Ice-rafted evidence of long-term North Atlantic circulation

Marine Geology, 64 (1985) 131--141

131

Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands

I C E- R AF TED EVIDENCE OF LONG-TERM N O R T H A T L A N T I C C IR C ULATI ON

F.W. S M Y T H E ,

Jr J, W.F. R U D D I M A N

~ and D.N. L L r M S D E N ~

~Department of Geology, Memphis State University, Memphis, TN 38152 (U.S.A.) 2Lamont-Doherty Geological Observatory and Department of Geological Sciences of Columbia University, Palisades, N Y 10964 (U.S.A.)

(Received March 15, 1984; revised and accepted May 22, 1984)

ABSTRACT Smythe Jr., F.W., Ruddiman, W.F. and Lumsden, D.N., 1985. Ice-rafted evidence of longterm North Atlantic circulation. Mar. Geol., 64: 131--141. The absolute and relative rates of deposition of ice-rafted volcanic and terrigenous clastic material were determined for the interval from the base of oxygen isotope stage 5 (-127,000 yrs B.P.) to the lower part of stage 3 (~50,000 yrs B.P.) in five subpolar North Atlantic deep-sea cores. During this time interval, global climate changed from a full interglaciation to nearly a full glaciation. For the sediment derived from the continents (dominantly quartz and feldspar), the eastward decrease in deposition rates during

interglacial intervals and eastward increase during glacial intervals is best explained by a southeastward jump in the locus of iceberg melting during climates colder than that of today. A similar model may explain the eastward increase in volcanogenic debris during glacial time intervals; however, the eastward increase of volcanogenic debris during interglacial stage 5 requires a stronger southeastward component of flow from Greenland than is evident in the modern circulation. INTRODUCTION Ice-rafted detritus forms a major c o m p o n e n t o f late Quaternary sediments in the subpolar N o r t h Atlantic. Bradley et al. (1942) first described the alternating carbonate oozes and glacial marine strata in the N ort h Atlantic and established that the mineralogic fraction deposited during late-glacial time was d o m i n a t e d by quartz and feldspar derived from the continents. T h e y also detected a smaller but significant c o m p o n e n t o f the sand-sized fraction consisting of volcanic glass and o t h e r volcanogenic debris derived f r o m Iceland and possibly Jan Mayen Island (see also Conolly and Ewing, 1965; R u d d ima n and Glover, 1972; Molnia, 1972). Ruddi m an (1977) discovered th at the locus of m a x i m u m absolute rate of deposition o f icerafted sand oscillates between an interglacial position like that of t o d a y along the coast o f Greenland and a glacial band trending east-northeast across th e No r th Atlantic between 45 ° and 53°N. In cores along the margins o f the Labrador Sea, Fillon et al. (1981) f o u n d considerable am ount s o f late Quaternary sand, some of which is ice-rafted. 0025-3227/85/$03.30

© 1 9 8 5 Elsevier Science Publishers B.V.

132 One question not resolved by those studies is whether there have been temporal variations in the composition of ice-rafted detritus that might provide information on changes in the locii of major continental glaciations or in the transit paths of icebergs through the ocean. Bradley et at. (1942) noted that the composition of the mineral sand fraction changed very little in a suite of cores taken along latitude 49°N; subsequent work has shown that their analyses were confined to the glacial maximum portions of the last climatic cycle (roughly oxygen isotopic stages 2, 3 and 4). In addition, there was a lack of time control adequate to convert relative compositional variations into absolute input rates for each mineral fraction. In the intervening years, extensive studies of North Atlantic sediments have provided a large number of piston cores with improved age dating and geographic coverage (Ruddiman, 1977). This enabled us to start a reconaissance study to determine whether compositional variations in mineral sands during the earlier phases of the last glacial cycle could provide paleoclimatic information n o t available from earlier efforts. CORE SELECTION AND TIME CONTROL Five subpolar North Atlantic deep-sea cores (Fig.1 and Table I) were investigated. Four intervals of differing climate were chosen: (1) the peak

~

70"

60 °

50 °

40 °

30 °

20 °

10 °

0 o

iO ~

Fig.l. Generalized circulation o f the modern North Atlantic subpolar gyre (after Ruddiman and Glover, 1975). NA C North Atlantic Current; EGC, WGC east, west Greenland Current; asterisks: polar front; dotted line: subarctic Convergence (SAC); open circles: uncertain northern extension of SAC; crosses: southern portion of SAC. Large boxes show location o f five cores used in this study.

