A. CONRAD NEUMANN MAHLON M. BALL School of Marine And

A. CONRAD NEUMANN School of Marine and Atmospheric Sciences, University of Miami, Miami, MAHLON M. BALL Florida 33149

Submersible Observations in the

Straits of Florida: Geology and Bottom Currents

ABSTRACT An interesting finding of the dives in the Straits of Florida is the observation, based on The submarine slopes chat border the Straits both current measurements and sedimentary of Florida off Miami and Bimini were traversed structures, that the bottom current on the by the submersible Aluminaut, in August and western side of the Straits flows southward at September of 1967. The Bimini escarpment is observed velocities of 2 to 50 cm/sec. This characterized by a three-part zonation con- southerly bottom flow is opposite to either the sisting of relatively strong northerly bottom northerly Florida Current above or to the currents in both deep and shallow zones, and an bottom current on the Bahama side of the intermediate zone of low northerly bottom Straits. The nature and orientation of the sedi- current velocities. From 538 m (the bottom of mentary structures, plus the combined observa- the traverse) to 222 m, there is a sloping, tions of several Aluminaut dives in the same smooth, rock surface veneered with sand, rip- area, indicate that the southward bottom ple-marked by northward bottom currents of counterflow is persistent and not a temporary 50 cm/sec or more. The middle, low velocity tidal reversal. An extensive sedimentary anti- zone, from 222 m to 76 m, exhibits a muddy cline in the west-central sector of the Straits slope of largely bank-derived material. The may have been built by this bottom counter sediment surface exhibits tracks, burrows and current bringing material from the north. mounds, which indicates that currents here are never as strong as in the rippled zones above INTRODUCTION and below. Observed current velocities in the In August and September of 1967, three middle zone were only 5 to 10 cm/sec. Above dives by the DSRV Aluminaut were made in 76 m, a steep, vertical to overhanging cliff with the Straits of Florida off Gun Cay, Bimini, and large talus blocks at its base rises to a crest at Miami (Fig. 1). A later dive was made off 30 m. Currents in the upper zone are north- Miami to 290 m in May, 1969. The 1967 ward at 50 to 150 cm/sec. The inverted situa- Bimini and Miami dives lasted about 10 hrs tion of higher energy bottom current condi- each and traversed the escarpments at the tions and associated sedimentary features and eastern and western margins of the Straits textures existing in the same area, but at a (Figs. 2 and 3). Observations and collections greater depth than low energy surface features were made of rock and sediments, and biologi- and fine sediments, is of significance to the cal and sedimentary processes were noted. Sedi- stratigraphic interpretation of ancient rocks. ment samples were taken with small flap- On the western side of the Straits, at the base fronted scoops. Bottom currents were measured of the Miami Terrace, is an elongate trough by timing the passage of dye or free-floating 825 m deep. The bottom here is characterized detritus past aim bar placed on the bottom by ridges and mounds of muddy sand capped by and oriented parallel with the current. Four to thickets of living deep-water branching coral. eight current measurements were made over a The eastward-facing escarpment of the Miami 10 to 20 minute interval at each locality. Terrace exhibits ledge-like outcroppings of Gravity measurements were made with two dark phosphatic limestone from depths of 719 Worden gravimeters; these data and their m to the crest at 457 m where the traverse geophysical significance have been reported ended. elsewhere (Ball and others, 1968).

Geological Society of America Bulletin, v. 81, p. 2861-2874, 9 figs., October 1970 2861

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Figure 1. Map of Straits of Florida showing location of 1967 Aluminum dives. The May 1969 dive was at 25°38.6' N./80°01.7' W., or near the "V" in "DIVE" on the left side of the Figure. "D" and "U" refer to the down and up locations, respectively. Among the observations of greatest oceano- Previously published observations of sub- graphic and geologic interest are the following: mersible dives in or near the Straits of Florida In the western Straits off Miami, there exists include a general narrative of an Aluminaut beneath the northward Florida Current a deep dive off Miami in November, 1965 (Neumann counter-flow to the south. On the Bahama side and Hull, 1966), and another in which the value off Bimini, the muddy talus sediments from of the submersible as a scientific instrument is the banktop appear to be restricted to a narrow discussed (Neumann, 1968). Emery and others zone of low current velocity at intermediate (1970) have described the observations of a 4.5 depths and do not accumulate at the base of the km traverse in depths of 32 to 165 m off West slope as the topography suggests. The base of Palm Beach made aboard the submersible Ben the slope exhibits, instead, a smooth, hard, bed- Franklin. A subsequent dive on the Miami rock surface, over which a veneer of rippled Terrace off Miami has been described by sand is moving northward under the influence Ballard and others (in prep.). of relatively strong bottom currents. A variety The physiography of the cliffs bordering the of sedimentary structures usually associated Tongue of the Ocean off Nassau and the nature with shallow water characterizes this thin, deep, of the sediments and rocks exposed there have sand blanket between 215 and 540m (Neumann been described by Gibson and Schlee (1967) and Ball, 1968). Thus, in making stratigraphic from the DSRV Alvin. Submarine lithification paleoenvironmental interpretation of the an- of slope sediments and subsequent fracture and cient counterpart of this sediment distribution, downfaulting were recognized as the processes it would be hazardous to assume that deeper responsible for the fact that the rocks that environments of deposition are usually charac- veneer the slope are younger than those at terized by relatively lower energy conditions. similar depths from the platform interior. The

