PALEOCEANOGRAPHY, VOL. 5, NO. 6, PAGES 1,025-1,039, DECEMBER 1990

THE RECORD OF DEGLACIATION IN THE SULU SEA: EVIDENCE FOR THE YOUNGER DRYAS EVENT IN THE TROPICAL WESTERN PACIFIC

Braddock K. Linsley

Department of Geology, University of New Mexico, Albuquerque

Robert C. Thunell

Department of Geological Sciences, University of South Carolina, Columbia

Abstract. A high-resolution, accelerator mass spectroscopy 14C dated sediment record from the Sulu Sea clearly indicates that the Younger Dryas event affected the western equatorial Pacific. Planktonic foraminiferal •)180 and abundance data both record significant changes during Younger Dryas time. In particular, a 0.4 %o increase in the 5•sO value of Globigerinoides ruber and the reappearance of the cool water planktonic foraminifera, Neoglobo- quadrina pachyderma, occur during the Younger Dryas at this location. These isotopic and faunal changes are a response to either surface water temperature or salinity changes, or some combination of the two. Changes in surface salinities could have been accomplished through either local or global processes. Intensi- fication of the monsoon climate system and increased precipitation at approximately 11 ka is one mechanism that may have resulted in local changes in salinity. A meltwater pulse derived from the Tibetan Plateau is another mechanism which may have caused local changes in salinity. The

Copyright 1990 by the American Geophysical Union.

Paper number 90PA01684. 0883- 8305/90/90PA- 01684510.00

presence of the Younger Dryas in the tropical western Pacific clearly indicates that this climatic event is not restricted to the North Atlantic or high latitudes, but rather is global in extent.

INTRODUCTION

High-accumulation-rate deep-marine sediments as found near continental

margins offer the opportunity to examine higher-frequency oceanographic and climatic change than open-ocean records with lower accumulation rates. The Sulu

Sea in the humid Indonesian tropics of the western Pacific is a deep basin (•4800 m) completely surrounded by a shelf most of which is less than 100 m deep (Figure 1). Tropical monsoonal climatic conditions and tectonic activity in the region have created one of the highest fluxes of river-borne sediment in the world [Milliman and Meade, 1983]; consequently sedimentation rates in the region are generally high. The isolated configuration of the Sulu Sea restricts circulation in the basin resulting in anomalously warm and dysaerobic deep waters. The low dissolved oxygen content of intermediate and deep waters combined with high sediment accumulation rates have resulted in reduced benthic mixing of sediments. In addition, the anomalously warm deep waters (10oC) have led to

1026 Linsley and Thunell: The Younger Dryas in the Sulu Sea

20øN

10 ø

Hong

South

China

Sea

U•

Mind•

PACIFIC

OCEAN

ingapore

.-•,

Java Sea Jakarta o

Celebes Sea

•.. ß

ß • -:•;•••, ' Banda

Sea ß

10 øs I 100 øE 110' 120 ø 130 ø

Fig. 1. Map of western tropical Pacific showing location map of Sulu Sea. Bathymetric contour is the 200 m isobath. Note location of Ocean Drilling Program hole 769.

enhanced carbonate preservation compared to the open western Pacific, also aiding in the preservation of high-resolution paleoclimatic records.

Oceanographic conditions in the Sulu Sea are strongly influenced by the monsoonal climate of the Indonesian region and by the quasi-permanent Indonesian low-pressure system. This low-pressure system is important both because of its influence on the Indonesian monsoon and for its role in the Southern Oscillation (SO). The Indonesian region has also recently been noted for its large contribution to the interocean exchange of thermocline water [Gordon, 1986; Murray and Arief, 1988; Street-Perrott and Perrott, 1990]. Large- scale exchange of upper ocean water from Indonesia, to the Indian Ocean and into the Atlantic may influence the production of North Atlantic Deep Water (NADW) [Street-

Perrott and Perrott, 1990]. In addition, the Indonesian region was climatically important during the Pleistocene because vast areas of shallow shelves were exposed during sea level lowstands. In particular, the emergence of these shallow shelf areas during glacial stages would have influenced the exchange of thermocline waters with the Indian Ocean [COHMAP, 1988]. Sea level changes would also have strongly influenced oceanographic conditions in the Sulu Sea where a sea level

drop of 120 m during the last glacial maximum [Curray, 1964: Milliman and Emory, 1968; Fairbanks, 1989] is sufficient to almost totally isolate the basin from the surrounding seas (Figure 2), potentially changing surface and deepwater conditions.

In order to elucidate high-frequency oceanographic and climate change in the

Linsley and Thunell: The Younger Dryas in the Sulu Sea 1027

117 ø 118 ø 119 ø 120 ø 121 ø 122 ø 123 ø

SULU

SEA

SITE

769

CELEBES

SEA

11 ø

10 ø

9 o

8 o

7 ø

6 o

Fig. 2. Detailed bathymetric map of the Sulu Sea (depths in meters). Shaded zone is th•.:•t.t portion of the shelf shallower than 100 m that would have been exposed during the last glacial low stand of sea level. Note location of hole 769, 3643'm water depth.

Sulu Sea the upper 3.5 m of sediment from Ocean Drilling Program (ODP) hole 769A was sampled at 2.5 cm intervals. Sedimentation rates in this portion of the record are - 11-16 cm/ka. The detailed

sampling (250 year intervals) spans approximately the last 22 ka. The high sedimentation rates and close sampling interval yield a record that is comparable in resolution to that of core V23-81, the "type section" for the Younger Dryas in the North Atlantic [Broecker et al., 1988a].

Detailed time series of planktonic foraminiferal 51sO and species abundances record the presence of the Younger Dryas event between -11 and 10 ka. In this paper we describe the evidence from site 769A for

oceanographic change in the Sulu Sea during Younger Dryas time and briefly discuss the significance of the Younger Dryas in the western tropical Pacific.

REGIONAL SETTING

The Sulu Sea is approximately 600 km long and 400 km wide, located between

northern Borneo (Sabah) to the west and the central Philippine Archipelago to the east. A distinct northeast trending bathymetric high, the Cagayan Ridge, subdivides the Sulu Sea into two subbasins (Figure 2). The northwest basin has water depths ranging from 1000 to 2000 m. The southeast Sulu

basin is much deeper, with maximum depths between 4500 m and 5500 m along the Sulu trench located off the Zamboanga Peninsula of Mindanao and Negros Island.

