PALEOCEANOGRAPHY, VOL. 14, NO. 3, PAGES 397-409, JUNE 1999

Nitrogen isotopic variations in the Gulf of California since the last deglaciation: Response to global climate change

Carol Pride, •'2 Robert Thunell, • Daniel Sigman, TM Lloyd Keigwin, 3 Mark Altabet, s

and Eric Tappa •

Abstract. High-resolution records of the nitrogen isotopic composition of organic matter (b•SNorg), opal content, and opal accumulation rates from the central Gulf of California reveal large and abrupt variations during deglaciation and gradual Holocene changes coincident with climatic changes recorded in the North Atlantic. Homogenous sediments with relatively low b lSNorg values and low opal content were deposited at the end of the last glacial period, during the Younger-Dryas event, and during the middle to late Holocene. In contrast, laminated sediments deposited in the two deglacial stages are characterized by very high b lSNorg values (>14%o) and opal accumulation rates (29-41 mg cm '2 yr-1). Abrupt shifts in b•SNorg were driven by widespread changes in the extent of suboxic subsurface waters supporting denitrification and were amplified in the central gulf record due to variations in upwelling, vertical mixing, and/or the latitudinal position of the Intertropical Convergence Zone.

1. Introduction

Observations from the eastern tropical North Pacific (ETNP) indicate that greater denitrification occurred during interglacial stages 1, 3, and 5 than during glacial stages 2, 4, and 6 [Ganeshram et al., 1995]. A similar pattern of increased interglacial denitrification and decreased glacial alenitrification has also been observed in the Arabian Sea

[Altabet et al., 1995]. Since, (1) nearly all water column alenitrification occurs within the eastern tropical Pacific (ETP) and the Arabian Sea, and (2) alenitrification results in a net loss of nitrate available to the biosphere, large and abrupt changes in alenitrification rates in these regions could have a significant impact on the nutrient inventory and productivity of the global ocean and hence atmospheric CO 2 [Codispoti, 1989; Altabet et al., 1995].

In this paper we present a high-resolution record of nitrogen isotopic variations in the central Gulf of California coincident with global climatic changes occurring from the late Holocene to the latter part of the last glacial period

1Marine Science Program and Department of Geological Sciences, University of South Carolina, Columbia.

2Now at Marine Science Institute, University of California, Santa Barbara.

3Woods Hole Oceanographic Institution, Woods Hole, Massachusetts.

4Now at Department of Geosciences, Princeton University, Princeton, New Jersey.

5Department of Chemistry and Biochemistry and Center for Marine Science and Technology, University of Massachusetts Dartmouth, North Dartmouth.

Copyright 1999 by the American Geophysical Union.

Paper number 1999PA900004. 0883-8305/99/1999PA900004512.00

(-17,000 calendar years (cal yr) B.P.). The abrupt changes present in this record are interpreted as deriving from variations in the rate of denitrification in the Gulf and ETP

and thus the •SN:•qN ratios of nitrate (b•SNso3-) available for primary production. The central Gulf of California is a particularly good location for studying past variation in nitrogen cycle dynamics in the subsurface water 02 minimum zone (OMZ) because of the delivery of nitrate to the euphotic zone through upwelling and/or water mass formation processes. Moreover, the sediments are excellent recorders of this signal because of the high accumulation rates and good preservation of organic matter.

2. Gulf of California: Climatic and

Oceanographic Setting

Unlike most midlatitude evaporative basins, the Gulf of Califomia has net outflow at the surface (above 250 m) and net inflow at depth [Bray, 1988] (Figure 1). Nutrient-rich subsurface water masses are relatively shallow and thus are able to sustain high rates of new production through upwelling [Alvarez-Borrego and Lara-Lara, 1991 ].

Coastal upwelling and seasonally reversing near-surface currents result from the monsoonal climate of the Gulf of

California region. In Winter the pressure field is dominated by the strong Great Basin high over the southwestern United States [Badan-Dangon et al., 1991 ]. The winter North Pacific high is relatively weak and is at its southernmost position while sea level pressure over Sonora is relatively low. The resulting winter mean pressure gradient induces strong northwesterly winds that are channeled down the axis of the Gulf of Califomia by surrounding mountains (Figure 2a) [Roden, 1958; Schrader and Baumgartner, 1983; Badan- Dangon et al., 1991]. Intensified winds, surface cooling, and high rates of evaporation in November weaken the winter

397

398 PRIDE ET AL.' GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD

500

lOOO

15oo

2000

2500

3000

Distance (km) 0 200 400 600 800

i i , I , t , ! i

'" CGW ( NGW(.....•

Nt'•W > OMZ

PDW

lOO

Carmen Basin

Temp. (øC) Salinity 0 15 30 34.4 35.0 35.6

0 _TSW_ l , I SSW

500 -

NPIW

1000

1500 PDW

2000

2500

(•N [air] NO•'

5 10 15

Guaymas Basin 0 15 30 34.4 35.0 35.6

CGW 250

ssw 500

5 10 15 I , ,I , I

Delfin Basin

500

15

I

Figure 1. Schematic of Gulf of California circulation from Bray [1988] and summer (July 1990) water column profiles of temperature, salinity, and •S15NNo 3_ from Carmen (26 ø 12.57'N, 110 ø 35.73'W), Guaymas (27 ø 54.40'N, 111 ø 39.28'W), and Delfin Basins (29 ø 50.42'N, 113 ø 53.08'W). Water masses are identified using temperature-salinity properties and definitions of Lavin et al. [1995]: TSW, tropical surface water; SSW, subtropical subsurface water; CCW, California current water; CGW, central gulf water; NGW, northern gulf water; NPIW, North Pacific Intermediate Water; and PDW, Pacific Deep Water.

