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Identification of density-stratified waters in the late-Pleistocene North Atlantic: A faunal derivation
QUATERNARY
RESEARCH
Identification
20, 374-386 (1983)
of Density-Stratified Waters in the Late-Pleistocene North Atlantic: A Fauna1 Derivation JOSEPHJ. MORLEY
Lamont-Doherty
Geological
Observatory
of Columbia
University,
Palisades,
New
York
10964
Received September 13, 1982 An expanded study of the radiolarian Cycladophora davisiana in late-Pleistocene North Atlantic marine sediments shows that over the last several hundred thousand years this species exhibits large variations in relative abundance. The C. davisiana curves in the North Atlantic cores are quite similar, with easily recognizable features common to all records. Minor deviations from the general pattern of this species’ abundance apparently reflect the response of C. davisiana to specific oceanographic conditions characteristic of a particular area within the North Atlantic. C. davisiana occurs today in high abundance (>20%) only in the Sea of Okhotsk. Extensive winter and early spring sea-ice cover coupled with low surface-water salinities during summer and fall is responsible for maintaining near-freezing subsurface temperatures in this northwest Pacific marginal sea as well as relatively stable temperatures and salinities at depths below a shallow subsurface temperature minimum. During periods in the late Pleistocene, high C. davisiana abundances (>20%) in the North Atlantic were probably associated with oceanographic properties similar to those that exist in the Sea of Okhotsk today. Because of the relationship between relatively stable subsurface temperatures and salinities and high abundance levels of C. davisiana, analysis of this species’ abundance pattern at several locations throughout the high-latitude North Atlantic should assist in identifying source areas of deep-water formation and determining the duration of deep convective processes at these sites.
INTRODUCTION
The cosmopolitan
radiolarian species Cycladophora (?) davisiana Ehrenberg exhibits high amplitude abundance variations in late-Pleistocene high-latitude (>40”) marine sediments. Comparison of this species’ abundance pattern with oxygen-isotope stratigraphies in records from the North Atlantic (Morley and Hays, 1979) and the subantarctic (Hays et al., 1976) showed that relative abundance variations of C. davisiana are not only similar within each of these high-latitude regions, but that changes in this species’ abundance appear to be synchronous between the two hemispheres. An expanded investigation of this species’ variations in late-Pleistocene and Holocene North Atlantic marine sediments was undertaken to acquire a better understanding of the stratigraphic and paleoclimatic significance of C. davisiana. Specific oceanographic conditions associated with various abundance levels of C. davisiana 374 0033-5894/83 $3.00 Copyright AU rights
@ 1983 by the University of Washington. of reproduction in any form reserved.
could only be proposed and tested after examining this species’ abundance pattern over a wider geographic area of the North Atlantic. This paper presents the latest results from this study. Cycladophora davisiana VARIATIONS THE NORTH ATLANTIC
IN
Radiolaria, when present in Pleistocene and Holocene North Atlantic sediments, usually occur at relatively low concentrations (10,000 radiolaria per gram of dry bulk sediment). Six cores that contain radiolaria throughout their entire length were selected for this phase of the project, in addition to another four cores in which radiolaria occur in the interval of the sedimentary record representing the last 30,000 yr (Fig. 1, Table 1). Samples were taken at 20-cm intervals or less in each core. After the sediment samples had been disaggregated and washed, slides of all radiolaria greater than 63 pm were prepared using the settling
LATE-PLEISTOCENE
FIG. study.
1. Map
showing
location
and identification
technique described by Moore (1973). A minimum of 300 radiolaria were counted on each slide. The C. duvisiana curves in three of the six cores which contained radiolaria throughout their entire length were initially presented by Morley and Hays (1979). For the present study, the percentage of C. duvisiunu relative to all other radiolaria in two
TABLE
Core
Latitude
V23-22 V23-23 V23-82 V23-83 V23-84 v27-20 V27-114 V27-116 V29-179 v30-105
54”12’N 56”05’N 52”35’N 49”.52’N 46”OO’N 54”OO’N 55”03’N 52”5O’N 44”Ol’N 54”31’N
1. CORE INFORMATION
Longitude
Depth (m)
45”58’W 44”33’W 21”56’W 24”lS’W 16”55’W 46”12’W 33”04’W 30”2O’W 24”32’W 36”3O’W
3669 3292 3974 3871 4513 3510 2532 3202 3331 2758
Length (cm) 1017 972 1022 1150 1075 904 1212 1014 699 944
NORTH
of cores
375
ATLANTIC
from
North
Atlantic
examined
in this
of these three records was calculated at additional levels increasing the sampling density over specific intervals in cores V27- 114 and V27-116. The C. duvisiunu abundance at these additional levels, along with abundances given by Morley and Hays (1979) in these three cores, is shown in Figure 2. Prominent maxima and minima in the C. duvisiunu pattern in each of these cores were given letter designations to facilitate comparison with other C. duvisiunu curves and oxygen-isotope records. Relative abundance variations of C. duvisiunu in the three remaining cores from the North Atlantic are shown in Figure 3 along with the C. duvisiunu record from core V27116. The same lettering system was used as in Figure 2 to designate specific C. duvisiunu maxima and minima in each core. Three reference levels were used to correlate the C. duvisiunu curves in these six North Atlantic cores. Distinctive volcanic ash layers (ash zone 1 and ash zone 2; Ruddiman and Glover, 1972) comprised the two
‘6 -?
5e
5a
L
+30
-
+25
~-3
+zo
PD B (%o)
6/S-
-L
!2
IO
-
5
f
%
e, e
15
C davisiano
0
il
,
50
40
30
C davisiana 20
V27-116 % IO
0 L
60 A
50 .
40 .
30
C davisiana -
20
% .
IO
4’
0
20
~~~~
A
15
FORAM
ASH
ASH
- ._
IO
C davisiano
.............’
-
NORTH ATLANTIC V27-114
9800 2 -57.500
1-y
6”O
SST MAXIMUM
ZONE
ZONE
%
V29-179
= 5/4
yr
yr
TRANSITION
B P
BP
PD.B (%.a)
1
1
‘7
FIG. 1. Abundance variations of c‘. drrr’isitrrlcr in three North Atlantic and one suhantarctic core with oxygen-isotope records from the subantarctic core and one North Atlantic core. Position of ash Lone> I and ? and a relative sea-surface temperature maximum at the isotopic 514 transition are indicated in three North Atlantic cores. Stage boundaries in core V?7-1 16 are based on abundance variations of foraminifera G. P~~CII).JCI.UILI.
