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Atmospheric water vapor is predominately sourced from the tropics, such that characterizing the link between the tropical water cycle and global climate is of critical importance. Studies of central Andean climate from Lake Junín (11 °S, Peru) show that tropical glacial extent tracks global ice volume at a ~100 ka periodicity for the last 6 glacial cycles, indicating a tight coupling between tropical water balance and high latitude climate. However, it can be difficult to decouple temperature, precipitation, and water balance histories from records of glacial extent, especially for older intervals. In this work, we focus on one such interval, MIS 15 (621–563 ka), when the connections between tropical Andean water balance and global climate seem different than the last glacial cycle. Globally, MIS 15 was a weak interglacial, with cool temperatures and low GHG concentrations, however, the Lake Junín glacial record suggests an amplified hydroclimate response to this interglacial, stronger than any other over the last 700 ka. Causes for this apparent tropical amplification may be due to large, precession-paced changes in meridional insolation gradients that exceed other interglacials owning to enhanced orbital eccentricity. Given that the role of precession on South American monsoon strength over the last glacial cycle is well established, we hypothesize that monsoon strength may have been highly variable during MIS 15 and forced changes in central Andean water balance and glacial extent. To test this, we reconstructed temperature and evaporation histories using carbonate clumped and triple oxygen isotopes of Lake Junín sediments. Preliminary results suggest temperatures were relatively stable, but possibly lower than both the present and Holocene, consistent with cool global climate at that time. Triple oxygen isotope values vary substantially, indicating massive swings in lake hydrology, between open and (nearly?) closed basin hydrology on a ~12 ka cycle that exactly match insolation variations. From this work, we conclude that hydrologic change in the central Andes was rapid and extreme during MIS 15, owning to profound changes in monsoon strength. Given that monsoons in other sectors are also sensitive to insolation changes, our work could suggest pervasive hydrologic variability throughout the tropics at this time. 

The δ18O of carbonate minerals that formed at Earth’s surface is widely used to investigate paleoclimates and paleo-elevations. However, a multitude of hydrologic processes can affect δ18O values, including mixing, evaporation, distillation of parent waters, and carbonate growth temperatures. We combined traditional carbon and oxygen isotope analyses with clumped (Δ47) and triple oxygen isotopes (Δ′17O) analyses in oyster shells (Acutostrea idriaensis) of the Goler Formation in southern California (USA) to obtain insights into surface temperatures and δ18O values of meteoric waters during the early Eocene hothouse climate. The Δ47-derived temperatures ranged from 9 °C to 20 °C. We found a correlation between the δ18O of growth water (δ18Ogw) (calculated using Δ47 temperatures and δ18O of carbonate) and the δ13C values of shells. The Δ′17O values of shell growth waters (0.006‰–0.013‰ relative to Vienna standard mean ocean water–standard light Antarctic precipitation [VSMOW-SLAP]) calculated from Δ′17O of carbonate (–0.087‰ to –0.078‰ VSMOW-SLAP) were lower than typical meteoric waters. These isotopic compositions are consistent with oyster habitation in an estuary. We present a new triple oxygen isotope mixing model to estimate the δ18O value of freshwater supplying the estuary (δ18Ofw). The reconstructed δ18Ofw of –11.3‰ to –14.7‰ (VSMOW) is significantly lower than the δ18Ogw of –4.4‰ to –9.9‰ that would have been calculated using “only” Δ47 and δ18O values of carbonate. This δ18Ofw estimate supports paleogeographic reconstructions of a Paleogene river fed by high-elevation catchments of the paleo–southern Sierra Nevada. Our study highlights the potential for paired Δ47 and Δ′17O analyses to improve reconstructions of meteoric water δ18O, with implications for understanding ancient climates and elevations. 

To assess thermal and kinetic influences on atomic mobility and mineral (neo)crystallization, clumped‐isotope abundances of calcite and dolomite were measured alongside dolomite cation ordering and U–Pb dates, across metamorphic grade within the c. 35–30 Ma Alta stock contact metamorphic aureole, Utah, USA. Average Δ47 values of dolomite inside the metamorphic aureole reflect the blocking temperature of dolomite (300°C–350°C) during cooling from peak temperatures. Dolomite Δ47 values outside the metamorphic aureole record a temperature of ~160°C. At the talc isograd, dolomite Δ47 values abruptly change, corresponding to a decrease of ~180°C over <50 m in the down‐temperature direction. This observed step in dolomite Δ47 values does not correlate with cation ordering in dolomite or U–Pb dates, neither of which correlate well with metamorphic grade. The short distance over which dolomite Δ47 values change indicates strong temperature sensitivity in the kinetics of dolomite clumped‐isotope reordering, and is consistent with a wide range of clumped‐isotope reequilibration modeling results. We hypothesize that clumped‐isotope reordering in dolomite precedes more extensive recrystallization or metamorphic reaction, such as the formation of talc. Dolomite U–Pb analyses from inside and outside the metamorphic aureole populate a single discordia ~60 Myr younger than depositional age (Mississippian), recording resetting in response to some older postdepositional, but premetamorphic process.