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(50-150 cm/sec) 7%'9'" WEST CURRENT /TNNORTH EAST U7 ^, ^ ^ BIMINI SHORT TERRACES ^ ~j Ft.

LARGE ROCKS FEW STREAKS NO RIPPLES BURROWS AND MOUNDS TRACKS AND TRAILS

I- 1200 RIPPLES STARVED RIPPLES CURRENT CRESCENTS "STREAKS" SAND RIDGES 3-5' HIGH NAUTICAL MILE

25'46'N 79'23'W Figure 2. Schematic profile of submersible dive traverse off Bimini, Bahamas, indicating major observations. Bottom current direction is north.

steep to overhanging, cave-studded cliffs of the shows a bimodal distribution of particle sizes upper 200 m off the east coast of Andros Island with peaks in the 250 to 62 micron and 8 to 4 have been described by Busby and others micron ranges. The coarse and medium sand (1966) from the DSRV Perry Cubmarine PC3- fractions are composed largely of planktonic B. pteropod and Foraminifera remains. The presence of calcareous algae, shallow benthonic OBSERVATIONS Foraminifera, and pelecypod fragments indicates some contribution from shallow bank- Gun Cay Dive top environments. Sedimentary structures During the dive off Gun Cay, Bahamas (Fig. called "current crescents" (Potter and Petti- 1), the density of suspended organic matter ap- john, 1963) were developed at the bases of peared to be high and increased with depth. obstacles, such as attached sponges and heavy Diving through these suspended aggregates fragments of deep water corals. Fragments of gave the impression of gliding in a snowstorm. Thalassia, a marine grass, were common and Visibility was so reduced that the bottom at drifted over the bottom in the current. The 800 m was not apparent until it was 6.1 m loose sand of the streaks was not moving at the (20 ft) away from the lower viewing port. The time of observation, despite a northward cur- bottom appeared flat, and exhibited a striped rent measured at 20 cm/sec (0.4 kts). appearance because of the north-south streaking out of a thin cover of mobile, dark colored Bimini Dive material over a firmer, finer, lighter colored The major observations of the Bimini dive substrate. Size analysis of a scooped sample traverse are summarized in Figure 2. The over-

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MIAMI 7 SEPT 67 Ft. M TERRACE 1000- "ALUMINAUT" DIVE -300 WESTERN STRAITS OF FLA. 1200 -400 1400-

VERTICAL ROCK LEDGES 1600- WALL BROKEN SLABS ON TOP STEEP 8 -500 ROCKY 525m. TRENCH - LIKE 1800 SILTY SED. AT \O BURROWS STEEP SLOPE CURRENT CRESCENTS -600 2000 RIPPLES COMMON 'AND STREAKS" ALL SOUTH SMALL DARK |DETRITUS MOVING SOUTH NODULES 2200- FIRST RIPPLES ALL FACE SOUTH -700 2400- ROCK BEGINS LARGE SLABS AND LEDGES 2600- SERIES OF MOUNDS -8OO CORAL THICKET ON CRESTS 2800- NAUTICAL MILES Figure 3. Schematic profile of submersible dive traverse off Miami, Florida, indicating major observations. Bottom current direction is south.

all concentration of suspended matter, or of lithified tubes resembling clay pipestems "snow," was lower than on the Gun Cay dive; were noted above 411 m. The mineralogy of the bottom came into view at a distance of 21.4 the sandy sediment—determined by X-ray dif- m (70 ft), in contrast to 6.1 m (20 ft) off Gun fraction peak height ratios—is aragonite, calcite Cay. The largest suspended aggregate particles and magnesian calcite in about equal pro- (10 to 15 cm in diameter) were noted on this portions. descent. The bottom, between 538 m and 222 The bottom current direction along the m, appeared to be a smooth rock surface, entire Bimini slope was to the north. At 538 m sloping about 5°, and covered with a thin it was 7.7 cm/sec (0.15 kt); at 366 m it was 20 veneer of rippled sand, which in places built to 25 cm/sec (0.5 kt) and reached a maximum up to sand waves 1 to 2 m high. Attempts to at 305 m where it was estimated at 50 cm/sec sample the flat rock surface with the pincer- (1.0 kt). At this velocity the sand on the east- like manipulators were unsuccessful; there were west-oriented ripple crests was moving. no loose fragments or sharp irregularities that The common benthic macro-organisms of could be gripped. Successful sampling would this deeper current-swept zone included numer- require a rock drill. Asymmetric ripples of 10 ous white fan-shaped sponges (Fig. 4) and cm wave length, starved ripples, sand waves, many echinoderms. Broken sand urchin tests current crescents and streaking characterized littered the bottom in some areas. Living sea the sand surface (Fig. 4). These sands are com- urchins and starfish were most commonly ob- posed of fragments of pteropods, Foraminifera, served at the bases of the sponges which pelecypods, gastropods, calcareous algae (Hali- probably served as protection from the sweep meda) and compound grains, including rock of the bottom currents. In several instances fragments, lightly cemented aggregate grains of where current velocities were high, the "paddle foram-pteropod sand, plus internal casts of spine" or cidaroid sea urchin was observed with pteropods and Foraminifera. Larger fragments its flat spines thrust downward into the sedi-