12
6-
5’
Ilt
+35
6’*0
SUBANTARCTIC--1 RC 11-120
LATE-PLEISTOCENE
NORTH
ATLANTIC
377
JOSEPH
378
J. MORLEY
younger datum levels. The younger ash (ash zone 1) has an estimated age of 9800 yr B .P. (based on recent 14C analyses; Ruddiman and McIntyre, 1981; Duplessy et al., 1981). and age estimates from most recent analyses of ash zone 2 center around 57,500 yr B.P. (W. F. Ruddiman, personal communication, 198 1). The third reference event was a short pulse of warm sea-surface temperatures, reflected in changes in foraminiferal assemblages, that occurred at the oxygenisotope stage 5/4 boundary (Ruddiman et al., 1980). Ruddiman and colleagues (Ruddiman and McIntyre, 1973, 198 1; Ruddiman and Glover, 1975; Ruddiman et al., 1980; W. F. Ruddiman, personal communication, 1981) located the position of these stratigraphic markers in the North Atlantic cores shown in Figures 2 and 3. Comparison of the C. davisianu records in the six cores (Figs. 2 and 3) indicates that marine sediments from a large portion of the North Atlantic contain an easily recognizable pattern of C. davisiunu abundance variations. For example, an interval
FIG. 4. Relative
abundance
of C. davisiana
marked by low abundances of C. duvisiuna (*a) occurs at the top of all six records and coincides with most of the Holocene. Ash zone 1 falls near the base of this low abundance interval. The maximum in relative abundance of C. duvisiuna (*b) is recorded in all six of the cores below this uppermost low abundance interval, with the northernmost core (V23-23) having the highest C. duvisiuna abundance value (79.2%). This abundance interval (*b) falls within oxygenisotope stage 2 in core V29-179. The second highest C. duvisiunu abundance maximum in most cores is designated *l and appears to correspond to middle- to late-glacial isotopic stage 8 based on correlation with foraminiferal data. Ash zone 2 occurs just above the C. duvisiunu maximum *d. The event marking the pulse of warm sea-surface temperature, which corresponds to the oxygen-isotope stage 5/4 boundary, falls between low abundance level *ei and high abundance zone “d. Despite the similarities in the six C. duvisiuna records, there are some noticeable
in surface-sediment
samples
from
the North
Atlantic.
LATE-PLEISTOCENE
differences in the various curves. These differences may result from varying sedimentation rates between cores, undetected small hiatuses, different sampling intervals in each core, fundamentally different regional responses of C. davisiuna, or a combination of these factors. For example, the record from core V23-23 contains a prominent abundance maximum between low abundance zone *a and high abundance level *b. This C. davisiana abundance maximum progressively decreases in amplitude with decreasing latitude, and in core V29-179 is recognizable as only a slight inflection point on the slope between low C. davisiana abundance interval *a and high abundance zone *b. A possible explanation for this specific abundance maximum is presented later. Figure 4 shows the abundance of C. davisiana in surface sediments reflecting present-day conditions as recorded in ten North Atlantic cores. The relative abundance levels of C. davisiana in sediments
NORTH ATLANTIC
379
recording conditions at the last glacial maximum (-18,000 yr BP.; CLIMAP, 1981) and at the time of maximum abundance interval *b (-20,000-30,000 yr B.P.) are plotted in Figures 5 and 6, respectively. The C. davisiana abundance in the North Atlantic generally increases from southeast to northwest during both of these Pleistocene intervals. Present-day abundances of C. davisiana never exceed 1% in any of the surface-sediment samples examined, whereas in many of the same cores, values ranging from 5 to 79% were recorded during the last glacial stage (either at the glacial maximum (Fig. 5) or at the time of maximum C. davisiana interval *b (Fig. 6)). OCEANOGRAPHIC PROPERTIES ASSOCIATED WITH HIGH ABUNDANCESOF Cycladophora davisiana Percentage abundance changes of a given variable are always subject to multiple interpretations. The specific parameter may
FIG. 5. Relative abundance of C. davisiana in North Atlantic sediments at the last glacial maximum (-18,000 yr B.P.).
380
JOSEPH J. MORLEY
4 30’
FIG. 6. Relative abundance of level.
C. davisiana
in North Atlantic records at the *b maximum abundance
actually be the component which varies in absolute abundance; the background against which it is compared may be the only varying component; or both variable and background may change. In the case of C. duvisiuna, absolute abundance curves (Fig. 7) show that both the specific parameter (C. duvisiunu) and the background (all other radiolaria) are variable. Because the environmental factor or factors responsible for producing abundance variations of C. duvisiunu appear to affect the entire radiolarian population, an examination of C. duvisiunu abundance in Holocene sediments and in the overlying water may identify specific oceanographic variables associated with high abundance levels of this species. The abundance of C. duvisiunu rarely exceeds 5% in Holocene surface-sediment samples except in the Sea of Okhotsk and areas of the Northwest Pacific adjacent to this marginal sea. Kruglikova (1975), Ling (1974), and Robertson (1975) all reported relatively high abundances of C. duvisiunu
in surface sediments from the Sea of Okhotsk. In a recent study Morley and Hays (in press) found that the relative abundance of C. duvisiunu in 22 surface-sediment samples from the Sea of Okhotsk ranged from 9.9 to 48.8%. It is possible to determine the probable water-depth distribution of C. duvisiunu by examining data on radiolaria in plankton tows and in surface sediments. In a review of plankton-tow data from temperate and polar regions, Petrushevskaya (1971) noted that C. duvisiunu was not found in tows from depths shallower than 100 m, but occurred occasionally in tows that sampled depths between 300 and 700 m. In a more recent study of radiolaria in plankton tows taken in the Antarctic in early spring 198 1, from near the ice edge and from within the ice Morley and Hays (in press) found that C. duvisiunu was present in all tows that sampled depths greater than 200 m. Only 7 of the 20 tows from depths shallower than 200 m from this same cruise contained C. duv-
LATE-PLEISTOCENE
NORTH
381
ATLANTIC
CO. m. . 0
-
N
10
d
(St1313W)
en
r-
(D
Hld3Q
3t103
m
m
382
JOSEPH J. MORLEY
isiana. The number of individual C. daris&a specimens in these 7 tows never exceeded two. In the Sea of Okhotsk, high C. davisiana abundances (> 16%) occur in the shallowest (300 m) as well as the deepest (3336 m) samples examined by Morley and Hays (in press). Although there are several subspecies of C. duvisiunu (Petrushevskaya, 1%7), Morley (1980) has shown that the abundance variations of C. duvisiunu var. duvisiunu account for most of the high-latitude abundance variations of this species. In the hundreds of surface-sediment samples examined from all parts of the world’s oceans, this particular subspecies is only present in samples from water depths greater than 150 m (Morley and Hays, in press). Available plankton-tow and surface-sediment data indicate that C. duvisiunu does not occur in high abundances in surface and near-surface waters (cl50 m). High abundances (> 12%) of this species are found in recent sediments only in the Sea of Okhotsk, where C. duvisianu appears to be living at subsurface depths. A unique set of physical and biological properties exists today in the Sea of Okhotsk, and results in large part from the extreme seasonal contrast in this region where intensely cold winters are associated with relatively warm summers. Sea ice begins to form in late November/early December and does not melt completely until June. Maximum sea-ice cover occurs in March. During this winter/spring period, the Sea of Okhotsk is frequently ice covered for 3 months at a time (Zenkevitch, 1963; Parkinson and Gratz, 1983; G. J. Kukla, personal communication, 1982). The rapid late spring/ summer melting of the sea ice establishes a summer through late fall water structure characterized by a low-salinity surface layer (30.0-32.8°/~~) with a temperature minimum of - 1.X within 140 m of the surface (Reid, 1965). Data from hydrocast stations occupied in late winter and summer in the Sea of Okhotsk (Reid, 1973) are shown in Figure 8.
