Source, timing, frequency and flux of ice‐rafted detritus to the Northeast Atlantic margin, 30–12 ka: testing the Heinrich precursor hypothesis

Abstract

Hall, I.R. 2010 (July): Source, timing, frequency and flux of ice-rafted detritus to the Northeast Atlantic margin, 30-12 ka: testing the Heinrich precursor hypothesis.Increased fluxes of ice-rafted detritus (IRD) from European ice sheets have been documented some 1000-1500 years before the arrival of Laurentide Ice Sheet (LIS)-sourced IRD during Heinrich (H) events. These early fluxes have become known as 'precursor events', and it has been suggested that they have mechanistic significance in the propagation of H events. Here we present a re-analysis of one of the main cores used to generate the precursor concept, OMEX-2K from the Goban Spur covering the last 30 ka, in order to identify whether the British-Irish Ice Sheet (BIIS) IRD fluxes occur only as precursors before H layers. IRD characterization and planktonic foraminiferal d 18 O measurements constrained by a new age model have enabled the generation of a continuous record of IRD sources, timing, frequency and flux, and of local contemporary hydrographic conditions. The evidence indicates that BIIS IRD precursors are not uniquely, or mechanistically, linked to H events, but are part of the pervasive millennial-scale cyclicity. Our results support an LIS source for the IRD comprising H layers, but the ambient glacial sections are dominated by assemblages typical of the Irish Sea Ice Stream. Light isotope excursions associated with H events are interpreted as resulting from the melting of the BIIS, with ice-sheet destabilization attributed to eustatic jumps generated by LIS discharge during H events. This positive-feedback mechanism probably caused similar responses in all circum-Atlantic ice-sheet margins, and the resulting gross freshwater flux contributed to the perturbation of the Atlantic Meridional Overturning Circulation during H events.

Fine scale sediment structure and geochemical signature between eastern and western North Atlantic during Heinrich events 1 and 2

“…7). Therefore, we suggest that the socalled precursor sediments are in fact the IRD produced by ambient icebergs and do not represent a "specific" iceberg-rafting event (Haapaniemi et al, 2010). Indeed, H-events are characterized by a preceding cooling trend in temperature, which would lead icebergs to survive longer in the ocean.…”

“…In the North Atlantic, these H-layers are identified by a sudden increase in the coarse lithic fraction (% >150 mm), a near absence of foraminifera, dominance of the polar planktonic foraminifera Neogloboquadrina pachyderma (s), and a lowering of inferred seasurface salinity (Heinrich, 1988;Bond et al, 1993;Cortijo et al, 2005). The presence of detrital carbonate grains within the IRDlayers and the geochemical signature of the non-carbonate fraction suggest that the LIS was the main source of icebergs during the H-events (Bond and Lotti, 1995;Grousset et al, 2001;Peck et al, 2008;Haapaniemi et al, 2010). In contrast, the lack of detrital carbonate grains within the other IRD-layers suggests that the icebergs were supplied by the European ice-sheet (EIS); namely, the Icelandic, Scandinavian, British, and East Greenlandic icesheets.…”

“…The D/O iceberg-rafting events in the North Atlantic have been a topic of research for the last decade (Bond et al, 1999;Peck et al, 2008;Scourse et al, 2009;Haapaniemi et al, 2010). However, it is noteworthy that these events are not found in the Labrador Sea (see Andrews and Barber, 2002).…”

“…In any event, at the start of H1 and H2, ice-stream margins probably extended to the Labrador trough edges, if not farther offshore (Andrews and MacLean, 2003;Rashid and Piper, 2007), and there may have been a peripheral floating ice-shelf (Hulbe et al, 2004). Considerable debate exists regarding whether the Quaternary ice-sheets of the Northern Hemisphere collapsed synchronously during H1 and H2 or whether these events were the result of sequential collapses of ice-sheets (Grousset et al, 2001;Haapaniemi et al, 2010). These questions arose after analysis of the ultra-fine structure of H-layers documented IRD sourced from various Northern Hemisphere ice sheets.…”

On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

“…The amount of meltwater increased toward the LGM, with a climax from 15 to 28 ka ( Figure 11) (Lekens et al, 2006;Haapaniemi et al, 2010; 26 Roger et al, 2013). Where ice persisted, sedimentation rates remained high until the beginning of the Holocene .…”