Surface Water Hydrography

Surface waters of southeast Asian seas

have high temperatures and low salinities typical of humid tropical regions. There is an open exchange of surface water between the Sulu Sea and surrounding seas over the shelf and through channels in the shelf. The large excess of rainfall over evaporation creates an average surface salinity of 34 %0. Annual temperature variations in the region are less than 2oC. Unlike temper- ature, salinity is extremely variable. This variability is a result of high rainfall and

1028 Linsley and Thunell' The Younger Dryas in the Sulu Sea

river runoff and the intricate geographical structure of the area [Wyrtki, 1961]. Surface water circulation in the western equatorial Pacific is driven by the annually reversing monsoon winds creating large volumetric changes in surface water flow. Although the total volumetric movement of surface water in the Sulu Sea is low, it does change direction in accord with wind stress.

High temperatures and low salinities create surface waters of relatively low density compared with open Pacific Ocean waters. The less dense surface water layer forms a strong contrast to the colder water masses at depth. The boundary between deep and surface waters occurs between 100 and 300 m and takes the form of a strong discontinuity layer permitting little vertical exchange of water. A thorough review of the surface water hydrography of the Indonesian region is given by Wyrtki [1961].

Deep Water Hydrograph y

A shallow shelf, most of which is less than 100 m deep, completely surrounds the Sulu Sea (Figure 2). Deepwater exchange is limited by the depth of the deepest sill

supplying water to the Sulu Sea from the surrounding basins. The Mindoro strait along the east coast of Panay connects the basin to the South China Sea and is the

deepest channel (420 m) into the Sulu Sea (Figures I and 2). China Sea Intermediate Water enters the Sulu Sea only through the Mindoro Strait, thus this channel controls the ventilation rate of the basin [Frische and Quadfasel, 1990; Wyrtki, 1961; Van Reil, 1943]. Currently, deep water below the thermocline is uniformly warm (10oC), dysaerobic, and has the same physical properties as the water entering from the China Sea. At 600 m depth, temperatures are -10.5oC and vary less than 0.1oC, while salinities are 34.5 %o and vary less than _+0.5%o. From 600 to 4800 m depth, temper- ature and salinity do not vary more than _+0.5oC and + 0.5 %o, respectively (Figure 3). The concentration of dissolved oxygen is low throughout the deep waters (<1.2 mL/L), corresponding to only 20% saturation at the prevailing salinity and temperature.

Warm bottom waters in the Sulu Sea

result in excellent biogenic carbonate preservation. The carbonate compensation depth (CCD) is currently at -4800 m, while

SU LU SEA

Fig. 3.

lOOO

2000

3000

4000

Temperature (øC) 0 10 20 30 33

/

Salinity (•) Oxygen (ml/I) 0 33.5 34.0 34.5 35.0 0.0 2.0 4.0 6.0

Temperature, salinity, and dissolved oxygen profiles from the Sulu Sea.

Linsley and Thunell: The Younger Dryas in the Sulu Sea 1029

the carbonate lysocline occurs near 4000 m [Linsley et al., 1985; Exon et al., 1981]. In contrast, the CCD and lysocline are at 4000 m and 3500 m, respectively, in the South China Sea [Rottman, 1979] which is open to the western Pacific and is well aerated at depth by cold bottom water.

METHODS

Ocean Drilling Program leg 124 recently cored site 769 at 3643 m water depth on a bathymetric high on the Cagayan ridge in the center of the Sulu Sea (Figure 2). The location of site 769 was chosen specifically to avoid turbidite sedimentation and to obtain

a continuous, high-resolution Neogene record. Holes 769A and 769B were hydrau- lically piston cored with overlapping sections to obtain a complete late Pleistocene record. Whole core paleomagnetic susceptibility results between 769A and 769B indicate excellent correlation in the late

Pleistocene portions of the record [Rangin et al., 1990]. The similarity of magnetic susceptibility between A and B holes attests to the continuity of the sediment record at site 769A.

Samples were taken at 2.5-cm intervals in the upper 3.5 m of core collected at hole 769A for a high-resolution study of the last glacial/interglacial cycle. The samples were wet sieved to remove the fraction < 63

[tm. The fraction >63 [tm was split in half, one half for faunal analysis and one half for isotopic analysis. The planktonic foraminifer Globigerinoides ruber (white variety: 250-400 •tm size fraction) was used to construct 5x80 and 5xaC time series records for 769A. Forty to fifty specimens were reacted at 50oC with 100% HsP04, and the C02 gas was analyzed in a semi- automated VG micromass 602E mass

spectrometer. Replicate sample analyses yielded a standard deviation from the mean of_+ 0.07 %o for oxygen and _+ 0.08 %0 for carbon. Data are presented relative to the PeeDee belemite (PDB) standard (Table 1).

Planktonic foraminifer species abundances at sample intervals of 5 cm were calculated by first tabulating the relative species abundance in a split of 300 or more planktonic foraminifera specimens greater than 150 gm and then calculating the number of individuals per gram of sediment (dry weight).

AGE MODEL

The chronology for the upper 350 cm of hole 769A is based on three AMS x4C ages. Specimens of Globigerinoides ruber from 107 cm, 128 cm, and 245 cm yielded corrected ages of 9,720 _+ 80 years, 11,100 _+ 185 years, and 18,320 _+ 155 years, respectively. A correction facter of-400 years was applied to each date to account for the difference in 14C between the

atmosphere and surface waters [Bard, 1988)].

Ages for individual samples were estimated assuming constant sedimen- tation rates between 14C dates. Sample ages for the upper part of the core (above the first x4C date) were derived assuming the top of the core has an age of zero. Likewise, sample ages below 245 cm (18,320 years) were extrapolated using the sedimentation rate calculated for the core interval between

128 and 245 cm. The x4C ages indicate that sedimentation rates in the core vary from -11 cm/1000 years during the Holocene to -16 cm/1000 years during the last glacial.

