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At least 235 records · Page 13Linked to original sources

USGS2016

Holocene climate variability and anthropogenic impacts from Lago Paixban, a perennial wetland in Peten, Guatemala

Analyses of an ~ 6 m sediment core from Lago Paixban in Peten, Guatemala, document the complex evolution of a perennial wetland over the last 10,300 years. The basal sediment is comprised of alluvial/colluvial fill deposited in the early Holocene. The absence of pollen and gastropods in the basal sediments suggests intermittently dry conditions until ~ 9000 cal yr. BP (henceforth BP) when the basin began to hold water perennially. Lowland tropical forest taxa dominated the local vegetation at this time. A distinct band of carbonate dating to ~ 8200 BP suggests regionally dry conditions, possibly associated with the 8.2 ka event. Wetter conditions during the Holocene Thermal Maximum are indicated by evidence of a raised water level and an open water lake. The timing of this interval coincides with strengthening of the Central American Monsoon. An abrupt change at 5500 BP involved the development of a sawgrass marsh and onset of peat deposition. The lowest recorded water levels date to 5500–4500 BP. Pollen, isotope, geochemical, and sedimentological data indicate that the coring site was near the edge of the marsh during this period. A rise in the water table after 4500 BP persisted until around 3500 BP. Clay marl deposition from 3500 to 210 BP corresponds to the period of Maya settlement. An increase in δ 13 C, the presence of Zea pollen, and a reduction in the percentage of forest taxa pollen indicate agricultural activity at this time. In contrast to several nearby paleoenvironmental studies, proxy evidence from Lago Paixban indicates human presence through the Classic/Postclassic period transition (~ 1000 BP) and persisting until the arrival of Europeans. Cessation of human activity around 210 BP resulted in local afforestation and the re-establishment of the current sawgrass marsh at Lago Paixban.

Peten↗

USGS2008

North polar region of Mars: Advances in stratigraphy, structure, and erosional modification

We have remapped the geology of the north polar plateau on Mars, Planum Boreum, and the surrounding plains of Vastitas Borealis using altimetry and image data along with thematic maps resulting from observations made by the Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter spacecraft. New and revised geographic and geologic terminologies assist with effectively discussing the various features of this region. We identify 7 geologic units making up Planum Boreum and at least 3 for the circumpolar plains, which collectively span the entire Amazonian Period. The Planum Boreum units resolve at least 6 distinct depositional and 5 erosional episodes. The first major stage of activity includes the Early Amazonian (∼3 to 1 Ga) deposition (and subsequent erosion) of the thick (locally exceeding 1000 m) and evenly-layered Rupes Tenuis unit (A b rt), which ultimately formed approximately half of the base of Planum Boreum. As previously suggested, this unit may be sourced by materials derived from the nearby Scandia region, and we interpret that it may correlate with the deposits that regionally underlie pedestal craters in the surrounding lowland plains. The second major episode of activity during the Middle to Late Amazonian ( ∼ < "> ∼< 1 Ga) began with a section of dark, sand-rich and light-toned ice-rich irregularly-bedded sequences (Planum Boreum cavi unit, A b b c ) along with deposition of evenly-bedded light-toned ice- and moderate-toned dust-rich layers (Planum Boreum 1 unit, A b b 1 ). These units have transgressive and gradational stratigraphic relationships. Materials in Olympia Planum underlying the dunes of Olympia Undae are interpreted to consist mostly of the Planum Boreum cavi unit (A b b c ). Planum Boreum materials were then deeply eroded to form spiral troughs, Chasma Boreale, and marginal scarps that define the major aspects of the polar plateau's current regional topography. Locally- to regionally-extensive (though vertically minor) episodes of deposition of evenly-bedded, light- and dark-toned layered materials and subsequent erosion of these materials persisted throughout the Late Amazonian. Sand saltation, including dune migration, is likely to account for much of the erosion of Planum Boreum, particularly at its margin, alluding to the lengthy sedimentological history of the circum-polar dune fields. Such erosion has been controlled largely by topographic effects on wind patterns and the variable resistance to erosion of materials (fresh and altered) and physiographic features. Some present-day dune fields may be hundreds of kilometers removed from possible sources along the margins of Planum Boreum, and dark materials, comprised of sand sheets, extend even farther downwind. These deposits also attest to the lengthy period of erosion following emplacement of the Planum Boreum 1 unit. We find no evidence for extensive glacial flow, topographic relaxation, or basal melting of Planum Boreum materials. However, minor development of normal faults and wrinkle ridges may suggest differential compaction of materials across buried scarps. Timing relations are poorly-defined mostly because resurfacing and other uncertainties prohibit precise determinations of surface impact crater densities. The majority of the stratigraphic record may predate the recent (<20 Ma) part of the orbitally-driven climate record that can be reliably calculated. Given the strong stratigraphic but loose temporal constraints of the north polar geologic record, a comparison of north and south polar stratigraphy permits a speculative scenario in which major Amazonian depositional and erosional episodes driven by global climate activity is plausible.

