Past climate variability in southwestern Oregon and relationships with regional and hemispheric climate

This dissertation presents the results of three studies that assessed climate variability on short and long timescales in western United States. The growth of carbonate formations in caves (speleothems) is used to infer the timing and amplitude of past climate variability. We first assess the controls on speleothem growth for the past 380 000 years by combining high-density U-Th dating with a regional climate model and an energy balance model. The majority of speleothem growth occurred overwhelmingly during interglacial periods and glacial periods were characterized by little or no growth. We assess the mechanisms responsible for growth cessation during the peak of the last glaciation (LGM), 21 000 years ago, with a regional climate model and determine that a combination of drier LGM conditions with a change in the seasonal cycle of the surface water balance provides a feedback mechanism that limited recharge to the deeper soil and reduced or eliminated drip water in the cave. The energy balance model supports this mechanism by indicating that cave temperatures did not drop below the freezing point of water at any time during the last glaciation. We then evaluate the climatic significance of stable isotopes of oxygen in rainwater collected in southwestern Oregon in order to accurately interpret the isotopic record of speleothems. We establish that temperature plays an important role in controlling the distribution of oxygen isotopes in rainwater and therefore speleothems in southwestern Oregon can preserve a record of temperature changes through time. Finally, we present a high-resolution paleoclimate record using speleothems from a cave in Oregon that grew during the last 9000 years and during the middle of the last interglaciation. We find the winter temperatures were sensitive to winter solar insolation and varied in a quasi-cyclical pattern on millennial timescales. This pattern was characteristic of both regional and hemispheric variability as indicated by similarities of our record with other regional and hemispheric climate reconstructions. It appears that higher climate variability in the past was associated with warming trends, with the implication that global warming may make the climate more unstable in the future. Climate variability during the last interglacial period was characterized by different scales of millennial and centennial climate variability compared with the Holocene

Exceptionally high levels of lead pollution in the Balkans from the Early Bronze Age to the Industrial Revolution

The Balkans are considered the birthplace of mineral resource exploitation and metalworking in Europe. However, since knowledge of the timing and extent of metallurgy in southeastern Europe is largely constrained by discontinuous archaeological findings, the long-term environmental impact of past mineral resource exploitation is not fully understood. Here we present a high resolution and continuous geochemical record from a peat bog in western Serbia, providing for the first time a clear indication of extent and magnitude of environmental pollution in this region, and a context in which to place archaeological findings. We observe initial evidence of anthropogenic lead (Pb) pollution during the earliest part of the Bronze Age (c.3600 yr before Common Era (BCE)), the earliest such evidence documented in European environmental records. A steady, almost linear increase in Pb concentration after 600 BCE, until circa 1600 CE is observed, documenting the development in both sophistication and extent of southeastern European metallurgical activity throughout Antiquity and the Medieval Period. This provides a new view on the history of mineral exploitation in Europe, with metal-related pollution not ceasing at the fall of the western Roman Empire, as was the case in western Europe. Further comparison with other Pb pollution records indicates the the amount of Pb deposited in the Balkans during the Medieval Period was if not greater, at least similar to records located close to western European mining regions, suggestive of the key role the Balkans have played in mineral resource exploitation in Europe over the last 5600 years

Detrital events and hydroclimate variability in the Romanian Carpathians during the Mid-to-Late Holocene

The Romanian Carpathians are located at the confluence of three major atmospheric pressure fields: the North Atlantic, the Mediterranean and the Siberian. Despite its importance for understanding past human impact and climate change, high-resolution palaeoenvironmental reconstructions of Holocene hydroclimate variability, and in particular records of extreme precipitation events in the area, are rare. Here we present a 7500-year-long high-resolution record of past climatic change and human impact recorded in a peatbog from the Southern Carpathians, integrating palynological, geochemical and sedimentological proxies. Natural climate fluctuations appear to be dominant until 4500 years before present (yr BP), followed by increasing importance of human impact. Sedimentological and geochemical analyses document regular minerogenic deposition within the bog, linked to periods of high precipitation. Such minerogenic depositional events began 4000 yr BP, with increased depositional rates during the Medieval Warm Period (MWP), the Little Ice Age (LIA) and during periods of societal upheaval (e.g. the Roman conquest of Dacia). The timing of minerogenic events appears to indicate a teleconnection between major shifts in North Atlantic Oscillation (NAO) and hydroclimate variability in southeastern Europe, with increased minerogenic deposition correlating to low NAO index values. By linking the minerogenic deposition to precipitation variability, we state that this link persists throughout the mid-to-late Holocene