PRIDE ET AL.: GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD 399

24 ø

18 ø

12 ø _

North Equatorial Current • i i i i

120 ø 114 ø 108 ø 102 ø

24 ø -

18 ø -

12 ø

North Equatorial Current •

Equatorial Counter Current I i i i

120 ø 114 ø 108 ø 102 ø

Figure 2. Climatology of the Gulf of California region: (a) winter upwelling conditions and weak northern equatorial gyre and (b) oligotrophic summer conditions and strong northern equatorial gyre. The thick arrow represents mean surface winds; the thin arrows represent simplified ocean currents (modified from Roden [1958]).

pycnocline [Robinson, 1973] and northwesterly winds induce coastal upwelling along the mainland margin resulting in nutrient transport to the euphotic zone [Roden and Groves, 1959; Roden, 1972; Soutar et al., 1981 ]. Primary production is high from the late fall through the early spring [Thunell et al., 1993] and is highest in Guaymas Basin [Alvarez-Borrego and Lara-Lara, 1991 ]. The northern equatorial gyre is weak in the winter and associated surface water masses (tropical surface water (TSW) and subtropical subsurface water (SSW)) remain at or near the mouth of the gulf (Figures 1 and 2).

In the summer the pressure field is dominated by a thermal low near the head of the gulf [Badan-Dangon et al., 1991 ]. This results in a reversal of gulf winds to weak southeasterlies and a cessation of upwelling (Figure 2b) [Roden, 1958; Roden and Groves, 1959; Schrader and Baurngartner, 1983; Badan- Dangon et al., 1991]. As surface waters warm, the water column becomes well stratified [Robinson, 1973], and primary production rates drop dramatically and remain low through the summer and early fall. At this time the northern equatorial gyre is strong and TSW and SSW penetrate northward toward the central gulf (Figures 1 and 2).

Water mass formation within the northern gulf significantly contributes to the high productivity of Guaymas Basin by producing surface waters rich in nutrients [Baurngartner, 1987]. Water mass formation occurs in the northern Gulf where SSW and Colorado delta water mix. by winter convection, tidal mixing, and buoyancy-driven horizontal circulation to form northern gulf water (NGW) (Figure 1) [Bray, 1988]. NGW is mixed with SSW and North Pacific Intermediate Water (NPIW) (Figure 1) in the island region where the water column is well mixed from 100 to 500

m during spring tides because of breaking internal waves [Bray et al., 1986]. This produces central gulf water (CGW). Since upwelling occurs within the depth range of this water mass the nutrient content of CGW largely determines the overall productivity of the Gulf of California [Baumgartner, 1987].

Today there is a distinct OMZ present in the Gulf of California from-• 500 to 1000 m water depth, the depth at which nutrient-rich NPIW [Alvarez-Borrego and Lara-Lara, 1991 ] enters the mouth of the gulf (Figure 1). Dysaerobic to anaerobic conditions within the OMZ prevent the colonization of benthos, and the resulting lack of bioturbation allows seasonal laminations to be preserved [Revelle, 1950; Calvert, 1964; Van Andel, 1964; Baumgartner et al., 1991; Thunell et al., 1993; Thunell, 1998]. The oxygen content of SSW (resident from 100 to 400 m water depth outside the Gulf) is also low, and the nitrogen isotopic composition of nitrate within both of these water masses shows evidence of

denitrification [Cline and Kaplan, 1975; Altabet et al., 1999; Brandes et al., 1998].

3. Methods

In July 1990, water samples were collected in Carmen, Guaymas, and Delfin Basins for nitrogen isotopic analysis of nitrate. The samples were preserved for nitrate isotopic analysis by acidification to a pH of 2 with 50% HC1. Water masses present at the time of sample collection were identified using conductivity-temperature-depth (CTD) profiles.

An automated PARFLUX (IV) sediment trap continuously collected a time series of sediment samples (each representing

400 PRIDE ET AL ß GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD

2 weeks) from Guaymas Basin. Sample cups were filled with buffered filtered seawater containing sodium azide as a poison. The trap was positioned-• 180 m off the seafloor at a water depth of 485 m on the eastern slope of the basin (Table 1). Data are presented for the period from March 1993 to November 1996. Trap samples were subsampled using a precision rotary splitter and were refrigerated until processed.

A series of piston, gravity, and box cores was collected in July 1990 from the eastern and western sides of Guaymas Basin (Table 1). Jumbo piston core 56 (JPC-56) (water depth = 818 m) is -• 19 m long, and the upper part of the core is missing because of overpenetration. Although cores JPC-44 (655 m) and JPC-48 (530 m) are shorter than JPC-56, they all contain records extending from the Holocene to the end of the last glacial. Giant gravity core 55 (GGC-55) (820 m) contains younger sediments and is -• 3.5 m long. Box core 57 (BC-57) (785 m) consists of recent bioturbated sediments, while BC- 43 (655 m) is composed of laminated sediments.

Age models for cores JPC-56 and' GGC-55 were constructed using 14 accelerator mass spectrometry (AMS) radiocarbon ages measured on Globigerina bulloides at the National Ocean Sciences AMS Facility at the Woods Hole Oceanographic Institution (Table 2 and Figure 3). Radiocarbon ages were converted to calendar years B.P. using the CALIB program [Stuiver and Reimer, 1993] and a reservoir correction of 520 + 40 years [Stuiver and Braziunas, 1993]. Ages of lithologic boundaries were determined by interpolation between radiocarbon dated intervals. The/5•80 values of benthic foraminifera (Bolivina spp.) were used to develop chronologies for cores JPC-44 and JPC-48 (Figure 3). The sedimentation rates for box cores used in this study were determined by 2•øpb dating as described by Kuehl et al. [1993].

The opal (biogenic silica) concentrations of core and sediment trap samples were determined by the Mortlock and Froelich [1989] technique. Opal accumulation rates for JPC- 56 were calculated as % opal x dry bulk density x sedimentation rate. Atomic carbon to nitrogen ratios were determined using a Perkin Elmer 2400 CHN elemental analyzer after dissolution of calcium carbonate with H3PO 4.

Isotopic analyses of seawater nitrate were performed by extraction as ammonium using the "passive ammonia diffusion" method [Sigrnan et al., 1997], combustion to N2 using a Europa Roboprep elemental analyzer, and on-line

isotopic analysis by a Finnigan MAT 251 stable isotope ratio mass spectrometer. The/5•SN and/513Corg values of sediment samples were determined on a VG OPTIMA stable isotope ratio mass spectrometer linked to a Carlo Erba elemental analyzer. Samples for/5•3C analysis were acidified to remove calcium carbonate and thus reflect the stable isotopic composition of the organic fraction. The /5•3Cor • values are reported in per mil relative to Peedee belemnite (PDB). Nitrogen isotopic ratios are reported in per mil relative to air N 2 with reproducibility less than or equal to 0.3%0.

