Deep-SeaResearch, 1972, Vol. 19, pp. 253 to 257. PergamonPress. Printed in Great Britain. SHORTER CONTRIBUTION Eastward-flowing bottom currents along the Clipperton Fracture Zone* DAVm A. JOHNSONt (Received 2 August 1971 ; in revisedform 5 November 1971 ; accepted 8 November 1971) ~ct~Bottom currents were measured at four locations within a small area in the equatorial Pacific during a study of ocean floor erosion. The observed current speeds were low (<10 cm/sec) during the period of observation, and exhibited a strong semi-diurnal tidal component. In spite of the fluctuations in speed, the direction of flow at each location (eastward to northeastward) was re- markably uniform. The data suggest that there is a pronounced eastward-flowing bottom current along the north side of the Clipperton Fracture Zone, and that the fracture zone may therefore serve as a major barrier to the abyssal circulation in the central Pacific. This observation is consistent with p~=vious studies of the regional circulation pattern. TH~ QE~gAL features of the circulation of bottom water in the central Pacific have become fairly well established as a result of recent hydrographic studies (WOOSTER and VOLKMA~, 1960; Rima, STOMMI~L, STROUe and WARREN, 1968; GOaVON and GretA,, 1971) and direct current measurements (REID, 1969; JOHNSON and JOHNSON,1970; GOnVONand GERARD,1971). Antarctic Bottom Water enters the Pacific Ocean south of New Zealand, and flows northwards as an intensified western boundary current (A, Fig. 1) between the western wall of the Tonga-Kormadec Trench and the Louisville Ridge (HAYES and Ewn~o, 1971). The current enters the North Pacific through a narrow passage northeast of Samoa (B, Fig. 1) and then apparently diverges. Within the North Pacific prominent topographic features such as the Hawaiian Ridge, Line Islands Ridge, and Mid-Pacific Mountains serve as major barriers to bottom water flow (EDMOND, CHUNG and SCLAT~, 1971). A western branch of the current passes between the Marshall Islands and the Mid-Pacific Mountains (D, Fig. 1). An eastern branch apparently flows northwestward along the western side of the Line Islands Ridge (C, Fig. 1 ), and turns eastwards across the ridge barrier at only two locations: Horizon Passage (E, Fig. 1) and Clarion Passaga (F, Fig. 1). In the region to the east of the Line Islands Ridge the abyssal circulation is less wvll-defined, Data from south and east of the Hawaiian Ridge suggest that bottom currents turn northward around the eastern and of the ridge (G, Fig. 1). In lower latitudes, however, the extremely small gradients of bottom potential temperature do not allow the determination of the flow direction, and there arc few direct current measurements. Recently a detailed investigation of the ocean floor was carried out within a small area in the equatorial Pacific near 07°40'N, 134°00'W (Figs. 1, 2) where substantial sediment erosion has occurred. The area was surveyed extensively using conventional reflection profiling techniques from a surface ship, and the deep-towed instxum~tation of the Marine Physical Laboratory (SPmss and Muvm, 1970). Closely-spaced sediment samples were obtained using frc~fall coring techniques. Results of these studies are reported by JOHNSON (1971). As a part of this study, bottom currents were measured at four locations within the survey area (Fig. 2), using free-vehicle current meters developed by the Marine Life Research Group of Scripps (ScmcK, ISAACSand SESSIONS,1968). Two of the instruments (#2 and #4) were located in an erosional channel at the base of a flat-toplmd plateau. A third instrument (#3) was located near the northern end of the plateau, and a fourth (# 5) was located in a wide trough to the east (Fig. 2). Each *Contribution of the Scripps Institution of Oceanography, new series. tUniversity of California, San Diego Marine Physical Laboratory of the Scripps Institution of Oceanography, La Jolla, California 92037, U.S.A. Present address: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543. 253
Transcript
Deep-Sea Research, 1972, Vol. 19, pp. 253 to 257. Pergamon Press. Printed in Great Britain.
S H O R T E R C O N T R I B U T I O N
Eastward-flowing bottom currents along the Clipperton Fracture Zone*
DAVm A. JOHNSONt
(Received 2 August 1971 ; in revised form 5 November 1971 ; accepted 8 November 1971)
~ c t ~ B o t t o m currents were measured at four locations within a small area in the equatorial Pacific during a study of ocean floor erosion. The observed current speeds were low (<10 cm/sec) during the period of observation, and exhibited a strong semi-diurnal tidal component. In spite of the fluctuations in speed, the direction of flow at each location (eastward to northeastward) was re- markably uniform. The data suggest that there is a pronounced eastward-flowing bottom current along the north side of the Clipperton Fracture Zone, and that the fracture zone may therefore serve as a major barrier to the abyssal circulation in the central Pacific. This observation is consistent with p~=vious studies of the regional circulation pattern.
