While there are disappointingly few high-resolution alkenone ocean cores with 20th century resolution, there are a few. Given the importance of this class of proxy in Marcott et al, one would have thought the performance of high-resolution alkenones in the 20th century would have been of interest to Marcott et al, but they were silent on the topic.
Before Climategate, I’d considered high-resolution ocean cores from time to time at Climate Audit (see alkenone tag). In 2007, I reported on a very high-resolution alkenone series offshore Morocco (about 30N) by Helen McGregor (see here and here). This dataset had a serious divergence problem, i.e. the water was getting colder. McGregor worried that fish in the area might need swimming lessons to cope with the rapid change.
McGregor’s series was cited in Leduc et al (2010), a specialist presentation of another high-resolution alkenone series taken in the Benguela upwelling zone offshore Nambia at about 30S. Like McGregor, they also found sharply cooler SST in late 20th century as measured by well dated alkenone data, indicating that the alkenone divergence problem was not unique to McGregor’s site:
Figure 1. From Leduc et al 2010.
The closest Marcott series to McGregor’s Morocco series was the Iberian Margin (#51) D13822 alkenone series (Abrantes et al.) The graphic below compares the two series: the shorter (warmer) high-resolution McGregor series in red and the longer series used by Marcott in black. The green “rug” marks at top are D13822 radiocarbon dates.
The next graphic shows the modern portion of these two series, both offset to facilitate comparison. In this graphic, I’ve both the published and Marcott dates for the Iberian Margin series, together with the high-resolution McGregor series. All show very pronounced closing downticks, with the well-dated McGregor series placing the Morocco decline in the 20th century and even the last half of the 20th century, Marcott dating the decline in the Iberian Margin to the 19th century (with a dating error of 100-150 years) and Rodrigues et al dating the decline to the 15th century. An obvious question is whether the downturns in the Iberian Margin and Morocco series are contemporary or phased. (Note that the removal of D13822 after the late 19th century contributed to the Marcott 20th century uptick.)
Both sites are very high accumulation. McGregor’s box core GeoB6008-1 accumulated 32 cm in less than a century (1912-1998), while the Iberian margin D13822 is estimated to have accumulated about 25 cm per century (this is about 20 times higher than many cores.) The top sample for D13822 is at 10 cm and dated 57BP by Marcott and 442 BP in the original publication (which I haven’t seen yet.) The closest radiocarbon date is at 257 cm (calibrated 1511 BP) i.e. not closely dated. For comparison, the 10 cm sample in closely-dated GeoB6008-1 is dated to 1981AD and to 1947AD in GeoB6008-2.
Trouet et al 2009
As an amusing sidebar, we also discussed the McGregor alkenone series as it was used in a multiproxy study by Trouet et al 2009 – see CA discussion here. Trouet “solved” the divergence problem in best Mann et al 2008 style: by turning the series upside down. The red series labelled “Cape Ghir” and going sharply upward in the excerpt from Trouet et al 2009 is the McGregor alkenone series with sharply colder temperatures.
Excerpt from Trouet et al 2009.
The practice of multiproxy authors turning indicators of cold SST upside down had been previously criticized at CA in connection with Moberg et al 2005, which used a proxy series showing increased presence of coldwater (polar) foraminifera (% G Bulloides) in the Arabian Sea as one of their most potent indicators of global warming.
The closest Marcott series to the Leduc et al 2010 Benguela series shown above is Farmer’s ODP1084B (also used in Loehle and McCulloch and discussed in Schmidt’s critique of Loehle and McCulloch.) I’ll discuss this interesting downspike on another occasions.
In general, 20th century downspikes in high-resolution alkenone series seem to be the rule, rather than the exception – a divergence problem that is not discussed in the multiproxy studies. The most plausible reason is that high-resolution in the 20th century requires high-resolution, which requires high biological productivity, which, in turn, is most characteristic of upwelling zones. Increased upwelling in upwelling zones seems to be a rather pronounced in the 20th century. This is not inconsistent with overall warming, but neither is it an issue that multiproxy jockeys can simply brush aside.