1. What Causes Heinrich Events?
A review of (Marcott et al., 2011)
Presented By: Kevin Cowell

2. Definition
Heinrich events represent the episodic discharge of icebergs from northern hemisphere ice sheets during the Pleistocene glaciations.
Many events have been directly linked to the Hudson Strait Ice Stream (HSIS)
(Marcott et al., 2011)

3. Reconstructing the Laurentide Ice Sheet
The majority of the LIS sat on hard crystalline bedrock.
High frictional resistance
At the Hudson Bay and Hudson Strait the LIS overlaid Paleozoic carbonates and Cretaceous mudstone.
Easily eroded into slippery lubricant
Many scientists wished to reconstruct the shape and size of the LIS at the last glacial maximum.
Two models were formed based off of two different solutions to deal with this slippery bed material.
Ignore layer  Large thick single domed ice sheet
Reduce basal stress  Multidomed thin ice sheet
Who Is Right? Both!!

4. 2-States of the Laurentide Ice Sheet
Binge State
Soft subglacial sediments are frozen
Basal stresses keep the Hudson Straight ice stream in place.
Growth of the Ice Sheet
Ice is very thick over the Hudson Bay
Reconstructed surface elevation = m
Purge State
Soft subglacial sediments thaw
Activation of Hudson Strait Ice Stream
Ice is thin as it spreads laterally
Extreme ice calving events
Reconstructed surface elevation <2000 m
(MacAyeal, 1993)

5. How do we know? Marine Core Records! Ice Rafted Debris layers
Delta-O-18 records
Several δ18O standards such as VSMOW

6. Opposing Hypotheses
A Heinrich event is a product of ice sheet dynamics and independent of external forcing (MacAyeal, 1993).
Heinrich events are a product of climate forcing associated with the reduction in the Atlantic Meridional Overturning Circulation (AMOC) (Marcott, 2011).
(MacAyeal, 1993)

7. Ice-shelf collapse from subsurface warming as a trigger for Heinrich events (Marcott et al., 2003)
Study was based off of the marine core EW9302-2JPC, which according to climate-model simulations, is at a depth and latitude that is ideal for monitoring subsurface warming associated with a reduction in the AMOC.
Extent of ice shelf derived from the Hudson Strait Ice Stream as reconstructed by Hulbe CL (1997)

8. Previous Work
Identified ice-rafted detrital carbonate layers that represent Heinrich events with associated changes in benthic faunas and the δ18O of their carbonate tests suggested intrusions of relatively warm water mass coincident with Heinrich events.
However, because the temperature transfer function for the benthic faunas is unknown, and ice-volume and hydrographic changes can mask the temperature signal in the δ18O of calcite, the inferred temperature changes were poorly constrained.

9. Methods
Analyzed the benthic species C. lobulatus (N=46), C.spp. (N=23), and M. barleeanum (N=44).
Used an automated flow-through system that cleans and dissolves the carbonate shells and thus minimizes the effects of secondary calcite and clay contamination.
Converted Mg/Ca ratios from foraminifera to benthic water temperatures (BWTs) following published calibration curves.
Determined ice-volume corrected benthic δ18O (δ18OIVC).
Mg is incorporated into calcite shells of benthic foraminifera as a trace element. Incorporation of Mg as an impurity in calcite is endothermic so more is incorporated into the growing crystal at higher temperatures. The calibration curves they used must be based off of empirical data.

10. Data Trends
Used several published ice core δ18O to compare to their Mg/Ca data.
Antarctic EPICA (European Project for Ice Coring in Antarctica)
North Greenland Ice Core Project
Ice corrected δ18O on benthic fauna from this core used a temperature dependent fractionation of 0.25‰/oC
The sample concentration can be seen as No. detrital CaCO3 g-1
Ages were determined by using stratigraphic relationships and published numerical dating of 14C, tephra layers, and marine isotope stage 5/4 boundary, and others.
The different data sets have close agreement.
Temperature
In ice the warmer the temperatures the more positive δ18O
In solution the warmer the temperature the more negative δ18O
δ18O is dependent on factors such as temperature, salinity, and ice sheet volume.
When an oxygen-bearing mineral precipitates chemically from an aqueous solution oxygen is segregated with some remaining in solution while others precipitate out forming the mineral. 18O is preferentially included in the mineral because they vibrate more slowly and move more sluggishly.
The colder the solution is (seawater), the more 18O is preferred and the more (+) the δ18O
The warmer the solution, the less δ18O is preferred and the more (–) the δ18O
Epstein et al. estimated (assuming salinity and ice volume change ignored) that a δ18O increase of 0.22 parts per mil = a decrease in temperature of 1K

11. Data Trends Cont…
Mg/Ca data determined that there were several systematic changes in temperature that gradually increased prior to the start of each Heinrich layer (event), with the start of the warming approx. 1-2 kyr before each Heinrich event.
This trend is replicated in the ice-volume corrected benthic δ18OIVC temperature record associated with H1.
Warming trend associated with a temperature oscillation of 3-4 oC around a mean value that is close to the present BWT of 3.4 oC and reaches a max BWT of 5-7 oC.

