Postglacial Iron-Rich Crusts in Hemipelagic Deep-Sea Sediment

Home , Abyssal fan

DAVID F. R. McGEARY Geology Department, California State University, Sacramento, Sacramento, California 95819 JOHN E. DAMUTH Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York New Yorl{ 10964

ABSTRACT layers and relate the origin of such layers to periods of slower deposition or nondeposition An iron-rich crust separates terrigenous of sediments. The precise origin of the metals hemipelagic lutites and turbidites from over- was not described in detail but was thought lying pelagic foraminiferal oozes and lutites to be related to scavenging reactions at the on the Amazon abyssal fan and adjacent con- sediment-water interface, perhaps in conjunc- tinental rise and abyssal plains. The crust tion with biogenic concentration. Watson and marks a sharp change in color and oxidation Angino also noted that many of these layers state of the sediments; the terrigenous sedi- seem to occur at the Pleistocene-Holocene ment below is generally gray and reduced, boundary. while the pelagic lutite above it is tan and This paper presents new evidence on the oxidized. The crust formed on top of hemi- distribution, composition, and origin of such pelagic lutite and turbidites after the post- layers. It results from the chance discovery glacial sea-level rise shut off the supply of that both authors had recognized rust-colored terrigenous sediment to deep water. Decaying indurated crusts while independently working organic material reduced the iron in the on the sediments of the equatorial Atlantic terrigenous minerals. The reduced iron dis- (McGeary, 1969, and in prep.; Damuth and solved in the interstitial water and was ex- Fairbridge, 1970) and the discovery that pressed upward during compaction. Upon similar crusts were well developed on ter- reaching the sediment-water interface, the rigenous sediments in the Coral Sea (E. iron was oxidized and precipitated, forming Winterer and J. Galehouse, 1968, oral com- an iron-cemented crust in the sediment that mun.). has been buried by postglacial foraminiferal The authors were in a position to simulta- lutite. This crust is found over wide areas of neously examine the entire deep-sea core the western equatorial Atlantic as well as in collections of Scripps Institution of Oceanogra- many other regions of the world. Its formation phy (by McGeary) and Lamont-Doherty marks the end of terrigenous hemipelagic and Geological Observatory (by Damuth) in 1969 turbidite deposition. The crust is always post- for further examples of such crusts. glacial, and occurs at or very near the Pleisto- cene-Holocene boundary. CRUSTS IN THE WESTERN EQUATORIAL ATLANTIC INTRODUCTION Description Thin yellow or rust-colored layers of sediment, occasionally well indurated, have been The relation of the rust-colored crust to described in abyssal sediments (Ewing and other sediment types in the western equatorial others, 1958; Broecker and others, 1960; Atlantic has been described by McGeary Heezen and others, 1966). A detailed descrip- (1969) in a study of cores taken from Atlantis II tion of several such layers from the Gulf of of the Woods Hole Oceanographic Institution, Mexico was given by Watson and Angino and by Damuth and Fairbridge (1970). (1969). They describe the concentration of The crust generally marks a sharp change iron, manganese, cobalt, and nickel in the in color and oxidation state within the sedi-

Geological Society of America Bulletin, v. 84, p. 1201-1212, 8 figs., April 1973 1201

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1201/3418059/i0016-7606-84-4-1201.pdf by guest on 29 September 2021 Figure 1. Location of cores in the western equa- locations of the 100-fathom contour and the axis and the location of cores containing the iron-rich crust in torial Atlantic which contain the iron-rich crust. The major fracture zones of the Mid-Atlantic Ridge (drawn the eastern equatorial Atlantic and the Mediterranean locations of other Lamont-Doherty cores in the region in part from Heezen and Tharp, 1962, and in part Sea (see Appendix for exact locations). are also shown. Dotted lines represent the approximate from original sounding records). The inset shows

