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Hurricane magnitude and frequency have been linked to numerous mechanisms, including the steady rise in annual sea-surface temperatures, El Nino--Southern Oscillation (ENSO) variations, and atmospheric changes. In order to better understand those factors that control hurricane magnitude and frequency, a long-term record spanning the entire Gulf of Mexico coast is needed. Here we present a detailed record from ca. 5300-900 yr B.P. of past intense hurricane impacts for cores collected from Laguna Madre, Texas, United States. Relative storm intensities were calculated for each event, and the average intense storm impact probability for south Texas was determined to be ~0.46% (annual landfall probability). Previous field studies in Western Lake, Florida, and Lake Shelby, Alabama, reveal similar probability intense hurricane strikes of ~0.39%. Although high-frequency oscillations between warm and dry and cool and wet climate conditions have occurred in Texas through the late Holocene, there has been no notable variation in intense storm impacts across the northwestern Gulf of Mexico coast during this time interval, implying no direct link between these changing climate conditions and annual hurricane impact probability. In addition, there have been no significant differences in the landfall probabilities of storms between the eastern and western Gulf of Mexico during the late Holocene, suggesting that storm steering mechanisms have not varied during this time. doi: 10.1130/G30729.1

Terrigenous sediment accumulation within nearshore marine environments is regarded as a major factor inhibiting carbonate production and coral reef accretion. While recent ecological and geological research into reef development under long-term terrigenous sediment influence questions the overly simplistic nature of such views, understanding of the time scales of reef initiation and growth and the morphodynamics of reef accretion in these settings remains limited. Here we present evidence to support recent suggestions that, once established, rapid reef accretion and progradation is possible, but that the restricted accommodation windows in which such reefs develop often result in short-lived (ephemeral) phases of reef building. Specifically, we describe two discrete periods of reef growth within one small (~600 m wide) coastal embayment around a high island on the terrigenous sediment-dominated inner shelf of the central Great Barrier Reef, Australia. These reef-building phases occurred at either end of the Holocene sea-level highstand, the first during the late postglacial marine transgression and early highstand (~6900-4500 calibrated (cal) yr B.P.), the second following the late Holocene regression and stillstand (~1600 cal yr B.P. to present). An ~3000 yr hiatus occurred between these events, probably as a function of subtle changes in sea level and associated shoreline morphodynamics. doi: 10.1130/G30444.1

The end Pleistocene to early Holocene drying of the North American midcontinent is an important case study for understanding regional drought, its drivers, and its impacts. This paper synthesizes pollen, [delta][sup.13]C, eolian, and paleohydrological proxies to document the spatial and temporal patterns in the onset and rate of drying. The timing of onset ranged from 14 to 6 ka with most sites drying between 10 and 8 ka (calendar years ago). The dominant spatial pattern is time transgressive, with interior Great Plains sites beginning to dry before peripheral sites. This time-transgressive trend can be explained as a region-wide drying that caused ecotones to shift eastward, or could indicate a progressive shift in atmospheric drying from west to east. One-third of sites responded rapidly to drying; a cluster of rapid responses occurred ca. 8 ka. The combination of onsets between 14 and 6 ka and rapid responses at 8 ka strongly suggests that (1) the combination of high summer insolation, Laurentide Ice Sheet retreat, and Lake Agassiz drainage drove midcontinental drying, and (2) drying accelerated with Laurentide Ice Sheet collapse at 8.4 ka. Local factors strongly mediate the timing and rate of site responses to regional drying. Local responses to future drying in the Great Plains also should be highly variable, which challenges impact assessments of future climate changes. doi: 10.1130/G30413.1

Paleoclimate reconstructions suggest distinctive changes in the El Nino--Southern Oscillation (ENSO), the dominant mode of tropical climate variability, over the last glacial cycle and throughout the Holocene. However, to the best of our knowledge, no studies provide parallel reconstructions of warm (El Nino) and cold (La Nina) phase variability, thus precluding distinction between variations in ENSO activity and the mean state of the tropical Pacific. Here we provide the first such records, generated using molecular organic geochemical proxies in a sediment core from the Peru margin region. The opposing influences of El Nino and La Nina on coastal upwelling and phytoplankton community structure produce distinct sedimentary sterol records describing the evolution of each ENSO phase. We document changes in surface ocean productivity on the Peru margin over the past 16 ka that indicate enhanced La Nina--like conditions alone during deglaciation, but concomitant increases in both El Nino and La Nina activity during the past 2 ka. We propose that the preponderance of La Nina--like conditions prior to 11.5 ka reflects the influence of waning glacial boundary conditions. By contrast, warm and cold phase covariability during the late Holocene suggests centennial-scale changes in ENSO activity, rather than the Pacific mean state, driven by tropical insolation. doi: 10.1130/G30366.1

The contribution of compaction of Holocene strata to sea-level rise, shoreline erosion, and wetland loss is difficult to decipher because of a lack of long-term empirical data. We analyzed more than 360 radiocarbon-dated index points to constrain relative sea levels along the east coast of England during the Holocene and to pruvide estimates of sediment compaction. The isostatic effect of glacial rebound explains regional-scale differences in relative sea levels. Northeast England, the most northern study area, reveals a middle to late Holocene highstand, doi: 10.1130/G30042A.1

Although oolitic sands are widespread throughout the Phanerozoic carbonate rock record, and they are abundant in some modern shallow-marine carbonate systems, recent ooids have not been recognized from any locality in the Pacific Basin. On Aitutaki Atoll (Cook Islands, South Pacific), oolitic sands occur in water depths of 2.1-6.3 m within the lagoon and, less abundantly, on shallower reef aprons. Most ooids are between 150 and 300 [micro]m in diameter, have between 1 and 25 concentric to irregular laminae, and occur with peloids and skeletal grains. Cortical laminae include Mg-caicite crystals with radial, tangential, and random orientations. The occurrence and distribution of oolitic sands on Aitutaki are facilitated by a convergence of hydrodynamic and chemical factors. Wave-driven currents, combined either with flow separation over a sharp increase in depth at the reef apron--lagoon interface or with oceanward flow in the lee of islands, serve to transport ooids without flushing them from the system. Here, pH, alkalinity, and carbonate supersaturation are at some of the highest levels in the tropical Pacific. The restricted occurrence of Holocene oolitic sands in areas of the Pacific and Atlantic Basins with elevated pH and total alkalinity reflects the importance of carbonate saturation thresholds in limiting the spatial distribution of these sands in modern shallow-marine settings. These recent examples are consistent with interpretations of the importance of these variables in determining the distribution, abundance, and cortical mineralogy of oolitic sands throughout the Phanerozoic stratigraphic record of carbonate accumulation. doi: 10.1130/G30332A.1

