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At least 109 records · Page 6Linked to original sources
USGS2024

Pollen in polar ice implies eastern Canadian forest dynamics diverged from climate after European settlement

Rapid warming and human exploitation threaten boreal forests. Understanding links among vegetation, climate, and people in this vast biome requires highly resolved long-term records that integrate regional inputs. We developed an 850-year pollen-based record of supraregional vegetation change using a southern Greenland ice core and atmospheric modeling that identified the boreal and mixed-conifer forests of eastern Canada as the dominant pollen source regions. Conifer pollen increased ∼1400 CE at the onset of the cooler and drier Little Ice Age. A subsequent decline began ∼1650 CE and a statistically significant pollen change after 1760 CE suggests ecological consequences of the Little Ice Age cooling and initial human exploitation that persisted until recent decades. These supraregional changes are broadly consistent with local records and demonstrate intensification of human impacts on northern forests, suggesting a shift from a climate-modulated to an increasingly human-controlled system during recent centuries.

Geophysical Research Letters↗
USGS2001

Engineering implications of ground motions from the 1999 Turkey earthquakes

The August 17, 1999 Izmit (Turkey) earthquake ( M w =7.4) will be remembered as one of the largest earthquakes of recent times that affected a large urban environment. The shaking that caused the widespread damage and destruction was recorded by a handful of accelerographs operated by different networks in the earthquake area. As rebuilding in Turkey starts and picks up speed, the recorded ground motions that adversely affected the built-up environment are being studied intensively to explain the past disaster and for the implications for future earthquakes. New forecasts of future large earthquakes in the ever growing urban environment of Istanbul and its vicinities necessitates better preparation and extensive planning to upgrade and retrofit existing infrastructures. To successfully execute these endeavors, it is imperative to have better defined ground motion characteristics. This paper aims at discussing the issues related to the engineering implications of the recorded ground motions. The main shock records show that the peak accelerations, even those from near-field stations that exhibit the characteristic near-fault pulses that cause large displacements, are smaller than expected. On the other hand, smaller magnitude aftershocks yielded larger peak accelerations. This is attributed to the sparse networks, which possibly missed the recording of larger motions during the main shock - particularly in the heavily damaged areas of South Izmit Bay. Aftershocks recorded at such areas are used to estimate the missed main shock ground motions. Accordingly, the estimated motions in South Izmit Bay are approximately 1 g highly polarized in N-S direction. These attributes have significant engineering implications to be considered during the rebuilding phase and beyond.

Bollettino di Geofisica Teorica ed Applicata↗
USGS2022

S/P amplitude ratios derived from single-component seismograms and their potential use in constraining focal mechanisms for micro-earthquake sequences

Focal mechanisms, which reflect the sense of slip in earthquakes, provide important constraints for understanding crustal tectonics and earthquake source physics, including the interactions among earthquakes during mainshock–aftershock sequences or seismic swarms. Focal mechanisms of small (magnitude ≲3.5) earthquakes are usually determined by first‐motion P ‐wave polarities, sometimes supplemented by the ratio of S ‐wave to P ‐wave amplitudes ( S/P ). However, focal mechanisms of such events can be difficult to reliably constrain, particularly with sparse recording networks or very small magnitude events. Here, we describe a method for deriving S/P amplitude ratios from P/P and S/S amplitude ratios measured on single seismic components between pairs of nearby events, as is often performed during correlation‐based earthquake detection and relocation. These measurements can be transformed into relative S/P amplitude ratios, or they can be combined with a smaller number of traditional S/P amplitude ratios to provide a single‐channel estimation of full S/P ratios, even for low signal‐to‐noise‐ratio events not routinely cataloged and not amenable to traditional S/P ratio processing. This approach has the potential to greatly expand the applicability of S/P amplitude ratios, providing additional constraints for focal mechanisms of small earthquakes, particularly for spatially concentrated seismicity sequences.