Controls on dripwater chemistry of Oregon Caves National Monument, Northwestern United States

Cave dripwater chemistry of Oregon Caves National Monument (OCNM) was studied, where the parameters pH, total alkalinity, calcium, magnesium, strontium, sodium and barium were analyzed at quasi-monthly intervals from 2005 to 2007. Different statistical analyses have been used to investigate the variability of the chemical parameters in the different sites in the OCNM cave system. The dripwater varies in response to seasonal changes in rainfall. The drip rates range from zero in summer to continuous flow in winter, closely following the rainfall intensity. Spatial variations of dripwater chemistry, which is nonlinearly related to dripwater discharge likely, reflect the chemical composition of bedrock and overlying soil, and the residence time of the ground water within the aquifer. The residence time of infiltrated water in bedrock cracks control the dissolution carbonate bedrock, reprecipitation of calcium carbonate and the degree of saturation of dripwater with respect to calcium carbonate minerals. Spatiotemporal fluctuations of dripwater Mg/Ca and Sr/Ca ratios are controlled by dissolution of carbonate bedrock and the degree of calcite reprecipitation in bedrock cracks. This suggests that trace elements in speleothem deposits at the OCNM may serve as paleoclimatological proxies for precipitation, if interpreted within the context of understanding local bedrock chemistry

Constraining Holocene hydrological changes in the Carpathian–Balkan region using speleothem δ18O and pollen-based temperature reconstructions

Here we present a speleothem isotope record (POM2) from Ascunsă Cave (Romania) that provides new data on past climate changes in the Carpathian–Balkan region from 8.2 ka until the present. This paper describes an approach to constrain the effect of temperature changes on calcite δ18O values in stalagmite POM2 over the course of the middle Holocene (6–4 ka), and across the 8.2 and 3.2 ka rapid climate change events. Independent pollen temperature reconstructions are used to this purpose. The approach combines the temperature-dependent isotope fractionation of rain water during condensation and fractionation resulting from calcite precipitation at the given cave temperature. The only prior assumptions are that pollen-derived average annual temperature reflects average cave temperature, and that pollen-derived coldest and warmest month temperatures reflect the range of condensation temperatures of rain above the cave site. This approach constrains a range of values between which speleothem δ18O changes should be found if controlled only by surface temperature variations at the cave site. Deviations of the change in δ18Ocspel values from the calculated temperature-constrained range of change are interpreted towards large-scale variability of climate–hydrology. Following this approach, we show that an additional ∼0.6‰ enrichment of δ18Oc in the POM2 stalagmite was caused by changing hydrological patterns in SW Romania across the middle Holocene, most likely comprising local evaporation from the soil and an increase in Mediterranean moisture δ18O. Further, by extending the calculations to other speleothem records from around the entire Mediterranean basin, it appears that all eastern Mediterranean speleothems recorded a similar isotopic enrichment due to changing hydrology, whereas all changes recorded in speleothems from the western Mediterranean are fully explained by temperature variation alone. This highlights a different hydrological evolution between the two sides of the Mediterranean. Our results also demonstrate that during the 8.2 ka event, POM2 stable isotope data essentially fit the temperature-constrained isotopic variability. In the case of the 3.2 ka event, an additional climate-related hydrological factor is more evident. This implies a different rainfall pattern in the Southern Carpathian region during this event at the end of the Bronze Age

Caves as observatories for atmospheric thermal tides: an example from Ascunsă Cave, Romania

As part of a microclimate study at Ascunsă Cave, Romania, we used Gemini Tinytag Plus 2 data loggers to record cave air temperature variability. At one of the monitoring points we recognized the presence of semidiurnal cycles on the order of a few thousands of a degree Celsius that could be produced under the influence of the semidiurnal tidal components of the Sun (S2) or the Moon (M2). Using a Gemini Tinytag Plus 2 data logger with an external probe we measured core rock temperature and showed that it does not influence the cave air temperature on such short time scales. We thus rejected the possibility that Earth tides, mostly produced by the lunar tidal influence on the Earth’s crust, would have had a semidiurnal influence on cave air temperature. Moreover, time series analysis revealed a 12.00-hour periodicity in temperature data, specific for the S2, allowing us to assign these variations to the influence of the thermo-tidal action of the Sun. Using the Ideal Gas Law and assuming a constant volume and amount of air, we calculated that a theoretical change in atmospheric pressure of around 40 Pa was needed to produce the temperature changes we observed. This agrees with published values of atmospheric pressure changes induced by the semidiurnal solar component of the thermal tides (S2(t)). We thus can assign the observed temperature changes to semidiurnal atmospheric pressure changes (S2(p)) induced by the thermal excitation of the Sun. Our study signals the possibility that readily available data from cave monitoring studies around the world could be used in the study of atmospheric tides. Moreover, it appears that Ascunsă Cave acts as a natural meteorological filter on a short time scale, removing the direct thermal influences of the Sun (especially night and day differences) and preserving only the barometric information from the surface