4. Results

4.1. Water Column Results

A profile of/5•5NNo 3- from Carmen Basin, near the mouth of the gulf reveals a profile similar to that of the ETNP [Cline and Kaplan, 1975; Altabet et al., 1999; Brandes et al., 1998]. Nitrogen-15-enriched nitrate is present with values above 9%0 in the upper water column with a maximum of 14%o near the boundary of SSW and NPIW (Figure 1) [Altabet et al., 1999]. In contrast, profiles from Guaymas Basin and Delfin Basin of the northern gulf reveal an upper water column homogenous with respect to/5•5Nso 3- in July 1990 due to mixing processes during the formation of CGW (Figure 1) [Altabet et al., 1999]. CGW is the dominant surface water mass in the central Gulf, and surface /515NNo 3- values are high (11-12%o) where this water mass is present (Figure 1) [Altabet et al., 1999]. All measured /5•5Nso 3- values are significantly higher than the global ocean average of 4.5-5.0%0 [Sigman et al., 1997] and clearly show the influence of partial removal of NO3- by denitrification throughout the region.

4.2. Sediment Trap Results

Opal fluxes to the sediment traps varied seasonally with maxima during the winter phytoplankton bloom (Figure 4). A maximum opal flux of 0.89 g opal m-2 d -• occurred during the winter of 1993-1994. In the winter, opal composes > 50% of the total particulate flux, while in the summer, opal content drops below 30% (Figure 4). This seasonal variation produces alternating light and dark colored laminae that are preserved as vatyes within the OMZ [Calvert, 1964; Donegan and Schrader, 1982; Baumgartner et al., 1991; Thunell et al., 1993].

Table 1. Core and Sediment Trap Locations

Cores (AII125-8) and Water Depth, Sediment Trap Latitude Longitude m

Sediment Trap 27 ø 53'N 111 ø 40'W BC-43 27 ø 54.09'N 111 ø 39.35'W BC-50 27 ø 47.15'N 111 ø 42.5 I'W BC-57 27 ø 27.21'N 112 ø 7.20'W

GGC-55 27 ø 28.22'N 112 ø 6.33'W JPC-56 27 ø 28.16'N 112 ø 6.26'W JPC-48 27 ø 56.40'N 111 ø 47.90'W JPC-44 27 ø 54.09'N 111 ø 39.30'W

BC, box core; GGC, giant gravity core; JPC, jumbo piston core

485 655 745 785 820 818

530 655

PRIDE ET AL.: GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD 401

Table 2. Results of Accelerator Mass Spectrometry

Core

(All-125)

Radiocarbon Reservoir

Core Accession Age + Corrected Calibrated 1 Sigma Depth, Number Standard Radiocarbon Calendar Calendar Age

cm Error, years Age, years Age, years Range, years

GGC-55

JPC-56

60 OS-5173 3,420 + 35 2,900 2,701 210 OS-5174 4,710 + 40 4,190 4,190 320 OS-5175 5,610 + 45 5,090 5,090 100 AA12033 5,630 + 60 5,110 5,460 420 AA12043 9,355 + 70 8,835 9,450 470 AA12044 9,705 + 75 9,185 9,860 650 AA12034 10,125 + 80 9,605 10,290 900 AA12035 10,945 + 75 10,425 11,550 1000 AA12036 11,490+ 90 10,970 12,500 1100 AA12037 12,395 + 80 11,875 13,390 1400 AA12039 13,400 + 90 12,880 14,620 1600 AA12040 13,685 + 100 13,165 15,060 1700 AA12041 14,580+ 100 14,060 16,370 1850 AA'12042 14,870 + 100 14,350 16,730

2,565-2,728 4,168-4,350 5,328-5,528 5,337-5,562 9,385-9,507 9,798-9,933

10,151-10,387 11,383-11,776 12,366-12,616 13,276-13,519 14,420-14,832 14,821-15,279 16,203-16,535 16,573-16,888

From 1993 to 1996 the Guaymas •15Npo m record is inverse to that of opal content. The •515Npo m value is low during the highly productive winters and high during oligotrophic summer conditions with values ranging from 6.6 to 11.4%o and a mean of 9.6%o (Figure 4).

When the intercept of the regression line relating weight percent organic carbon to weight percent total nitrogen is extended to 0% organic carbon, there is negative percent total nitrogen. This indicates that although measured b•SN values are of total nitrogen, there is no significant inorganic component of the total nitrogen nor excess nitrogen derived from adsorption onto particles [Hedges et al., 1988]. This is also true for surface sediments and core samples. We therefore assume that essentially all nitrogen derives from

organic matter flux and that measured b•SN is actually blSNpo m (or b lSNorg for sedimentary values). Detailed analysis of the •515Npo m time series is presented elsewhere [Pride, 1997; Altabet et al., 1999].

4.3. Sediment Core Results

Most of the interpretations presented in this paper are based on data from JPC-56 since this core provides the longest well-dated record. Our correlation of cores GGC-55, JPC-44 and JPC-48 to JPC-56 is presented in Figure 3. The results of AMS radiocarbon dating and the existence of similar opal content (Figure 5) and benthic foraminiferal records (S. Poli, personal communication, 1997) in the neighboring cores GGC-55 (820 m) and JPC-56 (818 m)

GGC-55 J PC-56 J PC-48 J PC-44 West Guaymas West Guaymas East Guaymas East Guaymas

820 rn 818 rn 530 rn 655 rn

o

500

1000

1500

2000 --

b•80 b•80 (ø/oo [ PDB ]) (ø/oo [ PDB l) 5 4 3 2 5 4 3 2

•,///////• ,,o•

11875/

[ t14060 •;• laminated •14350

Figure 3. Stratigraphic correlation of cores used in this study using lithologic characteristics, reservoir-corrected accelerator mass spectrometry (AMS) radiocarbon dates for giant gravity core 55 (GGC-55) and jumbo piston core 56 (JPC-56), and oxygen isotopic results for cores JPC-48 and JPC-44 (Bolivina spp.). Depths of AMS •4C dates are labeled.

