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Icebergs and Iceberg Rafted Detritus (IRD) in the North Atlantic: Facts and Assumptions

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Icebergs and Iceberg Rafted Detritus (IRD) in the North Atlantic: Facts and Assumptions REGULAR ISSUE Icebergs and iceberg rafted detritus (IRD) in the North Atlantic: facts and assumptions John T. Andrews University ~ Colorado • Bouldel, Colorado USA Abstract Over the last decade considerable attention has been of IRD varies, e.g. Molnia, 1972; Ruddiman, 1977; Bond given to evidence for the iceberg rafting of sediments and Lotti, 1995). into the North Atlantic, especially during Heinrich (H) Glacially transported sediments (striated clasts and events (presumed to be associated with short-lived, far-travelled erratics) were first described in detail from abrupt intervals of increased iceberg and meltwater marine sediments in the North Atlantic by Bramlette flux). However, our knowledge of the present sediment and Bradley (1940). Little came from these investiga- distribution within icebergs and the fate of this sedi- tions until the 1970s (cf. Molnia, 1972) when downcore ment during iceberg drift is rudimentary. Debris is usu- changes in IRD were computed as fluxes over intervals ally concentrated in a 1 3 m layer at the base of a glacier associated with Marine Isotope Stages 1 to 5 (approxi- ; if present in calved icebergs, this layer is rapidly melt- mately the last 130 ka) (Ruddiman, 1977). Subsequently, ed within a few 10s to 100s of km from an ice margin. Heinrich (1988) systematically counted the numbers of Ice rafted debris (IRD) is frequently characterized by lithic clasts and foraminifera in a series of cores west of the weight percentage or counts of mineral grains Portugal, and showed that the percentages of lithics > within certain size fractions (i.e. > 63, > 250 Fun, etc.) 180 pm in these cores represented a series of IRD events. with an implicit assumption that the selection of the These discrete events were later named "Heinrich" or size limits does not bias the data nor their interpreta- H-events (Broecker et al., 1992) and interpreted as being tion. However, studies of glacial and glacial marine the result of "armadas" of icebergs being suddenly sediments indicate there is a poor correlation between released from the Laurentide Ice Sheet (Broecker, 1994; the weight percentage in different sand-size classes, Dowdeswell et al., 1995; MacAyeal, 1993). Since the moreover, the bulk of glacially derived sediments are early 1990s numerous papers have described and inter- typically silts and clays (< 63 Fun). In areas close to the preted evidence for H-events in the North Atlantic (e.g. probable sources of iceberg discharge during Heinrich Hillaire-Marcel et al., 1994; Keigwin and Lehman, 1994; events, the abrupt changes are poorly defined by the Hemming et al., 1998; for review see Andrews, 1998). H- sand-size IRD fraction but are dramatically highlighted events have durations of a few hundred to 1000 years by the input of fine-grained (< 63 Fun) detrital carbon- and recurrence intervals of approximately 7,000 years ate. During the last glaciation, abrupt changes in IRD on average (Bond and Lotti, 1995). concentrations (IRD spikes) appear to be an important The purpose of this review paper is to examine what feature of the mid-latitude belt of the North Atlantic, we know about the present distribution of icebergs, the whereas at higher latitudes, such as areas of the controls on the distribution of IRD into the oceans, the Labrador and Nordic seas, IRD input was more contin- characteristics of sediments released from iceberg melt- uous and individual H-events cannot be identified ing, and some issues that pertain to the interpretation of solely by their IRD signal. H-events. Warren (1992) noted that the interpretation of IRD records can be complex; however, the interest in Introduction millermial-scale climate change has emphasized the Over the past decade considerable interest and climatic interpretations of iceberg rafted sediment the excitement has been sparked by the identification in expense of understanding the glaciological and sedi- North Atlantic sediments of discrete core intervals with mentological complexities (cf. Dowdeswell et al., 1999; high concentrations of IRD. IRD from open marine sed- Clarke et al., 1999). iment cores is identified in the literature as mineral This paper will focus on problems associated with the grains > 63 pm (sand-size; although the size definition interpretation of changes in IRD concentrations and 100 Oceanography • VoL 13 • No. 3/2000 PRESENT DAY ~°N icebergs is the calving of outlet glaciers from the I[-•iceberg J ¢U:,~2.2 -' ,',',',',',','.'. ~- > Greenland Ice Sheet. A relatively small additional source is the glaciers of the Canadian Arctic Archipelago. Reeh (1985) has estimated the flux of icebergs at 310 km3/yr from various glaciers and sectors of the Greenland Ice I Sheet, based upon the ice sheet's mass balance (Figure 1). Most icebergs are released from outlet glaciers of the I. ~.. { ~4"C wGc ~;"I I ! ~ ~. '*1 • core Greenland Ice Sheet and their rate and direction of drift I Greenland | ;'\ ,~ , " "*~ ~- L~.] ~ ~, ! ~d'~ L.