Arthur L. Bloom

Quaternary sea level fluctuations on a tectonic coast: New 230Th234U dates from the Huon Peninsula, New Guinea

Emerged coral reef terraces on the Huon Peninsula in New Guinea were reported in a reconnaissance dating study by Veeh and Chappell 1970. Age definition achieved was not good for several important terraces, and we report here a series of new 230Th234U dates, which further clarify the history of late Quaternary eustatic sea level fluctuations. More than 20 reef complexes are present, ranging well beyond 250,000 yr old: we are concerned with the seven lowest complexes. Major reef-building episodes dated by 30Th234U are reef complex I at 5–9 ka (kilo anno = 1000 yr), r.c. IIIb at 41 ka (four dates), r.c. IV at 61 ka (four dates), r.c. V at 85 ka (two dates), r.c. VI at 107 ka (two dates), and r.c. VII at 118–142 ka. Complex II was previously dated by 14C at 29 ka: this age has not yet been confirmed, and may be only a lower limit. The reef crests were built during or immediately before intervals of sea level maxima, when rates of rising sea level and tectonic uplift briefly coincided. The culmination of each reef-building episode was only a few thousand years in duration, and multiple dates from the same reef complex generally group within the statistical errors of the individual dates. Several methods can be used to estimate the altitude of each sea level maximum relative to present sea level. The least complicated is to calculate mean tectonic uplift rate for each profile of the terraces, and use the mean rate to calculate the tectonic displacement of each dated reef complex on that profile. The difference between the present altitude of a reef complex and its calculated tectonic uplift gives the paleosea level at the time the reef grew. We estimate uplift rates for six surveyed sections by calibrating against published paleosea level estimates from Barbados and elsewhere, viz 125 ka, paleosea at +6 m; 103 ka, −15 m; 82 ka, −13 m. For each section the individual uplift rates for reefs V, VI, and VIIb are within 5% of their section means. Using the mean rates. paleosea level estimates for reef crests II, IIIB, and IV are made for each section. Consistency of estimates between sections is good, giving −28 m for the 60 ka paleosea level, around −38 m for the 42 ka level and −41 m for the 28 ka level (if the age is older the paleosea level would be lower. Using the mean uplift rates, the 82 ka and 103 ka paleosea levels are also estimated for each section: all individual estimates are plotted graphically, and a sea level curve drawn. The reef stratigraphy indicates sea level lowerings between each dated reef crest: the crests probably represent the interstadials of the Wisconsin (Wurm, Weichsel) Glaciation, and intervening lower levels correspond to stadials. Since the last time of eustatic sea level higher than the present (about 125 ka), five sea level maxima occurred at roughly 20-ka intervals, none being as high as the present.

A Large Drop in Atmospheric 14C/12C and Reduced Melting in the Younger Dryas, Documented with 230Th Ages of Corals.

Paired carbon-14 (14C) and thorium-230(230Th) ages were determined on fossil corals from the Huon Peninsula, Papua New Guinea. The ages were used to calibrate part of the 14C time scale and to estimate rates of sea-level rise during the last deglaciation. An abrupt offset between the 14C and 230Th ages suggests that the atmospheric 14C/12C ratio dropped by 15 percent during the latter part of and after the Younger Dryas (YD). This prominent drop coincides with greatly reduced rates of sea-level rise. Reduction of melting because of cooler conditions during the YD may have caused an increase in the rate of ocean ventilation, which caused the atmospheric 14C/12C ratio to fall. The record of sea-level rise also shows that globally averaged rates of melting were relatively high at the beginning of the YD. Thus, these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the Mississippi to the St. Lawrence River triggered the YD event.

Estimation of Discharge From Three Braided Rivers Using Synthetic Aperture Radar Satellite Imagery: Potential Application to Ungaged Basins

