental margin settings is most likely the result of better preservation of PON due to high export production and high bulk sediment accumulation rates [Altabet et al., 1999]. As a result, changes through time in Santa Barbara Basin bottom water oxygen concentrations should not have a significant effect on the 8•SN signal preserved in the basin sediments. Instead, the b lSN of sinking or buried PON in the Santa Barbara Basin should reflect the isotopic composition of the subsurface waters.

4. Materials and Methods

Santa Barbara Basin ODP hole 893-A is a 196.5 m sediment

sequence representing the last 160,000 years [Kennett et al., 1994]. The site is located ~20-km offshore at 34ø17.25'N, 120ø02.2'W in a water depth of 576.5 m (Figure 1). The preservation of varves and the high sedimentation rates (> 100 cm/kyr) make the site ideal for detailed late Pleistocene paleoceanographic and paleoclimate studies. Numerous studies have been conducted on hole 893-A, including analysis of benthic and planktonic foraminiferal stable isotopes [Kennett, 1995; Hendy and Kennett, 1999], organic carbon accumulation rates [Stein and Rack, 1995; Gardner and Dartnell, 1995], diatom, radialoarian, pollen, and foraminiferal abundances [Hemphill-


Haley and Fourtanier, 1995; Heusser, 1995; Berger et aL, 1997; Cannariato et al., 1999], alkenone temperature estimates [Her- bert et al., 1995], and the stratigraphic distribution of laminated sediments [Behl and Kennett, 1996]. These studies have fo- cused on climate-linked changes in sediment and geochemical properties of the core.

The age model used in this study was derived using 17 accel- erator mass spectrometry (AMS) C •4 dates for the upper 20kyr [Ingram and Kennett, 1995] and a benthic foraminiferal oxygen isotope stratigraphy for the section of hole 893-A below 20kyr [Kennett, 1995]. A reservoir correction of 825 years was ap- plied to all of the radiocarbon ages [Ingram and Kennett, 1995]. The corrected radiocarbon ages were then converted to calendar ages using the calibration equation of Stuiver and Braziunas [1993] for samples with radiocarbon ages younger than 10,500 years before present and the calibration equation of Bard et al. [1990] for all samples with radiocarbon ages older than 10,500 years before present. Ages for the various events in the oxygen isotope record were assigned using the Martinson et al. [1987] timescale. Linear sedimentation rates derived from this age model vary from 75 to 333 cm/kyr, with an average of 165 cm/kyr.

Samples from ODP hole 893-A were taken at---20 cm intervals for the upper 72 m of the core. With the above sedimentation rates each sample has a time spacing of 60-250 years. The samples were dried, ground into a powder, and stored in speci- men vials.

The nitrogen isotope composition of bulk organic matter was determined using a Carlo Erba Elemental Analyzer interfaced with a VG Optima Stable Isotope Ratio Mass Spectrometer. A portion of each powdered sample (-25 mg) was treated with a 10% solution of phosphoric acid in order to remove all carbonate and then placed in a tin capsule. Urea (•5•5N = 0.1096o) was used as a working standard. Samples and standards were corn- busted at 1020øC, with standards spaced approximately every 10-12 samples. Approximately 25% of the samples were run in duplicate. Results are reported as •5•5N relative to N2 for the atmosphere.

In order to aid our interpretation of the hole 893-A nitrogen isotope record, •5•5N analyses also were carried out on sediment trap samples collected from the Santa Barbara Basin between July 1993 and October 1994. This allows us to evaluate the relationship between seasonal changes in the isotopic composition of sinking particulate material and upper ocean conditions in the basin. A detailed description of the Santa Barbara Basin sediment trapping project is presented by Thunell [ 1998].

5. Results

Significant seasonal variability exists in the nitrogen isotope composition of the sediment trap samples, with values ranging from 6.0 to 9.09/oo (Figure 3). The lowest values (6-796o) tend to occur during the fall, when sea surface temperatures are warm and the surface layer is thermally stratified. Higher •5•1 values

0 I

20-

40 .•

60-

80-

100 - !

• j •A• S •O• N•DI j • F•M•A•M ' j • j' A• S • O •

1993 1994

9.00 --

8.00 -

7.00 -

6.00

5.00

J I AI s l OI N! D[ J I F I MI AI Mi J I J I AI s l OI 1993 1994

Figure 3. (top) Temperature changes in the upper 100 m of the water column at the sediment trap mooring location for the period from July 1993 through October 1994. (bottom) Seasonal changes in the/5•SN of particulate organic matter collected from the sediment trap between July 1993 and October 1994.