. SALINITY 320
(%.) 340
330
-
e.OREAS 160 RYOFU MAR” 46
,000 -
FIG. 8. Temperature and salinity profiles from the Sea of Okhotsk. Winter station (Boreas 160) taken at 47”47’N 1SVlO’E; summer station (Ryofu Maru 46) taken at 47”31’N 149”59’E (Reid, 1973).
The data indicate that the water column below 175 m is quite stable even though major variations in temperature and salinity occur in the upper 150 m between these two seasons. In March, the lowest temperatures are recorded at the surface ( - 1.56”C) with a temperature maximum of approximately 2.4”C occurring between 800- and 900-m depth. In July, surface temperatures increase to 9.4”C, with a minimum of -0.55”C recorded at a depth of 125 m. Both the late winter and summer hydrocasts show the existence of a thermocline between 130 and 180 m. Although summer surface salinities are much lower than their winter counterparts, the curves are similar below 100 m where they show increasing salinities with depth. A halocline is present between 100 and 200 m during both seasons. The hydrocast data show that the warm summer surface waters in the Sea of Okhotsk are cooled and subsequently mixed into the shallow, cold, intermediate-water layer during late fall/early winter (Yasuoka, 1%7). This convection is confined to the upper 100 to 150 m of the water column because of the relatively low surface-water salinities compared to salinities below the halocline.
LATE-PLEISTOCENE
The particular climatological and oceanographic characteristics of today’s Sea of Okhotsk make it possible for near-freezing temperatures to be maintained at water depths between 50 and 150 m throughout the year and for a halocline to become a permanent subsurface feature. Although summer surface waters of the high-latitude oceans are characterized by low salinities, and subsurface temperature minima have been reported in the Bering Sea (Reid, 1973), the northwest Pacific (Reid, 1973), and the Gulf of Alaska (?\llly, 1957), nowhere is the subsurface temperature minimum with its overlying low-salinity waters developed to the degree found in the Sea of Okhotsk. Morley and Hays (in press) propose that this unusual water structure affects the present-day radiolarian abundance and distribution in the Sea of Okhotsk. C. davisiana apparently thrives in the stable waters below the permanent halocline, whereas the relatively low-surface salinities and large temperature range restrict the diversity as well as the abundance of shallower-dwelling radiolaria. Results from studies of the vertical distribution of zooplankton in the Sea of Okhotsk (Vinogradov, 1954) would appear to support this proposed model in that they show significantly lower numbers of zooplankton living within the temperature minimum layer than above or below this interval. Therefore, although C. davisiana is probably confined to subsurface waters, its abundance variations are related not only to the water properties of the subsurface depth interval which it inhabits, but also to the specific conditions in the overlying surface waters. DISCUSSION
Identification of corresponding abundance maxima and minima of C. davisiana in the six North Atlantic records permits an examination of the abundance variations of C. davisiana in time and space in the latePleistocene North Atlantic. Furthermore, the oxygen-isotope record in core V29-179
NORTH
ATLANTIC
383
(Streeter and Shackleton, 1979), along with the approximate location of specific isotopic stage boundaries in core V27-116 based on sporadic isotopic values (Ruddiman et al., 1980; Ruddiman and McIntyre, 1981) and the abundance variations of the left-coiling foraminifera Globigerina pachyderma (W. F. Ruddiman, personal communication, 1981), make it possible to place the major changes in this species’ abundance within a specific time frame. According to the model developed by Morley and Hays presented in the previous section of this paper, high abundances of C. davisiana (>20%) reflect not only specific subsurface conditions but also certain characteristics of the overlying surface waters. Based on this hypothesis, high C. davisiana abundances (>20%) in North Atlantic sediments are indicative of periods when the water structure in the overlying surface and intermediate waters was similar to that which exists today in the Sea of Okhotsk. During these periods the intermediate waters of the North Atlantic would be characterized by relative stability. Conditions in the overlying surface waters would be affected by extensive sea-ice cover during winter and spring. Although summer and fall temperatures of the surface waters would be much higher than the near-freezing levels recorded during the remainder of the year, surface salinities would remain low because of the influx of sea-ice and/or iceberg meltwater. The temperatures and salinities at depths below the subsurface temperature minimum would remain nearly constant throughout the year with convection confined to the uppermost portion of the water column (~200 m). Based on this interpretation, the high C. davisiana abundance levels (>20%) in sediments recording conditions at the last glacial maximum (Fig. 5) in the northwestemmost cores (V23-23 and V27-20) would indicate that oceanographic conditions in this portion of the North Atlantic were dominated by prolonged periods of sea-ice cover during _ winter coupled with relatively low
384
JOSEPH
salinity levels in the surface waters during the summer and fall seasons. These conditions are similar to those proposed by McIntyre et al. (1976) in their description of the North Atlantic during the last glacial maximum. In their reconstruction, the sites of both these cores are within the zone of extensive pack ice from February through May and just south of the estimated summer sea-ice limit. The high C. davisiana abundances at these sites at this time would indicate that temperatures and salinities of the underlying waters were relatively stable with surface salinities rarely increasing to high enough levels for initiation of deep convection. Examination of the six abundance records along a southeast (V29-179) to northwest transect (V23-23) reveals that the proportion of the total record characterized by C. duvisiunu values greater than 20% becomes progressively larger with increasing latitude. In the northernmost core (V23-23), the oceanographic conditions for much of the last 300,000 yr appear to have been as extreme as those that exist today in the Sea of Okhotsk. If C. duvisiana abundances exceeding 20% are a measure of the intensity of the unusual conditions of prolonged winter/ spring sea-ice cover with low-salinity summer/fall surface waters coupled with a stable halocline, then the C. davisianu curves show that these conditions were most acute in the North Atlantic during glacial isotopic stage 8 (* 1-*n) and the interval from late in isotopic stage 3 through most of glacial isotopic stage 2 (*b). These two intervals probably mark times of maximum density stratification with coldest shallow subsurface temperature minima and lowest surface-water salinities. Secondary maxima in these C. davisianu records (*f and *h) correlate with early and late intervals in glacial isotopic stage 6. Two minor C. davisiana maxima within isotopic stage 5 appear to coincide with isotopic substages Sb and 5d which were relatively cool events (increased global ice volume) within stage 5.
