Ancient lake sediments deposited alongside former outlet and alpine glaciers in the Dry Valleys region are sensitive indicators of past climate and ecological change. We have identified 17 former lake sites that lie above 1000-m elevation on the north wall of central Taylor Valley and in north-facing valleys of the Asgard and Olympus Ranges. Chronological control comes from laser-fusion 40Ar/39Ar analyses of interbedded volcanic ashfall. Lake sediments >~13 million years (Ma) contain layers of exceptionally well-preserved pleurocarpous mosses, diatoms, woody stems, and insect remains; lake sediments <~13 Ma lack such fossils. The fossil-rich lacustrine sediments of the Dry Valleys contain the only known, in-situ tundra-type flora and fauna in the Transantarctic Mountains outside the Beardmore Glacier region (Meyer Desert Formation, Oliver Bluffs).
Our objectives include: 1) to develop a better characterization of the areal distribution of ancient lakes; 2) to secure a more refined lake chronology; 3) to develop a better characterization of the flora and fauna within each lake system; 4) to produce a geochemical signature for tephra within ice-marginal lakes and; 5) to provide a comparison for terrestrial vegetation mapped previously in the central Transantarctic Mountains.
Because the fossil preservation is excellent and because we can date precisely the evolution of these lakes, our work has several benefits for allied research in the Ross Sea region. First, it places modern lakes of the Dry Valleys into a long-term evolutionary framework. Second, it facilitates correlation and dating among glacial and non-glacial deposits across the Transantarctic Mountains; we can compare species diversity and faunal assemblages in fossil-rich deposits in two distant regions of the Transantarctic Mountains. Third, our focus on chronology through 40Ar/39Ar analyses and chemical fingerprinting of glass shards through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) will help improve dating within nearby marine cores (e.g., ANDRILL initiatives).
Along with our collaborators at North Dakota State University, Allan Ashworth and former BU student Adam Lewis, we are addressing the following: 1) When did the polar desert biota of today replace the Neogene tundra biota? 2) Was the climate change that caused the biotic turnover unidirectional and permanent? 3) Or, did short-lived, warmer-climatic conditions, supporting tundra, return to the Dry Valleys after the mid-Miocene? 4) Were the warmer conditions regional or continental in their extent? 5) What variation exists in species content and richness among Neogene fossiliferous sites in the Transantarctic Mountains?
Our results also bear on long-standing questions of biogeography. Was the extinction of relictual Gondwana biota gradual, associated with the cooling that followed the mid-Miocene warm interval at ~17 Ma, or were there dispersal episodes during warmer intervals that replenished the biota from South America, New Zealand, and Tasmania, delaying extinction until the Pliocene?
(* = Student Advisee)