133 TABLE I Core locations Core

Lat.

Long.

Water d e p t h (m)

v27-20 V27-19 K708-6 K708-1 V28-14

54°00'N 56°06'N 51°34'N 50°00'N 64°47'N

46°12Tq 38°48~V 29°34~ 23° 45~vV 29°34'W

3510 3466 2469 4053 1855

of the last interglaciation during oxygen isotopic substage 5e (~127,000-115,000 yrs B.P.); (2) an interval (~115,000--77,000 yrs B.P.) encompassing the rest of "interglacial" isotopic stage 5, but including two strong glacial pulses; (3) the major glacial pulse at oxygen isotopic stage 4 (~77,000-65,000 yrs B.P.); and (4) the partial recovery from glacial conditions in the lower part Of stage 3 (~65,000--53,000 yrs B.P.). These four intervals are equivalent to the first four of the seven intervals used by Ruddiman (1977) to study the absolute input rate of the total noncarbonate sand fraction; we have chosen five of the cores he studied but incorporated subsequent improvements of time control. Recent efforts to refine the absolute ages of the oxygen isotopic stage boundaries have shifted the stage 5/4 boundary from 75,000 to 71,000 yrs B.P. (Imbrie et al., in press). In the earlier study, Ruddiman (1977) had used the estimated sea-surface temperature (SST) curve in core V23-82 (Sancetta et al., 1973) as a proxy for the global oxygen isotopic curve because of first-order similarities in their shapes. Subsequent work, however, has shown that the SST curves lag behind the isotopic response by several thousand years in this part of the North Atlantic (Ruddiman and McIntyre, 1981). Taken together, these revisions alter the original age picks in the vicinity of the stage 5/4 transition by about 6000 years, with smaller effects at other levels. Table II lists the revised estimates of the ages of critical control levels used to interpolate the ages of individual samples in each core. Also shown are the depths o f these levels in each core (in cm). TABLE

II

Stratigraphic reference dates in the cores studied

Dated event

Ash zone 1

Ash zone 2 Stage 5/4 Stage 6/5

Date

Depth (cm)

(yrs B.P.)

V27-20

V27-19

K708-6

K708-1

V28-14

9800 57,500 70,000 127,000

48 256 324 505

16 137 188 320

28 202 279 445

62 389 470 858

108 380 435 530

134 METHODS One hundred and ten samples were examined from the five cores (Smythe, 1982). Preliminary tests indicated that the 0.125--0.250 mm fraction was best suited for this study. This was based on a compromise between ease of identification (smaller grains being more difficult) and the need to obtain statistical reliability (larger grains introducing noise). A second reason for the restricted size fraction was the need to merge the compositional counts with weight data without having to correct for large disparities in the mass of individual grains. First, the absolute influx rate of sand in the 0.125--0.250 mm fraction was determined. Samples of total coarse fraction (>0.062 mm) from the original data set reported by Ruddiman (1977) were weighed and treated with acetic acid to remove the biogenic components. The residue was sieved and the 0.125--0.250 mm fraction weighed to determine its weight percent relative to the total mineral coarse fraction. We then calculated the absolute input rate of the 0.125--0.250 mm fraction in mg/cm2/1000 yr across each of the four time intervals in the five cores by using: (1) coarse fraction percent data (CaCO3 and non-CaCO3) and bulk density assumptions from Ruddiman (1977); (2) the adjusted age model; (3) grain density values for the various mineral components (Smythe, 1982); and (4) a methodology otherwise identical to that explained in Ruddiman (1977) and Smythe (1982). The input rates are calculated from average values of each parameter calculated across each time interval rather than from individual samples because of the poor reproducibility of trends core-to-core as noted in Ruddiman (1977). At the average sample spacing of 3500 years, there is an average of five samples per time block per interval. Grains were then identified and counted using a petrographic microscope, and the counts were tallied using a modified " b a n d " system. Details of counting procedures are available in Smythe (1982). Initially, the composition of all grains in an aliquot of the 0.125--0.250 mm fraction was determined for several samples in cores V27-20 and K708-1. In the second phase of the study, we determined only the relative amounts of volcanogenic versus non-volcanogenic grains and the compositional breakdown of the volcanogenic grains in that fraction. After correcting for differences in grain density of the various mineral components, we used the compositional counts of the 0.125--0.250 mm fraction to separate the total input rates into absolute input rates of the major mineral components in each core during each time interval. RESULTS Initial exploratory counts in cores V27-20 and K708-1 showed that the composition of the non-volcanogenic fraction changed relatively little over the four intervals examined. As noted by Bradley et al. (1942) and Molnia (1972), quartz and feldspar contribute over 80~ of the continental input,

