Swindles et al. (2018)“identify a period of markedly reduced volcanic activity centered on 5.5-4.5 ka that was preceded by a major change in atmospheric circulation patterns, expressed in the North Atlantic as a deepening of the Icelandic Low, favoring glacial advance on Iceland.” And from this, “calculate an apparent time lag of ∼600 yr between the climate event and change in eruption frequency.” If supported by data sets, this conclusion would be an important constraint on the link between glaciation and volcanism. However, we find substantial flaws in the evidence.
Regarding the lull in volcanic activity, Swindles et al. do not recognize the limitations of the tephra data sets. First, the northern European volcanic ash (NEVA) record only captures tephra dispersed eastward. Icelandic ash plumes disperse in other directions; either due to seasonal changes in stratospheric currents or in the overall structure of atmospheric flow (Lacasse, 2001). Second, <50% of the 62 events on the NEVA list are verified Icelandic eruptions and most of those are silicic. Silicic events only represent ∼60 of the ∼1950 explosive Holocene eruptions in Iceland (∼3%; Thordarson and Höskuldsson, 2008) and, thus, are not a reliable measure of total explosive volcanism. Third, the Icelandic tephra and Global Volcanism Program records employed to verify the NEVA record are also incomplete. Swindles et al. ignore more recent and detailed tephra records from east Iceland (Gudmundsdóttir et al., 2016) and west Iceland (Jóhannsdóttir, 2007; Jennings et al., 2014), which would contribute to a more complete record of Icelandic volcanism. Fourth, although Óladóttir et al. (2011) have reasonable coverage of southeast Iceland, the limitations of this data set are not acknowledged. At best, ∼1 out of 3 tephra-producing events is preserved in Icelandic soils (Thordarson and Höskuldsson, 2008). The preservation potential is even lower at the periphery of Vatnajökull, where 5 out of 7 sites studied by Óladóttir et al. (2011) are located, because periglacial environments have the least stable soils in Iceland (Arnalds et al., 2001). Hence, variation in the frequency of tephra fall in isolated Icelandic soils is not a reliable measure of total eruption frequency. Finally, some of Iceland’s most active volcanoes are capped by glaciers and produce ash-rich events as a result of water-magma interaction. At times when that ice is absent, these volcanoes will produce ash-poor and lapilli-rich eruptions, where the lapilli fraction will fall close to source due to its greater size. Hence, the ash from these “dry” eruptions would not only be less likely to reach Europe, but also less likely to be preserved across Iceland. Collectively, these issues highlight that the NEVA/Icelandic tephra records cannot be used to infer reduced Icelandic volcanism between 5.5 and 4.5 ka.
Our second set of concerns relates to the misinterpretation of paleoclimate data sets in light of Iceland’s well-established Holocene climate history. First, Swindles et al. refer to mid-Holocene Icelandic glacier advances that are based on radiocarbon/tephra ages from moraines with large uncertainties (ca. 7–4.5 ka). This age constraint is far too tenuous to be invoked as evidence for substantial ice growth prior to 5.5 ka. Second, Mayewski et al. (1997) do not present the Greenland Ice Sheet Project Two (GISP2) K+/Na+ records as a proxy for the Siberian High/Icelandic Low during the Holocene. Moreover, the citation of reduced Icelandic lake productivity at ca. 6.4 ka to support this inferred cooling is inappropriate, as these studies explicitly link it to effusive volcanism in Iceland. Finally, more recent and better temporally constrained records of glacier advance in Iceland, based on varved lake sediment records (Larsen et al., 2012; Striberger et al., 2012), threshold lake sediment records, and emerging dead vegetation data sets (Harning et al., 2016a, 2016b) demonstrate a severe reduction in ice volume during the Holocene Thermal Maximum (7.9–5.5 ka), and that glaciers throughout Iceland were progressively advancing from ca. 5.5 ka to the Little Ice age, in association with increased sea ice on the North Iceland Shelf (Cabedo-Sanz et al., 2016). These lines of evidence all strongly oppose Swindles et al.’s arguments for the reduction (e.g., atmospheric circulation change and glacier growth) and renewal of volcanic activity (e.g., glacier and sea-ice recession) during the mid-Holocene.
In conclusion, no firm evidence exists for (1) a lull in mid-Holocene volcanic activity from tephra records, (2) well-constrained advances of Icelandic glaciers before 5.5 ka, or (3) a concurrent change in the Icelandic Low. Consequently, no reliable lag time can be calculated between climate events and past/future volcanic eruption frequency in Iceland.