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One implication of Milankovitch's orbital theory of ice ages is that glacial cycles should be asynchronous between the northern and southern mid-latitudes, because insolation forcing is out of phase between the two hemispheres. However, evidence at present points to a near-synchronous termination (a rapid transition from glacial to interglacial conditions) at the end of the last glacial maximum (LGM) in both hemispheres. One way to test hypotheses for the ice ages and their terminations is to create detailed glacial chronologies that record the timing of past ice fluctuations. Globally distributed records allow one to make geographic comparisons to isolate potential mechanisms that drive ice ages.
The glacial history of southernmost South America is not well-documented at present, but is important for understanding the characteristics of the Southern Hemisphere ice age and termination. During March and April of 2013, a field team from the University of Maine (along with colleagues from Lamont-Doherty Earth Observatory and the University of Cincinnati) traveled to Bahía Inútil, Chile, to collect data to improve the local glacial record, and that allow for the testing of larger hypotheses concerning the termination of the last ice age. The field work consisted of glacial mapping and the collection of rock samples from boulder surfaces for surface-exposure age dating. This technique results in precise dates of glacial landform construction. When a quartz-bearing boulder melts out of a receding ice lobe, the boulder surface is exposed to cosmogenic rays. When these rays strike the surface, spallation reactions occur within the quartz and produce 10Be atoms at known production rates.
Location and Setting
The Bahía Inútil field area lies at 53°S and 69ºW on the equatorial side of the Antarctic Frontal Zone. The Straights of Magellan and Bahía Inútil are large marine inlets surrounded on all sides by low elevation coastal areas. During the LGM (~26,000 -18,000 years before present) ice flowed through the Straits of Magellan from mountains to the west and south, such as Cordillera Darwin. The moraines adjacent to Bahía Inútil represent the fluctuations of a major ice lobe during the LGM until the start of the last termination. The moraines examined during this field season lie just to the south of Bahía Inútil, on the Chilean side of Tierra del Fuego. The landscape is covered with heath-like vegetation and few trees. The moraines here are subtle, appearing as boulder strings on low, curving ridges. The windward surfaces of the boulders are very weathered.
The field party arrived in Punta Arenas, Chile, on the 25th of March, and by the next day, we had 4x4 vehicles loaded to the brim with field team members and equipment. To get to the field site, we first took a ferry across the Straits of Magellan. We drove south through Tierra del Fuego until arrival in Bahía Inútil. Most of Tierra del Fuego is privately owned, and the field work completed during this expedition was done on two large estancias, Estancia Tres Hermanos, and Estancia Rosa Irene, with landowner permission.
On Estancia Tres Hermanos, we sampled ten ideal boulders from two closely-spaced moraines. Estancia Rosa Irene contained a younger moraine set than those found at Estancia Tres Hermanos, and we took twenty-three samples from this location.
For the purposes of cosmogenic surface exposure dating, an ideal boulder should be in place, and have its original surface. In addition, the boulders should be shielded only minimally. Corrections can be made for shielding effects, but to introduce the least error, our field crew tried to select boulders where shielding was minimal.
After each boulder was approved by the field team, a surface-slab was taken using either a small charge or a set of wedges and shims. The surface characteristics of the boulder and its geomorphological setting were then recorded in field notebooks, photographed, and logged as points on a Trimble GPS.
The samples collected at Bahía Inútil are currently being processed at Lamont-Doherty Earth Observatory for 10Be surface-exposure dating. When geochemical processing at LDEO is complete, the samples will be sent to Lawrence-Livermore National Laboratory, where the 10Be/9Be ratio will be measured. This dating method is highly precise, and should result in a new chronology of glacial recession in Tierra del Fuego, allowing for the testing of larger hypotheses that aim to solve the mystery of the ice ages.
This field excursion was made possible by the generous support of the Churchill Fund- Thank you!