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Historical stability of the Gulf Stream meander belt: foraminiferal evidence
Deep-SeaResearch, 1968,Vol. 15, pp. 137to 148. PergamonPrea. Printedin GreatBritain.
Historical stability of the Gulf Stream meander belt: foraminiferal evidence* WILLIAM F. RUDDIMAN~
(Received 5 January 1968) ~--The tests of four equatorial species of planktonic Foraminifera (Globigerinoides sacculifer, Pulleniatina obliquiloculata, Globorotalia menardii and Globorotalia tumida) found in surface sediments delimit by pexcentage abundance contours the meander belt of the late Holocene Gulf Stream eastward from Cape Hatteras to 40°W. This band of faunal influence is only slightly wider than the meander belt observed over the past two decades, and it terminates in the same area where the surface expression of the Gulf Stream markedly weakens. Three resistant Gulf Stream indicator species are apparently transported to the south along the continental margin and selectively concentrated by a near-bottom contour current (the Western Boundary Undercurrent). INTRODUCTION
THE GULF Stream marks the boundary between the warm mid-ocean waters of midlatitudes and the cooler waters of the northwest Atlantic. It transports a considerable volume of tropical water in a northeasterly direction, but to describe it as a river of warm water is an oversimplification. It extends to great depths, probably touching bottom, and it includes extremely cold water in its deepest flow. It meanders widely south of Nova Scotia, and masses of warm water often become entirely detached from the main flow. Although it is only about 20 miles wide at any instant, the Gulf Stream shifts continuously north and south through a meander belt approximately 2 ° wide in its eastern half. Equatorial species of planktonic organisms are found in the Gulf Stream, and thus the remains of shell-bearing plankton in bottom sediments might be expected to provide a reliable method of determining the limits of the late Holoeene meander belt of the Stream. PREVIOUS WORK PHLEGER, PARKERand PEIR$ON(1953) examined 53 short core tops in the Atlantic, of which 16 overlap the eastern portion of this study and are included for comparison. WmCOXON (1964) counted benthonic and planktonic species in shelf and upper slope samples along the continental margin of the southeastern United States. Plankton tow studies have in general shown that Foraminifera are valuable indicators of instantaneous water mass positions (B~, 1959; B[~ and HAMLIN, 1967; BOLTOVSKOY,1959, 1962; BRADSHAW,1959; CIFELLI, 1962, 1965, 1967; PARm~R, 1960), while surface sediments yield more generalized information on past and present water mass boundaries (BELYmVA, 1964; KUSTANOWlCH,1957; PARm~R, 1956, 1958 ; PHLEGER, PARKER a n d PEIRSON, 1953). *Lamont Geological Observatory of Columbia University Contribution No. 1164. tLamont Geological Observatory, Palisades, New York.
137
138
WILLIAM F. Rtr~V~AN
Gulf Stream temperature and salinity information averaged over considerable intervals of time (FUGLISTER, 1947, 1954; SCHROEDER, 1963, 1965, 1966)is relevant to the study of Foraminifera in surface sediments. W'ith the rough assumption of an average sedimentation rate of 2 cm per 1000 years and an approximate burrowing depth of 5 cm or greater (ARRHEYIUS, 1963), top samples may contain Foraminifera that settled to the bottom over 2500 years ago. PROCEDURE
Counts of between 250 and 500 individuals were made on 5 gram samples taken from the tops of trigger weight cores. Core tops with insufficient assemblages lie either in deep abyssal hill and abyssal plain provinces where Foraminifera are dissolved or in shallow shelf or slope areas where dilution or breakage limit their abundances (Fig. 1, Tables 1 and 2). 80.
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121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136
A173-3 A173-4 A173-12 A173-13 A173-14 A179-8 A179-12 A179-16 A179-17 A179-18 A179-19 A179-31 A179-24 A180-7 A180-13 A180-14
37°50N 33°52N 42° 15N 41°50N 41 °50N 20°29N 24°02N 26° 25N 28°00N 29°29N 29°56N 32°18N 35°46N 39°36N 39°08N 38° 41N
66°22W 4709 62°32w 3932 63°26w 2103 63°23w 2926 64°34W 2578 72°49W 4060 75°22W 1719 74° 59W 4499 73°47W 4390 72°45W 4618 69°59W 5209 68°51W 5118 69°05W 4663 50°51w 5285 42°39w 4880 40 ° 40W 5020
137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152
A180-15 A181-9 RHI3-6 RHI3-7 KM1-4 KMI-11 CA20-1 CA25-9 SPI2-3 EW41--65-39 EW41-65-57 EW2-66-1 EW2-66--2 EW27-66--1 EW27-66-3 EW27--66-4
39°16N 20°44N 33°33N 44 °33N 43°32N 42°44N 36°20N 43 ° 39N 43°04N 33°40N 32°20N 34°51N 35°59N 34°39N 34°27N 36° 39N
36°42W 59° 12W 65°26w 34°39w 59° 16W 49°59W 70°29W 48 ° 54W 60°08W 73°02W 76°23W 75°23W 74°47W 75°21W 74°39w 73° 59W
4610 5215 1573 3506 1825 715 4206 2268 2377 3585 2255 405 275 2150 3400 2500
Table 2. Location of cores from previous study. Designation in Fig. 1 P1 P2 P3 P4 P5 P6 P7 P8
Designationin
PHLEGERet al. (1953) 269A 270A 272A 279A 280A 281A 283A 284A
MEANDER
Designation Designationin in Fig. 1 PrmEo~ et al. (1953) P 9 PI0 P 11 P12 PI 3 PI4 P15 PI6
285A 286A 4631 4741 4742 4743 3-38 3-41
BELT
