The last deglaciation in orca basin, gulf of Mexico: High-resolution planktonic foraminiferal changes

Palaeogeography, Palaeoclimatology, Palaeoecology, 50 (1985) 189--216

189

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE LAST DEGLACIATION IN ORCA BASIN, GULF OF MEXICO: HIGH-RESOLUTION PLANKTONIC FORAMINIFERAL CHANGES

J. P. KENNETT, K. ELMSTROM and N. PENROSE 1

Graduate School of Oceanography, University of Rhode Island, Kingston, R.I. 02881 (U.S.A.) (Received January 17, 1985)

ABSTRACT Kennett, J. P., Elmstrom, K. and Penrose, N., 1985. The last deglaciation in Orca Basin, Gulf of Mexico: high-resolution planktonic foraminiferal changes. Palaeogeogr., Palaeoclimatol., Palaeoecol., 50: 189--216. Anoxic sediments in Orca Basin, northern Gulf of Mexico (water depth 2400 m) provide late Quaternary planktonic foraminiferal and paleoclimatic records of remarkably high resolution ( ~ 2 5 0 yrs). Black, organic-rich, strongly laminated, unbioturbated muds of Orca Basin contrast with grayish, organic-poor, bioturbated sediments immediately outside of the basin. Planktonic foraminiferal assemblages contained within totally anoxic sediments exhibit superb preservation and probably have not been changed much, by dissolution or other processes, from the original living assemblage. A quantitative comparison of surface-sediment assemblages (> 100 um and > 175 u m size fractions) from the anoxic area and from the oxygenated regime immediately outside the basin, nevertheless has revealed no significant differences between the assemblages. It is inferred, therefore, that the assemblages well above the lysocline in the northern Gulf have been subjected to little postmortem alteration, even though their preservation does not match those from the Orca Basin. Planktonic foraminiferal assemblages have been counted in two piston cores from Orca Basin that are as old as 29 kyrs B.P., and which contain continuous, high-resolution records of the last deglaciation. The location of these cores is 290 km S of the modern Mississippi Delta and is known to have been within the influence of meltwater discharge from the Mississippi River system during the last deglaciation of the Laurentide ice sheet, an event which significantly decreased surface-water salinities in the Gulf of Mexico. This meltwater effect is strongly recorded in the Orca Basin sequence. Faunal and stable-isotope changes have been evaluated in relation to the Pleistocene-Holocene transition and are associated with two major processes related to deglaciation: (1) the well-known glacial-to-interglacial surface-water changes as cool Gulf waters were replaced by warmer w a t e r masses; and (2) severe open-ocean surface-water salinity decrease resulting from the meltwater discharge into the Gulf between about 16.5 and 12 kyrs ago. Species associations were dynamically changing throughout the late Quaternary in the northern Gulf in response to oceanic and continental influences. Factor analysis of the down-core assemblages reveal three major faunal groupings which account for 95% of the total variance. Factor one is by far dominated by Globigerinoides ruber which shows 1Present address: Office of Development, Tulane University, New Orleans, LA 70118

(U.S.A.) 0031-0182/85/$03.30

© 1985 Elsevier Science Publishers B.V.

190 highest values closely associated with a major negative oxygen isotopic anomaly that resulted from the deglacial meltwater discharge to the Gulf. A severe surface-water salinity decrease created a harsh environment that was not favorable to the successful proliferation of most other planktonic foraminiferal species. This low-salinity fauna interferes with the normal faunal transition observed elsewhere between the latest Pleistocene and Holocene. Factor two is a warm-water (interglacial) assemblage dominated by Globorotalia menardii, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei. Factor three is a cool-water (glacial) assemblage dominated by Globorotalia inflata, Globorotalia crassaformis and Globigerinella aequilateralis. In the sequence, the glacial assemblage is replaced about 16.5 kyrs ago by the lowsalinity assemblage which itself persisted until about 12 kyrs ago when the meltwater ceased to flow into the Gulf and was replaced briefly (for 1.5 kyrs) by a return of the glacial assemblage until about 10.5 kyrs ago. This cool interval seems to be correlative with the Younger Dryas cold episode of Europe, and with the cold interval between Termination 1A and 1B in north Atlantic cores. After this, the warm-water (interglacial) assemblage migrated into the Gulf. Early Holocene (10.5--5 kyrs ago) assemblages are quantitatively distinct from late Holocene (5--0 kyrs ago) assemblages. Unlike previously documented associations, G. tuber rather than N. dutertrei, dominated during intervals of low surface-water salinity in the Orca Basin. We believe that surface-water masses were still too cool during the meltwater spike and salinities were too low for the proliferation of N. dutertrei. On the other hand G. ruber, a more euryhaline opportunistic species, survived the severe conditions, but did not prosper. N. dutertrei distinctly increased to moderate frequencies in the second half of the meltwater spike probably as surface waters warmed during deglaciation. In cores more distant from the Mississippi River outfall region, where oxygen isotopic evidence indicates the occurrence of less severe salinity reductions, N. dutertrei exhibits a distinct frequency peak, and G. ruber is less dominant. Species frequency oscillations provide intercore correlations at a resolution of only several hundred years. INTRODUCTION Historically, t h e first c r i t e r i o n e m p l o y e d in deep-sea s e d i m e n t a r y sequences t o i d e n t i f y the P l e i s t o c e n e / H o l o c e n e b o u n d a r y was the c h a n g e f r o m c o o l t o w a r m p l a n k t o n i c f o r a m i n i f e r a l faunas ( S c h o t t , 1 9 3 5 ) . Q u a n t i t a t i v e t a x o n o m i c changes in p l a n k t o n i c f o r a m i n i f e r a l assemblages still r e m a i n o n e o f the m o s t useful and w i d e l y e m p l o y e d criterion f o r u n d e r s t a n d i n g p a l e o e n v i r o n m e n t a l changes associated with the last deglaciation. O u r c o n t r i b u t i o n is an extension o f these studies and o n e o f its objectives is t o q u a n t i t a t i v e l y describe, at high resolution, the stratigraphic d i s t r i b u t i o n o f p l a n k t o n i c f o r a m i n i f e r a l assemblages d u r i n g deglaciation in t w o piston cores f r o m O r c a Basin, nort h e r n G u l f o f M e x i c o (Fig. 1; Table I). O r c a Basin is an a n o x i c basin ( S h o k e s et al., 1 9 7 7 ) l o c a t e d 2 9 0 k m S of the p r e s e n t - d a y Mississippi Delta. Piston cores f r o m t h e basin p r o v i d e an except i o n a l o p p o r t u n i t y f o r high-resolution studies o f p l a n k t o n i c foraminiferal assemblages d u r i n g the last deglaciation f o r several reasons: (1) preservation o f microfossfl g r o u p s in t h e s e d i m e n t s has been e n h a n c e d b y a n o x i c sediments ( K e n n e t t and Penrose, 1 9 7 8 ; L e v e n t e r et al., 1 9 8 3 ) ; (2) an absence o f bioturb a t i o n due t o the b o t t o m - w a t e r a n o x i a o r partial a n o x i a d u r i n g the entire late Q u a t e r n a r y ; (3) v e r y high s e d i m e n t a t i o n rates ( 2 9 - - 4 9 c m / 1 0 0 0 yrs);

191

50 °

25 °

20 °

95oW

90 °

85 °

80 °

Fig.1. Location of Orca Basin on continental slope of Louisiana shown by dot.

(4) better monitoring of the large-scale changes in planktonic foraminiferal assemblages that occurred between glacial and interglacial episodes (Kennett and Huddlestun, 1972a, b) is possible due to the location of the basin at latitudes intermediate between temperate and tropical water masses; (5) an ideal location close enough to the Mississippi Delta to have been under the influence of glacial meltwater discharge to the Gulf during the early melting stages of the Laurentide ice sheet (Kennett and Shackleton, 1975; Leventer et al., 1982; Fillon and Williams, 1983) and yet not to have received large volumes of potentially diluting terrigenous sediments transported by the river system. Previous oxygen isotopic analyses of planktonic foraminifera in late Quaternary Gulf of Mexico sequences (Emiliani et al., 1975; Kennett and Shackleton, 1975; Leventer et al., 1982) have recorded a negative anomaly of up to 3.6°/°° which has been dated between 16.5--11.6 kyrs ago (Leventer et al., 1982) and is indicative of a discharge phase of glacial meltwater from the ablating Laurentide ice sheet. A second objective of our contribution is to attempt to discriminate TABLE I Data for piston and trigger cores for the Orca Basin and vicinity Site

EN32-1 EN32-2 EN32-3 EN32-4 EN32-5 EN32-6

Position

26° 54.6'N; 26° 57.5'N; 26° 59.5'N; 26°56.1'N; 26° 59.1'N; 26° 56.8'N;

Water depth (m)

