Ice-Rafted Debris (IRD)
KeywordsOcean Heat Transport Planktic Foraminifera Rafting Process Iceberg Calve Greenland Fjord
Ice-rafted debris (IRD) is a terrigenous material transported within a matrix of ice and deposited in marine or lake sediments when the ice matrix melts (US National Climatic Data Center).
History of Observations
Coarse-grained clasts interpreted as ice-rafted debris were first recognized in seabed samples collected during a 1928 expedition of the US Coast Guard vessel “Marion” in the northern Labrador Sea and Baffin Bay (Ricketts and Trask, 1932). A decade later, Bramlette and Bradley (1940) described glacially transported striated clasts and erratics from North Atlantic seabed sediments. The more common use of IRD as a proxy of glacial variability commenced with the systematic sampling of deep-sea sediments in the early 1970s, notably by the international Deep Sea Drilling Programme (DSDP) and the “Vema” cruises operated from Lamont-Doherty Earth Observatory in the USA. Initially, IRD analyses were mainly applied to understand the long-term Cenozoic evolution of ice sheets (Berggren, 1972). This was highlighted by the study of Shackleton et al. (1984), who traced the development of major northern hemisphere glaciations back to about 2.5 Ma ago. A different approach demonstrated by, among others, Ruddiman (1977) was the application of ice-rafted sand-sized material measured in multiple core records to investigate late Quaternary ice dispersal patterns in the North Atlantic. In 1988, a milestone article by Hartmut Heinrich, reporting on the origin and consequences of late Quaternary cyclic ice rafting, set the stage for a new understanding of rapid ice sheet–ocean interactions during the last glacial in the North Atlantic region (Heinrich, 1988). Subsequent work by Andrews and Tedesco (1992) and Broecker et al. (1992) identified these ice rafting cycles (so-called Heinrich, H-events) as distinct layers rich in detrital carbonate derived from Paleozoic limestone formations in the Hudson Bay region, thus pointing to the Laurentide Ice Sheet (LIS) as the main iceberg source. Heinrich (1988) documented that these cyclic layers were marked by their high lithic-to-foraminifera ratio and increased relative abundance of the polar planktic foraminifera N. pachyderma sinistral with low δ18O values indicative of low-salinity polar water. Bond et al. (1997) noted the carbonate-rich layers to have a sharp basal contact suggesting short-lived, catastrophic discharges of icebergs from the LIS. These IRD horizons are widely distributed in late Quaternary North Atlantic sediments between 40o and 55° N, occurring with intervals of about 7,000 years that partly correspond to shorter (<1,000 year), but intense cooling periods in Greenland ice core records (Bond and Lotti, 1995). The geochemical composition of the Heinrich layers shows a Hudson Strait source for H1, H2, H4, and H5, but chemical and mineralogical analyses suggest another source for H3 (Gwiazda et al., 1996). Since the mid-1990s, numerous paleoceanographic studies have utilized IRD to document regional and temporal ice sheet variability and the effect of meltwater on ocean thermohaline circulation (e.g., Vidal et al., 1997; Knutz et al., 2002). In the past decade, focus on the topic has further increased due to the ongoing debate on climate warming, Antarctic and Greenland Ice Sheet stability, and implications for sea level rise and ocean circulation.
Ice Rafting Processes and IRD Analysis
Sources and Distribution of IRD
IRD is not only present close to formerly glaciated continental margins, but has been found widespread in the subpolar North Atlantic, and occasionally even further south, i.e., in the eastern North Atlantic off the Strait of Gibraltar. Studies of North Atlantic glacial IRD provenance have provided important information on the relative contributions of the Laurentide, Greenland, and NW European ice sheets. During H-events 1 and 2, ice discharge from Hudson Strait contributed with large amounts of detrital carbonate, at approx. 14,500 and 20,000 14C year BP, respectively. (Andrews and Tedesco, 1992; Broecker et al., 1992). Furthermore, H-event layers contain igneous fragments of hornblende and feldspar displaying a dominant Paleoproterozoic (1,600–1,800 Ma) provenance age (Hemming et al., 1998), which is consistent with a Laurentide Ice Sheet (LIS) origin. On the other hand, IRD showing younger isotopic ages and containing chalk coccoliths are inferred to have been derived from NW European glacial outlets (Scourse et al., 2000). These isotopic and mineralogical fingerprints of European/Scandinavian derived icebergs appear to have formed precursors to the main LIS response, thus adding to the complexity of H-events. A possible IRD contribution to H-events by icebergs from the Greenland Ice Sheet has been a matter of discussion. Despite the scarcity of high-resolution records from the Greenland margin and lack of consistent chronological framework, some studies show that Greenland Ice Sheet iceberg calving events occurred more frequently than H-events (Stein et al., 1996; Andrews, 2000). An independent behavior of the Greenland Ice Sheet during the last deglaciation has been shown by Knutz et al. (2011) and is illustrated by the vast amount of continental ice still present in Greenland today. Timing of iceberg surging stages on the NW European margin reflects also here different ice sheet behaviors before and during Heinrich events (Scourse et al., 2000; Knutz et al., 2002). The arrival of European and Icelandic detritus preceding the deposition of detrital carbonate-rich debris from the LIS (Bond et al., 1999; Grousset et al., 2001) further support the conclusions by Dowdeswell et al. (1999) that the dynamics of Quaternary ice sheets surrounding the Nordic Seas and North Atlantic were asynchronous. This implies that each glaciated margin may have behaved differently in response to external ocean–climate forcing.