Drying in the Middle East during Northern Hemisphere cold events of the early glacial period

Few paleoclimate records exist to assess the central Middle East’s response to natural forcing beyond the instrumental record. Here we present a multi-proxy stalagmite-based climate reconstruction from Iran’s semi-arid northeast that spans 100-70 thousand years before present(ka). During severe cold (stadial) events in the North Atlantic at ≈88, 77, and 73 ka, stalagmite trace-element data indicate anomalously dry periods at this location. Stadial event increases in the stalagmite oxygen isotopes mirror those in a published Iranian stalagmite 800 km to the west. A global climate model simulates drying across the Middle East region in response to stadial event forcing, in agreement with oxygen isotope enrichments in both Iranian records, caused by a smaller fractional loss of moisture on the trajectory upstream. The paleoproxies and model experiments are consistent in indicating a drier Middle East climate during the cold North Atlantic stadials

High variability between regional histories of long-term atmospheric Pb pollution

The advent of metal processing was one of the key technological evolutions presaging the development of modern society. However, the interplay between metal use and the long-term changes it induced in the development and functioning of past societies remains unclear. We present a compilation of global records of anthropogenic atmospheric lead (Pb) spanning the last 4000 years, an effective indirect proxy for reliably assessing Pb emissions directly linked to human activities. Separating this global Pb pollution signal into regionally representative clusters allows identification of regional differences in pollution output that reflect technological innovations, market demands, or demise of various human cultures for last 4000 years. Our European reconstruction traces well periods of intensive metal production such as the Roman and Medieval periods, in contrast to clusters from the Americas, which show low levels of atmospheric Pb until the Industrial Revolution. Further investigation of the European synthesis results displays clear regional variation in the timing and extent of past development of polluting activities. This indicates the challenges of using individual reconstructions to infer regional or global development in Pb output and related pollution

Stable hydrogen and oxygen isotope abundance of major bottled water brands sold in the United Kingdom

Bottled water in the UK has a ~20 % share of the soft drinks market with a sales value of >£1.5 billion. Bottled water is susceptible to fraud and it is important to characterise the chemical signature of aquifers used by the bottled water industry. Measuring 18O/16O and 2H/1H ratios in bottled water is one important step in fraud prevention and aquifer characterisation as these ratios in groundwater tend to be stable or change very slowly through time. Here we characterise the isotopic signature of 30 brands of bottled water sold in the UK. The average δ18O of bottled waters is –7.4 and –48.4 for δ2H. This isotopic composition is closely related to that of the annual rainfall and follows latitudinal and longitudinal gradients which combine to explain 77 % of the δ18O variance

Rainwater isotopes in central Vietnam controlled by two oceanic moisture sources and rainout effects

The interpretation of palaeoclimate archives based on oxygen isotopes depends critically on a detailed understanding of processes controlling the isotopic composition of precipitation. In the summer monsoonal realm, like Southeast Asia, seasonally and interannually depleted oxygen isotope ratios in precipitation have been linked to the summer monsoon strength. However, in some regions, such as central Vietnam, the majority of precipitation falls outside the summer monsoon period. We investigate processes controlling stable isotopes in precipitation from central Vietnam by combining moisture uptake calculations with monthly stable isotope data observed over five years. We find that the isotopic seasonal cycle in this region is driven by a shift in moisture source from the Indian Ocean to the South China Sea. This shift is reflected in oxygen isotope ratios with low values (− 8 to − 10‰) during summer and high values during spring/winter (0 to − 3‰), while 70% of the annual rainfall occurs during autumn. Interannual changes in precipitation isotopes in central Vietnam are governed by the timing of the seasonal onset and withdrawal of the Intertropical Convergence Zone, which controls the amount of vapour contributed from each source