Ronald J. Goble

A 10,000 year record of dune activity；dust storms；and severe drought in the central Great Plains

Dune fi elds and loess deposits of the Great Plains of North America contain stratigraphic records of eolian activity that can be used to extend the short observational record of drought. We present a 10,000 yr reconstruction of dune activity and dust production in the central Great Plains region, based on 95 optically stimulated luminescence ages. The integration of data from both eolian sand and loess is an important new aspect of this record. Clusters of ages defi ne episodes of extensive eolian activity, which we interpret as a response to frequent severe drought, at 1.0‐0.7 ka and 2.3‐4.5 ka (with peaks centered on 2.5 and 3.8 ka); sustained eolian activity occurred from 9.6 to 6.5 ka. Parts of this record may be consistent with hypotheses linking Holocene drought to sea surface temperature anomalies in the Pacifi c or Atlantic oceans, or to the El Nino-Southern Oscillation phenomenon, but the record as a whole is diffi cult to reconcile with any of these hypotheses.

Dune mobility and aridity at the desert margin of northern China at a time of peak monsoon strength

Wind-blown sands were mobile at many sites along the desert margin in northern China during the early Holocene (11.5–8 ka ago), based on extensive new numerical dating. This mobility implies low effective moisture at the desert margin, in contrast to growing evidence for greater than modern monsoon precipitation at the same time in central and southern China. Dry conditions in the early Holocene at the desert margin can be explained through a dynamic link between enhanced diabatic heating in the core region of the strengthened monsoon and increased subsidence in drylands to the north, combined with high evapotranspiration rates due to high summer temperatures. After 8 ka ago, as the monsoon weakened and lower temperatures reduced evapotranspiration, eolian sands were stabilized by vegetation. Aridity and dune mobility at the desert margin and a strengthened monsoon can both be explained as responses to high summer insolation in the early Holocene.

Fluvial evolution of the lower Mississippi River valley during the last 100 k.y. glacial cycle: Response to glaciation and sea-level change

The lower Mississippi valley contains multiple large braid belts for which age control has been limited. Application of the optically stimulated luminescence technique has produced a new chronology of lower Mississippi valley channel-belt formation and insight into the valley9s evolution during the last glacial cycle. Fluvial deposits range from last interglacial meander belts (85 ± 7 to 83 ± 7 ka) to multiple braid belts (64 ± 5 to 11 ± 1 ka) and record large-amplitude responses of the Mississippi River to glacially induced changes in discharge and sediment supply during the last glacial cycle. Slackwater deposits in buried tributary valleys from the middle Mississippi valley and northern lower Mississippi valley suggest that the river was flowing at a position 8–21 m below the present flood plain during the last interglacial, then rapidly aggraded and switched to a braided regime to form the highest and oldest braid belt by 64 ± 5 to 50 ± 4 ka, coincident with initial glaciation of the upper drainage basin. The Mississippi River remained braided until final meltwater withdrawal from its headwaters in the earliest Holocene. Braid-belt formation and incision was controlled by fluctuations in meltwater and sediment discharge, while glacio-eustatic sea level controlled the elevation to which the river was graded, causing late glacial braid belts to dip below the Holocene flood plain in the southern lower Mississippi valley. Moreover, avulsions in the middle Mississippi valley and northern lower Mississippi valley during the last glaciation have pinned the river over regions of shallow bedrock, preventing the modern river from incising to its last interglacial profile. The new chronology and longitudinal profiles presented here provide insight into the response of this continental-scale river system to climatic (glacial) and base-level forcing during the last 100 k.y. glacial cycle.