These f o u r i n d i c a t o r species can be used to m a p tongue-like p a t t e r n s extending o u t f r o m t h e N o r t h C a r o l i n a coast a t 76°W to a b o u t 40°W (Figs. 2-5). Except for Globigerinoides sacculifer, which is shifted a degree o r m o r e t o the south a n d restricted t o a n a r r o w e r b a n d , these tongue-like b a n d s center o n the fortieth parallel. N o t h i n g in these p a t t e r n s seriously suggests a different climatological m e a n G u l f S t r e a m p o s i t i o n f r o m t h a t o b s e r v e d at present. A few plots o f inferred i n s t a n t a n e o u s G u l f S t r e a m m e a n d e r p o s i t i o n s o b t a i n e d over t h e p a s t t w o decades (Fig. 6) define a b a n d only slightly n a r r o w e r t h a n the f a u n a l belt. ISELIN (1936), SVERDRtn', JOHNSON a n d FLEmNC (1942) a n d FUGLISrER (1951) c o n s i d e r e d t h a t t h e G u l f S t r e a m separates into b r a n c h e s o r multiple currents in the a r e a o f 40°N, 40-45°W. SOULE, MORRILL a n d FRANCrSCrmTrl (1961) suggested t w o branches, the n o r t h e r l y a n o r d e r o f m a g n i t u d e stronger t h a n the southerly. WORtHINGTON (1962) d e d u c e d a t w o - g y r a l circulation, in which a b o u t 15% o f the G u l f S t r e a m w a t e r crosses a t r o u g h into t h e n o r t h e r n gyral, the rest continuing southeastward. MAt~q (1967) a r g u e d for a b r a n c h i n g at a b o u t 44°W, with 40 ~o o f t h e G u l f S t r e a m water flowing t o the n o r t h a n d 60 % to the southeast.
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Fig. 3. Percentage abundance of Pulleniatina obliquiloculata as a n indication of Gulf Stream influence. Selective concentration of this resistant species by a deep corrosive contour current is suggested by the unusually high percentages over the Blake-Bahama Outer Ridge at 73°W.
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20"
143
Historical stability of the Gulf Stream meander belt: foramirtiferal evidence
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Fig. 6. Delimitation of band of faunal influence of the late Holocene Gulf Stream. Measured paths of the Gulf Stream meanders are shown (after FUGLISTERand WORa~INGTON, 1951). Gulf Stream influence is defined by a 3 % abundance for Globigerinoides sacculifer and a 1 ~ abundance for the other three species. Easternmost termination of the faunal effect occurs in the area of substantial dilution of Gulf Stream oceanographic characteristics.
DEEP CIRCULATION
Selective concentration of three Gulf Stream indicator species (Pulleniatina obliquiloculata, Globorotalia menardii and Globorotalia tumida) by the southerly flowing Western Boundary Undercurrent is suggested by the south-pointing tongues of species distribution near 73°W overlying the Blake-Bahama Outer Ridge (Figs. 3-5). This deep contour current, which crosses south beneath the Gulf Stream along the continental margin at velocities of the order of 15 cm/sec (SWALLOWand WORTHINCTON, 1961; VOLKMANN,1962; BARRETT, 1965), appears to have transported sufficient silt and lutite to construct and shape the sediment tongue known as the BlakeBahama Outer Ridge (HEEZEN, HOLLISTERand RUDDIMAN, 1966). The distribution patterns of these three species may indicate post-depositional transport of planktonic Foraminifera as well. Since foraminiferal tests are essentially hollow, calcitic spheres (Pulleniatina obliquiloculata) or hollow flattish circular plates (Globorotalia menardii and Globorotalia tumida), they will be transported by currents unable to entrain smaller but denser mineral sand grains. MENARD(1952) estimated that 12 cm/sec may transport Foraminifera, since their average size lies very close to the ideal 0-18 mm grain size most easily carried by currents (INMAN, 1949).
144
WILLIAM F. RUDDIMAN
Several cores lying along the axis of this deep contour current contain percent abundances of Globorotalia tumida and Pulleniatina obliquiloculata that are even higher than cores from the central Gulf Stream axis. These two species are particularly resistant to solution (HAMILTON, 1957) and show large percentage gains in deep equatorial basins as the more fragile species are selectively dissolved out (RUDDIMAN and HEEZEN, 1967). The less resistant Globorotalia menardii is only slightly increased within the deep contour current on the Blake-Bahama Outer Ridge, while the fragile Globigerinoides sacculifer is not increased at all, suggesting the selective action of differential solution. Many core tops from deep basins beneath the subtropical Sargasso Central Water contain small fractions of Foraminifera left after extensive solution. However, they indicate no marked increase in Globorotalia tumida and Pulleniatina obliquiloculata because the accompanying fauna is different. The Sargasso Sea is in part characterized by Globorotalia species (G. hirsuta, G. inflata, G. crassaformis and G. truncatulinoides) of roughly the same resistance to solution as Globorotalia tumida and Pulleniatina
obliquiloculata. The cores on the Blake-Bahama Outer Ridge beneath the Western Boundary Undercurrent contain a more equatorial fauna, including many easily dissolved species. The intensified flow of cold water selectively dissolves the fragile species, leaving residual concentrations of resistant Foraminifera. Some combination of southerly transport and differential solution best explains the anomalous Outer Ridge cores.
