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

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.