Analysis of 41 ERS 1 synthetic aperture radar images and simultaneous ground measurements of discharge for three large braided rivers indicates that the area of active flow on braided river floodplains is primarily a function of discharge. A power law correlation is found between satellite-derived effective width We and discharge Q, where We is the water surface area within a braided reach divided by the reach length. Synthetic values of We and Q generated from a cellular automata model of stream braiding display a similar power law correlation. Power functions that are fit through plots of We and Q represent satellite-derived rating curves that can subsequently be used to estimate instantaneous river discharge from space, with errors ranging from tens to hundreds of cubic meters per second. For ungaged rivers, changes in relative discharge can be determined from satellite data alone to determine the shape and timing of annual flows in glacierized basins. Absolute discharge can probably be estimated within a factor of 2. More accurate estimates will require either (1) one or more ground measurements of discharge acquired simultaneously with a satellite image acquisition, or (2) successful parameterization of known morphologic controls such as total sinuosity 5;P, valley slope, bank material and stability, and braid channel hydraulic geometry. Values of total sinuosity 5;P derived from satellite imagery and field measurements from two rivers of braid channel width, depth, velocity, water surface slope, and bed material grain size indicate that while the shape of satellite-derived We-Q rating curves may be influenced by all of these variables, the sensitivity of flow area to changing discharge is most dependent upon the degree of braiding. Efforts to monitor river discharge from space will be most successful for intensely braided rivers with high values of total sinuosity. Subsampling of existing daily discharge records from the Iskut River suggests that satellite return times of about 1 week are sufficient for approximating the shape and timing of the seasonal hydrograph in large, glacierized basins. Although errors are large, the presented technique represents the only currently available way to estimate discharge in ungauged braided rivers.

Pleistocene Shorelines: A New Test of Isostasy

Recent geophysical studies of glacial-isostatic deformation overlook the same fact that was overlooked in earlier studies: the loads of ice that were applied to continents during glacial ages were not loads added to the Earth9s crust, but were loads transferred from the oceanic 70 percent to the glaciated 5 percent of the crust by the hydrologic cycle. A realistic model of glacial isostasy must be represented by a balance, in which glaciated areas totaling about 5 percent of the Earth9s surface have loads of 140 to 170 bars added or removed on a time scale of 10 4 years, while synchronously the oceanic 70 percent of the Earth9s surface has a load of 10 to 12 bars or more removed or added on a similar time scale. The subcrustal mass transfer involved in such a balance is not well represented by harmonic equations, for the water and ice loads are not symmetrically disposed on the Earth9s surface. The suitability of the proposed balance model depends on whether the ocean floor will respond isostatically to a load of as little as 10 bars. Evidence from Lake Mead, Arizona, and Lake Bonneville, Utah, suggest that the continental crust, at least, does deform under a regional load of 10 bars or less. Pleistocene marine shorelines offer a means of testing the balance model of isostasy. If the ocean floor deforms under water loads, the amount of postglacial submergence of a coast should be in part a function of the regional proximity of deep ocean water. Coasts with nearby ocean water more than 100 m deep had the load of water from the postglacial rise of sea level added early and close; coasts bordering shallow seas had the load added late and generally far offshore. An averaging technique to show the regional water load on the Atlantic coast of the northeastern United States provides a basis for comparing the submergence histories of several localities with the average water depth offshore. In general, the amount of submergence is proportional to the proximity of deep water. Oceanic islands should record different Pleistocene shoreline levels than continental coasts. Local, detailed, late Pleistocene histories of a variety of coasts will provide a test of isostasy better than the familiar test of postglacial uplift.

Submergence of the Connecticut Coast

Radiocarbon-dated samples show that the Connecticut coast has submerged about 9 feet in the last 3000 years and about 33 feet in the last 7000 years. The rate of submergence is similar to rates reported from other coasts. The finding strengthens the hypothesis that a worldwide postglacial rise of sea level is the cause.

Peat Accumulation and Compaction in a Connecticut Coastal Marsh

ABSTRACT A sedge-peat bed, upon which a maximum of 35 feet of postglacial estuarine mud has been deposited, has been compressed in 7000 years to between 13 and 44 percent of its original thickness. By extrapolation an estimate may be made that the ultimate thickness will range between 10 and 20 percent of original thickness, depending on the amount of admixed sand. Such percentages are somewhat less than the reported ratio of compaction of peat to coal of about 3:1. However, these percentages represent total compaction beginning with original plant debris, whereas the peat-to-coal ratio of compaction does not include the earliest stage of alteration and consolidation.