EMMER AND THUNELL: EASTERN PACIFIC DENITRIFICATION 3 81

occur during the spring and summer, in association with upwelling.

The isotopic analyses of bulk sedimentary material from hole 893-A yielded •5•SN values ranging from 5.16 to 9.3396o (Figure 4), with an average •5•SN value for the past 50 kyr of--7.096o. The upper 15 kyr of the sediment record has higher •5•SN values than the rest of the core, averaging 7.88%o, whereas before 15 kyr, the average •5•SN value is 6.6096o. The nitrogen isotopic value for the uppermost sample, at a core depth of 0.05 m, is 7.2596o and is comparable to the average value of particulate material collected in our sediment trap (7.3896o). This similarity in nitrogen isotope composition between water column particu- lates and surface sediments further supports the recent findings that there is little diagenetic effect on the nitrogen isotope signal

along the continental margin of the eastern North Pacific [,,tita- bet et al., 1999].

In order to use 15•5N to reconstruct changes in denitrification through time, it first must be demonstrated that the nitrogen being analyzed is organic in nature. A comparison of total organic carbon (TOC) [Stein and Rack, 1995] to total nitrogen (TN) in the hole 893-A sediment record suggests that only a small portion of the nitrogen in the sediments may come from an inorganic source (Figure 5). If all of the nitrogen were organic, this plot would have intercepted the axes at zero. However, the fact that there is a positive intercept of TN indicates that a small amount of inorganic nitrogen is present in the sediments. As suggested by Stein and Rack [1995], the inorganic nitrogen could be fixed as ammonium ions in the interlayers of the clay

(• 15Norg 5.0 6.0 7.0

10-

20-

I I

8.0 9.0

30-

Figure 4. Time series of 15•q for the last 50,000 years for Santa Barbara Basin ODP hole 893-A.

10.0

I

382 EMMER AND THUNELL' EASTERN PACIFIC DENITRIFICATION

0.50

0.40

0.30

0.20

0.10

0.00

0.00

++

+ +

++

++

+

I I

2.00 4.00

Total Organic Carbon (wt %, carbonate free)

I 6.00

Figure 5. Plot of total organic carbon content versus total nitrogen content for the ODP hole 893-A samples used in this study.

minerals. However, since most of the samples have relatively high concentrations of total nitrogen (>0.1%), the effect of inorganic nitrogen on 5•SN should be negligible.

6. Discussion

6.1. Nitrogen Isotope Record' Evidence for Changes in ETNP Denitrification

The sediment trap nitrogen isotope results provide a basis for interpreting the downcore 15•SN record. For much of the year the particulate organic matter produced in Santa Barbara Basin surface waters has 15•SN values of between 7 and 99/00 (Figure 3). These values reflect the presence of isotopically enriched nitrate produced by denitrification and transported northward along the California margin from the subsurface oxygen-minimum zone in the ETNP. Sediment trap sample 15•SN values are highest (8-996o) during upwelling and accurately record the nitrogen isotopic composition of the subsurface waters in the basin (Figure 2). During the postupwelling period in the fall, 15•SN values for particulate matter drop to 6.0-6.59/00 and indicate less influence of waters affected by denitrification. On the basis of these observations, we interpret values of >7.096o in the hole 893-A 15•SN record as indicating the existence of a strong oxygen-minimum

zone and intense denitrification in the eastern tropical North Pa- cific. Conversely, values <7.096o are considered to reflect a better ventilated subsurface water mass and reduced denitrification in the ETNP.

The 15•SN record from Santa Barbara Basin ODP Hole 893-A shows distinct changes over the past 50 kyr (Figure 4). The timing of these changes suggests a strong relationship between global climate patterns and factors influencing the nitrogen isotopic composition of the water entering the basin. During the Holocene (the last 10 kyr) the /5•5N record decreases by over 2.096o, from 9.3 to 7.19/oo. The values >9.0096o at the beginning of the Holocene are significantly higher than the present-day average annual/5•SN value for particulate material (7.3896o) and suggest that denitrification in the ETNP was more intense at this

time. The decrease in/5•SN values from 10 ka to the present is mirrored in other nitrogen isotope records from the eastern tropical North Pacific [Ganeshrarn et al., 1995] and the Gulf of California [Pride et al., 1999] (Figure 6). Sigman et al. [2000] attribute this trend in the Holocene portion of the record to changes in the balance between denitrification and nitrogen fixation in the subtropical Pacific.