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Figure 2. Core location map for AII-31 cores in equatorial Atlantic near the Verna Fracture Zone (see Figs. 4 and 5). ment. The terrigenous hemipelagic lutites and formational conglomerate—shows fine-grained turbidites beneath it are dark gray and re- laminae from 0.1 to 1 mm thick, varying from duced, while pelagic foraminiferal lutite above light brown to brownish black, with sharp the crust is tan and oxidized. Figure 1 shows upper and lower contacts. No foraminifers the distribution of the crust in the western were sectioned, although several samples of equatorial Atlantic. the crust contain visible foraminifers. The crust varies in color from rusty yellow- Age and Correlation brown to black. The colors are commonly in the form of variegated laminae, with the darker Nineteen piston cores raised from the laminae being more indurated. The thickness Amazon abyssal fan, the adjacent continental of the well-indurated layers, which offer re- rise, and the Demerara and Ceara abyssal sistance to cutting as the cores are split, rarely plains which contain the iron-rich crust were exceeds 1 cm, although more than one such subjected to a micropaleontological analysis of layer may lie in a band of soft, variegated the Foraminifera present to determine the age laminae several centimeters thick. In the case of the crust. The simplest method for zoning of core AII-31-3 (Figs. 2 and 4), the crust Holocene and late Pleistocene sediments in occurs in a dark, variegated zone 8 cm thick, the equatorial Atlantic is to construct a separated by 8 cm of foraminiferal lutite from climatic curve based on the temperature- a 13-cm band of soft, variegated laminae above. sensitive Foraminifera of the Globorotalia The indurated layers of crust are commonly menardii complex. This complex consists of broken in the opened cores, and were mistaken three subspecies, Globorotalia menardii, G. m. for wood fragments in the first few cores that tumida, and G. m. flexuosa, and is abundant in were opened. sediments deposited during cycles of relatively A thin section of a well-indurated chip of warmer climate (Holocene and previous inter- crust (impregnated for sectioning) from pilot glacials), but is absent or rare in sediments core AII-31-2—a 19-cm core consisting of deposited during relatively cooler cycles of broken pieces of crust redeposited as an intta- climate (Wisconsin and previous glacials; Eric-

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A180-73 1 : i V

Figure 3. Location of the iron-rich crust in relation shown by the numbers above the samples. Relatively to the Holocene-Pleistocene boundary as defined by cool temperatures are to the left, warm to the right. Ericson and others (1961) for nineteen piston cores The dotted line connects tfcie Holocene-Pleistocene from the western equatorial Atlantic (Fig. 1). Climatic boundary. curves based on the presence or absence of the Globoro- Core A180-73 is a pelagic foraminiferal ooze core talia menardii complex are to the right of each core log from the crest of the Mid-Atlantic Ridge near the and were constructed by the frequency-to-weight equator (Fig. 1) and shows the Holocene-Wisconsin method of Ericson and Wollin (1956b, 1968). Sample climatic zonation of Ericson and Wollin (1956b). locations are denoted by small circles. The ratios are son and Wollin, 1956a, 1968; Ericson and from cool temperatures of the latest Wisconsin others, 1961, 1964). (Y zone) to the relatively warmer temperatures Climatic curves for the 19 cores were con- of the Holocene (Z zone) as indicated by the structed by using the Frequency-to-Weight- Globorotalia menardii complex occurred ap- Ratio Method devised by Ericson and Wollin proximately 11,000 yrs B.P. Ericson and (1956b; 1968; Fig. 3). When the frequency- others (1961) have subsequently defined the to-vveight ratio for tests of the Globorotalia age of the Pleistocene-Holocene boundary as menardii complex are plotted versus the depth 11,000 yrs B.P. of each sample in the core, a characteristic Figure 3 shows the location of the iron-rich climatic curve is obtained for that core which crust in the 19 cores in relation to the Pleisto- can be correlated with standard climatic curves cene-Holocene boundary as defined by Ericson derived by Ericson and Wollin (1968) for the and others (1961). Although the crust occurs entire Pleistocene. Figure 3 shows the climatic at or near the boundary, it is apparent that it curves obtained by this method for the upper is not isochronous and varies as much as 30 cm 150 cm of each of the 19 cores containing the above and below the boundary. The time of iron-rich crust as well as a typical late Pleisto- the crust formation in any area seems to be cene-Holocene climatic curve derived by Eric- dependent upon the remoteness of that area son and Wollin (1956b) for a foraminiferal from sources of terrigenous sediment on the ooze core (A180-73) from the equatorial portion South American continent (mainly the Ama- of the Mid-Atlantic Ridge. zon River). The crust is well below the bound- Broecker and others (1960) have demon- ary for distal areas of the abyssal plains and the strated by radiocarbon dating and lithic inter- lower continental rise, while the crust is at or pretation of piston cores (including A180-73) above the boundary in areas closer to the that the midpoint of the temperature change mouths of the Amazon and smaller rivers

