Heinrich Events

Session 2: Ocean Processes at High Latitudes

Session Description

This session will investigate the role of oceanographic processes at high latitudes in maintaining the AMOC slowdown during Heinrich events. In particular, this session will focus upon the stable oxygen isotope anomaly, which is evident over a vast area of the Nordic Seas and the Northern North Atlantic, and will question how this may have related to physical processes.

Session Talks
Keynote

"Heinrich Events" - A riddle, wrapped in a mystery, inside an enigma

Henning Bauch


Henning A. Bauch


Mainz Academy c/o GEOMAR, Kiel, Germany


On glacial-interglacial timescales variations in global temperatures correlate well with major changes in the size of northern hemisphere ice sheets. On much shorter timescales, the discharge of icebergs from the ice sheets surrounding the North Atlantic region directly reflected the rates of growth/decay/flow of the ice sheet margins at sea level. Although the mechanism(s) involved behind major iceberg surge events into the ocean (so-called Heinrich-events) during the last glaciation is still unknown (e.g., pinge/purge oscillations, sea-level changes, bipolar seesaw...etc.) it is very evident that once in the ocean and melting away these bergs must have had a profound impact on surface ocean properties such as temperature, salinity, stratification, sea-ice formation...etc. all in all important processes which can affectively change the meridional overturning circulation and climate. In this study, I will examine glacial-interglacial transitions as these have been shown to be particularly sensitive to such rapid climate changes in association with iceberg events.

Changes in Deep-water flow during Heinrich Events

Babette Hoogakker


Babette Hoogakker


University of Oxford


An understanding of the response of ocean circulation to rapid climate change in the North Atlantic can be gained through examining the period 60 to 30 ka ago; an interval characterized by a series of abrupt climatic deteriorations with a 1.5 to 4 ka pacing. During stadial (cold) events, northern hemisphere climate was cooler and dryer. Several severe stadial events are further characterized by massive surges of icebergs whose detrital layers in deep sea sediments are referred to as Heinrich layers. Colder temperatures during stadial events caused a shift from North Atlantic Deep Water (NADW) formation in the Nordic Seas to intermediate water formation south of Iceland, whereas surface ocean freshening during Heinrich events may have caused cessation of NADW formation and slowdown in ocean circulation, with Southern Source Waters (SSW) replacing North Atlantic deep and intermediate waters.


The Western Boundary Undercurrent (WBUC) of eastern America is a principal route for southward transport of NADW and southward return of SSW. Recent bottom water mass and physical flow speed reconstructions (3.5 km water depth) of the paleo-WBUC between 60 and 30 ka ago reveal that:

1- Southern Source Waters occupied the site and NADW were absent over the entire interval, and

2- the majority of slow flow speed events occurred when Antarctic temperatures were warming and not during Heinrich events.


This would suggest that Antarctic climate exerted a stronger control on deep north Atlantic flow speed by controlling the flux of SSW (Hoogakker et al., 2007; Gutjahr et al., 2010).

Heinrich events and major Laurentide, Inuitian and Greenland ice-margin instabilities in Baffin Bay and the Labrador Sea: an overview

Claude Hillaire-Marcel


Claude Hillaire-Marcel, Olivia Gibbs, Laurence Nuttin, Quentin Simon, Guillaume St-Onge and Anne de Vernal