J. MORLEY
In the previous discussion describing the abundance variations of C. duvisiunu in these six North Atlantic cores, a prominent abundance maximum between designated levels *a and *b was given as an example of a major difference in the various C. duvisiuna patterns since this feature could not be clearly identified in the southernmost record. The estimated age of this event centers around 13,000 yr BP. based on ash zone 1 and C. davisiunu stratigraphy. In a detailed description of the North Atlantic during the last deglaciation, Ruddiman and McIntyre (1981) describe an interval between 16,000 and 13,000 B.P. during which the North Atlantic was ice covered in winter south to at least 50”N but completely ice free during summer months. The major thinning of the ice sheets proposed by Denton and Hughes (1981) during this same time would provide large quantities of icebergs and thus meltwater to this portion of the Atlantic. This anomalous C. davisianu abundance maximum between low abundance zone *a and high abundance level *b in cores V23-23, V23-22, and V27-20 reflects extensive winter sea ice, increased levels of iceberg activity and summer meltwater, low surface-water salinities, and maximum density stratification in the northwest North Atlantic, conditions similar to those proposed by Ruddiman and McIntyre (1981). The lower C. duvisiunu abundances (~20%) in cores V27-114, V27116, and V29-179 during this same period probably indicate that although a lowsalinity surface layer may have existed throughout much of the summer/fall in this region, surface salinities in the central and southeastern North Atlantic were not sufficiently low to effectively inhibit convective, open-ocean mixing processes during times of the year when this region of the North Atlantic was ice free. These initial comparisons show that C. duvisiuna records in late-Pleistocene North Atlantic marine sediments can provide information vital to paleoclimatic reconstructions. Further study of the specific environ-
LATE-PLEISTOCENE
ment which promotes high productivity levels of C,. davisiana today in the Sea of Okhotsk may permit more-detailed paleoclimatic reconstructions based on variations in this species’ abundance. CONCLUSIONS
The results of analyses presented in this paper show the following: (1) The radiolarian species C. davisiana undergoes large variations in its abundance in Pleistocene sediments throughout a major portion of the North Atlantic, and comprises as much as 79% of the total radiolarian population at specific times during the last glacial period. (2) Although there are distinct amplitude differences, the C. davisiana patterns in all six North Atlantic cores are similar with easily recognizable features common to all records. Deviations from the general abundance pattern of this species probably reflect the response of C. davisiana to specific oceanographic conditions that occurred within a limited region of the North Atlantic. (3) If the oceanographic conditions associated with high C. davisiana abundances in today’s Sea of Okhotsk can be expanded to include high-latitude open-ocean regions, then high abundances of this species (~20%) in late-Pleistocene North Atlantic sediments are indicative of prolonged periods of winter/spring sea-ice cover, summer/fall low-salinity surface waters coinciding with increased levels of sea-ice and/or iceberg meltwater, a shallow subsurface temperature minimum, and relatively stable temperatures and salinities at depths below the temperature minimum. Therefore, although C. davisiana appears to inhabit subsurface waters, its abundance variations reflect specific oceanographic conditions in waters ranging from the surface to shallow intermediate depths. (4) The relationship between stable temperatures and salinities and high C. davisiana abundances (~20%) suggests that analyses of (7. davisiana abundance in high-
NORTH
ATLANTIC
385
latitude sediments may assist in identifying specific areas where deep-water formation could not have occurred during times throughout the late Pleistocene. ACKNOWLEDGMENTS I thank .I. Hays, W. Ruddiman, C. Sancetta, and T. Kellogg for their suggestions and criticisms. I am also grateful to D. Kintas, A. Pesanell, B. Dworetsky, and W. Fuller for their conscientious laboratory and technical assistance. Illustrations were drawn by B. Taylor. This research was directly supported by National Science Foundation Grant OCE79-24563 to Lamont-Doherty Geological Observatory. The core collection at Lamont-Doherty Geological Observatory of Columbia University was supported through funds from Grant OCE81-22083 from the National Science Foundation, Lamont-Doherty Geological Observatory of Columbia University Contribution 3497.
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Hays, J. D., Lozano, J. A., Shackleton, N., and Irving, G. (1976). Reconstruction of the Atlantic and western Indian Ocean sectors of the 18,000 yr BP Antarctic Ocean. In “Investigation of Late Quaternary Paleoceanography and Paleoclimatology” (R. M. Cline and J. D. Hays, Eds.), pp. 337-372. Geological Society
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Kruglikova, S. B. (1975). Radiolarians in the surface layer of the sediments of the Okhotsk Sea. Oceanology 15, 116-122. Ling, H.-Y. (1974). Polycystine radiolaria and silicoflagellates from the surface sediments of the Sea of Okhotsk. Bulletin of the Geological Survey of Japan l-11. McIntyre, A., Kipp, N. G., Be, A. W. H., Crowley, T., Kellogg, T., Gardner, J. V., Prell, W., and Ruddiman, W. F. (1976). Glacial North Atlantic 18,000 years ago: A CLIMAP reconstruction. In “Investigation of Late Quarternary Paleoceanography and
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Petrushevskaya. M. G. (1967). Radiolaria of orders spumellaria and nassellaria of the Antarctic region. In “Biological Reports of the Soviet Antarctic Expedition (1955-1958)” (A. P. Andriyashev and P. V. Ushakov, Eds.). Vol. 3, pp. l-186. Academy of Sciences of the USSR, Zoological Institute, Leningrad. [Transl. from Russian in 1968.1 Petrushevskaya, M. Cl. (1971). Radiolaria in the plankton and recent sediments from the Indian Ocean and Antarctic. In “The Micropaleontology of Oceans” (B. M. Funnel1 and W. R. Riedel, Eds.), pp. 319-329. Cambridge Univ. Press, Cambridge. Reid, J. L. (1965). Intermediate waters of the Pacific Ocean. In “Johns Hopkins Oceanographic Studies Series 2.” Johns Hopkins Press, Baltimore. Reid, J. L. (1973). Northwest Pacific Ocean waters in winter. In “Johns Hopkins Oceanographic Studies Series 5.” Johns Hopkins Press, Baltimore. Robertson, J. R. (1975). “Glacial to Interglacial
Oceanographic Changes in the Northwest Pacific, Including a Continuous Record of the Last 400,000 Years.” Doctoral dissertation. Columbia University, New York. Ruddiman, W. F.. and Glover, L. K. (1972). Vertical mixing of ice-rafted volcanic ash in North Atlantic sediments. Geological Socier?, of America Bulletin 83, 2817-2836. Ruddiman. W. F.. and Glover, L. K. (1975). Subpolar North Atlantic circulation at 9300 yr B.P.: Fauna1 evidence. Quaternary Research 5, 361-389. Ruddiman, W. F.. and McIntyre, A. (1973). Timetransgressive deglacial retreat of polar waters from the North Atlantic. Quaternary Research 3, 117130. Ruddiman, W. E, and McIntyre, A. (1981). The North Atlantic Ocean during the last deglaciation. Palaeogeography,
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Palaeoecology
35,
145-214. Ruddiman, W. F., McIntyre, A., Niebler-Hunt, V., and Durazzi, J. T. (1980). Oceanic evidence for the mechanism of rapid northern hemisphere glaciation. Quaternary Research 13, 33-64. Streeter. S. S., and Shackleton, N. J. (1979). Paleocirculation of the deep North Atlantic: A 150,000 year record of benthic foraminifera and oxygen-18. Science 203, 168-171. ‘Ihlly, J. P. (1957). Some characteristics of sea water structure. Proceedings of the 8th Pacific Science Congress of Oceanography 3, 643-661. Vinogradov, M. (1954). Diurnal vertical migrations of the zooplankton in the Far East Seas. Transactions of the Institute of Oceanology Sciences of the USSR 8.
of the Academy
of
Yasuoka, T. (1967). Hydrography in the Okhotsk Sea (1). Oceanographical Magazine 19, 61-72. Zenkevitch, L. A. (1963). “Biology of the Seas of the USSR.” Wiley-Interscience, New York. [Transl. from Russian by S. Botcharskaya.]