within 3,000 yrs of the Pleistocene-Holocene boundary. It generally occurs slightly below the boundary because sea-level rise had shut off the sediment supply to most areas just prior to the beginning of the Holocene (Fig. 3). Regionally the crust can be taken to mark the approximate location of the Pleistocene- Holocene boundary in the western equatorial Atlantic as well as a change in the sedimentary regime from dominantly hemipelagic and tur- •a Ci Za Ca Di Cr 1 f Î J ° < 1 0 S,0 0 , 100 SO ISO SO ISO 0 100 bidite to pelagic during the Pleistocene-Holo- rr I 1 cene transition. Lithic relations and sediment accumulation rates in the upper 150 cm of cores from other regions throughout the world where the crust is found (Appendix1 and Figs. 1, 7, and 8) are similar to those for the western equatorial Atlantic. Thus the iron-rich crust also marks the Pleistocene-Holocene transition Fe h Ca la Ca Ii Cr for those areas. Î ! Î i r ?.! ? , IfO O _ «O ^0 g IfO o 10 Occasionally one or more rust-colored crusts are seen in a core well below the crust at the Pleistocene-Holocene transition. These crusts are thinner and less developed than the upper crust and seem to be formed on top of turbidites and other sections of the core that are particularly rich in organic matter. One such crust at a depth of 474 cm in core AII-31-2 was chemically analyzed (Fig. 4). The ages of f« •> Ca la Ca Di Cr S 0 s * 7 o I OWOoiOOOlOAUOSSoalalAim these older crusts vary within the late Wis- consin, and the crusts are not correlative from ira core to core. oo&J «11-31-4 Chemistry Figure 4. Chemistry of crust in cores AII-31-2, One-gram samples of the crust were dried AII-31-3, and AII-31-4 (see Figs. 1, 2, and Appendix at 100°C, ground in a Diamonite mortar and for core locations). Iron and manganese reported in pestle, and then redried and cooled in a desic- percentages, others in parts per million. Black layer in cator. From 0.1 to 0.5 g of powdered sample core represents crust; parallel black lines in cores 2,3, and 4 are laminae. See Appendix for precise depth of was weighed and heated to 550°C in air to crust in each core. Tan foraminiferal lutite lies above remove organic material. The sample was then the crust, gray hemipelagic silty lutite (SL) below the dissolved for 24 hrs in 2 ml HC1 and 10 ml crust. Black bars on right of core indicate sampling HF over a hot-water bath and the HF scrubbed intervals. Depth-in-core scale in centimeters. from the sample by evaporation, followed by washing and evaporation of deionized water. (Figs. 1 and 3). The time of crust formation The last drops of solution and residue were seems to have been controlled by sea-level rise diluted and redissolved in deionized water, during the latest Wisconsin, which progres- adding additional HC1 as needed. The samples sively cut off the vast supply of terrigenous were diluted to usable concentrations and sediments to various areas of the western analyzed for iron, manganese, copper, nickel, equatorial Atlantic floor. Thus the crust is

regionally diachronous across the Pleistocene- 1 Holocene boundary in the western equatorial Appendices (NAPS no. 02009) may be obtained by writing to ASIS Microfiche Publications, Div. of Micro- Atlantic (Fig. 3). If the local rate of Holocene fiche Systems Corp., 305 East 46 Street, New York, New pelagic sediment accumulation at each core 3 York 10017. Enclose $5.00 for photocopies or $1.50 for location (3 to 8 cm/10 yrs) is considered, then microfiche, and make checks payable to Microfiche it is apparent that the crust always occurs Publications.