Icarus↗

USGS2008

Distribution of geogenic arsenic in hydrologic systems: Controls and challenges

The presence of elevated concentration of arsenic (As) in natural hydrologic systems is regarded as the most formidable environmental crisis in the contemporary world. With its substantial presence in the drinking water of more than thirty countries worldwide, and with an affected population of more than 100 million, it has been termed as the largest mass poisoning in human history. In this special issue, we have tried to provide the most recent research advances on controls and challenges of this severe groundwater contaminant. The articles in this issue, originally presented in the 2006 Geological Society of America Annual Meeting, address the distribution of As in various geologic and geographic settings, the controls of redox and other geochemical parameters on its spatial and temporal variability, the influence of sedimentology and stratigraphy on its occurrence, and mechanisms controlling its mobility. The knowledge available from these studies should provide a roadmap for future research in arsenic contamination hydrology.

Journal of Contaminant Hydrology↗

USGS2015

Estimation of historic flows and sediment loads to San Francisco Bay,1849–2011

River flow and sediment transport in estuaries influence morphological development over decadal and century time scales, but hydrological and sedimentological records are typically too short to adequately characterize long-term trends. In this study, we recover archival records and apply a rating curve approach to develop the first instrumental estimates of daily delta inflow and sediment loads to San Francisco Bay (1849–1929). The total sediment load is constrained using sedimentation/erosion estimated from bathymetric survey data to produce continuous daily sediment transport estimates from 1849 to 1955, the time period prior to sediment load measurements. We estimate that ∼55% (45–75%) of the ∼1500 ± 400 million tons (Mt) of sediment delivered to the estuary between 1849 and 2011 was the result of anthropogenic alteration in the watershed that increased sediment supply. Also, the seasonal timing of sediment flux events has shifted because significant spring-melt floods have decreased, causing estimated springtime transport (April 1st to June 30th) to decrease from ∼25% to ∼15% of the annual total. By contrast, wintertime sediment loads (December 1st to March 31st) have increased from ∼70% to ∼80%. A ∼35% reduction of annual flow since the 19th century along with decreased sediment supply has resulted in a ∼50% reduction in annual sediment delivery. The methods developed in this study can be applied to other systems for which unanalyzed historic data exist.

California↗

USGS2023

Ash aggregate-rich pyroclastic density currents of the 431 CE Tierra Blanca Joven eruption, Ilopango caldera, El Salvador