Displacement rates for normal and reverse faults (n = 57) are generally higher when averaged for the Holocene (~10 ka) than for the late Quaternary (~300 ka) and longer time scales. Holocene acceleration of displacement rates could be attributed to geological processes that produce increases of tectonic tempo. We propose an alternative model in which the observed rate changes arise from variability in earthquake slip and/or recurrence coupled with a sampling bias toward those faults that are best represented at the Earth's surface and that accrued displacement fastest during the Holocene. This model is consistent with displacement rates measured over time intervals of as long as -300 ka for 129 faults from the Taupo Rift, New Zealand. Departures of earthquake parameters and associated displacement rates from their long-term (>300 ka) averages are attributed to fault interactions and occur on time intervals inversely related to these long-term displacement rates and to regional strain rates.

During the last glacial period, when sea level was low, meltwater discharge drove incision of the lower Mississippi valley, with valley filling and delta construction during Holocene sea-level rise. Isostatic modeling shows that sediment volumes removed and replaced were sufficient to induce uplift of >9 m along valley margins followed by subsidence of the same magnitude, with effects dissipating only over distances of >100-150 km along the Gulf of Mexico coast. Recognition of cyclical uplift and subsidence refutes recent interpretations of delta stability, and suggests that late Holocene relative sea-level curves from the delta region are instead a record of subsidence of the pre-Holocene depocenter. More broadly, incised valley cutting and filling is a common fluvial response to glacio-eustasy, and cyclical uplift and subsidence should be common to large alluvial-deltaic systems elsewhere. Keywords: Mississippi delta, incised valley, isostatic uplift and subsidence, Holocene sea-level change.

A continuous decadal-scale resolution record of climate variability over the past 1400 yr in the northern Gulf of Mexico was constructed from a box core recovered in the Pigmy Basin, northern Gulf of Mexico. Proxies include paired analyses of Mg/Ca and [[delta].sup.18]O in the white variety of the planktic foraminifer Globigerinoides ruber and relative abundance variations of G. sacculifer in the foraminifer assemblages. Two multi-decadal intervals of sustained high Mg/Ca indicate that Gulf of Mexico sea surface temperatures (SSTs) were as warm or warmer than near-modern conditions between 1000 and 1400 yr B.P. Foraminiferal Mg/Ca during the coolest interval of the Little Ice Age (ca. 250 yr B.P.) indicate that SST was 2-2.5[degrees]C below modern SST. Four minima in the Mg/Ca record between 900 and 250 yr B.P. correspond with the Maunder, Sporer, Wolf, and Oort sunspot minima, suggesting a link between changes in solar insolation and SST variability in the Gulf of Mexico. An abrupt shift recorded in both [[delta].sup.18][O.sub.calcite] and relative abundance of G. sacculifer occurred ca. 600 yr B.P. The shift in the Pigmy Basin record corresponds with a shift in the sea-salt-sodium (ssNa) record from the Greenland Ice Sheet Project 2 ice core, linking changes in high-latitude atmospheric circulation with the subtropical Atlantic Ocean. Keywords: Little Ice Age, Medieval Warm Period, Mg/Ca, Pigmy Basin, Gulf of Mexico, Holocene.

We present well-dated high-resolution Holocene records of sea-surface temperature (SST) and upwelling intensity off northwest (NW) Africa. We identify long-term cooling trends over the Holocene in the subtropical North Atlantic in response to boreal summer insolation. A pronounced cooling event of ~1[degrees]C ca. 8.5 cal ka indicates a large-scale reorganization of the ocean current system possibly induced by melt-water from the northern North Atlantic. Our alkenone SST record off Cape Ghir provides strong evidence for the impact of ocean circulation changes on subtropical North Atlantic SSTs. It is likely that cold waters were propagated to the subtropics via the Canary Current in a way similar to Heinrich events and the Younger Dryas off Cape Blanc. We find 2-3 k.y. periodic variations in SST and upwelling intensity off NW Africa superimposed on the cooling trend. Such a cycle has been documented in various paleoclimate archives in phase with solar forcing. We show that these variations on millennial time scales are linked to the North Atlantic subtropical gyre circulation and the Northern Hemisphere atmospheric circulation, and in particular to changes in the pressure gradient between the Icelandic Low and the Azores High. This suggests that oceanic circulation, in response to solar forcing, played a more important role in the generation of 2-3 k.y. cyclicity than has been previously considered. Keywords: alkenone sea-surface temperature, Holocene, North Atlantic subtropical gyre circulation, upwelling intensity, trade winds.

Paired planktic foraminiferal [delta] so and Mg/Ca data reveal trends of increasing temperatures (~3 [degrees]C) and salinities in the subpolar North Atlantic over the course of the Holocene, which were punctuated by abrupt events. The trends likely reflect an insolation-forced northward retreat of the boundary between polar and North Atlantic subsurface waters. The superimposed variability does not appear to be periodic, but tends to recur within a broad millennial band. The records provide convincing evidence of open-ocean cooling (nearly 2 [degrees]C) and freshening during the 8.2 ka event, and suggest similar conditions at 9.3 ka. However, the two largest temperature oscillations in our record (~2 [degrees]C) occurred during the past 4 k.y., suggesting a recent increase in temperature variability relative to the mid-Holocene, perhaps in response to neoglaciation, which began at about this time. Keywords: Ocean Drilling Program Site 984, paleotemperature, Mg/Ca, Bjorn Drift, Neogloboquadrina pachyderma dextral.

