Bathyal ostracodes from the Florida-Hatteras slope, the Straits of Florida, and the Blake Plateau

Marine Micropaleontology, 8 ( 1 9 8 3 / 8 4 ) 89--119

89

Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands

BATHYAL OSTRACODES FROM THE FLORIDA--HATTERAS SLOPE, THE STRAITS OF FLORIDA, AND THE BLAKE PLATEAU

T H O M A S M. C R O N I N

U.S. Geological Survey, Reston, Virginia 22092 (U.S.A.) (Accepted for publication April 11, 1983)

Abstract Cronin, T.M., 1983. Bathyal ostracodes from the Florida-Hatteras slope, the Straits o f Florida, and the Blake Plateau. Mar. Micropaleontol., 8: 89--119. Epibathyal ostracodes from the Florida--Hatteras slope, the Blake Plateau and the Straits of Florida were studied to determine the relationship of numerous genera and species to b o t t o m - w a t e r e n v i r o n m e n t a l conditions such as dissolved o x y g e n and b o t t o m - w a t e r temperatures. F r o m a total of 100 samples, 44 samples evenly distributed b e t w e e n 200 and 1100 m water depth and having an average of 325 specimens were e x a m i n e d in detail. Using occurrence data from the adjacent continental shelf, carapace preservation, Rose Bengal staining and p o p u l a t i o n data, indigenous death assemblages were distinguished from transported or r e w o r k e d fossil specimens. The percent of transported specimens varied as follows: Blake Plateau < 1%; Straits of Florida 10--60%; Florida--Hatteras slope 1--15%. Indigenous death assemblages c o n t a i n e d b e t w e e n 10 and 61 species per sample, averaging 33.3 species. Krithe, ArgiUoecia and Pseudocythere occur in > 90% o f the samples and usually constitute 10 to 30% of each. Trachyleberidea, Bairdoppilata, Saida, Paranesidea, Ambocythere, Bythocypris, Cytherella, Bradleya, Henryhowella, and Polycopidae o c c u r in 45 to 80% of the samples in varying percentages. The upper depth limits o f 39 taxa occur at or just below the t h e r m o c l i n e suggesting a relationship to temperature. Australoecia, Quasibuntonia, Cytheropteron, Ruggieriella, Saida, Ambocythere, Trachyleberidea, Macrocypris, Krithe, "Thalassocythere", and Cytherelia are most c o m m o n or restricted to the O~ m i n i m u m zone. Conversely, Anchistrocheles, Bradleya, Henryhowelia, and Rockallia are most c o m m o n below 750 m in well o x y g e n a t e d water with temperatures below 8°C. The results show that: (1) ostracodes display a narrow depth z o n a t i o n controlled by dissolved o x y g e n and water t e m p e r a t u r e ; (2) species diversity is very high for a bathyal zone; (3) ostracodes can be used to identify the source of sediment that has been transported downslope; and (4) some taxa are useful in recognizing low o x y g e n a t e d water in C e n o z o i c deposits.

Introduction Continental slopes are defined physiographically as the region between the shelf break and the continental rise and cover a b o u t 5.6% of the Earth's surface. They are generally 20 to 100 km wide, have an average gradient of a b o u t 4.17 ° , and range in depth from a clearly defined upper boundary a b o u t 0377-8398/83/$03.00

100 to 200 m to their lower boundary between 1400 and 3200 m where they grade into the continental rise. Within these broad limits, a wide variety of shapes, gradients and depth ranges occur (Shepard, 1973) that often reflect different geological origins. Faunally, continental slopes contain the tightest depth zonation of organisms of any region in the oceans and are characterized

© 1983 Elsevier Science Publishers B.V.

90 by rapid faunal changes caused by the disappearance of shelf taxa and an increase in deep-sea taxa. However, to those who have studied the megafauna of continental slopes, this region is not simply a transition region but rather a distinct system characterized by taxa able to inhabit this dynamic province (Rowe and Haedrich, 1979). Ostracode studies on modern continental slopes and in Mesozoic and Cenozoic bathyal deposits have lagged behind work on ostracodes from continental shelves and, in recent years, deepsea abyssal environments. In 1967, Richard H. Benson made the following comments about our understanding of depth zonation of marine ostracodes at the Second International Symposium on Ostracoda: "The most difficult places to collect are those which are of the most interest in the upper bathyal zone. As yet we simply don't know what causes depth zonation." (Benson, 1969, p. 479.) Despite remarkable advances in our understanding of the ecology and evolution of many deep water ostracodes by Benson (1981, and references therein), Peypouquet {1977, 1979, 1980} and others, most deeper water taxa are not understood taxonomically or ecologically because of the lack of detailed sampling on continental slopes, small sample size, problems of downslope transport of shallow water specimens, the seemingly cosmopolitan distribution of

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genera and the poor understanding of the taxonomy of many groups. A detailed study of ostracodes from the Florida--Hatteras slope, the Blake Plateau and the Florida Straits {Fig. 1) was undertaken in order to determine the distribution of ostracodes in epibathyal environments off the southeastern United States and to relate this to environmental parameters such as substrate, oxygen and temperature. Emphasis is placed on establishing the upper depth limits and abundances of deeper water taxa. It is hoped that the distribution data and SEM photomicrographs from this part of the North Atlantic will serve as a reference point in future studies of bathyal ostracodes and will be directly applicable to Cenozoic paleoceanographic studies along continental margins.

