Carbonate-rich Deposits Research Articles

Laser ablation of polyimide at 308 nm: Characterization of ablation products.

XeCl laser ablation of polyimide (PI) films in air and in a vacuum (1.3× 10-4Pa) was studied in terms of etch depth and ablated surfaces. For the fluence conditions of 100-300mJ/cm2, it was found that etch depths obtained in the vacuum were 20-30% greater than those obtained in air. X- ray photoelectron spectroscopy (XPS) showed that carbon-rich surfaces were produced after the irradiation of a single laser pulse in both air and vacuum. However, there were some differences in atomic ratios and bonding environments between the two ablated surfaces. The formation mechanism of these surfaces is discussed with emphasis on the influence of the ambient atmosphere. Furthermore, XPS results showed that more carbon-rich deposits were formed from laser ablation of PI. By mass spectrometry, carbon atoms with/without hydrogen atoms were detected as the main fragments of PI molecules. The chemical structures of deposits suggest the difference in the contribution of the fragments for the deposition process.

Paleomagnetic calibration of Milankovitch cyclicity in Lower Cretaceous sediments

Bedded pelagic limestones of early to mid Cretaceous age may record cyclic climate variability driven by changes in the Earth's orbit (“Milankovitch cycles”). If properly calibrated, the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy. The lower Cretaceous Maiolica Formation of northern and central Italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper M-series marine anomalies. The magnetic reversals divide measured sections into 0.5–2 Myr increments in which sedimentation rate (assuming continous deposition) can be measured to 15–25% accuracy. Paleomagnetically estimated sedimentation rates over a number of polarity zones establish, with one exception, a narrow range in periods of the carbonate cycles. Carbonate couplets have an estimated mean period of 23.5 kyr, and prominent modulations in bedding thickness (“bundling”) have a mean period of 117 kyr (Kent and Gradstein 1985 time scale). The close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing.Comparison of measured sections from different locations shows that bed-to-bed correlations are possible regionally, cosistent with the Milankovitch hypothesis. Episodes of Barremian “black shale” deposition occur in the troughs of the short eccentricity cycle, as roughly one half of the carbonate-marl couplet, or about 10 kyr duration. It is possible to estimate both the timing (relative to the top of reversed chron M-0) and duration of a period of unusual organic carbon-rich deposition (“Selli Horizon”) in the early part of the Cretaceous Long Normal Magnetic Polarity Interval by tracing the sedimentary cycles up from the Barremian strata.

Marginal Seas as Source of an Oceanic Oxygen Minimum and Origin of Organic Carbon-Rich Deposits: ABSTRACT

Cretaceous marginal seas extending across America, Asia, Africa, and Australia connected oceanic areas in different climate zones. Water masses in such regions may have had the same density but very different salinities and temperatures, as is the case with the modern Gulf of Mexico and eastern Arctic Ocean. Cretaceous seaways permitted such different water masses to mix, resulting in a third water mass of greater density. Preliminary calculations indicate that this mechanism may have caused the Western Interior seaway of North America to become a source of intermediate water with a strength equal to the modern Weddell Sea, the source of Antarctic bottom water. Intermediate water production would be maximized at times of maximum transgression. The surface waters of the world ocean would be drawn into both ends of such a seaway and the plankton killed at the front, where mixing occurred, as environmental conditions changed drastically. This introduced large amounts of organic matter into the intermediate water, causing development of an intense oxygen minimum and deposition of large quantities of organic carbon in seas of intermediate depth to form extensive hydrocarbon source beds.