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Figure 4. Thin veneer of rippled sand over smooth rock surface at 538 m in Straits of Florida off Bimini, Ba- hamas. Ripple crests are 10 cm apart. Fan-shaped sponges, about 20 cm high, are attached to rock surface; note current crescents and sand shadows at base. Current is toward north (left) at 8 cm/sec.

ment, thus serving as the flukes of an anchor shallow current-swept bottom than that com- and preventing the organism from otherwise monly expected for a deep-sea environment being overturned by the current. Fragments of (Fig. 4). shallow water "turtle grass" (Thalassia) The submersible can add another dimension drifted over the bottom.1 Grass blades were to biological observation. Dredge sampling also observed entangled at the base of small from the surface would probably miss the small stalk-like organisms of unknown origin which stalked tufts attached to bedrock, and it would appeared to be attached to the rock surface. not have shown the interrelationship of This situation is responsible for the dark organisms such as the starfish and urchins at the clusters seen in the background of Figure 4. In base of the sponges, nor could it reveal the general, the lower zone, from 538 to 222 m, interaction of organism and environment such exhibited more of the surficial features of a as the cidaroid's use of its flat spines as anchors in the sand. 1 Menzies and others (1967) have noted detritus of The origin of the smooth, featureless rock shallow water "turtle grass" at bathyal and abyssal surface from 538 m to 222 m is difficult to depths off Florida and North Carolina and have dis- interpret. The relatively strong northerly cur- cussed its significance as a food source to deep-sea rents on the eastern side, plus the thin veneer benthic organisms. of rippled sediment moving north, would tend

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to suggest a scoured surface, yet it is doubtful porarily reducing visibility. Suspecting a sub- that the present sandy veneer, much of which marine spring, we sampled the water, but later is fragile tests of pteropods and planktonic analysis showed it to be of normal marine Foraminifera, could abrade a limestone surface salinity, suggesting that the density difference which was too hard for the manipulators to causing the "schlieren" effect was more likely penetrate. due to mixing of marine waters of different Above the zone of rippled sand and rela- temperature; perhaps cooler sea water was tively vigorous northerly currents, a middle seeping from the rocks. depth zone of low current velocities was en- From 76 to 30 m rose a vertical to over- countered from 222 to 76 m. Here, the bedrock hanging cliff 46 m (150 ft) high. No continuous surface is buried by a deposit which grades from notches or undercuts were apparent. It was not muddy sand at 222 m to a mud-filled breccia of possible to approach the cliffs too closely in the large blocks near the base of the vertical cliff at submersible because the strong northerly cur- 76 m. In this zone, especially on the lower rents (estimated at 3 kts) near the surface made portion of the slope from 222 to 152 m, current- maneuvering difficult. After passing over the formed structures are only rarely observed; in- cliff crest at 30 m we rose to the surface, stead, pits, mounds, trails and burrows of terminating the dive off Bimini. organisms characterize the sediment surface. A sample taken at 213 m is muddy sand (45 per- Miami Terrace Escarpment cent <62 n), which is mainly aragonite and The generalized traverse and major observa- magnesian calcite with minor amounts of cal- tions of the dive from the bottom of the cite. This mineralogy is characteristic of shal- elongate depression at the base of the Miami low-water-derived carbonate sediments (Chave, Terrace and up the scarp are summarized in 1962). The constituent composition of the Figure 3. sand fraction also reflects a higher percentage of During the descent off Miami it was noted possible shallow water components (that is, that the water had a yellowish tint, in contrast rock fragments, algae, pelecypods, peneroplid to the clearer blue water on the Bahama side. Foraminifera) and a smaller percentage of The density of marine snow was high and in- pelagic components than that of the deeper creased with depth. Because of the reduced zone of rippled sand and higher current visibility, the bottom came into view at a velocities. distance of 9.1 m (30 ft), as compared to 70 ft Between 222 and 183 m, cemented tube-like off Bimini. The bottom was reached at a depth casts were observed embedded in the sediment of 825 m (2708 ft) in an elongate depression at and oriented vertically, as if in situ. They appear the foot of the Miami Terrace (Hurley and to be the lithified burrow linings of a tube- others, 1962; Kofoed and Malloy, 1965). The building organism. The mineralogy of a eastern scarp of the Miami Terrace has a com- sampled tube is aragonite and magnesian cal- pletely different appearance than the slope on cite, as is the unconsolidated sediment in the Bimini side of the Straits of Florida. To a which it was embedded. depth of 719 m, the bottom was characterized Currents in this intermediate zone were evi- by mounds of muddy sand 0.5 m high and 3 to denced only by the slow drift of suspended 4 m long which were capped by thickets of particles past the viewing ports. Current ve- branching deepwater ahermatypic corals. The locity was estimated to be about one-tenth of a genera Lophelia, Madrepora and Dendrophylha knot (5 cm/sec) northward. The finer sediment have been described from this general area or texture, the tracked and trailed surface, and environment, or both (Teichert, 1958; Stetson the relative scarcity of current-generated and others, 1962; Zarudzki and Uchupi, 1968). structures, suggest that low current velocity in Sediment infilled the coral clusters (Fig. 5). this region is a permanent situation. The fine Proceeding westward up the slope, the sand mud and sand from the bank above accumu- ridges exhibited a north-south orientation and lates in this intermediate zone; the talus depos- some were 12 to 15 m high. Coral thickets also it also accumulates here rather than at depths capped their crests. The origin of these larger below 222 m where a veneer of rippled sand is sand ridges is unknown. They may be a result moved northward over a smooth rock surface of bottom currents and the trapping effect of by stronger bottom currents. the coral communities (Neumann and Hull, At a depth of 99 m, near the top of the talus 1966). slope, the water appeared to shimmer, tem- Bedrock exposure began at 719 m (Fig. 6).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/10/2861/3432638/i0016-7606-81-10-2861.pdf by guest on 29 September 2021 Figure 5. Small mounds of muddy sand capped with thickets of branching coral at a depth of 825 m in the western Straits of Florida off Miami. The larger white objects are sponges about 20 cm high.