General circulation model simulations suggest that during the Last Glacial Maximum, the northern circumpolar vortex intensified and enlarged, a glacial anticyclone developed over the Laurentide Ice Sheet, and the position of the jet stream was shifted southward. However, observations directly related to shifts in wind patterns across the North American continent have not yet been reported. We examined tree-ring cellulose from the Holocene and the last glacial period for: (1) covariation between precipitation [[delta].sup.18]O (and [delta]D) and relative humidity, and (2) variation of cellulose [[delta].sup.18]O and [delta]D with longitude. Holocene isotopic features are consistent with modern moisture trajectories. The isotopic features during the last glaciation are dissimilar to those in the Holocene, and constitute direct evidence for an expansion of the polar easterlies to latitudes as low as 40[degrees]N. This is the first time that moisture transport patterns have been inferred from covariation between isotopic composition in precipitation and relative humidity, a technique that holds much promise for future studies of atmospheric circulation. Keywords: North America, continental glaciation, atmospheric circulation, paleoclimate, humidity, isotope ratios.

Holocene climate change has likely had a profound influence on ecosystems and culture. A link between solar forcing and Holocene climate, such as the Asian monsoon, has been shown for some regions, although no mechanism for this relationship has been suggested. Here we present the first high-resolution complete Holocene climate record for the North American monsoon region of the southwestern United States (southwest) in order to address the nature and causes of Holocene climate change. We show that periods of increased solar radiation correlate with decreased rainfall, the opposite to that observed in the Asian monsoon, and suggest that a solar link to Holocene climate is through changes in the Walker circulation and the Pacific Decadal Oscillation and El Nino--Southern Oscillation systems of the tropical Pacific Ocean. Given the link between increased warming and aridity in the southwest, additional warming due to greenhouse forcing could potentially lead to persistent hyperarid conditions, similar to those seen in our record during periods of high solar activity. Keywords: Solar forcing, Holocene climate, monsoon, speothem, U-series, southwestern United States.

Large deltas are commonly believed to exhibit rapid rates of tectonic subsidence, largely due to sediment loading of the lithosphere. As a result, deltaic plains are prone to accelerated relative sea-level rise, coastal erosion, and wetland loss. Hurricane Katrina's devastation testifies to the severe threat that these processes pose to the Mississippi Delta, but the relative role of tectonics versus other mechanisms causing land subsidence remains elusive. Relative sea-level records derived from basal peat have the potential to quantify differential crustal movements over Holocene time scales with exceptionally high accuracy and precision. Here we present new sea-level index points from two study areas in the southwestern Mississippi Delta that essentially coincide with a recently published detailed relative sea-level record from the eastern part of the delta. Our results show that differential vertical movements among the three study areas have been only ~0.1 mm [yr.sup.-1] We compare our evidence with a recent sea-level compilation from the Caribbean, to a large extent based on data from areas that are tectonically stable. Our sea-level index points nearly coincide with the Caribbean data, showing surprising tectonic stability for considerable sections of the Mississippi Delta. However, the well-documented high subsidence rates in and near the birdfoot of the Mississippi Delta indicate that different conditions prevail there. The rapid wetland loss in coastal Louisiana is likely due, to a considerable extent, to the compaction of Holocene strata. Keywords: Mississippi Delta, sea-level change, subsidence, Holocene.

Lake-level variations preserved as beach ridges in the Laguna El Fresnal and Laguna Santa Maria subbasins, northern Mexico, record millennially spaced episodes of increased precipitation during the Holocene epoch. We find that the early, middle, and late Holocene were punctuated by periods wet enough to establish large pluvial lakes in currently dry basins in the Chihuahuan Desert; the largest dated pluvial lake covered ~5650 k[m.sup.2] during the early Holocene. Constructional beach ridges in these subbasins are 221 [+ or -] 33 [sup.14C] yr B.P. (Little Ice Age equivalent), 3815 [+ or -] 52 to 4251 [+ or -] 59 [sup.14C] yr B.P. (early Neoglacial), 6110 [+ or -] 80 to 6721 [+ or -] 68 [sup.14C] yr B.P. (mid-Holocene), and 8269 [+ or -] 64 to 8456 [+ or -] 97 [sup.14C] yr B.P. (early Holocene), dates that correlate with other millennially spaced wet or cold events in the Northern Hemisphere. We attribute these wet episodes to increased precipitation, cooler temperatures, and reduced evaporation following southward shifts in winter storm tracks, which are related to long-term El Nino--Southern Oscillation variability during the Holocene. Keywords: paleoclimate, beach ridge, playa, desert, Holocene, Chihuahua, Mexico.

There is increasing evidence that the preceding Holocene climate was as unstable as the last glacial period, although variations occurred at much lower amplitudes. However, low-latitude climate records that confirm this variability are sparse. Here we present a radiocarbon-dated Holocene marine record from the tropical western Atlantic. Aragonite dissolution derived from the degree of preservation of the pteropod Limacina inflata records changes in the corrosiveness of the bottom water at the core site due to the changing influence of northern versus southern water masses. The [delta][sup.18]O difference between the shallow-living planktonic foraminifera Globigerinoides sacculifer and the deep-living Globorotalia tumida is used as proxy for changes in the vertical stratification of the surface water, hence the trade wind strength at this latitude. We compared our data to high-latitude records of the North Atlantic region. A good agreement is found between the aragonite dissolution and the strength in the Island-Scotland Overflow Water, which contributes significantly to the North Atlantic Deep Water. This suggests that large-scale variations in the Atlantic thermohaline circulation occurred throughout the Holocene. Concurrently, the comparison of our [DELTA][delta][sup.18]O with the GISP2 glaciochemical records points to global Holocene atmospheric reorganizations seen in both the tropics and high northern latitudes. Keywords: Holocene, tropical Atlantic, oceanography, stable oxygen isotopes, carbonate preservation, thermohaline circulation.

