Astronomically-Forced Lake Expansion and Contraction Cycles: Sr Isotopic Evidence from the Eocene Green River Formation, Western USA

Abstract:

⁸⁷Sr/⁸⁶Sr ratio from ancient lake deposits offer a unique insight into the astronomical forcing of lake expansion and contraction, by recording changes in runoff/groundwater provenance. We present new high-resolution ⁸⁷Sr/⁸⁶Sr data from the upper Wilkins Peak Member, to investigate linkages between astronomical forcing, water sources, and lake level in a classic rhythmic succession. Fifty-one ⁸⁷Sr/⁸⁶Sr ratios from White Mountain core #1 were acquired with a sampling interval of ~30 cm starting from the top of alluvial “I” bed to the lower Laney Member. The ⁸⁷Sr/⁸⁶Sr data show a strong and significant negative correlation with oil-yield, a traditional proxy for paleolake level and organic productivity. Application of a radioisotopic time scale, using previously dated ash beds, reveals that both ⁸⁷Sr/⁸⁶Sr and oil yield have a strong 20 kyr rhythm. The ⁸⁷Sr/⁸⁶Sr data more clearly express a longer period 100 kyr signal, similar to the Laskar 10D eccentricity solution. Using our nominal radioisotopic time scale, the Laskar 10D solution and ⁸⁷Sr/⁸⁶Sr data suggest that highest lake levels and greatest organic enrichment are attained during greatest precession and eccentricity.

Regional geologic studies and modern river water analyses have shown that less radiogenic waters mostly originate west of the basin, where drainage is strongly influenced by thick Paleozoic and Mesozoic marine carbonate units. Decreased in ⁸⁷Sr/⁸⁶Sr therefore imply greater relative water contributions from the Sevier orogenic highlands, relative to lower relief, more radiogenic ranges lying to the east. We therefore propose that highstands of Lake Gosiute record increased penetration of Pacific moisture, related either to increased El Niño frequency or southward displacement of major storm tracks. We hypothesize that the occurrence of wetter winters caused expansion of Lake Gosiute, deposition of organic carbon rich facies, and decreased lake water ⁸⁷Sr/⁸⁶Sr.