Material In 1962, the U.S. Geological Survey (USGS) and Woods Hole Oceanographic Institution (WHOI) initiated a detailed study of the marine geology of the Atlantic Continental Margin (Emery, 1966}. More than 1600 bottom samples were obtained from Maine to the Florida Keys using Campbell and Smith-McIntyre grab samples. Ostracodes from the northern part of the continental margin were studied by Hazel (1970), those from the shelf of the central region by Valentine {1971) and Hazel {1975). One hundred and fifty five samples from the southern region between latitudes 24 ° and 33°N are presently under study. A list of location and depth of these samples is available from the author. Hathaway (1971) gives detailed information about the samples. Preliminary results for the shallow water ostracodes from the shelf off South Carolina, Georgia and northern Florida are given in King (1981). The present paper discusses the ostracodes from 100 samples from the slope and Blake Plateau off the southeastern U.S. Fig. 1 shows the study region and the location of bottom samples examined in this study. An attempt was made to sample evenly across the depth interval from 200 to 1100 m and to obtain

91

TABLE

i

Ostracode sample size and species diversity Sample No.

Water Latitude depth (N)

Longitude Total In situ In situ (W) specimens specimens diversity

(m) 1594 1626 1561 1623 1618 2437 1773 1635 2454 1730 1613 2462 2451 2450 1726 1595 2442 1728 2448 1617 2455 1608 2478 1734 1615 1580 1645 2452 2445 1647 2463 2441 1630 2370 2368 2378 1653 1733 2471 2342 2340 2457 2348 2354 Average

220 229 261 286 321 341 347 348 372 374 382 454 462 478 494 500 532 533 548 549 568 584 649 664 669 681 712 715 720 727 739 753 783 787 803 829 835 861 894 930 1029 1029 1034 1070

24°58.4 27040.0 25039.7 27°30.0 27002.3 25035.2 32020.8 28030.8 27°30.1 30°05.3 26040.0 28°26.6 27°21.5 27010.5 29°32.8 24°54.8 26°07.9 29043.3 27°01.0 27°02.2 27°32.5 26o12.5 31°08.0 30020.4 26o56.2 24010.0 29°20.6 27°24.5 26°38.8 29°41.5 28°33.3 26°00.0 28002.2 29°31.3 29o00.0 30°26.5 30°22.5 30°14.3 30004.0 31°00.0 31°29.0 27°52.6 29045.2 28°52.0

80°11.5 79°50.1 80002.7 79°50.0 79050.0 79059.8 78044.9 79052.0 79°03.5 80°04.2 79048.0 79°44.8 79°41.0 79039.8 80°00.0 80°11.5 79°19.4 79o57.8 79°15.8 79°39.6 78°44.5 79°46.0 79°07.5 79o44.0 79°36.7 81022.0 79°44.8 79°29.5 79°09.6 79°43.6 79°36.6 79°34.0 79°81.1 79°00.1 79001.2 78°06.0 79°26.5 79°39.0 79040.7 77°31.5 77°20.0 78°32.0 76o51.0 77°15.0

S

(S - - s )

523 320 399 249 194 223 435 473 668 173 742 421 376 330 524 207 283 252 260 615 511 945 135 383 290 231 273 334 206 211 928 274 285 215 293 144 515 200 590 103 179 440 472 256

523 218 356 248 175 200 298 468 432 168 655 413 359 319 472 194 261 239 231 560 482 829 135 347 135 231 261 243 123 208 788 97 207 214 293 144 499 184 579 101 177 429 471 255

61 10 23 13 20 36 32 19 43 19 41 20 38 30 53 27 28 26 27 37 37 44 24 39 35 22 32 35 22 31 58 24 33 37 28 34 51 40 49 32 32 43 47 35

51 8 15 9 15 28 21 17 37 14 33 19 31 20 40 22 20 18 22 29 29 36 18 32 19 14 21 28 13 21 50 17 22 28 21 22 37 26 42 16 23 31 37 25

365.5

323.2

33.3

24.7

No. of allochthonous species ? 2 19 1 10 16 17 4 17 5 28 7 8 8 18 7 7 8 8 19 3 18 0 10 34 0 7 12 11 2 35 18 21 1 0 0 5 7 5 2 2 4 1 1 9.5

92

sity of ostracodes. In general, valves are extremely abundant along the Florida--Hatteras slope and c o m m o n to often very rare in the Straits of Florida and on the Blake Plateau.

