Browsing by Author "Denniston, Rhawn F."
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- Late Pleistocene Landscape and Settlement Dynamics of Portuguese EstremaduraPublication . Haws, Jonathan; Benedetti, Michael; Funk, Caroline L.; Bicho, Nuno; Pereira, Telmo; Marreiros, Joao; Daniels, J. Michael; Forman, Steven L.; Minckley, Thomas A.; Denniston, Rhawn F.Here we report the results of an integrated geoarchaeological survey to study Palaeolithic human settlement dynamics in the coastal region of Portuguese Estremadura. The region has been an important focus of human occupation across multiple glacial-interglacial cycles, including periods of well-documented abrupt climate instability during MIS 3 and 2. The pedestrian survey covered a roughly 10 km wide strip of land between Sao Pedro de Muel and Peniche. The survey intensively targeted three landscape settings with Pleistocene-age surfaces and sediments: the coastal bluffs with exposed aeolian, fluvial, and colluvial sands; the Caldas da Rainha diapiric valley and associated fluvial/estuarine fills; and, Cretaceous chert-rich limestone uplands that bound the inland margin of the study area. We discovered dozens of new Palaeolithic sites, analyzed numerous Pleistocene sedimentary sections, and applied widespread OSL-dating to establish age control that allowed us to build a regional geomorphic history to contextualize Late Pleistocene human settlement across the region.
- Linking the karst record to atmospheric, precipitation, and vegetation dynamics in PortugalPublication . Thatcher, Diana L.; Wanamaker, Alan D.; Denniston, Rhawn F.; Ummenhofer, Caroline C.; Regala, Frederico T.; Jorge, Nuno; Haws, Jonathan; Chormann, Alaina; Gillikin, David P.Cave deposits can be valuable sources of paleoclimate data, especially when atmospheric circulation patterns, precipitation variability, vegetation changes, and the chemical evolution of waters moving through the karst environment can be mechanistically linked to speleothem proxies. In particular, an evaluation of the factors that control the isotopic composition of precipitation and the evolution of rainwater during migration from the land surface to the cave are needed to robustly develop speleothems as hydroclimate-sensitive proxies. One area in which precipitation and atmospheric variability are closely linked is western Iberia, where rainfall is strongly influenced by the Azores High, part of the North Atlantic Oscillation (NAO) dipole. Therefore, in order to better characterize the factors that influence the isotopic composition of precipitation in Portugal and to evaluate the potential of using stalagmites from this region as hydroclimate (and NAO-sensitive) proxies, we investigated Global Network of Isotopes in Precipitation (GNIP) data from ten mainland Portugal sites spanning multiple decades. In addition, we use more than one hydrologic year of precipitation amount and isotope data from Buraca Gloriosa (BG), a cave in western Portugal, the site of on-going speleothem analyses, as well as six years of environmental monitoring from BG. We present an integrated analysis of rainfall and vegetation through the normalized difference vegetation index (NDVI) following extremely wet and dry winters. Summer vegetation density, related to the amount of precipitation in the preceding winter wet season, as well as prior calcite precipitation (PCP), would largely control the stable carbon isotopic signature (delta C-13) in stalagmites at BG. Cool season recharge is likely the dominant factor for the oxygen isotopic composition (delta O-18) of water percolating through the cave system, while amount effects exert the primary control on precipitation delta O-18 values. Based on HYSPLIT modeling, moisture sources overwhelmingly originate from the Atlantic Ocean as opposed to the Mediterranean or elsewhere
- A stalagmite test of North Atlantic SST and Iberian hydroclimate linkages over the last two glacial cyclesPublication . Denniston, Rhawn F.; Houts, Amanda N.; Asmerom, Yemane; Wanamaker Jr., Alan D.; Haws, Jonathan; Polyak, Victor J.; Thatcher, Diana L.; Altan-Ochir, Setsen; Borowske, Alyssa C.; Breitenbach, Sebastian F. M.; Ummenhofer, Caroline C.; Regala, Frederico T.; Benedetti, Michael M.; Bicho, Nuno Gonçalo Viana Pereira FerreiraClose coupling of Iberian hydroclimate and North Atlantic sea surface temperature (SST) during recent glacial periods has been identified through the analysis of marine sediment and pollen grains co-deposited on the Portuguese continental margin. While offering precisely correlatable records, these time series have lacked a directly dated, site-specific record of continental Iberian climate spanning multiple glacial cycles as a point of comparison. Here we present a high-resolution, multi-proxy (growth dynamics and delta C-13, delta O-18, and delta U-234 values) composite stalagmite record of hydroclimate from two caves in western Portugal across the majority of the last two glacial cycles (similar to 220 ka). At orbital and millennial scales, stalagmite-based proxies for hydroclimate proxies covaried with SST, with elevated delta C-13, delta O-18, and delta U-234 values and/or growth hiatuses indicating re-duced effective moisture coincident with periods of lowered SST during major ice-rafted debris events, in agreement with changes in palynological reconstructions of continental climate. While in many cases the Portuguese stalagmite record can be scaled to SST, in some intervals the magnitudes of stalagmite isotopic shifts, and possibly hydroclimate, appear to have been somewhat decoupled from SST.