Figure 6. Ledge-like outcroppings of dark limestone at a depth of 533 m in the Straits of Florida off Miami. Ledge in background is about 1.5 m high.

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Rock ledges varied in height from several cm faint north-south streaking of the sediment to 15 m or more. The rock surface was dark surface was frequently noted from 825 to 680 colored and a thin cap often protruded at the m. Above this depth the current lineations be- top of the rock ledges. This suggested that the came more pronounced, and definite leeside exposed strata were capped with a crust rela- tails, sand shadows, scour marks and current tively more resistant to submarine erosion than crescents were observed. At 661 m, the first the material beneath. Previous analysis of patches of ripple-marked bottom were ob- dredged samples from this area indicate that served. The asymmetry of the ripples indicated the rock is a phosphatic limestone of Miocene a southerly flow. Areas of sand rippled by age capped with a manganic alteration rind southerly currents were seen frequently above (Bush, 1951; Milligan, 1962). It often appeared this depth (Fig. 7). At 649 m and above, as if the surface layer had fractured in place, particles of larger organic detritus, such as leaving an irregular polygonal joint pattern blades of Thalassia, were observed moving separating the large slab-like pieces. Mounds of southward along the bottom. From 600 m to muddy sand capped with branching coral and 550 m, several long north-south-trending sand coral debris covered the ledges. Above 652 m, ridges were traversed. Southward-facing asym- small smooth black nodules, the size of stove metric ripple marks were superimposed on the coal, littered the surface and filled the surface ridges and a few steep southward-facing fractures, producing a superficial similarity to crescentic sandy slopes about 0.3 to 1 m high gravel polygons of patterned ground seen in were also observed (Fig. 8). The sedimentary high latitudes. This feature was also noted structures everywhere along this traverse in- during an Alvin dive on the Blake Plateau dicated that they were the product of southerly (Milliman and others, 1967). bottom flow, and although they were not under The ledge-like morphology of bedded lime- active formation at the time of observation, stone, topped by dark crust broken into large their widespread occurrence in the western slab-like fragments, continued to a depth of straits did suggest that the dominant bottom 457 m, whereupon we went over a very sharp flow here is to the south, at least below 450 m crest and down a near vertical west-facing wall where the dive terminated at the seaward edge into a depression 67 m deep. The fine mud at of the Terrace. the bottom was generally featureless and there Aside from the branching coral, organisms of was no evidence of moving water. Large, dark, many kinds were observed along the Miami slab-like boulders 3 to 4 m across were piled Terrace traverse. Numerous large crabs, prob- and wedged against each other and made up the ably of the genus Geryon, were seen occupying material of the near vertical wall. Retracing our shallow elongate trenches (about 1 m by 30 cm course up the rocky wall, we again passed the and 15 cm deep), at the ends of which were sharp crest at 457 m and returned to the sur- piled excavated material. Fan-shaped sponges face. The morphology of the outer edge of the of the type seen off Bimini were also numerous. Miami Terrace, where this dive terminated, is The variety and density of benthic organisms described in more detail by Ballard and others and fish appeared greater here than off Bimini. (in prep.). A detailed biological survey by submersible The bottom currents and associated sedi- would be needed to describe this complex mentary features constitute the phenomena of community and its interrelationships more greatest impact and interest along this traverse fully, but special sampling techniques and tools of the Miami Terrace's seaward escarpment. for submersible work would first have to be From the very base of the slope at 825 m, to developed. Off Bimini we tried to sample one the crest of the ridge at 457 m, the bottom of the flat sponges but met with defeat when currents were everywhere observed to be the fragile organism was completely fragmented flowing south, opposite to the Florida Current by the manipulators. above. Current measurements were made at Observations of More Recent Dives on the bottom depths of 825 m, 620 m, and 466 m; Miami Terrace. On 25 May, 1969, dive 233 of the flow direction was south and the velocities the DSRV Aluminaut bottomed at 25°36.8' were 2.1 cm/sec, 5.4 cm/sec, and 5.8 cm/sec, N./80°01.7' W. at 290 m, or 158 fathoms (see respectively (Fig. 2). Fig. 1). This position is at about the same The sediments indicated that southerly flow latitude as the 1967 traverse but near the in- is the rule rather than the exception all along shore or western edge of the Miami Terrace. this seaward slope of the Miami Terrace. A While on the bottom, from 1800 hrs to 2100