ANALYTICAL RESULTS

Stable Isotopes

The results of the oxygen and carbon isotopic analyses of the planktonic foram- inifer Globigerinoides ruber and the three AMS •4C ages are plotted to depth in Figure 4a. Using the age model described above, the data were converted to the time domain

and replotted (Figure 4b). The hole 769A 5•sO record clearly contains the two-step deglaciation. Previous work [Duplessy et a1.,1986; Bard et al., 1987; Fairbanks, 1989] has demonstrated that terminations 1A and

lB are centered at approximately 13 ka and 9.5 ka, respectively, and this agrees well with our AMS •4C derived chronology.

The magnitude of the 5x80 shift from maximum glacial to Holocene conditions is 1.3 %o (Figures 4a and 4b), which is consis- tent with previous results from this region [Linsley et a1.,1985]. The ice volume effect on the 5180 record for the most recent glacial-interglacial transition is also estimated to be ~1.3 %o, assuming a 120-m sea level rise and that 10 m of sea level rise

is equivalent to a 0.11%o change in 5•sO [Chappell and Shackleton, 1986; Labeyrie et

1030 Linsley and Thunell: The Younger Dryas in the Sulu Sea

TABLE 1. Stable Isotopic Data From Upper 4 m of Site 769A

Depth, Age, 513C 5180

rn years

Depth,

TABLE 1. (continued)

Age,

years

1.1 9851

'1.1 9851

0.2 1817 1.60 -2.60 1.13 10,049 0.25 2271 1.62 -2.61 1.15 10,180 0.3 2725 1.64 -2.61 '1.15 10,180 0.35 3179 1.59 -2.48 1.18 10,377 0.39 3543 1.46 -2.57 1.2 10,509 0.4 3634 1.59 -2.56 '1.2 10,509 0.44 3997 1.31 -2.63 1.23 10,706 0.45 4088 1.49 -2.61 1.25 10,837 *0.45 4088 1.41 -2.66 '1.25 10,837 0.48 4360 1.43 -2.53 1.29 11,100 0.5 4542 1.39 -2.56 1.3 11,162 0.53 4815 1.47 -2.44 1.33 11,349 0.55 4996 1.5 7 - 2.58 1.35 11,473 0.59 5360 1.46 -2.36 1.38 11,660 0.6 5450 1.56 -2.49 1.41 11,847 0.64 5814 1.54 - 2.39 1.43 11,971 0.65 5905 1.28 -2.41 1.45 12,096 0.69 6268 1.62 -2.51 1.48 12,283 0.7 6359 1.44 -2.53 1.53 12,594 0.74 6722 1.66 -2.51 1.55 12,718 0.75 6813 1.38 -2.41 1.58 12,905 0.79 7176 1.61 -2.34 1.6 13,029 0.8 7267 1.40 -2.49 1.64 13,278 0.84 7631 1.34 -2.22 1.65 13,341 0.85 7721 1.17 -2.43 1.69 13,590 0.88 7994 1.26 -2.41 1.7 13,652 0.9 8176 1.22 -2.49 1.74 13,901 0.93 8448 1.15 -2.27 '1.74 13,901 0.95 • 1.25 -2.23 1.75 13,963 *0.95 • 1.20 -2.20 1.79 14,212 0.99 8993 0.98 -2.56 1.8 14,274 i 9084 0.92 -2.55 1.84 14,523 1.04 9447 1.08 -2.38 1.85 14,586 1.05 9538 0.90 -2.45 1.89 14,834 1.08 9720 1.16 -2.23 1.9 14,897

0.87 -2.02

0.80 -2.13

1.05 -2.08

0.93 -1.99

0.99 -1.90

0.81 -1.72

0.88 -1.95

0.80 -1.86

1.12 -1.90

1.05 -1.79

1.00 -1.70

0.93 -1.95

1.03 -2.11

0.98 -2.01

1.04 -2.12

0.96 -1.93

0.92 -1.96

0.92 -1.50

0.92 -1.92

0.99 -1.60

0.94 -1.47

1.05 -1.67

1.01 -1.42

0.93 -1.28

1.09 -1.40

1.23 -1.33

1.05 -1.32

1.02 -1.42

1.05 -1.28

1.25 -1.27

0.80 -1.50

1.07 -1.26

1.02 -1.42

0.99 -1.35

1.03 -1.25

1.20 -1.37

1.10 -1.30

Linsley and Thunell: The Younger Dryas in the Sulu Sea 1031

TABLE 1. (continued)

Depth, Age, 513C 5180 Depth, in years in

TABLE 1. (continued)

