Geological Sciences

The extensive flight of uplifted reef terraces which occurs along the Vitiaz strait on the northern flank of the Huon Peninsula in PNG (Papua New Guinea) contains a particularly good record of sea level changes in the last 250 ky. The Huon terraces were the target of an international expedition which took place in July–August 1988. In particular, we searched for suitable samples for U-series dating in a reef complex designated as VII, which is correlated with the last interglacial episode and high sea level stand. This complex is composed of a barrier reef (VIIb), a lagoon, and a fringing reef (VIIa). Twelve corals from these terraces and two corals from the older reef complex VIII were selected for analysis. The petrography, oxygen and carbon isotope compositions, and magnesium and strontium concentrations were determined along with the concentrations and isotopic compositions of uranium and thorium. 230Th-234U ages of the corals with > 99% aragonite, having primary textures, and which show ratios around 0.4 × 10−3 and initial δ234U values close to that of present seawater, appear to be reliable. The “most reliable” ages from complex VII corals fall in two tight groups centered at 118 ky and 134 ky. Corals with δ234U(T) values higher than 160 and ratios substantially lower than seawater are assumed to have undergone diagenetic alteration, which appears to be common in this area. The simplest model for sea level height for terrace VII is a continuous rise between 134 and 118 ky. Alternatively, there may have been two periods of rapid sea level rise. In contrast, in the Bahamas, there is evidence that sea level remained rather constant over the time interval 132 to 120 ky. The absence of ages between 132 and 120 ky in PNG could be the result of changes in the local tectonic uplift rates during that time, or erosion that disrupted the continuous record. In any event, we find no basis for accepting a single brief time for the age of the last interglacial and applying this age as a precise chronometer for worldwide correlation, or as a test of climatic models. The older ages reported here precede the Milankovitch solar insolation peak at 128 ky, and the younger ages are ~ 10 ky after this peak. If the present high-precision data are correct, then it will be necessary to reassess the validity of the Milankovitch theory of climatic changes. The fundamental issue which must now be resolved is a means of identifying coral samples that have not been disturbed by diagenetic processes.

Observations from the first submersible reconnaissance of the Blake Ridge Diapir provide the geological and ecological contexts for chemosynthetic communities established in close association with methane seeps. The seeps mark the loci of focused venting of methane from the gas hydrate reservoir, and, in one location (Hole 996D of the Ocean Drilling Program), methane emitted at the seafloor was observed forming gas hydrate on the underside of a carbonate overhang.

An ancient hydrocarbon seep province of 14 isolated, authigenic carbonate deposits has been identified in fine-grained, deep-marine siliciclastic strata of the Miocene East Coast Basin, North Island, New Zealand. These forearc sediments have been uplifted and complexly deformed into accretionary ridges, adjacent to the still-active Hikurangi convergent margin. Older active and passive margin strata (mid-Cretaceous to Oligocene in age) underlie the Neogene sequence, and contain oil-and gas-prone source rocks.

The Southeast and the US Gulf Coast in particular are notably lacking isotope data in the water cycle despite the fact that moisture transport from the Gulf of Mexico (GOM) has a considerable influence on both regional and continental rainfall patterns.

ABSTRACT The historical record of severe storms on the Gulf Coast extends back only about 100 years. To better predict how the Gulf Coast will be impacted by future climate change and associated changes in storm frequency it is necessary to extend the severe storm record back to the time when global climate was different than today. A 3200-year record of severe storm impacts was reported from Lake Shelby, AL, using sand layers as proxies of storm intensity and frequency (Liu and Fearn, 1993).

Sulfate reduction and anaerobic methane oxidation are the dominant microbial processes occurring in hydrate-bearing sediments at bathyal depths in the Gulf of Mexico where crude oil and methane are advecting through fault conduits to the seafloor. The oil and gas seeps are typically overlain by chemosynthetic communities consisting of thiotrophic bacterial mats (Beggiatoa spp.) and methanotrophic mussels (Bathymodiolus spp.), respectively.

