Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Quaternary”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,297 records · Page 72Linked to original sources
USGS2004

Magma generation at a large, hyperactive silicic volcano (Taupo, New Zealand) revealed by U-Th and U-Pb systematics in zircons

Young (<∼65 ka) explosive silicic volcanism at Taupo volcano, New Zealand, has involved the development and evacuation of several crustal magmatic systems. Up to and including the 26·5 ka 530 km 3 Oruanui eruption, magmatic systems were contemporaneous but geographically separated. Subsequently they have been separated in time and have vented from geographically overlapping areas. Single-crystal (secondary ionization mass spectrometry) and multiple-crystal (thermal ionization mass spectrometry) zircon model-age data are presented from nine representative eruption deposits from ∼45 to ∼3·5 ka. Zircon yields vary by three orders of magnitude, correlating with the degrees of zircon saturation in the magmas, and influencing the spectra of model ages. Two adjacent magma systems active up to 26·5 ka show wholly contrasting model-age spectra. The smaller system shows a simple unimodal distribution. The larger system, using data from three eruptions, shows bimodal model-age spectra. An older ∼100 ka peak is interpreted to represent zircons (antecrysts) derived from older silicic mush or plutonic rocks, and a younger peak to represent zircons (phenocrysts) that grew in the magma body immediately prior to eruption. Post-26·5 ka magma batches show contrasting age spectra, consistent with a mixture of antecrysts, phenocrysts and, in two examples, xenocrysts from Quaternary plutonic and Mesozoic–Palaeozoic metasedimentary rocks. The model-age spectra, coupled with zircon-dissolution modelling, highlight contrasts between short-term silicic magma generation at Taupo, by bulk remobilization of crystal mush and assimilation of metasediment and/or silicic plutonic basement rocks, and the longer-term processes of fractionation from crustally contaminated mafic melts. Contrasts between adjacent or successive magma systems are attributed to differences in positions of the source and root zones within contrasting domains in the quartzo-feldspathic (<15 km deep) crust below the volcano.

Taupo volcano↗
USGS2009

Magma evolution and ascent at the craters of the moon and neighboring volcanic fields, southern Idaho, USA: Implications for the evolution of polygenetic and monogenetic volcanic fields

The evolution of polygenetic and monogenetic volcanic fields must reflect differences in magma processing during ascent. To assess their evolution we use thermobarometry and geochemistry to evaluate ascent paths for neighboring, nearly coeval volcanic fields in the Snake River Plain, in south-central Idaho, derived from (1) dominantly Holocene polygenetic evolved lavas from the Craters of the Moon lava field (COME) and (2) Quaternary non-evolved, olivine tholeiites (NEOT) from nearby monogenetic volcanic fields. These data show that NEOT have high magmatic temperatures (1205 + or - 27 degrees C) and a narrow temperature range (< 25 degrees C) at any given depth; NEOT parent magmas partially crystallize within the middle crust (14-17 km), but with little time for cooling or assimilation. In contrast, COME magmas partially crystallize at similar depths, but at any given depth exhibit lower temperatures (by ~40 degrees C), and wider temperature ranges (>50 degrees C). Prolonged storage of COME magmas allows them to evolve to higher 87 Sr/ 86 Sr and SiO 2 , and lower MgO and 143 Nd/ 144 Nd. Most importantly, ascent paths control evolution: NEOT often erupt near the axis of the plain where high-flux (Yellowstone-related), pre-Holocene magmatic activity replaces granitic middle crust with basaltic sills, resulting in a net increase in NEOT magma buoyancy. COME flows erupt off-axis, where felsic crustal lithologies sometimes remain intact, providing a barrier to ascent and a source for crustal contamination. A three-stage ascent process explains the entire range of erupted compositions. Stage 1 (40-20 km): picrites are transported to the middle crust, undergoing partial crystallization of olivine + or - clinopyroxene. COME magmas pass through unarmored conduits and assimilate 1% or less of ancient gabbroic crust having high Sr and 87 Sr/ 86 Sr and low SiO 2 . Stage 2 (20-10 km): magmas are stored within the middle crust, and evolve to moderate MgO (10%). NEOT magmas, reaching 10% MgO, are positively buoyant and migrate through the middle crust. COME magmas remain negatively buoyant and so crystallize further and assimilate middle crust. Stage 3 (15-0 km): final ascent and eruption occurs when volatile contents, increased by differentiation, are sufficient (1-2 wt % H 2 O) to provide magma buoyancy through the middle (and upper) crust.