COMPARISON
WITH
PLANKTON
STUDIES
Published studies of plankton tow data from the Gulf Stream area (B~, 1959; B~ and HAMLIN, 1967; CI~LLI, 1962, 1965, 1967) generally indicate that the Gulf Stream fauna has a distinctive percentage composition. Although seasonal information is incomplete, Globigerinoides sacculifer and Globorotalia menardii tend to reach their percentage maxima in the Gulf Stream throughout the year, while Pulleniatina obliquiloculata varies seasonally from extreme Gulf Stream abundance (Nov.Jan.) to sporadic, irregular patterns (summer and fall). In contrast to our results, none of the plankton studies noted any Globorotalia tumida. The largely immature specimens of Globorotalia menardii and Globorotalia tumida collected in plankton tows from this area are virtully indistinguishable, and B~ has counted them as Globorotalia menardii (B~, personal communication). The surface-dwelling Globigerinoides sacculifer is mainly abundant in Gulf Stream waters during summer and early autumn (B~, 1965). Although it is carried north in latitude-crossing patterns, the strong circulation merely brings it into areas already receptive to its optimum presence, particularly in the warm core of Gulf Stream flow. It is therefore more pertinent to ask why smaller percentages are found in sediments beneath the warm Sargasso water to the southeast. Low absolute abundances are readily explained by the general nutrient impoverishment (CLARKE, 1940). Low percentage abundances of Globigerinoides sacculifer are probably a consequence of its scarcity during the short but very productive late winter overturn in the northern Sargasso Sea (MrNZEL and RYTrmR, 1960) due to the prevalence of 18°C water (WORTHINGTON, 1959) that is below its optimum range (B~DSnAW, 1959; PARKER,
Historical stabilityof the Gulf Stream meander belt: foraminiferalevidence
145
1960). Its greatest abundance in the plankton of the Sargasso area is during warm summer months when productivity is low (B~, 1960). Pulleniatina obliquiloculata reaches peak Gulf Stream abundances in early winter (CIr~LLI, 1962) at surface temperatures that are somewhat cooler than equatorial. Oxygen isotope studies also predict that this species may inhabit fairly cool waters, secreting its test beneath warmer surface waters (EMILIANI, 1954). Pulleniatina obliquiloculata, although an equatorial species, does not require full equatorial warmth to establish viable habitats in the Gulf Stream. The presence of the Globorotalia menardii-Globorotalia tumida group in the Gulf Stream throughout the year (CIFELLI, 1965) suggests tolerance of a wide range of temperatures for these species as well. EMILIANI(1954) indicated such cool average temperatures of shell secretion that neither species should be barred from the Gulf Stream even in the coldest winter months. The two Globorotalia species intergrade thoroughly in plankton tows as noted previously, but are reasonably distinct in Recent sediments of the Gulf Stream area. Either the sediment specimens of Globorotalia tumida are relic assemblages indicating a formerly increased extent of this species, or one must conclude that complete development of distinguishing adult characteristics only begins in the Gulf Stream area below the 300 m depth maximum reached by most plankton tows. WILCOXON (1964) found G. tumida only in surface sediments cored below 300 m along the adjacent southeastern United States coast. ORR (1967) recently found evidence of secondary calcification of these two species at considerable depth in the Gulf of Mexico. Such a process had previously been noted in other areas (WISEMANand TODD, 1959; B~ and ERICSON, 1963; B~ and LoTr, 1964; B~, 1965; B~, MCINTYRE and B~GER, 1966). The virtual absence of Pulleniatina obliquiloculata and the Globorotalia menardiiGloborotalia tumida group in sediments beneath the Sargasso Sea is difficult to explain. Although differential solution should increase their abundances on the sea floor, they are less abundant than the plankton data would predict. In the Sargasso Central water, temperatures are sufficently warm through much of the year to create a viable habitat for these three Gulf Stream indicators. Their relative paucity in the sediments suggests that either the low nutrient content or high salinities of the central water mass may form an effective barrier to higher percentages of the Gulf Stream indicator species. Subtropical species with lesser nutrient requirements or higher salinity tolerances are favored instead. BRAOSHAW (1959) noted that reproductive distributions are often more limited than those for simple survival, and that sterile distributions presumably typify areas such as the northern Kuroshio, where the vigorous currents carry individuals into areas in which they cannot reproduce. All four Gulf Stream indicator species are probably subject to at least seasonal periods of sterile distribution, with reproductive distributions more likely typical of the warm core during summer months. The deep-dwelling, cold-tolerant species in this study cannot be analyzed by surface isotherms alone, since there are considerable differences between the surface and 200 m temperature patterns. Except for the narrow warm core of the Gulf Stream, the surface trend is one of poleward cooling, with the Gulf Stream marking a boundary of very strong temperature gradients. At 200 m depth, however, the warmest average North Atlantic waters (15°C) are found in the middle latitudes of the
146
WILLIAM F. RUDDIMAN
Sargasso Sea, while equatorial and Gulf Stream waters are slightly cooler on the average (10°-15°C) and roughly isothermal, creating a disjunct pattern. Individuals living near the surface in this subtropical gyral are thus more likely to cut across isotherms than those living at 200 m, because the latter may travel in the deeper Gulf Stream flow from the tropics to higher latitudes at virtually the same temperature. CONCLUSIONS
The Gulf Stream indicating Foraminifera are equatorial species carried into northerly latitudes particularly during months of receptive temperature conditions in the warm core of the Gulf Stream. They define part of the enormous subtropical gyral, beginning with the Canaries Current, and extending into the Brazil Current, the Florida Current, and the Gulf Stream. The discrete tongues of species percentage abundance in surface sediments are evidence of the historical stability of the Gulf Stream meander belt. It may be possible to delimit the Pleistocene glacial subtropical gyral by examination of Foraminifera from deeper horizons in piston cores. In the Gulf Stream area, the northern faunal boundary is determined by the yearround temperature barrier of extremely cold slope and coastal waters, while the southern boundary is a more complex function of nutrient deficiency and seasonal temperature barriers that favor warm-temperate and subtropical species at the expense of equatorial Gulf Stream indicating species.