RESEARCH
Identification
20, 374-386 (1983)
of Density-Stratified Waters in the Late-Pleistocene North Atlantic: A Fauna1 Derivation JOSEPHJ. MORLEY
Lamont-Doherty
Geological
Observatory
of Columbia
University,
Palisades,
New
York
10964
Received September 13, 1982 An expanded study of the radiolarian Cycladophora davisiana in late-Pleistocene North Atlantic marine sediments shows that over the last several hundred thousand years this species exhibits large variations in relative abundance. The C. davisiana curves in the North Atlantic cores are quite similar, with easily recognizable features common to all records. Minor deviations from the general pattern of this species’ abundance apparently reflect the response of C. davisiana to specific oceanographic conditions characteristic of a particular area within the North Atlantic. C. davisiana occurs today in high abundance (>20%) only in the Sea of Okhotsk. Extensive winter and early spring sea-ice cover coupled with low surface-water salinities during summer and fall is responsible for maintaining near-freezing subsurface temperatures in this northwest Pacific marginal sea as well as relatively stable temperatures and salinities at depths below a shallow subsurface temperature minimum. During periods in the late Pleistocene, high C. davisiana abundances (>20%) in the North Atlantic were probably associated with oceanographic properties similar to those that exist in the Sea of Okhotsk today. Because of the relationship between relatively stable subsurface temperatures and salinities and high abundance levels of C. davisiana, analysis of this species’ abundance pattern at several locations throughout the high-latitude North Atlantic should assist in identifying source areas of deep-water formation and determining the duration of deep convective processes at these sites.
INTRODUCTION
The cosmopolitan
radiolarian species Cycladophora (?) davisiana Ehrenberg exhibits high amplitude abundance variations in late-Pleistocene high-latitude (>40”) marine sediments. Comparison of this species’ abundance pattern with oxygen-isotope stratigraphies in records from the North Atlantic (Morley and Hays, 1979) and the subantarctic (Hays et al., 1976) showed that relative abundance variations of C. davisiana are not only similar within each of these high-latitude regions, but that changes in this species’ abundance appear to be synchronous between the two hemispheres. An expanded investigation of this species’ variations in late-Pleistocene and Holocene North Atlantic marine sediments was undertaken to acquire a better understanding of the stratigraphic and paleoclimatic significance of C. davisiana. Specific oceanographic conditions associated with various abundance levels of C. davisiana 374 0033-5894/83 $3.00 Copyright AU rights
@ 1983 by the University of Washington. of reproduction in any form reserved.
could only be proposed and tested after examining this species’ abundance pattern over a wider geographic area of the North Atlantic. This paper presents the latest results from this study. Cycladophora davisiana VARIATIONS THE NORTH ATLANTIC
IN
Radiolaria, when present in Pleistocene and Holocene North Atlantic sediments, usually occur at relatively low concentrations (10,000 radiolaria per gram of dry bulk sediment). Six cores that contain radiolaria throughout their entire length were selected for this phase of the project, in addition to another four cores in which radiolaria occur in the interval of the sedimentary record representing the last 30,000 yr (Fig. 1, Table 1). Samples were taken at 20-cm intervals or less in each core. After the sediment samples had been disaggregated and washed, slides of all radiolaria greater than 63 pm were prepared using the settling
LATE-PLEISTOCENE
FIG. study.
1. Map
showing
location
and identification
technique described by Moore (1973). A minimum of 300 radiolaria were counted on each slide. The C. duvisiana curves in three of the six cores which contained radiolaria throughout their entire length were initially presented by Morley and Hays (1979). For the present study, the percentage of C. duvisiunu relative to all other radiolaria in two
TABLE
Core
Latitude
V23-22 V23-23 V23-82 V23-83 V23-84 v27-20 V27-114 V27-116 V29-179 v30-105
54”12’N 56”05’N 52”35’N 49”.52’N 46”OO’N 54”OO’N 55”03’N 52”5O’N 44”Ol’N 54”31’N
1. CORE INFORMATION
Longitude
Depth (m)
45”58’W 44”33’W 21”56’W 24”lS’W 16”55’W 46”12’W 33”04’W 30”2O’W 24”32’W 36”3O’W
3669 3292 3974 3871 4513 3510 2532 3202 3331 2758
Length (cm) 1017 972 1022 1150 1075 904 1212 1014 699 944
NORTH
of cores
375
ATLANTIC
from
North
Atlantic
examined
in this
of these three records was calculated at additional levels increasing the sampling density over specific intervals in cores V27- 114 and V27-116. The C. duvisiunu abundance at these additional levels, along with abundances given by Morley and Hays (1979) in these three cores, is shown in Figure 2. Prominent maxima and minima in the C. duvisiunu pattern in each of these cores were given letter designations to facilitate comparison with other C. duvisiunu curves and oxygen-isotope records. Relative abundance variations of C. duvisiunu in the three remaining cores from the North Atlantic are shown in Figure 3 along with the C. duvisiunu record from core V27116. The same lettering system was used as in Figure 2 to designate specific C. duvisiunu maxima and minima in each core. Three reference levels were used to correlate the C. duvisiunu curves in these six North Atlantic cores. Distinctive volcanic ash layers (ash zone 1 and ash zone 2; Ruddiman and Glover, 1972) comprised the two
‘6 -?
5e
5a
L
+30
-
+25
~-3
+zo
PD B (%o)
6/S-
-L
!2
IO
-
5
f
%
e, e
15
C davisiano
0
il
,
50
40
30
C davisiana 20
V27-116 % IO
0 L
60 A
50 .
40 .
30
C davisiana -
20
% .
IO
4’
0
20
~~~~
A
15
FORAM
ASH
ASH
- ._
IO
C davisiano
.............’
-
NORTH ATLANTIC V27-114
9800 2 -57.500
1-y
6”O
SST MAXIMUM
ZONE
ZONE
%
V29-179
= 5/4
yr
yr
TRANSITION
B P
BP
PD.B (%.a)
1
1
‘7
FIG. 1. Abundance variations of c‘. drrr’isitrrlcr in three North Atlantic and one suhantarctic core with oxygen-isotope records from the subantarctic core and one North Atlantic core. Position of ash Lone> I and ? and a relative sea-surface temperature maximum at the isotopic 514 transition are indicated in three North Atlantic cores. Stage boundaries in core V?7-1 16 are based on abundance variations of foraminifera G. P~~CII).JCI.UILI.