The VEI 6, Tierra Blanca Joven pyroclastic sequence (30–90 km 3 DRE volume), erupted from Ilopango caldera, El Salvador, in 431 CE, is the product of one of the largest eruptions of the last two millennia. The eruption devastated Central America's Mayan civilization. The eruption began with a short-lived phase of ash and pumice fall deposition and transitioned to a ‘wet’ explosive phase during which pyroclastic density currents flowed >40 km from the caldera. Detailed field and sedimentological analyses are provided for the deposits of ash-aggregate-rich pyroclastic density currents generated during early phases of the eruption. The first phase of pyroclastic density current inundation incinerated forests and deposited up to 30 m of, non-welded, ash-rich ignimbrite in proximal regions, along with ash fall layers of co-ignimbrite origin. Following fallout of a thin layer of pumice and lithic lapilli, a second phase of pyroclastic density current inundation and co-ignimbrite ash fall commenced. A range of ash aggregate types is present in the pyroclastic density current deposits and interbedded co-ignimbrite ash fall layers. Whole and broken concentrically layered ash aggregates (accretionary lapilli) reach >50 vol% in some horizons within some beds in the pyroclastic density current deposits. The evidence indicates that the ash aggregates grew within overriding co-ignimbrite ash plumes and subsequently fell into ground-hugging currents. Our findings suggest that the aggregate-rich nature of the pyroclastic density current deposits originated through incorporation of lake water into eruptive plumes, which in turn triggered rapid, pervasive aggregation within ash clouds and co-ignimbrite plumes.

Ilopango caldera↗

USGS2009

Timing of occurrence of large submarine landslides on the Atlantic Ocean margin

Submarine landslides are distributed unevenly both in space and time. Spatially, they occur most commonly in fjords, active river deltas, submarine canyon-fan systems, the open continental slope and on the flanks of oceanic volcanic islands. Temporally, they are influenced by the size, location, and sedimentology of migrating depocenters, changes in seafloor pressures and temperatures, variations in seismicity and volcanic activity, and changes in groundwater flow conditions. The dominant factor influencing the timing of submarine landslide occurrence is glaciation. A review of known ages of submarine landslides along the margins of the Atlantic Ocean, augmented by a few ages from other submarine locations shows a relatively even distribution of large landslides with time from the last glacial maximum until about five thousand years after the end of glaciation. During the past 5000??yr, the frequency of occurrence is less by a factor of 1.7 to 3.5 than during or shortly after the last glacial/deglaciation period. Such an association likely exists because of the formation of thick deposits of sediment on the upper continental slope during glacial periods and increased seismicity caused by isostatic readjustment during and following deglaciation. Hydrate dissociation may play a role, as suggested previously in the literature, but the connection is unclear.

Marine Geology↗

USGS2011

Sediment dynamics and the burial and exhumation of bedrock reefs along an emergent coastline as elucidated by repetitive sonar surveys: Northern Monterey Bay, CA

Two high-resolution bathymetric and acoustic backscatter sonar surveys were conducted along the energetic emergent inner shelf of northern Monterey Bay, CA, USA, in the fall of 2005 and the spring of 2006 to determine the impact of winter storm waves, beach erosion, and river floods on biologically-important siliclastic bedrock reef habitats. The surveys extended from water depths of 4 m to 22 m and covered an area of 3.14 km2, 45.8% of which was bedrock, gravel, and coarse-grained sand and 54.2% was fine-grained sand. Our analyses of the bathymetric and acoustic backscatter data demonstrates that during the 6 months between surveys, 11.4% of the study area was buried by fine-grained sand while erosion resulted in the exposure of bedrock or coarse-grained sand over 26.5% of the study area. The probability of burial decreased with increasing water depth and rugosity; the probability of exhumation increased with increasing wave-induced near-bed shear stress, seabed slope and rugosity. Much of the detected change was at the boundary between bedrock and unconsolidated sediment due to sedimentation and erosion burying or exhuming bedrock, respectively. In a number of cases, however, the change in seabed character was apparently due to changes in sediment grain size when scour exposed what appeared to be an underlying coarser-grained lag or the burial of coarser-grained sand and gravel by fine-grained sand. These findings suggest that, in some places, (a) burial and exhumation of nearshore bedrock reefs along rocky, energetic inner shelves occurs over seasonal timescales and appears related to intrinsic factors such as seabed morphology and extrinsic factors such as wave forces, and (b) single acoustic surveys typically employed for geologic characterization and/or habitat mapping may not adequately characterize the geomorphologic and sedimentologic nature of these types of environments that typify most of the Pacific Ocean and up to 50% of the world's coastlines.