Age, 513C 5180

years

1.94 15,146 1.09 -1.29

1.96 15,270 1.15 -1.38

1.99 15,457 1.25 -1.39

2 15,519 1.22 -1.46

2.05 15,830 1.25 -1.62

2.06 15,893 1.11 -1.38

2.09 16,079 1.05 -1.53

2.1 16,142 1.05 -1.65

2.14 16,391 1.11 -1.51

2.15 16,453 1.22 -1.56

2.19 16,702 1.19 -1.45

2.2 16,764 1.07 -1.46

2.24 17,013 1.11 -1.36

2.25 17,075 1.21 -1.57

2.29 17,324 1.22 -1.43

2.3 17,386 1.12 -1.36

2.34 17,635 1.27 -1.30

2.35 17,698 1.15 -1.37

*2.35 17,698 1.21 -1.31 2.39 17,947 0.98 -1.44

2.4 18,009 1.17 -1.55

2.44 18,258 1.00 -1.66

2.45 18,320 1.09 -1.62

2.49 18,569 1.17 -1.55

2.5 18,631 1.23 -1.59

2.54 18,880 1.16 -1.84

2.55 18,942 1.47 -1.82

2.59 19,191 1.12 -1.77

2.6 19,254 1.00 -1.94

2.64 19,503 1.25 -1.93

2.65 19,565 1.16 -1.73

2.7 19,876 1.11 -1.78 2.71 19,938 1.21 -1.88

2.74 20,125 0.90 -1.89

2.75 20,187 1.23 -1.89

2.79 20,436 1.15 -1.93

2.8 20,498 1.23 -1.96

2.84 20,747 1.14 -2.03

2.85 20,810 1.29 -2.02

2.89 21,059 1.21 -1.74

2.9 21,121 1.17 -1.99

2.94 21,370 1.13 -1.72

2.95 21,432 1.14 -1.93

2.99 21,681 1.17 -1.99

*2.99 21,681 1.10 -2.08

3.05 22,054 1.05 -2.05

3.1 22,366 1.00 -2.16

3.15 22,677 1.10 -2.00

3.2 22,988 1.28 -1.93

3.25 23,299 1.20 -1.85

3.3 23,611 1.20 -1.80

3.35 23,922 1.18 -1.90

3.4 24,233 1.17 -2.00

3.5 24,855 1.48 -2.00

3.6 25,478 1.24 -1.80

3.65 25,789 1.20 -1.80

3.7 26,100 1.24 -1.81

3.8 26,723 1.12 -1.98

3.9 27,345 1.16 -2.01

4 27,967 1.50 -1.98

*=replicate analyses

al., 1987; Fairbanks, 1989]. Thus the overall glacial-interglacial 5•sO change in the Sulu Sea is primarily recording the global ice volume signal and not local surface water temperature or salinity changes. Super- imposed on the deglaciation trend is an increase in 5•sO which occurs between ~11 ka and 10 ka, and appears to be congruent with the climatic event referred to as the Younger Dryas.

The 51sC data show a gradual decrease from approximately 22 ka to 10 ka, after which there is 0.7 %o increase up to present levels (figure 4b). The timing of the 5x3C

1032 Linsley and Thunell: The Younger Dryas in the Sulu Sea

A Site 769A (0-4.0m)

2.7 2.6

2.2 2.1

1.2 1,1

0.7 0.6

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

del•h (m)

2.7

2.2

0.7

Site 769A (0- 28 Ka) .... ' .... ' .... ' .... ' .... ' '' '2.6

Younger Dryas 2.1

1.6

1.1

ß .. , .... , .... , ß .. 0.6 0 5000 10000 15000 20000 25000

Age (years)

Fig. 4. Planktonic oxygen and carbon isotopic data from hole 769A plotted versus (a) depth and (b) age. The positions of the three uncorrected AMS 14C ages are shown in Figure 4a. Termination I A and I B, and the temporal position of the Younger Dryas are indicated in Figure 4b.

minimum in the Sulu Sea at 10 ka is

consistent with the deglacial 513C minimum found in many low-latitude planktonic records [Oppo and Fairbanks, 1989], suggesting that 51• changes in the Sulu Sea surface Water are the result of a

global low latitude phenomenon.

Planktonic Foraminifera

Downcore variations in the absolute

abundances (number/gm) of four planktonic foraminiferal species are presented relative to the 51sO record in Figure 5. Neogloboquadrina pachyderma

(right coiling) is moderately abundant during the last glacial, becomes greatly reduced at the beginning of deglaciation, and then rapidly increases during Younger Dryas time. Neogloboquadrina dutertrei also increases in abundance during the Younger Dryas and displays large abundance changes throughout the 22 ka record (Figure 5). Globigerinoides ruber shows a large increase in abundance at ~11 ka and then a decrease at ~10 ka (Figure 5). Both Pulleniatina obliquiloculata and Globigerinoides ruber show abrupt (<500 year) abundance increases at ~ 9 ka in conjunction with the rapid increase in 5•3C

Linsley and Thunell: The Younger Dryas in the Sulu Sea 1033

2.6

2.1

1.1 o

4O

35

30

25

20

15

10

5

0 0

140

120

100

8O

60

4O

20 0

80 ß

60

4O

20

0 0

5000 10000 15000 20000 25000

5000 10000 15000 20000 25000

ß

,

5000 10000 15000 20000 25000

........ i ...... P• oi•.•uilo•uiat• '

, , .

5000 10000 15000 20000 25000

140 120 100

,•o 20

0 0 5000 10000 15000 20000 25000

Age (years)

Fig. 5. Variations in the abundances of N. pachyderma, N. dutertrei, P. obliqui- loculata, and G. ruber in hole 769A over the last ~22,000 years. The 5lsO record has been included for reference.

(see Figure 4). P. obliquiloculata, which is virtually absent prior to 13 ka, reaches maximum abundances at the base of the

Holocene immediately following the Younger Dryas and then decreases in abundance after ~7 ka.

DISCUSSION

Younger Dryas Climatic Event

During the transition from the last glacial climatic regime to the Holocene it has been suggested that a brief return to glacial-like conditions, referred to as the

Younger Dryas, occurred between ~11 and 10 ka. It has also been proposed that during deglaciation, maximum ice-melting rates occurred from ~14 to 12 ka and from ~10 to 7

ka [Fairbanks, 1989]. The Younger Dryas chronozone is thus positioned between these periods of rapid ice decay and sea level rise. The Younger Dryas has primarily been identified in records from Europe and the North Atlantic, although a number of recent studies have documented the

presence of a Younger Dryas-like climatic event in areas far removed from the North

Atlantic. These findings are briefly summarized below.

A thorough review of the European and North American evidence for the Younger Dryas is given by Wright [1989]. In Norway the Younger Dryas has been tied to a glacial readvance and a temperature drop of ~8- 10oC [Mangerud, 1987]. In southern Sweden, detailed glacial and paleoecological studies have documented a slowing of ice retreat and vegetation changes associated with the Younger Dryas [Bjorck and Moller, 1987; Lundquist, 1987]. The Grande Pile pollen record from France also shows strong evidence for a climatic cooling during the Younger Dryas [Woillard, 1978].

Planktonic foraminiferal data from the

North Atlantic have been interpreted as representing a shift of the polar front to a more southerly position during the Younger Dryas due to increased ice sheet elevation [Ruddiman et al., 1977; Ruddiman and Mcintyre, 1981a, b]. Other scenarios for the origin of this "cold" interval have focused on meltwater discharge to the North Atlantic and its possible control on the production of North Atlantic Deep Water [Broecker et al., 1988a, 1989; Boyle and Keigwin, 1987; Jansen and Veum, 1990].