135 with other minerals (pyroxenes, amphiboles, etc.) each adding at most a few percent. Calcareous minerals show spotty one-point maxima, b u t otherwise form a fairly constant 1--4% of the total mineral fraction. Combined with the findings of Bradley et al. (1942), the lack of significant compositional variations in the terrigenous fraction suggested to us that further detailed investigation of this c o m p o n e n t would n o t be fruitful. We also noted that the selected counts from core K708-1 indicated significantly higher percentages of volcanogenic sand than those in V27-20. Molnia (1972) noted a similar geographic trend in the coarser (~>0.5 ram) fraction of several North Atlantic cores. The major constituent of this volcanogenic debris is basaltic rock fragments, with basaltic glass a secondary c o m p o n e n t (Ruddiman and Glover, 1972). Silicic glass and other volcanic c o m p o n e n t s each account for only a few percent. The increased relative abundance of volcanogenic debris in the east-central relative to the western North Atlantic pointed the way to a new strategy: counts of total volcanogenic versus non-volcanogenic sand in the 0.125--0.250 mm fraction. We thus extended the study to five cores to improve the geographic coverage. The results, averaged across each of the four time intervals in the five cores studied, are shown in Fig.2. These maps show an overall increase in the total amount o f ice-rafted debris from the interglacial (substage 5e) to the glacial intervals (stages 4 and 3). This reflects the first-order positive correlation of global ice volume with ice-rafted deposition in the North Atlantic (Ruddiman, 1977). As for the two individual components, the non-volcanic (terrigenous) fraction shows an incoherent geographic trend during the two earlier intervals (oxygen isotopic stage 5). Except for a tendency toward higher input rates in the westernmost core V27-20, the rates of deposition are relatively low and the pattern diffuse. The t w o younger glacial intervals show a more definite trend, with a depositional increase from west to east in the four southern cores and substantial delivery to core V28-14 in the north. These patterns are very similar to those measured for the total 0.063--2.0 mm (non-biogenic) fraction by Ruddiman (1977), as expected from the large contribution of the 0.125--0.250 mm terrigenous fraction to the total mineral sand c o m p o n e n t analyzed in that study. Although a minor component, the volcanogenic fraction displays fairly coherent map patterns in all time intervals. Like the terrigenous fraction, the volcanogenic fraction increases eastward in the two glacial intervals; however, unlike the terrigenous debris, it does so as well during interglacial stage 5. In addition, the volcanogenic input in the northern core was roughly an order of magnitude higher than in the southern cores in all time intervals, whereas the terrigenous input to the north was comparable to that in the south during the interglacial intervals and no more than a factor of four higher during the glacial intervals.

136

ISOTOPIC SUBSTAGE 5e (INTERGLACIAL)

(b) ISOTOPIC SUBSTAGES 5d- 5(:] (,INTERGLACIAL AND GLACIAL)

(c) ISOTOPIC STAGE 4 (GLACIAL)

(d) EARLY ISOTOPIC STAGE 3 (GLACIAL)

NON-VOLCANOGENIC

VOLCANOGENIC

Fig.2. Absolute deposition rates in mg/cm2/1000 yrs of terrigenous and volcanogenic sand (0.125--0.250 ram) for four intervals of time in five North Atlantic cores. MODELS OF DEPOSITION OF ICE-RAFTED DETRITUS

Two very different models (Fig.3) can be proposed to explain the depositional patterns in Fig.2. The first assumes that the patterns primarily reflect changes in the locus of melting of the ice which carries the detrital sands. This model does not require significant changes in circulation between