Monterey Bay↗

USGS2014

Refining the link between the Holocene development of the Mississippi River Delta and the geologic evolution of Cat Island, MS: implications for delta-associated barrier islands

The geologic evolution of barrier islands is profoundly influenced by the nature of the deposits underlying them. Many researchers have speculated on the origin and evolution of Cat Island in Mississippi, but uncertainty remains about whether or not the island is underlain completely or in part by deposits associated with the past growth of the Mississippi River delta. In part, this is due to a lack of comprehensive geological information offshore of the island that could augment previous stratigraphic interpretations based on terrestrial borings. An extensive survey of Cat Island and its surrounding waters was conducted, including shallow-water geophysics (e.g., high-resolution chirp seismic, side-scan sonar, and swath and single-beam bathymetry) and both terrestrial and marine vibracoring. High-resolution seismic data and vibracores from south and east of the island show two horizontally laminated silt units; marine radiocarbon dates indicate that they are St. Bernard delta complex (SBDC) deposits. Furthermore, seismic data reveal that the SBDC deposits taper off toward the southern shoreline of Cat Island and to the west, morphology consistent with the distal edge of a delta complex. The sedimentology and extent of each unit suggest that the lower unit may have been deposited during an earlier period of continuous river flow while the upper unit may represent reduced or sporadic river flow. OSL dates from the island platform (beneath beach ridge complexes) indicate three stages of terrestrial evolution: island emergence resulting from relative sea-level rise (~ 5400 ybp) island aggradation via littoral transport (~ 2500–4000 ybp) and island degradation due to delta-mediated changes in wave direction (present– ~ 3600 ybp). Finally, the combination of terrestrial and marine data shows that portions of Cat Island that are lower in elevation than the central part of the island are younger and are likely underlain by a thin layer of deltaic sediments. This underscores the potential for increased future vulnerability of barrier islands that develop adjacent to major river delta complexes.

Mississippi↗

USGS2022

Primary deposition and early diagenetic effects on the high saturation accumulation of gas hydrate in a silt dominated reservoir in the Gulf of Mexico

On continental margins, high saturation gas hydrate systems (>60% pore volume) are common in canyon and channel environments within the gas hydrate stability zone, where reservoirs are dominated by coarse-grained, high porosity sand deposits. Recent studies, including the results presented here, suggest that rapidly deposited, silt-dominated channel-levee environments can also host high saturation gas hydrate accumulations. Here we present several sedimentological data sets, including sediment composition, biostratigraphic age from calcareous nannofossils, grain size, total organic carbon (TOC), C/N elemental ratio, δ 13 C-TOC , CaCO 3 , total sulfur (TS), and δ 34 S-TS from sediments collected with pressure cores from a gas hydrate rich, turbidite channel-levee system in the Gulf of Mexico during the 2017 UT-GOM2-1 Hydrate Pressure Coring Expedition. Our results indicate the reservoir is composed of three main lithofacies, which have distinct sediment grain size distributions (type A-silty clay to clayey silt, type B-clayey silt, and type C-sandy silt to silty sand) that are characteristic of variable turbidity current energy regimes within a Pleistocene (< 0.91 Ma) channel-levee environment. We document that the TOC in the sediments of the reservoir is terrestrial in origin and contained within the fine fraction of each lithofacies, while the CaCO 3 fraction is composed of primarily reworked grains, including Cretaceous calcareous nannofossils, and part of the detrital load. The lack of biogenic grains within the finest grained sediment intervals throughout the reservoir suggests interevent hemipelagic sediments are not preserved, resulting in a reservoir sequence of silt dominated, stacked turbidites. We observe two zones of enhanced TS at the top and bottom of the reservoir that correspond with enriched bulk sediment δ 34 S, indicating stalled or slowly advancing paleo-sulfate-methane transition zone (SMTZ) positions likely driven by relative decreases in sedimentation rate. Despite these two diagenetic zones, the low abundance of diagenetic precipitates throughout the reservoir allowed the primary porosity to remain largely intact, thus better preserving primary porosity for subsequent pore-filling gas hydrate. In canyon, channel, and levee environments, early diagenesis may be regulated via sedimentation rates, where high rates result in rapid progression through the SMTZ and minimal diagenetic mineralization and low rates result in the stalling of the SMTZ, enhancing diagenetic mineralization. Here, we observed some enhanced pyritization to implicate potential sedimentation rate changes, but not enough to consume primary porosity, resulting in a high saturation gas hydrate reservoir. These results emphasize the important implications of sedimentary processes, sedimentation rates, and early diagenesis on the distribution of gas hydrate in marine sediments along continental margins.