91° 21.0'W 91° 17.2'W 91° _23.2'W 91°21.7'W 91° 31.5'W 91° 21.0'W

2415 2310 1820 2260 2240 2280

Piston core length

Trigger core length

(cm)

(cm)

1165 929 933 765 767 1148

127 130 109 137 127 118

192

27000

'

26°55

'

] 91o50

'

9lo25

'

91o20

'

91ol5

'

Fig.2. L o c a t i o n of p i s t o n a n d trigger cores ( E N 3 2 ) f r o m w i t h i n a n d close t o Orca Basin. S h a d e d area i n d i c a t e s areal e x t e n t of brine. C o n t o u r s in meters.

between faunal changes that resulted from glacial-to-interglacial biogeographic change in the Gulf from those that resulted from more local severe surfacewater salinity changes. This, in turn, should provide insights into the response o f planktonic foraminiferal assemblages to salinity changes. Significant changes in surface-water salinity have been invoked in models to explain bottom-water anoxia in enclosed basins such as the eastern Mediterranean (Olausson, 1961; Thunell et al., 1977). A third objective of this study is to quantitatively evaluate and compare the preservational state of planktonic foraminiferal assemblages of anoxic surface sediments from Orca Basin and carbonate oozes from well above the foraminiferal lysocline immediately outside the basin (Fig.2). We wish to determine what, if any, faunal differences exist between almost perfectly preserved assemblages from the basin and carbonate oozes of the Gulf. OCEANOGRAPHIC SETTING OF ORCA BASIN

Orca Basin (McKee and Sidner, 1976; Trabant and Presley, 1978) is a 400 km 2 depression in the continental slope to the S of Louisiana which contains anoxic hypersaline (about 250 g kg-~ ; 260%o) water in the b o t t o m 200 m (Shokes et al., 1977; Tompkins and Shephard, 1979). This water has a density of 1.185 g/ml and a limited oxygen diffusion so that the water and underlying sediments are anoxic. The highly reduced surface sediments are 2--3 times richer in organic carbon (2--3%) than non-basin sediments (Northam et al., 1981). The brine is considered to be derived by lateral leakage from a nearsurface salt deposit. The basin has a maximum depth of 2400 m with a surrounding depth of 1800 m and consists of two depressions, separated b y a gentle swell (Fig.2). The hypersaline brine/seawater interface occurs at 2230 m.

193

Bottom sediments within the brine are black anoxic clays containing abundant Sargassum (Kennett and Penrose, 1978; McKee et al., 1978). Because of the severe environmental conditions within the Orca Basin brine layer, benthic life is essentially absent. No macrobenthic organisms have been reported from the basin. Also we have found that sediments contain only a single, rare, but persistent benthic foraminiferai species -- Osangularia culter, which has succeeded in adapting to the anoxic conditions of the basin. Preservation of planktonic foraminifera, pteropods and radiolaria is excellent. Late Quaternary piston cores from Orca Basin generally consist of an upper section of black mud underlain by a lower section of gray mud with intervals of dark laminae and streaks (Addy and Behrens, 1980; Leventer et al., 1983). The deposition of the upper black, anoxic sediment and hence the initiation of the brine has been dated at 7900 yrs B.P. (Addy and Behrens, 1980) based on radiocarbon dating of the sharp interface between gray mud, deposited under the more normal, but still partially anoxic conditions, and the black muds. M E T H O D S A N D CHRONOLOGY

Samples of approximately 5.5 cc were taken from the tops of six trigger cores (EN32 TC1--6) from within Orca Basin and immediately outside the basin (Fig.2; Table I). These samples were oven dried at 50°C, weighed, disaggregated in a hot Calgon solution, wet sieved over a 63 gm Tyler screen, oven dried and weighed again. These samples were ignited at 450°C to remove the abundant organic material to expedite counting (Sachs et al., 1964). About 300 specimens of planktonic foraminifera were counted in each sample in the >100 pm and >175 pm size fractions and the percent frequencies calculated. The piston cores (EN32-PC6 and PC4) were sampled at 10-cm intervals throughout and the samples were processed as with the trigger core samples. The samples were split using a modified Otto microsplitter and about 300 specimens of planktonic foraminifera were counted in the >150 pm size fraction. Relative percent frequencies were determined for each species. These data were processed using Q-mode factor analysis (Imbrie and Kipp, 1971; Klovan and Imbrie, 1971), a multivariate statistical technique. This technique examines the relative proportions of species within each sample in a data set, groups the data into a predetermined number of factors (assemblages; in this study three factors have been employed) and ranks each sample (provides a factor value) within each assemblage indicating its compositional similarity to that assemblage. Thus changes in faunal compositions are revealed by the changes in the factor scores. The chronology employed in this study largely follows that of Leventer et al. (1983) for EN32-PC6. This chronology is based upon linear interpolation between assumed zero-age at the core top, a carbonate radiocarbon date of 3915 + 105 yrs B.P. from 175--210 cm and the Y/Z biostratigraphic

194 boundary at 400 cm taken as 10.5 kyrs B.P. Broecker et al. (1960) and Ericson and WoUin (1968) dated the Y/Z boundary at 11 kyrs B.P., but we adopt a slightly younger age to allow for a diachronous appearance of this tropical form into the cooler Gulf of Mexico region. Further details, including a discussion of the limitations of the chronology are provided in Leventer et al. (1983). The Y/Z biostratigraphic boundary marks the first consistent appearance of the Globorotalia menardii group (Ericson and WoUin, 1968; Kennett and Huddlestun, 1972a). In EN32-PC6 and other cores from the Gulf (Kennett and Shackleton, 1975; Falls, 1980), the Y/Z boundary is located immediately above the oxygen isotopic enrichment indicative of the cessation of meltwater influx to the Gulf, but just prior to the oxygen isotopic depletion indicative of Termination I (Broecker and Van Donk, 1970). Using this chronology, sedimentation rates are shown to vary between about 30 and 50 cm/1000 yrs in EN32-PC6. This core contains a continuous record for the last 29 kyrs, with a resolution of about 250 yrs at a 10-cm sampling interval. The record of EN32-PC4 is quite easily correlated at high resolution with t h a t of EN32-PC6 because of similarity of faunal changes in both cores.

RESULTS

Micro fossil preservation Orca Basin anoxic sediments contain very well-preserved planktonic foraminifera, pteropods and radiolaria (Kennett and Penrose, 1978). We have n o t observed better preservation of planktonic foraminifera in any other sediments. Assemblages contain thin-wailed specimens some of which still have their spines intact around apertures and preservation is similar to specimens collected in plankton tows. TABLE II Comparison of general microfossil characteristics between anoxic and oxygenated surface sediments in the vicinity of Orca Basin Anoxic

Oxygenated

Pteropods

abundant

absent

Benthic foraminifera Planktonic foraminifera

absent

persistent, diverse

abundant, very well preserved, delicate forms

moderately well preserved, delicate forms rare

Radiolarians

common, very well preserved, delicate forms

common, only robust forms

Other groups preserved

seaweed, attached polychaetes, attached bryozoa, fish debris

benthonic ostracods, other group absent

195

General differences in microfossil assemblages between the anoxic and oxygenated surface sediments are summarized in Table II. Pteropods are abundant in the anoxic sediments but are almost completely lacking in the oxygenated surface sediments. Pteropods, being aragonitic in composition are highly susceptible to dissolution. Pteropods occur persistently throughout Orca Basin piston cores, often in large numbers and with good preservation, which is rare even for the late Quaternary of the Gulf of Mexico. Radiolaria are common in both anoxic and oxygenated surface sediments, but in the latter, assemblages are marked by specimen robustness, compared with the delicateness of forms preserved in the anoxic sediments. Also, unlike the planktonic foraminifera, many of the delicate radiolarian taxa present in anoxic sediments are absent, because of dissolution, in the oxygenated sediments.

PERCENTAGE 0

I I i I I I ~ I I

(>I00 F) 20

I0 I IIIIIIIII

30 IIIIII

IIII

IIII

#

6, MENARDII G, TUMIDA

?

6. CRASSAFORMI$ G, TRUNCATULINOIDES G, SCITULA G, FIMBRIATA G, RUBER G, SACCULIFER 6, CONGLOBATUS G, TENELLUS

i

6, BOLLII G, BULLOIDES 6, CAL]DA 6, FALCONENSIS

6, QUINQUELDBA G, RUBESCENS G, DIGITATA

(

6, GLUTINATA H, AEQUILAT~RALIS

I;

ANOXIC

H, PELAGICA N, DUTERTREI

-°

OXYGENATED

O, UNIVERSA P, OBLIQUILOCULATA S, DEHISCENS

I

I

I

I

Fig.3. Comparison of average percent-frequencies of planktonic foraminiferal species bet w e e n anoxic (heavy line) and o x y g e n a t e d (dashed-line) surface sediments (trigger-core tops) o f the Orca Basin area in the > 100 u m size-fraction.