Rivers are the main source of terrigenous sediment delivered to continental margins and thus exert a major control on coastal evolution and sequence development. However, little is known about past changes in fluvial sediment loads despite the recognition of significant variation under changing climatic regimes. In this study we present the first quantified estimate of sediment discharge for a major river system under conditions of an intensified early Holocene monsoon. Development of the Ganges-Brahmaputra River delta began ca. 11000 yr B.P., when rising sea level flooded the Bengal basin, thereby trapping most of the river's discharge on the inner margin. Chronostratigraphic data from these deltaic deposits are used to calculate the rates of sediment storage on the margin, which provide a minimum estimate of the river's past sediment load. Results reveal that ~5 x [10.sup.12] [m.sup.3] of sediment was stored in the Bengal basin from ca. 11000 to 7000 yr B.P., which corresponds to a mean load of 2.3 x [10.sup.9] t/yr. In comparison, modern sediment load of the Ganges-Brahmaputra is ~1 x [10.sup.9] t/yr, ranking it first among the world's rivers and underscoring the significance of a two-fold increase sustained over 4 k.y. Furthermore, the timing of immense discharge in the early Holocene strongly suggests its relation to a stronger than present southwest monsoon in South Asia. Similar patterns of high monsoon-related sediment discharge have been noted throughout the tropics and subtropics, suggesting a widespread fluviosedimentary response, the potential magnitude of which is showcased by the Ganges-Brahmaputra system. Keywords: climate effects, deltaic sedimentation, river discharge, sediment budget, GangesBrahmaputra River.

The diversion of the Colorado River by dams and irrigation projects, started in the 1930s, triggered the collapse of the Colorado delta ecosystem. Paleontological, ecological, geochronological, stable isotope, field, and satellite image data provide estimates of the delta's benthic productivity during the 1 k.y. directly preceding the artificial shutdown of the river. At least 2 x [10.sup.12] shells of bivalve mollusks make up the current beaches and islands of the delta. The 125 individual valves dated using [sup.14]C-calibrated amino acid racemization indicate that these shells range in age from A.D. 950 to 1950. Seasonal intrashell cycles in [[delta].sup.18] 0 values indicate that average-sized bivalves lived at least 3 yr. The most conservative calculation based on these numbers indicates that during the time of natural river flow, an average standing population of ~6 x [10.sup.9] bivalve mollusks (population density ~50/[m.sup.2) thrived on the delta. In contrast, the present abundance of shelly benthic macroinvertebrates is ~94% lower (3/[m.sup.2] in 1999-2000). The dramatic decrease in abundance testifies to the severe loss of benthic productivity resulting from diversion of the river's flow and the inadequacy of its partial resumption (1981 to present). An integration of paleontological records with geomorphological, geochemical, and geochronological data can provide quantitative insights into human impact on coastal ecosystems. Keywords: paleoecology, productivity, estuary, Colorado River, Holocene.

Old carbon from Holocene deposits was most likely incorporated directly into the peat-forming vegetation through carbon dioxide fixation. Methanogenesis was a significant process in forming peat but there was no evidence for methanotrophic activity.

We present a new glacial chronology demonstrating asynchroneity between advances of Himalayan glaciers and Northern Hemisphere ice-sheet volumes. Glaciers at Nanga Parbat expanded during the early to middle Holocene ca. 9.0-5.5 ka. No major advances at Nanga Parbat during the last global glacial stage of marine oxygen isotope stage 2 (MIS-2) between 24 and 11 ka were identified. Preliminary evidence also indicates advances between ca. 60 and 30 ka. These periods of high ice volume coincide with warm, wet regional climates dominated by a strong southwest Asian summer monsoon. The general lack of deposits dating from MIS-2 suggests that Nanga Parbat was too arid to support expanded ice during this period of low monsoon intensity. Advances during warm, wet periods are possible for the high-altitude summer accumulation glaciers typical of the Himalayas, and explain asynchronous behavior. However, the Holocene advances at Nanga Parbat appear to have been forced by an abrupt drop in temperature ca. 8.4-8.0 ka and an increase in winter precipitation ca. 7-5.5 ka. These results highlight the overall sensitivity of Himalayan glaciation to orbital forcing of monsoon intensity, and on millennial or shorter time scales, to changes in North Atlantic circulation. Keywords: Himalaya Mountains, Asian monsoon, Holocene, glaciation.

A new, widely applicable method to obtain well-constrained relative sea-level records, presented here, relies on optical dating of inorganic coastal landforms and sediments. Eighteen optical ages of basal dune sand from a barrier in Massachusetts, United States, indicate that local relative sea level rose 8 m during the past 5.5 k.y. A sea-level curve drawn from these optical ages shows good agreement with independent evidence from salt-marsh peat sampled behind the barrier, demonstrating the reliability of the method. Optical dating will be particularly useful in analyzing coastal records that contain too few organic indicators to allow high-resolution, (super 14)C-based analyses. Keywords: barriers, Cape Cod, coastal environment, Holocene, luminescence.

Analyses of oxygen isotopes in biogenic silica, carbon isotopes of organic matter, organic carbon content, and gray-scale density from radiocarbon-dated sediment cores retrieved from a lake located on the island of South Georgia (54 degrees S, 36 degrees W) provide the first late glacial to middle Holocene paleoclimatic record from this area of the Southern Ocean. The deglaciation on South Georgia, today 350 km south of the Antarctic convergence, commenced prior to 18.6 ka. Colder conditions were reinstated shortly after 14 ka and lasted across the Younger Dryas chronozone (12.7-11.5 ka) without significant change. The recorded transition to postglacial conditions between 8.4 and 6.5 ka was interrupted by a 400 yr cold event that began ca. 7.8 ka. Our results provide evidence of climate change from sub-Antarctic latitudes that helps determine the existence and timing of late Pleistocene and Holocene millennial-scale climatic events in the Southern Hemisphere.

Isotopic analyses of three stalagmites from Cold Water Cave, northeast Iowa, United States, reveal nearly identical [[Delta].sup.13]C trends from ca. 7 to 2 ka. However, [[Delta].sup.18]O patterns differ by as much as 3[per thousand] from 5.7 to 3.2 ka. These disparate [[Delta].sup.18]O values reflect evaporative 18O enrichment in meteoric water prior to infiltration, suggesting that previously calculated temperatures based on a single Cold Water Cave stalagmite overestimated middle Holocene warming. The coincidence of elevated middle Holocene growth rates in the stalagmites with the lowest oxygen isotopic compositions indicates that the middle Holocene was marked by a predominance of cool-weather precipitation during a period of increased overall aridity.

