Evolution of Iceberg Melting, Biological Productivity, and the Record of Icelandic Volcanism in the Irminger Basin Since 630 Ka

Marine Geology 212 (2004) 133–152 www.elsevier.com/locate/margeo

Evolution of iceberg melting, biological productivity, and the record of Icelandic volcanism in the Irminger basin since 630 ka

Kristen St. Johna,*, Benjamin P. Flowerb,1, Lawrence Krissekc,2

aDepartment of Geology, Appalachian State University, Boone, NC, USA bCollege of Marine Sciences, University of South Florida, St. Petersburg, FL, USA cDepartment of Geological Sciences, Ohio State University, Columbus, OH, USA Received 8 September 2003; received in revised form 27 August 2004; accepted 17 September 2004

Abstract

Planktic y18O and y13C records and point count records of biogenic, volcanic, and nonvolcanic terrigenous [ice-rafted debris (IRD)] sediment components from Hole 919A in the Irminger basin, northern North Atlantic provide a comprehensive dataset from which a paleoceanographic reconstruction for the last 630 kyr has been developed. The paleoceanographic evolution of the Irminger basin during this time contains both long-term patterns and significant developmental steps. One long-term pattern observed is the persistent deposition of hematite-stained ice-rafted debris. This record suggests that the modern and late Pleistocene discharges of icebergs from northern redbed regions to the Irminger Sea lie in the low end of the range observed over the last 630 kyr. In addition, Arctic front fluctuations appear to have been the main controlling factor on the long-term accumulation patterns of IRD and planktic biogenic groups. The Hole 919A sediment record also contains a long-term association between felsic volcanic ash abundances and light y18O excursions in both interglacial and glacial stages, which suggests a causal link between deglaciations and explosive Icelandic eruptions. A significant developmental step in the paleoceanographic reconstruction based on benthic evidence was for diminished supply of Denmark Strait Overflow Water (DSOW) beginning at ~380 ka, possibly initiated by the influx of meltwater from broad-scale iceberg discharges along the east Greenland coast. There is also planktic evidence of a two-step cooling of sea surface conditions in the Irminger basin, first at ~338–309 ka and later at ~211–190 ka, after which both glacials and interglacials were colder as the Arctic front migrated southeast of Site 919. In addition to offering

* Corresponding author. New address: Department of Geology and Environmental Science, MSC 7703, James Madison University, Harrisonburg, VA, USA. Fax: +1 540 568 8058. E-mail addresses: [email protected] (K. St. John)8 [email protected] (B.P. Flower)8 [email protected] (L. Krissek). 1 Fax: +1 727 553 1189. 2 Fax: +1 614 292 1496.

0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2004.09.004 134 K. St. John et al. / Marine Geology 212 (2004) 133–152 these findings, this reconstruction provides a longer-term geologic context for the interpretation of more recent paleoceanographic events and patterns of deposition from this region. D 2004 Elsevier B.V. All rights reserved.

Keywords: Irminger basin; Pleistocene; marine sediments; ice-rafting; foraminifera; volcanic ash

1. Introduction This study aims to fill in some of that context by reconstructing the paleoceanographic and paleo- A key question in understanding North Atlantic climatic history of the western Irminger basin since paleoceanography is whether the climatic events and 630 ka through a comprehensive examination of the cycles of deposition of the last glaciation were typical isotopic and biogenic, volcanic, and nonvolcanic or atypical of the Pleistocene. While it is established terrigenous [ice-rafted debris (IRD)] sediment that an interplay of orbital and suborbital variables records from Ocean Drilling Program Hole 919A largely controlled marine sedimentation since at least (Fig. 1). With high sedimentation rates of iceberg- the last glacial maximum (e.g., Bond et al., 1997; transported debris, biogenic remains, and occasional Shackleton et al., 2000; Elliot et al., 2001), the longer- volcaniclastic material and due to its subpolar setting term paleoceanographic record of the Pleistocene is south of the Denmark Strait and near the Arctic not as well developed, and therefore the broader front, the Irminger basin is sensitive to climate context is lacking when interpreting recent millennial change and well suited for paleoceanographic scale climate variability. studies.

Fig. 1. Map showing the location of ODP Site 919 in the western Irminger basin, the general locations of potential IRD source areas, major modern surface currents, and the position of the modern Arctic front. K. St. John et al. / Marine Geology 212 (2004) 133–152 135