Idaho↗
USGS2016

A semi-structured MODFLOW-USG model to evaluate local water sources to wells for decision support

In order to better represent the configuration of the stream network and simulate local groundwater-surface water interactions, a version of MODFLOW with refined spacing in the topmost layer was applied to a Lake Michigan Basin (LMB) regional groundwater-flow model developed by the U.S. Geological. Regional MODFLOW models commonly use coarse grids over large areas; this coarse spacing precludes model application to local management issues (e.g., surface-water depletion by wells) without recourse to labor-intensive inset models. Implementation of an unstructured formulation within the MODFLOW framework (MODFLOW-USG) allows application of regional models to address local problems. A &ldquo;semi-structured&rdquo; approach (uniform lateral spacing within layers, different lateral spacing among layers) was tested using the LMB regional model. The parent 20-layer model with uniform 5000-foot (1524-m) lateral spacing was converted to 4 layers with 500-foot (152-m) spacing in the top glacial (Quaternary) layer, where surface water features are located, overlying coarser resolution layers representing deeper deposits. This semi-structured version of the LMB model reproduces regional flow conditions, whereas the finer resolution in the top layer improves the accuracy of the simulated response of surface water to shallow wells. One application of the semi-structured LMB model is to provide statistical measures of the correlation between modeled inputs and the simulated amount of water that wells derive from local surface water. The relations identified in this paper serve as the basis for metamodels to predict (with uncertainty) surface-water depletion in response to shallow pumping within and potentially beyond the modeled area, see Fienen et al. (2015a).

Illinois, Indiana, Michigan, Wisconsin↗
USGS1971

Palaeomagnetism and potassium-argon ages of volcanic rocks of Ngorongoro caldera, Tanzania

Natural remanent magnetization (NRM) has been measured in 21 lava flows and 1 tuff in the south-west wall of Ngorongoro caldera, Tanzania. The lowest three lavas are normally magnetized, the next two have intermediate directions, and the remainder are reversed; potassium-argon dating places the reversal at 2.45 My, corresponding to the Gauss Matuyama epoch boundary. Alternating-field demagnetization was effective in removing the ubiquitous lightning-produced secondary magnetizations, but partial thermal demagnetization was not. Curie temperatures of the lavas fell into two groups: 250°–350°C and 570°–585°C, probably corresponding to unoxidized and oxidized titanomagnetites respectively. NRM directions in the lavas occurred in several distinct and stratigraphically continuous groups, suggesting intermittent eruption of groups of lavas. No correspondence was found between NRM groups and other parameters, such as Curie temperatures. The dispersion of mean NRM directions is large in comparison with late Quaternary lavas from low latitudes in the Pacific basin, but probably is not large relative to the proportion of the geomagnetic field in Africa that is non-dipolar. The mean palaeomagnetic pole is offset 10.5° from the geographic pole, but because of the large dispersion, the difference is not considered significant.

Ngorongoro caldera↗
USGS2011

Slip rate and slip magnitudes of past earthquakes along the Bogd left-lateral strike-slip fault (Mongolia)

