Jökull


Jökull - 01.12.2006, Page 70

Jökull - 01.12.2006, Page 70
David W. McGarvie et al. Figure 5. Five new Ar-Ar ages, plot- ted against past global mean temper- atures. Ages are from either rhyo- lite whole-rock samples (r) or from feldspars (f). The error bars repre- sent 1σ uncertainties. The temper- ature curve is based on a smoothed dataset from Vostok ice core (Petit et al., 1999). – Samanburður á 5 aldurs- greiningum af Torfajökulssvæðinu við fornhitastig samkvæmt Vostok ískjarn- anum af Suðurskautslandinu. eruption wholly during OI stage 11 and by implica- tion the Holsteinian interglacial, this implication is the subject of ongoing debate (e.g. Nitychoruk et al., 2006). Taking the above into account plus information from preceding sections, it can be concluded that Ar-Ar dating of sustained tuya-forming eruptions, even when they have been eroded, are likely to pro- vide the most unequivocal palaeoclimatic informa- tion. Smaller rhyolite eruptions are less useful, be- cause glaciovolcanic relationships are more equivo- cal. Finally, a powerful means of understanding the rapidity with which land-based ice accumulates and melts would be to correlate the Ar-Ar ages of vol- canic rocks with the subdivisions of the oxygen iso- tope record, such as the specific substages within OI stage 5. However this will only be achieved if uncer- tainties can be reduced to ±2 ka or less, which will require a breakthrough in the Ar-Ar dating method. Geochemical evolution Geochemical data for the five Ar-Ar dated samples (Table 3) plus one sample from each of the two youngest rhyolite eruptions at Torfajökull (1477 AD and c. 874 AD) are plotted on Figure 6, which uses Zn (which is strongly incompatible) as an index of mag- matic evolution against which are plotted data for two incompatible elements (Nb and Y) and one compat- ible element (Sr). Near-linear trends are apparent for all three trace elements (especially Nb and Y), with Sr showing a less regular trend due to variable feldspar contents in the porphyritic rhyolites (especially those with <220 ppm Zn). These near-linear trends have been noted before (e.g. McGarvie et al., 1990), and have been interpreted as indicating a co-genetic re- lationship between the rhyolites erupted at Torfajök- ull, and also, by using what relative stratigraphy ex- ists, as indicating that (in general) older rhyolites at Torfajökull are geochemically more evolved than the younger rhyolites. 68 JÖKULL No. 56
Page 1
Page 2
Page 3
Page 4
Page 5
Page 6
Page 7
Page 8
Page 9
Page 10
Page 11
Page 12
Page 13
Page 14
Page 15
Page 16
Page 17
Page 18
Page 19
Page 20
Page 21
Page 22
Page 23
Page 24
Page 25
Page 26
Page 27
Page 28
Page 29
Page 30
Page 31
Page 32
Page 33
Page 34
Page 35
Page 36
Page 37
Page 38
Page 39
Page 40
Page 41
Page 42
Page 43
Page 44
Page 45
Page 46
Page 47
Page 48
Page 49
Page 50
Page 51
Page 52
Page 53
Page 54
Page 55
Page 56
Page 57
Page 58
Page 59
Page 60
Page 61
Page 62
Page 63
Page 64
Page 65
Page 66
Page 67
Page 68
Page 69
Page 70
Page 71
Page 72
Page 73
Page 74
Page 75
Page 76
Page 77
Page 78
Page 79
Page 80
Page 81
Page 82
Page 83
Page 84
Page 85
Page 86
Page 87
Page 88
Page 89
Page 90
Page 91
Page 92
Page 93
Page 94
Page 95
Page 96
Page 97
Page 98
Page 99
Page 100
Page 101
Page 102
Page 103
Page 104
Page 105
Page 106
Page 107
Page 108

x

Jökull

Direct Links

If you want to link to this newspaper/magazine, please use these links:

Link to this newspaper/magazine: Jökull
https://timarit.is/publication/1155

Link to this issue:

Link to this page:

Link to this article:

Please do not link directly to images or PDFs on Timarit.is as such URLs may change without warning. Please use the URLs provided above for linking to the website.