Robert Stuckenrath

Acropora palmata reef framework: A reliable indicator of sea level in the western atlantic for the past 10,000 years

SummaryA minimum sea-level curve for the past 10,000 years has been constructed on the basis of radiocarbon dates of Acropora palmata (Lamarck) samples from the shallow-water framework of both relict and modern reefs of the tropical western Atlantic. A. palmata framework is a reliable reference for reconstructing the history of late Quaternary sea levels owing to its restricted depth range (<1 to 5 m), the lack of postdepositional transport of A. palmata framework, the ease of obtaining uncontaminated samples, and the minimal compaction of A. palmata reef facies. The minimum sea-level curve constructed in this study is useful not only in evaluating the reliability of present and future Holocene sea-level curves for the western Atlantic, but also in estimating paleo-water depths in the study of Holocene reef history of this area.

Late Cenozoic paleoclimates of the Gaap Escarpment, Kalahari margin, South Africa

Abstract The Gaap Escarpment is a dolomite cuesta demarcating the southeast margin of the Kalahari. Since Miocene-Pliocene times, thick masses of lime tufa have repeatedly accumulated at several points along this escarpment, and four regional sequences are described. These allow discrimination of six major depositional complexes, commonly characterized by basal cryoclastic breccias or coarse conglomerates that reflect frost shattering and torrential runoff, followed by sheets and lobes of tufa generated in an environment substantially wetter than today. A chronostratigraphy for the last 30,000 yr is provided by 14 C dating, with direct or indirect correlations to the Vaal River sequence. The regional stratigraphy as well as faunal dating indicate an early Pleistocene age for Australopithecus africanus at Taung. Repeated episodes of protracted cold or wetter climate or both begin in terminal Miocene times, and the last Pleistocene cold-moist interval began after 35,000 yr B.P. and ended 14,000 yr B.P. Early and late Holocene times were mainly wetter, whereas the middle Holocene was drier than today. The paleoclimatic sequence differs from that of the southern and southwestern Cape or that of East Africa, but close parallels are evident throughout the lower Vaal Basin and the southern Kalahari. The tufa cycles provide a unique, 5,000,000-yr record of climatic variation in the Kalahari.summer-rainfall belt that can be related to complex anomalies of the general atmospheric circulation.

Vegetation and associated environments during the past 14,000 years near Moulton Pond, Maine

Abstract Pollen influx and percentage diagrams were prepared from an 11.4 m core from Moulton Pond, Maine. The pond basin was deglaciated about 14,000 y. a., after which it was located on an island in a sea of subarctic character until about 12,400 y. a. when the surrounding area emerged from the sea. The terrestrial vegetation was tundra until about 10,000 y. a. A change in the tundra vegetation is synchronous with the emergence from the sea, but synchroneity with the Pineo Ridge glacial readvance, which reached its maximum 50 km to the east of the pond about 12,700 y. a., is also possible because of imprecision in the dating. Comparisons of the Moulton Pond results with late-glacial pollen sequences elsewhere in eastern United States and adjacent Canada reveal a lack of synchroneity in vegetational changes casting doubt on claims of major broad-scale climatic shifts over the entire area. The tundra period at Moulton Pond ended with a transition of a few hundred years to partly open, relatively xeric forests of low diversity dominated by white pine, oak, and birch trees. There was no intervening boreal forest. In the postglacial period the vegetation was continually changing, including in the early portion a series of immigrations of temperate tree taxa which later became important in the forests. The transient nature of these assemblages is further indicated by their differences from the closest modern analogs. From about 7100 y. a. until settlement by Europeans 200 y. a., the forests were closed. A major decline of conifers centering about 4700 y. a. was followed by maxima of mesic hardwoods about a thousand years later. In the most recent 2000 yr, the pollen record suggests greater environmental severity, evidenced by increasing spruce. But for the entire postglacial period, the closest modern vegetational analogs are all in the conifer-hardwood region. Much of the postglacial pollen sequence is inexplicable in climatic terms, as evidenced by nonsynchronous behavior of hemlock and beech. The pollen influx diagram is useful for distinguishing tundra from forest, but for the postglacial period it is difficult to interpret. Pollen influx data are strongly affected by shifts in the pattern of sedimentation in lakes. We propose that such shifts account for the major changes in influx in mid- and late-postglacial time at Moulton Pond and at Rogers Lake, Connecticut. This complicates the interpretation of influx data which otherwise are superior to percentage data.

Thickest recorded Holocene reef section, Isla Pérez core hole, Alacran Reef, Mexico

The Isla Perez core hole (Alacran Reef, Mexico) records the thickest Holocene section (33.5 m) known from either Atlantic or Indo-Pacific reefs. The high rate of deposition of reef material—a maximum of 12m/l,000 yr—is attributed to accumulation of the open framework constructed by the rapidly growing coral Acropora cervicornis . Mineralogic and radiocarbon analyses indicate that in areas protected from frequent high-energy agitation, this fragile branching coral, which thrives in moderate to shallow water depths, is capable of constructing extensive Holocene biohermal structures having more than 15m relief.