~ is determined by the surface currents. The main currents JCurrent / 2 :" ,-~b'lSO0-30o0 ~ ~.~ 4S°N / : ! Labrador ~'~ ~ ~;,- / responsible for bringing icebergs into the mid-latitude North Atlantic are the Labrador Current (Greenland and J Current |~ "~ ~ "~' : %,Lc ~ - " ~ I limit of Canadian icebergs), and the East Iceland Current (East Greenland). The International Ice Patrol has published a / " ~/ ..... * .'p,o..rs map showing the historic maximum limits of icebergs in l r~ ,Y': i 'it" l (max s. limit of icebergs 45 ° W the North Atlantic (Figure 1). It would be na'fve to ca. 28°N, 48°W) associate this limit with the extent of IRD sediment LAST GLACIAL PERIOD unless there is explicit information that all icebergs 65 = N retain measurable sediment concentrations along their Main entire transport route. debris transport source8 2. Sediment distribution in outlet glaciers The melting and destruction of icebergs releases freshwater to the oceans. However, the quantity and pattern of sediment release is controlled by the distri- Cores with bution of sediment within an iceberg, which in turn H-events relates to the patterns of sediments within glaciers and outside main IRD beJt ice sheets, and by the disintegration history of each [] iceberg. The main control on sediment distribution Coreswith within glacier ice depends on the basal thermal regime no H-coeval events of the ice mass (Boulton, 1972; Dreimanis, 1976; Drewry, 1986; Alley and MacAyeal, 1994). 45°W There are two primary sources of sediment for glaciers and ice sheets; the first is from the subaerial Figure 1. Upper: Map of the North Atlantic region showing the present weathering of exposed cliffs and nunataks that project volume of icebergs calved each year from Greenland (from Reeh, 1985), the above glacier surfaces, and the second is from erosion at fluxes along the Labrador~Newfoundland margin, and the maximum observed limits of iceberg drift (from International Ice Patrol). The present the base of the ice. Material that falls onto the ice sur- day extent of the Greenland Ice Sheet is shown schematically. face, depending upon whether this happens above or Lower: Ice sheet extent around the North Atlantic during the last below the Equilibrium Line Altitude (ELA) of the ice glacial maximum, showing the major sources for icebergs, IRD belt, and sheet, is transported in three distinct positions (Figure cores with ~nd without coeval Heinrich events (e.g. Dowdeswell et al., 2). The first is as supraglacial debris, the second is 1999). The extent of the ice sheets is shown schematically. englacial, and the third is basal debris. This first source of sediment is, however, rather restricted and at the their relationship to iceberg discharge. The four major continental scale will only make up a small fraction of issues that need to be addressed are: 1) what is the past the total sediment load. and present volume of icebergs calved into the North However, for large ice sheets which completely cover Atlantic; 2) what is the probable distribution of all outcrops, the major source of sediment is from the sediments within these icebergs; 3) what are the con- erosion and transport of bedrock at their base. trois on the trajectory and melt histories of icebergs; and However, this will only produce a debris-charged basal 4) what are the characteristics of iceberg transported unit if the glacier is at the pressure melting point, and sediments? The major source of data and ideas for this the thickness of this basal unit is generally limited to 1 review is the information provided by the International to a few meters (Drewry, 1986). Measurements of sedi- Ice Patrol in their web site (www.uscg.mil/lantarea- ment concentrations in glaciers frequently indicate a /iip/home.html), a series of reports from the Scott basal, sediment-rich layer 1-3 m thick with virtually Polar Institute, University of Cambridge (e.g. debris-clear ice above (e.g. Dowdeswell, 1986). Macqueen, 1991), and the numerous core studies In my experience thick sequences of subglacial and reported in the literature (Andrews and Matsch, 1983). englacial sediment within a glacier's margins are rare for 1. Modern iceberg distribution Canadian or Greenland calving fronts. However, thick In the northern North Atlantic the major source for sequences might be produced at the margins of subpolar Oceanography • Vol. 13 • No. 3/2000 101 : ~TEMPERATURE>PRES~URE MELllNG TEMPERATURE ." TEMPERATURE < PRESSURE MELTING TEMPERATURE Figure 2. Schematic distribution of an ice stream moving to the open ocean through an area such as Hudson Strait show- ing the location of supraglacial, englaciaI, and subglacial debris. In fast-flowing outlets most debris is located in the lowermost 1-3 m of ice. The Equilibrium Line Altitude (ELA) on a glacier divides the acccumulation zone from the lower ablation zone. Debris which falls onto the glacier in the abla- tion zone remains on the surface. WARM ~D COLD BA,SED W~ 8~-~0 glaciers, possibly due to a change in basal thermal con- what is more critical is the flux of icebergs at certain ditions over time from cold-based to warm-based transects.
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