Climatic Extremes and Human Resilience: An Examination of Two Hydrographic Basins in the Great Basin (northern Nevada, USA)

<p>The purpose of this paper is to look at the prehistoric human settlement patterns in the northern Great Basin of the United States<br>in light of a variety of climate proxies.  The intent is to look at the response of Great Basin hunter-gatherers in response to extreme climatic events.  <br>Focus will be on two US Geological Survey designated hydrographic basins: the Black Rock Basin and <br>the Truckee Basin.  The Black Rock Basin contains the Quinn River which originates in the Montanna Mountains and terminates into a seasonal lake<br>on the Black Rock playa.  The Truckee Basin contains the Truckee River which flows from Lake Tahoe in the Sierra Nevada range <br>to the terminal Pyramid Lake.  </p><p>Radiocarbon dates from excavated archaeological sites in the two basins are used as a demographic and settlement proxy.  Climate proxies<br>from the two basins include: oxygen isotope data from Pyramid Lake, pollen cores from Mud Meadows spring and Summit Lake, and tree ring <br>data from the Jackson Mountains. </p><p>Both basins see initial human settlement during the Younger Dryas period, with a growth in population/settlements through 8000 BP. After<br>approximately 7800 BP, there is a paucity of dated sites until approximately 4000 BP.  Whether this is due to the 8.2 kya BP climatic event and/or<br>the Mount Mazama volcanic eruption, is uncertain.  Oxygen isotope data from Pyramid Lake does indicate a period of hyper-aridity throughout the<br>northern Great Basin between ca. 8-4 kya BP.  The aridity declines after 4 kya based on the oxygen isotope data, and settlement in the <br>two basins increases.  With the onset of the Late Holocene Drought, ca. 2500 BP, population/settlement declines are seen except around <br>major lakes, north of 42N latitude, and elevations above 2000m. After 2000 BP, population/settlement increases throughout both basins.  Notable<br>increases of population/settlement occurs in the Late Antique Little Ice Age and continues throughout the Medieval Climatic Anomaly (MCA). Environmental proxy data indicates the MCA was a period of extreme aridity in the northern Great Basin. Despite ameorilating conditions in both basins after the MCA and in the Little Ice Age, population/settlement declines after circa 700 BP.    </p>

A probabilistic approach to oxygen isotope modelling of speleothem data with age uncertainties

<p>Due to age uncertainties and poor resolution in speleothems, age-depth modelling techniques are often implemented for cross-examinations. In this study, we use a variation of the analogue method to perform a pseudo-proxy reconstruction of the large-scale variability in Indian and East Asian monsoon precipitation using synthetic oxygen isotope records from speleothem sites and an isotope-enabled climate model.</p><p>We present a probabilistic approach to synchronize speleothems through oxygen isotope data and individual independent age constraints, achieved by co-estimating the regional δ18O variations through time. The δ18O variability is modelled using Gaussian processes, and an adaptation of BACON age-depth model is further used for the individual speleothem chronologies.</p><p>The method is tested using synthetic speleothem data generated from the ECHAM/MPI-OM climate model and corrupted through realistic noise from speleothems from the SISAL database.</p><p>By incorporating accurate and realistic depth-dependent age-uncertainties rather than shifting, stretching or compressing the time-series of oxygen isotope data, this modelling approach may lead to advancements of handling speleothem data for regional to global evaluations on variability between speleothems and timescales.</p>

Insights into the magma plumbing system of La Fossa di Vulcano (Aeolian Islands, Italy) using oxygen isotopes and clinopyroxene crystal structure

<p>The presently active La Fossa cone, Vulcano, widely considered the most hazardous volcano in the Aeolian Islands, is characterised by alternating periods of Vulcanian to subplinian explosive events and lava flow effusion. It has formed over 5.5 kyr, last erupting in 1888-90 [1], and presently behaves in a quiescent, fumarolic stage. The volcanic deposits from the cone comprise 7 major formations: Punte Nere, Grotta dei Palizzi 1, 2, and 3, Caruggi, Pietre Cotte and Gran Cratere. Many of these commence with dilute pyroclastic density current (PDC) deposits and tephra fallout capped by lava flows, with a compositional range from shoshonite to rhyolite (52-74 wt.% SiO<sub>2</sub>) [1]. Crustal xenoliths in some of the lava flows and PDC deposits signify the importance of crustal contamination in the La Fossa magmatic system [1]. Here, we present new oxygen isotope data of mineral (clinopyroxene, plagioclase) and glass separates and combine these with petrological and textural analyses as well as clinopyroxene crystal chemistry and thermobarometry to constrain the extent of crustal contamination and to determine if and where crustal contamination took place in the magmatic system of La Fossa.</p><p>Oxygen isotope data are presented for pumice, scoriae, breadcrust bombs, lavas and mafic magmatic enclaves of all formations of La Fossa. δ<sup>18</sup>O values range from +6.0‰ to +6.7‰ (SMOW) for clinopyroxene (n=19), from +7.0‰ to +8.1‰ for feldspar (n=15) and from +8.3 ‰ to +8.7 ‰ for obsidian glass (n=2). Estimated δ<sup>18</sup>O<sub>melt</sub> values are higher than that of mantle-derived magmas, indicating that crustal contamination is ubiquitous in the La Fossa magma plumbing system. δ<sup>18</sup>O<sub>fsp</sub> increases with the degree of magmatic differentiation, indicating feldspar is more contaminated in the more evolved products of La Fossa. However, no systematic variation is observed between δ<sup>18</sup>O<sub>px</sub> and whole-rock SiO<sub>2</sub>, indicating disequilibrium between clinopyroxene and plagioclase. The disequilibrium observed at La Fossa suggests that clinopyroxene is mostly xenocrystic in the more evolved samples. This is supported by clinopyroxene equilibrium tests. Single-crystal X-ray diffraction to determine clinopyroxene crystal structures is presented to constrain crystallisation pressures. Crystallisation pressure of magmas feeding explosive eruptions to between approximately 2 and 6 kbar, while magmas feeding effusive eruptions appear to have crystallised at a narrower pressure range. Our results indicate that crustal contamination is an important process at La Fossa that accompanies fractional crystallisation and magma mixing/mingling processes throughout the entire (deep to shallow) crustal magma plumbing system.</p><p>References:</p><p>[1] De Astis et al. 2013. Geol. Soc. London Memoirs. 37. 281-349.</p>

Oxygen isotope composition of zircons from the Talnakh economic intrusion of the Noril’sk province: first data

This study presents the first results of oxygen isotope analyses (18O) collected on zircons from the Talnakh economic intrusion within the Norilsk province. Zircons from gabbro-diorite, gabbroic rocks of the layered series and plagioclase-bearing wehrlite have similar mantle-like mean 18O values (5,39 0,49; 5,64 0,48 and of 5,28 0,34, respectively), which differ from 18O in zircons from sulfide-bearing melanocratic troctolite with a taxitic texture in the lower part of the intrusion (mean 18O = 6,50 0,98). These new oxygen isotope data support (i) the mantle-derived origin of the primary magma(s), parental to the Talnakh intrusion, and (ii) possible involvement of a crustal component during the formation of sulfide-bearing taxitic-textured rocks.