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Stratigraphic hiatuses and solution unconformities in the subsurface of Enewetak Atoll, northern Marshall Islands, record periods of atoll emergence during low stands of sea level. Changes in sea level are also recorded in the atoll subsurface by variations in the rate of sediment accumulation relative to the subsidence rate of the underlying volcanic edifice. Past sea levels can be derived from atoll stratigraphy by correcting the present depth of dated subsurface horizons for thermal subsidence and lithospheric flexure since the time of deposition. A correction for depositional paleodepth may also be necessary. As a result of erosion and nondeposition during periods of emergence, the history of sea level derived in this manner is discontinuous. Past sea levels derived from atoll stratigraphy can only be estimated to within ±50 m relative to present sea level owing to uncertainties in the corrections for subsidence and flexure; however, the minimum magnitude of sea level falls estimated from stratigraphic hiatuses can be estimated to within ±10 m. Owing to limited fossil-based age resolution, only long-term sea level trends can be deduced from sediments dated by means of biostratigraphy. Based on the biostratigraphic ages of subsurface horizons at Enewetak, we can discern very little long-term change in sea level from late Eocene through late Oligocene, a rise to ∼110 m above present sea level in the early Miocene, a long-term fall of ∼170 m through middle and late Miocene time, and a long-term rise of ∼60 m from the end of the Miocene to present. Resolution of the sea level history recorded beneath mid-ocean atolls may be improved by determining the age of shallow-marine carbonates by means of strontium isotope stratigraphy. Our interpretation of past sea levels based on 87Sr/86Sr chronostratigraphy from Enewetak confirms the long-term sea level trends inferred from biostratigraphic subsurface ages. In addition, we interpret three Oligocene sea level falls with minimum magnitudes of 30–50 m at ∼35, 33–30, and ∼25 Ma and four Miocene sea level falls greater than 30–95 m at 16–14, ∼12, ∼10, and ∼5 Ma. These estimates of the timing and magnitude of Oligocene and Miocene sea level changes, derived from Enewetak Atoll stratigraphy, are compared with sea level histories derived from continental margin stratigraphy and from ice volume changes inferred from deep-sea foraminifera δ18O records.

The Ontong Java Plateau, with its thick, capping sequence of Cretaceous and Cenozoic pure pelagic carbonate sediments, forms an ideal setting for the study of the acoustic properties of this lithology on an oceanic rise. Borehole logs, recorded on Deep Sea Drilling Project leg 89 at site 586, provided detailed data on in situ acoustic properties of Pleistocene to early Miocene sediments to a depth of 623 m below seafloor. Comparison of these logging results and the sonobuoy-based results of Johnson et al. (1978) with previous laboratory measurements from the Ontong Java Plateau shows that velocity/depth functions determined from the logging and sonobuoy methods are concordant but diverge significantly from functions derived from laboratory measurements. Log densities and compressional velocities exceed those measured by laboratory techniques; the density discrepancy is strongly influenced by laboratory method. The differences between log and laboratory compressional velocities are greater than and extend to greater depths than those between densities. These differences can be attributed to reductions in the frame bulk modulus and dynamic rigidity, caused by the removal of overburden pressure in the absence of significant porosity rebound. Agreement of site 586 log velocities with velocities derived from the earlier sonobuoy measurements across the plateau argues for the interpretation that both methods measure in situ values. The disagreement between the site 586 log results and the sonobuoy results with both the empirical velocity/depth function of Carlson et al. (1986) and the empirical velocity/porosity function of Raymer et al. (1980) supports the conclusion that pelagic carbonate sediments on oceanic plateaus and rises have unique acoustic properties, primarily arising from the presence of intraparticle porosity, and should not be grouped with other oceanic lithologies in acoustic modeling studies.

