Paleoceanography of the eastern equatorial Pacific during the Neogene: Synthesis of Leg 138 drilling results

The primary objective of Leg 138 was to provide detailed information about the ocean\u27s response to global climate change during the Neogene. Two north-south transects were drilled (95° and 110°W) within the region of equatorial divergence-driven upwelling (and thus high accumulation rates and resolution) and spanning the major equatorial ocean current boundaries (and thus recording a high-amplitude signal of the response of the sediment to climatically and/or tectonically driven changes in ocean circulation). The Neogene is marked by a number of well-known climatic and tectonic events (the closing of the Isthmus of Panama, the onset of North Atlantic Deep Water (NADW), the rapid uplift of the Himalayas, the major intensification of Northern Hemisphere glaciation), and the response of the ocean before and after these events was a key focus of Leg 138 drilling. To address these objectives at the highest resolution possible, the Leg 138 scientific staff developed a number of new shipboard strategies and analytical procedures. These included the real-time analysis of the near-continuous gamma ray attenuation porosity evaluator (GRAPE) and susceptibility profiles produced by the multisensor track (MST) on unsplit cores to monitor core recovery and, if necessary, to modify the drilling strategy to ensure proper offset of coring gaps; the collection of near-continuous color reflectance data on split cores; the logging of the first hole drilled at each site to optimize drilling and sampling strategies for subsequent holes; and the use of multiple continuous records to unambiguously construct complete composite sections for each site. The complete, continuous records provided by the GRAPE (with a temporal resolution of often yr), in conjunction with an excellent microfossil stratigraphy and often excellent magnetostratigraphy, allowed for astronomical tuning of the stratigraphic record and resulted in a set of internally consistent, high-resolution age models that provide a secure, absolute time scale for the past 6 m.y. For the period before 6 m.y., the absolute time calibration is less secure, but it is still better than any previously offered. The high-resolution stratigraphic framework of Leg 138 provided new insight into the previously ambiguous tectonic history of the region. By assuming that maximum sedimentation rates along the north-south transect would be expected at the equator, the Leg 138 stratigraphy supports the 1985 work of Cox and Engerbretson, which calls for two different poles of rotation of the Pacific Plate during the interval 0-20 Ma. The Leg 138 plate reconstructions also support several previously hypothesized ridge crest jumps and a slowing of the absolute motion of the Nazca Plate at about 5 Ma. Although Leg 138 data that predates about 13 Ma is limited, the impression that one can gain from these data is that the eastern equatorial Pacific was characterized by relatively high carbonate concentrations and accumulation rates before about 11 Ma. This pattern was interrupted occasionally by rapid massive outpourings of near-monospecific laminated diatom oozes that probably represent the formation of massive mats along strong surface-water fronts. The laminated diatom oozes (LDO) continue to be present in the Leg 138 record (many of them being expressed as seismic reflections) until about 4.4 Ma. Carbonate accumulation rates begin to decline slowly between 11 and 9.8 Ma, when, at about 9.5 Ma, a near-complete loss of carbonate (the carbonate crash ) takes place everywhere in the Leg 138 region (and beyond), except at the westernmost sites close to the equator. The carbonate crash was a time of fundamental change for the eastern equatorial Pacific, and perhaps for most of the ocean basins. Unlike many of the carbonate variations that precede and postdate it, this crash represents a major dissolution event whose effects can be traced seismically in the central and western Pacific. The changes in bottom-water chemistry associated with this event (or series of events) appear to be related to the early phases of the closing of the Panama Gateway. The role of NADW initiation and intensification for controlling carbonate accumulation in the eastern equatorial Pacific is still not resolved; however, ocean modeling demonstrates that the closing of the Panama Gateway may also have a direct influence on NADW production. Therefore, the effects of changes in the Panama Gateway sill depth and the production of NADW may be manifested in the history of eastern equatorial Pacific sedimentation. The carbonate crash was followed by a recovery of the carbonate system (except in the Guatemala and Peru basins, which never recovered) that led up to the late Miocene/early Pliocene sedimentation rate maxima, during which equatorial sedimentation rates are as much as five times greater than those of the late Pliocene or Pleistocene. Examination of modern productivity/preser vation relationships implies that the sedimentation rate maximum was the result of enhanced productivity. The distribution of eolian sediments and isotopic gradients, along with an analysis of the modes of variance in carbonate deposition over the last 6 m.y., suggest a more northerly position of the Intertropical Convergence Zone (ITCZ), a stronger north-south gradient across the equator, and a more zonal circulation focused along the equator during the time of maximum sedimentation. The mechanisms suggested for these changes in circulation patterns include the response of the eastern equatorial Pacific to the closing of the Isthmus of Panama, as well as a global increase in the flux of Ca and Si into the oceans, a possible response to evolution of the Himalayas and the Tibetan Plateau. In an effort to understand the response of the climate system to external (orbital) forcing, 6-m.y.-long, continuous records of carbonate (derived from GRAPE), δ 1 8 and insolation were analyzed and compared. Evolutionary spectral calculations of the variance and coherence among these records indicate that the insolation record is dominated by precessional frequencies, but that the relative importance of the two precessional frequencies has changed significantly over the last 6 m.y. In general, precessional forcing is not found in the carbonate or isotopic records. In the tilt band, however, a linear response is present between solar forcing and the carbonate and isotope records over some intervals. The carbonate record appears to be tightly coupled to the tilt component of insolation before about 1.9 Ma; however, the isotope record does not begin to show sensitivity to orbital tilt until about 4.5 Ma, the time of significant changes in sedimentation patterns in the eastern equatorial Pacific. Only during the last 500,000 yr do all frequencies respond in a similar manner; we also see a marked increase in the response of the isotopic record to orbital forcing (including 100,000- and 400,000-yr periods)

