Rhawn F. Denniston

Evidence for increased cool season moisture during the middle Holocene

Isotopic analyses of three stalagmites from Cold Water Cave, northeast Iowa, United States, reveal nearly identical δ 13 C trends from ca. 7 to 2 ka. However, δ 18 O patterns differ by as much as 3‰ from 5.7 to 3.2 ka. These disparate δ 18 O values reflect evaporative 18 O enrichment in meteoric water prior to infiltration, suggesting that previously calculated temperatures based on a single Cold Water Cave stalagmite overestimated middle Holocene warming. The coincidence of elevated middle Holocene growth rates in the stalagmites with the lowest oxygen isotopic compositions indicates that the middle Holocene was marked by a predominance of coolweather precipitation during a period of increased overall aridity.

Holocene paleoenvironments in southeastern Minnesota – chasing the prairie-forest ecotone

This paper uses a multi-proxy approach involving pollen, plant macrofossils, speleothem isotopes, and alluvial history of streams to reconstruct the history of prairie expansion and contraction along the prairie-forest border of southeastern Minnesota, USA. Early Holocene forests were replaced by prairie along this border, but eastward expansion of prairie stalled for 2000 yr when the prairie-forest ecotone stabilized. Prairie invaded the area from the west 8000^9000 yr B.P., but mesic forest remained less than 100 km to the east until about 6000 yr B.P. Changes in N 13 C values in speleothem calcite, that reflect the rise of C4 grasses, correlate well with the presence of C4 grass species identified in the plant macrofossil record. Periods of large floods correlate with speleothem evidence of dry summers, increased cool-season precipitation (both resulting in less plant cover to absorb moisture), and change to prairie vegetation. fl 2002 Elsevier Science B.V. All rights reserved.

Orbital control of western North America atmospheric circulation and climate over two glacial cycles

The now arid Great Basin of western North America hosted expansive late Quaternary pluvial lakes, yet the climate forcings that sustained large ice age hydrologic variations remain controversial. Here we present a 175,000 year oxygen isotope record from precisely-dated speleothems that documents a previously unrecognized and highly sensitive link between Great Basin climate and orbital forcing. Our data match the phasing and amplitudes of 65°N summer insolation, including the classic saw-tooth pattern of global ice volume and on-time terminations. Together with the observation of cold conditions during the marine isotope substage 5d glacial inception, our data document a strong precessional-scale Milankovitch forcing of southwestern paleoclimate. Because the expansion of pluvial lakes was associated with cold glacial conditions, the reappearance of large lakes in the Great Basin is unlikely until ca. 55,000 years into the future as climate remains in a mild non-glacial state over the next half eccentricity cycle.

Speleothem evidence for Holocene fluctuations of the prairie-forest ecotone, north-central USA

Carbon and oxygen isotopic trends from seven Midwestern speleothems record significant offsets in the timing of middle-Holocene vegetation change. Interactions of dry Pacific and moist Gulf of Mexico air masses maintained a sharp moisture gradient across Iowa, Minnesota, and Wisconsin such that the arrival of prairie was offset by 2000 years between caves and pollen sites located only 50 km apart. Oxygen isotopes shift concomitantly with carbon in most cases, although these changes are believed to represent increased evaporative enrichment of 18O prior to infiltration during the prairie period.

Extreme rainfall activity in the Australian tropics reflects changes in the El Niño/Southern Oscillation over the last two millennia

Significance Variations in tropical cyclone (TC) activity are poorly known prior to the twentieth century, complicating our ability to understand how cyclogenesis responds to different climate states. We used stalagmites to develop a near-annual record of cave flooding from the central Australian tropics, where TCs are responsible for the majority of extreme rainfall events. Our 2,200-year time series reveals shifts in the mean number of storms through time, similar to TC variability from the North Atlantic. This finding is consistent with modern relationships between El Niño/Southern Oscillation (ENSO) and cyclogenesis, as well as with the reconstructed state of ENSO over the past two millennia, suggesting that changes between La Niña- and El Niño-dominated periods drove multicentennial shifts in TC activity in both basins. Assessing temporal variability in extreme rainfall events before the historical era is complicated by the sparsity of long-term “direct” storm proxies. Here we present a 2,200-y-long, accurate, and precisely dated record of cave flooding events from the northwest Australian tropics that we interpret, based on an integrated analysis of meteorological data and sediment layers within stalagmites, as representing a proxy for extreme rainfall events derived primarily from tropical cyclones (TCs) and secondarily from the regional summer monsoon. This time series reveals substantial multicentennial variability in extreme rainfall, with elevated occurrence rates characterizing the twentieth century, 850–1450 CE (Common Era), and 50–400 CE; reduced activity marks 1450–1650 CE and 500–850 CE. These trends are similar to reconstructed numbers of TCs in the North Atlantic and Caribbean basins, and they form temporal and spatial patterns best explained by secular changes in the dominant mode of the El Niño/Southern Oscillation (ENSO), the primary driver of modern TC variability. We thus attribute long-term shifts in cyclogenesis in both the central Australian and North Atlantic sectors over the past two millennia to entrenched El Niño or La Niña states of the tropical Pacific. The influence of ENSO on monsoon precipitation in this region of northwest Australia is muted, but ENSO-driven changes to the monsoon may have complemented changes to TC activity.