402 PRIDE ET AL.: GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD

X • 0.8 •

0.4 !

0.0

90 /

g• 60

O•

E

12 /

a j , , \ I• r\ I x I• • I I

I I

1993 1994 1995 1996

b I• r •Jr•x I• e- ' 'I'\, /

993 1994 1995 1996

I I I t I t I \ /

993 1994 1995 1996

m2

- N

1 o

o 0

I

Pigment Conc.

Figure 4. Guaymas Basin sediment trap records of (a) opal flux, (b) opal content, and (c) (•15Npo m superimposed on a repeating cycle of long term monthly average surface pigment concentrations for waters overlying the Guaymas Basin sediment trap site derived from the coastal zone color scanner.

indicate that they overlap in time (Figure 3). However, GGC- 55 is laminated, while the uppermost section of JPC-56 is bioturbated. Either one of the radiocarbon dates for this interval is not accurate, or the Holocene bioturbation event evident at the top of JPC-56 did not occur basin-wide. The fact that recently deposited sediments are bioturbated at 785 m water depth (BC-57) near the core sites of GGC-55 and JPC-56, despite lying within the depth range of the modem OMZ suggests that oxygen concentrations could be more variable along the western margin than in other regions of Guaymas Basin. Benthic foraminiferal b•80 values suggest that full glacial sediments may be absent from JPC-48 and that late Holocene sediments are missing from JPC-44.

The nitrogen isotopic record for the last deglaciation is characterized by abrupt and large changes, while much of the Holocene record is characterized by a gradual decrease in b • (Figures 6 and 7) The b•SNo•g values increased from a SNorg . minimum of 8%o in the late glacial (JPC-48, Figure 6) to a maximum of 16%o (JPC-56, Figure 7) within the first stage of deglaciation. Similarly, abrupt shifts in b•SNo•g occurred at the initiation and termination of the Younger-Dryas interval

(Figures 5 and 7). A slow decrease in b•SNo•g occurred throughout the most of the early to middle Holocene laminated section, with minimum values occurring at- 4600 cal yr B.P. Results from GGC-55 suggest that b•SNo,g then remained near modem values (BC-57 •5•5No•g- 10.3 %o)with moderate oscillations possibly related to Neoglacial events (Figures 5 and 7). The b•SNo,g profiles of JPC-44 and JPC-48 from the eastern slope of Guaymas Basin are very similar to that of JPC-56, suggesting that isotopic variations reflect phenomena occurring throughout the central gulf (Figure 6).

Major downcore variations in JPC-56 and GGC-55 opal content coincide with changes in nitrogen isotopic composition (Figures 5 and 7). Opal content is high in the laminated Bolling-Allerod and Holocene sections and low in homogenous sediments representing the end of the last glaciation, the Younger Dryas, and the late Holocene (Figure 7).

The JPC-56 record of opal accumulation also closely resembles the nitrogen isotopic record with large abrupt changes throughout the last deglaciation (Figure 7). The primary differences are a rapid drop to low values below the

PRIDE ET AL.: GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD 403

Guaymas Basin •'SNo,. Cariaco Basin

(ø/oo [PDB]) Grey-Scale 9 13 17 160 180 200

2000 I • I GGC-5$

6000

10000

14000 ß

18000 I I I I ' I ' -42 -38 -34 0 25

GRIP (ø/oo [ PDB ])

N. Sargasso Sea % CaCO•

15 25 35

I , I , I ,

Neoglacial

,

Younger Dryas /

Last Glacial

I 50

Guaymas Basin % Opal

Figure 5. Combined piston (JPC-56) and gravity core (GGC-55) records of •]SNorg and opal content from Guaymas Basin are plotted with a Greenland Ice Core Project (GRIP) ice core b]gO record at 200-year intervals [Johnsen et al., 1993], a grey-scale record from Cariaco Basin in which dark sediments (high values) represent reduced upwelling and a northerly Intertropical Convergence Zone (ITCZ) [Hughen et al., 1995], and a weight percent carbonate record from the northem Sargasso Sea (giant piston core 5 (GPC-5)) in which reduced % CaCO3 reflects ice rafting [Keigwin, 1996].

GGC-55 JPC-56 JPC-48 JPC-44

West Guaymas West Guaymas East Guaymas East Guaymas 820 rn 818 rn 530 rn 655 rn

(•SNorg (•SNorg (•'•Nor. •5•SNorg 81216812168121681216

1000

• homogenous 1500

• laminated 2000 --

Figure 6. High-resolution profiles of bulk sediment •5•SNorg of cores GGC-55 and JPC-56 from the western slope of Guaymas Basin are presented with low-resolution nitrogen isotopic profiles of cores JPC-48 and JPC-44 from the eastern slope. All cores lie within the depth range of the modem oxygen minimum zone. Laminated intervals are shaded. Lines connecting the cores represent our tentative correlations.

404 PRIDE ET AL.: GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD

4000

8000

12000

16000

•15Nor . ( %o [air])

9 13

!

Opal A.R.

% Opal (mg cm -• yr -•) 17 0 25 50 0 50 100

I I 'IF I I , !

Neoglacial

Y-D

Late Glacial

Figure 7. Profiles of •lSNorg, opal content, and opal accumulation rate are plotted against time for the radiocarbon-dated JPC-56. Shaded regions represent laminated sediments. Modem surface sediment values for this region (box core 57 (BC-57)) are indicated by inverted triangles.

Younger Dryas interval where b•SNorg gradually decreases and a lack of significant Holocene variability. The latter could be an artifact of assuming constant sedimentation rates from 420 cm to the core top (Table 2 and Figure 3).

5. Discussion

5.1. Paleoceanographic Proxies for Upwelling, Denitrification, and NPIW Ventilation

This study relies on four paleoceanographic proxies: (1) sedimentary opal content, (2) opal accumulation rate, (3) b•SNorg, and (4) the presence of laminated or homogenous sediments at modem OMZ depths. Opal content is used to determine times when diatoms contributed significantly to overall sedimentation in Guaymas Basin. Opal accumulation rates are used more directly to identify periods in which upwelling of nutrient-rich waters and diatom blooms occurred in the central Gulf of California. Nitrogen isotopic values are used to identify periods when •5N-enriched nitrate resulting from subsurface denitrification becomes incorporated in phytoplankton biomass. The presence of laminated sediments within sediment cores used in this study is an indicator of suboxic conditions in the depth range of Pacific intermediate waters.