Acknowledgements--This study is mainly based on material provided by the Lamont Geological Observatory Core Library. The remainder of the cores were taken aboard R.V. Eastward of Duke University in 1965-1966 on cruises involving Lamont Observatory personnel. The research reported in this paper was supported by the National Science Foundation (GA 580). The marluscript was prepared while the author was the recipient of a Duke University Traineeship under the cooperative Research and Training program in Biological Oceanography. This program is supported through the National Science Foundation Grant GB 5529 to Duke University. The constant critical encouragement of Prof. BRUCEC. HEEZENthrough every phase of this study is greatfully acknowledged. REFERENCES
ARRHENIUSG. (1963) Pelagic sediments. In: The Sea, M. N. HILL, editor, Interscience, New York, 3, 655-727. BARRETT J. R. (1965) Subsurface Currents off Cape Hatteras. Deep-Sea Res., 12, 173-185. BE A. W. H. (1959) Ecology of Recent planktonic Foraminifera, Pt. 1. Areal distribution in the Western North Atlantic. Micropaleontology, 5, 77-100. BE A. W. H. 0960) Ecology of Recent planktonic Foraminifera, Pt. 2. Bathymetric and seasonal distributions in the Sargasso Sea off Bermuda. Micropaleontology, 6, 373-392. BE A. W. H. 0965) The influence of depth on shell growth in Globigerinoides sacculifer (Brady). Micropaleontology, 11, 81-97. BE A. W. H. and D. B. ERICSON (1963) Aspects of calcification in planktonic Foraminifera (Sarcodinia). Ann. N. Y. Acad. Sci., 109, 65-81. BE A. W. H. and W. I-L HAMLIN(1967) Ecology of Recent Planktonic Foraminifera. Pt. 3. Distribution in the north Atlantic during the summer of 1962. Micropaleontology, 13, 87-106. BE A. W. H. and L. LOTT (1964) Shell growth and structure of planktonic Foraminifera. Science, 145, 823-824. BE A. W. H., A. MCINTYREand D. L. BREGER(1966) Shell microstructure of a planktonic Foraminifer, Globorotalia menardii (d'Orbigny). Eeolgae Geol. Helv., 59, (2), 885-896. BELYAEVA N. V. (1964) Distribution of planktonic Foraminifera in the water and on the floor in the Indian Ocean. (In Russian) Trudy, Inst. Okeanol., Akad. Nauk USSR, 68, 12-83.
Historical stability of the Gulf Stream meander belt: foraminiferal evidence
147
BOLTOVSKOYE. (1959) Foraminifera as biological indicators in the study of ocean currents. Micropaleontology, 5, 473--481. BOLTOVSKOYE. (1962) Planktonic Foraminifera as indicators of different water masses in the South Atlantic. Micropaleontology, 8, 403--408. BRADSnAW J. S. (1959) Ecology of living planktonic Foraminifera in the North and Equatorial Pacific Ocean. Contr. Cush. Fd. foramin. Res., 10, 25-64. CIFELLIR. (1962) Some dynamic aspects of the distribution of planktonic Foraminifera in the western North Atlantic. J. mar. Res., 20, 201-213. CIFELLI R. (1965) Planktonic Foraminifera from the western North Atlantic. Publ. Smithsonian Inst., Misc. Coll. 148, (4599), 1-35. CIFELLI R. (1967) Distributional analysis of North Atlantic Foraminifera collected in 1961 during cruises 17 and 21 of the R.V. Chain. Contr. Cush. Fd. Foramin. Res., 18, (3), 118-127. CLARKE G. L. (1940) Comparative richness of zooplankton in coastal and offshore areas of the Atlantic. Biol. Bull. mar. biol. Lab., Woods Hole, 78, 226-255. EMILIANIC. (1954) Depth habitats of some species of pelagic Foraminifera as indicated by oxygen isotope ratios. Am. J. Sci., 252, 149-158. FUGLISTER F. C. (1947) Average monthly sea surface temperatures of the western North Atlantic Ocean. Pap. Phys. Oceanog. Meteorol., 10, (2), 25 pp. FUGLISTERF. C. (1951) Multiple currents in the Gulf Stream system. Tellus, 3, 230-233. FUGLISTER F. C. (1954) Average temperature and salinity at a depth of 200 metres in the North Atlantic. Tellus, 6, 46-58. FUGUSTER F. C. and L. V. WORTHINGTON,(1951) Some results of a multiple ship survey of the Gulf Stream. Tellus, 3, 1-14. HAMILTON E. L. (1957) Foraminifera from an Equatorial Pacific core. Micropaleontology, 3, 69-73. HEEZEN B. C., C. D. HOLLISTERand W. F. RUDDIMAN(1966) Shaping of the continental Rise by deep geostrophic contour currents. Science, 152 (3721), 502-508. HEEZENB. C., M. THARP and M. EWING (1959) The floors of the oceans, I: The North Atlantic. Spec. Pap. Geol. Soc. Am., 65, 126 pp. INMAN D. L. (1949) Sorting of sediments in the light of fluid mechanics. J. Sedim. Pet., 19, 51-70. ISELrN C. O'D. (1936) A study of the circulation of the western North Atlantic. Pap. phys. Oceanogr. Met., 4 (4), 101 pp. KUSTANOWICHS. (1963) Distribution of planktonic Foraminifera in surface sediments of the southwest Pacific Ocean. N. Z. Jl Geol. Geophys., 6, 534-565. MANN C. R. (1967) The termination of the Gulf Stream and the beginning of the North Atlantic Current. Deep-Sea Res., 14, 337-359. MENARDH. W. (1952) Deep ripple marks in the sea. J. sedim. Pet., 22, 3-9. MENZEL D. W. and J. H. RVTHER (1960) The annual cycle of primary production in the Sargasso Sea off Bermuda. Deep-Sea Res., 