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 Postglacial Iron-Rich Crusts in Hemipelagic Deep-Sea Sediment 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 sedi- ment, 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 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 POSTGLACIAL IRON-RICH CRUSTS IN DEEP-SEA SEDIMENT 1203 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- 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 1204 McGEARY AKD DAMUTH A180-73 1 : i V G if 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 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 POSTGLACIAL IRON-RICH CRUSTS IN DEEP-SEA SEDIMENT 1205 within 3,000 yrs of the Pleistocene-Holocene boundary.