Iceberg Surging Mechanisms
Since the early 1990s, several possible mechanisms responsible for forcing large-scale IRD events have been proposed (e.g., Alley and Clark, 1999). The binge–purge hypothesis of MacAyeal (1993) invoked internal ice sheet dynamics as a driver for H-events. However, this mechanism is not supported by the detailed structure of H-events and cannot explain North Atlantic records showing millennial and centennial scale IRD fluctuations that appear to be closely coupled to the ocean–atmosphere climate system (e.g., Bond and Lotti, 1995). Apart from internal ice sheet dynamics, ocean circulation changes, sea level fluctuations, variations in solar parameters, as well as ice-load-induced earthquakes have been proposed being responsible for iceberg surging. In addition, Hulbe et al. (2004) postulate catastrophic ice shelf breakup induced by climate-controlled meltwater infilling of surface crevasses as recently witnessed along the Antarctic Peninsula. Observations of recent iceberg discharge processes and ice rafting in an east Greenland fjord reported by Reeh et al. (1999) demonstrated that the transport and deposition of iceberg-derived IRD increased in periods of enhanced (subsurface) advection of “warm” Atlantic water. These findings were used in a study by Moros et al. (2002) and Kuijpers et al. (2005) who concluded that large-scale IRD events in the North Atlantic most likely were triggered by enhanced northward ocean heat transport which caused bottom melting of floating outlet glaciers and ice shelves, leading to ice sheet destabilization and iceberg surging. More recently, a sudden acceleration, thinning, and retreat of the Jakobshavn Isbræ, west Greenland, were observed, which have been attributed to the warming of the subsurface ocean currents off west Greenland (Holland et al., 2008). Studies of sedimentary records of the past ca. 100 years from this area and from another active glacier calving site on the southeast Greenland coast (Lloyd et al., 2011; Andresen et al., 2012) have meanwhile confirmed a correlation between enhanced ocean subsurface warming and associated glacier bottom melting and increase in glacier acceleration and iceberg IRD production. It is without doubt that also the other factors referred to above as, for instance, surface (air) temperature warming and ice sheet dynamics, influence iceberg calving activity. However, increasing evidence has demonstrated the important role of ocean (sub)surface warming when trying to explain iceberg surging over larger areas, such as during the Heinrich events (e.g., Alvares-Solas et al., 2010), or more locally, as recently observed in South Greenland fjords. Within this context, one should keep in mind that ocean heat transport pathways are influencing the temperature regime of various parts of the North Atlantic not everywhere in the same way, which may be one of the reasons for reported asynchronous behavior of glacial ice sheets. The occurrence of iceberg-derived IRD in sedimentary records thus not only provides a tool to reconstruct former ice sheet dynamics, glacier calving activity, and iceberg drift but also yields information on ocean current patterns. In addition, having a different origin and a distribution pattern more depending on atmospheric circulation, sea ice-derived IRD can provide additional information for assessing prevailing wind directions and albedo conditions.
IRD has been found to be widely distributed in the entire subpolar North Atlantic. Glacial sedimentary records from this region display discrete IRD layers at time intervals of about 7,000 years named “Heinrich” layers that witness large-scale iceberg surging of the North American Laurentide Ice Sheet. This conclusion was made based on the lithology of the IRD involved, showing, among others, a large contribution of detrital dolomitic carbonate derived from the sedimentary rocks around Hudson Bay and Strait. Mineral studies have provided also evidence for IRD originating from European and Greenland glaciers. Asynchronous deposition of IRD from these various sources suggests a different stability regime of ice sheets west and east of the North Atlantic. Although several mechanisms may have played a role, increasing evidence arises which demonstrates that (sub)surface ocean warming and associated bottom melting of floating glaciers and ice shelves have been an important mechanism triggering large-scale iceberg calving (“Heinrich”) events, both under glacial climate and under present-day warming conditions.
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