California↗
USGS2015

USGS Arctic Science Strategy

The United States is one of eight Arctic nations responsible for the stewardship of a polar region undergoing dramatic environmental, social, and economic changes. Although warming and cooling cycles have occurred over millennia in the Arctic region, the current warming trend is unlike anything recorded previously and is affecting the region faster than any other place on Earth, bringing dramatic reductions in sea ice extent, altered weather, and thawing permafrost. Implications of these changes include rapid coastal erosion threatening villages and critical infrastructure, potentially significant effects on subsistence activities and cultural resources, changes to wildlife habitat, increased greenhouse-gas emissions from thawing permafrost, threat of invasive species, and opening of the Arctic Ocean to oil and gas exploration and increased shipping. The Arctic science portfolio of the U.S. Geological Survey (USGS) and its response to climate-related changes focuses on landscapescale ecosystem and natural resource issues and provides scientific underpinning for understanding the physical processes that shape the Arctic. The science conducted by the USGS informs the Nation's resource management policies and improves the stewardship of the Arctic Region.

Fact Sheet↗
USGS2014

The digital global geologic map of Mars: Chronostratigraphic ages, topographic and crater morphologic characteristics, and updated resurfacing history

A new global geologic map of Mars has been completed in a digital, geographic information system (GIS) format using geospatially controlled altimetry and image data sets. The map reconstructs the geologic history of Mars, which includes many new findings collated in the quarter century since the previous, Viking-based global maps were published, as well as other discoveries that were made during the course of the mapping using new data sets. The technical approach enabled consistent and regulated mapping that is appropriate not only for the map's 1:20,000,000 scale but also for its widespread use by diverse audiences. Each geologic unit outcrop includes basic attributes regarding identity, location, area, crater densities, and chronostratigraphic age. In turn, units are grouped by geographic and lithologic types, which provide synoptic global views of material ages and resurfacing character for the Noachian, Hesperian, and Amazonian periods. As a consequence of more precise and better quality topographic and morphologic data and more complete crater-density dating, our statistical comparisons identify significant refinements for how Martian geologic terrains are characterized. Unit groups show trends in mean elevation and slope that relate to geographic occurrence and geologic origin. In comparison with the previous global geologic map series based on Viking data, the new mapping consists of half the number of units due to simpler, more conservative and globally based approaches to discriminating units. In particular, Noachian highland surfaces overall have high percentages of their areas now dated as an epoch older than in the Viking mapping. Minimally eroded (i.e., pristine) impact craters ≥3 km in diameter occur in greater proportion on Hesperian surfaces. This observation contrasts with a deficit of similarly sized craters on heavily cratered and otherwise degraded Noachian terrain as well as on young Amazonian surfaces. We interpret these as reflecting the relatively stronger, lava-rich, yet less-impacted materials making up much of the younger units. Reconstructions of resurfacing of Mars by its eight geologic epochs using the Hartmann and Neukum chronology models indicate high rates of highland resurfacing during the Noachian (peaking at 0.3 km 2 /yr during the Middle Noachian), modest rates of volcanism and transition zone and lowland resurfacing during the Hesperian (∼0.1 km 2 /yr), and low rates of mainly volcanic and polar resurfacing (∼0.01 km 2 /yr) for most of the Amazonian. Apparent resurfacing increased in the Late Amazonian (∼0.03 km 2 /yr), perhaps due to better preservation of this latest record.

Planetary and Space Science↗
USGS1954

Studies on avian malaria in vectors and hosts of encephalitis in Kern County, California. I. Infections in avian hosts