6, 351-367. ORR W. N. (1967) Secondary calcification in the foraminiferal genus iGloborotalia. Science, 157, 1554-1555. PARKER F. L. (1956) Distribution of planktonic Foraminifera in some Mediterranean sediments. In: Pap. Mar. Biol. Oceanog., Suppl. Vol. 3, Deep-Sea Res., 204-211. PARKER F. L. (1958) Eastern Mediterranean Foraminifera. Rep. Swedish deep Sea Exped., 8, 219-283. PARKER F. L. (1960) Living planktonic Foraminifera from the equatorial and southeast Pacific. Tokohu Univ., Sci. Repts., 4 (2), 71-82. Pr~LEGER F. B., F. L. PARKERand J. F. PEIRSON(1953) North Atlantic Foraminifera. Rep. Swedish deep Sea Exped., 7, 3-122. RUDDIMhN W. F. and B. C. HEEZEN (1967) Differential solution of planktonic Foraminifera. Deep-Sea Res., 14, 801-808. SCHROEDERE. H. (1963) North Atlantic temperatures at a depth of 200 meters. Serial Atlas of the Marine Environment, Am. Geogr. Soc., Folio 2. SCr~ROEDERE. H. (1965) Average monthly temperatures in the North Atlantic Ocean. DeepSea Res., 12, 323-343. SCHROEDERE. 14. (1966) Average surface temperature of the western North Atlantic. Bull. mar. Sci. Miami, 16, 302-323. SOULE F. M., P. A. MORRmL and A. P. FghNCESCI-mTTI(1961) Physical oceanography of the Grand Banks region and the Labrador Sea in 1960. U.S. Coast Guard Bull. 46, 31-114. SVERDRUP H. U., M. W. JOHNSON and R. H. FLEMrNG (1942) The Oceans. Prentice-Hall, Englewood Cliffs, 1087 pp.
148
WILLIAMF. RUDDIMAN
SWALLOW,J. C. and L. V. WORTHINGTON(1961) An observation of a deep countercurrent in the western North Atlantic. Deep-Sea Res., 8, 1-9. VOLKMANN,G. H. (1962) Deep current observations in the western North Atlantic. Deep-Sea Res,, 9, 493-500. WILCOXONJ. A. (1964) Distribution of Foraminifera off the southern Atlantic coast of the United States. Contr. Cush. Fd. foramin. Res., 15, 1-24. WISEMANJ. D. H. and I. TODD(1959) Signification des variations du taux d'accumulation de Globorotalia menardii menardii (d'Orbigny) dans une carrote de l'Atlantique Equatorial. Colloques int. Cent. natn. Rech. scient., 83, 193-208. WORTHINGTONL. V. (1959) The 18° water in the Sargasso Sea. Deep-Sea Res., 5, 297-305. WORTmNGTONL. V. (1962) Evidence for a 2-gyre circulation system in the North Atlantic. Deep-Sea Res., 9, 51-67.
Historical stability of the Gulf Stream meander belt: foraminiferal evidence* WILLIAM F. RUDDIMAN~
(Received 5 January 1968) ~--The tests of four equatorial species of planktonic Foraminifera (Globigerinoides sacculifer, Pulleniatina obliquiloculata, Globorotalia menardii and Globorotalia tumida) found in surface sediments delimit by pexcentage abundance contours the meander belt of the late Holocene Gulf Stream eastward from Cape Hatteras to 40°W. This band of faunal influence is only slightly wider than the meander belt observed over the past two decades, and it terminates in the same area where the surface expression of the Gulf Stream markedly weakens. Three resistant Gulf Stream indicator species are apparently transported to the south along the continental margin and selectively concentrated by a near-bottom contour current (the Western Boundary Undercurrent). INTRODUCTION
THE GULF Stream marks the boundary between the warm mid-ocean waters of midlatitudes and the cooler waters of the northwest Atlantic. It transports a considerable volume of tropical water in a northeasterly direction, but to describe it as a river of warm water is an oversimplification. It extends to great depths, probably touching bottom, and it includes extremely cold water in its deepest flow. It meanders widely south of Nova Scotia, and masses of warm water often become entirely detached from the main flow. Although it is only about 20 miles wide at any instant, the Gulf Stream shifts continuously north and south through a meander belt approximately 2 ° wide in its eastern half. Equatorial species of planktonic organisms are found in the Gulf Stream, and thus the remains of shell-bearing plankton in bottom sediments might be expected to provide a reliable method of determining the limits of the late Holoeene meander belt of the Stream. PREVIOUS WORK PHLEGER, PARKERand PEIR$ON(1953) examined 53 short core tops in the Atlantic, of which 16 overlap the eastern portion of this study and are included for comparison. WmCOXON (1964) counted benthonic and planktonic species in shelf and upper slope samples along the continental margin of the southeastern United States. Plankton tow studies have in general shown that Foraminifera are valuable indicators of instantaneous water mass positions (B~, 1959; B[~ and HAMLIN, 1967; BOLTOVSKOY,1959, 1962; BRADSHAW,1959; CIFELLI, 1962, 1965, 1967; PARm~R, 1960), while surface sediments yield more generalized information on past and present water mass boundaries (BELYmVA, 1964; KUSTANOWlCH,1957; PARm~R, 1956, 1958 ; PHLEGER, PARKER a n d PEIRSON, 1953). *Lamont Geological Observatory of Columbia University Contribution No. 1164. tLamont Geological Observatory, Palisades, New York.