30 Sample Size

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Depth (meters) Fig. 2. Number of samples studied for each 100 m depth interval Samples deeper than 200 m are the t o p i c o f this paper.

large samples sizes (300 specimens) so that upper depth limits (UDL's) of deeper water taxa could be accurately determined. Fig. 2 shows the distribution of samples for 100 m depth intervals. Most of the distribution data described below and plotted on the distribution maps were derived from 44 samples having average sample size of 323 specimens. Table I summarizes the ostracode data from these 44 samples. When collected, samples intended for faunal studies were stored in 5% buffered seawater solution of formaldehyde and stained with Rose Bengal dye. Between 20 and 100 cm 3 of sediment were washed through a 63 micron sieve, dried and various fractions examined for ostracodes. Ostracodes were c o m m o n on the 40, 60, 80, and 100 size sieves and an attempt was made to obtain juveniles of large species and adult specimens of small taxa that are n o t often described from deeper water sediments. Because of the extreme variability in ostracode abundance, no a t t e m p t was made to determine the den-

The study area consists of the Florida-Hatteras slope, the Blake Plateau, and the Straits of Florida and includes samples from depths of 200 to 1070 m (Fig. 1). The shelf/ slope break is generally shallow and is less than 60 m off Georgia and about 10 m off southern Florida. A wide shelf exists in the northern part of the study region off Georgia but narrows considerably to less than 3 km off Key West, Florida. The slope o f f the southeastern U.S. is atypical in that it is split by the Blake Plateau, a broad, very gently inclined surface of pelagic sedimentation extending from depths of 350 to 1000 m. Landward of the Blake Plateau is the Florida-Hatteras slope which has gradients of 2 to 5 ° . Seaward of the Blake Plateau is the Blake Escarpment which extends to below 2000 m. Fig. 3 shows the b o t t o m temperature and oxygen content versus depth and the UDLs for 39 taxa. The study region is considered subtropical and transitional to tropical south of Miami and on the continental shelf, bottom-water temperatures range from 18 to 30°C during the warmest months and from 12.5 to 25°C during the coldest months (Walford and Wicklund, 1968). Below 150 m this region has a strong thermocline; b o t t o m temperatures decrease with depth until reaching less than 8°C below 750 m. Fig. 3 is a composite temperature profile obtained from National Oceanographic and Atmospheric Administration ( N O A A ) d a t a files. The location of the thermocline is extremely important in determining the upper depth limit (UDL) of deep water species (see Benson and Sylvester-BracUey, 1971). Another factor strongly influencing ostracode distributions is the oxygen minimum layer. Bubnov (1966) determined the con-

93

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centration of dissolved 02 and the depth of the 02 minimum layer in the Atlantic Ocean and indicated that it lies between 200 and 800 m and is characterized by O2 concentrations less than 3 ml/1 in the study area. For comparison, the lowest O2 concentrations occur off southwestern Africa and are sometimes below 0.5 ml/1. The depth of the O2 minimum layer gradually increases from the Equator to the subtropics, reaches its greatest depths about 3 0 ° N and S, and gradually rises towards the poles. Specific dissolved oxygen measurements obtained from NOAA files are plotted in Fig. 3 and generally support the results given by Bubnov. Milliman (1972) studied the petrology of the sand fraction of sediments from the same

samples that were studied in the present paper; Hathaway (1972) studied the clay mineralogy. Mflliman found that sediments in the Straits of Florida and the Blake Plateau were >95% carbonate sands and muds, primarily planktic foraminifers and pteropods, having some phosphate and glauconite. Sediments from the slope o f f southern Florida consist of 50--95% carbonate mud and silt. The slope between Cape Hatteras and northern Florida contains glauconitic sand in which glauconite makes up an average of 45% of the non,carbonate fraction. Doyle et al. (1979) found that the upper slope had a high sand size fraction compared to the slope north of Cape Hatteras due to the Florida Current and that grain size decreased

94

downslope. They also found that shelf spillover is an important depositional process on the slope off the southeastern U.S., an observation that has important implications for recognizing transported ostracode specimens.

% SHALLOW WATER SPECIMENS

Ostracode biostratonomy t

5 Biostratonomy is a subfield of t a p h o n o m y concerned with the fate of organisms after death and before or during burial. Thus, this is a biostratonomic study of ostracodes in which we are trying to distinguish allochthonous specimens (transported, reworked fossils) from autochthonous specimens (those that lived in the area) in order to establish relationships between ostracode taxa and environmental parameters. A twofold division of specimens in each sample was made -those that appeared to be transported and those that appeared to be indigenous. Table I gives the total n u m b e r of specimens per sample, the n u m b e r considered to be autochthonous, that is, indigenous to the sample location, and the number of species considered transported. Fig. 4 plots the percentages of transported specimens in 44 samples. Several criteria were used to determine if a specimen was transported. Carapace TABLE

<1

1-5 5 - 1 0 1 0 - 2 5 ,25

Fig. 4. Percent of shallow water specimens out of total specimens or each sample. These represent amount of downslope transport.

preservation was extremely useful in determining whether a specimen had been transported after death because the preservation state of ostracode carapaces and valves is variable. Table II summarizes the types of preservation states and t h e frequency of each. It is doubtful that the light pink Rose Bengal stain taken up by the valves of cer-

II

Ostracode preservation Preservation state

Frequency

Comments

1.

1 specimen/sample

Most

2. 3.

Carapace w i t h s o f t parts, s t a i n e d vivid p i n k Valve or c a r a p a c e w i t h seta Valve w i t h light p i n k s t a i n *~

4.

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5. Well p r e s e r v e d valves 6a. B r o k e n or a b r a i d e d valves

b. c.

