A Pervasive Millennial-Scale Cycle in North Atlantic Holocene and Glacial Climates
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S. R. O'Brien et al. Science 270 1962 (1995).
Core samples were collected with precision cutting tools to remove material in narrow slots at centimeter intervals. We avoided sampling the core's outer rind and obvious disturbances such as burrowing. All samples (dry) were weighed then washed to separate the fine (<62 μm) from the coarse sediment fraction. All petrologic analyses were done in the 63- to 150-μm size range which consistently contains at least a few hundred lithic grains. A portion of that grain fraction was placed on a glass slide (with glycerin as a mounting medium) and 300 to 500 grains were counted per sample using a line rather than point counting method. A key factor in our ability to measure certain tracers accurately is a specially prepared microscope. We placed a white reflector on the condenser lens and illuminated the sample slide from above with a halogen light source. By moving the substage reflector up and down a position can be found that creates a strong impression of relief and brings into striking view details of surface textures and coatings on grains. The technique is especially useful for identifying hematite-stained grains even when the stain is quite small (Fig. 3). Counting of lithic grain concentrations was done in the >150 μm fraction. To avoid errors introduced by splitting small grain populations we did not split samples for this procedure. Counting of planktonic foraminiferal concentrations and abundances was done on splits of the >150 μm fraction. For the abundance measurements 300 to 500 individuals were counted. Species in the >150 μm fraction were selected for planktonic and benthic isotopic measurements. All isotopic measurements were made at North Carolina State University by W.S. The samples were crushed and washed with reversed-osmosis water and air-dried at 65°C and then 30- to 100-μg sample aliquots were isotopically analyzed in a Kiel Autocarbonate device attached to a FMAT 251 RMS. NBS-19 NBS-18 and the NCSU CM-1 marble standard were analyzed with every sample run for standard calibration and sample correction. During the period these samples were analyzed for samples in the 10- to 100-μg size range the standard reproducibility was 0.07 per mil for 13 C and 0.08 per mil for 18 O. Planktonic isotopic analyses for VM 28-14 were made on 18 to 20 individuals per sample and were replicated at a number of depths. For VM 29-191 10 replicate analyses were done on two or three individuals from each centimeter depth with the objective of investigating small-sample variability over time. Because those results showed no coherent relation to our other proxies we combined all measurements from each centimeter with a mass balance equation using the sample size (micromolar) and the sample isotopic composition per mil the results of which are shown in Fig. 2. The final result is equivalent to isotopically analyzing 20 planktonic individuals at once.
The counting error is the 2σ standard deviation for the mean of 20 replicate counts in each of two representative samples.
That we find any measurable record of drift ice as far south as 54°N is not as extraordinary as it might seem. Between about 1880 and the mid-1960s there were more than 20 iceberg sightings in the eastern North Atlantic between 45° and 60°N and icebergs have been reported from as far south as Bermuda and the Azores (40). Even so keeping in mind that the lithic grain concentrations are very low and that each sample integrates 50 to 100 years of time the amounts of drifting ice reaching either site even at peak concentrations must have been small and certainly much less than during the glaciation.
T. Johannessen E. Jansen A. Flatoy A. C. Raveo in Carbon Cycling in the Glacial Ocean: Constraints on the Ocean's Role in Global Change R. Zahn et al. Eds. (NATO ASI Series Vol. I 17 Springer-Verlag Berlin 1994) pp. 61–85.
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Most of the cores (indicated by solid dots in Fig. 1) have Holocene sections documented on the basis of radiocarbon dating isotopic measurements or the presence of ash layer 1 (∼10 000 years) at depths of tens of centimeters. The existence of Holocene sections in the remainder indicated by plus signs is inferred from distinct color patterns and bulk carbonate measurements (41). Core top ages likely vary from close to the present to perhaps a few thousand years; on average then the core top petrologic data reflect distributions of the tracers over time as well as over a large geographic area. The petrographic analyses of each core top lithic sample were done in the same way as for VM 29-191 and VM 28-14.
There probably were no tidewater glaciers in Iceland at least during the middle and late parts of the Holocene ruling out transport of debris to the sites by Icelandic icebergs; direct transport in eruption clouds is also ruled out by the large size of clasts (coarsest grains typically are 0.5 to 1 mm) and by evidence that only two of the six largest Holocene eruptions in Iceland are correlative with the petrologic events (42). The only plausible mechanism therefore is eruption of Icelandic volcanic material onto nearby sea ice (and probably glacier ice as well) and subsequent transport of that ice in surface currents to the coring sites. Explosive eruptions from volcanoes such as Hekla Katla and Grimsvotn occur once every 20 to 30 years or so in Iceland (43) and the Icelandic lithic grains at both sites are dominantly (70 to 95%) clear rhyolitic glass shards the characteristic product of explosive eruptions in silicic volcanoes. The importance of our interpretation of the Icelandic IRD is that its increases at VM 28-14 require increases in drift ice within reach of eruption clouds to the east. Hence changes in surface hydrography favoring preservation and transportation of drift ice in the western North Atlantic must have taken place at the very least from Denmark Strait to the vicinity of Iceland or Jan Mayen.
