Ice-rafted Detritus Research Articles

Paleoceanographic implications of terrigenous deposits on the Maurice Ewing Bank, southwest Atlantic Ocean

Grain size distributions of sediment samples from the Maurice Ewing Bank (MEB) were utilized to identify styles of sediment transport and to estimate current speeds within the benthic boundary layer. These data allow for examination of the relationship between sedimentation on the bank and circumpolar currents. Causal relationships between glacial episodes, current intensification, and bottom current scour were previously postulated. The intent of this study was to test the validity of these suggested relationships. Modern sediment distribution on the MEB is controlled by water column winnowing, bottom currents, and variations in the flux of ice-rafted detritus and biogenic material, and is modified by mass-flow processes. Indirect determinations of current speeds, using well established curves relating current velocity to sediment transport, suggest uniform, near-bottom current speeds of approximately 6–15 cm s −1 over the bank. This velocity range is considered relative, not absolute. Nonetheless, these moderate velocities are supported by geostrophic velocity calculations. Grain size distributions of down-core samples from Plio-Pleistocene sandy units were examined in order to investigate the history of bottom current flow over the bank during this time. The lack of significant variability of grain size distributions down-core indicates that no major fluctuations in circumpolar current intensification have occurred since Plio-Pleistocene time, and, therefore, argue against a causal relationship between glacial conditions and deep-sea (geostrophic) current intensification. In addition, mass movement, mainly in the form of debris flows and turbidites, has resulted in interruptions in the sedimentary record, which in some cases could have been misinterpreted as unconformities. Our scenario for sedimentation over the bank differs from previous ones in that it calls for less dramatic variations in bottom current speeds during Pliocene—Holocene time.

Late Pleistocene Eucampia antarctica Abundance Stratigraphy in the Atlantic Sector of the Southern Ocean

A stratigraphy based upon abundance changes in the marine diatom Eucampia antarctica (Castr.) Mangin is presented for Late Pleistocene sediments of the Atlantic sector of the Southern Ocean south of the Polar Front. This stratigraphy is directly tied to lithology and concentrations of ice-rafted detritus. We find a positive correlation between E. antarctica abundance, increased ice-rafted detritus, and silty diatomaceous sediment. Low E. antarctica counts are associated with little or no ice-rafted detritus and diatomaceous ooze. Tentative correlation with the oxygen isotope record indicates that low E. antarctica abundances are associated with Oxygen Isotope Stages 1, 5a, 5c, and 5e. Late Pleistocene Eucampia antarctica abundance stratigraphy in the Atlantic sector of the Southern Ocean INTRODUCTION The Antarctic region plays a significant role in the world climate system (Fletcher, 1969). Fletcher pointed out that ice cover in the Southern Ocean influences ocean/atmosphere heat exchange and that yearly variations in ice cover may be important in enhancing the climatic effects of minor changes in global heating. It is therefore important to be able to identify both shortand long-term climatic fluctuations in the Southern Ocean recorded in deep-sea sediments. Central to this theme is the development of high resolution stratigraphic schemes. Since foraminifera and coccoliths are largely absent from high southern latitude sediments, the responsibility for a suitable stratigraphic scheme rests with the radiolarians and diatoms. Regional stratigraphies in the deep-sea sediments based upon abundance changes of individual species (Ericson, 1961; Ericson et al., 1961; Hays et al., 1976; Morley and Hays, 1979) or morphologic changes of selected species [e.g. coiling direction in Globorotalia truncatulinoides (d'Orbigny) (Ericson and Wollin, 1956), size change in Coscinodiscus nodulifer A. Schmidt (Arrhenius, 1952; Burckle and McLaughlin, 1977)] have been used to resolve time intervals of less than 50,000 years. Within the past few years it has been possible to further refine the resolution of some of these stratigraphies by correlating them with the globally synchronous oxygen isotope record (Shackleton and Opdyke, 1973, 1976). Using such an approach, Hays et al. (1976) and Morley and Hays (1979) have resolved time intervals of less than 10,000 years with a stratigraphy based upon changes in abundance of the radiolarian Cycladophora davisiana Ehrenberg. In the Southern Ocean, and particularly south of the Polar Front, a number of diatom species show abundance variations during the Late Pleistocene (unpublished data L.H.B.). These include such well-known forms as Coscinodiscus lentiginosus Janisch, Nitzschia kerguelensis (O'Meara), and Eucampia antarctica (Castr.) Mangin. This paper describes variations in abundance of this latter species which is found in the Antarctic and sub-Antarctic regions of the Southern Ocean and is also a constituent of the displaced diatom flora entrained in the Antarctic Bottom Water (Burckle and Stanton, 1975). Five cores in the South Atlantic were selected for this study (see table 1). Four were located south of the Polar Front and one (V29-105) north of it. Although we have studied E. antarctica abundances in other sectors of the Southern Ocean, we have not yet assured ourselves that the fine details of each abundance curve are indeed correlative around the Antarctic. micropaleontology, vol. 29, no. 1, pp. 6-10, 1983 6 This content downloaded from 157.55.39.45 on Tue, 19 Jul 2016 05:50:29 UTC All use subject to http://about.jstor.org/terms