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/10/2861/3432638/i0016-7606-81-10-2861.pdf by guest on 29 September 2021 Figure 7. Ripple marks at a depth of 661 m in the western Straits of Florida off Miami. The short side of the ripples face south (left) indicating formation by a southward bottom current counter to the northward-flowing Florida Current above. The crests of the ripples are about 10 cm apart.

• f'gre 8' .S°ut,h'facinS «««ntic sandy slope about 0.5 m high at a depth of 595 m off Miami. Bottom current is to the south (lett).

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hrs, a steady southward bottom current was of the distance across the Straits. Bottom observed; its velocity was estimated at 8 to photographs of asymmetric ripple marks 10 cm/sec (about 0.2 kts). The bottom was formed by a southerly bottom current of an nearly flat and the sediments were composed of estimated 10 to 30 cm/sec were reported in what appeared to be a light gray clay or silt. 1963 by Hurley and Fink at 24°56' N./79°34' Mounds, as well as holes about 0.5 m deep, W. on the flat east-central floor of the Straits were frequent and appeared to be the work of 93 km south-southeast of the Miami Terrace large white crabs which were observed rework- scarp dive traverse reported here (see Fig. 1). ing the sediments. No ripple marks were The bottom counter current could extend for a reported.2 considerable distance eastward from the area On 13 and 14 February, 1970, dive 49 of the where the Aluminaut observations were made Grumman Aerospace submersible Ben Franklin on the western side of the Straits. If this is so, touched bottom in 356 m at 25°32.5' N./ the region of southerly bottom flow spans a 79°57.7' W. and traversed the top of the Ter- large north-south-oriented elongate mound of race in a northeasterly direction, reaching an unconsolidated sediment in the west-central easternmost excursion point of 25°38.1' N./ Straits recently described by Malloy (1968) as 79°55.4' W. at a depth of 434 m. At the point a "depositional anticline." The mound could where the dive reached bottom a current was be the depositional work of the southerly bot- measured at 0.35 kts (17 cm/sec) to the north. tom flow. Trace element distributions (Gassa- Northerly currents and associated sedimentary way, in prep.) and seismic profiles (Uchupi, features were observed throughout the north- 1966) have been interpreted to indicate bottom easterly course of the dive across the Terrace. sediment transport in the Straits in a southerly Neumann accompanied the dive as a guest direction. General sediment distribution in the observer. The detailed results are presented by Straits, based on a few samples, has also been Ballard and others (in prep.). interpreted as indicating a possible southerly transport direction (Gorsline, 1963). A con- DISCUSSION structional origin similar to that of a sea channel levee has been suggested for the deposi- The observed bottom current regimes and tional anticline in question (Malloy, 1968), the the sedimentary structures associated with channel in this case being the closed elongate them bear significantly upon our knowledge of depression at the base of the Miami Terrace. the Straits of Florida. In the latitude of Miami- Coarse material observed in bottom photos of Bimini, there appear to be four broad areas, the floor of this depression is interpreted as lag each characterized by a bottom flow opposite gravel and cited as evidence of stronger flow to the area adjacent. The observation by concentrated in the depression (Malloy, 1968). Bonatti (dive 233 of Aluminaut described Our observations in the same place indicate above), plus other studies (Broida, 1969), in- that this coarse material is a local product dicate a southerly and perhaps tidally in- formed in situ by the breakup of the deep- fluenced bottom flow along the shelf, the water branching coral, and that the material slope, and onto the western edge of the Miami between the fragments appears as fine or finer Terrace. The Ben Franklin observations (Bal- than elsewhere in the Straits. The measured lard and others, in prep.) record a northerly current flow in the depression, plus the dis- bottom flow along the top of the Miami Ter- tribution of current-produced sedimentary race extending out to its seaward or eastern structures, indicate that the elongate feature is edge. The Aluminaut traverses described here not a focus of stronger southerly flow but, if indicate a northerly flow along the Bahama anything, one of reduced flow. The deposi- side of the Straits, but a southerly bottom flow tional anticline in question could be building on the Florida side from the eastern edge of the and prograding southward under the influence Miami Terrace to at least the bottom of the of a broad southerly bottom flow regime ex- trench at its base. tending at least across the western half of the How far this deep southerly counterflow Straits. The elongate depression between the extends eastward out into the open Straits is Miami Terrace and the depositional mound not yet determined. It may extend two-thirds does appear to be a negative feature more by virtue of non-deposition than any obvious 2 Dr. Enrico Bonatti, University of Miami, supplied erosional process that we could observe from the information and observations of the May, 1969 dive. the submersible. Uchupi (1966) also questioned