TH~ QE~gAL features of the circulation of bottom water in the central Pacific have become fairly well established as a result of recent hydrographic studies (WOOSTER and VOLKMA~, 1960; Rima, STOMMI~L, STROUe and WARREN, 1968; GOaVON and GretA, , 1971) and direct current measurements (REID, 1969; JOHNSON and JOHNSON, 1970; GOnVON and GERARD, 1971). Antarctic Bottom Water enters the Pacific Ocean south of New Zealand, and flows northwards as an intensified western boundary current (A, Fig. 1) between the western wall of the Tonga-Kormadec Trench and the Louisville Ridge (HAYES and Ewn~o, 1971). The current enters the North Pacific through a narrow passage northeast of Samoa (B, Fig. 1) and then apparently diverges. Within the North Pacific prominent topographic features such as the Hawaiian Ridge, Line Islands Ridge, and Mid-Pacific Mountains serve as major barriers to bottom water flow (EDMOND, CHUNG and SCLAT~, 1971). A western branch of the current passes between the Marshall Islands and the Mid-Pacific Mountains (D, Fig. 1). An eastern branch apparently flows northwestward along the western side of the Line Islands Ridge (C, Fig. 1 ), and turns eastwards across the ridge barrier at only two locations: Horizon Passage (E, Fig. 1) and Clarion Passaga (F, Fig. 1).
In the region to the east of the Line Islands Ridge the abyssal circulation is less wvll-defined, Data from south and east of the Hawaiian Ridge suggest that bottom currents turn northward around the eastern and of the ridge (G, Fig. 1). In lower latitudes, however, the extremely small gradients of bottom potential temperature do not allow the determination of the flow direction, and there arc few direct current measurements.
Recently a detailed investigation of the ocean floor was carried out within a small area in the equatorial Pacific near 07°40'N, 134°00'W (Figs. 1, 2) where substantial sediment erosion has occurred. The area was surveyed extensively using conventional reflection profiling techniques from a surface ship, and the deep-towed instxum~tation of the Marine Physical Laboratory (SPmss and Muvm, 1970). Closely-spaced sediment samples were obtained using frc~fall coring techniques. Results of these studies are reported by JOHNSON (1971).
As a part of this study, bottom currents were measured at four locations within the survey area (Fig. 2), using free-vehicle current meters developed by the Marine Life Research Group of Scripps (ScmcK, ISAACS and SESSIONS, 1968). Two of the instruments (#2 and #4) were located in an erosional channel at the base of a flat-toplmd plateau. A third instrument (#3) was located near the northern end of the plateau, and a fourth (# 5) was located in a wide trough to the east (Fig. 2). Each
*Contribution of the Scripps Institution of Oceanography, new series.
tUniversity of California, San Diego Marine Physical Laboratory of the Scripps Institution of Oceanography, La Jolla, California 92037, U.S.A.
Fig. 1. Direction of flow of bottom water in the central Pacific Ocean. Shading indicates regions where water depth is less than approximately 2500 fm (4700 m). The asterisk indicates the location of current measurements reported in this paper. Arrows show the direction of flow of bottom water elsewhere in the central Pacific on the basis of hydrographic data and direct current measurements reported by the following authors: A: REIn, STOMMEL, STnotn' and WARREN, 1968; B: REID, 1969; C: JOHNSON and JOHNSON, 1970; D, E, F: EDMOND, Curing and
SCLATER, 1971 ; (3: GORDON and GERARD, 1971.
instrument was positioned several meters above the sea floor (Table 1), and recorded currents over a period of approximately four days.
The direction and speed of the currents were recorded internally on a strip-chart recorder (see Fig. 4 for sample of original records). Current d i r ~ i o n was monitored continuously, whereas the speed was commonly averaged over time increments of one hour. Current speed was designated by a sequence of tick marks along the record. The recorder was calibrated to produce a mark for a given numher of revolutions of a Savonius rotor, and the spacing between these marks is inversely pro-
Table 1. Summary of current meter data.
Instrument Water Height Mean* Mean* Maximum number depth above bottom direction speed speed
Fig. 2. Bathymetry of the region of detailed study (after JOHNSON, 1971). Contour interval 40 m. The four asterisks indicate the locations of bottom current measurements.
portional to the current speed. Current speed and direction wore measured by each instrument except #5, which recorded direction only. Data obtained from the current meters are summarized in Table I.
The observed current speeds were generally low (< I0 cm/sec) during the period of observation, but fluctuated significantly because of tidal effects. Figure 3 shows the strong semi-diurnal tidal component present in the current, as recorded by one of the instruments; similar fluctuations in speed were observed at each of the other instrument locations. In spite of the fluctuations in speed, the direction of flow at each location was remarkably uniform during the period of observation (Fig. 4). Eastward flowing currents were recorded at instruments # 3 and # 5, whereas in the depression to the west of the plateau, instruments # 2 and #4 recorded a northeastward flow. The data suggest that the~ is a net regional transport of bottom water toward the east, with minor modifications in direction due to local topographic effects.