5.1.1. Opal accumulation rates and content. Results of a time series sediment trap study in Guaymas Basin suggest that opal flux and content are good sedimentary proxies for the occurrence of upwelling-associated diatom blooms in the Gulf of California (Figure 4)[Pride, 1997; Thunell, 1998]. Opal fluxes are high during phytoplankton blooms because of the dominance of diatoms under conditions of high nutrient availability [Byrne, 1957; Schrader et al., 1980; Donegan and Schrader, 1982; $ancetta, 1995; Thunell et al., 1996]. Opal

fluxes are low in the summer because of oligotrophic conditions in the presence of a strong thermocline (Figure 4).

Use of opal accumulation rates as a paleoceanographic proxy for primary productivity can be problematic. It is used here not as a measure of absolute productivity but as an indicator of the occurrence of diatom blooms. The Gulf of

California under modern conditions possesses many of the characteristics identified by Nelson et al. [1995] as contributing to good opal preservation. These factors include being a coastal upwelling system with the occurrence of seasonal diatom blooms, aggregate formation, fecal pellet production, and rapid sedimentation in relatively shallow water. Although most of the biogenic silica produced in the gulf is dissolved within the upper water column [Thunell et al., 1994], seasonal extremes in silica production are recorded in variable sediment fluxes to the sediment trap at 485 m water depth (Figure 4). Dissolution of silica in the lower water column and in the surface sediments does not appear to be significant since the accumulation rates (8 mg cm -2 yr 4) of biogenic silica in a box core top (BC-43) underlying the Guaymas Basin sediment trap and in the Holocene homogenous section of JPC-56 (8 mg cm -2 yr 4) are similar to average opal fluxes to the sediment trap (7 mg cm '2 yr4).

5.1.2. Nitrogen isotopic composition. The nitrogen isotopic composition of organic matter preserved in gulf sediments potentially varies as a function of several parameters: (1) partial depletion of subsurface nitrate by denitrification in the OMZ of the Gulf and the ETP, (2) the process of surface water formation, (3) nitrate utilization in surface waters, (4) diagenesis, and (5) input of terrestrial organic matter. We believe that observed downcore b•SNorg changes are primarily caused by variability in the isotopic composition of the nitrate supply. Variability in surface water

PRIDE ET AL.: GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD 405

/515NNo 3- values is, in turn, a function of subsurface denitrification and transport of the resultant •SN-enriched nitrate to the photic zone. Fractionation during the denitrification process leads to preferential consumption of •4NNo 3- and enrichment of the residual nitrate in •SN [Miyake and Wada, 1971; Cline and Kaplan, 1975; Mariotti et al., 1982]. Results of Altabet et al. [1999] suggest simple Rayleigh fractionation occurs during denitrification, yielding a fractionation factor of-25% o. The water column b•SN

ß NO 3- profile from the Carmen Basin shows a marked isotopic enrichment of nitrate (14%o at 400 m water depth) due to denitrification in the OMZ (Figure 1) [Altabet et al., 1999].

In order for the effects of water column denitrification to

be recorded in the sedimentary record the isotopically heavy nitrate derived from denitrification must be supplied to the surface and incorporated in the phytoplankton biomass. A b •SNNo 3- profile for the upper 500 m of Guaymas Basin reveals uniformly high values (11-12%o) in a homogenous layer where CGW and SSW were present in the summer of 1990 [Altabet et al., 1999]. This record reflects the mixing processes that occur in the formation of CGW and the elevation of surface blSN?4o3- values in the central and northem gulf due to these processes. Sediment trap results show the effects of this isotopic enrichment with an average Guaymas Basin /5•SNpo m value of 9.6 %o [Pride, 1997; Altabet et al., 1999] relative to typical open-ocean blSNNo3 - values of 4.5- 5%o (Figure 4) [Liu and Kaplan, 1989; Sigman et al., 1997].

The nitrogen isotopic composition of sinking particulate organic matter (pom) varies seasonally about this high average value because of variable nitrate utilization in Guaymas Basin (Figure 4). When nutrients are abundant, nitrate is readily available for phytoplankton growth, and the lighter •4N isotope is preferentially incorporated into the biomass [Wada and Hattori, 1978; Altabet and McCarthy, 1985; Montoya, 1994; Altabet et al., 1991; Altabet and Francois, 1994]. The /5•SNpo m values thus tend to decrease during winter/spring phytoplankton blooms because of low nitrate utilization and maximal isotopic fractionation from the nitrogen source. When nutrient concentrations are low, as in summer conditions in the gulf, less fractionation occurs during photosynthesis (high utilization), resulting in /5•SNpo m values similar to the b•SN of nitrate [Altabet, 1988]. In July 1990, for example, the value of b•SNNo3 - at 60 m water depth in Carmen Basin was 9.73%o (Figure 1). The corresponding /5•SNpo m value from a sediment trap sample collected over the following 2 weeks in Carmen Basin was 9.76%o [Pride, 1997, Altabet et al., 1999].

The occurrence of low /5•SNpo m values in the winter and high values in the summer results in an inverse seasonal relationship between opal.content (or fluxes) and b•SNpom (Figure 4). If b•SNorg values of the sedimentary record were dominantly controlled by changes in nitrate utilization, we would expect to find an inverse relation between opal content and b•SNo•g similar to the seasonal pattem seen in the sediment trap record (Figure 4). In the downcore record, however, a strong covariance is observed between opal content and b•SNo•g (Figure 7). This leads us to the conclusion that varying rates of nitrate utilization are not a significant driving force behind the large and abrupt millenial-scale oscillations in /5•SNorg. On a annual basis, all upwelled NO3- is utilized and,

under these conditions, the average b•N of sinking pom must equal the b•N of upwelled NO3-. This equivalence has been shown at a number of sites in addition to the Gulf of

California [Altabet et al., 1999]. Our sediment trap and surface sediment results suggest that

the effect of diagenesis on the b•SNo•g record is minimal within gulf sediments. There is only a 1%o difference between water column /5•SNpo m and surface sediment b•SNorg values in Guaymas Basin [Pride, 1997] similar to other regions along the margin of the eastern North Pacific [Altabet et al., 1999]. This is likely due to good preservation of organic nitrogen in these regions [Altabet et al., 1999]. A comparison of surface samples from within the OMZ and below the OMZ also shows that the nitrogen isotopic record does not vary between aerobic and anaerobic depositional environments [Pride, 1997]. In addition, profiles from cores IPC-44 and IPC-48 show only a very gradual downcore decrease in N content, indicating that the large b•SNorg variations in these cores are not due to diagenetic processes.