Holocene environmental changes in the alpine zone, northern San Juan Mountains, Colorado: Evidence from bog stratigraphy and palynology

Abstract Cores from five high alpine basins in the northern San Juan Mountains show several fluctuations in lithology. Typically, peats are interbedded with coarser clastic sediments or else woody peats alternate with fibrous peat. Twenty 14 C dates provide radiometric control. Sediment rates averaged about 2.5 cm/100 yr but varied at the different sites between 1.19 and 50 cm/100 yr. Rates were lower during the middle of the Holocene. Basal radiocarbon dates indicate that these high (ca. 3600 m a.s.l.) northeasterly facing cirques were icefree by 9000 BP. There is some evidence in the cores for a short climatic reversal sometime between 8000 and 7000 BP. A major change occurred in the high basins very close to 5000 BP and thereafter there are several intervals of increased clastic sedimentation which may be related to Neoglacial climatic fluctuations. Analysis of a 2.15 m core near Hurricane Basin indicates significant fluctuation of pollen and macrofossils occurred during the 9000 ± year record. The Picea/Pinus ratios are used to delimit changes in the apparent elevation of the site: the ratios indicate that a short drop of “treeline” occurred about 8000 BP and then remained near present level until about ≥1800 BP when the apparent elevation of the site rose. Macrofossils indicate that spruce was present in the Hurricane Basin (and others) at specific periods and confirms the general results of the Picea/Pinus ratios. The San Juan Mountains do not possess a glacial Neoglacial record but the stratigraphy of these high cirque basins can be used to define glacial stades (cf. Jardine, 1972 ). The interpreted climatic response record on vegetation and sediment flux has both similarities and differences from other records in the western mountains of North America.

Dating and context of rock engravings in Southern Africa

Rock art is seldom recovered from sealed archeological contexts and is therefore difficult to date or integrate with other artifactual assemblages. South African engravings, found on low rocks at open-air sites, exemplify the problem. Multiscale spatial study of technique, thematic variation, faunal content, geoarcheological paterning, settlement history, and ethno-archeological setting provides coherent information on environment, time, and group identity. The major periods of naturalistic animal engravings coincide with wetter and warmer climates abot 3200 to 2500 and 2250 to 1800 years before present, but the earliest engravings may be older than 4000 years. Geometric designs were favored after 1300 years before present, when climate was drier and when one identity-conscious population of Bushman engravers first encountered domesticated animals.

Relict oysters on the United States Atlantic continental shelf: A reconsideration of their usefulness in understanding late Quaternary sea-level history

Relict oyster shells are thought to be unreliable references for reconstructing sea-level history owing to interrelationships between their age and present depth of occurrence off North Carolina. Radiocarbon dates were obtained from 44 relict oyster shells of Crassostrea virginica (Gmelin) collected within a relatively narrow area of the continental shelf from north of Cape Hatteras to Cape Fear (less than 40,000 km 2 ) in depths to 60 m. Data indicate significant postdepositional and landward transport of these shells, calling into question some previous interpretations of sea-level history that were based partly on the dating of relict oyster shells and other unreliable relict shoreline deposits.

Strait of Sicily depositional rates and patterns, and possible reversal of currents in the late quaternary

Abstract Late Quaternary sedimentation and lithostratigraphic patterns on the Strait of Sicily platform are readily discernible from those in the adjacent deep Mediterranean basins. Strait cores are unusually uniform, show a high degree of bioturbation and include an important coarse bioclastic fraction. Lithofacies are correlated with depth and proximity to the Strait narrows. Stratification (alternating sand and mud turbidites, hemipelagic mud and ash layers) is best developed in three deep narrow basins in the Strait where benthic populations and rates of bioturbation are significantly lower than on the shallower neritic platform and banks. 14 C dates show that the top of some cores are truncated in the early Holocene at a time when black, organic-rich sapropel layers formed in the central and eastern Mediterranean. No sapropels are recovered on the Strait and, further, bioturbation structures occur throughout late Quaternary sections. These and concurrent faunal changes indicate that: (1) the Strait remained ventilated and swept by currents while density stratification and anaerobic conditions prevailed to the east; and (2) circulation was not blocked at the Strait during the last glacial-postglacial evolution. Sedimentological and related observations are best interpreted in terms of short-lived but important water mass fluctuations, including a possible reversal of currents, which took place during the early warming phase of the climatic curve.

Turbid-layer bypassing model: The Corsican Trough, northwestern Mediterranean

Abstract Studies of Quaternary sediments on the Mediterranean margins indicate that: (1) erosion has prevailed on shelves, shallow ridges and upper slopes, while (2) erosional-depositional and resuspension regimes dominate slopes, and (3) ponding characterizes the deep, more distal basinal sectors. A sedimentation model involving gravity-induced transport of fine-grained material and downslope bypassing is developed on the basis of an examination of cores (petrologic and radiocarbon analyses) and a dense network of 3.5-kHz and seismic records in the Corsican Trough and associated margins and deep-sea fans of the northwest Mediterranean. This model emphasizes that silty mud accumulates at higher rates in more distal regions than do coarser grained sediments in more proximal settings. The principles highlighted here should be applicable to other continental margin settings in the Mediterranean and other ocean basins.

The Portwashingtonian warm interval in the northern Atlantic coastal plain

Stratified and deformed masses of marine sediment that range in age from 43,800 to 21,750 yr were deposited in the Woodfordian moraine on western Long Island, New York. Reconstruction of the geologic history of these sediments provides further evidence of a mid-Wisconsinan warm interval, the Portwashingtonian warm interval (new name), that is represented by warm climate, temperate forests, and relative sea level comparable to modern sea level.