UC Riverside

Stable isotopes ratios have yielded major geochemical insights since the mid-twentieth century, and yet there remains new ground to be broken when analyzing traditional isotopic systems in extreme environments. This dissertation relies on such analyses—including δ13C, δ18O, δ15N, and δ34S—at ancient and modern alkaline lakes. The specificity of these environments belies their broader significance, including for the earliest (> 3.5 Ga) Earth and Mars. For instance, impact craters, which were a vital feature of those systems, tend to produce alkaline lakes as a result of the weathering of mafic impact ejecta. Alkaline lakes are also among the most bioproductive natural aquatic environments, with chemistries favorable for accumulating bioessential compounds thought necessary for a de novo origin of life. The novel datasets within this dissertation investigate the capacity for alkaline lakes to exhibit distinct isotopic records. The exceptional nature of such signatures, particularly when combined with various contextual data, may allow them to act as effective proxies for past conditions. Notable examples include δ34S evidence of Rayleigh distillation during sulfate reduction within an impact-induced hydrothermal paleolake (Chapter 1), as well as the connection between pH and elevated δ15N via ammonia volatilization (Chapter 2); both of these topics are investigated at the Miocene Ries crater lake of southern Germany. To further elucidate δ15N increases via ammonia volatilization, multiple extant lakes of the Coorong lagoon are examined (Chapter 3). The results suggest hypersalinity is an important control on heightened δ15N dynamics and preservation in shallow alkaline systems. The dissertation work is then placed within a broader context (Chapter 4) by reviewing additional sites of ammonia volatilization; this includes a new stable isotope data set from the Eocene Green River Formation. A schematic for elevated δ15N in redox-stratified basins is proposed, based on the balance of ammonia volatilization and denitrification in response to spatial and temporal shifts in pH, salinity, and chemocline depth. In sum, alkaline lakes can exhibit remarkably distinct δ15N and δ34S signatures, which can help establish constraints on aqueous conditions at sites of high geologic and astrobiological value.