A 1500 yr record of North Atlantic storm activity based on optically dated relict beach scarps

Understanding of long-term dynamics of intense coastal storms is important for determining the frequency and impact of these events on sandy coasts. We use optically stimulated luminescence (OSL) dates on relict scarps within a prograded barrier sequence to reconstruct the chronology of large-magnitude erosional events in the western Gulf of Maine. OSL dates obtained on quartz-rich sediments immediately overlying relict scarps indicate severe beach erosion and retreat due to erosional events ca. 1550, 390, 290, and 150 cal yr B.P. Our data provide new evidence of increased storm activity (most likely frequency and/or intensity of extratropical storms) during the past 500 yr, which was preceded by a relatively calm period lasting ~1000 yr. The width of the coastal sequence preserved between successive paleoscarps shows strong correlation with the time interval elapsed between storms. Our fi ndings indicate that diagnostic geophysical and sedimentological signatures of severe erosional events offer new opportunities for assessing the impact and timing of major storms along sandy coasts.

An optical age chronology of Late Pleistocene fluvial deposits in the northern lower Mississippi valley

The lower Mississippi valley (LMV) contains many large braided channel belts that are preserved west of the Holocene floodplain. Previous efforts to establish geochronologic control on channel-belt construction have been hindered by the lack of organic material for radiocarbon dating. Luminescence techniques provide a burial date for the sediment itself and may prove useful in this context. Samples from three channel belts in the northern LMV were analyzed using the single aliquot-regenerative technique on 90–125mm quartz. Optical ages (19.7–17.8, 16.1–15.0 and 12.5–12.1 ka) are consistent with geomorphic relationships and indicate that channel belts were formed in the late Pleistocene under glacial conditions. These optical ages provide the first detailed chronology of LMV channel-belt formation and are the first step towards developing a chronology for the entire LMV. r 2003 Elsevier Science Ltd. All rights reserved.

Age and effects of the Odessa meteorite impact, western Texas, USA

The Odessa meteorite craters (Texas, United States) include a main crater (∼160 m diameter, ∼30 m deep) plus four smaller meteorite craters. The main crater was sampled by coring (to 22 m depth) to better understand its origin and history. Dating by optically stimulated luminescence indicates that it was produced immediately prior to ca. 63.5 ± 4.5 ka. Sediment filling the crater includes impact breccias produced at the time of impact; wind-dominated silts with minor amounts of pond sediments deposited ca. 63.5 ka, probably just after the impact, and ca. 53 ± 2 ka; wind-dominated silt ca. 38 ± 1.7 ka; and playa muds with a wind-blown silt component younger than 36 ka. The environment was arid or semiarid at the time of impact based on characteristics of soils on the surrounding landscape. The impact caused severe damage within 2 km and produced >1000 km/hr winds and thermal pulse. Animals within a 1–1.5-km-diameter area were probably killed. This is only the second well-dated Pleistocene hypervelocity impact crater in North America.

Geochronology of the Bolson sand sheet, New Mexico and Texas, and its archaeological significance

The Bolson sand sheet occurs in the Tularosa Valley, New Mexico, and the Hueco Bolson, Texas, and it consists of two principal eolian sand units. Optically stimulated luminescence (OSL) dating provides a new chronology of the sand sheet that relates as well to the formation, preservation, and visibility of the local archaeological record. The lower sand (unit Q2) (44.8 ± 2.9 ka) and the upper sand (unit Q3) (22.2 ± 1.6–5.2 ± 0.3 ka) have a combined thickness of less than 2 m. The Q2 sand is characterized by a red Bt paleosol, and the Q3 sand has a weak calcic paleosol with stage I carbonate morphology. Elevated amounts of airborne silt were incorporated in the Q3 sand during the period 24–14.5 ka, representing higher amounts of dust in the atmosphere during glacial and late-glacial time. Multiple OSL ages from the Q3 sand indicate a slow net sedimentation rate of 0.06–0.09 mm/yr, similar to other OSL-dated sand sheets in the region; sand deposits in dune fields have higher accumulation rates. The McGregor A horizon soil, radiocarbon-dated to younger than 0.5 ka, occurs at the top of the sand sheet and likely formed with desert grassland vegetation. Thousands of recent mesquite coppice dunes (unit Q4d) mantle the sand sheet, and two are dated to the twentieth century by OSL and 137 Cs. Archaeological sites that postdate 3000 B.C. are concentrated, sometimes together, on the surface of the sand sheet, while sites that predate 3000 B.C., although rare, may be buried within the Q3 eolian sand. The Q2 sand is too old to contain archaeological sites, although site features may intrude into the sand. Previous chronologies of the sand sheet are based on radiocarbon dates of charcoal from archaeological sites, on radiocarbon dates of soil carbonate, and on soil-geomorphology correlations with Rio Grande Valley alluvium. The optical chronology does not support these various correlations. We recommend that the alluvial names Isaacks9 Ranch, Fillmore, and Organ no longer be applied to the Bolson sand sheet.