196

The counts of the > 1 0 0 /~m and > 1 7 5 pm size fractions of planktonic foraminiferal assemblages show almost no quantitative differences between anoxic and oxygenated sediments (Figs.3 and 4). Average species frequency values are similar, differing by no more than 3% for any species. Species which do differ slightly between anoxic and oxygenated sediments do not reflect dissolution differences. The anoxic sediments do not contain more solution-susceptible species relative to the oxygenated sediments. Likewise, the oxygenated sediments do not contain higher proportions of the robust forms. This was unexpected because of the noticeably better specimen preservation in the anoxic assemblages. If it is a valid conclusion that the faunas from anoxic sediments have not changed much from the original living assemblage, it follows that the faunas well above the foraminiferal lysocline in the northern Gulf of Mexico also have changed little in taxonomic composition from the original living assemblages.

PERCENTAGE (>175~)

o

,,o

i

G,

MENARDI]

G,

TUMIDA

I

I

I

I

I

I

I

I

,20

I I

I

I I

I I

I

I

I

I

I

I

I

I

I

4,0 I

I

I

I

I

I

I

~/

G, CRASSAFORM£ I G, TRUNCATULN I OD I ES 5,

SC]TULA

G,

FIMBR]ATA

G, RUBER G, SACCULF I ER G,

CONGLOBATUS

5, TENELLUS ~,

BOLLII

G,

BULLOIDE£

G, CALIDA G. FALCONENSIS

G, QUN I QUELOBA G, RUBESCENS G, DIG]TATA B,

GLUTINATA

H, AEQU[LATERALIS H. PELAGICA N, DUTERTREI

~

--

ANOXIC

• "I

OXYGENATED

~I

O, UBIV ' ERSA P, O~LQ I UL IOCULATA S, DEHS I CENS

~.~,.

I

I

I

I

I

I

Fig.4. Comparison of average percent-frequencies of planktonic foraminiferal species bet w e e n anoxic (heavy line) and oxygenated (dashed-line) surface sediments (trigger-core tops) of the Orca Basin area in the > 1 7 5 / ~ m size-fraction.

197

Intercore correlations The data exhibit large downcore changes in percent frequency oscillations for most species that can be potentially exploited for paleoclimatic information. Can species oscillations in one core be recognized in another? At what stratigraphic resolution can intercore correlations be carried out? We present species frequency data from both piston cores for Globigerinoides ruber (Fig.5), Globorotalia inflata (Fig.6) and Neogloboquadrina dutertrei (Fig.7). Intercore correlations of species frequency oscillations have revealed that the upper part of EN32-PC4 is missing, equivalent to about 2.5 m of sediment. Comparison of the species frequency oscillations between the two cores clearly demonstrates that even the small-scale faunal oscillations can be correlated. To assist with intercore correlation, we have identified individual species frequency oscillations using numbers (Figs.5--7). Each species exhibits its own distinctive frequency oscillation; at times a species shows little change and hence is o f minimal value for intercore correlation. For example, N. dutertrei exhibits little frequency change during the last glacial episode (Fig.7). On the other hand, G. inflata exhibits large frequency oscillations during this interval that can be correlated between the cores (Fig.6). Use of all species can enhance the correlation resolution to within several hundred years at the sampling interval we have used. Closer sampling can potentially EN 32 PC 6 I0

I00

EN 32 PC 4

PERCENT

20

50

40

50

60

70

I0

20

30

40

50

60

I

I

I

I

I

I

I

I

I

I

I

I

t

-

MISSING -

HOLOCENE

200 G. tuber 300

-

400 --

E

t~ "r I-(1. L,J Q

m

r_Z_.

T

500 -600 -700800900-I000

-

GLACIAL r r6 r7

5

r6 r7

%

r8 r8

I100-

Fig.5. Correlation of percent-frequency oscillations of Globigerinoides ruber between EN32-PC6 and EN32-PC4. Individually correlated peaks and troughs are numbered. Shown is stratigraphic extent of the meltwater spike.

198 EN 5 2 PC 6

EN :52 PC 4. PERCENT

2

IO0

4

6

8

500

-

400

-

"E soo-

HOLOCENE

700 -

800

2

4

6

S

I0 12 14 16 18 20 22 24 26

MISSING

-~.

5:~-.i

t

MELT

,-',

0

-

200-

W

I0 12 14 16 18

. . . .

~

GLACIAL

~~i3

i2

-

900 -

.

.

.

.

.

.

~/nf/ato ~

i4

I000- ~

- - - - -7 . EZ~=:===-- 11

WATER SPIKE

i

5

~s

i5

i7 I100~

"-'-'---

i7

Fig.6. Correlation of percent-frequency oscillations of Globorotalia inflata between EN32-PC6 and EN32-PC4. Individually correlated peaks and troughs are numbered. Shown is stratigraphic extent of meltwater spike.

provide even greater resolution in these unbioturbated, rapidly deposited sediments. The obvious strong correlations in faunal oscillations between the two cores demonstrate that high-resolution paleoclimatic information of the last deglaciation is contained in these cores. Individual planktonic foraminiferal species are highly sensitive monitors o f paleoenvironmental change.

Meltwater spike and Y/Z boundary The highly distinctive negative isotopic anomaly (Fig.8) of about 3.6%o recorded between 640 and 430 cm in EN32-PC6 and dated between about 16.5--11.6 'kyrs B.P. (Leventer et al., 1982, 1983) represents a discharge phase of glacial meltwater from the early stages of the ablating Laurentide ice sheet in northern North America (Emiliani et al., 1975; Kennett and Shackleton, 1975; Leventer et al., 1982, 1983). We informally refer to this anomaly as the "meltwater spike" and its upper and lower limits are plotted on Figs.5--12. The details of the meltwater discharge history are discussed in Leventer et al. (1982), who describe two major discharges of magnitudes 2.0%o and 2.6%o which are superimposed upon several minor oxygen isotopic pulses. The oscillating nature of these isotopic changes indicates several

199

EN 52 PC6

EN 5 2 P C 4

PERCENT 0 2 4 6 8 I0 12 14 16 I

I

I

I

v-

Ioo- ~

i

I

I

HOLOCENE

0 '2 4 6 8 I0 12 I

I

I

~1~-

I

I000 - ~ d

I

l

d 2.,,~

ELT WATER SPIKE ~

IJJ 700 d9 t'~ ~ " d I0

900 - L , . / . / "

I

MISSING

5 .

~_. ouu._~d7

l

IT

°,

500 4.00--

I

.

.

.

.

P"d5 ,d6 ,d7 69 P'dlO

iV d u l e r l r e /

duterlre/

12

d13

h,..d 14

I100 -

Fig.7. Correlation of percent-frequency oscillations of Neogloboquadrina dutertrei between EN32-PC6 and EN32-PC4. Individually correlated peaks and troughs are numbered. Shown in stratigraphic extent of the meltwater spike.

retreats and advances of the southern margin of the Laurentide ice sheet during the interval between 16.5 and 11.6 kyrs B.P. after which most meltwater was diverted from the Mississippi River system to the E. The high-resolution oxygen isotopic data in EN32-PC6 (Fig.8) indicate that the greatest discharge of meltwater to the Gulf of Mexico occurred during the second major discharge between about 15 and 12 kyrs B.P. (Leventer et al., 1982). The first major discharge was about half the magnitude of the second. The Y/Z boundary in EN32-PC6 is placed at 400 cm and is defined (Ericson and Wollin, 1968; Kennett and Huddlestun, 1972a) by the first consistent appearance of Globorotalia menardii and has been radiocarbon dated at 11,000 + 500 yrs in the Caribbean (Broecker et al., 1960). We adopt an age of 10,500 yrs for the northern Gulf of Mexico. The Y/Z boundary is stratigraphically positioned slightly above the meltwater spike (Fig.9) and is generally interpreted to represent the time when a warm (interglacial) fauna effectively replaced the cool (glacial) assemblage in the Gulf of Mexico (Kennett and Huddlestun, 1972a).

200 EN 32 PC 6 PERCENT

JiB0 0

I

I00

-

200

-

-I

-2

[

-3

I

I

I0

20

30

I

I

I

40

50

60

70

I

J

I

I

300-

400

-

500 -

15 v

6 0 0 --

n-

a6

T I-n

700 -

"~

800 -

- 20

900

-

-25

I000

--

I100-50

A

B

Fig.8. Plots of: A. ~180 PDB %o for Globigerinoides tuber (left); and B. percent-frequencies of G. tuber in EN32-PC6. Depth scale shown at left; time scale at right. Shown is stratigraphic extent of the meltwater spike defined by the oxygen isotopic data, and the Y/Z planktonic foraminiferal boundary. Oxygen isotopic data is from Leventer et al. (1982).