Recently, Fairbanks [1989], using radio- carbon-dated submerged coral reefs from Barbados, showed that the transition from the last glacial maximum to the Holocene was indeed marked by two periods of extremely rapid sea level rise, centered at ~12 and 9.5 ka. This two-step deglaciation can not be attributed to insolation changes and is a good example of a nonlinear response of the climate system to orbital forcing. Perhaps more importantly, the Younger Dryas was shown to be a period of low rate of sea level rise between two large meltwater pulses. These results have a

1034 Linsley and Thunell: The Younger Dryas in the Sulu Sea

significant bearing on two aspects of the Younger Dryas story. First, does the Younger Dryas represent a brief return to glacial-like conditions, and second, was meltwater discharge to the North Atlantic the triggering mechanism for this event [Broecker et al., 1988a].

A substantial amount of evidence is

accumulating which suggests the presence of a Younger Dryas age climatic event outside the North Atlantic region. In the Orca Basin, Gulf of Mexico, Kennett et al. [1985] document planktonic foraminiferal 5xsO and abundance changes during Younger Dryas time which they interpret as indicating a cooling of surface waters. In the western Pacific, oxygen isotopic records from cores near Japan are marked by an interruption in the deglaciation trend and a brief return to heavier values between

approximately 11 and 10 ka [Chinzei and Oba, 1986; Chinzei et al., 1987; Kallel et al., 1988]. Based on microfossil evidence, Chinzei et al. [1987] concluded that a southward migration of the cold Oyashio Current occurred between 11 and 10 ka, and they associated this cold event with a brief lowering of sea level. According to Kallel et al. [1988], a southward shift of the polar front also occurred in the North Pacific

during Younger Dryas time. These authors concluded that the Younger Dryas cooling had a synchronous impact on high latitude climates throughout the northern hemisphere. Labracherie et al. [1989] also reported a Younger Dryas-like cooling in several cores from the Southern Ocean (44oS to 55øS), although the beginning of this cooling event appears to have been approximately 1000 years earlier than the start of the Northern Hemisphere Younger Dryas event.

In the tropics a decrease in lake levels during Younger Dryas time reflects the brief restablishment of cool, arid conditions [Street-Perrott and Perrott, 1990]. Drawing on the meltwater hypothesis of Broecker et al. [1988a and 1989], Street-Perrott and Perrott [1990] attribute this cooling in tropical regions to a decrease in North Atlantic Deep Water (NADW) production and an attendant reduction in the transfer

of heat across the equator by upper layer water. In the Northern Indian Ocean (7oN to 15oN), 5xsO data from planktonic foraminifera also record the presence of an

increase in 5xsO values during the most recent deglaciation that may be the Younger Dryas event [Pestiaux et al., 1987].

The study of Broecker et al. [1988b] on core V35-5 from the South China Sea is parti- cularly pertinent to our study in the Sulu Sea because of the close proximity of the two basins. The planktonic foraminiferal fauna in V35-5 records an abrupt and unin- terrupted termination of the last glacial period, and Broecker et al. [1988b] concluded that there was no evidence for a Younger Dryas event in this region. Based on this, they postulated that the Younger Dryas event did not have a global impact but was restricted to the North Atlantic.

EXPRESSION OF THE YOUNGER DRYAS IN THE SULU SEA

Stable Isotopic and Faunal Signature

The planktonic foraminiferal 5•sO and faunal records clearly identify the presence of a Younger Dryas event in the Sulu Sea (Figure 5) although the 0.4 %o increase in 5xsO in the hole 769A record during Younger Dryas time is smaller in magnitude than the 0.6-0.8 %o changes observed in high latitudes of the North Atlantic [Bard et al., 1987] and North Pacific [Kallel et al., 1988]. The question to be answered is whether this 5xsO change during Younger Dryas time in the Sulu Sea is primarily an expression of temperature or salinity changes in surface waters. Are the observed 5•sO values during the Younger Dryas really "heavy" or are the values on either side anomalously "light"? If the entire 0.4 %o change is attributed to temperature, this would represent a 1.5oC temperature drop during Younger Dryas time and would be nearly equal to the entire 2oC glacial-interglacial temperature change estimated for this region for both summer and winter [CLIM•, 1976]. Alternately, the low 5•s0 values which precede the Younger Dryas may reflect a lowering of surface water salinities.

Distinguishing between a temperature and a salinity control for the Younger Dryas 5•sO signature in the Sulu Sea is not straightforward, particularly since two fundamentally different scenarios have been proposed for the origin of the Younger Dryas. A number of authors have attrib-

Linsley and Thunell: The Younger Dryas in the Sulu Sea 1035

uted the Younger Dryas to a diversion of meltwater from the Mississippi to the St. Lawrence Seaway [Rooth, 1982; Broecker et al., 1988a, 1989]. According to this model, the cold, low-salinity meltwater would shut down the production of NADW and reduce the transfer of heat to the atmosphere over the North Atlantic, resulting in a brief climatic cooling. Fairbanks [1989] has interpreted conditions very differently for the Younger Dryas. Instead of the Younger Dryas being a time of high meltwater discharge to the North Atlantic, Fairbanks [1989] concludes that it was a time when meltwater discharge was reduced and the rate of sea level rise was low. The Barbados

sea level record [Fairbanks, 1989] indicates that two meltwater events occurred just before (~12 ka) and just after (~9.5 ka) the Younger Dryas. In this context, the Younger Dryas is distinguished not so much by the fact that it represents a unique event but rather that it was a brief interval

between two periods of lowered surface salinities. According to Fairbanks [1989], the extremely high discharge rates during these two meltwater pulses caused rapid decreases in the 5xsO of surface waters.

An examination of the hole 769A 5•80

record (Figure 4b) reveals that Younger Dryas time (~11 to 10 ka) is indeed bracketed by two brief intervals of light 5xsO values that may be equivalent to the two meltwater pulses identified by Fairbanks [1989]. Specifically, there is a 0.85 %0 decrease in 5•80 between 15 and 11.5 ka in our Sulu Sea record (Figure 4b). In comparison, the 5xso change of mean ocean water was only 0.4 to 0.5 %0 during this period [Fairbanks, 1989]. If this difference was due to a local salinity change and if the deglacial 51sO - salinity relationship was similar to that for the modern North Pacific [Craig and Gordon, 19651, then surface water salinities in the Sulu Sea at 11.5 ka would have been

approximately 1%o lower than during Younger Dryas time. Similarly, the difference in 5xsO between 15 ka and 10.5 ka (mid Younger Dryas time) in our Sulu Sea record is only 0.5 %o (Figure 4b), approximately equal to the 5xsO change of mean ocean water at this time interval.