Temporal and spatial variability in dune reactivation across the Nebraska Sand Hills, USA

The Nebraska Sand Hills is a stabilized dune field on the Great Plains of North America. Although it is well known that this dune field, like several others on the Great Plains, last experienced widespread activity during the Medieval Climatic Anomaly (MCA, ~AD 900–1300), spatial variation in the timing and nature of drought development is poorly constrained. To elucidate spatial trends in dune reactivation, samples potentially representing MCA activity across the Sand Hills were collected and dated using optically stimulated luminescence (OSL). Ages from the older part of the MCA were obtained from eolian sediments in the northwestern Sand Hills, while ages from later in the episode were obtained to the southeast, suggesting a geographic trend in the timing of revegetation of the dunes near the end of the drought. Revegetation likely occurred to the northwest initially as a result of renewed moisture availability from a rising water table in the interdunes, which serve as refugia for vegetation during times of drought. Vegetation then gradually spread to the southeastern Sand Hills. An additional spatial trend in ages is apparent in the chronology of linear dune mobilization across the Sand Hills. Linear dunes in the northwest are superimposed on megadunes and originated during the last reactivation, while linear dunes in the southeast are built around older cores of dunes and formed during several reactivations. Our geochronology reveals three episodes of eolian transport, including the MCA, in the formation of linear dunes in the southeast.

Climate-driven episodes of dune mobilization and barrier growth along the central coast of Portugal

Abstract Here, we explore the evolution of the coastal stretch between Mira Beach and Quiaios Beach in Portugal to understand how it adapted to climatic oscillations. To accomplish this, we integrate subsurface radar images, and sedimentological and chronological data, of the emerged coastal barrier. Our results show the installation and progradation of a stable barrier anchored to transgressive dunes 400 years ago. This is just the last pulse of barrier growth within a complex approximately 5000 year-long history of shoreline stability/instability. Episodes of inland dune mobility have been related to instabilities in the beach sediment budget driven by enhanced storminess and wave rotation around 4.25 and 1.14 ka ago. Conversely, lagoonal deposits documented in the literature suggest periods of relative barrier stability and growth around 4.3 and 2.7 cal ka BP. Wave and wind climate variability are driven by shifts in one of the major modes of atmospheric circulation in the North Atlantic, the North Atlantic Oscillation (NAO). Episodes of persistent positive mode of the NAO related to barrier growth and enhanced longshore sediment transport; those of persistent negative mode contributed to instabilities in the beach sediment budget and aeolian activity by enhancing storminess, but reduced effective longshore sediment transport.

Subsurface Images Shed Light on Past Tsunamis in India

The 2004 Indian Ocean tsunami caused massive devastation and left a lasting impact along many of the major coastal regions in South Asia, including the coast of Tamil Nadu, a state in the southeastern tip of India. Following the event, sand deposits draped the low-lying areas and buried the muddy sediments of the coastal plain [Babu et al., 2007; Srinivasalu et al., 2007]. In addition, erosional features related to the tsunami, such as channels and scarps, have been observed along many parts of the coast (Figure 1a). This tsunami, along with a recorded history of intense monsoons, has highlighted the need for focused research on the role of extreme events in shaping the geological character of Indias coastal plains.