Important and relevant paleoceanographic results provides a longer temporal record than that of Elliot have already come from Irminger basin sediment et al. (1998, 2001) (630 vs. 60 kyr), and it has records and from other nearby localities. For example, better age control, has more detailed temporal Bond and Lotti (1995) used a 35-kyr record of resolution, and is more comprehensive than previous provenance-distinct IRD (hematite-stained grains) studies of long-term paleoclimatic records from this from the Denmark Strait to support the conclusion area (e.g., Flower, 1998; Koç and Flower, 1998; that synchronous ice-rafting events in the North Spezzaferri, 1998; Krissek and St. John, 2002). In Atlantic correlated to the 2–3 kyr Dansgaard– addition, no previous studies of the Irminger Sea Oeschger (D–O) temperature oscillations in Green- examined the long-term sediment input records of land ice cores during marine isotope stage (MIS) 2–3. hematite-stained grains or dispersed (nonlayered) Elliot et al.’s (1998, 2001) high-resolution studies of volcanic ash. In sum, this study positions us to the iceberg discharges to the western Irminger basin address the question of how the last glacial cycle since 60 ka further established that sediment input to fits in longer-term patterns of Pleistocene climate the Irminger basin was affected by two oscillating and variability. interacting systems: Heinrich-type deposition due to the release of massive iceberg armadas from continental ice sheets every 5–10 kyr and more frequent 2. Methods D–O ice discharge events from northern coastal ice sheets. In addition, van Kreveld et al. (2000) made the 2.1. Age model argument that D–O cycles were driven by the internal dynamics of the east Greenland ice sheet based on Oxygen and carbon isotope data from Hole 919A surface and deepwater paleoclimate records from the were generated on Neogloboquadrina pachyderma Irminger Sea. (s) from the 150–250-Am size fraction (Flower, Other relevant Irminger basin studies consisted 1998; Koç et al., 2001; this study). Data are reported of longer-term (Plio–Pleistocene) but lower resolu- on a meters composite depth (mcd) scale based on tion paleoceanographic reconstructions, in which splicing Holes 919A and 919B using shipboard generally only single proxies were used. This magnetic susceptibility (Shipboard Scientific Party, primarily includes those studies related to Ocean 1994). This procedure accounts for known gaps Drilling Program Leg 152 (Saunders et al., 1998), between cores. Below core 919A 3H (29.02 mcd), such as Spezzaferri’s, (1998) planktic foraminifera no material is available to splice between cores, so biostratigraphy, Flower’s (1998) development of successive cores were placed below previous cores oxygen and carbon isotope stratigraphies for Hole on the mcd scale after sediment expansion. 919A, and Koç and Flower’s (1998) tracking of the N. pachyderma (s) oxygen isotope data were Arctic front through diatom abundance and preser- correlated to the orbitally tuned benthic y18O compo- vation records. In addition, Krissek and St. John site of cores V19–30 (Shackleton and Pisias, 1985) (2002) used the IRD mass accumulation record of and ODP Site 677 (Shackleton et al., 1990), updated to Hole 919A since 960 ka to identify temporal a retuned time scale (Shackleton, 2000). This time covariations of provenance-distinctive grain types; scale is similar but not identical to that found on this covariation suggested that glaciated areas in the Shackleton’s Delphi website (http://www.delphi.esc. Precambrian basement of SE Greenland and in the cam.ac.uk/coredata/v677846.html). For Hole 919A, Tertiary flood basalts south of Scoresby Sund transitions between oxygen isotope stages were used experienced similar iceberg release histories during exclusively to minimize bovertuningQ the record and the Pleistocene. introducing artificial sedimentation rate changes, By examining the abundances of multiple sedi- following Shackleton (2000). It is difficult to identify ment components at an average sample resolution of all the known isotopic stages and substages because of 1.3 kyr over the last 630 kyr, the dataset described coring gaps, insufficient data resolution, and over- here improves our understanding of the paleoceano- printing by isotopically light meltwater. For example, graphic evolution of the Irminger basin. This study the marine isotope stage (MIS) 5/4 boundary (5.0 in 136 K. St. John et al. / Marine Geology 212 (2004) 133–152 the nomenclature of Prell et al. (1986)) is subject to Wright and Flower, 2002). In Hole 919A, early interpretation. The MIS 5/4 boundary (74 ka) is y18O decreases (to values b3.0x) are often accom- interpreted to lie at 15.0 mcd but could lie at 10.95 panied by high % IRD and high % N. pachyderma mcd. However, the latter placement is inconsistent (s), indicative of glacial conditions. Interglacial with the interpretation of Ash Zone 2 (~55 ka) at 11.19 conditions commenced when % IRD decreased mcd (Lacasse et al., 1998). Furthermore, a well-known below about 25% and/or when % N. pachyderma minimum in planktic y13C occurs during MIS 4 in the (s) decreased below about b75%. Accordingly, we northwest Atlantic (Labeyrie and Duplessy, 1985; considered that glacial terminations were marked by Elliot et al., 1998). The presence of this minimum at transitions toward minimum values in % IRD and ~14 mcd supports our interpreted MIS 5/4 boundary at % N. pachyderma (s) after y18O reached minimum 15.0 mcd. The data do not allow unambiguous values (b3.0x). For example, at the MIS 12/11 definition of the MIS 4/3 and 3/2 boundaries. The transition, N. pachyderma (s) y18O decreased to resultant sedimentation rates range from 7.4 during 3.0x at 58 mcd, but % IRD and % N. pachyderma MIS 9 to 27.6 cm/kyr during MIS 13, with a mean of (s) reached interglacial levels at 57.7 mcd, indicat- 15.4 cm/kyr. Overall, the existing data allow adequate ing that MIS 12.0 should be placed at 57.7 mcd. age control for defining orbital-scale climate varia- Similar relationships at terminations I–VII allowed bility in the Irminger Sea (Table 1). refinement of the age model found in Flower Foraminiferal assemblage data and ice-rafted (1998). debris (IRD) data allowed refinement of the age model at glacial terminations by identifying when 2.2. Sediment counts interglacial conditions commenced relative to N. pachyderma (s) y18O. This is necessary because Sediment counts were completed every 20 cm low-salinity meltwater clearly influenced surface downcore to 96 mcd, yielding a total of 450 samples waters in the subpolar North Atlantic during some and an average sampling interval of 1.3 kyr. For each terminations, leading to an early y18O decrease of these counts, the N150-Am grain size fraction was relative to global ice volume (e.g., Lehman et al., split to about 500 grains, weighed, and examined 1993; Raymo et al., 1998; Oppo et al., 2001; under a binocular microscope. Specimens of each planktic foraminifera species, quartz mineral grains, hematite-stained mineral grains, mafic and felsic crystalline rock fragments, sedimentary rock frag- Table 1 Age-control points and calculated linear sedimentation rates (LSR) ments, dark (brown and black) and colorless volcanic for Hole 919A ash (i.e., tephra), benthic foraminifera, pyritized Event Age Depth LSR burrows, radiolarians, and diatoms were counted; the (ka) (mcd) (m/kyr) results of which are discussed here. Because volcanic 2.0 13 1.7 0.13 ash has multiple transport modes, IRD was defined on 5.0 74 15.0 0.22 an ash-free basis in this study; IRD includes the 6.0 132 20.3 0.09 nonvolcanic nonauthigenic terrigenous mineral and 7.0 190 27.6 0.13 rock fragments. 8.0 249 35.4 0.13 9.0 305 46.4 0.20 10.0 336 48.7 0.07 11.0 364 52.6 0.14 3. Results 12.0 425 57.7 0.08 13.0 480 65.9 0.15 3.1. Stable isotopes 14.0 526 78.6 0.28 15.0 567 89.6 0.27 16.0 622 94.7 0.09 Oxygen isotope data exhibit a series of variations x x The calculated LSR is to be read from the table as follows: from 0 to with amplitudes of 0.5 to 1.3 between 0 and 13 ka, the LSR is 0.13 m/ky, from 13 to 74 ka, the LSR is 0.22 m/ 48 mcd (331 ka), and individual values range ky, and so on. between 2.59x and 4.67x (Figs. 2 and 3). Higher K. St. John et al. / Marine Geology 212 (2004) 133–152 137