The large nitrogen isotopic shifts present in the gulf record also are not the result of variable contributions of terrestrial

organic matter. The refractory nitrogen, which is most likely to survive transport and burial in marine sediments, varies about a range of 0-3%o [Sweeney et al., 1978 ], much lighter than observed gulf /5•SNpo m values. Isotopic and elemental analysis of core-top sediments (b13Corg =-20.0 %o, [C/N]a-- 9.3) and sediment trap material (b•3Corg = -20.6 %o, [C/N]a = 8.3) suggests that organic matter accumulating in the modem Guaymas Basin is predominantly of marine origin [Pride, 1997; Thunell, 1998].

Carbon isotopic and [C/N]a records were produced for JPC-56 to determine whether past inputs of terrestrial matter significantly influenced the b•SNorg record of Guaymas Basin. Only within the homogenous late glacial section of this core do both b•3Corg and [C/N]a deviate slightly toward values suggestive of contributions of terrestrial organic matter (/513Corg --' -21.5%o, [C/N]a = 14.5). To explore the potential contribution of increased terrestrial carbon content to the

rapid 7.3%o change in/5•SNorg at the end of the last glacial, we used terrestrial (C3) and marine /5•3Corg end member values to estimate that at most 25% of the organic carbon in late glacial sediments was of terrestrial origin. Using a wide range of possible terrestrial/SlSNorg values, we then calculated that such a shift in terrestrial contribution to the total organic content could only account for 1%o of the observed 7.3%o change in /5•SNorg across the glacial-interglacial transition. The remainder of the nitrogen isotopic change is attributed to changes in the /5•SN of nitrate available in the euphotic zone.

5.1.3. Laminated sediments. The presence of laminated sediments in the Gulf of Califomia is indicative of oxygen concentrations below 5 [tM (Y. Zheng, A. van Geen, R. F. Anderson, J. V. Gardner, and W. E. Dean, Intensification of the noaheast Pacific oxygen-minimum zone during the B611ing-Aller6d warm period, submitted to Science, 1999, hereinafter referred to as Zheng et al., submitted manuscript, 1999) prohibiting burrowing by benthic organisms within the depth range of the modem OMZ. The intensity of the Gulf of Califomia OMZ is dependent on the oxygen content of infiowing NPIW and the demand for oxygen at depth. Although Guaymas Basin is silled, the OMZ and sediment

406 PRIDE ET AL.: GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD

cores used in this study lie above sill depth and are subjected to intermediate and deep water properties similar to those of the open Pacific. OMZ intensity is thus largely a function of the oxygen content and ventilation of NPIW. Opal records suggest that intense diatom blooms may have enhanced oxygen demand at times in the past and contributed to the central Gulf record of altemating laminated and homogenous sediments. However, the lack of correspondence between lithologic transitions and changes in the composition of JPC- 56 diatom assemblages suggests that variable bloom intensity was not the dominant control of OMZ intensity [Sancetta, 1995].

5.2. Coincidence of 8•SNorg Variations With Global Climate Changes

It has previously been shown that major sedimentary changes in the gulf during the last glacial-interglacial cycle coincided with global-scale climatic changes [Keigwin and Jones, 1990; Sancetta, 1995]. The current study improves upon the chronology presented for Deep Sea Drilling Project (DSDP) site 480, extends this record to the late Holocene, and documents changes in b•SNorg and opal. Intervals of reduced b•SNorg values, low opal content, and deposition of homogenous sediments in the central gulf coincide with times of reduced North Atlantic Deep Water (NADW) production [Boyle and Keigwin, 1987; Maier-Reirner and Mikolajewicz, 1989; Lehman and Keigwin, 1992], suggesting coincident climatic changes in the North Atlantic and North Pacific regions throughout the last glacial-interglacial cycle. The first major transition in lithology, opal content, and 6•SNorg in JPC- 56 is dated at 14,900 cal yr B.P. and coincides with similar transitions off central and southem Catifomia [Kennett and Ingram, 1995; Zheng et at., submitted manuscript, 1999] and with the first abrupt warming of the North Atlantic as recorded in Greenland ice cores (14,500 cal yr B.P.) (Figure 5) [Johnsen et al., 1992; Grootes et aL, 1993]. Deposition of laminated sediments ended and 6•SNorg values and opal content decreased at - 12,600 cal yr B.P., which marks the beginning of Younger Dryas conditions in the gulf. This transition was dated at 12,970 cal yr B.P. in Santa Barbara Basin [Kennett and Ingram, 1995]. The transition from Younger Dryas to Holocene deposition in the gulf occurred at-• 11,600 cat yr B.P. and is synchronous with the Greenland Ice Sheet Project (GISP)2 ice core record, which places this transition at 11,640 cal yr B.P. [Taylor et aL, 1993]. The final lithologic transition in JPC-56 is dated at 6400 cal yr B.P. Lower-amplitude changes in •SNorg and less abrupt changes in opal content in this Holocene homogenous section may represent Neoglacial climatic fluctuations. Neoglaciation is loosely defined as a series of century-scale climate variations observed in records of glacial advance [Porter, 1981], lake levels [Enzel et al., 1989], and deep sea sedimentation [Keigwin, 1996]. The largest of these events is the Little Ice Age [Overpeck, 1989].

5.3. Variations in the Gulf of California System Since the Late Glacial Period

Sediments presently accumulating within the Gulf of Califomia OMZ are marked by the. effects of high productivity and denitrification. Seasonal upwelling and

diatom blooms in the Gulf of Califomia result in high accumulation rates of opal. Denitrification within inflowing intermediate water masses (NPIW and SSW) and incorporation of •SN-enriched nitrate within the biomass result in high sedimentary /5•5Norg values. Although opal accumulation rates of 16 mg cm -2 yr 4 and 6•SNorg values of 10%o are high compared to typical open-ocean values, the records presented here show that they are moderate compared to past conditions in the central gulf (Figure 7).

