Geomorphic and stratigraphic studies of Mars prove extensive liquid water flowed and pooled on the surface early in Mars’ history. Martian paleoclimate models, however, have difficulty simulating climate conditions warm enough to maintain liquid water on early Mars. Reconciling the geologic record and paleoclimatic simulations of Mars is critical to understanding Mars’ early history, atmospheric conditions, and paleoclimate. This study uses an adapted lake energy balance model to investigate the connections between Martian geology and climate. The Lake Modeling on Mars for Atmospheric Reconstructions and Simulations (LakeM2ARS) model is modified from an earth-based lake model to function in Martian conditions. We use LakeM2ARS to investigate conditions necessary to simulate a lake in Gale crater. Working at a localized scale, we combine climate input from the Mars Weather Research & Forecasting general circulation model with geologic constraints from Curiosity rover observations; in doing so, we identify potential climatic conditions required to maintain a seasonal liquid lake. We successfully model lakes in Gale crater while varying initial climate conditions, lake size, and water salinity. Our results show that ice-free conditions in a plausible Gale crater lake are best supported when the lake is small, ~10 m deep, and air temperatures reach or are just above freezing seasonally during a Martian year. Continued use and iteration of LakeM2ARS will strengthen connections between Mars’ paleoclimate and geology to inform climate models and enhance our understanding of conditions on early Mars.

The El Niño-Southern Oscillation (ENSO) phenomena, originating in the tropical Pacific region, is an interannual climate variability driven by sea surface temperature and atmospheric pressure changes that affect weather patterns globally. In Mesoamerica, ENSO can cause significant changes in rainfall patterns with major impacts on water resources. This commentary presents results from a nearly 10-yr hydrometric and tracer monitoring network across north-central Costa Rica, a region known as a headwater-dependent system. This monitoring system has recorded different El Niño and La Niña events, as well as the direct/indirect effects of several hurricane and tropical storm passages. Our results show that ENSO exerts a significant but predictable impact on rainfall anomalies, groundwater recharge, and spring discharge, as evidenced by second-order water isotope parameters (e.g., line conditioned-excess or LC-excess). The Oceanic Niño Index (ONI) is correlated with a reduction in mean annual and cold front rainfall across the headwaters of north-central Costa Rica. During El Niño conditions, rainfall is substantially reduced (by up to 69.2%) during the critical cold fronts period, subsequently limiting groundwater recharge and promoting an early onset of baseflow conditions. In contrast, La Niña is associated with increased rainfall and groundwater recharge (by up to 94.7% during active cold front periods). During La Niña, the long-term mean spring discharge (39 Ls -1) is exceeded 63-80% of the time, whereas, during El Niño, the exceedance time ranges between 26% and 44%. These stark shifts in regional hydroclimatic variability are imprinted on the hydrogen and oxygen isotopic compositions of meteoric waters. Drier conditions favored lower LC-excess in rainfall (-17.3‰) and spring water (-6.5‰), whereas wetter conditions resulted in greater values (rainfall=+17.5‰; spring water=+10.7‰). The lower and higher LC-excess values in rainfall corresponded to the very strong 2014-16 El Niño and 2018 La Niña, respectively. During the recent triple-dip 2021-23 La Niña, LC-excess exhibited a significant and consistently increasing trend. These findings highlight the importance of combining hydrometric, synoptic, and isotopic monitoring as ENSO sentinels to advance our current understanding of ENSO impacts on hydrological systems across the humid Tropics. Such information is critical to constraining 21 st century projections of future water stress across this fragile region.

The oxygen and hydrogen isotopic composition in snow and ice have long been utilized to reconstruct past temperatures of polar regions, under the assumption that post-depositional processes such as sublimation do not fractionate snow. In low-accumulation (<0.01 m yr-1) areas near the McMurdo Dry Valleys in Antarctica surface snow and ice samples have negative deuterium excess values (δD - 8*δ18O). This unique phenomenon, only observed near the Dry Valleys, is not fully understood. Here we use both an isotope-enabled general circulation model and an ice physics model and establish that negative deuterium excess values can only arise from precipitation if the majority of the moisture is sourced from the Southern Ocean. However, the model results show that moisture sourced from oceans north of 55°S contributes significantly (>50%) to precipitation in Antarctica today. We thus propose that sublimation must have occurred to yield the negative deuterium excess values in snow observed in and near the Dry Valleys and that solid-phase-diffusion in ice grains is sufficiently fast to allow Rayleigh-like isotopic fractionation in similar environments. We calculate that under present-day conditions at the Allan Hills outside the Dry Valleys, 3 to 24% of the surface snow is lost due to sublimation. Because a higher fraction of snow is expected to be sublimed when accumulation rates are lower, the magnitude of δ18O and δD enrichment due to sublimation will be higher during past cold periods than at present, altering the relationship between the snow isotopic composition and polar temperatures.

Paleotempestology assists in extending the instrumental storm record through sedimentary-based, high-resolution records of storms over millennia. A fundamental understanding of the paleorecord provides essential context for modern climate models and, therefore, a broader understanding of our climate system. The Texas (TX) coastline receives the second largest number of hurricane landfalls per year in the United States; since 1900, 92 tropical storms and hurricanes have made landfall on the TX coast. During storm impacts, coastal downwelling storm channels deliver coarse sediment to the muddy shelf. This return flow or “backwash” process results in thin but expansive storm deposits in the region, making it ideal for paleotempestological reconstructions. In this work, three sediment cores from the central TX shelf, approximately six kilometers off the coast of Matagorda Island, were collected and analyzed. Several historic and Holocene storm events have been identified in cores by conducting detailed grain size analysis at one-centimeter intervals. Bayesian-based age models couple short-lived isotopic dating techniques (210-Pb and 137-Cs) with radiocarbon ages. X-ray fluorescence (XRF) analysis is used to determine geochemical signatures of the sediments and thus the material source for cross validating the depositional mechanism. Specifically, XRF is utilized to differentiate the effects of the 1929 Colorado River diversion relative to marine deposition. Our new record of tropical cyclone (TC) occurrence from the TX shelf is compared to paleoclimate models and proxy records of El Niño Southern Oscillation (ENSO) and Gulf of Mexico (GOM) sea surface temperature (SST). Preliminary results suggest that periods of decreased ENSO and increased GOM SST correspond with enhanced TX TC activity. Understanding these complex climatic interactions will help us to understand the changes in TC activity expected in the future against the background of accelerating climate change. Given that the frequency of extreme ENSO events is projected to increase, changes in the occurrence and severity of ENSO-TX TC events may prove detrimental to many coastal populations.