Geological and geophysical studies along the entire length of the Line Islands were undertaken in order to test the hot spot model for the origin of this major linear island chain. Volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls. Volcanic rocks from the Line Islands are alkalic basalts and hawaiites. In addition, a tholeiitic basalt and a phonolite have been recovered from the central part of the Line chain. Microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high TiO2 (1.0–4.0 wt %) and Al2O3 (3.4–9.1 wt %) and are of alkaline to peralkaline affinities. Major element compositions of the Line Islands volcanic rocks are very comparable to Hawaiian volcanic rocks. Trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas; the Line Islands lavas are very much unlike typical mid-ocean ridge or fracture zone basalts. Dating of these rocks by 40Ar-39Ar, K-Ar, and paleontological methods, combined with Deep Sea Drilling Project data from sites 165, 315, and 316 and previously dated dredged rocks, provide ages of volcanic events at 20 localities along the chain from 18°N to 9°S, a distance of almost 4000 km. All of these dates define mid-Cretaceous to late Eocene edifice or ridge-building volcanic events. Eocene volcanic events took place from 15°N to 9°S, and Late Cretaceous events took place from 18°N to 9°S. In the southern Line Islands both Cretaceous and Eocene events took place on the same edifice or ridge, indicating recurrent volcanism at a single locality. The irregular distribution of atolls in the chain, the fact that Late Cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern Line Islands, and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t1/2 related subsidence path. Magnetic surveys of 11 seamounts show that four of the seamounts, from the central Line chain, give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of Cretaceous seamounts. These four poles agree with other paleomagnetic data of middle-late Eocene-early Oligocene age from the Pacific. One of these four seamounts yielded a 40Ar-39Ar total fusion age of 39 Ma. Because the poles of all four seamounts fall into a tight group we infer that they are probably of middle-late Eocene age to early Oligocene age. The other three seamounts as well as one seamont from the Line Islands analyzed previously all give virtual geomagnetic poles which agree with Late Cretaceous paleomagnetic data from the Pacific. Of these four seamounts, three give Late Cretaceous 40Ar-39Ar ages ranging from 71 to 85 Ma; another, in the southern Line Islands, is interpreted, on paleontological evidence, to be of Late Cretaceous age. The origin of the Line Islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots. The single hot spot model cannot account for all of the volcanic edifices in the Line Islands, although it does explain a general age progression of 9.6±0.4 cm/yr from north to south along the chain derived from a number of dated edifices. The four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data. The petrologic data argue against a mid-ocean ridge or transform fault origin proposed by earlier workers. The complex volcanic province represented by the Line Islands remains a challenge to existing models for the origin of midplate volcanism in the Pacific. An atoll drilling program which could determine edifice building histories, paleolatitudes of seamount formation, and subsidence rates and patterns in the Line Islands is needed.

Laboratory studies have been carried out to determine the causes of velocity and attenuation anisotropy in pelagic carbonate sediments from Deep Sea Drilling Project site 288 on the Ontong-Java Plateau. Compressional velocity Vp was measured under independently controlled pore pressure and confining pressure. Velocity anisotropy for compressional waves (horizontal velocity > vertical velocity) is found to decrease with increasing effective pressure, i.e., with closure of pores. Previous studies have suggested that velocity anisotropy in pelagic carbonate sequences is due largely to preferred vertical orientation of calcite c axes. The present results indicate that preferred horizontal orientation of grain contacts and flat pores is also a significant cause of the observed anisotropy for many samples. Significant anisotropy in ultrasonic attenuation for compressional (Qp−1) and shear (Qs−1) waves is observed such that the losses are greatest for propagation modes that can generate relative motion across horizontal grain contacts or flat horizontal pores. The results are discussed in terms of the possible mechanisms of dissipation.

This volume brings together research carried out within the Cenozoic Paleoceanography Project (CENOP), a goal of which was to generate paleoceanographic reconstructions of the Miocene Epoch, 24 to 5 Ma. On the basis of stable isotope and quantitative micropaleontological studies, the authors have related these data bases to parameters and processes of the Miocene Ocean, including paleogeography, stratigraphy, circulation, water mass distributions, temperature variability, vertical water mass structure, biogeography, and biotic evolution. That these authors attained their goal is attested to by the quality and scope of the 14 chapters presented in this valuable treatise. The Miocene Epoch was chosen because it was in this interval of time that major ice sheets became a permanent feature of Earth's surface. The presentation of this stunning amount of data and interpretation is very well organized for the researcher as well as for the students and professors who will surely use the book as a basis for seminars.