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C o m m o n on Blake Plat. C o m m o n in some taxa on slope and Blake Plateau Moderately c o m m o n in all regions All samples Most c o m m o n on slope

C o m m o n o n Blake Plat. C o m m o n o n shallow water taxa transported downslope

common

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bairdiids

Krithe, Saida T a x a w i t h well d e v e l o p e d hinges Most c o m m o n p r e s e r v a t i o n s t a t e F r e q u e n t a m o n g t h i n shelled taxa s u c h as Pseudocythere, Paradoxostoma, o t h e r s

Paracypris, Pseudocythere

• 1 T h e significance o f p i n k s t a i n i n g o f o s t r a c o d e valves is n o t u n d e r s t o o d .

95

tain taxa signifies living specimens but this topic needs further investigation. Downslope transport of shallow water species is evident in many samples. Numerous studies of shelf ostracodes from the Atlantic and Gulf regions have given us a very good data base from which to infer transport (see Benson and Coleman, 1963; Van Morkhoven, 1972; Bold, 1977; Maddocks, 1978; Garbett and Maddocks, 1979; Cronin, 1979 and references therein). In addition to these published data, detailed study of ostracodes from the shallow-water samples indicate that clastic sediments of the shelf north of approximately Miami yield an ostracode fauna that is very distinct from carbonate assemblages off the Florida Keys, which show faunal similarities to other tropical carbonate assemblages such as those off Belize (Teeter, 1975). The presence of shallow-water shelf taxa and sometimes brackish-water species in deeper-water samples indicates transport, particularly when specimens are poorly preserved as described in Table II. The most difficult depth interval to judge whether specimens are transported is between 100 and 250 m because this is the area of the thermocline and the upperpart of the O2 minimum zone -- two major environmental boundaries that serve as barriers to shallow water taxa. The uppermost slope region is also very steep and is the site of very active sedimentary processes (Nardin et al., 1979) the effect of which is to mix shelf and upper slope faunas. Therefore estimates of the percent of downslope transport in this interval is necessarily subject to large errors and the lower depth limits (LDL's) of many shallow-water taxa remain unknown. An additional factor that helps demonstrate a species was indigenous is the presence of a complete population of adult valves and juvenile instars. The average percentage of species in each sample that had juvenile specimens was 72.3% for 44 samples in Table I. This simple measure of population structure indicates that most species are represented by adults and juveniles and which in turn suggests they are indigenous to the general area.

Fig. 4 shows zones of maximum transport of more than 10% of the total assemblage in the northern Straits of Florida, along the Florida--Hatteras slope off southern Florida, and in a single sample on the slope off Georgia. Moderate transport is in evidence off central Florida and the innermost Blake Plateau, whereas most of the Blake Plateau has fewer than 1% transported specimens. The overall pattern seems to show the strong influence of the Florida Current in transporting shallow-water shelf taxa from off southernmost Florida northward to the Miami area and somewhat less but still substantial downslope transport below the shelf/slope break throughout most of the region. Species diversity Species diversity of benthic marine organisms in deep-sea environments has received considerable attention ever since Hessler and Sanders (1967) discovered more species living in the deep oceans than previously suspected. Buzas and Gibson (1969) found peaks in benthic foraminifer species SPECIES

DIVERSITY

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Fig. 5. Ostracode species diversity. S = s i m p l e diversity, that is, the total number of species m i n u s transp o r t e d species• s is the number of species for which o n l y o n e s p e c i m e n was f o u n d • A. Species diversity versus s a m p l e size. B. Species diversity versus depth.

96 diversity at 35--45 m, 100--200 m and 2500 m off the eastern United States. Hazel {1975) found simple diversity, S {=number of species), for ostracodes to range from 8 to 81 on the continental shelf off North Carolina. Deeper-water environments (>500 m depth) have generally yielded ostracode species diversities about half those of continental shelf environments (Benson, 1972). Using the measure of diversity (S - - s ) in which the n u m b e r of species represented by a single specimen (s) is subtracted from the total n u m b e r of species (S), Benson (1983) found that most samples from depths > 5 0 0 m having at least 200 specimens contain 8 to 12 species. In the present study S and (S -- s) were plotted against sample size (Fig. 5A) and against depth (Fig. 5B) after transported species were subtracted from the total species list. The effect of obtaining larger sample size appears to be significant in obtaining higher diversity values as rare species are found more frequently in samples with more than 300 specimens (Fig. 5A). One interesting aspect of the data is the relatively high diversity encountered between 200 and 1100 m. Average S and ( S - - s ) for the 44 samples were 33.3 and 24.7 respectively (Table I). Once below the thermocline (about 150--200 m) there does not appear to be any relation between depth and diversity (Fig. 5B). Similarly, the oxygen minimum layer at depths of 200 to 750 m does not affect species diversity, although it does have an effect on which species inhabit certain depth intervals (see below). The areal distribution of species diversity shown in Fig. 6 shows a narrow zone of low diversity (<20 species) along the uppermost Florida--Hatteras slope in a region of glauconitic sand substrates. Just seaward of this zone slightly higher diversities (20--30 species) were found, except off central Florida where two samples had 53 and 58 species, respectively. Samples from most of the Blake Plateau and the Straits of Florida had 30 to 50 species per sample. Several factors might explain the relative-