The development of antarctic glaciation and the Neogene paleoenvironment of the Maurice Ewing Bank

A micropaleontologic, magnetostratigraphic, and sedimentologic analysis of 56 piston cores provides the basis of a geologic study of the late Miocene to Holocene depositional and erosional history of the intermediate-depth Maurice Ewing Bank (MEB) located at the eastern extremity of the Falkland (Malvinas) Plateau, southwest Atlantic Ocean. Sedimentation on the MEB is controlled presently by the position of the Polar Front and strong bottom currents beneath the axis of the Antarctic Circumpolar Current (ACC). Fluctuations in the position of the Polar Front and in the intensity of the ACC are inferred to have largely controlled deposition on the MEB since the initiation of ACC flow over the bank in the Miocene. The sedimentary record of the MEB is used to infer the late Miocene to Holocene oceanographic and climatic conditions in the South Atlantic sector of the Southern Ocean. The MEB suffered major erosion by intense ACC flow during late Miocene time which exposed Cretaceous—Miocene sediment near the surface and shaped the present configuration of the bank. This erosional event is inferred to have occurred during late Miocene to early Pliocene time (∼7.2–4.7 m.y.B.P.) with the major phase of erosion occurring between middle magnetostratigraphic Chron 7 and late Chron 6 (∼7.2–6.2 m.y.B.P.). We suggest that the Miocene sedimentary record of the MEB and other paleoenvironmental evidence from the circum-Antarctic region precludes the existence of a West Antarctic ice sheet or extensive ice shelves along the Antarctic margin prior to the late Miocene. During the late Miocene a West Antarctic ice shelf formed in the former West Antarctic Sea and rapidly thickened until it grounded below sea level to form the West Antarctic ice sheet. Formation of this ice sheet, with floating and partially grounded extensions (ice shelves) in the Ross and Weddell embayments, led to the first major production of true Antarctic Bottom Water (formed in a manner similar to today) which permanently altered global abyssal circulation. Late Miocene changes in oceanic CCD levels and a permanent shift in the oceanic 13 C 12 C composition may be attributed to this major change in abyssal circulation. A decrease in ACC velocity during an early Pliocene amelioration of climate led to resumption of deposition on the bank from ∼4.7 to 3.9 m.y.B.P. Intensification of the ACC during the late Gilbert and early Gauss Chrons again resulted in limited deposition and widespread erosion and/or non-deposition over most of the MEB from ∼4.0 to 3.2 m.y.B.P. ACC velocity apparently began to wane during the middle of the Gauss Chron and allowed widespread deposition on the bank during late Gauss time (2.9–2.48 m.y.B.P.) and over more limited areas during earliest Matuyama time (2.48 to about 2.2 m.y.B.P.). During middle to late Matuyama time bottom current intensity reached its greatest post-Miocene level forming a regional disconformity between sediments of about 2.0 and 1.0 m.y. in age. This increased circumpolar circulation and associated erosion is inferred to have peaked during the late Matuyama Chron (1.2–1.0 m.y.B.P.) and is approximately the same age as the tentatively dated greatest Patagonian Glaciation in nearby southern Argentina. Between ∼1.0 and 0.7 m.y.B.P., the bank was blanketed with a coarse, erosion-resistant layer of ice-rafted detritus which armored the older sediment thereby protecting it from major subsequent erosion. Sedimentation on the MEB during the Bruhnes Chron (720,000 yr B.P.–Present) was intermittent and finally culminated with the deposition of a veneer of carbonate ooze during or since latest Pleistocene time.

