A New Alaskan d18O Series

PAGES2K Arctic introduced a lake sediment d18O series from Kepler Lake, Alaska that hadn’t been used in previous studies. Although O18 data is a workhorse of paleoclimate, O18 data from Alaska (or, for that matter, anywhere in the Arctic hemisphere between 90E and 90W – going east) is very scarce. Thus, the appearance of a new d18O series from Alaska is of considerable interest. I’ll show why by comparing the new data to other O18 information:

The Mt Logan series, shown in the top panel below, is almost unique as a high-resolution “Pacific” hemisphere d18O series. As has been discussed in past CA posts, it goes the “wrong way” in the 19th and 20th century – attributed by the specialist authors (Fisher et al) to changes in regional circulation.

The Kepler Lake series is shown in the third panel/ Two cores were drilled. Both cores show a decline in O18 values comparable to the decline in O18 values in the nearby Mt Logan ice core. The resolution in the ice core is considerable greater, but the Kepler Lake values are sufficiently similar to provide a form of confirmation of the Mt Logan results. However, there are obviously differences in detail, both between the two sediment cores from Kepler Lake and between the sediment cores and the Mt Logan core. My own surmise is that some amount of the difference arises from dating errors in one or both sediment cores. If this data were given to an exploration geophysicist, my sense is that they would treat the Mt Logan data as the best-dated and try to use that as a dating model for the lower-quality sediment data (if they even bothered using the sediment data.)

The second panel shows an older and shorter Mt Logan ice core drilled by Holdsworth in the late 1980s (used in Mann et al 2008). It also shows a decline in d18O values in the 19th century, but the timing of the decline is somewhat later in the Holdsworth core: is this due to inconsistency in dating between the two cores? Or is it an authentic feature? Dunno. One wishes that multiproxy authors would engage in this sort of question.

Finally, the bottom panel shows an earlier lake sediment d18O series (Farewell Lake, Alaska), which was used in Ljungqvist 2010, a multiproxy study cited in AR5. Although Farewell Lake is reasonably close to Kepler Lake, the two series obviously have an entirely different appearance. The Farewell Lake data has a much lower resolution (about 52 years) and completely lacks even the gross features of the more highly resolved data. While the inclusion of “lower frequency” data has become somewhat fashionable in the more recent multiproxy reconstructions, the comparison shows the need for caution in including such muddy data: the Farewell Lake data is useless in a 1000-year reconstruction.

Figure 1. ALaska O18 series. Top – Mt Logan (Fisher et al 2006); second – earlier version of Mt Logan (HOldsworth); third – Kepler Lake (Gonyo et al, used in PAGES2K Arctic); bottom – Farewell Lake AK (Hu et al ) used in Ljungqvist 2010.

Despite the almost total absence of O18 data in the Pacific hemisphere, remarkably little interest has been shown by specialists in remedying the situation. Obviously many sediment cores have been taken in the eastern Canadian Arctic, but no d18O results have been reported for any of them. In 2002, Lonnie Thompson drilled an ice core on the Alaska-Yukon border (Bona-Churchill), the results of which should have been eagerly awaited by specialists. However, more than decade later, the core remains both unarchived and unpublished. I drew attention to the delay over six years ago in early CA posts. At the time, I speculated that Thompson would have promptly published Bona-Churchill if it had had “good” results; I therefore speculated that, like Mt Logan, Bona-Churchill results went the ‘wrong way”. Be that as it may, there is only a single d18O ice core series in the Pacific hemisphere, as compared to dozens in the north Atlantic hemisphere.

Remarkably, despite its uniqueness, the Mt Logan series has been excluded from the collection of Arctic ice cores used in two prominent recent multiproxy studies (TIngley and Huybers 2013; PAGES2K Arctic).

One of Tingley and Huybers’ “justifications” for excluding Mt Logan was that it was “out of phase with paleotemperature series”. The next figure compares Mt Logan O18 to Lomonosovfonna, Svalbard O18, one of the most easterly ice cores in the North Atlantic hemisphere. In a sense, the Mt Logan series is indeed “out of phase” with the Svalbard series, but this doesn’t justify deletion of the Pacific hemisphere series. Tingley and Huybers also observed that the original authors of the Mt Logan series (Fisher et al) had “explained” the decline in O18 values as due to a change in “source region”. If so (and it is entirely plausible), it opens up the prospect that anomalously high O18 values in other areas might also be due to a change in “source region”. If changes in source region are a source of error, there is all the more reason to closely examine and analyse all the d18O series, rather than expunging inconvenient series from the network, thereby, so to speak, hiding the decline in d18O values, as Tingley and Huybers did. Their intentions may well have been “good” but the effect is a form of ex post screening.

Figure 2. Top – Mt Logan; bottom – Lomosovonovfonna, Svalbard

Although PAGES2K Arctic also had an extensive collection of ice core isotope series, they also expunged the Mt Logan series from their network, again setting out seemingly objective selection criteria. Since Kepler Lake is comparable in its major features to Mt Logan, it’s hard to understand how one series “passed” the PAGES2K criteria, while the other one failed. However, these are the same folks who used the contaminated Igaliku series and used the Hvitarvatn data upside-down, so any search for precision is bound to be fruitless.

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