During the initial stage of deglaciation (Termination IA), nitrogen isotope values in Santa Barbara Basin increased by over


2.5•n, from -6.0960 at-17 ka to 8.7960 at -14 ka (Figure 4). This was followed by a rapid 3960 decrease during the Younger Dryas (YD) time (-12 ka) and an equally rapid increase to 8.8960 during the second stage of deglaciation (Termination IB). This sequence of abrupt changes in the •515N record during the most recent deglaciation is similar in character and timing to that seen in a variety of marine and terrestrial climate records for this time interval [Sowers and Bender, 1995]. This clearly indicates that conditions within the oxygen-minimum zone in the ETNP changed rapidly in response to the abrupt global climate changes that occurred during deglaciation. Specifically, the high •515N values (>8.796o) associated with Terminations IA and IB imply a greater level of denitrification and a more extensive oxygen- minimum zone than presently exists in the ETNP. Likewise, the

abrupt decrease in •515N during the Younger Dryas to values below 696o may indicate a significant reduction in the areal ex- panse of the oxygen-minimum zone and an associated decrease in denitrification in the ETNP at this time. A similar series of

changes during the most recent deglaciation is evident in the •5•SN record from the Gulf of California (Figure 6) [Pride et al., 1999].

During the Last Glacial Maximum (LGM), •5•$N values were significantly lower than during the Holocene (Figure 4). At 20 ka, •515N values are 5.996o, comparable to what is found presently in many open ocean regions [Liu and Kaplan, 1989; Sigman, 1997]. From the beginning of oxygen isotope stage 2 (24 ka) through 20 ka, values range from 4.86 to 6.7996o. Other nitrogen isotope records from the eastern Pacific also show low fi•-•N val-

10

9 Santa Barbara Basin

•:• 7

6

5

18 /

16 m

• 14--

•:• 12 --

10 m

/

Guaymas Basin (All 125-8 56)

10 /

/

/

/

Eastern Tropical North Pacific .

I I I I I I I ! I

0 10 20 30 40 50

Age (10 3 years) Figure 6. Comparison of the Santa Barbara Basin/5•-•1 time series with similar records from Guaymas Basin [Pride et al., 1999] and the eastern tropical North Pacific [Ganeshram et al., 1995].


ues during the last glacial (Figure 6). In the eastern tropical North Pacific, •515N values are ~7%o during the LGM in comparison to 9-1096o during the early Holocene [Ganeshram et al., 1995]. Likewise, in the Gulf of California, •515N values are ~1096o at the end of the last glacial and increase to nearly 159/oo at the beginning of the Holocene (Figure 4) [Pride et al., 1999]. These lower glacial values suggest that denitrification in the ETNP was reduced during this period relative to the Holocene.

From ~50 to 25 ka (isotope stage 3), the •515N record is marked by values that fall predominantly between 6 and 896o (Figure 4). These values are higher than those of the last glacial (isotope stage 2) and approach those of the late Holocene. In addition, the stage 3 section of the •515N record contains significantly more high-amplitude, high-frequency variability than found during stage 2 (Figure 4). This variability indicates rapid oscillations in the nitrogen isotopic composition of subsurface waters entering Santa Barbara Basin during stage 3 and suggests that the ETNP experienced millennial- and submillennial-scale fluctuations in the intensity of denitrification. It has been pro- posed that times of maximum ice volume (i.e., stage 2) are climatically more stable than times of intermediate ice volume (i.e., stage 3) [McManus et al., 1999], and this seems to be re- flected in the Santa Barbara Basin •515N record. On the basis of these observations, it is evident that the trends in the hole 893-A

•515N record are reflecting broad-scale regional changes in the •515N of subsurface nitrate.

What aspect of the ocean-climate system could be responsi- ble for the observed changes in denitrification in the ETNP? As previously mentioned, the present-day O2-minimum zone in the ETNP is maintained by high oxygen consumption rates related to high levels of surface productivity. Several studies [Ganeshram et al., 1995; Ganeshram and Pedersen, 1998] have demonstrated that productivity within the ETNP was lower during glacial times. According to these authors, a reduced land- ocean thermal gradient and a weakened North Pacific High during glacial conditions resulted in reduced upwelling and lower productivity. Furthermore, Ganeshram et al. [1995] concluded that this decrease in productivity resulted in a less intense O2-minimum zone and reduced denitrification within the ETNP during glacial periods. Similarly, Schulz et al. [1998] found that changes in the intensity of the oxygen-minimum zone in the Arabian Sea during the last 100 kyr were due to monsoonally driven changes in surface productivity.