12
6-
5’
Ilt
+35
6’*0
SUBANTARCTIC--1 RC 11-120
LATE-PLEISTOCENE
NORTH
ATLANTIC
377
JOSEPH
378
J. MORLEY
younger datum levels. The younger ash (ash zone 1) has an estimated age of 9800 yr B .P. (based on recent 14C analyses; Ruddiman and McIntyre, 1981; Duplessy et al., 1981). and age estimates from most recent analyses of ash zone 2 center around 57,500 yr B.P. (W. F. Ruddiman, personal communication, 198 1). The third reference event was a short pulse of warm sea-surface temperatures, reflected in changes in foraminiferal assemblages, that occurred at the oxygenisotope stage 5/4 boundary (Ruddiman et al., 1980). Ruddiman and colleagues (Ruddiman and McIntyre, 1973, 198 1; Ruddiman and Glover, 1975; Ruddiman et al., 1980; W. F. Ruddiman, personal communication, 1981) located the position of these stratigraphic markers in the North Atlantic cores shown in Figures 2 and 3. Comparison of the C. davisianu records in the six cores (Figs. 2 and 3) indicates that marine sediments from a large portion of the North Atlantic contain an easily recognizable pattern of C. davisiunu abundance variations. For example, an interval
FIG. 4. Relative
abundance
of C. davisiana
marked by low abundances of C. duvisiuna (*a) occurs at the top of all six records and coincides with most of the Holocene. Ash zone 1 falls near the base of this low abundance interval. The maximum in relative abundance of C. duvisiuna (*b) is recorded in all six of the cores below this uppermost low abundance interval, with the northernmost core (V23-23) having the highest C. duvisiuna abundance value (79.2%). This abundance interval (*b) falls within oxygenisotope stage 2 in core V29-179. The second highest C. duvisiunu abundance maximum in most cores is designated *l and appears to correspond to middle- to late-glacial isotopic stage 8 based on correlation with foraminiferal data. Ash zone 2 occurs just above the C. duvisiunu maximum *d. The event marking the pulse of warm sea-surface temperature, which corresponds to the oxygen-isotope stage 5/4 boundary, falls between low abundance level *ei and high abundance zone “d. Despite the similarities in the six C. duvisiuna records, there are some noticeable
in surface-sediment
samples
from
the North
Atlantic.
LATE-PLEISTOCENE
differences in the various curves. These differences may result from varying sedimentation rates between cores, undetected small hiatuses, different sampling intervals in each core, fundamentally different regional responses of C. davisiuna, or a combination of these factors. For example, the record from core V23-23 contains a prominent abundance maximum between low abundance zone *a and high abundance level *b. This C. davisiana abundance maximum progressively decreases in amplitude with decreasing latitude, and in core V29-179 is recognizable as only a slight inflection point on the slope between low C. davisiana abundance interval *a and high abundance zone *b. A possible explanation for this specific abundance maximum is presented later. Figure 4 shows the abundance of C. davisiana in surface sediments reflecting present-day conditions as recorded in ten North Atlantic cores. The relative abundance levels of C. davisiana in sediments
NORTH ATLANTIC
379
recording conditions at the last glacial maximum (-18,000 yr BP.; CLIMAP, 1981) and at the time of maximum abundance interval *b (-20,000-30,000 yr B.P.) are plotted in Figures 5 and 6, respectively. The C. davisiana abundance in the North Atlantic generally increases from southeast to northwest during both of these Pleistocene intervals. Present-day abundances of C. davisiana never exceed 1% in any of the surface-sediment samples examined, whereas in many of the same cores, values ranging from 5 to 79% were recorded during the last glacial stage (either at the glacial maximum (Fig. 5) or at the time of maximum C. davisiana interval *b (Fig. 6)). OCEANOGRAPHIC PROPERTIES ASSOCIATED WITH HIGH ABUNDANCESOF Cycladophora davisiana Percentage abundance changes of a given variable are always subject to multiple interpretations. The specific parameter may
FIG. 5. Relative abundance of C. davisiana in North Atlantic sediments at the last glacial maximum (-18,000 yr B.P.).
380
JOSEPH J. MORLEY
4 30’
FIG. 6. Relative abundance of level.
C. davisiana
in North Atlantic records at the *b maximum abundance
actually be the component which varies in absolute abundance; the background against which it is compared may be the only varying component; or both variable and background may change. In the case of C. duvisiuna, absolute abundance curves (Fig. 7) show that both the specific parameter (C. duvisiunu) and the background (all other radiolaria) are variable. Because the environmental factor or factors responsible for producing abundance variations of C. duvisiunu appear to affect the entire radiolarian population, an examination of C. duvisiunu abundance in Holocene sediments and in the overlying water may identify specific oceanographic variables associated with high abundance levels of this species. The abundance of C. duvisiunu rarely exceeds 5% in Holocene surface-sediment samples except in the Sea of Okhotsk and areas of the Northwest Pacific adjacent to this marginal sea. Kruglikova (1975), Ling (1974), and Robertson (1975) all reported relatively high abundances of C. duvisiunu
in surface sediments from the Sea of Okhotsk. In a recent study Morley and Hays (in press) found that the relative abundance of C. duvisiunu in 22 surface-sediment samples from the Sea of Okhotsk ranged from 9.9 to 48.8%. It is possible to determine the probable water-depth distribution of C. duvisiunu by examining data on radiolaria in plankton tows and in surface sediments. In a review of plankton-tow data from temperate and polar regions, Petrushevskaya (1971) noted that C. duvisiunu was not found in tows from depths shallower than 100 m, but occurred occasionally in tows that sampled depths between 300 and 700 m. In a more recent study of radiolaria in plankton tows taken in the Antarctic in early spring 198 1, from near the ice edge and from within the ice Morley and Hays (in press) found that C. duvisiunu was present in all tows that sampled depths greater than 200 m. Only 7 of the 20 tows from depths shallower than 200 m from this same cruise contained C. duv-
LATE-PLEISTOCENE
NORTH
381
ATLANTIC
CO. m. . 0
-
N
10
d
(St1313W)
en
r-
(D
Hld3Q
3t103
m
m
382
JOSEPH J. MORLEY
isiana. The number of individual C. daris&a specimens in these 7 tows never exceeded two. In the Sea of Okhotsk, high C. davisiana abundances (> 16%) occur in the shallowest (300 m) as well as the deepest (3336 m) samples examined by Morley and Hays (in press). Although there are several subspecies of C. duvisiunu (Petrushevskaya, 1%7), Morley (1980) has shown that the abundance variations of C. duvisiunu var. duvisiunu account for most of the high-latitude abundance variations of this species. In the hundreds of surface-sediment samples examined from all parts of the world’s oceans, this particular subspecies is only present in samples from water depths greater than 150 m (Morley and Hays, in press). Available plankton-tow and surface-sediment data indicate that C. duvisiunu does not occur in high abundances in surface and near-surface waters (cl50 m). High abundances (> 12%) of this species are found in recent sediments only in the Sea of Okhotsk, where C. duvisianu appears to be living at subsurface depths. A unique set of physical and biological properties exists today in the Sea of Okhotsk, and results in large part from the extreme seasonal contrast in this region where intensely cold winters are associated with relatively warm summers. Sea ice begins to form in late November/early December and does not melt completely until June. Maximum sea-ice cover occurs in March. During this winter/spring period, the Sea of Okhotsk is frequently ice covered for 3 months at a time (Zenkevitch, 1963; Parkinson and Gratz, 1983; G. J. Kukla, personal communication, 1982). The rapid late spring/ summer melting of the sea ice establishes a summer through late fall water structure characterized by a low-salinity surface layer (30.0-32.8°/~~) with a temperature minimum of - 1.X within 140 m of the surface (Reid, 1965). Data from hydrocast stations occupied in late winter and summer in the Sea of Okhotsk (Reid, 1973) are shown in Figure 8.