137 ~0,

60 °

70°

60 °

50 °

50 °

40 °

40 °

30 °

30 °

20 °

10 °

20 °

0o

10 o

10°

0o

Fig.3. Models to explain observed southeastward increase in volcanogenic and nonsand in the five cores used in this study (Fig.2). Locus-of-melting model: the core transect intersects the stippled ENE-trending region of maximum ice melting and dumping of glacial terrigenous debris (dashed lines show accumulation rates in mg/cm2/1000 yrs from Ruddiman, 1977, fig.4g). This model explains observed trends w i t h o u t altered circulation changes. Trajectory model: the core transect intersects the SE-trending t o n g u e o f ice-rafted ash delivered from Iceland (solid lines show ash abundance in sand-sized shards per cm 2 from Ruddiman and Glover, 1975). This model requires altered surface--ocean circulation. volcanogenic

glacial and interglacial periods. The second model assumes that the depositional patterns are direct tracers of the ice-flow trajectories and requires a substantial change in the surface circulation. The locus-of-melting m ode l derives primarily from concepts first presented b y Bradley et al. (1942) and later developed by Watkins et al. (1974) in the Antarctic. It assumes that icebergs will transit through very cold oceanic waters with relatively little melting and deposition of debris until t h e y e n c o u n t e r warmer waters at lower latitudes. Variations in ocean temperature can thus alter t he locus o f ice melting and detrital deposition w i t h o u t changes in the basic circulation trajectories. Ruddi m an (1977} argued t hat t h e depositional patterns o f total ice-rafted detritus in the N ort h Atlantic over the last 125,000 years are consistent with t he locus-of-melting model. Modern interglacial detritus is largely deposited along t he coasts of Greenland and Labrador because icebergs first e n c o u n t e r warm water in t hat area. Similar patterns marked t he last interglaciation. In contrast, with cold water filling th e subpolar N o r t h Atlantic during glacial times (McIntyre et al., 1972;

138

Ruddiman and McIntyre, 1981), icebergs and sea ice would travel much farther southward and eastward before disintegrating in the warmer waters encountered along the polar front. The locus of the band of maximum deposition of ice-rafted detritus during glacial times (Fig.3) corresponds closely to the locus of the glacial-age polar front, lending support to this model. The other model assumes that the deposition pattern of ice-rafted detritus is a direct reflection of the trajectory of ice flow. The model derives from studies o f two zones of ice-rafted silicic volcanic ash in North Atlantic sediments (Ruddiman and Glover, 1972, 1975). The ash zones were interpreted as the p r o d u c t of two discrete injection events in which Icelandic volcanoes erupted shards onto passing sea ice and icebergs, which subsequently rafted the ash into the North Atlantic. Both of these episodes left in the sedimentary record distinct tongues of ash that originate in the Denmark Straits between Greenland and Iceland and twist o u t into the North Atlantic in a cyclonic trajectory to the south and east (Fig.3). Both events occurred during times when ocean temperatures were neither fully glacial nor interglacial (roughly 9800 and 57,500 yrs B.P.). Ice melting in these conditions appears to have occurred both near the source and along the path of transport, leaving a pattern tracing the entire path of ice transit (Fig.3). DISCUSSION

Terrigenous fraction Each model appears to be relevant to different aspects of our data. The terrigenous input patterns (Fig.2) can be explained by the locus<~f-melting model. Deposition of the terrigenous fraction during the last interglaciation was higher at sites to the west and north in warm water which extended close to the ice sources. During the early part of the subsequent glaciation, the depositional maximum shifted to the south and east. The high rate of deposition of terrigenous sand in core V28-14 also follows the pattern shown in Ruddiman (1977). This appears to be related to several factors: proximity to detrital sources in Greenland; susceptibility to downslope displacement from nearby steep terrain; and possible jamming o f transiting ice in the Denmark Straits, causing unusually large amounts o f melting in this region. In the Precambrian shields and Paleozoic fold belts of North America, Greenland and Europe, there are many possible sources for the quartz, feldspar, and accessory continental minerals; our data cannot be used to choose among these sources. Larger rock fragments would be more definitive, b u t they are t o o scarce for precise statistics (Molnia, 1972), There are also many pathways that the icebergs rafting the continental debris may have traveled. Our data on the non-volcanic terrigenous fraction cannot be used to specify these transport paths. Our evidence on the continental fraction probably records changes in the ultimate locus of ice melting and accompanying detrital deposition, as well as local factors near core V28-14.