Alabama, Louisiana, Mississippi↗

USGS2023

Subaqueous clinoforms created by sandy wave-supported gravity flows: Lessons from the central California shelf

Subaqueous clinoforms are an important yet underappreciated shelf feature. Their origins are typically associated with subaerial deltas but recent work has identified similar features in settings without a significant fluvial source. These other studies have shown that such subaqueous clinoforms, also known as infralittoral prograding wedges (IPWs), are created largely by wave-induced processes. This study uses geophysical, sedimentological, and radiocarbon data to determine the sedimentary characteristics and genesis of a shore-parallel subaqueous clinoform developed far from any significant river on the central California continental shelf; a feature known locally as the Cross Hosgri Slope. Sediment cores through the feature reveal that it is composed of beds with an erosive base, followed by a thin coarsening upward sequence of shelly fine sands transitioning to a fining upward sequence marked by alternating parallel and ripple cross laminated very fine sands. The deposit is often capped by fine silts that are commonly interbedded with thin very fine sand beds. Radiocarbon dating of shells within the cores paired with seismic profiles indicate the subaqueous clinoform initiated progradation ~7 ka, nucleating on an older Younger Dryas relict shoreface. We suggest the CHS was created by winter-storm waves mobilizing sands in water depths up to ~ 70 m that transitioned into wave-supported gravity flows. The wave-supported gravity flows traveled downslope to water depths of up to ~85 m, corresponding to the foot of the subaqueous clinoform. They did not travel beyond this depth as wave influence at these depths is negligible and the shelf slope is insufficient to maintain movement of the load alone. Our work suggests that wave-supported gravity flows can entrain very fine sands and silts and build subaqueous clinoforms, even in the absence of a significant river source. Furthermore, we provide a facies model for sandy wave-supported gravity flow deposits.

California↗

USGS2024

Discerning sediment provenance in the Outer Banks (USA) through detrital zircon geochronology

Detrital zircon data from modern barrier island and estuarine environments in the Outer Banks (Atlantic Coast, USA) were statistically compared to sands from nearby rivers to assist in determining source-to-sink pathways. Fluvial samples, collected from near the Fall Line contact between the Appalachian Orogen and sediments of the coastal plain, all have age unique distributions, making them ideal for tracing provenance. Three samples from the Atlantic foreshore showed high similarities to one another, as well as to three samples from the estuarine (back-barrier) Pamlico and Albemarle Sounds. Mixture modeling with multiple data reduction methods and three different statistical tests for similarity consistently indicated that the nearby Potomac River was the primary source for all Atlantic foreshore and estuarine zircons, followed by minor contributions from the James River in some models. The models indicate little or no sediment contribution from the Susquehanna, Roanoke, Tar, Neuse, Cape Fear, and Peedee Rivers. Both Atlantic foreshore and estuarine sands are therefore interpreted to have initially originated from Appalachian bedrock to the north of their present-day location, and subsequently to have been transported southward through the Chesapeake Bay watershed before deposition in Virginia and North Carolina. Prior to barrier island formation in the last several thousand years, differing geomorphology of the Chesapeake Bay facilitated southward movement of sediments from its constituent rivers via longshore drift, where they were deposited in coastal settings on the mainland. The modern barrier islands, formed during the most recent post-glacial transgression, may be reworked from these deposits, but may also include a contribution from sediments that were derived more recently from relict deposits on the shelf. Oceanographic and sedimentological evidence suggests that movement of sand-sized grains from southern rivers across the back-barrier sounds is unlikely. These findings can assist with coastal resilience planning and resource management in a region under severe threat from climate change and rising sea levels.