Geochemical identification of Mount Mazama ash in the Greenland Ice Sheet Project 2 (GISP2) ice core gives a calendrical age of 7627 [+ or -] 150 cal yr B.P. (5677 [+ or -] 150 B.C.) for the eruption, thus providing a more accurate early Holocene stratigraphic time line than previously available. The GISP2 record of volcanically derived sulfate suggests a total stratospheric aerosol loading between 88 and 224 Mt spread over an [approximately]6 yr period following the eruption of Mount Mazama. Taking into account the likelihood of some tropospheric aerosol transport to Greenland, realistic estimates of the resulting atmospheric optical depth range from 0.6 to 1.5. These values may have produced a temperature depression of [approximately]0.6 to 0.7 [degrees] C at mid to high northern latitudes for 1-3 yr after the eruption. These results indicate that the 5677 B.C. eruption of Mount Mazama was one of the most climatically significant volcanic events of the Holocene in the Northern Hemisphere. We also calculate a maximum stratospheric [Cl.sup.-] release of 8.1 Mt by the eruption, which may have led to substantial stratospheric ozone depletion.

Synchronous changes in high-resolution loss on ignition profiles from widely separated shallow Arctic lakes in West Greenland suggest that sedimentary records of lake productivity provide a sensitive century-scale time series of variability in surface-air temperatures throughout the Holocene. Ice cover greatly reduces biological activity in arctic lakes, and the common productivity response among lakes is attributed to air-temperature-controlled variations in the length of the ice-cover period. Comparison with the Greenland oxygen isotope records from Summit ice cores demonstrates that the inferred temperature fluctuations in the lakes closely follow the century-scale fluctuations in the Greenland Ice Core Project [[Delta].sup.18]O record but correlate less well with the Greenland Ice Sheet Project 2 record. Low-altitude continental lake records thus not only confirm features of Holocene atmospheric temperature variability otherwise seen only in high-altitude ice cores from central Greenland, but also highlight discrepancies between the ice-core time series. The combined lake and ice-core data provide corroborative evidence for an intrinsically unstable Holocene climate in Greenland, including the 8200 yr B.P. event that is currently recognized as the most pronounced Holocene climatic cooling with possible global significance. Our finding of a link between paleoproductivity and surface-air temperature has important implications for reconstructing natural climate variability in areas where such information is still rare. The results illustrate the large potential of paleolimnologicai studies in the Arctic.

The partitioning of fluvial sediment load across continental margins is an important control on strata formation and sequence development; however, few quantitative sediment budgets that encompass entire dispersal systems exist. For the Ganges-Brahmaputra river system, sediment discharge is estimated to be [10.sup.9] t/yr at gauging stations [approximately] 300 km inland of the coast, but little has been known of the downstream fate of this material. Geochronological, geophysical, and stratigraphic investigations of the lowland flood plain, delta plain, and shelf help to delineate the extent of Holocene fill and allow calculation of a first-order sediment budget. Results reveal that 1500 x [10.sup.9] [m.sup.3] of sediment fill has been sequestered within the flood plain and delta plain since ca. 7000 yr B.P., or about one-third of the annual discharge. The remaining load appears to be apportioned between the prograding subaqueous delta (1970 x [10.sup.9] [m.sup.3]) and transport to the deep-sea Bengal fan via a nearshore canyon. Modern ([less than] 100 yr) budget estimates based on short-term accretion rates indicate a similar dispersal pattern and show that contemporaneous deposition continues within these disparate depocenters. The roughly equal partitioning of sediment among flood-plain, shelf, and deep-sea settings reflects the respective influence of an inland tectonic basin, a wide shelf, and a deeply incised canyon system. The findings also support new sequence stratigraphic models for these settings and indicate the important insight that modern river deltas can provide for ancient margin systems. Furthermore, results affirm that values of riverine sediment flux to the oceans may be considerably overestimated by not accounting for loss to the flood plains downstream of the gauging stations.

Carbon gases with younger 14C ages than those of the surrounding peat have been reported from continental boreal peatlands, a fact which suggests that significant movement of C[O.sub.2], C[H.sub.4], or DOC (dissolved organic carbon) and export of C via subsurface processes are not accounted for in most estimates of contributions to the C cycle. This paper tests the hypothesis that similar processes can occur in oceanic ombrotrophic mires where water and gas movement is theoretically minimal. Measurements of 14C and [Delta]13C in C[O.sub.2] C[H.sub.4], and DOC, and of tritium, are reported from depths to 250 cm at Tor Royal, a raised mire in southwest England. Radiocarbon ages of gases are 1460 to 500 yr younger than those of peat from the same depths, and C[O.sub.2] is consistently younger than C[H.sub.4]. DOC is 1260 to 830 yr younger than the peat, and significant amounts of tritium were found at all depths. Gas ages are mostly intermediate between the age of the peat and that of the DOC, which suggests that C is principally transported as DOC. However, some gases are younger than their associated DOC, which implies that movement of dissolved gases may also take place. [Delta]13C values in gases suggest that C[O.sub.2] reduction is the major pathway for C[H.sub.4] production. Transport of C in deep peats is likely to be a significant component in the overall C budget of ombrotrophic oceanic peatlands, and C export via discharge to ground or surface waters may be an important mechanism for gaseous C emissions.

Cosmogenic dating, using in situ 26Al and 10Be in quartz pebbles from alluvial terrace surfaces, constrains the late Holocene slip rate on the Xidatan segment of the Kunlun fault in northeastern Tibet. Two terrace risers offset by 24 [+ or -] 3 and 33 [+ or -] 4 m, having respective ages of 1788 [+ or -] 388 and 2914 [+ or -] 471 yr, imply a slip rate of 12.1 [+ or -] 2.6 mm/yr. The full range of ages obtained ([less than or equal to]22.8 k.y., most of them between 6.7 and 1.4 k.y.) confirm that terrace deposition and incision, hence landform evolution, are modulated by post-glacial climate change. Coupled with minimum offsets of 9-12 m, this slip rate implies that great earthquakes (M [approximately]8) with a recurrence time of 800-1000 yr, rupture the Kunlun fault near 94 [degrees] E.