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Eucytherura, Australoecia, Saida, and others were found. A third factor is the possible oversplitting of taxa while other ostracode workers may have adopted a more conservative approach and lumped taxa together. Finally, in a region of very low sedimentation rate such as the Blake Plateau, relict ostracode valves may be mixed with those recently living and this could increase apparent diversities. All these factors might suggest that diversity in other epibathyal zones are comparable to those off the southeastern U.S. and that the differences are an artifact of different sampling and processing procedures and philosophies of taxonomy. It is unlikely, however, that the taxonomic approaches differ so radically that they introduce diversities twice as high as is typical, especially because transported taxa were excluded from diversity measurements. Further (S -- s) values which omit species represented by only one specimen were surprisingly high. Although species level taxonomy is extremely difficult for many deep-water ostracodes, the electron photomicrographs given in this paper demonstrate morphotypes within genera that were consistently distinct from sample to sample and appear to represent separate species. The genera Ambocythere, Cytheropteron, and Pseudocythere are good examples. Further, the diversity values given may actually be biased towards the lower values because certain genera in which it is believed difficult to distinguish species on the basis of carapace features alone were not divided. For instance, Maddocks {1977) emphasized the difficulty of distinguishing species of Macrocypris on the basis of hard parts and, thus, only two species were counted in this study, although there are probably several others. Similar situations exist for Paranesidea, Paradoxostoma and the microcytherids. Breman (1978) studied ostracode species diversity in the Adriatic and attributed extxemely high diversities on parts of the continental slope at depths of 80 to 900 m to downslope transport of shallow-water species. Many of his species were obviously

shallow- or even brackish-water taxa and clearly allochthonous. Nevertheless, some of the diversity found by Breman may not have been a result of sedimentary processes but a real characteristic of the slope. The high diversities therefore, may well be real and an explanation might be found in the peculiar environmental conditions in the region. The region is a juncture between subtropical clastic and tropical carbonate shelf water environments with a sharp boundary occurring near Palm Beach, Florida. An analogous situation could exist in the bathyal environment where a mixing of Gulf of Mexico, Caribbean and North Atlantic Slope assemblages could occur, possibly resulting from the northward flow of the Florida Current through the study region. The zoogeography and taxonomy of bathyal ostracodes is not well enough known to determine if this is the case. Another factor might be the unusual morphology of the slope in the region in which the broad Blake Plateau interrupts the normal steep gradient of the slope and provides a large flat area at depths intermediate between continental shelves and abyssal environments. The Cenozoic history of the Blake Plateau consists of complex oscillations in the axis of the Gulf Stream in response to eustatic sea-level fluctuations (Pinet et al., 1982) and it is presently a region of erosion and scour {Pratt, 1968). It is possible that the frequent dynamic environmental fluctuations in this region have had an effect on increasing species diversity. Comparative studies of other epibathyal regions should help to resolve these questions. Distributions of ostracode genera and species Table III summarizes the depth distribution of 39 ostracode taxa in the study area and Fig. 3 shows the relationship of the UDL of these 39 taxa to 02 and bottom water temperature. Figs. 8--10 shows the percent of 20 of the most common species in 44 samples on depth profiles; Fig. 8 also shows the occurrence of five taxa that are very rare

98

TABLE

Ill

Depth ranges of 39 ostracode taxa off the Southeastern U.S. No. on Fig. 8 1.1

2 3 4 *] 5 .1 6 .1

7 8 .1

9 10 .1 11.1 12 .1 13 .1 14 15 16 17 18 .1 19 20 .1 21 22 .1 23 .1 24*2 25 26 27 .1 28 29 *~ 30 .1 31

32 33 .1 34 .1 35 . ! 36 37 38 .1 39 .1

Taxon

UDL (m)

LDL (m)

Number of samples .2

Cytherella sp. A Cytheropteron sp S Quasibuntonia sp. Argilloecia spp. Macrocypris spp. Pseudocythere spp. Cytheropteron sp. D and V Henryhowella asperrima ?R uggieriella sp. Saida sp. Bythocypris cf. reniformis Bythoceratina sp. A Krithe spp. Cytheropteron sp. C Cytherura sp. A Cytheropteron sp. talquinensis Loxoconcha sp. A Ambocythere spp. Loxoconcha sp. B Paranesidea sp. A

105 105 107 107 136 136 136 157 220 220 220 220 220 220 220 220 229 261 286 321 341 341 341 341 341 347 347 372 372 372 382 454 462 478 478 532 584 (382?) 584 669

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Cytheropteron sp. E Trachyleberidea pretiosa Bairdoppilata hirsuta "Thalassocythere" sp. Cytheropteron sp. B Cytheropteron sp. P Bythocypris cf. B. affinis Pedicythere spp. Polycope sp. A Polycope sp. B Ruggieriella sp. A ustraloecia sp. Cytherella sp. B Rockallia sp. Bradleya cf. B. dictyon Cytheropteron sp. R. Cytheropteron sp. Q Bythoceratl"na sp. B Anchistrocheles sp. A

• 2 Species for w h i c h % versus d e p t h profiles are given in Figs. 8--10. • 2 N u m b e r o f s a m p l e s in w h i c h t h e t a x o n was f o u n d o u t o f total 44 samples.