We carried out morphotectonic studies along the left-lateral strike-slip Bogd Fault, the principal structure involved in the Gobi-Altay earthquake of 1957 December 4 (published magnitudes range from 7.8 to 8.3). The Bogd Fault is 260 km long and can be subdivided into five main geometric segments, based on variation in strike direction. West to East these segments are, respectively: the West Ih Bogd (WIB), The North Ih Bogd (NIB), the West Ih Bogd (WIB), the West Baga Bogd (WBB) and the East Baga Bogd (EBB) segments. Morphological analysis of offset streams, ridges and alluvial fans—particularly well preserved in the arid environment of the Gobi region—allows evaluation of late Quaternary slip rates along the different faults segments. In this paper, we measure slip rates over the past 200 ka at four sites distributed across the three western segments of the Bogd Fault. Our results show that the left-lateral slip rate is ∼1 mm yr –1 along the WIB and EIB segments and ∼0.5 mm yr –1 along the NIB segment. These variations are consistent with the restraining bend geometry of the Bogd Fault. Our study also provides additional estimates of the horizontal offset associated with the 1957 earthquake along the western part of the Bogd rupture, complementing previously published studies. We show that the mean horizontal offset associated with the 1957 earthquake decreases progressively from 5.2 m in the west to 2.0 m in the east, reflecting the progressive change of kinematic style from pure left-lateral strike-slip faulting to left-lateral-reverse faulting. Along the three western segments, we measure cumulative displacements that are multiples of the 1957 coseismic offset, which may be consistent with a characteristic slip. Moreover, using these data, we re-estimate the moment magnitude of the Gobi-Altay earthquake at M w 7.78–7.95. Combining our slip rate estimates and the slip distribution per event we also determined a mean recurrence interval of ∼2500–5200 yr for past earthquakes along the different segments of the western Bogd Fault. This suggests that the three western segments of the Bogd Fault and the Gurvan Bulag thrust fault (a reverse fault bounding the southern side of the Ih Bogd range that ruptured during the 1957 earthquake) have similar average recurrence times, and therefore may have ruptured together in previous earthquakes as they did in 1957. These results suggest that the western part of the Bogd Fault system, including the Gurvan Bulag thrust fault, usually behaves in a ‘characteristic earthquake’ mode.

Bogd Fault↗
USGS2005

Inferring differential evolutionary processes of plant persistence traits in Northern Hemisphere Mediterranean fire-prone ecosystems

1 Resprouting capacity (R) and propagule-persistence (P) are traits that are often considered to have evolved where there are predictable crown fires. Because several indicators suggest a stronger selective pressure for such traits in California than in the Mediterranean Basin, we hypothesize that plant species should have evolved to become R+ and P+ more frequently in California than in the Mediterranean Basin. 2 To test this hypothesis we studied the phylogenetic association between R and P states in both California and the Mediterranean Basin using published molecular phylogenies. 3 The results suggest that R and P evolved differently in the two regions. The occurrence of the states differs significantly between regions for trait P, but not for trait R. The different patterns (towards R+ and P+ in California and towards R+ and P- in the Mediterranean Basin) are reflected in the higher abundance and the wider taxonomic distribution of species with both persistence traits (R+P+ species) in California. 4 The differential acquisition of fire persistence mechanisms at the propagule level (P+) supports the idea that fire selective pressures has been higher in California than in the Mediterranean Basin. 5 Our comparative phylogenetic-informed analysis contributes to an understanding of the differential role of the Quaternary climate in determining fire persistence traits in different Mediterranean-type ecosystems and, thus, to the debate on the evolutionary convergence of traits. ?? 2006 British Ecological Society.

Journal of Ecology↗
USGS2006

Pebble segregation and bed lenticularity in wave-worked versus alluvial gravel

Two aspects of bedding may permit visual discrimination between wave-worked and alluvial gravel. Pebbles in gravel worked by waves tend to be better segregated into discrete beds than those in alluvial deposits, and bedding in wave-worked gravel tends to be more laterally regular, or less lenticular, than that in stream gravel. Pebble segregation and lenticularity were quantitatively analysed in twenty-five different marine and fluvial deposits of Quaternary or Tertiary age in southwestern Oregon in order to evaluate their use as environmental indicators. The results indicate that these two factors combined provide a visual criterion for discriminating between many gravels of wave-worked and alluvial origin.