North Carolina, Virgina↗

USGS2009

Carbon isotope chemostratigraphy and precise dating of middle Frasnian (lower Upper Devonian) Alamo Breccia, Nevada, USA

At Hancock Summit West, Nevada, western USA, uppermost Givetian (upper Middle Devonian) and lower and middle Frasnian (lower Upper Devonian) rocks of the lower Guilmette Formation include, in stratigraphic sequence, carbonate-platform facies of the conodont falsiovalis , transitans , and punctata Zones; the type Alamo Breccia Member of the middle punctata Zone; and slope facies of the punctata and hassi Zones. The catastrophically deposited Alamo Breccia and related phenomena record the ~ 382 Ma Alamo event, produced by a km-scale bolide impact into a marine setting seaward of an extensive carbonate platform fringing western North America. Re-evaluation of conodonts from the lower Guilmette Formation and Alamo Breccia Member, together with regional sedimentologic and conodont biofacies comparisons, now firmly locates the onset of the Johnson et al. (1985) transgressive–regressive (T–R) cycle IIc, which occurred after the start of the punctata Zone, within a parautochthonous megablock low in the Alamo Breccia. Whole-rock carbon isotope analyses through the lower Guilmette Formation and Alamo Breccia Member reveal two positive δ 13 C carb excursions: (1) a small, 3‰ excursion, which is possibly correlative with the falsiovalis Event previously identified from sections in Western Europe and Australia, occurs below the breccia in the Upper falsiovalis Zone to early part of the transitans Zone; and (2) a large, multi-part excursion, dominated by a 6‰ positive shift, begins above the start of the punctata Zone and onset of T–R cycle IIc and continues above the Alamo Breccia, ending near the punctata – hassi zonal boundary. This large excursion correlates with the punctata Event, a major positive δ 13 C excursion previously recognized in eastern Laurussia and northern Gondwana. Consistent with previous studies, at Hancock Summit West the punctata Event is apparently not associated with any regional extinctions or ecosystem reorganizations. In the study area, onset of the main punctata Event began after the start of both the punctata Zone and T–R cycle IIc, and preceded the Alamo impact by less than 650 k.y., as inferred from conodont biochronologic and regional rock-accumulation rate estimates. Although complicated by the heterolithic, high-energy deposits of the Alamo Breccia, the carbon isotope record of the breccia and post-breccia beds does not indicate a major impact-correlative perturbation to the carbon cycle. This study extends recognition of the punctata Event to western Laurussia, further reinforcing the potential global scale of the event and its potential importance to understanding early to middle Frasnian marine geochemistry and palaeoenvironments. Based on previous models and our observations, increased tectonic activity, increased nutrient flux to oceans, increased marine bioproductivity, widespread anoxia, and increased organic carbon burial were all likely key factors in driving the punctata Event excursion. Furthermore, periodic eustatic and regional relative sea-level rises may have played an important role in promoting organic carbon burial and in maintaining a link between the primary open-marine geochemical signal and that recorded on the shallow-marine, lower Guilmette carbonate platform.

Nevada↗

USGS2009

Evidence for long-term climate change in Upper Devonian strata of the central Appalachians