This suggests that 5•sO values in the Sulu Sea during Younger Dryas time were not anomalously "heavy" but rather that the

preceding 1000 years was a time of anomalously "light" 51sO values.

Numerous rivers drain into the region, potentially affecting the 5xsO record. In particular, the Mekong River, which has its head waters in the Tibetan Plateau, empties into the southern end of the South China

Sea (Figure 1). The Mekong River has a discharge of 470 kma/yr, making it the ninth largest river in the world [Boreland, 1973]. By comparison, the Mississippi River is the sixth largest river in the world with a discharge of 580 kma/yr [Milliman and Meade, 1983]. In addition, mountain glaciers in Sabah (Borneo) could have added meltwater to the Sulu Sea, although these glaciers were volumetrically small (C. Muller, personal communication, 1989).

In contrast, the abundance distribution of the planktonic foraminifer N. pachyderma dextral in hole 769A (Figure 5) suggests a return of cooler surface water conditions to

the Sulu Sea during Younger Dryas time. A similar faunal response to the Younger Dryas was reported for the Sargasso Sea [Keigwin and Jones, 1989]. This species is characteristic of modern subpolar regions and occurs at present in the North Pacific when there is a well-developed thermocline [Reynolds-Sautter and Thunell, 1986]. N. pachyderma is present in the Sulu Sea throughout the last glacial and then disappears at the onset of deglaciation. This "cool water" species suddenly reappears during the Younger Dryas interval but is noticeably absent from what might be considered the two "meltwater pulses" that bracket the Younger Dryas. It is difficult to explain the presence of N. pachyderma dextral in the Sulu Sea during either the Younger Dryas or glacial times. This is a tropical region where surface waters supposedly cooled by no more than 2oC during the last glacial [CLIMAP, 1976]. It may be that we do not fully understand the ecology of this species and that its distribution is controlled by more than simply surface water temperatures.

N. dutertrei also increased in abundance

during Younger Dryas time (Figure 5). High abundances of N. dutertrei in late Pleistocene sediments have been used as an

indicator of low salinity and/or high productivity in the Mediterranean [Thunell,

1036 Linsley and Thunell: The Younger Dryas in the Sulu Sea

1978], and of low salinity in the Bay of Bengal [Cullen, 1981]. Cullen [1981], using increased N. dutertrei abundances during deglaciation, inferred that the southwest monsoon (rainy season) was more intense at this time than during the last glacial maximum. Based on modeling results, Kutzbach [1981] has shown that maximum summer insolation ~ 11 ka is associated

with maximum monsoonal precipitation in Africa and Asia. However, since the 5xsO value of precipitation in tropical regions is very close to that of seawater, it is unlikely that increased precipitation in the Sulu Sea would have greatly reduced the 5•sO of seawater approximately 11 ka. In the Sulu Sea it is unclear whether the high abun- dance of N. dutertrei during Younger Dryas time is the direct result of reduced surface salinities or productivity changes.

P. obliquiloculata, a tropical "deep- dwelling" species [Be', 1977], portrays a different picture of surface water conditions in this region during deglaciation (Figure 5). This species was virtually absent from the Sulu Sea during the last glacial, steadily increased in abundance during deglaci- ation, and reached maximum abundance at the beginning of the Holocene. There is no indication that the abundance of this

species was significantly reduced by a climatic cooling during Younger Dryas time. If the low 5x80 values just prior to Younger Dryas time were caused by a brief reduction in surface water salinities, we would not necessarily expect to see a response in the abundance of P. obliquilo- culata, since this species lives below the surface mixed layer.

CaC03 Preservation

The shallowly silled configuration of the Sulu Sea and the dependence of its deepwater properties on sill depth make the basin a potential monitor of relative sea level changes. An important question is how sea level fluctuations during deglaciation effected the levels of the aragonite and carbonate compensation depths (ACD and CCD, respectively). A sea level drop of 120 rn is sufficient to almost completely isolate the Sulu Sea from surrounding basins. The deepest water entering the basin would have originated

higher in the water column compared to the present and could have been as much as 5oC warmer if the glacial thermal structure of the upper water column in the South China Sea was similar to that of today. A shallowing of sill depth during glacial times could have caused a gradual warming of basin waters and potentially increased carbonate preservation.

The present day aragonite compensation depth (ACD) in the Sulu Sea occurs at approximately 1400 m water depth [Linsley et al., 1985]. However, pteropods were very abundant from ~20 to 13 ka at site 769A (3643 m), during the last glacial lowstand of sea level (Figure 6). In addition, piston cores RC14-78 at 1643 rn and RC12-357 at 2049 rn

in the basin record large increases in the abundance of pteropods during this time, while two deeper cores at 4102 m (V28-322) and 4480 m (V24-135) are devoid of pteropods during the last glacial maximum [Morgan, 1983]. Thus, during the last glacial lowstand of sea level, when sea level was ~120 rn below Holocene levels, the ACD in the Sulu Sea was at least 2200 m deeper than the present (between 3640 and 4100 rn water depth). We believe this indicates a sea level control on the ACD level in the Sulu

Sea, with lower sea level and shallower sill depths resulting in bottom water warming and/or increased productivity allowing enhanced carbonate preservation. It is important to note that the Younger Dryas was not a time of increased aragonite preservation at hole 769A; this would

Pteropod Fragments per gram 3.0 8000

'•• 2.0 4000 ,o 1.5 2000

1.0 0 0 5000 10000 15000 20000 25000

Age (years)

Fig. 6. Pteropod fragments per gram of dry sediment of hole 769A over the last 22,000 years.

Linsley and Thunell: The Younger Dryas in the Sulu Sea 1037

suggest that no appreciable drop in sea level occurred at this time.