but which appear to characterize the oxygen minimum zone in this region. When this approach is used the relative abundance of each taxon can be examined across the oxygen minimum layer, the thermocline and on different sediment types so that the environmental preference of each taxon can

be determined for this relatively small region. Figs. 11--15 are maps of the areal distribution of many of these taxa; Plates I--X are scanning electron photomicrographs of many taxa. All photographs are lateral views o f adult single valves unless otherwise stated. Locality numbers are given after U.S. Na-

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tional Museum Numbers. Morphologic variability in deep sea ostracodes has made species level taxonomy difficult, particularly given the relatively sparse modem data base available compared with that for shelf assemblages. Although significant advances have been made in understanding the morphologic adaptations of the ostracode carapace design to deep sea environments (Benson, 1981), much less is known about why particular species or genera inhabit certain regions. Two prerequisites for understanding ostracode zoogeography are a modem distribution data base and corresponding environmental data and an established taxonomy for deep water ostracodes. Because neither is yet available for epibathyal ostracodes, many specific identifications in Figs. 8--15 and Plates I--X are considered tentative until additional material is studied from other continental slopes. Some of the

more important results are briefly summarized below. It must be emphasized that the results apply only to the study region and the association of certain taxa with particular environmental conditions or depths need not hold true along other continental margins. Fig. 8 shows that the genera Ambocythere, Saida, Trachyleberidea, Cytherella, "Thalassocythere, " Krithe, and Macrocypris are common in the oxygen minimum zone, although some are also found below this zone. Figs. 11 and 14 show that these taxa are generally common along the Florida--Hatteras slope and inner Blake Plateau. Fig. 15 shows the occurrence of three species of Ambocythere, a genus that has potential for detailed biostratigzaphic and paleoceanographic study along continental margins. The large numbers of Cytherella sp. A and sp. C in the uppermost bathyal zone contrast with the

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abundance of Cytherella sp. B in deeper water below the oxygen minimum zone (Figs. 9, 14). Quasibuntonia, RuggierieUa, and Australoecia occur in very low numbers but are typically found in the 02 minimum zone. The species of Australoecia has a particularly interesting anteroventral morphology consisting of a large denticulate process (Plate I, G and H) similar to, but more strongly developed than that developed on a species illustrated by Maddocks (1969, fig. 35J). The species of Ruggieriella, a genus

recently described by Colalongo and Pasini (1980), does not appear to be conspecific with their species R. decemcostata from the Plio-Pleistocene Vrica section in Calabria, Italy. At depths of 600 to 1100 m, certain deep water taxa appear in large numbers; they include Rockallia, Bradleya, Anchistrocheles, and Henryhowella (Figs. 9, 12). Other genera such as Bythocypris, Bairdoppilata, Pseudocythere, Argilloecia, and Paranesidea are some of the most common taxa found in the study

101

area and constitute significant proportions of most samples across the entire depth interval from 200 to 1100 m (Figs. 10, 13 and 14). A few specimens of Bythoceratina scaberrima, a typical deep water species (Benson and Sylvester-Bradley, 1971), were

found in sample 1580. It is unknown what environmental conditions might control the distribution of many of these taxa within the study area but the upper depth limit of many is probably controlled by the location of the thermocline (Fig. 3).