Fig. 2. Total volcanic ash counts, nonvolcanic terrigenous (IRD) counts, N. pachyderma (s) carbon and oxygen isotope variations, the % N. pachyderma (s) record, and calculated linear sedimentation rates for Hole 919A plotted versus depth in meters composite depth (mcd). All sediment counts were on the N150-Am grain size fraction. Isotope variations are expressed as per mil. amplitude excursions are more common in the decrease at 96–95 mcd also defines the maximum record below 48 mcd and include high-amplitude range of y18O values in the 0–96 mcd record, from light isotope excursions of 1.41x to 3.2x. The 2.03x to 5.2x. 138 K. St. John et al. / Marine Geology 212 (2004) 133–152

Fig. 3. Hole 919A planktic foraminifera faunal records versus age. Due to their generally low individual relative abundances, percentages of Globoconella inflata, G. scitula, G. glutinata, and Orbulina species are grouped together as % of other nonpolar foraminifera groups. For reference, marine isotope interglacial stages are shaded and numbered. K. St. John et al. / Marine Geology 212 (2004) 133–152 139

Carbon isotope data exhibit a series of variations mcd. The greatest negative excursion in the carbon with amplitudes of 0.5x to 1.28x between 0 and isotope record (À1.0x) coincides with a sharp 96 mcd (631 ka), and individual values range from decrease in y18O at 59.6–57.9 mcd (438–426.5 ka). À1.0x to +0.28x (Figs. 2 and 4). Below 76 mcd (519 ka), y13C values are negative, with the 3.2. Sediment counts exception of a positive excursion of 0.17x at 82.3 mcd (539.9 ka). Between 75 and 36.6 mcd The biogenic fraction was dominated by planktic (255.1 ka), y13C values display sharp high-ampli- foraminifera, varying in abundance from 0 to tude variations. Above 36.6 mcd, carbon isotopic N176,000 counts/gram (Fig. 3). Neogloboquadrina variations are generally more gradual and less (s) was the most abundant planktic foraminifera negative than those observed between 75 and 36.6 species, comprising N88% of the total planktic

Fig. 4. Hole 919A biogenic counts of siliceous planktic taxa, benthic foraminifera, and pyritized burrows versus age. Marine isotope interglacial stages are shaded and numbered. 140 K. St. John et al. / Marine Geology 212 (2004) 133–152

Fig. 5. Hole 919A volcanic and nonvolcanic terrigenous (IRD) counts versus age and IRD component relative abundances. Marine isotope interglacial stages are shaded and numbered. K. St. John et al. / Marine Geology 212 (2004) 133–152 141 foraminifera specimens in the overall record and between 190.8 and 485 ka and smaller amplitude commonly forming N95% of the total foraminifera fluctuations between 486 and 630 ka. Other than the deposited since 190.8 ka. However, N. pachyderma brief disappearance of N. pachyderma (s) at 190.8 ka, (s) abundances exhibit high-amplitude fluctuations which coincided with a drop in the total foraminifera