Oregon↗
USGS2006

Groundwater dolocretes from the Upper Triassic of the Paris Basin, France: A case study of an arid, continental diagenetic facies

Thick dolomite-cemented horizons (dolocretes) occur within a fluvial sandstone-mudstone sequence of Late Triassic age in the western part of the Paris Basin, France. Two types of dolomites can be distinguished: (a) nodular dolomitic beds less than a few metres thick, which formed within mottled overbank siltstones and mudstones; and (b) massive dolomite up to 16 m thick, which occurs in coarse grained channel sandstones and conglomerates. The majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and, less commonly, dolomicrite. Glaebules, irregular spar-filled cracks, spheroidal dolomite, silicification and vuggy porosity are locally abundant in the massive dolomite. In contrast, biologically induced micromorphological features such as rhizocretions and alveolar-septal fabrics were observed in the thin, nodular dolomite beds. The dolomite is near stoichiometric, well ordered and non-ferroan. 18O values range from −7·7 to −0·4%o PDB and 18O values range from −5·1 to + 1·8%0 PDB and no obvious difference in the stable isotopic composition between both types of dolomites was observed. Sr isotope ratios range from 0·7101 to 0·7126 and are invariably higher than the contemporary Triassic sea water. A vadose—pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures. Several features, however, argue against a pedogenic origin for the massive carbonates, most notably the absence of biologically induced structures, the occurrence in coarse grained channel (and not overbank) deposits, and the great thickness. These units are thus interpreted as groundwater in origin. Phreatic calcretes of Quaternary age, widespread in inland Australia, are regarded as a modern analogue for the Triassic Paris Basin dolocretes. Petrographic observations argue in favour of primary (proto)dolomite precipitation, although early diagenetic replacement of calcite by (proto)dolomite cannot be ruled out. Strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non-marine, continental origin for the groundwaters. The poorly ordered and non-stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near-stoichiometric, well ordered dolomite that clearly lacks evidence for pervasive recrystallization.

Paris↗
USGS2006

Sea-level and tectonic control of middle to late Pleistocene turbidite systems in Santa Monica Basin, offshore California

Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity-current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high-resolution seismic-reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2-3 mm yr-1, with increases at times of extreme relative sea-level lowstand. Coarser-grained mid-fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea-level fall. These pulses of coarse-grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr-1 on the mid- and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr-1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer-term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level. ?? 2006 The Authors. Journal compilation 2006 International Association of Sedimentologists.

Sedimentology↗
USGS2011

Playa-lake sedimentation and organic matter accumulation in an Andean piggyback basin: The recent record from the Cuenca de Pozuelos, NW Argentina

Expansive playa‐lake systems situated in high‐altitude piggyback basins are important and conspicuous components of both modern and ancient cordilleran orogenic systems. Extant playa lakes provide vital habitat for numerous endemic species, whereas sediments from these deposystems may record signals of climate change or develop natural resources over geological time. Laguna de los Pozuelos (North‐west Argentina) provides the opportunity for an actualistic sedimentological and geochemical assessment of a piggyback basin playa lake in an area of critical interest for understanding Quaternary palaeoclimate dynamics. Silty clays and diatom ooze are the dominant playa‐lake centre microfacies, with concentrations of total organic carbon and biogenic silica commonly exceeding 1·5 wt% in this sub‐environment. Elemental and stable isotopic analyses point to a mixed organic matter composition in the playa‐lake centre, with substantial contributions from algae and transported aquatic macrophytes. Bulk sediment and organic mass accumulation rates in the southern playa‐lake centre approach 0.22g cm −2 year −1 and 2.89 mg cm −2 year −1 , respectively, indicating moderately rapid deposition with negligible deflation over historic time. Playa margin facies contain higher percentages of fragmented biogenic carbonate (ostracods and charophytes) and inorganically precipitated aragonite crusts due to seasonal pumping and evaporation of ground water. Organic matter accumulation is limited along these heavily bioturbated wet and dry mud flats. Fluvial–lacustrine transitional environments, which are key waterbird habitats, are either silty terminal splay (northern axis) or sandy deltas (southern axis) containing highly oxidized and partially allochthonous organic matter. Modern analogue data from Laguna de los Pozuelos provide key insights for: (i) environmental reconstructions of ancient lake sequences; and (ii) improving facies models for piggyback basins.

