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The climatic controls on the stable carbon isotopic composition (δ 13 C) of speleothem carbonate are less often discussed in the scientific literature in contrast to the frequently used stable oxygen isotopes. Various local processes influence speleothem δ 13 C values and confident and detailed interpretations of this proxy are often complex. A better understanding of speleothem δ 13 C values is critical to improving the amount of information that can be gained from existing and future records.This contribution aims to disentangle the various processes governing speleothem δ 13 C values and assess their relative importance. Using a large data set of previously published records we examine the spatial imprint of climate-related processes in speleothem δ 13 C values deposited post-1900 CE, a period during which global temperature and climate data is readily available. Additionally, we investigate the causes for differences in average δ 13 C values and growth rate under identical climatic conditions by analysing pairs of contemporaneously deposited speleothems from the same caves.This approach allows to focus on carbonate dissolution and fractionation processes during carbonate precipitation, which we evaluate using existing geochemical models. Our analysis of a large global data set of records reveals evidence for a temperature control, likely driven by vegetation and soil processes, on δ 13 C values in recently deposited speleothems. Moreover, datamodel intercomparison shows that calcite precipitation occurring along water flow paths prior to reaching the top of the speleothem can explain the wide δ 13 C range observed for concurrently deposited samples from the same cave. We demonstrate that using the combined information of contemporaneously growing speleothems is a powerful tool to decipher controls on δ 13 C values, which facilitates a more detailed discussion of speleothem δ 13 C values as a proxy for climate conditions and local soil-karst processes.

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Two types of El Niño events are distinguished by sea surface temperature (SST) anomalies centered in the central or eastern equatorial Pacific. The Central Pacific El Niño events (CP‐El Niño) are more highly correlated with weakening of the central Indian Summer Monsoon and linked to decadal Pacific climate variability. We present a 50 year, subannually resolved speleothem δ18O record from northeast India that exhibits a significant correlation with northern Pacific decadal variability and central equatorial Pacific SSTs. Accordingly, we suggest that δ18O time series in similar northeast Indian speleothems are effective tools for investigating preinstrumental changes in Pacific climate, including changes in El Niño dynamics. In contrast to central India, rainfall amounts in northeast India are relatively unaffected by El Niño. However, back trajectory analysis indicates that during CP‐El Niño events moisture transport distance to northeast India is reduced, suggesting that variations in moisture transport primarily control δ18O in the region.

The size distribution of lakes reflects the balance between the delivery of water through precipitation and its removal through evaporation. Lakes were abundant in the terminally draining basins of the western United States (U.S.) during both the colderthan-preindustrial latest Pleistocene glacial period (~14-29 ka) and the warmer-thanpreindustrial mid-Pliocene (~2.9-3.3 Ma). To understand the hydroclimatic conditions that permitted lakes, we couple lake mass balance equations with a water and energy balance framework (sensu Budyko). Further, we compare paleo-lake area distributions to forward-modeled lake areas using climate model simulations of the Last Glacial Maximum (LGM, 19-26 ka) and mid-Pliocene. We conclude that both warmer-and colder-than-modern periods of the Plio-Pleistocene resulted in wetter-than-modern conditions in the terminally draining basins of the western U.S. through similar mechanisms. Specifically, the presence of lakes during the LGM reflects increased precipitation in addition to decreased temperatures, which decrease evaporative demand. In the southern Great Basin, LGM lakes require large increases in precipitation across the region. During the mid-Pliocene, increased evaporative demand necessitates increased precipitation to maintain lakes. Further, the increase in precipitation and dominantly southwestern distribution of mid-Pliocene lake deposits is consistent with proposed mean 'El-Niño-like' conditions altering regional hydroclimate during this period. These observations suggest that during interglacial periods the western U.S. resides within a local aridity maximum, and both long-term increases and decreases in global temperatures have been associated with wetter conditions across much of the western U.S. in the past.

a b s t r a c tThe last deglaciation is marked by large and abrupt hydroclimatic changes in the Great Basin and the American Southwest. However, comparatively little is known about how hydroclimate varied on the western side of the Sierra Nevada. We present new evidence for abrupt changes in precipitation amount in the central Sierra Nevada during the last deglaciation. Our new record from McLean's Cave overlaps with a previously published record from nearby Moaning Cave (Oster et al., 2009), and extends the record of hydroclimatic change in the region through Heinrich Stadial 1. McLean's Cave speleothem d 18 O, d 13 C, Mg/Ca, and Sr/Ca indicate a shift to drier conditions at the beginning of Heinrich Stadial 1 from 16.1to 17.5 ka followed by wetter conditions at the end of Heinrich Stadial 1 when the majority of Great Basin lakes reached their deglacial highstands. During the last deglaciation, coincident shifts in the Moaning and McLean's Cave proxy records indicate drier conditions in the western Sierra Nevada during millennial-scale intervals of warming at high latitudes such as the B€ olling and Aller€ od, and wetter conditions during cooler intervals such as the Older and Younger Dryas. Thus, the Sierra Nevada speleothem records are consistent with other regional records that document increased winter rainfall during millennial-scale cold periods of the last deglaciation. However, regional differences exist in the hydroclimatic response to the Younger Dryas, with central Californian and interior southwestern sites indicating an increase in winter storms that is not apparent in southern California. Dynamic simulations of atmospheric circulation indicate that wetter conditions during the Younger Dryas relative to the B€ olling at McLean's Cave may have resulted from a stronger storm track in the north Pacific at the latitude of northern California.

Abstract. Characterizing the temporal uncertainty in palaeoclimate records is crucial for analysing past climate change, correlating climate events between records, assessing climate periodicities, identifying potential triggers and evaluating climate model simulations. The first global compilation of speleothem isotope records by the SISAL (Speleothem Isotope Synthesis and Analysis) working group showed that age model uncertainties are not systematically reported in the published literature, and these are only available for a limited number of records (ca. 15 %, n=107/691). To improve the usefulness of the SISAL database, we have (i) improved the database's spatio-temporal coverage and (ii) created new chronologies using seven different approaches for age–depth modelling. We have applied these alternative chronologies to the records from the first version of the SISAL database (SISALv1) and to new records compiled since the release of SISALv1. This paper documents the necessary changes in the structure of the SISAL database to accommodate the inclusion of the new age models and their uncertainties as well as the expansion of the database to include new records and the quality-control measures applied. This paper also documents the age–depth model approaches used to calculate the new chronologies. The updated version of the SISAL database (SISALv2) contains isotopic data from 691 speleothem records from 294 cave sites and new age–depth models, including age–depth temporal uncertainties for 512 speleothems. SISALv2 is available at https://doi.org/10.17864/1947.256 (Comas-Bru et al., 2020a).