Sedimentary cycles on the Exmouth Plateau

Sedimentary cycles are observed in the nearly complete Lower Cretaceous to Eocene pelagic carbonates at Site 762 on the Exmouth Plateau off northwest Australia. The high-frequency cycles of variable clay and foraminifers in nannofossil chalk appear as color cycles repeating on a scale of centimeters to meters in thickness. Measured cycle thickness indicate that the dominant cycles appear to be related to the precession and obliquity periods. To evaluate the high-frequency variance observed on the gamma-ray curve, spectral analysis of the log was performed on two intervals: 260 to 365 mbsf in the Cenozoic, and 555 to 685 mbsf in the Mesozoic. Average Cenozoic sedimentation rates of 10.5 m/m.y. are high enough to show that variance is present in the full suite of eccentricity bands (413-123-95 k.y.). Spectral analysis of the Mesozoic section failed to produce dominant peaks that could be correlated to predicted orbital periods. The bioturbation observed in the cores in this interval may be responsible for diluting the signal and producing high-frequency noise, which is manifested in the spectra as low, broad amplitude peaks. Orbital forcing may be affecting sedimentation on the Exmouth Plateau by influencing cycles of increased carbonate production or dissolution. Alternatively, clay abundance cycles may be related to eolian deposition during cycles of increased aridity in western Australia. Four low-frequency events were also identified at Site 762 from the core and log data. The duration of these events is approximately 13 m.y., and the conformable boundaries of these sedimentary cycles correlate with observed nondepositional surfaces in other wells in western Australia. The causal mechanism for the onset of these events may be eustatic, but alternatively may be regional tectonism with associated circulation pattern changes.

Ages, sedimentation rates and geochemical composition of ODP Site 162-983 sediments

A geochemical study of sediments from Ocean Drilling Program Site 983 was conducted to examine low-frequency variations in carbonate content as expressed by blue-band reflectance (450-500 nm) over the last 1.2 Ma. Sedimentary percent organic carbon, percent carbonate, and excess barium (Ba[ex]) were used as the primary tools to evaluate the factors responsible for these long-term changes. We observe positive correlation between the mass-accumulation rate of various biogenic components and the mass-accumulation rate of Ba(ex), especially in sediments younger than ~600 ka. Deeper in the section (~600-1200 ka), the correlation between Ba(ex) and the other biogenic tracers is weak. The lack of correlation between Ba(ex) and biogenic carbonate likely results either from a higher supply of terrigenous material at that time (which confounds Ba[ex] estimation), or remobilization of Ba resulting from low pore-water sulfate ion concentrations, or both. Nonbiogenic sediments at Site 983, represented by Th, K2O, and the molar Ti/Al ratio, exhibit cyclic variations that represent mixing between continental and oceanic (i.e., basaltic) terrigenous sources. The timing of these cycles matches that of the major glacial-interglacial cycles, which suggests that they result from the supply of continental material as ice-rafted debris during glacial periods and fine-grained basaltic material by bottom currents during interglacial periods. Given these observations, the most likely causes for the low-frequency carbonate variations observed in the Site 983 sediments are shifts in surface productivity and, to a lesser extent, dilution by the input of terrigenous material.

XRD mineralogy of sediments from the Greenland east coast

Quantitative X-Ray Diffraction (qXRD) analysis of the <2 mm sediment fraction from surface (sea floor) samples, and marine sediment cores that span the last 10-12 cal ka BP, are used to describe spatial and temporal variations in non-clay mineral compositions for an area between Kangerlussuaq Trough and Scoresby Sund (?67°-70°N), East Greenland. Bedrock consists primarily of an early Tertiary alkaline complex with high weight% of pyroxene and plagioclase. Farther inland and to the north, the bedrock is dominantly felsic with a high fraction of quartz and potassium feldspars. Principal Component (PC) analysis of the non-clay sediment compositions indicates the importance of quartz and pyroxene as compositional end members, with an abrupt shift from quartz and k-feldspar dominated sediments north of Scoresby Sund to sediments rich in pyroxene and plagioclase feldspars offshore from the early Tertiary basaltic outcrop. Coarse (<2 mm or <1 mm) ice-rafted sediments are largely absent from the trough sediments between ?8 and 5 cal ka BP, but then increase in the last 4 cal ka BP. Compositional unmixing of the sediments in Grivel Basin and Kangerlussuaq Trough indicate the dominance of local over long distance sediment sources, with pulses of sediment from tidewater glaciers in Kangerlussuaq and Nansen fjords reaching the inner shelf during the Neoglaciation. The change in IRD is more dramatic in the sediment grain-size proxies than in the quartz wt%. Forty to seventy percent of the variance in the quartz records from either side of Denmark Strait is explained by low frequency trends, but the data from the Grivel Basin, East Greenland, are distinctly different, with an approximate 2500 yr periodicity.

(Table 2) d13C from the Lusklint section

The carbon isotope composition (d13C) of bulk organic matter and two palynomorph groups (scolecodonts and chitinozoans) from the Llandovery-Wenlock strata of Gotland (E Sweden) are compared to gain knowledge about carbon cycling in the Silurian (sub)tropical shelf environment. The d13C values of the palynomorphs are mostly lower than the d13C values of the bulk organic matter, and the d13C values of the benthic scolecodonts are lower than those of the planktonic chitinozoans. While the difference between bulk and palynomorph d13C may be in part a function of trophic state, the lower values of the scolecodonts relative to those of chitinozoans, which are assumed to live in the well-mixed water column, might imply an infaunal mode of life for the polychaetes that carried the scolecodonts. Lower d13C for the scolecodonts in the middle of the section may represent variations in primary marine productivity (supported by acritarch abundance data), oxidation of organic matter in the bottom waters, or genera effects. In general, however, trends between the three data sets are parallel, indicating similarities in the low frequency, environmentally forced controls. The d13C data show a decreasing trend from the base of the section, up to a horizon well below the base of the Upper Visby Formation. At this level, and therefore probably several 10 kyr before the d13C increase in the carbonates, the d13C organic values increase by ~1 per mil. This perhaps is an expression of a changed composition of the bulk organic matter associated with the extinction events prior to the Llandovery-Wenlock boundary.