Fig. 6. Generalized stratigraphic column for Hole 919A, highlighting depth horizons of visually identified ash beds and the results of the total ash counts and the relative percentages of colorless ash of the total ash input. 142 K. St. John et al. / Marine Geology 212 (2004) 133–152 abundance, all of the decreases in N. pachyderma (s) either a bimodal mix of dominantly brown/black ash coincided with increases in one or more nonpolar with lesser amounts of colorless ash or were foraminifera groups, including N. pachyderma (d), comprised completely of brown/black ash. The tephra Turborotalia quinqueloba, Globigerina bulloides, event at 6.5 ka, however, was a bimodal mix of Globoconella inflata, Globorotalia scitula, Globiger- dominantly colorless ash and lesser amounts of inita glutinata, and Orbulina species. Collectively, brown/black ash. there was a distinct change in the planktic foraminifera record near the end of marine isotope stage (MIS) 7, which is marked by a significant decrease in 4. Discussion foraminiferal species-level diversity. Diatoms, radiolarians, benthic foraminifera, and pyritized burrows 4.1. Evolution of SST variability in the Irminger Sea were most abundant and diverse during MIS 11–14 but became rare or absent in younger sediments (Fig. The abundances of the polar species N. pachy- 4). derma (s) and of nonpolar foraminifera groups in the The nonvolcanic terrigenous component (IRD) Hole 919A sediment counts can be used as proxies for varied in abundance from 0 to 122,000 counts/gram, relative sea surface temperatures (SST) estimates in with a mean value of 11,438 counts/gram for the last the Irminger Sea at the time of deposition. These 630 kyr (Figs. 2 and 5). High IRD events (defined as proxies appear to delineate three intervals during the N50,000 counts/gram) were most common during past 630 kyr with distinct SST characteristics (Fig. 3). glacial stages, particularly during MIS 14, 12, 8, and First, between 630 and 486 ka, polar and nonpolar 6. High IRD events also occurred during interglacials, taxa experienced moderate variations in their abun- including those in MIS 11 and 9. An extended interval dances, although these variations were not always in of high IRD input occurred since the onset of MIS 4. tune with global climate trends as defined by marine Quartz is the dominant IRD component, forming an isotope stages. For example, prior to 514 ka, the average of 84% of the total IRD. Some of these quartz percentages of N. pachyderma (s) generally were grains are hematite-stained. Such grains vary widely higher in glacial stages and lower in interglacials, in abundance (0–100%); on average, however, 25% of however, from 514 to 482 ka, N. pachyderma (s) the total IRD input was hematite-stained. Crystalline percentages consistently were at or above 88%, felsic rock fragments, mafic rock fragments, and indicating that the surface ocean in the Irminger Sea sedimentary rock fragments comprised the remainder was cool during this time, in contrast to warm global of the IRD input to Hole 919A, averaging 6%, 3%, climate trends. and 7%, respectively, of the total IRD. Sedimentary The second distinct SST interval began at 485 ka rock fragments and crystalline mafic rock fragments and ended between 211 and 190 ka. This interval was displayed similar patterns of accumulation; they were characterized by high-amplitude and short-duration both more abundant between 630 and 255 ka than in SST changes, as indicated by percentages of N. sediments younger than 255 ka. Crystalline felsic rock pachyderma (s) that fluctuated between N90% and fragments were most abundant before 540 ka and also 20% within 1.5 to 7 kyr. These extreme and rapid within MIS 6 and 5. abundance fluctuations in relative abundance primar- Volcanic ash varied in abundance from 0 to 82,143 ily occurred during interglacials, but these also counts/gram and had a mean value of 5391 counts/ occurred throughout glacial stage 12. Interval 2 gram for the last 630 kyr (Figs. 2 and 5). High (485–190 ka) is also characterized by a two-step volcanic ash input (N25,000 counts/gram) did not decrease in the abundance and diversity of warm occur in this record until the end of MIS 11 (381.5 water taxa. The first step occurred by the end of MIS 9 ka). The majority (82% on average) of the ash was when decreases in SST appear to have been extreme brown or black. While colorless ash was less abundant enough to negatively impact many of the nonpolar overall, it periodically comprised ~60% to 100% of foraminifera groups. Between 338 and 309 ka, T. the total ash content (Fig. 6). With one exception, the quinqueloba, G. bulloides, G. scitula, and Orbulina tephra events with N25,000 counts/gram contained species all decreased in abundance or temporarily K. St. John et al. / Marine Geology 212 (2004) 133–152 143 disappeared from the record. Based on a diatom times, the position of the Arctic front sometimes extinction event at ~315 ka, Koc¸ et al. (2001) also shifted far to the southeast, resulting in increased argued for a significant cooling in sea surface IRD deposition in the North Atlantic (~508 N) and conditions in the Irminger Sea during MIS 9. Warm reduced heat and moisture transport to the polar water foraminifera made a temporary recovery at the region (Ruddiman and McIntyre, 1981; Smythe et transition to interglacial MIS 7 but experienced a al., 1985). Qualitative estimates of diatom abundance second drop in abundance and diversity between 211 and preservation have already demonstrated that Site and 190 ka. During this time, all nonpolar forami- 919 lies in a position to monitor fluctuations in the nifera groups, diatoms, and radiolarians virtually Arctic front (Koc¸ and Flower, 1998). Those results disappeared from the Hole 919A record (Figs. 3 and indicated that the Arctic front was east of Site 919 4). This change reflects the local effect of global during glacial stages 16, 12, 10, 6, and 2 and west of cooling at the MIS 7–6 transition. Irminger Sea Site 919 during glacial stages 20, 18, 8, and 4. To cooling is consistent with the interpretation by Funder further investigate the history of Arctic front, the et al. (1998) that ice sheets in the Scoresby Sund area percentages of N. pachyderma (s) and T. quinque- reached their maximum extent during MIS 6. loba for the 0–630-ka interval of the Hole 919A An additional observation of both intervals 1 (630– record are compared here (Fig. 7). This approach is 486 ka) and 2 (485–190 ka) is that light y18O based on observations made by Johannessen et al. excursions tend to precede both N. pachyderma (s) (1994) that the Arctic front is characterized by a abundance decreases and total foraminifera abundance faunal transition, with N. pachyderma (s) dominant increases in interglacial MIS 15, 13, 11, 9, and 7 in Arctic waters and T. quinqueloba more abundant (Fig. 3). This may mean that deglaciations during this near the sea ice edge in North Atlantic water. Wright time are marked by early meltwater input followed by and Flower (2002) used relative percentages of these later warming of SSTs and bioproductivity increases. two taxa in records from Feni Drift and Bjorn Drift, A similar observation was made by Elliot et al. (1998) south of Iceland to track the position of the Arctic for the last 60 kyr in the Irminger basin. front between 500–1000 ka; we have applied their The third SST interval was established since 190 approach to the Hole 919A record. ka and continued through the remainder of the In general, prior to 190 ka, there were high- Pleistocene. This interval is characterized by persis- amplitude variations in the abundances of both N. tently cold SSTs in the Irminger Sea during both pachyderma (s) and T. quinqueloba during intergla- glacials and interglacials. This is documented by cials. This suggests that the Arctic front and its consistently very high (N95%) percentages of N. associated sea ice edge repeatedly migrated to a pachyderma (s) from 190 ka to glacial MIS 4 at 69 position near but slightly to the northwest of Site 919 ka and high but slightly more variable percentages of during most interglacials. The abundances of both N. pachyderma (s) since 69 ka. This modest change in taxa were generally less variable during glacials. N. SST conditions interpreted at 69 ka is consistent with pachyderma (s) abundances were generally high, and the results of Funder et al. (1998) who recognized a T. quinqueloba abundances were generally low, shift to milder climates at 70 ka in the Scoresby Sund suggesting that the Arctic front migrated to a more region. stable position far to the southeast of Site 919 during most glacial periods. The primary exception is glacial 4.2. Evolution of Arctic front variability in the stage 12; the repeated high-amplitude variations of the Irminger Sea abundances of both taxa during MIS 12 imply a very dynamic Arctic front, more similar in character to the The modern-day Arctic front transects the typical interglacial Arctic front. Irminger Sea from the southwest to the northeast The general abundance patterns of N. pachyderma and defines the boundary between cold, ice-laden (s) and T. quinqueloba changes near the end of MIS 7. Arctic waters, and relatively warmer and more saline T. quinqueloba disappeared almost completely from North Atlantic waters (Fig. 1; Malmberg, 1985; the Hole 919A record in sediments younger than 205 Kuijpers et al., 2003). During late Pleistocene glacial ka, whereas abundances of N. pachyderma (s) 144 K. St. John et al. / Marine Geology 212 (2004) 133–152