137
138
WILLIAM F. Rtr~V~AN
Gulf Stream temperature and salinity information averaged over considerable intervals of time (FUGLISTER, 1947, 1954; SCHROEDER, 1963, 1965, 1966)is relevant to the study of Foraminifera in surface sediments. W'ith the rough assumption of an average sedimentation rate of 2 cm per 1000 years and an approximate burrowing depth of 5 cm or greater (ARRHEYIUS, 1963), top samples may contain Foraminifera that settled to the bottom over 2500 years ago. PROCEDURE
Counts of between 250 and 500 individuals were made on 5 gram samples taken from the tops of trigger weight cores. Core tops with insufficient assemblages lie either in deep abyssal hill and abyssal plain provinces where Foraminifera are dissolved or in shallow shelf or slope areas where dilution or breakage limit their abundances (Fig. 1, Tables 1 and 2). 80.
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<<<<<<<<<<<<<<<<<<<<<<<<<~~<<<<<<<<<<<<<<<<<<<<<<~<<<<<<-------------------------
~
(m)
Depth
(m)
Longitude
Latitude
Lamont Core No.
Core No. in Fig. 1
Depth
Longitude
Latitude
Lamont Core No.
CoreNo. in Fig. l
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Table 1--contd.
121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136
A173-3 A173-4 A173-12 A173-13 A173-14 A179-8 A179-12 A179-16 A179-17 A179-18 A179-19 A179-31 A179-24 A180-7 A180-13 A180-14
37°50N 33°52N 42° 15N 41°50N 41 °50N 20°29N 24°02N 26° 25N 28°00N 29°29N 29°56N 32°18N 35°46N 39°36N 39°08N 38° 41N
66°22W 4709 62°32w 3932 63°26w 2103 63°23w 2926 64°34W 2578 72°49W 4060 75°22W 1719 74° 59W 4499 73°47W 4390 72°45W 4618 69°59W 5209 68°51W 5118 69°05W 4663 50°51w 5285 42°39w 4880 40 ° 40W 5020
137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152
A180-15 A181-9 RHI3-6 RHI3-7 KM1-4 KMI-11 CA20-1 CA25-9 SPI2-3 EW41--65-39 EW41-65-57 EW2-66-1 EW2-66--2 EW27-66--1 EW27-66-3 EW27--66-4
39°16N 20°44N 33°33N 44 °33N 43°32N 42°44N 36°20N 43 ° 39N 43°04N 33°40N 32°20N 34°51N 35°59N 34°39N 34°27N 36° 39N
36°42W 59° 12W 65°26w 34°39w 59° 16W 49°59W 70°29W 48 ° 54W 60°08W 73°02W 76°23W 75°23W 74°47W 75°21W 74°39w 73° 59W
4610 5215 1573 3506 1825 715 4206 2268 2377 3585 2255 405 275 2150 3400 2500
Table 2. Location of cores from previous study. Designation in Fig. 1 P1 P2 P3 P4 P5 P6 P7 P8
Designationin
PHLEGERet al. (1953) 269A 270A 272A 279A 280A 281A 283A 284A
MEANDER
Designation Designationin in Fig. 1 PrmEo~ et al. (1953) P 9 PI0 P 11 P12 PI 3 PI4 P15 PI6
285A 286A 4631 4741 4742 4743 3-38 3-41
BELT
These f o u r i n d i c a t o r species can be used to m a p tongue-like p a t t e r n s extending o u t f r o m t h e N o r t h C a r o l i n a coast a t 76°W to a b o u t 40°W (Figs. 2-5). Except for Globigerinoides sacculifer, which is shifted a degree o r m o r e t o the south a n d restricted t o a n a r r o w e r b a n d , these tongue-like b a n d s center o n the fortieth parallel. N o t h i n g in these p a t t e r n s seriously suggests a different climatological m e a n G u l f S t r e a m p o s i t i o n f r o m t h a t o b s e r v e d at present. A few plots o f inferred i n s t a n t a n e o u s G u l f S t r e a m m e a n d e r p o s i t i o n s o b t a i n e d over t h e p a s t t w o decades (Fig. 6) define a b a n d only slightly n a r r o w e r t h a n the f a u n a l belt. ISELIN (1936), SVERDRtn', JOHNSON a n d FLEmNC (1942) a n d FUGLISrER (1951) c o n s i d e r e d t h a t t h e G u l f S t r e a m separates into b r a n c h e s o r multiple currents in the a r e a o f 40°N, 40-45°W. SOULE, MORRILL a n d FRANCrSCrmTrl (1961) suggested t w o branches, the n o r t h e r l y a n o r d e r o f m a g n i t u d e stronger t h a n the southerly. WORtHINGTON (1962) d e d u c e d a t w o - g y r a l circulation, in which a b o u t 15% o f the G u l f S t r e a m w a t e r crosses a t r o u g h into t h e n o r t h e r n gyral, the rest continuing southeastward. MAt~q (1967) a r g u e d for a b r a n c h i n g at a b o u t 44°W, with 40 ~o o f t h e G u l f S t r e a m water flowing t o the n o r t h a n d 60 % to the southeast.