Species frequency oscillations Species frequency oscillations of the more abundant planktonic foraminifera in EN32-PC6 are shown in Figs.8--11. Reference to the last glacial episode in this paper refers only to that part represented in the two cores studied, from a maximum age of 29 kyrs B.P. Glob~gerinoides ruber (Figs.5, 8) is the dominant faunal component throughout much of the latest Quaternary, ranging from 15 to almost 70% but averaging about 30-40% of the assemblage. During the last glacial episode, this species averaged about 35% of the assemblage which is slightly but noticeably more than the Holocene when it averaged 30%. Also, frequencies are distinctly lower during the early half of the Holocene. There are two distinct frequency abundance peaks in G. tuber: between 2 9 - 2 7 kyrs ago, within the last glaciation and between 16 and 12.5 kyrs ago, associated with the meltwater spike. Highest values occur during the earlier half of the meltwater spike centered at about 15 kyrs ago. The percent frequency of G. ruber diminishes rapidly and simultaneously with the cessation of meltwater inflow to the Gulf at about 12.5 kyrs ago (Fig.8). Globorotalia menardii (Figs.7, 9), a tropical/warm-subtropical species (B~ and Tolderlund, 1971), is absent during the last glaciation until 18 kyrs ago,

201

EN 52

PC6

PERCENT 0

I00

2 4

6

8 I0 12 0

2 4

6

8 I0 12 0

2

4 S 8 I0 12 14 16

--

-5

200

300

i~_~_

400

E "1I" n I.d r-,

Z Y

i G.menard/i

500 600

--

?00

--

'

t

~ ' MELT WATER , _~__ S P I K E . _~,'_

•is

r~ ~2

P

obliquloculoto -20

800

GLACIAL

900 -25 I000

I I00

3O

Fig.9. Percent frequency oscillations of planktonic foraminiferal species that quantitatively favor warm waters: Globorotalia menardii, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei in EN32 PC6. Depth scale is to the left; chronology to the right. Shown also are the lower and upper limits of the meltwater spike and the position of the Y/Z boundary.

and exhibits sporadic and very low frequencies during the end of the last glacial episode until about 10.5 kyrs ago (Y/Z boundary), after which it becomes a consistent faunal element. This species occurs in generally low frequencies during the early half of the Holocene and in high frequencies during the later half. It clearly marks Quaternary interglacial episodes in the Gulf of Mexico (Ericson and Wollin, 1968; Kennett and Huddlestun, 1972a), although its first and last consistent appearance is diachronous with the warmer Caribbean sequences (Kennett and Huddlestun, 1972b). Pulleniatina obliquiloculata (Fig.9) occurs only rarely and sporadically during the last glacial episode and first becomes consistently present near the beginning of the meltwater spike about 16 kyrs ago. Its frequency increases steadily during the latest Pleistocene. Within the Holocene, it reaches distinctly higher frequencies becoming an important faunal element during the later half of the Holocene. Neogloboquadrina dutertrei (Figs.7, 9) occurs consistently, but in low frequencies, during the last glacial episode except for a brief, high-frequency interval at 27 kyrs ago, most clearly shown in EN32-PC4 (Fig.7). This peak coincides, in part, with a high-frequency peak of G. ruber. Neogloboquadrina

202

dutertrei exhibits a marked increase in frequency in the middle of the meltwater spike at 14.5 kyrs ago; a brief temporary decrease between the end of the meltwater spike and the Y/Z boundary (10.5 kyrs B.P.); and a major frequency increase from the Y/Z boundary until the middle of the Holocene about 5 kyrs ago. The latter half of the Holocene is marked by only moderate frequencies of this species. Globorotalia inflata (Figs.6, 10) is a modern, temperate form (B~ and Tolderlund, 1971), and a clear marker of Quaternary glacial episodes in the Gulf o f Mexico (Kennett and Huddlestun, 1972a). Kennett and Huddlestun (1972a) have interpreted the occurrence of G. inflata in the Gulf of Mexico as a southern migration of its north Atlantic biogeographic range in response to cooler temperatures during glacial episodes. This species occurs consistently throughout the last glacial episode until near the beginning of the meltwater spike (16 kyrs ago) when it completely disappears. It also reappears very briefly upon the cessation of meltwater inflow to the Gulf about 12 kyrs ago and before the Holocene warming. Globorotalia inflata is entirely absent in Holocene and present-day Gulf assemblages. During the last glacial episode it exhibits a distinct decrease in frequencies between 28.5 and 26 kyrs ago during an interval of high frequencies of G. ruber (Fig.8). EN 52 PC 6 PERCENT 0

2

4

6

8

I0 12 ~4 16 18 0

2

4

6

8 I0 12

0

8 12 16 20 24 28

7 I00

-

HOLOCENE 200

-

~~G. aequi/a

300 -

teralis

400 -

-I0

500 -

~..~MELT WAitF'R _

r",

600 -

SPIKF.

n"

-m a~ >:

700 . G. i n f l o t o

-2G

800 900-25 i 000

-

IlO0-

-30

Fig.10. Percent frequency oscillations of planktonic foraminiferal species that quantitatively favor cool waters: Globorotalia inflata, Globorotalia crassaformis and Globigerinella aequilateralis in EN32-PC6. Depth scale is to the left; chronology to the right. Shown also are the lower and upper limits of the meltwater spike and the position of the Y/Z boundary.

203 Highest frequencies of G. inflata are recorded (Fig.6) between 23.5 and 26 kyrs ago, after which this species gradually declines in relative importance in Gulf assemblages, and disappears near the beginning of the meltwater spike. Globorotalia crassaformis (Fig.10) exhibits high but fluctuating frequencies during the glacial episode until near the beginning of the meltwater spike at 16 kyrs ago, when it sharply decreases in frequency during most of the meltwater spike. This species again increases to moderate frequencies immediately following the meltwater spike, and remains an important faunal element until 6 kyrs ago when it almost totally disappears from the Gulf assemblages. Frequencies of this species during the early Holocene are about half that of the late Pleistocene, supporting the conclusion of Kennett and Huddlestun (1972a) that this is a marginally cool species, rather than an extremely coolwater form such as G. inflata. In EN32-PC6, both G. crassaformis and G. inflata exhibit highest frequencies in the same intervals (Fig.10). GlobigerineUa aequilateralis (Fig.10) occurs as an important element throughout the entire latest Quaternary sequence, but exhibits highest frequencies during the glacial ePisode , until near the beginning of the meltwater spike about 16 kyrs ago, after which it exhibits large frequency fluctuations. Globigerina falconensis, a cool-water form (Kennett and Huddlestun, 1972a), exhibits highest frequencies during the glacial episode, decreases markedly at about 15 kyrs ago within the meltwater spike, and thence reappears between 11.5 and 10 kyrs ago to exhibit the highest frequency values. This species exhibits lowest percent frequencies during the Holocene. Globigerinita glutinata is a consistent faunal element during the entire late Quaternary sequence. It exhibits low frequencies (~ 5%) during the last glacial episode and increases to moderate values (~ 12%) from the late part of the meltwater spike to the late middle Holocene (13 to 3 kyrs B.P.). Globorotalia truncatulinoides (dextral coiling), a warm-water form (Kennett and Huddlestun, 1972a) exhibits highest frequencies during the Holocene, particularly during the last 2 kyrs. General faunal change and subzone correlations The changes in faunal assemblages (Fig.11) mark three principal paleoenvironmental episodes in this region. A glacial episode until 16 kyrs B.P. marked by high frequencies of G. inflata and other cool-water forms; a meltwater-spike assemblage marked by high frequencies of G. ruber until about 12 kyrs B.P.; and finally beginning at 10.5.kyrs B.P., an interglacial (Holocene) assemblage marked by high frequencies of G. menardii and other warm-water forms which are maintained in the modern Gulf. Changes in the late Quaternary assemblages in the Orca Basin sequence can be generally correlated to subzones (Fig.11) previously recognized by Kennett and Huddlestun (1972a):

204 EN 52 PC 6 PERCENT 2 I

4 I

6 I

8 I0 12 14 16 18 I I 1 l I I

I0 I

HOLOCENE

~=~ v

o

I I.-t,i

-

_

- _ -

_

20 I

~0 I

~

--_--_ _--_2 _

40 I

50 I

60 I

G. ruber

~-5

_-_-7_

MEL

~..

~ GLACIAL

~

~

IY21

so

Fig. 11. Percent frequency oscillations of the glacial marker Globorotalia inflata, the interglacial marker Globorotalia menardii and Globigerinoides tuber which exhibits high frequencies during the meltwater spike. Depth scale on the left; chronology to the right. Planktonic foraminiferal subzones of Kennett and Huddlestun, 1972a are shown to the right. Also shown are the lower and upper limits of the meltwater spike and the position of the Y/Z boundary.