The mode and mechanism of the last deglaciation: Oceanic evidence

Changes in ocean temperature, carbonate productivity, and ice-rafted detritus in the North Atlantic suggest that half of the Northern Hemisphere ice volume at the last glacial maximum had disappeared by 13,000 yr B.P., despite the still-extensive limits of the ice sheets. This early thinning of the ice sheets occurred during a time when summer insolation values were slowly rising but when pollen evidence south of the ice margins indicates cold, dry air masses. We infer that this rapid early ice disintegration (16,000–13,000 yr B.P.) was caused by oceanic mechanisms: (1) rising sea level, causing increased calving along ice margins; (2) the chilling of the sea-surface by icebergs and meltwater, reducing moisture extraction by the atmosphere and transport to the ice sheets; and (3) winter freezing of the low-salinity meltwater layer, suppressing local moisture extraction and the regional influx of moisture-bearing storms from lower latitudes in winter and hence starving the ice sheets. These oceanic feedback mechanisms were strongest from 16,000 to 13,000 yr B.P., and weaker but still active from that date until the end of deglaciation at 6000 yr B.P.

Sediment waves and other evidence of paleo-bottom currents at two locations in the deep Arctic ocean

Sediment waves and other evidence of strong paleo-bottom current activity were observed in the deep Arctic Ocean during the 1967 through 1970 drift of Fletcher's Ice Island (T-3). On the crest of the Alpha Cordillera, between depths of 1100 and 2500 m, a blanket of sediment waves approximately 100 m thick overlies the 200–800 m thick sequence of stratified sediments which cover the basement topography of the cordillera. Maximum wave amplitudes were 55 m, with 1000 m wavelength. These waves were observed over an area of approximately 30,000 km 2. In one instance a fortuitous loop in the drift track showed that at least two of these waves parallel the 150°W meridian. Many sediment cores and camera stations indicate that these sediment waves are themselves blanketed by at least several meters of pelagic lutite and ice-rafted detritus, and show that the wave-forming process has been inactive since at least mid to Upper Pliocene times (3.0–3.5 m.y. B.P.). However, nepheloid layer and current measurements suggest that the present bottom circulation, while too weak to produce such waves, might be capable of producing the drastic change in regimen represented by these waves if current speeds were increased considerably. Such a regimen probably existed for a certain period prior to the development of the present Arctic ice-cover. At a second location, buried waves appearing seismically as two sequences of hyperbolic echoes, were observed in a 300 m thick zone, 550 m beneath the 3260 m deep Mendeleyev Abyssal Plain. This zone in turn overlies at least another 1500 m of horizontally stratified sediments. Adjacent to this plain, by the Mendeleyev Fracture Zone, other evidence for strong current activity was found, in the form of a depositional scarp apparently maintained by current scour, a zone of present day bottom erosion, and a network of relict abyssal channels. These waves may mark various stages of ‘commotion in the ocean’ such as those suggested by Berggren and Hollister (1977) for the Pliocene, Cretaceous, or perhaps even earlier. Such drastic changes in the sedimentological regimen, inherent in the mechanics of plate tectonics, may provide additional clues to the origin of the Amerasia Basin in the Arctic Ocean.