CONCLUSIONS

The planktonic foraminiferal stable isotopic and faunal data presented here clearly demonstrate that significant oceanographic changes occurred in the Sulu Sea between approximately ~13 and 9 ka. This time interval includes the Younger Dryas chronozone (-10-11 ka) as well as the two episodes of rapid sea level rise reported by Fairbanks [1989]. From our data we can not be certain whether the

Younger Dryas event in the Sulu Sea is the result of surface water temperature or salinity changes, or a combination of the two. However, we do feel that salinity change is the more plausible explanation. Specifically, a comparison of the Sulu Sea 5•sO record with the estimated •180 change of mean ocean water [Fairbanks, 1989] suggests that the period immediately preceding the Younger Dryas was a time of lowered surface water salinities. Meltwater

from the Tibetan glaciers or increased precipitation in the Tibetan highlands related to an intensification of the monsoon

system - 11 ka could have produced the light 5•s0 values observed just proir to Younger Dryas time. Irrespective of the cause, our study clearly demonstrates that the Younger Dryas event is not restricted to the North Atlantic region but rather is global in extent.

Acknowledgments. We thank Robert Dunbar for use of the stable isotope facilities at Rice University. The manuscript has greatly benefited from reviews by L. Keigwin and H. Spero. The AMS 14C ages were provided by The University of Arizona, AMS facility. In addition we acknowledge the scientific members and crew of ODP leg 124 for their help. This research was supported by a grant from the JOI U.S. Science Support Program to B. Linsley and NSF grants OCE- 8614128 and OCE-9011846 to R. Thunell. The U.S. Science Program associated with the Ocean Drilling Program is sponsored by the National Science Foundation and the Joint Oceanographic Institutions, Inc.

REFERENCES

Bard, E., Correction of accelerator mass spectrometry 14C ages measured in planktonic foraminifera: Paleoceanographic implications. Paleoceanography, 3, (6), 635-645, 1988.

Bard, E., M. Arnold, P. Maurice, J. Dupray, J. Moyes, J.-C. Duplessy, Retreat velocity of the North Atlantic polar front during the last deglaciation determined by x4C accelerator mass spectrometry, Nature, 328, 791-794, 1987.

Be', A.W.H., An ecological, zoogeographical and taxonomic review of Recent planktonic foraminifera, in Oceanic Micropaleontology, edited by A.T.S. Ramsay, Academic, 1-100, San Diego, Calif., 1977.

Bjorck, S., and P. Moller, Late Weichselian environmental history in southeastern Sweden during the deglaciation of the Scandinavian ice sheet,Quat. Res., 28, 1- 37, 1987.

Boreland, W.M., Pa Mong phase II, Supplement to Main Report ( Hydraulics and Sediment Studies): U.S. Bur. Recl., v. 1,282p., v. 2,304 p., 1973.

Boyle, E.A., and L. Keigwin, North Atlantic thermohaline circulation during the past 20,000 years linked to high-latitude surface temperature, Nature, 330, 35-40, 1987.

Broecker, W.S., M. Andree, W. Wolfii, H. Oeschger, G. Bonani, J. Kennett, and D. Peteet, The chronology of the last deglaciation: Implication to the cause of the Younger Dryas Event, Paleoceanography, 3 (1), 1-19, 1988a.

Broecker, W.S., M. Andree, M. Klaus, G. Bonani, W. Wolff, and H. Oeschger, New evidence from the South China Sea for an

abrupt termination of the last glacial period, Nature, 333, 156-158, 1988b.

Broecker, W.S., J.P. Kennett, B.P. Flower, J.T. Teller, S. Trumbore, G. Bonani, W. Wolfii, Routing of meltwater from the Laurentide Ice Sheet during the Younger Dryas cold episode, Nature, 341, 318-312, 1989.

Chappell, J., and N.J. Shackleton, Oxygen isotopes and sea level, Nature, 324, 137- 140, 1986.

Chinzei, K., and T. Oba, Oxygen isotope studies of the deep sea sediments around

1038 Linsley and Thunell: The Younger Dryas in the Sulu Sea

Japan, in Recent Progress of Natural Sciences in Japan, Vol. II, Quaternary Ressearch, pp. 35-43, Science Council of Japan, Tokyo, 1986.

Chinzei, K., H. Fujioka, I. Kitazatom, T. Koizumi, M. Oba, H. Oda, T. Okada, T. Sakai, and Y. Tanimura, Postglacial environmental change of the Pacific Ocean off the coast of central Japan, Mar. Micropaleontol., 11, 273-291, 1987.

CLIMAP, The surface of the ice age earth, Science, 191, 1131-1137, 1976.

COHMAP, Climatic changes of the last 18,000 years: Observations and model simulations, Science, 241, 1043-1052, 1988.

Craig, H., and L. Gordon, Isotopic oceanography: Deuterium and oxygen •s variation in the ocean and marine

atmosphere, in Stable isotopes in oceanographic studies and paleo- temperatures, edited by E. Tongiorgo, Consiglio Nazionale delle Richerche, Laboratorio di Geologia Nucleare, Pisa, 9-103, 1965.

Cullen, J.L., Microfossil evidence for changing salinity patterns in the Bay of Bengal over the last 20,000 years, Palaeogeogr., Palaeoclimatol., Palaeoecol., 35, 315-356, 1981.

Curray, J.R., Transgressions and Regressions, in Papers on Marine Geology, Shephard Commemorative Volume, edited by R.C. Miller, pp. 175- 203, Macmillan, New York, 1964.

Duplessy, J.-C, M. Arnold, P. Maurice, E. Bard, J. Duprat, and J. Moyes, Direct dating of the oyygen-isotope record of the last deglaciation by 14C accelerator mass spectrometry, Nature, 320, 350-352, 1986.

Exon, M.F., F. -W. Haake, M. Hartmann, F.-C. Kogler, P.J. Muller, and M.J. Whiticar, Morphology, water characteristics and sedimentation in the

silled Sulu Sea, Southeast Asia, Mar. Geol.. 39, 165-195, 1981.

Fairbanks, R., A 17,000-year glacio-eustatic sea level record: Influence of glacial melting rates on the Younger Dryas event and deep ocean circulation, Nature, 342, 637-642, 1989.

Frische, A., and D. Quadfasel, Hydrography of the Sulu Sea, Proc. Ocean Drill. Program, Initial Rep., 124, 101-104, 1990.