Dated sediment cores from Cayuga Lake, New York State, document that biologically mediated precipitation of calcite has been controlled by environmental change, both natural and anthropogenic, over the past 10 000 yr. During the Holocene Hypsithermal ([approximately]9-4 ka [14C]), Milankovitch forcing of summer insolation in the Northern Hemisphere resulted in a broad increase (to 55%), then decrease (to

Massive fossil (outlier) reefs are preserved seaward of the modern shelf and reef tract along the southeast Florida margin. Thermal ionization mass-spectrometric (TIMS) U-Th dating of 16 pristine Acropora palmata and head corals cored from two transects document early Holocene reef growth from 8.9 to 5.0 ka, from approximately -13.5 to -7 m MSL (mean sea level). These samples fill a gap in the Florida Keys sea-level database and clarify the timing of a significant decrease in the rate of sea.level rise. A portion of this interval, represented by a gap in the Caribbean record of A. palmata reefs, has been interpreted as reef drowning during an inferred catastrophic sea-level rise event of >45 mm/yr, or a 6.5 m rise between 7.6 and 7.2 ka, attributed to West Antarctic Ice Sheet instability and changes in marine ice extent between 8 and 7 ka. Continuous in situ shallow-water reef growth in Florida during this interval precludes the occurrence of exceedingly rapid rates of sea-level rise and is consistent with the North Atlantic record of deglaciation from 9 to 7 ka. Gaps in the early Holocene sea-level records for Florida and the Caribbean are thus more likely to be artifacts of limited sampling and/or core coverage, and not necessarily a result of drowning.

Mount Rainier has produced numerous Holocene debris flows, the largest of which contain clays and other minerals derived from hydrothermally altered rocks on the volcano's edifice. Imagery from an advanced airborne sensor was used to map altered rocks at Mount Rainier and demonstrates their distinctly nonuniform distribution. The mapping of altered rocks helps to identify edifice failure surfaces and to recognize the source areas for the largest debris flow events. Remote sensing methods like those used at Mount Rainier can enhance ground-based mapping efforts and should prove useful for rapidly identifying hazardous sectors at other volcanoes.

The most prominent Holocene climatic event in Greenland ice-core proxies, with approximately half the amplitude of the Younger Dryas, occurred [approximately]8000 to 8400 yr ago. This Holocene event affected regions well beyond the North Atlantic basin, as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane. Widespread proxy records from the tropics to the north polar regions show a short-lived cool, dry, or windy event of similar age. The spatial pattern of terrestrial and marine changes is similar to that of the Younger Dryas event, suggesting a role for North Atlantic thermohaline circulation. Possible forcings identified thus far for this Holocene event are small, consistent with recent model results indicating high sensitivity and strong linkages in the climatic system.

Most of the sediment components that accumulated in Elk Lake, northwestern Minnesota, during the Holocene are autochthonous or biogenic, delivered to the sediment-water interface on a seasonal schedule, preserved in distinct annual laminae (varves). The main allochthonous component is detrital clastic material, as measured by bulk-sediment concentrations of aluminum, sodium, potassium, titanium, and quartz, that enters the lake mostly as eolian dust. The eolian clastic influx to Elk Lake was considerably greater during the mid-Holocene (8-4 ka) than it has been for the past 4000 yr, when periods of increased eolian activity correspond to the time of the Little Ice Age and the dust bowl. Geochemical records of eolian activity exhibit distinct cyclicities with dominant periodicities of 400 and 84 yr.

The [Delta]18O difference between shallow- and deep-living planktonic foraminifera is a proxy for the stratification of surface waters. In Holocene sediments from the Atlantic this difference increases from about 0[per thousand] in subpolar regions to [approximately]3[per thousand] in the tropics. The [Delta]18O values of the shallow dwellers Globigerinoides sacculifer and Neogloboquadrina pachyderma mainly reflect surface-water conditions. The calcification depth of the deep-living species Globorotalia truncatulinoides (right-coiling) is affected by the stratification of the water column. In the subtropics, shell formation of G. truncatulinoides (right-coiling) begins in weakly stratified surface waters. In the tropics, the initial shell is secreted below the mixed layer. This hydrographically induced difference in calcification depth allows the use of the isotopic difference between G. truncatulinoides and the shallow species as a proxy for the stratification of surface waters. The [Delta][Delta]18O between G. truncatulinoides and G. sacculifer from tropical gravity Core GeoB 1523-1 was significantly lower during isotope stage 2. This suggests that the glacial temperature stratification of western equatorial Atlantic surface waters was significantly reduced relative to the Holocene.

Sediment hiatuses, detritus layers, and inwashed terrestrial moss layers in five cores at Crawford Lake, Ontario, Canada, show that the lake level was low between ca. 4.8 and 2 ka (1 ka = 1000 14C yr B.P.). This low level is attributed to a dry and warm climate, which has also been documented at other sites in southern Ontario, southern Michigan, and southern Wisconsin. Oxygen isotope ([[Delta].sup.18]O) values from authigenic marl show a negative shift of 2.4[per thousand] from -9[per thousand] to -11.4[per thousand] (Vienna Peedee belemnite) between ca. 5 and 2 ka. Enhanced evaporation under dry and likely closed-basin conditions would lead to more enrichment in 18O, so we suggest that the trend to depleted 18O indicates a significant change in the [[Delta].sup.18]O value of the source meteoric water. The major moisture source for the Great Lakes region is the Gulf of Mexico, from which the amount and seasonality of precipitation are affected by the interplay of air masses from the Gulf of Mexico and North Pacific, probably controlled by jet stream positions and storm tracks. In the late middle Holocene, the isotopically heavy moisture from the gulf might have contributed less precipitation and/or a higher proportion in winter months, probably caused by more frequent eastward extension of dry Pacific air depleted in 18O. This hypothesis implies that the [[Delta].sup.18]O values of paleoprecipitation in the middle Holocene reflected moisture-source history more strongly than paleotemperature.