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cobalt, zinc, and chromium on an atomic to have been pelagically deposited during its absorption spectrophotometer. formation. Since the crust is indurated pelagic The concentrations of some metals in the sediment, the problem is the origin of the iron crust and sediment from the Atlantis II cores cement, not the origin of the sediment. are shown in Figures 4 and 5. Basically, it is an 4. Solution of iron from within the sediment iron-rich crust, with the iron content enriched with redeposition at the sediment-water inter- 2 to 5 times compared with the sediment above face appears to be the most probable origin. and below. Manganese is occasionally con- The turbidites contain iron-bearing minerals, centrated in the crust, but it does not approach notably biotite and chlorite. Reduced iron is values typical of manganese nodules (see Table much more soluble than oxidized iron. Plant 1). The concentrations of copper, zinc, cobalt, material decaying in the hemipelagic lutites nickel, and chromium show no apparent regu- and turbidites could reduce and dissolve the lar difference between the crust and the sur- iron from the terrigenous minerals. The gray rounding sediment. This is contrary to the sediments below the crust were strongly re- slight enrichment of cobalt and nickel in the duced when the cores were cut open, with a iron-rich layers found by Watson and Angino pronounced smell of hydrogen sulfide in a few (1969) in the Gulf of Mexico. cores. When opened, all of the cores had very abundant hydrotroilite staining below the Origin of Crust crust and many were stained completely black. The origin of the iron-enriched indurated The turbidites often contain coarse plant ma- layer could be due to: terial, and the gray hemipelagic lutites contain 1. Deposition of a manganese-nodule-like F Co Zl1 crust with subsequent leaching of the more >. .; i;. •. » 0"n 1 o 100 0 100 0Co 100 ,3N0i ISO Cor 100 soluble manganese. The lack of accessory metals does not support this. Table 1 compares the composition of the iron-rich crust with a local manganese crust. 2. A volcanic emanation. Lack of accessory metals, particularly chromium, suggests this is not likely (Table 1; Bostrom and Peterson, 1966). Although recent volcanism is indicated All-31-5 for the Mid-Atlantic Ridge crest by abundant- chips of glass and basalt in cores (such as AII- 31-11) from the crest, the distribution of the crust is not associated with the ridge crest. No pelagic late Quaternary cores from the ridge Ft *» C» Zn Co Ni Cr crest or flanks (including core A180-73, Figs. 1 and 3) contain the iron-rich crust, even though they are closer to the sources of volcanism than the cores containing the crust. 3. A terrigenous deposit. The absence of the crust in pelagic cores containing the Pleisto- cene-Holocene transition which are located on seamounts and ridges adjacent to areas containing the crust eliminates the possibility that the crust is a terrigenous sediment depositee

particle by particle. Thus, if it were derived by Fe Mn Co to Co Ni1 0 1 4 | ( ; 0 > I 0 , 104 f lfo ) ( 190 ? , erosion of the South American continent, it "»-I would have to have been transported to the deep sea by a bottom-seeking turbidity-type ( I ) { ) flow. However, the crust shows no stratigraphic relations characteristic of a turbidity deposit. Figure 5. Chemistry of crust in cores AII-31-5, It has no silt or sand, graded bedding, or other AII-31-8, and AII-31-1. See Figures 1, 2, and Appendix indicators of turbidity deposits. The crust also for core locations. Scales, symbols, and units as in contains occasional foraminifers which appear Figure 4.