Upper part o f Y2 Subzone (29--16 kyrs B.P.) T h e m o s t distinctive f e a t u r e o f this interval is t h e high frequencies o f G. crassaformis. In addition, G. inflata is c o n s i s t e n t l y present, a l t h o u g h dist i n c t l y decreases upwards. A n o t h e r c o o l - w a t e r f o r m , G. falconensis also exhibits high frequencies during this interval. T h e w a r m - w a t e r f o r m G. menardii is a b s e n t and P. obliquiloculata is sporadic and rare. Neogloboquadrina dutertrei exhibits low frequencies. T h e u p p e r p a r t o f Y2 is m a r k e d b y a distinctive faunal change at a b o u t 16 kyrs ago t h a t includes t h e last c o n s i s t e n t o c c u r r e n c e o f G. inflata, a m a j o r increase in G. ruber and N. dutertrei, dist i n c t decreases in G. crassaformis, G. aequilateralis and G. falconensis and t h e first c o n s i s t e n t a p p e a r a n c e o f P. obliquiloculata. Also G. menardii appears in l o w f r e q u e n c i e s at t h e b o u n d a r y .

Y1 Subzone (16--12 kyrs B.P.) Marked b y high f r e q u e n c i e s o f G. ruber and N. dutertrei and l o w e r frequencies o f G. crassaformis. This s u b z o n e is equivalent t o t h e m e l t w a t e r spike.

205

Y I A Subzone (12--10.5 kyrs B.P.) A brief zone previously unrecognized by Kennett and Huddlestun (1972a) because of the lower stratigraphic resolution provided in the cores examined. This interval is marked by high frequencies of G. inflata, G. aequilateralis and G. falconensis and distinctly lower frequencies of N. dutertrei. This interval is here designated as the Y1A Subzone. The assemblages represent a brief cool-water (glacial) interval between the termination of the meltwater spike and the migration of the warm (interglacial) assemblage to the Gulf. The upper part of this subzone is the Y/Z boundary marked by both the first consistent occurrence of G. menardii, a large frequency increase in N. dutertrei and consistent increases in G. crassaformis and P. obliquiloculata. In addition, G. truncatulinoides begins to increase in frequency upwards from this boundary. Z2 Subzone (10.5--6 kyrs B.P.) Exhibits high frequencies of N. dutertrei and G. crassaformis, moderate to low frequencies of G. falconensis, P. obliquiloculata, G. menardii and G. truncatulinoides, and markedly lower frequencies of G. tuber. The upper boundary of Z2, at 6 kyrs B.P. is marked by the upward, abrupt virtual elimination of G. crassaformis and a distinct decrease in N. dutertrei. In addition, distinct increases in frequencies occur in G. menardii, P. obliquiloculata, G. ruber and G. truncatulinoides (dextral coiling). Z1 Subzone (6 kyrs B.P.--Present) Marked by higher frequencies of G. menardii, P. obliquiloculata, G. truncatulinoides and G. ruber. Factor analysis o f assemblages Factor analysis of the assemblages in EN32-PC6 reveal three distinct groupings which account for 95% of the total variance. The overall communalities are good; all greater than 0.8 and most greater than 0.9. The factors distinguished are as follows: Factor 1 (34.4% of the faunal variance): this factor is totally dominated by G. tuber. Highest loadings of this factor occur between 29 and 27 kyrs ago and between about 16 and 12.5 kyrs ago where they are associated fairly closely with the meltwater spike. Factor 2 (32.3% of the faunal variance): this factor is dominated by G. menardii, P. obliquiloculata, N. dutertrei and G. sacculifera, all warmwater forms in the Gulf (Kennett and Huddlestun, 1972a), as well as G. glutinata and G. tuber. This factor exhibits highest loadings during the last 10.5 kyrs and, because of the importance of N. dutertrei in the assemblage, also briefly towards the end of the meltwater spike. As discussed later, this factor represents an interglacial assemblage during the Holocene and, in part, a low-salinity effect within the meltwater spike.

206

~I "8

I~ 0

. 0

0 rn r~

0

,

,

,

,

I

i

,

i

~ i

i

,

,

,

I

. . . .

I

. . . .

I

. . . .

,

r

6

I

~0

%

-

I

o-

o._ +

~S

,~

~ ~..~

- -

Q -

_

-~

0~-

<>

~-

~

~

<.: ~ ~,

~..~ ~>~-~ o~

© z f7 0

~ o

0

.

_J 0

oJ ~0

bJ

_~

Z

~-

W

"~ ~

I

~

I

i-

I

~

..

_I

rr" _l

0 Im

Z

W

7

0 7-

o m

I

,~ ~

I

~

~

~

~

~ . . : . . 4~ .~ ~

o~o.~

¢-)

m

~.~ "- -

Ii

~1+

0

o

~~.~ .~ =~, -

.~~

~ I

.4-

" ~

t,O

0 0

0 0

0 0

0 0

0 0

0 0

0 0

0 0

0 0

0 0

--

od

~r~

~-

~

tO

D--

~

O~

0

(u~o) H1cl3a

0 0

• "~

°

207

Factor 3 (28.1% of the faunal variance): this factor is dominated by G. infIata, G. crassaformis and G. aequilateralis, all cool-water forms, as well as G. ruber. This is a glacial assemblage, exhibiting highest Ioadings between 29 and 16 kyrs ago and briefly between 12 and 10.5 kyrs ago. DISCUSSION

The initiation of meltwater inflow to the Gulf of Mexico about 16.5 kyrs ago records the beginning of the retreat of the Laurentide ice sheet and the transformation from a glacial-to-interglacial global regime (Kennett and Shackleton, 1975; Leventer et al., 1982). The planktonic foraminiferal oxygen isotopic record from EN32-PC6 records a fairly detailed history of the early deglacial stages, against which the surface-water faunal changes also related to deglaciation can be compared. The planktonic foraminiferal faunas of the Orca Basin sequence changed both in response to the globally related glacial-to-interglacial migration of water masses to and within the Gulf, and to surface-water salinity changes resulting from meltwater discharge. The meltwater interfered with the more normal faunal transition from cool (glacial) to warm (interglacial) planktonic foraminiferal assemblages. The glacial assemblage disappeared near the beginning of the meltwater spike (at about 16.5 kyrs ago) in response to the large decrease in the salinity of surface waters, and the euryhaline form G. ruber became the dominant faunal element. The return to enriched 5180 values following the maximum meltwater discharge at 12.5 kyrs ago indicates that full interglacial conditions had not been reached, although meltwater input to the Gulf was diminished. The cessation of meltwater flow to the Gulf is attributed to the change in direction of meltwater flow from the south to the east into the St. Lawrence River Valley (Prest, 1970; Kennett and Shackleton, 1975; Leventer et al., 1983). By about 10.5 kyrs ago, the oxygen isotopic data indicate attainment of Holocene surface-water temperatures. This sequence of events is supported by changes in the planktonic foraminiferal assemblages. A distinct cold-water (glacial) assemblage returned to the northern Gulf of Mexico for 1.5 kyrs following cessation of the meltwater influence. In the absence of the low-salinities, the Gulf was still under the influence of cool-water masses and associated assemblages. This cool interval seems to correlate with a similar one identified in north Atlantic cores by Duplessy et al. (1981) located between Termination 1A and 1B and radiocarbon dated between 11.5 and 10.0 kyrs. A similar cold interval was also recognized by Ruddiman and McIntyre (1981) in north Atlantic cores. These workers correlate this interval with the Younger Dryas cold episode of Europe dated between about 11 and 10 kyrs B.P. (Eicher and Siegenthaler, 1976, 1982; Siegenthaler et al., 1984). By 10.5 kyrs ago cool assemblages were replaced by warm assemblages. Of particular interest in this study is the nature of the faunal response to