Six thousand years of sediment accumulation and autocompaction in a transgressive valley-fill deposit are simulated numerically. Rates of accumulation of freshwater organic-rich mud, subtidal mud, and organic-rich mud of the modern salt marsh are specified by using existing sedimentologic, paleontologic, and geochronologic data. Processes of pore-water expulsion and autocompaction are determined from finite-strain consolidation theory. Model parameters for subtidal mud facies are determined from laboratory consolidation tests. Model parameters for organic-rich deposits are determined by sampling equivalent modern sedimentary environments and by model calibration. Model calibration involves (1) reproducing the present distribution of void ratios measured in a 7.6 m Vibracore, (2) correctly calculating the preserved thickness of lithologic units, and (3) determining a history of elevation changes of the sediment surface that is consistent with the Delaware sea-level curve and the known sequence of paleoenvironments of Wolfe Glade, Delaware. The results indicate that preserved horizons in the 10 m Holocene section of Wolfe Glade have been lowered a maximum of 2.3 m. The rate of lowering due to autocompaction alone has been one-half to one-third of the rate of sea-level rise during most of the past 6000 yr. Numerical experiments suggest that organic-rich freshwater wetland deposits are primarily responsible for the extensive autocompaction at Wolfe Glade. Our approach may be used to correct Holocene coastal stratigraphic data for autocompaction, but careful model calibration (based in part on the stratigraphy) will usually be required.

Strontium isotope analysis of precisely dated calcite growth layers in Holocene speleothems from Barbados, West Indies, reveals high-resolution temporal variations in ground-water composition and may provide a new approach to documenting the links between climate variability and fluctuations in the hydrologic cycle such as recharge rates and flow paths. The speleothems grew in a cave that developed in a fresh-water aquifer in uplifted Pleistocene reef limestones. Three periods of ground-water Sr isotope evolution are observed: 87Sr/86Sr values decreased from 6 to 4 ka, increased from 4 to 1 ka, and decreased again after 1 ka. The Sr isotope oscillations appear to record periodic variations in the relative Sr fluxes to ground water from exchangeable soil sites vs. carbonate mineral reactions, as reflected in 87Sr/86Sr values of modern Barbados ground waters. A hydrologic model that explains changes in ground-water flow routes in karst aquifers as a function of amount of rainfall recharge can account for the speleothem Sr isotope record. Independent Holocene climate records that indicate a major period of aridity at around 1.3-1.1 ka in the American tropics correspond with periodic variations in rainfall on Barbados that are predicted by this hydrologic model.

Sedimentological studies of six Holocene lakes in the Coorong region of South Australia have revealed the presence of a calcrete layer at a similar stratigraphic level in all of these lakes. The calcrete layer lies at < 1.5 m subsurface depths within each of the lakes and is exposed along the marginal areas of some. Mud-crack polygon and desiccation cracks are well preserved in exposed parts of the calcrete surface, and the calcrete exhibits vadose-zone diagenetic fabrics. These features provide direct evidence for the existence of a late Holocene major Australian arid period during which the lakes in the Coorong region remained dry and the surfaces of lake sediments underwent calcretization. Radiocarbon dating of the calcrete layer indicates that this period existed from 3300 to 1300 yr B.P.

Radiocarbon-controlled stratigraphic data from Owasco Lake valley, New York, document two cycles of relative lake-level change during the Holocene in response to regional variations in effective precipitation. Lake highstands occurred at 10.5 and 6.9 ka, with an intervening lowstand at 9 ka. The 10.5 ka highstand occurred during the well-known Killarney-Younger Dryas cold interval (11.2-10.0 ka) in response to decreased evaporation and increased precipitation. The driest interval of the early-middle Holocene occurred [approximately]9 ka, at the time of maximum summer solar insolation. A second highstand occurred during the Hypsithermal ([approximately]8.5-5.5 ka) in response to increased precipitation at a time when the interior of North America underwent maximum aridity. This out-of-phase relationship between northeastern North America and the midcontinent during the Hypsithermal may have been a consequence of a northward shift of the jet stream and a change of precipitation patterns in response to increased global warmth. If the Hypsithermal can be used as an analog for potential global warming, the northeastern United States may experience future increases in regional precipitation.

The Holocene eruptive history of Popocatepetl volcano is characterized by recurrent voluminous Plinian eruptions every 1000 to 3000 yr, the most recent of which destroyed human settlements. Major eruptions occurred between 3195 and 2830 B.C., 800 and 215 B.C., and A.D. 675 and 1095. The three eruptions followed a similar pattern and started with minor ash fall and ash flows. The eruptions reached their peak with a main Plinian pulse that produced deposition of a pumice fall, the emplacement of hot ash flows, and finally extensive mudflows. Each time the area of devastation had become repopulated, before being devastated once again. During the last eruption several settlements, including Cholula (a major urban center), were inundated by lahars. A scenario of the possible recurrence of an eruption of similar magnitude, which would have disastrous consequences for the now highly populated areas around Popocatepetl, should be considered seriously in any volcano emergency contingency plan. This is especially important because more than one million people are living within a radius of 35 km around the volcano (the outskirts of Mexico City are at a distance of 40 km), and Popocatepetl resumed emitting ash on December 21, 1994, after decades of dormancy.

Earthquake-induced paleoliquefaction features in Holocene sediments in south-central Indiana are evidence for strong shaking in the recent prehistoric past in an area where only infrequent, low-magnitude earthquakes have occurred during the 200 yr of the historic record. Distribution of liquefaction features and geotechnical analyses of liquefied sediments are the basis for estimating that an earthquake with a magnitude of [approximately] 6.9 was centered near Vallonia, Indiana. Radiometric, archaeologic, and stratigraphic relations bracket the event to between [approximately] 3.7 ka and 4.2 ka B.P. Evidence for a local source for strong shaking requires reassessment of earthquakes hazards in the southern Indiana-northern Kentucky-southwestern Ohio area.

New radiocarbon-dated cores obtained by Vibracorers in the western Gulf of Maine confirm that a short-lived, relative sea-level lowstand of [approximately] -55 m occurred at 11-10.5 ka. These cores and younger salt-marsh data also reveal that rates of transgression varied throughout the Holocene, probably due to local variations in glacial isostasy. The isostatic component is resolved by subtracting published approximations of eustatic sea level from our well-determined observations of local relative sea level. A large peak in the isostatic curve coincides with the lowstand and is interpreted as a forebulge 20-25 m in amplitude. Forebulge migration is estimated at 7-11 km/100 yr, based on the timing of lowstands across the region.