Fig. 7. Arctic front (AF) indicator % N. pachyderma (s) and sea ice edge indicator % T. quinqueloba at Hole 919A for 0–630 ka. Arrows on % N. pachyderma (s) indicate that, when percentages were high, the AF was southeast of Site 919, and when low, the AF was northwest of Site 919. Arrows on % T. quinqueloba indicate that, when percentages were high, the AF was near Site 919, and when low, the AF was far from Site 919. Marine isotope interglacial stages are shaded and numbered. generally remained quite high since 190 ka. This impact of global climate changes on nutrient invento- suggests that the Arctic front primarily has been located ries in the Irminger Sea. The y13C pattern is in far to the southeast of Site 919 since ~205–190 ka, agreement with the interpretation of Flower (1998) migrating somewhat to the northwest only during MIS that the Hole 919A y13C signal since 960 ka was 4 and in the current interglacial. largely controlled by global inventory changes. Overall, our foraminifera-based interpretation of In addition to the glacial–interglacial pattern of Arctic front variability generally agrees with Koc¸ and productivity, the Hole 919A record also includes a Flower’s (1998) diatom-based interpretations. The distinct change in the abundance of siliceous planktic results of this study are also consistent with Wright taxa (diatoms and radiolarians), benthic foraminifera, and Flower’s (2002) interpretation that the Arctic and pyritized burrows after MIS 11. Prior to ~380 ka, front usually shifted to the northwest Atlantic during these groups all experienced extended episodes of interglacials of the past 620 ka. However, unlike high abundance. However, beginning within MIS 11, previous interpretations, this study indicates that the benthic foraminifera abundances decreased by a factor Arctic front was quite variable and frequently north- of 2 to 5, pyritized burrows disappeared until a brief west of Site 919 during glacial stage 12. In addition, reappearance around 200 ka, and diatoms and radio- the foraminifera data indicate that the Arctic front was larians decreased by a factor of 10 (Fig. 4). atypically positioned to the southeast of Site 919 There are at least two possible reasons for the during interglacial stages 5 and 13 (from 515–486 ka). change in the planktic and benthic abundances after MIS 11. One possibility is that, after ~350 ka, the 4.3. Evolution of nutrient supply to the Irminger Sea Arctic front moved farther away from Site 919 and did not return to a nearby position until a period from Interglacial stages in the Hole 919A record were ~262 to ~220 ka (Fig. 7). The migration of the Arctic generally characterized by relatively high abundances front presumably would also mean the migration of of planktic and benthic taxa and high y13C signals the high primary productivity zone in the surface (Fig. 4). The glacial–interglacial patterns of taxa waters associated with the Arctic front (Knudsen and abundances are interpreted to represent the local Eiriksson, 2002) and the sea ice edge (Sathyendranath K. St. John et al. / Marine Geology 212 (2004) 133–152 145 et al., 1995; Heinrich et al., 1996). Benthic produc- in the Greenland Sea during MIS 11 that negatively tivity would also decrease because the main food impacted the production of DSOW, and consequently source for benthic communities in the deep ocean is the Irminger Sea benthic community, as well as the production in the water column (Graf, 1992; Knudsen production of NADW. and Eiriksson, 2002). However, there are two prob- lems with this explanation: (1) the decreases in taxa 4.4. Evolution of iceberg discharges to the Irminger abundances appear to precede the shift in the Arctic Sea front by at least 30 kyr, and (2) when the Arctic front briefly returned to a position near Site 919 at 262 ka, 4.4.1. The temporal pattern of IRD accumulation the expected increases in taxa abundances did not IRD events in the Irminger basin since MIS 16 occur. were deposited under a range of climatic conditions, An alternate explanation for the pattern of benthic including glacial stages, interglacial stages, and productivity before and after MIS 11 may be related to climatic transitions (Fig. 5). IRD events since 190 changes in the chemistry and circulation of the ka generally occurred when SSTs in the Irminger Sea Denmark Strait Overflow Water (DSOW), which were cold, as indicated by high abundances (z80%) bathes Site 919. This subsurface water mass influen- of N. pachyderma concurrent with increased IRD ces thermohaline circulation and global climate (Fig. 8). In contrast, from 630 to 190 ka, IRD events because it contributes to the formation of North generally coincided with warm water pulses, as Atlantic Deep Water (NADW; Broecker and Denton, marked by decreased N. pachyderma (s) abundances 1989; Raymo et al., 1990; Kase and Oschlies, 2000). and increased abundances of nonpolar planktic The benthic abundances prior to MIS 11 may be foraminifera (Figs. 3 and 8). Inasmuch as the Arctic evidence of a strong influx of well-oxygenated front was positioned near (e.g., MIS 11 and 12) or to DSOW. As discussed above, increased productivity the northwest (e.g., during MIS 9) of Site 919 during near the Arctic front (Sathyendranath et al., 1995; much of this time (Fig. 7), the associated SST and Heinrich et al., 1996; Knudsen and Eiriksson, 2002) nutrient gradients (Sathyendranath et al., 1995; and/or increased nutrient supply from thermocline Heinrich et al., 1996) were probably responsible for depths may explain the concurrent changes in sili- the increases in iceberg melt and the changes in polar ceous planktic abundances. A decrease in oxygen to and nonpolar planktic productivity. Hole 919A, resulting from decreased flow of DSOW, could account for the decrease in benthic productivity 4.4.2. IRD composition, iceberg sources, and and diversity during and after MIS 11. dispersal Decreased DSOW during MIS 11 is consistent The presence of hematite-stained IRD in the with interpretations of concurrent events in the North Irminger basin reflects the discharge of debris-laden Atlantic and the Greenland–Iceland Sea. Thunell et al. icebergs from the east central coast of Greenland and (2002) described a decrease in NADW production possibly Svalbard (Bond and Lotti, 1995; Bond et al., during MIS 11 based on a benthic y13C record from 1997; van Kreveld et al., 2000). In this study, typically the western North Atlantic; diminished DSOW 20–40% of all the lithic (nonvolcanic) grains depo- presumably could have contributed to the decrease sited at Hole 919A were hematite-stained (Fig. 5). in NADW production. In addition, the Icelandic basin Modern values for this area do not exceed 15% experienced a period of extreme carbonate dissolution hematite-stained grains, and nearly all modern values and high IRD input at ~400 ka, which Huber et al. N15% are in the Greenland–Iceland sea (Bond et al., (2000) related to a meltwater pulse during the 1997). In addition, the abundance of hematite-stained interglacial. The largest IRD input to Hole 919A also grains at nearby site VM 28-14 was also low (10% or occurred in MIS 11 (419 ka; Fig. 5), just before the less) during the last glaciation (10–30 ka; Bond and decreases in benthic and siliceous planktic groups. Lotti, 1995). This indicates that the modern and late This set of observations raises the possibility that Pleistocene discharges of icebergs from northern red broad scale iceberg discharges from along the east bed regions to the Irminger Sea lie in the low end of Greenland coast may have created a freshwater pool the range observed over the last 630 kyr. 146 .S.Jh ta./Mrn elg 1 20)133–152 (2004) 212 Geology Marine / al. et John St. K.