TABLE 1. COMPARISON OF THE COMPOSITION OF THE IRON CRUST WITH VOLCANIC GLASS AND MANGANESE CRUST

Fe Mn Co Cu Zn Ni Cr (%) (%) (ppm) (ppm) (ppm) (ppm) (ppm)

Iron crust AII-31-2 pilot core cm 0 to 10 (hard) 11.1 1.2 70 50 100 80 60 (medium) 11.5 .60 100 40 100 90 60 (soft) 10.0 1.1 110 40 100 70 80 Volcanic glass 7.3 .14 220 60 90 250 220 AII-31-11 cm 0 to 10 Volcanic glass 7.9 .14 260 60 90 250 250 AII-31-11 cm 0 to 10 Manganese crust 24.3 16.0 7900 400 570 2700 150 AII-31-DC1 cm 0 to 1 Manganese crust 24.8 14.5 5500 490 620 2500 80 AII-31-DC1 cm 0 to 1

Note: Comparison of the composition of the iron crust, represented by three samples of varying hardness from AII-31-2 pilot core, with the local volcanic glass from the Ridge crest (core AII-31-11) and with manganese crust from the high south wall of the fracture zone. The iron crust is significantly different from the volcanic glass and the manganese crust. AII-31-2 pilot core is a 19-cm intraformational conglomerate of redeposited crust.

minute disseminated plant fragments. Table 2 The crust, therefore, marks a time of a shows that the organic carbon content of the sharp decrease in sedimentation rate. Average hemipelagic lutite is higher than in the over- accumulation rates for the pelagic foraminiferal lying foraminiferal lutite. Reduced iron would lutite above the iron-rich crust are 2 to 8 be carried upward in the interstitial water cm/103 yrs in the western equatorial Atlantic, during sediment compaction. Upon exposure to while rates for the gray hemipelagic lutites and the oxidized sea water lying above the sedi- turbidites below the crust vary from as low as 3 3 ment, the iron would be oxidized and repre- 5 cm/10 yrs to more than 100 cm/10 yrs cipitated, forming a cement that would bind (Bader and others, 1971). Bottom transport of the sediment into a hard iron-rich layer terrigenous sediments masked pelagic sedimen- (Fig. 6). tation before the crust formed. When the bottom transport ceased, the crust formed during normal pelagic sedimentation. Lamina- TABLE 2. CACOS AND ORGANIC CARBON tions of the crust are due to suddenly deposited CONTENTS OF GRAY HEMIPELAGIC SILTY LUTITE (SL) AND TAN FORAMINIFERAL laminae of pelagic sediments abruptly raising LUTITES (FL) IN VEMA FRACTURE ZONE the sediment-water interface, thus moving the locus of iron reprecipitation upward. The thick Organic layer of foraminiferal lutite separating the CaCOa carbon crust from the band of laminae above it in Cm Sediment (%) (%) core AII-31-3 (Fig. 4) was deposited suddenly, causing the iron deposition to move upward. AII-31-4 0 to 3 FL 47.6 0.0 20 to 30 SL 1.2 .52 The amount of induration of the sediment is 300 to 310 SL 0.0 .50 related to sedimentation rate, with slowly AII-31-8 0 to 10 FL 27.7 .18 depositing sediment receiving more iron and 15 to 25 FL 37.0 .02 becoming more indurated than rapidly de- 50 to 60 SL 0.0 .57 160 to 170 SL 0.0 .62 positing sediment. 380 to 390 SL 0.0 .76 Other factors influencing the degree of crust formation would be the quantity of plant See Figures 1, 2, 4, and 5. material and iron-bearing minerals in the