Observations on depositional environments and benthos of the continental slope and rise, east of Newfoundland

Sedimentologic, biologic, and morphologic criteria permit recognition of four depositional environments on the continental slope and rise, east of Newfoundland. The 'upper slope' (300–700 m) has a hummocky substrate with a mantle of terrigenous, gravelly muddy sand which is a mixture of ice-rafted detritus and sediment reworked from underlying glacial drift deposits. Reworking presumably took place during the last major lowering of sea level and it is continuing today under the influence of the Labrador Current and other oceanographic and biologically-related forces. The featureless bottom of the 'middle slope' (700–2000 m) is the principal depositional site of Recent mud. Fines, reworked from shelf and upper slope sediments, settle out together with fines transported to the area by the southeast-flowing Western Boundary Under-current (WBU). Compared to the upper slope this deeper environment receives less ice-rafted clasts, supports a richer macrofauna, and has a higher total species diversity of foraminifera. The 'lower slope' (2000–2500 m) is characterized by higher amounts of gravel and sand mixed with the mud, increasing numbers of current bedforms, and a more diverse foraminiferal assemblage, all of which correlate with the increasing power of the WBU with depth. The gravel was ice rafted probably at the end of the late Wisconsin to early Holocene and its presence on the seabed reflects the power of the WBU to inhibit deposition of Recent mud. The 'rise' (2500 to > 3000 m) is heralded by a subtle break in slope at about 2500 m. A high speed core of the undercurrent is situated in this area as indicated by the coarseness of the sediments (gravelly muddy sand) and the observed current bedforms. A marked increase in the numbers of benthonic and planktonic foraminifera is related primarily to the winnowing capacity of WBU. Numerous arenaceous deep sea forms first occur between 2500 and 3000 m and appear to reflect the reduced salinity, low temperature, high dissolved oxygen characteristics of the watermass that is associated with this depth interval.

Flow of Bottom Water Out of Weddell Sea, Antarctica--700,000 Years to Present: ABSTRACT

Bottom water produced at the Antarctic continental margin (loosely defined here as Antarctic bottom water, AABW) is thought to exert an important influence on calcium carbonate dissolution and terrigenous sedimentation in basins of the world ocean. Indeed it has been suggested that variation in AABW production and flow causes significant seafloor scouring in the Southern Hemisphere ocean basins. Today, most AABW flowing into the world ocean originates in the Weddell Sea. Piston cores from this vital region taken in the path of proposed present-day bottom-water flow, have been subjected to micropaleontologic, geochemical, and size analyses and their Th230 and paleomagnetic stratigraphy determined. These data have been used to reconstruct the history of AABW prod ction and flow in the Weddell Sea during the past 700,000 years. Variations in standard deviation ranging from 1.473 to 2.339, with sediments laminated and nonlaminated respectively, indicate fluctuations in apparent bottom-water-flow velocity and possibly, in turn, production in the west-central Weddell Sea. These fluctuations between peak and low velocities have occurred over periods of more than 100,000 years. Periods of peak-flow velocity also correspond to times when the calcite compensation depth (CCD) was elevated. Only in the past 300,000 years has the level of the CCD fallen below 4,000 m. Not yet fully understood is what seems to be a more rapid and chaotic periodicity in flow in the northwestern Weddell Sea and an apparent correlation between fluctuations and ice-rafted detritus (IRD). It does, however, seem apparent that during the past 700,000 years bottom-water flow in the Weddell Sea has not been strong enough to cause scouring. End_of_Article - Last_Page 467------------

Ice-rafted detritus and paleotemperature: Late Cenozoic relationships in the Ross Sea Region