Microstrip resonator for microwaves with controllable polarization

In this work the authors implemented a resonator based upon microstrip cavities that permits the generation of microwaves with arbitrary polarization. Design, simulation, and implementation of the resonators were performed using standard printed circuit boards. The electric field distribution was mapped using a scanning probe cavity perturbation technique. Electron spin resonance using a standard marker was carried out in order to verify the polarization control from linear to circular.Comment: 3 pages, 3 figures, submitted to Appl. Phys. Let

Sensor-assisted Video Mapping of the Seafloor

In recent years video surveys have become an increasingly important ground-truthing of acousticseafloor characterization and benthic habitat mapping studies. However, the ground-truthing and detailed characterization provided by video are still typically done using sparse sample imagery supplemented by physical samples. Combining single video frames in a seamless mosaic can provide a tool by which imagery has significant areal coverage, while at the same time showing small fauna and biological features at mm resolution. The generation of such a mosaic is a challenging task due to height variations of the imaged terrain and decimeter scale knowledge of camera position. This paper discusses the current role of underwater video survey, and the potential for generating consistent, quantitative image maps using video data, accompanied by data that can be measured by auxiliary sensors with sufficient accuracy, such as camera tilt and heading, and their use in automated mosaicking techniques. The camera attitude data also provide the necessary information to support the development of a video collage. The collage provides a quick look at the large spatial scale features in a scene and can be used to pinpoint regions that are likely to yield useful information when rendered into high-resolution mosaics. It is proposed that high quality mosaics can be produced using consumer-grade cameras and low-cost sensors, thereby allowing for the economical scientific video surveys. A case study is presented with the results from benthic habitat mapping and the ground-truthing ofseafloor acoustic data using both real underwater imagery and simulations. A computer modeling of the process of video data acquisition (in particular on a non-flat terrain) allows for a better understanding of the main sources of error in mosaic generation and for the choice of near-optimal processing strategies. Various spatial patterns of video survey coverage are compared and it is shown that some patterns have certain advantages in the sense of accumulated error and overall mosaic accuracy

Lande g-tensor in semiconductor nanostructures

Understanding the electronic structure of semiconductor nanostructures is not complete without a detailed description of their corresponding spin-related properties. Here we explore the response of the shell structure of InAs self-assembled quantum dots to magnetic fields oriented in several directions, allowing the mapping of the g-tensor modulus for the s and p shells. We found that the g-tensors for the s and p shells show a very different behavior. The s-state in being more localized allows the probing of the confining potential details by sweeping the magnetic field orientation from the growth direction towards the in-plane direction. As for the p-state, we found that the g-tensor modulus is closer to that of the surrounding GaAs, consistent with a larger delocalization. These results reveal further details of the confining potentials of self-assembled quantum dots that have not yet been probed, in addition to the assessment of the g-tensor, which is of fundamental importance for the implementation of spin related applications.Comment: 4 pages, 4 figure

1995, Spatial and temporal variability of late Neogene equatorial Pacific carbonate