Sedimentation Rates on Tidal Salt Marshes in Connecticut

ABSTRACT Rates of sediment accretion from 1963-1973 on five high marsh sites on the Connecticut coast of Long Island Sound ranged from 2.0 mm/yr to 6.6 mm/yr. The rates are correlated with tidal range; the highest sedimentation rates are on marshes with the greatest tidal range. The greater the tidal range, the larger are the deviations of high-tide level. Thus, greater net flooding occurs on high marshes with greater tide ranges and may cause the observed high accretion rates. Over ten years no measurable compaction has taken place within the near-surface sediment. Years with fewer than average storms show less sediment accretion. A sedimentation rate of 17.1 mm/yr from 1963-1973 was measured where Spartina patens salt marsh is giving way to Phragmites communis.

Estimation of discharge from braided glacial rivers using ERS 1 synthetic aperture radar: first results

Using multitemporal ERS 1 synthetic aperture radar (SAR) satellite imagery and simultaneous ground measurements of streamflow, a strong correlation (R2 = 0.89) was found between water surface area and discharge for a braided glacial river in British Columbia, Canada. Satellite-derived effective width (We) was found to vary with discharge (Q) as We = 27.5Q0.42, where We is defined as the total water surface area within a 10 km × 3 km control section, divided by the section length. This “area/discharge rating curve” yields instantaneous discharge estimates with a mean error of ±275 m3/s for ground-measured flows that ranged from 242 to 6350 m3/s.

Paludal Stratigraphy of Truk, Ponape, and Kusaie, Eastern Caroline Islands

Swampy coastal plains up to one-half a kilometer in width that fringe the high islands of the Eastern Carolines have been cited as evidence for recent emergence, but the stratigraphy and morphology of the tidal swamps are more consistent with a history of shoreline progradation during decelerating submergence. The mangrove, Nipa Phragmites, and taro swamps have accumulated no more than 3 m of peat, and generally only about 2 m. Beneath the peat or muck is either: (a) a level coral bench at approximate low-tide level, (b) coarse, well-sorted, shelly beach sand, sometimes in the form of low beach ridges across the shallowest marshes, or (c) brown, organic-rich mud with concentrations of immature pelecypod shells, deposited in former protected bays or estuaries. Estuarine mud was cored to a depth of 11.6 m in many swamps without finding bottom. Radiocarbon dates from intertidal peat layers that overlie former hillslopes of weathered volcanic rock demonstrate submergence of 6.2 m in the last 6500 years. The rate of submergence decreased abruptly during the final 1.7 m of submergence, which permitted extensive progradation by tidal swamps over former reef flats or into former muddy estuaries. By this interpretation, submergence has averaged only about 0.4 cm per 1000 years since 4100 years B.P., in contrast to the rate of 1.9 m per 1000 years between 4100 and 6500 years B.P. The high basaltic islands of Truk, Ponape, and Kusaie are conveniently spaced among the atolls of the Eastern Caroline Islands, across a total distance of nearly 1300 km. The uniformity of their paludal stratigraphy indicates a common late-Pleistocene and recent history for the entire region of a million sq mi or more, independent of the contradictory evidence about postglacial sea levels that has been reported from various atolls.

The Reef Flat and ‘Two-Meter Eustatic Terrace’ of Some Pacific Atolls

Characteristics of Indo-Pacific coral reefs long cited as evidence of a recent decline in sea level include: (1) the reef “flat” or pavement, (2) intertidal and supratidal flat erosion remnants of cemented coral rubble, and (3) erosion of the shores of reef islets. Our examination of 25 low islands and 8 high islands in the Caroline and Marshall Islands during the 1967 Scripps Institution of Oceanography Expedition CARMARSEL leads us to conclude that the reef flats in the western Pacific generally are not simply erosional platforms but represent an equilibrium surface between upward accretion by reef-building organisms and erosion at the mean level of low tides. Intertidal and supratidal coral rubble and calcareous sand accumulate above low-tide level during storms forming all of the visible land in the visited low islands. This rubble is currently being cemented and welded to the reef flats between tidal limits, and probably is also being cemented as beachrock and island conglomerate below the fluctuating water table of the cays. Topographically highest cemented carbonate rocks of the Caroline and Marshall Islands are uniformly less than 2 m above the adjacent reef flat, and entirely in the intertidal zone, hence, within the tidal range of present sea level. Flat horizontal surfaces of small extent occur on shore rocks, locally truncating inclined beds of beach rock. These surfaces roughly correspond to the upper limit of contemporary cementation somewhat modified by gravel scour. Comparison with rock platforms of a slightly elevated atoll of the Tuamotus, Raroia, indicates that the Marshall and Caroline Islands do not display expectable features of uplift or of subsiding sea level.