Talk by Myriam Guillevic, Centre for Ice and Climate – Niels Bohr Institute - University of Copenhagen

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Talk by Myriam Guillevic, Centre for Ice and Climate

Heinrich event 4 as seen in Greenland ice cores

Abstract
The last glacial period was recorded in Northern Hemisphere archives as a cold period marked by climatic instabilities. In the NGRIP ice core from Greenland, water isotopes, a proxy for local temperature variations, record a succession of 25 events called Dansgaard-Oescher events (DO) (NGRIP members, 2004). Each event is characterized by a rapid temperature increase (up to +16°C in a few decades, Huber et al., 2006) leading to a warm phase or Greenland interstadial (GI). Then the temperature slowly decreases, with sometimes an abrupt drop at the end, leading to a cold period or Greenland stadial (GS). This succession of warm/cold periods is also imprinted in e.g. pollens and planktonic foraminifers data from North Atlantic marine cores (40-55°N) (e.g., Sachez-Goni et al., 2000) as well as speleothems records from Europe (Gentyet al., 2010, Bochet al., 2011). InAntarctic ice cores, an Antarctic counterpart to each GI of the Last Glacial Period has been identified (EPICA c. m., 2006).

Marine cores between 40-55 °N also record layers of ice-rafted detritus (IRD, Ruddiman, 1977), interpreted as the signature of massive icebergs discharges in the North Atlantic Ocean originating from the Laurentide and European ice sheets (Heinrich 1988, Bond 1993, Grousset 2000). Such IRD layers are also known as Heinrich events (HE). These HE events cause local changes in oceanic temperature and salinity, while changes in temperature and humidity in Western Europe are evidenced in pollens records (e.g. Sanchez Goniet al., 2009). Six HE are recorded in the last glacial period, occurring during MIS2 to 4. These IRD inputs cause abrupt changes in the sedimentation rate of marine cores and makes any dating of the event (onset, duration) very challenging (e.g., Hemming, 2004).
So far, it has been impossible to detect a signature of HE in Greenland water isotopes or reconstructed temperature: GS with or without the occurrence of a HE are recorded with similar d18O and temperature levels, which is still not satisfactory explained. To place HE and DO events on a common timescale remains challenging. For this reason, the effect of Heinrich event on Greenland climate is still unknown: is H4 causing the end of DO 9? On the contrary, if H4 occurs during GS 9, why do water isotopes not record any further cooling in Greenland?

Here, we focus in Heinrich event 4, occurring between GI 8 and 9. This period is typical of MIS3, with short DO events marked by large temperature increases in Greenland. H4 is a large IRD event with European and Laurentide origins (Grousset, 2000). We first report how this event is recorded in marine cores. We then present a new high resolution (70yrs) multi-proxy record from Greenland and Antarctica ice cores, including the first 17Oxs data measured over a sequence of DO events. Based on this ice cores data, we propose a sequence of events for high and low latitudes covering H4. We reconstruct the effect of H4 on Greenland climate, European climate and greenhouse gases concentration. We finally propose a synchronization strategy between ice cores and North Atlantic marine cores over this sequence of events.