I've been looking to date the oldest ice being flushed out via the Jakobshavn Isbrae. Ice-penetrating radar is by far the easiest way to 'drill' a core -- in fact, a flight track amounts to thousands of virtual cores.
Since radar horizons have been dated with reference to annual ice layer counting at all of the conventional ice cores (NEEM, NGRIP, GRIP and GISP2), simply follow the lowest continuous radar horizon up the hill until it ties in. That gives the age of the ice at JI at that horizon.
Note an annual ice layer at NGRIP is ~13 millimeters thick at a depth of 2500 m which exceeds the resolution of the radar (~ 3.9 meters, number of vertical pixels representing a 1000 m section of a Cresis jpeg) by a factor of 300. In addition, the striations appear several pixels thick even though they do in fact correspond in some cases to an event wholly confined to one year (eg volcanic ashfall).
But there's more. Deformations to the radar horizons almost always come from below (ignoring firn). If the horizon being followed is not deformed, then neither are the ones above. That implies all the other horizons are present, possibly thinned and not distinctly visible, meaning all the properties logged on the fiducial tie-in core are transferable all along the track, even properties like beryllium-10, temperature, or methane that don't have anything directly to do with bumping dielectric above that of pure ice.
Because the thinning is known from surface to horizon depth, this can actually be verified via radar track synthesis, a 'dry lab' computation that takes properties giving rise to radar reflectors (ie conductance, dust, sulfate) measured along the core and figures out what the radar return from them should look like when thinned.
Looking at the Panton and Spuz radar horizon graphics above, the last easily traceable striations are a triple whose top unit hits the NGRIP core at about 2558 meters (resp 2000 m for NEEM). Those depths correspond to calendar dates of ~ 74,000 as determined by annual ice layer counting. There are still older striations but those are intermittant and tracing them soon becomes problematic.
So the issue is, does this get down to where the ice sheet feeds into Jakobshavn Isbrae or even to the coast by Swiss Camp? The lower ice stream itself has undergone a lot of turmoil in conforming to sills and troughs of overdeepenings. There may be one identifiable radar striation but I suspect this is just the Bølling–Allerød at 14100-12900 B2K at a depth of 1576 m up at NGRIP.
I'm interested in the ~ 74,000 triple though because it may correspond to the enormous erruption of Toba, the largest event by far in the Pleistocene. That left its mark all over the world in form of tephra and sulfuric acid depostion. Better yet, that event was very carefully studied by Svensson and 26 colleagues on multiple Greenland and Antarctic cores.
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The table below describes 9 closely spaced sub-events spanning 1,980 calendar years (25.39 m of NGRIP core) observed isochronically at both poles. (I've added the NEEM dates using the correspondence with NGRIP depths provided by the latest GICC05 timeline.)
Event Acronym Year B2K NEEM NGRIP GRIP GISP2
Toba tephra 1 T1 74057 2015 2547 2565 2591
Toba tephra 2 T2 74156 2016 2548 2565 2592
Toba tephra 3 T3 74358 2016 2550 2567 2593
Toba tephra 4 T4 74484 2017 2551 2567 2594
Toba tephra 5 T5 75039 2020 2558 2572 2599
Toba tephra 6 T6 75064 2020 2559 --- ---
Toba tephra 7 T7 75479 2023 2564 --- ---
Toba tephra 8 T8 75505 2023 2565 --- ---
Toba tephra 9 T9 76037 2026 2573 2581 2608The accompanying graphic shows annual core values for delta 18O (~temperature), ECM (direct current electrical conductivity measurement of acidity), DEP (dielectric profile), sulfate, and conductivity. There is no mention of radar horizons in the article even though the items that peaked are key players in generating those.
The overall timing corresponds closely to certain Greenland stadials (cold) and Dansgaard-Oeschger interstadials (warm), GS-21, GI-20, GS-20 and GI-19 though Toba is not put forth as an explanation for any of them. For reasons unknown, these AMOC cycles are in a separate graphic on a longer time scale, which I've adjusted below. The radar track running over NGRIP at this depth was enhanced by Panton as discussed above.
Toba was a supervolcano, two orders of magnitude more extreme than Mount Tambora, with a volume of some 2800 cubic km. Its ashfall has a diagnostic horizon called the Youngest Toba Tuff (YTT) that has not yet been identified in polar ice cores. The huge crater is located in Indonesia a couple of degrees north of the equator. There are pros and cons to various theories of nuclear winters and human genetic bottlenecks associated with it.