12. How to explain this increase in BWT?
A number of proxy records show that the Atlantic Meridional Overturning Current began to decrease 1-2 kyr prior to Heinrich events.
Decrease in AMOC strength believed related to climatically induced increase in freshwater fluxes from the Northern Hemisphere ice sheets associated with onset of deglaciation from the last glacial maximum at 19 ka.
Without an active AMOC and associated cooling of the ocean interior by convection, continued downward mixing of heat at low latitudes warms subsurface waters to a depth of approx. 2,500 m.
Some of this heat is then transported poleward causing a temperature inversion in the northern North Atlantic

13. Support for AMOC Reduction
A – Comparison between the 231Pa/230Th record from the Bermuda Rise, a proxy of AMOC strength, and strength of maximum AMOC transport simulated by NCAR CCSM3 model.
B – Evolution of temperature as function time and depth simulated by the NCAR CCSM3 at the location of study marine core.
C- Mg/Ca derived BWT (grey line) for study marine core as well as reconstructed δ18OIVC (blue line) and NCAR CCM3 calculated temperature (red line)
Also Heinrich events during (60-26ka) interval occurred only when Greenland was at its coldest and Antarctica at its warmest. Maximum expression of reduction in AMOC
National Center for Atmospheric Research Community Climate System Model version 3 (NCAR CCSM3)
Used to evaluate the response of the BWT at study core site to a reduction in the AMOC during the last glaciation
CCSM3 model further suggests that the δ18OIVC signal represents a dominant temperature control reflecting basin-wide subsurface warming.

14. δ18OIVC – A Dominant Temperature Control
Changes in δ18O in the Nordic Seas had been interpreted as recording increased brine formation beneath expanded sea ice.
They also sampled foraminifera from these locations.
Found 1.5 per mil δ18OIVC at this site can alternatively be explained by 6oC of warming found at their other locations
CCSM3 model simulated small changes in salinity at intermediate depths as freshwater added to the surface was convected downward.
We can see δ18OIVC is dominated by temperature not salinity.

15. Data Consistency
Proxy records show a gradual AMOC reduction prior to the times of Heinrich events.
The 1- to 2-kyr interval of gradual subsurface warming suggested by the Mg/Ca data that peak at the same time as H3, H5a, and H6 is thus consistent with a response to a max reduction in the AMOC

16. Sensitivity Test Cold State (Strong AMOC) Warm State (Weak AMOC)
Corresponds to years ka
Shelf average basal melt rate is m a-1
Integrated volume loss from the ice shelf by basal melt is 10% of estimated ice flux of 660 km3 a-1 across the HSIS grounding line
Corresponds to years ka
Shelf average basal melt rate is m a-1
Also performed three additional intervening simulations based on the simulated temperature evolution of water depths of m.
Max basal melt rates along the HSIS grounding line increased sixfold from 6 m a-1 to m a-1

17. Sensitivity Test Cont..
The computed time history of ice-shelf thinning in response to the warming of intermediate-depth waters indicates an approximate 1,000 year time scale for collapse of the ice shelf.
It is likely that this time scale is a maximum because the ice shelf would collapse before it thinned to zero.

18. Sources of Potential Error
Analytical and calibration uncertainties could contribute to an error of up to 1.3 oC for the Mg/Ca-derived BWT reconstructions.
Recent work suggests that the carbonate ion may also affect Mg/Ca in some benthic foraminifera at temperatures below approximately 3 oC, where carbonate ion saturation decreases rapidly (cold temps), and at low saturation levels.
However they were certain that at intermediate-water depths (site location) measured Mg/Ca values were not influenced by past carbonate ion concentrations.

19. (Marcott et al. 2013) Conclusion
Basin-wide subsurface warming occurred in the North Atlantic in response to a reduction in the AMOC prior to Heinrich events and that Heinrich events did not occur until the AMOC was at its weakest and subsurface temperatures were near their maximum values.

20. Sources
MacAyeal, D. R. "Binge/purge oscillations of the Laurentide ice sheet as a cause of the North Atlantic's Heinrich events." Paleoceanography 8.6 (1993):
Marcott, Shaun A., et al. "Ice-shelf collapse from subsurface warming as a trigger for Heinrich events." Proceedings of the National Academy of Sciences (2011):