Calcic dolomite cements compose an average of 5% of the upper 4.3 m of subtidal deposits

Fischer plots are graphic representations of cyclic carbonate deposits showing cumulative departure from average cycle thickness plotted against cycle number and corrected for assumed subsidence during each cycle. Observed cycle thicknesses in excess of subsidence have been interpreted to represent depositional accommodation space formed by eustatic sea-level rise. However, implicit in this interpretation is the assumption that preserved cycle thickness is a proxy for accommodation. To test this assumption, a survey of carbonate-sediment accumulation patterns (i.e., cycle thicknesses) developing during a single transgressive event (the Holocene postglacial sea-level rise) was conducted on a shallow carbonate platform (Great Bahama Bank) where accommodation was created primarily by sea-level rise over the platform margin (i.e., subsidence was minimal). This survey demonstrates that Holocene sediment (cycle) thickness and accommodation are uncorrelated ([r.sup.2] = 0.03). Consequently, Fischer plots constructed by using Holocene cycle thicknesses are poor representations of the Holocene transgression. In extreme examples, Holocene Fischer plots would be interpreted to show relative sea-level fall during the Holocene on Great Bahama Bank because Holocene subsidence currently exceeds sediment thickness. In addition, a simple sensitivity test shows that eustatic sea-level interpretations based on interbasinal correlation of Fischer plots are equivocal. The gross form of Fischer plots appears so overly robust as to be insensitive to broad variations in stratigraphic completeness, cycle duration, or subsidence. Because cycle thickness apparently is uncorrelated with accommodation and the gross form of Fischer plots is relatively invariant, it seems prudent to reevaluate the practice of interpreting Fischer plots as sea-level curves per se in the analysis of ancient carbonate cycles.

The 1.4 Ma to Holocene volcanic eruptive centers and flows in the Chyulu Hills, southeastern Kenya, range in composition between foidites and alkali basalts; they record the relation between linear volcanic chains, preexisting basement structures, geochemical evolution, and the effects of a changing regional tectonic stress field. Pleistocene volcanic centers follow northwest-trending Proterozoic joints and are compatible with the early to middle Pleistocene east-northeast-west-southwest-oriented regional position of the least compressive regional stress ([S.sub.hmin]) documented for the Kenya Rift. Because the angle between the joints and [S.sub.hmin] was high, the ascent of magmas was facilitated by the pre-existing basement structures. In contrast, Holocene north-northeast-trending vents and associated normal faults cut older basement structures. The youngest faults and vents are approximately perpendicular to the present regional [S.sub.hmin] position of 135 [degrees] that has prevailed in Kenya for [approximately]0.5 m.y. The position of the neotectonic stress field has resulted in a decreased angle between [S.sub.hmin] and the joints and may have led to more difficult magma ascent, longer crustal magma residence, a trend toward more rift-typical volcanic rocks, and finally new alignments of the volcanic fields.

Reconstructions of late Weichselian glacier coverage on the continental shelves of the Russian Arctic range from a large ice sheet terminating in northern Siberia to isolated ice caps restricted to Arctic archipelagos. This disparity in glacier reconstructions reflects the lack of chronological control on glacial and deglacial landforms. We present new Holocene relative sea-level data from Franz Josef Land and northern Novaya Zemlya, Russia, that place the thickest glacier loads in the northern Barents Sea and not over Novaya Zemlya. Radiocarbon ages from shelf and terrestrial areas at the former ice-sheet margin support deglaciation of the northern Barents Sea between 13.0 and 10.3 ka, considerably later than inferred from isotopic records for the Arctic Ocean. This analysis indicates that the Barents Sea ice sheet was a dominant sea-level reservoir in northern Eurasia, and that glacier loading of Novaya Zemlya was comparatively limited during the last glaciation.

Penguins are sensitive indicators of the Antarctic climate and of the environmental parameters that limit their presence and distribution. Paleoenvironmental data, obtained from the study of abandoned penguin rookeries (Pygoscelis adeliae) along the Victoria Land coast in Antarctica, indicate a 14C date of 11-13 ka for the oldest abandoned rookery and supply new information about the timing of glacier retreat in southern Victoria Land after the last glacial maximum. The continuous presence of the Adelie penguins is documented from 7 ka. According to our data, the limiting factors that control the presence of penguins along the coast of Victoria Land changed during the Holocene. Whereas several colonies were occupied for very long periods, other sites were used for more or less extended periods and then abandoned. The greatest diffusion of rookeries occurred between 3 and 4 ka, a period of particularly favorable environmental conditions that has never been repeated. It was followed by a sudden decrease in the number of penguin rookeries shortly after 3 ka. This event has been attributed to an increase of the sea-ice extension and may have been correlated to a worldwide phase of climate change near the Subboreal-Subatlantic boundary. A minor phase of penguin reoccupation occurred locally in the eighth to fourteenth centuries (A.D.). Because the presence and number of penguins reflect the state of health of the Antarctic marine ecosystem, it is important to evaluate the variations in their distribution in the past, in the absence of human-induced changes.

The Honey Lake fault zone, a major right-lateral fault in the Basin and Range province of eastern California, is one of a broad system of faults that accommodate some of the relative motion between the Pacific and North American plates. These right-lateral faults may have significantly higher slip rates, and thus greater earthquake hazards, than the normal faults for which the Basin and Range is more commonly known. In the Honey Lake Valley, the fault forms a 50-km-long zone of landforms typical of active strike-slip faults. Right-lateral offset of an incised creek channel is used to estimate a Holocene slip rate of between 1.1 and 2.6 mm/yr. A fault exposure in Holocene alluvium shows evidence for at least four late Holocene surface-faulting earthquakes.

Coastal lake sediments contain a stratigraphically and chronologically distinct record of major hurricane strikes during late Holocene time. Frederic--a category 3 hurricane that struck the Alabama coast on the Gulf of Mexico in 1979--left a distinct sand layer in the nearshore sediments of Lake Shelby as a result of storm-tide overwash of beaches and dunes. Sediment cores taken from the center of Lake Shelby contain multiple sand layers, suggesting that major hurricanes of category 4 or 5 intensity directly struck the Alabama coast at ca. 3.2-3.0, 2.6, 2.2, 1.4, and 0.8 ka (14C yr), with an average recurrence interval of |similar to~600 yr. The Alabama coast is likely to be struck by a category 4 or 5 hurricane within the next century.