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PLATE

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PLATE I A--C. Bairdoppilata hirsuta Brady, 1880: A. male right valve, interior, USNM 363093, 1617, X 37.5; B. female? left valve, interior view, USNM 363094, 2450, × 41.3 ; C. female? left valve, USNM 363095, 1595. x 41.3. D--F. Paranesidea sp. A: D. female left valve, late instar, USNM 363096, 2450, x 45; E. female? right valve, interior view, USNM 363097, 2450, x 41.3; F. female right valve, USNM 363098, 1734, × 37.5. G--H. Australoecia sp.: G. female carapace, left lateral view, USNM 363099, 1726, x 180; H. male left valve, interior view, USNM 363100, 1617, × 150. P L A T E IV A, C, D. Macrocypris bathyalensis Hulings, 1967: A. female right valve, internal view, U S N M 363117, 1734, x 35.3; C. male left valve, U S N M 363118, 1617, x 45; D. female left valve, internal view, U S N M 363119, 1734, x 48.8. B, E, G. "Thalassocythere" sp. B. left valve, instar, USNM 363120, 1635, X 120. E. female right valve, internal view, USNM 363121, 1580, x 75. G. female right valve, USNM 363122, 1726, ×67.5. F. Henryhowella asperrima Reuss, female left valve, USNM 363123, 2368, x 75. H, I. Pedicythere: H. P. sp. A, female right valve, USNM 363124, 2463, x 202.5; I. P. sp. B, ?male right valve, USNM 363125, 2451, x 225. PLATE V A, B. RuggierieUa sp. A: A. female carapace, left lateral view, USNM 363126, 2348, × 165; B. female left valve, internal view, USNM 363127, 2348, x 150. C. Loxoconcha sp. B, female carapace, left lateral view, USNM 363128, 2340, x 135. D. ?Ruggieriella sp. female left valve, USNM 363129, 2463, × 150. E. Loxoconcha sp. A, female carapace left lateral view, USNM 363130, 1726, x 150. F--H. indet, gen.: F. ?female right valve, USNM 363131, 2463, × 180; G. ?female left valve, USNM 363132, 2463, x 165; H. ?female right valve, internal view, USNM 363133, 2463, × 165. PLATE VI A. ?Tubercuiocythere sp., ?female right valve, USNM 363134, 2457, × 202.5. B. Eucytherura sp., female left valve, USNM 363135, 2348, × 270. C. Typhioeucytherura sp., female left valve, USNM 363136, 2348, × 202.5. D. ?Eucytherura sp., ?female left valve, USNM 363137, 2348, × 202.5. E. Cytherella sp. B, female carapace, left lateral view, USNM 363138, 2368, × 67.5. F. ?Bythoceratina sp. F, ?female left valve, USNM 363139, 2457, × 135. G. Bythoceratina sp. A, female left valve, USNM 363140, 1608, x 135. H. Bythoceratina sp. B, female left valve, USNM 363141, 2370, x 112.5. PLATE VII A, C, E. RockaUia sp. A: A. female left valve, USNM 363142, 2348, x 135; C. female right valve, internal view, USNM 363143, 2348, X 135; E. ?female right valve, USNM 363144, 2348, x 135. B, D, F. Saida sp. A: B. female left valve, USNM 363145, 1608, x 150; D. female right valve, internal view, USNM 363146, 1608, x 150; F. ?female right valve, USNM 363147, 1608, x 150. G, I. Argilloecia sp. A: G. ?male right valve, USNM 363148, 1726, x 97.5; I. ?male carapace, left lateral view, USNM 363149, 1726, x 97.5. H, J. Argiiloecia sp. B: H. ?male carapace, left lateral view, USNM 363150, 1608, x 112.5; J. ?male right valve, USNM 363151, 1608, X 112.5. PLATE VIII A. Cytheropteron sp. R, female left valve, USNM 363152, 2340, x 165. B. Cytheropteron sp. P, female left valve, USNM 363153, 2457, × 135. C. Cytheropteron sp. S, ?female left valve, USNM 363154, 2340, × 165. D. Cytheropteron sp. Q, ?male left valve, late instar, USNM 363155, 2457, x 120. E. Cytheropteron sp. D, female left valve, USNM 363156, 2348, x 97.5. F. Cytheropteron sp. E, ?male right valve, USNM 363157, 2348, x 180. G. Cytheropteron sp. V, ?male left valve, USNM 363158, 2455, × 135. H. Cytheropteron sp. B, female left valve, USNM 363159, 2348, x 150.

108 PLATE II

A--C. Pseudocythere: A. P. sp. B, Female left valve, USNM 363101, 1608, x97.5; B. P. sp. A, male left valve, USNM 363102, 2471, × 120; C. P. sp. C, male left valve, USNM 363103, 2471, x 135. D. Indet. gen. left valve, USNM 363104, 2455, x 75. E--H. Bythocypris: ~ B. cf. B. affinis Brady, 1886. female right valve, USNM 363105, 1647, x67.5; F. B. cf. B. reniformis Brady, 1880. female carapace right lateral view, USNM 363106, 1643, × 48.8. G. B. cf. B. affinis, female right valve, internal view, USNM 363107, 1615, x 56.3; H. B. cf. B. reniformis, female left valve, internal view, USNM 363108, 1653, x 48.8.

109

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A. "Costa" beUipulex Levinson,1974, female right valve, late i n s t a l USNM 363109, 1595, x 90. B. T~achyleberidea pretiosa Levinson, 1974. female carapace, left lateral view, USNM 363110, 1595, x75. C, E. Anchistrocheles sp. A: C. male right valve, USNM 363111, 2354, x 90; E. male carapace, left lateral view, USNM 363112, 2354, X 82.5. D. Bradleya cf. B. dictyon Brady, 1880, male left valve, USNM 363113, 2368, x 75. F--H. Ambocythere: F. A. sp. C, male carapace, left lateral view, USNM 363114, 1608, x112.5; G. A. sp. A, male left valve, USNM 363115, 2471, x 120; H. A. sp. B, male carapace, left lateral view, USNM 363116, 2455, x 135.

110

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115 PLATE IX

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117

PLATE IX A. Cytheropteron sp. C, female left valve, USNM 363160, 1617, × 135. B. ?Bythocythere sp., male right valve, USNM 363161, 2354, × 97.5. C. ?Digmocythere sp., female left valve, USNM 363162, 2442, × 48.8 (specimen may be reworked from Tertiary deposits). D. Cytherura sp. A, male carapace, left lateral view, USNM 363163, 2340, × 165. E. ?Loxoconcha sp., female right valve, USNM 363164, 2448, × 97.5. F. Cytherura sp. B, female carapace, right lateral view, USNM 363165, × 180. G. Quasibuntonia sp., female carapace, left lateral view, USNM 363166, 1730, × 135. H. Eucythere sp., female left valve, USNM 363167, 2448, × 82.5.