Four “Eltanin” piston cores spanning 66°S to 71°S along a meridian which passes through the Ross Sea have been used to compare paleotemperature records based on radiolarian faunas with ice-rafted detritus (IRD) accumulation rates. Biostratigraphic/magnetostratigraphic core-bottom ages range from 0.7 m.y.B.P. to 3.2 m.y.B.P. Paleotemperature variations are clearly correlatable from core to core, while fluctuations in IRD accumulation rates at these sites show slight correspondence. Paleotemperature and IRD values in each core were compared to determine the succession of intervals of obviously positive, negative and nil correlation. This information was compared with a two-part model of IRD deposition derived from a consideration of the important principles of glaciology, iceberg dynamics, oceanography, paleoclimatology, paleo-oceanography and glacial history which have a probable bearing on IRD production and distribution. Case I of the model includes the present-day environment. It assumes the presence of a large ice shelf in the Ross Sea and unimportant winter/summer differences in the location of IRD accumulation zones. Regions of maximum interglacial IRD accumulation occurred beneath the ice shelf and near the Antarctic Convergence. During glacial expansion, the zone beneath the ice shelf diminished in importance and large amounts of IRD were deposited seaward of the ice-shelf margin as well as in the vicinity of a more northerly, glacial Antarctic Convergence. Case II reflects conditions prior to ice-shelf development with a significant seasonal variation in the distribution of IRD. The warmer episodes were probably marked by uniformly high rates of IRD accumulation between the continent and a region of high winter sea-surface temperature gradient. Cool periods saw two separate zones develop, one close to the continent and another close to the region of strong thermal contrast. Model comparison of IRD-paleotemperature correlations established for cores in this study reveals a gradual but irregular migration of the zones of greatest average glacial and interglacial IRD accumulation northward over about 5° of latitude from 3.2. m.y.B.P. to approximately 2.4 m.y.B.P. followed by a southward retreat to about their present position by 1.6 m.y.B.P. This was succeeded by a lengthy northward migration which culminated by approximately 0.3 m.y.B.P. and a final recovery to the current glacial-interglacial configuration. Case II “ice-shelf-free” conditions best explain core data in the interval between 3.2 and 2.6 m.y.B.P., suggesting an episode of significantly reduced Antarctic glaciation. Extensive perennial sea ice events are indicated at about 2.3, 2.1, 1.7, 1.1, and 0.3 m.y.B.P.

Late Quaternary deposition of ice-rafted sand in the subpolar North Atlantic (lat 40° to 65°N)

A major change in the North Atlantic pattern of ice-rafting deposition, during the last interglacial-glacial cycle, occurred approximately 75,000 B.P. Prior to this time, deposition for a period of almost 50,000 yr during isotopic stage 5 was greatest in the northwest near Greenland and Newfoundland. The main glacial pattern was very different; the main depositional axis shifted abruptly to a zonal axis along lat 46° to 50°N, reflecting the passage of ice farther from the pole before reaching water warm enough in which to melt. This pattern remained essentially unchanged for 65,000 yr during the main Wurm glaciation. The peak interglacial depositional pattern can best be explained by analogy with the modern oceanic flow, except for the addition of a concentrated eastward component along lat 50°N. The glacial pattern is also best explained by counterclockwise flow. Laurentide and Greenlandic ice entering the western North Atlantic from the Labrador Sea moved to the east and southeast directly into the glacial depositional maximum. Scandinavian ice dropped part of its bed load near Norway, looped to the southwest into the North Atlantic in a counterclockwise passage south of Iceland, and finally melted along the primary depositional maximum. Total input rates of ice-rafted sediment to the Atlantic and Norwegian Sea increased slightly at 115,000 B.P. (the glacial inception), rose markedly at 75,000 B.P. (the major glacial transition), and continued to rise late in the Wurm toward the late-glacial maximum. North Atlantic ice-rafting deposition is thus positively correlated with ice-sheet size. During Quaternary time, roughly 70% of ice-rafted deposition of continental detritus in the world9s oceans has occurred in the subpolar North Atlantic south of Iceland. During the past 3 m.y., a mass of wet unconsolidated drift estimated at 200,000 km 3 has been moved from the continents to the deep Atlantic by ice-rafting alone. This is equivalent to a layer of drift 16 m thick over all parts of the continents thought to have supplied ice-rafted detritus to the North Atlantic.

The discovery Tablemount chain

A high carbonate ooze core of Lower Pleistocene age from the Discovery Tablemount contains planktonic foraminifera, coccoliths, ice-rafted detritus and little clay. Thin-shelled planktonic foraminifera indicate a shoaling to 200 m during the Lower Pleistocene. Changes in the foraminiferal population indicate two cold episodes during which the Antarctic Convergence was considerably further north than at present. An abundance of ice-rafted quartz sand grains coincides with the latter part of a warm period; the grains are ice and wind erosion products of exposed Antarctic rocks. Dredged volcanic rocks indicate volcanism occurred on the tablemount at 25 m.y. BP (K/Ar) and 6.5 to 7 m.y. BP(induration of ooze).