Gordon, A.L., Interocean exchange of thermocline water, J. Geophys. Res., 91 (C4), 5037-5046, 1986.

Jansen, E. and T. Veum, Evidence for two- step deglaciation and its impact on North Atlantic deep-water circulation, Nature, 343, 612-616, 1990.

Kallel, N., L.D. Labeyrie, M. Arnold, H. Okada, W. C. Dudley, and J.-C. Duplessy, Evidence of cooling during the Younger Dryas in the western North Pacific, Oceanok Acta, 11 (4), 369-375, 1988.

Keigwin, L.D., and G.A. Jones, Glacial- Holocene stratigraphy, chronology and paleoceanographic observations of some North Atlantic sediment drifts. Deep Sea Res., 36, 845-867, 1989.

Kennett, J.P., K. Elmstrom, and N. Penrose, The last deglaciation in Orca Basin, Gulf of Mexico: High-resolution planktonic foraminiferal changes, Palaeogeogr. Palaeoclimatol. Palaeoecol., 50, 189-216, 1985.

Kutzbach, J.E., Monsoon climate of the early Holocene: Climate experiment with earth's orbital parameters for 9,000 years ago, Science, 214, 59-61, 1981.

Labeyrie, L.D., J.C. Duplessy, and P.L. Blanc, Variations in mode of formation and temperature of oceanic deep waters over the past 125,000 years, Nature, 327, 477-482, 1987.

Labracherie, M., L. D. Labeyrie, J. Duprat, E. Bard, M. Arnold, J-Jacques Pichon, and J. C. Duplessy, The last deglaciation in the southern ocean, Paleoceanography, 4 (6), 629-638, 1989.

Linsley, B.K., R.C. Thunell, C. Morgan, and D.F. Williams, Oxygen minimum expansion in the Sulu Sea, western equatorial Pacific, during the last glacial low stand of sea level, Mar. Micropaleontol. 9, 395-418, 1985.

Lundquist, J., Glaciodynamics of the Younger Dryas marginal zone in Scandinavia, Geog. Ann., 69A, 305-320, 1987.

Mangerud, J., The Allerod/Younger Dryas boundry, in Abrupt Climatic Change, edited by W.H. Berger, and L.D. Labeyrie, pp. 163-171, D. Reidel, Hingham, Mass., 1987.

Milliman, J.D., and K.O. Emory, Sea levels during the past 35,000 years, Science, 162, 1121, 1968.

Linsley and Thunell: The Younger Dryas in the Sulu Sea 1039

Milliman, J.D., and R.H. Meade, World wide delivery of river sediment to the oceans, J. Geol. 91 (1), 1-21, 1983.

Morgan, C., The response of the Sulu Sea to late Quaternary Climatic Oscillations, M.S. thesis, 96 pp., Univ. of S.C., Columbia, 1983.

Murray, S.P., and D. Arief, Throughflow into the Indian Ocean through the Lombok Strait, January 1985-January 1986, Nature, 333, 444-447, 1988.

Oppo, D.W., and R.G. Fairbanks, Carbon isotope composition of tropical surface water during the past 22,000 years, Paleoceanography, 4 (4), 333-351, 1989.

Pestiaux, P., J.C. Duplessy, and A. Berger, Paleoclimatic variability at frequencies from 10 -4 _ 10 -3 cycles per year, in Climate--History, periodicity and predictability, edited by M.R. Rampino, J.E. Sanders, W.S. Newman, and L.K. Konigsson (eds.), pp. 285-299, 1987.

Rangin C., et al., Proc. Ocean Drill. Program, Initial Rep., 124, 321-326, 1990.

Reynolds-Sautter, L., and R.C. Thunell, Seasonal succession of planktonic foraminifera: Results from a four-year time series sediment trap experiment in the northeast Pacific, J. Foraminiferal. Res., 9 (4), 253-267, 1989.

Rooth, C., Hydrology and ocean circulation, Prog. Oceanogr., 11,131-149, 1982.

Rottman, M.L., Dissolution of Planktonic foraminifera and Pteropods in the South China Sea sediments, J. Foraminiferal Res., 9 (1), 41-49, 1979.

Ruddiman, W., C.D. Sancetta, and A. Mcintyre, Glacial/interglacial response rate of subpolar North Atlantic waters to climatic change: The record in ocean sediments, Philos. Trans. R. Soc. London, Ser. B, 280, 119-142, 1977.

Ruddiman, W.F., and A. Mcintyre, The North Atlantic Ocean during the last

deglaciation, Palaeogeogr. Palaeoclimatol. Palaeoecol., 35, 145-214, 1981a.

Ruddiman, W.F., and A. Mcintyre, The mode and mechanism of the last

deglaciation: Oceanic evidence, Quat. Res., 16, 125-134, 1981b.

Street-Perrott, F.A., and R.A. Perrott, Abrupt climate fluctuations in the tropics: The influence of Atlantic Ocean Circulation, Nature, 343, 607-612, 1990.

Thunell, R.C., Distribution of recent planktonic foraminifera in sediments of the Mediterranean Sea, Mar. Micropaleontol. 3, 147-173, 1978.

Van Riel, P.M., The Snellius Expedition 1929-1930, oceanographic results, Part V, The Bottom Water, Introductory Remarks and Oxygen Content, 2, Brill, Leiden, 76pp., 1943.

Woillard, G.M., Grande Pile peat bog: A continuous pollen record for the last 140,000 years, Quat. Res,. 9, 1-21, 1978.

Wright, H.E., Jr., The amphi-Atlantic distribution of the Younger Dryas paleoclimatic oscillation, Quat. Sci. Rev, 8, 295-306, 1989.

Wyrtki, K., Scientific results of marine investigations of the South China Sea and the Gulf of Thailand, physical oceanography of the Southeast Asian waters, Univ. of Calif., Scripps Inst. of Oceanogr., La Jolla, Calif., 195pp., 1961.

B.K. Linsley, Department of Geology, University of New Mexico, Albuquerque, NM 87131.

R.C. Thunell, Department of Geological Sciences, University of South Carolina, Columbia, SC 29208.

(Received April 1, 1990; accepted July 30, 1990.)