Finely laminated sediments within Bainbridge Crater Lake, Galápagos, provide a record of El Niño–Southern Oscillation (ENSO) events over the Holocene. Despite the importance of this sediment record, hypotheses for how climate variability is preserved in the lake sediments have not been tested. Here we present results of long‐term monitoring of the local climate and limnology and a revised interpretation of the sediment record. Brown‐green, organic‐rich, siliciclastic laminae reflect warm, wet conditions typical of El Niño events, whereas carbonate and gypsum precipitate during cool, dry La Niña events and persistent dry periods, respectively. Applying this new interpretation, we find that ENSO events of both phases were generally less frequent during the mid‐Holocene (~6100–4000 calendar years B.P.) relative to the last ~1500 calendar years. Abundant carbonate laminations between 3500 and 3000 calendar years B.P. imply that conditions in the Galápagos region were cool and dry during this period when the tropical Pacific E‐W sea surface temperature (SST) gradient likely strengthened. The frequency of El Niño and La Niña events then intensified dramatically around 1750–2000 calendar years B.P., consistent with a weaker SST gradient and an increased frequency of ENSO events in other regional records. This strong interannual variability persisted until ~700 calendar years B.P., when ENSO‐related variability at the lake decreased as the SST gradient strengthened. Persistent, dry conditions then dominated between 300 and 50 calendar years B.P. (A.D. 1650–1900, ± ~100 years), whereas wetter conditions and frequent El Niño events dominated in the most recent century.

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Molybdenum (Mo) isotopic signature and concentrations can be a geologic indicator of changes in molecular oxygen concentrations in Earth's oceans and atmosphere. However, in natural waters that are weakly to strongly sulfidic, few studies have been able to determine the exact mechanisms controlling partitioning and speciation related to isotopic fractionation of Mo. To understand the isotopic contribution of each Mo species to the overall bulk Mo isotope signature, we report a method that measures both the concentration and the isotopic composition of each Mo-bearing species during thiolation using synthesized sulfidic water. We accomplish this by using reversed-phase ion chromatography to isolate individual Mo species and then measure the isotopic ratio of each species with a multicollector inductively coupled plasma mass spectrometer. Our findings show that progressively thiolated species contribute to an increasingly negative isotopic value (i.e., −3.16 to +3.14) compared to the precursor stock solution. Additionally, these results corroborate Tossell's theoretical predictions, where lower Mo isotope values correlate to increasing sulfide presence. Furthermore, the formation of thiolated species and evolution of Mo isotopic signatures in our experiments do not occur via a stepwise formation or a quantitative conversion model for molybdate to tetrathiomolybdate. Overall, our study improves our understanding of Mo systematics, shows the potential of measuring specific Mo species, and thereby strengthens the application of Mo paleoproxy through consideration of all Mo species.

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Molybdenum (Mo) is a widely used trace metal for investigating redox conditions. However, unanswered questions remain that concentration and bulk isotopic analysis cannot specially answer. Improvements can be made by combining new geochemical techniques to traditional methods of Mo analysis. In this Element, we propose a refinement of Mo geochemistry within aquatic systems, ancient rocks, and modern sediments through molecular geochemistry (systematically combining concentration, isotope ratio, elemental mapping, and speciation analyses). Specifically, to intermediate sulfide concentrations governing Mo behavior below the "switch-point" and dominant sequestration pathways in low-oxygen conditions. The aim of this work is to (1) aid and improve the breadth of Mo paleoproxy interpretations by considering Mo speciation and (2) address outstanding research gaps concerning Mo systematics (cycling, partitioning, sequestration, etc.). The Mo paleoproxy has potential to solve ever-complex research questions. By using molecular geochemical recommendations, improved Mo paleoproxy interpretations and reconstruction can be achieved.