High-resolution, continuous records of GRAPE wet bulk density (a carbonate proxy) from Ocean Drilling Program Leg 138 provide one the opportunity for a detailed study of eastern equatorial Pacific Ocean carbonate sedimentation during the last 6 m.y. The transect of sites drilled spans both latitude and longitude in the eastern equatorial Pacific from 90° to 110°W and from 5°S to 10°N. Two modes of variability are resolved through the use of Empirical Orthogonal Function (EOF) analysis. In the presence of large tectonic and climatic boundary condition changes over the last 6 m.y., the dominant mode of spatial variability in carbonate sedimentation is remarkably constant. The first mode accounts for over 50% of the variance in the data, and is consistent with forcing by equatorial divergence. This mode characterizes both carbonate concentration and carbonate mass accumulation rate time series. Variability in the first mode is highly coherent with insolation, indicating a strong linear relationship between equatorial Pacific car bonate sedimentation and Milankovitch variability. Frequency domain analysis indicates that the coupling to equatorial divergence in carbonate sedimentation is strongest in the precession band (19-23 k.y.) and weakest though present at lower frequencies. The second mode of variability has a consistent spatial pattern of east-west asymmetry over the past 4 m.y. only; prior to 4 Ma, a different mode of spatial variability may have been present, possibly suggesting influence by closure of the Isthmus of Panama or other tectonic changes. The second mode of variability may indicate influence by CaCO3 dissolution. The second mode of variability is not highly coherent with insolation. Comparison of the modes of carbonate variability to a 4 m.y. record of benthic δ 1 8 indicates that although overall correlation between carbonate and δ 1 8 is low, both modes of variability in carbonate sedimentation are coherent with δ 1 8 changes at some frequencies. The first mode of carbonate variability is coherent with Sites 846/849 δ 1 8 at the dominant insolation periods, and the second mode is coherent at 100 k.y. during the last 2 m.y. The coherence between carbonate sedimentation and δ 1 8 in both EOF modes suggests that multiple uncorrelated modes of variability operated within the climate system during the late Neogene

On the spectrum of Farey and Gauss maps

In this paper we introduce Hilbert spaces of holomorphic functions given by generalized Borel and Laplace transforms which are left invariant by the transfer operators of the Farey map and its induced version, the Gauss map, respectively. By means of a suitable operator-valued power series we are able to study simultaneously the spectrum of both these operators along with the analytic properties of the associated dynamical zeta functions.Comment: 23 page

On the Use of Historical Bathymetric Data to Determine Changes in Bathymetry: An Analysis of Errors and Application to Great Bay Estuary, NH

The depth measurements that are incorporated into bathymetric charts have associated errors with magnitudes depending on the survey circumstances and applied techniques. For this reason, combining and comparing depth measurements collected over many years with different techniques and standards is a difficult task which must be done with great caution. In this study we have developed an approach for comparing historical bathymetric surveys. Our methodology uses Monte Carlo modelling to account for the random error components inherited in the data due to positioning and depth measurement uncertainties

Dynamic heterogeneities in critical coarsening: Exact results for correlation and response fluctuations in finite-sized spherical models

We study dynamic heterogeneities in the out-of-equilibrium coarsening dynamics of the spherical ferromagnet after a quench from infinite temperature to its critical point. A standard way of probing such heterogeneities is by monitoring the fluctuations of correlation and susceptibility, coarse-grained over mesoscopic regions. We discuss how to define fluctuating coarse-grained correlations (C) and susceptibilities (Chi) in models where no quenched disorder is present. Our focus for the spherical model is on coarse-graining over the whole volume of $N$ spins, which requires accounting for N^{-1/2} non-Gaussian fluctuations of the spin. The latter are treated as a perturbation about the leading order Gaussian statistics. We obtain exact results for these quantities, which enable us to characterise the joint distribution of C and Chi fluctuations. We find that this distribution is qualitatively different, even for equilibrium above criticality, from the spin-glass scenario where C and Chi fluctuations are linked in a manner akin to the fluctuation-dissipation relation between the average C and Chi. Our results show that coarsening at criticality is clearly heterogeneous for d>4 and suggest that, as in other glassy systems, there is a well-defined timescale on which fluctuations across thermal histories are largest. Surprisingly, however, neither this timescale nor the amplitude of the heterogeneities increase with the age of the system, as would be expected from the growing correlation length. For d<4, the strength of the fluctuations varies on a timescale proportional to the age of the system; the corresponding amplitude also grows with age, but does not scale with the correlation volume as might have been expected naively.Comment: 39 pages, 9 figures, version for publication in J. Stat. Mech. Shortened by cutting all technical details in section 6, with minor corrections elsewher