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turbidites and hemipelagic lutite, and the similar sedimentary ::egime to the Amazon amount of compaction of these beds, which abyssal fan. The Ganges fan in the Bay of influences the amount of interstitial water Bengal, the Mississippi fan in the Gulf of expressed upward. Hemipelagic lutite and Mexico, and the Magdalena fan in the Carib- high-lutite turbidites will be compacted Tiore bean lie off major rivers, and the Delgada fan than coarse-grained sediments and should carry near the Mendocino Fracture Zone, the Coral more iron upward. Sea, the Sierra Leone Basin, and the Mediter- Diffusion of iron does not contribute sig- ranean have smooth bottom profiles and nearby nificantly to the crust formation, as evidenced rivers, strongly suggesting turbidite deposition. by the sharp contacts of the laminae and the In each of these areas the crust indicates a lack of crusts below the turbidites. A crust general cessation of terrigenous deposition at occurs within the lutite of core AII-31-2 at 474 the end of the last glacial period (Wisconsin) cm, well below the turbidite (Fig. 4). This when the sea level rose and covered the conti- crust is not related to the turbidite above it, nental shelf, moving river sedimentation away but formed during a period of very slow deposi- from tne shelf edge and thereby cutting off tion of hemipelagic lutite. terrigenous sediment transport to the deep sea. Such a history has been previously sug- Other Areas in Which a Similar gested for the Gulf of Mexico (Ewing and Crust Is Found others, 1958; Broecker and others, 1960) and A search through most of the cores and core the Coral Sea (E. Winterer and J. Galehouse, description collections of Scripps Institution 1968, oral commun.). of Oceanography (by McGeary) and Lamont- The Blake-Bahama Outer Ridge has few Doherty Geological Observatory (by Damuth) turbidites, but is made up largely of hemi- revealed several other areas of the sea floor in pelagic gray silty lutite that is terrigenous and which a similar iron crust occurs in the sedi- seems to have been carried out to form the ment (Figs. 1, 7, 8, and Appendix). In nearly ridge by geostrophic contour currents (Heezen all cores the crust was less than 1 m deep in and others, 1966). the sediment, and at: or near the contact be- The cores east of Japan and south of Java tween lower gray hemipelagic silty lutite and are separated from the nearest land by oceanic turbidites and the upper tan foraminileral lutite. Thus, the crust elsewhere seems to have NORMAL formed at the same time and under the same PELAGIC conditions as the crust in the western equa- SEDIMENTATION torial Atlantic. Chemical analyses were made of several samples from the Scripps Institution of Ocean- A ography's core collection (Table 3). Crusts similar to those of the western equatorial Atlantic are found in the following places: the Ganges abyssal fan in the Bay of Bengal; an area in the northwest Pacific Ocean, east of Japan; the Sierra Leone Basin off western Africa; the western Mediterranean Sea; the Coral Sea; a location near the Mendocino Fracture Zone; and south of Hawaii (Figs. 1, 8, and Appendix). In addition, the crust has been visually identified, though not chemically confirmed, in the following areas: the Magdalena River abyssal fan in the Caribbean Sea, the Gulf of Mexico, the Blake-Bahama Outer Ridge off Figure 6. Mechanism of crust formation. (1) Deposition of plant-bearing turbidite. (2) Decaying the southeastern United States, the eastern plants reduce iron, which dissolves; compaction carries Mediterranean Sea, a location south of Java in reduced Fe upward in solution. (3) Reduced Fe meets the eastern Indian Ocean, and possibly off sea water, oxidizes, and precipitates, cementing pelagic Norway (Figs. 1, 7, 8, and Appendix). ooze. (4) Compaction over, normal pelagic sedimenta- Most of the areas with iron crusts have a tion buries crust.

9Ct 80- Figure 7. Location of cores containing the iron- western North Atlantic Ocean (Blake-Bahama Outer rich crust in the Caribbean Sea, Gulf of Mexico, and Ridge area). See Appendix for exact locations. trenches (Fig. 8). The crust on these cores con- Holocene transition. The width of the shelf sists of many individual rusty laminae within would have a large effect in timing the shutting gray silty lutite, bearing no relation to the off of turbidites to the deep sea. The wide contact with tan sediment above. It is possible Amazon shelf would lead to a rapid transgres- that these iron-rich laminae formed at many sion of the shoreline seaward, shutting off intervals of nondeposition and are unrelated to turbidites early in the rise of sea level. The the change in sedimentary regime at the narrow shelf off the Magdalena River, how- Pleistocene-Holocene transition. ever, would cause a much slower transgression Some variations of composition occur in resulting in slowed turbidite deposition con- crusts from different areas. The crust from the tinuing during a longer period of sea-level rise. western Mediterranean is high in nickel and The depth of the shelf edge, the volume and chromium, and the crust south of Hawaii is velocity of river flow, and the amount of ero- high in most accessory metals including chro- sion on the continents would also control the mium. Neither crust has the high concentra- time of turbidite shutoff. Thus, the precise tions of manganese or accessory metals charac- age of the crust may vary by a few thousand teristic of manganese crusts (Table 3). How- years from region to region. ever, the high chromium values suggest that The crust is not continuous over a single volcanism may have some effect on crust region. Several cores from the western equa- formation in these areas (Bostrom and Peter- torial Atlantic have tan Holocene foraminiferal son, 1966), either as a primary contributor of lutite over late Wisconsin hemipelagic lutite iron and chromium or as a supplier of accessory but do not have the crust. The crust probably metals that were adsorbed onto an existing failed to form in these areas because the sedi- crust. ments contained less organic material and iron The time of crust formation in different and were less compacted than sediments con- areas seems to consistently be the Pleistocene- taining the crust. An abyssal fan builds up by