Climate Change in the North Pacific Using Ice-Rafted Detritus as a Climatic Indicator

The variations in weight percent of the grain size fraction greater than 250 μ in nine cores from the North Pacific were determined using sampling intervals of 5 to 20 cm. Material in this size fraction is interpreted as transported by icebergs, and fluctuations are attributed to the waxing and waning of glaciers on the surrounding continents. At least eleven periods of increased ice rafting are detected in the cores during the time from 1.2 m.y. ago to the present, whereas only about four are identified from 1.2 m.y. to 2.5 m.y. B.P. The dating and time correlations are based on the magnetic stratigraphy, ash falls, and faunal extinctions. The ice-rafted detritus indicates a cooling beginning about 1.2 m.y. ago and becoming very intense between the Jaramillo event and the Brunhes-Matuyama boundary. This time may correspond to the initiation of mid-latitude glaciations of Europe and North America. At least six zones of ice-rafted sediment are present in the Brunhes normal polarity series. The correlations between these and the carbonate fluctuations of the central Pacific are good. Evidence for a marked interglacial ranging from about 460,000 to 530,000 yrs B.P. occurs within these cores. This interglacial may be worldwide in extent.

Paleomagnetic Study of Antarctic Deep-Sea Cores

The magnetic inclinations and inten sities of about 650 samples from seven deepsea cores taken in the Antarctic were measured on a spinner magnetometer. This series of measurements provided a magnetic stratigraphy, based on zones of normally or reversally polar ized specimens for each core, which was then correlated with the magnetic stra tigraphy of Cox et al. (1). One core (V16-134) gave a continuous record of the paleomagnetic field back to about 3.5 million years. When selected samples were subject ed to alternatingfield demagnetization, most were found to have an unstable component that was removed by fields of 150 oersteds; all samples from two cores were partially demagnetized in a field of 150 oersteds. The average inclination in these two cores was then in good agreement with the average inclination of the ambient field for the latitude of the core site. It was also found that the intensities of the samples decreased at the points of reversal; this finding is to be expected if, as has been postulated by the dynamo theory, the intensity of the dipole field decreases to zero and builds again with opposite polarity. We believe that the magnetiza tion of the cores results from the pres ence of detrital magnetite, although other magnetic minerals also may be present. Four faunal zones (, X, , and ) have been recognized in these Antarctic cores on the basis of upward sequential disappearance of Radiolaria. The faunal boundaries and reversals consistently have the same relations to one another, indicating that they are both timedependent phenomena. Using previously determined times of reversal, one may date the following events in the cores: 1) Radiolarian faunal boundaries:-X, 2 million years; X-, 0.7 million years; -, 0.4 to 0.5 million years. These dates are in good agreement with ages previously extrapolated from radio metric dates. 2) Initiation of Antarctic diatom ooze deposition, approximately 2.0 mil-lion years ago. 3) First occurrence of ice- rafted detritus, approximately 2.5 million years ago. One can also calculate rates of sedi mentation, which vary in the cores studied from 1.1 to about 8.0 millimeters per 1000 years. Sedimentation rates for the Indian Ocean cores are higher than for the Bellingshausen Sea cores. The near coincidence of faunal changes and reversals in the cores suggests but does not prove a causal relation. We conclude from this study that paleomagnetic stratigraphy is a unique method for correlating and dating deep sea cores, and that future work with such cores may provide a complete or nearly complete record of the history of the earth's magnetic field beyond 4 million years.

Ice-Rafted Detritus as a Climatic Indicator in Antarctic Deep-Sea Cores

Ice-rafted detritus is readily identified in sediment cores raised from the deep ocean floor around Antarctica. A few cores have reached a depth below which no ice-rafted material is found. This depth is interpreted as indicating the establishment of earliest Pleistocene glaciation in the Southern Hemisphere. It is just below a depth where there is a change in assemblages of Radiolaria which Hays associates with the Pliocene-Pleistocene boundary. The presence of ice-rafted material throughout the upper zone in cores taken south of the Polar Front indicates continuity of glaciation in Antarctica. Further north, near 45 degrees S in the Argentine Basin, zonation of the ice-rafted detritus can be used to delineate glacial stages of the Pleistocene.