. SALINITY 320
(%.) 340
330
-
e.OREAS 160 RYOFU MAR” 46
,000 -
FIG. 8. Temperature and salinity profiles from the Sea of Okhotsk. Winter station (Boreas 160) taken at 47”47’N 1SVlO’E; summer station (Ryofu Maru 46) taken at 47”31’N 149”59’E (Reid, 1973).
The data indicate that the water column below 175 m is quite stable even though major variations in temperature and salinity occur in the upper 150 m between these two seasons. In March, the lowest temperatures are recorded at the surface ( - 1.56”C) with a temperature maximum of approximately 2.4”C occurring between 800- and 900-m depth. In July, surface temperatures increase to 9.4”C, with a minimum of -0.55”C recorded at a depth of 125 m. Both the late winter and summer hydrocasts show the existence of a thermocline between 130 and 180 m. Although summer surface salinities are much lower than their winter counterparts, the curves are similar below 100 m where they show increasing salinities with depth. A halocline is present between 100 and 200 m during both seasons. The hydrocast data show that the warm summer surface waters in the Sea of Okhotsk are cooled and subsequently mixed into the shallow, cold, intermediate-water layer during late fall/early winter (Yasuoka, 1%7). This convection is confined to the upper 100 to 150 m of the water column because of the relatively low surface-water salinities compared to salinities below the halocline.
LATE-PLEISTOCENE
The particular climatological and oceanographic characteristics of today’s Sea of Okhotsk make it possible for near-freezing temperatures to be maintained at water depths between 50 and 150 m throughout the year and for a halocline to become a permanent subsurface feature. Although summer surface waters of the high-latitude oceans are characterized by low salinities, and subsurface temperature minima have been reported in the Bering Sea (Reid, 1973), the northwest Pacific (Reid, 1973), and the Gulf of Alaska (?\llly, 1957), nowhere is the subsurface temperature minimum with its overlying low-salinity waters developed to the degree found in the Sea of Okhotsk. Morley and Hays (in press) propose that this unusual water structure affects the present-day radiolarian abundance and distribution in the Sea of Okhotsk. C. davisiana apparently thrives in the stable waters below the permanent halocline, whereas the relatively low-surface salinities and large temperature range restrict the diversity as well as the abundance of shallower-dwelling radiolaria. Results from studies of the vertical distribution of zooplankton in the Sea of Okhotsk (Vinogradov, 1954) would appear to support this proposed model in that they show significantly lower numbers of zooplankton living within the temperature minimum layer than above or below this interval. Therefore, although C. davisiana is probably confined to subsurface waters, its abundance variations are related not only to the water properties of the subsurface depth interval which it inhabits, but also to the specific conditions in the overlying surface waters. DISCUSSION
Identification of corresponding abundance maxima and minima of C. davisiana in the six North Atlantic records permits an examination of the abundance variations of C. davisiana in time and space in the latePleistocene North Atlantic. Furthermore, the oxygen-isotope record in core V29-179
NORTH
ATLANTIC
383
(Streeter and Shackleton, 1979), along with the approximate location of specific isotopic stage boundaries in core V27-116 based on sporadic isotopic values (Ruddiman et al., 1980; Ruddiman and McIntyre, 1981) and the abundance variations of the left-coiling foraminifera Globigerina pachyderma (W. F. Ruddiman, personal communication, 1981), make it possible to place the major changes in this species’ abundance within a specific time frame. According to the model developed by Morley and Hays presented in the previous section of this paper, high abundances of C. davisiana (>20%) reflect not only specific subsurface conditions but also certain characteristics of the overlying surface waters. Based on this hypothesis, high C. davisiana abundances (>20%) in North Atlantic sediments are indicative of periods when the water structure in the overlying surface and intermediate waters was similar to that which exists today in the Sea of Okhotsk. During these periods the intermediate waters of the North Atlantic would be characterized by relative stability. Conditions in the overlying surface waters would be affected by extensive sea-ice cover during winter and spring. Although summer and fall temperatures of the surface waters would be much higher than the near-freezing levels recorded during the remainder of the year, surface salinities would remain low because of the influx of sea-ice and/or iceberg meltwater. The temperatures and salinities at depths below the subsurface temperature minimum would remain nearly constant throughout the year with convection confined to the uppermost portion of the water column (~200 m). Based on this interpretation, the high C. davisiana abundance levels (>20%) in sediments recording conditions at the last glacial maximum (Fig. 5) in the northwestemmost cores (V23-23 and V27-20) would indicate that oceanographic conditions in this portion of the North Atlantic were dominated by prolonged periods of sea-ice cover during _ winter coupled with relatively low
384
JOSEPH
salinity levels in the surface waters during the summer and fall seasons. These conditions are similar to those proposed by McIntyre et al. (1976) in their description of the North Atlantic during the last glacial maximum. In their reconstruction, the sites of both these cores are within the zone of extensive pack ice from February through May and just south of the estimated summer sea-ice limit. The high C. davisiana abundances at these sites at this time would indicate that temperatures and salinities of the underlying waters were relatively stable with surface salinities rarely increasing to high enough levels for initiation of deep convection. Examination of the six abundance records along a southeast (V29-179) to northwest transect (V23-23) reveals that the proportion of the total record characterized by C. duvisiunu values greater than 20% becomes progressively larger with increasing latitude. In the northernmost core (V23-23), the oceanographic conditions for much of the last 300,000 yr appear to have been as extreme as those that exist today in the Sea of Okhotsk. If C. duvisiana abundances exceeding 20% are a measure of the intensity of the unusual conditions of prolonged winter/ spring sea-ice cover with low-salinity summer/fall surface waters coupled with a stable halocline, then the C. davisianu curves show that these conditions were most acute in the North Atlantic during glacial isotopic stage 8 (* 1-*n) and the interval from late in isotopic stage 3 through most of glacial isotopic stage 2 (*b). These two intervals probably mark times of maximum density stratification with coldest shallow subsurface temperature minima and lowest surface-water salinities. Secondary maxima in these C. davisianu records (*f and *h) correlate with early and late intervals in glacial isotopic stage 6. Two minor C. davisiana maxima within isotopic stage 5 appear to coincide with isotopic substages Sb and 5d which were relatively cool events (increased global ice volume) within stage 5.
