No abstract

No abstract

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.

No abstract

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.

A fast-growing stalagmite from the central California coast provides a high-resolution record of climatic changes synchronous with global perturbations resulting from the catastrophic drainage of proglacial Lake Agassiz at ca. 8.2 ka. High frequency, large amplitude variations in carbon isotopes during the 8.2 ka event, coupled with pulsed increases in phosphorus concentrations, indicate more frequent or intense winter storms on the California coast. Decreased magnesium-calcium ratios point toward a sustained increase in effective moisture during the event, however the magnitude of change in Mg/Ca suggests this event was not as pronounced on the western North American coast as anomalies seen in the high northern latitudes and monsoon-influenced areas. Nevertheless, shifts in the White Moon Cave record that are synchronous within age uncertainties with cooling of Greenland, and changes in global monsoon systems, suggest rapid changes in atmospheric circulation occurred in response to freshwater input and associated cooling in the North Atlantic region. Our record is consistent with intensification of the Pacific winter storm track in response to North Atlantic freshwater forcing, a mechanism suggested by simulations of the last deglaciation, and indicates this intensification led to increases in precipitation and infiltration along the California coast during the Holocene.