Sample descriptions, d-spacing and geochemistry at DSDP Holes 68-501 and 69-504B

Mössbauer analyses were conducted on a sample of saponite selected from DSDP Leg 69 basalt core. The sample was initially placed within a nitrogen-purged container on-board Glomar Challenger approximately three hours after recovery, where it remained until analysis. The Mössbauer data revealed an original, in situ Fe2O3/FeO ratio of 0.46, with both Fe**2+ and Fe**3+ in octahedral coordination. With controlled exposure to air under ambient laboratory storage conditions, the proportion of Fe**3+ increased from an original 30% to 51% over a period of about 11.5 months. The Fe**3+ thus produced remained in octahedral coordination, and no observable changes occurred in the physical appearance of the sample.

(Table 1) K-Ar age and inert-gas-isotopes at DSDP Holes 68-501, 69-504B and 69-505B

Conventional K-Ar ages have been determined and inert-gas abundances have been measured on representative samples of altered rocks from Deep Sea Drilling Project Holes 501, 504B, and 505B in an attempt to correlate their degree of alteration with inert-gas and K-Ar data. Samples taken from the first 60 meters below the sediment/basalt interface give significantly higher ages than would be expected from the magnetic stratigraphy, though at greater depths the calculated ages are in broad agreement with the expected age. The inert gas ratios 20Ne/36Ar, 36Ar/84Kr, and 84Kr/130Xe also show a marked discontinuity at the 60-meter depth, and all these effects are interpreted as being a consequence of low-temperature alteration produced by burial metamorphism and by interaction with sea water (halmyrolysis).

Beryllium-10, mineralogy and grain size distribution of DSDP Hole 68-502B (Table 1)

We present a high-resolution 10Be profile from deep sea sediments (sampled from Hole 502B in the Caribbean sea) that strongly resembles the 10Be record in ice core profiles, particularly the Vostok core from Antarctica. This high-resolution profile revealed occurrences of enhanced 10Be concentrations at about 23-24, 37-39 and 60-65 ka. The excellent match between these peaks appearing in a georeservoir profile other than in polar ice, strengthens the implications that can be inferred from 10Be and provide global markers for chronological correlation of climatic events. The position at low latitude of the studied sediment section is, unlike the case with the high latitude polar regions, excellent for exposing causes of modulation in 10Be production. We interpret the source of the pattern and enhancements, particularly the 37-39 ka peak, of 10Be to be global and do not strictly relate to climatic conditions and/or production rates specific to the polar regions.

Morphological variability of Globorotalia menardii in DSDP Sites 68-502 and 68-503

Variability in the test of Globorotalia menardii during the past 8 million years has been investigated at DSDP Site 502A (Caribbean Sea) and DSDP Site 503A (Eastern Equatorial Pacific). Measurements were made of spire height (delta x), maximum diameter (delta y), the tangent angles of the upper and lower peripheral keels (phi 1, phi 2, respectively), the number of chambers in the final whorl, and the area of the silhouette in keel view. Four morphotypes alpha, beta, gamma, and delta were distinguished. Morphotype alpha was found in strata ranging in age from the Late Miocene through the Holocene. It shows a continuous increase in delta x and delta y until the Late Pleistocene. During and after the final closure of the ancient Central American Seaway (between 2.4 Ma and 1.8 Ma) there was a rapid increase in the area of the test in keel view. At the Caribbean Sea site, morphotype beta evolved during the past 0.22 Ma. It is less inflated than alpha and has a more delicate test. In the morphospace of delta x vs. delta y, morphotypes alpha and beta can be distinguished by a separation line delta y = 3.2 * delta x - 160 (delta x and delta y in µm). Plots of morphotype alpha are below that line, those of beta are above it. Morphotype alpha is taken to be Globorotalia menardii menardii Parker, Jones & Brady (1865) and includes G. menardii 'A' Bolli (1970). Morphotype beta is identified as G. menardii cultrata (d'Orbigny). Morphotypes gamma and delta are extinct Upper Miocene to Pliocene forms which evolved from morphotype alpha. They have a narrower phi 1 angle and more chambers (>=7) than morphotype alpha commonly with 5 to 6 chambers (7 in transitional forms). In contemporaneous samples morphotype delta can be distinguished from gamma by a smaller value of phi 1 and 8 or more chambers in the final whorl. Morphotype gamma is taken to be G. limbata (Fornasini, 1902) and includes the junior synonym G. menardii 'B' Bolli (1970). Morphotype delta is G. multicamerata Cushman & Jarvis (1930). With the exception of the Late Pleistocene development of G. menardii cultrataonly in the Caribbean the morphological changes of G. menardii at DSDP Sites 502A and 503A are similar. The development from the ancestral G. menardii menardii of the G. limbata - G. multicamerata lineage during the Pliocene and of G. menardii cultrata during the Late Pleistocene suggests responses at the two sites to a changing palaeoceanography during and after the formation of the Isthmus of Panama.