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

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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; Koc¸ 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 con- tinental 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; Koc¸ 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 Koc¸ 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.
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