While the •515N record reflects the character of the intermediate waters (~100-400 m) flowing into Santa Barbara Basin, the lithology of the sediments (bioturbated versus laminated) accu- mulating in the center of the basin is a function of the oxygen content of the bottom waters (>475 m). As discussed, these bottom waters are derived from North Pacific Intermediate Wa- ter (NPIW) that periodically flows over the deepest sill. A bioturbation index has been established for this site and provides a detailed record of temporal changes from laminated to bioturbated sediments [Behl and Kennett, 1996]. This index thus serves as a proxy for changes in NPIW oxygen content through time. Additionally, it has been suggested that changes in the ventilation of NPIW are related to cooling in the high latitudes of the northwest Pacific [Keigwin et al., 1992; Keigwin, 1998]. Comparison of the Santa Barbara Basin •SlSN record with the bioturbation index allows us to determine whether changes in the nature of the subsurface oxygen-minimum zone in the ETNP are synchronous with changes in NPIW ventilation (Figure 7).

During the Holocene, Santa Barbara Basin sediments are primarily laminated, indicating poor ventilation. During the Younger Dryas, sediments become massive and then return to laminated facies during the early stage of deglaciation (12-15 ka). Glacial stage 2 consists predominantly of bioturbated sediments [Behl and Kennett, 1996]. These major lithologic changes in Santa Barbara Basin sediments during the last 20-25 ka are synchronous with the large-scale changes in the •5•5N record (Figure 7). Similar trends in both •515N values and lithology are found in Guaymas Basin, Gulf of California, where high •515N values also correspond to occurrences of laminated sediments [Pride et al., 1999]. As with the Santa Barbara Basin, Guaymas Basin also contains NPIW and thus should record similar variations through time in the oxygen content of this water mass. The strong similarity between the •515N records and sediment lithology in both Santa Barbara and Guaymas Basins during the last 25 kyr indicates that there were synchronous changes in the ventilation of both the ETNP subsurface 02- minimum zone and NPIW. While changes in the ventilation of NPIW are most likely tied to rapid reorganizations of the global thermohaline circulation system [Kennett and Ingram, 1995; Keigwin, 1998], the observed changes in the ETNP probably are related to changes in surface wind patterns and primary productivity [Ganeshram et al., 1995].

The laminated intervals that occur throughout isotope stage 3 (~24-50 ka) have been correlated with the Dansgaard-Oeschger interstadial events recorded in the Greenland Ice Project 2 (GISP2) ice core (Figure 7) [Behl and Kennett, 1996]. These interstadials are abrupt, millennial-scale warming events and are characterized by rapid initial warming occurring over the course of several decades, after which there is a slower return to glacial conditions over a period of several hundred years (Dansgaard et al., 1993]. The correlation between the timing of the interstadials and the occurrence of laminated sediments in Santa Bar-

bara Basin suggests that these brief climate events are global in nature and that they affected ocean thermohaline circulation in such a manner to cause changes in the ventilation of NPIW [Behl and Kennett, 1996].

Correlation between the nitrogen isotope record and the bioturbation index is not as distinctive or straightforward during isotope stage 3 (Figure 7). The generally higher •5•5N values during this period compared to isotope stage 2 suggest more oxygen-depleted conditions, though not as severe as during the Holocene. Also, the stage 3 interval of the •515N record is marked by significantly more high-amplitude, millennial-scale variability than is found during stage 2. In this sense, the •515N and bioturbation records are similar. However, it is difficult to establish a one for one correlation between the occurrence of laminated

sediments during stage 3 and increases in the/5•5N record. While some of the peaks in •515N clearly are associated with laminated sediments, this relationship is not consistent. The differences between the two records may be related partly to the fact that the bioturbation index reported by Behl and Kennett [1996] and il- lustrated in Figure 7 is a highly smoothed curve (49-point run- ning average) intended to dampen high-frequency variability. Despite this, a basic similarity exists between the nitrogen isotope record and the bioturbation index, suggesting that millennial-scale changes in the ventilation of subsurface waters were occurring in both the eastern tropical Pacific and the northwest Pacific during stage 3.


Site 893 Bioturbation Index

10

,•, 20

<I• 30

40

50

Massive Laminated 4 3 2 1

I I I

6 7 8 9 10 -50 -45 -40

•5•SN org •180 ice Site 893 GISP 2

I I -35 -30

Figure 7. Comparison of the/5•SN record and the bioturbation index [Behl and Kennett, 1996] for ODP hole 893-A with the b•80 record from the GISP2 Greenland ice core [Grootes et al., 1993].