Synchronous millennial-scale climatic changes in the Great Basin and the North Atlantic during the last interglacial

Stalagmites from Goshute Cave, located in the Great Basin of the western United States, preserve ~20,000 yr of millennial-scale oxygen isotopic variability during marine isotope stages 5c and 5b, similar in timing and structure to Dansgaard-Oeschger (D-O) events 23–21 from the Greenland Ice Sheet Project 2 record. That D-O interstades 23–21 were of longer duration than many of the later D-O events, coupled with the asymmetric shape of the D-O oxygen isotope curve, and the direct U-Th dating of the Goshute Cave stalagmites, allows for an improved understanding of the synchroneity of climatic changes between the western continental United States and the North Atlantic. Eastern Pacifi c–atmosphere interactions are a likely mechanism for transmission of millennial-scale climate variability into the Great Basin.

SIMS analyses of minor and trace element distributions in fracture calcite from Yucca Mountain, Nevada, USA

Abstract Fracture-lining calcite samples from Yucca Mountain, Nevada, obtained as part of the extensive vertical sampling in studies of this site as a potential high-level waste repository, have been characterized according to microbeam-scale (25–30 μm) trace and minor element chemistry, and cathodoluminescent zonation patterns. As bulk chemical analyses are limited in spatial resolution and are subject to contamination by intergrown phases, a technique for analysis by secondary ion mass spectrometry (SIMS) of minor (Mn, Fe, Sr) and trace (REE) elements in calcite was developed and applied to eighteen calcite samples from four boreholes and one trench. SIMS analyses of REE in calcite and dolomite have been shown to be quantitative to abundances Bulk chemical signatures noted by Vaniman (1994) allowed correlation of minor and trace element signatures in Yucca Mountain calcite with location of calcite precipitation (saturated vs. unsaturated zone). For example, upper unsaturated zone calcite exhibits pronounced negative Ce and Eu anomalies not observed in calcite collected below in the deep unsaturated zone. These chemical distinctions served as fingerprints which were applied to growth zones in order to examine temporal changes in calcite crystallization histories; analyses of such fine-scale zonal variations are unattainable using bulk analytical techniques. In addition, LREE (particularly Ce) scavenging of calcite-precipitating solutions by manganese oxide phases is discussed as the mechanism for Ce-depletion in unsaturated zone calcite.

87Sr/86Sr stratigraphy from the Early Triassic of Zal, Iran: Linking temperature to weathering rates and the tempo of ecosystem recovery

Recovery from the Late Permian mass extinction was slowed by continued environmental perturbations during the Early Triassic. Rapid fluctuations of the Early Triassic marine carbonate carbon isotope record indicate instability in the global carbon cycle, and recent δ 18 O apatite studies link elevated temperatures to the prolonged biotic recovery. High temperatures potentially caused enhanced continental weathering that was detrimental to marine ecosystems, but linking weathering rates to temperature has proven difficult. One proxy for weathering is the 87 Sr/ 86 Sr of marine carbonate; we present here an 87 Sr/ 86 Sr record from an upper Permian–lower Triassic succession near Zal, Iran, that is coupled to a δ 13 C carbonate record. An increase in the rate of 87 Sr/ 86 Sr rise from the Dienerian to the Smithian may be linked to elevated continental weathering rates caused by warming during the Smithian.

Plant remains, alluvial chronology, and cave speleothem isotopes indicate abrupt Holocene climatic change at 6 ka in midwestern USA

Abstract Sediments along 1st–5th-order streams in Midwestern USA contain excellent records of abrupt climatic change in the Holocene. Cutbank exposures provide “snapshots” of areal paleovegetation based on assemblages of well-preserved pollen and plant macrofossils; when these sites are radiocarbon dated and arranged chronologically, a detailed picture of Holocene vegetational change emerges that is consistent with regional patterns. Lithologically distinct alluvial sediments and periods of rapid change from aggradation to entrenchment occur at intervals of rapid vegetational change, and are coeval with changing values of carbon isotopes from both cave speleothems and stream alluvium, indicating that climate is the major forcing function.

Paleolithic Landscapes and Seascapes of the West Coast of Portugal

The antiquity of coastal adaptations has gained renewed attention in the last several years as archaeologists have recognized that coasts have long been important foci of human settlement (Bailey 2004; Bailey and Milner 2003; Erlandson and Fitzpatrick 2006; Fa 2008; Price 1995; Sauer 1962; Westley and Dix 2006).