An epizoological study of Plasmodium infections in wild birds of Kern County, California, in the years 1946 through 1951 greatly extended knowledge of the occurrence of these parasites and their behavior in nature. Examination of 10,459 blood smears from 8,674 birds representing 73 species resulted in the observation of Plasmodium spp. in 1,094 smears representing 888 individual birds of 27 species. Seven species of Plasmodium were found: relictum, elongatum, hexamerium, nucleophilum, polare, rouxi and vaughani. Plasmodium relictum was by far the most frequently observed species, occurring in at least 79 per cent of the infected birds. Twelve new host species are recorded for this parasite. Sufficient morphological variation was observed to indicate that two strains of this species probably exist in nature. Numerous new host records were made of plasmodia with elongate gametocytes. The finding of parasites believed to be P. rouxi in two new host species represents the first record of the occurrence of this Plasmodium outside of Algeria. Multiple smears were obtained from a number of individual birds over varying time periods. Evidence of prolonged parasitemia was unusual, but some individuals had parasitemia on consecutive months and even for three successive years. In most individuals, parasitemias were of short duration. The inoculation of blood from wild birds into canaries led to the demonstration of many infections not observed on blood smear examination of donors. Use of these two complementary techniques led to more complete host records and a truer picture of the prevalence of infection. Three age classes of birds were studied--nestling, immature (less than 1 year of age) and adult. Parasites were observed in all three groups but infections in the younger individuals were most susceptible to interpretation. As to time of onset, numerous records were obtained of infection in nestling birds. Prevalence rates in immature birds after a single season's exposure ranged from 64 to 100 per cent in the house finch and 17 to 68 per cent in the English sparrow in different areas and years. Marked differences were found in the prevalence rates in different summer months, years and areas. It is believed these differences reflect variation in a number of environmental factors. This study indicates the extensive distribution of Plasmodium infection in a wide range of wild avian hosts. The observations are of possible importance in epidemiological studies of other arthropod-borne diseases such as the viral encephalitides for which these birds serve as hosts.

California↗
USGS2005

Polar climate: Arctic sea ice

Recent decreases in snow and sea ice cover in the high northern latitudes are among the most notable indicators of climate change. Northern Hemisphere sea ice extent for the year as a whole was the third lowest on record dating back to 1973, behind 1995 (lowest) and 1990 (second lowest; Hadley Center–NCEP). September sea ice extent, which is at the end of the summer melt season and is typically the month with the lowest sea ice extent of the year, has decreased by about 19% since the late 1970s (Fig. 5.2), with a record minimum observed in 2002 (Serreze et al. 2003). A record low extent also occurred in spring (Chapman 2005, personal communication), and 2004 marked the third consecutive year of anomalously extreme sea ice retreat in the Arctic (Stroeve et al. 2005). Some model simulations indicate that ice-free summers will occur in the Arctic by the year 2070 (ACIA 2004).

Arctic↗
USGS2012

Shear-wave splitting from local earthquakes at the Geysers Geothermal Field, California

Shear-wave splitting from local microearth-quakes recorded in The Geysers geothermal field shows that seismic anisotropy is distributed in a complex geographic pattern. At stations within about 2 km of northwest-striking regional faults, the fast polarization direction is parallel to those faults. The geothermal field, lying between two such faults, has both northwest and northeast fast polarization directions, often at the same station. This pattern suggests at least two causes of splitting: (1) extensive dilatancy anisotropy (EDA) and (2) fault-produced fractures or rock fabric. The observed anisotropy may derive from the upper 1.5 km of the crust, averaging 4% there, or it may be heterogeneously distributed throughout the upper 5 km. Fast polarization directions coincide with fracture directions inferred from borehole data for one of the youngest rock types in the region, a felsite pluton of about 1 Ma, and with injectate pathways inferred from microseismicity and geochemistry. Including in reservoir models a permeability anisotropy with a pattern similar to seismic anisotropy may help in optimizing fluid injection and steam recovery.

California↗
USGS2000

Milestones in Antarctic Ice Sheet history: Preliminary results from Leg 188 drilling in Prydz Bay Antarctica

The Antarctic Ice Sheet is one of the great features of our planet. It plays a pivotal role in global atmospheric circulation and the sea-ice zone around it produces cold waters that control much of the ocean’s deep circulation. The Antarctic Ice Sheet is also the largest store of fresh water on earth and controls short-term sea level changes. The history of the Antarctic Ice Sheet has been pieced together from various sources. For the late Quaternary, ice cores contain a detailed record of accumulation, air temperature and atmospheric composition. For the Cenozoic, information has come from distal marine oxygen isotope records, records of detrital output from the continent and fragmentary outcrops in ice-free areas on the continent. These records have been augmented by drilling on the Antarctic continental margin to try and recover direct evidence of glacial ice and to investigatethe transition from pre-glacial to the full polar glacial conditions that we see today.