Abstract We developed a new method for reconstructing millennia-long hurricane records from coastal environments that uses Organic Geochemical Proxies (OGPs) of organic carbon and nitrogen concentrations and their δ 13 C and δ 15 N compositions. The new method is independent of presence/absence of sand layers and improves significantly the severe-storm history resolution.

Abstract The Cantwell Formation consists of a lower sedimentary sequence as much as 4000 m thick and an upper volcanic sequence with a maximum thickness of 3750 m that was deposited in the Cantwell basin, south-central Alaska. Previous to this study, the Cantwell basin was interpreted as a Paleogene, nonmarine (mainly fluvial), pull-apart basin that formed in response to dextral, strike-slip displacement on the Denali fault system.

Abstract D-alloisoleucine/L-isoleucine (D/L) ratios were measured in Tridacna gigas (the giant clam) whose ages were calibrated against radiometrically dated coral reef terraces from New Guinea and storm ridges on coral islands from the Great Barrier Reef.

ABSTRACT Stable isotopes of oxygen and hydrogen in water, conventionally reported as δ 18 O and δD, are important tracers of the global, regional, and local hydrologic cycle. The significance of the isotopes tracer aspect in water management has long been recognized by the International Atomic Energy Agency (IAEA) that maintains a global network of sampling stations. Surprisingly, however, there are no stations on the entire US Gulf Coast.

Evidence of hydrocarbon venting within slumped bodies associated with the siliciclastic, dominantly turbiditic, Marnoso-arenacea Formation (Umbria-Romagna structural domain, Romagna Apennine, northern Italy) is documented with sedimentological, faunal, and geochemical data.

A newly acquired, absolute U/Th dated, δ18O record archived in a stalagmite from DeSoto Caverns in Alabama renders highly resolved time series for the past four millennia. Two principal states of variability are discerned in the δ18O record:(1) stable state spanning the intervals before Common Era 2350 to AD 400 and AD 1700 to 2008 that exhibits significant periodicities of 30 and 60 years and (2) unstable state in between containing six major discontinuities alternating with rapid δ18O positive excursions.

Abstract Observations of Holocene sealevel fluctuations contain information on the melting history of the major past ice sheets, on the viscosity of the mantle, and on vertical motions of tectonic origin. Sealevel changes at eleven sites in the Southwest Pacific region have been examined with the following results.

North American freshwater runoff records have been used to support the case that climate flickers were caused by shutdowns of the ocean thermohaline circulation (THC) resulting from reversals of meltwater discharges. Inconsistencies in the documentation of these meltwater switches, however, continue to fuel the debate on the cause/s of the oscillatory nature of the deglacial climate.

Abstract Cores drilled through the carbonate cap of Niue, a mid-oceanic carbonate platform in the South Pacific, contain an unusually well resolved stratigraphic section of the latest Miocene at 35 to 140 m depth below sea level. Intensely leached and chalky intervals associated with pronounced 18 O and 13 C depletions mark the position of dissolution unconformities representing eight episodes of eustatic falls that outstripped the local rate of subsidence.

Abstract Massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid Bathymodiolus sp. containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and U-series isotopes. Stable carbon isotopes and 87 Sr/86 Sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon-rich fluids.

A Miocene methane-seep limestone from the Romagna Apennine (Pietralunga, Italy) was found to contain an extraordinarily well-preserved microbial mat consisting of filamentous fossils. Individual filaments of the lithified Pietralunga mat are 50 to 80 μ m in diameter and resemble the sulfide-oxidizing bacterium Beggiatoa. Mats of sulfur bacteria are common around modern methane-seeps, but have not yet been reported from ancient seep limestones.

Continuous, high-resolution, century to millennium-long records of the El-Nino/Southern Oscillation (ENSO) phenomenon are desired in order to improve the understanding of its mode switches and persistence through time. Niue Island (19º 00'S, 169º 50'W), an uplifted carbonate atoll located on the edge of the Pacific Warm Water Pool (PWWP) and lying close to the heart of the South Pacific Convergence Zone (SPCZ), has the potential to provide such records via speleothems.