PLATE X A, B, C. Krithe sp. C: A. female left valve, USNM 363168, 2348, ×67.5; B. female right valve, USNM 363169, 2354, × 60; C. female right valve, internal view, USNM 363170, 2354, × 67.5. D. Krithe cf. sp. C, male right valve USNM 363171, 2340, × 75. E. Parakrithe sp., female left valve, USNM 363172, 2463, × 135. F. Polycope sp. A, lateral view, USNM 363173, 2354, × 180. G, I. Krithe sp. A: G. female left valve, USNM 363174, 1595, × 75; I. female right valve, internal view, USNM 363175, 1595, × 75. H. Polycope sp. B, lateral view, USNM 363176, 2455, × 90.

Comparison with other areas Studies of m o d e m bathyal ostracodes are very rare and therefore the global or ocean wide zoogeographic distribution of typical slope taxa cannot y e t be determined. P e y p o u q u e t and Benson (1980) synthesized the relationship of ostracodes and depth and concluded that Krithinae with large vestibules,

Macrocypris, Bythocypris, Argilloecia, and Cytheropteron were markers for the epibathyal zone. In general, the Florida--Hatteras slope, Blake Plateau, Straits of Florida faunas include these taxa in large numbers b u t also many others. There is a strong similarity on a generic scale to the Pliocene and Pleistocene ostracodes from Calabria, Italy (Colalongo and Pasini, 1980), in sediments probably deposited at depths similar to those in the present study. It is particularly interesting to note the occurrence of Colalongo and Pasini's new genera Typhloeucytherura and Ruggieriella, and other genera that have n o t been c o m m o n l y reported in the literature. H o w e and Bold {1975) illustrated ostracodes from Mississippi mudlumps and some species such as their Buntonia n. sp., Ambocythere exilis, and Eucytherura sp. 1, m a y be

conspecific with those from the present study area. The fauna also contains the same species of Henryhowella, Trachyleberidea, Krithe, and "Costa" illustrated by Leroy and Levinson (1974) from Pleistocene deepwater deposits from the northern Gulf of Mexico. Deep-sea ostracodes have been studied from Cenozoic sediments from many parts of the worlds oceans (see Benson, 1979). Although a few genera from the present study are c o m m o n in deep-sea environments, many more are absent from the Blake Plateau at depths above 1100 m and their absence may have some significance. Some notable examples are Poseidonamicus,

Agrenocythere, Bathycythere, and Oblitacythereis. Further study from the Blake Escarpment and deeper waters is needed to determine the distribution of these and other taxa in regions adjacent to the Blake Plateau.

Acknowledgements M y sincere thanks go to J.E. Hazel (U.S. Geological Survey), R.H. Benson (SmithsonJan Institution, Washington, D.C.), R.F.

118

Maddocks (University of Houston), W.V. Sliter (U.S. Geological Survey), and E.M. Brouwers (U.S. Geological Survey) for helpful comments on the manuscript and discussion of bathyal ostracodes. T.G. Gibson and J. Hathaway (U.S. Geological Survey) provided assistance with the Woods Hole samples. E.E. Compton, S.L. King, L. Besse, C. Cecca and L.S. Weissleader assisted with sample processing and analyses. E.A. Martin typed the manuscript. References Benson, R.H., 1969. Preliminary report on the study of abyssal ostracodes. In: J.W. Neale (Editor), The Taxonomy, Morphology and Ecology of Recent Ostracoda. pp. 4 7 5 - 4 8 0 . Benson, R.H., 1972. The Bradleya problem, with descriptions of two new psychrospheric ostracode genera, Agrenocythere and Poseidonamicus (Ostracoda: Crustacea). Smithson. Contrib. Paleobiol., 1 2 : 1 3 8 pp. Benson, R.H., 1979. In search of lost oceans: a paradox in discovery. In: J. Gray and A.J. Boucot (Editors), Historical Biogeography, Plate Tectonics, and the Changing Environment. Oregon State University Press, Corvallis, Ore., pp. 379-389. Benson, R.H., 1981. Form, function, and architecture of ostracode shells. Ann. Rev. Earth Planet. Sci., 9 : 59--80. Benson, R.H., 1983. Estimating greater paleodepths with ostracodes, especially in past thermospheric oceans. Palaeogeograph., Palaeoclimatol., Palaeoecol. In press. Benson, R.H. and Coleman, G.L., 1963. Arthropoda, article 2, recent marine ostracodes from the eastern Gulf of Mexico. Univ. Kans. Paleontol. Contrib., p. 1--52. Benson, R.H. and Sylvester-Bradley, P.C., 1971. Deep-sea ostracodes and the transformation of ocean to sea in the Tethys. In: H.J. Oertli (Editor), Paleoecologie Ostracodes. Bulletin Centre Recherche PAU-SNPA, 5th Suppl., pp. 63--91. Bold, W.A. van den, 1977. Distribution of marine podocopid Ostracoda in the Gulf of Mexico and the Caribbean. In: H. Loftier and D.L. Danielopol (Editors), Aspects of Ecology and Zoogeography of Recent and Fossil Ostracoda. Proc. Int. Syrup. Ostracods, 6th. Saalfelden, pp. 175--186. Breman, E., 1978. The Distribution of Ostracodes in the Bottom Sediments of the Adriatic Sea. Ph.D. Thesis, University of Amsterdam, 165 pp. Bubnov, V.A., 1966. The distribution pattern of minimum oxygen concentrations in the Atlantic. Oceanology, 6 : 193--201.

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