JOIDES Journal↗
USGS2009

On the reported magnetic precursor of the 1993 guam earthquake

Using 1-second magnetometer data recorded 67 km from the epicenter of the 1993 M w 7.7 Guam earthquake, Hayakawa et al. (1996) and Miyahara et al. (1999) identify anomalous precursory changes in ultra-low frequency magnetic polarization (the ratio of vertical to horizontal field components). In a check of their results, we compare their data (GAM) with 1-second data from the Kakioka observatory (KAK) in Japan and the global magnetic activity index Kp. We also examine log books kept by USGS staff working on the Guam magnetic observatory. We find (1) analysis problems with both Hayakawa et al. and Miyahara et al., (2) significant correlation between the GAM, KAK, and Kp data, and (3) an absence of identifiable localized anomalous signals occurring prior to the earthquake. The changes we do find in polarization are part of normal global magnetic activity; they are unrelated to the earthquake. Copyright 2009 by the American Geophysical Union.

Geophysical Research Letters↗
USGS1985

Paleomagnetic investigations at Mahd adh Dhahab, Kingdom of Saudi Arabia

Paleomagnetic studies of 25 oriented bedrock specimens from Jabal Mahd adh Dhahab, located 160 km southeast of Al Madinah, Kingdom of Saudi Arabia, have yielded important structural information relating to the geologic history of this base and precious metal deposit. Samples were collected along one traverse away from the mineralized zones, across the northeast-plunging antiform that constitutes the dominant regional structure of the area, and along another traverse down the axis of the mineralized zones. Lithologies range from andesite flows through andesitic to rhyodacitic tuffs, lapilli tuffs, and volcaniclastic sediments. Measurements of remanent magnetization direction and intensity before and after stepwise alternating-field demagnetization ranging from 25 Oersted (Oe) to 1000 Oe were carried out on all samples. Three classes of demagnetization behavior were observed: A very stable class with little change in direction and intensity of magnetization; a class in which rapid changes were observed at first but which then settled on a high coercivity stable component of magnetization; and a class in which the magnetization was composed of a spectrum of low coervicity components, and continuous variation of direction and intensity of magnetization occurred. A structural correction restoring the bedding to horizontal resulted in a significant reduction in dispersion of magnetization directions, and implies that the magnetization was acquired before the northeast-trending deformation. At least one geomagnetic reversal is recorded and, together with drillhole electric logs, suggests structural repetition of some parts of the area. The determined paleopole location, after correction for Tertiary rotation of Arabia from Africa, falls on the apparent polar wander path for Gondwanaland at an age of 600-590 Ma, which is younger than the mineralization age of about 680 Ma and the age of the Mahd group of rocks (772 Ma) determined from geochronologic studies, and implies that the rocks have been remagnetized during the middle Pan African times. The results of this work suggest that more extensive paleomagnetic investigations of the felsic volcanic and volcanic-derived rocks of the northern Arabian shield should provide greater insight into the structural evolution of the shield, and these investigations are recommended.

Open-File Report↗
USGS2011

The rise and fall of Lake Bonneville between 45 and 10.5 ka

A sediment core taken from the western edge of the Bonneville Basin has provided high-resolution proxy records of relative lake-size change for the period 45.1–10.5 calendar ka (hereafter ka). Age control was provided by a paleomagnetic secular variation (PSV)-based age model for Blue Lake core BL04-4. Continuous records of δ18O and total inorganic carbon (TIC) generally match an earlier lake-level envelope based on outcrops and geomorphic features, but with differences in the timing of some hydrologic events/states. The Stansbury Oscillation was found to consist of two oscillations centered on 25 and 24 ka. Lake Bonneville appears to have reached its geomorphic highstand and began spilling at 18.5 ka. The fall from the highstand to the Provo level occurred at 17.0 ka and the lake intermittently overflowed at the Provo level until 15.2 ka, at which time the lake fell again, bottoming out at ∼14.7 ka. The lake also fell briefly below the Provo level at ∼15.9 ka. Carbonate and δ18O data indicate that between 14.7 and 13.1 ka the lake slowly rose to the Gilbert shoreline and remained at about that elevation until 11.6 ka, when it fell again. Chemical and sedimentological data indicate that a marsh formed in the Blue Lake area at 10.5 ka. Relatively dry periods in the BL04-4 records are associated with Heinrich events H1–H4, suggesting that either the warming that closely followed a Heinrich event increased the evaporation rate in the Bonneville Basin and (or) that the core of the polar jet stream (PJS) shifted north of the Bonneville Basin in response to massive losses of ice from the Laurentide Ice Sheet (LIS) during the Heinrich event. The second Stansbury Oscillation occurred during Heinrich event H2, and the Gilbert wet event occurred during the Younger Dryas cold interval. Several relatively wet events in BL04-4 occur during Dansgaard-Oeschger (DO) warm events. The growth of the Bear River glacier between 32 and 17 ka paralleled changes in the values of proxy indicators of Bonneville Basin wetness and terminal moraines on the western side of the Wasatch Mountains have ages ranging from 16.9 to 15.2 ka. This suggests a near synchroneity of change in the hydrologic and cryologic balances occurring in the Bonneville drainage system and that glacial extent was linked to lake size.