GEOTOP-UQAM, Montreal, Canada


ODP, IODP and CCGS-Hudson cores from the central Baffin Bay, the Hudson Strait, and the Orphan Knoll and Eirik ridge areas in the Labrador Sea provide a comprehensive view of Late Pleistocene Heinrich events (H-events) and other large scale ice rafting events linked to instabilities of the Laurentide Ice Sheet (LIS) margin in the Hudson Strait area, and to western Greenland and eastern Inuitian ice margin instabilities. Whereas ice surges in Hudson Strait resulted in the deposition of detrital carbonate (mostly calcite) into the NW North Atlantic, those from the Northern Baffin Bay ice margins were characterized by detrital dolomitic limestones. The two sets of events did not occur simultaneously, thus pointing to triggering mechanisms related to the ice dynamics rather than than to climatic forcing (eg. Clarke et al. 1999). Here, we give a closer look to high- vs low-detrital carbonate events recorded at IODP site 1302/03 during the last ~ 800 ka (Hillaire-Marcel et al., 2011; Channel et al., 2012) with focus of the last H-events (H2, H1, H0), based on exhaustive sedimentological, paleomagnetic and geochemical investigations. Radiocarbon and 230Th-excesses yield relatively coherent time constraints on the duration of these recent events (~ 1.5, 2 and <1 ka, respectively), which confirm estimates from earlier studies (e.g., Veiga-Pires & Hillaire-Marcel, 1998). Grain-size, organic/inorganic carbonate content and isotopic composition, and δ18O and δ13C values in planktic foraminifers highlight a repetitive pattern of deposition of detrital carbonate-rich H-Layers near the source area. The coarse fraction IRD material is split into two apparent peaks at bottom and near the top of the layer, due to the deposition of fine carbonate-rich material originating from Hudson Strait basal till and subglacial water-laid material. This fine material might have been partly flushed out from the Strait with subglacial meltwater and/or uptaken by the ice stream then active in the Strait. A characteristic light δ18O-excursion in planctik foraminifers marks the top of the layer. It is though to record mostly the release of isotopically light brines related to intense sea-ice production triggered by the freshwater pulse. These large scale ice-rafting events from the NE LIS-margin were responsible for a significant reduction in the Atlantic Meridional Overturning, as recorded by 230Th-231Pa isotopes in marine cores (McManus et al., 2004), and for the spreading of a dense sea-ice cover in the Northern North Atlantic (e.g., Hillaire-Marcel & de Vernal, 2008), both with major climate impacts. Worth of mention is the fact that the Younger Dryas predated H0 and differed from the above H-events, and was likely triggered by a fresh-water/sea-ice pulse from the Arctic Ocean (Not & Hillaire-Marcel, 2012).


References

Channell, J.E.T., Hodel, D.A., Romer, O., Hillaire-Marcel, C., de Vernal, A., Stoner, J.S., Mazaud, A., Röhl, U., 2012. A 750-kyr detrital-layer stratigraphy for the North Atlantic (IODP Sites U1302-U1303, Orphan Knoll, Labrador Sea). Earth and Planetary Science Letters 317-318, 218-230.

Clarke, G.K., Marshall, S.J., Hillaire-Marcel, C., 1999. A glaciological perspective on Heinrich events. Geophysical Monograph 112, American Geophysical Union, 243-163.

Hillaire-Marcel., C. de Vernal, A. and McKay, J., 2011. Foraminifer isotope study of the Pleistocene Labrador Sea, northwest North Atlantic (IODP Sites1302/03 and 1305), with emphasis on paleo-ceanographical differences between its "inner" and "outer" basins. Marine Geology 279, 188-198.

Hillaire-Marcel, C., de Vernal, A., 2008. Stable isotope clue to episodic sea ice formation in the glacial North Atlantic. Earth and Planetary Science Letters 268 (1-2), 43-150.

McManus, J. F., Francois, R., Gherardi, J. M., Kelgwin, L., and Brown-Leger, S. (2004). Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes. Nature 428, 834-837.

Not, C., Hillaire-Marcel, C., 2012. Enhanced sea-ice export from the Arctic during the Younger Dryas. Nature Communications, doi: 10.1038/ncomms1658.

Veiga-Pires, C., Hillaire-Marcel, C., 1998. U-Th isotope constraints on the duration of Heinrich events H0 to H4 in Southeastern Labrador Sea. Paleoceanography 14, 187-199.




Dr. Jennifer D. Stanford, Geography & Environment, University of Southampton