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20"
143
Historical stability of the Gulf Stream meander belt: foramirtiferal evidence
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Fig. 6. Delimitation of band of faunal influence of the late Holocene Gulf Stream. Measured paths of the Gulf Stream meanders are shown (after FUGLISTERand WORa~INGTON, 1951). Gulf Stream influence is defined by a 3 % abundance for Globigerinoides sacculifer and a 1 ~ abundance for the other three species. Easternmost termination of the faunal effect occurs in the area of substantial dilution of Gulf Stream oceanographic characteristics.
DEEP CIRCULATION
Selective concentration of three Gulf Stream indicator species (Pulleniatina obliquiloculata, Globorotalia menardii and Globorotalia tumida) by the southerly flowing Western Boundary Undercurrent is suggested by the south-pointing tongues of species distribution near 73°W overlying the Blake-Bahama Outer Ridge (Figs. 3-5). This deep contour current, which crosses south beneath the Gulf Stream along the continental margin at velocities of the order of 15 cm/sec (SWALLOWand WORTHINCTON, 1961; VOLKMANN,1962; BARRETT, 1965), appears to have transported sufficient silt and lutite to construct and shape the sediment tongue known as the BlakeBahama Outer Ridge (HEEZEN, HOLLISTERand RUDDIMAN, 1966). The distribution patterns of these three species may indicate post-depositional transport of planktonic Foraminifera as well. Since foraminiferal tests are essentially hollow, calcitic spheres (Pulleniatina obliquiloculata) or hollow flattish circular plates (Globorotalia menardii and Globorotalia tumida), they will be transported by currents unable to entrain smaller but denser mineral sand grains. MENARD(1952) estimated that 12 cm/sec may transport Foraminifera, since their average size lies very close to the ideal 0-18 mm grain size most easily carried by currents (INMAN, 1949).
144
WILLIAM F. RUDDIMAN
Several cores lying along the axis of this deep contour current contain percent abundances of Globorotalia tumida and Pulleniatina obliquiloculata that are even higher than cores from the central Gulf Stream axis. These two species are particularly resistant to solution (HAMILTON, 1957) and show large percentage gains in deep equatorial basins as the more fragile species are selectively dissolved out (RUDDIMAN and HEEZEN, 1967). The less resistant Globorotalia menardii is only slightly increased within the deep contour current on the Blake-Bahama Outer Ridge, while the fragile Globigerinoides sacculifer is not increased at all, suggesting the selective action of differential solution. Many core tops from deep basins beneath the subtropical Sargasso Central Water contain small fractions of Foraminifera left after extensive solution. However, they indicate no marked increase in Globorotalia tumida and Pulleniatina obliquiloculata because the accompanying fauna is different. The Sargasso Sea is in part characterized by Globorotalia species (G. hirsuta, G. inflata, G. crassaformis and G. truncatulinoides) of roughly the same resistance to solution as Globorotalia tumida and Pulleniatina
obliquiloculata. The cores on the Blake-Bahama Outer Ridge beneath the Western Boundary Undercurrent contain a more equatorial fauna, including many easily dissolved species. The intensified flow of cold water selectively dissolves the fragile species, leaving residual concentrations of resistant Foraminifera. Some combination of southerly transport and differential solution best explains the anomalous Outer Ridge cores.