Cuenca de Pozuelos↗
USGS2008

Recent shallow marine ostracods from high latitudes: Implications for late Pliocene and Quarternary palaeoclimatology

New information about modern high-latitude marine ostracod faunas and about their occurrence in late Pliocene and Quaternary sequences promotes the application of these microfossils to palaeoenvironmental, palaeo-biogeographical and palaeoclimatological problems. Five poorly known species ( Cluthia cluthae, Finmarchinella logani, Roundstonia globulifera, Roundstonia macchesneyi and Semicytherura complanata ) from North Atlantic, North Pacific and Arctic areas are confirmed as extant and are used to demonstrate the potential of marine ostracods as tools for palaeoclimate studies. The genus Bonnyannella Athersuch, 1982 is shown to be a junior synonym of Roundstonia Neale, 1973.

Boreas↗
USGS2009

Paleosols in central Illinois as potential sources of ammonium in groundwater

Glacially buried paleosols of pre-Holocene age were evaluated as potential sources for anomalously large concentrations of ammonium in groundwater in East Central Illinois. Ammonium has been detected at concentrations that are problematic to water treatment facilities (greater than 2.0 mg/L) in this region. Paleosols characterized for this study were of Quaternary age, specifically Robein Silt samples. Paleosol samples displayed significant capacity to both store and release ammonium through experiments measuring processes of sorption, ion exchange, and weathering. Bacteria and fungi within paleosols may significantly facilitate the leaching of ammonium into groundwater by the processes of assimilation and mineralization. Bacterial genetic material (DNA) was successfully extracted from the Robein Silt, purified, and amplified by polymerase chain reaction to produce 16S rRNA terminal restriction fragment length polymorphism (TRFLP) community analyses. The Robein Silt was found to have established diverse and viable bacterial communities. 16S rRNA TRFLP comparisons to well-known bacterial species yielded possible matches with facultative chemolithotrophs, cellulose consumers, nitrate reducers, and actinomycetes. It was concluded that the Robein Silt is both a source and reservoir for groundwater ammonium. Therefore, the occurrence of relatively large concentrations of ammonium in groundwater monitoring data may not necessarily be an indication of only anthropogenic contamination. The results of this study, however, need to be placed in a hydrological context to better understand whether paleosols can be a significant source of ammonium to drinking water supplies. ?? 2009 National Ground Water Association.

Ground Water Monitoring and Remediation↗
USGS2012

Temporal, spatial and ecological dynamics of speciation among amphi-Beringian small mammals

Quaternary climate cycles played an important role in promoting diversification across the Northern Hemisphere, although details of the mechanisms driving evolutionary change are still poorly resolved. In a comparative phylogeographical framework, we investigate temporal, spatial and ecological components of evolution within a suite of Holarctic small mammals. We test a hypothesis of simultaneous divergence among multiple taxon pairs, investigating time to coalescence and demographic change for each taxon in response to a combination of climate and geography.

Beringia↗
USGS2014

Evidence against a Pleistocene desert refugium in the Lower Colorado River Basin

Aim The absence of Sonoran Desert plants in late Pleistocene-aged packrat middens has led to speculation that they survived glacial episodes either in refugia as intact associations (Clementsian community concept) or in dry microsites within chaparral or woodland according to individualistic species responses (Gleasonian community concept). To test these hypotheses, we developed a midden record from one likely refugium in north-eastern Baja California, Mexico. We also measured stomatal guard cell size in fossil leaves to further evaluate site-level individualistic responses of Larrea tridentata (creosote bush) ploidy races to climatic changes, including monsoonal history, over the late Quaternary. Location Sierra Ju&aacute;rez, Lower Colorado River Basin, north-eastern Baja California, Mexico. Methods Packrat ( Neotoma ) middens were collected from <300 m elevation on the eastern piedmont of the Sierra Ju&aacute;rez. Plant macrofossils and pollen were analysed from 50 dated middens, including determination of Larrea tridentata ploidy races. Results Pleistocene middens dating back to >55,000 cal. yr BP contained a mix of extralocal species characteristic of chaparral and pinyon&ndash;juniper&ndash;oak woodland, along with some modern desert elements. Many other desert taxa were absent during the Pleistocene, although most had arrived by the beginning of the Holocene 11,700 years ago. Main conclusions The assemblage of chaparral, woodland and select desert elements refutes the hypothesis that the Lower Colorado River Basin served as a late Pleistocene refugium for Sonoran Desert flora. The rapid arrival of most missing desert species by the early Holocene suggests they did not have far to migrate. They probably survived the last glacial period as smaller, disparate populations in dry microsites within chaparral and pinyon&ndash;juniper&ndash;oak woodlands. Diploid and tetraploid races of Larrea tridentata were present during the Pleistocene, but hexaploids did not appear until the mid-Holocene. This demonstrates that individualistic responses to climate involved genetic variants, in this case cytotypes, and not just species.

