Daniel Voisin, Climate Change Research Center, University of New Hampshire
Andrei Kurbatov, Institute for Quaternary and Climate Studies, University of Maine
Nelia Dunbar, N.M.B.M.M.R./E&ES Department, New Mexico Tech
Greg Zielinski, Institute for Quaternary and Climate Studies, University of Maine
Using the SO42- and Cl- time series and a continuous scan for volcanic glass, a highly detailed record of past volcanism is being developed from the Siple Dome A (primary core) and B cores. The volcanic glaciochemical record has been developed at a 2-4 year resolution for the last 5000 years in collaboration with the chemical group at UMaine (Paul Mayewski, Eric Meyerson, Sharon Sneed). The technique used to identify volcanic peaks in SO42- is the same as that used for the GISP2 core allowing direct comparisons to be made between the two records. Concentrations were measured with an ion chromatograph, while volcanic signals were developed using a robust spline best fit of the raw data. Volcanic peaks in nss SO42- were identified as those 2s above the mean positive residual of the spline fit. In this case, volcanic peaks were those with concentrations above 48 ppb. An EOF analysis will be performed to determine if this technique also can identify volcanic signals when the complete glaciochemical record is developed. The search for volcanic glass was done on annual layers over the last 2000 years using a petrographic microscope. Samples containing glass were then subjected to analysis by scanning electron microscope and electron microprobe to determine major oxide compositions of the glass for comparison with compositions from known volcanic eruptions. One of the main goals of tephra studies is to verify the source of acid peaks to understand better the climatic impact of particular eruptions and to provide reliable time lines to assist in developing the downcore chronology. It was hoped that we could identify several well-dated equatorial or southern hemispheric eruptions in this process. However, as was presented at earlier WAISCORE meetings, the glass we found over the last 2000 years primarily matches sources from Antarctica eruptions. Nevertheless, the volcanic records developed in this study provide the most up-to-date chronology of past volcanism for the Antarctica continent.
Although glass that appears to originate from Antarctica volcanism has been found in several layers whose ages correspond to known equatorial or southern hemisphere eruptions, it is possible that part of the acidic signal could be from these larger, more distant eruptions. For instance, high acidity spikes are found in layers close in age to such recent eruptions as the Tarawera (1886 C.E.)/Krakatau (1883 C.E.) doublet, 1830s C.E. eruptions of Coseguina (1835 C.E.) and Babuyan (1831 C.E.), and the Tambora (1815 C.E.)/1809 C.E. doublet. Volcanic signals are also found close to the timing of the 1600 C.E. Huaynaputina and 1595 C.E. Nevado del Ruiz eruptions as well as the very large signals found in other ice cores thought to be related to the extremely large Kuwae eruption. The age of the Kuwae eruption is thought to be in the 1450s C.E. El Misti also is known to have erupted in the 1450s C.E., thus the several large spikes in the Siple Dome core during that decade could be related to several non-Antarctica eruptions. Interestingly, the present depth-age scale does not have a signal that is close to the ultra-plinian, 181 C.E. eruption of Taupo, N.Z. as was evaluated through a detailed tephra search over this part of the record. Largest signals occur around 1280 C.E. (140-160 ppb), as discussed below, -241 C.E. (2241 years ago; 270 ppb), and -2710 C.E. (4710 years ago; 188 ppb).
The detailed search for volcanic glass showed that there were several time periods over the last 2000 years when Antarctica volcanism appeared to be more active than the surrounding time frames. The periods with greater amounts of volcanic glass are from 1640 C.E. to the present, 1100-1350 C.E. and 170-650 C.E. Glass found in various samples within these time periods indicate that certain volcanic centers in Antarctica have been more active than others. Volcanoes within the McMurdo Volcanic Center (Victoria Land and the islands off its coast) including Mt. Melbourne, The Pleaides and Buckle Island appear to be the most active. Rhyolitic shards of a composition not found in Antarctica also are present in some layers, although they are not overly abundant. Nevertheless, the presence of this material suggests more distant transport to the Siple Dome site. The highly-silicic composition of these shards indicate either a New Zealand or South American source. As there are no known eruptions in New Zealand that correspond to the age of the layers in which these shards are found, it is suggested that South America is the more likely source. The presence of dust with a Patagonian origin in East Antarctica ice cores as well as the satellite trace of aerosols from the 1991 Cerro Hudson eruption, Argentina, into the Antarctica vortex indicate that material from this part of the southern hemisphere can reach various parts of Antarctica. Common circulation patterns around the Ross and Amundson Seas provide an easy mechanism for glass to reach Siple Dome from Victoria Land.
A very detailed search for tephra in a series of three acidity spikes at the 94-97 m depth in both the Siple A and Siple B cores was undertaken because the initial depth-age scale put these depths close to the 1259 C.E. age. It was hoped that glass could be found that matched glass found in the GISP2 and South Pole cores in the 1259 layer. An identical glass was found in both Siple Dome cores; however, the composition matches that of Mt. Melbourne, not the 1259 eruption. The 3 m separation in depth between the A and B cores is worth discussion. Ages of these layers is now put in the 1280s C.E. Furthermore, the glass in the Siple Dome cores is the same as the visible tephra layer at 79 m in the Taylor Dome core. The present Holocene depth-age scale of the Taylor Dome core as modified by vertical strain rates places that visible layer around 1280 C.E., as well. Thus, we have identified a regional stratigraphic time line for the Ross Sea region of Antarctica.
The volcanic records developed, to date, from the Siple Dome ice cores improves greatly on the record of Antarctica volcanism. These findings will be of great value in evaluating the volcanic record from the Inland Site, particularly by identifying signals that may be of a local source. Better estimates of the global impact from equatorial eruptions found in the Inland Site core may be attained by eliminating the contribution from local eruptions.