37 Dr. Allan M. Thompson: Honoring his Legacy as a Geologist and Educator

Monday, 24 March 2014
8:00 AM - 11:20 AM
Heritage Salon D

TRACKING PAST HYDROLOGY IN A WARMER, WETTER WORLD:  WHAT CAN WE LEARN FROM PLIOCENE LAKES?

Paper number: 37-9
Program: Theme Sessions
Time: 10:20 AM – 10:40 AM

Authors

Abstract

The Pliocene Epoch (5.3-2.6 mya) is a possible analog for global warming conditions, because the fossil record and paleoclimate modelling indicate global temperatures 2-3 degrees C higher (Lunt et al., 2012), with polar regions as much as 10 – 15 degrees C higher than today, and higher levels of CO2 (Haywood et al., 2010). Mid-latitudes experienced a very different hydroclimate during this time (Salzmann et al., 2008; 2009). This is also the last time in the geologic record when western North America had large, deep, permanent lakes that persisted on time scales of 105 and 106 years. How could these large lakes have persisted, and how can we understand their regional Pliocene hydrology, so different from that of today? Although tectonic uplift and rain shadow development played a role in curtailing these lakes, recent studies have proposed that the ENSO cycle was important in sustaining them. During the Pliocene, a permanent El Niño circulation is hypothesized to have been in place, providing substantial precipitation to the western U.S. (Federov et al., 2006; 2010), although this hypothesis has been challenged (Watanabe et al., 2011). The persistence of these large Pliocene lakes supports this hypothesis.

The fossil record of non-marine ostracodes is important in understanding the paleohydrology of these lakes and in testing the Pliocene ENSO hypothesis. Non-marine ostracodes have known biogeographic and ecological responses to temperature and hydrochemistry (www.kent.edu/NANODe ), and many of the Pliocene species are extant. In Pliocene Butte Valley Lake, California, and Plio-Pleistocene Lake Idaho (Glenns Ferry Lake), we can trace lake history through ostracode assemblages and oxygen isotopic signatures in the shells (Forester, 1991; Carter, 1994; Dennison-Budak et al., 2010; Mathias et al., 2012). Freshwater conditions dominated by Cytherissa lacustris in mid-Pliocene time support the hypothesis of a permanent El Niño system. Isotopic evidence from fossil ostracodes from Lake Idaho suggests that by Plio-Pleistocene time, seasonal precipitation patterns were established similar to modern regional conditions (Dennison-Budak, 2010). We speculate that a warmer world could drive a permanent El Niño cycle again, although modern mountain elevations will preclude formation of such large persistent lakes.