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the erosional origin of this feature on the basis OBSERVATIONS OF SOUTHERLY BOTTOM FLOW IN THE WESTERN STRAITS OF FLORIDA RELATIVE TO THE MIAMI-BIMIN! SEMI- of subbottom seismic profiling data. DIURNAL TIDE The southerly bottom current exists over a BAR LENGTH IS DURATION OF OBSER\ 41 depth range of at least 535 m and a horizontal CIRCLENUMBERS IREPRESENS DIVE NUMBET MEASUREMENTR S distance of perhaps 100 km. Paleocurrent analysis of the ancient counterpart of rippled ^^^m52 sediments of the Straits of Florida would in- 233 dicate a wide southerly current on the west side interrupted by a narrow band of northerly flow over the Miami Terrace. On the east, or Bahamian side, there would be another relatively narrow band of strongly rippled sand LOW with sedimentary features, such as current 0 2 4 6 8 10 12 crescents, indicating a shallow water environ- HOURS ment of deposition. In this segment of the GENERALIZED SEMI-DIURNAL TIDE, MIAMI HARBOR ENTRANCE future geologic record, the Gulf Stream might Figure 9. Time and duration of submersible observations of southerly bottom flow in the western Straits, well escape unnoticed, or at best appear as two of Florida shown in relation to the corresponding state narrow bands of shallow northerly flow within of the semi-diurnal tide at Miami and Bimini. The ob- a broader region marked by southerly paleo- servations completely span the semi-diurnal tidal cycle current direction. indicating the bottom flow is not a tidal reversal. The Aside from geologic observations, oceano- dive numbers are those listed in Table 1. graphic data, including both dynamic considerations and direct measurements made by bottom currents and, in each case, the current free-fall instruments between Miami and direction was to the south (Table 1). Bimini, have also indicated a deep southerly In order to test whether the soutberly direc- flow, but these data have been interpreted as tion of the bottom flow in this region was temporary tidal reversals (Richardson and periodic or permanent, the time and duration Schmitz, 1965; Richardson and others, 1969; of each submersible observation of southerly Broida, 1969). Several Aluminaut dives have bottom flow was plotted relative to the cor- been made in the vicinity of the depression at responding stage of the semi-diurnal tide at the base of the Miami Terrace in the western Miami and Bimini (see Fig. 9). The observa- Straits. Examination of the logs of these dives tions span the 12 hr cycle completely, in- and communication with the dive participants dicating that the southerly flow is not a semi- have revealed that most of the dives did note diurnal tidal reversal. TABLE 1. COMPILATION OF SUBMERSIBLE (Aluminaut) OBSERVATIONS OF BOTTOM CURRENTS IN WESTERN STRAITS OF FLORIDA FROM 1965 TO 1969 Current Time of Tide at Dive Bottom Current Velocity Type of Observation Miami No. Date Depth (m) Position Dir. cm/sec Observation EST EST 41 9 Nov. 65 550 25°50' N. S. 25 Estimated 1145-1235 0800 H 79°48' W. drift time 1407 L 42 9 Nov. 65 763 25°49' N. S. 50 Estimated 1530-1630 0800 H 79°47' W. motor speed 1407 L 44 19 Nov. 65 763 25°50' N. S. 20 Estimated 0905-1027 0458 H 79°49' W. sed. moving 1109 L 46 19 Nov. 65 763 25°56' N. S. 5-10 Estimated 1730-1930 1713 H 79°49' W. sed. moving 2332 L 52 19 Jan. 66 763 25°53.5' N. SSW. 17 Estimated 1200-1805 1254 H 79°48' W. 1848 L 169 7 Sept. 67 826 25°37' N. S. 2 Measured 1830 1618 L 702 79°50' W. S. Estimated 2042 696 S. slow Estimated 2100 651 S. slow Estimated 2238 2224 H 620 25°36' N. SE. 5 Measured 2322 467 79°54' W. S. 6 Measured 0204 0439 L 233 24 May 69 290 25°39' N. S. 6-10 Estimated 1900-2200- 2012 L 80°02' W.