Fig. 8. Comparison of N. pachyderma (s) abundances, N. pachyderma (s) oxygen isotope variations, colorless ash input, and total IRD input versus age. Marine isotope interglacial stages are shaded and numbered. Dashed lines linking the oxygen isotope and colorless ash curves mark times when explosive eruptions coincided with light y18O excursions. When these dash lies also are linked with the IRD curve, it indicates IRD events also occurred at these times. The values of 3000 and 20,000 grains/g were used as the respective lowest abundance levels marking significant colorless ash and IRD peaks in this comparison; both levels are indicated by horizontal dotted lines. Marine isotope interglacial stages are shaded and numbered. K. St. John et al. / Marine Geology 212 (2004) 133–152 147

In light of these conclusions, some IRD events central east Greenland coast (Bond and Lotti, 1995; elsewhere in the North Atlantic may require reinter- Krissek and St. John, 2002; St. John and Krissek, pretation. For example, Bond and Lotti (1995) 2002; Fig. 1). The results here show that ice concluded that lithic peaks in the eastern North discharges from glaciated regions north of the Atlantic (DSDP 609 and VM 23-81), which had high Irminger basin contributed a greater proportion of percentages (z15%) of hematite-stained grains and IRD to Hole 919A than did ice discharges directly correlated to D–O events in the GIS during MIS 2–3, to the west inasmuch as the inputs of sedimentary must have resulted from discharges of ice in the Gulf rock fragments, mafic crystalline rock fragments, of St. Lawrence. This conclusion is supported by good and hematite-stained grains are generally greater evidence for a Gulf of St. Lawrence source for lithic than the input of felsic crystalline rock fragments events in the western North Atlantic (e.g., Keigwin (Fig. 5). This interpretation is consistent with and Jones, 1995; Piper and Sleene, 1998) and by the previous findings that identified a greater input of judgment that the abundances of hematite-stained basaltic and sedimentary dropstones than gneissic grains transported through the Denmark Strait are too dropstones at Hole 919A (Shipboard Scientific Party, low to explain the high abundances of those grains in 1994). A significant change in the input of the eastern North Atlantic. However, this study shows provenance-specific IRD to Hole 919A occurred at that Pleistocene icebergs commonly carried signifi- ~255 ka. In sediments younger than 255 ka, IRD cant abundances of hematite-stained grains south- derived from glaciers draining mafic crystalline ward, out of the Greenland–Iceland Sea and at least as outcrops and sedimentary outcrops rarely was far south as Site 919 in the Irminger basin. Thus, deposited at Hole 919A, while input from glaciers discharges of ice from eastern Greenland may have draining felsic crystalline outcrops remained variably played a greater role in North Atlantic lithic (IRD) low, and input from glaciers in red bed regions events than previously thought. increased slightly. While hematite-stained grains commonly formed To account for this set of observations, we suggest 20–40% of the IRD input to Hole 919A, multiple that semipermanent sea ice may have been estab- episodes also were recorded when hematite-stained lished since 255 ka in the fjords along the Scoresby grains increased to 65–100% of the IRD input. At Sund coast, blocking or redirecting the discharge of each of the times when the proportion of hematite- icebergs from areas draining basaltic and some stained grains was high (65–100%), total IRD input sedimentary outcrops. An increased presence of sea was low (Fig. 5). Most of these high hematite–low ice in the Scoresby Sund region since 255 ka is total IRD events occurred during interglacials, but consistent with evidence that ice sheets in the one also occurred during glacial stage 14. This Scoresby Sund area reached their maximum size inverse relationship between total IRD and the during glacial MIS 6 and that long-lasting sea ice importance of hematite-stained IRD suggests that cover was present over the continental slope off the Irminger Sea received a constant rainout of ice- Scoresby Sund during that time (Funder et al., 1998). rafted debris derived from central east Greenland and Differences in the thermal regimes of the glaciers possibly Svalbard, but this input was often diluted by draining the basalts and non-red bed sedimentary IRD input from other glaciated regions closer to the units and those draining the red beds could also site of deposition. explain the major decrease in the input of mafic Other major IRD components identified in this crystalline and sedimentary rock fragments to Hole study were felsic and mafic crystalline rock frag- 919A since 255 ka, while the supply of hematite- ments and sedimentary rock fragments (Fig. 5). stained grains continued. The discharge of debris- Previous compositional work identified the dominant laden icebergs containing mafic and sedimentary respective source areas of such IRD in the Irminger rock fragments would have been reduced if the basin to be the Precambrian igneous and metaig- glaciers draining those source areas shifted from neous crystalline basement of SE Greenland, the being warm-based to cold-based at ~255 ka. More Tertiary flood basalts near Scoresby Sund, and information from land-based glaciological studies is exposures of Paleozoic sedimentary units along the needed, however, to evaluate this possibility. 148 K. St. John et al. / Marine Geology 212 (2004) 133–152