The highest 1 to 200 m of the Upper Devonian (Famennian) Catskill and equivalent Hampshire formations exhibit a noticeable vertical or stratigraphic change in color and a shift in lithologic character. The lower part of the unit is characterized by typically red, channel-phase sandstones and overbank siltstone and mudstone containing thin calcareous paleosols. These lithologies give way to greenish gray sandstone containing abundant coaly plant fragments, coalified logs, and pyrite, interbedded with thick intervals of non-calcareous paleo-vertisols. The increase in the prominence of preserved terrestrial organic matter suggests that there was a corresponding increase in the abundance of plants in terrestrial ecosystems. The stratigraphic change in lithology within the upper part of the Catskill-Hampshire succession suggests the onset of environmental conditions that became increasingly wet in response to elevated humid climatic conditions during the final stages of Catskill alluvial plain deposition. The sedimentological signature suggesting increased climatic wetness within the uppermost Catskill and Hampshire formations is nearly contemporaneous with the initiation of Late Devonian Gondwanan glaciation in the paleo-high-latitudes. The Appalachian climate record indicates that this change began during the Fa2c and continued through the latest Famennian, reaching its peak during the Fa2d when glacial deposits are recorded in the paleo-mid-latitudes of the Appalachian basin. Evidence of this late Famennian increase in precipitation also is recorded in the adjacent marine environments. Equivalent-age marine units in Ohio and Kentucky record progressive increases in both total organic carbon and the percentage of terrestrially-derived organic carbon. This suggests that there was a late Famennian increase in terrestrial organic matter productivity, and that during the late Famennian, there were elevated levels of runoff produced by the interpreted increase in precipitation that washed progressively higher amounts of terrestrial organic matter into the local marine environments. The late Famennian climate changes identified within the Appalachian basin strata have been recognizable globally, and appear to have had both positive and negative effects on the Earth's biota. Some marine groups exhibit sharp diversity drops or even extinction coincident with the maximum development of the late Famennian ice age. Conversely, terrestrial biota appears to have been more positively affected by the late Famennian increased wetness that accompanied this progressive climate change. Marked diversification and evolutionary innovation, which appear to coincide with this climatic deviation, can be recognized within terrestrial plant communities and early tetrapod amphibians. ?? 2009 Elsevier B.V. All rights reserved.

Palaeogeography, Palaeoclimatology, Palaeoecology↗

USGS2015

Uranium-series ages of fossil corals from Mallorca, Spain: The "Neotyrrhenian" high stand of the Mediterranean Sea revisited

The emergent marine deposits of the Mediterranean basin have been recognized as an important record of Quaternary sea level history for more than a century. Previous workers identified what have been interpreted to be two separate high stands of sea in the late Quaternary, namely the “Eutyrrhenian” (thought to be ~ 120 ka) and the “Neotyrrhenian” (thought to be either ~ 100 ka or ~ 80 ka). On Mallorca, Spain, both of these named deposits lie close to present sea level, implying paleo-sea levels slightly above present during both marine isotope stages (MIS) 5.5/5e and either 5.3/5c or 5.1/5a. If these interpretations are correct, they conflict, at least in part, with sea level records from far-field localities. We analyzed corals from the Neotyrrhenian beds on Mallorca, which gave U-series ages from ~ 126 ka to ~ 118 ka. These ages are consistent with previously published amino acid data that show that the Neotyrrhenian and Eutyrrhenian deposits are not significantly different in age. A fossil molluscan fauna from the Neotyrrhenian deposits on Mallorca has a warm-water paleozoogeographic aspect, with nine southward-ranging species and four extralimital southern species. When compared with sea surface temperatures obtained from planktonic foraminifera and alkenones from ODP core 977 in the nearby Alboran Sea, the only time period that shows comparable warmth is MIS 5.5/5e, consistent with the U-series ages of corals from the Neotyrrhenian deposits. We propose that the Neotyrrhenian deposits are a beachrock facies of the same age as the Eutyrrhenian deposits. This interpretation is consistent with the differences in physical sedimentology of the two deposits, explains the U-series and amino acid data indicating the same age, is consistent with the very slight elevation difference of the Neotyrrhenian and Eutyrrhenian beds, and explains the similar, though not identical paleozoogeographic aspects of their fossil faunas.

Mallorca↗

USGS2009

Mesoscale raised rim depressions (MRRDs) on Earth: A review of the characteristics, processes, and spatial distributions of analogs for Mars

