GEOCHRONOLOGY EVIDENCE FOR THE PROVENANCE OF DROPSTONES IN THE SCOTIA SEA

Icebergs calved from Antarctica travel counterclockwise around the continent in the Antarctic Coastal Current. When icebergs reach the Weddell Sea, they follow its circulation gyre and are exported north through “Iceberg Alley” in the Scotia Sea (Budge and Long, 2018, https://doi.org/10.1109/JSTARS.2017.2784186; see also https://www.youtube.com/watch?v=f1z5brn2iLU). Because icebergs melt and some drift north along the way, most of the Iceberg Alley icebergs are from the Weddell Sea embayment or Antarctic Peninsula sources. We seek to distinguish East and West Antarctic contributions to iceberg rafted detritus in the past. Because of the contrast in ages between East (old) and West (young) Antarctica, geochronology and thermochronology of ice rafted detritus provides important clues about the sectors that calved abundant icebergs. Dredge hauls in the Scotia Sea during NBP0805 and cores from IODP Expedition 382 sites U1536, U1537 and U1538 intercepted numerous dropstones in the Scotia Sea. These dropstones provide an integrated record of iceberg origins in their petrology and ages. Of five granite dropstones measured with 40Ar/39Ar so far from the NBP0805 dredges, the ages range from 67 to 216 Ma, and thus indicate West Antarctic and/or Peninsula sources. A large green sandstone dropstone from early Pliocene deposits in U1536 has a visual, U-Pb zircon, and apatite fission track match to the Wyatt Earp Formation of the Crashsite Group in the Ellsworth Mountains. This distinctive tracer may constrain times of West Antarctic Ice Sheet retreat. Ongoing work will add to the geochronology database of dropstone ages, and further petrological and geochemical proxies will also be used to fingerprint the sources and to evaluate changes in the dominant iceberg sources through the Pliocene and Pleistocene.