Holocene sea-level changes along the North Carolina Coastline and their implications for glacial isostatic adjustment models

We have synthesized new and existing relative sea-level (RSL) data to produce a quality-controlled, spatially comprehensive database from the North Carolina coastline. The RSL database consists of 54 sea-level index points that are quantitatively related to an appropriate tide level and assigned an error estimate, and a further 33 limiting dates that confine the maximum and minimum elevations of RSL. The temporal distribution of the index points is very uneven with only five index points older than 4000 cal a BP, but the form of the Holocene sea-level trend is constrained by both terrestrial and marine limiting dates. The data illustrate RSL rapidly rising during the early and mid Holocene from an observed elevation of ?35.7 ± 1.1 m MSL at 11062–10576 cal a BP to ?4.2 m ± 0.4 m MSL at 4240–3592 cal a BP.We restricted comparisons between observations and predictions from the ICE-5G(VM2) with rotational feedback Glacial Isostatic Adjustment (GIA) model to the Late Holocene RSL (last 4000 cal a BP) because of the wealth of sea-level data during this time interval. The ICE-5G(VM2) model predicts significant spatial variations in RSL across North Carolina, thus we subdivided the observations into two regions. The model forecasts an increase in the rate of sea-level rise in Region 1 (Albemarle, Currituck, Roanoke, Croatan, and northern Pamlico sounds) compared to Region 2 (southern Pamlico, Core and Bogue sounds, and farther south to Wilmington). The observations show Late Holocene sea-level rising at 1.14 ± 0.03 mm year^?1 and 0.82 ± 0.02 mm year^?1 in Regions 1 and 2, respectively. The ICE-5G(VM2) predictions capture the general temporal trend of the observations, although there is an apparent misfit for index points older than 2000 cal a BP. It is presently unknown whether these misfits are caused by possible tectonic uplift associated with the mid-Carolina Platform High or a flaw in the GIA model. A comparison of local tide gauge data with the Late Holocene RSL trends from Regions 1 and 2 support the spatial variation in RSL across North Carolina, and imply an additional increase of mean sea level of greater than 2 mm year^?1 during the latter half of the 20th century; this is in general agreement with historical tide gauge and satellite altimetry data.     

The Upper Crustal Evolution of a Large Silicic Magma Body: Evidence from Crystal-scale Rb Sr Isotopic Heterogeneities in the Fish Canyon Magmatic System, Colorado

Batholith-sized bodies of crystal-rich magmatic ‘mush’are widely inferred to represent the hidden sources of manylarge-volume high-silica rhyolite eruptive units. Occasionallythese mush bodies are ejected along with their trapped interstitialliquid, forming the distinctive crystal-rich ignimbrites knownas ‘monotonous intermediates’. These ignimbritesare notable for their combination of high crystal contents (35–55%),dacitic bulk compositions with interstitial high-silica rhyoliticglass, and general lack of compositional zonation. The 5000km³ Fish Canyon Tuff is an archetypal eruption deposit of thistype, and is the largest known silicic eruption on Earth. Ejectafrom the Fish Canyon magmatic system are notable for the limitedcompositional variation that they define on the basis of whole-rockchemistry, whereas 45 vol. % crystals in a matrix of high-silicarhyolite glass together span a large range of mineral-scaleisotopic variability (microns to millimetres). Rb/Sr isotopicanalyses of single crystals (sanidine, plagioclase, biotite,hornblende, apatite, titanite) and sampling by micromillingof selected zones within glass plus sanidine and plagioclasecrystals document widespread isotopic disequilibrium at manyscales. High and variable ⁸⁷Sr/⁸⁶Sr_i values for euhedral biotitegrains cannot be explained by any model involving closed-systemradiogenic ingrowth, and they are difficult to rationalize unlessmuch of this radiogenic Sr has been introduced at a late stagevia assimilation of local Proterozoic crust. Hornblende is theonly phase that approaches isotopic equilibrium with the surroundingmelt, but the melt (glass) was isotopically heterogeneous atthe millimetre scale, and was therefore apparently contaminatedwith radiogenic Sr shortly prior to eruption. The other mineralphases (plagioclase, sanidine, titanite, and apatite) have significantlylower ⁸⁷Sr/⁸⁶Sr_i values than whole-rock values (as much as –0·0005).Such isotopic disequilibrium implies that feldspars, titaniteand apatite are antecrysts that crystallized from less radiogenicmelt compositions at earlier stages of magma evolution, whereashighly radiogenic biotite xenocrysts and the development ofisotopic heterogeneity in matrix melt glass appear to coincidewith the final stage of the evolution of the Fish Canyon magmabody in the upper crust. Integrated petrographic and geochemicalevidence is consistent with pre-eruptive thermal rejuvenationof a near-solidus mineral assemblage from 720 to 760°C (i.e.partial dissolution of feldspars + quartz while hornblende +titanite + biotite were crystallizing). Assimilation and blendingof phenocrysts, antecrysts and xenocrysts reflects chamber-wide,low Reynolds number convection that occurred within the last10 000 years before eruption. KEY WORDS: Fish Canyon Tuff; Rb–Sr isotopes; microsampling; magmatic processes; crystal mush     

Quantifying the Tetrad Effect,Shape Components,and Ce–Eu–Gd Anomalies in Rare Earth Element Patterns

Mathematical Geosciences - Plots of chondrite-normalised rare earth element (REE) patterns often appear as smooth curves. These curves can be decomposed into orthogonal polynomial functions (shape...     