Quaternary International↗
USGS2003

Lead isotopes in North Pacific deep water - Implications for past changes in input sources and circulation patterns

The sources of non-anthropogenic Pb in seawater have been the subject of debate. Here we present Pb isotope time-series that indicate that the non-anthropogenic Pb budget of the northernmost Pacific Ocean has been governed by ocean circulation and riverine inputs, which in turn have ultimately been controlled by tectonic processes. Despite the fact that the investigated locations are situated within the Asian dust plume, and proximal to extensive arc volcanism, eolian contributions have had little impact. We have obtained the first high-resolution and high-precision Pb isotope time-series of North Pacific deep water from two ferromanganese crusts from the Gulf of Alaska in the NE Pacific Ocean, and from the Detroit Seamount in the NW Pacific Ocean. Both crusts were dated applying 10 Be/9Be ratios and yield continuous time-series for the past 13.5 and 9.6 Myr, respectively. Lead isotopes show a monotonic evolution in 206Pb/204Pb from low values in the Miocene (??? 18.57) to high values at present day (??? 18.84) in both crusts, even though they are separated by more than 3000 km along the Aleutian Arc. The variation exceeds the amplitude found in Equatorial Pacific deep water records by about three-fold. There also is a striking similarity in 207Pb/204Pb and 208Pb/ 204Pb ratios of the two crusts, indicating the existence of a local circulation cell in the sub-polar North Pacific, where efficient lateral mixing has taken place but only limited exchange (in terms of Pb) with deep water from the Equatorial Pacific has occurred. Both crusts display well-defined trends with age in Pb-Pb isotope mixing plots, which require the involvement of at least four distinct Pb sources for North Pacific deep water. The Pb isotope time-series reveal that eolian supplies (volcanic ash and continent-derived loess) have only been of minor importance for the dissolved Pb budget of marginal sites in the deep North Pacific over the past 6 Myr. The two predominant sources have been young volcanic arcs, one located in the northeastern part and one located in the northwestern part of the Pacific margin, from where material has been eroded and delivered to the ocean, most likely via riverine pathways. ?? 2003 Elsevier Science B.V. All rights reserved.

Earth and Planetary Science Letters↗
USGS2021

Global Changes in 20-year, 50-year and 100-year River Floods

Concepts like the 100-year flood event can be misleading if they are not updated to reflect significant changes over time. Here, we model observed annual maximum daily streamflow using a nonstationary approach to provide the first global picture of changes in: (a) the magnitudes of the 20-, 50-, and 100-year floods (i.e., flows of a given exceedance probability in each year ); (b) the return periods of the 20-, 50-, and 100-year floods, as assessed in 1970 (i.e., flows of a fixed magnitude ); and (c) corresponding flood probabilities. Empirically, we find the 20-/50-year floods have mostly increased in temperate climate zones, but decreased in arid, tropical, polar, and cold zones. In contrast, 100-year floods have mostly decreased in arid/temperate zones and exhibit mixed trends in cold zones, but results are influenced by the small number of stations with long records, and highlight the need for continued updating of hazard assessments.