COMPARISON
WITH
PLANKTON
STUDIES
Published studies of plankton tow data from the Gulf Stream area (B~, 1959; B~ and HAMLIN, 1967; CI~LLI, 1962, 1965, 1967) generally indicate that the Gulf Stream fauna has a distinctive percentage composition. Although seasonal information is incomplete, Globigerinoides sacculifer and Globorotalia menardii tend to reach their percentage maxima in the Gulf Stream throughout the year, while Pulleniatina obliquiloculata varies seasonally from extreme Gulf Stream abundance (Nov.Jan.) to sporadic, irregular patterns (summer and fall). In contrast to our results, none of the plankton studies noted any Globorotalia tumida. The largely immature specimens of Globorotalia menardii and Globorotalia tumida collected in plankton tows from this area are virtully indistinguishable, and B~ has counted them as Globorotalia menardii (B~, personal communication). The surface-dwelling Globigerinoides sacculifer is mainly abundant in Gulf Stream waters during summer and early autumn (B~, 1965). Although it is carried north in latitude-crossing patterns, the strong circulation merely brings it into areas already receptive to its optimum presence, particularly in the warm core of Gulf Stream flow. It is therefore more pertinent to ask why smaller percentages are found in sediments beneath the warm Sargasso water to the southeast. Low absolute abundances are readily explained by the general nutrient impoverishment (CLARKE, 1940). Low percentage abundances of Globigerinoides sacculifer are probably a consequence of its scarcity during the short but very productive late winter overturn in the northern Sargasso Sea (MrNZEL and RYTrmR, 1960) due to the prevalence of 18°C water (WORTHINGTON, 1959) that is below its optimum range (B~DSnAW, 1959; PARKER,
Historical stabilityof the Gulf Stream meander belt: foraminiferalevidence
145
1960). Its greatest abundance in the plankton of the Sargasso area is during warm summer months when productivity is low (B~, 1960). Pulleniatina obliquiloculata reaches peak Gulf Stream abundances in early winter (CIr~LLI, 1962) at surface temperatures that are somewhat cooler than equatorial. Oxygen isotope studies also predict that this species may inhabit fairly cool waters, secreting its test beneath warmer surface waters (EMILIANI, 1954). Pulleniatina obliquiloculata, although an equatorial species, does not require full equatorial warmth to establish viable habitats in the Gulf Stream. The presence of the Globorotalia menardii-Globorotalia tumida group in the Gulf Stream throughout the year (CIFELLI, 1965) suggests tolerance of a wide range of temperatures for these species as well. EMILIANI(1954) indicated such cool average temperatures of shell secretion that neither species should be barred from the Gulf Stream even in the coldest winter months. The two Globorotalia species intergrade thoroughly in plankton tows as noted previously, but are reasonably distinct in Recent sediments of the Gulf Stream area. Either the sediment specimens of Globorotalia tumida are relic assemblages indicating a formerly increased extent of this species, or one must conclude that complete development of distinguishing adult characteristics only begins in the Gulf Stream area below the 300 m depth maximum reached by most plankton tows. WILCOXON (1964) found G. tumida only in surface sediments cored below 300 m along the adjacent southeastern United States coast. ORR (1967) recently found evidence of secondary calcification of these two species at considerable depth in the Gulf of Mexico. Such a process had previously been noted in other areas (WISEMANand TODD, 1959; B~ and ERICSON, 1963; B~ and LoTr, 1964; B~, 1965; B~, MCINTYRE and B~GER, 1966). The virtual absence of Pulleniatina obliquiloculata and the Globorotalia menardiiGloborotalia tumida group in sediments beneath the Sargasso Sea is difficult to explain. Although differential solution should increase their abundances on the sea floor, they are less abundant than the plankton data would predict. In the Sargasso Central water, temperatures are sufficently warm through much of the year to create a viable habitat for these three Gulf Stream indicators. Their relative paucity in the sediments suggests that either the low nutrient content or high salinities of the central water mass may form an effective barrier to higher percentages of the Gulf Stream indicator species. Subtropical species with lesser nutrient requirements or higher salinity tolerances are favored instead. BRAOSHAW (1959) noted that reproductive distributions are often more limited than those for simple survival, and that sterile distributions presumably typify areas such as the northern Kuroshio, where the vigorous currents carry individuals into areas in which they cannot reproduce. All four Gulf Stream indicator species are probably subject to at least seasonal periods of sterile distribution, with reproductive distributions more likely typical of the warm core during summer months. The deep-dwelling, cold-tolerant species in this study cannot be analyzed by surface isotherms alone, since there are considerable differences between the surface and 200 m temperature patterns. Except for the narrow warm core of the Gulf Stream, the surface trend is one of poleward cooling, with the Gulf Stream marking a boundary of very strong temperature gradients. At 200 m depth, however, the warmest average North Atlantic waters (15°C) are found in the middle latitudes of the
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Sargasso Sea, while equatorial and Gulf Stream waters are slightly cooler on the average (10°-15°C) and roughly isothermal, creating a disjunct pattern. Individuals living near the surface in this subtropical gyral are thus more likely to cut across isotherms than those living at 200 m, because the latter may travel in the deeper Gulf Stream flow from the tropics to higher latitudes at virtually the same temperature. CONCLUSIONS
The Gulf Stream indicating Foraminifera are equatorial species carried into northerly latitudes particularly during months of receptive temperature conditions in the warm core of the Gulf Stream. They define part of the enormous subtropical gyral, beginning with the Canaries Current, and extending into the Brazil Current, the Florida Current, and the Gulf Stream. The discrete tongues of species percentage abundance in surface sediments are evidence of the historical stability of the Gulf Stream meander belt. It may be possible to delimit the Pleistocene glacial subtropical gyral by examination of Foraminifera from deeper horizons in piston cores. In the Gulf Stream area, the northern faunal boundary is determined by the yearround temperature barrier of extremely cold slope and coastal waters, while the southern boundary is a more complex function of nutrient deficiency and seasonal temperature barriers that favor warm-temperate and subtropical species at the expense of equatorial Gulf Stream indicating species.
Acknowledgements--This study is mainly based on material provided by the Lamont Geological Observatory Core Library. The remainder of the cores were taken aboard R.V. Eastward of Duke University in 1965-1966 on cruises involving Lamont Observatory personnel. The research reported in this paper was supported by the National Science Foundation (GA 580). The marluscript was prepared while the author was the recipient of a Duke University Traineeship under the cooperative Research and Training program in Biological Oceanography. This program is supported through the National Science Foundation Grant GB 5529 to Duke University. The constant critical encouragement of Prof. BRUCEC. HEEZENthrough every phase of this study is greatfully acknowledged. REFERENCES
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