It has been suggested that during periods of cool climates such as the LGM, the Younger Dryas, and the stadial events of stage 3 NADW production was shut off or diminished owing to reduced heat transport to the northern latitudes [Keigwin and Jones, 1994; Oppo and Lehman, 1995; Curry and Oppo, 1997].

One possible consequence of such a change in the thermohaline conveyor system might be the generation of an intermediate water mass at the surface in the northwest Pacific [Duplessy et al., 1989; Kennett and Ingram, 1995; Van Geen et al., 1996; Keigwin, 1998], and this seems to be borne out by the bioturba-


tion index [Behl and Kennett, 1996]. In addition, our nitrogen isotope results indicate that ventilation changes also occurred in the eastern tropical North Pacific O2-minimum zone, most likely in response to changes in atmospheric circulation and surface productivity.

6.2. Glacial-Interglacial Changes in Denitrification and Effect on Atmospheric CO2

The oceanic reservoir of fixed nitrogen is controlled by the interplay between nitrogen fixation and denitrification. It is be- lieved that the oceans are currently losing fixed nitrogen owing to denitrification occurring in excess of nitrogen fixation [McEl- roy, 1983; Codispoti and Christensen, 1985; Codispoti, 1995; Galloway et al., 1995; Gruber and Sarrniento, 1997]. This is important since primary productivity levels are controlled by the availability of fixed nitrogen. The present-day net loss of fixed nitrogen from the oceans is driven largely by the intense denitrification occurring within the expansive oxygen-minimUm zones of the eastern tropical Pacific and the Arabian Sea.

Glacial-interglacial changes in the balance between nitrogen fixation and denitrification have important implications for ocean productivity and the exchange of carbon dioxide between the ocean and atmosphere. Our •jlSN results from Santa Barbara Basin, together with similar records from the Gulf of California [Pride et al., 1999], the eastern tropical North Pacific [Ganeshrarn et al., 1995], and the Arabian Sea [Altabet et al., 1995] clearly indicate that denitrification in the global ocean was reduced significantly during the last glacial (Figure 6). This would have increased the pool of available fixed nitrogen and led to higher rates of primary productivity. Indeed, studies from many different regions have found that marine productivity was higher during the last glacial than during the Holocene [Pealer- sen, 1983; Pedersen et al., 1988; $arnthein et al., 1987; Lyle, 1988; Thunell et al., 1992; Paytan et al., 1996; Falkowski, 1997]. Furthermore, higher rates of photosynthesis during glacial periods would have resulted in a drawdown of atmospheric CO2, and this would account for, at least in part, the lower atmospheric CO2 levels observed in the glacial sections of ice core

records [Oeschger et al., 1985; Barnola et al., 1987; Neffel et al., 1988]. In this model, the oceanic nitrogen cycle plays an important role in regulating glacial-interglacial changes in atmospheric CO2. This is in agreement with the recent findings of Broecker and Henderson [1998], although they emphasize the importance of enhanced nitrogen fixation during glacial times as the primary mechanism for increasing productivity.

7. Conclusions

A sediment •5•N record from Santa Barbara Basin off the coast of southern California provides evidence for high- frequency (millennial-scale) changes in the isotope composition of intermediate waters (---100-400 m) entering the basin during the past 50 kyr. These changes arc interpreted as reflecting variations in the intensity of denitrification occurring within the eastern tropical North Pacific oxygen-minimum zone and are attributed to climatically induced changes in surface productivity in the ETNP. The visual correlation between •5•N and the Santa Barbara Basin bioturbation index suggests that ventilation changes in the ETNP tend to bc synchronous with changes in NPIW ventilation occurring at high latitudes. The correlation between the •5•N and bioturbation records is particularly strong for the last 24 kyr, with high •5•N values corresponding to the deposition of laminated sediments during warm climate intervals.

The glacial-interglacial changes in •5•5N in the Santa Barbara Basin are consistent with those previously reported for the Ara- bian Sea, the eastern tropical Pacific, and the Gulf of California. Taken together, these data indicate that there was a significant reduction in dcnitrification in the glacial ocean during the last glacial period. This should have resulted in increased availability of nitrate and higher rates of productivity outside the ETNP, with the latter resulting in a drawdown of atmospheric CO2.

Acknowledgments. We thank E. Tappa for technical assistance. Sarnpl½s were provided by the Ocean Drilling Program through support from the National Science Foundation. This research was supported by grants from NSF (ATM-9622684 and ATM-9808508).

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(Received June 4, 1999; revised November 15, 1999; accepted February 24, 2000.)

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