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depositing turbidites more or less randomly back and forth across its surface, with the crust being formed locally after the last turbidite was deposited. Since some areas received fewer turbidites and terrigenous sediments than others, there is less organic material and iron available to form the crust. These factors caused local variations in crustal thickness (from zero to several centimeters) and age.

Summary and Conclusions

A rust-colored, iron-rich crust separates Figure 8. Location of cores containing the iron-rich oxidized tan foraminiferal lutites from under- crust in the western Pacific east of Japan, the Coral lying gray reduced hemipelagic lutites and Sea, and the Indian Ocean (Ganges abyssal fan and turbidites, rich in terrigenous minerals and south of Java). See Appendix for exact locations. organic particles, over much of the Amazon Symbols are the same as in Figure I. abyssal fan and the adjacent continental rise and abyssal plains of the western equatorial that has been buried by postglacial pelagic Atlantic. Foraminiferal evidence indicates that sediments. the crust occurs at or near the Pleistocene- Although the crust is best developed and Holocene boundary and was formed on top of most widely distributed over the western equa- the terrigenous sediments when postglacial sea- torial Atlantic, crusts of similar age are found level rise shut off the supply of terrigenous sedi- in several other regions including the Carib- ment to the deep-sea floor. bean Sea, the Gulf of Mexico, the Blake- Geochemical and stratigraphie evidence seem Bahama Outer Ridge, the Coral Sea, the to indicate that the iron-rich crust was formed Mediterranean Sea, the Sierra Leone Basin, when decaying organic material reduced the and the Ganges abyssal fan. The crust shows iron in the terrigenous minerals. The reduced stratigraphic and lithic relations in each region iron dissolved in the interstitial water, which similar to those in the western equatorial was then expressed upward during sediment Atlantic and appears to owe its origin to a compaction. Upon reaching the sediment- general world-wide cessation of terrigenous water interface, the iron was oxidized and re- sedimentation in the deep sea due to post- precipitated, forming an iron-cemented crust glacial sea-level rise.

TABLE 3. COMPOSITION OF CRUSTS FROM AR AS OTHER THAN THE VEMA FRACTURE ZONE

Fe Mn Co Cu Zn Ni Cr Area Core (%) (%) (ppm) (ppm) (l>pm) (ppm) (ppm;

Bay of Bengal Circe 30G 9.6 .07 200 100 150 200 300 (Ganges fan) cm 12 Bay of Bengal Circe 38 13.8 .014 40 65 50 200 100 (Ganges fan) cm 19.5 Northwest Pacific JYN II 13 8.8 .21 80 210 210 190 tr East of Japan cm 58 Sierra Leone LSDA 207G 13.1 .22 130 110 160 310 130 Basin cm 62 Western ZEPH I 31G 10.0 .037 140 110 70 800 730 Mediterranean cm 40 Coral Sea Nova AIV31GP 13.9 .37 80 40 70 200 50 cm 63-65 North of Mendocino LFGS 47 6.0 .85 50 40 100 210 160 Fracture Zone cm 23.5 South of Hawaii TET 18 8.5 .34 280 430 260 710 570 cm 0

For latitude and longitude, see Appendix (Figs. 1 and 8).