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Recent considerations of transport in the 1967). The exact nature of the flow regime in Straits (Schmitz and Richardson, 1968) have the Straits of Florida remains unclear, but the revealed a strong diurnal component, however, collective submersible observations do point and because there are no tables of predicted out that it is not entirely a northerly flow with diurnal tides for the Straits, a plot similar to superimposed tidal oscillations but that per- Figure 9 cannot be constructed to test the manent southerly elements do exist. relationship between the observation of south- Submersibles present the total subsea en- erly flow and diurnal tidal behavior. It would vironment to the observer in a way that cannot be quite fortuitous, however, if these same ob- be matched by the specific data access approach servations that span a complete 12 hr semi- of surface ships. They are particularly valuable diurnal cycle would all happen to fall on one 12 for initial surveys to acquaint investigators hr limb of a 24 hr diurnal cycle. with the over-all setting of the study area and The southward-oriented sedimentary struc- to bring out unforeseen features and processes. tures, plus the observation of both surface They are also valuable for specific detailed ships and submersibles, provide sufficient data studies and for the operation of in situ experi- to argue that the southerly bottom flow ob- ments. Their employment in oceanographic served in a variety of ways over a considerable research will widen as a supporting tool lateral, vertical, and temporal range in the technology develops, but this in turn must be region of the western Straits off Miami is a generated through increased use. The dive permanent situation and was not happened series reported here, as limited as it is, has, upon in each case during one similar phase of a nevertheless, indicated the value of submersible reversing tidal oscillation. The sedimentary techniques in oceanography by providing data structures themselves constitute a record of a of geophysical, geological, biological and physi- dominant southerly bottom flow with no clear cal oceanographic significance. indications of temporarily reversed flow. A recent detailed survey of transport in the ACKNOWLEDGMENTS Straits by free fall instruments indicates that We are grateful to Reynolds Submarine the northerly flow of the Florida Current Services, Inc., the operators of the Aluminaut, extends to the bottom in the latitude of Fort for their cooperation and expertise. We thank Pierce, north of Miami, and that the southerly Dr. E. Bonatti for the observations of the flow observed off Miami and described here May, 1969 Aluminaut dive, and Robert Ballard could not be a significant factor in the dominant for the data from the 1970 Ben Franklin dives northern transport of water in the Straits of on the Miami Terrace. Doctors S. Broida and Florida (Richardson and others, 1969). If this R. Hurley (University of Miami School of is the case, and if this bottom counterflow does Marine and Atmospheric Sciences), W. J. not extend beyond Fort Pierce, then these Richardson (Nova University), R. Malloy and submersible observations could be interpreted R. Dietz (ESSA) and K. O. Emery and R. to mean that the southerly bottom flow of the Ballard (Woods Hole Oceanographic Institu- west-central Straits of Florida constitutes the tion) read and discussed the manuscript. J. western limb of a large and continuous counter- Neumann provided valuable editorial services. clockwise gyre operating within the confines of National Science Foundation Grant GA-1222 the Straits in the general latitude of Miami. supported the work. The floor of the Straits rises northward to a sill crest of about 720 m at the latitude of 27° N. REFERENCES CITED (Hurley and others, 1962). It has been sug- Ball, M. M., Supko, P. R., and Neumann, A. C., gested (S. Broida, 1970, oral commun.) that a 1968, Gravity measurements on the floor of deep gyre might result south of this sill, in the Florida Straits (abs.): Am. Geophys. Union some way due to the volume adjustments Trans., v. 49, p. 196. caused by restricted northward flow across this Broida, S., 1969, Geostrophy and direct measure- sill. The situation is complicated by the fact ments in the Straits of Florida: Jour. Marine that the Northwest Providence Channel enters Research, v. 27, p. 278-292. Busby, R. F., Bright, C. V., and Pruna, A., 1966, the Straits between the latitude of the sill and Ocean bottom reconnaissance off the east the position of the observed southerly bottom coast of Andros Island, Bahamas: U.S. Naval flow. The net flow in the Northwest Providence Oceanog. Office Tech. Rept. TR-189, p. 58. Channel is to the west and thus adds to the Bush, J., 1951, Rock from Straits of Florida: Geol. Florida Current (Richardson and Finlen, Soc. America Bull., v. 35, p. 102-107.