4.5. Evolution of Icelandic explosive volcanism as transported as IRD, or (3) ash can be transported recorded in the Irminger basin through the atmosphere and fallout onto glacial surfaces, subsequently being transported to a marine 4.5.1. Source and composition site as IRD. Additionally, in the special case of Tephra events at Hole 919A commonly were subglacial volcanic eruptions, ash may be carried to dominated by brown/black ash with lesser amounts the adjacent sea via jokulhlaups (glacial floods) and of colorless ash (Fig. 6). Geochemical analysis related turbidity currents (Geirsdottir et al., 2000; (Lacasse et al., 1998) of Hole 919A tephra layers, Maria et al., 2000) but probably would not be zones, or pods indicated that each tephra event had an dispersed into the atmosphere for any long distance affinity with either a basaltic or a rhyolitic source in transport. Iceland. We assume that the dispersed ash events For at least the last 300 kyr, the primary dispersal identified in this study have similar Icelandic affi- direction for Icelandic tephra has been to the north liations, with brown/black ash indicating input from a and northwest under the influence of a prevailing basaltic eruption, and colorless ash indicating input wind and ocean current system similar to that of from a rhyolitic eruption. today (Haflidason et al., 2000; Wallrabe-Adams and Lackschewitz, 2003). Therefore, it is likely that most 4.5.2. Stratigraphic distribution ash transported to the Irminger Sea arrived under the Shipboard scientists visually identified six vol- influence of the southward-flowing East Greenland canic ash beds in sediments from Hole 919A (Fig. 6; Current (Fig. 1). Most of the tephra events identified Shipboard Scientific Party, 1994). Lacasse et al. at Hole 919A were diluted by other marine sediment (1998) suggested that the apparent lack of ash layers components, and most of these dispersed tephra between ~14 and 60 mcd in Hole 919A either events cooccurred with increased IRD (Fig. 5). reflected a decrease in the volcanic/rifting activity Assuming past wind regimes similar to those of as no ash fall events were recorded in the Icelandic today, this suggests that the Icelandic ash initially basin (site 907) sediments between MIS 8 and 15 was transported through the atmosphere to glaciated (245–620 ka) or may have resulted from an increase coastal regions of Greenland or to sea ice along the in sea ice during that time, which prevented the Greenland coast and subsequently was ice-rafted to deposition of ash layers on the sea floor. However, the western Irminger basin under the influence of the the data presented here show multiple tephra East Greenland Current. Direct atmospheric transport (N25,000 counts/gram) events within the 14 to 60 was probably a secondary mode of transport for this mcd interval, including the six largest tephra events ash. Such transport must have occurred at least six in our record. This suggests that ash input to the times to explain the texture and bedding character- western Irminger basin was significant throughout the istics of the discrete ash beds (including chronostrati- mid- to late Pleistocene. These events are dispersed graphic marker beds, Ash Zones 1 and 2) in Hole and cannot be identified by visual core descriptions 919A sediments (Fig. 6; Shipboard Scientific Party, primarily because of dilution by glaciomarine and 1994; Lacasse et al., 1998). Any subglacially erupted biogenic sediment components as well as mixing by ash transported to the Irminger Sea via jokulhlaups bioturbation. These tephra events can be identified, and associated turbidity currents presumably would however, by point count analysis of the total sedi- be concentrated in the northeastern part of the basin, ment population. adjacent to the Icelandic margin, and less so at Site 919 in the western part of the basin. 4.5.3. Transport modes Explaining the input of volcanic ash by surface Long-distant transport of ash from an explosive currents (iceberg or sea ice-rafting) brings into volcanic eruptions to a deep marine site of deposition question the time that could have elapsed between can occur primarily in three ways: (1) ash can be eruption and ultimate deposition at Hole 919A. transported directly through the atmosphere to open Lacasse and van den Bogaard (2002) considered this surface water, or (2) ash can be transported through question for Plio–Pleistocene ice-rafted tephras reco- the atmosphere and fallout onto sea ice and be vered from the marine basins surrounding Iceland K. St. John et al. / Marine Geology 212 (2004) 133–152 149

(including the Irminger basin). They concluded that these colorless ash events (N3000 counts/gram) also the time involved was short, ranging from a few years coincide with IRD events (N20,000 counts/gram), to several hundred years, and was below the but approximately 50% do not, eliminating the resolution of current y18O stratigraphy. Larsen et al. possibility that the concurrence between colorless (1998) have shown that the residence time of volcanic ash events and light y18O excursions simply reflects material in some Icelandic glaciers to be as much as the positive relationship between increased ice- 800 years or more. Thus, a reasonable upper time limit rafting and y18O meltwater signals. Instead, the Hole for ash transported to Site 919 via ice-rafting is 919A data supports the hypothesis that episodic several hundred years, and perhaps 1000 years, deglaciations in Iceland and the associated glacio- whereas the time elapse was probably only on the isostatic changes resulted in an increase of explosive order of days to weeks for ash transported to the volcanism in Iceland (Hall, 1982; Sigvaldason et al., seafloor site via atmospheric fallout. 1992; Maclennan et al., 2002). Unlike previous datasets (Sejrup et al., 1989; Haflidason et al., 4.5.4. Timing of tephra events 2000), however, the data presented here shows that A final topic to consider is the temporal relation- this causal relationship between deglaciations and ship between tephra events and evidence of climatic explosive eruptions was not restricted to interglacial change. We focus this discussion on the rhyolitic marine isotope stages but also occurred within tephra events because rhyolitic ash is more likely to glacial stages. The difference in conclusions may be erupted explosively, and explosive eruptions are reflect the difference in sampling strategy between more likely to be either a driver of climate change this study and previous studies; examining dispersed or a consequence of climate change. Airborne ash ash input using point count data produced a more from explosive eruptions may drive climate change continuous record of the relationship between by increasing the reflectivity of the atmosphere, explosive volcanism and y18O than could be causing cooling. For example, increased explosive developed from a study restricted to discrete ash eruptions along the North Pacific rim may have beds. helped rapidly propel that region into large-scale glaciation at 2.67 Ma (Pruher and Rea, 1998, 2001). An increase in explosive eruptions may also be a 5. Conclusions consequence of climate change, particularly for ice- covered Icelandic volcanoes. Ice unloading from The paleoceanographic evolution of the Irminger Icelandic volcanoes during deglaciations that has basin since 630 ka contains both long-term patterns been shown to have influenced both eruption rates and significant developmental steps, which are (Hall, 1982; Sigvaldason et al., 1992; Jull and interpreted from Hole 919A sediment counts and McKenzie, 1996) and compositions of Icelandic isotopic data. The evolutionary development of magma (Maclennan et al., 2002). Lacasse and oceanic and climatic conditions in the Irminger basin Garbe-Scho¨nberg (2001) established that rhyolitic during this time span provides a history and there- tephra in the northern North Atlantic (including Site fore a context in which the millennial-scale cycles 919) and Arctic oceans records the sources and and events of the last glacial cycle can be timing of explosive volcanism in Iceland and the Jan considered. Mayen area. Therefore, the record of rhyolitic tephra The primary long-term depositional patterns that at Hole 919A may provide some insight on the persisted throughout all or most of the last 630 kyr in relationship between Icelandic explosive volcanism the Irminger basin include the following: and climate change over the last 630 kyr. The abundance pattern of colorless ash documents (1) Persistent deposition of ice-rafted debris derived numerous explosive eruptions since 630 ka. The from red beds in central east Greenland and majority of the explosive eruptions coincide with possibly Svalbard, with abundances more typi- light y18O excursions during both glacial and cal of sites in the Greenland Sea and more interglacial marine isotope stages (Fig. 8). Many of abundant than modern Irminger basin values, 150 K. St. John et al. / Marine Geology 212 (2004) 133–152