The two stages of deglaciation were characterized by an amplification of modem conditions. During the Bolling- Allemd and Preboreal intervals, opal accumulation rates averaged 12-25 mg cm -2 yr 4 higher than modem values. Diatom assemblages from JPC-56 are indicative of strong upwelling during deposition of the Bolling-Allemd laminated interval but reduced spring upwelling during the Preboreal, with most of the production occurring from the summer through the winter [Sancetta, 1995]. In addition to variable upwelling conditions, the nutrient content of upwelled waters likely increased during deglaciation because of the inflow of older more nutrient rich subsurface waters and/or the year- round presence of SSW in the central and northem gulf regions.

In contrast, our opal records suggest that nutrient supply and diatom production during the late glacial period and the Younger Dryas event were lower than today. These results are consistent with other observations of reduced productivity associated with homogenous sediments in the gulf [Schrader, 1982; Murray and Schrader, 1982; Kelts and Niemitz, 1982; Crawford and Schrader, 1982; Sancetta, 1995]. Ganeshram and Pederson [ 1998] attributed reduced primary productivity in glacial intervals off northwest Mexico over the past 140 kyr in part to reduced land-ocean thermal contrast and reduced wind-induced upwelling. Similar processes could operate during brief cooling events such as during the Younger Dryas. In addition to variable upwelling conditions, the nutrient content of upwelled waters may have decreased during cool periods because of the inflow of younger nutrient- poor subsurface waters. Better ventilated subsurface waters and reduced productivity facilitated oxic conditions within the depth range of the modem OMZ.

Opal content records suggest potential reductions in diatom blooms during late Holocene Neoglacial oscillations that may not show up in the opal accumulation rate records because of the assumption of constant sedimentation rates over this interval (Figures 5 and 7). Deposition of low opal content sediments in Guaymas Basin in the Holocene coincided with Neoglacial variations observed in the North Atlantic [Keigwin, 1996] and elsewhere [Denton and Karlen, 1973; Porter, 1981; Grove, 1988] (Figure 5). These climatic variations include enhanced ice rafting in the northern Sargasso Sea indicated by reduced sedimentary CaCO 3 content (Figure 5) [Keigwin, 1996], a cooling event recorded in the eastern subtropical Atlantic [Maslin and Tzedakis, 1996], and glacial readvances in southern Norway [Matthews, 1991] and western North America [Porter, 1981]. Although the implications and geographic extent of Holocene fluctuations in opal content (Figures 5 and 7) in the gulf are yet to be determined, their coincidence with other Northern Hemisphere climatic oscillations suggests the functioning of

PRIDE ET AL.: GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD 407

climatic processes similar to those of the Younger Dryas and the last glaciation. Such Holocene climatic variability in the eastern North Pacific deserves further investigation.

Reduced b•SNorg values in the late glacial and Younger Dryas intervals support lithologic evidence in the central gulf of enhanced ventilation of intermediate waters from the North

Pacific. Enhanced glacial intermediate water ventilation has also been observed in the eastern tropical North Pacific [Ganeshram et al., 1995], in Santa Barbara Basin [Kennett and Ingram, 1995; Behl and Kennett, 1996], off Central California [Van Geen et al., 1996; Zheng et al., submitted manuscript, 1999], and in the northwest Pacific [Duplessy et al., 1989; Keigwin, 1998]. Evidence of enhanced intermediate depth ventilation during the Younger Dryas includes bioturbated sediments along the California margin [Kennett and Ingram, 1995; Van Geen et al., 1996] as well as in the Gulf of California [Keigwin and Jones, 1990] and relatively low radiocarbon-based ventilation ages at intermediate depths in the North Pacific [Duplessy et al., 1989]. While the Younger Dryas and glacial episodes of enhanced intermediate water oxygen content and bioturbation occurred basin wide (at least within the depth range of our cores), the extent of oxic sediments at intermediate water depths in the late Holocene (Neoglacial) Guaymas Basin is uncertain.

Nitrogen isotopic records from the central Gulf of Califomia are consistent with those from the ETNP

[Ganeshram et al., 1995] and Arabian Sea [Altabet et al., 1995] and suggestive of an overall increase of water column denitrification during interglacial stages and a decrease in glacial stages (Figs. 5 and 7). Such a pattern would result in a net loss of nitrate available to the biosphere during interglacials due to widespread denitrification and could lead ' to reduced primary production and increased atmospheric CO2 concentrations [Codispoti, 1989; Altabet and Curry, 1989; Altabet et al., 1995; Ganeshram et al., 1995]. In addition, our records suggest that the trend of reduced denitrification during cool periods extends to shorter timescale events such as the Younger Dryas and perhaps the Neoglaciation. Since marine nitrate has a relatively short residence time of 3 kyr, even these brief fluctuations in &nitrification can have a significant impact on the marine N inventory.

There are two major differences between the Guaymas Basin glacial-interglacial nitrogen isotopic record and a record from a more open ocean setting off northwest Mexico [Ganeshram et al., 1995]. The first is that deglacial and Holocene b•SNorg values never dropped below 9%o in IPC-56 and GGC-55 from Guaymas Basin (Figures 5 and 7). This result is in contrast to a longer record from outside the gulf showing •5•SNorg values dropping to values of 6 or 7%o in the last glacial period [Ganeshram et al., 1995]. Glacial b•SNorg values comparable to those of modem gulf sediments suggest that •SN-enriched nitrate from denitrification was always present in the gulf whether transported in from the ETP or formed locally. It seems that any denitrification occurring within the gulf would had to have occurred at a depth other than that of the modem OMZ since these sediments were oxic

over the late glacial interval. However, it is possible that suboxic conditions favorable for denitrification occurred only seasonally, allowing for bioturbation during the more oxic

season. Alternatively, subsurface waters moderately enriched in •SN because of limited glacial-aged denitrification may have been efficiently transported to the surface during water mass formation inside the Gulf of California, but not outside the gulf. Localized gulf mixing processes would not have had a significant effect on pom formed in the more open ocean setting of the ETNP [Ganeshram et al., 1995].