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Chave, K. E., 1962, Factors influencing the miner- of Florida (abs.): Am. Geophys. Union Trans., alogy of carbonate sediments: Limnology and v. 49, p. 196. Oceanography, v. 7, p. 218-233. Neumann, A. C., and Hull, E. W. S., 1966, Half Emery, K. O., Ballard, R. D., and Wigley, R. L., mile down to the floor of the Gulf Stream: 1970, A dive aboard Ben Franklin off West Geomarine Technology, v. 2, p. 16-27. Palm Beach, Florida: Marine Tech. Soc. Potter, P. E., and Pettijohn, F. J., 1963, Paleocur- Jour., v. 4, p. 7-16. rents and Basin Analysis: New York, Aca- Gibson, T. G., and Schlee, J., 1967, Sediments and demic Press, p. 121. fossiliferous rocks from the eastern side of the Richardson, W. S., and Finlen, J. R., 1967, The Tongue of the Ocean, Bahamas: Deep-Sea transport of the Northwest Providence Chan- Research, v. 14, p. 691-702. nel: Deep-Sea Research, v. 14, p. 361-367. Gorsline, D. S., 1963, Environments of carbonate Richardson, W. S., and Schmitz, W. J., Jr., 1965, deposition Florida Bay and the Florida Straits, A technique for the direct measurement of A Symposium: Four Corners Geol. Soc., p. transport with application to the Straits of 130-143. Florida: Jour. Marine Research, v. 23, p. Hurley, R. J., and Fink, L. K., 1963, Ripple marks 172-185. show that counter-current exists in Florida Richardson, W. S., Schmitz, W. J., Jr., andNiiler, Straits: Science, v. 139, p. 603-605. P. P., 1969, The velocity structure of the Hurley, R. J., Siegler, V. B., and Fink, L. K., Florida Current from the Straits of Florida to 1962, Bathymetry of the Straits of Florida and Cape Fear: Deep-Sea Research, supp., v. 16, the Bahama Islands; Part 1, Northern Straits p. 225-231. of Florida: Bull. Marine Sci., v. 12, p. 313-321. Schmitz, W. J., Jr., and Richardson, W. S., 1968, Kofoed, J. W., and Malloy, R. J., 1965, Bathymetry On the transport of the Florida Current: of the Miami Terrace: Southeastern Geology, Deep-Sea Research, v. 15, p. 679-693. v. 6, p. 159-165. Stetson, T. R., Squires, D. F., and Pratt, R. M., Malloy, R. J., 1968, Depositional anticlines versus 1962, Coral banks occurring in deep water on tectonic "reverse drag": Gulf Coast Assoc. the Blake Plateau: Am. Mus. Novitates, no. Geol. Soc. Trans., v. 18, p. 114-123. 2114, p. 39. Menzies, R. J., Zaneveld, J. S., and Pratt, R. M., Teichert, C., 1958, Cold and deep-water coral 1967, Transported turtle grass as a source of banks: Am. Assoc. Petroleum Geologists Bull., organic enrichment of abyssal sediments off v. 42, p. 1064-1082. North Carolina: Deep-Sea Research, v. 14, Uchupi, E., 1966, Shallow structure of the Straits p. 111-112. of Florida: Science, v. 153, p. 529-531. Milligan, D. B., 1962, Marine geology of the Zarudzki, E. F. K., and Uchupi, E., 1968, Organic Florida Straits: M.S. thesis, Florida State reef alignments on the Continental margin Univ., p. 120. south of Cape Hatteras: Geol. Soc. America Milliman, J. D., Manheim, F. T., Pratt, R. M., Bull., v. 79, p. 1867-1870. and Zarudzki, E. F. K., 1967, Alvin dives on the Continental margin off the southeastern United States: Woods Hole Oceanog. Inst., MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 10, unpub. Tech. Rept., ref. no. 67-80, p. 64. 1970 Neumann, A. C., 1968, The submersible as a CONTRIBUTION No. 1236 FROM THE ROSENSTIEL scientific instrument: Oceanology Internal., SCHOOL OF MARINE AND ATMOSPHERIC SCI- v. 3, p. 39-42. ENCES, UNIVERSITY OF MIAMI Neumann, A. C., and Ball, M. M., 1968, Sub- PRESENT ADDRESS: (NEUMANN) DIRECTOR, SUB- mersible observation of sediment movement, MARINE GEOLOGY AND GEOPHYSICS PROGRAM, bottom currents, and bedrock over the Ba- NATIONAL SCIENCE FOUNDATION, WASHING- hamian and Floridian escarpments of the Straits TON, D.C.20550

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