suggesting that the modern and late Pleistocene Acknowledgments discharges of icebergs from northern red bed regions to the Irminger Sea lie in the low end of We wish to thank Terri King for assistance in the range observed over the last 630 kyr. generating the mcd scale and students Callie Rowe, (2) IRD deposition between 630 and 190 ka that Jennifer Milliken, and Trey Kendrick for assistance in appears to have been largely controlled by the sample processing. Jon Eiriksson and Bill Ruddiman position of the Arctic front. provided insightful and thorough reviews for which (3) An extensive record of Icelandic volcanism we are most appreciative. based on dispersed ash, which links explosive eruptions to deglaciations (i.e., light y18O excursions) in both interglacial and glacial References stages during the last 630 kyr. (4) A pattern of deglaciations between 630 and 190 Bond, G., Lotti, R., 1995. Iceberg discharges into the North Atlantic ka marked by early meltwater input followed by on millennial time scales during the last glaciation. Science 267, later SST warming and bioproductivity increases 1005–1010. Bond, G., Showers, W., Cheeseby, M., Lotti, R., Almasi, P., in interglacial stages, similar to what was deMenocal, P., Priore, P., Cullen, H., Hajdas, I., Bonani, G., concluded for the last 60 kyr in Irminger basin 1997. A pervasive millennial-scale cycle in North Atlantic (Elliot et al., 1998). Holocene and glacial climates. Science 278, 1257–1266. Broecker, W.S., Denton, G.H., 1989. The role of ocean-atmosphere Significant developmental steps in the paleoceano- reorganizations in glacial cycles. Geochimica et Cosmochimica Acta 53, 2465–2501. graphic evolution of the Irminger basin during the past Elliot, M., Labeyrie, L., Bond, G., Cortijo, E., Turon, J.L., Tisnerat, 630 kyr include the following: N., Duplessy, J.C., 1998. Millennial-scale iceberg discharges in the Irminger basin during the last glacial period: relations with (1) Between 630–380 ka, increased supply of the Heinrich events and environmental setting. Paleoceano- DSOW (relative to today) based on benthic graphy 13, 433–446. Elliot, M., Labeyrie, L., Dokken, T., Manthe, S., 2001. Coherent assemblage changes. patterns of ice-rafted debris deposited in the Nordic regions (2) Between 485 and 190 ka, extreme and rapidly during the last glacial (10–60 ka). Earth and Planetary Science fluctuating interglacial (and glacial stage 12) Letters 194, 151–163. SSTs, related to a proximal but migrating Arctic Flower, B.P., 1998. Mid- to Late Quaternary stable isotopic front. stratigraphy and paleoceanography at Site 919 in the Irminger basin. In: Saunders, A.D., Larsen, H.C., Wise, S.W. (Eds,), (3) At ~380 ka, diminished supply of DSOW, Proceedings of the Ocean Drilling Program. Scientific Results possibly initiated by the influx of meltwater vol. 152. pp. 243–248. from broad-scale iceberg discharges along east Funder, S., Hjort, C., Landvik, J.Y., Nam, S., Reeh, N., Stein, R., Greenland. This negatively impacted benthic 1998. History of a stable ice margin—east Greenland during the communities in the Irminger basin and may middle and upper Pleistocene. Quaternary Science Reviews 17, 77–123. have contributed to the diminished supply of Geirsdottir, A., Hardardottir, J., Sveinbjornsdottir, A., 2000. Glacial NADW. extent and catastrophic meltwater events during deglaciation of (4) First at ~338–309 ka and again at ~211–190 ka, southern Iceland. Quaternary Science Reviews 19, 1749–1761. a two-step cooling of sea surface conditions in Graf, G., 1992. Benthic–pelagic coupling: a benthic view. Oceano- the Irminger basin, after which both glacials and graphy and Marine Biology Annual Reviews 30, 149–190. Haflidason, H., Eiriksson, J., Van Kreveld, S., 2000. The interglacials were colder as the Arctic front tephrachronology of Iceland and the North Atlantic region migrated to the southeast. during the Middle and Late Quaternary: a review. Journal of (5) After ~225 ka, significantly reduced iceberg Quaternary Science 15, 3–22. discharge to the Irminger basin from glaciers Hall, K., 1982. Rapid deglaciation as an initiator of volcanic draining mafic crystalline outcrops and sedi- activity—a hypothesis. Earth Surface Processes and Landforms 7, 45–51. mentary outcrops in east central Greenland Heinrich, R., Freiwald, A., Wehrmann, A., Schaefer, P., Samtle- possibly due to semipermanent sea ice along ben, C., Zankl, H., 1996. Nordic cold-water carbonates; the Scoresby Sund coast. occurrences and controls. In: Reitner, L., Neuweiler, F., K. St. John et al. / Marine Geology 212 (2004) 133–152 151

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