Geophysical Research Letters↗
USGS1979

A single-degree-of-freedom model for non-linear soil amplification

For proper understanding of soil behavior during earthquakes and assessment of a realistic surface motion, studies of the large-strain dynamic response of non-linear hysteretic soil systems are indispensable. Most of the presently available studies are based on the assumption that the response of a soil deposit is mainly due to the upward propagation of horizontally polarized shear waves from the underlying bedrock. Equivalent-linear procedures, currently in common use in non-linear soil response analysis, provide a simple approach and have been favorably compared with the actual recorded motions in some particular cases. Strain compatibility in these equivalent-linear approaches is maintained by selecting values of shear moduli and damping ratios in accordance with the average soil strains, in an iterative manner. Truly non-linear constitutive models with complete strain compatibility have also been employed. The equivalent-linear approaches often raise some doubt as to the reliability of their results concerning the system response in high frequency regions. In these frequency regions the equivalent-linear methods may underestimate the surface motion by as much as a factor of two or more. Although studies are complete in their methods of analysis, they inevitably provide applications pertaining only to a few specific soil systems, and do not lead to general conclusions about soil behavior. This report attempts to provide a general picture of the soil response through the use of a single-degree-of-freedom non-linear-hysteretic model. Although the investigation is based on a specific type of nonlinearity and a set of dynamic soil properties, the method described does not limit itself to these assumptions and is equally applicable to other types of nonlinearity and soil parameters.

Open-File Report↗
USGS1997

An 800,000-year pollen record from Owens Lake, California: Preliminary analyses

A long sequence of fossil palynomorph assemblages from a 323-m-long core taken at Owens Lake has enabled us to evaluate the gross vegetational trends for the Owens Valley region of California over the past ~800,000 years. Shifts in vegetation composition and abundance in the study area during the Pleistocene were indicated in core sediments by marked fluctuations in the pollen frequencies of pines, junipers, and, to a lesser extent, of big sagebrush, composites, and chenopods/amaranths. The modern vegetation distribution and modern pollen rain on the eastern flank of the Sierra Nevada indicate that maximal abundances of these taxa generally characterize higher elevation subalpine and montane coniferous forests, lower elevation coniferous woodland, steppe, and desert scrub environments. Pollen frequencies in the upper part of core OL-92 corroborate vegetational trends documented previously from late Wisconsin and Holocene Neotoma middens in the Great Basin. These trends and evidence from this study suggest that woodland taxa expanded their range down the slope of the eastern flank of the Sierra Nevada and were established in (and immediately adjacent to) Owens Valley during moderated climates of the late Wisconsin, apparently in response to decreases in temperature and increases in precipitation, but retreated upslope toward their present position starting as long ago as ca. 20 ka. More importantly, pollen evidence from core OL-92 documents that the southern Sierra Nevada has experienced nine major cool-to-warm vegetation shifts (in addition to the late Wisconsin-early Holocene warming) during the time interval spanning the middle Pleistocene to early Holocene (Brunhes Normal Polarity Chron). We believe that at least six consecutive cool-to-warm shifts (the most recent ones) represent transitions from full-glacial to full-interglacial conditions on the basis of the magnitude of vegetation change in this portion of the pollen record. These marked changes in the frequency curves of dominant palynomorph taxa enabled us to identify boundaries that define 19 (?20) pollen zones in OL-92. The excursions of the pollen frequency curves within and across zone boundaries approximate the nature, duration, and timing of the middle and late Pleistocene climatic trends documented by geochemical (δ 18 O) evidence from OL-92 and from Devils Hole (DH-11) in the Amargosa Desert of Nevada.

California↗
USGS2001

Constraining controls on carbonate sequences with high-resolution chronostratigraphy: Upper Miocene, Cabo de Gata region, SE Spain