139

Volcanogenic fraction The depositional patterns of the volcanogenic fraction show b o t h significant similarities with and differences from those of the terrigenous detritus. For the two glacial intervals, there is an exactly proportional eastward increase in absolute depositional rates of the t w o components in the southeru transect, suggesting that the locus-of-melting model is a reasonable explanation for the volcanogenic c o m p o n e n t as well as for the terrigenous fraction. One prominent difference in the distribution of the two components is that the volcanogenic input in the northern core is proportionately much larger for all time intervals than it is in the four southern cores. This is probably due to the close proximity of this core to the major sources of volcanogenic debris. In this respect, the volcanogenic fraction is quite different from the terrigenous continental component; it enters the ocean from a small number of sources (Iceland, Jan Mayen, and East Greenland}, all of which feed icebergs into the North Atlantic at a virtual point source through the Denmark Straits. Several processes thus create the volcanogenic maximum in V28-14: erosion of volcanic debris b y ice at these various volcanic regions and calving of the debris-laden ice into the ocean; jamming and melting of this ice in the Denmark Straits; downslope displacement o f volcanic debris from the Icelandic slopes, particularly during times of lowered sea level; and westward spreading of a small fraction of the Icelandic ash normally erupted into a prevailing westerlies flow. In addition, volcanic eruptions can occur beneath and into ice, causing catastrophic melting events called " J o k u l h a u p s " that may send volcanogenic products onto nearby sea ice or directly into the ocean. The most critical difference between the terrigenous and volcanogenic components is that the rate of deposition of volcanogenic debris increased to the east in the two interglacial intervals (substages 5e and 5d-5a), whereas the terrigenous input decreased to the east. This suggests that the locusof-melting model used to explain the terrigenous trend does not explain the volcanogenic pattern, and that it is necessary to call on an additional factor. The most likely explanation is an altered trajectory of surface--ocean circulation similar to that shown by the ash zones (Fig.3). The data suggest that the counterclockwise pathway of ice to the southeast deduced by Ruddiman and Glover (1972, 1975) may be typical of much broader spans of time than the few years or tens of years required to redistribute the silicic ash from the t w o eruptions. Such a flow pattern would differ substantially from the modern circulation. Today, icebergs traveling south through the Denmark Straits move along the coast o f Greenland to Cape Farewell and then turn into the Labrador Sea, where they move northwestward in the West Greenland Current and join icebergs entering the Labrador Sea from the west coast of Greenland. The surviving icebergs winter over in the Northern Labrador Sea and then move to the southeast in the Labrador Current and melt near the Grand Banks the following summer.

140 In contrast, the pattern shown by solid contours in Fig.3 requires that much of the ice arriving through the Denmark Straits turn southeastward before reaching or upon reaching Cape Farewell. In the case of ash zone 1, Fillon et al. {1981) showed that very little of the ash was deposited in the Labrador Sea. This suggests that most of the ash-bearing ice during this episode turned to the southeast at Cape Farewell (Fig.3). This kind of circulation trend is a viable explanation for our long-term data only if the ice transiting through the Denmark Straits during isotopic stage 5 was largely free of terrigenous sand, or if it carried a very small terrigenous c o m p o n e n t relative to that arriving from sources around the margins of the Labrador Sea. Otherwise, there would be no difference between the long-term patterns of terrigenous and volcanogenic detritus in the four southern cores. There are several possible causal mechanisms to explain why the ice would have followed such a path. These include: (1) stronger westerly flow in the atmosphere driving the mixed-layer flow more vigorously toward the southeast; (2) much larger amounts of sea ice in the Labrador Sea, effectively blocking ice entering the Atlantic through the Denmark Straits from turning to the east; and (3) reduced formation of upper North Atlantic Deep Water in the Labrador Sea, including a diminished influx of the warmer and saltier North Atlantic Drift Water that sinks during this process. Other possible explanations exist. Our evidence suggests that this altered flow path was in effect both during climates as warm as today (Fig.2a) and during intervals when there were climatic episodes (such as isotopic substages 5d and 5b) that were considerably cooler than t o d a y (Fig.2b). It is also possible that this circulation pattern occurred during the glacial episodes (Fig.2c and 2d) but was masked by the strong overprint from the locus-of-melting effect. In any case, these lines of evidence suggest that the modern circulation exchange between the Labrador Sea and central North Atlantic may not be characteristic of the longer history of this region. Even in the modern ocean, tendencies toward increased flux of water away from the Labrador and Greenland coasts and into the central Atlantic are occasionally evident in lowered surface salinities (Brewer et al., 1983). ACKNOWLEDGEMENTS This research was funded by grant OCE80-18177 from the Submarine Geology and Geophysics Program of the National Science Foundation. National Science Foundation Grants OCE82-16061 and OCE81-22083 supporting the Lamont-Doherty Geological Observatory Marine Sample Repository are acknowledged. This is Lamont--Doherty Geological Observatory Contribution No.0000.

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