208 the decrease in surface-water salinities during deglaciation. To what extent did surface-water salinities decrease in the Orca Basin region and other areas of the Gulf?. In the m o d e r n Gulf, Mississippi River discharge has a more local influence and lowers salinity as far as 150 km from the delta at depths of 50 m (Nowlin and McLellan, 1967; Thurman, 1978). Modern surface-water salinities at Orca Basin are 36°/°° and well outside the effect of Mississippi freshwater discharge. Near the beginning of deglaciation, when sea levels were about 100 m lower than the present day, Orca Basin would have been located 17 km closer to the freshwater outfall region. The oxygen isotopic data from EN32-PC6 records a maximum lightening of 3.6°/°0 during the meltwater spike, as recorded in the surface-dwelling species G. ruber (Leventer et al., 1982, 1983). An average salinity of 37°/°° was determined by Brunner and Cooley (1976) for the Gulf during the last glacial m a x i m u m (18 kyrs ago). The m a x i m u m oxygen isotopic lightening of 3.6°/°0 in the Orca Basin area includes about 0.6°/°0 in temperature increase (equal to about 2°C). The remaining anomaly can be attributed to salinity decrease. The magnitude of the anomaly suggests that salinities during the m a x i m u m meltwater discharge were as low as 34%°, assuming a 1°/oo change in the isotopic composition to a 1% change in salinity. A salinity decrease of 3%° is large for open-ocean planktonic species and was not conducive to the success of many planktonic foraminiferal species. Jones {1967) observed that salinity variations as low as 0.2--0.5%o affected the relative abundances of N. dutertrei in the Atlantic Equatorial Undercurrent. Oxygen isotopic data for G. tuber indicate that the magnitude of the meltwater anomaly in cores decreases with increasing distance from the Mississippi Delta, probably reflecting a diminishing effect (Leventer et al., 1982). The largest recorded anomaly is in EN32-PC6 (--3.6%) and GS7102-7 (--3.4%°; Emiliani et al., 1975) which are both closest to the outfall. Core K97, in the central western Gulf exhibits an anomaly of--2.4%o (Kennett and Shackleton, 1975), while TR126-23 farthest from the source in the southwest Gulf has a m a x i m u m anomaly of --1.4%o (Falls, 1980). These values may be up to about--0.4°/oo too light because of the disequilibrium calcification of G. ruber (Fairbanks et al., 1982). During the meltwater spike in Orca Basin, G. ruber increased in frequencies to values as high as 70%, with associated decreases of other species. During other intervals, G. tuber averaged about 35% of the assemblage and decreased at times to as low as 15--20%. Globigerinoides ruber is an extremely successful surface-water dweller, containing zooxanthellae and inhabiting the tropical and warm-subtropical water masses (B~ and Tolderlund, 1971). It favors areas where the permanent thermocline is deep and well developed. It seems to be an opportunistic species with a wide salinity tolerance. B~ and Tolderlund (1971) observed that G. ruber is a euryhaline species that proliferates in low-salinity waters of less than 34.5%0 and high salinities of greater than 36%0 such as the Sargasso Sea (Ruddiman, 1969) and the eastern Mediterranean Sea (B~ and Tolderlund, 1971). The central

209 Sargasso Sea is an area of high salinities (37.3%) and low nutrient availability and is an extreme surface environment for the open ocean. In this environment, G. ruber attained a position of dominance by default (Ruddiman, 1969). In eastern Mediterranean Quaternary sequences, G. ruber is an abundant form associated with several, but not all, sapropel layers (Muerdter et al., 1984) which have been linked to decreased surface-water salinities (for summary of references refer to Muerdter et al., 1984). In other sapropel layers, G. ruber actually decreased in frequency at times of inferred lower surface-water salinities (Thunell et al., 1977). In the present-day surface sediments of the Gulf of Mexico, G. ruber is the dominant species, comprising 30--70% of the assemblage (Brunner, 1979). Highest modern values (50--70%) occur in the isolated southwestern Gulf (Gulf of Campeche) and in the slope waters of the extreme northwestern Gulf off Louisiana and eastern Texas where salinity is low. Lowest modern values occur in the east where the changing position of the Loop Current (Leipper, 1970; Maul, 1977) creates instabilities in the depth of the thermocline (Brunner, 1979). It seems that G. ruber dominated during the meltwater spike by default. Other species simply could not easily survive in the lowsalinity conditions. Calculations of rates of planktonic foraminiferal accumulation (>150 pm fraction) in EN32-PC6 (Fig.13) shows that values did not change much, on average, during the meltwater spike (= ~Y1 Subzone) relative to the last glacial episode (Y2 Subzone). Average rates of accumulation for the last glacial episode and the meltwater spike are about 23 X 103 cm-~ 1 0 -3 yrs. During the early part of the meltwater spike, planktonic foraminiferal accumulations are by far dominated by individuals of G. ruber. Fluctuations in the rates of accumulation of planktonic foraminifera in this sequence, must, in part, be due to preservational differences (Leventer et al., 1983). The amount of accumulation rate change that can be attributed to changes in productivity cannot be determined. However, there is no evidence to indicate that the total production of planktonic foraminifera changed much during the time of low salinity associated with the meltwater spike. Instead, a conspicuous increase in accumulation rates of planktonic foraminifera occurs just below the Z/Y (Holocene/Pleistocene) boundary within the Y1A subzone about 11,000 years ago (Fig.13). Rates remained high during the early Holocene until about 4000 yrs B.P. when they again decreased to about 35 X 103 cm -2 10 -3 yrs. During the Holocene, foraminiferal accumulations included much higher proportions of other species relative to G. ruber compared with the Pleistocene. Another species that is more generally known to increase in abundance at times of decreased surface-water salinity is N. dutertrei. Unusual assemblages marked by high frequencies (20--55%) of N. dutertrei are associated with many, but not all Quaternary sapropel layers in the eastern Mediterranean basin (KuUenberg, 1952; Parker, 1958; Ryan, 1972; Cita et al., 1977, 1982; ThuneU et al., 1977; Muerdter et al., 1984) and has been termed the "sapropel-related" assemblage (Thunell et al., 1977). Kullenberg (1952) first pro-

210

"d "El "A "N o

0 i~.

O ,Vl >

,7-

;

~

o

"

"

o 8 ~

~1

~_

,

o o

O_

O.

u2

o_

0

0

"~e~

O-

~t x

t

.~,~ ~ o ~ E

0 r0-

•~ =

..=

o_

•~ ~,~

0

X

O-

~

O

,

, , 'I ~|

•

_o/

,.., ~.,,, '~ 0 0

X-

X

0 0-

rn

,,.o ..o

I

r--.ll

V

I

,

;" I

0:

1 I --~

I- b.I ~

" ~ o ~ o ~'~ o

t

J o

~/

o_

~

~ '

I V "~,,,

~',

•

~'~

o~

o_ 0

X

0

~7~

0

z

N N o,-~ L O-

~~WJIIlIHII O

0 0

--

0 0 ~

0 0

ro

i

I

II/llJlll~J[ltllIIJllll/llll I 0 0 ~

I

I

I

I

I

0 0

0 0

0 0

0 0 co

0 0 0~

~

~.~

i' ~

( ~ O ) HJ_d3a

III IIIIIII 1

I

0 0

0 0

0

--

211

posed that N. dutertrei might respond primarily to salinity changes in the eastern Mediterranean. Later, B~ and Tolderlund (1971) and Ruddiman (1969) found N. dutertrei to be most abundant in low-salinity surface waters of the north Atlantic and the Mediterranean (Thunell, 1978; Loubere, 1981). Likewise, in the Gulf of Mexico, high frequencies of N. dutertrei have previously been recognized in association with the meltwater spike (Kennett and Shackleton, 1975; Thunell, 1976). The salinity optimum of N. dutertrei in the modern ocean is 35--35.5°/oo(Jones, 1967; Kipp, 1976). Since it is well established, therefore, that N. dutertrei frequencies increase during low salinity, why did this species not dominate during the meltwater spike in Orca Basin rather than G. ruber? Actually, N. dutertrei does increase in frequencies in the upper part of the meltwater spike (Figs.7, 9), but occurs only with maximum frequencies of 8%. In the earlier half of the Holocene when conditions were warmer (10.5--5 kyrs B.P.) N. dutertrei exhibits higher frequencies (up to 16%) than during the meltwater spike. B~ and Tolderlund (1971) associated N. dutertrei with areas of upweUing, near continental margins and in strong currents. Kipp (1976) found N. dutertrei to be a member of the Gyre Margin assemblage in the north Atlantic, which is in close agreement with the modern distribution data of B~ and Tolderlund (1971). Jones (1967) found N. dutertrei to be closely associated with the Equatorial Atlantic Undercurrent. Fairbanks et al. (1982) showed that living N. dutertrei in the Panama Basin exhibits a pronounced abundance peak between 25 and 50 m, precisely the interval corresponding to the steep thermocline, highest primary production and associated deep chlorophyll maximum. Thunell et al. (1983) also observed that seasonal high abundances of N. dutertrei are closely linked to shallowing of the thermocline to within the photic zone which enhances productivity of this species. Oxygen isotopic evidence indicates that N. dutertrei was living at greater depths than G. ruber during the meltwater spike. In K97, the oxygen isotopic anomaly, based on analysis of G. ruber is 1.18%o lighter than the oxygen isotopic anomaly based u p o n N. dutertrei during the inferred peak of meltwater inflow (Kennett and Shackleton, 1975). We believe that the combination of temperature and salinity of surface waters was not optimal for the successful proliferation of N. dutertrei, and that G. ruber dominated by default. During the early part of the meltwater spike, salinities would have been closer to the optimal values preferred by N. dutertrei (35--35.5%0). However, temperatures were too cool during the early stages of deglaciation for N. dutertrei to proliferate. During the later part of the meltwater spike, warmer conditions began to prevail as indicated by increased frequencies of P. obliquiloculata and rare appearances of G. menardii. However, at this time, salinities were probably too low (33-34%0) in the Orca Basin area for successful exploitation by N. dutertrei. On the other hand, in Gulf cores further to the south, at greater distances from the freshwater outfall, assemblages contain distinctly higher frequencies of N. dutertrei during the meltwater spike (Subzone Y1A). In the central