Fields of mesoscale raised rim depressions (MRRDs) of various origins are found on Earth and Mars. Examples include rootless cones, mud volcanoes, collapsed pingos, rimmed kettle holes, and basaltic ring structures. Correct identification of MRRDs on Mars is valuable because different MRRD types have different geologic and/or climatic implications and are often associated with volcanism and/or water, which may provide locales for biotic or prebiotic activity. In order to facilitate correct identification of fields of MRRDs on Mars and their implications, this work provides a review of common terrestrial MRRD types that occur in fields. In this review, MRRDs by formation mechanism, including hydrovolcanic (phreatomagmatic cones, basaltic ring structures), sedimentological (mud volcanoes), and ice-related (pingos, volatile ice-block forms) mechanisms. For each broad mechanism, we present a comparative synopsis of (i) morphology and observations, (ii) physical formation processes, and (iii) published hypothesized locations on Mars. Because the morphology for MRRDs may be ambiguous, an additional tool is provided for distinguishing fields of MRRDs by origin on Mars, namely, spatial distribution analyses for MRRDs within fields on Earth. We find that MRRDs have both distinguishing and similar characteristics, and observation that applies both to their mesoscale morphology and to their spatial distribution statistics. Thus, this review provides tools for distinguishing between various MRRDs, while highlighting the utility of the multiple working hypotheses approach. ?? 2008 Elsevier Ltd.

Planetary and Space Science↗

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↗

USGS2016

Differences in coastal subsidence in southern Oregon (USA) during at least six prehistoric megathrust earthquakes

Stratigraphic, sedimentologic (including CT 3D X-ray tomography scans), foraminiferal, and radiocarbon analyses show that at least six of seven abrupt peat-to-mud contacts in cores from a tidal marsh at Talbot Creek (South Slough, Coos Bay), record sudden subsidence (relative sea-level rise) during great megathrust earthquakes at the Cascadia subduction zone. Data for one contact are insufficient to infer whether or not it records a great earthquake—it may also have formed through local, non-seismic, hydrographic processes. To estimate the amount of subsidence marked by each contact, we expanded a previous regional modern foraminiferal dataset to 174 samples from six Oregon estuaries. Using a transfer function derived from the new dataset, estimates of coseismic subsidence across the six earthquake contacts vary from 0.31 m to 0.75 m. Comparison of subsidence estimates for three contacts in adjacent cores shows within-site differences of ≤0.10 m, about half the ±0.22 m error, although some estimates may be minimums due to uncertain ecological preferences for Balticammina pseudomacrescens in brackish environments and almost monospecific assemblages of Miliammina fusca on tidal flats. We also account for the influence of taphonomic processes, such as infiltration of mud with mixed foraminiferal assemblages into peat, on subsidence estimates. Comparisons of our subsidence estimates with values for correlative contacts at other Oregon sites suggest that some of our estimates are minimums and that Cascadia's megathrust earthquake ruptures have been heterogeneous over the past 3500 years.

Oregon↗

USGS2018

Aeolian stratigraphy describes ice-age paleoenvironments in unglaciated Arctic Alaska

Terrestrial paleoenvironmental records with high dating resolution extending into the last ice age are rare from the western Arctic. Such records can test the synchronicity and extent of ice-age climatic events and define how Arctic landscapes respond to rapid climate changes. Here we describe the stratigraphy and sedimentology of a yedoma deposit in Arctic Alaska (the Carter Section) dating to between 37,000 and 9000 calibrated radiocarbon years BP (37–9 ka) and containing detailed records of loess and sand-sheet sedimentation, soil development, carbon storage, and permafrost dynamics. Alternation between sand-sheet and loess deposition provides a proxy for the extent and activity of the Ikpikpuk Sand Sea (ISS), a large dune field located immediately upwind. Warm, moist interstadial times (ca. 37, 36.3–32.5, and 15–13 ka) triggered floodplain aggradation, permafrost thaw, reduced loess deposition, increased vegetation cover, and rapid soil development accompanied by enhanced carbon storage. During the Last Glacial Maximum (LGM, ca. 28–18 ka), rapid loess deposition took place on a landscape where vegetation was sparse and non-woody. The most intense aeolian activity occurred after the LGM between ca. 18 and 15 ka when sand sheets fringing the ISS expanded over the site, possibly in response to increasingly droughty conditions as summers warmed and active layers deepened. With the exception of this lagged LGM response, the record of aeolian activity at the Carter Section correlates with other paleoenvironmental records from unglaciated Siberia and Alaska. Overall, rapid shifts in geomorphology, soils, vegetation, and permafrost portray an ice-age landscape where, in contrast to the Holocene, environmental change was chronic and dominated by aeolian processes.

Alaska↗