Baja California↗
USGS2017

Discriminating between natural vs induced seismicity from long-term deformation history of intraplate faults

To assess whether recent seismicity is induced by human activity or is of natural origin, we analyze fault displacements on high-resolution seismic reflection profiles for two regions in the central United States (CUS): the Fort Worth Basin (FWB) of Texas, and the northern Mississippi embayment (NME). Since 2009 earthquake activity in the CUS has increased dramatically, and numerous publications suggest that this increase is primarily due to induced earthquakes caused by deep-well injection of wastewater, both flowback water from hydrofracturing operations and produced water accompanying hydrocarbon production. Alternatively, some argue that these earthquakes are natural, and that the seismicity increase is a normal variation that occurs over millions of years. Our analysis shows that within the NME, faults deform both Quaternary alluvium and underlying sediments dating from Paleozoic through Tertiary, with displacement increasing with geologic unit age, documenting a long history of natural activity. In the FWB, a region of ongoing wastewater injection, basement faults show deformation of the Proterozoic and Paleozoic units, but little or no deformation of younger strata. Specifically, vertical displacements in the post-Pennsylvanian formations, if any, are below the resolution (~15 m) of the seismic data, far less than expected had these faults accumulated deformation over millions of years. Our results support the assertion that recent FWB earthquakes are of induced origin; this conclusion is entirely independent of analyses correlating seismicity and wastewater injection practices. To our knowledge this is the first study to discriminate natural and induced seismicity using classical structural geology analysis techniques.

Arkansas, Illinois, Mississippi, Missouri, Tenness↗
USGS2021

High- and low-latitude forcings drive Atacama Desert rainfall variations over the past 16,000 years

Late Quaternary precipitation dynamics in the central Andes have been linked to both high- and low-latitude atmospheric teleconnections. We use present-day relationships between fecal pellet diameters from ashy chinchilla rats ( Abrocoma cinerea ) and mean annual rainfall to reconstruct the timing and magnitude of pluvials (wet episodes) spanning the past 16,000 years in the Atacama Desert based on 81 14 C-dated A. cinerea paleomiddens. A transient climate simulation shows that pluvials identified at 15.9 to 14.8, 13.0 to 8.6, and 8.1 to 7.6 ka B.P. can be linked to North Atlantic (high-latitude) forcing (e.g., Heinrich Stadial 1, Younger Dryas, and Bond cold events). Holocene pluvials at 5.0 to 4.6, 3.2 to 2.1, and 1.4 to 0.7 ka B.P. are not simulated, implying low-latitude internal variability forcing (i.e., ENSO regime shifts). These results help constrain future central Andean hydroclimatic variability and hold promise for reconstructing past climates from rodent middens in desert ecosystems worldwide.

Science Advances↗
USGS1969

Methane-derived marine carbonates of pleistocene age

In some calcium carbonate-bearing sandstones from the edge of the continental shelf off the northeast United States, the δC 13 range is from -30 and -60 per mil for both aragonite and high-magnesium calcite. The δC 13 of co-existing shells of Modiolus sp. is normal (+ 1.7 to -2.7 per mil). The δO 18 values of around + 3.5 per mil in all samples suggest deposition at temperatures around 0°C. Quaternary methane oxidized either chemically or microbiologically to carbon dioxide is the probable source of carbon in these carbonates.

Science↗