The carbon isotopic composition of C. wuellerstorfi appears to record the 13 C of total CO 2 of sea water and therefore is taken as a proxy of the nutrient content of that water (44). Today VM 29-191 lies within NADW which is nutrient-depleted and hence δ 13 C-enriched; below the depth of the core are modified waters of Southern Ocean origin which are nutrient-enriched and δ 13 C-depleted (45). Variations in the δ 13 C of that foraminifera are related to shifts in the rate of production of NADW and hence in the rate of convective overturning and thermohaline circulation in the North Atlantic (34 44).
The calendar age model in the composite record for the interval between the Younger Dryas and 18 000 radiocarbon years was constructed by linear interpolation between radiocarbon dates from VM 23-81 calibrated following Bard et al. (46) (Table 1). For the remainder of the record we transferred the GISP2 ages of Heinrich events 2 and 3 (H2 and H3) (47) into their equivalent levels in VM 23-81 (Table 1) (48) and interpolated linearly between the two ages. Our estimate of the age of H3 is corroborated by calibrated radiocarbon measurements in Barbados corals (Table 1) and by paired U-Th– 14 C measurements in aragonite from Lake Lisan in the Dead Sea Rift (49). Detrital carbonate at the Younger Dryas level was deposited rapidly and is widespread extending from the eastern North Atlantic to sources in Hudson Strait and Cumberland Strait (50); it serves as an excellent chronostratigraphic marker. Calibrations of radiocarbon ages in VM 19-30 (Table 1) at 100 and 150 cm are from (46). Remaining calibrations are from the age model for the Holocene-glacial composite record as described above.
The Heinrich events are tied to well-documented lowerings of atmospheric and ocean surface temperatures and to large reductions in the rate of NADW formation (Fig. 6) (34). The two distinct ice-rafting peaks between interstadial 1 and the Younger Dryas dated at ∼13 200 and ∼14 000 calendar years (∼11 300 and 12 000 14 C years) are accompanied by increases in N. pachyderma (s.) measured in the same core (51) and they are coincident with prominent decreases in δ 18 O in Greenland ice (Fig. 6). Evidence of coolings at about the same times have been found in other deep sea cores from the North Atlantic (52) and the Nordic Seas (53) and in pollen and glacial records in Europe and North America (53). From 23 000 calendar years to the end of the record the IRD-petrologic episodes between Heinrich events also occur at times of maxima in abundances of N. pachyderma (s.) (7) and at times of prominent cold phases in the ice core δ 18 O records (Fig. 6). We also find distinct IRD events punctuating prolonged stadials such as between interstadials 2 and 3 and interstadials 4 and 5. There the IRD-petrologic peaks correspond to prominent increases in dissolved Ca (Fig. 6) and in the polar circulation index (23) suggesting a close association of the marine events with expansion of polar circulation above the ice cap. Within the long interval between interstadials 1 and 2 we have identified four other IRD-petrologic events in addition to H1. The N. pachyderma (s.) abundances are saturated through the entire interval preventing reliable estimates of changes in sea surface temperatures. The youngest of the four events dated at ∼15 200 calendar years (∼12 900 14 C years) however is correlative with glacial advances in the U.S. midwest [for example Port Huron Stade (54)] with a glacial readvance on the Scotian shelf (55) and with a Heinrich-like cooling and ice-rafting event identified on the Scotian Slope (56). The other three events a b and c previously identified by Bond and Lotti (7) and dated at about 19 000 21 000 and 22 000 calendar years (∼16 500 17 500 and ∼18 600 14 C years) are present in IRD records from a number of other sites in the subpolar North Atlantic and GIN Seas (57) and they also appear in magnetic susceptibility records from the Labrador Sea (58). Two of these widespread events a and c are correlative with prominent coolings identified in pollen records from a long lacustrine sequence in northern Norway (59). There is evidence that the two older events b and c may even be correlative with glacial advances in the Southern Hemisphere implying a bihemispheric symmetry of the abrupt changes (60).
We define event pacing as the time separating adjacent peaks placed at the midpoints between the peaks (Fig. 6). We define peaks as maxima with at least two points and amplitudes equal to or exceeding the 2σ counting error [4% (8)]; the measure of separation is made between maxima in peak percentage. At the levels of H1 and H2 the sharp decreases in percentages of the two petrologic tracers reflect dilution by massive amounts of IRD discharged from the Labrador Sea (7) and we place the peaks for those two events at maxima in the lithic grain concentrations (Fig. 6).
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We thank G. Kukla J. Lynch-Stieglitz and A. Van Geen for comments on the manuscript. Supported in part by grants from NSF and the National Oceanic and Atmospheric Administration. Support for the core curation facilities of the Lamont-Doherty Earth Observatory Deep-Sea Sample Repository is provided by NSF through grant OCE94-02150 and the Office of Naval Research through grant N00014-96-I-0186. This is L-DEO contribution 5714.