A high-resolution chronostratigraphy has been developed for Miocene shallow-water carbonate strata in the Cabo de Gata region of SE Spain for evaluation of local, regional and global factors that controlled platform architecture prior to and during the Messinian salinity crisis. Paleomagnetic data were collected from strata at three localities. Mean natural remanent magnetization (NRM) ranges between 1.53 ?? 10-8 and 5.2 ?? 10-3 Am2/kg. Incremental thermal and alternating field demagnetization isolated the characteristic remanent magnetization (ChRM). Rock magnetic studies show that the dominant magnetic mineral is magnetite, but mixtures of magnetite and hematite occur. A composite chronostratigraphy was derived from five stratigraphic sections. Regional stratigraphic data, biostratigraphic data, and an 40Ar/39Ar date of 8.5 ?? 0.1 Ma, for an interbedded volcanic flow, place the strata in geomagnetic polarity Chrons C4r to C3r. Sequence-stratigraphic and diagenetic evidence indicate a major unconformity at the base of depositional sequence (DS)3 that contains a prograding reef complex, suggesting that approximately 250 000 yr of record (Subchrons C3Br.2r to 3Br.1r) are missing near the Messinian-Tortonian boundary. Correlation to the GPTS shows that the studied strata represent five third- to fourth-order DSs. Basal units are temperate to subtropical ramps (DS1A, DS1B, DS2); these are overlain by subtropical to tropical reefal platforms (DS3), which are capped by subtropical to tropical cyclic carbonates (Terminal Carbonate Complex, TCC). Correlation of the Cabo de Gata record to the Melilla area of Morocco, and the Sorbas basin of Spain indicate that early - Late Tortonian ramp strata from these areas are partially time-equivalent. Similar strata are extensively developed in the Western Mediterranean and likely were influenced by a cool climate or influx of nutrients during an overall rise in global sea-level. After ramp deposition, a sequence boundary (SB3) in Cabo de Gata correlates with a sequence boundary in Morocco and a published third-order eustatic fall suggesting at least a partial eustatic control for the sequence boundary. Coral reefs began to develop earlier in Cabo de Gata than at Melilla or Sorbas, arguing for local factors affecting this major environmental transition. Later Messinian reefs (DS3) from all areas are time-equivalent, suggesting a regional or global control on their formation. Some Halimeda-rich horizons in the Western Mediterranean are not time-equivalent event strata as hypothesized by others. Correlation of the relative sea-level curve for the fringing reef complex (DS3) with a published eustatic curve suggests at least a partial third-order global eustatic control for the highstand part of the sequence. Downstepping DS3 reefs and initial subaerial exposure of earlier DS3 reef strata approximately correlate with initiation of a series of subaerial unconformities in the South Pacific. The longer-term relative fall in sea-level during DS3 downstepping reef progradation does not correlate with a published third-order eustatic fall. Eustatic sea-level fluctuations may have been associated with initiation of the Mediterranean Messinian salinity crisis, but the longer-term fall may have been linked to tectonic uplift in the Mediterranean region. Widespread distribution of 'TCC-style' cycles of approximately the same age suggests a regional (Western Mediterranean) or global control on sea-level change responsible for TCC cycles. In addition, four subaerial exposure-capped TCC cycles may correlate with similar subaerial unconformities in the South Pacific, suggesting at least a partial eustatic control on TCC cyclicity. The high rates of relative sea-level change needed to generate a minimum of 25-30 m sea-level changes associated with each cycle are consistent with glacio-eustacy along with rapid evaporitic drawdown in the Mediterranean. ?? 2001 Elsevier Science B.V. All rights reserved.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1975

Morphology and phylogeny of the coccolithophycean family Ceratolithaceae

The family Ceratolithaceae includes a group of horseshoe-shaped calcareous nannofossils and contains ten species which are assignable to two genera: Amaurolithus n. gen. and Ceratolithus . Species of Amaurolithus are characterized by showing faint or no birefringence in cross-polarized light when viewed in preferred orientation. Included in Amaurolithus are A. amplificus (Bukry and Percival), A. Mzzarus (Bukry), A. delicatus n. sp., A. primus (Bukry and Percival), and A. tricorniculatus (Gartner). Species of Ceratolithus are characterized by their strong birefringence in cross-polarized light when viewed in preferred orientation. Ceratolithus includes C. acutus Gartner and Bukry, C. armatus Miiller, C. cristatus Kamptner, C. rugosus Bukry and Bramlette, and C. telesmus Norris. The family first appears in the geologic record during the late Miocene, represented by the nonbirefringent-appearing species that constitute the genus Amaurolithus . A succession of these species persists into the early Pliocene. The distinctly birefringent forms assigned to the genus Ceratolithus first appear near the base of the Pliocene; the succession has persisted to modern time.

Journal of Research of the U.S. Geological Survey↗