ACKNOWLEDGMENTS torial Atlantic deep-sea arkosic sands and ice- age aridity in tropical South America: Geol. Tj. H. van Andel, M.N. A. Peterson, and Soc. America Bull., v. 81, p. 189-206. E. L. Winterer supervised McGeary's work at Ericson, D. B., and Wollin, G., 1956a, Correlation Scripps Institution of Oceanography. R. P. of six cores from the equatorial Atlantic and Von Herzen and J. Burke of Woods Hole the Caribbean: Deep-Sea Research, v. 3, no. 2, Oceanographic Institution helped during core p. 104-125. collection on cruise 32 of Atlantis II. R. Goll, 1956b, Micropaleontological and isotopic de- F. McCoy, F. Jones, and T. Walsh helped terminations of Pleistocene climates: Micro- paleontology, v. 2, no. 3, p. 257-270. during core sampling; J. Edmond, D. Z. Piper, 1968, Pleistocene climates and chronology in and G. R. Heath suggested laboratory tech- deep-sea sediments: Science, v. 162, p. 1227— niques; and J. Kahn and R. Williams helped 1234. with chemical analyses. M. E. McGeary helped Ericson, D. B., Ewing, M., Wollin, G., and with the manuscript. Financial support was Heezen, B. C., 1961, Atlantic deep-sea sedi- provided by contracts N00014-66-C0241 ment cores: Geol. Soc. America Bull., v. 72, and NONR-1841(74) of the Office of Naval p. 193-286. Research, GA-1077 of the National Science Ericson, D. B., Ewing, M., and Wollin, G., 1964, Foundation, and by the Kennecott Copper The Pleistocene epoch in deep-sea sediments: Corporation. Science, v. 146, p. 723-732. Ewing, M., Ericson, D. B., and Heezen, B. C., Damuth was supported during this research 1958, Sediments and topography of the Gulf by the Office of Naval Research (N00014-67- of Mexico: Habitat of oil symposium: Am. A-0108-0004) and the National Science Foun- Assoc. Petroleum Geologists Bull., p. 995- dation (GA-27281 and GA-29460), who 1053. financially maintain the Lamont-Doherty Heezen, B. C., and Tharp, Marie, 1962, Physio- Geological Observatory Core Library. Piston graphic diagram of the South Atlantic Ocean, cores studied by Damuth were from the core the Caribbean Sea, the Scotia Sea, and the library. eastern margin of the South Pacific Ocean: Geol. Soc. America, scale 1:11,000,000. We thank B. C. Heezen for instructive dis- Heezen, B. C., Hollister, C. D., and Ruddiman, cussion. This manuscript has been read and W. F., 1966, Shaping of the continental rise criticized by D. E. Hayes and L. H. Burckle. by deep geostrophic contour currents: Sci- ence, v. 152, p. 502-508. McGeary, D.F.R., 1969, Sediments of the Vema REFERENCES CITED Fracture Zone [Ph.D. thesis]: La Jolla, Univ. Bader, R. G., and others, 1971, Leg 4 of the Deep California, San Diego, 62 p. Sea Drilling Project: Science, v. 172, p. 1197- Watson, J. A., and Angino, E. A., 1969, Iron-rich 1205. layers in sediments from the Gulf of Mexico: Bostrom, K., and Peterson, M.N.A., 1966, Pre- Jour. Sed. Petrology, v. 39, p. 1412-1419. cipitates from hydrothermal exhalations on the East Pacific Rise: Econ. Geology, v. 61, p. 1258-1265. MANUSCRIPT RECEIVED BY THE SOCIETY MAY 12, Broecker, W. S., Ewing, M., and Heezen, B. C., 1972 1960, Evidence for an abrupt change in REVISED MANUSCRIPT RECEIVED OCTOBER 18, climate close to 11,000 years age: Am. Jour. 1972 Sci., v. 258, p. 429-448. LAMONT-DOHERTY GEOLOGICAL OBSERVATORY Damuth, J. E., and Fairbridge, R. W., 1970, Equa- CONTRIBUTION NO. 1928

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