The second major difference between existing b•SNor• records from the ETNP region [Ganeshram et al., 1995] and those from the central gulf is the amplitude of variability. Increases in b•SNo•g of 2-3%o from the last glacial to the Holocene were observed along the northwestern Mexican margin south of the Gulf of California and were attributed to enhanced denitrification within SSW [Ganeshram et al., 1995]. This nitrogen isotopic event was amplified within the gulf into a •5•SNorg increase of 7%o (Figure 7). As mentioned earlier, only 1%o of this isotopic change can be attributed to changes in terrestrial carbon inputs.

Since b•SNNo3 - can be used as a tracer of waters deriving from zones of denitrification [Brandes et al., 1998] the nitrogen isotopic composition of nitrate available to phytoplankton is a function of both denitrification within subsurface waters and of vertical and horizontal transport of these subsurface waters. The large amplitude of glacial- interglacial b•SNorg variability in the central gulf relative to that of the ETNP is likely due to variability in water column mixing processes and water mass intrusion into this marginal basin in addition to actual variations in the extent of suboxic

waters in which denitrification occurred. Higher interglacial sedimentary opal content is suggestive of enhanced coastal upwelling during warm intervals. In addition, CGW, the water mass present in the central gulf for most of the year, may have received a greater contribution of •SN-enriched Pacific subsurface waters during its formation during deglaciation than it did during the glacial. This process could result from a northerly migration of the Intertropical Convergence Zone (ITCZ) during deglaciation which would displace the northern equatorial gyre northward in the Pacific and make SSW prominent within the central gulf year-round (Figures 1 and 2). Such a northerly migration of the ITCZ was documented for the Bolling-Allerod and Preboreal stages by evidence of weak trade winds in the southern Caribbean

[Hughen et al., 1996] (Figure 5). Since SSW becomes incorporated in CGW [Baumgartner, 1987] and is both nutrient-rich [Warsh et al., 1973; Baumgartner, 1987] and •SN-rich [Cline and Kaplan, 1975; Liu and Kaplan, 1989; Brandes et al., 1998; Altabet et al., 1999] its presence in the central gulf during the two stages of deglaciation and its withdrawal in the late glacial and Younger Dryas could contribute significantly to the large amplitude of deglacial nitrogen isotopic variability.

The Holocene nitrogen isotopic record of JPC-56 differs significantly from the deglacial record. Unlike the abrupt isotopic changes observed prior to 10,000 cal yr B.P., there is a gradual early to middle Holocene decrease in b•SNorg that we suggest is controlled by the dynamics of the nitrogen cycle (Figure 7). This will be discussed in a future paper by Sigman and others. The middle to late Holocene record of GGC-55 is

characterized by 1-2%o variations in fi•SNorg , which approach the magnitude of glacial-interglacial nitrogen isotopic

408 PRIDE ET AL.: GULF OF CALIFORNIA NITROGEN ISOTOPIC RECORD

variability observed in the ETNP [Ganeshram et al., 1995]. The lower-amplitude variability suggests that the record of subsurface denitrification was not locally amplified in the gulf during the middle to late Holocene as it was during deglaciation, possibly because of a lack of changes in the position of the ITCZ.

6. Conclusions

1. Deglacial 15•SNorg changes in the central Gulf of California predominantly reflect variable denitrification within subsurface waters.

2. The processes of water mass formation (CGW) and upwelling in the northern and central Gulf amplify nitrogen isotopic variations observed in the ETNP. 3. The nitrogen isotopic record of Guaymas Basin indicates the occurrence of significant (although reduced) denitrification in the late glacial period. 4. The nitrogen stable isotopic and opal records from the central gulf provide additional evidence of climatic and thermohaline changes occurring in the North Pacific coincident with thermohaline changes in the North Atlantic. 5. The abrupt Younger Dryas fluctuation in •tSNorg provides evidence of short-term fluctuations in denitrification which

could significantly impact nitrogen availability.

6. Gulf of California sediments provide evidence of significant Holocene climatic variability in addition to a two- staged deglaciation. Significant variability in the late Holocene records of nitrogen isotopic composition, opal content, and intermediate water oxygen content are suggestive of Neoglacial climate variability in the subtropics of the eastern North Pacific.

Acknowledgments. We thank the director and crew of B/O El Puma, the director and staff of the Centro Regional de Investigacion Pesquera, the staff of the Centro de Investigaciones Biologicas, and the crews of BIP XI and Atlantis II for their assistance in coring and maintaining the sediment trapping program. We appreciate the valuable reviews contributed by A. Van Geen and one anonymous reviewer. We would also like to acknowledge J. Pike and A. Kemp for sharing their detailed core description of JPC-56. This research was supported in part by NSF Grants OCE-9301413, OCE-8917699, OCE-9201255, OCE-9526356 (MAA), and OPP-9530714 (MAA). Radiocarbon dating and •5180 analyses were supported by an NSF grant to L.Keigwin. C. Pride's research was performed under appointment to the Graduate Fellowships for Global Change program administered by Oak Ridge Associated Universities for the U.S. Department of Energy, Office of Health and Environmental Research, Atmospheric and Climate Research Division. Data archived at the World Data Center-A for Paleoclimatology, NOAA- NGDC, 325 Broadway, Boulder, Colorado (e-mail: paleo•mail.ngdc.noa.gov; URL: http://www.ngdc.noaa.gov/paleo).

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M. Altabet, Department of Chemistry and Biochemistry and Center for Marine Science and Technology, University of Massachusetts Dartmouth, North Dartmouth, MA 02747. (e-mail: maltabet•umassd.edu)

L. Keigwin, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543. (e-mail: lkeigwin•mail.whoi.edu)

C. Pride, Marine Science Institute, University of California, Santa Barbara, CA 93106. (e-mail: pride•lifesci.ucsb.edu)

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E. Tappa •d R. Thunell, Departn•nt of Geologi- cal Sciences, University of SouthCarolina, Colum- bia, SC 29208. (e-mail: tappa•geol.sc.edu; thunell• geol.sc.edu)

(Received December 23, 1997; revised January 15, 1999; accepted January 20, 1999.)