The longest very high-resolution alkenone core that I’m aware of is Sicre et al’s MD99-2275 (plus splices) from offshore Iceland (67N 18W). It is 4550 years long, its most recent value is 2001AD and its resolution is 4 years. Marcott used nearby core JR51GC-35 (also at 67N 18W), also an alkenone record, which had a resolution of 110 years and a most recent Marcott date of 1836AD.
Here is how the two series compare over the 4500 years covered by the Sicre et al record (originally published in 2008, but updated in 2012). (The NOAA archive is unfortunately inadequate as it does not include depth or identify splice points.)
Taking a longer view, here are the two series compared over the Holocene.
In geophysical surveying, one tries to use the best quality surveys where available and benchmark lower quality surveys against the highest quality ones. If this methodology were used here, the errors in JR51GC-35 are obviously very large, much higher than arising from the alkenone calibration equation by itself, though other factors could be at work as well.
MD99-2275 has well-dated core going deep into the Holocene. Hopefully, Sicre and other specialists will continue their commendable program. Seeing if these results can be replicated in another core would also do much to increase confidence.
The alkenone divergence problem is clearly present in this data. ALkenone-estimated temperatures in the 20th century continued to decline. In the Marcott reconstruction, JR51GC-35 makes its last (very negative) contribution in the 1820 step. By the act of no longer participating, it causes the Marcott composite to go up in the next period, even though it appears that the “true” alkenone estimated temperature in the area continues to decline.
Postscript: I mentioned MD99-2275 as a high-resolution core in my notes on AGU 2006. Like McGregor’s Cape Ghir (used inverted), it was one of the proxies in Trouet et al 2009 discussed at CA here. Here is a figure showing the updated Sicre version against the Trouet et al illustration. The Sicre version is in cyan (versus the “Iceland” series in blue). The divergence in the present series continues further than in the Trouet version.