212 Gulf, frequencies of N. dutertrei average about 10% while in the far southwest (Gulf of Campeche) frequencies of N. dutertrei at this time average a b o u t 15% (Kennett and Huddlestun, 1972a). Thunell (1976) also noted increased frequencies of N. dutertrei in Y1 in southwest Gulf cores which he attributed to a salinity decrease in surface waters of the Gulf at the end of the last glacial episode. We believe that, during the later part of the meltwater spike in areas further to the south in the Gulf, where salinity decrease was less severe and temperatures were perhaps slightly warmer, N. dutertrei did increase significantly in abundance. At the same time, G. tuber does not form a distinct high-frequency peak and other species generally exhibit higher frequencies than in the more northern cores. During the early part of the Holocene, N. dutertrei became a consistent and abundant faunal element throughout the Gulf when warmer water masses existed in the Gulf and salinities were optimal for this species. In the Orca Basin sequence there exists a distinct frequency increase exhibited by G. ruber from 29--27 kyrs ago (Fig.8) which we cannot explain because it is n o t associated with any lightening in the 5'sO record. This peak in G. ruber is also associated with a brief frequency peak in N. dutertrei (Fig.7) and a decrease in G. inflata. The faunas seemed to respond in a manner similar to their response during the upper part of the meltwater spike, suggesting a salinity decrease 29--27 kyrs ago, b u t this is not supported by the oxygen isotopic data. CONCLUSIONS (1) Orca Basin, a late Quaternary anoxic to partly anoxic basin, 290 km S of the Mississippi Delta, contains a sequence of unbioturbated, rapidly deposited sediments, rich in well-preserved microfossils that provide a high-resolution biostratigraphic, paleoclimatic and stable oxygen isotopic record during the last deglaciation. (2) The stable oxygen isotopic data (Leventer et al., 1983) show that the glacial-to-interglacial (Holocene) transition is complicated by meltwater discharge from the Mississippi River system during the last deglaciation of the Laurentide ice sheet. This significantly reduced surface-water salinities of the Gulf of Mexico between a b o u t 16.5 and 11.6 kyrs ago by an inferred maxim u m o f 3%o. This is referred to as the meltwater spike. (3) Quantitative studies of planktonic foraminiferal assemblages in two late Quaternary cores from Orca Basin, spanning the last 29 kyrs provide a high-resolution record at sampling resolution approximately equal to every 250 yrs. Percent-frequency oscillations of individual species exhibit strong similarities between the cores and provide a basis for high-resolution intercore correlations. (4) Planktonic foraminiferal faunas of Orca Basin changed both in response to globally related glacial-to-interglacial migration of water masses to and within the Gulf, and to the surface-water salinity changes resulting from the

213

meltwater discharge as shown by the oxygen isotopic record. The meltwater interfered with an otherwise more normal faunal transition from cool-water (glacial) to warm-water (interglacial) assemblages. Factor analysis of the assemblages reveals three distinct assemblages interpreted as a meltwater related assemblage; a cool-water (glacial) assemblage and a warm-water (interglacial) assemblage. (5) In the sequence, the glacial assemblage is replaced about 16 kyrs ago by the low-salinity assemblage which itself persisted until about 12 kyrs ago when the meltwater ceased to flow into the Gulf and was replaced briefly (for 1.5 kyrs) by a return of the glacial assemblage until 10.5 kyrs ago. This cool interval seems to be correlative with the Younger Dryas cold episode of Europe and with the cold interval between Termination 1A and 1B in north Atlantic cores. After this, the warm-water (interglacial) assemblage migrated into the Gulf. Early Holocene (10.5--5 kyrs) assemblages of the Gulf are quantitatively distinct from late Holocene (5--0 kyrs) assemblages indicating that the late Holocene was warmer than the early Holocene. (6) The meltwater spike, as defined by the major negative oxygen isotopic anomaly, is closely associated with very high frequencies of G. ruber, a known euryhaline, surface-dwelling species. Surface-water salinities were too low in the northern Gulf for the successful proliferation of most species and G. ruber dominated by default. Another species known to have low-salinity tolerance, N. dutertrei, was not able to exploit the low salinities. During the early part of the meltwater spike, surface-water temperatures were still too low. In the later part of the meltwater spike, salinities may have been too low. (7) In sequences further to the south, N. dutertrei does exhibit higher frequencies during the meltwater spike. Isotopic evidence in these areas indicate less severe salinity reduction of surface waters. Here G. ruber does not exhibit a high-frequency peak associated with the meltwater spike. (8) Unmixed sedimentary sequences such as in Orca Basin can reveal interspecies relationships which in turn can provide useful data about species changes within a complex set of changing paleoenvironmental parameters. (9) A quantitative comparison of planktonic foraminiferal assemblages in the anoxic surface sediments of Orca Basin with those from oxygenated surface sediments immediately outside of the basin reveals no significant taxonomic or quantitative faunal differences. It is possible that the superbly preserved assemblages from the Orca Basin sediments are quantitatively a good representation of the living original assemblage. If so, then the planktonic foraminiferal assemblages in the oxygenated surface sediments outside the basin (water depths 1800--2200 m) have a taxonomic composition also little altered from the original living assemblage. This contrasts with the radiolarian assemblage which displays large differences in taxonomic composition between the well-preserved faunas in anoxic sediments and those in oxygenated sediments outside of the basin which are dominated by more robust taxa.

214 ACKNOWLEDGMENTS This r e s e a r c h w a s s u p p o r t e d b y U n i t e d S t a t e s N a t i o n a l S c i e n c e F o u n d a t i o n G r a n t N o . A T M 8 0 - 1 9 0 5 9 (Division o f A t m o s p h e r i c Sciences). We t h a n k A. D o h e r t y f o r d r a f t i n g t h e figures a n d N. M e a d e r f o r t y p i n g t h e m a n u s c r i p t . REFERENCES Addy, S. K. and Behrens, E. W., 1980. Time of accumulation of hypersaline brine in Orca Basin (Gulf of Mexico). Mar. Geol., 37: 241--252. B~, A. W. H. and Tolderlund, D. S., 1971. Distribution and ecology of living planktonic foraminifera in surface waters of the Atlantic and Indian Oceans. In: B. M. Funnell and W. R. Reidel (Editors), The Micropaleontology of Oceans. Cambridge Univ. Press, London, pp. 105--149. Broecker, W. S., Ewing, M. and Heezen, B. C., 1960. Evidence for an abrupt change in climate close to 11,000 years ago. Am. J. Sci., 258: 429--448. Broecker, W. S. and Van Donk, J., 1970. Insolation changes, ice volumes and the 0 TM record in deep-sea cores. Rev. Geophys. Space Phys., 8: 1 6 9 - 1 9 8 . Brunner, C. A., 1979. Distribution of planktonic foraminifera in surface sediments of the Gulf of Mexico. Micropaleontology, 25(3): 325--335. Brunner, C. A. and Cooley, J. F., 1976. Circulation in the Gulf of Mexico during the last glacial maximum 18,000 yr. ago. Geol. Soc. Am. Bull., 87: 681--686. Cita, M. B., Broglia, C., Malinverno, A., Spezzibottiani, G., Tomadin, L. and Violanti, D., 1982. Late Quaternary pelagic sedimentation on the southern Calabrian Ridge and the western Mediterranean Ridge, eastern Mediterranean. Mar. Micropaleontol., 7: 135-162. Cita, M. B., Vergnaud-Grazzini, C., Robert, C., Chamley, H., Ciaranfi, N. and d'Orofrio, S., 1977. Paleoclimatic record of a long deep-sea core from the eastern Mediterranean. Quat. Res., 8: 205--235. Duplessy, J. C., Delibrias, G., Turon, J. L., Pujol, C. and Duprat, J., 1981. Deglacial warming of the northeastern Atlantic Ocean: correlation with the paleoclimatic evolution of the European continent. Palaeogeogr., Palaeoclimatol., Palaeoecol., 35: 121--144. Eicher, U. and Siegenthaler, U., 1976. Palynological and oxygen isotope investigations on Late-Glacial sediment cores from Swiss lakes. Boreas, 5: 1 0 9 - 1 1 7 . Eicher, U. and Siegenthaler, U., 1982. Klimatische informationen aus sauerstoff-isotopenverh~iltnissen in seesedimenten. Phys. Geogr., 1: 103--110. Emiliani, C., Gartner, S., Lidz, B., Eldridge, K., Elvey, D.K., Huang, T.L., Stipp, J.J. and Swanson, M. F., 1975. Paleoclimatological analysis of late Quaternary cores from the northeastern Gulf of Mexico. Science, 189: 1083--1088. Ericson, D. B. and WoUln, G., 1968. Pleistocene climates and chronology in deep-sea sediments. Science, 162: 1227--1234. Fairbanks, R. G., Sverdlove, M., Free, R., Wiebe, P. H. and B~, A. W. H., 1982. Vertical distribution and isotopic fractionation of living planktonic foraminifera from the Panama Basin. Nature, 298: 841--844. Falls, W. F., 1980. Glacial meltwater in-flow into the Gulf of Mexico during the last 150,000 years. Thesis. Univ. South Carolina, Columbia, S.C., 20 pp. Fillon, R. H. and Williams, D. F., 1983. Meltwater-induced 1gO anomalies in marginal seas during the last deglaciation of the Laurentide ice sheet. Geol. Soc. Am. Abstr. with Programs, 15: 572. Imbrie, J. and Kipp, N. G., 1971. New Method for quantitative paleoclimatology. In: K. K. Turekian (Editor), The Late Cenozoic Glacial Ages. Yale Univ., New Haven, Conn., pp. 71--181.