J. MORLEY
In the previous discussion describing the abundance variations of C. duvisiunu in these six North Atlantic cores, a prominent abundance maximum between designated levels *a and *b was given as an example of a major difference in the various C. duvisiuna patterns since this feature could not be clearly identified in the southernmost record. The estimated age of this event centers around 13,000 yr BP. based on ash zone 1 and C. davisiunu stratigraphy. In a detailed description of the North Atlantic during the last deglaciation, Ruddiman and McIntyre (1981) describe an interval between 16,000 and 13,000 B.P. during which the North Atlantic was ice covered in winter south to at least 50”N but completely ice free during summer months. The major thinning of the ice sheets proposed by Denton and Hughes (1981) during this same time would provide large quantities of icebergs and thus meltwater to this portion of the Atlantic. This anomalous C. davisianu abundance maximum between low abundance zone *a and high abundance level *b in cores V23-23, V23-22, and V27-20 reflects extensive winter sea ice, increased levels of iceberg activity and summer meltwater, low surface-water salinities, and maximum density stratification in the northwest North Atlantic, conditions similar to those proposed by Ruddiman and McIntyre (1981). The lower C. duvisiunu abundances (~20%) in cores V27-114, V27116, and V29-179 during this same period probably indicate that although a lowsalinity surface layer may have existed throughout much of the summer/fall in this region, surface salinities in the central and southeastern North Atlantic were not sufficiently low to effectively inhibit convective, open-ocean mixing processes during times of the year when this region of the North Atlantic was ice free. These initial comparisons show that C. duvisiuna records in late-Pleistocene North Atlantic marine sediments can provide information vital to paleoclimatic reconstructions. Further study of the specific environ-
LATE-PLEISTOCENE
ment which promotes high productivity levels of C,. davisiana today in the Sea of Okhotsk may permit more-detailed paleoclimatic reconstructions based on variations in this species’ abundance. CONCLUSIONS
The results of analyses presented in this paper show the following: (1) The radiolarian species C. davisiana undergoes large variations in its abundance in Pleistocene sediments throughout a major portion of the North Atlantic, and comprises as much as 79% of the total radiolarian population at specific times during the last glacial period. (2) Although there are distinct amplitude differences, the C. davisiana patterns in all six North Atlantic cores are similar with easily recognizable features common to all records. Deviations from the general abundance pattern of this species probably reflect the response of C. davisiana to specific oceanographic conditions that occurred within a limited region of the North Atlantic. (3) If the oceanographic conditions associated with high C. davisiana abundances in today’s Sea of Okhotsk can be expanded to include high-latitude open-ocean regions, then high abundances of this species (~20%) in late-Pleistocene North Atlantic sediments are indicative of prolonged periods of winter/spring sea-ice cover, summer/fall low-salinity surface waters coinciding with increased levels of sea-ice and/or iceberg meltwater, a shallow subsurface temperature minimum, and relatively stable temperatures and salinities at depths below the temperature minimum. Therefore, although C. davisiana appears to inhabit subsurface waters, its abundance variations reflect specific oceanographic conditions in waters ranging from the surface to shallow intermediate depths. (4) The relationship between stable temperatures and salinities and high C. davisiana abundances (~20%) suggests that analyses of (7. davisiana abundance in high-
NORTH
ATLANTIC
385
latitude sediments may assist in identifying specific areas where deep-water formation could not have occurred during times throughout the late Pleistocene. ACKNOWLEDGMENTS I thank .I. Hays, W. Ruddiman, C. Sancetta, and T. Kellogg for their suggestions and criticisms. I am also grateful to D. Kintas, A. Pesanell, B. Dworetsky, and W. Fuller for their conscientious laboratory and technical assistance. Illustrations were drawn by B. Taylor. This research was directly supported by National Science Foundation Grant OCE79-24563 to Lamont-Doherty Geological Observatory. The core collection at Lamont-Doherty Geological Observatory of Columbia University was supported through funds from Grant OCE81-22083 from the National Science Foundation, Lamont-Doherty Geological Observatory of Columbia University Contribution 3497.
REFERENCES CLIMAP Project Members, McIntyre, A., Leader LGM Project (1981). “Seasonal Reconstructions of the Earth’s Surface at the Last Glacial Maximum.” Geological Society of America Map and Chart Series MC-36. Denton, G. H., and Hughes, T. J. (1981). The Arctic Ice Sheet: An outrageous hypothesis. In “The Last Great Ice Sheets” (G. H. Denton and T. J. Hughes. Eds.), pp. 440-467. Wiley-Interscience, New York. 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. Palaeogeography, Palaeoclimatology, ecology 35, 121-144.
Palaeo-
Hays, J. D., Lozano, J. A., Shackleton, N., and Irving, G. (1976). Reconstruction of the Atlantic and western Indian Ocean sectors of the 18,000 yr BP Antarctic Ocean. In “Investigation of Late Quaternary Paleoceanography and Paleoclimatology” (R. M. Cline and J. D. Hays, Eds.), pp. 337-372. Geological Society
of America
Memoir
145.
Kruglikova, S. B. (1975). Radiolarians in the surface layer of the sediments of the Okhotsk Sea. Oceanology 15, 116-122. Ling, H.-Y. (1974). Polycystine radiolaria and silicoflagellates from the surface sediments of the Sea of Okhotsk. Bulletin of the Geological Survey of Japan l-11. McIntyre, A., Kipp, N. G., Be, A. W. H., Crowley, T., Kellogg, T., Gardner, J. V., Prell, W., and Ruddiman, W. F. (1976). Glacial North Atlantic 18,000 years ago: A CLIMAP reconstruction. In “Investigation of Late Quarternary Paleoceanography and
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(R. M. Cline and J. D. Hays, Geological
Society
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145.
Moore, T. C., Jr. (1973). Method of randomly distributing grains for microscopic examination. Journal of Sedimentary
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43, 904-906.
Morley, J. J. (1980). Analysis of the abundance variations of the subspecies of Cycladophora davisiana. Marine Micropaleontology 5, 205-214. Morley, J. J., and Hays, J. D. (1979). Cycladophora davisiana: A stratigraphic tool for Pleistocene North Atlantic and interhemispheric correlation. Earth and Planetary
Science
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44, 383-389.
Morley, J. J., and Hays, J. D. (in press). Oceanographic conditions associated with high abundances of the radiolarian Cycladophora davisiana. Earth and Planetary
Science
Letters.
Parkinson, C. L., and Gratz, A. J. (1983). On the seasonal sea ice cover of the Sea of Okhotsk. Journal of Geophysical
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