The Clipperton Fracture Zone, which lies 50 to I00 km south of the study area (Fig. I), is among the most prominent bathymetdc and structural features in the eastern equatorial Pacific (MY.NArD and FISH~, 1958). The fracture zone is commonly a well-defined north-facing escarpment (H~TH and MooPJE, 1965; EWtNO, EWTNG, AITKEN and LUDWIG, 1968) with up to several hundred meters of relief. In the region nearest the study area the fracture zone is not an obvious topographic feature, but it can be identified by seismic reflection profiling (JohNsoN, 1971).
Bottom current measurements have recently been obtained at one other location on the north side of the Clipperton Fracture Zone. M. Evans (personal communication), using free-vehicle current meters near 09°N, 115eW, reports that the mean flow is toward the northeast at this location. These data, together with the results presented in this report, suggest that the Clipperton Fracture Zone may control the dii'ection of transport of bottom water in the eastern equatorial Pacific, and may prevent a southward return flow along the west flank of the East Pacific Rise.
256 Shorter Contribution
C u r r e n t
V e l o c i t y
(cm/$ec)
#2
• • l l ",, ;.°o ; L i e , ,
i
e ° ~ t
,..-.,
I
I ?
... '
/
/
,1!
I r l
I r
i e
, v *
\ j
I t
?
i
! I
J I
; ,t
s
~10 I I I ] I I 0 6 1200 1800 t 0 6 0 0 1200 IO00
20 October I 21 October
I 0 6 o o
2 2 O c t o b e r
Fig. 3. Summary of bottom current measureanents obtained by instrument #2 over a period of approximately two days, illustrating the pronounced tidal effect upon deep ocean current
velocities. Similar tidal variations in speed ware recorded by other instruments.
Acknowledgements--This research was supported under grants from the U.S. Office of Naval Research, National Science Foundation, and the International Nickel Company. I thank F. B. Pm.WaEl% F. N. SvlESS and J. L. REID for reviewing portions of the manuscript.
R E F E R E N C E S
EDMOND J. ]V[, Y. CMtmo and J. G. SCLA~m (1971) Pacific Bottom Water: penetration east around Hawaii. J. geophys. Res., 76, 8089-8097.
EWlNO J., M. EWING, T. ArrKeN and W. J. LtnavaG (1968) North Pacific sediment layers measured by seismic profiling. In: The crust and upper mantle o f the Pae~fe area. L. Kt~eovv, C. L. DItAKE and P. J. HART, editors, American ~ Union, 147-173.
GORDON A. L. and R. D. GIm/mD (1971) North Pacifk bottom potential texapemture. In: Geological investigation o f the North Pacific, J. D. HAYs, editor, Mere. geol. Soc. Am., 126, 23-29.
HAVES D. E. and M. E w I ~ (1971) The Louhville Ridge - - a possible extension of the Eltanin Fracture Zone. In: Antarctic ocecaolo~ I, J. L. Rim), editor, American Geophysical Union, 223-228.
HEATH G. R. and T. C. Mooms, JR. (1965) Sub-bottom profile of abyssal sediments in the central equatorial Pacific. $c/ence, 149, 744--746.
JOHNSON D. A. (1971) Studies of deep-sea erosion using deep.towed instrumeatation. Ph. D. dis- sertation, University of California, Sea Dieso, 170 pp.
3omoaN D. A. and T. C. JowmoN (1970) Sediment redistribution by bottom currents in the ceatral Pacific. Deep-Sea Res., 17, 157-169.
Mm,tARD H. W. and R. L. Ftmnm (1958) O ipp~on Fracture Zone in the northeastern equatorial PaCific. J. Geol., 66, 239-253.
R~m J. L., JR. (1969) Preliminary results of measurements of deep currents in the Pacific Ocean. Nature, Lond., 221, 848.
J. L., JR., H. STOMMlgL, E. D. SrRouP and B. A. W~m~N (1968) Detection of a deep boundary current in the western South Pa~qc. Nature, Lond., 217, 937.
Fig. 4. Original current meter records over a 14-hour interval, illustrating the relative uni- formity in flow direction at each site. The continuous trace on each record indicates the direction of flow relative to magnetic north. Current speed is indicated by the sequence of tick marks along the right-hand edge of each record. Eastward-flowing currents were recorded at instruments #3
and #5, whereas the flow direction was generally northeastward at instruments #2 and #4.
Shorter Contribution 257
Scmclc G. B., J. D. Is~,cs and M. H. SEssioNs (1968) Autonomous instruments in oceanographic research. In: Marine sciences instrumentation, Plenum Press, 4, 203-230.
Spmss F. N. and J. D. Mu'Dm (1970) Small-scale topographic and magnetic features. In: The sea, A. E. MAXWELL, editor, Interscience, 4, (1), pp. 205-250.
W o o s r ~ W. S. and G. H. VOLKMA~ (1960) Indieations of deep Pacific circulation from the dis- tribution of properties at five kilometers. J. geophys. ICes., 65, 1239-1249.
