skip to main content

Search for: All records

Creators/Authors contains: "Koppers, A. A."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Abstract

    Breakup of the proposed greater Ontong Java Nui large igneous province during the Cretaceous Normal Superchron can be constrained by the opening of the Ellice Basin (EB) separating the Ontong Java and Manihiki Plateaus and the Osbourn Basin separating the Manihiki and Hikurangi Plateaus. Dating of recovered dredged samples using plagioclase40Ar/39Ar and zircon U/Pb geochronology methods indicates that spreading was well underway in the EB by 118 Ma with full spreading rates up to 3X faster than any observed today of 30–45 cm/yr and spreading likely continued until 112‐108 Ma. Ellice Basin samples show diverse geochemical affinities ranging from mid‐ocean ridge basalt (MORB) or Ontong Java‐like to more enriched OIB‐like. Pb and Nd isotopes from six samples contain varying influences from Pacific MORB and possibly Ontong Java. The geochemistry shows a lack of a clear mantle plume influence despite EB's close temporal and spatial relationship to Ontong Java, while some data resemble the Louisville Seamounts. This compositional diversity complements morphological differences among dredge sites and shows that both in situ MORB and younger overprinted features related to the nearby Tuvalu Seamounts were sampled.40Ar/39Ar geochronology confirms the age of International Ocean Discovery Program Site U1365 near the Osbourn Trough (OT) to be 102.60 ± 0.26 Ma (2σ,n = 18). This age constrains the timing of a spreading reorientation event observed in the OB to coincide with a global plate reorganization event around 105 Ma and estimates the cessation of spreading at the OT to 96 Ma.

    more » « less
  2. Abstract

    Twenty‐two sites, subjected to an IZZI‐modified Thellier‐Thellier experiment and strict selection criteria, recover a paleomagnetic axial dipole moment (PADM) of 62.2 ± 30.6 ZAm2in Northern Israel over the Pleistocene (0.012–2.58 Ma). Pleistocene data from comparable studies from Antarctica, Iceland, and Hawaii, re‐analyzed using the same criteria and age range, show that the Northern Israeli data are on average slightly higher than those from Iceland (PADM = 53.8 ± 23 ZAm2,n = 51 sites) and even higher than the Antarctica average (PADM = 40.3 ± 17.3 ZAm2,n = 42 sites). Also, the data from the Hawaiian drill core, HSDP2, spanning the last half million years (PADM = 76.7 ± 21.3 ZAm2,n = 59 sites) are higher than those from Northern Israel. These results, when compared to Pleistocene results filtered from the PINT database ( suggest that data from the Northern hemisphere mid‐latitudes are on average higher than those from the southern hemisphere and than those from latitudes higher than 60°N. The weaker intensities found at high (northern and southern) latitudes therefore, cannot be attributed to inadequate spatiotemporal sampling of a time‐varying dipole moment or low quality data. The high fields in mid‐latitude northern hemisphere could result from long‐lived non‐axial dipole terms in the geomagnetic field with episodes of high field intensities occurring at different times in different longitudes. This hypothesis is supported by an asymmetry predicted from the Holocene, 100 kyr, and 5 million year time‐averaged geomagnetic field models.

    more » « less