The Depiction of Coronal Structure in White-Light Images

The very steep decrease in density with heliocentric distance makes imaging of coronal density structures out to a few solar radii challenging. The radial gradient in brightness can be reduced using numerous image processing techniques, thus quantitative data are manipulated to provide qualitative images. We introduce a new normalizing-radial-graded filter (NRGF): a simple filter for removing the radial gradient to reveal coronal structure. Applied to polarized brightness observations of the corona, the NRGF produces images which are striking in their detail. Total-brightness, white-light images include contributions from the F corona, stray light, and other instrumental contributions which need to be removed as effectively as possible to properly reveal the electron corona structure. A new procedure for subtracting this background from LASCO C2 white-light, total-brightness images is introduced. The background is created from the unpolarized component of total-brightness images and is found to be remarkably time-invariant, remaining virtually unchanged over the solar cycle. By direct comparison with polarized-brightness data, we show that the new background-subtracting procedure is superior in depicting coronal structure accurately, particularly when used in conjunction with the NRGF. The effectiveness of the procedures is demonstrated on a series of LASCO C2 observations of a coronal mass ejection (CME).     

Geological evolution of the Ishpeming Greenstone Belt,Michigan, U.S.A.

The Ishpeming Greenstone Belt is an Archean belt in the southern part of the Canadian Shield in the Upper Peninsula of Michigan, U.S.A. Two volcanic cycles are preserved in it. The oldest formation, and basal to the first cycle (the Kitchi Schist), consists of mafic metavolcanics, has a major serpentinized ultramafic body near its base, and grades upward to a coarse felsic volcanic breccia at the top of the cycle. This unit in turn is overlain by a sequence of mafic flows that grades upward to interbedded mafic flows and exhalites of the Mona Schist. This sequence has been intruded by the Dead River Pluton.The Ishpeming Greenstone Belt probably represents the keel of a previously much more extensive Greenstone Belt.Gold mineralization occurs associated with mafic basaltic volcanic rocks and serpentinized ultramafics low in the succession, and with carbonate-rich quartz-chlorite-sericite schists and exhalites higher in the sequence. No mineral deposits are now being exploited here.     

Intertidal macroalgae and macroinvertebrates: Seasonal and spatial abundance patterns along an estuarine gradient

Quantitative sampling of the dominant intertidal epibiota was conducted seasonally along an estuarine gradient within the Great Bay Estuary System, New Hampshire, U.S.A. The abundance and zonation of the dominant macroorganisms varied with distance into the estuary. Replacement of marine by estuarine species occurred, and overall abundance and species richness decreased along the estuarine gradient. Zonation patterns within the inner estuary were primarily allied with substrata. Maximum abundance of invertebrates occurred in the mid-intertidal zone where a dense fucoid canopy provided habitat heterogeneity. Densities of epibiotic organisms decreased toward low water, especially in the inner estuary where hard substratum was limiting. Settlement blocks, introduced into the low intertidal zone, were dominated by barnacles and fucoid algae; after 16 months, the species composition on the settlement blocks resembled the adjacent community. Semibalanus balanoides settled in the spring, while Fucus vesiculosus var. spiralis exhibited low but constant settlement. Despite the physical rigors of the estuarine environment, only Semibalanus balanoides, Ilyanassa obsoleta and Spartina alterniflora showed significant seasonal changes in density. Thus, there are predictable and persistent epibiotic species assemblages within the intertidal zone of the Great Bay Estuary System.     

VIEWS OF THE AAG: RESULTS OF THE LONG RANGE PLANNING MEMBERSHIP SURVEY

In this issue THE PROFESSIONAL GEOGRAPHER presents the first two reports in a series on the Association's long-range planning process, which was described by Past President Marcus in May. David Morgan outlines here the results of a survey of members' attitudes on a number of important questions.     

Contrasting interactions of sodium and potassium with H<Subscript>2</Subscript>O in haplogranitic liquids and glasses at 200 MPa from hydration–diffusion experiments

This study examines hydration–diffusion in the metaluminous haplogranite system at 200 MPa H₂O and 800–300°C. At 800°C hydration is accompanied by melting and uphill diffusion of sodium from anhydrous glass toward the region of hydration and melting, whereas potassium diffuses away from the hydration front and into anhydrous glass. Silicon and aluminum are simply diluted upon hydration. There is no change in molecular Al/(Na + K) throughout the entire hydration-diffusion aureole and, therefore, (1) there is no loss of alkalis to the vapor, and (2) K migrates to replace Na in order to maintain local charge balance required by ^IVAl. Alkali diffusion occurs over a viscosity contrast from 10^4.1 Pa s in hydrous liquid to 10^11.8–10^13.5 Pa s in anhydrous glass. From these results, we interpret that: (1) Na is structurally or energetically favored over K as a charge-balancing cation for ^IVAl in hydrous granitic liquids, whereas the opposite behavior has been observed for anhydrous melts, and (2) the diffusion of alkalis through silicate melts is largely independent of viscosity. Results from 600°C are similar to those at 800°C, but hydration at 300°C involves a loss of Na and concomitant increase in molar Al/(Na + K) in the hydration zone due to hydrogen-alkali exchange between fluid and glass. Hydration behavior at 400°C is transitional between those at 300°C and 600°C, suggesting that the change in hydration mechanism occurs near the glass transition.