215 Jones, J. I., 1967. Significance of distribution of planktonic foraminifera in the Equatorial Atlantic Undercurrent. Micropaleontology, 13(4): 489--501. Kennett, J.P. and Huddlestun, P., 1972a. Late Pleistocene paleoclimatology, foraminiferal biostratigraphy and tephrochronology, western Gulf of Mexico. Quat. Res., 2: 38--69. Kennett, J. P. and Huddlestun, P., 1972b. Abrupt climatic change at 90,000 yr. B.P.: Faunal evidence from Gulf o f Mexico cores. Quat. Res., 2: 384--395. Kennett, J. P. and Penrose, N., 1978. Fossil Holocene seaweed and attached calcareous polychaetes in an anoxic basin, Gulf of Mexico. Nature, 276(5784): 172--173. Kennett, J. P. and Shackleton, N. J., 1975. Laurentide ice sheet meltwater recorded in Gulf of Mexico deep-sea cores. Science, 188: 147--50. Kipp, N. G., 1976. New transfer function for estimating past sea-surface conditions from the sea-bed distribution of planktonic foraminiferal assemblages in the North Atlantic. Geol. Soc. Am. Mem., 145: 3--42. Klovan, J. E. and Imbrie, J., 1971. A logarithm and Fortran IV program for large scale Q-mode factor analysis. Int. Assoc. Math. Geol. J., 31(1): 61--67. Kullenberg, B., 1952. On the salinity of the water contained in marine sediments. Goteborg K. Vet. Vitterh. Sam. Handl., Set. (B) 6: 1--37. Leipper, D. F., 1970. A sequence of current patterns in the Gulf of Mexico. J. Geophys. Res., 75(3): 637--657. Leventer, A., Williams, D. F. and Kennett, J. P., 1982. Dynamics of the Laurentide ice sheet during the last deglaciation: Evidence from the Gulf of Mexico. Earth Planet. Sci. Lett., 59: 11--17. Leventer, A., Williams, D. F. and Kennett, J. P., 1983. Relationships between anoxia, glacial meltwater and microfossil preservation in the Orca Basin, Gulf of Mexico. Mar. Geol., 53: 23--40. Loubere, P., 1981. Oceanographic parameters reflected in the seabed distribution of planktonic foraminifera from the North Atlantic and Mediterranean Sea. J. Foraminiferal Res., 11: 137--158. Maul, G. A., 1977. The annual cycle of the Gulf Loop Current Part I: observations during a one-year time series. J. Mar. Res., 35(1): 2 9 - 4 7 . McKee, T. R., Jeffrey, L. M., Presley, B. J. and Whitehouse, U. G., 1978. Holocene sediment geochemistry of continental slope and intraslope basin areas, northwest Gulf of Mexico. In: A. H. Bouma, G. T. Moore and J. M. Coleman (Editors), Framework, Facies, and Oil-Trapping Characteristics of the Upper Continental Margin (Stud. in Geol., 7) Am. Assoc. Pet. Geol., Tulsa, Okla., pp. 313--326. McKee, T. R. and Sidner, B. R., 1976. An anoxic high salinity intraslope basin in the northwest Gulf of Mexico. In: A.H. Bouma (Editor), Beyond the Shelf Break. AAPG Mar. Geol. Comm. Short Course, 2, p. 125. Muerdter, D.R., Kennett, J.P. and Thunell, R.C., 1984. Late Quaternary sapropel sediments in the Eastern Mediterranean Sea: Faunal variations and chronology. Quat. Res., 21: 385--403. Northam, M. A., Curry, D. J., Scalan, R. S. and Parker, P. L., 1981. Stable carbon isotope ratio variations of organic matter in Orca Basin sediments. Geochim. Cosmochim. Acta, 45: 257--260. Nowlin Jr., W. D. and McLellan, H. J., 1967. A characterization of the Gulf of Mexico waters in winter. J. Mar. Res., 25: 2 9 - 5 9 . Olausson, E., 1961. Studies of deep sea cores. Rep. Swed. Deep-Sea Exped. 1947--1948, 8: 335--391. Parker, F. L., 1958. Eastern Mediterranean foraminifera. Rep. Swed. Deep-Sea Exped. 1947--1948, 8: 217--283. Prest, V. K., 1970. Quaternary geology. In: R. J. Douglas (Editor), Geology and Economic Minerals of Canada. Dep. Energy, Mines, Res., Ottawa, Ont., pp. 6 7 6 - - 7 6 4 . Ruddiman, W. F., 1969. Planktonic foraminifera of the subtropical North Atlantic gyre. Thesis. Columbia Univ., New York, N.Y.

216 Ruddiman, W. F. and McIntyre, A., 1981. The North Atlantic Ocean during the last deglaciation. Palaeogeogr., Palaeoclimatol., Palaeoecol., 35: 145--214. Ryan, W. B. F., 1972. Stratigraphy of the Late Quaternary sediments in the eastern Mediterranean. In: D. J. Stanley (Editor), The Mediterranean Sea. Dowden, Hutchinson and Ross, Stroudsburg, P.A., pp. 149--169. Sachs Jr., K. N., Cifelli, R. and Bowen, V. T., 1964. Ignition to concentrate shelled organisms in plankton samples. Deep-Sea Res., 11: 621--622. Schott, W., 1935. Die Foraminiferen in dem iiquatorialen Teil des Atlantischen Ozeans. Dtsch. Atl. Exped. "Meteor," 1925--1927. Wiss. Ergeb., 3, pt. 3: 43--134. Shokes, R. F., Trabant, P. K., Presley, B. J. and Reid, D. F., 1977. Anoxic hypersaline basin in the northern Gulf of Mexico. Science, 196: 1443--1446. Siegenthaler, U., Either, U., Oeschger, H. and Dansgaard, W., 1984. Lake sediments as continental 180 records from the transition Glacial--Postglacial. Ann. Glaciol., 5: 1-243. Thunell, R. C., 1976. Calcium carbonate dissolution history in Late Quaternary deep-sea sediments, Western Gulf of Mexico. J. Quat. Res., 6: 281--297. Thunell, R. C., 1978. Distribution of Recent planktonic foraminifera in surface sediments of the Mediterranean Sea. Mar. Micropaleontol., 3: 147--173. Thunell, R. C., Curry, W. B. and Honjo, S., 1983. Seasonal variation in the flux of planktonic foraminifera: time series sediment trap results from the Panama Basin. Earth Planet. Sci. Lett., 64: 44--56. Thunell, R. C., Williams, D. F. and Kennett, J. P., 1977. Late Quaternary paleoclimatology, stratigraphy and sapropel history in Eastern Mediterranean deep-sea sediments. Mar. Micropaleontol., 2: 371--388. Thurman, H. V., 1978. Introductory Oceanography. Merrill, Columbus, Ohio, 506 pp. Tompkins, R. E. and Shephard, L. E., 1979. Orca Basin: Depositional processes, geotechnical properties, and clay mineralogy of Holocene sediments within an anoxic, hypersaline basin, northwest Gulf of Mexico. Mar. Geol., 33: 221--238. Trabant, P. K. and Presley, B. J., 1978. Orca Basin, an anoxic depression on the continental slope, northwest Gulf of Mexico. In: A. H. Bouma, G. T. Moore and J. M. Coleman (Editors), Framework, Facies and Oil-Trapping Characteristics o f the Upper Continental Margin (Stud. in Geol., 7) Am. Assoc. Pet. Geol., Tulsa, Okla., pp. 289--303.